diff --git a/cmd/golangorg/server.go b/cmd/golangorg/server.go
index 41d2c6b..88c8d32 100644
--- a/cmd/golangorg/server.go
+++ b/cmd/golangorg/server.go
@@ -14,7 +14,6 @@
 	"errors"
 	"flag"
 	"fmt"
-	"go/format"
 	"io/fs"
 	"io/ioutil"
 	"log"
@@ -32,6 +31,7 @@
 	"cloud.google.com/go/datastore"
 	"golang.org/x/build/repos"
 	"golang.org/x/website"
+	"golang.org/x/website/internal/backport/go/format"
 	"golang.org/x/website/internal/backport/html/template"
 	"golang.org/x/website/internal/blog"
 	"golang.org/x/website/internal/codewalk"
diff --git a/internal/api/api_test.go b/internal/api/api_test.go
index 98b1dd4..7928627 100644
--- a/internal/api/api_test.go
+++ b/internal/api/api_test.go
@@ -5,10 +5,11 @@
 package api
 
 import (
-	"go/build"
 	"os"
 	"runtime"
 	"testing"
+
+	"golang.org/x/website/internal/backport/go/build"
 )
 
 func TestParseVersionRow(t *testing.T) {
diff --git a/internal/backport/go/ast/ast.go b/internal/backport/go/ast/ast.go
new file mode 100644
index 0000000..279e58e
--- /dev/null
+++ b/internal/backport/go/ast/ast.go
@@ -0,0 +1,1063 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package ast declares the types used to represent syntax trees for Go
+// packages.
+package ast
+
+import (
+	"golang.org/x/website/internal/backport/go/token"
+	"strings"
+)
+
+// ----------------------------------------------------------------------------
+// Interfaces
+//
+// There are 3 main classes of nodes: Expressions and type nodes,
+// statement nodes, and declaration nodes. The node names usually
+// match the corresponding Go spec production names to which they
+// correspond. The node fields correspond to the individual parts
+// of the respective productions.
+//
+// All nodes contain position information marking the beginning of
+// the corresponding source text segment; it is accessible via the
+// Pos accessor method. Nodes may contain additional position info
+// for language constructs where comments may be found between parts
+// of the construct (typically any larger, parenthesized subpart).
+// That position information is needed to properly position comments
+// when printing the construct.
+
+// All node types implement the Node interface.
+type Node interface {
+	Pos() token.Pos // position of first character belonging to the node
+	End() token.Pos // position of first character immediately after the node
+}
+
+// All expression nodes implement the Expr interface.
+type Expr interface {
+	Node
+	exprNode()
+}
+
+// All statement nodes implement the Stmt interface.
+type Stmt interface {
+	Node
+	stmtNode()
+}
+
+// All declaration nodes implement the Decl interface.
+type Decl interface {
+	Node
+	declNode()
+}
+
+// ----------------------------------------------------------------------------
+// Comments
+
+// A Comment node represents a single //-style or /*-style comment.
+//
+// The Text field contains the comment text without carriage returns (\r) that
+// may have been present in the source. Because a comment's end position is
+// computed using len(Text), the position reported by End() does not match the
+// true source end position for comments containing carriage returns.
+type Comment struct {
+	Slash token.Pos // position of "/" starting the comment
+	Text  string    // comment text (excluding '\n' for //-style comments)
+}
+
+func (c *Comment) Pos() token.Pos { return c.Slash }
+func (c *Comment) End() token.Pos { return token.Pos(int(c.Slash) + len(c.Text)) }
+
+// A CommentGroup represents a sequence of comments
+// with no other tokens and no empty lines between.
+type CommentGroup struct {
+	List []*Comment // len(List) > 0
+}
+
+func (g *CommentGroup) Pos() token.Pos { return g.List[0].Pos() }
+func (g *CommentGroup) End() token.Pos { return g.List[len(g.List)-1].End() }
+
+func isWhitespace(ch byte) bool { return ch == ' ' || ch == '\t' || ch == '\n' || ch == '\r' }
+
+func stripTrailingWhitespace(s string) string {
+	i := len(s)
+	for i > 0 && isWhitespace(s[i-1]) {
+		i--
+	}
+	return s[0:i]
+}
+
+// Text returns the text of the comment.
+// Comment markers (//, /*, and */), the first space of a line comment, and
+// leading and trailing empty lines are removed.
+// Comment directives like "//line" and "//go:noinline" are also removed.
+// Multiple empty lines are reduced to one, and trailing space on lines is trimmed.
+// Unless the result is empty, it is newline-terminated.
+func (g *CommentGroup) Text() string {
+	if g == nil {
+		return ""
+	}
+	comments := make([]string, len(g.List))
+	for i, c := range g.List {
+		comments[i] = c.Text
+	}
+
+	lines := make([]string, 0, 10) // most comments are less than 10 lines
+	for _, c := range comments {
+		// Remove comment markers.
+		// The parser has given us exactly the comment text.
+		switch c[1] {
+		case '/':
+			//-style comment (no newline at the end)
+			c = c[2:]
+			if len(c) == 0 {
+				// empty line
+				break
+			}
+			if c[0] == ' ' {
+				// strip first space - required for Example tests
+				c = c[1:]
+				break
+			}
+			if isDirective(c) {
+				// Ignore //go:noinline, //line, and so on.
+				continue
+			}
+		case '*':
+			/*-style comment */
+			c = c[2 : len(c)-2]
+		}
+
+		// Split on newlines.
+		cl := strings.Split(c, "\n")
+
+		// Walk lines, stripping trailing white space and adding to list.
+		for _, l := range cl {
+			lines = append(lines, stripTrailingWhitespace(l))
+		}
+	}
+
+	// Remove leading blank lines; convert runs of
+	// interior blank lines to a single blank line.
+	n := 0
+	for _, line := range lines {
+		if line != "" || n > 0 && lines[n-1] != "" {
+			lines[n] = line
+			n++
+		}
+	}
+	lines = lines[0:n]
+
+	// Add final "" entry to get trailing newline from Join.
+	if n > 0 && lines[n-1] != "" {
+		lines = append(lines, "")
+	}
+
+	return strings.Join(lines, "\n")
+}
+
+// isDirective reports whether c is a comment directive.
+func isDirective(c string) bool {
+	// "//line " is a line directive.
+	// (The // has been removed.)
+	if strings.HasPrefix(c, "line ") {
+		return true
+	}
+
+	// "//[a-z0-9]+:[a-z0-9]"
+	// (The // has been removed.)
+	colon := strings.Index(c, ":")
+	if colon <= 0 || colon+1 >= len(c) {
+		return false
+	}
+	for i := 0; i <= colon+1; i++ {
+		if i == colon {
+			continue
+		}
+		b := c[i]
+		if !('a' <= b && b <= 'z' || '0' <= b && b <= '9') {
+			return false
+		}
+	}
+	return true
+}
+
+// ----------------------------------------------------------------------------
+// Expressions and types
+
+// A Field represents a Field declaration list in a struct type,
+// a method list in an interface type, or a parameter/result declaration
+// in a signature.
+// Field.Names is nil for unnamed parameters (parameter lists which only contain types)
+// and embedded struct fields. In the latter case, the field name is the type name.
+type Field struct {
+	Doc     *CommentGroup // associated documentation; or nil
+	Names   []*Ident      // field/method/(type) parameter names; or nil
+	Type    Expr          // field/method/parameter type; or nil
+	Tag     *BasicLit     // field tag; or nil
+	Comment *CommentGroup // line comments; or nil
+}
+
+func (f *Field) Pos() token.Pos {
+	if len(f.Names) > 0 {
+		return f.Names[0].Pos()
+	}
+	if f.Type != nil {
+		return f.Type.Pos()
+	}
+	return token.NoPos
+}
+
+func (f *Field) End() token.Pos {
+	if f.Tag != nil {
+		return f.Tag.End()
+	}
+	if f.Type != nil {
+		return f.Type.End()
+	}
+	if len(f.Names) > 0 {
+		return f.Names[len(f.Names)-1].End()
+	}
+	return token.NoPos
+}
+
+// A FieldList represents a list of Fields, enclosed by parentheses,
+// curly braces, or square brackets.
+type FieldList struct {
+	Opening token.Pos // position of opening parenthesis/brace/bracket, if any
+	List    []*Field  // field list; or nil
+	Closing token.Pos // position of closing parenthesis/brace/bracket, if any
+}
+
+func (f *FieldList) Pos() token.Pos {
+	if f.Opening.IsValid() {
+		return f.Opening
+	}
+	// the list should not be empty in this case;
+	// be conservative and guard against bad ASTs
+	if len(f.List) > 0 {
+		return f.List[0].Pos()
+	}
+	return token.NoPos
+}
+
+func (f *FieldList) End() token.Pos {
+	if f.Closing.IsValid() {
+		return f.Closing + 1
+	}
+	// the list should not be empty in this case;
+	// be conservative and guard against bad ASTs
+	if n := len(f.List); n > 0 {
+		return f.List[n-1].End()
+	}
+	return token.NoPos
+}
+
+// NumFields returns the number of parameters or struct fields represented by a FieldList.
+func (f *FieldList) NumFields() int {
+	n := 0
+	if f != nil {
+		for _, g := range f.List {
+			m := len(g.Names)
+			if m == 0 {
+				m = 1
+			}
+			n += m
+		}
+	}
+	return n
+}
+
+// An expression is represented by a tree consisting of one
+// or more of the following concrete expression nodes.
+type (
+	// A BadExpr node is a placeholder for an expression containing
+	// syntax errors for which a correct expression node cannot be
+	// created.
+	//
+	BadExpr struct {
+		From, To token.Pos // position range of bad expression
+	}
+
+	// An Ident node represents an identifier.
+	Ident struct {
+		NamePos token.Pos // identifier position
+		Name    string    // identifier name
+		Obj     *Object   // denoted object; or nil
+	}
+
+	// An Ellipsis node stands for the "..." type in a
+	// parameter list or the "..." length in an array type.
+	//
+	Ellipsis struct {
+		Ellipsis token.Pos // position of "..."
+		Elt      Expr      // ellipsis element type (parameter lists only); or nil
+	}
+
+	// A BasicLit node represents a literal of basic type.
+	BasicLit struct {
+		ValuePos token.Pos   // literal position
+		Kind     token.Token // token.INT, token.FLOAT, token.IMAG, token.CHAR, or token.STRING
+		Value    string      // literal string; e.g. 42, 0x7f, 3.14, 1e-9, 2.4i, 'a', '\x7f', "foo" or `\m\n\o`
+	}
+
+	// A FuncLit node represents a function literal.
+	FuncLit struct {
+		Type *FuncType  // function type
+		Body *BlockStmt // function body
+	}
+
+	// A CompositeLit node represents a composite literal.
+	CompositeLit struct {
+		Type       Expr      // literal type; or nil
+		Lbrace     token.Pos // position of "{"
+		Elts       []Expr    // list of composite elements; or nil
+		Rbrace     token.Pos // position of "}"
+		Incomplete bool      // true if (source) expressions are missing in the Elts list
+	}
+
+	// A ParenExpr node represents a parenthesized expression.
+	ParenExpr struct {
+		Lparen token.Pos // position of "("
+		X      Expr      // parenthesized expression
+		Rparen token.Pos // position of ")"
+	}
+
+	// A SelectorExpr node represents an expression followed by a selector.
+	SelectorExpr struct {
+		X   Expr   // expression
+		Sel *Ident // field selector
+	}
+
+	// An IndexExpr node represents an expression followed by an index.
+	IndexExpr struct {
+		X      Expr      // expression
+		Lbrack token.Pos // position of "["
+		Index  Expr      // index expression
+		Rbrack token.Pos // position of "]"
+	}
+
+	// An IndexListExpr node represents an expression followed by multiple
+	// indices.
+	IndexListExpr struct {
+		X       Expr      // expression
+		Lbrack  token.Pos // position of "["
+		Indices []Expr    // index expressions
+		Rbrack  token.Pos // position of "]"
+	}
+
+	// A SliceExpr node represents an expression followed by slice indices.
+	SliceExpr struct {
+		X      Expr      // expression
+		Lbrack token.Pos // position of "["
+		Low    Expr      // begin of slice range; or nil
+		High   Expr      // end of slice range; or nil
+		Max    Expr      // maximum capacity of slice; or nil
+		Slice3 bool      // true if 3-index slice (2 colons present)
+		Rbrack token.Pos // position of "]"
+	}
+
+	// A TypeAssertExpr node represents an expression followed by a
+	// type assertion.
+	//
+	TypeAssertExpr struct {
+		X      Expr      // expression
+		Lparen token.Pos // position of "("
+		Type   Expr      // asserted type; nil means type switch X.(type)
+		Rparen token.Pos // position of ")"
+	}
+
+	// A CallExpr node represents an expression followed by an argument list.
+	CallExpr struct {
+		Fun      Expr      // function expression
+		Lparen   token.Pos // position of "("
+		Args     []Expr    // function arguments; or nil
+		Ellipsis token.Pos // position of "..." (token.NoPos if there is no "...")
+		Rparen   token.Pos // position of ")"
+	}
+
+	// A StarExpr node represents an expression of the form "*" Expression.
+	// Semantically it could be a unary "*" expression, or a pointer type.
+	//
+	StarExpr struct {
+		Star token.Pos // position of "*"
+		X    Expr      // operand
+	}
+
+	// A UnaryExpr node represents a unary expression.
+	// Unary "*" expressions are represented via StarExpr nodes.
+	//
+	UnaryExpr struct {
+		OpPos token.Pos   // position of Op
+		Op    token.Token // operator
+		X     Expr        // operand
+	}
+
+	// A BinaryExpr node represents a binary expression.
+	BinaryExpr struct {
+		X     Expr        // left operand
+		OpPos token.Pos   // position of Op
+		Op    token.Token // operator
+		Y     Expr        // right operand
+	}
+
+	// A KeyValueExpr node represents (key : value) pairs
+	// in composite literals.
+	//
+	KeyValueExpr struct {
+		Key   Expr
+		Colon token.Pos // position of ":"
+		Value Expr
+	}
+)
+
+// The direction of a channel type is indicated by a bit
+// mask including one or both of the following constants.
+type ChanDir int
+
+const (
+	SEND ChanDir = 1 << iota
+	RECV
+)
+
+// A type is represented by a tree consisting of one
+// or more of the following type-specific expression
+// nodes.
+type (
+	// An ArrayType node represents an array or slice type.
+	ArrayType struct {
+		Lbrack token.Pos // position of "["
+		Len    Expr      // Ellipsis node for [...]T array types, nil for slice types
+		Elt    Expr      // element type
+	}
+
+	// A StructType node represents a struct type.
+	StructType struct {
+		Struct     token.Pos  // position of "struct" keyword
+		Fields     *FieldList // list of field declarations
+		Incomplete bool       // true if (source) fields are missing in the Fields list
+	}
+
+	// Pointer types are represented via StarExpr nodes.
+
+	// A FuncType node represents a function type.
+	FuncType struct {
+		Func       token.Pos  // position of "func" keyword (token.NoPos if there is no "func")
+		TypeParams *FieldList // type parameters; or nil
+		Params     *FieldList // (incoming) parameters; non-nil
+		Results    *FieldList // (outgoing) results; or nil
+	}
+
+	// An InterfaceType node represents an interface type.
+	InterfaceType struct {
+		Interface  token.Pos  // position of "interface" keyword
+		Methods    *FieldList // list of embedded interfaces, methods, or types
+		Incomplete bool       // true if (source) methods or types are missing in the Methods list
+	}
+
+	// A MapType node represents a map type.
+	MapType struct {
+		Map   token.Pos // position of "map" keyword
+		Key   Expr
+		Value Expr
+	}
+
+	// A ChanType node represents a channel type.
+	ChanType struct {
+		Begin token.Pos // position of "chan" keyword or "<-" (whichever comes first)
+		Arrow token.Pos // position of "<-" (token.NoPos if there is no "<-")
+		Dir   ChanDir   // channel direction
+		Value Expr      // value type
+	}
+)
+
+// Pos and End implementations for expression/type nodes.
+
+func (x *BadExpr) Pos() token.Pos  { return x.From }
+func (x *Ident) Pos() token.Pos    { return x.NamePos }
+func (x *Ellipsis) Pos() token.Pos { return x.Ellipsis }
+func (x *BasicLit) Pos() token.Pos { return x.ValuePos }
+func (x *FuncLit) Pos() token.Pos  { return x.Type.Pos() }
+func (x *CompositeLit) Pos() token.Pos {
+	if x.Type != nil {
+		return x.Type.Pos()
+	}
+	return x.Lbrace
+}
+func (x *ParenExpr) Pos() token.Pos      { return x.Lparen }
+func (x *SelectorExpr) Pos() token.Pos   { return x.X.Pos() }
+func (x *IndexExpr) Pos() token.Pos      { return x.X.Pos() }
+func (x *IndexListExpr) Pos() token.Pos  { return x.X.Pos() }
+func (x *SliceExpr) Pos() token.Pos      { return x.X.Pos() }
+func (x *TypeAssertExpr) Pos() token.Pos { return x.X.Pos() }
+func (x *CallExpr) Pos() token.Pos       { return x.Fun.Pos() }
+func (x *StarExpr) Pos() token.Pos       { return x.Star }
+func (x *UnaryExpr) Pos() token.Pos      { return x.OpPos }
+func (x *BinaryExpr) Pos() token.Pos     { return x.X.Pos() }
+func (x *KeyValueExpr) Pos() token.Pos   { return x.Key.Pos() }
+func (x *ArrayType) Pos() token.Pos      { return x.Lbrack }
+func (x *StructType) Pos() token.Pos     { return x.Struct }
+func (x *FuncType) Pos() token.Pos {
+	if x.Func.IsValid() || x.Params == nil { // see issue 3870
+		return x.Func
+	}
+	return x.Params.Pos() // interface method declarations have no "func" keyword
+}
+func (x *InterfaceType) Pos() token.Pos { return x.Interface }
+func (x *MapType) Pos() token.Pos       { return x.Map }
+func (x *ChanType) Pos() token.Pos      { return x.Begin }
+
+func (x *BadExpr) End() token.Pos { return x.To }
+func (x *Ident) End() token.Pos   { return token.Pos(int(x.NamePos) + len(x.Name)) }
+func (x *Ellipsis) End() token.Pos {
+	if x.Elt != nil {
+		return x.Elt.End()
+	}
+	return x.Ellipsis + 3 // len("...")
+}
+func (x *BasicLit) End() token.Pos       { return token.Pos(int(x.ValuePos) + len(x.Value)) }
+func (x *FuncLit) End() token.Pos        { return x.Body.End() }
+func (x *CompositeLit) End() token.Pos   { return x.Rbrace + 1 }
+func (x *ParenExpr) End() token.Pos      { return x.Rparen + 1 }
+func (x *SelectorExpr) End() token.Pos   { return x.Sel.End() }
+func (x *IndexExpr) End() token.Pos      { return x.Rbrack + 1 }
+func (x *IndexListExpr) End() token.Pos  { return x.Rbrack + 1 }
+func (x *SliceExpr) End() token.Pos      { return x.Rbrack + 1 }
+func (x *TypeAssertExpr) End() token.Pos { return x.Rparen + 1 }
+func (x *CallExpr) End() token.Pos       { return x.Rparen + 1 }
+func (x *StarExpr) End() token.Pos       { return x.X.End() }
+func (x *UnaryExpr) End() token.Pos      { return x.X.End() }
+func (x *BinaryExpr) End() token.Pos     { return x.Y.End() }
+func (x *KeyValueExpr) End() token.Pos   { return x.Value.End() }
+func (x *ArrayType) End() token.Pos      { return x.Elt.End() }
+func (x *StructType) End() token.Pos     { return x.Fields.End() }
+func (x *FuncType) End() token.Pos {
+	if x.Results != nil {
+		return x.Results.End()
+	}
+	return x.Params.End()
+}
+func (x *InterfaceType) End() token.Pos { return x.Methods.End() }
+func (x *MapType) End() token.Pos       { return x.Value.End() }
+func (x *ChanType) End() token.Pos      { return x.Value.End() }
+
+// exprNode() ensures that only expression/type nodes can be
+// assigned to an Expr.
+func (*BadExpr) exprNode()        {}
+func (*Ident) exprNode()          {}
+func (*Ellipsis) exprNode()       {}
+func (*BasicLit) exprNode()       {}
+func (*FuncLit) exprNode()        {}
+func (*CompositeLit) exprNode()   {}
+func (*ParenExpr) exprNode()      {}
+func (*SelectorExpr) exprNode()   {}
+func (*IndexExpr) exprNode()      {}
+func (*IndexListExpr) exprNode()  {}
+func (*SliceExpr) exprNode()      {}
+func (*TypeAssertExpr) exprNode() {}
+func (*CallExpr) exprNode()       {}
+func (*StarExpr) exprNode()       {}
+func (*UnaryExpr) exprNode()      {}
+func (*BinaryExpr) exprNode()     {}
+func (*KeyValueExpr) exprNode()   {}
+
+func (*ArrayType) exprNode()     {}
+func (*StructType) exprNode()    {}
+func (*FuncType) exprNode()      {}
+func (*InterfaceType) exprNode() {}
+func (*MapType) exprNode()       {}
+func (*ChanType) exprNode()      {}
+
+// ----------------------------------------------------------------------------
+// Convenience functions for Idents
+
+// NewIdent creates a new Ident without position.
+// Useful for ASTs generated by code other than the Go parser.
+func NewIdent(name string) *Ident { return &Ident{token.NoPos, name, nil} }
+
+// IsExported reports whether name starts with an upper-case letter.
+func IsExported(name string) bool { return token.IsExported(name) }
+
+// IsExported reports whether id starts with an upper-case letter.
+func (id *Ident) IsExported() bool { return token.IsExported(id.Name) }
+
+func (id *Ident) String() string {
+	if id != nil {
+		return id.Name
+	}
+	return "<nil>"
+}
+
+// ----------------------------------------------------------------------------
+// Statements
+
+// A statement is represented by a tree consisting of one
+// or more of the following concrete statement nodes.
+type (
+	// A BadStmt node is a placeholder for statements containing
+	// syntax errors for which no correct statement nodes can be
+	// created.
+	//
+	BadStmt struct {
+		From, To token.Pos // position range of bad statement
+	}
+
+	// A DeclStmt node represents a declaration in a statement list.
+	DeclStmt struct {
+		Decl Decl // *GenDecl with CONST, TYPE, or VAR token
+	}
+
+	// An EmptyStmt node represents an empty statement.
+	// The "position" of the empty statement is the position
+	// of the immediately following (explicit or implicit) semicolon.
+	//
+	EmptyStmt struct {
+		Semicolon token.Pos // position of following ";"
+		Implicit  bool      // if set, ";" was omitted in the source
+	}
+
+	// A LabeledStmt node represents a labeled statement.
+	LabeledStmt struct {
+		Label *Ident
+		Colon token.Pos // position of ":"
+		Stmt  Stmt
+	}
+
+	// An ExprStmt node represents a (stand-alone) expression
+	// in a statement list.
+	//
+	ExprStmt struct {
+		X Expr // expression
+	}
+
+	// A SendStmt node represents a send statement.
+	SendStmt struct {
+		Chan  Expr
+		Arrow token.Pos // position of "<-"
+		Value Expr
+	}
+
+	// An IncDecStmt node represents an increment or decrement statement.
+	IncDecStmt struct {
+		X      Expr
+		TokPos token.Pos   // position of Tok
+		Tok    token.Token // INC or DEC
+	}
+
+	// An AssignStmt node represents an assignment or
+	// a short variable declaration.
+	//
+	AssignStmt struct {
+		Lhs    []Expr
+		TokPos token.Pos   // position of Tok
+		Tok    token.Token // assignment token, DEFINE
+		Rhs    []Expr
+	}
+
+	// A GoStmt node represents a go statement.
+	GoStmt struct {
+		Go   token.Pos // position of "go" keyword
+		Call *CallExpr
+	}
+
+	// A DeferStmt node represents a defer statement.
+	DeferStmt struct {
+		Defer token.Pos // position of "defer" keyword
+		Call  *CallExpr
+	}
+
+	// A ReturnStmt node represents a return statement.
+	ReturnStmt struct {
+		Return  token.Pos // position of "return" keyword
+		Results []Expr    // result expressions; or nil
+	}
+
+	// A BranchStmt node represents a break, continue, goto,
+	// or fallthrough statement.
+	//
+	BranchStmt struct {
+		TokPos token.Pos   // position of Tok
+		Tok    token.Token // keyword token (BREAK, CONTINUE, GOTO, FALLTHROUGH)
+		Label  *Ident      // label name; or nil
+	}
+
+	// A BlockStmt node represents a braced statement list.
+	BlockStmt struct {
+		Lbrace token.Pos // position of "{"
+		List   []Stmt
+		Rbrace token.Pos // position of "}", if any (may be absent due to syntax error)
+	}
+
+	// An IfStmt node represents an if statement.
+	IfStmt struct {
+		If   token.Pos // position of "if" keyword
+		Init Stmt      // initialization statement; or nil
+		Cond Expr      // condition
+		Body *BlockStmt
+		Else Stmt // else branch; or nil
+	}
+
+	// A CaseClause represents a case of an expression or type switch statement.
+	CaseClause struct {
+		Case  token.Pos // position of "case" or "default" keyword
+		List  []Expr    // list of expressions or types; nil means default case
+		Colon token.Pos // position of ":"
+		Body  []Stmt    // statement list; or nil
+	}
+
+	// A SwitchStmt node represents an expression switch statement.
+	SwitchStmt struct {
+		Switch token.Pos  // position of "switch" keyword
+		Init   Stmt       // initialization statement; or nil
+		Tag    Expr       // tag expression; or nil
+		Body   *BlockStmt // CaseClauses only
+	}
+
+	// A TypeSwitchStmt node represents a type switch statement.
+	TypeSwitchStmt struct {
+		Switch token.Pos  // position of "switch" keyword
+		Init   Stmt       // initialization statement; or nil
+		Assign Stmt       // x := y.(type) or y.(type)
+		Body   *BlockStmt // CaseClauses only
+	}
+
+	// A CommClause node represents a case of a select statement.
+	CommClause struct {
+		Case  token.Pos // position of "case" or "default" keyword
+		Comm  Stmt      // send or receive statement; nil means default case
+		Colon token.Pos // position of ":"
+		Body  []Stmt    // statement list; or nil
+	}
+
+	// A SelectStmt node represents a select statement.
+	SelectStmt struct {
+		Select token.Pos  // position of "select" keyword
+		Body   *BlockStmt // CommClauses only
+	}
+
+	// A ForStmt represents a for statement.
+	ForStmt struct {
+		For  token.Pos // position of "for" keyword
+		Init Stmt      // initialization statement; or nil
+		Cond Expr      // condition; or nil
+		Post Stmt      // post iteration statement; or nil
+		Body *BlockStmt
+	}
+
+	// A RangeStmt represents a for statement with a range clause.
+	RangeStmt struct {
+		For        token.Pos   // position of "for" keyword
+		Key, Value Expr        // Key, Value may be nil
+		TokPos     token.Pos   // position of Tok; invalid if Key == nil
+		Tok        token.Token // ILLEGAL if Key == nil, ASSIGN, DEFINE
+		X          Expr        // value to range over
+		Body       *BlockStmt
+	}
+)
+
+// Pos and End implementations for statement nodes.
+
+func (s *BadStmt) Pos() token.Pos        { return s.From }
+func (s *DeclStmt) Pos() token.Pos       { return s.Decl.Pos() }
+func (s *EmptyStmt) Pos() token.Pos      { return s.Semicolon }
+func (s *LabeledStmt) Pos() token.Pos    { return s.Label.Pos() }
+func (s *ExprStmt) Pos() token.Pos       { return s.X.Pos() }
+func (s *SendStmt) Pos() token.Pos       { return s.Chan.Pos() }
+func (s *IncDecStmt) Pos() token.Pos     { return s.X.Pos() }
+func (s *AssignStmt) Pos() token.Pos     { return s.Lhs[0].Pos() }
+func (s *GoStmt) Pos() token.Pos         { return s.Go }
+func (s *DeferStmt) Pos() token.Pos      { return s.Defer }
+func (s *ReturnStmt) Pos() token.Pos     { return s.Return }
+func (s *BranchStmt) Pos() token.Pos     { return s.TokPos }
+func (s *BlockStmt) Pos() token.Pos      { return s.Lbrace }
+func (s *IfStmt) Pos() token.Pos         { return s.If }
+func (s *CaseClause) Pos() token.Pos     { return s.Case }
+func (s *SwitchStmt) Pos() token.Pos     { return s.Switch }
+func (s *TypeSwitchStmt) Pos() token.Pos { return s.Switch }
+func (s *CommClause) Pos() token.Pos     { return s.Case }
+func (s *SelectStmt) Pos() token.Pos     { return s.Select }
+func (s *ForStmt) Pos() token.Pos        { return s.For }
+func (s *RangeStmt) Pos() token.Pos      { return s.For }
+
+func (s *BadStmt) End() token.Pos  { return s.To }
+func (s *DeclStmt) End() token.Pos { return s.Decl.End() }
+func (s *EmptyStmt) End() token.Pos {
+	if s.Implicit {
+		return s.Semicolon
+	}
+	return s.Semicolon + 1 /* len(";") */
+}
+func (s *LabeledStmt) End() token.Pos { return s.Stmt.End() }
+func (s *ExprStmt) End() token.Pos    { return s.X.End() }
+func (s *SendStmt) End() token.Pos    { return s.Value.End() }
+func (s *IncDecStmt) End() token.Pos {
+	return s.TokPos + 2 /* len("++") */
+}
+func (s *AssignStmt) End() token.Pos { return s.Rhs[len(s.Rhs)-1].End() }
+func (s *GoStmt) End() token.Pos     { return s.Call.End() }
+func (s *DeferStmt) End() token.Pos  { return s.Call.End() }
+func (s *ReturnStmt) End() token.Pos {
+	if n := len(s.Results); n > 0 {
+		return s.Results[n-1].End()
+	}
+	return s.Return + 6 // len("return")
+}
+func (s *BranchStmt) End() token.Pos {
+	if s.Label != nil {
+		return s.Label.End()
+	}
+	return token.Pos(int(s.TokPos) + len(s.Tok.String()))
+}
+func (s *BlockStmt) End() token.Pos {
+	if s.Rbrace.IsValid() {
+		return s.Rbrace + 1
+	}
+	if n := len(s.List); n > 0 {
+		return s.List[n-1].End()
+	}
+	return s.Lbrace + 1
+}
+func (s *IfStmt) End() token.Pos {
+	if s.Else != nil {
+		return s.Else.End()
+	}
+	return s.Body.End()
+}
+func (s *CaseClause) End() token.Pos {
+	if n := len(s.Body); n > 0 {
+		return s.Body[n-1].End()
+	}
+	return s.Colon + 1
+}
+func (s *SwitchStmt) End() token.Pos     { return s.Body.End() }
+func (s *TypeSwitchStmt) End() token.Pos { return s.Body.End() }
+func (s *CommClause) End() token.Pos {
+	if n := len(s.Body); n > 0 {
+		return s.Body[n-1].End()
+	}
+	return s.Colon + 1
+}
+func (s *SelectStmt) End() token.Pos { return s.Body.End() }
+func (s *ForStmt) End() token.Pos    { return s.Body.End() }
+func (s *RangeStmt) End() token.Pos  { return s.Body.End() }
+
+// stmtNode() ensures that only statement nodes can be
+// assigned to a Stmt.
+func (*BadStmt) stmtNode()        {}
+func (*DeclStmt) stmtNode()       {}
+func (*EmptyStmt) stmtNode()      {}
+func (*LabeledStmt) stmtNode()    {}
+func (*ExprStmt) stmtNode()       {}
+func (*SendStmt) stmtNode()       {}
+func (*IncDecStmt) stmtNode()     {}
+func (*AssignStmt) stmtNode()     {}
+func (*GoStmt) stmtNode()         {}
+func (*DeferStmt) stmtNode()      {}
+func (*ReturnStmt) stmtNode()     {}
+func (*BranchStmt) stmtNode()     {}
+func (*BlockStmt) stmtNode()      {}
+func (*IfStmt) stmtNode()         {}
+func (*CaseClause) stmtNode()     {}
+func (*SwitchStmt) stmtNode()     {}
+func (*TypeSwitchStmt) stmtNode() {}
+func (*CommClause) stmtNode()     {}
+func (*SelectStmt) stmtNode()     {}
+func (*ForStmt) stmtNode()        {}
+func (*RangeStmt) stmtNode()      {}
+
+// ----------------------------------------------------------------------------
+// Declarations
+
+// A Spec node represents a single (non-parenthesized) import,
+// constant, type, or variable declaration.
+type (
+	// The Spec type stands for any of *ImportSpec, *ValueSpec, and *TypeSpec.
+	Spec interface {
+		Node
+		specNode()
+	}
+
+	// An ImportSpec node represents a single package import.
+	ImportSpec struct {
+		Doc     *CommentGroup // associated documentation; or nil
+		Name    *Ident        // local package name (including "."); or nil
+		Path    *BasicLit     // import path
+		Comment *CommentGroup // line comments; or nil
+		EndPos  token.Pos     // end of spec (overrides Path.Pos if nonzero)
+	}
+
+	// A ValueSpec node represents a constant or variable declaration
+	// (ConstSpec or VarSpec production).
+	//
+	ValueSpec struct {
+		Doc     *CommentGroup // associated documentation; or nil
+		Names   []*Ident      // value names (len(Names) > 0)
+		Type    Expr          // value type; or nil
+		Values  []Expr        // initial values; or nil
+		Comment *CommentGroup // line comments; or nil
+	}
+
+	// A TypeSpec node represents a type declaration (TypeSpec production).
+	TypeSpec struct {
+		Doc        *CommentGroup // associated documentation; or nil
+		Name       *Ident        // type name
+		TypeParams *FieldList    // type parameters; or nil
+		Assign     token.Pos     // position of '=', if any
+		Type       Expr          // *Ident, *ParenExpr, *SelectorExpr, *StarExpr, or any of the *XxxTypes
+		Comment    *CommentGroup // line comments; or nil
+	}
+)
+
+// Pos and End implementations for spec nodes.
+
+func (s *ImportSpec) Pos() token.Pos {
+	if s.Name != nil {
+		return s.Name.Pos()
+	}
+	return s.Path.Pos()
+}
+func (s *ValueSpec) Pos() token.Pos { return s.Names[0].Pos() }
+func (s *TypeSpec) Pos() token.Pos  { return s.Name.Pos() }
+
+func (s *ImportSpec) End() token.Pos {
+	if s.EndPos != 0 {
+		return s.EndPos
+	}
+	return s.Path.End()
+}
+
+func (s *ValueSpec) End() token.Pos {
+	if n := len(s.Values); n > 0 {
+		return s.Values[n-1].End()
+	}
+	if s.Type != nil {
+		return s.Type.End()
+	}
+	return s.Names[len(s.Names)-1].End()
+}
+func (s *TypeSpec) End() token.Pos { return s.Type.End() }
+
+// specNode() ensures that only spec nodes can be
+// assigned to a Spec.
+func (*ImportSpec) specNode() {}
+func (*ValueSpec) specNode()  {}
+func (*TypeSpec) specNode()   {}
+
+// A declaration is represented by one of the following declaration nodes.
+type (
+	// A BadDecl node is a placeholder for a declaration containing
+	// syntax errors for which a correct declaration node cannot be
+	// created.
+	//
+	BadDecl struct {
+		From, To token.Pos // position range of bad declaration
+	}
+
+	// A GenDecl node (generic declaration node) represents an import,
+	// constant, type or variable declaration. A valid Lparen position
+	// (Lparen.IsValid()) indicates a parenthesized declaration.
+	//
+	// Relationship between Tok value and Specs element type:
+	//
+	//	token.IMPORT  *ImportSpec
+	//	token.CONST   *ValueSpec
+	//	token.TYPE    *TypeSpec
+	//	token.VAR     *ValueSpec
+	//
+	GenDecl struct {
+		Doc    *CommentGroup // associated documentation; or nil
+		TokPos token.Pos     // position of Tok
+		Tok    token.Token   // IMPORT, CONST, TYPE, or VAR
+		Lparen token.Pos     // position of '(', if any
+		Specs  []Spec
+		Rparen token.Pos // position of ')', if any
+	}
+
+	// A FuncDecl node represents a function declaration.
+	FuncDecl struct {
+		Doc  *CommentGroup // associated documentation; or nil
+		Recv *FieldList    // receiver (methods); or nil (functions)
+		Name *Ident        // function/method name
+		Type *FuncType     // function signature: type and value parameters, results, and position of "func" keyword
+		Body *BlockStmt    // function body; or nil for external (non-Go) function
+	}
+)
+
+// Pos and End implementations for declaration nodes.
+
+func (d *BadDecl) Pos() token.Pos  { return d.From }
+func (d *GenDecl) Pos() token.Pos  { return d.TokPos }
+func (d *FuncDecl) Pos() token.Pos { return d.Type.Pos() }
+
+func (d *BadDecl) End() token.Pos { return d.To }
+func (d *GenDecl) End() token.Pos {
+	if d.Rparen.IsValid() {
+		return d.Rparen + 1
+	}
+	return d.Specs[0].End()
+}
+func (d *FuncDecl) End() token.Pos {
+	if d.Body != nil {
+		return d.Body.End()
+	}
+	return d.Type.End()
+}
+
+// declNode() ensures that only declaration nodes can be
+// assigned to a Decl.
+func (*BadDecl) declNode()  {}
+func (*GenDecl) declNode()  {}
+func (*FuncDecl) declNode() {}
+
+// ----------------------------------------------------------------------------
+// Files and packages
+
+// A File node represents a Go source file.
+//
+// The Comments list contains all comments in the source file in order of
+// appearance, including the comments that are pointed to from other nodes
+// via Doc and Comment fields.
+//
+// For correct printing of source code containing comments (using packages
+// go/format and go/printer), special care must be taken to update comments
+// when a File's syntax tree is modified: For printing, comments are interspersed
+// between tokens based on their position. If syntax tree nodes are
+// removed or moved, relevant comments in their vicinity must also be removed
+// (from the File.Comments list) or moved accordingly (by updating their
+// positions). A CommentMap may be used to facilitate some of these operations.
+//
+// Whether and how a comment is associated with a node depends on the
+// interpretation of the syntax tree by the manipulating program: Except for Doc
+// and Comment comments directly associated with nodes, the remaining comments
+// are "free-floating" (see also issues #18593, #20744).
+type File struct {
+	Doc        *CommentGroup   // associated documentation; or nil
+	Package    token.Pos       // position of "package" keyword
+	Name       *Ident          // package name
+	Decls      []Decl          // top-level declarations; or nil
+	Scope      *Scope          // package scope (this file only)
+	Imports    []*ImportSpec   // imports in this file
+	Unresolved []*Ident        // unresolved identifiers in this file
+	Comments   []*CommentGroup // list of all comments in the source file
+}
+
+func (f *File) Pos() token.Pos { return f.Package }
+func (f *File) End() token.Pos {
+	if n := len(f.Decls); n > 0 {
+		return f.Decls[n-1].End()
+	}
+	return f.Name.End()
+}
+
+// A Package node represents a set of source files
+// collectively building a Go package.
+type Package struct {
+	Name    string             // package name
+	Scope   *Scope             // package scope across all files
+	Imports map[string]*Object // map of package id -> package object
+	Files   map[string]*File   // Go source files by filename
+}
+
+func (p *Package) Pos() token.Pos { return token.NoPos }
+func (p *Package) End() token.Pos { return token.NoPos }
diff --git a/internal/backport/go/ast/ast_test.go b/internal/backport/go/ast/ast_test.go
new file mode 100644
index 0000000..71b2d6c
--- /dev/null
+++ b/internal/backport/go/ast/ast_test.go
@@ -0,0 +1,79 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"testing"
+)
+
+var comments = []struct {
+	list []string
+	text string
+}{
+	{[]string{"//"}, ""},
+	{[]string{"//   "}, ""},
+	{[]string{"//", "//", "//   "}, ""},
+	{[]string{"// foo   "}, "foo\n"},
+	{[]string{"//", "//", "// foo"}, "foo\n"},
+	{[]string{"// foo  bar  "}, "foo  bar\n"},
+	{[]string{"// foo", "// bar"}, "foo\nbar\n"},
+	{[]string{"// foo", "//", "//", "//", "// bar"}, "foo\n\nbar\n"},
+	{[]string{"// foo", "/* bar */"}, "foo\n bar\n"},
+	{[]string{"//", "//", "//", "// foo", "//", "//", "//"}, "foo\n"},
+
+	{[]string{"/**/"}, ""},
+	{[]string{"/*   */"}, ""},
+	{[]string{"/**/", "/**/", "/*   */"}, ""},
+	{[]string{"/* Foo   */"}, " Foo\n"},
+	{[]string{"/* Foo  Bar  */"}, " Foo  Bar\n"},
+	{[]string{"/* Foo*/", "/* Bar*/"}, " Foo\n Bar\n"},
+	{[]string{"/* Foo*/", "/**/", "/**/", "/**/", "// Bar"}, " Foo\n\nBar\n"},
+	{[]string{"/* Foo*/", "/*\n*/", "//", "/*\n*/", "// Bar"}, " Foo\n\nBar\n"},
+	{[]string{"/* Foo*/", "// Bar"}, " Foo\nBar\n"},
+	{[]string{"/* Foo\n Bar*/"}, " Foo\n Bar\n"},
+
+	{[]string{"// foo", "//go:noinline", "// bar", "//:baz"}, "foo\nbar\n:baz\n"},
+	{[]string{"// foo", "//lint123:ignore", "// bar"}, "foo\nbar\n"},
+}
+
+func TestCommentText(t *testing.T) {
+	for i, c := range comments {
+		list := make([]*Comment, len(c.list))
+		for i, s := range c.list {
+			list[i] = &Comment{Text: s}
+		}
+
+		text := (&CommentGroup{list}).Text()
+		if text != c.text {
+			t.Errorf("case %d: got %q; expected %q", i, text, c.text)
+		}
+	}
+}
+
+var isDirectiveTests = []struct {
+	in string
+	ok bool
+}{
+	{"abc", false},
+	{"go:inline", true},
+	{"Go:inline", false},
+	{"go:Inline", false},
+	{":inline", false},
+	{"lint:ignore", true},
+	{"lint:1234", true},
+	{"1234:lint", true},
+	{"go: inline", false},
+	{"go:", false},
+	{"go:*", false},
+	{"go:x*", true},
+}
+
+func TestIsDirective(t *testing.T) {
+	for _, tt := range isDirectiveTests {
+		if ok := isDirective(tt.in); ok != tt.ok {
+			t.Errorf("isDirective(%q) = %v, want %v", tt.in, ok, tt.ok)
+		}
+	}
+}
diff --git a/internal/backport/go/ast/commentmap.go b/internal/backport/go/ast/commentmap.go
new file mode 100644
index 0000000..5e63605
--- /dev/null
+++ b/internal/backport/go/ast/commentmap.go
@@ -0,0 +1,329 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"bytes"
+	"fmt"
+	"golang.org/x/website/internal/backport/go/token"
+	"sort"
+)
+
+type byPos []*CommentGroup
+
+func (a byPos) Len() int           { return len(a) }
+func (a byPos) Less(i, j int) bool { return a[i].Pos() < a[j].Pos() }
+func (a byPos) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+
+// sortComments sorts the list of comment groups in source order.
+func sortComments(list []*CommentGroup) {
+	// TODO(gri): Does it make sense to check for sorted-ness
+	//            first (because we know that sorted-ness is
+	//            very likely)?
+	if orderedList := byPos(list); !sort.IsSorted(orderedList) {
+		sort.Sort(orderedList)
+	}
+}
+
+// A CommentMap maps an AST node to a list of comment groups
+// associated with it. See NewCommentMap for a description of
+// the association.
+type CommentMap map[Node][]*CommentGroup
+
+func (cmap CommentMap) addComment(n Node, c *CommentGroup) {
+	list := cmap[n]
+	if len(list) == 0 {
+		list = []*CommentGroup{c}
+	} else {
+		list = append(list, c)
+	}
+	cmap[n] = list
+}
+
+type byInterval []Node
+
+func (a byInterval) Len() int { return len(a) }
+func (a byInterval) Less(i, j int) bool {
+	pi, pj := a[i].Pos(), a[j].Pos()
+	return pi < pj || pi == pj && a[i].End() > a[j].End()
+}
+func (a byInterval) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
+
+// nodeList returns the list of nodes of the AST n in source order.
+func nodeList(n Node) []Node {
+	var list []Node
+	Inspect(n, func(n Node) bool {
+		// don't collect comments
+		switch n.(type) {
+		case nil, *CommentGroup, *Comment:
+			return false
+		}
+		list = append(list, n)
+		return true
+	})
+	// Note: The current implementation assumes that Inspect traverses the
+	//       AST in depth-first and thus _source_ order. If AST traversal
+	//       does not follow source order, the sorting call below will be
+	//       required.
+	// sort.Sort(byInterval(list))
+	return list
+}
+
+// A commentListReader helps iterating through a list of comment groups.
+type commentListReader struct {
+	fset     *token.FileSet
+	list     []*CommentGroup
+	index    int
+	comment  *CommentGroup  // comment group at current index
+	pos, end token.Position // source interval of comment group at current index
+}
+
+func (r *commentListReader) eol() bool {
+	return r.index >= len(r.list)
+}
+
+func (r *commentListReader) next() {
+	if !r.eol() {
+		r.comment = r.list[r.index]
+		r.pos = r.fset.Position(r.comment.Pos())
+		r.end = r.fset.Position(r.comment.End())
+		r.index++
+	}
+}
+
+// A nodeStack keeps track of nested nodes.
+// A node lower on the stack lexically contains the nodes higher on the stack.
+type nodeStack []Node
+
+// push pops all nodes that appear lexically before n
+// and then pushes n on the stack.
+func (s *nodeStack) push(n Node) {
+	s.pop(n.Pos())
+	*s = append((*s), n)
+}
+
+// pop pops all nodes that appear lexically before pos
+// (i.e., whose lexical extent has ended before or at pos).
+// It returns the last node popped.
+func (s *nodeStack) pop(pos token.Pos) (top Node) {
+	i := len(*s)
+	for i > 0 && (*s)[i-1].End() <= pos {
+		top = (*s)[i-1]
+		i--
+	}
+	*s = (*s)[0:i]
+	return top
+}
+
+// NewCommentMap creates a new comment map by associating comment groups
+// of the comments list with the nodes of the AST specified by node.
+//
+// A comment group g is associated with a node n if:
+//
+//   - g starts on the same line as n ends
+//   - g starts on the line immediately following n, and there is
+//     at least one empty line after g and before the next node
+//   - g starts before n and is not associated to the node before n
+//     via the previous rules
+//
+// NewCommentMap tries to associate a comment group to the "largest"
+// node possible: For instance, if the comment is a line comment
+// trailing an assignment, the comment is associated with the entire
+// assignment rather than just the last operand in the assignment.
+func NewCommentMap(fset *token.FileSet, node Node, comments []*CommentGroup) CommentMap {
+	if len(comments) == 0 {
+		return nil // no comments to map
+	}
+
+	cmap := make(CommentMap)
+
+	// set up comment reader r
+	tmp := make([]*CommentGroup, len(comments))
+	copy(tmp, comments) // don't change incoming comments
+	sortComments(tmp)
+	r := commentListReader{fset: fset, list: tmp} // !r.eol() because len(comments) > 0
+	r.next()
+
+	// create node list in lexical order
+	nodes := nodeList(node)
+	nodes = append(nodes, nil) // append sentinel
+
+	// set up iteration variables
+	var (
+		p     Node           // previous node
+		pend  token.Position // end of p
+		pg    Node           // previous node group (enclosing nodes of "importance")
+		pgend token.Position // end of pg
+		stack nodeStack      // stack of node groups
+	)
+
+	for _, q := range nodes {
+		var qpos token.Position
+		if q != nil {
+			qpos = fset.Position(q.Pos()) // current node position
+		} else {
+			// set fake sentinel position to infinity so that
+			// all comments get processed before the sentinel
+			const infinity = 1 << 30
+			qpos.Offset = infinity
+			qpos.Line = infinity
+		}
+
+		// process comments before current node
+		for r.end.Offset <= qpos.Offset {
+			// determine recent node group
+			if top := stack.pop(r.comment.Pos()); top != nil {
+				pg = top
+				pgend = fset.Position(pg.End())
+			}
+			// Try to associate a comment first with a node group
+			// (i.e., a node of "importance" such as a declaration);
+			// if that fails, try to associate it with the most recent
+			// node.
+			// TODO(gri) try to simplify the logic below
+			var assoc Node
+			switch {
+			case pg != nil &&
+				(pgend.Line == r.pos.Line ||
+					pgend.Line+1 == r.pos.Line && r.end.Line+1 < qpos.Line):
+				// 1) comment starts on same line as previous node group ends, or
+				// 2) comment starts on the line immediately after the
+				//    previous node group and there is an empty line before
+				//    the current node
+				// => associate comment with previous node group
+				assoc = pg
+			case p != nil &&
+				(pend.Line == r.pos.Line ||
+					pend.Line+1 == r.pos.Line && r.end.Line+1 < qpos.Line ||
+					q == nil):
+				// same rules apply as above for p rather than pg,
+				// but also associate with p if we are at the end (q == nil)
+				assoc = p
+			default:
+				// otherwise, associate comment with current node
+				if q == nil {
+					// we can only reach here if there was no p
+					// which would imply that there were no nodes
+					panic("internal error: no comments should be associated with sentinel")
+				}
+				assoc = q
+			}
+			cmap.addComment(assoc, r.comment)
+			if r.eol() {
+				return cmap
+			}
+			r.next()
+		}
+
+		// update previous node
+		p = q
+		pend = fset.Position(p.End())
+
+		// update previous node group if we see an "important" node
+		switch q.(type) {
+		case *File, *Field, Decl, Spec, Stmt:
+			stack.push(q)
+		}
+	}
+
+	return cmap
+}
+
+// Update replaces an old node in the comment map with the new node
+// and returns the new node. Comments that were associated with the
+// old node are associated with the new node.
+func (cmap CommentMap) Update(old, new Node) Node {
+	if list := cmap[old]; len(list) > 0 {
+		delete(cmap, old)
+		cmap[new] = append(cmap[new], list...)
+	}
+	return new
+}
+
+// Filter returns a new comment map consisting of only those
+// entries of cmap for which a corresponding node exists in
+// the AST specified by node.
+func (cmap CommentMap) Filter(node Node) CommentMap {
+	umap := make(CommentMap)
+	Inspect(node, func(n Node) bool {
+		if g := cmap[n]; len(g) > 0 {
+			umap[n] = g
+		}
+		return true
+	})
+	return umap
+}
+
+// Comments returns the list of comment groups in the comment map.
+// The result is sorted in source order.
+func (cmap CommentMap) Comments() []*CommentGroup {
+	list := make([]*CommentGroup, 0, len(cmap))
+	for _, e := range cmap {
+		list = append(list, e...)
+	}
+	sortComments(list)
+	return list
+}
+
+func summary(list []*CommentGroup) string {
+	const maxLen = 40
+	var buf bytes.Buffer
+
+	// collect comments text
+loop:
+	for _, group := range list {
+		// Note: CommentGroup.Text() does too much work for what we
+		//       need and would only replace this innermost loop.
+		//       Just do it explicitly.
+		for _, comment := range group.List {
+			if buf.Len() >= maxLen {
+				break loop
+			}
+			buf.WriteString(comment.Text)
+		}
+	}
+
+	// truncate if too long
+	if buf.Len() > maxLen {
+		buf.Truncate(maxLen - 3)
+		buf.WriteString("...")
+	}
+
+	// replace any invisibles with blanks
+	bytes := buf.Bytes()
+	for i, b := range bytes {
+		switch b {
+		case '\t', '\n', '\r':
+			bytes[i] = ' '
+		}
+	}
+
+	return string(bytes)
+}
+
+func (cmap CommentMap) String() string {
+	// print map entries in sorted order
+	var nodes []Node
+	for node := range cmap {
+		nodes = append(nodes, node)
+	}
+	sort.Sort(byInterval(nodes))
+
+	var buf bytes.Buffer
+	fmt.Fprintln(&buf, "CommentMap {")
+	for _, node := range nodes {
+		comment := cmap[node]
+		// print name of identifiers; print node type for other nodes
+		var s string
+		if ident, ok := node.(*Ident); ok {
+			s = ident.Name
+		} else {
+			s = fmt.Sprintf("%T", node)
+		}
+		fmt.Fprintf(&buf, "\t%p  %20s:  %s\n", node, s, summary(comment))
+	}
+	fmt.Fprintln(&buf, "}")
+	return buf.String()
+}
diff --git a/internal/backport/go/ast/commentmap_test.go b/internal/backport/go/ast/commentmap_test.go
new file mode 100644
index 0000000..67c2daf
--- /dev/null
+++ b/internal/backport/go/ast/commentmap_test.go
@@ -0,0 +1,170 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// To avoid a cyclic dependency with go/parser, this file is in a separate package.
+
+package ast_test
+
+import (
+	"bytes"
+	"fmt"
+	"sort"
+	"testing"
+
+	. "golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+const src = `
+// the very first comment
+
+// package p
+package p /* the name is p */
+
+// imports
+import (
+	"bytes"     // bytes
+	"fmt"       // fmt
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+)
+
+// T
+type T struct {
+	a, b, c int // associated with a, b, c
+	// associated with x, y
+	x, y float64    // float values
+	z    complex128 // complex value
+}
+// also associated with T
+
+// x
+var x = 0 // x = 0
+// also associated with x
+
+// f1
+func f1() {
+	/* associated with s1 */
+	s1()
+	// also associated with s1
+
+	// associated with s2
+
+	// also associated with s2
+	s2() // line comment for s2
+}
+// associated with f1
+// also associated with f1
+
+// associated with f2
+
+// f2
+func f2() {
+}
+
+func f3() {
+	i := 1 /* 1 */ + 2 // addition
+	_ = i
+}
+
+// the very last comment
+`
+
+// res maps a key of the form "line number: node type"
+// to the associated comments' text.
+var res = map[string]string{
+	" 5: *ast.File":       "the very first comment\npackage p\n",
+	" 5: *ast.Ident":      " the name is p\n",
+	" 8: *ast.GenDecl":    "imports\n",
+	" 9: *ast.ImportSpec": "bytes\n",
+	"10: *ast.ImportSpec": "fmt\n",
+	"16: *ast.GenDecl":    "T\nalso associated with T\n",
+	"17: *ast.Field":      "associated with a, b, c\n",
+	"19: *ast.Field":      "associated with x, y\nfloat values\n",
+	"20: *ast.Field":      "complex value\n",
+	"25: *ast.GenDecl":    "x\nx = 0\nalso associated with x\n",
+	"29: *ast.FuncDecl":   "f1\nassociated with f1\nalso associated with f1\n",
+	"31: *ast.ExprStmt":   " associated with s1\nalso associated with s1\n",
+	"37: *ast.ExprStmt":   "associated with s2\nalso associated with s2\nline comment for s2\n",
+	"45: *ast.FuncDecl":   "associated with f2\nf2\n",
+	"49: *ast.AssignStmt": "addition\n",
+	"49: *ast.BasicLit":   " 1\n",
+	"50: *ast.Ident":      "the very last comment\n",
+}
+
+func ctext(list []*CommentGroup) string {
+	var buf bytes.Buffer
+	for _, g := range list {
+		buf.WriteString(g.Text())
+	}
+	return buf.String()
+}
+
+func TestCommentMap(t *testing.T) {
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	cmap := NewCommentMap(fset, f, f.Comments)
+
+	// very correct association of comments
+	for n, list := range cmap {
+		key := fmt.Sprintf("%2d: %T", fset.Position(n.Pos()).Line, n)
+		got := ctext(list)
+		want := res[key]
+		if got != want {
+			t.Errorf("%s: got %q; want %q", key, got, want)
+		}
+	}
+
+	// verify that no comments got lost
+	if n := len(cmap.Comments()); n != len(f.Comments) {
+		t.Errorf("got %d comment groups in map; want %d", n, len(f.Comments))
+	}
+
+	// support code to update test:
+	// set genMap to true to generate res map
+	const genMap = false
+	if genMap {
+		out := make([]string, 0, len(cmap))
+		for n, list := range cmap {
+			out = append(out, fmt.Sprintf("\t\"%2d: %T\":\t%q,", fset.Position(n.Pos()).Line, n, ctext(list)))
+		}
+		sort.Strings(out)
+		for _, s := range out {
+			fmt.Println(s)
+		}
+	}
+}
+
+func TestFilter(t *testing.T) {
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	cmap := NewCommentMap(fset, f, f.Comments)
+
+	// delete variable declaration
+	for i, decl := range f.Decls {
+		if gen, ok := decl.(*GenDecl); ok && gen.Tok == token.VAR {
+			copy(f.Decls[i:], f.Decls[i+1:])
+			f.Decls = f.Decls[:len(f.Decls)-1]
+			break
+		}
+	}
+
+	// check if comments are filtered correctly
+	cc := cmap.Filter(f)
+	for n, list := range cc {
+		key := fmt.Sprintf("%2d: %T", fset.Position(n.Pos()).Line, n)
+		got := ctext(list)
+		want := res[key]
+		if key == "25: *ast.GenDecl" || got != want {
+			t.Errorf("%s: got %q; want %q", key, got, want)
+		}
+	}
+}
diff --git a/internal/backport/go/ast/example_test.go b/internal/backport/go/ast/example_test.go
new file mode 100644
index 0000000..ae92618
--- /dev/null
+++ b/internal/backport/go/ast/example_test.go
@@ -0,0 +1,206 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast_test
+
+import (
+	"bytes"
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/format"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+// This example demonstrates how to inspect the AST of a Go program.
+func ExampleInspect() {
+	// src is the input for which we want to inspect the AST.
+	src := `
+package p
+const c = 1.0
+var X = f(3.14)*2 + c
+`
+
+	// Create the AST by parsing src.
+	fset := token.NewFileSet() // positions are relative to fset
+	f, err := parser.ParseFile(fset, "src.go", src, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	// Inspect the AST and print all identifiers and literals.
+	ast.Inspect(f, func(n ast.Node) bool {
+		var s string
+		switch x := n.(type) {
+		case *ast.BasicLit:
+			s = x.Value
+		case *ast.Ident:
+			s = x.Name
+		}
+		if s != "" {
+			fmt.Printf("%s:\t%s\n", fset.Position(n.Pos()), s)
+		}
+		return true
+	})
+
+	// Output:
+	// src.go:2:9:	p
+	// src.go:3:7:	c
+	// src.go:3:11:	1.0
+	// src.go:4:5:	X
+	// src.go:4:9:	f
+	// src.go:4:11:	3.14
+	// src.go:4:17:	2
+	// src.go:4:21:	c
+}
+
+// This example shows what an AST looks like when printed for debugging.
+func ExamplePrint() {
+	// src is the input for which we want to print the AST.
+	src := `
+package main
+func main() {
+	println("Hello, World!")
+}
+`
+
+	// Create the AST by parsing src.
+	fset := token.NewFileSet() // positions are relative to fset
+	f, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	// Print the AST.
+	ast.Print(fset, f)
+
+	// Output:
+	//      0  *ast.File {
+	//      1  .  Package: 2:1
+	//      2  .  Name: *ast.Ident {
+	//      3  .  .  NamePos: 2:9
+	//      4  .  .  Name: "main"
+	//      5  .  }
+	//      6  .  Decls: []ast.Decl (len = 1) {
+	//      7  .  .  0: *ast.FuncDecl {
+	//      8  .  .  .  Name: *ast.Ident {
+	//      9  .  .  .  .  NamePos: 3:6
+	//     10  .  .  .  .  Name: "main"
+	//     11  .  .  .  .  Obj: *ast.Object {
+	//     12  .  .  .  .  .  Kind: func
+	//     13  .  .  .  .  .  Name: "main"
+	//     14  .  .  .  .  .  Decl: *(obj @ 7)
+	//     15  .  .  .  .  }
+	//     16  .  .  .  }
+	//     17  .  .  .  Type: *ast.FuncType {
+	//     18  .  .  .  .  Func: 3:1
+	//     19  .  .  .  .  Params: *ast.FieldList {
+	//     20  .  .  .  .  .  Opening: 3:10
+	//     21  .  .  .  .  .  Closing: 3:11
+	//     22  .  .  .  .  }
+	//     23  .  .  .  }
+	//     24  .  .  .  Body: *ast.BlockStmt {
+	//     25  .  .  .  .  Lbrace: 3:13
+	//     26  .  .  .  .  List: []ast.Stmt (len = 1) {
+	//     27  .  .  .  .  .  0: *ast.ExprStmt {
+	//     28  .  .  .  .  .  .  X: *ast.CallExpr {
+	//     29  .  .  .  .  .  .  .  Fun: *ast.Ident {
+	//     30  .  .  .  .  .  .  .  .  NamePos: 4:2
+	//     31  .  .  .  .  .  .  .  .  Name: "println"
+	//     32  .  .  .  .  .  .  .  }
+	//     33  .  .  .  .  .  .  .  Lparen: 4:9
+	//     34  .  .  .  .  .  .  .  Args: []ast.Expr (len = 1) {
+	//     35  .  .  .  .  .  .  .  .  0: *ast.BasicLit {
+	//     36  .  .  .  .  .  .  .  .  .  ValuePos: 4:10
+	//     37  .  .  .  .  .  .  .  .  .  Kind: STRING
+	//     38  .  .  .  .  .  .  .  .  .  Value: "\"Hello, World!\""
+	//     39  .  .  .  .  .  .  .  .  }
+	//     40  .  .  .  .  .  .  .  }
+	//     41  .  .  .  .  .  .  .  Ellipsis: -
+	//     42  .  .  .  .  .  .  .  Rparen: 4:25
+	//     43  .  .  .  .  .  .  }
+	//     44  .  .  .  .  .  }
+	//     45  .  .  .  .  }
+	//     46  .  .  .  .  Rbrace: 5:1
+	//     47  .  .  .  }
+	//     48  .  .  }
+	//     49  .  }
+	//     50  .  Scope: *ast.Scope {
+	//     51  .  .  Objects: map[string]*ast.Object (len = 1) {
+	//     52  .  .  .  "main": *(obj @ 11)
+	//     53  .  .  }
+	//     54  .  }
+	//     55  .  Unresolved: []*ast.Ident (len = 1) {
+	//     56  .  .  0: *(obj @ 29)
+	//     57  .  }
+	//     58  }
+}
+
+// This example illustrates how to remove a variable declaration
+// in a Go program while maintaining correct comment association
+// using an ast.CommentMap.
+func ExampleCommentMap() {
+	// src is the input for which we create the AST that we
+	// are going to manipulate.
+	src := `
+// This is the package comment.
+package main
+
+// This comment is associated with the hello constant.
+const hello = "Hello, World!" // line comment 1
+
+// This comment is associated with the foo variable.
+var foo = hello // line comment 2
+
+// This comment is associated with the main function.
+func main() {
+	fmt.Println(hello) // line comment 3
+}
+`
+
+	// Create the AST by parsing src.
+	fset := token.NewFileSet() // positions are relative to fset
+	f, err := parser.ParseFile(fset, "src.go", src, parser.ParseComments)
+	if err != nil {
+		panic(err)
+	}
+
+	// Create an ast.CommentMap from the ast.File's comments.
+	// This helps keeping the association between comments
+	// and AST nodes.
+	cmap := ast.NewCommentMap(fset, f, f.Comments)
+
+	// Remove the first variable declaration from the list of declarations.
+	for i, decl := range f.Decls {
+		if gen, ok := decl.(*ast.GenDecl); ok && gen.Tok == token.VAR {
+			copy(f.Decls[i:], f.Decls[i+1:])
+			f.Decls = f.Decls[:len(f.Decls)-1]
+			break
+		}
+	}
+
+	// Use the comment map to filter comments that don't belong anymore
+	// (the comments associated with the variable declaration), and create
+	// the new comments list.
+	f.Comments = cmap.Filter(f).Comments()
+
+	// Print the modified AST.
+	var buf bytes.Buffer
+	if err := format.Node(&buf, fset, f); err != nil {
+		panic(err)
+	}
+	fmt.Printf("%s", buf.Bytes())
+
+	// Output:
+	// // This is the package comment.
+	// package main
+	//
+	// // This comment is associated with the hello constant.
+	// const hello = "Hello, World!" // line comment 1
+	//
+	// // This comment is associated with the main function.
+	// func main() {
+	// 	fmt.Println(hello) // line comment 3
+	// }
+}
diff --git a/internal/backport/go/ast/filter.go b/internal/backport/go/ast/filter.go
new file mode 100644
index 0000000..8c2ed9d
--- /dev/null
+++ b/internal/backport/go/ast/filter.go
@@ -0,0 +1,488 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"golang.org/x/website/internal/backport/go/token"
+	"sort"
+)
+
+// ----------------------------------------------------------------------------
+// Export filtering
+
+// exportFilter is a special filter function to extract exported nodes.
+func exportFilter(name string) bool {
+	return IsExported(name)
+}
+
+// FileExports trims the AST for a Go source file in place such that
+// only exported nodes remain: all top-level identifiers which are not exported
+// and their associated information (such as type, initial value, or function
+// body) are removed. Non-exported fields and methods of exported types are
+// stripped. The File.Comments list is not changed.
+//
+// FileExports reports whether there are exported declarations.
+func FileExports(src *File) bool {
+	return filterFile(src, exportFilter, true)
+}
+
+// PackageExports trims the AST for a Go package in place such that
+// only exported nodes remain. The pkg.Files list is not changed, so that
+// file names and top-level package comments don't get lost.
+//
+// PackageExports reports whether there are exported declarations;
+// it returns false otherwise.
+func PackageExports(pkg *Package) bool {
+	return filterPackage(pkg, exportFilter, true)
+}
+
+// ----------------------------------------------------------------------------
+// General filtering
+
+type Filter func(string) bool
+
+func filterIdentList(list []*Ident, f Filter) []*Ident {
+	j := 0
+	for _, x := range list {
+		if f(x.Name) {
+			list[j] = x
+			j++
+		}
+	}
+	return list[0:j]
+}
+
+// fieldName assumes that x is the type of an anonymous field and
+// returns the corresponding field name. If x is not an acceptable
+// anonymous field, the result is nil.
+func fieldName(x Expr) *Ident {
+	switch t := x.(type) {
+	case *Ident:
+		return t
+	case *SelectorExpr:
+		if _, ok := t.X.(*Ident); ok {
+			return t.Sel
+		}
+	case *StarExpr:
+		return fieldName(t.X)
+	}
+	return nil
+}
+
+func filterFieldList(fields *FieldList, filter Filter, export bool) (removedFields bool) {
+	if fields == nil {
+		return false
+	}
+	list := fields.List
+	j := 0
+	for _, f := range list {
+		keepField := false
+		if len(f.Names) == 0 {
+			// anonymous field
+			name := fieldName(f.Type)
+			keepField = name != nil && filter(name.Name)
+		} else {
+			n := len(f.Names)
+			f.Names = filterIdentList(f.Names, filter)
+			if len(f.Names) < n {
+				removedFields = true
+			}
+			keepField = len(f.Names) > 0
+		}
+		if keepField {
+			if export {
+				filterType(f.Type, filter, export)
+			}
+			list[j] = f
+			j++
+		}
+	}
+	if j < len(list) {
+		removedFields = true
+	}
+	fields.List = list[0:j]
+	return
+}
+
+func filterCompositeLit(lit *CompositeLit, filter Filter, export bool) {
+	n := len(lit.Elts)
+	lit.Elts = filterExprList(lit.Elts, filter, export)
+	if len(lit.Elts) < n {
+		lit.Incomplete = true
+	}
+}
+
+func filterExprList(list []Expr, filter Filter, export bool) []Expr {
+	j := 0
+	for _, exp := range list {
+		switch x := exp.(type) {
+		case *CompositeLit:
+			filterCompositeLit(x, filter, export)
+		case *KeyValueExpr:
+			if x, ok := x.Key.(*Ident); ok && !filter(x.Name) {
+				continue
+			}
+			if x, ok := x.Value.(*CompositeLit); ok {
+				filterCompositeLit(x, filter, export)
+			}
+		}
+		list[j] = exp
+		j++
+	}
+	return list[0:j]
+}
+
+func filterParamList(fields *FieldList, filter Filter, export bool) bool {
+	if fields == nil {
+		return false
+	}
+	var b bool
+	for _, f := range fields.List {
+		if filterType(f.Type, filter, export) {
+			b = true
+		}
+	}
+	return b
+}
+
+func filterType(typ Expr, f Filter, export bool) bool {
+	switch t := typ.(type) {
+	case *Ident:
+		return f(t.Name)
+	case *ParenExpr:
+		return filterType(t.X, f, export)
+	case *ArrayType:
+		return filterType(t.Elt, f, export)
+	case *StructType:
+		if filterFieldList(t.Fields, f, export) {
+			t.Incomplete = true
+		}
+		return len(t.Fields.List) > 0
+	case *FuncType:
+		b1 := filterParamList(t.Params, f, export)
+		b2 := filterParamList(t.Results, f, export)
+		return b1 || b2
+	case *InterfaceType:
+		if filterFieldList(t.Methods, f, export) {
+			t.Incomplete = true
+		}
+		return len(t.Methods.List) > 0
+	case *MapType:
+		b1 := filterType(t.Key, f, export)
+		b2 := filterType(t.Value, f, export)
+		return b1 || b2
+	case *ChanType:
+		return filterType(t.Value, f, export)
+	}
+	return false
+}
+
+func filterSpec(spec Spec, f Filter, export bool) bool {
+	switch s := spec.(type) {
+	case *ValueSpec:
+		s.Names = filterIdentList(s.Names, f)
+		s.Values = filterExprList(s.Values, f, export)
+		if len(s.Names) > 0 {
+			if export {
+				filterType(s.Type, f, export)
+			}
+			return true
+		}
+	case *TypeSpec:
+		if f(s.Name.Name) {
+			if export {
+				filterType(s.Type, f, export)
+			}
+			return true
+		}
+		if !export {
+			// For general filtering (not just exports),
+			// filter type even if name is not filtered
+			// out.
+			// If the type contains filtered elements,
+			// keep the declaration.
+			return filterType(s.Type, f, export)
+		}
+	}
+	return false
+}
+
+func filterSpecList(list []Spec, f Filter, export bool) []Spec {
+	j := 0
+	for _, s := range list {
+		if filterSpec(s, f, export) {
+			list[j] = s
+			j++
+		}
+	}
+	return list[0:j]
+}
+
+// FilterDecl trims the AST for a Go declaration in place by removing
+// all names (including struct field and interface method names, but
+// not from parameter lists) that don't pass through the filter f.
+//
+// FilterDecl reports whether there are any declared names left after
+// filtering.
+func FilterDecl(decl Decl, f Filter) bool {
+	return filterDecl(decl, f, false)
+}
+
+func filterDecl(decl Decl, f Filter, export bool) bool {
+	switch d := decl.(type) {
+	case *GenDecl:
+		d.Specs = filterSpecList(d.Specs, f, export)
+		return len(d.Specs) > 0
+	case *FuncDecl:
+		return f(d.Name.Name)
+	}
+	return false
+}
+
+// FilterFile trims the AST for a Go file in place by removing all
+// names from top-level declarations (including struct field and
+// interface method names, but not from parameter lists) that don't
+// pass through the filter f. If the declaration is empty afterwards,
+// the declaration is removed from the AST. Import declarations are
+// always removed. The File.Comments list is not changed.
+//
+// FilterFile reports whether there are any top-level declarations
+// left after filtering.
+func FilterFile(src *File, f Filter) bool {
+	return filterFile(src, f, false)
+}
+
+func filterFile(src *File, f Filter, export bool) bool {
+	j := 0
+	for _, d := range src.Decls {
+		if filterDecl(d, f, export) {
+			src.Decls[j] = d
+			j++
+		}
+	}
+	src.Decls = src.Decls[0:j]
+	return j > 0
+}
+
+// FilterPackage trims the AST for a Go package in place by removing
+// all names from top-level declarations (including struct field and
+// interface method names, but not from parameter lists) that don't
+// pass through the filter f. If the declaration is empty afterwards,
+// the declaration is removed from the AST. The pkg.Files list is not
+// changed, so that file names and top-level package comments don't get
+// lost.
+//
+// FilterPackage reports whether there are any top-level declarations
+// left after filtering.
+func FilterPackage(pkg *Package, f Filter) bool {
+	return filterPackage(pkg, f, false)
+}
+
+func filterPackage(pkg *Package, f Filter, export bool) bool {
+	hasDecls := false
+	for _, src := range pkg.Files {
+		if filterFile(src, f, export) {
+			hasDecls = true
+		}
+	}
+	return hasDecls
+}
+
+// ----------------------------------------------------------------------------
+// Merging of package files
+
+// The MergeMode flags control the behavior of MergePackageFiles.
+type MergeMode uint
+
+const (
+	// If set, duplicate function declarations are excluded.
+	FilterFuncDuplicates MergeMode = 1 << iota
+	// If set, comments that are not associated with a specific
+	// AST node (as Doc or Comment) are excluded.
+	FilterUnassociatedComments
+	// If set, duplicate import declarations are excluded.
+	FilterImportDuplicates
+)
+
+// nameOf returns the function (foo) or method name (foo.bar) for
+// the given function declaration. If the AST is incorrect for the
+// receiver, it assumes a function instead.
+func nameOf(f *FuncDecl) string {
+	if r := f.Recv; r != nil && len(r.List) == 1 {
+		// looks like a correct receiver declaration
+		t := r.List[0].Type
+		// dereference pointer receiver types
+		if p, _ := t.(*StarExpr); p != nil {
+			t = p.X
+		}
+		// the receiver type must be a type name
+		if p, _ := t.(*Ident); p != nil {
+			return p.Name + "." + f.Name.Name
+		}
+		// otherwise assume a function instead
+	}
+	return f.Name.Name
+}
+
+// separator is an empty //-style comment that is interspersed between
+// different comment groups when they are concatenated into a single group
+var separator = &Comment{token.NoPos, "//"}
+
+// MergePackageFiles creates a file AST by merging the ASTs of the
+// files belonging to a package. The mode flags control merging behavior.
+func MergePackageFiles(pkg *Package, mode MergeMode) *File {
+	// Count the number of package docs, comments and declarations across
+	// all package files. Also, compute sorted list of filenames, so that
+	// subsequent iterations can always iterate in the same order.
+	ndocs := 0
+	ncomments := 0
+	ndecls := 0
+	filenames := make([]string, len(pkg.Files))
+	i := 0
+	for filename, f := range pkg.Files {
+		filenames[i] = filename
+		i++
+		if f.Doc != nil {
+			ndocs += len(f.Doc.List) + 1 // +1 for separator
+		}
+		ncomments += len(f.Comments)
+		ndecls += len(f.Decls)
+	}
+	sort.Strings(filenames)
+
+	// Collect package comments from all package files into a single
+	// CommentGroup - the collected package documentation. In general
+	// there should be only one file with a package comment; but it's
+	// better to collect extra comments than drop them on the floor.
+	var doc *CommentGroup
+	var pos token.Pos
+	if ndocs > 0 {
+		list := make([]*Comment, ndocs-1) // -1: no separator before first group
+		i := 0
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			if f.Doc != nil {
+				if i > 0 {
+					// not the first group - add separator
+					list[i] = separator
+					i++
+				}
+				for _, c := range f.Doc.List {
+					list[i] = c
+					i++
+				}
+				if f.Package > pos {
+					// Keep the maximum package clause position as
+					// position for the package clause of the merged
+					// files.
+					pos = f.Package
+				}
+			}
+		}
+		doc = &CommentGroup{list}
+	}
+
+	// Collect declarations from all package files.
+	var decls []Decl
+	if ndecls > 0 {
+		decls = make([]Decl, ndecls)
+		funcs := make(map[string]int) // map of func name -> decls index
+		i := 0                        // current index
+		n := 0                        // number of filtered entries
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			for _, d := range f.Decls {
+				if mode&FilterFuncDuplicates != 0 {
+					// A language entity may be declared multiple
+					// times in different package files; only at
+					// build time declarations must be unique.
+					// For now, exclude multiple declarations of
+					// functions - keep the one with documentation.
+					//
+					// TODO(gri): Expand this filtering to other
+					//            entities (const, type, vars) if
+					//            multiple declarations are common.
+					if f, isFun := d.(*FuncDecl); isFun {
+						name := nameOf(f)
+						if j, exists := funcs[name]; exists {
+							// function declared already
+							if decls[j] != nil && decls[j].(*FuncDecl).Doc == nil {
+								// existing declaration has no documentation;
+								// ignore the existing declaration
+								decls[j] = nil
+							} else {
+								// ignore the new declaration
+								d = nil
+							}
+							n++ // filtered an entry
+						} else {
+							funcs[name] = i
+						}
+					}
+				}
+				decls[i] = d
+				i++
+			}
+		}
+
+		// Eliminate nil entries from the decls list if entries were
+		// filtered. We do this using a 2nd pass in order to not disturb
+		// the original declaration order in the source (otherwise, this
+		// would also invalidate the monotonically increasing position
+		// info within a single file).
+		if n > 0 {
+			i = 0
+			for _, d := range decls {
+				if d != nil {
+					decls[i] = d
+					i++
+				}
+			}
+			decls = decls[0:i]
+		}
+	}
+
+	// Collect import specs from all package files.
+	var imports []*ImportSpec
+	if mode&FilterImportDuplicates != 0 {
+		seen := make(map[string]bool)
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			for _, imp := range f.Imports {
+				if path := imp.Path.Value; !seen[path] {
+					// TODO: consider handling cases where:
+					// - 2 imports exist with the same import path but
+					//   have different local names (one should probably
+					//   keep both of them)
+					// - 2 imports exist but only one has a comment
+					// - 2 imports exist and they both have (possibly
+					//   different) comments
+					imports = append(imports, imp)
+					seen[path] = true
+				}
+			}
+		}
+	} else {
+		// Iterate over filenames for deterministic order.
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			imports = append(imports, f.Imports...)
+		}
+	}
+
+	// Collect comments from all package files.
+	var comments []*CommentGroup
+	if mode&FilterUnassociatedComments == 0 {
+		comments = make([]*CommentGroup, ncomments)
+		i := 0
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			i += copy(comments[i:], f.Comments)
+		}
+	}
+
+	// TODO(gri) need to compute unresolved identifiers!
+	return &File{doc, pos, NewIdent(pkg.Name), decls, pkg.Scope, imports, nil, comments}
+}
diff --git a/internal/backport/go/ast/filter_test.go b/internal/backport/go/ast/filter_test.go
new file mode 100644
index 0000000..fa5da6a
--- /dev/null
+++ b/internal/backport/go/ast/filter_test.go
@@ -0,0 +1,85 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// To avoid a cyclic dependency with go/parser, this file is in a separate package.
+
+package ast_test
+
+import (
+	"bytes"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/format"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
+	"testing"
+)
+
+const input = `package p
+
+type t1 struct{}
+type t2 struct{}
+
+func f1() {}
+func f1() {}
+func f2() {}
+
+func (*t1) f1() {}
+func (t1) f1() {}
+func (t1) f2() {}
+
+func (t2) f1() {}
+func (t2) f2() {}
+func (x *t2) f2() {}
+`
+
+// Calling ast.MergePackageFiles with ast.FilterFuncDuplicates
+// keeps a duplicate entry with attached documentation in favor
+// of one without, and it favors duplicate entries appearing
+// later in the source over ones appearing earlier. This is why
+// (*t2).f2 is kept and t2.f2 is eliminated in this test case.
+const golden = `package p
+
+type t1 struct{}
+type t2 struct{}
+
+func f1() {}
+func f2() {}
+
+func (t1) f1() {}
+func (t1) f2() {}
+
+func (t2) f1() {}
+
+func (x *t2) f2() {}
+`
+
+func TestFilterDuplicates(t *testing.T) {
+	// parse input
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "", input, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// create package
+	files := map[string]*ast.File{"": file}
+	pkg, err := ast.NewPackage(fset, files, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// filter
+	merged := ast.MergePackageFiles(pkg, ast.FilterFuncDuplicates)
+
+	// pretty-print
+	var buf bytes.Buffer
+	if err := format.Node(&buf, fset, merged); err != nil {
+		t.Fatal(err)
+	}
+	output := buf.String()
+
+	if output != golden {
+		t.Errorf("incorrect output:\n%s", output)
+	}
+}
diff --git a/internal/backport/go/ast/import.go b/internal/backport/go/ast/import.go
new file mode 100644
index 0000000..7c70ef6
--- /dev/null
+++ b/internal/backport/go/ast/import.go
@@ -0,0 +1,230 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"golang.org/x/website/internal/backport/go/token"
+	"sort"
+	"strconv"
+)
+
+// SortImports sorts runs of consecutive import lines in import blocks in f.
+// It also removes duplicate imports when it is possible to do so without data loss.
+func SortImports(fset *token.FileSet, f *File) {
+	for _, d := range f.Decls {
+		d, ok := d.(*GenDecl)
+		if !ok || d.Tok != token.IMPORT {
+			// Not an import declaration, so we're done.
+			// Imports are always first.
+			break
+		}
+
+		if !d.Lparen.IsValid() {
+			// Not a block: sorted by default.
+			continue
+		}
+
+		// Identify and sort runs of specs on successive lines.
+		i := 0
+		specs := d.Specs[:0]
+		for j, s := range d.Specs {
+			if j > i && lineAt(fset, s.Pos()) > 1+lineAt(fset, d.Specs[j-1].End()) {
+				// j begins a new run. End this one.
+				specs = append(specs, sortSpecs(fset, f, d.Specs[i:j])...)
+				i = j
+			}
+		}
+		specs = append(specs, sortSpecs(fset, f, d.Specs[i:])...)
+		d.Specs = specs
+
+		// Deduping can leave a blank line before the rparen; clean that up.
+		if len(d.Specs) > 0 {
+			lastSpec := d.Specs[len(d.Specs)-1]
+			lastLine := lineAt(fset, lastSpec.Pos())
+			rParenLine := lineAt(fset, d.Rparen)
+			for rParenLine > lastLine+1 {
+				rParenLine--
+				fset.File(d.Rparen).MergeLine(rParenLine)
+			}
+		}
+	}
+}
+
+func lineAt(fset *token.FileSet, pos token.Pos) int {
+	return fset.PositionFor(pos, false).Line
+}
+
+func importPath(s Spec) string {
+	t, err := strconv.Unquote(s.(*ImportSpec).Path.Value)
+	if err == nil {
+		return t
+	}
+	return ""
+}
+
+func importName(s Spec) string {
+	n := s.(*ImportSpec).Name
+	if n == nil {
+		return ""
+	}
+	return n.Name
+}
+
+func importComment(s Spec) string {
+	c := s.(*ImportSpec).Comment
+	if c == nil {
+		return ""
+	}
+	return c.Text()
+}
+
+// collapse indicates whether prev may be removed, leaving only next.
+func collapse(prev, next Spec) bool {
+	if importPath(next) != importPath(prev) || importName(next) != importName(prev) {
+		return false
+	}
+	return prev.(*ImportSpec).Comment == nil
+}
+
+type posSpan struct {
+	Start token.Pos
+	End   token.Pos
+}
+
+type cgPos struct {
+	left bool // true if comment is to the left of the spec, false otherwise.
+	cg   *CommentGroup
+}
+
+func sortSpecs(fset *token.FileSet, f *File, specs []Spec) []Spec {
+	// Can't short-circuit here even if specs are already sorted,
+	// since they might yet need deduplication.
+	// A lone import, however, may be safely ignored.
+	if len(specs) <= 1 {
+		return specs
+	}
+
+	// Record positions for specs.
+	pos := make([]posSpan, len(specs))
+	for i, s := range specs {
+		pos[i] = posSpan{s.Pos(), s.End()}
+	}
+
+	// Identify comments in this range.
+	begSpecs := pos[0].Start
+	endSpecs := pos[len(pos)-1].End
+	beg := fset.File(begSpecs).LineStart(lineAt(fset, begSpecs))
+	endLine := lineAt(fset, endSpecs)
+	endFile := fset.File(endSpecs)
+	var end token.Pos
+	if endLine == endFile.LineCount() {
+		end = endSpecs
+	} else {
+		end = endFile.LineStart(endLine + 1) // beginning of next line
+	}
+	first := len(f.Comments)
+	last := -1
+	for i, g := range f.Comments {
+		if g.End() >= end {
+			break
+		}
+		// g.End() < end
+		if beg <= g.Pos() {
+			// comment is within the range [beg, end[ of import declarations
+			if i < first {
+				first = i
+			}
+			if i > last {
+				last = i
+			}
+		}
+	}
+
+	var comments []*CommentGroup
+	if last >= 0 {
+		comments = f.Comments[first : last+1]
+	}
+
+	// Assign each comment to the import spec on the same line.
+	importComments := map[*ImportSpec][]cgPos{}
+	specIndex := 0
+	for _, g := range comments {
+		for specIndex+1 < len(specs) && pos[specIndex+1].Start <= g.Pos() {
+			specIndex++
+		}
+		var left bool
+		// A block comment can appear before the first import spec.
+		if specIndex == 0 && pos[specIndex].Start > g.Pos() {
+			left = true
+		} else if specIndex+1 < len(specs) && // Or it can appear on the left of an import spec.
+			lineAt(fset, pos[specIndex].Start)+1 == lineAt(fset, g.Pos()) {
+			specIndex++
+			left = true
+		}
+		s := specs[specIndex].(*ImportSpec)
+		importComments[s] = append(importComments[s], cgPos{left: left, cg: g})
+	}
+
+	// Sort the import specs by import path.
+	// Remove duplicates, when possible without data loss.
+	// Reassign the import paths to have the same position sequence.
+	// Reassign each comment to the spec on the same line.
+	// Sort the comments by new position.
+	sort.Slice(specs, func(i, j int) bool {
+		ipath := importPath(specs[i])
+		jpath := importPath(specs[j])
+		if ipath != jpath {
+			return ipath < jpath
+		}
+		iname := importName(specs[i])
+		jname := importName(specs[j])
+		if iname != jname {
+			return iname < jname
+		}
+		return importComment(specs[i]) < importComment(specs[j])
+	})
+
+	// Dedup. Thanks to our sorting, we can just consider
+	// adjacent pairs of imports.
+	deduped := specs[:0]
+	for i, s := range specs {
+		if i == len(specs)-1 || !collapse(s, specs[i+1]) {
+			deduped = append(deduped, s)
+		} else {
+			p := s.Pos()
+			fset.File(p).MergeLine(lineAt(fset, p))
+		}
+	}
+	specs = deduped
+
+	// Fix up comment positions
+	for i, s := range specs {
+		s := s.(*ImportSpec)
+		if s.Name != nil {
+			s.Name.NamePos = pos[i].Start
+		}
+		s.Path.ValuePos = pos[i].Start
+		s.EndPos = pos[i].End
+		for _, g := range importComments[s] {
+			for _, c := range g.cg.List {
+				if g.left {
+					c.Slash = pos[i].Start - 1
+				} else {
+					// An import spec can have both block comment and a line comment
+					// to its right. In that case, both of them will have the same pos.
+					// But while formatting the AST, the line comment gets moved to
+					// after the block comment.
+					c.Slash = pos[i].End
+				}
+			}
+		}
+	}
+
+	sort.Slice(comments, func(i, j int) bool {
+		return comments[i].Pos() < comments[j].Pos()
+	})
+
+	return specs
+}
diff --git a/internal/backport/go/ast/issues_test.go b/internal/backport/go/ast/issues_test.go
new file mode 100644
index 0000000..3fb3205
--- /dev/null
+++ b/internal/backport/go/ast/issues_test.go
@@ -0,0 +1,42 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast_test
+
+import (
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
+	"testing"
+)
+
+func TestIssue33649(t *testing.T) {
+	for _, src := range []string{
+		`package p; func _()`,
+		`package p; func _() {`,
+		`package p; func _() { _ = 0`,
+		`package p; func _() { _ = 0 }`,
+	} {
+		fset := token.NewFileSet()
+		f, _ := parser.ParseFile(fset, "", src, parser.AllErrors)
+		if f == nil {
+			panic("invalid test setup: parser didn't return an AST")
+		}
+
+		// find corresponding token.File
+		var tf *token.File
+		fset.Iterate(func(f *token.File) bool {
+			tf = f
+			return true
+		})
+		tfEnd := tf.Base() + tf.Size()
+
+		fd := f.Decls[len(f.Decls)-1].(*ast.FuncDecl)
+		fdEnd := int(fd.End())
+
+		if fdEnd != tfEnd {
+			t.Errorf("%q: got fdEnd = %d; want %d (base = %d, size = %d)", src, fdEnd, tfEnd, tf.Base(), tf.Size())
+		}
+	}
+}
diff --git a/internal/backport/go/ast/print.go b/internal/backport/go/ast/print.go
new file mode 100644
index 0000000..98eeddc
--- /dev/null
+++ b/internal/backport/go/ast/print.go
@@ -0,0 +1,254 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains printing support for ASTs.
+
+package ast
+
+import (
+	"fmt"
+	"golang.org/x/website/internal/backport/go/token"
+	"io"
+	"os"
+	"reflect"
+)
+
+// A FieldFilter may be provided to Fprint to control the output.
+type FieldFilter func(name string, value reflect.Value) bool
+
+// NotNilFilter returns true for field values that are not nil;
+// it returns false otherwise.
+func NotNilFilter(_ string, v reflect.Value) bool {
+	switch v.Kind() {
+	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
+		return !v.IsNil()
+	}
+	return true
+}
+
+// Fprint prints the (sub-)tree starting at AST node x to w.
+// If fset != nil, position information is interpreted relative
+// to that file set. Otherwise positions are printed as integer
+// values (file set specific offsets).
+//
+// A non-nil FieldFilter f may be provided to control the output:
+// struct fields for which f(fieldname, fieldvalue) is true are
+// printed; all others are filtered from the output. Unexported
+// struct fields are never printed.
+func Fprint(w io.Writer, fset *token.FileSet, x interface{}, f FieldFilter) error {
+	return fprint(w, fset, x, f)
+}
+
+func fprint(w io.Writer, fset *token.FileSet, x interface{}, f FieldFilter) (err error) {
+	// setup printer
+	p := printer{
+		output: w,
+		fset:   fset,
+		filter: f,
+		ptrmap: make(map[interface{}]int),
+		last:   '\n', // force printing of line number on first line
+	}
+
+	// install error handler
+	defer func() {
+		if e := recover(); e != nil {
+			err = e.(localError).err // re-panics if it's not a localError
+		}
+	}()
+
+	// print x
+	if x == nil {
+		p.printf("nil\n")
+		return
+	}
+	p.print(reflect.ValueOf(x))
+	p.printf("\n")
+
+	return
+}
+
+// Print prints x to standard output, skipping nil fields.
+// Print(fset, x) is the same as Fprint(os.Stdout, fset, x, NotNilFilter).
+func Print(fset *token.FileSet, x interface{}) error {
+	return Fprint(os.Stdout, fset, x, NotNilFilter)
+}
+
+type printer struct {
+	output io.Writer
+	fset   *token.FileSet
+	filter FieldFilter
+	ptrmap map[interface{}]int // *T -> line number
+	indent int                 // current indentation level
+	last   byte                // the last byte processed by Write
+	line   int                 // current line number
+}
+
+var indent = []byte(".  ")
+
+func (p *printer) Write(data []byte) (n int, err error) {
+	var m int
+	for i, b := range data {
+		// invariant: data[0:n] has been written
+		if b == '\n' {
+			m, err = p.output.Write(data[n : i+1])
+			n += m
+			if err != nil {
+				return
+			}
+			p.line++
+		} else if p.last == '\n' {
+			_, err = fmt.Fprintf(p.output, "%6d  ", p.line)
+			if err != nil {
+				return
+			}
+			for j := p.indent; j > 0; j-- {
+				_, err = p.output.Write(indent)
+				if err != nil {
+					return
+				}
+			}
+		}
+		p.last = b
+	}
+	if len(data) > n {
+		m, err = p.output.Write(data[n:])
+		n += m
+	}
+	return
+}
+
+// localError wraps locally caught errors so we can distinguish
+// them from genuine panics which we don't want to return as errors.
+type localError struct {
+	err error
+}
+
+// printf is a convenience wrapper that takes care of print errors.
+func (p *printer) printf(format string, args ...interface{}) {
+	if _, err := fmt.Fprintf(p, format, args...); err != nil {
+		panic(localError{err})
+	}
+}
+
+// Implementation note: Print is written for AST nodes but could be
+// used to print arbitrary data structures; such a version should
+// probably be in a different package.
+//
+// Note: This code detects (some) cycles created via pointers but
+// not cycles that are created via slices or maps containing the
+// same slice or map. Code for general data structures probably
+// should catch those as well.
+
+func (p *printer) print(x reflect.Value) {
+	if !NotNilFilter("", x) {
+		p.printf("nil")
+		return
+	}
+
+	switch x.Kind() {
+	case reflect.Interface:
+		p.print(x.Elem())
+
+	case reflect.Map:
+		p.printf("%s (len = %d) {", x.Type(), x.Len())
+		if x.Len() > 0 {
+			p.indent++
+			p.printf("\n")
+			for _, key := range x.MapKeys() {
+				p.print(key)
+				p.printf(": ")
+				p.print(x.MapIndex(key))
+				p.printf("\n")
+			}
+			p.indent--
+		}
+		p.printf("}")
+
+	case reflect.Ptr:
+		p.printf("*")
+		// type-checked ASTs may contain cycles - use ptrmap
+		// to keep track of objects that have been printed
+		// already and print the respective line number instead
+		ptr := x.Interface()
+		if line, exists := p.ptrmap[ptr]; exists {
+			p.printf("(obj @ %d)", line)
+		} else {
+			p.ptrmap[ptr] = p.line
+			p.print(x.Elem())
+		}
+
+	case reflect.Array:
+		p.printf("%s {", x.Type())
+		if x.Len() > 0 {
+			p.indent++
+			p.printf("\n")
+			for i, n := 0, x.Len(); i < n; i++ {
+				p.printf("%d: ", i)
+				p.print(x.Index(i))
+				p.printf("\n")
+			}
+			p.indent--
+		}
+		p.printf("}")
+
+	case reflect.Slice:
+		if s, ok := x.Interface().([]byte); ok {
+			p.printf("%#q", s)
+			return
+		}
+		p.printf("%s (len = %d) {", x.Type(), x.Len())
+		if x.Len() > 0 {
+			p.indent++
+			p.printf("\n")
+			for i, n := 0, x.Len(); i < n; i++ {
+				p.printf("%d: ", i)
+				p.print(x.Index(i))
+				p.printf("\n")
+			}
+			p.indent--
+		}
+		p.printf("}")
+
+	case reflect.Struct:
+		t := x.Type()
+		p.printf("%s {", t)
+		p.indent++
+		first := true
+		for i, n := 0, t.NumField(); i < n; i++ {
+			// exclude non-exported fields because their
+			// values cannot be accessed via reflection
+			if name := t.Field(i).Name; IsExported(name) {
+				value := x.Field(i)
+				if p.filter == nil || p.filter(name, value) {
+					if first {
+						p.printf("\n")
+						first = false
+					}
+					p.printf("%s: ", name)
+					p.print(value)
+					p.printf("\n")
+				}
+			}
+		}
+		p.indent--
+		p.printf("}")
+
+	default:
+		v := x.Interface()
+		switch v := v.(type) {
+		case string:
+			// print strings in quotes
+			p.printf("%q", v)
+			return
+		case token.Pos:
+			// position values can be printed nicely if we have a file set
+			if p.fset != nil {
+				p.printf("%s", p.fset.Position(v))
+				return
+			}
+		}
+		// default
+		p.printf("%v", v)
+	}
+}
diff --git a/internal/backport/go/ast/print_test.go b/internal/backport/go/ast/print_test.go
new file mode 100644
index 0000000..210f164
--- /dev/null
+++ b/internal/backport/go/ast/print_test.go
@@ -0,0 +1,97 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+var tests = []struct {
+	x interface{} // x is printed as s
+	s string
+}{
+	// basic types
+	{nil, "0  nil"},
+	{true, "0  true"},
+	{42, "0  42"},
+	{3.14, "0  3.14"},
+	{1 + 2.718i, "0  (1+2.718i)"},
+	{"foobar", "0  \"foobar\""},
+
+	// maps
+	{map[Expr]string{}, `0  map[ast.Expr]string (len = 0) {}`},
+	{map[string]int{"a": 1},
+		`0  map[string]int (len = 1) {
+		1  .  "a": 1
+		2  }`},
+
+	// pointers
+	{new(int), "0  *0"},
+
+	// arrays
+	{[0]int{}, `0  [0]int {}`},
+	{[3]int{1, 2, 3},
+		`0  [3]int {
+		1  .  0: 1
+		2  .  1: 2
+		3  .  2: 3
+		4  }`},
+	{[...]int{42},
+		`0  [1]int {
+		1  .  0: 42
+		2  }`},
+
+	// slices
+	{[]int{}, `0  []int (len = 0) {}`},
+	{[]int{1, 2, 3},
+		`0  []int (len = 3) {
+		1  .  0: 1
+		2  .  1: 2
+		3  .  2: 3
+		4  }`},
+
+	// structs
+	{struct{}{}, `0  struct {} {}`},
+	{struct{ x int }{007}, `0  struct { x int } {}`},
+	{struct{ X, y int }{42, 991},
+		`0  struct { X int; y int } {
+		1  .  X: 42
+		2  }`},
+	{struct{ X, Y int }{42, 991},
+		`0  struct { X int; Y int } {
+		1  .  X: 42
+		2  .  Y: 991
+		3  }`},
+}
+
+// Split s into lines, trim whitespace from all lines, and return
+// the concatenated non-empty lines.
+func trim(s string) string {
+	lines := strings.Split(s, "\n")
+	i := 0
+	for _, line := range lines {
+		line = strings.TrimSpace(line)
+		if line != "" {
+			lines[i] = line
+			i++
+		}
+	}
+	return strings.Join(lines[0:i], "\n")
+}
+
+func TestPrint(t *testing.T) {
+	var buf bytes.Buffer
+	for _, test := range tests {
+		buf.Reset()
+		if err := Fprint(&buf, nil, test.x, nil); err != nil {
+			t.Errorf("Fprint failed: %s", err)
+		}
+		if s, ts := trim(buf.String()), trim(test.s); s != ts {
+			t.Errorf("got:\n%s\nexpected:\n%s\n", s, ts)
+		}
+	}
+}
diff --git a/internal/backport/go/ast/resolve.go b/internal/backport/go/ast/resolve.go
new file mode 100644
index 0000000..4d3d3fd
--- /dev/null
+++ b/internal/backport/go/ast/resolve.go
@@ -0,0 +1,173 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements NewPackage.
+
+package ast
+
+import (
+	"fmt"
+	"golang.org/x/website/internal/backport/go/scanner"
+	"golang.org/x/website/internal/backport/go/token"
+	"strconv"
+)
+
+type pkgBuilder struct {
+	fset   *token.FileSet
+	errors scanner.ErrorList
+}
+
+func (p *pkgBuilder) error(pos token.Pos, msg string) {
+	p.errors.Add(p.fset.Position(pos), msg)
+}
+
+func (p *pkgBuilder) errorf(pos token.Pos, format string, args ...interface{}) {
+	p.error(pos, fmt.Sprintf(format, args...))
+}
+
+func (p *pkgBuilder) declare(scope, altScope *Scope, obj *Object) {
+	alt := scope.Insert(obj)
+	if alt == nil && altScope != nil {
+		// see if there is a conflicting declaration in altScope
+		alt = altScope.Lookup(obj.Name)
+	}
+	if alt != nil {
+		prevDecl := ""
+		if pos := alt.Pos(); pos.IsValid() {
+			prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", p.fset.Position(pos))
+		}
+		p.error(obj.Pos(), fmt.Sprintf("%s redeclared in this block%s", obj.Name, prevDecl))
+	}
+}
+
+func resolve(scope *Scope, ident *Ident) bool {
+	for ; scope != nil; scope = scope.Outer {
+		if obj := scope.Lookup(ident.Name); obj != nil {
+			ident.Obj = obj
+			return true
+		}
+	}
+	return false
+}
+
+// An Importer resolves import paths to package Objects.
+// The imports map records the packages already imported,
+// indexed by package id (canonical import path).
+// An Importer must determine the canonical import path and
+// check the map to see if it is already present in the imports map.
+// If so, the Importer can return the map entry. Otherwise, the
+// Importer should load the package data for the given path into
+// a new *Object (pkg), record pkg in the imports map, and then
+// return pkg.
+type Importer func(imports map[string]*Object, path string) (pkg *Object, err error)
+
+// NewPackage creates a new Package node from a set of File nodes. It resolves
+// unresolved identifiers across files and updates each file's Unresolved list
+// accordingly. If a non-nil importer and universe scope are provided, they are
+// used to resolve identifiers not declared in any of the package files. Any
+// remaining unresolved identifiers are reported as undeclared. If the files
+// belong to different packages, one package name is selected and files with
+// different package names are reported and then ignored.
+// The result is a package node and a scanner.ErrorList if there were errors.
+func NewPackage(fset *token.FileSet, files map[string]*File, importer Importer, universe *Scope) (*Package, error) {
+	var p pkgBuilder
+	p.fset = fset
+
+	// complete package scope
+	pkgName := ""
+	pkgScope := NewScope(universe)
+	for _, file := range files {
+		// package names must match
+		switch name := file.Name.Name; {
+		case pkgName == "":
+			pkgName = name
+		case name != pkgName:
+			p.errorf(file.Package, "package %s; expected %s", name, pkgName)
+			continue // ignore this file
+		}
+
+		// collect top-level file objects in package scope
+		for _, obj := range file.Scope.Objects {
+			p.declare(pkgScope, nil, obj)
+		}
+	}
+
+	// package global mapping of imported package ids to package objects
+	imports := make(map[string]*Object)
+
+	// complete file scopes with imports and resolve identifiers
+	for _, file := range files {
+		// ignore file if it belongs to a different package
+		// (error has already been reported)
+		if file.Name.Name != pkgName {
+			continue
+		}
+
+		// build file scope by processing all imports
+		importErrors := false
+		fileScope := NewScope(pkgScope)
+		for _, spec := range file.Imports {
+			if importer == nil {
+				importErrors = true
+				continue
+			}
+			path, _ := strconv.Unquote(spec.Path.Value)
+			pkg, err := importer(imports, path)
+			if err != nil {
+				p.errorf(spec.Path.Pos(), "could not import %s (%s)", path, err)
+				importErrors = true
+				continue
+			}
+			// TODO(gri) If a local package name != "." is provided,
+			// global identifier resolution could proceed even if the
+			// import failed. Consider adjusting the logic here a bit.
+
+			// local name overrides imported package name
+			name := pkg.Name
+			if spec.Name != nil {
+				name = spec.Name.Name
+			}
+
+			// add import to file scope
+			if name == "." {
+				// merge imported scope with file scope
+				for _, obj := range pkg.Data.(*Scope).Objects {
+					p.declare(fileScope, pkgScope, obj)
+				}
+			} else if name != "_" {
+				// declare imported package object in file scope
+				// (do not re-use pkg in the file scope but create
+				// a new object instead; the Decl field is different
+				// for different files)
+				obj := NewObj(Pkg, name)
+				obj.Decl = spec
+				obj.Data = pkg.Data
+				p.declare(fileScope, pkgScope, obj)
+			}
+		}
+
+		// resolve identifiers
+		if importErrors {
+			// don't use the universe scope without correct imports
+			// (objects in the universe may be shadowed by imports;
+			// with missing imports, identifiers might get resolved
+			// incorrectly to universe objects)
+			pkgScope.Outer = nil
+		}
+		i := 0
+		for _, ident := range file.Unresolved {
+			if !resolve(fileScope, ident) {
+				p.errorf(ident.Pos(), "undeclared name: %s", ident.Name)
+				file.Unresolved[i] = ident
+				i++
+			}
+
+		}
+		file.Unresolved = file.Unresolved[0:i]
+		pkgScope.Outer = universe // reset universe scope
+	}
+
+	p.errors.Sort()
+	return &Package{pkgName, pkgScope, imports, files}, p.errors.Err()
+}
diff --git a/internal/backport/go/ast/scope.go b/internal/backport/go/ast/scope.go
new file mode 100644
index 0000000..165c935
--- /dev/null
+++ b/internal/backport/go/ast/scope.go
@@ -0,0 +1,156 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements scopes and the objects they contain.
+
+package ast
+
+import (
+	"bytes"
+	"fmt"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+// A Scope maintains the set of named language entities declared
+// in the scope and a link to the immediately surrounding (outer)
+// scope.
+type Scope struct {
+	Outer   *Scope
+	Objects map[string]*Object
+}
+
+// NewScope creates a new scope nested in the outer scope.
+func NewScope(outer *Scope) *Scope {
+	const n = 4 // initial scope capacity
+	return &Scope{outer, make(map[string]*Object, n)}
+}
+
+// Lookup returns the object with the given name if it is
+// found in scope s, otherwise it returns nil. Outer scopes
+// are ignored.
+func (s *Scope) Lookup(name string) *Object {
+	return s.Objects[name]
+}
+
+// Insert attempts to insert a named object obj into the scope s.
+// If the scope already contains an object alt with the same name,
+// Insert leaves the scope unchanged and returns alt. Otherwise
+// it inserts obj and returns nil.
+func (s *Scope) Insert(obj *Object) (alt *Object) {
+	if alt = s.Objects[obj.Name]; alt == nil {
+		s.Objects[obj.Name] = obj
+	}
+	return
+}
+
+// Debugging support
+func (s *Scope) String() string {
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, "scope %p {", s)
+	if s != nil && len(s.Objects) > 0 {
+		fmt.Fprintln(&buf)
+		for _, obj := range s.Objects {
+			fmt.Fprintf(&buf, "\t%s %s\n", obj.Kind, obj.Name)
+		}
+	}
+	fmt.Fprintf(&buf, "}\n")
+	return buf.String()
+}
+
+// ----------------------------------------------------------------------------
+// Objects
+
+// An Object describes a named language entity such as a package,
+// constant, type, variable, function (incl. methods), or label.
+//
+// The Data fields contains object-specific data:
+//
+//	Kind    Data type         Data value
+//	Pkg     *Scope            package scope
+//	Con     int               iota for the respective declaration
+type Object struct {
+	Kind ObjKind
+	Name string      // declared name
+	Decl interface{} // corresponding Field, XxxSpec, FuncDecl, LabeledStmt, AssignStmt, Scope; or nil
+	Data interface{} // object-specific data; or nil
+	Type interface{} // placeholder for type information; may be nil
+}
+
+// NewObj creates a new object of a given kind and name.
+func NewObj(kind ObjKind, name string) *Object {
+	return &Object{Kind: kind, Name: name}
+}
+
+// Pos computes the source position of the declaration of an object name.
+// The result may be an invalid position if it cannot be computed
+// (obj.Decl may be nil or not correct).
+func (obj *Object) Pos() token.Pos {
+	name := obj.Name
+	switch d := obj.Decl.(type) {
+	case *Field:
+		for _, n := range d.Names {
+			if n.Name == name {
+				return n.Pos()
+			}
+		}
+	case *ImportSpec:
+		if d.Name != nil && d.Name.Name == name {
+			return d.Name.Pos()
+		}
+		return d.Path.Pos()
+	case *ValueSpec:
+		for _, n := range d.Names {
+			if n.Name == name {
+				return n.Pos()
+			}
+		}
+	case *TypeSpec:
+		if d.Name.Name == name {
+			return d.Name.Pos()
+		}
+	case *FuncDecl:
+		if d.Name.Name == name {
+			return d.Name.Pos()
+		}
+	case *LabeledStmt:
+		if d.Label.Name == name {
+			return d.Label.Pos()
+		}
+	case *AssignStmt:
+		for _, x := range d.Lhs {
+			if ident, isIdent := x.(*Ident); isIdent && ident.Name == name {
+				return ident.Pos()
+			}
+		}
+	case *Scope:
+		// predeclared object - nothing to do for now
+	}
+	return token.NoPos
+}
+
+// ObjKind describes what an object represents.
+type ObjKind int
+
+// The list of possible Object kinds.
+const (
+	Bad ObjKind = iota // for error handling
+	Pkg                // package
+	Con                // constant
+	Typ                // type
+	Var                // variable
+	Fun                // function or method
+	Lbl                // label
+)
+
+var objKindStrings = [...]string{
+	Bad: "bad",
+	Pkg: "package",
+	Con: "const",
+	Typ: "type",
+	Var: "var",
+	Fun: "func",
+	Lbl: "label",
+}
+
+func (kind ObjKind) String() string { return objKindStrings[kind] }
diff --git a/internal/backport/go/ast/walk.go b/internal/backport/go/ast/walk.go
new file mode 100644
index 0000000..0c64d27
--- /dev/null
+++ b/internal/backport/go/ast/walk.go
@@ -0,0 +1,398 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import "fmt"
+
+// A Visitor's Visit method is invoked for each node encountered by Walk.
+// If the result visitor w is not nil, Walk visits each of the children
+// of node with the visitor w, followed by a call of w.Visit(nil).
+type Visitor interface {
+	Visit(node Node) (w Visitor)
+}
+
+// Helper functions for common node lists. They may be empty.
+
+func walkIdentList(v Visitor, list []*Ident) {
+	for _, x := range list {
+		Walk(v, x)
+	}
+}
+
+func walkExprList(v Visitor, list []Expr) {
+	for _, x := range list {
+		Walk(v, x)
+	}
+}
+
+func walkStmtList(v Visitor, list []Stmt) {
+	for _, x := range list {
+		Walk(v, x)
+	}
+}
+
+func walkDeclList(v Visitor, list []Decl) {
+	for _, x := range list {
+		Walk(v, x)
+	}
+}
+
+// TODO(gri): Investigate if providing a closure to Walk leads to
+//            simpler use (and may help eliminate Inspect in turn).
+
+// Walk traverses an AST in depth-first order: It starts by calling
+// v.Visit(node); node must not be nil. If the visitor w returned by
+// v.Visit(node) is not nil, Walk is invoked recursively with visitor
+// w for each of the non-nil children of node, followed by a call of
+// w.Visit(nil).
+func Walk(v Visitor, node Node) {
+	if v = v.Visit(node); v == nil {
+		return
+	}
+
+	// walk children
+	// (the order of the cases matches the order
+	// of the corresponding node types in ast.go)
+	switch n := node.(type) {
+	// Comments and fields
+	case *Comment:
+		// nothing to do
+
+	case *CommentGroup:
+		for _, c := range n.List {
+			Walk(v, c)
+		}
+
+	case *Field:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		walkIdentList(v, n.Names)
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
+		if n.Tag != nil {
+			Walk(v, n.Tag)
+		}
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
+
+	case *FieldList:
+		for _, f := range n.List {
+			Walk(v, f)
+		}
+
+	// Expressions
+	case *BadExpr, *Ident, *BasicLit:
+		// nothing to do
+
+	case *Ellipsis:
+		if n.Elt != nil {
+			Walk(v, n.Elt)
+		}
+
+	case *FuncLit:
+		Walk(v, n.Type)
+		Walk(v, n.Body)
+
+	case *CompositeLit:
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
+		walkExprList(v, n.Elts)
+
+	case *ParenExpr:
+		Walk(v, n.X)
+
+	case *SelectorExpr:
+		Walk(v, n.X)
+		Walk(v, n.Sel)
+
+	case *IndexExpr:
+		Walk(v, n.X)
+		Walk(v, n.Index)
+
+	case *IndexListExpr:
+		Walk(v, n.X)
+		for _, index := range n.Indices {
+			Walk(v, index)
+		}
+
+	case *SliceExpr:
+		Walk(v, n.X)
+		if n.Low != nil {
+			Walk(v, n.Low)
+		}
+		if n.High != nil {
+			Walk(v, n.High)
+		}
+		if n.Max != nil {
+			Walk(v, n.Max)
+		}
+
+	case *TypeAssertExpr:
+		Walk(v, n.X)
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
+
+	case *CallExpr:
+		Walk(v, n.Fun)
+		walkExprList(v, n.Args)
+
+	case *StarExpr:
+		Walk(v, n.X)
+
+	case *UnaryExpr:
+		Walk(v, n.X)
+
+	case *BinaryExpr:
+		Walk(v, n.X)
+		Walk(v, n.Y)
+
+	case *KeyValueExpr:
+		Walk(v, n.Key)
+		Walk(v, n.Value)
+
+	// Types
+	case *ArrayType:
+		if n.Len != nil {
+			Walk(v, n.Len)
+		}
+		Walk(v, n.Elt)
+
+	case *StructType:
+		Walk(v, n.Fields)
+
+	case *FuncType:
+		if n.TypeParams != nil {
+			Walk(v, n.TypeParams)
+		}
+		if n.Params != nil {
+			Walk(v, n.Params)
+		}
+		if n.Results != nil {
+			Walk(v, n.Results)
+		}
+
+	case *InterfaceType:
+		Walk(v, n.Methods)
+
+	case *MapType:
+		Walk(v, n.Key)
+		Walk(v, n.Value)
+
+	case *ChanType:
+		Walk(v, n.Value)
+
+	// Statements
+	case *BadStmt:
+		// nothing to do
+
+	case *DeclStmt:
+		Walk(v, n.Decl)
+
+	case *EmptyStmt:
+		// nothing to do
+
+	case *LabeledStmt:
+		Walk(v, n.Label)
+		Walk(v, n.Stmt)
+
+	case *ExprStmt:
+		Walk(v, n.X)
+
+	case *SendStmt:
+		Walk(v, n.Chan)
+		Walk(v, n.Value)
+
+	case *IncDecStmt:
+		Walk(v, n.X)
+
+	case *AssignStmt:
+		walkExprList(v, n.Lhs)
+		walkExprList(v, n.Rhs)
+
+	case *GoStmt:
+		Walk(v, n.Call)
+
+	case *DeferStmt:
+		Walk(v, n.Call)
+
+	case *ReturnStmt:
+		walkExprList(v, n.Results)
+
+	case *BranchStmt:
+		if n.Label != nil {
+			Walk(v, n.Label)
+		}
+
+	case *BlockStmt:
+		walkStmtList(v, n.List)
+
+	case *IfStmt:
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		Walk(v, n.Cond)
+		Walk(v, n.Body)
+		if n.Else != nil {
+			Walk(v, n.Else)
+		}
+
+	case *CaseClause:
+		walkExprList(v, n.List)
+		walkStmtList(v, n.Body)
+
+	case *SwitchStmt:
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		if n.Tag != nil {
+			Walk(v, n.Tag)
+		}
+		Walk(v, n.Body)
+
+	case *TypeSwitchStmt:
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		Walk(v, n.Assign)
+		Walk(v, n.Body)
+
+	case *CommClause:
+		if n.Comm != nil {
+			Walk(v, n.Comm)
+		}
+		walkStmtList(v, n.Body)
+
+	case *SelectStmt:
+		Walk(v, n.Body)
+
+	case *ForStmt:
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		if n.Cond != nil {
+			Walk(v, n.Cond)
+		}
+		if n.Post != nil {
+			Walk(v, n.Post)
+		}
+		Walk(v, n.Body)
+
+	case *RangeStmt:
+		if n.Key != nil {
+			Walk(v, n.Key)
+		}
+		if n.Value != nil {
+			Walk(v, n.Value)
+		}
+		Walk(v, n.X)
+		Walk(v, n.Body)
+
+	// Declarations
+	case *ImportSpec:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		if n.Name != nil {
+			Walk(v, n.Name)
+		}
+		Walk(v, n.Path)
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
+
+	case *ValueSpec:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		walkIdentList(v, n.Names)
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
+		walkExprList(v, n.Values)
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
+
+	case *TypeSpec:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		Walk(v, n.Name)
+		if n.TypeParams != nil {
+			Walk(v, n.TypeParams)
+		}
+		Walk(v, n.Type)
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
+
+	case *BadDecl:
+		// nothing to do
+
+	case *GenDecl:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		for _, s := range n.Specs {
+			Walk(v, s)
+		}
+
+	case *FuncDecl:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		if n.Recv != nil {
+			Walk(v, n.Recv)
+		}
+		Walk(v, n.Name)
+		Walk(v, n.Type)
+		if n.Body != nil {
+			Walk(v, n.Body)
+		}
+
+	// Files and packages
+	case *File:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		Walk(v, n.Name)
+		walkDeclList(v, n.Decls)
+		// don't walk n.Comments - they have been
+		// visited already through the individual
+		// nodes
+
+	case *Package:
+		for _, f := range n.Files {
+			Walk(v, f)
+		}
+
+	default:
+		panic(fmt.Sprintf("ast.Walk: unexpected node type %T", n))
+	}
+
+	v.Visit(nil)
+}
+
+type inspector func(Node) bool
+
+func (f inspector) Visit(node Node) Visitor {
+	if f(node) {
+		return f
+	}
+	return nil
+}
+
+// Inspect traverses an AST in depth-first order: It starts by calling
+// f(node); node must not be nil. If f returns true, Inspect invokes f
+// recursively for each of the non-nil children of node, followed by a
+// call of f(nil).
+func Inspect(node Node, f func(Node) bool) {
+	Walk(inspector(f), node)
+}
diff --git a/internal/backport/go/build/build.go b/internal/backport/go/build/build.go
new file mode 100644
index 0000000..12866e7
--- /dev/null
+++ b/internal/backport/go/build/build.go
@@ -0,0 +1,1973 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"go/build"
+	"go/build/constraint"
+	"golang.org/x/website/internal/backport/go/doc"
+	"io"
+	"io/fs"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	pathpkg "path"
+	"path/filepath"
+	"runtime"
+	"sort"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+// A Context specifies the supporting context for a build.
+type Context struct {
+	GOARCH string // target architecture
+	GOOS   string // target operating system
+	GOROOT string // Go root
+	GOPATH string // Go path
+
+	// Dir is the caller's working directory, or the empty string to use
+	// the current directory of the running process. In module mode, this is used
+	// to locate the main module.
+	//
+	// If Dir is non-empty, directories passed to Import and ImportDir must
+	// be absolute.
+	Dir string
+
+	CgoEnabled  bool   // whether cgo files are included
+	UseAllFiles bool   // use files regardless of +build lines, file names
+	Compiler    string // compiler to assume when computing target paths
+
+	// The build, tool, and release tags specify build constraints
+	// that should be considered satisfied when processing +build lines.
+	// Clients creating a new context may customize BuildTags, which
+	// defaults to empty, but it is usually an error to customize ToolTags or ReleaseTags.
+	// ToolTags defaults to build tags appropriate to the current Go toolchain configuration.
+	// ReleaseTags defaults to the list of Go releases the current release is compatible with.
+	// BuildTags is not set for the Default build Context.
+	// In addition to the BuildTags, ToolTags, and ReleaseTags, build constraints
+	// consider the values of GOARCH and GOOS as satisfied tags.
+	// The last element in ReleaseTags is assumed to be the current release.
+	BuildTags   []string
+	ToolTags    []string
+	ReleaseTags []string
+
+	// The install suffix specifies a suffix to use in the name of the installation
+	// directory. By default it is empty, but custom builds that need to keep
+	// their outputs separate can set InstallSuffix to do so. For example, when
+	// using the race detector, the go command uses InstallSuffix = "race", so
+	// that on a Linux/386 system, packages are written to a directory named
+	// "linux_386_race" instead of the usual "linux_386".
+	InstallSuffix string
+
+	// By default, Import uses the operating system's file system calls
+	// to read directories and files. To read from other sources,
+	// callers can set the following functions. They all have default
+	// behaviors that use the local file system, so clients need only set
+	// the functions whose behaviors they wish to change.
+
+	// JoinPath joins the sequence of path fragments into a single path.
+	// If JoinPath is nil, Import uses filepath.Join.
+	JoinPath func(elem ...string) string
+
+	// SplitPathList splits the path list into a slice of individual paths.
+	// If SplitPathList is nil, Import uses filepath.SplitList.
+	SplitPathList func(list string) []string
+
+	// IsAbsPath reports whether path is an absolute path.
+	// If IsAbsPath is nil, Import uses filepath.IsAbs.
+	IsAbsPath func(path string) bool
+
+	// IsDir reports whether the path names a directory.
+	// If IsDir is nil, Import calls os.Stat and uses the result's IsDir method.
+	IsDir func(path string) bool
+
+	// HasSubdir reports whether dir is lexically a subdirectory of
+	// root, perhaps multiple levels below. It does not try to check
+	// whether dir exists.
+	// If so, HasSubdir sets rel to a slash-separated path that
+	// can be joined to root to produce a path equivalent to dir.
+	// If HasSubdir is nil, Import uses an implementation built on
+	// filepath.EvalSymlinks.
+	HasSubdir func(root, dir string) (rel string, ok bool)
+
+	// ReadDir returns a slice of fs.FileInfo, sorted by Name,
+	// describing the content of the named directory.
+	// If ReadDir is nil, Import uses ioutil.ReadDir.
+	ReadDir func(dir string) ([]fs.FileInfo, error)
+
+	// OpenFile opens a file (not a directory) for reading.
+	// If OpenFile is nil, Import uses os.Open.
+	OpenFile func(path string) (io.ReadCloser, error)
+}
+
+// joinPath calls ctxt.JoinPath (if not nil) or else filepath.Join.
+func (ctxt *Context) joinPath(elem ...string) string {
+	if f := ctxt.JoinPath; f != nil {
+		return f(elem...)
+	}
+	return filepath.Join(elem...)
+}
+
+// splitPathList calls ctxt.SplitPathList (if not nil) or else filepath.SplitList.
+func (ctxt *Context) splitPathList(s string) []string {
+	if f := ctxt.SplitPathList; f != nil {
+		return f(s)
+	}
+	return filepath.SplitList(s)
+}
+
+// isAbsPath calls ctxt.IsAbsPath (if not nil) or else filepath.IsAbs.
+func (ctxt *Context) isAbsPath(path string) bool {
+	if f := ctxt.IsAbsPath; f != nil {
+		return f(path)
+	}
+	return filepath.IsAbs(path)
+}
+
+// isDir calls ctxt.IsDir (if not nil) or else uses os.Stat.
+func (ctxt *Context) isDir(path string) bool {
+	if f := ctxt.IsDir; f != nil {
+		return f(path)
+	}
+	fi, err := os.Stat(path)
+	return err == nil && fi.IsDir()
+}
+
+// hasSubdir calls ctxt.HasSubdir (if not nil) or else uses
+// the local file system to answer the question.
+func (ctxt *Context) hasSubdir(root, dir string) (rel string, ok bool) {
+	if f := ctxt.HasSubdir; f != nil {
+		return f(root, dir)
+	}
+
+	// Try using paths we received.
+	if rel, ok = hasSubdir(root, dir); ok {
+		return
+	}
+
+	// Try expanding symlinks and comparing
+	// expanded against unexpanded and
+	// expanded against expanded.
+	rootSym, _ := filepath.EvalSymlinks(root)
+	dirSym, _ := filepath.EvalSymlinks(dir)
+
+	if rel, ok = hasSubdir(rootSym, dir); ok {
+		return
+	}
+	if rel, ok = hasSubdir(root, dirSym); ok {
+		return
+	}
+	return hasSubdir(rootSym, dirSym)
+}
+
+// hasSubdir reports if dir is within root by performing lexical analysis only.
+func hasSubdir(root, dir string) (rel string, ok bool) {
+	const sep = string(filepath.Separator)
+	root = filepath.Clean(root)
+	if !strings.HasSuffix(root, sep) {
+		root += sep
+	}
+	dir = filepath.Clean(dir)
+	if !strings.HasPrefix(dir, root) {
+		return "", false
+	}
+	return filepath.ToSlash(dir[len(root):]), true
+}
+
+// readDir calls ctxt.ReadDir (if not nil) or else ioutil.ReadDir.
+func (ctxt *Context) readDir(path string) ([]fs.FileInfo, error) {
+	if f := ctxt.ReadDir; f != nil {
+		return f(path)
+	}
+	// TODO: use os.ReadDir
+	return ioutil.ReadDir(path)
+}
+
+// openFile calls ctxt.OpenFile (if not nil) or else os.Open.
+func (ctxt *Context) openFile(path string) (io.ReadCloser, error) {
+	if fn := ctxt.OpenFile; fn != nil {
+		return fn(path)
+	}
+
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err // nil interface
+	}
+	return f, nil
+}
+
+// isFile determines whether path is a file by trying to open it.
+// It reuses openFile instead of adding another function to the
+// list in Context.
+func (ctxt *Context) isFile(path string) bool {
+	f, err := ctxt.openFile(path)
+	if err != nil {
+		return false
+	}
+	f.Close()
+	return true
+}
+
+// gopath returns the list of Go path directories.
+func (ctxt *Context) gopath() []string {
+	var all []string
+	for _, p := range ctxt.splitPathList(ctxt.GOPATH) {
+		if p == "" || p == ctxt.GOROOT {
+			// Empty paths are uninteresting.
+			// If the path is the GOROOT, ignore it.
+			// People sometimes set GOPATH=$GOROOT.
+			// Do not get confused by this common mistake.
+			continue
+		}
+		if strings.HasPrefix(p, "~") {
+			// Path segments starting with ~ on Unix are almost always
+			// users who have incorrectly quoted ~ while setting GOPATH,
+			// preventing it from expanding to $HOME.
+			// The situation is made more confusing by the fact that
+			// bash allows quoted ~ in $PATH (most shells do not).
+			// Do not get confused by this, and do not try to use the path.
+			// It does not exist, and printing errors about it confuses
+			// those users even more, because they think "sure ~ exists!".
+			// The go command diagnoses this situation and prints a
+			// useful error.
+			// On Windows, ~ is used in short names, such as c:\progra~1
+			// for c:\program files.
+			continue
+		}
+		all = append(all, p)
+	}
+	return all
+}
+
+// SrcDirs returns a list of package source root directories.
+// It draws from the current Go root and Go path but omits directories
+// that do not exist.
+func (ctxt *Context) SrcDirs() []string {
+	var all []string
+	if ctxt.GOROOT != "" && ctxt.Compiler != "gccgo" {
+		dir := ctxt.joinPath(ctxt.GOROOT, "src")
+		if ctxt.isDir(dir) {
+			all = append(all, dir)
+		}
+	}
+	for _, p := range ctxt.gopath() {
+		dir := ctxt.joinPath(p, "src")
+		if ctxt.isDir(dir) {
+			all = append(all, dir)
+		}
+	}
+	return all
+}
+
+// Default is the default Context for builds.
+// It uses the GOARCH, GOOS, GOROOT, and GOPATH environment variables
+// if set, or else the compiled code's GOARCH, GOOS, and GOROOT.
+var Default Context = defaultContext()
+
+func defaultContext() Context {
+	d := build.Default
+	return Context{
+		GOARCH:        d.GOARCH,
+		GOOS:          d.GOOS,
+		GOROOT:        d.GOROOT,
+		GOPATH:        d.GOPATH,
+		Dir:           d.Dir,
+		CgoEnabled:    d.CgoEnabled,
+		UseAllFiles:   d.UseAllFiles,
+		Compiler:      d.Compiler,
+		BuildTags:     d.BuildTags,
+		ReleaseTags:   d.ReleaseTags,
+		InstallSuffix: d.InstallSuffix,
+		JoinPath:      d.JoinPath,
+		SplitPathList: d.SplitPathList,
+		IsAbsPath:     d.IsAbsPath,
+		IsDir:         d.IsDir,
+		HasSubdir:     d.HasSubdir,
+		ReadDir:       d.ReadDir,
+		OpenFile:      d.OpenFile,
+	}
+}
+
+func defaultGOPATH() string {
+	env := "HOME"
+	if runtime.GOOS == "windows" {
+		env = "USERPROFILE"
+	} else if runtime.GOOS == "plan9" {
+		env = "home"
+	}
+	if home := os.Getenv(env); home != "" {
+		def := filepath.Join(home, "go")
+		if filepath.Clean(def) == filepath.Clean(runtime.GOROOT()) {
+			// Don't set the default GOPATH to GOROOT,
+			// as that will trigger warnings from the go tool.
+			return ""
+		}
+		return def
+	}
+	return ""
+}
+
+var defaultReleaseTags = build.Default.ReleaseTags
+
+func envOr(name, def string) string {
+	s := os.Getenv(name)
+	if s == "" {
+		return def
+	}
+	return s
+}
+
+// An ImportMode controls the behavior of the Import method.
+type ImportMode uint
+
+const (
+	// If FindOnly is set, Import stops after locating the directory
+	// that should contain the sources for a package. It does not
+	// read any files in the directory.
+	FindOnly ImportMode = 1 << iota
+
+	// If AllowBinary is set, Import can be satisfied by a compiled
+	// package object without corresponding sources.
+	//
+	// Deprecated:
+	// The supported way to create a compiled-only package is to
+	// write source code containing a //go:binary-only-package comment at
+	// the top of the file. Such a package will be recognized
+	// regardless of this flag setting (because it has source code)
+	// and will have BinaryOnly set to true in the returned Package.
+	AllowBinary
+
+	// If ImportComment is set, parse import comments on package statements.
+	// Import returns an error if it finds a comment it cannot understand
+	// or finds conflicting comments in multiple source files.
+	// See golang.org/s/go14customimport for more information.
+	ImportComment
+
+	// By default, Import searches vendor directories
+	// that apply in the given source directory before searching
+	// the GOROOT and GOPATH roots.
+	// If an Import finds and returns a package using a vendor
+	// directory, the resulting ImportPath is the complete path
+	// to the package, including the path elements leading up
+	// to and including "vendor".
+	// For example, if Import("y", "x/subdir", 0) finds
+	// "x/vendor/y", the returned package's ImportPath is "x/vendor/y",
+	// not plain "y".
+	// See golang.org/s/go15vendor for more information.
+	//
+	// Setting IgnoreVendor ignores vendor directories.
+	//
+	// In contrast to the package's ImportPath,
+	// the returned package's Imports, TestImports, and XTestImports
+	// are always the exact import paths from the source files:
+	// Import makes no attempt to resolve or check those paths.
+	IgnoreVendor
+)
+
+// A Package describes the Go package found in a directory.
+type Package struct {
+	Dir           string   // directory containing package sources
+	Name          string   // package name
+	ImportComment string   // path in import comment on package statement
+	Doc           string   // documentation synopsis
+	ImportPath    string   // import path of package ("" if unknown)
+	Root          string   // root of Go tree where this package lives
+	SrcRoot       string   // package source root directory ("" if unknown)
+	PkgRoot       string   // package install root directory ("" if unknown)
+	PkgTargetRoot string   // architecture dependent install root directory ("" if unknown)
+	BinDir        string   // command install directory ("" if unknown)
+	Goroot        bool     // package found in Go root
+	PkgObj        string   // installed .a file
+	AllTags       []string // tags that can influence file selection in this directory
+	ConflictDir   string   // this directory shadows Dir in $GOPATH
+	BinaryOnly    bool     // cannot be rebuilt from source (has //go:binary-only-package comment)
+
+	// Source files
+	GoFiles           []string // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
+	CgoFiles          []string // .go source files that import "C"
+	IgnoredGoFiles    []string // .go source files ignored for this build (including ignored _test.go files)
+	InvalidGoFiles    []string // .go source files with detected problems (parse error, wrong package name, and so on)
+	IgnoredOtherFiles []string // non-.go source files ignored for this build
+	CFiles            []string // .c source files
+	CXXFiles          []string // .cc, .cpp and .cxx source files
+	MFiles            []string // .m (Objective-C) source files
+	HFiles            []string // .h, .hh, .hpp and .hxx source files
+	FFiles            []string // .f, .F, .for and .f90 Fortran source files
+	SFiles            []string // .s source files
+	SwigFiles         []string // .swig files
+	SwigCXXFiles      []string // .swigcxx files
+	SysoFiles         []string // .syso system object files to add to archive
+
+	// Cgo directives
+	CgoCFLAGS    []string // Cgo CFLAGS directives
+	CgoCPPFLAGS  []string // Cgo CPPFLAGS directives
+	CgoCXXFLAGS  []string // Cgo CXXFLAGS directives
+	CgoFFLAGS    []string // Cgo FFLAGS directives
+	CgoLDFLAGS   []string // Cgo LDFLAGS directives
+	CgoPkgConfig []string // Cgo pkg-config directives
+
+	// Test information
+	TestGoFiles  []string // _test.go files in package
+	XTestGoFiles []string // _test.go files outside package
+
+	// Dependency information
+	Imports        []string                    // import paths from GoFiles, CgoFiles
+	ImportPos      map[string][]token.Position // line information for Imports
+	TestImports    []string                    // import paths from TestGoFiles
+	TestImportPos  map[string][]token.Position // line information for TestImports
+	XTestImports   []string                    // import paths from XTestGoFiles
+	XTestImportPos map[string][]token.Position // line information for XTestImports
+
+	// //go:embed patterns found in Go source files
+	// For example, if a source file says
+	//	//go:embed a* b.c
+	// then the list will contain those two strings as separate entries.
+	// (See package embed for more details about //go:embed.)
+	EmbedPatterns        []string                    // patterns from GoFiles, CgoFiles
+	EmbedPatternPos      map[string][]token.Position // line information for EmbedPatterns
+	TestEmbedPatterns    []string                    // patterns from TestGoFiles
+	TestEmbedPatternPos  map[string][]token.Position // line information for TestEmbedPatterns
+	XTestEmbedPatterns   []string                    // patterns from XTestGoFiles
+	XTestEmbedPatternPos map[string][]token.Position // line information for XTestEmbedPatternPos
+}
+
+// IsCommand reports whether the package is considered a
+// command to be installed (not just a library).
+// Packages named "main" are treated as commands.
+func (p *Package) IsCommand() bool {
+	return p.Name == "main"
+}
+
+// ImportDir is like Import but processes the Go package found in
+// the named directory.
+func (ctxt *Context) ImportDir(dir string, mode ImportMode) (*Package, error) {
+	return ctxt.Import(".", dir, mode)
+}
+
+// NoGoError is the error used by Import to describe a directory
+// containing no buildable Go source files. (It may still contain
+// test files, files hidden by build tags, and so on.)
+type NoGoError struct {
+	Dir string
+}
+
+func (e *NoGoError) Error() string {
+	return "no buildable Go source files in " + e.Dir
+}
+
+// MultiplePackageError describes a directory containing
+// multiple buildable Go source files for multiple packages.
+type MultiplePackageError struct {
+	Dir      string   // directory containing files
+	Packages []string // package names found
+	Files    []string // corresponding files: Files[i] declares package Packages[i]
+}
+
+func (e *MultiplePackageError) Error() string {
+	// Error string limited to two entries for compatibility.
+	return fmt.Sprintf("found packages %s (%s) and %s (%s) in %s", e.Packages[0], e.Files[0], e.Packages[1], e.Files[1], e.Dir)
+}
+
+func nameExt(name string) string {
+	i := strings.LastIndex(name, ".")
+	if i < 0 {
+		return ""
+	}
+	return name[i:]
+}
+
+// Import returns details about the Go package named by the import path,
+// interpreting local import paths relative to the srcDir directory.
+// If the path is a local import path naming a package that can be imported
+// using a standard import path, the returned package will set p.ImportPath
+// to that path.
+//
+// In the directory containing the package, .go, .c, .h, and .s files are
+// considered part of the package except for:
+//
+//   - .go files in package documentation
+//   - files starting with _ or . (likely editor temporary files)
+//   - files with build constraints not satisfied by the context
+//
+// If an error occurs, Import returns a non-nil error and a non-nil
+// *Package containing partial information.
+func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Package, error) {
+	p := &Package{
+		ImportPath: path,
+	}
+	if path == "" {
+		return p, fmt.Errorf("import %q: invalid import path", path)
+	}
+
+	var pkgtargetroot string
+	var pkga string
+	var pkgerr error
+	suffix := ""
+	if ctxt.InstallSuffix != "" {
+		suffix = "_" + ctxt.InstallSuffix
+	}
+	switch ctxt.Compiler {
+	case "gccgo":
+		pkgtargetroot = "pkg/gccgo_" + ctxt.GOOS + "_" + ctxt.GOARCH + suffix
+	case "gc":
+		pkgtargetroot = "pkg/" + ctxt.GOOS + "_" + ctxt.GOARCH + suffix
+	default:
+		// Save error for end of function.
+		pkgerr = fmt.Errorf("import %q: unknown compiler %q", path, ctxt.Compiler)
+	}
+	setPkga := func() {
+		switch ctxt.Compiler {
+		case "gccgo":
+			dir, elem := pathpkg.Split(p.ImportPath)
+			pkga = pkgtargetroot + "/" + dir + "lib" + elem + ".a"
+		case "gc":
+			pkga = pkgtargetroot + "/" + p.ImportPath + ".a"
+		}
+	}
+	setPkga()
+
+	binaryOnly := false
+	if IsLocalImport(path) {
+		pkga = "" // local imports have no installed path
+		if srcDir == "" {
+			return p, fmt.Errorf("import %q: import relative to unknown directory", path)
+		}
+		if !ctxt.isAbsPath(path) {
+			p.Dir = ctxt.joinPath(srcDir, path)
+		}
+		// p.Dir directory may or may not exist. Gather partial information first, check if it exists later.
+		// Determine canonical import path, if any.
+		// Exclude results where the import path would include /testdata/.
+		inTestdata := func(sub string) bool {
+			return strings.Contains(sub, "/testdata/") || strings.HasSuffix(sub, "/testdata") || strings.HasPrefix(sub, "testdata/") || sub == "testdata"
+		}
+		if ctxt.GOROOT != "" {
+			root := ctxt.joinPath(ctxt.GOROOT, "src")
+			if sub, ok := ctxt.hasSubdir(root, p.Dir); ok && !inTestdata(sub) {
+				p.Goroot = true
+				p.ImportPath = sub
+				p.Root = ctxt.GOROOT
+				setPkga() // p.ImportPath changed
+				goto Found
+			}
+		}
+		all := ctxt.gopath()
+		for i, root := range all {
+			rootsrc := ctxt.joinPath(root, "src")
+			if sub, ok := ctxt.hasSubdir(rootsrc, p.Dir); ok && !inTestdata(sub) {
+				// We found a potential import path for dir,
+				// but check that using it wouldn't find something
+				// else first.
+				if ctxt.GOROOT != "" && ctxt.Compiler != "gccgo" {
+					if dir := ctxt.joinPath(ctxt.GOROOT, "src", sub); ctxt.isDir(dir) {
+						p.ConflictDir = dir
+						goto Found
+					}
+				}
+				for _, earlyRoot := range all[:i] {
+					if dir := ctxt.joinPath(earlyRoot, "src", sub); ctxt.isDir(dir) {
+						p.ConflictDir = dir
+						goto Found
+					}
+				}
+
+				// sub would not name some other directory instead of this one.
+				// Record it.
+				p.ImportPath = sub
+				p.Root = root
+				setPkga() // p.ImportPath changed
+				goto Found
+			}
+		}
+		// It's okay that we didn't find a root containing dir.
+		// Keep going with the information we have.
+	} else {
+		if strings.HasPrefix(path, "/") {
+			return p, fmt.Errorf("import %q: cannot import absolute path", path)
+		}
+
+		if err := ctxt.importGo(p, path, srcDir, mode); err == nil {
+			goto Found
+		} else if err != errNoModules {
+			return p, err
+		}
+
+		gopath := ctxt.gopath() // needed twice below; avoid computing many times
+
+		// tried records the location of unsuccessful package lookups
+		var tried struct {
+			vendor []string
+			goroot string
+			gopath []string
+		}
+
+		// Vendor directories get first chance to satisfy import.
+		if mode&IgnoreVendor == 0 && srcDir != "" {
+			searchVendor := func(root string, isGoroot bool) bool {
+				sub, ok := ctxt.hasSubdir(root, srcDir)
+				if !ok || !strings.HasPrefix(sub, "src/") || strings.Contains(sub, "/testdata/") {
+					return false
+				}
+				for {
+					vendor := ctxt.joinPath(root, sub, "vendor")
+					if ctxt.isDir(vendor) {
+						dir := ctxt.joinPath(vendor, path)
+						if ctxt.isDir(dir) && hasGoFiles(ctxt, dir) {
+							p.Dir = dir
+							p.ImportPath = strings.TrimPrefix(pathpkg.Join(sub, "vendor", path), "src/")
+							p.Goroot = isGoroot
+							p.Root = root
+							setPkga() // p.ImportPath changed
+							return true
+						}
+						tried.vendor = append(tried.vendor, dir)
+					}
+					i := strings.LastIndex(sub, "/")
+					if i < 0 {
+						break
+					}
+					sub = sub[:i]
+				}
+				return false
+			}
+			if ctxt.Compiler != "gccgo" && searchVendor(ctxt.GOROOT, true) {
+				goto Found
+			}
+			for _, root := range gopath {
+				if searchVendor(root, false) {
+					goto Found
+				}
+			}
+		}
+
+		// Determine directory from import path.
+		if ctxt.GOROOT != "" {
+			// If the package path starts with "vendor/", only search GOROOT before
+			// GOPATH if the importer is also within GOROOT. That way, if the user has
+			// vendored in a package that is subsequently included in the standard
+			// distribution, they'll continue to pick up their own vendored copy.
+			gorootFirst := srcDir == "" || !strings.HasPrefix(path, "vendor/")
+			if !gorootFirst {
+				_, gorootFirst = ctxt.hasSubdir(ctxt.GOROOT, srcDir)
+			}
+			if gorootFirst {
+				dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
+				if ctxt.Compiler != "gccgo" {
+					isDir := ctxt.isDir(dir)
+					binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
+					if isDir || binaryOnly {
+						p.Dir = dir
+						p.Goroot = true
+						p.Root = ctxt.GOROOT
+						goto Found
+					}
+				}
+				tried.goroot = dir
+			}
+		}
+		for _, root := range gopath {
+			dir := ctxt.joinPath(root, "src", path)
+			isDir := ctxt.isDir(dir)
+			binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(root, pkga))
+			if isDir || binaryOnly {
+				p.Dir = dir
+				p.Root = root
+				goto Found
+			}
+			tried.gopath = append(tried.gopath, dir)
+		}
+
+		// If we tried GOPATH first due to a "vendor/" prefix, fall back to GOPATH.
+		// That way, the user can still get useful results from 'go list' for
+		// standard-vendored paths passed on the command line.
+		if ctxt.GOROOT != "" && tried.goroot == "" {
+			dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
+			if ctxt.Compiler != "gccgo" {
+				isDir := ctxt.isDir(dir)
+				binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
+				if isDir || binaryOnly {
+					p.Dir = dir
+					p.Goroot = true
+					p.Root = ctxt.GOROOT
+					goto Found
+				}
+			}
+			tried.goroot = dir
+		}
+
+		// package was not found
+		var paths []string
+		format := "\t%s (vendor tree)"
+		for _, dir := range tried.vendor {
+			paths = append(paths, fmt.Sprintf(format, dir))
+			format = "\t%s"
+		}
+		if tried.goroot != "" {
+			paths = append(paths, fmt.Sprintf("\t%s (from $GOROOT)", tried.goroot))
+		} else {
+			paths = append(paths, "\t($GOROOT not set)")
+		}
+		format = "\t%s (from $GOPATH)"
+		for _, dir := range tried.gopath {
+			paths = append(paths, fmt.Sprintf(format, dir))
+			format = "\t%s"
+		}
+		if len(tried.gopath) == 0 {
+			paths = append(paths, "\t($GOPATH not set. For more details see: 'go help gopath')")
+		}
+		return p, fmt.Errorf("cannot find package %q in any of:\n%s", path, strings.Join(paths, "\n"))
+	}
+
+Found:
+	if p.Root != "" {
+		p.SrcRoot = ctxt.joinPath(p.Root, "src")
+		p.PkgRoot = ctxt.joinPath(p.Root, "pkg")
+		p.BinDir = ctxt.joinPath(p.Root, "bin")
+		if pkga != "" {
+			p.PkgTargetRoot = ctxt.joinPath(p.Root, pkgtargetroot)
+			p.PkgObj = ctxt.joinPath(p.Root, pkga)
+		}
+	}
+
+	// If it's a local import path, by the time we get here, we still haven't checked
+	// that p.Dir directory exists. This is the right time to do that check.
+	// We can't do it earlier, because we want to gather partial information for the
+	// non-nil *Package returned when an error occurs.
+	// We need to do this before we return early on FindOnly flag.
+	if IsLocalImport(path) && !ctxt.isDir(p.Dir) {
+		if ctxt.Compiler == "gccgo" && p.Goroot {
+			// gccgo has no sources for GOROOT packages.
+			return p, nil
+		}
+
+		// package was not found
+		return p, fmt.Errorf("cannot find package %q in:\n\t%s", p.ImportPath, p.Dir)
+	}
+
+	if mode&FindOnly != 0 {
+		return p, pkgerr
+	}
+	if binaryOnly && (mode&AllowBinary) != 0 {
+		return p, pkgerr
+	}
+
+	if ctxt.Compiler == "gccgo" && p.Goroot {
+		// gccgo has no sources for GOROOT packages.
+		return p, nil
+	}
+
+	dirs, err := ctxt.readDir(p.Dir)
+	if err != nil {
+		return p, err
+	}
+
+	var badGoError error
+	badFiles := make(map[string]bool)
+	badFile := func(name string, err error) {
+		if badGoError == nil {
+			badGoError = err
+		}
+		if !badFiles[name] {
+			p.InvalidGoFiles = append(p.InvalidGoFiles, name)
+			badFiles[name] = true
+		}
+	}
+
+	var Sfiles []string // files with ".S"(capital S)/.sx(capital s equivalent for case insensitive filesystems)
+	var firstFile, firstCommentFile string
+	embedPos := make(map[string][]token.Position)
+	testEmbedPos := make(map[string][]token.Position)
+	xTestEmbedPos := make(map[string][]token.Position)
+	importPos := make(map[string][]token.Position)
+	testImportPos := make(map[string][]token.Position)
+	xTestImportPos := make(map[string][]token.Position)
+	allTags := make(map[string]bool)
+	fset := token.NewFileSet()
+	for _, d := range dirs {
+		if d.IsDir() {
+			continue
+		}
+		if d.Mode()&fs.ModeSymlink != 0 {
+			if ctxt.isDir(ctxt.joinPath(p.Dir, d.Name())) {
+				// Symlinks to directories are not source files.
+				continue
+			}
+		}
+
+		name := d.Name()
+		ext := nameExt(name)
+
+		info, err := ctxt.matchFile(p.Dir, name, allTags, &p.BinaryOnly, fset)
+		if err != nil {
+			badFile(name, err)
+			continue
+		}
+		if info == nil {
+			if strings.HasPrefix(name, "_") || strings.HasPrefix(name, ".") {
+				// not due to build constraints - don't report
+			} else if ext == ".go" {
+				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
+			} else if fileListForExt(p, ext) != nil {
+				p.IgnoredOtherFiles = append(p.IgnoredOtherFiles, name)
+			}
+			continue
+		}
+		data, filename := info.header, info.name
+
+		// Going to save the file. For non-Go files, can stop here.
+		switch ext {
+		case ".go":
+			// keep going
+		case ".S", ".sx":
+			// special case for cgo, handled at end
+			Sfiles = append(Sfiles, name)
+			continue
+		default:
+			if list := fileListForExt(p, ext); list != nil {
+				*list = append(*list, name)
+			}
+			continue
+		}
+
+		if info.parseErr != nil {
+			badFile(name, info.parseErr)
+			// Fall through: we might still have a partial AST in info.parsed,
+			// and we want to list files with parse errors anyway.
+		}
+
+		var pkg string
+		if info.parsed != nil {
+			pkg = info.parsed.Name.Name
+			if pkg == "documentation" {
+				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
+				continue
+			}
+		}
+
+		isTest := strings.HasSuffix(name, "_test.go")
+		isXTest := false
+		if isTest && strings.HasSuffix(pkg, "_test") && p.Name != pkg {
+			isXTest = true
+			pkg = pkg[:len(pkg)-len("_test")]
+		}
+
+		if p.Name == "" {
+			p.Name = pkg
+			firstFile = name
+		} else if pkg != p.Name {
+			// TODO(#45999): The choice of p.Name is arbitrary based on file iteration
+			// order. Instead of resolving p.Name arbitrarily, we should clear out the
+			// existing name and mark the existing files as also invalid.
+			badFile(name, &MultiplePackageError{
+				Dir:      p.Dir,
+				Packages: []string{p.Name, pkg},
+				Files:    []string{firstFile, name},
+			})
+		}
+		// Grab the first package comment as docs, provided it is not from a test file.
+		if info.parsed != nil && info.parsed.Doc != nil && p.Doc == "" && !isTest && !isXTest {
+			p.Doc = doc.Synopsis(info.parsed.Doc.Text())
+		}
+
+		if mode&ImportComment != 0 {
+			qcom, line := findImportComment(data)
+			if line != 0 {
+				com, err := strconv.Unquote(qcom)
+				if err != nil {
+					badFile(name, fmt.Errorf("%s:%d: cannot parse import comment", filename, line))
+				} else if p.ImportComment == "" {
+					p.ImportComment = com
+					firstCommentFile = name
+				} else if p.ImportComment != com {
+					badFile(name, fmt.Errorf("found import comments %q (%s) and %q (%s) in %s", p.ImportComment, firstCommentFile, com, name, p.Dir))
+				}
+			}
+		}
+
+		// Record imports and information about cgo.
+		isCgo := false
+		for _, imp := range info.imports {
+			if imp.path == "C" {
+				if isTest {
+					badFile(name, fmt.Errorf("use of cgo in test %s not supported", filename))
+					continue
+				}
+				isCgo = true
+				if imp.doc != nil {
+					if err := ctxt.saveCgo(filename, p, imp.doc); err != nil {
+						badFile(name, err)
+					}
+				}
+			}
+		}
+
+		var fileList *[]string
+		var importMap, embedMap map[string][]token.Position
+		switch {
+		case isCgo:
+			allTags["cgo"] = true
+			if ctxt.CgoEnabled {
+				fileList = &p.CgoFiles
+				importMap = importPos
+				embedMap = embedPos
+			} else {
+				// Ignore imports and embeds from cgo files if cgo is disabled.
+				fileList = &p.IgnoredGoFiles
+			}
+		case isXTest:
+			fileList = &p.XTestGoFiles
+			importMap = xTestImportPos
+			embedMap = xTestEmbedPos
+		case isTest:
+			fileList = &p.TestGoFiles
+			importMap = testImportPos
+			embedMap = testEmbedPos
+		default:
+			fileList = &p.GoFiles
+			importMap = importPos
+			embedMap = embedPos
+		}
+		*fileList = append(*fileList, name)
+		if importMap != nil {
+			for _, imp := range info.imports {
+				importMap[imp.path] = append(importMap[imp.path], fset.Position(imp.pos))
+			}
+		}
+		if embedMap != nil {
+			for _, emb := range info.embeds {
+				embedMap[emb.pattern] = append(embedMap[emb.pattern], emb.pos)
+			}
+		}
+	}
+
+	for tag := range allTags {
+		p.AllTags = append(p.AllTags, tag)
+	}
+	sort.Strings(p.AllTags)
+
+	p.EmbedPatterns, p.EmbedPatternPos = cleanDecls(embedPos)
+	p.TestEmbedPatterns, p.TestEmbedPatternPos = cleanDecls(testEmbedPos)
+	p.XTestEmbedPatterns, p.XTestEmbedPatternPos = cleanDecls(xTestEmbedPos)
+
+	p.Imports, p.ImportPos = cleanDecls(importPos)
+	p.TestImports, p.TestImportPos = cleanDecls(testImportPos)
+	p.XTestImports, p.XTestImportPos = cleanDecls(xTestImportPos)
+
+	// add the .S/.sx files only if we are using cgo
+	// (which means gcc will compile them).
+	// The standard assemblers expect .s files.
+	if len(p.CgoFiles) > 0 {
+		p.SFiles = append(p.SFiles, Sfiles...)
+		sort.Strings(p.SFiles)
+	} else {
+		p.IgnoredOtherFiles = append(p.IgnoredOtherFiles, Sfiles...)
+		sort.Strings(p.IgnoredOtherFiles)
+	}
+
+	if badGoError != nil {
+		return p, badGoError
+	}
+	if len(p.GoFiles)+len(p.CgoFiles)+len(p.TestGoFiles)+len(p.XTestGoFiles) == 0 {
+		return p, &NoGoError{p.Dir}
+	}
+	return p, pkgerr
+}
+
+func fileListForExt(p *Package, ext string) *[]string {
+	switch ext {
+	case ".c":
+		return &p.CFiles
+	case ".cc", ".cpp", ".cxx":
+		return &p.CXXFiles
+	case ".m":
+		return &p.MFiles
+	case ".h", ".hh", ".hpp", ".hxx":
+		return &p.HFiles
+	case ".f", ".F", ".for", ".f90":
+		return &p.FFiles
+	case ".s", ".S", ".sx":
+		return &p.SFiles
+	case ".swig":
+		return &p.SwigFiles
+	case ".swigcxx":
+		return &p.SwigCXXFiles
+	case ".syso":
+		return &p.SysoFiles
+	}
+	return nil
+}
+
+func uniq(list []string) []string {
+	if list == nil {
+		return nil
+	}
+	out := make([]string, len(list))
+	copy(out, list)
+	sort.Strings(out)
+	uniq := out[:0]
+	for _, x := range out {
+		if len(uniq) == 0 || uniq[len(uniq)-1] != x {
+			uniq = append(uniq, x)
+		}
+	}
+	return uniq
+}
+
+var errNoModules = errors.New("not using modules")
+
+// importGo checks whether it can use the go command to find the directory for path.
+// If using the go command is not appropriate, importGo returns errNoModules.
+// Otherwise, importGo tries using the go command and reports whether that succeeded.
+// Using the go command lets build.Import and build.Context.Import find code
+// in Go modules. In the long term we want tools to use go/packages (currently golang.org/x/tools/go/packages),
+// which will also use the go command.
+// Invoking the go command here is not very efficient in that it computes information
+// about the requested package and all dependencies and then only reports about the requested package.
+// Then we reinvoke it for every dependency. But this is still better than not working at all.
+// See golang.org/issue/26504.
+func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode) error {
+	// To invoke the go command,
+	// we must not being doing special things like AllowBinary or IgnoreVendor,
+	// and all the file system callbacks must be nil (we're meant to use the local file system).
+	if mode&AllowBinary != 0 || mode&IgnoreVendor != 0 ||
+		ctxt.JoinPath != nil || ctxt.SplitPathList != nil || ctxt.IsAbsPath != nil || ctxt.IsDir != nil || ctxt.HasSubdir != nil || ctxt.ReadDir != nil || ctxt.OpenFile != nil || !equal(ctxt.ReleaseTags, defaultReleaseTags) {
+		return errNoModules
+	}
+
+	// Predict whether module aware mode is enabled by checking the value of
+	// GO111MODULE and looking for a go.mod file in the source directory or
+	// one of its parents. Running 'go env GOMOD' in the source directory would
+	// give a canonical answer, but we'd prefer not to execute another command.
+	go111Module := os.Getenv("GO111MODULE")
+	switch go111Module {
+	case "off":
+		return errNoModules
+	default: // "", "on", "auto", anything else
+		// Maybe use modules.
+	}
+
+	if srcDir != "" {
+		var absSrcDir string
+		if filepath.IsAbs(srcDir) {
+			absSrcDir = srcDir
+		} else if ctxt.Dir != "" {
+			return fmt.Errorf("go/build: Dir is non-empty, so relative srcDir is not allowed: %v", srcDir)
+		} else {
+			// Find the absolute source directory. hasSubdir does not handle
+			// relative paths (and can't because the callbacks don't support this).
+			var err error
+			absSrcDir, err = filepath.Abs(srcDir)
+			if err != nil {
+				return errNoModules
+			}
+		}
+
+		// If the source directory is in GOROOT, then the in-process code works fine
+		// and we should keep using it. Moreover, the 'go list' approach below doesn't
+		// take standard-library vendoring into account and will fail.
+		if _, ok := ctxt.hasSubdir(filepath.Join(ctxt.GOROOT, "src"), absSrcDir); ok {
+			return errNoModules
+		}
+	}
+
+	// For efficiency, if path is a standard library package, let the usual lookup code handle it.
+	if ctxt.GOROOT != "" {
+		dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
+		if ctxt.isDir(dir) {
+			return errNoModules
+		}
+	}
+
+	// If GO111MODULE=auto, look to see if there is a go.mod.
+	// Since go1.13, it doesn't matter if we're inside GOPATH.
+	if go111Module == "auto" {
+		var (
+			parent string
+			err    error
+		)
+		if ctxt.Dir == "" {
+			parent, err = os.Getwd()
+			if err != nil {
+				// A nonexistent working directory can't be in a module.
+				return errNoModules
+			}
+		} else {
+			parent, err = filepath.Abs(ctxt.Dir)
+			if err != nil {
+				// If the caller passed a bogus Dir explicitly, that's materially
+				// different from not having modules enabled.
+				return err
+			}
+		}
+		for {
+			if f, err := ctxt.openFile(ctxt.joinPath(parent, "go.mod")); err == nil {
+				buf := make([]byte, 100)
+				_, err := f.Read(buf)
+				f.Close()
+				if err == nil || err == io.EOF {
+					// go.mod exists and is readable (is a file, not a directory).
+					break
+				}
+			}
+			d := filepath.Dir(parent)
+			if len(d) >= len(parent) {
+				return errNoModules // reached top of file system, no go.mod
+			}
+			parent = d
+		}
+	}
+
+	cmd := exec.Command("go", "list", "-e", "-compiler="+ctxt.Compiler, "-tags="+strings.Join(ctxt.BuildTags, ","), "-installsuffix="+ctxt.InstallSuffix, "-f={{.Dir}}\n{{.ImportPath}}\n{{.Root}}\n{{.Goroot}}\n{{if .Error}}{{.Error}}{{end}}\n", "--", path)
+
+	if ctxt.Dir != "" {
+		cmd.Dir = ctxt.Dir
+	}
+
+	var stdout, stderr strings.Builder
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+
+	cgo := "0"
+	if ctxt.CgoEnabled {
+		cgo = "1"
+	}
+	cmd.Env = append(os.Environ(),
+		"GOOS="+ctxt.GOOS,
+		"GOARCH="+ctxt.GOARCH,
+		"GOROOT="+ctxt.GOROOT,
+		"GOPATH="+ctxt.GOPATH,
+		"CGO_ENABLED="+cgo,
+	)
+
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("go/build: go list %s: %v\n%s\n", path, err, stderr.String())
+	}
+
+	f := strings.SplitN(stdout.String(), "\n", 5)
+	if len(f) != 5 {
+		return fmt.Errorf("go/build: importGo %s: unexpected output:\n%s\n", path, stdout.String())
+	}
+	dir := f[0]
+	errStr := strings.TrimSpace(f[4])
+	if errStr != "" && dir == "" {
+		// If 'go list' could not locate the package (dir is empty),
+		// return the same error that 'go list' reported.
+		return errors.New(errStr)
+	}
+
+	// If 'go list' did locate the package, ignore the error.
+	// It was probably related to loading source files, and we'll
+	// encounter it ourselves shortly if the FindOnly flag isn't set.
+	p.Dir = dir
+	p.ImportPath = f[1]
+	p.Root = f[2]
+	p.Goroot = f[3] == "true"
+	return nil
+}
+
+func equal(x, y []string) bool {
+	if len(x) != len(y) {
+		return false
+	}
+	for i, xi := range x {
+		if xi != y[i] {
+			return false
+		}
+	}
+	return true
+}
+
+// hasGoFiles reports whether dir contains any files with names ending in .go.
+// For a vendor check we must exclude directories that contain no .go files.
+// Otherwise it is not possible to vendor just a/b/c and still import the
+// non-vendored a/b. See golang.org/issue/13832.
+func hasGoFiles(ctxt *Context, dir string) bool {
+	ents, _ := ctxt.readDir(dir)
+	for _, ent := range ents {
+		if !ent.IsDir() && strings.HasSuffix(ent.Name(), ".go") {
+			return true
+		}
+	}
+	return false
+}
+
+func findImportComment(data []byte) (s string, line int) {
+	// expect keyword package
+	word, data := parseWord(data)
+	if string(word) != "package" {
+		return "", 0
+	}
+
+	// expect package name
+	_, data = parseWord(data)
+
+	// now ready for import comment, a // or /* */ comment
+	// beginning and ending on the current line.
+	for len(data) > 0 && (data[0] == ' ' || data[0] == '\t' || data[0] == '\r') {
+		data = data[1:]
+	}
+
+	var comment []byte
+	switch {
+	case bytes.HasPrefix(data, slashSlash):
+		comment, _, _ = bytesCut(data[2:], newline)
+	case bytes.HasPrefix(data, slashStar):
+		var ok bool
+		comment, _, ok = bytesCut(data[2:], starSlash)
+		if !ok {
+			// malformed comment
+			return "", 0
+		}
+		if bytes.Contains(comment, newline) {
+			return "", 0
+		}
+	}
+	comment = bytes.TrimSpace(comment)
+
+	// split comment into `import`, `"pkg"`
+	word, arg := parseWord(comment)
+	if string(word) != "import" {
+		return "", 0
+	}
+
+	line = 1 + bytes.Count(data[:cap(data)-cap(arg)], newline)
+	return strings.TrimSpace(string(arg)), line
+}
+
+var (
+	slashSlash = []byte("//")
+	slashStar  = []byte("/*")
+	starSlash  = []byte("*/")
+	newline    = []byte("\n")
+)
+
+// skipSpaceOrComment returns data with any leading spaces or comments removed.
+func skipSpaceOrComment(data []byte) []byte {
+	for len(data) > 0 {
+		switch data[0] {
+		case ' ', '\t', '\r', '\n':
+			data = data[1:]
+			continue
+		case '/':
+			if bytes.HasPrefix(data, slashSlash) {
+				i := bytes.Index(data, newline)
+				if i < 0 {
+					return nil
+				}
+				data = data[i+1:]
+				continue
+			}
+			if bytes.HasPrefix(data, slashStar) {
+				data = data[2:]
+				i := bytes.Index(data, starSlash)
+				if i < 0 {
+					return nil
+				}
+				data = data[i+2:]
+				continue
+			}
+		}
+		break
+	}
+	return data
+}
+
+// parseWord skips any leading spaces or comments in data
+// and then parses the beginning of data as an identifier or keyword,
+// returning that word and what remains after the word.
+func parseWord(data []byte) (word, rest []byte) {
+	data = skipSpaceOrComment(data)
+
+	// Parse past leading word characters.
+	rest = data
+	for {
+		r, size := utf8.DecodeRune(rest)
+		if unicode.IsLetter(r) || '0' <= r && r <= '9' || r == '_' {
+			rest = rest[size:]
+			continue
+		}
+		break
+	}
+
+	word = data[:len(data)-len(rest)]
+	if len(word) == 0 {
+		return nil, nil
+	}
+
+	return word, rest
+}
+
+// MatchFile reports whether the file with the given name in the given directory
+// matches the context and would be included in a Package created by ImportDir
+// of that directory.
+//
+// MatchFile considers the name of the file and may use ctxt.OpenFile to
+// read some or all of the file's content.
+func (ctxt *Context) MatchFile(dir, name string) (match bool, err error) {
+	info, err := ctxt.matchFile(dir, name, nil, nil, nil)
+	return info != nil, err
+}
+
+var dummyPkg Package
+
+// fileInfo records information learned about a file included in a build.
+type fileInfo struct {
+	name     string // full name including dir
+	header   []byte
+	fset     *token.FileSet
+	parsed   *ast.File
+	parseErr error
+	imports  []fileImport
+	embeds   []fileEmbed
+	embedErr error
+}
+
+type fileImport struct {
+	path string
+	pos  token.Pos
+	doc  *ast.CommentGroup
+}
+
+type fileEmbed struct {
+	pattern string
+	pos     token.Position
+}
+
+// matchFile determines whether the file with the given name in the given directory
+// should be included in the package being constructed.
+// If the file should be included, matchFile returns a non-nil *fileInfo (and a nil error).
+// Non-nil errors are reserved for unexpected problems.
+//
+// If name denotes a Go program, matchFile reads until the end of the
+// imports and returns that section of the file in the fileInfo's header field,
+// even though it only considers text until the first non-comment
+// for +build lines.
+//
+// If allTags is non-nil, matchFile records any encountered build tag
+// by setting allTags[tag] = true.
+func (ctxt *Context) matchFile(dir, name string, allTags map[string]bool, binaryOnly *bool, fset *token.FileSet) (*fileInfo, error) {
+	if strings.HasPrefix(name, "_") ||
+		strings.HasPrefix(name, ".") {
+		return nil, nil
+	}
+
+	i := strings.LastIndex(name, ".")
+	if i < 0 {
+		i = len(name)
+	}
+	ext := name[i:]
+
+	if !ctxt.goodOSArchFile(name, allTags) && !ctxt.UseAllFiles {
+		return nil, nil
+	}
+
+	if ext != ".go" && fileListForExt(&dummyPkg, ext) == nil {
+		// skip
+		return nil, nil
+	}
+
+	info := &fileInfo{name: ctxt.joinPath(dir, name), fset: fset}
+	if ext == ".syso" {
+		// binary, no reading
+		return info, nil
+	}
+
+	f, err := ctxt.openFile(info.name)
+	if err != nil {
+		return nil, err
+	}
+
+	if strings.HasSuffix(name, ".go") {
+		err = readGoInfo(f, info)
+		if strings.HasSuffix(name, "_test.go") {
+			binaryOnly = nil // ignore //go:binary-only-package comments in _test.go files
+		}
+	} else {
+		binaryOnly = nil // ignore //go:binary-only-package comments in non-Go sources
+		info.header, err = readComments(f)
+	}
+	f.Close()
+	if err != nil {
+		return nil, fmt.Errorf("read %s: %v", info.name, err)
+	}
+
+	// Look for +build comments to accept or reject the file.
+	ok, sawBinaryOnly, err := ctxt.shouldBuild(info.header, allTags)
+	if err != nil {
+		return nil, fmt.Errorf("%s: %v", name, err)
+	}
+	if !ok && !ctxt.UseAllFiles {
+		return nil, nil
+	}
+
+	if binaryOnly != nil && sawBinaryOnly {
+		*binaryOnly = true
+	}
+
+	return info, nil
+}
+
+func cleanDecls(m map[string][]token.Position) ([]string, map[string][]token.Position) {
+	all := make([]string, 0, len(m))
+	for path := range m {
+		all = append(all, path)
+	}
+	sort.Strings(all)
+	return all, m
+}
+
+// Import is shorthand for Default.Import.
+func Import(path, srcDir string, mode ImportMode) (*Package, error) {
+	return Default.Import(path, srcDir, mode)
+}
+
+// ImportDir is shorthand for Default.ImportDir.
+func ImportDir(dir string, mode ImportMode) (*Package, error) {
+	return Default.ImportDir(dir, mode)
+}
+
+var (
+	bSlashSlash = []byte(slashSlash)
+	bStarSlash  = []byte(starSlash)
+	bSlashStar  = []byte(slashStar)
+	bPlusBuild  = []byte("+build")
+
+	goBuildComment = []byte("//go:build")
+
+	errGoBuildWithoutBuild = errors.New("//go:build comment without // +build comment")
+	errMultipleGoBuild     = errors.New("multiple //go:build comments")
+)
+
+func isGoBuildComment(line []byte) bool {
+	if !bytes.HasPrefix(line, goBuildComment) {
+		return false
+	}
+	line = bytes.TrimSpace(line)
+	rest := line[len(goBuildComment):]
+	return len(rest) == 0 || len(bytes.TrimSpace(rest)) < len(rest)
+}
+
+// Special comment denoting a binary-only package.
+// See https://golang.org/design/2775-binary-only-packages
+// for more about the design of binary-only packages.
+var binaryOnlyComment = []byte("//go:binary-only-package")
+
+// shouldBuild reports whether it is okay to use this file,
+// The rule is that in the file's leading run of // comments
+// and blank lines, which must be followed by a blank line
+// (to avoid including a Go package clause doc comment),
+// lines beginning with '// +build' are taken as build directives.
+//
+// The file is accepted only if each such line lists something
+// matching the file. For example:
+//
+//	// +build windows linux
+//
+// marks the file as applicable only on Windows and Linux.
+//
+// For each build tag it consults, shouldBuild sets allTags[tag] = true.
+//
+// shouldBuild reports whether the file should be built
+// and whether a //go:binary-only-package comment was found.
+func (ctxt *Context) shouldBuild(content []byte, allTags map[string]bool) (shouldBuild, binaryOnly bool, err error) {
+	// Identify leading run of // comments and blank lines,
+	// which must be followed by a blank line.
+	// Also identify any //go:build comments.
+	content, goBuild, sawBinaryOnly, err := parseFileHeader(content)
+	if err != nil {
+		return false, false, err
+	}
+
+	// If //go:build line is present, it controls.
+	// Otherwise fall back to +build processing.
+	switch {
+	case goBuild != nil:
+		x, err := constraint.Parse(string(goBuild))
+		if err != nil {
+			return false, false, fmt.Errorf("parsing //go:build line: %v", err)
+		}
+		shouldBuild = ctxt.eval(x, allTags)
+
+	default:
+		shouldBuild = true
+		p := content
+		for len(p) > 0 {
+			line := p
+			if i := bytes.IndexByte(line, '\n'); i >= 0 {
+				line, p = line[:i], p[i+1:]
+			} else {
+				p = p[len(p):]
+			}
+			line = bytes.TrimSpace(line)
+			if !bytes.HasPrefix(line, bSlashSlash) || !bytes.Contains(line, bPlusBuild) {
+				continue
+			}
+			text := string(line)
+			if !constraint.IsPlusBuild(text) {
+				continue
+			}
+			if x, err := constraint.Parse(text); err == nil {
+				if !ctxt.eval(x, allTags) {
+					shouldBuild = false
+				}
+			}
+		}
+	}
+
+	return shouldBuild, sawBinaryOnly, nil
+}
+
+func parseFileHeader(content []byte) (trimmed, goBuild []byte, sawBinaryOnly bool, err error) {
+	end := 0
+	p := content
+	ended := false       // found non-blank, non-// line, so stopped accepting // +build lines
+	inSlashStar := false // in /* */ comment
+
+Lines:
+	for len(p) > 0 {
+		line := p
+		if i := bytes.IndexByte(line, '\n'); i >= 0 {
+			line, p = line[:i], p[i+1:]
+		} else {
+			p = p[len(p):]
+		}
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 && !ended { // Blank line
+			// Remember position of most recent blank line.
+			// When we find the first non-blank, non-// line,
+			// this "end" position marks the latest file position
+			// where a // +build line can appear.
+			// (It must appear _before_ a blank line before the non-blank, non-// line.
+			// Yes, that's confusing, which is part of why we moved to //go:build lines.)
+			// Note that ended==false here means that inSlashStar==false,
+			// since seeing a /* would have set ended==true.
+			end = len(content) - len(p)
+			continue Lines
+		}
+		if !bytes.HasPrefix(line, slashSlash) { // Not comment line
+			ended = true
+		}
+
+		if !inSlashStar && isGoBuildComment(line) {
+			if goBuild != nil {
+				return nil, nil, false, errMultipleGoBuild
+			}
+			goBuild = line
+		}
+		if !inSlashStar && bytes.Equal(line, binaryOnlyComment) {
+			sawBinaryOnly = true
+		}
+
+	Comments:
+		for len(line) > 0 {
+			if inSlashStar {
+				if i := bytes.Index(line, starSlash); i >= 0 {
+					inSlashStar = false
+					line = bytes.TrimSpace(line[i+len(starSlash):])
+					continue Comments
+				}
+				continue Lines
+			}
+			if bytes.HasPrefix(line, bSlashSlash) {
+				continue Lines
+			}
+			if bytes.HasPrefix(line, bSlashStar) {
+				inSlashStar = true
+				line = bytes.TrimSpace(line[len(bSlashStar):])
+				continue Comments
+			}
+			// Found non-comment text.
+			break Lines
+		}
+	}
+
+	return content[:end], goBuild, sawBinaryOnly, nil
+}
+
+// saveCgo saves the information from the #cgo lines in the import "C" comment.
+// These lines set CFLAGS, CPPFLAGS, CXXFLAGS and LDFLAGS and pkg-config directives
+// that affect the way cgo's C code is built.
+func (ctxt *Context) saveCgo(filename string, di *Package, cg *ast.CommentGroup) error {
+	text := cg.Text()
+	for _, line := range strings.Split(text, "\n") {
+		orig := line
+
+		// Line is
+		//	#cgo [GOOS/GOARCH...] LDFLAGS: stuff
+		//
+		line = strings.TrimSpace(line)
+		if len(line) < 5 || line[:4] != "#cgo" || (line[4] != ' ' && line[4] != '\t') {
+			continue
+		}
+
+		// Split at colon.
+		line, argstr, ok := stringsCut(strings.TrimSpace(line[4:]), ":")
+		if !ok {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+
+		// Parse GOOS/GOARCH stuff.
+		f := strings.Fields(line)
+		if len(f) < 1 {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+
+		cond, verb := f[:len(f)-1], f[len(f)-1]
+		if len(cond) > 0 {
+			ok := false
+			for _, c := range cond {
+				if ctxt.matchAuto(c, nil) {
+					ok = true
+					break
+				}
+			}
+			if !ok {
+				continue
+			}
+		}
+
+		args, err := splitQuoted(argstr)
+		if err != nil {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+		for i, arg := range args {
+			if arg, ok = expandSrcDir(arg, di.Dir); !ok {
+				return fmt.Errorf("%s: malformed #cgo argument: %s", filename, arg)
+			}
+			args[i] = arg
+		}
+
+		switch verb {
+		case "CFLAGS", "CPPFLAGS", "CXXFLAGS", "FFLAGS", "LDFLAGS":
+			// Change relative paths to absolute.
+			ctxt.makePathsAbsolute(args, di.Dir)
+		}
+
+		switch verb {
+		case "CFLAGS":
+			di.CgoCFLAGS = append(di.CgoCFLAGS, args...)
+		case "CPPFLAGS":
+			di.CgoCPPFLAGS = append(di.CgoCPPFLAGS, args...)
+		case "CXXFLAGS":
+			di.CgoCXXFLAGS = append(di.CgoCXXFLAGS, args...)
+		case "FFLAGS":
+			di.CgoFFLAGS = append(di.CgoFFLAGS, args...)
+		case "LDFLAGS":
+			di.CgoLDFLAGS = append(di.CgoLDFLAGS, args...)
+		case "pkg-config":
+			di.CgoPkgConfig = append(di.CgoPkgConfig, args...)
+		default:
+			return fmt.Errorf("%s: invalid #cgo verb: %s", filename, orig)
+		}
+	}
+	return nil
+}
+
+// expandSrcDir expands any occurrence of ${SRCDIR}, making sure
+// the result is safe for the shell.
+func expandSrcDir(str string, srcdir string) (string, bool) {
+	// "\" delimited paths cause safeCgoName to fail
+	// so convert native paths with a different delimiter
+	// to "/" before starting (eg: on windows).
+	srcdir = filepath.ToSlash(srcdir)
+
+	chunks := strings.Split(str, "${SRCDIR}")
+	if len(chunks) < 2 {
+		return str, safeCgoName(str)
+	}
+	ok := true
+	for _, chunk := range chunks {
+		ok = ok && (chunk == "" || safeCgoName(chunk))
+	}
+	ok = ok && (srcdir == "" || safeCgoName(srcdir))
+	res := strings.Join(chunks, srcdir)
+	return res, ok && res != ""
+}
+
+// makePathsAbsolute looks for compiler options that take paths and
+// makes them absolute. We do this because through the 1.8 release we
+// ran the compiler in the package directory, so any relative -I or -L
+// options would be relative to that directory. In 1.9 we changed to
+// running the compiler in the build directory, to get consistent
+// build results (issue #19964). To keep builds working, we change any
+// relative -I or -L options to be absolute.
+//
+// Using filepath.IsAbs and filepath.Join here means the results will be
+// different on different systems, but that's OK: -I and -L options are
+// inherently system-dependent.
+func (ctxt *Context) makePathsAbsolute(args []string, srcDir string) {
+	nextPath := false
+	for i, arg := range args {
+		if nextPath {
+			if !filepath.IsAbs(arg) {
+				args[i] = filepath.Join(srcDir, arg)
+			}
+			nextPath = false
+		} else if strings.HasPrefix(arg, "-I") || strings.HasPrefix(arg, "-L") {
+			if len(arg) == 2 {
+				nextPath = true
+			} else {
+				if !filepath.IsAbs(arg[2:]) {
+					args[i] = arg[:2] + filepath.Join(srcDir, arg[2:])
+				}
+			}
+		}
+	}
+}
+
+// NOTE: $ is not safe for the shell, but it is allowed here because of linker options like -Wl,$ORIGIN.
+// We never pass these arguments to a shell (just to programs we construct argv for), so this should be okay.
+// See golang.org/issue/6038.
+// The @ is for OS X. See golang.org/issue/13720.
+// The % is for Jenkins. See golang.org/issue/16959.
+// The ! is because module paths may use them. See golang.org/issue/26716.
+// The ~ and ^ are for sr.ht. See golang.org/issue/32260.
+const safeString = "+-.,/0123456789=ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz:$@%! ~^"
+
+func safeCgoName(s string) bool {
+	if s == "" {
+		return false
+	}
+	for i := 0; i < len(s); i++ {
+		if c := s[i]; c < utf8.RuneSelf && strings.IndexByte(safeString, c) < 0 {
+			return false
+		}
+	}
+	return true
+}
+
+// splitQuoted splits the string s around each instance of one or more consecutive
+// white space characters while taking into account quotes and escaping, and
+// returns an array of substrings of s or an empty list if s contains only white space.
+// Single quotes and double quotes are recognized to prevent splitting within the
+// quoted region, and are removed from the resulting substrings. If a quote in s
+// isn't closed err will be set and r will have the unclosed argument as the
+// last element. The backslash is used for escaping.
+//
+// For example, the following string:
+//
+//	a b:"c d" 'e''f'  "g\""
+//
+// Would be parsed as:
+//
+//	[]string{"a", "b:c d", "ef", `g"`}
+func splitQuoted(s string) (r []string, err error) {
+	var args []string
+	arg := make([]rune, len(s))
+	escaped := false
+	quoted := false
+	quote := '\x00'
+	i := 0
+	for _, rune := range s {
+		switch {
+		case escaped:
+			escaped = false
+		case rune == '\\':
+			escaped = true
+			continue
+		case quote != '\x00':
+			if rune == quote {
+				quote = '\x00'
+				continue
+			}
+		case rune == '"' || rune == '\'':
+			quoted = true
+			quote = rune
+			continue
+		case unicode.IsSpace(rune):
+			if quoted || i > 0 {
+				quoted = false
+				args = append(args, string(arg[:i]))
+				i = 0
+			}
+			continue
+		}
+		arg[i] = rune
+		i++
+	}
+	if quoted || i > 0 {
+		args = append(args, string(arg[:i]))
+	}
+	if quote != 0 {
+		err = errors.New("unclosed quote")
+	} else if escaped {
+		err = errors.New("unfinished escaping")
+	}
+	return args, err
+}
+
+// matchAuto interprets text as either a +build or //go:build expression (whichever works),
+// reporting whether the expression matches the build context.
+//
+// matchAuto is only used for testing of tag evaluation
+// and in #cgo lines, which accept either syntax.
+func (ctxt *Context) matchAuto(text string, allTags map[string]bool) bool {
+	if strings.ContainsAny(text, "&|()") {
+		text = "//go:build " + text
+	} else {
+		text = "// +build " + text
+	}
+	x, err := constraint.Parse(text)
+	if err != nil {
+		return false
+	}
+	return ctxt.eval(x, allTags)
+}
+
+func (ctxt *Context) eval(x constraint.Expr, allTags map[string]bool) bool {
+	return x.Eval(func(tag string) bool { return ctxt.matchTag(tag, allTags) })
+}
+
+// matchTag reports whether the name is one of:
+//
+//	cgo (if cgo is enabled)
+//	$GOOS
+//	$GOARCH
+//	ctxt.Compiler
+//	linux (if GOOS = android)
+//	solaris (if GOOS = illumos)
+//	tag (if tag is listed in ctxt.BuildTags or ctxt.ReleaseTags)
+//
+// It records all consulted tags in allTags.
+func (ctxt *Context) matchTag(name string, allTags map[string]bool) bool {
+	if allTags != nil {
+		allTags[name] = true
+	}
+
+	// special tags
+	if ctxt.CgoEnabled && name == "cgo" {
+		return true
+	}
+	if name == ctxt.GOOS || name == ctxt.GOARCH || name == ctxt.Compiler {
+		return true
+	}
+	if ctxt.GOOS == "android" && name == "linux" {
+		return true
+	}
+	if ctxt.GOOS == "illumos" && name == "solaris" {
+		return true
+	}
+	if ctxt.GOOS == "ios" && name == "darwin" {
+		return true
+	}
+
+	// other tags
+	for _, tag := range ctxt.BuildTags {
+		if tag == name {
+			return true
+		}
+	}
+	for _, tag := range ctxt.ToolTags {
+		if tag == name {
+			return true
+		}
+	}
+	for _, tag := range ctxt.ReleaseTags {
+		if tag == name {
+			return true
+		}
+	}
+
+	return false
+}
+
+// goodOSArchFile returns false if the name contains a $GOOS or $GOARCH
+// suffix which does not match the current system.
+// The recognized name formats are:
+//
+//	name_$(GOOS).*
+//	name_$(GOARCH).*
+//	name_$(GOOS)_$(GOARCH).*
+//	name_$(GOOS)_test.*
+//	name_$(GOARCH)_test.*
+//	name_$(GOOS)_$(GOARCH)_test.*
+//
+// Exceptions:
+// if GOOS=android, then files with GOOS=linux are also matched.
+// if GOOS=illumos, then files with GOOS=solaris are also matched.
+// if GOOS=ios, then files with GOOS=darwin are also matched.
+func (ctxt *Context) goodOSArchFile(name string, allTags map[string]bool) bool {
+	name, _, _ = stringsCut(name, ".")
+
+	// Before Go 1.4, a file called "linux.go" would be equivalent to having a
+	// build tag "linux" in that file. For Go 1.4 and beyond, we require this
+	// auto-tagging to apply only to files with a non-empty prefix, so
+	// "foo_linux.go" is tagged but "linux.go" is not. This allows new operating
+	// systems, such as android, to arrive without breaking existing code with
+	// innocuous source code in "android.go". The easiest fix: cut everything
+	// in the name before the initial _.
+	i := strings.Index(name, "_")
+	if i < 0 {
+		return true
+	}
+	name = name[i:] // ignore everything before first _
+
+	l := strings.Split(name, "_")
+	if n := len(l); n > 0 && l[n-1] == "test" {
+		l = l[:n-1]
+	}
+	n := len(l)
+	if n >= 2 && knownOS[l[n-2]] && knownArch[l[n-1]] {
+		return ctxt.matchTag(l[n-1], allTags) && ctxt.matchTag(l[n-2], allTags)
+	}
+	if n >= 1 && (knownOS[l[n-1]] || knownArch[l[n-1]]) {
+		return ctxt.matchTag(l[n-1], allTags)
+	}
+	return true
+}
+
+var knownOS = make(map[string]bool)
+var knownArch = make(map[string]bool)
+
+func init() {
+	for _, v := range strings.Fields(goosList) {
+		knownOS[v] = true
+	}
+	for _, v := range strings.Fields(goarchList) {
+		knownArch[v] = true
+	}
+}
+
+// ToolDir is the directory containing build tools.
+var ToolDir = getToolDir()
+
+// IsLocalImport reports whether the import path is
+// a local import path, like ".", "..", "./foo", or "../foo".
+func IsLocalImport(path string) bool {
+	return path == "." || path == ".." ||
+		strings.HasPrefix(path, "./") || strings.HasPrefix(path, "../")
+}
+
+// ArchChar returns "?" and an error.
+// In earlier versions of Go, the returned string was used to derive
+// the compiler and linker tool names, the default object file suffix,
+// and the default linker output name. As of Go 1.5, those strings
+// no longer vary by architecture; they are compile, link, .o, and a.out, respectively.
+func ArchChar(goarch string) (string, error) {
+	return "?", errors.New("architecture letter no longer used")
+}
+
+func bytesCut(s, sep []byte) (before, after []byte, found bool) {
+	if i := bytes.Index(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
+	}
+	return s, nil, false
+}
+
+func stringsCut(s, sep string) (before, after string, found bool) {
+	if i := strings.Index(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
+	}
+	return s, "", false
+}
diff --git a/internal/backport/go/build/doc.go b/internal/backport/go/build/doc.go
new file mode 100644
index 0000000..262f670
--- /dev/null
+++ b/internal/backport/go/build/doc.go
@@ -0,0 +1,97 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package build gathers information about Go packages.
+//
+// # Go Path
+//
+// The Go path is a list of directory trees containing Go source code.
+// It is consulted to resolve imports that cannot be found in the standard
+// Go tree. The default path is the value of the GOPATH environment
+// variable, interpreted as a path list appropriate to the operating system
+// (on Unix, the variable is a colon-separated string;
+// on Windows, a semicolon-separated string;
+// on Plan 9, a list).
+//
+// Each directory listed in the Go path must have a prescribed structure:
+//
+// The src/ directory holds source code. The path below 'src' determines
+// the import path or executable name.
+//
+// The pkg/ directory holds installed package objects.
+// As in the Go tree, each target operating system and
+// architecture pair has its own subdirectory of pkg
+// (pkg/GOOS_GOARCH).
+//
+// If DIR is a directory listed in the Go path, a package with
+// source in DIR/src/foo/bar can be imported as "foo/bar" and
+// has its compiled form installed to "DIR/pkg/GOOS_GOARCH/foo/bar.a"
+// (or, for gccgo, "DIR/pkg/gccgo/foo/libbar.a").
+//
+// The bin/ directory holds compiled commands.
+// Each command is named for its source directory, but only
+// using the final element, not the entire path. That is, the
+// command with source in DIR/src/foo/quux is installed into
+// DIR/bin/quux, not DIR/bin/foo/quux. The foo/ is stripped
+// so that you can add DIR/bin to your PATH to get at the
+// installed commands.
+//
+// Here's an example directory layout:
+//
+//	GOPATH=/home/user/gocode
+//
+//	/home/user/gocode/
+//	    src/
+//	        foo/
+//	            bar/               (go code in package bar)
+//	                x.go
+//	            quux/              (go code in package main)
+//	                y.go
+//	    bin/
+//	        quux                   (installed command)
+//	    pkg/
+//	        linux_amd64/
+//	            foo/
+//	                bar.a          (installed package object)
+//
+// # Build Constraints
+//
+// A build constraint, also known as a build tag, is a line comment that begins
+//
+//	//go:build
+//
+// that lists the conditions under which a file should be included in the
+// package. Build constraints may also be part of a file's name
+// (for example, source_windows.go will only be included if the target
+// operating system is windows).
+//
+// See 'go help buildconstraint'
+// (https://golang.org/cmd/go/#hdr-Build_constraints) for details.
+//
+// # Binary-Only Packages
+//
+// In Go 1.12 and earlier, it was possible to distribute packages in binary
+// form without including the source code used for compiling the package.
+// The package was distributed with a source file not excluded by build
+// constraints and containing a "//go:binary-only-package" comment. Like a
+// build constraint, this comment appeared at the top of a file, preceded
+// only by blank lines and other line comments and with a blank line
+// following the comment, to separate it from the package documentation.
+// Unlike build constraints, this comment is only recognized in non-test
+// Go source files.
+//
+// The minimal source code for a binary-only package was therefore:
+//
+//	//go:binary-only-package
+//
+//	package mypkg
+//
+// The source code could include additional Go code. That code was never
+// compiled but would be processed by tools like godoc and might be useful
+// as end-user documentation.
+//
+// "go build" and other commands no longer support binary-only-packages.
+// Import and ImportDir will still set the BinaryOnly flag in packages
+// containing these comments for use in tools and error messages.
+package build
diff --git a/internal/backport/go/build/gc.go b/internal/backport/go/build/gc.go
new file mode 100644
index 0000000..e16e186
--- /dev/null
+++ b/internal/backport/go/build/gc.go
@@ -0,0 +1,18 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gc
+// +build gc
+
+package build
+
+import (
+	"path/filepath"
+	"runtime"
+)
+
+// getToolDir returns the default value of ToolDir.
+func getToolDir() string {
+	return filepath.Join(runtime.GOROOT(), "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH)
+}
diff --git a/internal/backport/go/build/gccgo.go b/internal/backport/go/build/gccgo.go
new file mode 100644
index 0000000..c8ec704
--- /dev/null
+++ b/internal/backport/go/build/gccgo.go
@@ -0,0 +1,15 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gccgo
+// +build gccgo
+
+package build
+
+import "runtime"
+
+// getToolDir returns the default value of ToolDir.
+func getToolDir() string {
+	return envOr("GCCGOTOOLDIR", runtime.GCCGOTOOLDIR)
+}
diff --git a/internal/backport/go/build/read.go b/internal/backport/go/build/read.go
new file mode 100644
index 0000000..e8fe9a0
--- /dev/null
+++ b/internal/backport/go/build/read.go
@@ -0,0 +1,578 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
+	"io"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+type importReader struct {
+	b    *bufio.Reader
+	buf  []byte
+	peek byte
+	err  error
+	eof  bool
+	nerr int
+	pos  token.Position
+}
+
+var bom = []byte{0xef, 0xbb, 0xbf}
+
+func newImportReader(name string, r io.Reader) *importReader {
+	b := bufio.NewReader(r)
+	// Remove leading UTF-8 BOM.
+	// Per https://golang.org/ref/spec#Source_code_representation:
+	// a compiler may ignore a UTF-8-encoded byte order mark (U+FEFF)
+	// if it is the first Unicode code point in the source text.
+	if leadingBytes, err := b.Peek(3); err == nil && bytes.Equal(leadingBytes, bom) {
+		b.Discard(3)
+	}
+	return &importReader{
+		b: b,
+		pos: token.Position{
+			Filename: name,
+			Line:     1,
+			Column:   1,
+		},
+	}
+}
+
+func isIdent(c byte) bool {
+	return 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z' || '0' <= c && c <= '9' || c == '_' || c >= utf8.RuneSelf
+}
+
+var (
+	errSyntax = errors.New("syntax error")
+	errNUL    = errors.New("unexpected NUL in input")
+)
+
+// syntaxError records a syntax error, but only if an I/O error has not already been recorded.
+func (r *importReader) syntaxError() {
+	if r.err == nil {
+		r.err = errSyntax
+	}
+}
+
+// readByte reads the next byte from the input, saves it in buf, and returns it.
+// If an error occurs, readByte records the error in r.err and returns 0.
+func (r *importReader) readByte() byte {
+	c, err := r.b.ReadByte()
+	if err == nil {
+		r.buf = append(r.buf, c)
+		if c == 0 {
+			err = errNUL
+		}
+	}
+	if err != nil {
+		if err == io.EOF {
+			r.eof = true
+		} else if r.err == nil {
+			r.err = err
+		}
+		c = 0
+	}
+	return c
+}
+
+// readByteNoBuf is like readByte but doesn't buffer the byte.
+// It exhausts r.buf before reading from r.b.
+func (r *importReader) readByteNoBuf() byte {
+	var c byte
+	var err error
+	if len(r.buf) > 0 {
+		c = r.buf[0]
+		r.buf = r.buf[1:]
+	} else {
+		c, err = r.b.ReadByte()
+		if err == nil && c == 0 {
+			err = errNUL
+		}
+	}
+
+	if err != nil {
+		if err == io.EOF {
+			r.eof = true
+		} else if r.err == nil {
+			r.err = err
+		}
+		return 0
+	}
+	r.pos.Offset++
+	if c == '\n' {
+		r.pos.Line++
+		r.pos.Column = 1
+	} else {
+		r.pos.Column++
+	}
+	return c
+}
+
+// peekByte returns the next byte from the input reader but does not advance beyond it.
+// If skipSpace is set, peekByte skips leading spaces and comments.
+func (r *importReader) peekByte(skipSpace bool) byte {
+	if r.err != nil {
+		if r.nerr++; r.nerr > 10000 {
+			panic("go/build: import reader looping")
+		}
+		return 0
+	}
+
+	// Use r.peek as first input byte.
+	// Don't just return r.peek here: it might have been left by peekByte(false)
+	// and this might be peekByte(true).
+	c := r.peek
+	if c == 0 {
+		c = r.readByte()
+	}
+	for r.err == nil && !r.eof {
+		if skipSpace {
+			// For the purposes of this reader, semicolons are never necessary to
+			// understand the input and are treated as spaces.
+			switch c {
+			case ' ', '\f', '\t', '\r', '\n', ';':
+				c = r.readByte()
+				continue
+
+			case '/':
+				c = r.readByte()
+				if c == '/' {
+					for c != '\n' && r.err == nil && !r.eof {
+						c = r.readByte()
+					}
+				} else if c == '*' {
+					var c1 byte
+					for (c != '*' || c1 != '/') && r.err == nil {
+						if r.eof {
+							r.syntaxError()
+						}
+						c, c1 = c1, r.readByte()
+					}
+				} else {
+					r.syntaxError()
+				}
+				c = r.readByte()
+				continue
+			}
+		}
+		break
+	}
+	r.peek = c
+	return r.peek
+}
+
+// nextByte is like peekByte but advances beyond the returned byte.
+func (r *importReader) nextByte(skipSpace bool) byte {
+	c := r.peekByte(skipSpace)
+	r.peek = 0
+	return c
+}
+
+var goEmbed = []byte("go:embed")
+
+// findEmbed advances the input reader to the next //go:embed comment.
+// It reports whether it found a comment.
+// (Otherwise it found an error or EOF.)
+func (r *importReader) findEmbed(first bool) bool {
+	// The import block scan stopped after a non-space character,
+	// so the reader is not at the start of a line on the first call.
+	// After that, each //go:embed extraction leaves the reader
+	// at the end of a line.
+	startLine := !first
+	var c byte
+	for r.err == nil && !r.eof {
+		c = r.readByteNoBuf()
+	Reswitch:
+		switch c {
+		default:
+			startLine = false
+
+		case '\n':
+			startLine = true
+
+		case ' ', '\t':
+			// leave startLine alone
+
+		case '"':
+			startLine = false
+			for r.err == nil {
+				if r.eof {
+					r.syntaxError()
+				}
+				c = r.readByteNoBuf()
+				if c == '\\' {
+					r.readByteNoBuf()
+					if r.err != nil {
+						r.syntaxError()
+						return false
+					}
+					continue
+				}
+				if c == '"' {
+					c = r.readByteNoBuf()
+					goto Reswitch
+				}
+			}
+			goto Reswitch
+
+		case '`':
+			startLine = false
+			for r.err == nil {
+				if r.eof {
+					r.syntaxError()
+				}
+				c = r.readByteNoBuf()
+				if c == '`' {
+					c = r.readByteNoBuf()
+					goto Reswitch
+				}
+			}
+
+		case '\'':
+			startLine = false
+			for r.err == nil {
+				if r.eof {
+					r.syntaxError()
+				}
+				c = r.readByteNoBuf()
+				if c == '\\' {
+					r.readByteNoBuf()
+					if r.err != nil {
+						r.syntaxError()
+						return false
+					}
+					continue
+				}
+				if c == '\'' {
+					c = r.readByteNoBuf()
+					goto Reswitch
+				}
+			}
+
+		case '/':
+			c = r.readByteNoBuf()
+			switch c {
+			default:
+				startLine = false
+				goto Reswitch
+
+			case '*':
+				var c1 byte
+				for (c != '*' || c1 != '/') && r.err == nil {
+					if r.eof {
+						r.syntaxError()
+					}
+					c, c1 = c1, r.readByteNoBuf()
+				}
+				startLine = false
+
+			case '/':
+				if startLine {
+					// Try to read this as a //go:embed comment.
+					for i := range goEmbed {
+						c = r.readByteNoBuf()
+						if c != goEmbed[i] {
+							goto SkipSlashSlash
+						}
+					}
+					c = r.readByteNoBuf()
+					if c == ' ' || c == '\t' {
+						// Found one!
+						return true
+					}
+				}
+			SkipSlashSlash:
+				for c != '\n' && r.err == nil && !r.eof {
+					c = r.readByteNoBuf()
+				}
+				startLine = true
+			}
+		}
+	}
+	return false
+}
+
+// readKeyword reads the given keyword from the input.
+// If the keyword is not present, readKeyword records a syntax error.
+func (r *importReader) readKeyword(kw string) {
+	r.peekByte(true)
+	for i := 0; i < len(kw); i++ {
+		if r.nextByte(false) != kw[i] {
+			r.syntaxError()
+			return
+		}
+	}
+	if isIdent(r.peekByte(false)) {
+		r.syntaxError()
+	}
+}
+
+// readIdent reads an identifier from the input.
+// If an identifier is not present, readIdent records a syntax error.
+func (r *importReader) readIdent() {
+	c := r.peekByte(true)
+	if !isIdent(c) {
+		r.syntaxError()
+		return
+	}
+	for isIdent(r.peekByte(false)) {
+		r.peek = 0
+	}
+}
+
+// readString reads a quoted string literal from the input.
+// If an identifier is not present, readString records a syntax error.
+func (r *importReader) readString() {
+	switch r.nextByte(true) {
+	case '`':
+		for r.err == nil {
+			if r.nextByte(false) == '`' {
+				break
+			}
+			if r.eof {
+				r.syntaxError()
+			}
+		}
+	case '"':
+		for r.err == nil {
+			c := r.nextByte(false)
+			if c == '"' {
+				break
+			}
+			if r.eof || c == '\n' {
+				r.syntaxError()
+			}
+			if c == '\\' {
+				r.nextByte(false)
+			}
+		}
+	default:
+		r.syntaxError()
+	}
+}
+
+// readImport reads an import clause - optional identifier followed by quoted string -
+// from the input.
+func (r *importReader) readImport() {
+	c := r.peekByte(true)
+	if c == '.' {
+		r.peek = 0
+	} else if isIdent(c) {
+		r.readIdent()
+	}
+	r.readString()
+}
+
+// readComments is like io.ReadAll, except that it only reads the leading
+// block of comments in the file.
+func readComments(f io.Reader) ([]byte, error) {
+	r := newImportReader("", f)
+	r.peekByte(true)
+	if r.err == nil && !r.eof {
+		// Didn't reach EOF, so must have found a non-space byte. Remove it.
+		r.buf = r.buf[:len(r.buf)-1]
+	}
+	return r.buf, r.err
+}
+
+// readGoInfo expects a Go file as input and reads the file up to and including the import section.
+// It records what it learned in *info.
+// If info.fset is non-nil, readGoInfo parses the file and sets info.parsed, info.parseErr,
+// info.imports, info.embeds, and info.embedErr.
+//
+// It only returns an error if there are problems reading the file,
+// not for syntax errors in the file itself.
+func readGoInfo(f io.Reader, info *fileInfo) error {
+	r := newImportReader(info.name, f)
+
+	r.readKeyword("package")
+	r.readIdent()
+	for r.peekByte(true) == 'i' {
+		r.readKeyword("import")
+		if r.peekByte(true) == '(' {
+			r.nextByte(false)
+			for r.peekByte(true) != ')' && r.err == nil {
+				r.readImport()
+			}
+			r.nextByte(false)
+		} else {
+			r.readImport()
+		}
+	}
+
+	info.header = r.buf
+
+	// If we stopped successfully before EOF, we read a byte that told us we were done.
+	// Return all but that last byte, which would cause a syntax error if we let it through.
+	if r.err == nil && !r.eof {
+		info.header = r.buf[:len(r.buf)-1]
+	}
+
+	// If we stopped for a syntax error, consume the whole file so that
+	// we are sure we don't change the errors that go/parser returns.
+	if r.err == errSyntax {
+		r.err = nil
+		for r.err == nil && !r.eof {
+			r.readByte()
+		}
+		info.header = r.buf
+	}
+	if r.err != nil {
+		return r.err
+	}
+
+	if info.fset == nil {
+		return nil
+	}
+
+	// Parse file header & record imports.
+	info.parsed, info.parseErr = parser.ParseFile(info.fset, info.name, info.header, parser.ImportsOnly|parser.ParseComments)
+	if info.parseErr != nil {
+		return nil
+	}
+
+	hasEmbed := false
+	for _, decl := range info.parsed.Decls {
+		d, ok := decl.(*ast.GenDecl)
+		if !ok {
+			continue
+		}
+		for _, dspec := range d.Specs {
+			spec, ok := dspec.(*ast.ImportSpec)
+			if !ok {
+				continue
+			}
+			quoted := spec.Path.Value
+			path, err := strconv.Unquote(quoted)
+			if err != nil {
+				return fmt.Errorf("parser returned invalid quoted string: <%s>", quoted)
+			}
+			if path == "embed" {
+				hasEmbed = true
+			}
+
+			doc := spec.Doc
+			if doc == nil && len(d.Specs) == 1 {
+				doc = d.Doc
+			}
+			info.imports = append(info.imports, fileImport{path, spec.Pos(), doc})
+		}
+	}
+
+	// If the file imports "embed",
+	// we have to look for //go:embed comments
+	// in the remainder of the file.
+	// The compiler will enforce the mapping of comments to
+	// declared variables. We just need to know the patterns.
+	// If there were //go:embed comments earlier in the file
+	// (near the package statement or imports), the compiler
+	// will reject them. They can be (and have already been) ignored.
+	if hasEmbed {
+		var line []byte
+		for first := true; r.findEmbed(first); first = false {
+			line = line[:0]
+			pos := r.pos
+			for {
+				c := r.readByteNoBuf()
+				if c == '\n' || r.err != nil || r.eof {
+					break
+				}
+				line = append(line, c)
+			}
+			// Add args if line is well-formed.
+			// Ignore badly-formed lines - the compiler will report them when it finds them,
+			// and we can pretend they are not there to help go list succeed with what it knows.
+			embs, err := parseGoEmbed(string(line), pos)
+			if err == nil {
+				info.embeds = append(info.embeds, embs...)
+			}
+		}
+	}
+
+	return nil
+}
+
+// parseGoEmbed parses the text following "//go:embed" to extract the glob patterns.
+// It accepts unquoted space-separated patterns as well as double-quoted and back-quoted Go strings.
+// This is based on a similar function in cmd/compile/internal/gc/noder.go;
+// this version calculates position information as well.
+func parseGoEmbed(args string, pos token.Position) ([]fileEmbed, error) {
+	trimBytes := func(n int) {
+		pos.Offset += n
+		pos.Column += utf8.RuneCountInString(args[:n])
+		args = args[n:]
+	}
+	trimSpace := func() {
+		trim := strings.TrimLeftFunc(args, unicode.IsSpace)
+		trimBytes(len(args) - len(trim))
+	}
+
+	var list []fileEmbed
+	for trimSpace(); args != ""; trimSpace() {
+		var path string
+		pathPos := pos
+	Switch:
+		switch args[0] {
+		default:
+			i := len(args)
+			for j, c := range args {
+				if unicode.IsSpace(c) {
+					i = j
+					break
+				}
+			}
+			path = args[:i]
+			trimBytes(i)
+
+		case '`':
+			var ok bool
+			path, _, ok = stringsCut(args[1:], "`")
+			if !ok {
+				return nil, fmt.Errorf("invalid quoted string in //go:embed: %s", args)
+			}
+			trimBytes(1 + len(path) + 1)
+
+		case '"':
+			i := 1
+			for ; i < len(args); i++ {
+				if args[i] == '\\' {
+					i++
+					continue
+				}
+				if args[i] == '"' {
+					q, err := strconv.Unquote(args[:i+1])
+					if err != nil {
+						return nil, fmt.Errorf("invalid quoted string in //go:embed: %s", args[:i+1])
+					}
+					path = q
+					trimBytes(i + 1)
+					break Switch
+				}
+			}
+			if i >= len(args) {
+				return nil, fmt.Errorf("invalid quoted string in //go:embed: %s", args)
+			}
+		}
+
+		if args != "" {
+			r, _ := utf8.DecodeRuneInString(args)
+			if !unicode.IsSpace(r) {
+				return nil, fmt.Errorf("invalid quoted string in //go:embed: %s", args)
+			}
+		}
+		list = append(list, fileEmbed{path, pathPos})
+	}
+	return list, nil
+}
diff --git a/internal/backport/go/build/read_test.go b/internal/backport/go/build/read_test.go
new file mode 100644
index 0000000..f15b153
--- /dev/null
+++ b/internal/backport/go/build/read_test.go
@@ -0,0 +1,360 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"fmt"
+	"io"
+	"strings"
+	"testing"
+
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+const quote = "`"
+
+type readTest struct {
+	// Test input contains ℙ where readGoInfo should stop.
+	in  string
+	err string
+}
+
+var readGoInfoTests = []readTest{
+	{
+		`package p`,
+		"",
+	},
+	{
+		`package p; import "x"`,
+		"",
+	},
+	{
+		`package p; import . "x"`,
+		"",
+	},
+	{
+		`package p; import "x";ℙvar x = 1`,
+		"",
+	},
+	{
+		`package p
+
+		// comment
+
+		import "x"
+		import _ "x"
+		import a "x"
+
+		/* comment */
+
+		import (
+			"x" /* comment */
+			_ "x"
+			a "x" // comment
+			` + quote + `x` + quote + `
+			_ /*comment*/ ` + quote + `x` + quote + `
+			a ` + quote + `x` + quote + `
+		)
+		import (
+		)
+		import ()
+		import()import()import()
+		import();import();import()
+
+		ℙvar x = 1
+		`,
+		"",
+	},
+	{
+		"\ufeff𝔻" + `package p; import "x";ℙvar x = 1`,
+		"",
+	},
+}
+
+var readCommentsTests = []readTest{
+	{
+		`ℙpackage p`,
+		"",
+	},
+	{
+		`ℙpackage p; import "x"`,
+		"",
+	},
+	{
+		`ℙpackage p; import . "x"`,
+		"",
+	},
+	{
+		"\ufeff𝔻" + `ℙpackage p; import . "x"`,
+		"",
+	},
+	{
+		`// foo
+
+		/* bar */
+
+		/* quux */ // baz
+
+		/*/ zot */
+
+		// asdf
+		ℙHello, world`,
+		"",
+	},
+	{
+		"\ufeff𝔻" + `// foo
+
+		/* bar */
+
+		/* quux */ // baz
+
+		/*/ zot */
+
+		// asdf
+		ℙHello, world`,
+		"",
+	},
+}
+
+func testRead(t *testing.T, tests []readTest, read func(io.Reader) ([]byte, error)) {
+	for i, tt := range tests {
+		beforeP, afterP, _ := stringsCut1(tt.in, "ℙ")
+		in := beforeP + afterP
+		testOut := beforeP
+
+		if beforeD, afterD, ok := stringsCut1(beforeP, "𝔻"); ok {
+			in = beforeD + afterD + afterP
+			testOut = afterD
+		}
+
+		r := strings.NewReader(in)
+		buf, err := read(r)
+		if err != nil {
+			if tt.err == "" {
+				t.Errorf("#%d: err=%q, expected success (%q)", i, err, string(buf))
+			} else if !strings.Contains(err.Error(), tt.err) {
+				t.Errorf("#%d: err=%q, expected %q", i, err, tt.err)
+			}
+			continue
+		}
+		if tt.err != "" {
+			t.Errorf("#%d: success, expected %q", i, tt.err)
+			continue
+		}
+
+		out := string(buf)
+		if out != testOut {
+			t.Errorf("#%d: wrong output:\nhave %q\nwant %q\n", i, out, testOut)
+		}
+	}
+}
+
+func TestReadGoInfo(t *testing.T) {
+	testRead(t, readGoInfoTests, func(r io.Reader) ([]byte, error) {
+		var info fileInfo
+		err := readGoInfo(r, &info)
+		return info.header, err
+	})
+}
+
+func TestReadComments(t *testing.T) {
+	testRead(t, readCommentsTests, readComments)
+}
+
+var readFailuresTests = []readTest{
+	{
+		`package`,
+		"syntax error",
+	},
+	{
+		"package p\n\x00\nimport `math`\n",
+		"unexpected NUL in input",
+	},
+	{
+		`package p; import`,
+		"syntax error",
+	},
+	{
+		`package p; import "`,
+		"syntax error",
+	},
+	{
+		"package p; import ` \n\n",
+		"syntax error",
+	},
+	{
+		`package p; import "x`,
+		"syntax error",
+	},
+	{
+		`package p; import _`,
+		"syntax error",
+	},
+	{
+		`package p; import _ "`,
+		"syntax error",
+	},
+	{
+		`package p; import _ "x`,
+		"syntax error",
+	},
+	{
+		`package p; import .`,
+		"syntax error",
+	},
+	{
+		`package p; import . "`,
+		"syntax error",
+	},
+	{
+		`package p; import . "x`,
+		"syntax error",
+	},
+	{
+		`package p; import (`,
+		"syntax error",
+	},
+	{
+		`package p; import ("`,
+		"syntax error",
+	},
+	{
+		`package p; import ("x`,
+		"syntax error",
+	},
+	{
+		`package p; import ("x"`,
+		"syntax error",
+	},
+}
+
+func TestReadFailuresIgnored(t *testing.T) {
+	// Syntax errors should not be reported (false arg to readImports).
+	// Instead, entire file should be the output and no error.
+	// Convert tests not to return syntax errors.
+	tests := make([]readTest, len(readFailuresTests))
+	copy(tests, readFailuresTests)
+	for i := range tests {
+		tt := &tests[i]
+		if !strings.Contains(tt.err, "NUL") {
+			tt.err = ""
+		}
+	}
+	testRead(t, tests, func(r io.Reader) ([]byte, error) {
+		var info fileInfo
+		err := readGoInfo(r, &info)
+		return info.header, err
+	})
+}
+
+var readEmbedTests = []struct {
+	in, out string
+}{
+	{
+		"package p\n",
+		"",
+	},
+	{
+		"package p\nimport \"embed\"\nvar i int\n//go:embed x y z\nvar files embed.FS",
+		`test:4:12:x
+		 test:4:14:y
+		 test:4:16:z`,
+	},
+	{
+		"package p\nimport \"embed\"\nvar i int\n//go:embed x \"\\x79\" `z`\nvar files embed.FS",
+		`test:4:12:x
+		 test:4:14:y
+		 test:4:21:z`,
+	},
+	{
+		"package p\nimport \"embed\"\nvar i int\n//go:embed x y\n//go:embed z\nvar files embed.FS",
+		`test:4:12:x
+		 test:4:14:y
+		 test:5:12:z`,
+	},
+	{
+		"package p\nimport \"embed\"\nvar i int\n\t //go:embed x y\n\t //go:embed z\n\t var files embed.FS",
+		`test:4:14:x
+		 test:4:16:y
+		 test:5:14:z`,
+	},
+	{
+		"package p\nimport \"embed\"\n//go:embed x y z\nvar files embed.FS",
+		`test:3:12:x
+		 test:3:14:y
+		 test:3:16:z`,
+	},
+	{
+		"\ufeffpackage p\nimport \"embed\"\n//go:embed x y z\nvar files embed.FS",
+		`test:3:12:x
+		 test:3:14:y
+		 test:3:16:z`,
+	},
+	{
+		"package p\nimport \"embed\"\nvar s = \"/*\"\n//go:embed x\nvar files embed.FS",
+		`test:4:12:x`,
+	},
+	{
+		`package p
+		 import "embed"
+		 var s = "\"\\\\"
+		 //go:embed x
+		 var files embed.FS`,
+		`test:4:15:x`,
+	},
+	{
+		"package p\nimport \"embed\"\nvar s = `/*`\n//go:embed x\nvar files embed.FS",
+		`test:4:12:x`,
+	},
+	{
+		"package p\nimport \"embed\"\nvar s = z/ *y\n//go:embed pointer\nvar pointer embed.FS",
+		"test:4:12:pointer",
+	},
+	{
+		"package p\n//go:embed x y z\n", // no import, no scan
+		"",
+	},
+	{
+		"package p\n//go:embed x y z\nvar files embed.FS", // no import, no scan
+		"",
+	},
+	{
+		"\ufeffpackage p\n//go:embed x y z\nvar files embed.FS", // no import, no scan
+		"",
+	},
+}
+
+func TestReadEmbed(t *testing.T) {
+	fset := token.NewFileSet()
+	for i, tt := range readEmbedTests {
+		info := fileInfo{
+			name: "test",
+			fset: fset,
+		}
+		err := readGoInfo(strings.NewReader(tt.in), &info)
+		if err != nil {
+			t.Errorf("#%d: %v", i, err)
+			continue
+		}
+		b := &strings.Builder{}
+		sep := ""
+		for _, emb := range info.embeds {
+			fmt.Fprintf(b, "%s%v:%s", sep, emb.pos, emb.pattern)
+			sep = "\n"
+		}
+		got := b.String()
+		want := strings.Join(strings.Fields(tt.out), "\n")
+		if got != want {
+			t.Errorf("#%d: embeds:\n%s\nwant:\n%s", i, got, want)
+		}
+	}
+}
+
+func stringsCut1(s, sep string) (before, after string, found bool) {
+	if i := strings.Index(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
+	}
+	return s, "", false
+}
diff --git a/internal/backport/go/build/syslist.go b/internal/backport/go/build/syslist.go
new file mode 100644
index 0000000..0f6e336
--- /dev/null
+++ b/internal/backport/go/build/syslist.go
@@ -0,0 +1,11 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+// List of past, present, and future known GOOS and GOARCH values.
+// Do not remove from this list, as these are used for go/build filename matching.
+
+const goosList = "aix android darwin dragonfly freebsd hurd illumos ios js linux nacl netbsd openbsd plan9 solaris windows zos "
+const goarchList = "386 amd64 amd64p32 arm armbe arm64 arm64be loong64 mips mipsle mips64 mips64le mips64p32 mips64p32le ppc ppc64 ppc64le riscv riscv64 s390 s390x sparc sparc64 wasm "
diff --git a/internal/backport/go/build/syslist_test.go b/internal/backport/go/build/syslist_test.go
new file mode 100644
index 0000000..2b7b4c7
--- /dev/null
+++ b/internal/backport/go/build/syslist_test.go
@@ -0,0 +1,62 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"runtime"
+	"testing"
+)
+
+var (
+	thisOS    = runtime.GOOS
+	thisArch  = runtime.GOARCH
+	otherOS   = anotherOS()
+	otherArch = anotherArch()
+)
+
+func anotherOS() string {
+	if thisOS != "darwin" && thisOS != "ios" {
+		return "darwin"
+	}
+	return "linux"
+}
+
+func anotherArch() string {
+	if thisArch != "amd64" {
+		return "amd64"
+	}
+	return "386"
+}
+
+type GoodFileTest struct {
+	name   string
+	result bool
+}
+
+var tests = []GoodFileTest{
+	{"file.go", true},
+	{"file.c", true},
+	{"file_foo.go", true},
+	{"file_" + thisArch + ".go", true},
+	{"file_" + otherArch + ".go", false},
+	{"file_" + thisOS + ".go", true},
+	{"file_" + otherOS + ".go", false},
+	{"file_" + thisOS + "_" + thisArch + ".go", true},
+	{"file_" + otherOS + "_" + thisArch + ".go", false},
+	{"file_" + thisOS + "_" + otherArch + ".go", false},
+	{"file_" + otherOS + "_" + otherArch + ".go", false},
+	{"file_foo_" + thisArch + ".go", true},
+	{"file_foo_" + otherArch + ".go", false},
+	{"file_" + thisOS + ".c", true},
+	{"file_" + otherOS + ".c", false},
+}
+
+func TestGoodOSArch(t *testing.T) {
+	for _, test := range tests {
+		if Default.goodOSArchFile(test.name, make(map[string]bool)) != test.result {
+			t.Fatalf("goodOSArchFile(%q) != %v", test.name, test.result)
+		}
+	}
+}
diff --git a/internal/backport/go/doc/Makefile b/internal/backport/go/doc/Makefile
new file mode 100644
index 0000000..ca4948f
--- /dev/null
+++ b/internal/backport/go/doc/Makefile
@@ -0,0 +1,7 @@
+# Copyright 2009 The Go Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+# Script to test heading detection heuristic
+headscan: headscan.go
+	go build headscan.go
diff --git a/internal/backport/go/doc/comment.go b/internal/backport/go/doc/comment.go
new file mode 100644
index 0000000..6060fcf
--- /dev/null
+++ b/internal/backport/go/doc/comment.go
@@ -0,0 +1,518 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Godoc comment extraction and comment -> HTML formatting.
+
+package doc
+
+import (
+	"bytes"
+	"io"
+	"regexp"
+	"strings"
+	"text/template" // for HTMLEscape
+	"unicode"
+	"unicode/utf8"
+)
+
+const (
+	ldquo = "&ldquo;"
+	rdquo = "&rdquo;"
+	ulquo = "“"
+	urquo = "”"
+)
+
+var (
+	htmlQuoteReplacer    = strings.NewReplacer(ulquo, ldquo, urquo, rdquo)
+	unicodeQuoteReplacer = strings.NewReplacer("``", ulquo, "''", urquo)
+)
+
+// Escape comment text for HTML. If nice is set,
+// also turn “ into &ldquo; and ” into &rdquo;.
+func commentEscape(w io.Writer, text string, nice bool) {
+	if nice {
+		// In the first pass, we convert `` and '' into their unicode equivalents.
+		// This prevents them from being escaped in HTMLEscape.
+		text = convertQuotes(text)
+		var buf bytes.Buffer
+		template.HTMLEscape(&buf, []byte(text))
+		// Now we convert the unicode quotes to their HTML escaped entities to maintain old behavior.
+		// We need to use a temp buffer to read the string back and do the conversion,
+		// otherwise HTMLEscape will escape & to &amp;
+		htmlQuoteReplacer.WriteString(w, buf.String())
+		return
+	}
+	template.HTMLEscape(w, []byte(text))
+}
+
+func convertQuotes(text string) string {
+	return unicodeQuoteReplacer.Replace(text)
+}
+
+const (
+	// Regexp for Go identifiers
+	identRx = `[\pL_][\pL_0-9]*`
+
+	// Regexp for URLs
+	// Match parens, and check later for balance - see #5043, #22285
+	// Match .,:;?! within path, but not at end - see #18139, #16565
+	// This excludes some rare yet valid urls ending in common punctuation
+	// in order to allow sentences ending in URLs.
+
+	// protocol (required) e.g. http
+	protoPart = `(https?|ftp|file|gopher|mailto|nntp)`
+	// host (required) e.g. www.example.com or [::1]:8080
+	hostPart = `([a-zA-Z0-9_@\-.\[\]:]+)`
+	// path+query+fragment (optional) e.g. /path/index.html?q=foo#bar
+	pathPart = `([.,:;?!]*[a-zA-Z0-9$'()*+&#=@~_/\-\[\]%])*`
+
+	urlRx = protoPart + `://` + hostPart + pathPart
+)
+
+var matchRx = regexp.MustCompile(`(` + urlRx + `)|(` + identRx + `)`)
+
+var (
+	html_a      = []byte(`<a href="`)
+	html_aq     = []byte(`">`)
+	html_enda   = []byte("</a>")
+	html_i      = []byte("<i>")
+	html_endi   = []byte("</i>")
+	html_p      = []byte("<p>\n")
+	html_endp   = []byte("</p>\n")
+	html_pre    = []byte("<pre>")
+	html_endpre = []byte("</pre>\n")
+	html_h      = []byte(`<h3 id="`)
+	html_hq     = []byte(`">`)
+	html_endh   = []byte("</h3>\n")
+)
+
+// Emphasize and escape a line of text for HTML. URLs are converted into links;
+// if the URL also appears in the words map, the link is taken from the map (if
+// the corresponding map value is the empty string, the URL is not converted
+// into a link). Go identifiers that appear in the words map are italicized; if
+// the corresponding map value is not the empty string, it is considered a URL
+// and the word is converted into a link. If nice is set, the remaining text's
+// appearance is improved where it makes sense (e.g., “ is turned into &ldquo;
+// and ” into &rdquo;).
+func emphasize(w io.Writer, line string, words map[string]string, nice bool) {
+	for {
+		m := matchRx.FindStringSubmatchIndex(line)
+		if m == nil {
+			break
+		}
+		// m >= 6 (two parenthesized sub-regexps in matchRx, 1st one is urlRx)
+
+		// write text before match
+		commentEscape(w, line[0:m[0]], nice)
+
+		// adjust match for URLs
+		match := line[m[0]:m[1]]
+		if strings.Contains(match, "://") {
+			m0, m1 := m[0], m[1]
+			for _, s := range []string{"()", "{}", "[]"} {
+				open, close := s[:1], s[1:] // E.g., "(" and ")"
+				// require opening parentheses before closing parentheses (#22285)
+				if i := strings.Index(match, close); i >= 0 && i < strings.Index(match, open) {
+					m1 = m0 + i
+					match = line[m0:m1]
+				}
+				// require balanced pairs of parentheses (#5043)
+				for i := 0; strings.Count(match, open) != strings.Count(match, close) && i < 10; i++ {
+					m1 = strings.LastIndexAny(line[:m1], s)
+					match = line[m0:m1]
+				}
+			}
+			if m1 != m[1] {
+				// redo matching with shortened line for correct indices
+				m = matchRx.FindStringSubmatchIndex(line[:m[0]+len(match)])
+			}
+		}
+
+		// analyze match
+		url := ""
+		italics := false
+		if words != nil {
+			url, italics = words[match]
+		}
+		if m[2] >= 0 {
+			// match against first parenthesized sub-regexp; must be match against urlRx
+			if !italics {
+				// no alternative URL in words list, use match instead
+				url = match
+			}
+			italics = false // don't italicize URLs
+		}
+
+		// write match
+		if len(url) > 0 {
+			w.Write(html_a)
+			template.HTMLEscape(w, []byte(url))
+			w.Write(html_aq)
+		}
+		if italics {
+			w.Write(html_i)
+		}
+		commentEscape(w, match, nice)
+		if italics {
+			w.Write(html_endi)
+		}
+		if len(url) > 0 {
+			w.Write(html_enda)
+		}
+
+		// advance
+		line = line[m[1]:]
+	}
+	commentEscape(w, line, nice)
+}
+
+func indentLen(s string) int {
+	i := 0
+	for i < len(s) && (s[i] == ' ' || s[i] == '\t') {
+		i++
+	}
+	return i
+}
+
+func isBlank(s string) bool {
+	return len(s) == 0 || (len(s) == 1 && s[0] == '\n')
+}
+
+func commonPrefix(a, b string) string {
+	i := 0
+	for i < len(a) && i < len(b) && a[i] == b[i] {
+		i++
+	}
+	return a[0:i]
+}
+
+func unindent(block []string) {
+	if len(block) == 0 {
+		return
+	}
+
+	// compute maximum common white prefix
+	prefix := block[0][0:indentLen(block[0])]
+	for _, line := range block {
+		if !isBlank(line) {
+			prefix = commonPrefix(prefix, line[0:indentLen(line)])
+		}
+	}
+	n := len(prefix)
+
+	// remove
+	for i, line := range block {
+		if !isBlank(line) {
+			block[i] = line[n:]
+		}
+	}
+}
+
+// heading returns the trimmed line if it passes as a section heading;
+// otherwise it returns the empty string.
+func heading(line string) string {
+	line = strings.TrimSpace(line)
+	if len(line) == 0 {
+		return ""
+	}
+
+	// a heading must start with an uppercase letter
+	r, _ := utf8.DecodeRuneInString(line)
+	if !unicode.IsLetter(r) || !unicode.IsUpper(r) {
+		return ""
+	}
+
+	// it must end in a letter or digit:
+	r, _ = utf8.DecodeLastRuneInString(line)
+	if !unicode.IsLetter(r) && !unicode.IsDigit(r) {
+		return ""
+	}
+
+	// exclude lines with illegal characters. we allow "(),"
+	if strings.ContainsAny(line, ";:!?+*/=[]{}_^°&§~%#@<\">\\") {
+		return ""
+	}
+
+	// allow "'" for possessive "'s" only
+	for b := line; ; {
+		var ok bool
+		if _, b, ok = stringsCut(b, "'"); !ok {
+			break
+		}
+		if b != "s" && !strings.HasPrefix(b, "s ") {
+			return "" // ' not followed by s and then end-of-word
+		}
+	}
+
+	// allow "." when followed by non-space
+	for b := line; ; {
+		var ok bool
+		if _, b, ok = stringsCut(b, "."); !ok {
+			break
+		}
+		if b == "" || strings.HasPrefix(b, " ") {
+			return "" // not followed by non-space
+		}
+	}
+
+	return line
+}
+
+type op int
+
+const (
+	opPara op = iota
+	opHead
+	opPre
+)
+
+type block struct {
+	op    op
+	lines []string
+}
+
+var nonAlphaNumRx = regexp.MustCompile(`[^a-zA-Z0-9]`)
+
+func anchorID(line string) string {
+	// Add a "hdr-" prefix to avoid conflicting with IDs used for package symbols.
+	return "hdr-" + nonAlphaNumRx.ReplaceAllString(line, "_")
+}
+
+// ToHTML converts comment text to formatted HTML.
+// The comment was prepared by DocReader,
+// so it is known not to have leading, trailing blank lines
+// nor to have trailing spaces at the end of lines.
+// The comment markers have already been removed.
+//
+// Each span of unindented non-blank lines is converted into
+// a single paragraph. There is one exception to the rule: a span that
+// consists of a single line, is followed by another paragraph span,
+// begins with a capital letter, and contains no punctuation
+// other than parentheses and commas is formatted as a heading.
+//
+// A span of indented lines is converted into a <pre> block,
+// with the common indent prefix removed.
+//
+// URLs in the comment text are converted into links; if the URL also appears
+// in the words map, the link is taken from the map (if the corresponding map
+// value is the empty string, the URL is not converted into a link).
+//
+// A pair of (consecutive) backticks (`) is converted to a unicode left quote (“), and a pair of (consecutive)
+// single quotes (') is converted to a unicode right quote (”).
+//
+// Go identifiers that appear in the words map are italicized; if the corresponding
+// map value is not the empty string, it is considered a URL and the word is converted
+// into a link.
+func ToHTML(w io.Writer, text string, words map[string]string) {
+	for _, b := range blocks(text) {
+		switch b.op {
+		case opPara:
+			w.Write(html_p)
+			for _, line := range b.lines {
+				emphasize(w, line, words, true)
+			}
+			w.Write(html_endp)
+		case opHead:
+			w.Write(html_h)
+			id := ""
+			for _, line := range b.lines {
+				if id == "" {
+					id = anchorID(line)
+					w.Write([]byte(id))
+					w.Write(html_hq)
+				}
+				commentEscape(w, line, true)
+			}
+			if id == "" {
+				w.Write(html_hq)
+			}
+			w.Write(html_endh)
+		case opPre:
+			w.Write(html_pre)
+			for _, line := range b.lines {
+				emphasize(w, line, nil, false)
+			}
+			w.Write(html_endpre)
+		}
+	}
+}
+
+func blocks(text string) []block {
+	var (
+		out  []block
+		para []string
+
+		lastWasBlank   = false
+		lastWasHeading = false
+	)
+
+	close := func() {
+		if para != nil {
+			out = append(out, block{opPara, para})
+			para = nil
+		}
+	}
+
+	lines := strings.SplitAfter(text, "\n")
+	unindent(lines)
+	for i := 0; i < len(lines); {
+		line := lines[i]
+		if isBlank(line) {
+			// close paragraph
+			close()
+			i++
+			lastWasBlank = true
+			continue
+		}
+		if indentLen(line) > 0 {
+			// close paragraph
+			close()
+
+			// count indented or blank lines
+			j := i + 1
+			for j < len(lines) && (isBlank(lines[j]) || indentLen(lines[j]) > 0) {
+				j++
+			}
+			// but not trailing blank lines
+			for j > i && isBlank(lines[j-1]) {
+				j--
+			}
+			pre := lines[i:j]
+			i = j
+
+			unindent(pre)
+
+			// put those lines in a pre block
+			out = append(out, block{opPre, pre})
+			lastWasHeading = false
+			continue
+		}
+
+		if lastWasBlank && !lastWasHeading && i+2 < len(lines) &&
+			isBlank(lines[i+1]) && !isBlank(lines[i+2]) && indentLen(lines[i+2]) == 0 {
+			// current line is non-blank, surrounded by blank lines
+			// and the next non-blank line is not indented: this
+			// might be a heading.
+			if head := heading(line); head != "" {
+				close()
+				out = append(out, block{opHead, []string{head}})
+				i += 2
+				lastWasHeading = true
+				continue
+			}
+		}
+
+		// open paragraph
+		lastWasBlank = false
+		lastWasHeading = false
+		para = append(para, lines[i])
+		i++
+	}
+	close()
+
+	return out
+}
+
+// ToText prepares comment text for presentation in textual output.
+// It wraps paragraphs of text to width or fewer Unicode code points
+// and then prefixes each line with the indent. In preformatted sections
+// (such as program text), it prefixes each non-blank line with preIndent.
+//
+// A pair of (consecutive) backticks (`) is converted to a unicode left quote (“), and a pair of (consecutive)
+// single quotes (') is converted to a unicode right quote (”).
+func ToText(w io.Writer, text string, indent, preIndent string, width int) {
+	l := lineWrapper{
+		out:    w,
+		width:  width,
+		indent: indent,
+	}
+	for _, b := range blocks(text) {
+		switch b.op {
+		case opPara:
+			// l.write will add leading newline if required
+			for _, line := range b.lines {
+				line = convertQuotes(line)
+				l.write(line)
+			}
+			l.flush()
+		case opHead:
+			w.Write(nl)
+			for _, line := range b.lines {
+				line = convertQuotes(line)
+				l.write(line + "\n")
+			}
+			l.flush()
+		case opPre:
+			w.Write(nl)
+			for _, line := range b.lines {
+				if isBlank(line) {
+					w.Write([]byte("\n"))
+				} else {
+					w.Write([]byte(preIndent))
+					w.Write([]byte(line))
+				}
+			}
+		}
+	}
+}
+
+type lineWrapper struct {
+	out       io.Writer
+	printed   bool
+	width     int
+	indent    string
+	n         int
+	pendSpace int
+}
+
+var nl = []byte("\n")
+var space = []byte(" ")
+var prefix = []byte("// ")
+
+func (l *lineWrapper) write(text string) {
+	if l.n == 0 && l.printed {
+		l.out.Write(nl) // blank line before new paragraph
+	}
+	l.printed = true
+
+	needsPrefix := false
+	isComment := strings.HasPrefix(text, "//")
+	for _, f := range strings.Fields(text) {
+		w := utf8.RuneCountInString(f)
+		// wrap if line is too long
+		if l.n > 0 && l.n+l.pendSpace+w > l.width {
+			l.out.Write(nl)
+			l.n = 0
+			l.pendSpace = 0
+			needsPrefix = isComment && !strings.HasPrefix(f, "//")
+		}
+		if l.n == 0 {
+			l.out.Write([]byte(l.indent))
+		}
+		if needsPrefix {
+			l.out.Write(prefix)
+			needsPrefix = false
+		}
+		l.out.Write(space[:l.pendSpace])
+		l.out.Write([]byte(f))
+		l.n += l.pendSpace + w
+		l.pendSpace = 1
+	}
+}
+
+func (l *lineWrapper) flush() {
+	if l.n == 0 {
+		return
+	}
+	l.out.Write(nl)
+	l.pendSpace = 0
+	l.n = 0
+}
+
+func stringsCut(s, sep string) (before, after string, found bool) {
+	if i := strings.Index(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
+	}
+	return s, "", false
+}
diff --git a/internal/backport/go/doc/comment_test.go b/internal/backport/go/doc/comment_test.go
new file mode 100644
index 0000000..6d1b209
--- /dev/null
+++ b/internal/backport/go/doc/comment_test.go
@@ -0,0 +1,247 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import (
+	"bytes"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+var headingTests = []struct {
+	line string
+	ok   bool
+}{
+	{"Section", true},
+	{"A typical usage", true},
+	{"ΔΛΞ is Greek", true},
+	{"Foo 42", true},
+	{"", false},
+	{"section", false},
+	{"A typical usage:", false},
+	{"This code:", false},
+	{"δ is Greek", false},
+	{"Foo §", false},
+	{"Fermat's Last Sentence", true},
+	{"Fermat's", true},
+	{"'sX", false},
+	{"Ted 'Too' Bar", false},
+	{"Use n+m", false},
+	{"Scanning:", false},
+	{"N:M", false},
+}
+
+func TestIsHeading(t *testing.T) {
+	for _, tt := range headingTests {
+		if h := heading(tt.line); (len(h) > 0) != tt.ok {
+			t.Errorf("isHeading(%q) = %v, want %v", tt.line, h, tt.ok)
+		}
+	}
+}
+
+var blocksTests = []struct {
+	in   string
+	out  []block
+	text string
+}{
+	{
+		in: `Para 1.
+Para 1 line 2.
+
+Para 2.
+
+Section
+
+Para 3.
+
+	pre
+	pre1
+
+Para 4.
+
+	pre
+	pre1
+
+	pre2
+
+Para 5.
+
+
+	pre
+
+
+	pre1
+	pre2
+
+Para 6.
+	pre
+	pre2
+`,
+		out: []block{
+			{opPara, []string{"Para 1.\n", "Para 1 line 2.\n"}},
+			{opPara, []string{"Para 2.\n"}},
+			{opHead, []string{"Section"}},
+			{opPara, []string{"Para 3.\n"}},
+			{opPre, []string{"pre\n", "pre1\n"}},
+			{opPara, []string{"Para 4.\n"}},
+			{opPre, []string{"pre\n", "pre1\n", "\n", "pre2\n"}},
+			{opPara, []string{"Para 5.\n"}},
+			{opPre, []string{"pre\n", "\n", "\n", "pre1\n", "pre2\n"}},
+			{opPara, []string{"Para 6.\n"}},
+			{opPre, []string{"pre\n", "pre2\n"}},
+		},
+		text: `.   Para 1. Para 1 line 2.
+
+.   Para 2.
+
+
+.   Section
+
+.   Para 3.
+
+$	pre
+$	pre1
+
+.   Para 4.
+
+$	pre
+$	pre1
+
+$	pre2
+
+.   Para 5.
+
+$	pre
+
+
+$	pre1
+$	pre2
+
+.   Para 6.
+
+$	pre
+$	pre2
+`,
+	},
+	{
+		in: "Para.\n\tshould not be ``escaped''",
+		out: []block{
+			{opPara, []string{"Para.\n"}},
+			{opPre, []string{"should not be ``escaped''"}},
+		},
+		text: ".   Para.\n\n$	should not be ``escaped''",
+	},
+	{
+		in: "// A very long line of 46 char for line wrapping.",
+		out: []block{
+			{opPara, []string{"// A very long line of 46 char for line wrapping."}},
+		},
+		text: `.   // A very long line of 46 char for line
+.   // wrapping.
+`,
+	},
+	{
+		in: `/* A very long line of 46 char for line wrapping.
+A very long line of 46 char for line wrapping. */`,
+		out: []block{
+			{opPara, []string{"/* A very long line of 46 char for line wrapping.\n", "A very long line of 46 char for line wrapping. */"}},
+		},
+		text: `.   /* A very long line of 46 char for line
+.   wrapping. A very long line of 46 char
+.   for line wrapping. */
+`,
+	},
+	{
+		in: `A line of 36 char for line wrapping.
+//Another line starting with //`,
+		out: []block{
+			{opPara, []string{"A line of 36 char for line wrapping.\n",
+				"//Another line starting with //"}},
+		},
+		text: `.   A line of 36 char for line wrapping.
+.   //Another line starting with //
+`,
+	},
+}
+
+func TestBlocks(t *testing.T) {
+	for i, tt := range blocksTests {
+		b := blocks(tt.in)
+		if !reflect.DeepEqual(b, tt.out) {
+			t.Errorf("#%d: mismatch\nhave: %v\nwant: %v", i, b, tt.out)
+		}
+	}
+}
+
+func TestToText(t *testing.T) {
+	var buf bytes.Buffer
+	for i, tt := range blocksTests {
+		ToText(&buf, tt.in, ".   ", "$\t", 40)
+		if have := buf.String(); have != tt.text {
+			t.Errorf("#%d: mismatch\nhave: %s\nwant: %s\nhave vs want:\n%q\n%q", i, have, tt.text, have, tt.text)
+		}
+		buf.Reset()
+	}
+}
+
+var emphasizeTests = []struct {
+	in, out string
+}{
+	{"", ""},
+	{"http://[::1]:8080/foo.txt", `<a href="http://[::1]:8080/foo.txt">http://[::1]:8080/foo.txt</a>`},
+	{"before (https://www.google.com) after", `before (<a href="https://www.google.com">https://www.google.com</a>) after`},
+	{"before https://www.google.com:30/x/y/z:b::c. After", `before <a href="https://www.google.com:30/x/y/z:b::c">https://www.google.com:30/x/y/z:b::c</a>. After`},
+	{"http://www.google.com/path/:;!-/?query=%34b#093124", `<a href="http://www.google.com/path/:;!-/?query=%34b#093124">http://www.google.com/path/:;!-/?query=%34b#093124</a>`},
+	{"http://www.google.com/path/:;!-/?query=%34bar#093124", `<a href="http://www.google.com/path/:;!-/?query=%34bar#093124">http://www.google.com/path/:;!-/?query=%34bar#093124</a>`},
+	{"http://www.google.com/index.html! After", `<a href="http://www.google.com/index.html">http://www.google.com/index.html</a>! After`},
+	{"http://www.google.com/", `<a href="http://www.google.com/">http://www.google.com/</a>`},
+	{"https://www.google.com/", `<a href="https://www.google.com/">https://www.google.com/</a>`},
+	{"http://www.google.com/path.", `<a href="http://www.google.com/path">http://www.google.com/path</a>.`},
+	{"http://en.wikipedia.org/wiki/Camellia_(cipher)", `<a href="http://en.wikipedia.org/wiki/Camellia_(cipher)">http://en.wikipedia.org/wiki/Camellia_(cipher)</a>`},
+	{"(http://www.google.com/)", `(<a href="http://www.google.com/">http://www.google.com/</a>)`},
+	{"http://gmail.com)", `<a href="http://gmail.com">http://gmail.com</a>)`},
+	{"((http://gmail.com))", `((<a href="http://gmail.com">http://gmail.com</a>))`},
+	{"http://gmail.com ((http://gmail.com)) ()", `<a href="http://gmail.com">http://gmail.com</a> ((<a href="http://gmail.com">http://gmail.com</a>)) ()`},
+	{"Foo bar http://example.com/ quux!", `Foo bar <a href="http://example.com/">http://example.com/</a> quux!`},
+	{"Hello http://example.com/%2f/ /world.", `Hello <a href="http://example.com/%2f/">http://example.com/%2f/</a> /world.`},
+	{"Lorem http: ipsum //host/path", "Lorem http: ipsum //host/path"},
+	{"javascript://is/not/linked", "javascript://is/not/linked"},
+	{"http://foo", `<a href="http://foo">http://foo</a>`},
+	{"art by [[https://www.example.com/person/][Person Name]]", `art by [[<a href="https://www.example.com/person/">https://www.example.com/person/</a>][Person Name]]`},
+	{"please visit (http://golang.org/)", `please visit (<a href="http://golang.org/">http://golang.org/</a>)`},
+	{"please visit http://golang.org/hello())", `please visit <a href="http://golang.org/hello()">http://golang.org/hello()</a>)`},
+	{"http://git.qemu.org/?p=qemu.git;a=blob;f=qapi-schema.json;hb=HEAD", `<a href="http://git.qemu.org/?p=qemu.git;a=blob;f=qapi-schema.json;hb=HEAD">http://git.qemu.org/?p=qemu.git;a=blob;f=qapi-schema.json;hb=HEAD</a>`},
+	{"https://foo.bar/bal/x(])", `<a href="https://foo.bar/bal/x(">https://foo.bar/bal/x(</a>])`}, // inner ] causes (]) to be cut off from URL
+	{"foo [ http://bar(])", `foo [ <a href="http://bar(">http://bar(</a>])`},                      // outer [ causes ]) to be cut off from URL
+}
+
+func TestEmphasize(t *testing.T) {
+	for i, tt := range emphasizeTests {
+		var buf bytes.Buffer
+		emphasize(&buf, tt.in, nil, true)
+		out := buf.String()
+		if out != tt.out {
+			t.Errorf("#%d: mismatch\nhave: %v\nwant: %v", i, out, tt.out)
+		}
+	}
+}
+
+func TestCommentEscape(t *testing.T) {
+	commentTests := []struct {
+		in, out string
+	}{
+		{"typically invoked as ``go tool asm'',", "typically invoked as " + ldquo + "go tool asm" + rdquo + ","},
+		{"For more detail, run ``go help test'' and ``go help testflag''", "For more detail, run " + ldquo + "go help test" + rdquo + " and " + ldquo + "go help testflag" + rdquo},
+	}
+	for i, tt := range commentTests {
+		var buf strings.Builder
+		commentEscape(&buf, tt.in, true)
+		out := buf.String()
+		if out != tt.out {
+			t.Errorf("#%d: mismatch\nhave: %q\nwant: %q", i, out, tt.out)
+		}
+	}
+}
diff --git a/internal/backport/go/doc/doc.go b/internal/backport/go/doc/doc.go
new file mode 100644
index 0000000..43246d0
--- /dev/null
+++ b/internal/backport/go/doc/doc.go
@@ -0,0 +1,220 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package doc extracts source code documentation from a Go AST.
+package doc
+
+import (
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/token"
+	"strings"
+)
+
+// Package is the documentation for an entire package.
+type Package struct {
+	Doc        string
+	Name       string
+	ImportPath string
+	Imports    []string
+	Filenames  []string
+	Notes      map[string][]*Note
+
+	// Deprecated: For backward compatibility Bugs is still populated,
+	// but all new code should use Notes instead.
+	Bugs []string
+
+	// declarations
+	Consts []*Value
+	Types  []*Type
+	Vars   []*Value
+	Funcs  []*Func
+
+	// Examples is a sorted list of examples associated with
+	// the package. Examples are extracted from _test.go files
+	// provided to NewFromFiles.
+	Examples []*Example
+}
+
+// Value is the documentation for a (possibly grouped) var or const declaration.
+type Value struct {
+	Doc   string
+	Names []string // var or const names in declaration order
+	Decl  *ast.GenDecl
+
+	order int
+}
+
+// Type is the documentation for a type declaration.
+type Type struct {
+	Doc  string
+	Name string
+	Decl *ast.GenDecl
+
+	// associated declarations
+	Consts  []*Value // sorted list of constants of (mostly) this type
+	Vars    []*Value // sorted list of variables of (mostly) this type
+	Funcs   []*Func  // sorted list of functions returning this type
+	Methods []*Func  // sorted list of methods (including embedded ones) of this type
+
+	// Examples is a sorted list of examples associated with
+	// this type. Examples are extracted from _test.go files
+	// provided to NewFromFiles.
+	Examples []*Example
+}
+
+// Func is the documentation for a func declaration.
+type Func struct {
+	Doc  string
+	Name string
+	Decl *ast.FuncDecl
+
+	// methods
+	// (for functions, these fields have the respective zero value)
+	Recv  string // actual   receiver "T" or "*T"
+	Orig  string // original receiver "T" or "*T"
+	Level int    // embedding level; 0 means not embedded
+
+	// Examples is a sorted list of examples associated with this
+	// function or method. Examples are extracted from _test.go files
+	// provided to NewFromFiles.
+	Examples []*Example
+}
+
+// A Note represents a marked comment starting with "MARKER(uid): note body".
+// Any note with a marker of 2 or more upper case [A-Z] letters and a uid of
+// at least one character is recognized. The ":" following the uid is optional.
+// Notes are collected in the Package.Notes map indexed by the notes marker.
+type Note struct {
+	Pos, End token.Pos // position range of the comment containing the marker
+	UID      string    // uid found with the marker
+	Body     string    // note body text
+}
+
+// Mode values control the operation of New and NewFromFiles.
+type Mode int
+
+const (
+	// AllDecls says to extract documentation for all package-level
+	// declarations, not just exported ones.
+	AllDecls Mode = 1 << iota
+
+	// AllMethods says to show all embedded methods, not just the ones of
+	// invisible (unexported) anonymous fields.
+	AllMethods
+
+	// PreserveAST says to leave the AST unmodified. Originally, pieces of
+	// the AST such as function bodies were nil-ed out to save memory in
+	// godoc, but not all programs want that behavior.
+	PreserveAST
+)
+
+// New computes the package documentation for the given package AST.
+// New takes ownership of the AST pkg and may edit or overwrite it.
+// To have the Examples fields populated, use NewFromFiles and include
+// the package's _test.go files.
+func New(pkg *ast.Package, importPath string, mode Mode) *Package {
+	var r reader
+	r.readPackage(pkg, mode)
+	r.computeMethodSets()
+	r.cleanupTypes()
+	return &Package{
+		Doc:        r.doc,
+		Name:       pkg.Name,
+		ImportPath: importPath,
+		Imports:    sortedKeys(r.imports),
+		Filenames:  r.filenames,
+		Notes:      r.notes,
+		Bugs:       noteBodies(r.notes["BUG"]),
+		Consts:     sortedValues(r.values, token.CONST),
+		Types:      sortedTypes(r.types, mode&AllMethods != 0),
+		Vars:       sortedValues(r.values, token.VAR),
+		Funcs:      sortedFuncs(r.funcs, true),
+	}
+}
+
+// NewFromFiles computes documentation for a package.
+//
+// The package is specified by a list of *ast.Files and corresponding
+// file set, which must not be nil.
+// NewFromFiles uses all provided files when computing documentation,
+// so it is the caller's responsibility to provide only the files that
+// match the desired build context. "go/build".Context.MatchFile can
+// be used for determining whether a file matches a build context with
+// the desired GOOS and GOARCH values, and other build constraints.
+// The import path of the package is specified by importPath.
+//
+// Examples found in _test.go files are associated with the corresponding
+// type, function, method, or the package, based on their name.
+// If the example has a suffix in its name, it is set in the
+// Example.Suffix field. Examples with malformed names are skipped.
+//
+// Optionally, a single extra argument of type Mode can be provided to
+// control low-level aspects of the documentation extraction behavior.
+//
+// NewFromFiles takes ownership of the AST files and may edit them,
+// unless the PreserveAST Mode bit is on.
+func NewFromFiles(fset *token.FileSet, files []*ast.File, importPath string, opts ...interface{}) (*Package, error) {
+	// Check for invalid API usage.
+	if fset == nil {
+		panic(fmt.Errorf("doc.NewFromFiles: no token.FileSet provided (fset == nil)"))
+	}
+	var mode Mode
+	switch len(opts) { // There can only be 0 or 1 options, so a simple switch works for now.
+	case 0:
+		// Nothing to do.
+	case 1:
+		m, ok := opts[0].(Mode)
+		if !ok {
+			panic(fmt.Errorf("doc.NewFromFiles: option argument type must be doc.Mode"))
+		}
+		mode = m
+	default:
+		panic(fmt.Errorf("doc.NewFromFiles: there must not be more than 1 option argument"))
+	}
+
+	// Collect .go and _test.go files.
+	var (
+		goFiles     = make(map[string]*ast.File)
+		testGoFiles []*ast.File
+	)
+	for i := range files {
+		f := fset.File(files[i].Pos())
+		if f == nil {
+			return nil, fmt.Errorf("file files[%d] is not found in the provided file set", i)
+		}
+		switch name := f.Name(); {
+		case strings.HasSuffix(name, ".go") && !strings.HasSuffix(name, "_test.go"):
+			goFiles[name] = files[i]
+		case strings.HasSuffix(name, "_test.go"):
+			testGoFiles = append(testGoFiles, files[i])
+		default:
+			return nil, fmt.Errorf("file files[%d] filename %q does not have a .go extension", i, name)
+		}
+	}
+
+	// TODO(dmitshur,gri): A relatively high level call to ast.NewPackage with a simpleImporter
+	// ast.Importer implementation is made below. It might be possible to short-circuit and simplify.
+
+	// Compute package documentation.
+	pkg, _ := ast.NewPackage(fset, goFiles, simpleImporter, nil) // Ignore errors that can happen due to unresolved identifiers.
+	p := New(pkg, importPath, mode)
+	classifyExamples(p, Examples(testGoFiles...))
+	return p, nil
+}
+
+// simpleImporter returns a (dummy) package object named by the last path
+// component of the provided package path (as is the convention for packages).
+// This is sufficient to resolve package identifiers without doing an actual
+// import. It never returns an error.
+func simpleImporter(imports map[string]*ast.Object, path string) (*ast.Object, error) {
+	pkg := imports[path]
+	if pkg == nil {
+		// note that strings.LastIndex returns -1 if there is no "/"
+		pkg = ast.NewObj(ast.Pkg, path[strings.LastIndex(path, "/")+1:])
+		pkg.Data = ast.NewScope(nil) // required by ast.NewPackage for dot-import
+		imports[path] = pkg
+	}
+	return pkg, nil
+}
diff --git a/internal/backport/go/doc/doc_test.go b/internal/backport/go/doc/doc_test.go
new file mode 100644
index 0000000..a4a427a
--- /dev/null
+++ b/internal/backport/go/doc/doc_test.go
@@ -0,0 +1,246 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+	"text/template"
+
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/printer"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+var update = flag.Bool("update", false, "update golden (.out) files")
+var files = flag.String("files", "", "consider only Go test files matching this regular expression")
+
+const dataDir = "testdata"
+
+var templateTxt = readTemplate("template.txt")
+
+func readTemplate(filename string) *template.Template {
+	t := template.New(filename)
+	t.Funcs(template.FuncMap{
+		"node":     nodeFmt,
+		"synopsis": synopsisFmt,
+		"indent":   indentFmt,
+	})
+	return template.Must(t.ParseFiles(filepath.Join(dataDir, filename)))
+}
+
+func nodeFmt(node interface{}, fset *token.FileSet) string {
+	var buf bytes.Buffer
+	printer.Fprint(&buf, fset, node)
+	return strings.ReplaceAll(strings.TrimSpace(buf.String()), "\n", "\n\t")
+}
+
+func synopsisFmt(s string) string {
+	const n = 64
+	if len(s) > n {
+		// cut off excess text and go back to a word boundary
+		s = s[0:n]
+		if i := strings.LastIndexAny(s, "\t\n "); i >= 0 {
+			s = s[0:i]
+		}
+		s = strings.TrimSpace(s) + " ..."
+	}
+	return "// " + strings.ReplaceAll(s, "\n", " ")
+}
+
+func indentFmt(indent, s string) string {
+	end := ""
+	if strings.HasSuffix(s, "\n") {
+		end = "\n"
+		s = s[:len(s)-1]
+	}
+	return indent + strings.ReplaceAll(s, "\n", "\n"+indent) + end
+}
+
+func isGoFile(fi fs.FileInfo) bool {
+	name := fi.Name()
+	return !fi.IsDir() &&
+		len(name) > 0 && name[0] != '.' && // ignore .files
+		filepath.Ext(name) == ".go"
+}
+
+type bundle struct {
+	*Package
+	FSet *token.FileSet
+}
+
+func test(t *testing.T, mode Mode) {
+	// determine file filter
+	filter := isGoFile
+	if *files != "" {
+		rx, err := regexp.Compile(*files)
+		if err != nil {
+			t.Fatal(err)
+		}
+		filter = func(fi fs.FileInfo) bool {
+			return isGoFile(fi) && rx.MatchString(fi.Name())
+		}
+	}
+
+	// get packages
+	fset := token.NewFileSet()
+	pkgs, err := parser.ParseDir(fset, dataDir, filter, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// test packages
+	for _, pkg := range pkgs {
+		t.Run(pkg.Name, func(t *testing.T) {
+			importPath := dataDir + "/" + pkg.Name
+			var files []*ast.File
+			for _, f := range pkg.Files {
+				files = append(files, f)
+			}
+			doc, err := NewFromFiles(fset, files, importPath, mode)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			// golden files always use / in filenames - canonicalize them
+			for i, filename := range doc.Filenames {
+				doc.Filenames[i] = filepath.ToSlash(filename)
+			}
+
+			// print documentation
+			var buf bytes.Buffer
+			if err := templateTxt.Execute(&buf, bundle{doc, fset}); err != nil {
+				t.Fatal(err)
+			}
+			got := buf.Bytes()
+
+			// update golden file if necessary
+			golden := filepath.Join(dataDir, fmt.Sprintf("%s.%d.golden", pkg.Name, mode))
+			if *update {
+				err := os.WriteFile(golden, got, 0644)
+				if err != nil {
+					t.Fatal(err)
+				}
+			}
+
+			// get golden file
+			want, err := os.ReadFile(golden)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			// compare
+			if !bytes.Equal(got, want) {
+				t.Errorf("package %s\n\tgot:\n%s\n\twant:\n%s", pkg.Name, got, want)
+			}
+		})
+	}
+}
+
+func Test(t *testing.T) {
+	t.Run("default", func(t *testing.T) { test(t, 0) })
+	t.Run("AllDecls", func(t *testing.T) { test(t, AllDecls) })
+	t.Run("AllMethods", func(t *testing.T) { test(t, AllMethods) })
+}
+
+func TestAnchorID(t *testing.T) {
+	const in = "Important Things 2 Know & Stuff"
+	const want = "hdr-Important_Things_2_Know___Stuff"
+	got := anchorID(in)
+	if got != want {
+		t.Errorf("anchorID(%q) = %q; want %q", in, got, want)
+	}
+}
+
+const funcsTestFile = `
+package funcs
+
+func F() {}
+
+type S1 struct {
+	S2  // embedded, exported
+	s3  // embedded, unexported
+}
+
+func NewS1()  S1 {return S1{} }
+func NewS1p() *S1 { return &S1{} }
+
+func (S1) M1() {}
+func (r S1) M2() {}
+func(S1) m3() {}		// unexported not shown
+func (*S1) P1() {}		// pointer receiver
+
+type S2 int
+func (S2) M3() {}		// shown on S2
+
+type s3 int
+func (s3) M4() {}		// shown on S1
+
+type G1[T any] struct {
+	*s3
+}
+
+func NewG1[T any]() G1[T] { return G1[T]{} }
+
+func (G1[T]) MG1() {}
+func (*G1[U]) MG2() {}
+
+type G2[T, U any] struct {}
+
+func NewG2[T, U any]() G2[T, U] { return G2[T, U]{} }
+
+func (G2[T, U]) MG3() {}
+func (*G2[A, B]) MG4() {}
+
+
+`
+
+var funcsPackage = &Package{
+	Funcs: []*Func{{Name: "F"}},
+	Types: []*Type{
+		{
+			Name:  "G1",
+			Funcs: []*Func{{Name: "NewG1"}},
+			Methods: []*Func{
+				{Name: "M4", Recv: "G1", // TODO: synthesize a param for G1?
+					Orig: "s3", Level: 1},
+				{Name: "MG1", Recv: "G1[T]", Orig: "G1[T]", Level: 0},
+				{Name: "MG2", Recv: "*G1[U]", Orig: "*G1[U]", Level: 0},
+			},
+		},
+		{
+			Name:  "G2",
+			Funcs: []*Func{{Name: "NewG2"}},
+			Methods: []*Func{
+				{Name: "MG3", Recv: "G2[T, U]", Orig: "G2[T, U]", Level: 0},
+				{Name: "MG4", Recv: "*G2[A, B]", Orig: "*G2[A, B]", Level: 0},
+			},
+		},
+		{
+			Name:  "S1",
+			Funcs: []*Func{{Name: "NewS1"}, {Name: "NewS1p"}},
+			Methods: []*Func{
+				{Name: "M1", Recv: "S1", Orig: "S1", Level: 0},
+				{Name: "M2", Recv: "S1", Orig: "S1", Level: 0},
+				{Name: "M4", Recv: "S1", Orig: "s3", Level: 1},
+				{Name: "P1", Recv: "*S1", Orig: "*S1", Level: 0},
+			},
+		},
+		{
+			Name: "S2",
+			Methods: []*Func{
+				{Name: "M3", Recv: "S2", Orig: "S2", Level: 0},
+			},
+		},
+	},
+}
diff --git a/internal/backport/go/doc/example.go b/internal/backport/go/doc/example.go
new file mode 100644
index 0000000..f4aaa83
--- /dev/null
+++ b/internal/backport/go/doc/example.go
@@ -0,0 +1,550 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Extract example functions from file ASTs.
+
+package doc
+
+import (
+	"path"
+	"regexp"
+	"sort"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+// An Example represents an example function found in a test source file.
+type Example struct {
+	Name        string // name of the item being exemplified (including optional suffix)
+	Suffix      string // example suffix, without leading '_' (only populated by NewFromFiles)
+	Doc         string // example function doc string
+	Code        ast.Node
+	Play        *ast.File // a whole program version of the example
+	Comments    []*ast.CommentGroup
+	Output      string // expected output
+	Unordered   bool
+	EmptyOutput bool // expect empty output
+	Order       int  // original source code order
+}
+
+// Examples returns the examples found in testFiles, sorted by Name field.
+// The Order fields record the order in which the examples were encountered.
+// The Suffix field is not populated when Examples is called directly, it is
+// only populated by NewFromFiles for examples it finds in _test.go files.
+//
+// Playable Examples must be in a package whose name ends in "_test".
+// An Example is "playable" (the Play field is non-nil) in either of these
+// circumstances:
+//   - The example function is self-contained: the function references only
+//     identifiers from other packages (or predeclared identifiers, such as
+//     "int") and the test file does not include a dot import.
+//   - The entire test file is the example: the file contains exactly one
+//     example function, zero test, fuzz test, or benchmark function, and at
+//     least one top-level function, type, variable, or constant declaration
+//     other than the example function.
+func Examples(testFiles ...*ast.File) []*Example {
+	var list []*Example
+	for _, file := range testFiles {
+		hasTests := false // file contains tests, fuzz test, or benchmarks
+		numDecl := 0      // number of non-import declarations in the file
+		var flist []*Example
+		for _, decl := range file.Decls {
+			if g, ok := decl.(*ast.GenDecl); ok && g.Tok != token.IMPORT {
+				numDecl++
+				continue
+			}
+			f, ok := decl.(*ast.FuncDecl)
+			if !ok || f.Recv != nil {
+				continue
+			}
+			numDecl++
+			name := f.Name.Name
+			if isTest(name, "Test") || isTest(name, "Benchmark") || isTest(name, "Fuzz") {
+				hasTests = true
+				continue
+			}
+			if !isTest(name, "Example") {
+				continue
+			}
+			if params := f.Type.Params; len(params.List) != 0 {
+				continue // function has params; not a valid example
+			}
+			if f.Body == nil { // ast.File.Body nil dereference (see issue 28044)
+				continue
+			}
+			var doc string
+			if f.Doc != nil {
+				doc = f.Doc.Text()
+			}
+			output, unordered, hasOutput := exampleOutput(f.Body, file.Comments)
+			flist = append(flist, &Example{
+				Name:        name[len("Example"):],
+				Doc:         doc,
+				Code:        f.Body,
+				Play:        playExample(file, f),
+				Comments:    file.Comments,
+				Output:      output,
+				Unordered:   unordered,
+				EmptyOutput: output == "" && hasOutput,
+				Order:       len(flist),
+			})
+		}
+		if !hasTests && numDecl > 1 && len(flist) == 1 {
+			// If this file only has one example function, some
+			// other top-level declarations, and no tests or
+			// benchmarks, use the whole file as the example.
+			flist[0].Code = file
+			flist[0].Play = playExampleFile(file)
+		}
+		list = append(list, flist...)
+	}
+	// sort by name
+	sort.Slice(list, func(i, j int) bool {
+		return list[i].Name < list[j].Name
+	})
+	return list
+}
+
+var outputPrefix = regexp.MustCompile(`(?i)^[[:space:]]*(unordered )?output:`)
+
+// Extracts the expected output and whether there was a valid output comment
+func exampleOutput(b *ast.BlockStmt, comments []*ast.CommentGroup) (output string, unordered, ok bool) {
+	if _, last := lastComment(b, comments); last != nil {
+		// test that it begins with the correct prefix
+		text := last.Text()
+		if loc := outputPrefix.FindStringSubmatchIndex(text); loc != nil {
+			if loc[2] != -1 {
+				unordered = true
+			}
+			text = text[loc[1]:]
+			// Strip zero or more spaces followed by \n or a single space.
+			text = strings.TrimLeft(text, " ")
+			if len(text) > 0 && text[0] == '\n' {
+				text = text[1:]
+			}
+			return text, unordered, true
+		}
+	}
+	return "", false, false // no suitable comment found
+}
+
+// isTest tells whether name looks like a test, example, fuzz test, or
+// benchmark. It is a Test (say) if there is a character after Test that is not
+// a lower-case letter. (We don't want Testiness.)
+func isTest(name, prefix string) bool {
+	if !strings.HasPrefix(name, prefix) {
+		return false
+	}
+	if len(name) == len(prefix) { // "Test" is ok
+		return true
+	}
+	rune, _ := utf8.DecodeRuneInString(name[len(prefix):])
+	return !unicode.IsLower(rune)
+}
+
+// playExample synthesizes a new *ast.File based on the provided
+// file with the provided function body as the body of main.
+func playExample(file *ast.File, f *ast.FuncDecl) *ast.File {
+	body := f.Body
+
+	if !strings.HasSuffix(file.Name.Name, "_test") {
+		// We don't support examples that are part of the
+		// greater package (yet).
+		return nil
+	}
+
+	// Collect top-level declarations in the file.
+	topDecls := make(map[*ast.Object]ast.Decl)
+	typMethods := make(map[string][]ast.Decl)
+
+	for _, decl := range file.Decls {
+		switch d := decl.(type) {
+		case *ast.FuncDecl:
+			if d.Recv == nil {
+				topDecls[d.Name.Obj] = d
+			} else {
+				if len(d.Recv.List) == 1 {
+					t := d.Recv.List[0].Type
+					tname, _ := baseTypeName(t)
+					typMethods[tname] = append(typMethods[tname], d)
+				}
+			}
+		case *ast.GenDecl:
+			for _, spec := range d.Specs {
+				switch s := spec.(type) {
+				case *ast.TypeSpec:
+					topDecls[s.Name.Obj] = d
+				case *ast.ValueSpec:
+					for _, name := range s.Names {
+						topDecls[name.Obj] = d
+					}
+				}
+			}
+		}
+	}
+
+	// Find unresolved identifiers and uses of top-level declarations.
+	unresolved := make(map[string]bool)
+	var depDecls []ast.Decl
+	hasDepDecls := make(map[ast.Decl]bool)
+
+	var inspectFunc func(ast.Node) bool
+	inspectFunc = func(n ast.Node) bool {
+		switch e := n.(type) {
+		case *ast.Ident:
+			if e.Obj == nil && e.Name != "_" {
+				unresolved[e.Name] = true
+			} else if d := topDecls[e.Obj]; d != nil {
+				if !hasDepDecls[d] {
+					hasDepDecls[d] = true
+					depDecls = append(depDecls, d)
+				}
+			}
+			return true
+		case *ast.SelectorExpr:
+			// For selector expressions, only inspect the left hand side.
+			// (For an expression like fmt.Println, only add "fmt" to the
+			// set of unresolved names, not "Println".)
+			ast.Inspect(e.X, inspectFunc)
+			return false
+		case *ast.KeyValueExpr:
+			// For key value expressions, only inspect the value
+			// as the key should be resolved by the type of the
+			// composite literal.
+			ast.Inspect(e.Value, inspectFunc)
+			return false
+		}
+		return true
+	}
+	ast.Inspect(body, inspectFunc)
+	for i := 0; i < len(depDecls); i++ {
+		switch d := depDecls[i].(type) {
+		case *ast.FuncDecl:
+			// Inspect types of parameters and results. See #28492.
+			if d.Type.Params != nil {
+				for _, p := range d.Type.Params.List {
+					ast.Inspect(p.Type, inspectFunc)
+				}
+			}
+			if d.Type.Results != nil {
+				for _, r := range d.Type.Results.List {
+					ast.Inspect(r.Type, inspectFunc)
+				}
+			}
+
+			// Functions might not have a body. See #42706.
+			if d.Body != nil {
+				ast.Inspect(d.Body, inspectFunc)
+			}
+		case *ast.GenDecl:
+			for _, spec := range d.Specs {
+				switch s := spec.(type) {
+				case *ast.TypeSpec:
+					ast.Inspect(s.Type, inspectFunc)
+
+					depDecls = append(depDecls, typMethods[s.Name.Name]...)
+				case *ast.ValueSpec:
+					if s.Type != nil {
+						ast.Inspect(s.Type, inspectFunc)
+					}
+					for _, val := range s.Values {
+						ast.Inspect(val, inspectFunc)
+					}
+				}
+			}
+		}
+	}
+
+	// Remove predeclared identifiers from unresolved list.
+	for n := range unresolved {
+		if predeclaredTypes[n] || predeclaredConstants[n] || predeclaredFuncs[n] {
+			delete(unresolved, n)
+		}
+	}
+
+	// Use unresolved identifiers to determine the imports used by this
+	// example. The heuristic assumes package names match base import
+	// paths for imports w/o renames (should be good enough most of the time).
+	namedImports := make(map[string]string) // [name]path
+	var blankImports []ast.Spec             // _ imports
+	for _, s := range file.Imports {
+		p, err := strconv.Unquote(s.Path.Value)
+		if err != nil {
+			continue
+		}
+		if p == "syscall/js" {
+			// We don't support examples that import syscall/js,
+			// because the package syscall/js is not available in the playground.
+			return nil
+		}
+		n := path.Base(p)
+		if s.Name != nil {
+			n = s.Name.Name
+			switch n {
+			case "_":
+				blankImports = append(blankImports, s)
+				continue
+			case ".":
+				// We can't resolve dot imports (yet).
+				return nil
+			}
+		}
+		if unresolved[n] {
+			namedImports[n] = p
+			delete(unresolved, n)
+		}
+	}
+
+	// If there are other unresolved identifiers, give up because this
+	// synthesized file is not going to build.
+	if len(unresolved) > 0 {
+		return nil
+	}
+
+	// Include documentation belonging to blank imports.
+	var comments []*ast.CommentGroup
+	for _, s := range blankImports {
+		if c := s.(*ast.ImportSpec).Doc; c != nil {
+			comments = append(comments, c)
+		}
+	}
+
+	// Include comments that are inside the function body.
+	for _, c := range file.Comments {
+		if body.Pos() <= c.Pos() && c.End() <= body.End() {
+			comments = append(comments, c)
+		}
+	}
+
+	// Strip the "Output:" or "Unordered output:" comment and adjust body
+	// end position.
+	body, comments = stripOutputComment(body, comments)
+
+	// Include documentation belonging to dependent declarations.
+	for _, d := range depDecls {
+		switch d := d.(type) {
+		case *ast.GenDecl:
+			if d.Doc != nil {
+				comments = append(comments, d.Doc)
+			}
+		case *ast.FuncDecl:
+			if d.Doc != nil {
+				comments = append(comments, d.Doc)
+			}
+		}
+	}
+
+	// Synthesize import declaration.
+	importDecl := &ast.GenDecl{
+		Tok:    token.IMPORT,
+		Lparen: 1, // Need non-zero Lparen and Rparen so that printer
+		Rparen: 1, // treats this as a factored import.
+	}
+	for n, p := range namedImports {
+		s := &ast.ImportSpec{Path: &ast.BasicLit{Value: strconv.Quote(p)}}
+		if path.Base(p) != n {
+			s.Name = ast.NewIdent(n)
+		}
+		importDecl.Specs = append(importDecl.Specs, s)
+	}
+	importDecl.Specs = append(importDecl.Specs, blankImports...)
+
+	// Synthesize main function.
+	funcDecl := &ast.FuncDecl{
+		Name: ast.NewIdent("main"),
+		Type: f.Type,
+		Body: body,
+	}
+
+	decls := make([]ast.Decl, 0, 2+len(depDecls))
+	decls = append(decls, importDecl)
+	decls = append(decls, depDecls...)
+	decls = append(decls, funcDecl)
+
+	sort.Slice(decls, func(i, j int) bool {
+		return decls[i].Pos() < decls[j].Pos()
+	})
+
+	sort.Slice(comments, func(i, j int) bool {
+		return comments[i].Pos() < comments[j].Pos()
+	})
+
+	// Synthesize file.
+	return &ast.File{
+		Name:     ast.NewIdent("main"),
+		Decls:    decls,
+		Comments: comments,
+	}
+}
+
+// playExampleFile takes a whole file example and synthesizes a new *ast.File
+// such that the example is function main in package main.
+func playExampleFile(file *ast.File) *ast.File {
+	// Strip copyright comment if present.
+	comments := file.Comments
+	if len(comments) > 0 && strings.HasPrefix(comments[0].Text(), "Copyright") {
+		comments = comments[1:]
+	}
+
+	// Copy declaration slice, rewriting the ExampleX function to main.
+	var decls []ast.Decl
+	for _, d := range file.Decls {
+		if f, ok := d.(*ast.FuncDecl); ok && isTest(f.Name.Name, "Example") {
+			// Copy the FuncDecl, as it may be used elsewhere.
+			newF := *f
+			newF.Name = ast.NewIdent("main")
+			newF.Body, comments = stripOutputComment(f.Body, comments)
+			d = &newF
+		}
+		decls = append(decls, d)
+	}
+
+	// Copy the File, as it may be used elsewhere.
+	f := *file
+	f.Name = ast.NewIdent("main")
+	f.Decls = decls
+	f.Comments = comments
+	return &f
+}
+
+// stripOutputComment finds and removes the "Output:" or "Unordered output:"
+// comment from body and comments, and adjusts the body block's end position.
+func stripOutputComment(body *ast.BlockStmt, comments []*ast.CommentGroup) (*ast.BlockStmt, []*ast.CommentGroup) {
+	// Do nothing if there is no "Output:" or "Unordered output:" comment.
+	i, last := lastComment(body, comments)
+	if last == nil || !outputPrefix.MatchString(last.Text()) {
+		return body, comments
+	}
+
+	// Copy body and comments, as the originals may be used elsewhere.
+	newBody := &ast.BlockStmt{
+		Lbrace: body.Lbrace,
+		List:   body.List,
+		Rbrace: last.Pos(),
+	}
+	newComments := make([]*ast.CommentGroup, len(comments)-1)
+	copy(newComments, comments[:i])
+	copy(newComments[i:], comments[i+1:])
+	return newBody, newComments
+}
+
+// lastComment returns the last comment inside the provided block.
+func lastComment(b *ast.BlockStmt, c []*ast.CommentGroup) (i int, last *ast.CommentGroup) {
+	if b == nil {
+		return
+	}
+	pos, end := b.Pos(), b.End()
+	for j, cg := range c {
+		if cg.Pos() < pos {
+			continue
+		}
+		if cg.End() > end {
+			break
+		}
+		i, last = j, cg
+	}
+	return
+}
+
+// classifyExamples classifies examples and assigns them to the Examples field
+// of the relevant Func, Type, or Package that the example is associated with.
+//
+// The classification process is ambiguous in some cases:
+//
+//   - ExampleFoo_Bar matches a type named Foo_Bar
+//     or a method named Foo.Bar.
+//   - ExampleFoo_bar matches a type named Foo_bar
+//     or Foo (with a "bar" suffix).
+//
+// Examples with malformed names are not associated with anything.
+func classifyExamples(p *Package, examples []*Example) {
+	if len(examples) == 0 {
+		return
+	}
+
+	// Mapping of names for funcs, types, and methods to the example listing.
+	ids := make(map[string]*[]*Example)
+	ids[""] = &p.Examples // package-level examples have an empty name
+	for _, f := range p.Funcs {
+		if !token.IsExported(f.Name) {
+			continue
+		}
+		ids[f.Name] = &f.Examples
+	}
+	for _, t := range p.Types {
+		if !token.IsExported(t.Name) {
+			continue
+		}
+		ids[t.Name] = &t.Examples
+		for _, f := range t.Funcs {
+			if !token.IsExported(f.Name) {
+				continue
+			}
+			ids[f.Name] = &f.Examples
+		}
+		for _, m := range t.Methods {
+			if !token.IsExported(m.Name) {
+				continue
+			}
+			ids[strings.TrimPrefix(m.Recv, "*")+"_"+m.Name] = &m.Examples
+		}
+	}
+
+	// Group each example with the associated func, type, or method.
+	for _, ex := range examples {
+		// Consider all possible split points for the suffix
+		// by starting at the end of string (no suffix case),
+		// then trying all positions that contain a '_' character.
+		//
+		// An association is made on the first successful match.
+		// Examples with malformed names that match nothing are skipped.
+		for i := len(ex.Name); i >= 0; i = strings.LastIndexByte(ex.Name[:i], '_') {
+			prefix, suffix, ok := splitExampleName(ex.Name, i)
+			if !ok {
+				continue
+			}
+			exs, ok := ids[prefix]
+			if !ok {
+				continue
+			}
+			ex.Suffix = suffix
+			*exs = append(*exs, ex)
+			break
+		}
+	}
+
+	// Sort list of example according to the user-specified suffix name.
+	for _, exs := range ids {
+		sort.Slice((*exs), func(i, j int) bool {
+			return (*exs)[i].Suffix < (*exs)[j].Suffix
+		})
+	}
+}
+
+// splitExampleName attempts to split example name s at index i,
+// and reports if that produces a valid split. The suffix may be
+// absent. Otherwise, it must start with a lower-case letter and
+// be preceded by '_'.
+//
+// One of i == len(s) or s[i] == '_' must be true.
+func splitExampleName(s string, i int) (prefix, suffix string, ok bool) {
+	if i == len(s) {
+		return s, "", true
+	}
+	if i == len(s)-1 {
+		return "", "", false
+	}
+	prefix, suffix = s[:i], s[i+1:]
+	return prefix, suffix, isExampleSuffix(suffix)
+}
+
+func isExampleSuffix(s string) bool {
+	r, size := utf8.DecodeRuneInString(s)
+	return size > 0 && unicode.IsLower(r)
+}
diff --git a/internal/backport/go/doc/example_test.go b/internal/backport/go/doc/example_test.go
new file mode 100644
index 0000000..fc5b83c
--- /dev/null
+++ b/internal/backport/go/doc/example_test.go
@@ -0,0 +1,749 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc_test
+
+import (
+	"bytes"
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/doc"
+	"golang.org/x/website/internal/backport/go/format"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+const exampleTestFile = `
+package foo_test
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"sort"
+	"os/exec"
+)
+
+func ExampleHello() {
+	fmt.Println("Hello, world!")
+	// Output: Hello, world!
+}
+
+func ExampleImport() {
+	out, err := exec.Command("date").Output()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("The date is %s\n", out)
+}
+
+func ExampleKeyValue() {
+	v := struct {
+		a string
+		b int
+	}{
+		a: "A",
+		b: 1,
+	}
+	fmt.Print(v)
+	// Output: a: "A", b: 1
+}
+
+func ExampleKeyValueImport() {
+	f := flag.Flag{
+		Name: "play",
+	}
+	fmt.Print(f)
+	// Output: Name: "play"
+}
+
+var keyValueTopDecl = struct {
+	a string
+	b int
+}{
+	a: "B",
+	b: 2,
+}
+
+func ExampleKeyValueTopDecl() {
+	fmt.Print(keyValueTopDecl)
+	// Output: a: "B", b: 2
+}
+
+// Person represents a person by name and age.
+type Person struct {
+    Name string
+    Age  int
+}
+
+// String returns a string representation of the Person.
+func (p Person) String() string {
+    return fmt.Sprintf("%s: %d", p.Name, p.Age)
+}
+
+// ByAge implements sort.Interface for []Person based on
+// the Age field.
+type ByAge []Person
+
+// Len returns the number of elements in ByAge.
+func (a (ByAge)) Len() int { return len(a) }
+
+// Swap swaps the elements in ByAge.
+func (a ByAge) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a ByAge) Less(i, j int) bool { return a[i].Age < a[j].Age }
+
+// people is the array of Person
+var people = []Person{
+	{"Bob", 31},
+	{"John", 42},
+	{"Michael", 17},
+	{"Jenny", 26},
+}
+
+func ExampleSort() {
+    fmt.Println(people)
+    sort.Sort(ByAge(people))
+    fmt.Println(people)
+    // Output:
+    // [Bob: 31 John: 42 Michael: 17 Jenny: 26]
+    // [Michael: 17 Jenny: 26 Bob: 31 John: 42]
+}
+`
+
+var exampleTestCases = []struct {
+	Name, Play, Output string
+}{
+	{
+		Name:   "Hello",
+		Play:   exampleHelloPlay,
+		Output: "Hello, world!\n",
+	},
+	{
+		Name: "Import",
+		Play: exampleImportPlay,
+	},
+	{
+		Name:   "KeyValue",
+		Play:   exampleKeyValuePlay,
+		Output: "a: \"A\", b: 1\n",
+	},
+	{
+		Name:   "KeyValueImport",
+		Play:   exampleKeyValueImportPlay,
+		Output: "Name: \"play\"\n",
+	},
+	{
+		Name:   "KeyValueTopDecl",
+		Play:   exampleKeyValueTopDeclPlay,
+		Output: "a: \"B\", b: 2\n",
+	},
+	{
+		Name:   "Sort",
+		Play:   exampleSortPlay,
+		Output: "[Bob: 31 John: 42 Michael: 17 Jenny: 26]\n[Michael: 17 Jenny: 26 Bob: 31 John: 42]\n",
+	},
+}
+
+const exampleHelloPlay = `package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	fmt.Println("Hello, world!")
+}
+`
+const exampleImportPlay = `package main
+
+import (
+	"fmt"
+	"log"
+	"os/exec"
+)
+
+func main() {
+	out, err := exec.Command("date").Output()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("The date is %s\n", out)
+}
+`
+
+const exampleKeyValuePlay = `package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	v := struct {
+		a string
+		b int
+	}{
+		a: "A",
+		b: 1,
+	}
+	fmt.Print(v)
+}
+`
+
+const exampleKeyValueImportPlay = `package main
+
+import (
+	"flag"
+	"fmt"
+)
+
+func main() {
+	f := flag.Flag{
+		Name: "play",
+	}
+	fmt.Print(f)
+}
+`
+
+const exampleKeyValueTopDeclPlay = `package main
+
+import (
+	"fmt"
+)
+
+var keyValueTopDecl = struct {
+	a string
+	b int
+}{
+	a: "B",
+	b: 2,
+}
+
+func main() {
+	fmt.Print(keyValueTopDecl)
+}
+`
+
+const exampleSortPlay = `package main
+
+import (
+	"fmt"
+	"sort"
+)
+
+// Person represents a person by name and age.
+type Person struct {
+	Name string
+	Age  int
+}
+
+// String returns a string representation of the Person.
+func (p Person) String() string {
+	return fmt.Sprintf("%s: %d", p.Name, p.Age)
+}
+
+// ByAge implements sort.Interface for []Person based on
+// the Age field.
+type ByAge []Person
+
+// Len returns the number of elements in ByAge.
+func (a ByAge) Len() int { return len(a) }
+
+// Swap swaps the elements in ByAge.
+func (a ByAge) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a ByAge) Less(i, j int) bool { return a[i].Age < a[j].Age }
+
+// people is the array of Person
+var people = []Person{
+	{"Bob", 31},
+	{"John", 42},
+	{"Michael", 17},
+	{"Jenny", 26},
+}
+
+func main() {
+	fmt.Println(people)
+	sort.Sort(ByAge(people))
+	fmt.Println(people)
+}
+`
+
+func TestExamples(t *testing.T) {
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "test.go", strings.NewReader(exampleTestFile), parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, e := range doc.Examples(file) {
+		c := exampleTestCases[i]
+		if e.Name != c.Name {
+			t.Errorf("got Name == %q, want %q", e.Name, c.Name)
+		}
+		if w := c.Play; w != "" {
+			g := formatFile(t, fset, e.Play)
+			if g != w {
+				t.Errorf("%s: got Play == %q, want %q", c.Name, g, w)
+			}
+		}
+		if g, w := e.Output, c.Output; g != w {
+			t.Errorf("%s: got Output == %q, want %q", c.Name, g, w)
+		}
+	}
+}
+
+const exampleWholeFile = `package foo_test
+
+type X int
+
+func (X) Foo() {
+}
+
+func (X) TestBlah() {
+}
+
+func (X) BenchmarkFoo() {
+}
+
+func (X) FuzzFoo() {
+}
+
+func Example() {
+	fmt.Println("Hello, world!")
+	// Output: Hello, world!
+}
+`
+
+const exampleWholeFileOutput = `package main
+
+type X int
+
+func (X) Foo() {
+}
+
+func (X) TestBlah() {
+}
+
+func (X) BenchmarkFoo() {
+}
+
+func (X) FuzzFoo() {
+}
+
+func main() {
+	fmt.Println("Hello, world!")
+}
+`
+
+const exampleWholeFileFunction = `package foo_test
+
+func Foo(x int) {
+}
+
+func Example() {
+	fmt.Println("Hello, world!")
+	// Output: Hello, world!
+}
+`
+
+const exampleWholeFileFunctionOutput = `package main
+
+func Foo(x int) {
+}
+
+func main() {
+	fmt.Println("Hello, world!")
+}
+`
+
+const exampleWholeFileExternalFunction = `package foo_test
+
+func foo(int)
+
+func Example() {
+	foo(42)
+	// Output:
+}
+`
+
+const exampleWholeFileExternalFunctionOutput = `package main
+
+func foo(int)
+
+func main() {
+	foo(42)
+}
+`
+
+var exampleWholeFileTestCases = []struct {
+	Title, Source, Play, Output string
+}{
+	{
+		"Methods",
+		exampleWholeFile,
+		exampleWholeFileOutput,
+		"Hello, world!\n",
+	},
+	{
+		"Function",
+		exampleWholeFileFunction,
+		exampleWholeFileFunctionOutput,
+		"Hello, world!\n",
+	},
+	{
+		"ExternalFunction",
+		exampleWholeFileExternalFunction,
+		exampleWholeFileExternalFunctionOutput,
+		"",
+	},
+}
+
+func TestExamplesWholeFile(t *testing.T) {
+	for _, c := range exampleWholeFileTestCases {
+		fset := token.NewFileSet()
+		file, err := parser.ParseFile(fset, "test.go", strings.NewReader(c.Source), parser.ParseComments)
+		if err != nil {
+			t.Fatal(err)
+		}
+		es := doc.Examples(file)
+		if len(es) != 1 {
+			t.Fatalf("%s: wrong number of examples; got %d want 1", c.Title, len(es))
+		}
+		e := es[0]
+		if e.Name != "" {
+			t.Errorf("%s: got Name == %q, want %q", c.Title, e.Name, "")
+		}
+		if g, w := formatFile(t, fset, e.Play), c.Play; g != w {
+			t.Errorf("%s: got Play == %q, want %q", c.Title, g, w)
+		}
+		if g, w := e.Output, c.Output; g != w {
+			t.Errorf("%s: got Output == %q, want %q", c.Title, g, w)
+		}
+	}
+}
+
+const exampleInspectSignature = `package foo_test
+
+import (
+	"bytes"
+	"io"
+)
+
+func getReader() io.Reader { return nil }
+
+func do(b bytes.Reader) {}
+
+func Example() {
+	getReader()
+	do()
+	// Output:
+}
+
+func ExampleIgnored() {
+}
+`
+
+const exampleInspectSignatureOutput = `package main
+
+import (
+	"bytes"
+	"io"
+)
+
+func getReader() io.Reader { return nil }
+
+func do(b bytes.Reader) {}
+
+func main() {
+	getReader()
+	do()
+}
+`
+
+func TestExampleInspectSignature(t *testing.T) {
+	// Verify that "bytes" and "io" are imported. See issue #28492.
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "test.go", strings.NewReader(exampleInspectSignature), parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	es := doc.Examples(file)
+	if len(es) != 2 {
+		t.Fatalf("wrong number of examples; got %d want 2", len(es))
+	}
+	// We are interested in the first example only.
+	e := es[0]
+	if e.Name != "" {
+		t.Errorf("got Name == %q, want %q", e.Name, "")
+	}
+	if g, w := formatFile(t, fset, e.Play), exampleInspectSignatureOutput; g != w {
+		t.Errorf("got Play == %q, want %q", g, w)
+	}
+	if g, w := e.Output, ""; g != w {
+		t.Errorf("got Output == %q, want %q", g, w)
+	}
+}
+
+const exampleEmpty = `
+package p
+func Example() {}
+func Example_a()
+`
+
+const exampleEmptyOutput = `package main
+
+func main() {}
+func main()
+`
+
+func TestExampleEmpty(t *testing.T) {
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "test.go", strings.NewReader(exampleEmpty), parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	es := doc.Examples(file)
+	if len(es) != 1 {
+		t.Fatalf("wrong number of examples; got %d want 1", len(es))
+	}
+	e := es[0]
+	if e.Name != "" {
+		t.Errorf("got Name == %q, want %q", e.Name, "")
+	}
+	if g, w := formatFile(t, fset, e.Play), exampleEmptyOutput; g != w {
+		t.Errorf("got Play == %q, want %q", g, w)
+	}
+	if g, w := e.Output, ""; g != w {
+		t.Errorf("got Output == %q, want %q", g, w)
+	}
+}
+
+func formatFile(t *testing.T, fset *token.FileSet, n *ast.File) string {
+	if n == nil {
+		return "<nil>"
+	}
+	var buf bytes.Buffer
+	if err := format.Node(&buf, fset, n); err != nil {
+		t.Fatal(err)
+	}
+	return buf.String()
+}
+
+// This example illustrates how to use NewFromFiles
+// to compute package documentation with examples.
+func ExampleNewFromFiles() {
+	// src and test are two source files that make up
+	// a package whose documentation will be computed.
+	const src = `
+// This is the package comment.
+package p
+
+import "fmt"
+
+// This comment is associated with the Greet function.
+func Greet(who string) {
+	fmt.Printf("Hello, %s!\n", who)
+}
+`
+	const test = `
+package p_test
+
+// This comment is associated with the ExampleGreet_world example.
+func ExampleGreet_world() {
+	Greet("world")
+}
+`
+
+	// Create the AST by parsing src and test.
+	fset := token.NewFileSet()
+	files := []*ast.File{
+		mustParse(fset, "src.go", src),
+		mustParse(fset, "src_test.go", test),
+	}
+
+	// Compute package documentation with examples.
+	p, err := doc.NewFromFiles(fset, files, "example.com/p")
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("package %s - %s", p.Name, p.Doc)
+	fmt.Printf("func %s - %s", p.Funcs[0].Name, p.Funcs[0].Doc)
+	fmt.Printf(" ⤷ example with suffix %q - %s", p.Funcs[0].Examples[0].Suffix, p.Funcs[0].Examples[0].Doc)
+
+	// Output:
+	// package p - This is the package comment.
+	// func Greet - This comment is associated with the Greet function.
+	//  ⤷ example with suffix "world" - This comment is associated with the ExampleGreet_world example.
+}
+
+func TestClassifyExamples(t *testing.T) {
+	const src = `
+package p
+
+const Const1 = 0
+var   Var1   = 0
+
+type (
+	Type1     int
+	Type1_Foo int
+	Type1_foo int
+	type2     int
+
+	Embed struct { Type1 }
+	Uembed struct { type2 }
+)
+
+func Func1()     {}
+func Func1_Foo() {}
+func Func1_foo() {}
+func func2()     {}
+
+func (Type1) Func1() {}
+func (Type1) Func1_Foo() {}
+func (Type1) Func1_foo() {}
+func (Type1) func2() {}
+
+func (type2) Func1() {}
+
+type (
+	Conflict          int
+	Conflict_Conflict int
+	Conflict_conflict int
+)
+
+func (Conflict) Conflict() {}
+`
+	const test = `
+package p_test
+
+func ExampleConst1() {} // invalid - no support for consts and vars
+func ExampleVar1()   {} // invalid - no support for consts and vars
+
+func Example()               {}
+func Example_()              {} // invalid - suffix must start with a lower-case letter
+func Example_suffix()        {}
+func Example_suffix_xX_X_x() {}
+func Example_世界()           {} // invalid - suffix must start with a lower-case letter
+func Example_123()           {} // invalid - suffix must start with a lower-case letter
+func Example_BadSuffix()     {} // invalid - suffix must start with a lower-case letter
+
+func ExampleType1()               {}
+func ExampleType1_()              {} // invalid - suffix must start with a lower-case letter
+func ExampleType1_suffix()        {}
+func ExampleType1_BadSuffix()     {} // invalid - suffix must start with a lower-case letter
+func ExampleType1_Foo()           {}
+func ExampleType1_Foo_suffix()    {}
+func ExampleType1_Foo_BadSuffix() {} // invalid - suffix must start with a lower-case letter
+func ExampleType1_foo()           {}
+func ExampleType1_foo_suffix()    {}
+func ExampleType1_foo_Suffix()    {} // matches Type1, instead of Type1_foo
+func Exampletype2()               {} // invalid - cannot match unexported
+
+func ExampleFunc1()               {}
+func ExampleFunc1_()              {} // invalid - suffix must start with a lower-case letter
+func ExampleFunc1_suffix()        {}
+func ExampleFunc1_BadSuffix()     {} // invalid - suffix must start with a lower-case letter
+func ExampleFunc1_Foo()           {}
+func ExampleFunc1_Foo_suffix()    {}
+func ExampleFunc1_Foo_BadSuffix() {} // invalid - suffix must start with a lower-case letter
+func ExampleFunc1_foo()           {}
+func ExampleFunc1_foo_suffix()    {}
+func ExampleFunc1_foo_Suffix()    {} // matches Func1, instead of Func1_foo
+func Examplefunc1()               {} // invalid - cannot match unexported
+
+func ExampleType1_Func1()               {}
+func ExampleType1_Func1_()              {} // invalid - suffix must start with a lower-case letter
+func ExampleType1_Func1_suffix()        {}
+func ExampleType1_Func1_BadSuffix()     {} // invalid - suffix must start with a lower-case letter
+func ExampleType1_Func1_Foo()           {}
+func ExampleType1_Func1_Foo_suffix()    {}
+func ExampleType1_Func1_Foo_BadSuffix() {} // invalid - suffix must start with a lower-case letter
+func ExampleType1_Func1_foo()           {}
+func ExampleType1_Func1_foo_suffix()    {}
+func ExampleType1_Func1_foo_Suffix()    {} // matches Type1.Func1, instead of Type1.Func1_foo
+func ExampleType1_func2()               {} // matches Type1, instead of Type1.func2
+
+func ExampleEmbed_Func1()         {} // invalid - no support for forwarded methods from embedding exported type
+func ExampleUembed_Func1()        {} // methods from embedding unexported types are OK
+func ExampleUembed_Func1_suffix() {}
+
+func ExampleConflict_Conflict()        {} // ambiguous with either Conflict or Conflict_Conflict type
+func ExampleConflict_conflict()        {} // ambiguous with either Conflict or Conflict_conflict type
+func ExampleConflict_Conflict_suffix() {} // ambiguous with either Conflict or Conflict_Conflict type
+func ExampleConflict_conflict_suffix() {} // ambiguous with either Conflict or Conflict_conflict type
+`
+
+	// Parse literal source code as a *doc.Package.
+	fset := token.NewFileSet()
+	files := []*ast.File{
+		mustParse(fset, "src.go", src),
+		mustParse(fset, "src_test.go", test),
+	}
+	p, err := doc.NewFromFiles(fset, files, "example.com/p")
+	if err != nil {
+		t.Fatalf("doc.NewFromFiles: %v", err)
+	}
+
+	// Collect the association of examples to top-level identifiers.
+	got := map[string][]string{}
+	got[""] = exampleNames(p.Examples)
+	for _, f := range p.Funcs {
+		got[f.Name] = exampleNames(f.Examples)
+	}
+	for _, t := range p.Types {
+		got[t.Name] = exampleNames(t.Examples)
+		for _, f := range t.Funcs {
+			got[f.Name] = exampleNames(f.Examples)
+		}
+		for _, m := range t.Methods {
+			got[t.Name+"."+m.Name] = exampleNames(m.Examples)
+		}
+	}
+
+	want := map[string][]string{
+		"": {"", "suffix", "suffix_xX_X_x"}, // Package-level examples.
+
+		"Type1":     {"", "foo_Suffix", "func2", "suffix"},
+		"Type1_Foo": {"", "suffix"},
+		"Type1_foo": {"", "suffix"},
+
+		"Func1":     {"", "foo_Suffix", "suffix"},
+		"Func1_Foo": {"", "suffix"},
+		"Func1_foo": {"", "suffix"},
+
+		"Type1.Func1":     {"", "foo_Suffix", "suffix"},
+		"Type1.Func1_Foo": {"", "suffix"},
+		"Type1.Func1_foo": {"", "suffix"},
+
+		"Uembed.Func1": {"", "suffix"},
+
+		// These are implementation dependent due to the ambiguous parsing.
+		"Conflict_Conflict": {"", "suffix"},
+		"Conflict_conflict": {"", "suffix"},
+	}
+
+	for id := range got {
+		if !reflect.DeepEqual(got[id], want[id]) {
+			t.Errorf("classification mismatch for %q:\ngot  %q\nwant %q", id, got[id], want[id])
+		}
+	}
+}
+
+func exampleNames(exs []*doc.Example) (out []string) {
+	for _, ex := range exs {
+		out = append(out, ex.Suffix)
+	}
+	return out
+}
+
+func mustParse(fset *token.FileSet, filename, src string) *ast.File {
+	f, err := parser.ParseFile(fset, filename, src, parser.ParseComments)
+	if err != nil {
+		panic(err)
+	}
+	return f
+}
diff --git a/internal/backport/go/doc/exports.go b/internal/backport/go/doc/exports.go
new file mode 100644
index 0000000..c2b91d5
--- /dev/null
+++ b/internal/backport/go/doc/exports.go
@@ -0,0 +1,324 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements export filtering of an AST.
+
+package doc
+
+import (
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+// filterIdentList removes unexported names from list in place
+// and returns the resulting list.
+func filterIdentList(list []*ast.Ident) []*ast.Ident {
+	j := 0
+	for _, x := range list {
+		if token.IsExported(x.Name) {
+			list[j] = x
+			j++
+		}
+	}
+	return list[0:j]
+}
+
+var underscore = ast.NewIdent("_")
+
+func filterCompositeLit(lit *ast.CompositeLit, filter Filter, export bool) {
+	n := len(lit.Elts)
+	lit.Elts = filterExprList(lit.Elts, filter, export)
+	if len(lit.Elts) < n {
+		lit.Incomplete = true
+	}
+}
+
+func filterExprList(list []ast.Expr, filter Filter, export bool) []ast.Expr {
+	j := 0
+	for _, exp := range list {
+		switch x := exp.(type) {
+		case *ast.CompositeLit:
+			filterCompositeLit(x, filter, export)
+		case *ast.KeyValueExpr:
+			if x, ok := x.Key.(*ast.Ident); ok && !filter(x.Name) {
+				continue
+			}
+			if x, ok := x.Value.(*ast.CompositeLit); ok {
+				filterCompositeLit(x, filter, export)
+			}
+		}
+		list[j] = exp
+		j++
+	}
+	return list[0:j]
+}
+
+// updateIdentList replaces all unexported identifiers with underscore
+// and reports whether at least one exported name exists.
+func updateIdentList(list []*ast.Ident) (hasExported bool) {
+	for i, x := range list {
+		if token.IsExported(x.Name) {
+			hasExported = true
+		} else {
+			list[i] = underscore
+		}
+	}
+	return hasExported
+}
+
+// hasExportedName reports whether list contains any exported names.
+func hasExportedName(list []*ast.Ident) bool {
+	for _, x := range list {
+		if x.IsExported() {
+			return true
+		}
+	}
+	return false
+}
+
+// removeAnonymousField removes anonymous fields named name from an interface.
+func removeAnonymousField(name string, ityp *ast.InterfaceType) {
+	list := ityp.Methods.List // we know that ityp.Methods != nil
+	j := 0
+	for _, field := range list {
+		keepField := true
+		if n := len(field.Names); n == 0 {
+			// anonymous field
+			if fname, _ := baseTypeName(field.Type); fname == name {
+				keepField = false
+			}
+		}
+		if keepField {
+			list[j] = field
+			j++
+		}
+	}
+	if j < len(list) {
+		ityp.Incomplete = true
+	}
+	ityp.Methods.List = list[0:j]
+}
+
+// filterFieldList removes unexported fields (field names) from the field list
+// in place and reports whether fields were removed. Anonymous fields are
+// recorded with the parent type. filterType is called with the types of
+// all remaining fields.
+func (r *reader) filterFieldList(parent *namedType, fields *ast.FieldList, ityp *ast.InterfaceType) (removedFields bool) {
+	if fields == nil {
+		return
+	}
+	list := fields.List
+	j := 0
+	for _, field := range list {
+		keepField := false
+		if n := len(field.Names); n == 0 {
+			// anonymous field or embedded type or union element
+			fname := r.recordAnonymousField(parent, field.Type)
+			if fname != "" {
+				if token.IsExported(fname) {
+					keepField = true
+				} else if ityp != nil && predeclaredTypes[fname] {
+					// possibly an embedded predeclared type; keep it for now but
+					// remember this interface so that it can be fixed if name is also
+					// defined locally
+					keepField = true
+					r.remember(fname, ityp)
+				}
+			} else {
+				// If we're operating on an interface, assume that this is an embedded
+				// type or union element.
+				//
+				// TODO(rfindley): consider traversing into approximation/unions
+				// elements to see if they are entirely unexported.
+				keepField = ityp != nil
+			}
+		} else {
+			field.Names = filterIdentList(field.Names)
+			if len(field.Names) < n {
+				removedFields = true
+			}
+			if len(field.Names) > 0 {
+				keepField = true
+			}
+		}
+		if keepField {
+			r.filterType(nil, field.Type)
+			list[j] = field
+			j++
+		}
+	}
+	if j < len(list) {
+		removedFields = true
+	}
+	fields.List = list[0:j]
+	return
+}
+
+// filterParamList applies filterType to each parameter type in fields.
+func (r *reader) filterParamList(fields *ast.FieldList) {
+	if fields != nil {
+		for _, f := range fields.List {
+			r.filterType(nil, f.Type)
+		}
+	}
+}
+
+// filterType strips any unexported struct fields or method types from typ
+// in place. If fields (or methods) have been removed, the corresponding
+// struct or interface type has the Incomplete field set to true.
+func (r *reader) filterType(parent *namedType, typ ast.Expr) {
+	switch t := typ.(type) {
+	case *ast.Ident:
+		// nothing to do
+	case *ast.ParenExpr:
+		r.filterType(nil, t.X)
+	case *ast.StarExpr: // possibly an embedded type literal
+		r.filterType(nil, t.X)
+	case *ast.UnaryExpr:
+		if t.Op == token.TILDE { // approximation element
+			r.filterType(nil, t.X)
+		}
+	case *ast.BinaryExpr:
+		if t.Op == token.OR { // union
+			r.filterType(nil, t.X)
+			r.filterType(nil, t.Y)
+		}
+	case *ast.ArrayType:
+		r.filterType(nil, t.Elt)
+	case *ast.StructType:
+		if r.filterFieldList(parent, t.Fields, nil) {
+			t.Incomplete = true
+		}
+	case *ast.FuncType:
+		r.filterParamList(t.TypeParams)
+		r.filterParamList(t.Params)
+		r.filterParamList(t.Results)
+	case *ast.InterfaceType:
+		if r.filterFieldList(parent, t.Methods, t) {
+			t.Incomplete = true
+		}
+	case *ast.MapType:
+		r.filterType(nil, t.Key)
+		r.filterType(nil, t.Value)
+	case *ast.ChanType:
+		r.filterType(nil, t.Value)
+	}
+}
+
+func (r *reader) filterSpec(spec ast.Spec) bool {
+	switch s := spec.(type) {
+	case *ast.ImportSpec:
+		// always keep imports so we can collect them
+		return true
+	case *ast.ValueSpec:
+		s.Values = filterExprList(s.Values, token.IsExported, true)
+		if len(s.Values) > 0 || s.Type == nil && len(s.Values) == 0 {
+			// If there are values declared on RHS, just replace the unexported
+			// identifiers on the LHS with underscore, so that it matches
+			// the sequence of expression on the RHS.
+			//
+			// Similarly, if there are no type and values, then this expression
+			// must be following an iota expression, where order matters.
+			if updateIdentList(s.Names) {
+				r.filterType(nil, s.Type)
+				return true
+			}
+		} else {
+			s.Names = filterIdentList(s.Names)
+			if len(s.Names) > 0 {
+				r.filterType(nil, s.Type)
+				return true
+			}
+		}
+	case *ast.TypeSpec:
+		// Don't filter type parameters here, by analogy with function parameters
+		// which are not filtered for top-level function declarations.
+		if name := s.Name.Name; token.IsExported(name) {
+			r.filterType(r.lookupType(s.Name.Name), s.Type)
+			return true
+		} else if IsPredeclared(name) {
+			if r.shadowedPredecl == nil {
+				r.shadowedPredecl = make(map[string]bool)
+			}
+			r.shadowedPredecl[name] = true
+		}
+	}
+	return false
+}
+
+// copyConstType returns a copy of typ with position pos.
+// typ must be a valid constant type.
+// In practice, only (possibly qualified) identifiers are possible.
+func copyConstType(typ ast.Expr, pos token.Pos) ast.Expr {
+	switch typ := typ.(type) {
+	case *ast.Ident:
+		return &ast.Ident{Name: typ.Name, NamePos: pos}
+	case *ast.SelectorExpr:
+		if id, ok := typ.X.(*ast.Ident); ok {
+			// presumably a qualified identifier
+			return &ast.SelectorExpr{
+				Sel: ast.NewIdent(typ.Sel.Name),
+				X:   &ast.Ident{Name: id.Name, NamePos: pos},
+			}
+		}
+	}
+	return nil // shouldn't happen, but be conservative and don't panic
+}
+
+func (r *reader) filterSpecList(list []ast.Spec, tok token.Token) []ast.Spec {
+	if tok == token.CONST {
+		// Propagate any type information that would get lost otherwise
+		// when unexported constants are filtered.
+		var prevType ast.Expr
+		for _, spec := range list {
+			spec := spec.(*ast.ValueSpec)
+			if spec.Type == nil && len(spec.Values) == 0 && prevType != nil {
+				// provide current spec with an explicit type
+				spec.Type = copyConstType(prevType, spec.Pos())
+			}
+			if hasExportedName(spec.Names) {
+				// exported names are preserved so there's no need to propagate the type
+				prevType = nil
+			} else {
+				prevType = spec.Type
+			}
+		}
+	}
+
+	j := 0
+	for _, s := range list {
+		if r.filterSpec(s) {
+			list[j] = s
+			j++
+		}
+	}
+	return list[0:j]
+}
+
+func (r *reader) filterDecl(decl ast.Decl) bool {
+	switch d := decl.(type) {
+	case *ast.GenDecl:
+		d.Specs = r.filterSpecList(d.Specs, d.Tok)
+		return len(d.Specs) > 0
+	case *ast.FuncDecl:
+		// ok to filter these methods early because any
+		// conflicting method will be filtered here, too -
+		// thus, removing these methods early will not lead
+		// to the false removal of possible conflicts
+		return token.IsExported(d.Name.Name)
+	}
+	return false
+}
+
+// fileExports removes unexported declarations from src in place.
+func (r *reader) fileExports(src *ast.File) {
+	j := 0
+	for _, d := range src.Decls {
+		if r.filterDecl(d) {
+			src.Decls[j] = d
+			j++
+		}
+	}
+	src.Decls = src.Decls[0:j]
+}
diff --git a/internal/backport/go/doc/filter.go b/internal/backport/go/doc/filter.go
new file mode 100644
index 0000000..ddf0368
--- /dev/null
+++ b/internal/backport/go/doc/filter.go
@@ -0,0 +1,106 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import "golang.org/x/website/internal/backport/go/ast"
+
+type Filter func(string) bool
+
+func matchFields(fields *ast.FieldList, f Filter) bool {
+	if fields != nil {
+		for _, field := range fields.List {
+			for _, name := range field.Names {
+				if f(name.Name) {
+					return true
+				}
+			}
+		}
+	}
+	return false
+}
+
+func matchDecl(d *ast.GenDecl, f Filter) bool {
+	for _, d := range d.Specs {
+		switch v := d.(type) {
+		case *ast.ValueSpec:
+			for _, name := range v.Names {
+				if f(name.Name) {
+					return true
+				}
+			}
+		case *ast.TypeSpec:
+			if f(v.Name.Name) {
+				return true
+			}
+			// We don't match ordinary parameters in filterFuncs, so by analogy don't
+			// match type parameters here.
+			switch t := v.Type.(type) {
+			case *ast.StructType:
+				if matchFields(t.Fields, f) {
+					return true
+				}
+			case *ast.InterfaceType:
+				if matchFields(t.Methods, f) {
+					return true
+				}
+			}
+		}
+	}
+	return false
+}
+
+func filterValues(a []*Value, f Filter) []*Value {
+	w := 0
+	for _, vd := range a {
+		if matchDecl(vd.Decl, f) {
+			a[w] = vd
+			w++
+		}
+	}
+	return a[0:w]
+}
+
+func filterFuncs(a []*Func, f Filter) []*Func {
+	w := 0
+	for _, fd := range a {
+		if f(fd.Name) {
+			a[w] = fd
+			w++
+		}
+	}
+	return a[0:w]
+}
+
+func filterTypes(a []*Type, f Filter) []*Type {
+	w := 0
+	for _, td := range a {
+		n := 0 // number of matches
+		if matchDecl(td.Decl, f) {
+			n = 1
+		} else {
+			// type name doesn't match, but we may have matching consts, vars, factories or methods
+			td.Consts = filterValues(td.Consts, f)
+			td.Vars = filterValues(td.Vars, f)
+			td.Funcs = filterFuncs(td.Funcs, f)
+			td.Methods = filterFuncs(td.Methods, f)
+			n += len(td.Consts) + len(td.Vars) + len(td.Funcs) + len(td.Methods)
+		}
+		if n > 0 {
+			a[w] = td
+			w++
+		}
+	}
+	return a[0:w]
+}
+
+// Filter eliminates documentation for names that don't pass through the filter f.
+// TODO(gri): Recognize "Type.Method" as a name.
+func (p *Package) Filter(f Filter) {
+	p.Consts = filterValues(p.Consts, f)
+	p.Vars = filterValues(p.Vars, f)
+	p.Types = filterTypes(p.Types, f)
+	p.Funcs = filterFuncs(p.Funcs, f)
+	p.Doc = "" // don't show top-level package doc
+}
diff --git a/internal/backport/go/doc/reader.go b/internal/backport/go/doc/reader.go
new file mode 100644
index 0000000..45f1280
--- /dev/null
+++ b/internal/backport/go/doc/reader.go
@@ -0,0 +1,953 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import (
+	"fmt"
+	"regexp"
+	"sort"
+	"strconv"
+	"strings"
+
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+// ----------------------------------------------------------------------------
+// function/method sets
+//
+// Internally, we treat functions like methods and collect them in method sets.
+
+// A methodSet describes a set of methods. Entries where Decl == nil are conflict
+// entries (more than one method with the same name at the same embedding level).
+type methodSet map[string]*Func
+
+// recvString returns a string representation of recv of the form "T", "*T",
+// "T[A, ...]", "*T[A, ...]" or "BADRECV" (if not a proper receiver type).
+func recvString(recv ast.Expr) string {
+	switch t := recv.(type) {
+	case *ast.Ident:
+		return t.Name
+	case *ast.StarExpr:
+		return "*" + recvString(t.X)
+	case *ast.IndexExpr:
+		// Generic type with one parameter.
+		return fmt.Sprintf("%s[%s]", recvString(t.X), recvParam(t.Index))
+	case *ast.IndexListExpr:
+		// Generic type with multiple parameters.
+		if len(t.Indices) > 0 {
+			var b strings.Builder
+			b.WriteString(recvString(t.X))
+			b.WriteByte('[')
+			b.WriteString(recvParam(t.Indices[0]))
+			for _, e := range t.Indices[1:] {
+				b.WriteString(", ")
+				b.WriteString(recvParam(e))
+			}
+			b.WriteByte(']')
+			return b.String()
+		}
+	}
+	return "BADRECV"
+}
+
+func recvParam(p ast.Expr) string {
+	if id, ok := p.(*ast.Ident); ok {
+		return id.Name
+	}
+	return "BADPARAM"
+}
+
+// set creates the corresponding Func for f and adds it to mset.
+// If there are multiple f's with the same name, set keeps the first
+// one with documentation; conflicts are ignored. The boolean
+// specifies whether to leave the AST untouched.
+func (mset methodSet) set(f *ast.FuncDecl, preserveAST bool) {
+	name := f.Name.Name
+	if g := mset[name]; g != nil && g.Doc != "" {
+		// A function with the same name has already been registered;
+		// since it has documentation, assume f is simply another
+		// implementation and ignore it. This does not happen if the
+		// caller is using go/build.ScanDir to determine the list of
+		// files implementing a package.
+		return
+	}
+	// function doesn't exist or has no documentation; use f
+	recv := ""
+	if f.Recv != nil {
+		var typ ast.Expr
+		// be careful in case of incorrect ASTs
+		if list := f.Recv.List; len(list) == 1 {
+			typ = list[0].Type
+		}
+		recv = recvString(typ)
+	}
+	mset[name] = &Func{
+		Doc:  f.Doc.Text(),
+		Name: name,
+		Decl: f,
+		Recv: recv,
+		Orig: recv,
+	}
+	if !preserveAST {
+		f.Doc = nil // doc consumed - remove from AST
+	}
+}
+
+// add adds method m to the method set; m is ignored if the method set
+// already contains a method with the same name at the same or a higher
+// level than m.
+func (mset methodSet) add(m *Func) {
+	old := mset[m.Name]
+	if old == nil || m.Level < old.Level {
+		mset[m.Name] = m
+		return
+	}
+	if m.Level == old.Level {
+		// conflict - mark it using a method with nil Decl
+		mset[m.Name] = &Func{
+			Name:  m.Name,
+			Level: m.Level,
+		}
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Named types
+
+// baseTypeName returns the name of the base type of x (or "")
+// and whether the type is imported or not.
+func baseTypeName(x ast.Expr) (name string, imported bool) {
+	switch t := x.(type) {
+	case *ast.Ident:
+		return t.Name, false
+	case *ast.IndexExpr:
+		return baseTypeName(t.X)
+	case *ast.IndexListExpr:
+		return baseTypeName(t.X)
+	case *ast.SelectorExpr:
+		if _, ok := t.X.(*ast.Ident); ok {
+			// only possible for qualified type names;
+			// assume type is imported
+			return t.Sel.Name, true
+		}
+	case *ast.ParenExpr:
+		return baseTypeName(t.X)
+	case *ast.StarExpr:
+		return baseTypeName(t.X)
+	}
+	return "", false
+}
+
+// An embeddedSet describes a set of embedded types.
+type embeddedSet map[*namedType]bool
+
+// A namedType represents a named unqualified (package local, or possibly
+// predeclared) type. The namedType for a type name is always found via
+// reader.lookupType.
+type namedType struct {
+	doc  string       // doc comment for type
+	name string       // type name
+	decl *ast.GenDecl // nil if declaration hasn't been seen yet
+
+	isEmbedded bool        // true if this type is embedded
+	isStruct   bool        // true if this type is a struct
+	embedded   embeddedSet // true if the embedded type is a pointer
+
+	// associated declarations
+	values  []*Value // consts and vars
+	funcs   methodSet
+	methods methodSet
+}
+
+// ----------------------------------------------------------------------------
+// AST reader
+
+// reader accumulates documentation for a single package.
+// It modifies the AST: Comments (declaration documentation)
+// that have been collected by the reader are set to nil
+// in the respective AST nodes so that they are not printed
+// twice (once when printing the documentation and once when
+// printing the corresponding AST node).
+type reader struct {
+	mode Mode
+
+	// package properties
+	doc       string // package documentation, if any
+	filenames []string
+	notes     map[string][]*Note
+
+	// declarations
+	imports   map[string]int
+	hasDotImp bool     // if set, package contains a dot import
+	values    []*Value // consts and vars
+	order     int      // sort order of const and var declarations (when we can't use a name)
+	types     map[string]*namedType
+	funcs     methodSet
+
+	// support for package-local shadowing of predeclared types
+	shadowedPredecl map[string]bool
+	fixmap          map[string][]*ast.InterfaceType
+}
+
+func (r *reader) isVisible(name string) bool {
+	return r.mode&AllDecls != 0 || token.IsExported(name)
+}
+
+// lookupType returns the base type with the given name.
+// If the base type has not been encountered yet, a new
+// type with the given name but no associated declaration
+// is added to the type map.
+func (r *reader) lookupType(name string) *namedType {
+	if name == "" || name == "_" {
+		return nil // no type docs for anonymous types
+	}
+	if typ, found := r.types[name]; found {
+		return typ
+	}
+	// type not found - add one without declaration
+	typ := &namedType{
+		name:     name,
+		embedded: make(embeddedSet),
+		funcs:    make(methodSet),
+		methods:  make(methodSet),
+	}
+	r.types[name] = typ
+	return typ
+}
+
+// recordAnonymousField registers fieldType as the type of an
+// anonymous field in the parent type. If the field is imported
+// (qualified name) or the parent is nil, the field is ignored.
+// The function returns the field name.
+func (r *reader) recordAnonymousField(parent *namedType, fieldType ast.Expr) (fname string) {
+	fname, imp := baseTypeName(fieldType)
+	if parent == nil || imp {
+		return
+	}
+	if ftype := r.lookupType(fname); ftype != nil {
+		ftype.isEmbedded = true
+		_, ptr := fieldType.(*ast.StarExpr)
+		parent.embedded[ftype] = ptr
+	}
+	return
+}
+
+func (r *reader) readDoc(comment *ast.CommentGroup) {
+	// By convention there should be only one package comment
+	// but collect all of them if there are more than one.
+	text := comment.Text()
+	if r.doc == "" {
+		r.doc = text
+		return
+	}
+	r.doc += "\n" + text
+}
+
+func (r *reader) remember(predecl string, typ *ast.InterfaceType) {
+	if r.fixmap == nil {
+		r.fixmap = make(map[string][]*ast.InterfaceType)
+	}
+	r.fixmap[predecl] = append(r.fixmap[predecl], typ)
+}
+
+func specNames(specs []ast.Spec) []string {
+	names := make([]string, 0, len(specs)) // reasonable estimate
+	for _, s := range specs {
+		// s guaranteed to be an *ast.ValueSpec by readValue
+		for _, ident := range s.(*ast.ValueSpec).Names {
+			names = append(names, ident.Name)
+		}
+	}
+	return names
+}
+
+// readValue processes a const or var declaration.
+func (r *reader) readValue(decl *ast.GenDecl) {
+	// determine if decl should be associated with a type
+	// Heuristic: For each typed entry, determine the type name, if any.
+	//            If there is exactly one type name that is sufficiently
+	//            frequent, associate the decl with the respective type.
+	domName := ""
+	domFreq := 0
+	prev := ""
+	n := 0
+	for _, spec := range decl.Specs {
+		s, ok := spec.(*ast.ValueSpec)
+		if !ok {
+			continue // should not happen, but be conservative
+		}
+		name := ""
+		switch {
+		case s.Type != nil:
+			// a type is present; determine its name
+			if n, imp := baseTypeName(s.Type); !imp {
+				name = n
+			}
+		case decl.Tok == token.CONST && len(s.Values) == 0:
+			// no type or value is present but we have a constant declaration;
+			// use the previous type name (possibly the empty string)
+			name = prev
+		}
+		if name != "" {
+			// entry has a named type
+			if domName != "" && domName != name {
+				// more than one type name - do not associate
+				// with any type
+				domName = ""
+				break
+			}
+			domName = name
+			domFreq++
+		}
+		prev = name
+		n++
+	}
+
+	// nothing to do w/o a legal declaration
+	if n == 0 {
+		return
+	}
+
+	// determine values list with which to associate the Value for this decl
+	values := &r.values
+	const threshold = 0.75
+	if domName != "" && r.isVisible(domName) && domFreq >= int(float64(len(decl.Specs))*threshold) {
+		// typed entries are sufficiently frequent
+		if typ := r.lookupType(domName); typ != nil {
+			values = &typ.values // associate with that type
+		}
+	}
+
+	*values = append(*values, &Value{
+		Doc:   decl.Doc.Text(),
+		Names: specNames(decl.Specs),
+		Decl:  decl,
+		order: r.order,
+	})
+	if r.mode&PreserveAST == 0 {
+		decl.Doc = nil // doc consumed - remove from AST
+	}
+	// Note: It's important that the order used here is global because the cleanupTypes
+	// methods may move values associated with types back into the global list. If the
+	// order is list-specific, sorting is not deterministic because the same order value
+	// may appear multiple times (was bug, found when fixing #16153).
+	r.order++
+}
+
+// fields returns a struct's fields or an interface's methods.
+func fields(typ ast.Expr) (list []*ast.Field, isStruct bool) {
+	var fields *ast.FieldList
+	switch t := typ.(type) {
+	case *ast.StructType:
+		fields = t.Fields
+		isStruct = true
+	case *ast.InterfaceType:
+		fields = t.Methods
+	}
+	if fields != nil {
+		list = fields.List
+	}
+	return
+}
+
+// readType processes a type declaration.
+func (r *reader) readType(decl *ast.GenDecl, spec *ast.TypeSpec) {
+	typ := r.lookupType(spec.Name.Name)
+	if typ == nil {
+		return // no name or blank name - ignore the type
+	}
+
+	// A type should be added at most once, so typ.decl
+	// should be nil - if it is not, simply overwrite it.
+	typ.decl = decl
+
+	// compute documentation
+	doc := spec.Doc
+	if doc == nil {
+		// no doc associated with the spec, use the declaration doc, if any
+		doc = decl.Doc
+	}
+	if r.mode&PreserveAST == 0 {
+		spec.Doc = nil // doc consumed - remove from AST
+		decl.Doc = nil // doc consumed - remove from AST
+	}
+	typ.doc = doc.Text()
+
+	// record anonymous fields (they may contribute methods)
+	// (some fields may have been recorded already when filtering
+	// exports, but that's ok)
+	var list []*ast.Field
+	list, typ.isStruct = fields(spec.Type)
+	for _, field := range list {
+		if len(field.Names) == 0 {
+			r.recordAnonymousField(typ, field.Type)
+		}
+	}
+}
+
+// isPredeclared reports whether n denotes a predeclared type.
+func (r *reader) isPredeclared(n string) bool {
+	return predeclaredTypes[n] && r.types[n] == nil
+}
+
+// readFunc processes a func or method declaration.
+func (r *reader) readFunc(fun *ast.FuncDecl) {
+	// strip function body if requested.
+	if r.mode&PreserveAST == 0 {
+		fun.Body = nil
+	}
+
+	// associate methods with the receiver type, if any
+	if fun.Recv != nil {
+		// method
+		if len(fun.Recv.List) == 0 {
+			// should not happen (incorrect AST); (See issue 17788)
+			// don't show this method
+			return
+		}
+		recvTypeName, imp := baseTypeName(fun.Recv.List[0].Type)
+		if imp {
+			// should not happen (incorrect AST);
+			// don't show this method
+			return
+		}
+		if typ := r.lookupType(recvTypeName); typ != nil {
+			typ.methods.set(fun, r.mode&PreserveAST != 0)
+		}
+		// otherwise ignore the method
+		// TODO(gri): There may be exported methods of non-exported types
+		// that can be called because of exported values (consts, vars, or
+		// function results) of that type. Could determine if that is the
+		// case and then show those methods in an appropriate section.
+		return
+	}
+
+	// Associate factory functions with the first visible result type, as long as
+	// others are predeclared types.
+	if fun.Type.Results.NumFields() >= 1 {
+		var typ *namedType // type to associate the function with
+		numResultTypes := 0
+		for _, res := range fun.Type.Results.List {
+			factoryType := res.Type
+			if t, ok := factoryType.(*ast.ArrayType); ok {
+				// We consider functions that return slices or arrays of type
+				// T (or pointers to T) as factory functions of T.
+				factoryType = t.Elt
+			}
+			if n, imp := baseTypeName(factoryType); !imp && r.isVisible(n) && !r.isPredeclared(n) {
+				if lookupTypeParam(n, fun.Type.TypeParams) != nil {
+					// Issue #49477: don't associate fun with its type parameter result.
+					// A type parameter is not a defined type.
+					continue
+				}
+				if t := r.lookupType(n); t != nil {
+					typ = t
+					numResultTypes++
+					if numResultTypes > 1 {
+						break
+					}
+				}
+			}
+		}
+		// If there is exactly one result type,
+		// associate the function with that type.
+		if numResultTypes == 1 {
+			typ.funcs.set(fun, r.mode&PreserveAST != 0)
+			return
+		}
+	}
+
+	// just an ordinary function
+	r.funcs.set(fun, r.mode&PreserveAST != 0)
+}
+
+// lookupTypeParam searches for type parameters named name within the tparams
+// field list, returning the relevant identifier if found, or nil if not.
+func lookupTypeParam(name string, tparams *ast.FieldList) *ast.Ident {
+	if tparams == nil {
+		return nil
+	}
+	for _, field := range tparams.List {
+		for _, id := range field.Names {
+			if id.Name == name {
+				return id
+			}
+		}
+	}
+	return nil
+}
+
+var (
+	noteMarker    = `([A-Z][A-Z]+)\(([^)]+)\):?`                    // MARKER(uid), MARKER at least 2 chars, uid at least 1 char
+	noteMarkerRx  = regexp.MustCompile(`^[ \t]*` + noteMarker)      // MARKER(uid) at text start
+	noteCommentRx = regexp.MustCompile(`^/[/*][ \t]*` + noteMarker) // MARKER(uid) at comment start
+)
+
+// readNote collects a single note from a sequence of comments.
+func (r *reader) readNote(list []*ast.Comment) {
+	text := (&ast.CommentGroup{List: list}).Text()
+	if m := noteMarkerRx.FindStringSubmatchIndex(text); m != nil {
+		// The note body starts after the marker.
+		// We remove any formatting so that we don't
+		// get spurious line breaks/indentation when
+		// showing the TODO body.
+		body := clean(text[m[1]:], keepNL)
+		if body != "" {
+			marker := text[m[2]:m[3]]
+			r.notes[marker] = append(r.notes[marker], &Note{
+				Pos:  list[0].Pos(),
+				End:  list[len(list)-1].End(),
+				UID:  text[m[4]:m[5]],
+				Body: body,
+			})
+		}
+	}
+}
+
+// readNotes extracts notes from comments.
+// A note must start at the beginning of a comment with "MARKER(uid):"
+// and is followed by the note body (e.g., "// BUG(gri): fix this").
+// The note ends at the end of the comment group or at the start of
+// another note in the same comment group, whichever comes first.
+func (r *reader) readNotes(comments []*ast.CommentGroup) {
+	for _, group := range comments {
+		i := -1 // comment index of most recent note start, valid if >= 0
+		list := group.List
+		for j, c := range list {
+			if noteCommentRx.MatchString(c.Text) {
+				if i >= 0 {
+					r.readNote(list[i:j])
+				}
+				i = j
+			}
+		}
+		if i >= 0 {
+			r.readNote(list[i:])
+		}
+	}
+}
+
+// readFile adds the AST for a source file to the reader.
+func (r *reader) readFile(src *ast.File) {
+	// add package documentation
+	if src.Doc != nil {
+		r.readDoc(src.Doc)
+		if r.mode&PreserveAST == 0 {
+			src.Doc = nil // doc consumed - remove from AST
+		}
+	}
+
+	// add all declarations but for functions which are processed in a separate pass
+	for _, decl := range src.Decls {
+		switch d := decl.(type) {
+		case *ast.GenDecl:
+			switch d.Tok {
+			case token.IMPORT:
+				// imports are handled individually
+				for _, spec := range d.Specs {
+					if s, ok := spec.(*ast.ImportSpec); ok {
+						if import_, err := strconv.Unquote(s.Path.Value); err == nil {
+							r.imports[import_] = 1
+							if s.Name != nil && s.Name.Name == "." {
+								r.hasDotImp = true
+							}
+						}
+					}
+				}
+			case token.CONST, token.VAR:
+				// constants and variables are always handled as a group
+				r.readValue(d)
+			case token.TYPE:
+				// types are handled individually
+				if len(d.Specs) == 1 && !d.Lparen.IsValid() {
+					// common case: single declaration w/o parentheses
+					// (if a single declaration is parenthesized,
+					// create a new fake declaration below, so that
+					// go/doc type declarations always appear w/o
+					// parentheses)
+					if s, ok := d.Specs[0].(*ast.TypeSpec); ok {
+						r.readType(d, s)
+					}
+					break
+				}
+				for _, spec := range d.Specs {
+					if s, ok := spec.(*ast.TypeSpec); ok {
+						// use an individual (possibly fake) declaration
+						// for each type; this also ensures that each type
+						// gets to (re-)use the declaration documentation
+						// if there's none associated with the spec itself
+						fake := &ast.GenDecl{
+							Doc: d.Doc,
+							// don't use the existing TokPos because it
+							// will lead to the wrong selection range for
+							// the fake declaration if there are more
+							// than one type in the group (this affects
+							// src/cmd/godoc/godoc.go's posLink_urlFunc)
+							TokPos: s.Pos(),
+							Tok:    token.TYPE,
+							Specs:  []ast.Spec{s},
+						}
+						r.readType(fake, s)
+					}
+				}
+			}
+		}
+	}
+
+	// collect MARKER(...): annotations
+	r.readNotes(src.Comments)
+	if r.mode&PreserveAST == 0 {
+		src.Comments = nil // consumed unassociated comments - remove from AST
+	}
+}
+
+func (r *reader) readPackage(pkg *ast.Package, mode Mode) {
+	// initialize reader
+	r.filenames = make([]string, len(pkg.Files))
+	r.imports = make(map[string]int)
+	r.mode = mode
+	r.types = make(map[string]*namedType)
+	r.funcs = make(methodSet)
+	r.notes = make(map[string][]*Note)
+
+	// sort package files before reading them so that the
+	// result does not depend on map iteration order
+	i := 0
+	for filename := range pkg.Files {
+		r.filenames[i] = filename
+		i++
+	}
+	sort.Strings(r.filenames)
+
+	// process files in sorted order
+	for _, filename := range r.filenames {
+		f := pkg.Files[filename]
+		if mode&AllDecls == 0 {
+			r.fileExports(f)
+		}
+		r.readFile(f)
+	}
+
+	// process functions now that we have better type information
+	for _, f := range pkg.Files {
+		for _, decl := range f.Decls {
+			if d, ok := decl.(*ast.FuncDecl); ok {
+				r.readFunc(d)
+			}
+		}
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Types
+
+func customizeRecv(f *Func, recvTypeName string, embeddedIsPtr bool, level int) *Func {
+	if f == nil || f.Decl == nil || f.Decl.Recv == nil || len(f.Decl.Recv.List) != 1 {
+		return f // shouldn't happen, but be safe
+	}
+
+	// copy existing receiver field and set new type
+	newField := *f.Decl.Recv.List[0]
+	origPos := newField.Type.Pos()
+	_, origRecvIsPtr := newField.Type.(*ast.StarExpr)
+	newIdent := &ast.Ident{NamePos: origPos, Name: recvTypeName}
+	var typ ast.Expr = newIdent
+	if !embeddedIsPtr && origRecvIsPtr {
+		newIdent.NamePos++ // '*' is one character
+		typ = &ast.StarExpr{Star: origPos, X: newIdent}
+	}
+	newField.Type = typ
+
+	// copy existing receiver field list and set new receiver field
+	newFieldList := *f.Decl.Recv
+	newFieldList.List = []*ast.Field{&newField}
+
+	// copy existing function declaration and set new receiver field list
+	newFuncDecl := *f.Decl
+	newFuncDecl.Recv = &newFieldList
+
+	// copy existing function documentation and set new declaration
+	newF := *f
+	newF.Decl = &newFuncDecl
+	newF.Recv = recvString(typ)
+	// the Orig field never changes
+	newF.Level = level
+
+	return &newF
+}
+
+// collectEmbeddedMethods collects the embedded methods of typ in mset.
+func (r *reader) collectEmbeddedMethods(mset methodSet, typ *namedType, recvTypeName string, embeddedIsPtr bool, level int, visited embeddedSet) {
+	visited[typ] = true
+	for embedded, isPtr := range typ.embedded {
+		// Once an embedded type is embedded as a pointer type
+		// all embedded types in those types are treated like
+		// pointer types for the purpose of the receiver type
+		// computation; i.e., embeddedIsPtr is sticky for this
+		// embedding hierarchy.
+		thisEmbeddedIsPtr := embeddedIsPtr || isPtr
+		for _, m := range embedded.methods {
+			// only top-level methods are embedded
+			if m.Level == 0 {
+				mset.add(customizeRecv(m, recvTypeName, thisEmbeddedIsPtr, level))
+			}
+		}
+		if !visited[embedded] {
+			r.collectEmbeddedMethods(mset, embedded, recvTypeName, thisEmbeddedIsPtr, level+1, visited)
+		}
+	}
+	delete(visited, typ)
+}
+
+// computeMethodSets determines the actual method sets for each type encountered.
+func (r *reader) computeMethodSets() {
+	for _, t := range r.types {
+		// collect embedded methods for t
+		if t.isStruct {
+			// struct
+			r.collectEmbeddedMethods(t.methods, t, t.name, false, 1, make(embeddedSet))
+		} else {
+			// interface
+			// TODO(gri) fix this
+		}
+	}
+
+	// For any predeclared names that are declared locally, don't treat them as
+	// exported fields anymore.
+	for predecl := range r.shadowedPredecl {
+		for _, ityp := range r.fixmap[predecl] {
+			removeAnonymousField(predecl, ityp)
+		}
+	}
+}
+
+// cleanupTypes removes the association of functions and methods with
+// types that have no declaration. Instead, these functions and methods
+// are shown at the package level. It also removes types with missing
+// declarations or which are not visible.
+func (r *reader) cleanupTypes() {
+	for _, t := range r.types {
+		visible := r.isVisible(t.name)
+		predeclared := predeclaredTypes[t.name]
+
+		if t.decl == nil && (predeclared || visible && (t.isEmbedded || r.hasDotImp)) {
+			// t.name is a predeclared type (and was not redeclared in this package),
+			// or it was embedded somewhere but its declaration is missing (because
+			// the AST is incomplete), or we have a dot-import (and all bets are off):
+			// move any associated values, funcs, and methods back to the top-level so
+			// that they are not lost.
+			// 1) move values
+			r.values = append(r.values, t.values...)
+			// 2) move factory functions
+			for name, f := range t.funcs {
+				// in a correct AST, package-level function names
+				// are all different - no need to check for conflicts
+				r.funcs[name] = f
+			}
+			// 3) move methods
+			if !predeclared {
+				for name, m := range t.methods {
+					// don't overwrite functions with the same name - drop them
+					if _, found := r.funcs[name]; !found {
+						r.funcs[name] = m
+					}
+				}
+			}
+		}
+		// remove types w/o declaration or which are not visible
+		if t.decl == nil || !visible {
+			delete(r.types, t.name)
+		}
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Sorting
+
+type data struct {
+	n    int
+	swap func(i, j int)
+	less func(i, j int) bool
+}
+
+func (d *data) Len() int           { return d.n }
+func (d *data) Swap(i, j int)      { d.swap(i, j) }
+func (d *data) Less(i, j int) bool { return d.less(i, j) }
+
+// sortBy is a helper function for sorting
+func sortBy(less func(i, j int) bool, swap func(i, j int), n int) {
+	sort.Sort(&data{n, swap, less})
+}
+
+func sortedKeys(m map[string]int) []string {
+	list := make([]string, len(m))
+	i := 0
+	for key := range m {
+		list[i] = key
+		i++
+	}
+	sort.Strings(list)
+	return list
+}
+
+// sortingName returns the name to use when sorting d into place.
+func sortingName(d *ast.GenDecl) string {
+	if len(d.Specs) == 1 {
+		if s, ok := d.Specs[0].(*ast.ValueSpec); ok {
+			return s.Names[0].Name
+		}
+	}
+	return ""
+}
+
+func sortedValues(m []*Value, tok token.Token) []*Value {
+	list := make([]*Value, len(m)) // big enough in any case
+	i := 0
+	for _, val := range m {
+		if val.Decl.Tok == tok {
+			list[i] = val
+			i++
+		}
+	}
+	list = list[0:i]
+
+	sortBy(
+		func(i, j int) bool {
+			if ni, nj := sortingName(list[i].Decl), sortingName(list[j].Decl); ni != nj {
+				return ni < nj
+			}
+			return list[i].order < list[j].order
+		},
+		func(i, j int) { list[i], list[j] = list[j], list[i] },
+		len(list),
+	)
+
+	return list
+}
+
+func sortedTypes(m map[string]*namedType, allMethods bool) []*Type {
+	list := make([]*Type, len(m))
+	i := 0
+	for _, t := range m {
+		list[i] = &Type{
+			Doc:     t.doc,
+			Name:    t.name,
+			Decl:    t.decl,
+			Consts:  sortedValues(t.values, token.CONST),
+			Vars:    sortedValues(t.values, token.VAR),
+			Funcs:   sortedFuncs(t.funcs, true),
+			Methods: sortedFuncs(t.methods, allMethods),
+		}
+		i++
+	}
+
+	sortBy(
+		func(i, j int) bool { return list[i].Name < list[j].Name },
+		func(i, j int) { list[i], list[j] = list[j], list[i] },
+		len(list),
+	)
+
+	return list
+}
+
+func removeStar(s string) string {
+	if len(s) > 0 && s[0] == '*' {
+		return s[1:]
+	}
+	return s
+}
+
+func sortedFuncs(m methodSet, allMethods bool) []*Func {
+	list := make([]*Func, len(m))
+	i := 0
+	for _, m := range m {
+		// determine which methods to include
+		switch {
+		case m.Decl == nil:
+			// exclude conflict entry
+		case allMethods, m.Level == 0, !token.IsExported(removeStar(m.Orig)):
+			// forced inclusion, method not embedded, or method
+			// embedded but original receiver type not exported
+			list[i] = m
+			i++
+		}
+	}
+	list = list[0:i]
+	sortBy(
+		func(i, j int) bool { return list[i].Name < list[j].Name },
+		func(i, j int) { list[i], list[j] = list[j], list[i] },
+		len(list),
+	)
+	return list
+}
+
+// noteBodies returns a list of note body strings given a list of notes.
+// This is only used to populate the deprecated Package.Bugs field.
+func noteBodies(notes []*Note) []string {
+	var list []string
+	for _, n := range notes {
+		list = append(list, n.Body)
+	}
+	return list
+}
+
+// ----------------------------------------------------------------------------
+// Predeclared identifiers
+
+// IsPredeclared reports whether s is a predeclared identifier.
+func IsPredeclared(s string) bool {
+	return predeclaredTypes[s] || predeclaredFuncs[s] || predeclaredConstants[s]
+}
+
+var predeclaredTypes = map[string]bool{
+	"any":        true,
+	"bool":       true,
+	"byte":       true,
+	"comparable": true,
+	"complex64":  true,
+	"complex128": true,
+	"error":      true,
+	"float32":    true,
+	"float64":    true,
+	"int":        true,
+	"int8":       true,
+	"int16":      true,
+	"int32":      true,
+	"int64":      true,
+	"rune":       true,
+	"string":     true,
+	"uint":       true,
+	"uint8":      true,
+	"uint16":     true,
+	"uint32":     true,
+	"uint64":     true,
+	"uintptr":    true,
+}
+
+var predeclaredFuncs = map[string]bool{
+	"append":  true,
+	"cap":     true,
+	"close":   true,
+	"complex": true,
+	"copy":    true,
+	"delete":  true,
+	"imag":    true,
+	"len":     true,
+	"make":    true,
+	"new":     true,
+	"panic":   true,
+	"print":   true,
+	"println": true,
+	"real":    true,
+	"recover": true,
+}
+
+var predeclaredConstants = map[string]bool{
+	"false": true,
+	"iota":  true,
+	"nil":   true,
+	"true":  true,
+}
diff --git a/internal/backport/go/doc/synopsis.go b/internal/backport/go/doc/synopsis.go
new file mode 100644
index 0000000..ca607cc
--- /dev/null
+++ b/internal/backport/go/doc/synopsis.go
@@ -0,0 +1,81 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import (
+	"strings"
+	"unicode"
+)
+
+// firstSentenceLen returns the length of the first sentence in s.
+// The sentence ends after the first period followed by space and
+// not preceded by exactly one uppercase letter.
+func firstSentenceLen(s string) int {
+	var ppp, pp, p rune
+	for i, q := range s {
+		if q == '\n' || q == '\r' || q == '\t' {
+			q = ' '
+		}
+		if q == ' ' && p == '.' && (!unicode.IsUpper(pp) || unicode.IsUpper(ppp)) {
+			return i
+		}
+		if p == '。' || p == '．' {
+			return i
+		}
+		ppp, pp, p = pp, p, q
+	}
+	return len(s)
+}
+
+const (
+	keepNL = 1 << iota
+)
+
+// clean replaces each sequence of space, \n, \r, or \t characters
+// with a single space and removes any trailing and leading spaces.
+// If the keepNL flag is set, newline characters are passed through
+// instead of being change to spaces.
+func clean(s string, flags int) string {
+	var b []byte
+	p := byte(' ')
+	for i := 0; i < len(s); i++ {
+		q := s[i]
+		if (flags&keepNL) == 0 && q == '\n' || q == '\r' || q == '\t' {
+			q = ' '
+		}
+		if q != ' ' || p != ' ' {
+			b = append(b, q)
+			p = q
+		}
+	}
+	// remove trailing blank, if any
+	if n := len(b); n > 0 && p == ' ' {
+		b = b[0 : n-1]
+	}
+	return string(b)
+}
+
+// Synopsis returns a cleaned version of the first sentence in s.
+// That sentence ends after the first period followed by space and
+// not preceded by exactly one uppercase letter. The result string
+// has no \n, \r, or \t characters and uses only single spaces between
+// words. If s starts with any of the IllegalPrefixes, the result
+// is the empty string.
+func Synopsis(s string) string {
+	s = clean(s[0:firstSentenceLen(s)], 0)
+	for _, prefix := range IllegalPrefixes {
+		if strings.HasPrefix(strings.ToLower(s), prefix) {
+			return ""
+		}
+	}
+	s = convertQuotes(s)
+	return s
+}
+
+var IllegalPrefixes = []string{
+	"copyright",
+	"all rights",
+	"author",
+}
diff --git a/internal/backport/go/doc/synopsis_test.go b/internal/backport/go/doc/synopsis_test.go
new file mode 100644
index 0000000..3f443dc
--- /dev/null
+++ b/internal/backport/go/doc/synopsis_test.go
@@ -0,0 +1,52 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import "testing"
+
+var tests = []struct {
+	txt string
+	fsl int
+	syn string
+}{
+	{"", 0, ""},
+	{"foo", 3, "foo"},
+	{"foo.", 4, "foo."},
+	{"foo.bar", 7, "foo.bar"},
+	{"  foo.  ", 6, "foo."},
+	{"  foo\t  bar.\n", 12, "foo bar."},
+	{"  foo\t  bar.\n", 12, "foo bar."},
+	{"a  b\n\nc\r\rd\t\t", 12, "a b c d"},
+	{"a  b\n\nc\r\rd\t\t  . BLA", 15, "a b c d ."},
+	{"Package poems by T.S.Eliot. To rhyme...", 27, "Package poems by T.S.Eliot."},
+	{"Package poems by T. S. Eliot. To rhyme...", 29, "Package poems by T. S. Eliot."},
+	{"foo implements the foo ABI. The foo ABI is...", 27, "foo implements the foo ABI."},
+	{"Package\nfoo. ..", 12, "Package foo."},
+	{"P . Q.", 3, "P ."},
+	{"P. Q.   ", 8, "P. Q."},
+	{"Package Καλημέρα κόσμε.", 36, "Package Καλημέρα κόσμε."},
+	{"Package こんにちは 世界\n", 31, "Package こんにちは 世界"},
+	{"Package こんにちは。世界", 26, "Package こんにちは。"},
+	{"Package 안녕．世界", 17, "Package 안녕．"},
+	{"Package foo does bar.", 21, "Package foo does bar."},
+	{"Copyright 2012 Google, Inc. Package foo does bar.", 27, ""},
+	{"All Rights reserved. Package foo does bar.", 20, ""},
+	{"All rights reserved. Package foo does bar.", 20, ""},
+	{"Authors: foo@bar.com. Package foo does bar.", 21, ""},
+	{"typically invoked as ``go tool asm'',", 37, "typically invoked as " + ulquo + "go tool asm" + urquo + ","},
+}
+
+func TestSynopsis(t *testing.T) {
+	for _, e := range tests {
+		fsl := firstSentenceLen(e.txt)
+		if fsl != e.fsl {
+			t.Errorf("got fsl = %d; want %d for %q\n", fsl, e.fsl, e.txt)
+		}
+		syn := Synopsis(e.txt)
+		if syn != e.syn {
+			t.Errorf("got syn = %q; want %q for %q\n", syn, e.syn, e.txt)
+		}
+	}
+}
diff --git a/internal/backport/go/doc/testdata/a.0.golden b/internal/backport/go/doc/testdata/a.0.golden
new file mode 100644
index 0000000..7e680b8
--- /dev/null
+++ b/internal/backport/go/doc/testdata/a.0.golden
@@ -0,0 +1,52 @@
+// comment 0  comment 1 
+PACKAGE a
+
+IMPORTPATH
+	testdata/a
+
+FILENAMES
+	testdata/a0.go
+	testdata/a1.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	bug0
+
+	bug1
+
+
+BUGS
+BUG(uid)	bug0
+
+BUG(uid)	bug1
+
+
+NOTES
+NOTE(uid)	
+
+NOTE(foo)	1 of 4 - this is the first line of note 1
+	- note 1 continues on this 2nd line
+	- note 1 continues on this 3rd line
+
+NOTE(foo)	2 of 4
+
+NOTE(bar)	3 of 4
+
+NOTE(bar)	4 of 4
+	- this is the last line of note 4
+
+NOTE(bam)	This note which contains a (parenthesized) subphrase
+	 must appear in its entirety.
+
+NOTE(xxx)	The ':' after the marker and uid is optional.
+
+
+SECBUGS
+SECBUG(uid)	sec hole 0
+	need to fix asap
+
+
+TODOS
+TODO(uid)	todo0
+
+TODO(uid)	todo1
+
diff --git a/internal/backport/go/doc/testdata/a.1.golden b/internal/backport/go/doc/testdata/a.1.golden
new file mode 100644
index 0000000..7e680b8
--- /dev/null
+++ b/internal/backport/go/doc/testdata/a.1.golden
@@ -0,0 +1,52 @@
+// comment 0  comment 1 
+PACKAGE a
+
+IMPORTPATH
+	testdata/a
+
+FILENAMES
+	testdata/a0.go
+	testdata/a1.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	bug0
+
+	bug1
+
+
+BUGS
+BUG(uid)	bug0
+
+BUG(uid)	bug1
+
+
+NOTES
+NOTE(uid)	
+
+NOTE(foo)	1 of 4 - this is the first line of note 1
+	- note 1 continues on this 2nd line
+	- note 1 continues on this 3rd line
+
+NOTE(foo)	2 of 4
+
+NOTE(bar)	3 of 4
+
+NOTE(bar)	4 of 4
+	- this is the last line of note 4
+
+NOTE(bam)	This note which contains a (parenthesized) subphrase
+	 must appear in its entirety.
+
+NOTE(xxx)	The ':' after the marker and uid is optional.
+
+
+SECBUGS
+SECBUG(uid)	sec hole 0
+	need to fix asap
+
+
+TODOS
+TODO(uid)	todo0
+
+TODO(uid)	todo1
+
diff --git a/internal/backport/go/doc/testdata/a.2.golden b/internal/backport/go/doc/testdata/a.2.golden
new file mode 100644
index 0000000..7e680b8
--- /dev/null
+++ b/internal/backport/go/doc/testdata/a.2.golden
@@ -0,0 +1,52 @@
+// comment 0  comment 1 
+PACKAGE a
+
+IMPORTPATH
+	testdata/a
+
+FILENAMES
+	testdata/a0.go
+	testdata/a1.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	bug0
+
+	bug1
+
+
+BUGS
+BUG(uid)	bug0
+
+BUG(uid)	bug1
+
+
+NOTES
+NOTE(uid)	
+
+NOTE(foo)	1 of 4 - this is the first line of note 1
+	- note 1 continues on this 2nd line
+	- note 1 continues on this 3rd line
+
+NOTE(foo)	2 of 4
+
+NOTE(bar)	3 of 4
+
+NOTE(bar)	4 of 4
+	- this is the last line of note 4
+
+NOTE(bam)	This note which contains a (parenthesized) subphrase
+	 must appear in its entirety.
+
+NOTE(xxx)	The ':' after the marker and uid is optional.
+
+
+SECBUGS
+SECBUG(uid)	sec hole 0
+	need to fix asap
+
+
+TODOS
+TODO(uid)	todo0
+
+TODO(uid)	todo1
+
diff --git a/internal/backport/go/doc/testdata/a0.go b/internal/backport/go/doc/testdata/a0.go
new file mode 100644
index 0000000..2420c8a
--- /dev/null
+++ b/internal/backport/go/doc/testdata/a0.go
@@ -0,0 +1,40 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// comment 0
+package a
+
+//BUG(uid): bug0
+
+//TODO(uid): todo0
+
+// A note with some spaces after it, should be ignored (watch out for
+// emacs modes that remove trailing whitespace).
+//NOTE(uid):
+
+// SECBUG(uid): sec hole 0
+// need to fix asap
+
+// Multiple notes may be in the same comment group and should be
+// recognized individually. Notes may start in the middle of a
+// comment group as long as they start at the beginning of an
+// individual comment.
+//
+// NOTE(foo): 1 of 4 - this is the first line of note 1
+// - note 1 continues on this 2nd line
+// - note 1 continues on this 3rd line
+// NOTE(foo): 2 of 4
+// NOTE(bar): 3 of 4
+/* NOTE(bar): 4 of 4 */
+// - this is the last line of note 4
+//
+//
+
+// NOTE(bam): This note which contains a (parenthesized) subphrase
+//            must appear in its entirety.
+
+// NOTE(xxx) The ':' after the marker and uid is optional.
+
+// NOTE(): NO uid - should not show up.
+// NOTE()  NO uid - should not show up.
diff --git a/internal/backport/go/doc/testdata/a1.go b/internal/backport/go/doc/testdata/a1.go
new file mode 100644
index 0000000..9fad1e0
--- /dev/null
+++ b/internal/backport/go/doc/testdata/a1.go
@@ -0,0 +1,12 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// comment 1
+package a
+
+//BUG(uid): bug1
+
+//TODO(uid): todo1
+
+//TODO(): ignored
diff --git a/internal/backport/go/doc/testdata/b.0.golden b/internal/backport/go/doc/testdata/b.0.golden
new file mode 100644
index 0000000..c06246a
--- /dev/null
+++ b/internal/backport/go/doc/testdata/b.0.golden
@@ -0,0 +1,74 @@
+// 
+PACKAGE b
+
+IMPORTPATH
+	testdata/b
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/b.go
+
+CONSTANTS
+	// 
+	const (
+		C1	notExported	= iota
+		C2
+	
+		C4
+		C5
+	)
+
+	// 
+	const C notExported = 0
+
+	// 
+	const Pi = 3.14	// Pi
+
+
+VARIABLES
+	// 
+	var (
+		U1, U2, U4, U5	notExported
+	
+		U7	notExported	= 7
+	)
+
+	// 
+	var MaxInt int	// MaxInt
+
+	// 
+	var V notExported
+
+	// 
+	var V1, V2, V4, V5 notExported
+
+
+FUNCTIONS
+	// Associated with comparable type if AllDecls is set. 
+	func ComparableFactory() comparable
+
+	// 
+	func F(x int) int
+
+	// 
+	func F1() notExported
+
+	// Always under the package functions list. 
+	func NotAFactory() int
+
+	// Associated with uint type if AllDecls is set. 
+	func UintFactory() uint
+
+
+TYPES
+	// 
+	type T struct{}	// T
+
+	// 
+	var V T	// v
+
+	// 
+	func (x *T) M()
+
diff --git a/internal/backport/go/doc/testdata/b.1.golden b/internal/backport/go/doc/testdata/b.1.golden
new file mode 100644
index 0000000..2b62c34
--- /dev/null
+++ b/internal/backport/go/doc/testdata/b.1.golden
@@ -0,0 +1,89 @@
+// 
+PACKAGE b
+
+IMPORTPATH
+	testdata/b
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/b.go
+
+CONSTANTS
+	// 
+	const Pi = 3.14	// Pi
+
+
+VARIABLES
+	// 
+	var MaxInt int	// MaxInt
+
+
+FUNCTIONS
+	// 
+	func F(x int) int
+
+	// Always under the package functions list. 
+	func NotAFactory() int
+
+
+TYPES
+	// 
+	type T struct{}	// T
+
+	// 
+	var V T	// v
+
+	// 
+	func (x *T) M()
+
+	// Should only appear if AllDecls is set. 
+	type comparable struct{}	// overrides a predeclared type comparable
+
+	// Associated with comparable type if AllDecls is set. 
+	func ComparableFactory() comparable
+
+	// 
+	type notExported int
+
+	// 
+	const (
+		C1	notExported	= iota
+		C2
+		c3
+		C4
+		C5
+	)
+
+	// 
+	const C notExported = 0
+
+	// 
+	var (
+		U1, U2, u3, U4, U5	notExported
+		u6			notExported
+		U7			notExported	= 7
+	)
+
+	// 
+	var V notExported
+
+	// 
+	var V1, V2, v3, V4, V5 notExported
+
+	// 
+	func F1() notExported
+
+	// 
+	func f2() notExported
+
+	// Should only appear if AllDecls is set. 
+	type uint struct{}	// overrides a predeclared type uint
+
+	// Associated with uint type if AllDecls is set. 
+	func UintFactory() uint
+
+	// Associated with uint type if AllDecls is set. 
+	func uintFactory() uint
+
diff --git a/internal/backport/go/doc/testdata/b.2.golden b/internal/backport/go/doc/testdata/b.2.golden
new file mode 100644
index 0000000..c06246a
--- /dev/null
+++ b/internal/backport/go/doc/testdata/b.2.golden
@@ -0,0 +1,74 @@
+// 
+PACKAGE b
+
+IMPORTPATH
+	testdata/b
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/b.go
+
+CONSTANTS
+	// 
+	const (
+		C1	notExported	= iota
+		C2
+	
+		C4
+		C5
+	)
+
+	// 
+	const C notExported = 0
+
+	// 
+	const Pi = 3.14	// Pi
+
+
+VARIABLES
+	// 
+	var (
+		U1, U2, U4, U5	notExported
+	
+		U7	notExported	= 7
+	)
+
+	// 
+	var MaxInt int	// MaxInt
+
+	// 
+	var V notExported
+
+	// 
+	var V1, V2, V4, V5 notExported
+
+
+FUNCTIONS
+	// Associated with comparable type if AllDecls is set. 
+	func ComparableFactory() comparable
+
+	// 
+	func F(x int) int
+
+	// 
+	func F1() notExported
+
+	// Always under the package functions list. 
+	func NotAFactory() int
+
+	// Associated with uint type if AllDecls is set. 
+	func UintFactory() uint
+
+
+TYPES
+	// 
+	type T struct{}	// T
+
+	// 
+	var V T	// v
+
+	// 
+	func (x *T) M()
+
diff --git a/internal/backport/go/doc/testdata/b.go b/internal/backport/go/doc/testdata/b.go
new file mode 100644
index 0000000..61b512b
--- /dev/null
+++ b/internal/backport/go/doc/testdata/b.go
@@ -0,0 +1,64 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package b
+
+import "a"
+
+// ----------------------------------------------------------------------------
+// Basic declarations
+
+const Pi = 3.14   // Pi
+var MaxInt int    // MaxInt
+type T struct{}   // T
+var V T           // v
+func F(x int) int {} // F
+func (x *T) M()   {} // M
+
+// Corner cases: association with (presumed) predeclared types
+
+// Always under the package functions list.
+func NotAFactory() int {}
+
+// Associated with uint type if AllDecls is set.
+func UintFactory() uint {}
+
+// Associated with uint type if AllDecls is set.
+func uintFactory() uint {}
+
+// Associated with comparable type if AllDecls is set.
+func ComparableFactory() comparable {}
+
+// Should only appear if AllDecls is set.
+type uint struct{} // overrides a predeclared type uint
+
+// Should only appear if AllDecls is set.
+type comparable struct{} // overrides a predeclared type comparable
+
+// ----------------------------------------------------------------------------
+// Exported declarations associated with non-exported types must always be shown.
+
+type notExported int
+
+const C notExported = 0
+
+const (
+	C1 notExported = iota
+	C2
+	c3
+	C4
+	C5
+)
+
+var V notExported
+var V1, V2, v3, V4, V5 notExported
+
+var (
+	U1, U2, u3, U4, U5 notExported
+	u6                 notExported
+	U7                 notExported = 7
+)
+
+func F1() notExported {}
+func f2() notExported {}
diff --git a/internal/backport/go/doc/testdata/benchmark.go b/internal/backport/go/doc/testdata/benchmark.go
new file mode 100644
index 0000000..d27bf11
--- /dev/null
+++ b/internal/backport/go/doc/testdata/benchmark.go
@@ -0,0 +1,293 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"runtime"
+	"time"
+)
+
+var matchBenchmarks = flag.String("test.bench", "", "regular expression to select benchmarks to run")
+var benchTime = flag.Duration("test.benchtime", 1*time.Second, "approximate run time for each benchmark")
+
+// An internal type but exported because it is cross-package; part of the implementation
+// of go test.
+type InternalBenchmark struct {
+	Name string
+	F    func(b *B)
+}
+
+// B is a type passed to Benchmark functions to manage benchmark
+// timing and to specify the number of iterations to run.
+type B struct {
+	common
+	N         int
+	benchmark InternalBenchmark
+	bytes     int64
+	timerOn   bool
+	result    BenchmarkResult
+}
+
+// StartTimer starts timing a test. This function is called automatically
+// before a benchmark starts, but it can also used to resume timing after
+// a call to StopTimer.
+func (b *B) StartTimer() {
+	if !b.timerOn {
+		b.start = time.Now()
+		b.timerOn = true
+	}
+}
+
+// StopTimer stops timing a test. This can be used to pause the timer
+// while performing complex initialization that you don't
+// want to measure.
+func (b *B) StopTimer() {
+	if b.timerOn {
+		b.duration += time.Now().Sub(b.start)
+		b.timerOn = false
+	}
+}
+
+// ResetTimer sets the elapsed benchmark time to zero.
+// It does not affect whether the timer is running.
+func (b *B) ResetTimer() {
+	if b.timerOn {
+		b.start = time.Now()
+	}
+	b.duration = 0
+}
+
+// SetBytes records the number of bytes processed in a single operation.
+// If this is called, the benchmark will report ns/op and MB/s.
+func (b *B) SetBytes(n int64) { b.bytes = n }
+
+func (b *B) nsPerOp() int64 {
+	if b.N <= 0 {
+		return 0
+	}
+	return b.duration.Nanoseconds() / int64(b.N)
+}
+
+// runN runs a single benchmark for the specified number of iterations.
+func (b *B) runN(n int) {
+	// Try to get a comparable environment for each run
+	// by clearing garbage from previous runs.
+	runtime.GC()
+	b.N = n
+	b.ResetTimer()
+	b.StartTimer()
+	b.benchmark.F(b)
+	b.StopTimer()
+}
+
+func min(x, y int) int {
+	if x > y {
+		return y
+	}
+	return x
+}
+
+func max(x, y int) int {
+	if x < y {
+		return y
+	}
+	return x
+}
+
+// roundDown10 rounds a number down to the nearest power of 10.
+func roundDown10(n int) int {
+	var tens = 0
+	// tens = floor(log_10(n))
+	for n > 10 {
+		n = n / 10
+		tens++
+	}
+	// result = 10^tens
+	result := 1
+	for i := 0; i < tens; i++ {
+		result *= 10
+	}
+	return result
+}
+
+// roundUp rounds x up to a number of the form [1eX, 2eX, 5eX].
+func roundUp(n int) int {
+	base := roundDown10(n)
+	if n < (2 * base) {
+		return 2 * base
+	}
+	if n < (5 * base) {
+		return 5 * base
+	}
+	return 10 * base
+}
+
+// run times the benchmark function in a separate goroutine.
+func (b *B) run() BenchmarkResult {
+	go b.launch()
+	<-b.signal
+	return b.result
+}
+
+// launch launches the benchmark function. It gradually increases the number
+// of benchmark iterations until the benchmark runs for a second in order
+// to get a reasonable measurement. It prints timing information in this form
+//		testing.BenchmarkHello	100000		19 ns/op
+// launch is run by the fun function as a separate goroutine.
+func (b *B) launch() {
+	// Run the benchmark for a single iteration in case it's expensive.
+	n := 1
+
+	// Signal that we're done whether we return normally
+	// or by FailNow's runtime.Goexit.
+	defer func() {
+		b.signal <- b
+	}()
+
+	b.runN(n)
+	// Run the benchmark for at least the specified amount of time.
+	d := *benchTime
+	for !b.failed && b.duration < d && n < 1e9 {
+		last := n
+		// Predict iterations/sec.
+		if b.nsPerOp() == 0 {
+			n = 1e9
+		} else {
+			n = int(d.Nanoseconds() / b.nsPerOp())
+		}
+		// Run more iterations than we think we'll need for a second (1.5x).
+		// Don't grow too fast in case we had timing errors previously.
+		// Be sure to run at least one more than last time.
+		n = max(min(n+n/2, 100*last), last+1)
+		// Round up to something easy to read.
+		n = roundUp(n)
+		b.runN(n)
+	}
+	b.result = BenchmarkResult{b.N, b.duration, b.bytes}
+}
+
+// The results of a benchmark run.
+type BenchmarkResult struct {
+	N     int           // The number of iterations.
+	T     time.Duration // The total time taken.
+	Bytes int64         // Bytes processed in one iteration.
+}
+
+func (r BenchmarkResult) NsPerOp() int64 {
+	if r.N <= 0 {
+		return 0
+	}
+	return r.T.Nanoseconds() / int64(r.N)
+}
+
+func (r BenchmarkResult) mbPerSec() float64 {
+	if r.Bytes <= 0 || r.T <= 0 || r.N <= 0 {
+		return 0
+	}
+	return (float64(r.Bytes) * float64(r.N) / 1e6) / r.T.Seconds()
+}
+
+func (r BenchmarkResult) String() string {
+	mbs := r.mbPerSec()
+	mb := ""
+	if mbs != 0 {
+		mb = fmt.Sprintf("\t%7.2f MB/s", mbs)
+	}
+	nsop := r.NsPerOp()
+	ns := fmt.Sprintf("%10d ns/op", nsop)
+	if r.N > 0 && nsop < 100 {
+		// The format specifiers here make sure that
+		// the ones digits line up for all three possible formats.
+		if nsop < 10 {
+			ns = fmt.Sprintf("%13.2f ns/op", float64(r.T.Nanoseconds())/float64(r.N))
+		} else {
+			ns = fmt.Sprintf("%12.1f ns/op", float64(r.T.Nanoseconds())/float64(r.N))
+		}
+	}
+	return fmt.Sprintf("%8d\t%s%s", r.N, ns, mb)
+}
+
+// An internal function but exported because it is cross-package; part of the implementation
+// of go test.
+func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark) {
+	// If no flag was specified, don't run benchmarks.
+	if len(*matchBenchmarks) == 0 {
+		return
+	}
+	for _, Benchmark := range benchmarks {
+		matched, err := matchString(*matchBenchmarks, Benchmark.Name)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: invalid regexp for -test.bench: %s\n", err)
+			os.Exit(1)
+		}
+		if !matched {
+			continue
+		}
+		for _, procs := range cpuList {
+			runtime.GOMAXPROCS(procs)
+			b := &B{
+				common: common{
+					signal: make(chan any),
+				},
+				benchmark: Benchmark,
+			}
+			benchName := Benchmark.Name
+			if procs != 1 {
+				benchName = fmt.Sprintf("%s-%d", Benchmark.Name, procs)
+			}
+			fmt.Printf("%s\t", benchName)
+			r := b.run()
+			if b.failed {
+				// The output could be very long here, but probably isn't.
+				// We print it all, regardless, because we don't want to trim the reason
+				// the benchmark failed.
+				fmt.Printf("--- FAIL: %s\n%s", benchName, b.output)
+				continue
+			}
+			fmt.Printf("%v\n", r)
+			// Unlike with tests, we ignore the -chatty flag and always print output for
+			// benchmarks since the output generation time will skew the results.
+			if len(b.output) > 0 {
+				b.trimOutput()
+				fmt.Printf("--- BENCH: %s\n%s", benchName, b.output)
+			}
+			if p := runtime.GOMAXPROCS(-1); p != procs {
+				fmt.Fprintf(os.Stderr, "testing: %s left GOMAXPROCS set to %d\n", benchName, p)
+			}
+		}
+	}
+}
+
+// trimOutput shortens the output from a benchmark, which can be very long.
+func (b *B) trimOutput() {
+	// The output is likely to appear multiple times because the benchmark
+	// is run multiple times, but at least it will be seen. This is not a big deal
+	// because benchmarks rarely print, but just in case, we trim it if it's too long.
+	const maxNewlines = 10
+	for nlCount, j := 0, 0; j < len(b.output); j++ {
+		if b.output[j] == '\n' {
+			nlCount++
+			if nlCount >= maxNewlines {
+				b.output = append(b.output[:j], "\n\t... [output truncated]\n"...)
+				break
+			}
+		}
+	}
+}
+
+// Benchmark benchmarks a single function. Useful for creating
+// custom benchmarks that do not use go test.
+func Benchmark(f func(b *B)) BenchmarkResult {
+	b := &B{
+		common: common{
+			signal: make(chan any),
+		},
+		benchmark: InternalBenchmark{"", f},
+	}
+	return b.run()
+}
diff --git a/internal/backport/go/doc/testdata/blank.0.golden b/internal/backport/go/doc/testdata/blank.0.golden
new file mode 100644
index 0000000..70f2929
--- /dev/null
+++ b/internal/backport/go/doc/testdata/blank.0.golden
@@ -0,0 +1,62 @@
+// Package blank is a go/doc test for the handling of _. See issue ...
+PACKAGE blank
+
+IMPORTPATH
+	testdata/blank
+
+IMPORTS
+	os
+
+FILENAMES
+	testdata/blank.go
+
+CONSTANTS
+	// T constants counting from unexported constants. 
+	const (
+		C1	T
+		C2
+	
+		C3
+	
+		C4	int
+	)
+
+	// Constants with a single type that is not propagated. 
+	const (
+		Default		= 0644
+		Useless		= 0312
+		WideOpen	= 0777
+	)
+
+	// Constants with an imported type that is propagated. 
+	const (
+		M1	os.FileMode
+		M2
+		M3
+	)
+
+	// Package constants. 
+	const (
+		I1	int
+		I2
+	)
+
+
+TYPES
+	// S has a padding field. 
+	type S struct {
+		H	uint32
+	
+		A	uint8
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T int
+
+	// T constants counting from a blank constant. 
+	const (
+		T1	T
+		T2
+	)
+
diff --git a/internal/backport/go/doc/testdata/blank.1.golden b/internal/backport/go/doc/testdata/blank.1.golden
new file mode 100644
index 0000000..8098cb6
--- /dev/null
+++ b/internal/backport/go/doc/testdata/blank.1.golden
@@ -0,0 +1,83 @@
+// Package blank is a go/doc test for the handling of _. See issue ...
+PACKAGE blank
+
+IMPORTPATH
+	testdata/blank
+
+IMPORTS
+	os
+
+FILENAMES
+	testdata/blank.go
+
+CONSTANTS
+	// T constants counting from unexported constants. 
+	const (
+		tweedledee	T	= iota
+		tweedledum
+		C1
+		C2
+		alice
+		C3
+		redQueen	int	= iota
+		C4
+	)
+
+	// Constants with a single type that is not propagated. 
+	const (
+		zero		os.FileMode	= 0
+		Default				= 0644
+		Useless				= 0312
+		WideOpen			= 0777
+	)
+
+	// Constants with an imported type that is propagated. 
+	const (
+		zero	os.FileMode	= 0
+		M1
+		M2
+		M3
+	)
+
+	// Package constants. 
+	const (
+		_	int	= iota
+		I1
+		I2
+	)
+
+	// Unexported constants counting from blank iota. See issue 9615. 
+	const (
+		_	= iota
+		one	= iota + 1
+	)
+
+
+VARIABLES
+	// 
+	var _ = T(55)
+
+
+FUNCTIONS
+	// 
+	func _()
+
+
+TYPES
+	// S has a padding field. 
+	type S struct {
+		H	uint32
+		_	uint8
+		A	uint8
+	}
+
+	// 
+	type T int
+
+	// T constants counting from a blank constant. 
+	const (
+		_	T	= iota
+		T1
+		T2
+	)
+
diff --git a/internal/backport/go/doc/testdata/blank.2.golden b/internal/backport/go/doc/testdata/blank.2.golden
new file mode 100644
index 0000000..70f2929
--- /dev/null
+++ b/internal/backport/go/doc/testdata/blank.2.golden
@@ -0,0 +1,62 @@
+// Package blank is a go/doc test for the handling of _. See issue ...
+PACKAGE blank
+
+IMPORTPATH
+	testdata/blank
+
+IMPORTS
+	os
+
+FILENAMES
+	testdata/blank.go
+
+CONSTANTS
+	// T constants counting from unexported constants. 
+	const (
+		C1	T
+		C2
+	
+		C3
+	
+		C4	int
+	)
+
+	// Constants with a single type that is not propagated. 
+	const (
+		Default		= 0644
+		Useless		= 0312
+		WideOpen	= 0777
+	)
+
+	// Constants with an imported type that is propagated. 
+	const (
+		M1	os.FileMode
+		M2
+		M3
+	)
+
+	// Package constants. 
+	const (
+		I1	int
+		I2
+	)
+
+
+TYPES
+	// S has a padding field. 
+	type S struct {
+		H	uint32
+	
+		A	uint8
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T int
+
+	// T constants counting from a blank constant. 
+	const (
+		T1	T
+		T2
+	)
+
diff --git a/internal/backport/go/doc/testdata/blank.go b/internal/backport/go/doc/testdata/blank.go
new file mode 100644
index 0000000..5ea6186
--- /dev/null
+++ b/internal/backport/go/doc/testdata/blank.go
@@ -0,0 +1,75 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package blank is a go/doc test for the handling of _.
+// See issue 5397.
+package blank
+
+import "os"
+
+type T int
+
+// T constants counting from a blank constant.
+const (
+	_ T = iota
+	T1
+	T2
+)
+
+// T constants counting from unexported constants.
+const (
+	tweedledee T = iota
+	tweedledum
+	C1
+	C2
+	alice
+	C3
+	redQueen int = iota
+	C4
+)
+
+// Constants with a single type that is not propagated.
+const (
+	zero     os.FileMode = 0
+	Default              = 0644
+	Useless              = 0312
+	WideOpen             = 0777
+)
+
+// Constants with an imported type that is propagated.
+const (
+	zero os.FileMode = 0
+	M1
+	M2
+	M3
+)
+
+// Package constants.
+const (
+	_ int = iota
+	I1
+	I2
+)
+
+// Unexported constants counting from blank iota.
+// See issue 9615.
+const (
+	_   = iota
+	one = iota + 1
+)
+
+// Blanks not in doc output:
+
+// S has a padding field.
+type S struct {
+	H uint32
+	_ uint8
+	A uint8
+}
+
+func _() {}
+
+type _ T
+
+var _ = T(55)
diff --git a/internal/backport/go/doc/testdata/bugpara.0.golden b/internal/backport/go/doc/testdata/bugpara.0.golden
new file mode 100644
index 0000000..5804859
--- /dev/null
+++ b/internal/backport/go/doc/testdata/bugpara.0.golden
@@ -0,0 +1,20 @@
+// 
+PACKAGE bugpara
+
+IMPORTPATH
+	testdata/bugpara
+
+FILENAMES
+	testdata/bugpara.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
+
+BUGS
+BUG(rsc)	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
diff --git a/internal/backport/go/doc/testdata/bugpara.1.golden b/internal/backport/go/doc/testdata/bugpara.1.golden
new file mode 100644
index 0000000..5804859
--- /dev/null
+++ b/internal/backport/go/doc/testdata/bugpara.1.golden
@@ -0,0 +1,20 @@
+// 
+PACKAGE bugpara
+
+IMPORTPATH
+	testdata/bugpara
+
+FILENAMES
+	testdata/bugpara.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
+
+BUGS
+BUG(rsc)	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
diff --git a/internal/backport/go/doc/testdata/bugpara.2.golden b/internal/backport/go/doc/testdata/bugpara.2.golden
new file mode 100644
index 0000000..5804859
--- /dev/null
+++ b/internal/backport/go/doc/testdata/bugpara.2.golden
@@ -0,0 +1,20 @@
+// 
+PACKAGE bugpara
+
+IMPORTPATH
+	testdata/bugpara
+
+FILENAMES
+	testdata/bugpara.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
+
+BUGS
+BUG(rsc)	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
diff --git a/internal/backport/go/doc/testdata/bugpara.go b/internal/backport/go/doc/testdata/bugpara.go
new file mode 100644
index 0000000..0360a6f
--- /dev/null
+++ b/internal/backport/go/doc/testdata/bugpara.go
@@ -0,0 +1,9 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bugpara
+
+// BUG(rsc): Sometimes bugs have multiple paragraphs.
+//
+// Like this one.
diff --git a/internal/backport/go/doc/testdata/c.0.golden b/internal/backport/go/doc/testdata/c.0.golden
new file mode 100644
index 0000000..e21959b
--- /dev/null
+++ b/internal/backport/go/doc/testdata/c.0.golden
@@ -0,0 +1,48 @@
+// 
+PACKAGE c
+
+IMPORTPATH
+	testdata/c
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/c.go
+
+TYPES
+	// A (should see this) 
+	type A struct{}
+
+	// B (should see this) 
+	type B struct{}
+
+	// C (should see this) 
+	type C struct{}
+
+	// D (should see this) 
+	type D struct{}
+
+	// E1 (should see this) 
+	type E1 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E2 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E3 struct{}
+
+	// E4 (should see this) 
+	type E4 struct{}
+
+	// 
+	type T1 struct{}
+
+	// 
+	func (t1 *T1) M()
+
+	// T2 must not show methods of local T1 
+	type T2 struct {
+		a.T1	// not the same as locally declared T1
+	}
+
diff --git a/internal/backport/go/doc/testdata/c.1.golden b/internal/backport/go/doc/testdata/c.1.golden
new file mode 100644
index 0000000..e21959b
--- /dev/null
+++ b/internal/backport/go/doc/testdata/c.1.golden
@@ -0,0 +1,48 @@
+// 
+PACKAGE c
+
+IMPORTPATH
+	testdata/c
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/c.go
+
+TYPES
+	// A (should see this) 
+	type A struct{}
+
+	// B (should see this) 
+	type B struct{}
+
+	// C (should see this) 
+	type C struct{}
+
+	// D (should see this) 
+	type D struct{}
+
+	// E1 (should see this) 
+	type E1 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E2 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E3 struct{}
+
+	// E4 (should see this) 
+	type E4 struct{}
+
+	// 
+	type T1 struct{}
+
+	// 
+	func (t1 *T1) M()
+
+	// T2 must not show methods of local T1 
+	type T2 struct {
+		a.T1	// not the same as locally declared T1
+	}
+
diff --git a/internal/backport/go/doc/testdata/c.2.golden b/internal/backport/go/doc/testdata/c.2.golden
new file mode 100644
index 0000000..e21959b
--- /dev/null
+++ b/internal/backport/go/doc/testdata/c.2.golden
@@ -0,0 +1,48 @@
+// 
+PACKAGE c
+
+IMPORTPATH
+	testdata/c
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/c.go
+
+TYPES
+	// A (should see this) 
+	type A struct{}
+
+	// B (should see this) 
+	type B struct{}
+
+	// C (should see this) 
+	type C struct{}
+
+	// D (should see this) 
+	type D struct{}
+
+	// E1 (should see this) 
+	type E1 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E2 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E3 struct{}
+
+	// E4 (should see this) 
+	type E4 struct{}
+
+	// 
+	type T1 struct{}
+
+	// 
+	func (t1 *T1) M()
+
+	// T2 must not show methods of local T1 
+	type T2 struct {
+		a.T1	// not the same as locally declared T1
+	}
+
diff --git a/internal/backport/go/doc/testdata/c.go b/internal/backport/go/doc/testdata/c.go
new file mode 100644
index 0000000..e0f3919
--- /dev/null
+++ b/internal/backport/go/doc/testdata/c.go
@@ -0,0 +1,62 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package c
+
+import "a"
+
+// ----------------------------------------------------------------------------
+// Test that empty declarations don't cause problems
+
+const ()
+
+type ()
+
+var ()
+
+// ----------------------------------------------------------------------------
+// Test that types with documentation on both, the Decl and the Spec node
+// are handled correctly.
+
+// A (should see this)
+type A struct{}
+
+// B (should see this)
+type (
+	B struct{}
+)
+
+type (
+	// C (should see this)
+	C struct{}
+)
+
+// D (should not see this)
+type (
+	// D (should see this)
+	D struct{}
+)
+
+// E (should see this for E2 and E3)
+type (
+	// E1 (should see this)
+	E1 struct{}
+	E2 struct{}
+	E3 struct{}
+	// E4 (should see this)
+	E4 struct{}
+)
+
+// ----------------------------------------------------------------------------
+// Test that local and imported types are different when
+// handling anonymous fields.
+
+type T1 struct{}
+
+func (t1 *T1) M() {}
+
+// T2 must not show methods of local T1
+type T2 struct {
+	a.T1 // not the same as locally declared T1
+}
diff --git a/internal/backport/go/doc/testdata/d.0.golden b/internal/backport/go/doc/testdata/d.0.golden
new file mode 100644
index 0000000..c005199
--- /dev/null
+++ b/internal/backport/go/doc/testdata/d.0.golden
@@ -0,0 +1,104 @@
+// 
+PACKAGE d
+
+IMPORTPATH
+	testdata/d
+
+FILENAMES
+	testdata/d1.go
+	testdata/d2.go
+
+CONSTANTS
+	// CBx constants should appear before CAx constants. 
+	const (
+		CB2	= iota	// before CB1
+		CB1		// before CB0
+		CB0		// at end
+	)
+
+	// CAx constants should appear after CBx constants. 
+	const (
+		CA2	= iota	// before CA1
+		CA1		// before CA0
+		CA0		// at end
+	)
+
+	// C0 should be first. 
+	const C0 = 0
+
+	// C1 should be second. 
+	const C1 = 1
+
+	// C2 should be third. 
+	const C2 = 2
+
+	// 
+	const (
+		// Single const declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Cungrouped = 0
+	)
+
+
+VARIABLES
+	// VBx variables should appear before VAx variables. 
+	var (
+		VB2	int	// before VB1
+		VB1	int	// before VB0
+		VB0	int	// at end
+	)
+
+	// VAx variables should appear after VBx variables. 
+	var (
+		VA2	int	// before VA1
+		VA1	int	// before VA0
+		VA0	int	// at end
+	)
+
+	// V0 should be first. 
+	var V0 uintptr
+
+	// V1 should be second. 
+	var V1 uint
+
+	// V2 should be third. 
+	var V2 int
+
+	// 
+	var (
+		// Single var declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Vungrouped = 0
+	)
+
+
+FUNCTIONS
+	// F0 should be first. 
+	func F0()
+
+	// F1 should be second. 
+	func F1()
+
+	// F2 should be third. 
+	func F2()
+
+
+TYPES
+	// T0 should be first. 
+	type T0 struct{}
+
+	// T1 should be second. 
+	type T1 struct{}
+
+	// T2 should be third. 
+	type T2 struct{}
+
+	// TG0 should be first. 
+	type TG0 struct{}
+
+	// TG1 should be second. 
+	type TG1 struct{}
+
+	// TG2 should be third. 
+	type TG2 struct{}
+
diff --git a/internal/backport/go/doc/testdata/d.1.golden b/internal/backport/go/doc/testdata/d.1.golden
new file mode 100644
index 0000000..c005199
--- /dev/null
+++ b/internal/backport/go/doc/testdata/d.1.golden
@@ -0,0 +1,104 @@
+// 
+PACKAGE d
+
+IMPORTPATH
+	testdata/d
+
+FILENAMES
+	testdata/d1.go
+	testdata/d2.go
+
+CONSTANTS
+	// CBx constants should appear before CAx constants. 
+	const (
+		CB2	= iota	// before CB1
+		CB1		// before CB0
+		CB0		// at end
+	)
+
+	// CAx constants should appear after CBx constants. 
+	const (
+		CA2	= iota	// before CA1
+		CA1		// before CA0
+		CA0		// at end
+	)
+
+	// C0 should be first. 
+	const C0 = 0
+
+	// C1 should be second. 
+	const C1 = 1
+
+	// C2 should be third. 
+	const C2 = 2
+
+	// 
+	const (
+		// Single const declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Cungrouped = 0
+	)
+
+
+VARIABLES
+	// VBx variables should appear before VAx variables. 
+	var (
+		VB2	int	// before VB1
+		VB1	int	// before VB0
+		VB0	int	// at end
+	)
+
+	// VAx variables should appear after VBx variables. 
+	var (
+		VA2	int	// before VA1
+		VA1	int	// before VA0
+		VA0	int	// at end
+	)
+
+	// V0 should be first. 
+	var V0 uintptr
+
+	// V1 should be second. 
+	var V1 uint
+
+	// V2 should be third. 
+	var V2 int
+
+	// 
+	var (
+		// Single var declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Vungrouped = 0
+	)
+
+
+FUNCTIONS
+	// F0 should be first. 
+	func F0()
+
+	// F1 should be second. 
+	func F1()
+
+	// F2 should be third. 
+	func F2()
+
+
+TYPES
+	// T0 should be first. 
+	type T0 struct{}
+
+	// T1 should be second. 
+	type T1 struct{}
+
+	// T2 should be third. 
+	type T2 struct{}
+
+	// TG0 should be first. 
+	type TG0 struct{}
+
+	// TG1 should be second. 
+	type TG1 struct{}
+
+	// TG2 should be third. 
+	type TG2 struct{}
+
diff --git a/internal/backport/go/doc/testdata/d.2.golden b/internal/backport/go/doc/testdata/d.2.golden
new file mode 100644
index 0000000..c005199
--- /dev/null
+++ b/internal/backport/go/doc/testdata/d.2.golden
@@ -0,0 +1,104 @@
+// 
+PACKAGE d
+
+IMPORTPATH
+	testdata/d
+
+FILENAMES
+	testdata/d1.go
+	testdata/d2.go
+
+CONSTANTS
+	// CBx constants should appear before CAx constants. 
+	const (
+		CB2	= iota	// before CB1
+		CB1		// before CB0
+		CB0		// at end
+	)
+
+	// CAx constants should appear after CBx constants. 
+	const (
+		CA2	= iota	// before CA1
+		CA1		// before CA0
+		CA0		// at end
+	)
+
+	// C0 should be first. 
+	const C0 = 0
+
+	// C1 should be second. 
+	const C1 = 1
+
+	// C2 should be third. 
+	const C2 = 2
+
+	// 
+	const (
+		// Single const declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Cungrouped = 0
+	)
+
+
+VARIABLES
+	// VBx variables should appear before VAx variables. 
+	var (
+		VB2	int	// before VB1
+		VB1	int	// before VB0
+		VB0	int	// at end
+	)
+
+	// VAx variables should appear after VBx variables. 
+	var (
+		VA2	int	// before VA1
+		VA1	int	// before VA0
+		VA0	int	// at end
+	)
+
+	// V0 should be first. 
+	var V0 uintptr
+
+	// V1 should be second. 
+	var V1 uint
+
+	// V2 should be third. 
+	var V2 int
+
+	// 
+	var (
+		// Single var declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Vungrouped = 0
+	)
+
+
+FUNCTIONS
+	// F0 should be first. 
+	func F0()
+
+	// F1 should be second. 
+	func F1()
+
+	// F2 should be third. 
+	func F2()
+
+
+TYPES
+	// T0 should be first. 
+	type T0 struct{}
+
+	// T1 should be second. 
+	type T1 struct{}
+
+	// T2 should be third. 
+	type T2 struct{}
+
+	// TG0 should be first. 
+	type TG0 struct{}
+
+	// TG1 should be second. 
+	type TG1 struct{}
+
+	// TG2 should be third. 
+	type TG2 struct{}
+
diff --git a/internal/backport/go/doc/testdata/d1.go b/internal/backport/go/doc/testdata/d1.go
new file mode 100644
index 0000000..ebd6941
--- /dev/null
+++ b/internal/backport/go/doc/testdata/d1.go
@@ -0,0 +1,57 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test cases for sort order of declarations.
+
+package d
+
+// C2 should be third.
+const C2 = 2
+
+// V2 should be third.
+var V2 int
+
+// CBx constants should appear before CAx constants.
+const (
+	CB2 = iota // before CB1
+	CB1        // before CB0
+	CB0        // at end
+)
+
+// VBx variables should appear before VAx variables.
+var (
+	VB2 int // before VB1
+	VB1 int // before VB0
+	VB0 int // at end
+)
+
+const (
+	// Single const declarations inside ()'s are considered ungrouped
+	// and show up in sorted order.
+	Cungrouped = 0
+)
+
+var (
+	// Single var declarations inside ()'s are considered ungrouped
+	// and show up in sorted order.
+	Vungrouped = 0
+)
+
+// T2 should be third.
+type T2 struct{}
+
+// Grouped types are sorted nevertheless.
+type (
+	// TG2 should be third.
+	TG2 struct{}
+
+	// TG1 should be second.
+	TG1 struct{}
+
+	// TG0 should be first.
+	TG0 struct{}
+)
+
+// F2 should be third.
+func F2() {}
diff --git a/internal/backport/go/doc/testdata/d2.go b/internal/backport/go/doc/testdata/d2.go
new file mode 100644
index 0000000..2f56f4f
--- /dev/null
+++ b/internal/backport/go/doc/testdata/d2.go
@@ -0,0 +1,45 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test cases for sort order of declarations.
+
+package d
+
+// C1 should be second.
+const C1 = 1
+
+// C0 should be first.
+const C0 = 0
+
+// V1 should be second.
+var V1 uint
+
+// V0 should be first.
+var V0 uintptr
+
+// CAx constants should appear after CBx constants.
+const (
+	CA2 = iota // before CA1
+	CA1        // before CA0
+	CA0        // at end
+)
+
+// VAx variables should appear after VBx variables.
+var (
+	VA2 int // before VA1
+	VA1 int // before VA0
+	VA0 int // at end
+)
+
+// T1 should be second.
+type T1 struct{}
+
+// T0 should be first.
+type T0 struct{}
+
+// F1 should be second.
+func F1() {}
+
+// F0 should be first.
+func F0() {}
diff --git a/internal/backport/go/doc/testdata/e.0.golden b/internal/backport/go/doc/testdata/e.0.golden
new file mode 100644
index 0000000..6987e58
--- /dev/null
+++ b/internal/backport/go/doc/testdata/e.0.golden
@@ -0,0 +1,109 @@
+// The package e is a go/doc test for embedded methods. 
+PACKAGE e
+
+IMPORTPATH
+	testdata/e
+
+FILENAMES
+	testdata/e.go
+
+TYPES
+	// T1 has no embedded (level 1) M method due to conflict. 
+	type T1 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T2 has only M as top-level method. 
+	type T2 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T2.M should appear as method of T2. 
+	func (T2) M()
+
+	// T3 has only M as top-level method. 
+	type T3 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T3.M should appear as method of T3. 
+	func (T3) M()
+
+	// 
+	type T4 struct{}
+
+	// T4.M should appear as method of T5 only if AllMethods is set. 
+	func (*T4) M()
+
+	// 
+	type T5 struct {
+		T4
+	}
+
+	// 
+	type U1 struct {
+		*U1
+	}
+
+	// U1.M should appear as method of U1. 
+	func (*U1) M()
+
+	// 
+	type U2 struct {
+		*U3
+	}
+
+	// U2.M should appear as method of U2 and as method of U3 only if ...
+	func (*U2) M()
+
+	// 
+	type U3 struct {
+		*U2
+	}
+
+	// U3.N should appear as method of U3 and as method of U2 only if ...
+	func (*U3) N()
+
+	// 
+	type U4 struct {
+		// contains filtered or unexported fields
+	}
+
+	// U4.M should appear as method of U4. 
+	func (*U4) M()
+
+	// 
+	type V1 struct {
+		*V2
+		*V5
+	}
+
+	// 
+	type V2 struct {
+		*V3
+	}
+
+	// 
+	type V3 struct {
+		*V4
+	}
+
+	// 
+	type V4 struct {
+		*V5
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (*V4) M()
+
+	// 
+	type V5 struct {
+		*V6
+	}
+
+	// 
+	type V6 struct{}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (*V6) M()
+
diff --git a/internal/backport/go/doc/testdata/e.1.golden b/internal/backport/go/doc/testdata/e.1.golden
new file mode 100644
index 0000000..cbe22e0
--- /dev/null
+++ b/internal/backport/go/doc/testdata/e.1.golden
@@ -0,0 +1,144 @@
+// The package e is a go/doc test for embedded methods. 
+PACKAGE e
+
+IMPORTPATH
+	testdata/e
+
+FILENAMES
+	testdata/e.go
+
+TYPES
+	// T1 has no embedded (level 1) M method due to conflict. 
+	type T1 struct {
+		t1
+		t2
+	}
+
+	// T2 has only M as top-level method. 
+	type T2 struct {
+		t1
+	}
+
+	// T2.M should appear as method of T2. 
+	func (T2) M()
+
+	// T3 has only M as top-level method. 
+	type T3 struct {
+		t1e
+		t2e
+	}
+
+	// T3.M should appear as method of T3. 
+	func (T3) M()
+
+	// 
+	type T4 struct{}
+
+	// T4.M should appear as method of T5 only if AllMethods is set. 
+	func (*T4) M()
+
+	// 
+	type T5 struct {
+		T4
+	}
+
+	// 
+	type U1 struct {
+		*U1
+	}
+
+	// U1.M should appear as method of U1. 
+	func (*U1) M()
+
+	// 
+	type U2 struct {
+		*U3
+	}
+
+	// U2.M should appear as method of U2 and as method of U3 only if ...
+	func (*U2) M()
+
+	// 
+	type U3 struct {
+		*U2
+	}
+
+	// U3.N should appear as method of U3 and as method of U2 only if ...
+	func (*U3) N()
+
+	// 
+	type U4 struct {
+		*u5
+	}
+
+	// U4.M should appear as method of U4. 
+	func (*U4) M()
+
+	// 
+	type V1 struct {
+		*V2
+		*V5
+	}
+
+	// 
+	type V2 struct {
+		*V3
+	}
+
+	// 
+	type V3 struct {
+		*V4
+	}
+
+	// 
+	type V4 struct {
+		*V5
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (*V4) M()
+
+	// 
+	type V5 struct {
+		*V6
+	}
+
+	// 
+	type V6 struct{}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (*V6) M()
+
+	// 
+	type t1 struct{}
+
+	// t1.M should not appear as method in a Tx type. 
+	func (t1) M()
+
+	// 
+	type t1e struct {
+		t1
+	}
+
+	// t1.M should not appear as method in a Tx type. 
+	func (t1e) M()
+
+	// 
+	type t2 struct{}
+
+	// t2.M should not appear as method in a Tx type. 
+	func (t2) M()
+
+	// 
+	type t2e struct {
+		t2
+	}
+
+	// t2.M should not appear as method in a Tx type. 
+	func (t2e) M()
+
+	// 
+	type u5 struct {
+		*U4
+	}
+
diff --git a/internal/backport/go/doc/testdata/e.2.golden b/internal/backport/go/doc/testdata/e.2.golden
new file mode 100644
index 0000000..e7b05e8
--- /dev/null
+++ b/internal/backport/go/doc/testdata/e.2.golden
@@ -0,0 +1,130 @@
+// The package e is a go/doc test for embedded methods. 
+PACKAGE e
+
+IMPORTPATH
+	testdata/e
+
+FILENAMES
+	testdata/e.go
+
+TYPES
+	// T1 has no embedded (level 1) M method due to conflict. 
+	type T1 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T2 has only M as top-level method. 
+	type T2 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T2.M should appear as method of T2. 
+	func (T2) M()
+
+	// T3 has only M as top-level method. 
+	type T3 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T3.M should appear as method of T3. 
+	func (T3) M()
+
+	// 
+	type T4 struct{}
+
+	// T4.M should appear as method of T5 only if AllMethods is set. 
+	func (*T4) M()
+
+	// 
+	type T5 struct {
+		T4
+	}
+
+	// T4.M should appear as method of T5 only if AllMethods is set. 
+	func (*T5) M()
+
+	// 
+	type U1 struct {
+		*U1
+	}
+
+	// U1.M should appear as method of U1. 
+	func (*U1) M()
+
+	// 
+	type U2 struct {
+		*U3
+	}
+
+	// U2.M should appear as method of U2 and as method of U3 only if ...
+	func (*U2) M()
+
+	// U3.N should appear as method of U3 and as method of U2 only if ...
+	func (U2) N()
+
+	// 
+	type U3 struct {
+		*U2
+	}
+
+	// U2.M should appear as method of U2 and as method of U3 only if ...
+	func (U3) M()
+
+	// U3.N should appear as method of U3 and as method of U2 only if ...
+	func (*U3) N()
+
+	// 
+	type U4 struct {
+		// contains filtered or unexported fields
+	}
+
+	// U4.M should appear as method of U4. 
+	func (*U4) M()
+
+	// 
+	type V1 struct {
+		*V2
+		*V5
+	}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (V1) M()
+
+	// 
+	type V2 struct {
+		*V3
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (V2) M()
+
+	// 
+	type V3 struct {
+		*V4
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (V3) M()
+
+	// 
+	type V4 struct {
+		*V5
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (*V4) M()
+
+	// 
+	type V5 struct {
+		*V6
+	}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (V5) M()
+
+	// 
+	type V6 struct{}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (*V6) M()
+
diff --git a/internal/backport/go/doc/testdata/e.go b/internal/backport/go/doc/testdata/e.go
new file mode 100644
index 0000000..ec432e3
--- /dev/null
+++ b/internal/backport/go/doc/testdata/e.go
@@ -0,0 +1,147 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The package e is a go/doc test for embedded methods.
+package e
+
+// ----------------------------------------------------------------------------
+// Conflicting methods M must not show up.
+
+type t1 struct{}
+
+// t1.M should not appear as method in a Tx type.
+func (t1) M() {}
+
+type t2 struct{}
+
+// t2.M should not appear as method in a Tx type.
+func (t2) M() {}
+
+// T1 has no embedded (level 1) M method due to conflict.
+type T1 struct {
+	t1
+	t2
+}
+
+// ----------------------------------------------------------------------------
+// Higher-level method M wins over lower-level method M.
+
+// T2 has only M as top-level method.
+type T2 struct {
+	t1
+}
+
+// T2.M should appear as method of T2.
+func (T2) M() {}
+
+// ----------------------------------------------------------------------------
+// Higher-level method M wins over lower-level conflicting methods M.
+
+type t1e struct {
+	t1
+}
+
+type t2e struct {
+	t2
+}
+
+// T3 has only M as top-level method.
+type T3 struct {
+	t1e
+	t2e
+}
+
+// T3.M should appear as method of T3.
+func (T3) M() {}
+
+// ----------------------------------------------------------------------------
+// Don't show conflicting methods M embedded via an exported and non-exported
+// type.
+
+// T1 has no embedded (level 1) M method due to conflict.
+type T4 struct {
+	t2
+	T2
+}
+
+// ----------------------------------------------------------------------------
+// Don't show embedded methods of exported anonymous fields unless AllMethods
+// is set.
+
+type T4 struct{}
+
+// T4.M should appear as method of T5 only if AllMethods is set.
+func (*T4) M() {}
+
+type T5 struct {
+	T4
+}
+
+// ----------------------------------------------------------------------------
+// Recursive type declarations must not lead to endless recursion.
+
+type U1 struct {
+	*U1
+}
+
+// U1.M should appear as method of U1.
+func (*U1) M() {}
+
+type U2 struct {
+	*U3
+}
+
+// U2.M should appear as method of U2 and as method of U3 only if AllMethods is set.
+func (*U2) M() {}
+
+type U3 struct {
+	*U2
+}
+
+// U3.N should appear as method of U3 and as method of U2 only if AllMethods is set.
+func (*U3) N() {}
+
+type U4 struct {
+	*u5
+}
+
+// U4.M should appear as method of U4.
+func (*U4) M() {}
+
+type u5 struct {
+	*U4
+}
+
+// ----------------------------------------------------------------------------
+// A higher-level embedded type (and its methods) wins over the same type (and
+// its methods) embedded at a lower level.
+
+type V1 struct {
+	*V2
+	*V5
+}
+
+type V2 struct {
+	*V3
+}
+
+type V3 struct {
+	*V4
+}
+
+type V4 struct {
+	*V5
+}
+
+type V5 struct {
+	*V6
+}
+
+type V6 struct{}
+
+// V4.M should appear as method of V2 and V3 if AllMethods is set.
+func (*V4) M() {}
+
+// V6.M should appear as method of V1 and V5 if AllMethods is set.
+func (*V6) M() {}
diff --git a/internal/backport/go/doc/testdata/error1.0.golden b/internal/backport/go/doc/testdata/error1.0.golden
new file mode 100644
index 0000000..6c6fe5d
--- /dev/null
+++ b/internal/backport/go/doc/testdata/error1.0.golden
@@ -0,0 +1,30 @@
+// 
+PACKAGE error1
+
+IMPORTPATH
+	testdata/error1
+
+FILENAMES
+	testdata/error1.go
+
+TYPES
+	// 
+	type I0 interface {
+		// When embedded, the predeclared error interface
+		// must remain visible in interface types.
+		error
+	}
+
+	// 
+	type S0 struct {
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// error should be visible
+			error
+		}
+	}
+
diff --git a/internal/backport/go/doc/testdata/error1.1.golden b/internal/backport/go/doc/testdata/error1.1.golden
new file mode 100644
index 0000000..a8dc2e7
--- /dev/null
+++ b/internal/backport/go/doc/testdata/error1.1.golden
@@ -0,0 +1,32 @@
+// 
+PACKAGE error1
+
+IMPORTPATH
+	testdata/error1
+
+FILENAMES
+	testdata/error1.go
+
+TYPES
+	// 
+	type I0 interface {
+		// When embedded, the predeclared error interface
+		// must remain visible in interface types.
+		error
+	}
+
+	// 
+	type S0 struct {
+		// In struct types, an embedded error must only be visible
+		// if AllDecls is set.
+		error
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// error should be visible
+			error
+		}
+	}
+
diff --git a/internal/backport/go/doc/testdata/error1.2.golden b/internal/backport/go/doc/testdata/error1.2.golden
new file mode 100644
index 0000000..6c6fe5d
--- /dev/null
+++ b/internal/backport/go/doc/testdata/error1.2.golden
@@ -0,0 +1,30 @@
+// 
+PACKAGE error1
+
+IMPORTPATH
+	testdata/error1
+
+FILENAMES
+	testdata/error1.go
+
+TYPES
+	// 
+	type I0 interface {
+		// When embedded, the predeclared error interface
+		// must remain visible in interface types.
+		error
+	}
+
+	// 
+	type S0 struct {
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// error should be visible
+			error
+		}
+	}
+
diff --git a/internal/backport/go/doc/testdata/error1.go b/internal/backport/go/doc/testdata/error1.go
new file mode 100644
index 0000000..3c777a7
--- /dev/null
+++ b/internal/backport/go/doc/testdata/error1.go
@@ -0,0 +1,24 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package error1
+
+type I0 interface {
+	// When embedded, the predeclared error interface
+	// must remain visible in interface types.
+	error
+}
+
+type T0 struct {
+	ExportedField interface {
+		// error should be visible
+		error
+	}
+}
+
+type S0 struct {
+	// In struct types, an embedded error must only be visible
+	// if AllDecls is set.
+	error
+}
diff --git a/internal/backport/go/doc/testdata/error2.0.golden b/internal/backport/go/doc/testdata/error2.0.golden
new file mode 100644
index 0000000..dedfe41
--- /dev/null
+++ b/internal/backport/go/doc/testdata/error2.0.golden
@@ -0,0 +1,27 @@
+// 
+PACKAGE error2
+
+IMPORTPATH
+	testdata/error2
+
+FILENAMES
+	testdata/error2.go
+
+TYPES
+	// 
+	type I0 interface {
+		// contains filtered or unexported methods
+	}
+
+	// 
+	type S0 struct {
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// contains filtered or unexported methods
+		}
+	}
+
diff --git a/internal/backport/go/doc/testdata/error2.1.golden b/internal/backport/go/doc/testdata/error2.1.golden
new file mode 100644
index 0000000..dbcc1b0
--- /dev/null
+++ b/internal/backport/go/doc/testdata/error2.1.golden
@@ -0,0 +1,37 @@
+// 
+PACKAGE error2
+
+IMPORTPATH
+	testdata/error2
+
+FILENAMES
+	testdata/error2.go
+
+TYPES
+	// 
+	type I0 interface {
+		// When embedded, the locally-declared error interface
+		// is only visible if all declarations are shown.
+		error
+	}
+
+	// 
+	type S0 struct {
+		// In struct types, an embedded error must only be visible
+		// if AllDecls is set.
+		error
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// error should not be visible
+			error
+		}
+	}
+
+	// This error declaration shadows the predeclared error type. 
+	type error interface {
+		Error() string
+	}
+
diff --git a/internal/backport/go/doc/testdata/error2.2.golden b/internal/backport/go/doc/testdata/error2.2.golden
new file mode 100644
index 0000000..dedfe41
--- /dev/null
+++ b/internal/backport/go/doc/testdata/error2.2.golden
@@ -0,0 +1,27 @@
+// 
+PACKAGE error2
+
+IMPORTPATH
+	testdata/error2
+
+FILENAMES
+	testdata/error2.go
+
+TYPES
+	// 
+	type I0 interface {
+		// contains filtered or unexported methods
+	}
+
+	// 
+	type S0 struct {
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// contains filtered or unexported methods
+		}
+	}
+
diff --git a/internal/backport/go/doc/testdata/error2.go b/internal/backport/go/doc/testdata/error2.go
new file mode 100644
index 0000000..6ee96c2
--- /dev/null
+++ b/internal/backport/go/doc/testdata/error2.go
@@ -0,0 +1,29 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package error2
+
+type I0 interface {
+	// When embedded, the locally-declared error interface
+	// is only visible if all declarations are shown.
+	error
+}
+
+type T0 struct {
+	ExportedField interface {
+		// error should not be visible
+		error
+	}
+}
+
+type S0 struct {
+	// In struct types, an embedded error must only be visible
+	// if AllDecls is set.
+	error
+}
+
+// This error declaration shadows the predeclared error type.
+type error interface {
+	Error() string
+}
diff --git a/internal/backport/go/doc/testdata/example.go b/internal/backport/go/doc/testdata/example.go
new file mode 100644
index 0000000..fdeda13
--- /dev/null
+++ b/internal/backport/go/doc/testdata/example.go
@@ -0,0 +1,81 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"time"
+)
+
+type InternalExample struct {
+	Name   string
+	F      func()
+	Output string
+}
+
+func RunExamples(examples []InternalExample) (ok bool) {
+	ok = true
+
+	var eg InternalExample
+
+	stdout, stderr := os.Stdout, os.Stderr
+	defer func() {
+		os.Stdout, os.Stderr = stdout, stderr
+		if e := recover(); e != nil {
+			fmt.Printf("--- FAIL: %s\npanic: %v\n", eg.Name, e)
+			os.Exit(1)
+		}
+	}()
+
+	for _, eg = range examples {
+		if *chatty {
+			fmt.Printf("=== RUN: %s\n", eg.Name)
+		}
+
+		// capture stdout and stderr
+		r, w, err := os.Pipe()
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+		os.Stdout, os.Stderr = w, w
+		outC := make(chan string)
+		go func() {
+			buf := new(bytes.Buffer)
+			_, err := io.Copy(buf, r)
+			if err != nil {
+				fmt.Fprintf(stderr, "testing: copying pipe: %v\n", err)
+				os.Exit(1)
+			}
+			outC <- buf.String()
+		}()
+
+		// run example
+		t0 := time.Now()
+		eg.F()
+		dt := time.Now().Sub(t0)
+
+		// close pipe, restore stdout/stderr, get output
+		w.Close()
+		os.Stdout, os.Stderr = stdout, stderr
+		out := <-outC
+
+		// report any errors
+		tstr := fmt.Sprintf("(%.2f seconds)", dt.Seconds())
+		if g, e := strings.TrimSpace(out), strings.TrimSpace(eg.Output); g != e {
+			fmt.Printf("--- FAIL: %s %s\ngot:\n%s\nwant:\n%s\n",
+				eg.Name, tstr, g, e)
+			ok = false
+		} else if *chatty {
+			fmt.Printf("--- PASS: %s %s\n", eg.Name, tstr)
+		}
+	}
+
+	return
+}
diff --git a/internal/backport/go/doc/testdata/f.0.golden b/internal/backport/go/doc/testdata/f.0.golden
new file mode 100644
index 0000000..8175901
--- /dev/null
+++ b/internal/backport/go/doc/testdata/f.0.golden
@@ -0,0 +1,13 @@
+// The package f is a go/doc test for functions and factory ...
+PACKAGE f
+
+IMPORTPATH
+	testdata/f
+
+FILENAMES
+	testdata/f.go
+
+FUNCTIONS
+	// Exported must always be visible. Was issue 2824. 
+	func Exported() private
+
diff --git a/internal/backport/go/doc/testdata/f.1.golden b/internal/backport/go/doc/testdata/f.1.golden
new file mode 100644
index 0000000..ba68e88
--- /dev/null
+++ b/internal/backport/go/doc/testdata/f.1.golden
@@ -0,0 +1,16 @@
+// The package f is a go/doc test for functions and factory ...
+PACKAGE f
+
+IMPORTPATH
+	testdata/f
+
+FILENAMES
+	testdata/f.go
+
+TYPES
+	// 
+	type private struct{}
+
+	// Exported must always be visible. Was issue 2824. 
+	func Exported() private
+
diff --git a/internal/backport/go/doc/testdata/f.2.golden b/internal/backport/go/doc/testdata/f.2.golden
new file mode 100644
index 0000000..8175901
--- /dev/null
+++ b/internal/backport/go/doc/testdata/f.2.golden
@@ -0,0 +1,13 @@
+// The package f is a go/doc test for functions and factory ...
+PACKAGE f
+
+IMPORTPATH
+	testdata/f
+
+FILENAMES
+	testdata/f.go
+
+FUNCTIONS
+	// Exported must always be visible. Was issue 2824. 
+	func Exported() private
+
diff --git a/internal/backport/go/doc/testdata/f.go b/internal/backport/go/doc/testdata/f.go
new file mode 100644
index 0000000..7e9add9
--- /dev/null
+++ b/internal/backport/go/doc/testdata/f.go
@@ -0,0 +1,14 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The package f is a go/doc test for functions and factory methods.
+package f
+
+// ----------------------------------------------------------------------------
+// Factory functions for non-exported types must not get lost.
+
+type private struct{}
+
+// Exported must always be visible. Was issue 2824.
+func Exported() private {}
diff --git a/internal/backport/go/doc/testdata/g.0.golden b/internal/backport/go/doc/testdata/g.0.golden
new file mode 100644
index 0000000..487cf06
--- /dev/null
+++ b/internal/backport/go/doc/testdata/g.0.golden
@@ -0,0 +1,32 @@
+// The package g is a go/doc test for mixed exported/unexported ...
+PACKAGE g
+
+IMPORTPATH
+	testdata/g
+
+FILENAMES
+	testdata/g.go
+
+CONSTANTS
+	// 
+	const (
+		A, _	= iota, iota
+		_, D
+		E, _
+		G, H
+	)
+
+
+VARIABLES
+	// 
+	var (
+		_, C2, _	= 1, 2, 3
+		C4, _, C6	= 4, 5, 6
+		_, C8, _	= 7, 8, 9
+	)
+
+	// 
+	var (
+		_, X = f()
+	)
+
diff --git a/internal/backport/go/doc/testdata/g.1.golden b/internal/backport/go/doc/testdata/g.1.golden
new file mode 100644
index 0000000..438441a
--- /dev/null
+++ b/internal/backport/go/doc/testdata/g.1.golden
@@ -0,0 +1,34 @@
+// The package g is a go/doc test for mixed exported/unexported ...
+PACKAGE g
+
+IMPORTPATH
+	testdata/g
+
+FILENAMES
+	testdata/g.go
+
+CONSTANTS
+	// 
+	const (
+		A, b	= iota, iota
+		c, D
+		E, f
+		G, H
+	)
+
+
+VARIABLES
+	// 
+	var (
+		c1, C2, c3	= 1, 2, 3
+		C4, c5, C6	= 4, 5, 6
+		c7, C8, c9	= 7, 8, 9
+		xx, yy, zz	= 0, 0, 0	// all unexported and hidden
+	)
+
+	// 
+	var (
+		x, X	= f()
+		y, z	= f()
+	)
+
diff --git a/internal/backport/go/doc/testdata/g.2.golden b/internal/backport/go/doc/testdata/g.2.golden
new file mode 100644
index 0000000..487cf06
--- /dev/null
+++ b/internal/backport/go/doc/testdata/g.2.golden
@@ -0,0 +1,32 @@
+// The package g is a go/doc test for mixed exported/unexported ...
+PACKAGE g
+
+IMPORTPATH
+	testdata/g
+
+FILENAMES
+	testdata/g.go
+
+CONSTANTS
+	// 
+	const (
+		A, _	= iota, iota
+		_, D
+		E, _
+		G, H
+	)
+
+
+VARIABLES
+	// 
+	var (
+		_, C2, _	= 1, 2, 3
+		C4, _, C6	= 4, 5, 6
+		_, C8, _	= 7, 8, 9
+	)
+
+	// 
+	var (
+		_, X = f()
+	)
+
diff --git a/internal/backport/go/doc/testdata/g.go b/internal/backport/go/doc/testdata/g.go
new file mode 100644
index 0000000..ceeb417
--- /dev/null
+++ b/internal/backport/go/doc/testdata/g.go
@@ -0,0 +1,25 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The package g is a go/doc test for mixed exported/unexported values.
+package g
+
+const (
+	A, b = iota, iota
+	c, D
+	E, f
+	G, H
+)
+
+var (
+	c1, C2, c3 = 1, 2, 3
+	C4, c5, C6 = 4, 5, 6
+	c7, C8, c9 = 7, 8, 9
+	xx, yy, zz = 0, 0, 0 // all unexported and hidden
+)
+
+var (
+	x, X = f()
+	y, z = f()
+)
diff --git a/internal/backport/go/doc/testdata/generics.0.golden b/internal/backport/go/doc/testdata/generics.0.golden
new file mode 100644
index 0000000..91c874c
--- /dev/null
+++ b/internal/backport/go/doc/testdata/generics.0.golden
@@ -0,0 +1,76 @@
+// Package generics contains the new syntax supporting generic ...
+PACKAGE generics
+
+IMPORTPATH
+	testdata/generics
+
+FILENAMES
+	testdata/generics.go
+
+FUNCTIONS
+	// AnotherFunc has an implicit constraint interface.  Neither type ...
+	func AnotherFunc[T ~struct{ f int }](_ struct{ f int })
+
+	// Func has an instantiated constraint. 
+	func Func[T Constraint[string, Type[int]]]()
+
+	// Single is not a factory function. 
+	func Single[T any]() *T
+
+	// Slice is not a factory function. 
+	func Slice[T any]() []T
+
+
+TYPES
+	// AFuncType demonstrates filtering of parameters and type ...
+	type AFuncType[T ~struct{ f int }] func(_ struct {
+		// contains filtered or unexported fields
+	})
+
+	// Constraint is a constraint interface with two type parameters. 
+	type Constraint[P, Q interface{ string | ~int | Type[int] }] interface {
+		~int | ~byte | Type[string]
+		M() P
+	}
+
+	// NewEmbeddings demonstrates how we filter the new embedded ...
+	type NewEmbeddings interface {
+		string	// should not be filtered
+	
+		struct {
+			// contains filtered or unexported fields
+		}
+		~struct {
+			// contains filtered or unexported fields
+		}
+		*struct {
+			// contains filtered or unexported fields
+		}
+		struct {
+			// contains filtered or unexported fields
+		} | ~struct {
+			// contains filtered or unexported fields
+		}
+		// contains filtered or unexported methods
+	}
+
+	// Parameterized types should be shown. 
+	type Type[P any] struct {
+		Field P
+	}
+
+	// Variables with an instantiated type should be shown. 
+	var X Type[int]
+
+	// Constructors for parameterized types should be shown. 
+	func Constructor[lowerCase any]() Type[lowerCase]
+
+	// MethodA uses a different name for its receiver type parameter. 
+	func (t Type[A]) MethodA(p A)
+
+	// MethodB has a blank receiver type parameter. 
+	func (t Type[_]) MethodB()
+
+	// MethodC has a lower-case receiver type parameter. 
+	func (t Type[c]) MethodC()
+
diff --git a/internal/backport/go/doc/testdata/generics.1.golden b/internal/backport/go/doc/testdata/generics.1.golden
new file mode 100644
index 0000000..923a4ce
--- /dev/null
+++ b/internal/backport/go/doc/testdata/generics.1.golden
@@ -0,0 +1,66 @@
+// Package generics contains the new syntax supporting generic ...
+PACKAGE generics
+
+IMPORTPATH
+	testdata/generics
+
+FILENAMES
+	testdata/generics.go
+
+FUNCTIONS
+	// AnotherFunc has an implicit constraint interface.  Neither type ...
+	func AnotherFunc[T ~struct{ f int }](_ struct{ f int })
+
+	// Func has an instantiated constraint. 
+	func Func[T Constraint[string, Type[int]]]()
+
+	// Single is not a factory function. 
+	func Single[T any]() *T
+
+	// Slice is not a factory function. 
+	func Slice[T any]() []T
+
+
+TYPES
+	// AFuncType demonstrates filtering of parameters and type ...
+	type AFuncType[T ~struct{ f int }] func(_ struct{ f int })
+
+	// Constraint is a constraint interface with two type parameters. 
+	type Constraint[P, Q interface{ string | ~int | Type[int] }] interface {
+		~int | ~byte | Type[string]
+		M() P
+	}
+
+	// NewEmbeddings demonstrates how we filter the new embedded ...
+	type NewEmbeddings interface {
+		string	// should not be filtered
+		int16
+		struct{ f int }
+		~struct{ f int }
+		*struct{ f int }
+		struct{ f int } | ~struct{ f int }
+	}
+
+	// Parameterized types should be shown. 
+	type Type[P any] struct {
+		Field P
+	}
+
+	// Variables with an instantiated type should be shown. 
+	var X Type[int]
+
+	// Constructors for parameterized types should be shown. 
+	func Constructor[lowerCase any]() Type[lowerCase]
+
+	// MethodA uses a different name for its receiver type parameter. 
+	func (t Type[A]) MethodA(p A)
+
+	// MethodB has a blank receiver type parameter. 
+	func (t Type[_]) MethodB()
+
+	// MethodC has a lower-case receiver type parameter. 
+	func (t Type[c]) MethodC()
+
+	// int16 shadows the predeclared type int16. 
+	type int16 int
+
diff --git a/internal/backport/go/doc/testdata/generics.2.golden b/internal/backport/go/doc/testdata/generics.2.golden
new file mode 100644
index 0000000..91c874c
--- /dev/null
+++ b/internal/backport/go/doc/testdata/generics.2.golden
@@ -0,0 +1,76 @@
+// Package generics contains the new syntax supporting generic ...
+PACKAGE generics
+
+IMPORTPATH
+	testdata/generics
+
+FILENAMES
+	testdata/generics.go
+
+FUNCTIONS
+	// AnotherFunc has an implicit constraint interface.  Neither type ...
+	func AnotherFunc[T ~struct{ f int }](_ struct{ f int })
+
+	// Func has an instantiated constraint. 
+	func Func[T Constraint[string, Type[int]]]()
+
+	// Single is not a factory function. 
+	func Single[T any]() *T
+
+	// Slice is not a factory function. 
+	func Slice[T any]() []T
+
+
+TYPES
+	// AFuncType demonstrates filtering of parameters and type ...
+	type AFuncType[T ~struct{ f int }] func(_ struct {
+		// contains filtered or unexported fields
+	})
+
+	// Constraint is a constraint interface with two type parameters. 
+	type Constraint[P, Q interface{ string | ~int | Type[int] }] interface {
+		~int | ~byte | Type[string]
+		M() P
+	}
+
+	// NewEmbeddings demonstrates how we filter the new embedded ...
+	type NewEmbeddings interface {
+		string	// should not be filtered
+	
+		struct {
+			// contains filtered or unexported fields
+		}
+		~struct {
+			// contains filtered or unexported fields
+		}
+		*struct {
+			// contains filtered or unexported fields
+		}
+		struct {
+			// contains filtered or unexported fields
+		} | ~struct {
+			// contains filtered or unexported fields
+		}
+		// contains filtered or unexported methods
+	}
+
+	// Parameterized types should be shown. 
+	type Type[P any] struct {
+		Field P
+	}
+
+	// Variables with an instantiated type should be shown. 
+	var X Type[int]
+
+	// Constructors for parameterized types should be shown. 
+	func Constructor[lowerCase any]() Type[lowerCase]
+
+	// MethodA uses a different name for its receiver type parameter. 
+	func (t Type[A]) MethodA(p A)
+
+	// MethodB has a blank receiver type parameter. 
+	func (t Type[_]) MethodB()
+
+	// MethodC has a lower-case receiver type parameter. 
+	func (t Type[c]) MethodC()
+
diff --git a/internal/backport/go/doc/testdata/generics.go b/internal/backport/go/doc/testdata/generics.go
new file mode 100644
index 0000000..ba7187e
--- /dev/null
+++ b/internal/backport/go/doc/testdata/generics.go
@@ -0,0 +1,74 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package generics contains the new syntax supporting generic programming in
+// Go.
+package generics
+
+// Variables with an instantiated type should be shown.
+var X Type[int]
+
+// Parameterized types should be shown.
+type Type[P any] struct {
+	Field P
+}
+
+// Constructors for parameterized types should be shown.
+func Constructor[lowerCase any]() Type[lowerCase] {
+	return Type[lowerCase]{}
+}
+
+// MethodA uses a different name for its receiver type parameter.
+func (t Type[A]) MethodA(p A) {}
+
+// MethodB has a blank receiver type parameter.
+func (t Type[_]) MethodB() {}
+
+// MethodC has a lower-case receiver type parameter.
+func (t Type[c]) MethodC() {}
+
+// Constraint is a constraint interface with two type parameters.
+type Constraint[P, Q interface{ string | ~int | Type[int] }] interface {
+	~int | ~byte | Type[string]
+	M() P
+}
+
+// int16 shadows the predeclared type int16.
+type int16 int
+
+// NewEmbeddings demonstrates how we filter the new embedded elements.
+type NewEmbeddings interface {
+	string // should not be filtered
+	int16
+	struct{ f int }
+	~struct{ f int }
+	*struct{ f int }
+	struct{ f int } | ~struct{ f int }
+}
+
+// Func has an instantiated constraint.
+func Func[T Constraint[string, Type[int]]]() {}
+
+// AnotherFunc has an implicit constraint interface.
+//
+// Neither type parameters nor regular parameters should be filtered.
+func AnotherFunc[T ~struct{ f int }](_ struct{ f int }) {}
+
+// AFuncType demonstrates filtering of parameters and type parameters. Here we
+// don't filter type parameters (to be consistent with function declarations),
+// but DO filter the RHS.
+type AFuncType[T ~struct{ f int }] func(_ struct{ f int })
+
+// See issue #49477: type parameters should not be interpreted as named types
+// for the purpose of determining whether a function is a factory function.
+
+// Slice is not a factory function.
+func Slice[T any]() []T {
+	return nil
+}
+
+// Single is not a factory function.
+func Single[T any]() *T {
+	return nil
+}
diff --git a/internal/backport/go/doc/testdata/issue12839.0.golden b/internal/backport/go/doc/testdata/issue12839.0.golden
new file mode 100644
index 0000000..6b59774
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue12839.0.golden
@@ -0,0 +1,51 @@
+// Package issue12839 is a go/doc test to test association of a ...
+PACKAGE issue12839
+
+IMPORTPATH
+	testdata/issue12839
+
+IMPORTS
+	p
+
+FILENAMES
+	testdata/issue12839.go
+
+FUNCTIONS
+	// F1 should not be associated with T1 
+	func F1() (*T1, *T2)
+
+	// F10 should not be associated with T1. 
+	func F10() (T1, T2, error)
+
+	// F4 should not be associated with a type (same as F1) 
+	func F4() (a T1, b T2)
+
+	// F9 should not be associated with T1. 
+	func F9() (int, T1, T2)
+
+
+TYPES
+	// 
+	type T1 struct{}
+
+	// F2 should be associated with T1 
+	func F2() (a, b, c T1)
+
+	// F3 should be associated with T1 because b.T3 is from a ...
+	func F3() (a T1, b p.T3)
+
+	// F5 should be associated with T1. 
+	func F5() (T1, error)
+
+	// F6 should be associated with T1. 
+	func F6() (*T1, error)
+
+	// F7 should be associated with T1. 
+	func F7() (T1, string)
+
+	// F8 should be associated with T1. 
+	func F8() (int, T1, string)
+
+	// 
+	type T2 struct{}
+
diff --git a/internal/backport/go/doc/testdata/issue12839.1.golden b/internal/backport/go/doc/testdata/issue12839.1.golden
new file mode 100644
index 0000000..4b9b9f6
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue12839.1.golden
@@ -0,0 +1,54 @@
+// Package issue12839 is a go/doc test to test association of a ...
+PACKAGE issue12839
+
+IMPORTPATH
+	testdata/issue12839
+
+IMPORTS
+	p
+
+FILENAMES
+	testdata/issue12839.go
+
+FUNCTIONS
+	// F1 should not be associated with T1 
+	func F1() (*T1, *T2)
+
+	// F10 should not be associated with T1. 
+	func F10() (T1, T2, error)
+
+	// F4 should not be associated with a type (same as F1) 
+	func F4() (a T1, b T2)
+
+	// F9 should not be associated with T1. 
+	func F9() (int, T1, T2)
+
+
+TYPES
+	// 
+	type T1 struct{}
+
+	// F2 should be associated with T1 
+	func F2() (a, b, c T1)
+
+	// F3 should be associated with T1 because b.T3 is from a ...
+	func F3() (a T1, b p.T3)
+
+	// F5 should be associated with T1. 
+	func F5() (T1, error)
+
+	// F6 should be associated with T1. 
+	func F6() (*T1, error)
+
+	// F7 should be associated with T1. 
+	func F7() (T1, string)
+
+	// F8 should be associated with T1. 
+	func F8() (int, T1, string)
+
+	// 
+	func (t T1) hello() string
+
+	// 
+	type T2 struct{}
+
diff --git a/internal/backport/go/doc/testdata/issue12839.2.golden b/internal/backport/go/doc/testdata/issue12839.2.golden
new file mode 100644
index 0000000..6b59774
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue12839.2.golden
@@ -0,0 +1,51 @@
+// Package issue12839 is a go/doc test to test association of a ...
+PACKAGE issue12839
+
+IMPORTPATH
+	testdata/issue12839
+
+IMPORTS
+	p
+
+FILENAMES
+	testdata/issue12839.go
+
+FUNCTIONS
+	// F1 should not be associated with T1 
+	func F1() (*T1, *T2)
+
+	// F10 should not be associated with T1. 
+	func F10() (T1, T2, error)
+
+	// F4 should not be associated with a type (same as F1) 
+	func F4() (a T1, b T2)
+
+	// F9 should not be associated with T1. 
+	func F9() (int, T1, T2)
+
+
+TYPES
+	// 
+	type T1 struct{}
+
+	// F2 should be associated with T1 
+	func F2() (a, b, c T1)
+
+	// F3 should be associated with T1 because b.T3 is from a ...
+	func F3() (a T1, b p.T3)
+
+	// F5 should be associated with T1. 
+	func F5() (T1, error)
+
+	// F6 should be associated with T1. 
+	func F6() (*T1, error)
+
+	// F7 should be associated with T1. 
+	func F7() (T1, string)
+
+	// F8 should be associated with T1. 
+	func F8() (int, T1, string)
+
+	// 
+	type T2 struct{}
+
diff --git a/internal/backport/go/doc/testdata/issue12839.go b/internal/backport/go/doc/testdata/issue12839.go
new file mode 100644
index 0000000..51c7ac1
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue12839.go
@@ -0,0 +1,69 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package issue12839 is a go/doc test to test association of a function
+// that returns multiple types.
+// See golang.org/issue/12839.
+// (See also golang.org/issue/27928.)
+package issue12839
+
+import "p"
+
+type T1 struct{}
+
+type T2 struct{}
+
+func (t T1) hello() string {
+	return "hello"
+}
+
+// F1 should not be associated with T1
+func F1() (*T1, *T2) {
+	return &T1{}, &T2{}
+}
+
+// F2 should be associated with T1
+func F2() (a, b, c T1) {
+	return T1{}, T1{}, T1{}
+}
+
+// F3 should be associated with T1 because b.T3 is from a different package
+func F3() (a T1, b p.T3) {
+	return T1{}, p.T3{}
+}
+
+// F4 should not be associated with a type (same as F1)
+func F4() (a T1, b T2) {
+	return T1{}, T2{}
+}
+
+// F5 should be associated with T1.
+func F5() (T1, error) {
+	return T1{}, nil
+}
+
+// F6 should be associated with T1.
+func F6() (*T1, error) {
+	return &T1{}, nil
+}
+
+// F7 should be associated with T1.
+func F7() (T1, string) {
+	return T1{}, nil
+}
+
+// F8 should be associated with T1.
+func F8() (int, T1, string) {
+	return 0, T1{}, nil
+}
+
+// F9 should not be associated with T1.
+func F9() (int, T1, T2) {
+	return 0, T1{}, T2{}
+}
+
+// F10 should not be associated with T1.
+func F10() (T1, T2, error) {
+	return T1{}, T2{}, nil
+}
diff --git a/internal/backport/go/doc/testdata/issue13742.0.golden b/internal/backport/go/doc/testdata/issue13742.0.golden
new file mode 100644
index 0000000..8dee9aa
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue13742.0.golden
@@ -0,0 +1,25 @@
+// 
+PACKAGE issue13742
+
+IMPORTPATH
+	testdata/issue13742
+
+IMPORTS
+	go/ast
+
+FILENAMES
+	testdata/issue13742.go
+
+FUNCTIONS
+	// Both F0 and G0 should appear as functions. 
+	func F0(Node)
+
+	// Both F1 and G1 should appear as functions. 
+	func F1(ast.Node)
+
+	// 
+	func G0() Node
+
+	// 
+	func G1() ast.Node
+
diff --git a/internal/backport/go/doc/testdata/issue13742.1.golden b/internal/backport/go/doc/testdata/issue13742.1.golden
new file mode 100644
index 0000000..8dee9aa
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue13742.1.golden
@@ -0,0 +1,25 @@
+// 
+PACKAGE issue13742
+
+IMPORTPATH
+	testdata/issue13742
+
+IMPORTS
+	go/ast
+
+FILENAMES
+	testdata/issue13742.go
+
+FUNCTIONS
+	// Both F0 and G0 should appear as functions. 
+	func F0(Node)
+
+	// Both F1 and G1 should appear as functions. 
+	func F1(ast.Node)
+
+	// 
+	func G0() Node
+
+	// 
+	func G1() ast.Node
+
diff --git a/internal/backport/go/doc/testdata/issue13742.2.golden b/internal/backport/go/doc/testdata/issue13742.2.golden
new file mode 100644
index 0000000..8dee9aa
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue13742.2.golden
@@ -0,0 +1,25 @@
+// 
+PACKAGE issue13742
+
+IMPORTPATH
+	testdata/issue13742
+
+IMPORTS
+	go/ast
+
+FILENAMES
+	testdata/issue13742.go
+
+FUNCTIONS
+	// Both F0 and G0 should appear as functions. 
+	func F0(Node)
+
+	// Both F1 and G1 should appear as functions. 
+	func F1(ast.Node)
+
+	// 
+	func G0() Node
+
+	// 
+	func G1() ast.Node
+
diff --git a/internal/backport/go/doc/testdata/issue13742.go b/internal/backport/go/doc/testdata/issue13742.go
new file mode 100644
index 0000000..dbc1941
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue13742.go
@@ -0,0 +1,18 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue13742
+
+import (
+	"go/ast"
+	. "go/ast"
+)
+
+// Both F0 and G0 should appear as functions.
+func F0(Node)  {}
+func G0() Node { return nil }
+
+// Both F1 and G1 should appear as functions.
+func F1(ast.Node)  {}
+func G1() ast.Node { return nil }
diff --git a/internal/backport/go/doc/testdata/issue16153.0.golden b/internal/backport/go/doc/testdata/issue16153.0.golden
new file mode 100644
index 0000000..189260b
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue16153.0.golden
@@ -0,0 +1,32 @@
+// 
+PACKAGE issue16153
+
+IMPORTPATH
+	testdata/issue16153
+
+FILENAMES
+	testdata/issue16153.go
+
+CONSTANTS
+	// 
+	const (
+		X3	int64	= iota
+		Y3		= 1
+	)
+
+	// 
+	const (
+		X4	int64	= iota
+		Y4
+	)
+
+	// original test case 
+	const (
+		Y1 = 256
+	)
+
+	// variations 
+	const (
+		Y2 uint8
+	)
+
diff --git a/internal/backport/go/doc/testdata/issue16153.1.golden b/internal/backport/go/doc/testdata/issue16153.1.golden
new file mode 100644
index 0000000..803df3e
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue16153.1.golden
@@ -0,0 +1,34 @@
+// 
+PACKAGE issue16153
+
+IMPORTPATH
+	testdata/issue16153
+
+FILENAMES
+	testdata/issue16153.go
+
+CONSTANTS
+	// original test case 
+	const (
+		x1	uint8	= 255
+		Y1		= 256
+	)
+
+	// variations 
+	const (
+		x2	uint8	= 255
+		Y2
+	)
+
+	// 
+	const (
+		X3	int64	= iota
+		Y3		= 1
+	)
+
+	// 
+	const (
+		X4	int64	= iota
+		Y4
+	)
+
diff --git a/internal/backport/go/doc/testdata/issue16153.2.golden b/internal/backport/go/doc/testdata/issue16153.2.golden
new file mode 100644
index 0000000..189260b
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue16153.2.golden
@@ -0,0 +1,32 @@
+// 
+PACKAGE issue16153
+
+IMPORTPATH
+	testdata/issue16153
+
+FILENAMES
+	testdata/issue16153.go
+
+CONSTANTS
+	// 
+	const (
+		X3	int64	= iota
+		Y3		= 1
+	)
+
+	// 
+	const (
+		X4	int64	= iota
+		Y4
+	)
+
+	// original test case 
+	const (
+		Y1 = 256
+	)
+
+	// variations 
+	const (
+		Y2 uint8
+	)
+
diff --git a/internal/backport/go/doc/testdata/issue16153.go b/internal/backport/go/doc/testdata/issue16153.go
new file mode 100644
index 0000000..528be42
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue16153.go
@@ -0,0 +1,27 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue16153
+
+// original test case
+const (
+	x1 uint8 = 255
+	Y1       = 256
+)
+
+// variations
+const (
+	x2 uint8 = 255
+	Y2
+)
+
+const (
+	X3 int64 = iota
+	Y3       = 1
+)
+
+const (
+	X4 int64 = iota
+	Y4
+)
diff --git a/internal/backport/go/doc/testdata/issue17788.0.golden b/internal/backport/go/doc/testdata/issue17788.0.golden
new file mode 100644
index 0000000..42c00da
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue17788.0.golden
@@ -0,0 +1,8 @@
+// 
+PACKAGE issue17788
+
+IMPORTPATH
+	testdata/issue17788
+
+FILENAMES
+	testdata/issue17788.go
diff --git a/internal/backport/go/doc/testdata/issue17788.1.golden b/internal/backport/go/doc/testdata/issue17788.1.golden
new file mode 100644
index 0000000..42c00da
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue17788.1.golden
@@ -0,0 +1,8 @@
+// 
+PACKAGE issue17788
+
+IMPORTPATH
+	testdata/issue17788
+
+FILENAMES
+	testdata/issue17788.go
diff --git a/internal/backport/go/doc/testdata/issue17788.2.golden b/internal/backport/go/doc/testdata/issue17788.2.golden
new file mode 100644
index 0000000..42c00da
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue17788.2.golden
@@ -0,0 +1,8 @@
+// 
+PACKAGE issue17788
+
+IMPORTPATH
+	testdata/issue17788
+
+FILENAMES
+	testdata/issue17788.go
diff --git a/internal/backport/go/doc/testdata/issue17788.go b/internal/backport/go/doc/testdata/issue17788.go
new file mode 100644
index 0000000..883ad5f
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue17788.go
@@ -0,0 +1,8 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue17788
+
+func ( /* receiver type */ ) f0() {
+}
diff --git a/internal/backport/go/doc/testdata/issue22856.0.golden b/internal/backport/go/doc/testdata/issue22856.0.golden
new file mode 100644
index 0000000..a88f43f
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue22856.0.golden
@@ -0,0 +1,45 @@
+// 
+PACKAGE issue22856
+
+IMPORTPATH
+	testdata/issue22856
+
+FILENAMES
+	testdata/issue22856.go
+
+FUNCTIONS
+	// NewPointerSliceOfSlice is not a factory function because slices ...
+	func NewPointerSliceOfSlice() [][]*T
+
+	// NewSlice3 is not a factory function because 3 nested slices of ...
+	func NewSlice3() [][][]T
+
+	// NewSliceOfSlice is not a factory function because slices of a ...
+	func NewSliceOfSlice() [][]T
+
+
+TYPES
+	// 
+	type T struct{}
+
+	// 
+	func New() T
+
+	// 
+	func NewArray() [1]T
+
+	// 
+	func NewPointer() *T
+
+	// 
+	func NewPointerArray() [1]*T
+
+	// 
+	func NewPointerOfPointer() **T
+
+	// 
+	func NewPointerSlice() []*T
+
+	// 
+	func NewSlice() []T
+
diff --git a/internal/backport/go/doc/testdata/issue22856.1.golden b/internal/backport/go/doc/testdata/issue22856.1.golden
new file mode 100644
index 0000000..a88f43f
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue22856.1.golden
@@ -0,0 +1,45 @@
+// 
+PACKAGE issue22856
+
+IMPORTPATH
+	testdata/issue22856
+
+FILENAMES
+	testdata/issue22856.go
+
+FUNCTIONS
+	// NewPointerSliceOfSlice is not a factory function because slices ...
+	func NewPointerSliceOfSlice() [][]*T
+
+	// NewSlice3 is not a factory function because 3 nested slices of ...
+	func NewSlice3() [][][]T
+
+	// NewSliceOfSlice is not a factory function because slices of a ...
+	func NewSliceOfSlice() [][]T
+
+
+TYPES
+	// 
+	type T struct{}
+
+	// 
+	func New() T
+
+	// 
+	func NewArray() [1]T
+
+	// 
+	func NewPointer() *T
+
+	// 
+	func NewPointerArray() [1]*T
+
+	// 
+	func NewPointerOfPointer() **T
+
+	// 
+	func NewPointerSlice() []*T
+
+	// 
+	func NewSlice() []T
+
diff --git a/internal/backport/go/doc/testdata/issue22856.2.golden b/internal/backport/go/doc/testdata/issue22856.2.golden
new file mode 100644
index 0000000..a88f43f
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue22856.2.golden
@@ -0,0 +1,45 @@
+// 
+PACKAGE issue22856
+
+IMPORTPATH
+	testdata/issue22856
+
+FILENAMES
+	testdata/issue22856.go
+
+FUNCTIONS
+	// NewPointerSliceOfSlice is not a factory function because slices ...
+	func NewPointerSliceOfSlice() [][]*T
+
+	// NewSlice3 is not a factory function because 3 nested slices of ...
+	func NewSlice3() [][][]T
+
+	// NewSliceOfSlice is not a factory function because slices of a ...
+	func NewSliceOfSlice() [][]T
+
+
+TYPES
+	// 
+	type T struct{}
+
+	// 
+	func New() T
+
+	// 
+	func NewArray() [1]T
+
+	// 
+	func NewPointer() *T
+
+	// 
+	func NewPointerArray() [1]*T
+
+	// 
+	func NewPointerOfPointer() **T
+
+	// 
+	func NewPointerSlice() []*T
+
+	// 
+	func NewSlice() []T
+
diff --git a/internal/backport/go/doc/testdata/issue22856.go b/internal/backport/go/doc/testdata/issue22856.go
new file mode 100644
index 0000000..f456998
--- /dev/null
+++ b/internal/backport/go/doc/testdata/issue22856.go
@@ -0,0 +1,27 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue22856
+
+type T struct{}
+
+func New() T                   { return T{} }
+func NewPointer() *T           { return &T{} }
+func NewPointerSlice() []*T    { return []*T{&T{}} }
+func NewSlice() []T            { return []T{T{}} }
+func NewPointerOfPointer() **T { x := &T{}; return &x }
+func NewArray() [1]T           { return [1]T{T{}} }
+func NewPointerArray() [1]*T   { return [1]*T{&T{}} }
+
+// NewSliceOfSlice is not a factory function because slices of a slice of
+// type *T are not factory functions of type T.
+func NewSliceOfSlice() [][]T { return []T{[]T{}} }
+
+// NewPointerSliceOfSlice is not a factory function because slices of a
+// slice of type *T are not factory functions of type T.
+func NewPointerSliceOfSlice() [][]*T { return []*T{[]*T{}} }
+
+// NewSlice3 is not a factory function because 3 nested slices of type T
+// are not factory functions of type T.
+func NewSlice3() [][][]T { return []T{[]T{[]T{}}} }
diff --git a/internal/backport/go/doc/testdata/predeclared.0.golden b/internal/backport/go/doc/testdata/predeclared.0.golden
new file mode 100644
index 0000000..9f37b06
--- /dev/null
+++ b/internal/backport/go/doc/testdata/predeclared.0.golden
@@ -0,0 +1,8 @@
+// Package predeclared is a go/doc test for handling of exported ...
+PACKAGE predeclared
+
+IMPORTPATH
+	testdata/predeclared
+
+FILENAMES
+	testdata/predeclared.go
diff --git a/internal/backport/go/doc/testdata/predeclared.1.golden b/internal/backport/go/doc/testdata/predeclared.1.golden
new file mode 100644
index 0000000..2ff8ee6
--- /dev/null
+++ b/internal/backport/go/doc/testdata/predeclared.1.golden
@@ -0,0 +1,22 @@
+// Package predeclared is a go/doc test for handling of exported ...
+PACKAGE predeclared
+
+IMPORTPATH
+	testdata/predeclared
+
+FILENAMES
+	testdata/predeclared.go
+
+TYPES
+	// 
+	type bool int
+
+	// Must not be visible. 
+	func (b bool) String() string
+
+	// 
+	type error struct{}
+
+	// Must not be visible. 
+	func (e error) Error() string
+
diff --git a/internal/backport/go/doc/testdata/predeclared.2.golden b/internal/backport/go/doc/testdata/predeclared.2.golden
new file mode 100644
index 0000000..9f37b06
--- /dev/null
+++ b/internal/backport/go/doc/testdata/predeclared.2.golden
@@ -0,0 +1,8 @@
+// Package predeclared is a go/doc test for handling of exported ...
+PACKAGE predeclared
+
+IMPORTPATH
+	testdata/predeclared
+
+FILENAMES
+	testdata/predeclared.go
diff --git a/internal/backport/go/doc/testdata/predeclared.go b/internal/backport/go/doc/testdata/predeclared.go
new file mode 100644
index 0000000..c6dd806
--- /dev/null
+++ b/internal/backport/go/doc/testdata/predeclared.go
@@ -0,0 +1,22 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package predeclared is a go/doc test for handling of
+// exported methods on locally-defined predeclared types.
+// See issue 9860.
+package predeclared
+
+type error struct{}
+
+// Must not be visible.
+func (e error) Error() string {
+	return ""
+}
+
+type bool int
+
+// Must not be visible.
+func (b bool) String() string {
+	return ""
+}
diff --git a/internal/backport/go/doc/testdata/template.txt b/internal/backport/go/doc/testdata/template.txt
new file mode 100644
index 0000000..1b07382
--- /dev/null
+++ b/internal/backport/go/doc/testdata/template.txt
@@ -0,0 +1,68 @@
+{{synopsis .Doc}}
+PACKAGE {{.Name}}
+
+IMPORTPATH
+	{{.ImportPath}}
+
+{{with .Imports}}IMPORTS
+{{range .}}	{{.}}
+{{end}}
+{{end}}{{/*
+
+*/}}FILENAMES
+{{range .Filenames}}	{{.}}
+{{end}}{{/*
+
+*/}}{{with .Consts}}
+CONSTANTS
+{{range .}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{end}}{{/*
+
+*/}}{{with .Vars}}
+VARIABLES
+{{range .}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{end}}{{/*
+
+*/}}{{with .Funcs}}
+FUNCTIONS
+{{range .}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{end}}{{/*
+
+*/}}{{with .Types}}
+TYPES
+{{range .}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{range .Consts}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{/*
+
+*/}}{{range .Vars}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{/*
+
+*/}}{{range .Funcs}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{/*
+
+*/}}{{range .Methods}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{end}}{{end}}{{/*
+
+*/}}{{with .Bugs}}
+BUGS .Bugs is now deprecated, please use .Notes instead
+{{range .}}{{indent "\t" .}}
+{{end}}{{end}}{{with .Notes}}{{range $marker, $content := .}}
+{{$marker}}S
+{{range $content}}{{$marker}}({{.UID}}){{indent "\t" .Body}}
+{{end}}{{end}}{{end}}
\ No newline at end of file
diff --git a/internal/backport/go/doc/testdata/testing.0.golden b/internal/backport/go/doc/testdata/testing.0.golden
new file mode 100644
index 0000000..61dac8b
--- /dev/null
+++ b/internal/backport/go/doc/testdata/testing.0.golden
@@ -0,0 +1,156 @@
+// Package testing provides support for automated testing of Go ...
+PACKAGE testing
+
+IMPORTPATH
+	testdata/testing
+
+IMPORTS
+	bytes
+	flag
+	fmt
+	io
+	os
+	runtime
+	runtime/pprof
+	strconv
+	strings
+	time
+
+FILENAMES
+	testdata/benchmark.go
+	testdata/example.go
+	testdata/testing.go
+
+FUNCTIONS
+	// An internal function but exported because it is cross-package; ...
+	func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample)
+
+	// An internal function but exported because it is cross-package; ...
+	func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark)
+
+	// 
+	func RunExamples(examples []InternalExample) (ok bool)
+
+	// 
+	func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool)
+
+	// Short reports whether the -test.short flag is set. 
+	func Short() bool
+
+
+TYPES
+	// B is a type passed to Benchmark functions to manage benchmark ...
+	type B struct {
+		N int
+		// contains filtered or unexported fields
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *B) Error(args ...any)
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *B) Errorf(format string, args ...any)
+
+	// Fail marks the function as having failed but continues ...
+	func (c *B) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *B) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *B) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *B) Fatal(args ...any)
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *B) Fatalf(format string, args ...any)
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *B) Log(args ...any)
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *B) Logf(format string, args ...any)
+
+	// ResetTimer sets the elapsed benchmark time to zero. It does not ...
+	func (b *B) ResetTimer()
+
+	// SetBytes records the number of bytes processed in a single ...
+	func (b *B) SetBytes(n int64)
+
+	// StartTimer starts timing a test. This function is called ...
+	func (b *B) StartTimer()
+
+	// StopTimer stops timing a test. This can be used to pause the ...
+	func (b *B) StopTimer()
+
+	// The results of a benchmark run. 
+	type BenchmarkResult struct {
+		N	int		// The number of iterations.
+		T	time.Duration	// The total time taken.
+		Bytes	int64		// Bytes processed in one iteration.
+	}
+
+	// Benchmark benchmarks a single function. Useful for creating ...
+	func Benchmark(f func(b *B)) BenchmarkResult
+
+	// 
+	func (r BenchmarkResult) NsPerOp() int64
+
+	// 
+	func (r BenchmarkResult) String() string
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalBenchmark struct {
+		Name	string
+		F	func(b *B)
+	}
+
+	// 
+	type InternalExample struct {
+		Name	string
+		F	func()
+		Output	string
+	}
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalTest struct {
+		Name	string
+		F	func(*T)
+	}
+
+	// T is a type passed to Test functions to manage test state and ...
+	type T struct {
+		// contains filtered or unexported fields
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *T) Error(args ...any)
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *T) Errorf(format string, args ...any)
+
+	// Fail marks the function as having failed but continues ...
+	func (c *T) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *T) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *T) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *T) Fatal(args ...any)
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *T) Fatalf(format string, args ...any)
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *T) Log(args ...any)
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *T) Logf(format string, args ...any)
+
+	// Parallel signals that this test is to be run in parallel with ...
+	func (t *T) Parallel()
+
diff --git a/internal/backport/go/doc/testdata/testing.1.golden b/internal/backport/go/doc/testdata/testing.1.golden
new file mode 100644
index 0000000..1655af1
--- /dev/null
+++ b/internal/backport/go/doc/testdata/testing.1.golden
@@ -0,0 +1,298 @@
+// Package testing provides support for automated testing of Go ...
+PACKAGE testing
+
+IMPORTPATH
+	testdata/testing
+
+IMPORTS
+	bytes
+	flag
+	fmt
+	io
+	os
+	runtime
+	runtime/pprof
+	strconv
+	strings
+	time
+
+FILENAMES
+	testdata/benchmark.go
+	testdata/example.go
+	testdata/testing.go
+
+VARIABLES
+	// 
+	var (
+		// The short flag requests that tests run more quickly, but its functionality
+		// is provided by test writers themselves. The testing package is just its
+		// home. The all.bash installation script sets it to make installation more
+		// efficient, but by default the flag is off so a plain "go test" will do a
+		// full test of the package.
+		short	= flag.Bool("test.short", false, "run smaller test suite to save time")
+	
+		// Report as tests are run; default is silent for success.
+		chatty		= flag.Bool("test.v", false, "verbose: print additional output")
+		match		= flag.String("test.run", "", "regular expression to select tests to run")
+		memProfile	= flag.String("test.memprofile", "", "write a memory profile to the named file after execution")
+		memProfileRate	= flag.Int("test.memprofilerate", 0, "if >=0, sets runtime.MemProfileRate")
+		cpuProfile	= flag.String("test.cpuprofile", "", "write a cpu profile to the named file during execution")
+		timeout		= flag.Duration("test.timeout", 0, "if positive, sets an aggregate time limit for all tests")
+		cpuListStr	= flag.String("test.cpu", "", "comma-separated list of number of CPUs to use for each test")
+		parallel	= flag.Int("test.parallel", runtime.GOMAXPROCS(0), "maximum test parallelism")
+	
+		cpuList	[]int
+	)
+
+	// 
+	var benchTime = flag.Duration("test.benchtime", 1*time.Second, "approximate run time for each benchmark")
+
+	// 
+	var matchBenchmarks = flag.String("test.bench", "", "regular expression to select benchmarks to run")
+
+	// 
+	var timer *time.Timer
+
+
+FUNCTIONS
+	// An internal function but exported because it is cross-package; ...
+	func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample)
+
+	// An internal function but exported because it is cross-package; ...
+	func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark)
+
+	// 
+	func RunExamples(examples []InternalExample) (ok bool)
+
+	// 
+	func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool)
+
+	// Short reports whether the -test.short flag is set. 
+	func Short() bool
+
+	// after runs after all testing. 
+	func after()
+
+	// alarm is called if the timeout expires. 
+	func alarm()
+
+	// before runs before all testing. 
+	func before()
+
+	// decorate inserts the final newline if needed and indentation ...
+	func decorate(s string, addFileLine bool) string
+
+	// 
+	func max(x, y int) int
+
+	// 
+	func min(x, y int) int
+
+	// 
+	func parseCpuList()
+
+	// roundDown10 rounds a number down to the nearest power of 10. 
+	func roundDown10(n int) int
+
+	// roundUp rounds x up to a number of the form [1eX, 2eX, 5eX]. 
+	func roundUp(n int) int
+
+	// startAlarm starts an alarm if requested. 
+	func startAlarm()
+
+	// stopAlarm turns off the alarm. 
+	func stopAlarm()
+
+	// 
+	func tRunner(t *T, test *InternalTest)
+
+
+TYPES
+	// B is a type passed to Benchmark functions to manage benchmark ...
+	type B struct {
+		common
+		N		int
+		benchmark	InternalBenchmark
+		bytes		int64
+		timerOn		bool
+		result		BenchmarkResult
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *B) Error(args ...any)
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *B) Errorf(format string, args ...any)
+
+	// Fail marks the function as having failed but continues ...
+	func (c *B) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *B) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *B) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *B) Fatal(args ...any)
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *B) Fatalf(format string, args ...any)
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *B) Log(args ...any)
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *B) Logf(format string, args ...any)
+
+	// ResetTimer sets the elapsed benchmark time to zero. It does not ...
+	func (b *B) ResetTimer()
+
+	// SetBytes records the number of bytes processed in a single ...
+	func (b *B) SetBytes(n int64)
+
+	// StartTimer starts timing a test. This function is called ...
+	func (b *B) StartTimer()
+
+	// StopTimer stops timing a test. This can be used to pause the ...
+	func (b *B) StopTimer()
+
+	// launch launches the benchmark function. It gradually increases ...
+	func (b *B) launch()
+
+	// log generates the output. It's always at the same stack depth. 
+	func (c *B) log(s string)
+
+	// 
+	func (b *B) nsPerOp() int64
+
+	// run times the benchmark function in a separate goroutine. 
+	func (b *B) run() BenchmarkResult
+
+	// runN runs a single benchmark for the specified number of ...
+	func (b *B) runN(n int)
+
+	// trimOutput shortens the output from a benchmark, which can be ...
+	func (b *B) trimOutput()
+
+	// The results of a benchmark run. 
+	type BenchmarkResult struct {
+		N	int		// The number of iterations.
+		T	time.Duration	// The total time taken.
+		Bytes	int64		// Bytes processed in one iteration.
+	}
+
+	// Benchmark benchmarks a single function. Useful for creating ...
+	func Benchmark(f func(b *B)) BenchmarkResult
+
+	// 
+	func (r BenchmarkResult) NsPerOp() int64
+
+	// 
+	func (r BenchmarkResult) String() string
+
+	// 
+	func (r BenchmarkResult) mbPerSec() float64
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalBenchmark struct {
+		Name	string
+		F	func(b *B)
+	}
+
+	// 
+	type InternalExample struct {
+		Name	string
+		F	func()
+		Output	string
+	}
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalTest struct {
+		Name	string
+		F	func(*T)
+	}
+
+	// T is a type passed to Test functions to manage test state and ...
+	type T struct {
+		common
+		name		string		// Name of test.
+		startParallel	chan bool	// Parallel tests will wait on this.
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *T) Error(args ...any)
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *T) Errorf(format string, args ...any)
+
+	// Fail marks the function as having failed but continues ...
+	func (c *T) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *T) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *T) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *T) Fatal(args ...any)
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *T) Fatalf(format string, args ...any)
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *T) Log(args ...any)
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *T) Logf(format string, args ...any)
+
+	// Parallel signals that this test is to be run in parallel with ...
+	func (t *T) Parallel()
+
+	// log generates the output. It's always at the same stack depth. 
+	func (c *T) log(s string)
+
+	// 
+	func (t *T) report()
+
+	// common holds the elements common between T and B and captures ...
+	type common struct {
+		output		[]byte		// Output generated by test or benchmark.
+		failed		bool		// Test or benchmark has failed.
+		start		time.Time	// Time test or benchmark started
+		duration	time.Duration
+		self		any		// To be sent on signal channel when done.
+		signal		chan any	// Output for serial tests.
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *common) Error(args ...any)
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *common) Errorf(format string, args ...any)
+
+	// Fail marks the function as having failed but continues ...
+	func (c *common) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *common) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *common) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *common) Fatal(args ...any)
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *common) Fatalf(format string, args ...any)
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *common) Log(args ...any)
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *common) Logf(format string, args ...any)
+
+	// log generates the output. It's always at the same stack depth. 
+	func (c *common) log(s string)
+
diff --git a/internal/backport/go/doc/testdata/testing.2.golden b/internal/backport/go/doc/testdata/testing.2.golden
new file mode 100644
index 0000000..61dac8b
--- /dev/null
+++ b/internal/backport/go/doc/testdata/testing.2.golden
@@ -0,0 +1,156 @@
+// Package testing provides support for automated testing of Go ...
+PACKAGE testing
+
+IMPORTPATH
+	testdata/testing
+
+IMPORTS
+	bytes
+	flag
+	fmt
+	io
+	os
+	runtime
+	runtime/pprof
+	strconv
+	strings
+	time
+
+FILENAMES
+	testdata/benchmark.go
+	testdata/example.go
+	testdata/testing.go
+
+FUNCTIONS
+	// An internal function but exported because it is cross-package; ...
+	func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample)
+
+	// An internal function but exported because it is cross-package; ...
+	func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark)
+
+	// 
+	func RunExamples(examples []InternalExample) (ok bool)
+
+	// 
+	func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool)
+
+	// Short reports whether the -test.short flag is set. 
+	func Short() bool
+
+
+TYPES
+	// B is a type passed to Benchmark functions to manage benchmark ...
+	type B struct {
+		N int
+		// contains filtered or unexported fields
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *B) Error(args ...any)
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *B) Errorf(format string, args ...any)
+
+	// Fail marks the function as having failed but continues ...
+	func (c *B) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *B) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *B) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *B) Fatal(args ...any)
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *B) Fatalf(format string, args ...any)
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *B) Log(args ...any)
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *B) Logf(format string, args ...any)
+
+	// ResetTimer sets the elapsed benchmark time to zero. It does not ...
+	func (b *B) ResetTimer()
+
+	// SetBytes records the number of bytes processed in a single ...
+	func (b *B) SetBytes(n int64)
+
+	// StartTimer starts timing a test. This function is called ...
+	func (b *B) StartTimer()
+
+	// StopTimer stops timing a test. This can be used to pause the ...
+	func (b *B) StopTimer()
+
+	// The results of a benchmark run. 
+	type BenchmarkResult struct {
+		N	int		// The number of iterations.
+		T	time.Duration	// The total time taken.
+		Bytes	int64		// Bytes processed in one iteration.
+	}
+
+	// Benchmark benchmarks a single function. Useful for creating ...
+	func Benchmark(f func(b *B)) BenchmarkResult
+
+	// 
+	func (r BenchmarkResult) NsPerOp() int64
+
+	// 
+	func (r BenchmarkResult) String() string
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalBenchmark struct {
+		Name	string
+		F	func(b *B)
+	}
+
+	// 
+	type InternalExample struct {
+		Name	string
+		F	func()
+		Output	string
+	}
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalTest struct {
+		Name	string
+		F	func(*T)
+	}
+
+	// T is a type passed to Test functions to manage test state and ...
+	type T struct {
+		// contains filtered or unexported fields
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *T) Error(args ...any)
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *T) Errorf(format string, args ...any)
+
+	// Fail marks the function as having failed but continues ...
+	func (c *T) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *T) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *T) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *T) Fatal(args ...any)
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *T) Fatalf(format string, args ...any)
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *T) Log(args ...any)
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *T) Logf(format string, args ...any)
+
+	// Parallel signals that this test is to be run in parallel with ...
+	func (t *T) Parallel()
+
diff --git a/internal/backport/go/doc/testdata/testing.go b/internal/backport/go/doc/testdata/testing.go
new file mode 100644
index 0000000..80238df
--- /dev/null
+++ b/internal/backport/go/doc/testdata/testing.go
@@ -0,0 +1,404 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package testing provides support for automated testing of Go packages.
+// It is intended to be used in concert with the ``go test'' utility, which automates
+// execution of any function of the form
+//     func TestXxx(*testing.T)
+// where Xxx can be any alphanumeric string (but the first letter must not be in
+// [a-z]) and serves to identify the test routine.
+// These TestXxx routines should be declared within the package they are testing.
+//
+// Functions of the form
+//     func BenchmarkXxx(*testing.B)
+// are considered benchmarks, and are executed by go test when the -test.bench
+// flag is provided.
+//
+// A sample benchmark function looks like this:
+//     func BenchmarkHello(b *testing.B) {
+//         for i := 0; i < b.N; i++ {
+//             fmt.Sprintf("hello")
+//         }
+//     }
+// The benchmark package will vary b.N until the benchmark function lasts
+// long enough to be timed reliably. The output
+//     testing.BenchmarkHello    10000000    282 ns/op
+// means that the loop ran 10000000 times at a speed of 282 ns per loop.
+//
+// If a benchmark needs some expensive setup before running, the timer
+// may be stopped:
+//     func BenchmarkBigLen(b *testing.B) {
+//         b.StopTimer()
+//         big := NewBig()
+//         b.StartTimer()
+//         for i := 0; i < b.N; i++ {
+//             big.Len()
+//         }
+//     }
+package testing
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"runtime"
+	"runtime/pprof"
+	"strconv"
+	"strings"
+	"time"
+)
+
+var (
+	// The short flag requests that tests run more quickly, but its functionality
+	// is provided by test writers themselves. The testing package is just its
+	// home. The all.bash installation script sets it to make installation more
+	// efficient, but by default the flag is off so a plain "go test" will do a
+	// full test of the package.
+	short = flag.Bool("test.short", false, "run smaller test suite to save time")
+
+	// Report as tests are run; default is silent for success.
+	chatty         = flag.Bool("test.v", false, "verbose: print additional output")
+	match          = flag.String("test.run", "", "regular expression to select tests to run")
+	memProfile     = flag.String("test.memprofile", "", "write a memory profile to the named file after execution")
+	memProfileRate = flag.Int("test.memprofilerate", 0, "if >=0, sets runtime.MemProfileRate")
+	cpuProfile     = flag.String("test.cpuprofile", "", "write a cpu profile to the named file during execution")
+	timeout        = flag.Duration("test.timeout", 0, "if positive, sets an aggregate time limit for all tests")
+	cpuListStr     = flag.String("test.cpu", "", "comma-separated list of number of CPUs to use for each test")
+	parallel       = flag.Int("test.parallel", runtime.GOMAXPROCS(0), "maximum test parallelism")
+
+	cpuList []int
+)
+
+// common holds the elements common between T and B and
+// captures common methods such as Errorf.
+type common struct {
+	output   []byte    // Output generated by test or benchmark.
+	failed   bool      // Test or benchmark has failed.
+	start    time.Time // Time test or benchmark started
+	duration time.Duration
+	self     any      // To be sent on signal channel when done.
+	signal   chan any // Output for serial tests.
+}
+
+// Short reports whether the -test.short flag is set.
+func Short() bool {
+	return *short
+}
+
+// decorate inserts the final newline if needed and indentation tabs for formatting.
+// If addFileLine is true, it also prefixes the string with the file and line of the call site.
+func decorate(s string, addFileLine bool) string {
+	if addFileLine {
+		_, file, line, ok := runtime.Caller(3) // decorate + log + public function.
+		if ok {
+			// Truncate file name at last file name separator.
+			if index := strings.LastIndex(file, "/"); index >= 0 {
+				file = file[index+1:]
+			} else if index = strings.LastIndex(file, "\\"); index >= 0 {
+				file = file[index+1:]
+			}
+		} else {
+			file = "???"
+			line = 1
+		}
+		s = fmt.Sprintf("%s:%d: %s", file, line, s)
+	}
+	s = "\t" + s // Every line is indented at least one tab.
+	n := len(s)
+	if n > 0 && s[n-1] != '\n' {
+		s += "\n"
+		n++
+	}
+	for i := 0; i < n-1; i++ { // -1 to avoid final newline
+		if s[i] == '\n' {
+			// Second and subsequent lines are indented an extra tab.
+			return s[0:i+1] + "\t" + decorate(s[i+1:n], false)
+		}
+	}
+	return s
+}
+
+// T is a type passed to Test functions to manage test state and support formatted test logs.
+// Logs are accumulated during execution and dumped to standard error when done.
+type T struct {
+	common
+	name          string    // Name of test.
+	startParallel chan bool // Parallel tests will wait on this.
+}
+
+// Fail marks the function as having failed but continues execution.
+func (c *common) Fail() { c.failed = true }
+
+// Failed reports whether the function has failed.
+func (c *common) Failed() bool { return c.failed }
+
+// FailNow marks the function as having failed and stops its execution.
+// Execution will continue at the next Test.
+func (c *common) FailNow() {
+	c.Fail()
+
+	// Calling runtime.Goexit will exit the goroutine, which
+	// will run the deferred functions in this goroutine,
+	// which will eventually run the deferred lines in tRunner,
+	// which will signal to the test loop that this test is done.
+	//
+	// A previous version of this code said:
+	//
+	//	c.duration = ...
+	//	c.signal <- c.self
+	//	runtime.Goexit()
+	//
+	// This previous version duplicated code (those lines are in
+	// tRunner no matter what), but worse the goroutine teardown
+	// implicit in runtime.Goexit was not guaranteed to complete
+	// before the test exited. If a test deferred an important cleanup
+	// function (like removing temporary files), there was no guarantee
+	// it would run on a test failure. Because we send on c.signal during
+	// a top-of-stack deferred function now, we know that the send
+	// only happens after any other stacked defers have completed.
+	runtime.Goexit()
+}
+
+// log generates the output. It's always at the same stack depth.
+func (c *common) log(s string) {
+	c.output = append(c.output, decorate(s, true)...)
+}
+
+// Log formats its arguments using default formatting, analogous to Println(),
+// and records the text in the error log.
+func (c *common) Log(args ...any) { c.log(fmt.Sprintln(args...)) }
+
+// Logf formats its arguments according to the format, analogous to Printf(),
+// and records the text in the error log.
+func (c *common) Logf(format string, args ...any) { c.log(fmt.Sprintf(format, args...)) }
+
+// Error is equivalent to Log() followed by Fail().
+func (c *common) Error(args ...any) {
+	c.log(fmt.Sprintln(args...))
+	c.Fail()
+}
+
+// Errorf is equivalent to Logf() followed by Fail().
+func (c *common) Errorf(format string, args ...any) {
+	c.log(fmt.Sprintf(format, args...))
+	c.Fail()
+}
+
+// Fatal is equivalent to Log() followed by FailNow().
+func (c *common) Fatal(args ...any) {
+	c.log(fmt.Sprintln(args...))
+	c.FailNow()
+}
+
+// Fatalf is equivalent to Logf() followed by FailNow().
+func (c *common) Fatalf(format string, args ...any) {
+	c.log(fmt.Sprintf(format, args...))
+	c.FailNow()
+}
+
+// Parallel signals that this test is to be run in parallel with (and only with)
+// other parallel tests in this CPU group.
+func (t *T) Parallel() {
+	t.signal <- (*T)(nil) // Release main testing loop
+	<-t.startParallel     // Wait for serial tests to finish
+}
+
+// An internal type but exported because it is cross-package; part of the implementation
+// of go test.
+type InternalTest struct {
+	Name string
+	F    func(*T)
+}
+
+func tRunner(t *T, test *InternalTest) {
+	t.start = time.Now()
+
+	// When this goroutine is done, either because test.F(t)
+	// returned normally or because a test failure triggered
+	// a call to runtime.Goexit, record the duration and send
+	// a signal saying that the test is done.
+	defer func() {
+		t.duration = time.Now().Sub(t.start)
+		t.signal <- t
+	}()
+
+	test.F(t)
+}
+
+// An internal function but exported because it is cross-package; part of the implementation
+// of go test.
+func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample) {
+	flag.Parse()
+	parseCpuList()
+
+	before()
+	startAlarm()
+	testOk := RunTests(matchString, tests)
+	exampleOk := RunExamples(examples)
+	if !testOk || !exampleOk {
+		fmt.Println("FAIL")
+		os.Exit(1)
+	}
+	fmt.Println("PASS")
+	stopAlarm()
+	RunBenchmarks(matchString, benchmarks)
+	after()
+}
+
+func (t *T) report() {
+	tstr := fmt.Sprintf("(%.2f seconds)", t.duration.Seconds())
+	format := "--- %s: %s %s\n%s"
+	if t.failed {
+		fmt.Printf(format, "FAIL", t.name, tstr, t.output)
+	} else if *chatty {
+		fmt.Printf(format, "PASS", t.name, tstr, t.output)
+	}
+}
+
+func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool) {
+	ok = true
+	if len(tests) == 0 {
+		fmt.Fprintln(os.Stderr, "testing: warning: no tests to run")
+		return
+	}
+	for _, procs := range cpuList {
+		runtime.GOMAXPROCS(procs)
+		// We build a new channel tree for each run of the loop.
+		// collector merges in one channel all the upstream signals from parallel tests.
+		// If all tests pump to the same channel, a bug can occur where a test
+		// kicks off a goroutine that Fails, yet the test still delivers a completion signal,
+		// which skews the counting.
+		var collector = make(chan any)
+
+		numParallel := 0
+		startParallel := make(chan bool)
+
+		for i := 0; i < len(tests); i++ {
+			matched, err := matchString(*match, tests[i].Name)
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "testing: invalid regexp for -test.run: %s\n", err)
+				os.Exit(1)
+			}
+			if !matched {
+				continue
+			}
+			testName := tests[i].Name
+			if procs != 1 {
+				testName = fmt.Sprintf("%s-%d", tests[i].Name, procs)
+			}
+			t := &T{
+				common: common{
+					signal: make(chan any),
+				},
+				name:          testName,
+				startParallel: startParallel,
+			}
+			t.self = t
+			if *chatty {
+				fmt.Printf("=== RUN %s\n", t.name)
+			}
+			go tRunner(t, &tests[i])
+			out := (<-t.signal).(*T)
+			if out == nil { // Parallel run.
+				go func() {
+					collector <- <-t.signal
+				}()
+				numParallel++
+				continue
+			}
+			t.report()
+			ok = ok && !out.failed
+		}
+
+		running := 0
+		for numParallel+running > 0 {
+			if running < *parallel && numParallel > 0 {
+				startParallel <- true
+				running++
+				numParallel--
+				continue
+			}
+			t := (<-collector).(*T)
+			t.report()
+			ok = ok && !t.failed
+			running--
+		}
+	}
+	return
+}
+
+// before runs before all testing.
+func before() {
+	if *memProfileRate > 0 {
+		runtime.MemProfileRate = *memProfileRate
+	}
+	if *cpuProfile != "" {
+		f, err := os.Create(*cpuProfile)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: %s", err)
+			return
+		}
+		if err := pprof.StartCPUProfile(f); err != nil {
+			fmt.Fprintf(os.Stderr, "testing: can't start cpu profile: %s", err)
+			f.Close()
+			return
+		}
+		// Could save f so after can call f.Close; not worth the effort.
+	}
+
+}
+
+// after runs after all testing.
+func after() {
+	if *cpuProfile != "" {
+		pprof.StopCPUProfile() // flushes profile to disk
+	}
+	if *memProfile != "" {
+		f, err := os.Create(*memProfile)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: %s", err)
+			return
+		}
+		if err = pprof.WriteHeapProfile(f); err != nil {
+			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s", *memProfile, err)
+		}
+		f.Close()
+	}
+}
+
+var timer *time.Timer
+
+// startAlarm starts an alarm if requested.
+func startAlarm() {
+	if *timeout > 0 {
+		timer = time.AfterFunc(*timeout, alarm)
+	}
+}
+
+// stopAlarm turns off the alarm.
+func stopAlarm() {
+	if *timeout > 0 {
+		timer.Stop()
+	}
+}
+
+// alarm is called if the timeout expires.
+func alarm() {
+	panic("test timed out")
+}
+
+func parseCpuList() {
+	if len(*cpuListStr) == 0 {
+		cpuList = append(cpuList, runtime.GOMAXPROCS(-1))
+	} else {
+		for _, val := range strings.Split(*cpuListStr, ",") {
+			cpu, err := strconv.Atoi(val)
+			if err != nil || cpu <= 0 {
+				fmt.Fprintf(os.Stderr, "testing: invalid value %q for -test.cpu", val)
+				os.Exit(1)
+			}
+			cpuList = append(cpuList, cpu)
+		}
+	}
+}
diff --git a/internal/backport/go/format/benchmark_test.go b/internal/backport/go/format/benchmark_test.go
new file mode 100644
index 0000000..e2a6491
--- /dev/null
+++ b/internal/backport/go/format/benchmark_test.go
@@ -0,0 +1,91 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file provides a simple framework to add benchmarks
+// based on generated input (source) files.
+
+package format_test
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"golang.org/x/website/internal/backport/go/format"
+	"os"
+	"testing"
+)
+
+var debug = flag.Bool("debug", false, "write .src files containing formatting input; for debugging")
+
+// array1 generates an array literal with n elements of the form:
+//
+// var _ = [...]byte{
+//
+//	// 0
+//	0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+//	0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
+//	0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
+//	0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
+//	0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27,
+//	// 40
+//	0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
+//	0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
+//	...
+func array1(buf *bytes.Buffer, n int) {
+	buf.WriteString("var _ = [...]byte{\n")
+	for i := 0; i < n; {
+		if i%10 == 0 {
+			fmt.Fprintf(buf, "\t// %d\n", i)
+		}
+		buf.WriteByte('\t')
+		for j := 0; j < 8; j++ {
+			fmt.Fprintf(buf, "0x%02x, ", byte(i))
+			i++
+		}
+		buf.WriteString("\n")
+	}
+	buf.WriteString("}\n")
+}
+
+var tests = []struct {
+	name string
+	gen  func(*bytes.Buffer, int)
+	n    int
+}{
+	{"array1", array1, 10000},
+	// add new test cases here as needed
+}
+
+func BenchmarkFormat(b *testing.B) {
+	var src bytes.Buffer
+	for _, t := range tests {
+		src.Reset()
+		src.WriteString("package p\n")
+		t.gen(&src, t.n)
+		data := src.Bytes()
+
+		if *debug {
+			filename := t.name + ".src"
+			err := os.WriteFile(filename, data, 0660)
+			if err != nil {
+				b.Fatalf("couldn't write %s: %v", filename, err)
+			}
+		}
+
+		b.Run(fmt.Sprintf("%s-%d", t.name, t.n), func(b *testing.B) {
+			b.SetBytes(int64(len(data)))
+			b.ReportAllocs()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				var err error
+				sink, err = format.Source(data)
+				if err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}
+
+var sink []byte
diff --git a/internal/backport/go/format/example_test.go b/internal/backport/go/format/example_test.go
new file mode 100644
index 0000000..5e7ceee
--- /dev/null
+++ b/internal/backport/go/format/example_test.go
@@ -0,0 +1,39 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package format_test
+
+import (
+	"bytes"
+	"fmt"
+	"golang.org/x/website/internal/backport/go/format"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
+	"log"
+)
+
+func ExampleNode() {
+	const expr = "(6+2*3)/4"
+
+	// parser.ParseExpr parses the argument and returns the
+	// corresponding ast.Node.
+	node, err := parser.ParseExpr(expr)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Create a FileSet for node. Since the node does not come
+	// from a real source file, fset will be empty.
+	fset := token.NewFileSet()
+
+	var buf bytes.Buffer
+	err = format.Node(&buf, fset, node)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Println(buf.String())
+
+	// Output: (6 + 2*3) / 4
+}
diff --git a/internal/backport/go/format/format.go b/internal/backport/go/format/format.go
new file mode 100644
index 0000000..0268883
--- /dev/null
+++ b/internal/backport/go/format/format.go
@@ -0,0 +1,133 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package format implements standard formatting of Go source.
+//
+// Note that formatting of Go source code changes over time, so tools relying on
+// consistent formatting should execute a specific version of the gofmt binary
+// instead of using this package. That way, the formatting will be stable, and
+// the tools won't need to be recompiled each time gofmt changes.
+//
+// For example, pre-submit checks that use this package directly would behave
+// differently depending on what Go version each developer uses, causing the
+// check to be inherently fragile.
+package format
+
+import (
+	"bytes"
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/printer"
+	"golang.org/x/website/internal/backport/go/token"
+	"io"
+)
+
+// Keep these in sync with cmd/gofmt/gofmt.go.
+const (
+	tabWidth    = 8
+	printerMode = printer.UseSpaces | printer.TabIndent | printerNormalizeNumbers
+
+	// printerNormalizeNumbers means to canonicalize number literal prefixes
+	// and exponents while printing. See https://golang.org/doc/go1.13#gofmt.
+	//
+	// This value is defined in go/printer specifically for go/format and cmd/gofmt.
+	printerNormalizeNumbers = 1 << 30
+)
+
+var config = printer.Config{Mode: printerMode, Tabwidth: tabWidth}
+
+const parserMode = parser.ParseComments
+
+// Node formats node in canonical gofmt style and writes the result to dst.
+//
+// The node type must be *ast.File, *printer.CommentedNode, []ast.Decl,
+// []ast.Stmt, or assignment-compatible to ast.Expr, ast.Decl, ast.Spec,
+// or ast.Stmt. Node does not modify node. Imports are not sorted for
+// nodes representing partial source files (for instance, if the node is
+// not an *ast.File or a *printer.CommentedNode not wrapping an *ast.File).
+//
+// The function may return early (before the entire result is written)
+// and return a formatting error, for instance due to an incorrect AST.
+func Node(dst io.Writer, fset *token.FileSet, node interface{}) error {
+	// Determine if we have a complete source file (file != nil).
+	var file *ast.File
+	var cnode *printer.CommentedNode
+	switch n := node.(type) {
+	case *ast.File:
+		file = n
+	case *printer.CommentedNode:
+		if f, ok := n.Node.(*ast.File); ok {
+			file = f
+			cnode = n
+		}
+	}
+
+	// Sort imports if necessary.
+	if file != nil && hasUnsortedImports(file) {
+		// Make a copy of the AST because ast.SortImports is destructive.
+		// TODO(gri) Do this more efficiently.
+		var buf bytes.Buffer
+		err := config.Fprint(&buf, fset, file)
+		if err != nil {
+			return err
+		}
+		file, err = parser.ParseFile(fset, "", buf.Bytes(), parserMode)
+		if err != nil {
+			// We should never get here. If we do, provide good diagnostic.
+			return fmt.Errorf("format.Node internal error (%s)", err)
+		}
+		ast.SortImports(fset, file)
+
+		// Use new file with sorted imports.
+		node = file
+		if cnode != nil {
+			node = &printer.CommentedNode{Node: file, Comments: cnode.Comments}
+		}
+	}
+
+	return config.Fprint(dst, fset, node)
+}
+
+// Source formats src in canonical gofmt style and returns the result
+// or an (I/O or syntax) error. src is expected to be a syntactically
+// correct Go source file, or a list of Go declarations or statements.
+//
+// If src is a partial source file, the leading and trailing space of src
+// is applied to the result (such that it has the same leading and trailing
+// space as src), and the result is indented by the same amount as the first
+// line of src containing code. Imports are not sorted for partial source files.
+func Source(src []byte) ([]byte, error) {
+	fset := token.NewFileSet()
+	file, sourceAdj, indentAdj, err := parse(fset, "", src, true)
+	if err != nil {
+		return nil, err
+	}
+
+	if sourceAdj == nil {
+		// Complete source file.
+		// TODO(gri) consider doing this always.
+		ast.SortImports(fset, file)
+	}
+
+	return format(fset, file, sourceAdj, indentAdj, src, config)
+}
+
+func hasUnsortedImports(file *ast.File) bool {
+	for _, d := range file.Decls {
+		d, ok := d.(*ast.GenDecl)
+		if !ok || d.Tok != token.IMPORT {
+			// Not an import declaration, so we're done.
+			// Imports are always first.
+			return false
+		}
+		if d.Lparen.IsValid() {
+			// For now assume all grouped imports are unsorted.
+			// TODO(gri) Should check if they are sorted already.
+			return true
+		}
+		// Ungrouped imports are sorted by default.
+	}
+	return false
+}
diff --git a/internal/backport/go/format/format_test.go b/internal/backport/go/format/format_test.go
new file mode 100644
index 0000000..d984eec
--- /dev/null
+++ b/internal/backport/go/format/format_test.go
@@ -0,0 +1,187 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package format
+
+import (
+	"bytes"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
+	"os"
+	"strings"
+	"testing"
+)
+
+const testfile = "format_test.go"
+
+func diff(t *testing.T, dst, src []byte) {
+	line := 1
+	offs := 0 // line offset
+	for i := 0; i < len(dst) && i < len(src); i++ {
+		d := dst[i]
+		s := src[i]
+		if d != s {
+			t.Errorf("dst:%d: %s\n", line, dst[offs:i+1])
+			t.Errorf("src:%d: %s\n", line, src[offs:i+1])
+			return
+		}
+		if s == '\n' {
+			line++
+			offs = i + 1
+		}
+	}
+	if len(dst) != len(src) {
+		t.Errorf("len(dst) = %d, len(src) = %d\nsrc = %q", len(dst), len(src), src)
+	}
+}
+
+func TestNode(t *testing.T) {
+	src, err := os.ReadFile(testfile)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, testfile, src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+
+	if err = Node(&buf, fset, file); err != nil {
+		t.Fatal("Node failed:", err)
+	}
+
+	diff(t, buf.Bytes(), src)
+}
+
+// Node is documented to not modify the AST.
+// Test that it is so even when numbers are normalized.
+func TestNodeNoModify(t *testing.T) {
+	const (
+		src    = "package p\n\nconst _ = 0000000123i\n"
+		golden = "package p\n\nconst _ = 123i\n"
+	)
+
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Capture original address and value of a BasicLit node
+	// which will undergo formatting changes during printing.
+	wantLit := file.Decls[0].(*ast.GenDecl).Specs[0].(*ast.ValueSpec).Values[0].(*ast.BasicLit)
+	wantVal := wantLit.Value
+
+	var buf bytes.Buffer
+	if err = Node(&buf, fset, file); err != nil {
+		t.Fatal("Node failed:", err)
+	}
+	diff(t, buf.Bytes(), []byte(golden))
+
+	// Check if anything changed after Node returned.
+	gotLit := file.Decls[0].(*ast.GenDecl).Specs[0].(*ast.ValueSpec).Values[0].(*ast.BasicLit)
+	gotVal := gotLit.Value
+
+	if gotLit != wantLit {
+		t.Errorf("got *ast.BasicLit address %p, want %p", gotLit, wantLit)
+	}
+	if gotVal != wantVal {
+		t.Errorf("got *ast.BasicLit value %q, want %q", gotVal, wantVal)
+	}
+}
+
+func TestSource(t *testing.T) {
+	src, err := os.ReadFile(testfile)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	res, err := Source(src)
+	if err != nil {
+		t.Fatal("Source failed:", err)
+	}
+
+	diff(t, res, src)
+}
+
+// Test cases that are expected to fail are marked by the prefix "ERROR".
+// The formatted result must look the same as the input for successful tests.
+var tests = []string{
+	// declaration lists
+	`import "golang.org/x/website/internal/backport/go/format"`,
+	"var x int",
+	"var x int\n\ntype T struct{}",
+
+	// statement lists
+	"x := 0",
+	"f(a, b, c)\nvar x int = f(1, 2, 3)",
+
+	// indentation, leading and trailing space
+	"\tx := 0\n\tgo f()",
+	"\tx := 0\n\tgo f()\n\n\n",
+	"\n\t\t\n\n\tx := 0\n\tgo f()\n\n\n",
+	"\n\t\t\n\n\t\t\tx := 0\n\t\t\tgo f()\n\n\n",
+	"\n\t\t\n\n\t\t\tx := 0\n\t\t\tconst s = `\nfoo\n`\n\n\n",     // no indentation added inside raw strings
+	"\n\t\t\n\n\t\t\tx := 0\n\t\t\tconst s = `\n\t\tfoo\n`\n\n\n", // no indentation removed inside raw strings
+
+	// comments
+	"/* Comment */",
+	"\t/* Comment */ ",
+	"\n/* Comment */ ",
+	"i := 5 /* Comment */",         // issue #5551
+	"\ta()\n//line :1",             // issue #11276
+	"\t//xxx\n\ta()\n//line :2",    // issue #11276
+	"\ta() //line :1\n\tb()\n",     // issue #11276
+	"x := 0\n//line :1\n//line :2", // issue #11276
+
+	// whitespace
+	"",     // issue #11275
+	" ",    // issue #11275
+	"\t",   // issue #11275
+	"\t\t", // issue #11275
+	"\n",   // issue #11275
+	"\n\n", // issue #11275
+	"\t\n", // issue #11275
+
+	// erroneous programs
+	"ERROR1 + 2 +",
+	"ERRORx :=  0",
+
+	// build comments
+	"// copyright\n\n//go:build x\n\npackage p\n",
+	"// copyright\n\n//go:build x\n// +build x\n\npackage p\n",
+}
+
+func String(s string) (string, error) {
+	res, err := Source([]byte(s))
+	if err != nil {
+		return "", err
+	}
+	return string(res), nil
+}
+
+func TestPartial(t *testing.T) {
+	for _, src := range tests {
+		if strings.HasPrefix(src, "ERROR") {
+			// test expected to fail
+			src = src[5:] // remove ERROR prefix
+			res, err := String(src)
+			if err == nil && res == src {
+				t.Errorf("formatting succeeded but was expected to fail:\n%q", src)
+			}
+		} else {
+			// test expected to succeed
+			res, err := String(src)
+			if err != nil {
+				t.Errorf("formatting failed (%s):\n%q", err, src)
+			} else if res != src {
+				t.Errorf("formatting incorrect:\nsource: %q\nresult: %q", src, res)
+			}
+		}
+	}
+}
diff --git a/internal/backport/go/format/internal.go b/internal/backport/go/format/internal.go
new file mode 100644
index 0000000..c83432d
--- /dev/null
+++ b/internal/backport/go/format/internal.go
@@ -0,0 +1,176 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// TODO(gri): This file and the file src/cmd/gofmt/internal.go are
+// the same (but for this comment and the package name). Do not modify
+// one without the other. Determine if we can factor out functionality
+// in a public API. See also #11844 for context.
+
+package format
+
+import (
+	"bytes"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/printer"
+	"golang.org/x/website/internal/backport/go/token"
+	"strings"
+)
+
+// parse parses src, which was read from the named file,
+// as a Go source file, declaration, or statement list.
+func parse(fset *token.FileSet, filename string, src []byte, fragmentOk bool) (
+	file *ast.File,
+	sourceAdj func(src []byte, indent int) []byte,
+	indentAdj int,
+	err error,
+) {
+	// Try as whole source file.
+	file, err = parser.ParseFile(fset, filename, src, parserMode)
+	// If there's no error, return. If the error is that the source file didn't begin with a
+	// package line and source fragments are ok, fall through to
+	// try as a source fragment. Stop and return on any other error.
+	if err == nil || !fragmentOk || !strings.Contains(err.Error(), "expected 'package'") {
+		return
+	}
+
+	// If this is a declaration list, make it a source file
+	// by inserting a package clause.
+	// Insert using a ';', not a newline, so that the line numbers
+	// in psrc match the ones in src.
+	psrc := append([]byte("package p;"), src...)
+	file, err = parser.ParseFile(fset, filename, psrc, parserMode)
+	if err == nil {
+		sourceAdj = func(src []byte, indent int) []byte {
+			// Remove the package clause.
+			// Gofmt has turned the ';' into a '\n'.
+			src = src[indent+len("package p\n"):]
+			return bytes.TrimSpace(src)
+		}
+		return
+	}
+	// If the error is that the source file didn't begin with a
+	// declaration, fall through to try as a statement list.
+	// Stop and return on any other error.
+	if !strings.Contains(err.Error(), "expected declaration") {
+		return
+	}
+
+	// If this is a statement list, make it a source file
+	// by inserting a package clause and turning the list
+	// into a function body. This handles expressions too.
+	// Insert using a ';', not a newline, so that the line numbers
+	// in fsrc match the ones in src. Add an extra '\n' before the '}'
+	// to make sure comments are flushed before the '}'.
+	fsrc := append(append([]byte("package p; func _() {"), src...), '\n', '\n', '}')
+	file, err = parser.ParseFile(fset, filename, fsrc, parserMode)
+	if err == nil {
+		sourceAdj = func(src []byte, indent int) []byte {
+			// Cap adjusted indent to zero.
+			if indent < 0 {
+				indent = 0
+			}
+			// Remove the wrapping.
+			// Gofmt has turned the "; " into a "\n\n".
+			// There will be two non-blank lines with indent, hence 2*indent.
+			src = src[2*indent+len("package p\n\nfunc _() {"):]
+			// Remove only the "}\n" suffix: remaining whitespaces will be trimmed anyway
+			src = src[:len(src)-len("}\n")]
+			return bytes.TrimSpace(src)
+		}
+		// Gofmt has also indented the function body one level.
+		// Adjust that with indentAdj.
+		indentAdj = -1
+	}
+
+	// Succeeded, or out of options.
+	return
+}
+
+// format formats the given package file originally obtained from src
+// and adjusts the result based on the original source via sourceAdj
+// and indentAdj.
+func format(
+	fset *token.FileSet,
+	file *ast.File,
+	sourceAdj func(src []byte, indent int) []byte,
+	indentAdj int,
+	src []byte,
+	cfg printer.Config,
+) ([]byte, error) {
+	if sourceAdj == nil {
+		// Complete source file.
+		var buf bytes.Buffer
+		err := cfg.Fprint(&buf, fset, file)
+		if err != nil {
+			return nil, err
+		}
+		return buf.Bytes(), nil
+	}
+
+	// Partial source file.
+	// Determine and prepend leading space.
+	i, j := 0, 0
+	for j < len(src) && isSpace(src[j]) {
+		if src[j] == '\n' {
+			i = j + 1 // byte offset of last line in leading space
+		}
+		j++
+	}
+	var res []byte
+	res = append(res, src[:i]...)
+
+	// Determine and prepend indentation of first code line.
+	// Spaces are ignored unless there are no tabs,
+	// in which case spaces count as one tab.
+	indent := 0
+	hasSpace := false
+	for _, b := range src[i:j] {
+		switch b {
+		case ' ':
+			hasSpace = true
+		case '\t':
+			indent++
+		}
+	}
+	if indent == 0 && hasSpace {
+		indent = 1
+	}
+	for i := 0; i < indent; i++ {
+		res = append(res, '\t')
+	}
+
+	// Format the source.
+	// Write it without any leading and trailing space.
+	cfg.Indent = indent + indentAdj
+	var buf bytes.Buffer
+	err := cfg.Fprint(&buf, fset, file)
+	if err != nil {
+		return nil, err
+	}
+	out := sourceAdj(buf.Bytes(), cfg.Indent)
+
+	// If the adjusted output is empty, the source
+	// was empty but (possibly) for white space.
+	// The result is the incoming source.
+	if len(out) == 0 {
+		return src, nil
+	}
+
+	// Otherwise, append output to leading space.
+	res = append(res, out...)
+
+	// Determine and append trailing space.
+	i = len(src)
+	for i > 0 && isSpace(src[i-1]) {
+		i--
+	}
+	return append(res, src[i:]...), nil
+}
+
+// isSpace reports whether the byte is a space character.
+// isSpace defines a space as being among the following bytes: ' ', '\t', '\n' and '\r'.
+func isSpace(b byte) bool {
+	return b == ' ' || b == '\t' || b == '\n' || b == '\r'
+}
diff --git a/internal/backport/go/internal/typeparams/common.go b/internal/backport/go/internal/typeparams/common.go
new file mode 100644
index 0000000..9b82e60
--- /dev/null
+++ b/internal/backport/go/internal/typeparams/common.go
@@ -0,0 +1,15 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package typeparams provides functions to work with type parameter data
+// stored in the AST, while these AST changes are guarded by a build
+// constraint.
+package typeparams
+
+// 'Hidden' parser modes to control the parsing of type-parameter related
+// features.
+const (
+	DisallowTypeSets = 1 << 29 // Disallow eliding 'interface' in constraint type sets.
+	DisallowParsing  = 1 << 30 // Disallow type parameters entirely.
+)
diff --git a/internal/backport/go/internal/typeparams/typeparams.go b/internal/backport/go/internal/typeparams/typeparams.go
new file mode 100644
index 0000000..2f7a5ce
--- /dev/null
+++ b/internal/backport/go/internal/typeparams/typeparams.go
@@ -0,0 +1,54 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package typeparams
+
+import (
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+func PackIndexExpr(x ast.Expr, lbrack token.Pos, exprs []ast.Expr, rbrack token.Pos) ast.Expr {
+	switch len(exprs) {
+	case 0:
+		panic("internal error: PackIndexExpr with empty expr slice")
+	case 1:
+		return &ast.IndexExpr{
+			X:      x,
+			Lbrack: lbrack,
+			Index:  exprs[0],
+			Rbrack: rbrack,
+		}
+	default:
+		return &ast.IndexListExpr{
+			X:       x,
+			Lbrack:  lbrack,
+			Indices: exprs,
+			Rbrack:  rbrack,
+		}
+	}
+}
+
+// IndexExpr wraps an ast.IndexExpr or ast.IndexListExpr.
+//
+// Orig holds the original ast.Expr from which this IndexExpr was derived.
+type IndexExpr struct {
+	Orig ast.Expr // the wrapped expr, which may be distinct from the IndexListExpr below.
+	*ast.IndexListExpr
+}
+
+func UnpackIndexExpr(n ast.Node) *IndexExpr {
+	switch e := n.(type) {
+	case *ast.IndexExpr:
+		return &IndexExpr{e, &ast.IndexListExpr{
+			X:       e.X,
+			Lbrack:  e.Lbrack,
+			Indices: []ast.Expr{e.Index},
+			Rbrack:  e.Rbrack,
+		}}
+	case *ast.IndexListExpr:
+		return &IndexExpr{e, e}
+	}
+	return nil
+}
diff --git a/internal/backport/go/parser/error_test.go b/internal/backport/go/parser/error_test.go
new file mode 100644
index 0000000..38658c2
--- /dev/null
+++ b/internal/backport/go/parser/error_test.go
@@ -0,0 +1,202 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements a parser test harness. The files in the testdata
+// directory are parsed and the errors reported are compared against the
+// error messages expected in the test files. The test files must end in
+// .src rather than .go so that they are not disturbed by gofmt runs.
+//
+// Expected errors are indicated in the test files by putting a comment
+// of the form /* ERROR "rx" */ immediately following an offending token.
+// The harness will verify that an error matching the regular expression
+// rx is reported at that source position.
+//
+// For instance, the following test file indicates that a "not declared"
+// error should be reported for the undeclared variable x:
+//
+//	package p
+//	func f() {
+//		_ = x /* ERROR "not declared" */ + 1
+//	}
+
+package parser
+
+import (
+	"flag"
+	"golang.org/x/website/internal/backport/go/internal/typeparams"
+	"golang.org/x/website/internal/backport/go/scanner"
+	"golang.org/x/website/internal/backport/go/token"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+var traceErrs = flag.Bool("trace_errs", false, "whether to enable tracing for error tests")
+
+const testdata = "testdata"
+
+// getFile assumes that each filename occurs at most once
+func getFile(fset *token.FileSet, filename string) (file *token.File) {
+	fset.Iterate(func(f *token.File) bool {
+		if f.Name() == filename {
+			if file != nil {
+				panic(filename + " used multiple times")
+			}
+			file = f
+		}
+		return true
+	})
+	return file
+}
+
+func getPos(fset *token.FileSet, filename string, offset int) token.Pos {
+	if f := getFile(fset, filename); f != nil {
+		return f.Pos(offset)
+	}
+	return token.NoPos
+}
+
+// ERROR comments must be of the form /* ERROR "rx" */ and rx is
+// a regular expression that matches the expected error message.
+// The special form /* ERROR HERE "rx" */ must be used for error
+// messages that appear immediately after a token, rather than at
+// a token's position.
+var errRx = regexp.MustCompile(`^/\* *ERROR *(HERE)? *"([^"]*)" *\*/$`)
+
+// expectedErrors collects the regular expressions of ERROR comments found
+// in files and returns them as a map of error positions to error messages.
+func expectedErrors(fset *token.FileSet, filename string, src []byte) map[token.Pos]string {
+	errors := make(map[token.Pos]string)
+
+	var s scanner.Scanner
+	// file was parsed already - do not add it again to the file
+	// set otherwise the position information returned here will
+	// not match the position information collected by the parser
+	s.Init(getFile(fset, filename), src, nil, scanner.ScanComments)
+	var prev token.Pos // position of last non-comment, non-semicolon token
+	var here token.Pos // position immediately after the token at position prev
+
+	for {
+		pos, tok, lit := s.Scan()
+		switch tok {
+		case token.EOF:
+			return errors
+		case token.COMMENT:
+			s := errRx.FindStringSubmatch(lit)
+			if len(s) == 3 {
+				pos := prev
+				if s[1] == "HERE" {
+					pos = here
+				}
+				errors[pos] = s[2]
+			}
+		case token.SEMICOLON:
+			// don't use the position of auto-inserted (invisible) semicolons
+			if lit != ";" {
+				break
+			}
+			fallthrough
+		default:
+			prev = pos
+			var l int // token length
+			if tok.IsLiteral() {
+				l = len(lit)
+			} else {
+				l = len(tok.String())
+			}
+			here = prev + token.Pos(l)
+		}
+	}
+}
+
+// compareErrors compares the map of expected error messages with the list
+// of found errors and reports discrepancies.
+func compareErrors(t *testing.T, fset *token.FileSet, expected map[token.Pos]string, found scanner.ErrorList) {
+	t.Helper()
+	for _, error := range found {
+		// error.Pos is a token.Position, but we want
+		// a token.Pos so we can do a map lookup
+		pos := getPos(fset, error.Pos.Filename, error.Pos.Offset)
+		if msg, found := expected[pos]; found {
+			// we expect a message at pos; check if it matches
+			rx, err := regexp.Compile(msg)
+			if err != nil {
+				t.Errorf("%s: %v", error.Pos, err)
+				continue
+			}
+			if match := rx.MatchString(error.Msg); !match {
+				t.Errorf("%s: %q does not match %q", error.Pos, error.Msg, msg)
+				continue
+			}
+			// we have a match - eliminate this error
+			delete(expected, pos)
+		} else {
+			// To keep in mind when analyzing failed test output:
+			// If the same error position occurs multiple times in errors,
+			// this message will be triggered (because the first error at
+			// the position removes this position from the expected errors).
+			t.Errorf("%s: unexpected error: %s", error.Pos, error.Msg)
+		}
+	}
+
+	// there should be no expected errors left
+	if len(expected) > 0 {
+		t.Errorf("%d errors not reported:", len(expected))
+		for pos, msg := range expected {
+			t.Errorf("%s: %s\n", fset.Position(pos), msg)
+		}
+	}
+}
+
+func checkErrors(t *testing.T, filename string, input interface{}, mode Mode, expectErrors bool) {
+	t.Helper()
+	src, err := readSource(filename, input)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	fset := token.NewFileSet()
+	_, err = ParseFile(fset, filename, src, mode)
+	found, ok := err.(scanner.ErrorList)
+	if err != nil && !ok {
+		t.Error(err)
+		return
+	}
+	found.RemoveMultiples()
+
+	expected := map[token.Pos]string{}
+	if expectErrors {
+		// we are expecting the following errors
+		// (collect these after parsing a file so that it is found in the file set)
+		expected = expectedErrors(fset, filename, src)
+	}
+
+	// verify errors returned by the parser
+	compareErrors(t, fset, expected, found)
+}
+
+func TestErrors(t *testing.T) {
+	list, err := os.ReadDir(testdata)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, d := range list {
+		name := d.Name()
+		t.Run(name, func(t *testing.T) {
+			if !d.IsDir() && !strings.HasPrefix(name, ".") && (strings.HasSuffix(name, ".src") || strings.HasSuffix(name, ".go2")) {
+				mode := DeclarationErrors | AllErrors
+				if !strings.HasSuffix(name, ".go2") {
+					mode |= typeparams.DisallowParsing
+				}
+				if *traceErrs {
+					mode |= Trace
+				}
+				checkErrors(t, filepath.Join(testdata, name), nil, mode, true)
+			}
+		})
+	}
+}
diff --git a/internal/backport/go/parser/example_test.go b/internal/backport/go/parser/example_test.go
new file mode 100644
index 0000000..35fdd33
--- /dev/null
+++ b/internal/backport/go/parser/example_test.go
@@ -0,0 +1,43 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parser_test
+
+import (
+	"fmt"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+func ExampleParseFile() {
+	fset := token.NewFileSet() // positions are relative to fset
+
+	src := `package foo
+
+import (
+	"fmt"
+	"time"
+)
+
+func bar() {
+	fmt.Println(time.Now())
+}`
+
+	// Parse src but stop after processing the imports.
+	f, err := parser.ParseFile(fset, "", src, parser.ImportsOnly)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	// Print the imports from the file's AST.
+	for _, s := range f.Imports {
+		fmt.Println(s.Path.Value)
+	}
+
+	// output:
+	//
+	// "fmt"
+	// "time"
+}
diff --git a/internal/backport/go/parser/interface.go b/internal/backport/go/parser/interface.go
new file mode 100644
index 0000000..d0fc6e8
--- /dev/null
+++ b/internal/backport/go/parser/interface.go
@@ -0,0 +1,232 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains the exported entry points for invoking the parser.
+
+package parser
+
+import (
+	"bytes"
+	"errors"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/token"
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// If src != nil, readSource converts src to a []byte if possible;
+// otherwise it returns an error. If src == nil, readSource returns
+// the result of reading the file specified by filename.
+func readSource(filename string, src interface{}) ([]byte, error) {
+	if src != nil {
+		switch s := src.(type) {
+		case string:
+			return []byte(s), nil
+		case []byte:
+			return s, nil
+		case *bytes.Buffer:
+			// is io.Reader, but src is already available in []byte form
+			if s != nil {
+				return s.Bytes(), nil
+			}
+		case io.Reader:
+			return io.ReadAll(s)
+		}
+		return nil, errors.New("invalid source")
+	}
+	return os.ReadFile(filename)
+}
+
+// A Mode value is a set of flags (or 0).
+// They control the amount of source code parsed and other optional
+// parser functionality.
+type Mode uint
+
+const (
+	PackageClauseOnly    Mode             = 1 << iota // stop parsing after package clause
+	ImportsOnly                                       // stop parsing after import declarations
+	ParseComments                                     // parse comments and add them to AST
+	Trace                                             // print a trace of parsed productions
+	DeclarationErrors                                 // report declaration errors
+	SpuriousErrors                                    // same as AllErrors, for backward-compatibility
+	SkipObjectResolution                              // don't resolve identifiers to objects - see ParseFile
+	AllErrors            = SpuriousErrors             // report all errors (not just the first 10 on different lines)
+)
+
+// ParseFile parses the source code of a single Go source file and returns
+// the corresponding ast.File node. The source code may be provided via
+// the filename of the source file, or via the src parameter.
+//
+// If src != nil, ParseFile parses the source from src and the filename is
+// only used when recording position information. The type of the argument
+// for the src parameter must be string, []byte, or io.Reader.
+// If src == nil, ParseFile parses the file specified by filename.
+//
+// The mode parameter controls the amount of source text parsed and other
+// optional parser functionality. If the SkipObjectResolution mode bit is set,
+// the object resolution phase of parsing will be skipped, causing File.Scope,
+// File.Unresolved, and all Ident.Obj fields to be nil.
+//
+// Position information is recorded in the file set fset, which must not be
+// nil.
+//
+// If the source couldn't be read, the returned AST is nil and the error
+// indicates the specific failure. If the source was read but syntax
+// errors were found, the result is a partial AST (with ast.Bad* nodes
+// representing the fragments of erroneous source code). Multiple errors
+// are returned via a scanner.ErrorList which is sorted by source position.
+func ParseFile(fset *token.FileSet, filename string, src interface{}, mode Mode) (f *ast.File, err error) {
+	if fset == nil {
+		panic("parser.ParseFile: no token.FileSet provided (fset == nil)")
+	}
+
+	// get source
+	text, err := readSource(filename, src)
+	if err != nil {
+		return nil, err
+	}
+
+	var p parser
+	defer func() {
+		if e := recover(); e != nil {
+			// resume same panic if it's not a bailout
+			if _, ok := e.(bailout); !ok {
+				panic(e)
+			}
+		}
+
+		// set result values
+		if f == nil {
+			// source is not a valid Go source file - satisfy
+			// ParseFile API and return a valid (but) empty
+			// *ast.File
+			f = &ast.File{
+				Name:  new(ast.Ident),
+				Scope: ast.NewScope(nil),
+			}
+		}
+
+		p.errors.Sort()
+		err = p.errors.Err()
+	}()
+
+	// parse source
+	p.init(fset, filename, text, mode)
+	f = p.parseFile()
+
+	return
+}
+
+// ParseDir calls ParseFile for all files with names ending in ".go" in the
+// directory specified by path and returns a map of package name -> package
+// AST with all the packages found.
+//
+// If filter != nil, only the files with fs.FileInfo entries passing through
+// the filter (and ending in ".go") are considered. The mode bits are passed
+// to ParseFile unchanged. Position information is recorded in fset, which
+// must not be nil.
+//
+// If the directory couldn't be read, a nil map and the respective error are
+// returned. If a parse error occurred, a non-nil but incomplete map and the
+// first error encountered are returned.
+func ParseDir(fset *token.FileSet, path string, filter func(fs.FileInfo) bool, mode Mode) (pkgs map[string]*ast.Package, first error) {
+	list, err := os.ReadDir(path)
+	if err != nil {
+		return nil, err
+	}
+
+	pkgs = make(map[string]*ast.Package)
+	for _, d := range list {
+		if d.IsDir() || !strings.HasSuffix(d.Name(), ".go") {
+			continue
+		}
+		if filter != nil {
+			info, err := d.Info()
+			if err != nil {
+				return nil, err
+			}
+			if !filter(info) {
+				continue
+			}
+		}
+		filename := filepath.Join(path, d.Name())
+		if src, err := ParseFile(fset, filename, nil, mode); err == nil {
+			name := src.Name.Name
+			pkg, found := pkgs[name]
+			if !found {
+				pkg = &ast.Package{
+					Name:  name,
+					Files: make(map[string]*ast.File),
+				}
+				pkgs[name] = pkg
+			}
+			pkg.Files[filename] = src
+		} else if first == nil {
+			first = err
+		}
+	}
+
+	return
+}
+
+// ParseExprFrom is a convenience function for parsing an expression.
+// The arguments have the same meaning as for ParseFile, but the source must
+// be a valid Go (type or value) expression. Specifically, fset must not
+// be nil.
+//
+// If the source couldn't be read, the returned AST is nil and the error
+// indicates the specific failure. If the source was read but syntax
+// errors were found, the result is a partial AST (with ast.Bad* nodes
+// representing the fragments of erroneous source code). Multiple errors
+// are returned via a scanner.ErrorList which is sorted by source position.
+func ParseExprFrom(fset *token.FileSet, filename string, src interface{}, mode Mode) (expr ast.Expr, err error) {
+	if fset == nil {
+		panic("parser.ParseExprFrom: no token.FileSet provided (fset == nil)")
+	}
+
+	// get source
+	text, err := readSource(filename, src)
+	if err != nil {
+		return nil, err
+	}
+
+	var p parser
+	defer func() {
+		if e := recover(); e != nil {
+			// resume same panic if it's not a bailout
+			if _, ok := e.(bailout); !ok {
+				panic(e)
+			}
+		}
+		p.errors.Sort()
+		err = p.errors.Err()
+	}()
+
+	// parse expr
+	p.init(fset, filename, text, mode)
+	expr = p.parseRhsOrType()
+
+	// If a semicolon was inserted, consume it;
+	// report an error if there's more tokens.
+	if p.tok == token.SEMICOLON && p.lit == "\n" {
+		p.next()
+	}
+	p.expect(token.EOF)
+
+	return
+}
+
+// ParseExpr is a convenience function for obtaining the AST of an expression x.
+// The position information recorded in the AST is undefined. The filename used
+// in error messages is the empty string.
+//
+// If syntax errors were found, the result is a partial AST (with ast.Bad* nodes
+// representing the fragments of erroneous source code). Multiple errors are
+// returned via a scanner.ErrorList which is sorted by source position.
+func ParseExpr(x string) (ast.Expr, error) {
+	return ParseExprFrom(token.NewFileSet(), "", []byte(x), 0)
+}
diff --git a/internal/backport/go/parser/parser.go b/internal/backport/go/parser/parser.go
new file mode 100644
index 0000000..b258c0b
--- /dev/null
+++ b/internal/backport/go/parser/parser.go
@@ -0,0 +1,2894 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package parser implements a parser for Go source files. Input may be
+// provided in a variety of forms (see the various Parse* functions); the
+// output is an abstract syntax tree (AST) representing the Go source. The
+// parser is invoked through one of the Parse* functions.
+//
+// The parser accepts a larger language than is syntactically permitted by
+// the Go spec, for simplicity, and for improved robustness in the presence
+// of syntax errors. For instance, in method declarations, the receiver is
+// treated like an ordinary parameter list and thus may contain multiple
+// entries where the spec permits exactly one. Consequently, the corresponding
+// field in the AST (ast.FuncDecl.Recv) field is not restricted to one entry.
+package parser
+
+import (
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/internal/typeparams"
+	"golang.org/x/website/internal/backport/go/scanner"
+	"golang.org/x/website/internal/backport/go/token"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+// The parser structure holds the parser's internal state.
+type parser struct {
+	file    *token.File
+	errors  scanner.ErrorList
+	scanner scanner.Scanner
+
+	// Tracing/debugging
+	mode   Mode // parsing mode
+	trace  bool // == (mode&Trace != 0)
+	indent int  // indentation used for tracing output
+
+	// Comments
+	comments    []*ast.CommentGroup
+	leadComment *ast.CommentGroup // last lead comment
+	lineComment *ast.CommentGroup // last line comment
+
+	// Next token
+	pos token.Pos   // token position
+	tok token.Token // one token look-ahead
+	lit string      // token literal
+
+	// Error recovery
+	// (used to limit the number of calls to parser.advance
+	// w/o making scanning progress - avoids potential endless
+	// loops across multiple parser functions during error recovery)
+	syncPos token.Pos // last synchronization position
+	syncCnt int       // number of parser.advance calls without progress
+
+	// Non-syntactic parser control
+	exprLev int  // < 0: in control clause, >= 0: in expression
+	inRhs   bool // if set, the parser is parsing a rhs expression
+
+	imports []*ast.ImportSpec // list of imports
+}
+
+func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode Mode) {
+	p.file = fset.AddFile(filename, -1, len(src))
+	var m scanner.Mode
+	if mode&ParseComments != 0 {
+		m = scanner.ScanComments
+	}
+	eh := func(pos token.Position, msg string) { p.errors.Add(pos, msg) }
+	p.scanner.Init(p.file, src, eh, m)
+
+	p.mode = mode
+	p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)
+	p.next()
+}
+
+func (p *parser) allowGenerics() bool { return p.mode&typeparams.DisallowParsing == 0 }
+func (p *parser) allowTypeSets() bool { return p.mode&typeparams.DisallowTypeSets == 0 }
+
+// ----------------------------------------------------------------------------
+// Parsing support
+
+func (p *parser) printTrace(a ...interface{}) {
+	const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
+	const n = len(dots)
+	pos := p.file.Position(p.pos)
+	fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
+	i := 2 * p.indent
+	for i > n {
+		fmt.Print(dots)
+		i -= n
+	}
+	// i <= n
+	fmt.Print(dots[0:i])
+	fmt.Println(a...)
+}
+
+func trace(p *parser, msg string) *parser {
+	p.printTrace(msg, "(")
+	p.indent++
+	return p
+}
+
+// Usage pattern: defer un(trace(p, "..."))
+func un(p *parser) {
+	p.indent--
+	p.printTrace(")")
+}
+
+// Advance to the next token.
+func (p *parser) next0() {
+	// Because of one-token look-ahead, print the previous token
+	// when tracing as it provides a more readable output. The
+	// very first token (!p.pos.IsValid()) is not initialized
+	// (it is token.ILLEGAL), so don't print it.
+	if p.trace && p.pos.IsValid() {
+		s := p.tok.String()
+		switch {
+		case p.tok.IsLiteral():
+			p.printTrace(s, p.lit)
+		case p.tok.IsOperator(), p.tok.IsKeyword():
+			p.printTrace("\"" + s + "\"")
+		default:
+			p.printTrace(s)
+		}
+	}
+
+	p.pos, p.tok, p.lit = p.scanner.Scan()
+}
+
+// Consume a comment and return it and the line on which it ends.
+func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
+	// /*-style comments may end on a different line than where they start.
+	// Scan the comment for '\n' chars and adjust endline accordingly.
+	endline = p.file.Line(p.pos)
+	if p.lit[1] == '*' {
+		// don't use range here - no need to decode Unicode code points
+		for i := 0; i < len(p.lit); i++ {
+			if p.lit[i] == '\n' {
+				endline++
+			}
+		}
+	}
+
+	comment = &ast.Comment{Slash: p.pos, Text: p.lit}
+	p.next0()
+
+	return
+}
+
+// Consume a group of adjacent comments, add it to the parser's
+// comments list, and return it together with the line at which
+// the last comment in the group ends. A non-comment token or n
+// empty lines terminate a comment group.
+func (p *parser) consumeCommentGroup(n int) (comments *ast.CommentGroup, endline int) {
+	var list []*ast.Comment
+	endline = p.file.Line(p.pos)
+	for p.tok == token.COMMENT && p.file.Line(p.pos) <= endline+n {
+		var comment *ast.Comment
+		comment, endline = p.consumeComment()
+		list = append(list, comment)
+	}
+
+	// add comment group to the comments list
+	comments = &ast.CommentGroup{List: list}
+	p.comments = append(p.comments, comments)
+
+	return
+}
+
+// Advance to the next non-comment token. In the process, collect
+// any comment groups encountered, and remember the last lead and
+// line comments.
+//
+// A lead comment is a comment group that starts and ends in a
+// line without any other tokens and that is followed by a non-comment
+// token on the line immediately after the comment group.
+//
+// A line comment is a comment group that follows a non-comment
+// token on the same line, and that has no tokens after it on the line
+// where it ends.
+//
+// Lead and line comments may be considered documentation that is
+// stored in the AST.
+func (p *parser) next() {
+	p.leadComment = nil
+	p.lineComment = nil
+	prev := p.pos
+	p.next0()
+
+	if p.tok == token.COMMENT {
+		var comment *ast.CommentGroup
+		var endline int
+
+		if p.file.Line(p.pos) == p.file.Line(prev) {
+			// The comment is on same line as the previous token; it
+			// cannot be a lead comment but may be a line comment.
+			comment, endline = p.consumeCommentGroup(0)
+			if p.file.Line(p.pos) != endline || p.tok == token.EOF {
+				// The next token is on a different line, thus
+				// the last comment group is a line comment.
+				p.lineComment = comment
+			}
+		}
+
+		// consume successor comments, if any
+		endline = -1
+		for p.tok == token.COMMENT {
+			comment, endline = p.consumeCommentGroup(1)
+		}
+
+		if endline+1 == p.file.Line(p.pos) {
+			// The next token is following on the line immediately after the
+			// comment group, thus the last comment group is a lead comment.
+			p.leadComment = comment
+		}
+	}
+}
+
+// A bailout panic is raised to indicate early termination.
+type bailout struct{}
+
+func (p *parser) error(pos token.Pos, msg string) {
+	if p.trace {
+		defer un(trace(p, "error: "+msg))
+	}
+
+	epos := p.file.Position(pos)
+
+	// If AllErrors is not set, discard errors reported on the same line
+	// as the last recorded error and stop parsing if there are more than
+	// 10 errors.
+	if p.mode&AllErrors == 0 {
+		n := len(p.errors)
+		if n > 0 && p.errors[n-1].Pos.Line == epos.Line {
+			return // discard - likely a spurious error
+		}
+		if n > 10 {
+			panic(bailout{})
+		}
+	}
+
+	p.errors.Add(epos, msg)
+}
+
+func (p *parser) errorExpected(pos token.Pos, msg string) {
+	msg = "expected " + msg
+	if pos == p.pos {
+		// the error happened at the current position;
+		// make the error message more specific
+		switch {
+		case p.tok == token.SEMICOLON && p.lit == "\n":
+			msg += ", found newline"
+		case p.tok.IsLiteral():
+			// print 123 rather than 'INT', etc.
+			msg += ", found " + p.lit
+		default:
+			msg += ", found '" + p.tok.String() + "'"
+		}
+	}
+	p.error(pos, msg)
+}
+
+func (p *parser) expect(tok token.Token) token.Pos {
+	pos := p.pos
+	if p.tok != tok {
+		p.errorExpected(pos, "'"+tok.String()+"'")
+	}
+	p.next() // make progress
+	return pos
+}
+
+// expect2 is like expect, but it returns an invalid position
+// if the expected token is not found.
+func (p *parser) expect2(tok token.Token) (pos token.Pos) {
+	if p.tok == tok {
+		pos = p.pos
+	} else {
+		p.errorExpected(p.pos, "'"+tok.String()+"'")
+	}
+	p.next() // make progress
+	return
+}
+
+// expectClosing is like expect but provides a better error message
+// for the common case of a missing comma before a newline.
+func (p *parser) expectClosing(tok token.Token, context string) token.Pos {
+	if p.tok != tok && p.tok == token.SEMICOLON && p.lit == "\n" {
+		p.error(p.pos, "missing ',' before newline in "+context)
+		p.next()
+	}
+	return p.expect(tok)
+}
+
+func (p *parser) expectSemi() {
+	// semicolon is optional before a closing ')' or '}'
+	if p.tok != token.RPAREN && p.tok != token.RBRACE {
+		switch p.tok {
+		case token.COMMA:
+			// permit a ',' instead of a ';' but complain
+			p.errorExpected(p.pos, "';'")
+			fallthrough
+		case token.SEMICOLON:
+			p.next()
+		default:
+			p.errorExpected(p.pos, "';'")
+			p.advance(stmtStart)
+		}
+	}
+}
+
+func (p *parser) atComma(context string, follow token.Token) bool {
+	if p.tok == token.COMMA {
+		return true
+	}
+	if p.tok != follow {
+		msg := "missing ','"
+		if p.tok == token.SEMICOLON && p.lit == "\n" {
+			msg += " before newline"
+		}
+		p.error(p.pos, msg+" in "+context)
+		return true // "insert" comma and continue
+	}
+	return false
+}
+
+func assert(cond bool, msg string) {
+	if !cond {
+		panic("golang.org/x/website/internal/backport/go/parser internal error: " + msg)
+	}
+}
+
+// advance consumes tokens until the current token p.tok
+// is in the 'to' set, or token.EOF. For error recovery.
+func (p *parser) advance(to map[token.Token]bool) {
+	for ; p.tok != token.EOF; p.next() {
+		if to[p.tok] {
+			// Return only if parser made some progress since last
+			// sync or if it has not reached 10 advance calls without
+			// progress. Otherwise consume at least one token to
+			// avoid an endless parser loop (it is possible that
+			// both parseOperand and parseStmt call advance and
+			// correctly do not advance, thus the need for the
+			// invocation limit p.syncCnt).
+			if p.pos == p.syncPos && p.syncCnt < 10 {
+				p.syncCnt++
+				return
+			}
+			if p.pos > p.syncPos {
+				p.syncPos = p.pos
+				p.syncCnt = 0
+				return
+			}
+			// Reaching here indicates a parser bug, likely an
+			// incorrect token list in this function, but it only
+			// leads to skipping of possibly correct code if a
+			// previous error is present, and thus is preferred
+			// over a non-terminating parse.
+		}
+	}
+}
+
+var stmtStart = map[token.Token]bool{
+	token.BREAK:       true,
+	token.CONST:       true,
+	token.CONTINUE:    true,
+	token.DEFER:       true,
+	token.FALLTHROUGH: true,
+	token.FOR:         true,
+	token.GO:          true,
+	token.GOTO:        true,
+	token.IF:          true,
+	token.RETURN:      true,
+	token.SELECT:      true,
+	token.SWITCH:      true,
+	token.TYPE:        true,
+	token.VAR:         true,
+}
+
+var declStart = map[token.Token]bool{
+	token.CONST: true,
+	token.TYPE:  true,
+	token.VAR:   true,
+}
+
+var exprEnd = map[token.Token]bool{
+	token.COMMA:     true,
+	token.COLON:     true,
+	token.SEMICOLON: true,
+	token.RPAREN:    true,
+	token.RBRACK:    true,
+	token.RBRACE:    true,
+}
+
+// safePos returns a valid file position for a given position: If pos
+// is valid to begin with, safePos returns pos. If pos is out-of-range,
+// safePos returns the EOF position.
+//
+// This is hack to work around "artificial" end positions in the AST which
+// are computed by adding 1 to (presumably valid) token positions. If the
+// token positions are invalid due to parse errors, the resulting end position
+// may be past the file's EOF position, which would lead to panics if used
+// later on.
+func (p *parser) safePos(pos token.Pos) (res token.Pos) {
+	defer func() {
+		if recover() != nil {
+			res = token.Pos(p.file.Base() + p.file.Size()) // EOF position
+		}
+	}()
+	_ = p.file.Offset(pos) // trigger a panic if position is out-of-range
+	return pos
+}
+
+// ----------------------------------------------------------------------------
+// Identifiers
+
+func (p *parser) parseIdent() *ast.Ident {
+	pos := p.pos
+	name := "_"
+	if p.tok == token.IDENT {
+		name = p.lit
+		p.next()
+	} else {
+		p.expect(token.IDENT) // use expect() error handling
+	}
+	return &ast.Ident{NamePos: pos, Name: name}
+}
+
+func (p *parser) parseIdentList() (list []*ast.Ident) {
+	if p.trace {
+		defer un(trace(p, "IdentList"))
+	}
+
+	list = append(list, p.parseIdent())
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseIdent())
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Common productions
+
+// If lhs is set, result list elements which are identifiers are not resolved.
+func (p *parser) parseExprList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ExpressionList"))
+	}
+
+	list = append(list, p.checkExpr(p.parseExpr()))
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.checkExpr(p.parseExpr()))
+	}
+
+	return
+}
+
+func (p *parser) parseList(inRhs bool) []ast.Expr {
+	old := p.inRhs
+	p.inRhs = inRhs
+	list := p.parseExprList()
+	p.inRhs = old
+	return list
+}
+
+// ----------------------------------------------------------------------------
+// Types
+
+func (p *parser) parseType() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Type"))
+	}
+
+	typ := p.tryIdentOrType()
+
+	if typ == nil {
+		pos := p.pos
+		p.errorExpected(pos, "type")
+		p.advance(exprEnd)
+		return &ast.BadExpr{From: pos, To: p.pos}
+	}
+
+	return typ
+}
+
+func (p *parser) parseQualifiedIdent(ident *ast.Ident) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "QualifiedIdent"))
+	}
+
+	typ := p.parseTypeName(ident)
+	if p.tok == token.LBRACK && p.allowGenerics() {
+		typ = p.parseTypeInstance(typ)
+	}
+
+	return typ
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) parseTypeName(ident *ast.Ident) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "TypeName"))
+	}
+
+	if ident == nil {
+		ident = p.parseIdent()
+	}
+
+	if p.tok == token.PERIOD {
+		// ident is a package name
+		p.next()
+		sel := p.parseIdent()
+		return &ast.SelectorExpr{X: ident, Sel: sel}
+	}
+
+	return ident
+}
+
+// "[" has already been consumed, and lbrack is its position.
+// If len != nil it is the already consumed array length.
+func (p *parser) parseArrayType(lbrack token.Pos, len ast.Expr) *ast.ArrayType {
+	if p.trace {
+		defer un(trace(p, "ArrayType"))
+	}
+
+	if len == nil {
+		p.exprLev++
+		// always permit ellipsis for more fault-tolerant parsing
+		if p.tok == token.ELLIPSIS {
+			len = &ast.Ellipsis{Ellipsis: p.pos}
+			p.next()
+		} else if p.tok != token.RBRACK {
+			len = p.parseRhs()
+		}
+		p.exprLev--
+	}
+	if p.tok == token.COMMA {
+		// Trailing commas are accepted in type parameter
+		// lists but not in array type declarations.
+		// Accept for better error handling but complain.
+		p.error(p.pos, "unexpected comma; expecting ]")
+		p.next()
+	}
+	p.expect(token.RBRACK)
+	elt := p.parseType()
+	return &ast.ArrayType{Lbrack: lbrack, Len: len, Elt: elt}
+}
+
+func (p *parser) parseArrayFieldOrTypeInstance(x *ast.Ident) (*ast.Ident, ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ArrayFieldOrTypeInstance"))
+	}
+
+	// TODO(gri) Should we allow a trailing comma in a type argument
+	//           list such as T[P,]? (We do in parseTypeInstance).
+	lbrack := p.expect(token.LBRACK)
+	var args []ast.Expr
+	var firstComma token.Pos
+	// TODO(rfindley): consider changing parseRhsOrType so that this function variable
+	// is not needed.
+	argparser := p.parseRhsOrType
+	if !p.allowGenerics() {
+		argparser = p.parseRhs
+	}
+	if p.tok != token.RBRACK {
+		p.exprLev++
+		args = append(args, argparser())
+		for p.tok == token.COMMA {
+			if !firstComma.IsValid() {
+				firstComma = p.pos
+			}
+			p.next()
+			args = append(args, argparser())
+		}
+		p.exprLev--
+	}
+	rbrack := p.expect(token.RBRACK)
+
+	if len(args) == 0 {
+		// x []E
+		elt := p.parseType()
+		return x, &ast.ArrayType{Lbrack: lbrack, Elt: elt}
+	}
+
+	// x [P]E or x[P]
+	if len(args) == 1 {
+		elt := p.tryIdentOrType()
+		if elt != nil {
+			// x [P]E
+			return x, &ast.ArrayType{Lbrack: lbrack, Len: args[0], Elt: elt}
+		}
+		if !p.allowGenerics() {
+			p.error(rbrack, "missing element type in array type expression")
+			return nil, &ast.BadExpr{From: args[0].Pos(), To: args[0].End()}
+		}
+	}
+
+	if !p.allowGenerics() {
+		p.error(firstComma, "expected ']', found ','")
+		return x, &ast.BadExpr{From: args[0].Pos(), To: args[len(args)-1].End()}
+	}
+
+	// x[P], x[P1, P2], ...
+	return nil, typeparams.PackIndexExpr(x, lbrack, args, rbrack)
+}
+
+func (p *parser) parseFieldDecl() *ast.Field {
+	if p.trace {
+		defer un(trace(p, "FieldDecl"))
+	}
+
+	doc := p.leadComment
+
+	var names []*ast.Ident
+	var typ ast.Expr
+	if p.tok == token.IDENT {
+		name := p.parseIdent()
+		if p.tok == token.PERIOD || p.tok == token.STRING || p.tok == token.SEMICOLON || p.tok == token.RBRACE {
+			// embedded type
+			typ = name
+			if p.tok == token.PERIOD {
+				typ = p.parseQualifiedIdent(name)
+			}
+		} else {
+			// name1, name2, ... T
+			names = []*ast.Ident{name}
+			for p.tok == token.COMMA {
+				p.next()
+				names = append(names, p.parseIdent())
+			}
+			// Careful dance: We don't know if we have an embedded instantiated
+			// type T[P1, P2, ...] or a field T of array type []E or [P]E.
+			if len(names) == 1 && p.tok == token.LBRACK {
+				name, typ = p.parseArrayFieldOrTypeInstance(name)
+				if name == nil {
+					names = nil
+				}
+			} else {
+				// T P
+				typ = p.parseType()
+			}
+		}
+	} else {
+		// embedded, possibly generic type
+		// (using the enclosing parentheses to distinguish it from a named field declaration)
+		// TODO(rFindley) confirm that this doesn't allow parenthesized embedded type
+		typ = p.parseType()
+	}
+
+	var tag *ast.BasicLit
+	if p.tok == token.STRING {
+		tag = &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
+		p.next()
+	}
+
+	p.expectSemi() // call before accessing p.linecomment
+
+	field := &ast.Field{Doc: doc, Names: names, Type: typ, Tag: tag, Comment: p.lineComment}
+	return field
+}
+
+func (p *parser) parseStructType() *ast.StructType {
+	if p.trace {
+		defer un(trace(p, "StructType"))
+	}
+
+	pos := p.expect(token.STRUCT)
+	lbrace := p.expect(token.LBRACE)
+	var list []*ast.Field
+	for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
+		// a field declaration cannot start with a '(' but we accept
+		// it here for more robust parsing and better error messages
+		// (parseFieldDecl will check and complain if necessary)
+		list = append(list, p.parseFieldDecl())
+	}
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.StructType{
+		Struct: pos,
+		Fields: &ast.FieldList{
+			Opening: lbrace,
+			List:    list,
+			Closing: rbrace,
+		},
+	}
+}
+
+func (p *parser) parsePointerType() *ast.StarExpr {
+	if p.trace {
+		defer un(trace(p, "PointerType"))
+	}
+
+	star := p.expect(token.MUL)
+	base := p.parseType()
+
+	return &ast.StarExpr{Star: star, X: base}
+}
+
+func (p *parser) parseDotsType() *ast.Ellipsis {
+	if p.trace {
+		defer un(trace(p, "DotsType"))
+	}
+
+	pos := p.expect(token.ELLIPSIS)
+	elt := p.parseType()
+
+	return &ast.Ellipsis{Ellipsis: pos, Elt: elt}
+}
+
+type field struct {
+	name *ast.Ident
+	typ  ast.Expr
+}
+
+func (p *parser) parseParamDecl(name *ast.Ident, typeSetsOK bool) (f field) {
+	// TODO(rFindley) refactor to be more similar to paramDeclOrNil in the syntax
+	// package
+	if p.trace {
+		defer un(trace(p, "ParamDeclOrNil"))
+	}
+
+	ptok := p.tok
+	if name != nil {
+		p.tok = token.IDENT // force token.IDENT case in switch below
+	} else if typeSetsOK && p.tok == token.TILDE {
+		// "~" ...
+		return field{nil, p.embeddedElem(nil)}
+	}
+
+	switch p.tok {
+	case token.IDENT:
+		// name
+		if name != nil {
+			f.name = name
+			p.tok = ptok
+		} else {
+			f.name = p.parseIdent()
+		}
+		switch p.tok {
+		case token.IDENT, token.MUL, token.ARROW, token.FUNC, token.CHAN, token.MAP, token.STRUCT, token.INTERFACE, token.LPAREN:
+			// name type
+			f.typ = p.parseType()
+
+		case token.LBRACK:
+			// name "[" type1, ..., typeN "]" or name "[" n "]" type
+			f.name, f.typ = p.parseArrayFieldOrTypeInstance(f.name)
+
+		case token.ELLIPSIS:
+			// name "..." type
+			f.typ = p.parseDotsType()
+			return // don't allow ...type "|" ...
+
+		case token.PERIOD:
+			// name "." ...
+			f.typ = p.parseQualifiedIdent(f.name)
+			f.name = nil
+
+		case token.TILDE:
+			if typeSetsOK {
+				f.typ = p.embeddedElem(nil)
+				return
+			}
+
+		case token.OR:
+			if typeSetsOK {
+				// name "|" typeset
+				f.typ = p.embeddedElem(f.name)
+				f.name = nil
+				return
+			}
+		}
+
+	case token.MUL, token.ARROW, token.FUNC, token.LBRACK, token.CHAN, token.MAP, token.STRUCT, token.INTERFACE, token.LPAREN:
+		// type
+		f.typ = p.parseType()
+
+	case token.ELLIPSIS:
+		// "..." type
+		// (always accepted)
+		f.typ = p.parseDotsType()
+		return // don't allow ...type "|" ...
+
+	default:
+		// TODO(rfindley): this looks incorrect in the case of type parameter
+		// lists.
+		p.errorExpected(p.pos, ")")
+		p.advance(exprEnd)
+	}
+
+	// [name] type "|"
+	if typeSetsOK && p.tok == token.OR && f.typ != nil {
+		f.typ = p.embeddedElem(f.typ)
+	}
+
+	return
+}
+
+func (p *parser) parseParameterList(name0 *ast.Ident, typ0 ast.Expr, closing token.Token) (params []*ast.Field) {
+	if p.trace {
+		defer un(trace(p, "ParameterList"))
+	}
+
+	// Type parameters are the only parameter list closed by ']'.
+	tparams := closing == token.RBRACK
+	// Type set notation is ok in type parameter lists.
+	typeSetsOK := tparams && p.allowTypeSets()
+
+	pos := p.pos
+	if name0 != nil {
+		pos = name0.Pos()
+	}
+
+	var list []field
+	var named int // number of parameters that have an explicit name and type
+
+	for name0 != nil || p.tok != closing && p.tok != token.EOF {
+		var par field
+		if typ0 != nil {
+			if typeSetsOK {
+				typ0 = p.embeddedElem(typ0)
+			}
+			par = field{name0, typ0}
+		} else {
+			par = p.parseParamDecl(name0, typeSetsOK)
+		}
+		name0 = nil // 1st name was consumed if present
+		typ0 = nil  // 1st typ was consumed if present
+		if par.name != nil || par.typ != nil {
+			list = append(list, par)
+			if par.name != nil && par.typ != nil {
+				named++
+			}
+		}
+		if !p.atComma("parameter list", closing) {
+			break
+		}
+		p.next()
+	}
+
+	if len(list) == 0 {
+		return // not uncommon
+	}
+
+	// TODO(gri) parameter distribution and conversion to []*ast.Field
+	//           can be combined and made more efficient
+
+	// distribute parameter types
+	if named == 0 {
+		// all unnamed => found names are type names
+		for i := 0; i < len(list); i++ {
+			par := &list[i]
+			if typ := par.name; typ != nil {
+				par.typ = typ
+				par.name = nil
+			}
+		}
+		if tparams {
+			p.error(pos, "all type parameters must be named")
+		}
+	} else if named != len(list) {
+		// some named => all must be named
+		ok := true
+		var typ ast.Expr
+		missingName := pos
+		for i := len(list) - 1; i >= 0; i-- {
+			if par := &list[i]; par.typ != nil {
+				typ = par.typ
+				if par.name == nil {
+					ok = false
+					missingName = par.typ.Pos()
+					n := ast.NewIdent("_")
+					n.NamePos = typ.Pos() // correct position
+					par.name = n
+				}
+			} else if typ != nil {
+				par.typ = typ
+			} else {
+				// par.typ == nil && typ == nil => we only have a par.name
+				ok = false
+				missingName = par.name.Pos()
+				par.typ = &ast.BadExpr{From: par.name.Pos(), To: p.pos}
+			}
+		}
+		if !ok {
+			if tparams {
+				p.error(missingName, "all type parameters must be named")
+			} else {
+				p.error(pos, "mixed named and unnamed parameters")
+			}
+		}
+	}
+
+	// convert list []*ast.Field
+	if named == 0 {
+		// parameter list consists of types only
+		for _, par := range list {
+			assert(par.typ != nil, "nil type in unnamed parameter list")
+			params = append(params, &ast.Field{Type: par.typ})
+		}
+		return
+	}
+
+	// parameter list consists of named parameters with types
+	var names []*ast.Ident
+	var typ ast.Expr
+	addParams := func() {
+		assert(typ != nil, "nil type in named parameter list")
+		field := &ast.Field{Names: names, Type: typ}
+		params = append(params, field)
+		names = nil
+	}
+	for _, par := range list {
+		if par.typ != typ {
+			if len(names) > 0 {
+				addParams()
+			}
+			typ = par.typ
+		}
+		names = append(names, par.name)
+	}
+	if len(names) > 0 {
+		addParams()
+	}
+	return
+}
+
+func (p *parser) parseParameters(acceptTParams bool) (tparams, params *ast.FieldList) {
+	if p.trace {
+		defer un(trace(p, "Parameters"))
+	}
+
+	if p.allowGenerics() && acceptTParams && p.tok == token.LBRACK {
+		opening := p.pos
+		p.next()
+		// [T any](params) syntax
+		list := p.parseParameterList(nil, nil, token.RBRACK)
+		rbrack := p.expect(token.RBRACK)
+		tparams = &ast.FieldList{Opening: opening, List: list, Closing: rbrack}
+		// Type parameter lists must not be empty.
+		if tparams.NumFields() == 0 {
+			p.error(tparams.Closing, "empty type parameter list")
+			tparams = nil // avoid follow-on errors
+		}
+	}
+
+	opening := p.expect(token.LPAREN)
+
+	var fields []*ast.Field
+	if p.tok != token.RPAREN {
+		fields = p.parseParameterList(nil, nil, token.RPAREN)
+	}
+
+	rparen := p.expect(token.RPAREN)
+	params = &ast.FieldList{Opening: opening, List: fields, Closing: rparen}
+
+	return
+}
+
+func (p *parser) parseResult() *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Result"))
+	}
+
+	if p.tok == token.LPAREN {
+		_, results := p.parseParameters(false)
+		return results
+	}
+
+	typ := p.tryIdentOrType()
+	if typ != nil {
+		list := make([]*ast.Field, 1)
+		list[0] = &ast.Field{Type: typ}
+		return &ast.FieldList{List: list}
+	}
+
+	return nil
+}
+
+func (p *parser) parseFuncType() *ast.FuncType {
+	if p.trace {
+		defer un(trace(p, "FuncType"))
+	}
+
+	pos := p.expect(token.FUNC)
+	tparams, params := p.parseParameters(true)
+	if tparams != nil {
+		p.error(tparams.Pos(), "function type must have no type parameters")
+	}
+	results := p.parseResult()
+
+	return &ast.FuncType{Func: pos, Params: params, Results: results}
+}
+
+func (p *parser) parseMethodSpec() *ast.Field {
+	if p.trace {
+		defer un(trace(p, "MethodSpec"))
+	}
+
+	doc := p.leadComment
+	var idents []*ast.Ident
+	var typ ast.Expr
+	x := p.parseTypeName(nil)
+	if ident, _ := x.(*ast.Ident); ident != nil {
+		switch {
+		case p.tok == token.LBRACK && p.allowGenerics():
+			// generic method or embedded instantiated type
+			lbrack := p.pos
+			p.next()
+			p.exprLev++
+			x := p.parseExpr()
+			p.exprLev--
+			if name0, _ := x.(*ast.Ident); name0 != nil && p.tok != token.COMMA && p.tok != token.RBRACK {
+				// generic method m[T any]
+				//
+				// Interface methods do not have type parameters. We parse them for a
+				// better error message and improved error recovery.
+				_ = p.parseParameterList(name0, nil, token.RBRACK)
+				_ = p.expect(token.RBRACK)
+				p.error(lbrack, "interface method must have no type parameters")
+
+				// TODO(rfindley) refactor to share code with parseFuncType.
+				_, params := p.parseParameters(false)
+				results := p.parseResult()
+				idents = []*ast.Ident{ident}
+				typ = &ast.FuncType{
+					Func:    token.NoPos,
+					Params:  params,
+					Results: results,
+				}
+			} else {
+				// embedded instantiated type
+				// TODO(rfindley) should resolve all identifiers in x.
+				list := []ast.Expr{x}
+				if p.atComma("type argument list", token.RBRACK) {
+					p.exprLev++
+					p.next()
+					for p.tok != token.RBRACK && p.tok != token.EOF {
+						list = append(list, p.parseType())
+						if !p.atComma("type argument list", token.RBRACK) {
+							break
+						}
+						p.next()
+					}
+					p.exprLev--
+				}
+				rbrack := p.expectClosing(token.RBRACK, "type argument list")
+				typ = typeparams.PackIndexExpr(ident, lbrack, list, rbrack)
+			}
+		case p.tok == token.LPAREN:
+			// ordinary method
+			// TODO(rfindley) refactor to share code with parseFuncType.
+			_, params := p.parseParameters(false)
+			results := p.parseResult()
+			idents = []*ast.Ident{ident}
+			typ = &ast.FuncType{Func: token.NoPos, Params: params, Results: results}
+		default:
+			// embedded type
+			typ = x
+		}
+	} else {
+		// embedded, possibly instantiated type
+		typ = x
+		if p.tok == token.LBRACK && p.allowGenerics() {
+			// embedded instantiated interface
+			typ = p.parseTypeInstance(typ)
+		}
+	}
+
+	// Comment is added at the callsite: the field below may joined with
+	// additional type specs using '|'.
+	// TODO(rfindley) this should be refactored.
+	// TODO(rfindley) add more tests for comment handling.
+	return &ast.Field{Doc: doc, Names: idents, Type: typ}
+}
+
+func (p *parser) embeddedElem(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "EmbeddedElem"))
+	}
+	if x == nil {
+		x = p.embeddedTerm()
+	}
+	for p.tok == token.OR {
+		t := new(ast.BinaryExpr)
+		t.OpPos = p.pos
+		t.Op = token.OR
+		p.next()
+		t.X = x
+		t.Y = p.embeddedTerm()
+		x = t
+	}
+	return x
+}
+
+func (p *parser) embeddedTerm() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "EmbeddedTerm"))
+	}
+	if p.tok == token.TILDE {
+		t := new(ast.UnaryExpr)
+		t.OpPos = p.pos
+		t.Op = token.TILDE
+		p.next()
+		t.X = p.parseType()
+		return t
+	}
+
+	t := p.tryIdentOrType()
+	if t == nil {
+		pos := p.pos
+		p.errorExpected(pos, "~ term or type")
+		p.advance(exprEnd)
+		return &ast.BadExpr{From: pos, To: p.pos}
+	}
+
+	return t
+}
+
+func (p *parser) parseInterfaceType() *ast.InterfaceType {
+	if p.trace {
+		defer un(trace(p, "InterfaceType"))
+	}
+
+	pos := p.expect(token.INTERFACE)
+	lbrace := p.expect(token.LBRACE)
+
+	var list []*ast.Field
+
+parseElements:
+	for {
+		switch {
+		case p.tok == token.IDENT:
+			f := p.parseMethodSpec()
+			if f.Names == nil && p.allowGenerics() {
+				f.Type = p.embeddedElem(f.Type)
+			}
+			p.expectSemi()
+			f.Comment = p.lineComment
+			list = append(list, f)
+		case p.tok == token.TILDE && p.allowGenerics():
+			typ := p.embeddedElem(nil)
+			p.expectSemi()
+			comment := p.lineComment
+			list = append(list, &ast.Field{Type: typ, Comment: comment})
+		case p.allowGenerics():
+			if t := p.tryIdentOrType(); t != nil {
+				typ := p.embeddedElem(t)
+				p.expectSemi()
+				comment := p.lineComment
+				list = append(list, &ast.Field{Type: typ, Comment: comment})
+			} else {
+				break parseElements
+			}
+		default:
+			break parseElements
+		}
+	}
+
+	// TODO(rfindley): the error produced here could be improved, since we could
+	// accept a identifier, 'type', or a '}' at this point.
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.InterfaceType{
+		Interface: pos,
+		Methods: &ast.FieldList{
+			Opening: lbrace,
+			List:    list,
+			Closing: rbrace,
+		},
+	}
+}
+
+func (p *parser) parseMapType() *ast.MapType {
+	if p.trace {
+		defer un(trace(p, "MapType"))
+	}
+
+	pos := p.expect(token.MAP)
+	p.expect(token.LBRACK)
+	key := p.parseType()
+	p.expect(token.RBRACK)
+	value := p.parseType()
+
+	return &ast.MapType{Map: pos, Key: key, Value: value}
+}
+
+func (p *parser) parseChanType() *ast.ChanType {
+	if p.trace {
+		defer un(trace(p, "ChanType"))
+	}
+
+	pos := p.pos
+	dir := ast.SEND | ast.RECV
+	var arrow token.Pos
+	if p.tok == token.CHAN {
+		p.next()
+		if p.tok == token.ARROW {
+			arrow = p.pos
+			p.next()
+			dir = ast.SEND
+		}
+	} else {
+		arrow = p.expect(token.ARROW)
+		p.expect(token.CHAN)
+		dir = ast.RECV
+	}
+	value := p.parseType()
+
+	return &ast.ChanType{Begin: pos, Arrow: arrow, Dir: dir, Value: value}
+}
+
+func (p *parser) parseTypeInstance(typ ast.Expr) ast.Expr {
+	assert(p.allowGenerics(), "parseTypeInstance while not parsing type params")
+	if p.trace {
+		defer un(trace(p, "TypeInstance"))
+	}
+
+	opening := p.expect(token.LBRACK)
+	p.exprLev++
+	var list []ast.Expr
+	for p.tok != token.RBRACK && p.tok != token.EOF {
+		list = append(list, p.parseType())
+		if !p.atComma("type argument list", token.RBRACK) {
+			break
+		}
+		p.next()
+	}
+	p.exprLev--
+
+	closing := p.expectClosing(token.RBRACK, "type argument list")
+
+	if len(list) == 0 {
+		p.errorExpected(closing, "type argument list")
+		return &ast.IndexExpr{
+			X:      typ,
+			Lbrack: opening,
+			Index:  &ast.BadExpr{From: opening + 1, To: closing},
+			Rbrack: closing,
+		}
+	}
+
+	return typeparams.PackIndexExpr(typ, opening, list, closing)
+}
+
+func (p *parser) tryIdentOrType() ast.Expr {
+	switch p.tok {
+	case token.IDENT:
+		typ := p.parseTypeName(nil)
+		if p.tok == token.LBRACK && p.allowGenerics() {
+			typ = p.parseTypeInstance(typ)
+		}
+		return typ
+	case token.LBRACK:
+		lbrack := p.expect(token.LBRACK)
+		return p.parseArrayType(lbrack, nil)
+	case token.STRUCT:
+		return p.parseStructType()
+	case token.MUL:
+		return p.parsePointerType()
+	case token.FUNC:
+		typ := p.parseFuncType()
+		return typ
+	case token.INTERFACE:
+		return p.parseInterfaceType()
+	case token.MAP:
+		return p.parseMapType()
+	case token.CHAN, token.ARROW:
+		return p.parseChanType()
+	case token.LPAREN:
+		lparen := p.pos
+		p.next()
+		typ := p.parseType()
+		rparen := p.expect(token.RPAREN)
+		return &ast.ParenExpr{Lparen: lparen, X: typ, Rparen: rparen}
+	}
+
+	// no type found
+	return nil
+}
+
+// ----------------------------------------------------------------------------
+// Blocks
+
+func (p *parser) parseStmtList() (list []ast.Stmt) {
+	if p.trace {
+		defer un(trace(p, "StatementList"))
+	}
+
+	for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
+		list = append(list, p.parseStmt())
+	}
+
+	return
+}
+
+func (p *parser) parseBody() *ast.BlockStmt {
+	if p.trace {
+		defer un(trace(p, "Body"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	list := p.parseStmtList()
+	rbrace := p.expect2(token.RBRACE)
+
+	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
+}
+
+func (p *parser) parseBlockStmt() *ast.BlockStmt {
+	if p.trace {
+		defer un(trace(p, "BlockStmt"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	list := p.parseStmtList()
+	rbrace := p.expect2(token.RBRACE)
+
+	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
+}
+
+// ----------------------------------------------------------------------------
+// Expressions
+
+func (p *parser) parseFuncTypeOrLit() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "FuncTypeOrLit"))
+	}
+
+	typ := p.parseFuncType()
+	if p.tok != token.LBRACE {
+		// function type only
+		return typ
+	}
+
+	p.exprLev++
+	body := p.parseBody()
+	p.exprLev--
+
+	return &ast.FuncLit{Type: typ, Body: body}
+}
+
+// parseOperand may return an expression or a raw type (incl. array
+// types of the form [...]T. Callers must verify the result.
+func (p *parser) parseOperand() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Operand"))
+	}
+
+	switch p.tok {
+	case token.IDENT:
+		x := p.parseIdent()
+		return x
+
+	case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
+		x := &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
+		p.next()
+		return x
+
+	case token.LPAREN:
+		lparen := p.pos
+		p.next()
+		p.exprLev++
+		x := p.parseRhsOrType() // types may be parenthesized: (some type)
+		p.exprLev--
+		rparen := p.expect(token.RPAREN)
+		return &ast.ParenExpr{Lparen: lparen, X: x, Rparen: rparen}
+
+	case token.FUNC:
+		return p.parseFuncTypeOrLit()
+	}
+
+	if typ := p.tryIdentOrType(); typ != nil { // do not consume trailing type parameters
+		// could be type for composite literal or conversion
+		_, isIdent := typ.(*ast.Ident)
+		assert(!isIdent, "type cannot be identifier")
+		return typ
+	}
+
+	// we have an error
+	pos := p.pos
+	p.errorExpected(pos, "operand")
+	p.advance(stmtStart)
+	return &ast.BadExpr{From: pos, To: p.pos}
+}
+
+func (p *parser) parseSelector(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Selector"))
+	}
+
+	sel := p.parseIdent()
+
+	return &ast.SelectorExpr{X: x, Sel: sel}
+}
+
+func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "TypeAssertion"))
+	}
+
+	lparen := p.expect(token.LPAREN)
+	var typ ast.Expr
+	if p.tok == token.TYPE {
+		// type switch: typ == nil
+		p.next()
+	} else {
+		typ = p.parseType()
+	}
+	rparen := p.expect(token.RPAREN)
+
+	return &ast.TypeAssertExpr{X: x, Type: typ, Lparen: lparen, Rparen: rparen}
+}
+
+func (p *parser) parseIndexOrSliceOrInstance(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "parseIndexOrSliceOrInstance"))
+	}
+
+	lbrack := p.expect(token.LBRACK)
+	if p.tok == token.RBRACK {
+		// empty index, slice or index expressions are not permitted;
+		// accept them for parsing tolerance, but complain
+		p.errorExpected(p.pos, "operand")
+		rbrack := p.pos
+		p.next()
+		return &ast.IndexExpr{
+			X:      x,
+			Lbrack: lbrack,
+			Index:  &ast.BadExpr{From: rbrack, To: rbrack},
+			Rbrack: rbrack,
+		}
+	}
+	p.exprLev++
+
+	const N = 3 // change the 3 to 2 to disable 3-index slices
+	var args []ast.Expr
+	var index [N]ast.Expr
+	var colons [N - 1]token.Pos
+	var firstComma token.Pos
+	if p.tok != token.COLON {
+		// We can't know if we have an index expression or a type instantiation;
+		// so even if we see a (named) type we are not going to be in type context.
+		index[0] = p.parseRhsOrType()
+	}
+	ncolons := 0
+	switch p.tok {
+	case token.COLON:
+		// slice expression
+		for p.tok == token.COLON && ncolons < len(colons) {
+			colons[ncolons] = p.pos
+			ncolons++
+			p.next()
+			if p.tok != token.COLON && p.tok != token.RBRACK && p.tok != token.EOF {
+				index[ncolons] = p.parseRhs()
+			}
+		}
+	case token.COMMA:
+		firstComma = p.pos
+		// instance expression
+		args = append(args, index[0])
+		for p.tok == token.COMMA {
+			p.next()
+			if p.tok != token.RBRACK && p.tok != token.EOF {
+				args = append(args, p.parseType())
+			}
+		}
+	}
+
+	p.exprLev--
+	rbrack := p.expect(token.RBRACK)
+
+	if ncolons > 0 {
+		// slice expression
+		slice3 := false
+		if ncolons == 2 {
+			slice3 = true
+			// Check presence of 2nd and 3rd index here rather than during type-checking
+			// to prevent erroneous programs from passing through gofmt (was issue 7305).
+			if index[1] == nil {
+				p.error(colons[0], "2nd index required in 3-index slice")
+				index[1] = &ast.BadExpr{From: colons[0] + 1, To: colons[1]}
+			}
+			if index[2] == nil {
+				p.error(colons[1], "3rd index required in 3-index slice")
+				index[2] = &ast.BadExpr{From: colons[1] + 1, To: rbrack}
+			}
+		}
+		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Slice3: slice3, Rbrack: rbrack}
+	}
+
+	if len(args) == 0 {
+		// index expression
+		return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}
+	}
+
+	if !p.allowGenerics() {
+		p.error(firstComma, "expected ']' or ':', found ','")
+		return &ast.BadExpr{From: args[0].Pos(), To: args[len(args)-1].End()}
+	}
+
+	// instance expression
+	return typeparams.PackIndexExpr(x, lbrack, args, rbrack)
+}
+
+func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
+	if p.trace {
+		defer un(trace(p, "CallOrConversion"))
+	}
+
+	lparen := p.expect(token.LPAREN)
+	p.exprLev++
+	var list []ast.Expr
+	var ellipsis token.Pos
+	for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
+		list = append(list, p.parseRhsOrType()) // builtins may expect a type: make(some type, ...)
+		if p.tok == token.ELLIPSIS {
+			ellipsis = p.pos
+			p.next()
+		}
+		if !p.atComma("argument list", token.RPAREN) {
+			break
+		}
+		p.next()
+	}
+	p.exprLev--
+	rparen := p.expectClosing(token.RPAREN, "argument list")
+
+	return &ast.CallExpr{Fun: fun, Lparen: lparen, Args: list, Ellipsis: ellipsis, Rparen: rparen}
+}
+
+func (p *parser) parseValue() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Element"))
+	}
+
+	if p.tok == token.LBRACE {
+		return p.parseLiteralValue(nil)
+	}
+
+	x := p.checkExpr(p.parseExpr())
+
+	return x
+}
+
+func (p *parser) parseElement() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Element"))
+	}
+
+	x := p.parseValue()
+	if p.tok == token.COLON {
+		colon := p.pos
+		p.next()
+		x = &ast.KeyValueExpr{Key: x, Colon: colon, Value: p.parseValue()}
+	}
+
+	return x
+}
+
+func (p *parser) parseElementList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ElementList"))
+	}
+
+	for p.tok != token.RBRACE && p.tok != token.EOF {
+		list = append(list, p.parseElement())
+		if !p.atComma("composite literal", token.RBRACE) {
+			break
+		}
+		p.next()
+	}
+
+	return
+}
+
+func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "LiteralValue"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	var elts []ast.Expr
+	p.exprLev++
+	if p.tok != token.RBRACE {
+		elts = p.parseElementList()
+	}
+	p.exprLev--
+	rbrace := p.expectClosing(token.RBRACE, "composite literal")
+	return &ast.CompositeLit{Type: typ, Lbrace: lbrace, Elts: elts, Rbrace: rbrace}
+}
+
+// checkExpr checks that x is an expression (and not a type).
+func (p *parser) checkExpr(x ast.Expr) ast.Expr {
+	switch unparen(x).(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.BasicLit:
+	case *ast.FuncLit:
+	case *ast.CompositeLit:
+	case *ast.ParenExpr:
+		panic("unreachable")
+	case *ast.SelectorExpr:
+	case *ast.IndexExpr:
+	case *ast.IndexListExpr:
+	case *ast.SliceExpr:
+	case *ast.TypeAssertExpr:
+		// If t.Type == nil we have a type assertion of the form
+		// y.(type), which is only allowed in type switch expressions.
+		// It's hard to exclude those but for the case where we are in
+		// a type switch. Instead be lenient and test this in the type
+		// checker.
+	case *ast.CallExpr:
+	case *ast.StarExpr:
+	case *ast.UnaryExpr:
+	case *ast.BinaryExpr:
+	default:
+		// all other nodes are not proper expressions
+		p.errorExpected(x.Pos(), "expression")
+		x = &ast.BadExpr{From: x.Pos(), To: p.safePos(x.End())}
+	}
+	return x
+}
+
+// If x is of the form (T), unparen returns unparen(T), otherwise it returns x.
+func unparen(x ast.Expr) ast.Expr {
+	if p, isParen := x.(*ast.ParenExpr); isParen {
+		x = unparen(p.X)
+	}
+	return x
+}
+
+// checkExprOrType checks that x is an expression or a type
+// (and not a raw type such as [...]T).
+func (p *parser) checkExprOrType(x ast.Expr) ast.Expr {
+	switch t := unparen(x).(type) {
+	case *ast.ParenExpr:
+		panic("unreachable")
+	case *ast.ArrayType:
+		if len, isEllipsis := t.Len.(*ast.Ellipsis); isEllipsis {
+			p.error(len.Pos(), "expected array length, found '...'")
+			x = &ast.BadExpr{From: x.Pos(), To: p.safePos(x.End())}
+		}
+	}
+
+	// all other nodes are expressions or types
+	return x
+}
+
+func (p *parser) parsePrimaryExpr(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "PrimaryExpr"))
+	}
+
+	if x == nil {
+		x = p.parseOperand()
+	}
+	for {
+		switch p.tok {
+		case token.PERIOD:
+			p.next()
+			switch p.tok {
+			case token.IDENT:
+				x = p.parseSelector(p.checkExprOrType(x))
+			case token.LPAREN:
+				x = p.parseTypeAssertion(p.checkExpr(x))
+			default:
+				pos := p.pos
+				p.errorExpected(pos, "selector or type assertion")
+				// TODO(rFindley) The check for token.RBRACE below is a targeted fix
+				//                to error recovery sufficient to make the x/tools tests to
+				//                pass with the new parsing logic introduced for type
+				//                parameters. Remove this once error recovery has been
+				//                more generally reconsidered.
+				if p.tok != token.RBRACE {
+					p.next() // make progress
+				}
+				sel := &ast.Ident{NamePos: pos, Name: "_"}
+				x = &ast.SelectorExpr{X: x, Sel: sel}
+			}
+		case token.LBRACK:
+			x = p.parseIndexOrSliceOrInstance(p.checkExpr(x))
+		case token.LPAREN:
+			x = p.parseCallOrConversion(p.checkExprOrType(x))
+		case token.LBRACE:
+			// operand may have returned a parenthesized complit
+			// type; accept it but complain if we have a complit
+			t := unparen(x)
+			// determine if '{' belongs to a composite literal or a block statement
+			switch t.(type) {
+			case *ast.BadExpr, *ast.Ident, *ast.SelectorExpr:
+				if p.exprLev < 0 {
+					return x
+				}
+				// x is possibly a composite literal type
+			case *ast.IndexExpr, *ast.IndexListExpr:
+				if p.exprLev < 0 {
+					return x
+				}
+				// x is possibly a composite literal type
+			case *ast.ArrayType, *ast.StructType, *ast.MapType:
+				// x is a composite literal type
+			default:
+				return x
+			}
+			if t != x {
+				p.error(t.Pos(), "cannot parenthesize type in composite literal")
+				// already progressed, no need to advance
+			}
+			x = p.parseLiteralValue(x)
+		default:
+			return x
+		}
+	}
+}
+
+func (p *parser) parseUnaryExpr() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "UnaryExpr"))
+	}
+
+	switch p.tok {
+	case token.ADD, token.SUB, token.NOT, token.XOR, token.AND:
+		pos, op := p.pos, p.tok
+		p.next()
+		x := p.parseUnaryExpr()
+		return &ast.UnaryExpr{OpPos: pos, Op: op, X: p.checkExpr(x)}
+
+	case token.ARROW:
+		// channel type or receive expression
+		arrow := p.pos
+		p.next()
+
+		// If the next token is token.CHAN we still don't know if it
+		// is a channel type or a receive operation - we only know
+		// once we have found the end of the unary expression. There
+		// are two cases:
+		//
+		//   <- type  => (<-type) must be channel type
+		//   <- expr  => <-(expr) is a receive from an expression
+		//
+		// In the first case, the arrow must be re-associated with
+		// the channel type parsed already:
+		//
+		//   <- (chan type)    =>  (<-chan type)
+		//   <- (chan<- type)  =>  (<-chan (<-type))
+
+		x := p.parseUnaryExpr()
+
+		// determine which case we have
+		if typ, ok := x.(*ast.ChanType); ok {
+			// (<-type)
+
+			// re-associate position info and <-
+			dir := ast.SEND
+			for ok && dir == ast.SEND {
+				if typ.Dir == ast.RECV {
+					// error: (<-type) is (<-(<-chan T))
+					p.errorExpected(typ.Arrow, "'chan'")
+				}
+				arrow, typ.Begin, typ.Arrow = typ.Arrow, arrow, arrow
+				dir, typ.Dir = typ.Dir, ast.RECV
+				typ, ok = typ.Value.(*ast.ChanType)
+			}
+			if dir == ast.SEND {
+				p.errorExpected(arrow, "channel type")
+			}
+
+			return x
+		}
+
+		// <-(expr)
+		return &ast.UnaryExpr{OpPos: arrow, Op: token.ARROW, X: p.checkExpr(x)}
+
+	case token.MUL:
+		// pointer type or unary "*" expression
+		pos := p.pos
+		p.next()
+		x := p.parseUnaryExpr()
+		return &ast.StarExpr{Star: pos, X: p.checkExprOrType(x)}
+	}
+
+	return p.parsePrimaryExpr(nil)
+}
+
+func (p *parser) tokPrec() (token.Token, int) {
+	tok := p.tok
+	if p.inRhs && tok == token.ASSIGN {
+		tok = token.EQL
+	}
+	return tok, tok.Precedence()
+}
+
+// parseBinaryExpr parses a (possibly) binary expression.
+// If x is non-nil, it is used as the left operand.
+// If check is true, operands are checked to be valid expressions.
+//
+// TODO(rfindley): parseBinaryExpr has become overloaded. Consider refactoring.
+func (p *parser) parseBinaryExpr(x ast.Expr, prec1 int, check bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "BinaryExpr"))
+	}
+
+	if x == nil {
+		x = p.parseUnaryExpr()
+	}
+	for {
+		op, oprec := p.tokPrec()
+		if oprec < prec1 {
+			return x
+		}
+		pos := p.expect(op)
+		y := p.parseBinaryExpr(nil, oprec+1, check)
+		if check {
+			x = p.checkExpr(x)
+			y = p.checkExpr(y)
+		}
+		x = &ast.BinaryExpr{X: x, OpPos: pos, Op: op, Y: y}
+	}
+}
+
+// checkBinaryExpr checks binary expressions that were not already checked by
+// parseBinaryExpr, because the latter was called with check=false.
+func (p *parser) checkBinaryExpr(x ast.Expr) {
+	bx, ok := x.(*ast.BinaryExpr)
+	if !ok {
+		return
+	}
+
+	bx.X = p.checkExpr(bx.X)
+	bx.Y = p.checkExpr(bx.Y)
+
+	// parseBinaryExpr checks x and y for each binary expr in a tree, so we
+	// traverse the tree of binary exprs starting from x.
+	p.checkBinaryExpr(bx.X)
+	p.checkBinaryExpr(bx.Y)
+}
+
+// The result may be a type or even a raw type ([...]int). Callers must
+// check the result (using checkExpr or checkExprOrType), depending on
+// context.
+func (p *parser) parseExpr() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Expression"))
+	}
+
+	return p.parseBinaryExpr(nil, token.LowestPrec+1, true)
+}
+
+func (p *parser) parseRhs() ast.Expr {
+	old := p.inRhs
+	p.inRhs = true
+	x := p.checkExpr(p.parseExpr())
+	p.inRhs = old
+	return x
+}
+
+func (p *parser) parseRhsOrType() ast.Expr {
+	old := p.inRhs
+	p.inRhs = true
+	x := p.checkExprOrType(p.parseExpr())
+	p.inRhs = old
+	return x
+}
+
+// ----------------------------------------------------------------------------
+// Statements
+
+// Parsing modes for parseSimpleStmt.
+const (
+	basic = iota
+	labelOk
+	rangeOk
+)
+
+// parseSimpleStmt returns true as 2nd result if it parsed the assignment
+// of a range clause (with mode == rangeOk). The returned statement is an
+// assignment with a right-hand side that is a single unary expression of
+// the form "range x". No guarantees are given for the left-hand side.
+func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
+	if p.trace {
+		defer un(trace(p, "SimpleStmt"))
+	}
+
+	x := p.parseList(false)
+
+	switch p.tok {
+	case
+		token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
+		token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
+		token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,
+		token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
+		// assignment statement, possibly part of a range clause
+		pos, tok := p.pos, p.tok
+		p.next()
+		var y []ast.Expr
+		isRange := false
+		if mode == rangeOk && p.tok == token.RANGE && (tok == token.DEFINE || tok == token.ASSIGN) {
+			pos := p.pos
+			p.next()
+			y = []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
+			isRange = true
+		} else {
+			y = p.parseList(true)
+		}
+		as := &ast.AssignStmt{Lhs: x, TokPos: pos, Tok: tok, Rhs: y}
+		if tok == token.DEFINE {
+			p.checkAssignStmt(as)
+		}
+		return as, isRange
+	}
+
+	if len(x) > 1 {
+		p.errorExpected(x[0].Pos(), "1 expression")
+		// continue with first expression
+	}
+
+	switch p.tok {
+	case token.COLON:
+		// labeled statement
+		colon := p.pos
+		p.next()
+		if label, isIdent := x[0].(*ast.Ident); mode == labelOk && isIdent {
+			// Go spec: The scope of a label is the body of the function
+			// in which it is declared and excludes the body of any nested
+			// function.
+			stmt := &ast.LabeledStmt{Label: label, Colon: colon, Stmt: p.parseStmt()}
+			return stmt, false
+		}
+		// The label declaration typically starts at x[0].Pos(), but the label
+		// declaration may be erroneous due to a token after that position (and
+		// before the ':'). If SpuriousErrors is not set, the (only) error
+		// reported for the line is the illegal label error instead of the token
+		// before the ':' that caused the problem. Thus, use the (latest) colon
+		// position for error reporting.
+		p.error(colon, "illegal label declaration")
+		return &ast.BadStmt{From: x[0].Pos(), To: colon + 1}, false
+
+	case token.ARROW:
+		// send statement
+		arrow := p.pos
+		p.next()
+		y := p.parseRhs()
+		return &ast.SendStmt{Chan: x[0], Arrow: arrow, Value: y}, false
+
+	case token.INC, token.DEC:
+		// increment or decrement
+		s := &ast.IncDecStmt{X: x[0], TokPos: p.pos, Tok: p.tok}
+		p.next()
+		return s, false
+	}
+
+	// expression
+	return &ast.ExprStmt{X: x[0]}, false
+}
+
+func (p *parser) checkAssignStmt(as *ast.AssignStmt) {
+	for _, x := range as.Lhs {
+		if _, isIdent := x.(*ast.Ident); !isIdent {
+			p.errorExpected(x.Pos(), "identifier on left side of :=")
+		}
+	}
+}
+
+func (p *parser) parseCallExpr(callType string) *ast.CallExpr {
+	x := p.parseRhsOrType() // could be a conversion: (some type)(x)
+	if call, isCall := x.(*ast.CallExpr); isCall {
+		return call
+	}
+	if _, isBad := x.(*ast.BadExpr); !isBad {
+		// only report error if it's a new one
+		p.error(p.safePos(x.End()), fmt.Sprintf("function must be invoked in %s statement", callType))
+	}
+	return nil
+}
+
+func (p *parser) parseGoStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "GoStmt"))
+	}
+
+	pos := p.expect(token.GO)
+	call := p.parseCallExpr("go")
+	p.expectSemi()
+	if call == nil {
+		return &ast.BadStmt{From: pos, To: pos + 2} // len("go")
+	}
+
+	return &ast.GoStmt{Go: pos, Call: call}
+}
+
+func (p *parser) parseDeferStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "DeferStmt"))
+	}
+
+	pos := p.expect(token.DEFER)
+	call := p.parseCallExpr("defer")
+	p.expectSemi()
+	if call == nil {
+		return &ast.BadStmt{From: pos, To: pos + 5} // len("defer")
+	}
+
+	return &ast.DeferStmt{Defer: pos, Call: call}
+}
+
+func (p *parser) parseReturnStmt() *ast.ReturnStmt {
+	if p.trace {
+		defer un(trace(p, "ReturnStmt"))
+	}
+
+	pos := p.pos
+	p.expect(token.RETURN)
+	var x []ast.Expr
+	if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
+		x = p.parseList(true)
+	}
+	p.expectSemi()
+
+	return &ast.ReturnStmt{Return: pos, Results: x}
+}
+
+func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
+	if p.trace {
+		defer un(trace(p, "BranchStmt"))
+	}
+
+	pos := p.expect(tok)
+	var label *ast.Ident
+	if tok != token.FALLTHROUGH && p.tok == token.IDENT {
+		label = p.parseIdent()
+	}
+	p.expectSemi()
+
+	return &ast.BranchStmt{TokPos: pos, Tok: tok, Label: label}
+}
+
+func (p *parser) makeExpr(s ast.Stmt, want string) ast.Expr {
+	if s == nil {
+		return nil
+	}
+	if es, isExpr := s.(*ast.ExprStmt); isExpr {
+		return p.checkExpr(es.X)
+	}
+	found := "simple statement"
+	if _, isAss := s.(*ast.AssignStmt); isAss {
+		found = "assignment"
+	}
+	p.error(s.Pos(), fmt.Sprintf("expected %s, found %s (missing parentheses around composite literal?)", want, found))
+	return &ast.BadExpr{From: s.Pos(), To: p.safePos(s.End())}
+}
+
+// parseIfHeader is an adjusted version of parser.header
+// in cmd/compile/internal/syntax/parser.go, which has
+// been tuned for better error handling.
+func (p *parser) parseIfHeader() (init ast.Stmt, cond ast.Expr) {
+	if p.tok == token.LBRACE {
+		p.error(p.pos, "missing condition in if statement")
+		cond = &ast.BadExpr{From: p.pos, To: p.pos}
+		return
+	}
+	// p.tok != token.LBRACE
+
+	prevLev := p.exprLev
+	p.exprLev = -1
+
+	if p.tok != token.SEMICOLON {
+		// accept potential variable declaration but complain
+		if p.tok == token.VAR {
+			p.next()
+			p.error(p.pos, "var declaration not allowed in 'IF' initializer")
+		}
+		init, _ = p.parseSimpleStmt(basic)
+	}
+
+	var condStmt ast.Stmt
+	var semi struct {
+		pos token.Pos
+		lit string // ";" or "\n"; valid if pos.IsValid()
+	}
+	if p.tok != token.LBRACE {
+		if p.tok == token.SEMICOLON {
+			semi.pos = p.pos
+			semi.lit = p.lit
+			p.next()
+		} else {
+			p.expect(token.SEMICOLON)
+		}
+		if p.tok != token.LBRACE {
+			condStmt, _ = p.parseSimpleStmt(basic)
+		}
+	} else {
+		condStmt = init
+		init = nil
+	}
+
+	if condStmt != nil {
+		cond = p.makeExpr(condStmt, "boolean expression")
+	} else if semi.pos.IsValid() {
+		if semi.lit == "\n" {
+			p.error(semi.pos, "unexpected newline, expecting { after if clause")
+		} else {
+			p.error(semi.pos, "missing condition in if statement")
+		}
+	}
+
+	// make sure we have a valid AST
+	if cond == nil {
+		cond = &ast.BadExpr{From: p.pos, To: p.pos}
+	}
+
+	p.exprLev = prevLev
+	return
+}
+
+func (p *parser) parseIfStmt() *ast.IfStmt {
+	if p.trace {
+		defer un(trace(p, "IfStmt"))
+	}
+
+	pos := p.expect(token.IF)
+
+	init, cond := p.parseIfHeader()
+	body := p.parseBlockStmt()
+
+	var else_ ast.Stmt
+	if p.tok == token.ELSE {
+		p.next()
+		switch p.tok {
+		case token.IF:
+			else_ = p.parseIfStmt()
+		case token.LBRACE:
+			else_ = p.parseBlockStmt()
+			p.expectSemi()
+		default:
+			p.errorExpected(p.pos, "if statement or block")
+			else_ = &ast.BadStmt{From: p.pos, To: p.pos}
+		}
+	} else {
+		p.expectSemi()
+	}
+
+	return &ast.IfStmt{If: pos, Init: init, Cond: cond, Body: body, Else: else_}
+}
+
+func (p *parser) parseTypeList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "TypeList"))
+	}
+
+	list = append(list, p.parseType())
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseType())
+	}
+
+	return
+}
+
+func (p *parser) parseCaseClause(typeSwitch bool) *ast.CaseClause {
+	if p.trace {
+		defer un(trace(p, "CaseClause"))
+	}
+
+	pos := p.pos
+	var list []ast.Expr
+	if p.tok == token.CASE {
+		p.next()
+		if typeSwitch {
+			list = p.parseTypeList()
+		} else {
+			list = p.parseList(true)
+		}
+	} else {
+		p.expect(token.DEFAULT)
+	}
+
+	colon := p.expect(token.COLON)
+	body := p.parseStmtList()
+
+	return &ast.CaseClause{Case: pos, List: list, Colon: colon, Body: body}
+}
+
+func isTypeSwitchAssert(x ast.Expr) bool {
+	a, ok := x.(*ast.TypeAssertExpr)
+	return ok && a.Type == nil
+}
+
+func (p *parser) isTypeSwitchGuard(s ast.Stmt) bool {
+	switch t := s.(type) {
+	case *ast.ExprStmt:
+		// x.(type)
+		return isTypeSwitchAssert(t.X)
+	case *ast.AssignStmt:
+		// v := x.(type)
+		if len(t.Lhs) == 1 && len(t.Rhs) == 1 && isTypeSwitchAssert(t.Rhs[0]) {
+			switch t.Tok {
+			case token.ASSIGN:
+				// permit v = x.(type) but complain
+				p.error(t.TokPos, "expected ':=', found '='")
+				fallthrough
+			case token.DEFINE:
+				return true
+			}
+		}
+	}
+	return false
+}
+
+func (p *parser) parseSwitchStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "SwitchStmt"))
+	}
+
+	pos := p.expect(token.SWITCH)
+
+	var s1, s2 ast.Stmt
+	if p.tok != token.LBRACE {
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok != token.SEMICOLON {
+			s2, _ = p.parseSimpleStmt(basic)
+		}
+		if p.tok == token.SEMICOLON {
+			p.next()
+			s1 = s2
+			s2 = nil
+			if p.tok != token.LBRACE {
+				// A TypeSwitchGuard may declare a variable in addition
+				// to the variable declared in the initial SimpleStmt.
+				// Introduce extra scope to avoid redeclaration errors:
+				//
+				//	switch t := 0; t := x.(T) { ... }
+				//
+				// (this code is not valid Go because the first t
+				// cannot be accessed and thus is never used, the extra
+				// scope is needed for the correct error message).
+				//
+				// If we don't have a type switch, s2 must be an expression.
+				// Having the extra nested but empty scope won't affect it.
+				s2, _ = p.parseSimpleStmt(basic)
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	typeSwitch := p.isTypeSwitchGuard(s2)
+	lbrace := p.expect(token.LBRACE)
+	var list []ast.Stmt
+	for p.tok == token.CASE || p.tok == token.DEFAULT {
+		list = append(list, p.parseCaseClause(typeSwitch))
+	}
+	rbrace := p.expect(token.RBRACE)
+	p.expectSemi()
+	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
+
+	if typeSwitch {
+		return &ast.TypeSwitchStmt{Switch: pos, Init: s1, Assign: s2, Body: body}
+	}
+
+	return &ast.SwitchStmt{Switch: pos, Init: s1, Tag: p.makeExpr(s2, "switch expression"), Body: body}
+}
+
+func (p *parser) parseCommClause() *ast.CommClause {
+	if p.trace {
+		defer un(trace(p, "CommClause"))
+	}
+
+	pos := p.pos
+	var comm ast.Stmt
+	if p.tok == token.CASE {
+		p.next()
+		lhs := p.parseList(false)
+		if p.tok == token.ARROW {
+			// SendStmt
+			if len(lhs) > 1 {
+				p.errorExpected(lhs[0].Pos(), "1 expression")
+				// continue with first expression
+			}
+			arrow := p.pos
+			p.next()
+			rhs := p.parseRhs()
+			comm = &ast.SendStmt{Chan: lhs[0], Arrow: arrow, Value: rhs}
+		} else {
+			// RecvStmt
+			if tok := p.tok; tok == token.ASSIGN || tok == token.DEFINE {
+				// RecvStmt with assignment
+				if len(lhs) > 2 {
+					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
+					// continue with first two expressions
+					lhs = lhs[0:2]
+				}
+				pos := p.pos
+				p.next()
+				rhs := p.parseRhs()
+				as := &ast.AssignStmt{Lhs: lhs, TokPos: pos, Tok: tok, Rhs: []ast.Expr{rhs}}
+				if tok == token.DEFINE {
+					p.checkAssignStmt(as)
+				}
+				comm = as
+			} else {
+				// lhs must be single receive operation
+				if len(lhs) > 1 {
+					p.errorExpected(lhs[0].Pos(), "1 expression")
+					// continue with first expression
+				}
+				comm = &ast.ExprStmt{X: lhs[0]}
+			}
+		}
+	} else {
+		p.expect(token.DEFAULT)
+	}
+
+	colon := p.expect(token.COLON)
+	body := p.parseStmtList()
+
+	return &ast.CommClause{Case: pos, Comm: comm, Colon: colon, Body: body}
+}
+
+func (p *parser) parseSelectStmt() *ast.SelectStmt {
+	if p.trace {
+		defer un(trace(p, "SelectStmt"))
+	}
+
+	pos := p.expect(token.SELECT)
+	lbrace := p.expect(token.LBRACE)
+	var list []ast.Stmt
+	for p.tok == token.CASE || p.tok == token.DEFAULT {
+		list = append(list, p.parseCommClause())
+	}
+	rbrace := p.expect(token.RBRACE)
+	p.expectSemi()
+	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
+
+	return &ast.SelectStmt{Select: pos, Body: body}
+}
+
+func (p *parser) parseForStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "ForStmt"))
+	}
+
+	pos := p.expect(token.FOR)
+
+	var s1, s2, s3 ast.Stmt
+	var isRange bool
+	if p.tok != token.LBRACE {
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok != token.SEMICOLON {
+			if p.tok == token.RANGE {
+				// "for range x" (nil lhs in assignment)
+				pos := p.pos
+				p.next()
+				y := []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
+				s2 = &ast.AssignStmt{Rhs: y}
+				isRange = true
+			} else {
+				s2, isRange = p.parseSimpleStmt(rangeOk)
+			}
+		}
+		if !isRange && p.tok == token.SEMICOLON {
+			p.next()
+			s1 = s2
+			s2 = nil
+			if p.tok != token.SEMICOLON {
+				s2, _ = p.parseSimpleStmt(basic)
+			}
+			p.expectSemi()
+			if p.tok != token.LBRACE {
+				s3, _ = p.parseSimpleStmt(basic)
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	body := p.parseBlockStmt()
+	p.expectSemi()
+
+	if isRange {
+		as := s2.(*ast.AssignStmt)
+		// check lhs
+		var key, value ast.Expr
+		switch len(as.Lhs) {
+		case 0:
+			// nothing to do
+		case 1:
+			key = as.Lhs[0]
+		case 2:
+			key, value = as.Lhs[0], as.Lhs[1]
+		default:
+			p.errorExpected(as.Lhs[len(as.Lhs)-1].Pos(), "at most 2 expressions")
+			return &ast.BadStmt{From: pos, To: p.safePos(body.End())}
+		}
+		// parseSimpleStmt returned a right-hand side that
+		// is a single unary expression of the form "range x"
+		x := as.Rhs[0].(*ast.UnaryExpr).X
+		return &ast.RangeStmt{
+			For:    pos,
+			Key:    key,
+			Value:  value,
+			TokPos: as.TokPos,
+			Tok:    as.Tok,
+			X:      x,
+			Body:   body,
+		}
+	}
+
+	// regular for statement
+	return &ast.ForStmt{
+		For:  pos,
+		Init: s1,
+		Cond: p.makeExpr(s2, "boolean or range expression"),
+		Post: s3,
+		Body: body,
+	}
+}
+
+func (p *parser) parseStmt() (s ast.Stmt) {
+	if p.trace {
+		defer un(trace(p, "Statement"))
+	}
+
+	switch p.tok {
+	case token.CONST, token.TYPE, token.VAR:
+		s = &ast.DeclStmt{Decl: p.parseDecl(stmtStart)}
+	case
+		// tokens that may start an expression
+		token.IDENT, token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operands
+		token.LBRACK, token.STRUCT, token.MAP, token.CHAN, token.INTERFACE, // composite types
+		token.ADD, token.SUB, token.MUL, token.AND, token.XOR, token.ARROW, token.NOT: // unary operators
+		s, _ = p.parseSimpleStmt(labelOk)
+		// because of the required look-ahead, labeled statements are
+		// parsed by parseSimpleStmt - don't expect a semicolon after
+		// them
+		if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
+			p.expectSemi()
+		}
+	case token.GO:
+		s = p.parseGoStmt()
+	case token.DEFER:
+		s = p.parseDeferStmt()
+	case token.RETURN:
+		s = p.parseReturnStmt()
+	case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
+		s = p.parseBranchStmt(p.tok)
+	case token.LBRACE:
+		s = p.parseBlockStmt()
+		p.expectSemi()
+	case token.IF:
+		s = p.parseIfStmt()
+	case token.SWITCH:
+		s = p.parseSwitchStmt()
+	case token.SELECT:
+		s = p.parseSelectStmt()
+	case token.FOR:
+		s = p.parseForStmt()
+	case token.SEMICOLON:
+		// Is it ever possible to have an implicit semicolon
+		// producing an empty statement in a valid program?
+		// (handle correctly anyway)
+		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: p.lit == "\n"}
+		p.next()
+	case token.RBRACE:
+		// a semicolon may be omitted before a closing "}"
+		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: true}
+	default:
+		// no statement found
+		pos := p.pos
+		p.errorExpected(pos, "statement")
+		p.advance(stmtStart)
+		s = &ast.BadStmt{From: pos, To: p.pos}
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Declarations
+
+type parseSpecFunction func(doc *ast.CommentGroup, pos token.Pos, keyword token.Token, iota int) ast.Spec
+
+func isValidImport(lit string) bool {
+	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
+	s, _ := strconv.Unquote(lit) // go/scanner returns a legal string literal
+	for _, r := range s {
+		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
+			return false
+		}
+	}
+	return s != ""
+}
+
+func (p *parser) parseImportSpec(doc *ast.CommentGroup, _ token.Pos, _ token.Token, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "ImportSpec"))
+	}
+
+	var ident *ast.Ident
+	switch p.tok {
+	case token.PERIOD:
+		ident = &ast.Ident{NamePos: p.pos, Name: "."}
+		p.next()
+	case token.IDENT:
+		ident = p.parseIdent()
+	}
+
+	pos := p.pos
+	var path string
+	if p.tok == token.STRING {
+		path = p.lit
+		if !isValidImport(path) {
+			p.error(pos, "invalid import path: "+path)
+		}
+		p.next()
+	} else {
+		p.expect(token.STRING) // use expect() error handling
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// collect imports
+	spec := &ast.ImportSpec{
+		Doc:     doc,
+		Name:    ident,
+		Path:    &ast.BasicLit{ValuePos: pos, Kind: token.STRING, Value: path},
+		Comment: p.lineComment,
+	}
+	p.imports = append(p.imports, spec)
+
+	return spec
+}
+
+func (p *parser) parseValueSpec(doc *ast.CommentGroup, _ token.Pos, keyword token.Token, iota int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, keyword.String()+"Spec"))
+	}
+
+	pos := p.pos
+	idents := p.parseIdentList()
+	typ := p.tryIdentOrType()
+	var values []ast.Expr
+	// always permit optional initialization for more tolerant parsing
+	if p.tok == token.ASSIGN {
+		p.next()
+		values = p.parseList(true)
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	switch keyword {
+	case token.VAR:
+		if typ == nil && values == nil {
+			p.error(pos, "missing variable type or initialization")
+		}
+	case token.CONST:
+		if values == nil && (iota == 0 || typ != nil) {
+			p.error(pos, "missing constant value")
+		}
+	}
+
+	spec := &ast.ValueSpec{
+		Doc:     doc,
+		Names:   idents,
+		Type:    typ,
+		Values:  values,
+		Comment: p.lineComment,
+	}
+	return spec
+}
+
+func (p *parser) parseGenericType(spec *ast.TypeSpec, openPos token.Pos, name0 *ast.Ident, typ0 ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "parseGenericType"))
+	}
+
+	list := p.parseParameterList(name0, typ0, token.RBRACK)
+	closePos := p.expect(token.RBRACK)
+	spec.TypeParams = &ast.FieldList{Opening: openPos, List: list, Closing: closePos}
+	// Let the type checker decide whether to accept type parameters on aliases:
+	// see issue #46477.
+	if p.tok == token.ASSIGN {
+		// type alias
+		spec.Assign = p.pos
+		p.next()
+	}
+	spec.Type = p.parseType()
+}
+
+func (p *parser) parseTypeSpec(doc *ast.CommentGroup, _ token.Pos, _ token.Token, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "TypeSpec"))
+	}
+
+	ident := p.parseIdent()
+	spec := &ast.TypeSpec{Doc: doc, Name: ident}
+
+	if p.tok == token.LBRACK && p.allowGenerics() {
+		lbrack := p.pos
+		p.next()
+		if p.tok == token.IDENT {
+			// We may have an array type or a type parameter list.
+			// In either case we expect an expression x (which may
+			// just be a name, or a more complex expression) which
+			// we can analyze further.
+			//
+			// A type parameter list may have a type bound starting
+			// with a "[" as in: P []E. In that case, simply parsing
+			// an expression would lead to an error: P[] is invalid.
+			// But since index or slice expressions are never constant
+			// and thus invalid array length expressions, if we see a
+			// "[" following a name it must be the start of an array
+			// or slice constraint. Only if we don't see a "[" do we
+			// need to parse a full expression.
+
+			// Index or slice expressions are never constant and thus invalid
+			// array length expressions. Thus, if we see a "[" following name
+			// we can safely assume that "[" name starts a type parameter list.
+			var x ast.Expr = p.parseIdent()
+			if p.tok != token.LBRACK {
+				// To parse the expression starting with name, expand
+				// the call sequence we would get by passing in name
+				// to parser.expr, and pass in name to parsePrimaryExpr.
+				p.exprLev++
+				lhs := p.parsePrimaryExpr(x)
+				x = p.parseBinaryExpr(lhs, token.LowestPrec+1, false)
+				p.exprLev--
+			}
+
+			// analyze the cases
+			var pname *ast.Ident // pname != nil means pname is the type parameter name
+			var ptype ast.Expr   // ptype != nil means ptype is the type parameter type; pname != nil in this case
+
+			switch t := x.(type) {
+			case *ast.Ident:
+				// Unless we see a "]", we are at the start of a type parameter list.
+				if p.tok != token.RBRACK {
+					// d.Name "[" name ...
+					pname = t
+					// no ptype
+				}
+			case *ast.BinaryExpr:
+				// If we have an expression of the form name*T, and T is a (possibly
+				// parenthesized) type literal or the next token is a comma, we are
+				// at the start of a type parameter list.
+				if name, _ := t.X.(*ast.Ident); name != nil {
+					if t.Op == token.MUL && (isTypeLit(t.Y) || p.tok == token.COMMA) {
+						// d.Name "[" name "*" t.Y
+						// d.Name "[" name "*" t.Y ","
+						// convert t into unary *t.Y
+						pname = name
+						ptype = &ast.StarExpr{Star: t.OpPos, X: t.Y}
+					}
+				}
+				if pname == nil {
+					// A normal binary expression. Since we passed check=false, we must
+					// now check its operands.
+					p.checkBinaryExpr(t)
+				}
+			case *ast.CallExpr:
+				// If we have an expression of the form name(T), and T is a (possibly
+				// parenthesized) type literal or the next token is a comma, we are
+				// at the start of a type parameter list.
+				if name, _ := t.Fun.(*ast.Ident); name != nil {
+					if len(t.Args) == 1 && !t.Ellipsis.IsValid() && (isTypeLit(t.Args[0]) || p.tok == token.COMMA) {
+						// d.Name "[" name "(" t.ArgList[0] ")"
+						// d.Name "[" name "(" t.ArgList[0] ")" ","
+						pname = name
+						ptype = t.Args[0]
+					}
+				}
+			}
+
+			if pname != nil {
+				// d.Name "[" pname ...
+				// d.Name "[" pname ptype ...
+				// d.Name "[" pname ptype "," ...
+				p.parseGenericType(spec, lbrack, pname, ptype)
+			} else {
+				// d.Name "[" x ...
+				spec.Type = p.parseArrayType(lbrack, x)
+			}
+		} else {
+			// array type
+			spec.Type = p.parseArrayType(lbrack, nil)
+		}
+	} else {
+		// no type parameters
+		if p.tok == token.ASSIGN {
+			// type alias
+			spec.Assign = p.pos
+			p.next()
+		}
+		spec.Type = p.parseType()
+	}
+
+	p.expectSemi() // call before accessing p.linecomment
+	spec.Comment = p.lineComment
+
+	return spec
+}
+
+// isTypeLit reports whether x is a (possibly parenthesized) type literal.
+func isTypeLit(x ast.Expr) bool {
+	switch x := x.(type) {
+	case *ast.ArrayType, *ast.StructType, *ast.FuncType, *ast.InterfaceType, *ast.MapType, *ast.ChanType:
+		return true
+	case *ast.StarExpr:
+		// *T may be a pointer dereferenciation.
+		// Only consider *T as type literal if T is a type literal.
+		return isTypeLit(x.X)
+	case *ast.ParenExpr:
+		return isTypeLit(x.X)
+	}
+	return false
+}
+
+func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
+	if p.trace {
+		defer un(trace(p, "GenDecl("+keyword.String()+")"))
+	}
+
+	doc := p.leadComment
+	pos := p.expect(keyword)
+	var lparen, rparen token.Pos
+	var list []ast.Spec
+	if p.tok == token.LPAREN {
+		lparen = p.pos
+		p.next()
+		for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
+			list = append(list, f(p.leadComment, pos, keyword, iota))
+		}
+		rparen = p.expect(token.RPAREN)
+		p.expectSemi()
+	} else {
+		list = append(list, f(nil, pos, keyword, 0))
+	}
+
+	return &ast.GenDecl{
+		Doc:    doc,
+		TokPos: pos,
+		Tok:    keyword,
+		Lparen: lparen,
+		Specs:  list,
+		Rparen: rparen,
+	}
+}
+
+func (p *parser) parseFuncDecl() *ast.FuncDecl {
+	if p.trace {
+		defer un(trace(p, "FunctionDecl"))
+	}
+
+	doc := p.leadComment
+	pos := p.expect(token.FUNC)
+
+	var recv *ast.FieldList
+	if p.tok == token.LPAREN {
+		_, recv = p.parseParameters(false)
+	}
+
+	ident := p.parseIdent()
+
+	tparams, params := p.parseParameters(true)
+	if recv != nil && tparams != nil {
+		// Method declarations do not have type parameters. We parse them for a
+		// better error message and improved error recovery.
+		p.error(tparams.Opening, "method must have no type parameters")
+		tparams = nil
+	}
+	results := p.parseResult()
+
+	var body *ast.BlockStmt
+	switch p.tok {
+	case token.LBRACE:
+		body = p.parseBody()
+		p.expectSemi()
+	case token.SEMICOLON:
+		p.next()
+		if p.tok == token.LBRACE {
+			// opening { of function declaration on next line
+			p.error(p.pos, "unexpected semicolon or newline before {")
+			body = p.parseBody()
+			p.expectSemi()
+		}
+	default:
+		p.expectSemi()
+	}
+
+	decl := &ast.FuncDecl{
+		Doc:  doc,
+		Recv: recv,
+		Name: ident,
+		Type: &ast.FuncType{
+			Func:       pos,
+			TypeParams: tparams,
+			Params:     params,
+			Results:    results,
+		},
+		Body: body,
+	}
+	return decl
+}
+
+func (p *parser) parseDecl(sync map[token.Token]bool) ast.Decl {
+	if p.trace {
+		defer un(trace(p, "Declaration"))
+	}
+
+	var f parseSpecFunction
+	switch p.tok {
+	case token.CONST, token.VAR:
+		f = p.parseValueSpec
+
+	case token.TYPE:
+		f = p.parseTypeSpec
+
+	case token.FUNC:
+		return p.parseFuncDecl()
+
+	default:
+		pos := p.pos
+		p.errorExpected(pos, "declaration")
+		p.advance(sync)
+		return &ast.BadDecl{From: pos, To: p.pos}
+	}
+
+	return p.parseGenDecl(p.tok, f)
+}
+
+// ----------------------------------------------------------------------------
+// Source files
+
+func (p *parser) parseFile() *ast.File {
+	if p.trace {
+		defer un(trace(p, "File"))
+	}
+
+	// Don't bother parsing the rest if we had errors scanning the first token.
+	// Likely not a Go source file at all.
+	if p.errors.Len() != 0 {
+		return nil
+	}
+
+	// package clause
+	doc := p.leadComment
+	pos := p.expect(token.PACKAGE)
+	// Go spec: The package clause is not a declaration;
+	// the package name does not appear in any scope.
+	ident := p.parseIdent()
+	if ident.Name == "_" && p.mode&DeclarationErrors != 0 {
+		p.error(p.pos, "invalid package name _")
+	}
+	p.expectSemi()
+
+	// Don't bother parsing the rest if we had errors parsing the package clause.
+	// Likely not a Go source file at all.
+	if p.errors.Len() != 0 {
+		return nil
+	}
+
+	var decls []ast.Decl
+	if p.mode&PackageClauseOnly == 0 {
+		// import decls
+		for p.tok == token.IMPORT {
+			decls = append(decls, p.parseGenDecl(token.IMPORT, p.parseImportSpec))
+		}
+
+		if p.mode&ImportsOnly == 0 {
+			// rest of package body
+			for p.tok != token.EOF {
+				decls = append(decls, p.parseDecl(declStart))
+			}
+		}
+	}
+
+	f := &ast.File{
+		Doc:      doc,
+		Package:  pos,
+		Name:     ident,
+		Decls:    decls,
+		Imports:  p.imports,
+		Comments: p.comments,
+	}
+	var declErr func(token.Pos, string)
+	if p.mode&DeclarationErrors != 0 {
+		declErr = p.error
+	}
+	if p.mode&SkipObjectResolution == 0 {
+		resolveFile(f, p.file, declErr)
+	}
+
+	return f
+}
diff --git a/internal/backport/go/parser/parser_test.go b/internal/backport/go/parser/parser_test.go
new file mode 100644
index 0000000..feb05ea
--- /dev/null
+++ b/internal/backport/go/parser/parser_test.go
@@ -0,0 +1,579 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parser
+
+import (
+	"bytes"
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/token"
+	"io/fs"
+	"strings"
+	"testing"
+)
+
+var validFiles = []string{
+	"parser.go",
+	"parser_test.go",
+	"error_test.go",
+	"short_test.go",
+}
+
+func TestParse(t *testing.T) {
+	for _, filename := range validFiles {
+		_, err := ParseFile(token.NewFileSet(), filename, nil, DeclarationErrors)
+		if err != nil {
+			t.Fatalf("ParseFile(%s): %v", filename, err)
+		}
+	}
+}
+
+func nameFilter(filename string) bool {
+	switch filename {
+	case "parser.go", "interface.go", "parser_test.go":
+		return true
+	case "parser.go.orig":
+		return true // permit but should be ignored by ParseDir
+	}
+	return false
+}
+
+func dirFilter(f fs.FileInfo) bool { return nameFilter(f.Name()) }
+
+func TestParseFile(t *testing.T) {
+	src := "package p\nvar _=s[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]"
+	_, err := ParseFile(token.NewFileSet(), "", src, 0)
+	if err == nil {
+		t.Errorf("ParseFile(%s) succeeded unexpectedly", src)
+	}
+}
+
+func TestParseExprFrom(t *testing.T) {
+	src := "s[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]"
+	_, err := ParseExprFrom(token.NewFileSet(), "", src, 0)
+	if err == nil {
+		t.Errorf("ParseExprFrom(%s) succeeded unexpectedly", src)
+	}
+}
+
+func TestParseDir(t *testing.T) {
+	path := "."
+	pkgs, err := ParseDir(token.NewFileSet(), path, dirFilter, 0)
+	if err != nil {
+		t.Fatalf("ParseDir(%s): %v", path, err)
+	}
+	if n := len(pkgs); n != 1 {
+		t.Errorf("got %d packages; want 1", n)
+	}
+	pkg := pkgs["parser"]
+	if pkg == nil {
+		t.Errorf(`package "parser" not found`)
+		return
+	}
+	if n := len(pkg.Files); n != 3 {
+		t.Errorf("got %d package files; want 3", n)
+	}
+	for filename := range pkg.Files {
+		if !nameFilter(filename) {
+			t.Errorf("unexpected package file: %s", filename)
+		}
+	}
+}
+
+func TestIssue42951(t *testing.T) {
+	path := "./testdata/issue42951"
+	_, err := ParseDir(token.NewFileSet(), path, nil, 0)
+	if err != nil {
+		t.Errorf("ParseDir(%s): %v", path, err)
+	}
+}
+
+func TestParseExpr(t *testing.T) {
+	// just kicking the tires:
+	// a valid arithmetic expression
+	src := "a + b"
+	x, err := ParseExpr(src)
+	if err != nil {
+		t.Errorf("ParseExpr(%q): %v", src, err)
+	}
+	// sanity check
+	if _, ok := x.(*ast.BinaryExpr); !ok {
+		t.Errorf("ParseExpr(%q): got %T, want *ast.BinaryExpr", src, x)
+	}
+
+	// a valid type expression
+	src = "struct{x *int}"
+	x, err = ParseExpr(src)
+	if err != nil {
+		t.Errorf("ParseExpr(%q): %v", src, err)
+	}
+	// sanity check
+	if _, ok := x.(*ast.StructType); !ok {
+		t.Errorf("ParseExpr(%q): got %T, want *ast.StructType", src, x)
+	}
+
+	// an invalid expression
+	src = "a + *"
+	x, err = ParseExpr(src)
+	if err == nil {
+		t.Errorf("ParseExpr(%q): got no error", src)
+	}
+	if x == nil {
+		t.Errorf("ParseExpr(%q): got no (partial) result", src)
+	}
+	if _, ok := x.(*ast.BinaryExpr); !ok {
+		t.Errorf("ParseExpr(%q): got %T, want *ast.BinaryExpr", src, x)
+	}
+
+	// a valid expression followed by extra tokens is invalid
+	src = "a[i] := x"
+	if _, err := ParseExpr(src); err == nil {
+		t.Errorf("ParseExpr(%q): got no error", src)
+	}
+
+	// a semicolon is not permitted unless automatically inserted
+	src = "a + b\n"
+	if _, err := ParseExpr(src); err != nil {
+		t.Errorf("ParseExpr(%q): got error %s", src, err)
+	}
+	src = "a + b;"
+	if _, err := ParseExpr(src); err == nil {
+		t.Errorf("ParseExpr(%q): got no error", src)
+	}
+
+	// various other stuff following a valid expression
+	const validExpr = "a + b"
+	const anything = "dh3*#D)#_"
+	for _, c := range "!)]};," {
+		src := validExpr + string(c) + anything
+		if _, err := ParseExpr(src); err == nil {
+			t.Errorf("ParseExpr(%q): got no error", src)
+		}
+	}
+
+	// ParseExpr must not crash
+	for _, src := range valids {
+		ParseExpr(src)
+	}
+}
+
+func TestColonEqualsScope(t *testing.T) {
+	f, err := ParseFile(token.NewFileSet(), "", `package p; func f() { x, y, z := x, y, z }`, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// RHS refers to undefined globals; LHS does not.
+	as := f.Decls[0].(*ast.FuncDecl).Body.List[0].(*ast.AssignStmt)
+	for _, v := range as.Rhs {
+		id := v.(*ast.Ident)
+		if id.Obj != nil {
+			t.Errorf("rhs %s has Obj, should not", id.Name)
+		}
+	}
+	for _, v := range as.Lhs {
+		id := v.(*ast.Ident)
+		if id.Obj == nil {
+			t.Errorf("lhs %s does not have Obj, should", id.Name)
+		}
+	}
+}
+
+func TestVarScope(t *testing.T) {
+	f, err := ParseFile(token.NewFileSet(), "", `package p; func f() { var x, y, z = x, y, z }`, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// RHS refers to undefined globals; LHS does not.
+	as := f.Decls[0].(*ast.FuncDecl).Body.List[0].(*ast.DeclStmt).Decl.(*ast.GenDecl).Specs[0].(*ast.ValueSpec)
+	for _, v := range as.Values {
+		id := v.(*ast.Ident)
+		if id.Obj != nil {
+			t.Errorf("rhs %s has Obj, should not", id.Name)
+		}
+	}
+	for _, id := range as.Names {
+		if id.Obj == nil {
+			t.Errorf("lhs %s does not have Obj, should", id.Name)
+		}
+	}
+}
+
+func TestObjects(t *testing.T) {
+	const src = `
+package p
+import fmt "fmt"
+const pi = 3.14
+type T struct{}
+var x int
+func f() { L: }
+`
+
+	f, err := ParseFile(token.NewFileSet(), "", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	objects := map[string]ast.ObjKind{
+		"p":   ast.Bad, // not in a scope
+		"fmt": ast.Bad, // not resolved yet
+		"pi":  ast.Con,
+		"T":   ast.Typ,
+		"x":   ast.Var,
+		"int": ast.Bad, // not resolved yet
+		"f":   ast.Fun,
+		"L":   ast.Lbl,
+	}
+
+	ast.Inspect(f, func(n ast.Node) bool {
+		if ident, ok := n.(*ast.Ident); ok {
+			obj := ident.Obj
+			if obj == nil {
+				if objects[ident.Name] != ast.Bad {
+					t.Errorf("no object for %s", ident.Name)
+				}
+				return true
+			}
+			if obj.Name != ident.Name {
+				t.Errorf("names don't match: obj.Name = %s, ident.Name = %s", obj.Name, ident.Name)
+			}
+			kind := objects[ident.Name]
+			if obj.Kind != kind {
+				t.Errorf("%s: obj.Kind = %s; want %s", ident.Name, obj.Kind, kind)
+			}
+		}
+		return true
+	})
+}
+
+func TestUnresolved(t *testing.T) {
+	f, err := ParseFile(token.NewFileSet(), "", `
+package p
+//
+func f1a(int)
+func f2a(byte, int, float)
+func f3a(a, b int, c float)
+func f4a(...complex)
+func f5a(a s1a, b ...complex)
+//
+func f1b(*int)
+func f2b([]byte, (int), *float)
+func f3b(a, b *int, c []float)
+func f4b(...*complex)
+func f5b(a s1a, b ...[]complex)
+//
+type s1a struct { int }
+type s2a struct { byte; int; s1a }
+type s3a struct { a, b int; c float }
+//
+type s1b struct { *int }
+type s2b struct { byte; int; *float }
+type s3b struct { a, b *s3b; c []float }
+`, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want := "int " + // f1a
+		"byte int float " + // f2a
+		"int float " + // f3a
+		"complex " + // f4a
+		"complex " + // f5a
+		//
+		"int " + // f1b
+		"byte int float " + // f2b
+		"int float " + // f3b
+		"complex " + // f4b
+		"complex " + // f5b
+		//
+		"int " + // s1a
+		"byte int " + // s2a
+		"int float " + // s3a
+		//
+		"int " + // s1a
+		"byte int float " + // s2a
+		"float " // s3a
+
+	// collect unresolved identifiers
+	var buf bytes.Buffer
+	for _, u := range f.Unresolved {
+		buf.WriteString(u.Name)
+		buf.WriteByte(' ')
+	}
+	got := buf.String()
+
+	if got != want {
+		t.Errorf("\ngot:  %s\nwant: %s", got, want)
+	}
+}
+
+var imports = map[string]bool{
+	`"a"`:        true,
+	"`a`":        true,
+	`"a/b"`:      true,
+	`"a.b"`:      true,
+	`"m\x61th"`:  true,
+	`"greek/αβ"`: true,
+	`""`:         false,
+
+	// Each of these pairs tests both `` vs "" strings
+	// and also use of invalid characters spelled out as
+	// escape sequences and written directly.
+	// For example `"\x00"` tests import "\x00"
+	// while "`\x00`" tests import `<actual-NUL-byte>`.
+	`"\x00"`:     false,
+	"`\x00`":     false,
+	`"\x7f"`:     false,
+	"`\x7f`":     false,
+	`"a!"`:       false,
+	"`a!`":       false,
+	`"a b"`:      false,
+	"`a b`":      false,
+	`"a\\b"`:     false,
+	"`a\\b`":     false,
+	"\"`a`\"":    false,
+	"`\"a\"`":    false,
+	`"\x80\x80"`: false,
+	"`\x80\x80`": false,
+	`"\xFFFD"`:   false,
+	"`\xFFFD`":   false,
+}
+
+func TestImports(t *testing.T) {
+	for path, isValid := range imports {
+		src := fmt.Sprintf("package p; import %s", path)
+		_, err := ParseFile(token.NewFileSet(), "", src, 0)
+		switch {
+		case err != nil && isValid:
+			t.Errorf("ParseFile(%s): got %v; expected no error", src, err)
+		case err == nil && !isValid:
+			t.Errorf("ParseFile(%s): got no error; expected one", src)
+		}
+	}
+}
+
+func TestCommentGroups(t *testing.T) {
+	f, err := ParseFile(token.NewFileSet(), "", `
+package p /* 1a */ /* 1b */      /* 1c */ // 1d
+/* 2a
+*/
+// 2b
+const pi = 3.1415
+/* 3a */ // 3b
+/* 3c */ const e = 2.7182
+
+// Example from issue 3139
+func ExampleCount() {
+	fmt.Println(strings.Count("cheese", "e"))
+	fmt.Println(strings.Count("five", "")) // before & after each rune
+	// Output:
+	// 3
+	// 5
+}
+`, ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected := [][]string{
+		{"/* 1a */", "/* 1b */", "/* 1c */", "// 1d"},
+		{"/* 2a\n*/", "// 2b"},
+		{"/* 3a */", "// 3b", "/* 3c */"},
+		{"// Example from issue 3139"},
+		{"// before & after each rune"},
+		{"// Output:", "// 3", "// 5"},
+	}
+	if len(f.Comments) != len(expected) {
+		t.Fatalf("got %d comment groups; expected %d", len(f.Comments), len(expected))
+	}
+	for i, exp := range expected {
+		got := f.Comments[i].List
+		if len(got) != len(exp) {
+			t.Errorf("got %d comments in group %d; expected %d", len(got), i, len(exp))
+			continue
+		}
+		for j, exp := range exp {
+			got := got[j].Text
+			if got != exp {
+				t.Errorf("got %q in group %d; expected %q", got, i, exp)
+			}
+		}
+	}
+}
+
+func getField(file *ast.File, fieldname string) *ast.Field {
+	parts := strings.Split(fieldname, ".")
+	for _, d := range file.Decls {
+		if d, ok := d.(*ast.GenDecl); ok && d.Tok == token.TYPE {
+			for _, s := range d.Specs {
+				if s, ok := s.(*ast.TypeSpec); ok && s.Name.Name == parts[0] {
+					if s, ok := s.Type.(*ast.StructType); ok {
+						for _, f := range s.Fields.List {
+							for _, name := range f.Names {
+								if name.Name == parts[1] {
+									return f
+								}
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+	return nil
+}
+
+// Don't use ast.CommentGroup.Text() - we want to see exact comment text.
+func commentText(c *ast.CommentGroup) string {
+	var buf bytes.Buffer
+	if c != nil {
+		for _, c := range c.List {
+			buf.WriteString(c.Text)
+		}
+	}
+	return buf.String()
+}
+
+func checkFieldComments(t *testing.T, file *ast.File, fieldname, lead, line string) {
+	f := getField(file, fieldname)
+	if f == nil {
+		t.Fatalf("field not found: %s", fieldname)
+	}
+	if got := commentText(f.Doc); got != lead {
+		t.Errorf("got lead comment %q; expected %q", got, lead)
+	}
+	if got := commentText(f.Comment); got != line {
+		t.Errorf("got line comment %q; expected %q", got, line)
+	}
+}
+
+func TestLeadAndLineComments(t *testing.T) {
+	f, err := ParseFile(token.NewFileSet(), "", `
+package p
+type T struct {
+	/* F1 lead comment */
+	//
+	F1 int  /* F1 */ // line comment
+	// F2 lead
+	// comment
+	F2 int  // F2 line comment
+	// f3 lead comment
+	f3 int  // f3 line comment
+}
+`, ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	checkFieldComments(t, f, "T.F1", "/* F1 lead comment *///", "/* F1 */// line comment")
+	checkFieldComments(t, f, "T.F2", "// F2 lead// comment", "// F2 line comment")
+	checkFieldComments(t, f, "T.f3", "// f3 lead comment", "// f3 line comment")
+	ast.FileExports(f)
+	checkFieldComments(t, f, "T.F1", "/* F1 lead comment *///", "/* F1 */// line comment")
+	checkFieldComments(t, f, "T.F2", "// F2 lead// comment", "// F2 line comment")
+	if getField(f, "T.f3") != nil {
+		t.Error("not expected to find T.f3")
+	}
+}
+
+// TestIssue9979 verifies that empty statements are contained within their enclosing blocks.
+func TestIssue9979(t *testing.T) {
+	for _, src := range []string{
+		"package p; func f() {;}",
+		"package p; func f() {L:}",
+		"package p; func f() {L:;}",
+		"package p; func f() {L:\n}",
+		"package p; func f() {L:\n;}",
+		"package p; func f() { ; }",
+		"package p; func f() { L: }",
+		"package p; func f() { L: ; }",
+		"package p; func f() { L: \n}",
+		"package p; func f() { L: \n; }",
+	} {
+		fset := token.NewFileSet()
+		f, err := ParseFile(fset, "", src, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		var pos, end token.Pos
+		ast.Inspect(f, func(x ast.Node) bool {
+			switch s := x.(type) {
+			case *ast.BlockStmt:
+				pos, end = s.Pos()+1, s.End()-1 // exclude "{", "}"
+			case *ast.LabeledStmt:
+				pos, end = s.Pos()+2, s.End() // exclude "L:"
+			case *ast.EmptyStmt:
+				// check containment
+				if s.Pos() < pos || s.End() > end {
+					t.Errorf("%s: %T[%d, %d] not inside [%d, %d]", src, s, s.Pos(), s.End(), pos, end)
+				}
+				// check semicolon
+				offs := fset.Position(s.Pos()).Offset
+				if ch := src[offs]; ch != ';' != s.Implicit {
+					want := "want ';'"
+					if s.Implicit {
+						want = "but ';' is implicit"
+					}
+					t.Errorf("%s: found %q at offset %d; %s", src, ch, offs, want)
+				}
+			}
+			return true
+		})
+	}
+}
+
+// TestIncompleteSelection ensures that an incomplete selector
+// expression is parsed as a (blank) *ast.SelectorExpr, not a
+// *ast.BadExpr.
+func TestIncompleteSelection(t *testing.T) {
+	for _, src := range []string{
+		"package p; var _ = fmt.",             // at EOF
+		"package p; var _ = fmt.\ntype X int", // not at EOF
+	} {
+		fset := token.NewFileSet()
+		f, err := ParseFile(fset, "", src, 0)
+		if err == nil {
+			t.Errorf("ParseFile(%s) succeeded unexpectedly", src)
+			continue
+		}
+
+		const wantErr = "expected selector or type assertion"
+		if !strings.Contains(err.Error(), wantErr) {
+			t.Errorf("ParseFile returned wrong error %q, want %q", err, wantErr)
+		}
+
+		var sel *ast.SelectorExpr
+		ast.Inspect(f, func(n ast.Node) bool {
+			if n, ok := n.(*ast.SelectorExpr); ok {
+				sel = n
+			}
+			return true
+		})
+		if sel == nil {
+			t.Error("found no *ast.SelectorExpr")
+			continue
+		}
+		const wantSel = "&{fmt _}"
+		if fmt.Sprint(sel) != wantSel {
+			t.Errorf("found selector %s, want %s", sel, wantSel)
+			continue
+		}
+	}
+}
+
+func TestLastLineComment(t *testing.T) {
+	const src = `package main
+type x int // comment
+`
+	fset := token.NewFileSet()
+	f, err := ParseFile(fset, "", src, ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	comment := f.Decls[0].(*ast.GenDecl).Specs[0].(*ast.TypeSpec).Comment.List[0].Text
+	if comment != "// comment" {
+		t.Errorf("got %q, want %q", comment, "// comment")
+	}
+}
diff --git a/internal/backport/go/parser/performance_test.go b/internal/backport/go/parser/performance_test.go
new file mode 100644
index 0000000..6e7243f
--- /dev/null
+++ b/internal/backport/go/parser/performance_test.go
@@ -0,0 +1,57 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parser
+
+import (
+	"golang.org/x/website/internal/backport/go/token"
+	"os"
+	"testing"
+)
+
+// TODO(rFindley): use a testdata file or file from another package here, to
+//
+//	avoid a moving target.
+var src = readFile("parser.go")
+
+func readFile(filename string) []byte {
+	data, err := os.ReadFile(filename)
+	if err != nil {
+		panic(err)
+	}
+	return data
+}
+
+func BenchmarkParse(b *testing.B) {
+	b.SetBytes(int64(len(src)))
+	for i := 0; i < b.N; i++ {
+		if _, err := ParseFile(token.NewFileSet(), "", src, ParseComments); err != nil {
+			b.Fatalf("benchmark failed due to parse error: %s", err)
+		}
+	}
+}
+
+func BenchmarkParseOnly(b *testing.B) {
+	b.SetBytes(int64(len(src)))
+	for i := 0; i < b.N; i++ {
+		if _, err := ParseFile(token.NewFileSet(), "", src, ParseComments|SkipObjectResolution); err != nil {
+			b.Fatalf("benchmark failed due to parse error: %s", err)
+		}
+	}
+}
+
+func BenchmarkResolve(b *testing.B) {
+	b.SetBytes(int64(len(src)))
+	for i := 0; i < b.N; i++ {
+		b.StopTimer()
+		fset := token.NewFileSet()
+		file, err := ParseFile(fset, "", src, SkipObjectResolution)
+		if err != nil {
+			b.Fatalf("benchmark failed due to parse error: %s", err)
+		}
+		b.StartTimer()
+		handle := fset.File(file.Package)
+		resolveFile(file, handle, nil)
+	}
+}
diff --git a/internal/backport/go/parser/resolver.go b/internal/backport/go/parser/resolver.go
new file mode 100644
index 0000000..b648836
--- /dev/null
+++ b/internal/backport/go/parser/resolver.go
@@ -0,0 +1,607 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parser
+
+import (
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/token"
+	"strings"
+)
+
+const debugResolve = false
+
+// resolveFile walks the given file to resolve identifiers within the file
+// scope, updating ast.Ident.Obj fields with declaration information.
+//
+// If declErr is non-nil, it is used to report declaration errors during
+// resolution. tok is used to format position in error messages.
+func resolveFile(file *ast.File, handle *token.File, declErr func(token.Pos, string)) {
+	pkgScope := ast.NewScope(nil)
+	r := &resolver{
+		handle:   handle,
+		declErr:  declErr,
+		topScope: pkgScope,
+		pkgScope: pkgScope,
+		depth:    1,
+	}
+
+	for _, decl := range file.Decls {
+		ast.Walk(r, decl)
+	}
+
+	r.closeScope()
+	assert(r.topScope == nil, "unbalanced scopes")
+	assert(r.labelScope == nil, "unbalanced label scopes")
+
+	// resolve global identifiers within the same file
+	i := 0
+	for _, ident := range r.unresolved {
+		// i <= index for current ident
+		assert(ident.Obj == unresolved, "object already resolved")
+		ident.Obj = r.pkgScope.Lookup(ident.Name) // also removes unresolved sentinel
+		if ident.Obj == nil {
+			r.unresolved[i] = ident
+			i++
+		} else if debugResolve {
+			pos := ident.Obj.Decl.(interface{ Pos() token.Pos }).Pos()
+			r.trace("resolved %s@%v to package object %v", ident.Name, ident.Pos(), pos)
+		}
+	}
+	file.Scope = r.pkgScope
+	file.Unresolved = r.unresolved[0:i]
+}
+
+type resolver struct {
+	handle  *token.File
+	declErr func(token.Pos, string)
+
+	// Ordinary identifier scopes
+	pkgScope   *ast.Scope   // pkgScope.Outer == nil
+	topScope   *ast.Scope   // top-most scope; may be pkgScope
+	unresolved []*ast.Ident // unresolved identifiers
+	depth      int          // scope depth
+
+	// Label scopes
+	// (maintained by open/close LabelScope)
+	labelScope  *ast.Scope     // label scope for current function
+	targetStack [][]*ast.Ident // stack of unresolved labels
+}
+
+func (r *resolver) trace(format string, args ...interface{}) {
+	fmt.Println(strings.Repeat(". ", r.depth) + r.sprintf(format, args...))
+}
+
+func (r *resolver) sprintf(format string, args ...interface{}) string {
+	for i, arg := range args {
+		switch arg := arg.(type) {
+		case token.Pos:
+			args[i] = r.handle.Position(arg)
+		}
+	}
+	return fmt.Sprintf(format, args...)
+}
+
+func (r *resolver) openScope(pos token.Pos) {
+	if debugResolve {
+		r.trace("opening scope @%v", pos)
+		r.depth++
+	}
+	r.topScope = ast.NewScope(r.topScope)
+}
+
+func (r *resolver) closeScope() {
+	if debugResolve {
+		r.depth--
+		r.trace("closing scope")
+	}
+	r.topScope = r.topScope.Outer
+}
+
+func (r *resolver) openLabelScope() {
+	r.labelScope = ast.NewScope(r.labelScope)
+	r.targetStack = append(r.targetStack, nil)
+}
+
+func (r *resolver) closeLabelScope() {
+	// resolve labels
+	n := len(r.targetStack) - 1
+	scope := r.labelScope
+	for _, ident := range r.targetStack[n] {
+		ident.Obj = scope.Lookup(ident.Name)
+		if ident.Obj == nil && r.declErr != nil {
+			r.declErr(ident.Pos(), fmt.Sprintf("label %s undefined", ident.Name))
+		}
+	}
+	// pop label scope
+	r.targetStack = r.targetStack[0:n]
+	r.labelScope = r.labelScope.Outer
+}
+
+func (r *resolver) declare(decl, data interface{}, scope *ast.Scope, kind ast.ObjKind, idents ...*ast.Ident) {
+	for _, ident := range idents {
+		if ident.Obj != nil {
+			panic(fmt.Sprintf("%v: identifier %s already declared or resolved", ident.Pos(), ident.Name))
+		}
+		obj := ast.NewObj(kind, ident.Name)
+		// remember the corresponding declaration for redeclaration
+		// errors and global variable resolution/typechecking phase
+		obj.Decl = decl
+		obj.Data = data
+		// Identifiers (for receiver type parameters) are written to the scope, but
+		// never set as the resolved object. See issue #50956.
+		if _, ok := decl.(*ast.Ident); !ok {
+			ident.Obj = obj
+		}
+		if ident.Name != "_" {
+			if debugResolve {
+				r.trace("declaring %s@%v", ident.Name, ident.Pos())
+			}
+			if alt := scope.Insert(obj); alt != nil && r.declErr != nil {
+				prevDecl := ""
+				if pos := alt.Pos(); pos.IsValid() {
+					prevDecl = r.sprintf("\n\tprevious declaration at %v", pos)
+				}
+				r.declErr(ident.Pos(), fmt.Sprintf("%s redeclared in this block%s", ident.Name, prevDecl))
+			}
+		}
+	}
+}
+
+func (r *resolver) shortVarDecl(decl *ast.AssignStmt) {
+	// Go spec: A short variable declaration may redeclare variables
+	// provided they were originally declared in the same block with
+	// the same type, and at least one of the non-blank variables is new.
+	n := 0 // number of new variables
+	for _, x := range decl.Lhs {
+		if ident, isIdent := x.(*ast.Ident); isIdent {
+			assert(ident.Obj == nil, "identifier already declared or resolved")
+			obj := ast.NewObj(ast.Var, ident.Name)
+			// remember corresponding assignment for other tools
+			obj.Decl = decl
+			ident.Obj = obj
+			if ident.Name != "_" {
+				if debugResolve {
+					r.trace("declaring %s@%v", ident.Name, ident.Pos())
+				}
+				if alt := r.topScope.Insert(obj); alt != nil {
+					ident.Obj = alt // redeclaration
+				} else {
+					n++ // new declaration
+				}
+			}
+		}
+	}
+	if n == 0 && r.declErr != nil {
+		r.declErr(decl.Lhs[0].Pos(), "no new variables on left side of :=")
+	}
+}
+
+// The unresolved object is a sentinel to mark identifiers that have been added
+// to the list of unresolved identifiers. The sentinel is only used for verifying
+// internal consistency.
+var unresolved = new(ast.Object)
+
+// If x is an identifier, resolve attempts to resolve x by looking up
+// the object it denotes. If no object is found and collectUnresolved is
+// set, x is marked as unresolved and collected in the list of unresolved
+// identifiers.
+func (r *resolver) resolve(ident *ast.Ident, collectUnresolved bool) {
+	if ident.Obj != nil {
+		panic(r.sprintf("%v: identifier %s already declared or resolved", ident.Pos(), ident.Name))
+	}
+	// '_' should never refer to existing declarations, because it has special
+	// handling in the spec.
+	if ident.Name == "_" {
+		return
+	}
+	for s := r.topScope; s != nil; s = s.Outer {
+		if obj := s.Lookup(ident.Name); obj != nil {
+			if debugResolve {
+				r.trace("resolved %v:%s to %v", ident.Pos(), ident.Name, obj)
+			}
+			assert(obj.Name != "", "obj with no name")
+			// Identifiers (for receiver type parameters) are written to the scope,
+			// but never set as the resolved object. See issue #50956.
+			if _, ok := obj.Decl.(*ast.Ident); !ok {
+				ident.Obj = obj
+			}
+			return
+		}
+	}
+	// all local scopes are known, so any unresolved identifier
+	// must be found either in the file scope, package scope
+	// (perhaps in another file), or universe scope --- collect
+	// them so that they can be resolved later
+	if collectUnresolved {
+		ident.Obj = unresolved
+		r.unresolved = append(r.unresolved, ident)
+	}
+}
+
+func (r *resolver) walkExprs(list []ast.Expr) {
+	for _, node := range list {
+		ast.Walk(r, node)
+	}
+}
+
+func (r *resolver) walkLHS(list []ast.Expr) {
+	for _, expr := range list {
+		expr := unparen(expr)
+		if _, ok := expr.(*ast.Ident); !ok && expr != nil {
+			ast.Walk(r, expr)
+		}
+	}
+}
+
+func (r *resolver) walkStmts(list []ast.Stmt) {
+	for _, stmt := range list {
+		ast.Walk(r, stmt)
+	}
+}
+
+func (r *resolver) Visit(node ast.Node) ast.Visitor {
+	if debugResolve && node != nil {
+		r.trace("node %T@%v", node, node.Pos())
+	}
+
+	switch n := node.(type) {
+
+	// Expressions.
+	case *ast.Ident:
+		r.resolve(n, true)
+
+	case *ast.FuncLit:
+		r.openScope(n.Pos())
+		defer r.closeScope()
+		r.walkFuncType(n.Type)
+		r.walkBody(n.Body)
+
+	case *ast.SelectorExpr:
+		ast.Walk(r, n.X)
+		// Note: don't try to resolve n.Sel, as we don't support qualified
+		// resolution.
+
+	case *ast.StructType:
+		r.openScope(n.Pos())
+		defer r.closeScope()
+		r.walkFieldList(n.Fields, ast.Var)
+
+	case *ast.FuncType:
+		r.openScope(n.Pos())
+		defer r.closeScope()
+		r.walkFuncType(n)
+
+	case *ast.CompositeLit:
+		if n.Type != nil {
+			ast.Walk(r, n.Type)
+		}
+		for _, e := range n.Elts {
+			if kv, _ := e.(*ast.KeyValueExpr); kv != nil {
+				// See issue #45160: try to resolve composite lit keys, but don't
+				// collect them as unresolved if resolution failed. This replicates
+				// existing behavior when resolving during parsing.
+				if ident, _ := kv.Key.(*ast.Ident); ident != nil {
+					r.resolve(ident, false)
+				} else {
+					ast.Walk(r, kv.Key)
+				}
+				ast.Walk(r, kv.Value)
+			} else {
+				ast.Walk(r, e)
+			}
+		}
+
+	case *ast.InterfaceType:
+		r.openScope(n.Pos())
+		defer r.closeScope()
+		r.walkFieldList(n.Methods, ast.Fun)
+
+	// Statements
+	case *ast.LabeledStmt:
+		r.declare(n, nil, r.labelScope, ast.Lbl, n.Label)
+		ast.Walk(r, n.Stmt)
+
+	case *ast.AssignStmt:
+		r.walkExprs(n.Rhs)
+		if n.Tok == token.DEFINE {
+			r.shortVarDecl(n)
+		} else {
+			r.walkExprs(n.Lhs)
+		}
+
+	case *ast.BranchStmt:
+		// add to list of unresolved targets
+		if n.Tok != token.FALLTHROUGH && n.Label != nil {
+			depth := len(r.targetStack) - 1
+			r.targetStack[depth] = append(r.targetStack[depth], n.Label)
+		}
+
+	case *ast.BlockStmt:
+		r.openScope(n.Pos())
+		defer r.closeScope()
+		r.walkStmts(n.List)
+
+	case *ast.IfStmt:
+		r.openScope(n.Pos())
+		defer r.closeScope()
+		if n.Init != nil {
+			ast.Walk(r, n.Init)
+		}
+		ast.Walk(r, n.Cond)
+		ast.Walk(r, n.Body)
+		if n.Else != nil {
+			ast.Walk(r, n.Else)
+		}
+
+	case *ast.CaseClause:
+		r.walkExprs(n.List)
+		r.openScope(n.Pos())
+		defer r.closeScope()
+		r.walkStmts(n.Body)
+
+	case *ast.SwitchStmt:
+		r.openScope(n.Pos())
+		defer r.closeScope()
+		if n.Init != nil {
+			ast.Walk(r, n.Init)
+		}
+		if n.Tag != nil {
+			// The scope below reproduces some unnecessary behavior of the parser,
+			// opening an extra scope in case this is a type switch. It's not needed
+			// for expression switches.
+			// TODO: remove this once we've matched the parser resolution exactly.
+			if n.Init != nil {
+				r.openScope(n.Tag.Pos())
+				defer r.closeScope()
+			}
+			ast.Walk(r, n.Tag)
+		}
+		if n.Body != nil {
+			r.walkStmts(n.Body.List)
+		}
+
+	case *ast.TypeSwitchStmt:
+		if n.Init != nil {
+			r.openScope(n.Pos())
+			defer r.closeScope()
+			ast.Walk(r, n.Init)
+		}
+		r.openScope(n.Assign.Pos())
+		defer r.closeScope()
+		ast.Walk(r, n.Assign)
+		// s.Body consists only of case clauses, so does not get its own
+		// scope.
+		if n.Body != nil {
+			r.walkStmts(n.Body.List)
+		}
+
+	case *ast.CommClause:
+		r.openScope(n.Pos())
+		defer r.closeScope()
+		if n.Comm != nil {
+			ast.Walk(r, n.Comm)
+		}
+		r.walkStmts(n.Body)
+
+	case *ast.SelectStmt:
+		// as for switch statements, select statement bodies don't get their own
+		// scope.
+		if n.Body != nil {
+			r.walkStmts(n.Body.List)
+		}
+
+	case *ast.ForStmt:
+		r.openScope(n.Pos())
+		defer r.closeScope()
+		if n.Init != nil {
+			ast.Walk(r, n.Init)
+		}
+		if n.Cond != nil {
+			ast.Walk(r, n.Cond)
+		}
+		if n.Post != nil {
+			ast.Walk(r, n.Post)
+		}
+		ast.Walk(r, n.Body)
+
+	case *ast.RangeStmt:
+		r.openScope(n.Pos())
+		defer r.closeScope()
+		ast.Walk(r, n.X)
+		var lhs []ast.Expr
+		if n.Key != nil {
+			lhs = append(lhs, n.Key)
+		}
+		if n.Value != nil {
+			lhs = append(lhs, n.Value)
+		}
+		if len(lhs) > 0 {
+			if n.Tok == token.DEFINE {
+				// Note: we can't exactly match the behavior of object resolution
+				// during the parsing pass here, as it uses the position of the RANGE
+				// token for the RHS OpPos. That information is not contained within
+				// the AST.
+				as := &ast.AssignStmt{
+					Lhs:    lhs,
+					Tok:    token.DEFINE,
+					TokPos: n.TokPos,
+					Rhs:    []ast.Expr{&ast.UnaryExpr{Op: token.RANGE, X: n.X}},
+				}
+				// TODO(rFindley): this walkLHS reproduced the parser resolution, but
+				// is it necessary? By comparison, for a normal AssignStmt we don't
+				// walk the LHS in case there is an invalid identifier list.
+				r.walkLHS(lhs)
+				r.shortVarDecl(as)
+			} else {
+				r.walkExprs(lhs)
+			}
+		}
+		ast.Walk(r, n.Body)
+
+	// Declarations
+	case *ast.GenDecl:
+		switch n.Tok {
+		case token.CONST, token.VAR:
+			for i, spec := range n.Specs {
+				spec := spec.(*ast.ValueSpec)
+				kind := ast.Con
+				if n.Tok == token.VAR {
+					kind = ast.Var
+				}
+				r.walkExprs(spec.Values)
+				if spec.Type != nil {
+					ast.Walk(r, spec.Type)
+				}
+				r.declare(spec, i, r.topScope, kind, spec.Names...)
+			}
+		case token.TYPE:
+			for _, spec := range n.Specs {
+				spec := spec.(*ast.TypeSpec)
+				// Go spec: The scope of a type identifier declared inside a function begins
+				// at the identifier in the TypeSpec and ends at the end of the innermost
+				// containing block.
+				r.declare(spec, nil, r.topScope, ast.Typ, spec.Name)
+				if spec.TypeParams != nil {
+					r.openScope(spec.Pos())
+					defer r.closeScope()
+					r.walkTParams(spec.TypeParams)
+				}
+				ast.Walk(r, spec.Type)
+			}
+		}
+
+	case *ast.FuncDecl:
+		// Open the function scope.
+		r.openScope(n.Pos())
+		defer r.closeScope()
+
+		r.walkRecv(n.Recv)
+
+		// Type parameters are walked normally: they can reference each other, and
+		// can be referenced by normal parameters.
+		if n.Type.TypeParams != nil {
+			r.walkTParams(n.Type.TypeParams)
+			// TODO(rFindley): need to address receiver type parameters.
+		}
+
+		// Resolve and declare parameters in a specific order to get duplicate
+		// declaration errors in the correct location.
+		r.resolveList(n.Type.Params)
+		r.resolveList(n.Type.Results)
+		r.declareList(n.Recv, ast.Var)
+		r.declareList(n.Type.Params, ast.Var)
+		r.declareList(n.Type.Results, ast.Var)
+
+		r.walkBody(n.Body)
+		if n.Recv == nil && n.Name.Name != "init" {
+			r.declare(n, nil, r.pkgScope, ast.Fun, n.Name)
+		}
+
+	default:
+		return r
+	}
+
+	return nil
+}
+
+func (r *resolver) walkFuncType(typ *ast.FuncType) {
+	// typ.TypeParams must be walked separately for FuncDecls.
+	r.resolveList(typ.Params)
+	r.resolveList(typ.Results)
+	r.declareList(typ.Params, ast.Var)
+	r.declareList(typ.Results, ast.Var)
+}
+
+func (r *resolver) resolveList(list *ast.FieldList) {
+	if list == nil {
+		return
+	}
+	for _, f := range list.List {
+		if f.Type != nil {
+			ast.Walk(r, f.Type)
+		}
+	}
+}
+
+func (r *resolver) declareList(list *ast.FieldList, kind ast.ObjKind) {
+	if list == nil {
+		return
+	}
+	for _, f := range list.List {
+		r.declare(f, nil, r.topScope, kind, f.Names...)
+	}
+}
+
+func (r *resolver) walkRecv(recv *ast.FieldList) {
+	// If our receiver has receiver type parameters, we must declare them before
+	// trying to resolve the rest of the receiver, and avoid re-resolving the
+	// type parameter identifiers.
+	if recv == nil || len(recv.List) == 0 {
+		return // nothing to do
+	}
+	typ := recv.List[0].Type
+	if ptr, ok := typ.(*ast.StarExpr); ok {
+		typ = ptr.X
+	}
+
+	var declareExprs []ast.Expr // exprs to declare
+	var resolveExprs []ast.Expr // exprs to resolve
+	switch typ := typ.(type) {
+	case *ast.IndexExpr:
+		declareExprs = []ast.Expr{typ.Index}
+		resolveExprs = append(resolveExprs, typ.X)
+	case *ast.IndexListExpr:
+		declareExprs = typ.Indices
+		resolveExprs = append(resolveExprs, typ.X)
+	default:
+		resolveExprs = append(resolveExprs, typ)
+	}
+	for _, expr := range declareExprs {
+		if id, _ := expr.(*ast.Ident); id != nil {
+			r.declare(expr, nil, r.topScope, ast.Typ, id)
+		} else {
+			// The receiver type parameter expression is invalid, but try to resolve
+			// it anyway for consistency.
+			resolveExprs = append(resolveExprs, expr)
+		}
+	}
+	for _, expr := range resolveExprs {
+		if expr != nil {
+			ast.Walk(r, expr)
+		}
+	}
+	// The receiver is invalid, but try to resolve it anyway for consistency.
+	for _, f := range recv.List[1:] {
+		if f.Type != nil {
+			ast.Walk(r, f.Type)
+		}
+	}
+}
+
+func (r *resolver) walkFieldList(list *ast.FieldList, kind ast.ObjKind) {
+	if list == nil {
+		return
+	}
+	r.resolveList(list)
+	r.declareList(list, kind)
+}
+
+// walkTParams is like walkFieldList, but declares type parameters eagerly so
+// that they may be resolved in the constraint expressions held in the field
+// Type.
+func (r *resolver) walkTParams(list *ast.FieldList) {
+	r.declareList(list, ast.Typ)
+	r.resolveList(list)
+}
+
+func (r *resolver) walkBody(body *ast.BlockStmt) {
+	if body == nil {
+		return
+	}
+	r.openLabelScope()
+	defer r.closeLabelScope()
+	r.walkStmts(body.List)
+}
diff --git a/internal/backport/go/parser/resolver_test.go b/internal/backport/go/parser/resolver_test.go
new file mode 100644
index 0000000..8dd9172
--- /dev/null
+++ b/internal/backport/go/parser/resolver_test.go
@@ -0,0 +1,176 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parser
+
+import (
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/internal/typeparams"
+	"golang.org/x/website/internal/backport/go/scanner"
+	"golang.org/x/website/internal/backport/go/token"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// TestResolution checks that identifiers are resolved to the declarations
+// annotated in the source, by comparing the positions of the resulting
+// Ident.Obj.Decl to positions marked in the source via special comments.
+//
+// In the test source, any comment prefixed with '=' or '@' (or both) marks the
+// previous token position as the declaration ('=') or a use ('@') of an
+// identifier. The text following '=' and '@' in the comment string is the
+// label to use for the location.  Declaration labels must be unique within the
+// file, and use labels must refer to an existing declaration label. It's OK
+// for a comment to denote both the declaration and use of a label (e.g.
+// '=@foo'). Leading and trailing whitespace is ignored. Any comment not
+// beginning with '=' or '@' is ignored.
+func TestResolution(t *testing.T) {
+	dir := filepath.Join("testdata", "resolution")
+	fis, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, fi := range fis {
+		t.Run(fi.Name(), func(t *testing.T) {
+			fset := token.NewFileSet()
+			path := filepath.Join(dir, fi.Name())
+			src := readFile(path) // panics on failure
+			var mode Mode
+			if !strings.HasSuffix(path, ".go2") {
+				mode |= typeparams.DisallowParsing
+			}
+			file, err := ParseFile(fset, path, src, mode)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			// Compare the positions of objects resolved during parsing (fromParser)
+			// to those annotated in source comments (fromComments).
+
+			handle := fset.File(file.Package)
+			fromParser := declsFromParser(file)
+			fromComments := declsFromComments(handle, src)
+
+			pos := func(pos token.Pos) token.Position {
+				p := handle.Position(pos)
+				// The file name is implied by the subtest, so remove it to avoid
+				// clutter in error messages.
+				p.Filename = ""
+				return p
+			}
+			for k, want := range fromComments {
+				if got := fromParser[k]; got != want {
+					t.Errorf("%s resolved to %s, want %s", pos(k), pos(got), pos(want))
+				}
+				delete(fromParser, k)
+			}
+			// What remains in fromParser are unexpected resolutions.
+			for k, got := range fromParser {
+				t.Errorf("%s resolved to %s, want no object", pos(k), pos(got))
+			}
+		})
+	}
+}
+
+// declsFromParser walks the file and collects the map associating an
+// identifier position with its declaration position.
+func declsFromParser(file *ast.File) map[token.Pos]token.Pos {
+	objmap := map[token.Pos]token.Pos{}
+	ast.Inspect(file, func(node ast.Node) bool {
+		// Ignore blank identifiers to reduce noise.
+		if ident, _ := node.(*ast.Ident); ident != nil && ident.Obj != nil && ident.Name != "_" {
+			objmap[ident.Pos()] = ident.Obj.Pos()
+		}
+		return true
+	})
+	return objmap
+}
+
+// declsFromComments looks at comments annotating uses and declarations, and
+// maps each identifier use to its corresponding declaration. See the
+// description of these annotations in the documentation for TestResolution.
+func declsFromComments(handle *token.File, src []byte) map[token.Pos]token.Pos {
+	decls, uses := positionMarkers(handle, src)
+
+	objmap := make(map[token.Pos]token.Pos)
+	// Join decls and uses on name, to build the map of use->decl.
+	for name, posns := range uses {
+		declpos, ok := decls[name]
+		if !ok {
+			panic(fmt.Sprintf("missing declaration for %s", name))
+		}
+		for _, pos := range posns {
+			objmap[pos] = declpos
+		}
+	}
+	return objmap
+}
+
+// positionMarkers extracts named positions from the source denoted by comments
+// prefixed with '=' (declarations) and '@' (uses): for example '@foo' or
+// '=@bar'. It returns a map of name->position for declarations, and
+// name->position(s) for uses.
+func positionMarkers(handle *token.File, src []byte) (decls map[string]token.Pos, uses map[string][]token.Pos) {
+	var s scanner.Scanner
+	s.Init(handle, src, nil, scanner.ScanComments)
+	decls = make(map[string]token.Pos)
+	uses = make(map[string][]token.Pos)
+	var prev token.Pos // position of last non-comment, non-semicolon token
+
+scanFile:
+	for {
+		pos, tok, lit := s.Scan()
+		switch tok {
+		case token.EOF:
+			break scanFile
+		case token.COMMENT:
+			name, decl, use := annotatedObj(lit)
+			if len(name) > 0 {
+				if decl {
+					if _, ok := decls[name]; ok {
+						panic(fmt.Sprintf("duplicate declaration markers for %s", name))
+					}
+					decls[name] = prev
+				}
+				if use {
+					uses[name] = append(uses[name], prev)
+				}
+			}
+		case token.SEMICOLON:
+			// ignore automatically inserted semicolon
+			if lit == "\n" {
+				continue scanFile
+			}
+			fallthrough
+		default:
+			prev = pos
+		}
+	}
+	return decls, uses
+}
+
+func annotatedObj(lit string) (name string, decl, use bool) {
+	if lit[1] == '*' {
+		lit = lit[:len(lit)-2] // strip trailing */
+	}
+	lit = strings.TrimSpace(lit[2:])
+
+scanLit:
+	for idx, r := range lit {
+		switch r {
+		case '=':
+			decl = true
+		case '@':
+			use = true
+		default:
+			name = lit[idx:]
+			break scanLit
+		}
+	}
+	return
+}
diff --git a/internal/backport/go/parser/short_test.go b/internal/backport/go/parser/short_test.go
new file mode 100644
index 0000000..ada2caa
--- /dev/null
+++ b/internal/backport/go/parser/short_test.go
@@ -0,0 +1,284 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains test cases for short valid and invalid programs.
+
+package parser
+
+import (
+	"golang.org/x/website/internal/backport/go/internal/typeparams"
+	"testing"
+)
+
+var valids = []string{
+	"package p\n",
+	`package p;`,
+	`package p; import "fmt"; func f() { fmt.Println("Hello, World!") };`,
+	`package p; func f() { if f(T{}) {} };`,
+	`package p; func f() { _ = <-chan int(nil) };`,
+	`package p; func f() { _ = (<-chan int)(nil) };`,
+	`package p; func f() { _ = (<-chan <-chan int)(nil) };`,
+	`package p; func f() { _ = <-chan <-chan <-chan <-chan <-int(nil) };`,
+	`package p; func f(func() func() func());`,
+	`package p; func f(...T);`,
+	`package p; func f(float, ...int);`,
+	`package p; func f(x int, a ...int) { f(0, a...); f(1, a...,) };`,
+	`package p; func f(int,) {};`,
+	`package p; func f(...int,) {};`,
+	`package p; func f(x ...int,) {};`,
+	`package p; type T []int; var a []bool; func f() { if a[T{42}[0]] {} };`,
+	`package p; type T []int; func g(int) bool { return true }; func f() { if g(T{42}[0]) {} };`,
+	`package p; type T []int; func f() { for _ = range []int{T{42}[0]} {} };`,
+	`package p; var a = T{{1, 2}, {3, 4}}`,
+	`package p; func f() { select { case <- c: case c <- d: case c <- <- d: case <-c <- d: } };`,
+	`package p; func f() { select { case x := (<-c): } };`,
+	`package p; func f() { if ; true {} };`,
+	`package p; func f() { switch ; {} };`,
+	`package p; func f() { for _ = range "foo" + "bar" {} };`,
+	`package p; func f() { var s []int; g(s[:], s[i:], s[:j], s[i:j], s[i:j:k], s[:j:k]) };`,
+	`package p; var ( _ = (struct {*T}).m; _ = (interface {T}).m )`,
+	`package p; func ((T),) m() {}`,
+	`package p; func ((*T),) m() {}`,
+	`package p; func (*(T),) m() {}`,
+	`package p; func _(x []int) { for range x {} }`,
+	`package p; func _() { if [T{}.n]int{} {} }`,
+	`package p; func _() { map[int]int{}[0]++; map[int]int{}[0] += 1 }`,
+	`package p; func _(x interface{f()}) { interface{f()}(x).f() }`,
+	`package p; func _(x chan int) { chan int(x) <- 0 }`,
+	`package p; const (x = 0; y; z)`, // issue 9639
+	`package p; var _ = map[P]int{P{}:0, {}:1}`,
+	`package p; var _ = map[*P]int{&P{}:0, {}:1}`,
+	`package p; type T = int`,
+	`package p; type (T = p.T; _ = struct{}; x = *T)`,
+	`package p; type T (*int)`,
+	`package p; type _ struct{ ((int)) }`,
+	`package p; type _ struct{ (*(int)) }`,
+	`package p; type _ struct{ ([]byte) }`, // disallowed by type-checker
+	`package p; var _ = func()T(nil)`,
+	`package p; func _(T (P))`,
+	`package p; func _(T []E)`,
+	`package p; func _(T [P]E)`,
+	`package p; type _ [A+B]struct{}`,
+	`package p; func (R) _()`,
+	`package p; type _ struct{ f [n]E }`,
+	`package p; type _ struct{ f [a+b+c+d]E }`,
+	`package p; type I1 interface{}; type I2 interface{ I1 }`,
+}
+
+// validWithTParamsOnly holds source code examples that are valid if
+// parseTypeParams is set, but invalid if not. When checking with the
+// parseTypeParams set, errors are ignored.
+var validWithTParamsOnly = []string{
+	`package p; type _ []T[ /* ERROR "expected ';', found '\['" */ int]`,
+	`package p; type T[P any /* ERROR "expected ']', found any" */ ] struct { P }`,
+	`package p; type T[P comparable /* ERROR "expected ']', found comparable" */ ] struct { P }`,
+	`package p; type T[P comparable /* ERROR "expected ']', found comparable" */ [P]] struct { P }`,
+	`package p; type T[P1, /* ERROR "unexpected comma" */ P2 any] struct { P1; f []P2 }`,
+	`package p; func _[ /* ERROR "expected '\(', found '\['" */ T any]()()`,
+	`package p; func _(T (P))`,
+	`package p; func f[ /* ERROR "expected '\(', found '\['" */ A, B any](); func _() { _ = f[int, int] }`,
+	`package p; func _(x /* ERROR "mixed named and unnamed parameters" */ T[P1, P2, P3])`,
+	`package p; func _(x /* ERROR "mixed named and unnamed parameters" */ p.T[Q])`,
+	`package p; func _(p.T[ /* ERROR "missing ',' in parameter list" */ Q])`,
+	`package p; type _[A interface /* ERROR "expected ']', found 'interface'" */ {},] struct{}`,
+	`package p; type _[A interface /* ERROR "expected ']', found 'interface'" */ {}] struct{}`,
+	`package p; type _[A, /* ERROR "unexpected comma" */  B any,] struct{}`,
+	`package p; type _[A, /* ERROR "unexpected comma" */ B any] struct{}`,
+	`package p; type _[A any /* ERROR "expected ']', found any" */,] struct{}`,
+	`package p; type _[A any /* ERROR "expected ']', found any" */ ]struct{}`,
+	`package p; type _[A any /* ERROR "expected ']', found any" */ ] struct{ A }`,
+	`package p; func _[ /* ERROR "expected '\(', found '\['" */ T any]()`,
+	`package p; func _[ /* ERROR "expected '\(', found '\['" */ T any](x T)`,
+	`package p; func _[ /* ERROR "expected '\(', found '\['" */ T1, T2 any](x T)`,
+	`package p; func _[ /* ERROR "expected '\(', found '\['" */ A, B any](a A) B`,
+	`package p; func _[ /* ERROR "expected '\(', found '\['" */ A, B C](a A) B`,
+	`package p; func _[ /* ERROR "expected '\(', found '\['" */ A, B C[A, B]](a A) B`,
+
+	`package p; type _[A, /* ERROR "unexpected comma" */ B any] interface { _(a A) B }`,
+	`package p; type _[A, /* ERROR "unexpected comma" */ B C[A, B]] interface { _(a A) B }`,
+	`package p; func _[ /* ERROR "expected '\(', found '\['" */ T1, T2 interface{}](x T1) T2`,
+	`package p; func _[ /* ERROR "expected '\(', found '\['" */ T1 interface{ m() }, T2, T3 interface{}](x T1, y T3) T2`,
+	`package p; var _ = [ /* ERROR "expected expression" */ ]T[int]{}`,
+	`package p; var _ = [ /* ERROR "expected expression" */ 10]T[int]{}`,
+	`package p; var _ = func /* ERROR "expected expression" */ ()T[int]{}`,
+	`package p; var _ = map /* ERROR "expected expression" */ [T[int]]T[int]{}`,
+	`package p; var _ = chan /* ERROR "expected expression" */ T[int](x)`,
+	`package p; func _(_ T[ /* ERROR "missing ',' in parameter list" */ P], T P) T[P]`,
+	`package p; var _ T[ /* ERROR "expected ';', found '\['" */ chan int]`,
+
+	// TODO(rfindley) this error message could be improved.
+	`package p; func (_ /* ERROR "mixed named and unnamed parameters" */ R[P]) _(x T)`,
+	`package p; func (_ /* ERROR "mixed named and unnamed parameters" */ R[ P, Q]) _(x T)`,
+
+	`package p; func (R[P] /* ERROR "missing element type" */ ) _()`,
+	`package p; func _(T[P] /* ERROR "missing element type" */ )`,
+	`package p; func _(T[P1, /* ERROR "expected ']', found ','" */ P2, P3 ])`,
+	`package p; func _(T[P] /* ERROR "missing element type" */ ) T[P]`,
+	`package p; type _ struct{ T[P] /* ERROR "missing element type" */ }`,
+	`package p; type _ struct{ T[struct /* ERROR "expected expression" */ {a, b, c int}] }`,
+	`package p; type _ interface{int| /* ERROR "expected ';'" */ float32; bool; m(); string;}`,
+	`package p; type I1[T any /* ERROR "expected ']', found any" */ ] interface{}; type I2 interface{ I1[int] }`,
+	`package p; type I1[T any /* ERROR "expected ']', found any" */ ] interface{}; type I2[T any] interface{ I1[T] }`,
+	`package p; type _ interface { N[ /* ERROR "expected ';', found '\['" */ T] }`,
+	`package p; type T[P any /* ERROR "expected ']'" */ ] = T0`,
+}
+
+func TestValid(t *testing.T) {
+	t.Run("no tparams", func(t *testing.T) {
+		for _, src := range valids {
+			checkErrors(t, src, src, DeclarationErrors|AllErrors, false)
+		}
+	})
+	t.Run("tparams", func(t *testing.T) {
+		for _, src := range valids {
+			checkErrors(t, src, src, DeclarationErrors|AllErrors, false)
+		}
+		for _, src := range validWithTParamsOnly {
+			checkErrors(t, src, src, DeclarationErrors|AllErrors, false)
+		}
+	})
+}
+
+// TestSingle is useful to track down a problem with a single short test program.
+func TestSingle(t *testing.T) {
+	const src = `package p; var _ = T{}`
+	checkErrors(t, src, src, DeclarationErrors|AllErrors, true)
+}
+
+var invalids = []string{
+	`foo /* ERROR "expected 'package'" */ !`,
+	`package p; func f() { if { /* ERROR "missing condition" */ } };`,
+	`package p; func f() { if ; /* ERROR "missing condition" */ {} };`,
+	`package p; func f() { if f(); /* ERROR "missing condition" */ {} };`,
+	`package p; func f() { if _ = range /* ERROR "expected operand" */ x; true {} };`,
+	`package p; func f() { switch _ /* ERROR "expected switch expression" */ = range x; true {} };`,
+	`package p; func f() { for _ = range x ; /* ERROR "expected '{'" */ ; {} };`,
+	`package p; func f() { for ; ; _ = range /* ERROR "expected operand" */ x {} };`,
+	`package p; func f() { for ; _ /* ERROR "expected boolean or range expression" */ = range x ; {} };`,
+	`package p; func f() { switch t = /* ERROR "expected ':=', found '='" */ t.(type) {} };`,
+	`package p; func f() { switch t /* ERROR "expected switch expression" */ , t = t.(type) {} };`,
+	`package p; func f() { switch t /* ERROR "expected switch expression" */ = t.(type), t {} };`,
+	`package p; var a = [ /* ERROR "expected expression" */ 1]int;`,
+	`package p; var a = [ /* ERROR "expected expression" */ ...]int;`,
+	`package p; var a = struct /* ERROR "expected expression" */ {}`,
+	`package p; var a = func /* ERROR "expected expression" */ ();`,
+	`package p; var a = interface /* ERROR "expected expression" */ {}`,
+	`package p; var a = [ /* ERROR "expected expression" */ ]int`,
+	`package p; var a = map /* ERROR "expected expression" */ [int]int`,
+	`package p; var a = chan /* ERROR "expected expression" */ int;`,
+	`package p; var a = []int{[ /* ERROR "expected expression" */ ]int};`,
+	`package p; var a = ( /* ERROR "expected expression" */ []int);`,
+	`package p; var a = <- /* ERROR "expected expression" */ chan int;`,
+	`package p; func f() { select { case _ <- chan /* ERROR "expected expression" */ int: } };`,
+	`package p; func f() { _ = (<-<- /* ERROR "expected 'chan'" */ chan int)(nil) };`,
+	`package p; func f() { _ = (<-chan<-chan<-chan<-chan<-chan<- /* ERROR "expected channel type" */ int)(nil) };`,
+	`package p; func f() { var t []int; t /* ERROR "expected identifier on left side of :=" */ [0] := 0 };`,
+	`package p; func f() { if x := g(); x /* ERROR "expected boolean expression" */ = 0 {}};`,
+	`package p; func f() { _ = x = /* ERROR "expected '=='" */ 0 {}};`,
+	`package p; func f() { _ = 1 == func()int { var x bool; x = x = /* ERROR "expected '=='" */ true; return x }() };`,
+	`package p; func f() { var s []int; _ = s[] /* ERROR "expected operand" */ };`,
+	`package p; func f() { var s []int; _ = s[i:j: /* ERROR "3rd index required" */ ] };`,
+	`package p; func f() { var s []int; _ = s[i: /* ERROR "2nd index required" */ :k] };`,
+	`package p; func f() { var s []int; _ = s[i: /* ERROR "2nd index required" */ :] };`,
+	`package p; func f() { var s []int; _ = s[: /* ERROR "2nd index required" */ :] };`,
+	`package p; func f() { var s []int; _ = s[: /* ERROR "2nd index required" */ ::] };`,
+	`package p; func f() { var s []int; _ = s[i:j:k: /* ERROR "expected ']'" */ l] };`,
+	`package p; func f() { for x /* ERROR "boolean or range expression" */ = []string {} }`,
+	`package p; func f() { for x /* ERROR "boolean or range expression" */ := []string {} }`,
+	`package p; func f() { for i /* ERROR "boolean or range expression" */ , x = []string {} }`,
+	`package p; func f() { for i /* ERROR "boolean or range expression" */ , x := []string {} }`,
+	`package p; func f() { go f /* ERROR HERE "function must be invoked" */ }`,
+	`package p; func f() { defer func() {} /* ERROR HERE "function must be invoked" */ }`,
+	`package p; func f() { go func() { func() { f(x func /* ERROR "missing ','" */ (){}) } } }`,
+	`package p; func _() (type /* ERROR "found 'type'" */ T)(T)`,
+	`package p; func (type /* ERROR "found 'type'" */ T)(T) _()`,
+	`package p; type _[A+B, /* ERROR "unexpected comma" */ ] int`,
+
+	// TODO(rfindley): this error should be positioned on the ':'
+	`package p; var a = a[[]int:[ /* ERROR "expected expression" */ ]int];`,
+
+	// TODO(rfindley): the compiler error is better here: "cannot parenthesize embedded type"
+	// TODO(rfindley): confirm that parenthesized types should now be accepted.
+	// `package p; type I1 interface{}; type I2 interface{ (/* ERROR "expected '}', found '\('" */ I1) }`,
+
+	// issue 8656
+	`package p; func f() (a b string /* ERROR "missing ','" */ , ok bool)`,
+
+	// issue 9639
+	`package p; var x /* ERROR "missing variable type or initialization" */ , y, z;`,
+	`package p; const x /* ERROR "missing constant value" */ ;`,
+	`package p; const x /* ERROR "missing constant value" */ int;`,
+	`package p; const (x = 0; y; z /* ERROR "missing constant value" */ int);`,
+
+	// issue 12437
+	`package p; var _ = struct { x int, /* ERROR "expected ';', found ','" */ }{};`,
+	`package p; var _ = struct { x int, /* ERROR "expected ';', found ','" */ y float }{};`,
+
+	// issue 11611
+	`package p; type _ struct { int, } /* ERROR "expected 'IDENT', found '}'" */ ;`,
+	`package p; type _ struct { int, float } /* ERROR "expected type, found '}'" */ ;`,
+
+	// issue 13475
+	`package p; func f() { if true {} else ; /* ERROR "expected if statement or block" */ }`,
+	`package p; func f() { if true {} else defer /* ERROR "expected if statement or block" */ f() }`,
+}
+
+// invalidNoTParamErrs holds invalid source code examples annotated with the
+// error messages produced when ParseTypeParams is not set.
+var invalidNoTParamErrs = []string{
+	`package p; type _[_ any /* ERROR "expected ']', found any" */ ] int; var _ = T[]{}`,
+	`package p; type T[P any /* ERROR "expected ']', found any" */ ] = T0`,
+	`package p; var _ func[ /* ERROR "expected '\(', found '\['" */ T any](T)`,
+	`package p; func _[ /* ERROR "expected '\(', found '\['" */ ]()`,
+	`package p; type _[A, /* ERROR "unexpected comma" */] struct{ A }`,
+	`package p; func _[ /* ERROR "expected '\(', found '\['" */ type P, *Q interface{}]()`,
+
+	`package p; func (T) _[ /* ERROR "expected '\(', found '\['" */ A, B any](a A) B`,
+	`package p; func (T) _[ /* ERROR "expected '\(', found '\['" */ A, B C](a A) B`,
+	`package p; func (T) _[ /* ERROR "expected '\(', found '\['" */ A, B C[A, B]](a A) B`,
+
+	`package p; func(*T[ /* ERROR "missing ',' in parameter list" */ e, e]) _()`,
+}
+
+// invalidTParamErrs holds invalid source code examples annotated with the
+// error messages produced when ParseTypeParams is set.
+var invalidTParamErrs = []string{
+	`package p; type _[_ any] int; var _ = T[] /* ERROR "expected operand" */ {}`,
+	`package p; var _ func[ /* ERROR "must have no type parameters" */ T any](T)`,
+	`package p; func _[]/* ERROR "empty type parameter list" */()`,
+
+	// TODO(rfindley) a better location would be after the ']'
+	`package p; type _[A /* ERROR "all type parameters must be named" */ ,] struct{ A }`,
+
+	// TODO(rfindley) this error is confusing.
+	`package p; func _[type /* ERROR "all type parameters must be named" */ P, *Q interface{}]()`,
+
+	`package p; func (T) _[ /* ERROR "must have no type parameters" */ A, B any](a A) B`,
+	`package p; func (T) _[ /* ERROR "must have no type parameters" */ A, B C](a A) B`,
+	`package p; func (T) _[ /* ERROR "must have no type parameters" */ A, B C[A, B]](a A) B`,
+
+	`package p; func(*T[e, e /* ERROR "e redeclared" */ ]) _()`,
+}
+
+func TestInvalid(t *testing.T) {
+	t.Run("no tparams", func(t *testing.T) {
+		for _, src := range invalids {
+			checkErrors(t, src, src, DeclarationErrors|AllErrors|typeparams.DisallowParsing, true)
+		}
+		for _, src := range validWithTParamsOnly {
+			checkErrors(t, src, src, DeclarationErrors|AllErrors|typeparams.DisallowParsing, true)
+		}
+		for _, src := range invalidNoTParamErrs {
+			checkErrors(t, src, src, DeclarationErrors|AllErrors|typeparams.DisallowParsing, true)
+		}
+	})
+	t.Run("tparams", func(t *testing.T) {
+		for _, src := range invalids {
+			checkErrors(t, src, src, DeclarationErrors|AllErrors, true)
+		}
+		for _, src := range invalidTParamErrs {
+			checkErrors(t, src, src, DeclarationErrors|AllErrors, true)
+		}
+	})
+}
diff --git a/internal/backport/go/parser/testdata/chans.go2 b/internal/backport/go/parser/testdata/chans.go2
new file mode 100644
index 0000000..fad2bce
--- /dev/null
+++ b/internal/backport/go/parser/testdata/chans.go2
@@ -0,0 +1,62 @@
+package chans
+
+import "runtime"
+
+// Ranger returns a Sender and a Receiver. The Receiver provides a
+// Next method to retrieve values. The Sender provides a Send method
+// to send values and a Close method to stop sending values. The Next
+// method indicates when the Sender has been closed, and the Send
+// method indicates when the Receiver has been freed.
+//
+// This is a convenient way to exit a goroutine sending values when
+// the receiver stops reading them.
+func Ranger[T any]() (*Sender[T], *Receiver[T]) {
+	c := make(chan T)
+	d := make(chan bool)
+	s := &Sender[T]{values: c, done: d}
+	r := &Receiver[T]{values: c, done: d}
+	runtime.SetFinalizer(r, r.finalize)
+	return s, r
+}
+
+// A sender is used to send values to a Receiver.
+type Sender[T any] struct {
+	values chan<- T
+	done <-chan bool
+}
+
+// Send sends a value to the receiver. It returns whether any more
+// values may be sent; if it returns false the value was not sent.
+func (s *Sender[T]) Send(v T) bool {
+	select {
+	case s.values <- v:
+		return true
+	case <-s.done:
+		return false
+	}
+}
+
+// Close tells the receiver that no more values will arrive.
+// After Close is called, the Sender may no longer be used.
+func (s *Sender[T]) Close() {
+	close(s.values)
+}
+
+// A Receiver receives values from a Sender.
+type Receiver[T any] struct {
+	values <-chan T
+	done chan<- bool
+}
+
+// Next returns the next value from the channel. The bool result
+// indicates whether the value is valid, or whether the Sender has
+// been closed and no more values will be received.
+func (r *Receiver[T]) Next() (T, bool) {
+	v, ok := <-r.values
+	return v, ok
+}
+
+// finalize is a finalizer for the receiver.
+func (r *Receiver[T]) finalize() {
+	close(r.done)
+}
diff --git a/internal/backport/go/parser/testdata/commas.src b/internal/backport/go/parser/testdata/commas.src
new file mode 100644
index 0000000..e0603cf
--- /dev/null
+++ b/internal/backport/go/parser/testdata/commas.src
@@ -0,0 +1,19 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for error messages/parser synchronization
+// after missing commas.
+
+package p
+
+var _ = []int{
+	0/* ERROR HERE "missing ','" */
+}
+
+var _ = []int{
+	0,
+	1,
+	2,
+	3/* ERROR HERE "missing ','" */
+}
diff --git a/internal/backport/go/parser/testdata/interface.go2 b/internal/backport/go/parser/testdata/interface.go2
new file mode 100644
index 0000000..2ed9339
--- /dev/null
+++ b/internal/backport/go/parser/testdata/interface.go2
@@ -0,0 +1,76 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains test cases for interfaces containing
+// constraint elements.
+
+package p
+
+type _ interface {
+	m()
+	~int
+	~int|string
+	E
+}
+
+type _ interface {
+	m()
+	~int
+	int | string
+	int | ~string
+	~int | ~string
+}
+
+type _ interface {
+	m()
+	~int
+	T[int, string] | string
+	int | ~T[string, struct{}]
+	~int | ~string
+}
+
+type _ interface {
+	int
+	[]byte
+	[10]int
+	struct{}
+	*int
+	func()
+	interface{}
+	map[string]int
+	chan T
+	chan<- T
+	<-chan T
+	T[int]
+}
+
+type _ interface {
+	int | string
+	[]byte | string
+	[10]int | string
+	struct{} | string
+	*int | string
+	func() | string
+	interface{} | string
+	map[string]int | string
+	chan T | string
+	chan<- T | string
+	<-chan T | string
+	T[int] | string
+}
+
+type _ interface {
+	~int | string
+	~[]byte | string
+	~[10]int | string
+	~struct{} | string
+	~*int | string
+	~func() | string
+	~interface{} | string
+	~map[string]int | string
+	~chan T | string
+	~chan<- T | string
+	~<-chan T | string
+	~T[int] | string
+}
diff --git a/internal/backport/go/parser/testdata/issue11377.src b/internal/backport/go/parser/testdata/issue11377.src
new file mode 100644
index 0000000..1c43800
--- /dev/null
+++ b/internal/backport/go/parser/testdata/issue11377.src
@@ -0,0 +1,27 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 11377: Better synchronization of
+// parser after certain syntax errors.
+
+package p
+
+func bad1() {
+    if f()) /* ERROR "expected ';', found '\)'" */ {
+        return
+    }
+}
+
+// There shouldn't be any errors down below.
+
+func F1() {}
+func F2() {}
+func F3() {}
+func F4() {}
+func F5() {}
+func F6() {}
+func F7() {}
+func F8() {}
+func F9() {}
+func F10() {}
diff --git a/internal/backport/go/parser/testdata/issue23434.src b/internal/backport/go/parser/testdata/issue23434.src
new file mode 100644
index 0000000..24a0832
--- /dev/null
+++ b/internal/backport/go/parser/testdata/issue23434.src
@@ -0,0 +1,25 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 23434: Better synchronization of
+// parser after missing type. There should be exactly
+// one error each time, with now follow errors.
+
+package p
+
+func g() {
+	m := make(map[string]! /* ERROR "expected type, found '!'" */ )
+	for {
+		x := 1
+		print(x)
+	}
+}
+
+func f() {
+	m := make(map[string]) /* ERROR "expected type, found '\)'" */
+	for {
+		x := 1
+		print(x)
+	}
+}
diff --git a/internal/backport/go/parser/testdata/issue3106.src b/internal/backport/go/parser/testdata/issue3106.src
new file mode 100644
index 0000000..2db10be
--- /dev/null
+++ b/internal/backport/go/parser/testdata/issue3106.src
@@ -0,0 +1,46 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 3106: Better synchronization of
+// parser after certain syntax errors.
+
+package main
+
+func f() {
+	var m Mutex
+	c := MakeCond(&m)
+	percent := 0
+	const step = 10
+	for i := 0; i < 5; i++ {
+		go func() {
+			for {
+				// Emulates some useful work.
+				time.Sleep(1e8)
+				m.Lock()
+				defer
+				if /* ERROR "expected ';', found 'if'" */ percent == 100 {
+					m.Unlock()
+					break
+				}
+				percent++
+				if percent % step == 0 {
+					//c.Signal()
+				}
+				m.Unlock()
+			}
+		}()
+	}
+	for {
+		m.Lock()
+		if percent == 0 || percent % step != 0 {
+			c.Wait()
+		}
+		fmt.Print(",")
+		if percent == 100 {
+			m.Unlock()
+			break
+		}
+		m.Unlock()
+	}
+}
diff --git a/internal/backport/go/parser/testdata/issue34946.src b/internal/backport/go/parser/testdata/issue34946.src
new file mode 100644
index 0000000..6bb15e1
--- /dev/null
+++ b/internal/backport/go/parser/testdata/issue34946.src
@@ -0,0 +1,22 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 34946: Better synchronization of
+// parser for function declarations that start their
+// body's opening { on a new line.
+
+package p
+
+// accept Allman/BSD-style declaration but complain
+// (implicit semicolon between signature and body)
+func _() int
+{ /* ERROR "unexpected semicolon or newline before {" */
+	{ return 0 }
+}
+
+func _() {}
+
+func _(); { /* ERROR "unexpected semicolon or newline before {" */ }
+
+func _() {}
diff --git a/internal/backport/go/parser/testdata/issue42951/not_a_file.go/invalid.go b/internal/backport/go/parser/testdata/issue42951/not_a_file.go/invalid.go
new file mode 100644
index 0000000..bb698be
--- /dev/null
+++ b/internal/backport/go/parser/testdata/issue42951/not_a_file.go/invalid.go
@@ -0,0 +1 @@
+This file should not be parsed by ParseDir.
diff --git a/internal/backport/go/parser/testdata/issue44504.src b/internal/backport/go/parser/testdata/issue44504.src
new file mode 100644
index 0000000..7791f4a
--- /dev/null
+++ b/internal/backport/go/parser/testdata/issue44504.src
@@ -0,0 +1,13 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 44504: panic due to duplicate resolution of slice/index
+// operands. We should not try to resolve a LHS expression with invalid syntax.
+
+package p
+
+func _() {
+  var items []bool
+  items[] /* ERROR "operand" */ = false
+}
diff --git a/internal/backport/go/parser/testdata/issue49174.go2 b/internal/backport/go/parser/testdata/issue49174.go2
new file mode 100644
index 0000000..77c1950
--- /dev/null
+++ b/internal/backport/go/parser/testdata/issue49174.go2
@@ -0,0 +1,8 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[_ []int | int]() {}
+func _[_ int | []int]() {}
diff --git a/internal/backport/go/parser/testdata/issue49175.go2 b/internal/backport/go/parser/testdata/issue49175.go2
new file mode 100644
index 0000000..a5ad30f
--- /dev/null
+++ b/internal/backport/go/parser/testdata/issue49175.go2
@@ -0,0 +1,13 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type _[_ []t]t
+type _[_ [1]t]t
+
+func _[_ []t]() {}
+func _[_ [1]t]() {}
+
+type t [t /* ERROR "all type parameters must be named" */ [0]]t
diff --git a/internal/backport/go/parser/testdata/issue49482.go2 b/internal/backport/go/parser/testdata/issue49482.go2
new file mode 100644
index 0000000..50de651
--- /dev/null
+++ b/internal/backport/go/parser/testdata/issue49482.go2
@@ -0,0 +1,35 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type (
+        // these need a comma to disambiguate
+        _[P *T,] struct{}
+        _[P *T, _ any] struct{}
+        _[P (*T),] struct{}
+        _[P (*T), _ any] struct{}
+        _[P (T),] struct{}
+        _[P (T), _ any] struct{}
+
+        // these parse as name followed by type
+        _[P *struct{}] struct{}
+        _[P (*struct{})] struct{}
+        _[P ([]int)] struct{}
+
+        // array declarations
+        _ [P(T)]struct{}
+        _ [P((T))]struct{}
+        _ [P * *T]struct{}
+        _ [P * T]struct{}
+        _ [P(*T)]struct{}
+        _ [P(**T)]struct{}
+        _ [P * T - T]struct{}
+        _ [P*T-T, /* ERROR "unexpected comma" */ ]struct{}
+        _ [10, /* ERROR "unexpected comma" */ ]struct{}
+
+        // These should be parsed as generic type declarations.
+        _[P *struct /* ERROR "expected expression" */ {}|int] struct{}
+        _[P *struct /* ERROR "expected expression" */ {}|int|string] struct{}
+)
diff --git a/internal/backport/go/parser/testdata/issue50427.go2 b/internal/backport/go/parser/testdata/issue50427.go2
new file mode 100644
index 0000000..1521459
--- /dev/null
+++ b/internal/backport/go/parser/testdata/issue50427.go2
@@ -0,0 +1,19 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T interface{ m[ /* ERROR "must have no type parameters" */ P any]() }
+
+func _(t T) {
+	var _ interface{ m[ /* ERROR "must have no type parameters" */ P any](); n() } = t
+}
+
+type S struct{}
+
+func (S) m[ /* ERROR "must have no type parameters" */ P any]() {}
+
+func _(s S) {
+	var _ interface{ m[ /* ERROR "must have no type parameters" */ P any](); n() } = s
+}
diff --git a/internal/backport/go/parser/testdata/linalg.go2 b/internal/backport/go/parser/testdata/linalg.go2
new file mode 100644
index 0000000..7ccb19c
--- /dev/null
+++ b/internal/backport/go/parser/testdata/linalg.go2
@@ -0,0 +1,83 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package linalg
+
+import "math"
+
+// Numeric is type bound that matches any numeric type.
+// It would likely be in a constraints package in the standard library.
+type Numeric interface {
+	~int|~int8|~int16|~int32|~int64|
+		~uint|~uint8|~uint16|~uint32|~uint64|~uintptr|
+		~float32|~float64|
+		~complex64|~complex128
+}
+
+func DotProduct[T Numeric](s1, s2 []T) T {
+	if len(s1) != len(s2) {
+		panic("DotProduct: slices of unequal length")
+	}
+	var r T
+	for i := range s1 {
+		r += s1[i] * s2[i]
+	}
+	return r
+}
+
+// NumericAbs matches numeric types with an Abs method.
+type NumericAbs[T any] interface {
+	Numeric
+
+	Abs() T
+}
+
+// AbsDifference computes the absolute value of the difference of
+// a and b, where the absolute value is determined by the Abs method.
+func AbsDifference[T NumericAbs](a, b T) T {
+	d := a - b
+	return d.Abs()
+}
+
+// OrderedNumeric is a type bound that matches numeric types that support the < operator.
+type OrderedNumeric interface {
+	~int|~int8|~int16|~int32|~int64|
+		~uint|~uint8|~uint16|~uint32|~uint64|~uintptr|
+		~float32|~float64
+}
+
+// Complex is a type bound that matches the two complex types, which do not have a < operator.
+type Complex interface {
+	~complex64|~complex128
+}
+
+// OrderedAbs is a helper type that defines an Abs method for
+// ordered numeric types.
+type OrderedAbs[T OrderedNumeric] T
+
+func (a OrderedAbs[T]) Abs() OrderedAbs[T] {
+	if a < 0 {
+		return -a
+	}
+	return a
+}
+
+// ComplexAbs is a helper type that defines an Abs method for
+// complex types.
+type ComplexAbs[T Complex] T
+
+func (a ComplexAbs[T]) Abs() ComplexAbs[T] {
+	r := float64(real(a))
+	i := float64(imag(a))
+	d := math.Sqrt(r * r + i * i)
+	return ComplexAbs[T](complex(d, 0))
+}
+
+func OrderedAbsDifference[T OrderedNumeric](a, b T) T {
+	return T(AbsDifference(OrderedAbs[T](a), OrderedAbs[T](b)))
+}
+
+func ComplexAbsDifference[T Complex](a, b T) T {
+	return T(AbsDifference(ComplexAbs[T](a), ComplexAbs[T](b)))
+}
diff --git a/internal/backport/go/parser/testdata/map.go2 b/internal/backport/go/parser/testdata/map.go2
new file mode 100644
index 0000000..74c79ae
--- /dev/null
+++ b/internal/backport/go/parser/testdata/map.go2
@@ -0,0 +1,109 @@
+// Package orderedmap provides an ordered map, implemented as a binary tree.
+package orderedmap
+
+import "chans"
+
+// Map is an ordered map.
+type Map[K, V any] struct {
+	root    *node[K, V]
+	compare func(K, K) int
+}
+
+// node is the type of a node in the binary tree.
+type node[K, V any] struct {
+	key         K
+	val         V
+	left, right *node[K, V]
+}
+
+// New returns a new map.
+func New[K, V any](compare func(K, K) int) *Map[K, V] {
+        return &Map[K, V]{compare: compare}
+}
+
+// find looks up key in the map, and returns either a pointer
+// to the node holding key, or a pointer to the location where
+// such a node would go.
+func (m *Map[K, V]) find(key K) **node[K, V] {
+	pn := &m.root
+	for *pn != nil {
+		switch cmp := m.compare(key, (*pn).key); {
+		case cmp < 0:
+			pn = &(*pn).left
+		case cmp > 0:
+			pn = &(*pn).right
+		default:
+			return pn
+		}
+	}
+	return pn
+}
+
+// Insert inserts a new key/value into the map.
+// If the key is already present, the value is replaced.
+// Returns true if this is a new key, false if already present.
+func (m *Map[K, V]) Insert(key K, val V) bool {
+	pn := m.find(key)
+	if *pn != nil {
+		(*pn).val = val
+		return false
+	}
+        *pn = &node[K, V]{key: key, val: val}
+	return true
+}
+
+// Find returns the value associated with a key, or zero if not present.
+// The found result reports whether the key was found.
+func (m *Map[K, V]) Find(key K) (V, bool) {
+	pn := m.find(key)
+	if *pn == nil {
+		var zero V // see the discussion of zero values, above
+		return zero, false
+	}
+	return (*pn).val, true
+}
+
+// keyValue is a pair of key and value used when iterating.
+type keyValue[K, V any] struct {
+	key K
+	val V
+}
+
+// InOrder returns an iterator that does an in-order traversal of the map.
+func (m *Map[K, V]) InOrder() *Iterator[K, V] {
+	sender, receiver := chans.Ranger[keyValue[K, V]]()
+	var f func(*node[K, V]) bool
+	f = func(n *node[K, V]) bool {
+		if n == nil {
+			return true
+		}
+		// Stop sending values if sender.Send returns false,
+		// meaning that nothing is listening at the receiver end.
+		return f(n.left) &&
+                        // TODO
+			// sender.Send(keyValue[K, V]{n.key, n.val}) &&
+			f(n.right)
+	}
+	go func() {
+		f(m.root)
+		sender.Close()
+	}()
+	return &Iterator{receiver}
+}
+
+// Iterator is used to iterate over the map.
+type Iterator[K, V any] struct {
+	r *chans.Receiver[keyValue[K, V]]
+}
+
+// Next returns the next key and value pair, and a boolean indicating
+// whether they are valid or whether we have reached the end.
+func (it *Iterator[K, V]) Next() (K, V, bool) {
+	keyval, ok := it.r.Next()
+	if !ok {
+		var zerok K
+		var zerov V
+		return zerok, zerov, false
+	}
+	return keyval.key, keyval.val, true
+}
diff --git a/internal/backport/go/parser/testdata/metrics.go2 b/internal/backport/go/parser/testdata/metrics.go2
new file mode 100644
index 0000000..ef1c66b
--- /dev/null
+++ b/internal/backport/go/parser/testdata/metrics.go2
@@ -0,0 +1,58 @@
+package metrics
+
+import "sync"
+
+type Metric1[T comparable] struct {
+	mu sync.Mutex
+	m  map[T]int
+}
+
+func (m *Metric1[T]) Add(v T) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if m.m == nil {
+		m.m = make(map[T]int)
+	}
+	m[v]++
+}
+
+type key2[T1, T2 comparable] struct {
+	f1 T1
+	f2 T2
+}
+
+type Metric2[T1, T2 cmp2] struct {
+	mu sync.Mutex
+	m  map[key2[T1, T2]]int
+}
+
+func (m *Metric2[T1, T2]) Add(v1 T1, v2 T2) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if m.m == nil {
+		m.m = make(map[key2[T1, T2]]int)
+	}
+	m[key[T1, T2]{v1, v2}]++
+}
+
+type key3[T1, T2, T3 comparable] struct {
+	f1 T1
+	f2 T2
+	f3 T3
+}
+
+type Metric3[T1, T2, T3 comparable] struct {
+	mu sync.Mutex
+	m  map[key3[T1, T2, T3]]int
+}
+
+func (m *Metric3[T1, T2, T3]) Add(v1 T1, v2 T2, v3 T3) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if m.m == nil {
+		m.m = make(map[key3]int)
+	}
+	m[key[T1, T2, T3]{v1, v2, v3}]++
+}
+
+// Repeat for the maximum number of permitted arguments.
diff --git a/internal/backport/go/parser/testdata/resolution/issue45136.src b/internal/backport/go/parser/testdata/resolution/issue45136.src
new file mode 100644
index 0000000..e1d63d8
--- /dev/null
+++ b/internal/backport/go/parser/testdata/resolution/issue45136.src
@@ -0,0 +1,27 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue45136
+
+type obj /* =@obj */ struct {
+	name /*=@name */ string
+}
+
+func _() {
+	var foo /* =@foo */ = "foo"
+	obj /* @obj */ ["foo"]
+	obj /* @obj */ .run()
+	obj /* @obj */ {
+		name: foo /* @foo */,
+	}
+	obj /* @obj */ {
+		name: "bar",
+	}.run()
+
+	var _ = File{key: obj /* @obj */ {}}
+	var _ = File{obj /* @obj */ {}}
+
+	[]obj /* @obj */ {foo /* @foo */}
+	x /* =@x1 */ := obj /* @obj */{}
+}
diff --git a/internal/backport/go/parser/testdata/resolution/issue45160.src b/internal/backport/go/parser/testdata/resolution/issue45160.src
new file mode 100644
index 0000000..6be933b
--- /dev/null
+++ b/internal/backport/go/parser/testdata/resolution/issue45160.src
@@ -0,0 +1,25 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue45160
+
+func mklink1 /* =@mklink1func */() {}
+
+func _() {
+	var tests /* =@tests */ = []dirLinkTest /* @dirLinkTest */ {
+		{
+			mklink1 /* @mklink1func */: func() {},
+			mklink2: func(link /* =@link */, target /* =@target */ string) error {
+				return nil
+			},
+		},
+	}
+}
+
+type dirLinkTest /* =@dirLinkTest */ struct {
+	mklink1 /* =@mklink1field */ func(string, string) error
+	mklink2 /* =@mklink2field */ func(string, string) error
+}
+
+func mklink2 /* =@mklink2func */() {}
diff --git a/internal/backport/go/parser/testdata/resolution/resolution.src b/internal/backport/go/parser/testdata/resolution/resolution.src
new file mode 100644
index 0000000..a880dd1
--- /dev/null
+++ b/internal/backport/go/parser/testdata/resolution/resolution.src
@@ -0,0 +1,63 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package resolution
+
+func f /* =@fdecl */(n /* =@narg */ ast.Node) bool {
+		if n /* =@ninit */, ok /* =@ok */ := n /* @narg */ .(*ast.SelectorExpr); ok /* @ok */ {
+			sel = n /* @ninit */
+	}
+}
+
+type c /* =@cdecl */ map[token.Pos]resolvedObj
+
+func (v /* =@vdecl */ c /* @cdecl */) Visit(node /* =@nodearg */ ast.Node) (w /* =@w */ ast.Visitor) {}
+
+const (
+	basic /* =@basic */ = iota
+	labelOk // =@labelOk
+)
+
+type T /* =@T */ int
+
+func _(count /* =@count */ T /* @T */) {
+	x /* =@x1 */ := c /* @cdecl */{}
+	switch x /* =@x2 */ := x /* @x1 */; x /* =@x3 */ := x /* @x2 */.(type) {
+	case c /* @cdecl */:
+	default:
+	}
+loop /* =@loop */:
+	for {
+		if true {
+			break loop /* @loop */
+		}
+	}
+	select {
+	case err /* =@err1 */ := <-_:
+		return err /* @err1 */
+	case err /* =@err2 */ := <-_:
+		return err /* @err2 */
+	}
+
+	_ = func(p1 /* =@p1 */ int, p2 /* =@p2 */ p1) {
+		closed /* =@closed */ := p1 // @p1
+		shadowed /* =@shadowed1 */ := p2 // @p2
+		_ = func(shadowed /* =@shadowed2 */ p2 /* @p2 */) {
+			closed /* @closed */ = 1
+			shadowed /* @shadowed2 */ = 2
+		}
+	}
+}
+
+func (r /* =@r */ c /* @cdecl */) m(_ r) c /* @cdecl */ { return r /* @r */ }
+
+var cycle /* =@cycle */ = cycle /* @cycle */ + 1
+
+type chain /* =@chain */ struct {
+	next /* =@next */ *chain /* @chain */
+}
+
+func recursive /* =@recursive */() {
+	recursive /* @recursive */ ()
+}
diff --git a/internal/backport/go/parser/testdata/resolution/typeparams.go2 b/internal/backport/go/parser/testdata/resolution/typeparams.go2
new file mode 100644
index 0000000..7395ca2
--- /dev/null
+++ b/internal/backport/go/parser/testdata/resolution/typeparams.go2
@@ -0,0 +1,51 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package resolution
+
+type List /* =@List */ [E /* =@E */ any] []E // @E
+
+type Pair /* =@Pair */ [L /* =@L */, R /* =@R */ any] struct {
+	Left /* =@Left */ L // @L
+	Right /* =@Right */ R // @R
+	L /* =@Lfield */ int
+}
+
+var _ = Pair /* @Pair */ [int, string]{}
+
+type Addable /* =@Addable */ interface {
+	~int64|~float64
+}
+
+func Add /* =@AddDecl */[T /* =@T */ Addable /* @Addable */](l /* =@l */, r /* =@r */ T /* @T */) T /* @T */ {
+	var t /* =@t */ T /* @T */
+	return l /* @l */ + r /* @r */ + t /* @t */
+}
+
+type Receiver /* =@Receiver */[P /* =@P */ any] struct {}
+
+type RP /* =@RP1 */ struct{}
+
+// TODO(rFindley): make a decision on how/whether to resolve identifiers that
+// refer to receiver type parameters, as is the case for the 'P' result
+// parameter below.
+//
+// For now, we ensure that types are not incorrectly resolved when receiver
+// type parameters are in scope.
+func (r /* =@recv */ Receiver /* @Receiver */ [RP]) m(RP) RP {}
+
+func f /* =@f */[T1 /* =@T1 */ interface{~[]T2 /* @T2 */}, T2 /* =@T2 */ any](
+  x /* =@x */ T1 /* @T1 */, T1 /* =@T1_duplicate */ y,  // Note that this is a bug:
+                                                        // the duplicate T1 should
+							// not be allowed.
+  ){
+  // Note that duplicate short var declarations resolve to their alt declaration.
+  x /* @x */ := 0
+  y /* =@y */ := 0
+  T1 /* @T1 */ := 0
+  var t1var /* =@t1var */ T1 /* @T1 */
+}
+
+// From issue #39634
+func(*ph1[e, e])h(d)
diff --git a/internal/backport/go/parser/testdata/set.go2 b/internal/backport/go/parser/testdata/set.go2
new file mode 100644
index 0000000..0da6377
--- /dev/null
+++ b/internal/backport/go/parser/testdata/set.go2
@@ -0,0 +1,31 @@
+// Package set implements sets of any type.
+package set
+
+type Set[Elem comparable] map[Elem]struct{}
+
+func Make[Elem comparable]() Set[Elem] {
+	return make(Set(Elem))
+}
+
+func (s Set[Elem]) Add(v Elem) {
+	s[v] = struct{}{}
+}
+
+func (s Set[Elem]) Delete(v Elem) {
+	delete(s, v)
+}
+
+func (s Set[Elem]) Contains(v Elem) bool {
+	_, ok := s[v]
+	return ok
+}
+
+func (s Set[Elem]) Len() int {
+	return len(s)
+}
+
+func (s Set[Elem]) Iterate(f func(Elem)) {
+	for v := range s {
+		f(v)
+	}
+}
diff --git a/internal/backport/go/parser/testdata/slices.go2 b/internal/backport/go/parser/testdata/slices.go2
new file mode 100644
index 0000000..e060212
--- /dev/null
+++ b/internal/backport/go/parser/testdata/slices.go2
@@ -0,0 +1,31 @@
+// Package slices implements various slice algorithms.
+package slices
+
+// Map turns a []T1 to a []T2 using a mapping function.
+func Map[T1, T2 any](s []T1, f func(T1) T2) []T2 {
+	r := make([]T2, len(s))
+	for i, v := range s {
+		r[i] = f(v)
+	}
+	return r
+}
+
+// Reduce reduces a []T1 to a single value using a reduction function.
+func Reduce[T1, T2 any](s []T1, initializer T2, f func(T2, T1) T2) T2 {
+	r := initializer
+	for _, v := range s {
+		r = f(r, v)
+	}
+	return r
+}
+
+// Filter filters values from a slice using a filter function.
+func Filter[T any](s []T, f func(T) bool) []T {
+	var r []T
+	for _, v := range s {
+		if f(v) {
+			r = append(r, v)
+		}
+	}
+	return r
+}
diff --git a/internal/backport/go/parser/testdata/sort.go2 b/internal/backport/go/parser/testdata/sort.go2
new file mode 100644
index 0000000..88be79f
--- /dev/null
+++ b/internal/backport/go/parser/testdata/sort.go2
@@ -0,0 +1,27 @@
+package sort
+
+type orderedSlice[Elem comparable] []Elem
+
+func (s orderedSlice[Elem]) Len() int           { return len(s) }
+func (s orderedSlice[Elem]) Less(i, j int) bool { return s[i] < s[j] }
+func (s orderedSlice[Elem]) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
+
+// OrderedSlice sorts the slice s in ascending order.
+// The elements of s must be ordered using the < operator.
+func OrderedSlice[Elem comparable](s []Elem) {
+	sort.Sort(orderedSlice[Elem](s))
+}
+
+type sliceFn[Elem any] struct {
+	s []Elem
+	f func(Elem, Elem) bool
+}
+
+func (s sliceFn[Elem]) Len() int           { return len(s.s) }
+func (s sliceFn[Elem]) Less(i, j int) bool { return s.f(s.s[i], s.s[j]) }
+func (s sliceFn[Elem]) Swap(i, j int)      { s.s[i], s.s[j] = s.s[j], s.s[i] }
+
+// SliceFn sorts the slice s according to the function f.
+func SliceFn[Elem any](s []Elem, f func(Elem, Elem) bool) {
+	Sort(sliceFn[Elem]{s, f})
+}
diff --git a/internal/backport/go/parser/testdata/typeparams.src b/internal/backport/go/parser/testdata/typeparams.src
new file mode 100644
index 0000000..479cb96
--- /dev/null
+++ b/internal/backport/go/parser/testdata/typeparams.src
@@ -0,0 +1,17 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test cases for error messages produced while parsing code that uses type
+// parameters, without ParseTypeParams being enabled.
+
+package p
+
+type List[E any /* ERROR "expected ']', found any" */ ] []E
+
+type Pair[L, /* ERROR "unexpected comma" */ R any] struct {
+	Left L
+	Right R
+}
+
+var _ = Pair[int, /* ERROR "expected ']' or ':', found ','" */ string]{}
diff --git a/internal/backport/go/parser/testdata/typeset.go2 b/internal/backport/go/parser/testdata/typeset.go2
new file mode 100644
index 0000000..aa18e8c
--- /dev/null
+++ b/internal/backport/go/parser/testdata/typeset.go2
@@ -0,0 +1,75 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains test cases for typeset-only constraint elements.
+// TODO(gri) gofmt once/if gofmt supports this notation.
+
+package p
+
+type (
+        _[_ t] t
+        _[_ ~t] t
+        _[_ t|t] t
+        _[_ ~t|t] t
+        _[_ t|~t] t
+        _[_ ~t|~t] t
+
+        _[_ t, _, _ t|t] t
+        _[_ t, _, _ ~t|t] t
+        _[_ t, _, _ t|~t] t
+        _[_ t, _, _ ~t|~t] t
+
+        _[_ t.t] t
+        _[_ ~t.t] t
+        _[_ t.t|t.t] t
+        _[_ ~t.t|t.t] t
+        _[_ t.t|~t.t] t
+        _[_ ~t.t|~t.t] t
+
+        _[_ t, _, _ t.t|t.t] t
+        _[_ t, _, _ ~t.t|t.t] t
+        _[_ t, _, _ t.t|~t.t] t
+        _[_ t, _, _ ~t.t|~t.t] t
+
+        _[_ struct{}] t
+        _[_ ~struct{}] t
+
+        _[_ struct{}|t] t
+        _[_ ~struct{}|t] t
+        _[_ struct{}|~t] t
+        _[_ ~struct{}|~t] t
+
+        _[_ t|struct{}] t
+        _[_ ~t|struct{}] t
+        _[_ t|~struct{}] t
+        _[_ ~t|~struct{}] t
+)
+
+// Single-expression type parameter lists and those that don't start
+// with a (type parameter) name are considered array sizes.
+// The term must be a valid expression (it could be a type - and then
+// a type-checker will complain - but we don't allow ~ in the expr).
+// TODO(rfindley): Improve error recover here. In these cases go/parser error
+// recovery is worse than cmd/compile/internal/syntax, and unnecessary type
+// declarations had to be inserted to force synchronization.
+type _[t] t
+type _[~ /* ERROR "expected operand" */ t] t
+type /* ERROR "expected ']'" */ Sync int  // placeholder to synchronize the parser
+type _[t|t] t
+type _[~ /* ERROR "expected operand" */ t|t] t
+type /* ERROR "expected ']'" */ Sync int  // placeholder to synchronize the parser
+type _[t| ~ /* ERROR "expected operand" */ t] t
+type /* ERROR "expected ']'" */ Sync int  // placeholder to synchronize the parser
+type _[~ /* ERROR "expected operand" */ t|~t] t
+type /* ERROR "expected ']'" */ Sync int  // placeholder to synchronize the parser
+
+type _[_ t, t /* ERROR "type parameters must be named" */ ] t
+type _[_ ~t, t /* ERROR "type parameters must be named" */ ] t
+type _[_ t, ~ /* ERROR "type parameters must be named" */ t] t
+type _[_ ~t, ~ /* ERROR "type parameters must be named" */ t] t
+
+type _[_ t|t, t /* ERROR "type parameters must be named" */ |t] t
+type _[_ ~t|t, t /* ERROR "type parameters must be named" */ |t] t
+type _[_ t|t, ~ /* ERROR "type parameters must be named" */ t|t] t
+type _[_ ~t|t, ~ /* ERROR "type parameters must be named" */ t|t] t
diff --git a/internal/backport/go/printer/example_test.go b/internal/backport/go/printer/example_test.go
new file mode 100644
index 0000000..0df644d
--- /dev/null
+++ b/internal/backport/go/printer/example_test.go
@@ -0,0 +1,67 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package printer_test
+
+import (
+	"bytes"
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/printer"
+	"golang.org/x/website/internal/backport/go/token"
+	"strings"
+	"testing"
+)
+
+// Dummy test function so that godoc does not use the entire file as example.
+func Test(*testing.T) {}
+
+func parseFunc(filename, functionname string) (fun *ast.FuncDecl, fset *token.FileSet) {
+	fset = token.NewFileSet()
+	if file, err := parser.ParseFile(fset, filename, nil, 0); err == nil {
+		for _, d := range file.Decls {
+			if f, ok := d.(*ast.FuncDecl); ok && f.Name.Name == functionname {
+				fun = f
+				return
+			}
+		}
+	}
+	panic("function not found")
+}
+
+func ExampleFprint() {
+	// Parse source file and extract the AST without comments for
+	// this function, with position information referring to the
+	// file set fset.
+	funcAST, fset := parseFunc("example_test.go", "ExampleFprint")
+
+	// Print the function body into buffer buf.
+	// The file set is provided to the printer so that it knows
+	// about the original source formatting and can add additional
+	// line breaks where they were present in the source.
+	var buf bytes.Buffer
+	printer.Fprint(&buf, fset, funcAST.Body)
+
+	// Remove braces {} enclosing the function body, unindent,
+	// and trim leading and trailing white space.
+	s := buf.String()
+	s = s[1 : len(s)-1]
+	s = strings.TrimSpace(strings.ReplaceAll(s, "\n\t", "\n"))
+
+	// Print the cleaned-up body text to stdout.
+	fmt.Println(s)
+
+	// output:
+	// funcAST, fset := parseFunc("example_test.go", "ExampleFprint")
+	//
+	// var buf bytes.Buffer
+	// printer.Fprint(&buf, fset, funcAST.Body)
+	//
+	// s := buf.String()
+	// s = s[1 : len(s)-1]
+	// s = strings.TrimSpace(strings.ReplaceAll(s, "\n\t", "\n"))
+	//
+	// fmt.Println(s)
+}
diff --git a/internal/backport/go/printer/gobuild.go b/internal/backport/go/printer/gobuild.go
new file mode 100644
index 0000000..f00492d
--- /dev/null
+++ b/internal/backport/go/printer/gobuild.go
@@ -0,0 +1,170 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package printer
+
+import (
+	"go/build/constraint"
+	"sort"
+	"text/tabwriter"
+)
+
+func (p *printer) fixGoBuildLines() {
+	if len(p.goBuild)+len(p.plusBuild) == 0 {
+		return
+	}
+
+	// Find latest possible placement of //go:build and // +build comments.
+	// That's just after the last blank line before we find a non-comment.
+	// (We'll add another blank line after our comment block.)
+	// When we start dropping // +build comments, we can skip over /* */ comments too.
+	// Note that we are processing tabwriter input, so every comment
+	// begins and ends with a tabwriter.Escape byte.
+	// And some newlines have turned into \f bytes.
+	insert := 0
+	for pos := 0; ; {
+		// Skip leading space at beginning of line.
+		blank := true
+		for pos < len(p.output) && (p.output[pos] == ' ' || p.output[pos] == '\t') {
+			pos++
+		}
+		// Skip over // comment if any.
+		if pos+3 < len(p.output) && p.output[pos] == tabwriter.Escape && p.output[pos+1] == '/' && p.output[pos+2] == '/' {
+			blank = false
+			for pos < len(p.output) && !isNL(p.output[pos]) {
+				pos++
+			}
+		}
+		// Skip over \n at end of line.
+		if pos >= len(p.output) || !isNL(p.output[pos]) {
+			break
+		}
+		pos++
+
+		if blank {
+			insert = pos
+		}
+	}
+
+	// If there is a //go:build comment before the place we identified,
+	// use that point instead. (Earlier in the file is always fine.)
+	if len(p.goBuild) > 0 && p.goBuild[0] < insert {
+		insert = p.goBuild[0]
+	} else if len(p.plusBuild) > 0 && p.plusBuild[0] < insert {
+		insert = p.plusBuild[0]
+	}
+
+	var x constraint.Expr
+	switch len(p.goBuild) {
+	case 0:
+		// Synthesize //go:build expression from // +build lines.
+		for _, pos := range p.plusBuild {
+			y, err := constraint.Parse(p.commentTextAt(pos))
+			if err != nil {
+				x = nil
+				break
+			}
+			if x == nil {
+				x = y
+			} else {
+				x = &constraint.AndExpr{X: x, Y: y}
+			}
+		}
+	case 1:
+		// Parse //go:build expression.
+		x, _ = constraint.Parse(p.commentTextAt(p.goBuild[0]))
+	}
+
+	var block []byte
+	if x == nil {
+		// Don't have a valid //go:build expression to treat as truth.
+		// Bring all the lines together but leave them alone.
+		// Note that these are already tabwriter-escaped.
+		for _, pos := range p.goBuild {
+			block = append(block, p.lineAt(pos)...)
+		}
+		for _, pos := range p.plusBuild {
+			block = append(block, p.lineAt(pos)...)
+		}
+	} else {
+		block = append(block, tabwriter.Escape)
+		block = append(block, "//go:build "...)
+		block = append(block, x.String()...)
+		block = append(block, tabwriter.Escape, '\n')
+		if len(p.plusBuild) > 0 {
+			lines, err := constraint.PlusBuildLines(x)
+			if err != nil {
+				lines = []string{"// +build error: " + err.Error()}
+			}
+			for _, line := range lines {
+				block = append(block, tabwriter.Escape)
+				block = append(block, line...)
+				block = append(block, tabwriter.Escape, '\n')
+			}
+		}
+	}
+	block = append(block, '\n')
+
+	// Build sorted list of lines to delete from remainder of output.
+	toDelete := append(p.goBuild, p.plusBuild...)
+	sort.Ints(toDelete)
+
+	// Collect output after insertion point, with lines deleted, into after.
+	var after []byte
+	start := insert
+	for _, end := range toDelete {
+		if end < start {
+			continue
+		}
+		after = appendLines(after, p.output[start:end])
+		start = end + len(p.lineAt(end))
+	}
+	after = appendLines(after, p.output[start:])
+	if n := len(after); n >= 2 && isNL(after[n-1]) && isNL(after[n-2]) {
+		after = after[:n-1]
+	}
+
+	p.output = p.output[:insert]
+	p.output = append(p.output, block...)
+	p.output = append(p.output, after...)
+}
+
+// appendLines is like append(x, y...)
+// but it avoids creating doubled blank lines,
+// which would not be gofmt-standard output.
+// It assumes that only whole blocks of lines are being appended,
+// not line fragments.
+func appendLines(x, y []byte) []byte {
+	if len(y) > 0 && isNL(y[0]) && // y starts in blank line
+		(len(x) == 0 || len(x) >= 2 && isNL(x[len(x)-1]) && isNL(x[len(x)-2])) { // x is empty or ends in blank line
+		y = y[1:] // delete y's leading blank line
+	}
+	return append(x, y...)
+}
+
+func (p *printer) lineAt(start int) []byte {
+	pos := start
+	for pos < len(p.output) && !isNL(p.output[pos]) {
+		pos++
+	}
+	if pos < len(p.output) {
+		pos++
+	}
+	return p.output[start:pos]
+}
+
+func (p *printer) commentTextAt(start int) string {
+	if start < len(p.output) && p.output[start] == tabwriter.Escape {
+		start++
+	}
+	pos := start
+	for pos < len(p.output) && p.output[pos] != tabwriter.Escape && !isNL(p.output[pos]) {
+		pos++
+	}
+	return string(p.output[start:pos])
+}
+
+func isNL(b byte) bool {
+	return b == '\n' || b == '\f'
+}
diff --git a/internal/backport/go/printer/nodes.go b/internal/backport/go/printer/nodes.go
new file mode 100644
index 0000000..82b6aed
--- /dev/null
+++ b/internal/backport/go/printer/nodes.go
@@ -0,0 +1,1904 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements printing of AST nodes; specifically
+// expressions, statements, declarations, and files. It uses
+// the print functionality implemented in printer.go.
+
+package printer
+
+import (
+	"bytes"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/token"
+	"math"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// Formatting issues:
+// - better comment formatting for /*-style comments at the end of a line (e.g. a declaration)
+//   when the comment spans multiple lines; if such a comment is just two lines, formatting is
+//   not idempotent
+// - formatting of expression lists
+// - should use blank instead of tab to separate one-line function bodies from
+//   the function header unless there is a group of consecutive one-liners
+
+// ----------------------------------------------------------------------------
+// Common AST nodes.
+
+// Print as many newlines as necessary (but at least min newlines) to get to
+// the current line. ws is printed before the first line break. If newSection
+// is set, the first line break is printed as formfeed. Returns 0 if no line
+// breaks were printed, returns 1 if there was exactly one newline printed,
+// and returns a value > 1 if there was a formfeed or more than one newline
+// printed.
+//
+// TODO(gri): linebreak may add too many lines if the next statement at "line"
+//
+//	is preceded by comments because the computation of n assumes
+//	the current position before the comment and the target position
+//	after the comment. Thus, after interspersing such comments, the
+//	space taken up by them is not considered to reduce the number of
+//	linebreaks. At the moment there is no easy way to know about
+//	future (not yet interspersed) comments in this function.
+func (p *printer) linebreak(line, min int, ws whiteSpace, newSection bool) (nbreaks int) {
+	n := nlimit(line - p.pos.Line)
+	if n < min {
+		n = min
+	}
+	if n > 0 {
+		p.print(ws)
+		if newSection {
+			p.print(formfeed)
+			n--
+			nbreaks = 2
+		}
+		nbreaks += n
+		for ; n > 0; n-- {
+			p.print(newline)
+		}
+	}
+	return
+}
+
+// setComment sets g as the next comment if g != nil and if node comments
+// are enabled - this mode is used when printing source code fragments such
+// as exports only. It assumes that there is no pending comment in p.comments
+// and at most one pending comment in the p.comment cache.
+func (p *printer) setComment(g *ast.CommentGroup) {
+	if g == nil || !p.useNodeComments {
+		return
+	}
+	if p.comments == nil {
+		// initialize p.comments lazily
+		p.comments = make([]*ast.CommentGroup, 1)
+	} else if p.cindex < len(p.comments) {
+		// for some reason there are pending comments; this
+		// should never happen - handle gracefully and flush
+		// all comments up to g, ignore anything after that
+		p.flush(p.posFor(g.List[0].Pos()), token.ILLEGAL)
+		p.comments = p.comments[0:1]
+		// in debug mode, report error
+		p.internalError("setComment found pending comments")
+	}
+	p.comments[0] = g
+	p.cindex = 0
+	// don't overwrite any pending comment in the p.comment cache
+	// (there may be a pending comment when a line comment is
+	// immediately followed by a lead comment with no other
+	// tokens between)
+	if p.commentOffset == infinity {
+		p.nextComment() // get comment ready for use
+	}
+}
+
+type exprListMode uint
+
+const (
+	commaTerm exprListMode = 1 << iota // list is optionally terminated by a comma
+	noIndent                           // no extra indentation in multi-line lists
+)
+
+// If indent is set, a multi-line identifier list is indented after the
+// first linebreak encountered.
+func (p *printer) identList(list []*ast.Ident, indent bool) {
+	// convert into an expression list so we can re-use exprList formatting
+	xlist := make([]ast.Expr, len(list))
+	for i, x := range list {
+		xlist[i] = x
+	}
+	var mode exprListMode
+	if !indent {
+		mode = noIndent
+	}
+	p.exprList(token.NoPos, xlist, 1, mode, token.NoPos, false)
+}
+
+const filteredMsg = "contains filtered or unexported fields"
+
+// Print a list of expressions. If the list spans multiple
+// source lines, the original line breaks are respected between
+// expressions.
+//
+// TODO(gri) Consider rewriting this to be independent of []ast.Expr
+//
+//	so that we can use the algorithm for any kind of list
+//	(e.g., pass list via a channel over which to range).
+func (p *printer) exprList(prev0 token.Pos, list []ast.Expr, depth int, mode exprListMode, next0 token.Pos, isIncomplete bool) {
+	if len(list) == 0 {
+		if isIncomplete {
+			prev := p.posFor(prev0)
+			next := p.posFor(next0)
+			if prev.IsValid() && prev.Line == next.Line {
+				p.print("/* " + filteredMsg + " */")
+			} else {
+				p.print(newline)
+				p.print(indent, "// "+filteredMsg, unindent, newline)
+			}
+		}
+		return
+	}
+
+	prev := p.posFor(prev0)
+	next := p.posFor(next0)
+	line := p.lineFor(list[0].Pos())
+	endLine := p.lineFor(list[len(list)-1].End())
+
+	if prev.IsValid() && prev.Line == line && line == endLine {
+		// all list entries on a single line
+		for i, x := range list {
+			if i > 0 {
+				// use position of expression following the comma as
+				// comma position for correct comment placement
+				p.print(x.Pos(), token.COMMA, blank)
+			}
+			p.expr0(x, depth)
+		}
+		if isIncomplete {
+			p.print(token.COMMA, blank, "/* "+filteredMsg+" */")
+		}
+		return
+	}
+
+	// list entries span multiple lines;
+	// use source code positions to guide line breaks
+
+	// Don't add extra indentation if noIndent is set;
+	// i.e., pretend that the first line is already indented.
+	ws := ignore
+	if mode&noIndent == 0 {
+		ws = indent
+	}
+
+	// The first linebreak is always a formfeed since this section must not
+	// depend on any previous formatting.
+	prevBreak := -1 // index of last expression that was followed by a linebreak
+	if prev.IsValid() && prev.Line < line && p.linebreak(line, 0, ws, true) > 0 {
+		ws = ignore
+		prevBreak = 0
+	}
+
+	// initialize expression/key size: a zero value indicates expr/key doesn't fit on a single line
+	size := 0
+
+	// We use the ratio between the geometric mean of the previous key sizes and
+	// the current size to determine if there should be a break in the alignment.
+	// To compute the geometric mean we accumulate the ln(size) values (lnsum)
+	// and the number of sizes included (count).
+	lnsum := 0.0
+	count := 0
+
+	// print all list elements
+	prevLine := prev.Line
+	for i, x := range list {
+		line = p.lineFor(x.Pos())
+
+		// Determine if the next linebreak, if any, needs to use formfeed:
+		// in general, use the entire node size to make the decision; for
+		// key:value expressions, use the key size.
+		// TODO(gri) for a better result, should probably incorporate both
+		//           the key and the node size into the decision process
+		useFF := true
+
+		// Determine element size: All bets are off if we don't have
+		// position information for the previous and next token (likely
+		// generated code - simply ignore the size in this case by setting
+		// it to 0).
+		prevSize := size
+		const infinity = 1e6 // larger than any source line
+		size = p.nodeSize(x, infinity)
+		pair, isPair := x.(*ast.KeyValueExpr)
+		if size <= infinity && prev.IsValid() && next.IsValid() {
+			// x fits on a single line
+			if isPair {
+				size = p.nodeSize(pair.Key, infinity) // size <= infinity
+			}
+		} else {
+			// size too large or we don't have good layout information
+			size = 0
+		}
+
+		// If the previous line and the current line had single-
+		// line-expressions and the key sizes are small or the
+		// ratio between the current key and the geometric mean
+		// if the previous key sizes does not exceed a threshold,
+		// align columns and do not use formfeed.
+		if prevSize > 0 && size > 0 {
+			const smallSize = 40
+			if count == 0 || prevSize <= smallSize && size <= smallSize {
+				useFF = false
+			} else {
+				const r = 2.5                               // threshold
+				geomean := math.Exp(lnsum / float64(count)) // count > 0
+				ratio := float64(size) / geomean
+				useFF = r*ratio <= 1 || r <= ratio
+			}
+		}
+
+		needsLinebreak := 0 < prevLine && prevLine < line
+		if i > 0 {
+			// Use position of expression following the comma as
+			// comma position for correct comment placement, but
+			// only if the expression is on the same line.
+			if !needsLinebreak {
+				p.print(x.Pos())
+			}
+			p.print(token.COMMA)
+			needsBlank := true
+			if needsLinebreak {
+				// Lines are broken using newlines so comments remain aligned
+				// unless useFF is set or there are multiple expressions on
+				// the same line in which case formfeed is used.
+				nbreaks := p.linebreak(line, 0, ws, useFF || prevBreak+1 < i)
+				if nbreaks > 0 {
+					ws = ignore
+					prevBreak = i
+					needsBlank = false // we got a line break instead
+				}
+				// If there was a new section or more than one new line
+				// (which means that the tabwriter will implicitly break
+				// the section), reset the geomean variables since we are
+				// starting a new group of elements with the next element.
+				if nbreaks > 1 {
+					lnsum = 0
+					count = 0
+				}
+			}
+			if needsBlank {
+				p.print(blank)
+			}
+		}
+
+		if len(list) > 1 && isPair && size > 0 && needsLinebreak {
+			// We have a key:value expression that fits onto one line
+			// and it's not on the same line as the prior expression:
+			// Use a column for the key such that consecutive entries
+			// can align if possible.
+			// (needsLinebreak is set if we started a new line before)
+			p.expr(pair.Key)
+			p.print(pair.Colon, token.COLON, vtab)
+			p.expr(pair.Value)
+		} else {
+			p.expr0(x, depth)
+		}
+
+		if size > 0 {
+			lnsum += math.Log(float64(size))
+			count++
+		}
+
+		prevLine = line
+	}
+
+	if mode&commaTerm != 0 && next.IsValid() && p.pos.Line < next.Line {
+		// Print a terminating comma if the next token is on a new line.
+		p.print(token.COMMA)
+		if isIncomplete {
+			p.print(newline)
+			p.print("// " + filteredMsg)
+		}
+		if ws == ignore && mode&noIndent == 0 {
+			// unindent if we indented
+			p.print(unindent)
+		}
+		p.print(formfeed) // terminating comma needs a line break to look good
+		return
+	}
+
+	if isIncomplete {
+		p.print(token.COMMA, newline)
+		p.print("// "+filteredMsg, newline)
+	}
+
+	if ws == ignore && mode&noIndent == 0 {
+		// unindent if we indented
+		p.print(unindent)
+	}
+}
+
+type paramMode int
+
+const (
+	funcParam paramMode = iota
+	funcTParam
+	typeTParam
+)
+
+func (p *printer) parameters(fields *ast.FieldList, mode paramMode) {
+	openTok, closeTok := token.LPAREN, token.RPAREN
+	if mode != funcParam {
+		openTok, closeTok = token.LBRACK, token.RBRACK
+	}
+	p.print(fields.Opening, openTok)
+	if len(fields.List) > 0 {
+		prevLine := p.lineFor(fields.Opening)
+		ws := indent
+		for i, par := range fields.List {
+			// determine par begin and end line (may be different
+			// if there are multiple parameter names for this par
+			// or the type is on a separate line)
+			parLineBeg := p.lineFor(par.Pos())
+			parLineEnd := p.lineFor(par.End())
+			// separating "," if needed
+			needsLinebreak := 0 < prevLine && prevLine < parLineBeg
+			if i > 0 {
+				// use position of parameter following the comma as
+				// comma position for correct comma placement, but
+				// only if the next parameter is on the same line
+				if !needsLinebreak {
+					p.print(par.Pos())
+				}
+				p.print(token.COMMA)
+			}
+			// separator if needed (linebreak or blank)
+			if needsLinebreak && p.linebreak(parLineBeg, 0, ws, true) > 0 {
+				// break line if the opening "(" or previous parameter ended on a different line
+				ws = ignore
+			} else if i > 0 {
+				p.print(blank)
+			}
+			// parameter names
+			if len(par.Names) > 0 {
+				// Very subtle: If we indented before (ws == ignore), identList
+				// won't indent again. If we didn't (ws == indent), identList will
+				// indent if the identList spans multiple lines, and it will outdent
+				// again at the end (and still ws == indent). Thus, a subsequent indent
+				// by a linebreak call after a type, or in the next multi-line identList
+				// will do the right thing.
+				p.identList(par.Names, ws == indent)
+				p.print(blank)
+			}
+			// parameter type
+			p.expr(stripParensAlways(par.Type))
+			prevLine = parLineEnd
+		}
+
+		// if the closing ")" is on a separate line from the last parameter,
+		// print an additional "," and line break
+		if closing := p.lineFor(fields.Closing); 0 < prevLine && prevLine < closing {
+			p.print(token.COMMA)
+			p.linebreak(closing, 0, ignore, true)
+		} else if mode == typeTParam && fields.NumFields() == 1 {
+			// Otherwise, if we are in a type parameter list that could be confused
+			// with the constant array length expression [P*C], print a comma so that
+			// parsing is unambiguous.
+			//
+			// Note that while ParenExprs can also be ambiguous (issue #49482), the
+			// printed type is never parenthesized (stripParensAlways is used above).
+			if t, _ := fields.List[0].Type.(*ast.StarExpr); t != nil && !isTypeLit(t.X) {
+				p.print(token.COMMA)
+			}
+		}
+
+		// unindent if we indented
+		if ws == ignore {
+			p.print(unindent)
+		}
+	}
+
+	p.print(fields.Closing, closeTok)
+}
+
+// isTypeLit reports whether x is a (possibly parenthesized) type literal.
+func isTypeLit(x ast.Expr) bool {
+	switch x := x.(type) {
+	case *ast.ArrayType, *ast.StructType, *ast.FuncType, *ast.InterfaceType, *ast.MapType, *ast.ChanType:
+		return true
+	case *ast.StarExpr:
+		// *T may be a pointer dereferenciation.
+		// Only consider *T as type literal if T is a type literal.
+		return isTypeLit(x.X)
+	case *ast.ParenExpr:
+		return isTypeLit(x.X)
+	}
+	return false
+}
+
+func (p *printer) signature(sig *ast.FuncType) {
+	if sig.TypeParams != nil {
+		p.parameters(sig.TypeParams, funcTParam)
+	}
+	if sig.Params != nil {
+		p.parameters(sig.Params, funcParam)
+	} else {
+		p.print(token.LPAREN, token.RPAREN)
+	}
+	res := sig.Results
+	n := res.NumFields()
+	if n > 0 {
+		// res != nil
+		p.print(blank)
+		if n == 1 && res.List[0].Names == nil {
+			// single anonymous res; no ()'s
+			p.expr(stripParensAlways(res.List[0].Type))
+			return
+		}
+		p.parameters(res, funcParam)
+	}
+}
+
+func identListSize(list []*ast.Ident, maxSize int) (size int) {
+	for i, x := range list {
+		if i > 0 {
+			size += len(", ")
+		}
+		size += utf8.RuneCountInString(x.Name)
+		if size >= maxSize {
+			break
+		}
+	}
+	return
+}
+
+func (p *printer) isOneLineFieldList(list []*ast.Field) bool {
+	if len(list) != 1 {
+		return false // allow only one field
+	}
+	f := list[0]
+	if f.Tag != nil || f.Comment != nil {
+		return false // don't allow tags or comments
+	}
+	// only name(s) and type
+	const maxSize = 30 // adjust as appropriate, this is an approximate value
+	namesSize := identListSize(f.Names, maxSize)
+	if namesSize > 0 {
+		namesSize = 1 // blank between names and types
+	}
+	typeSize := p.nodeSize(f.Type, maxSize)
+	return namesSize+typeSize <= maxSize
+}
+
+func (p *printer) setLineComment(text string) {
+	p.setComment(&ast.CommentGroup{List: []*ast.Comment{{Slash: token.NoPos, Text: text}}})
+}
+
+func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool) {
+	lbrace := fields.Opening
+	list := fields.List
+	rbrace := fields.Closing
+	hasComments := isIncomplete || p.commentBefore(p.posFor(rbrace))
+	srcIsOneLine := lbrace.IsValid() && rbrace.IsValid() && p.lineFor(lbrace) == p.lineFor(rbrace)
+
+	if !hasComments && srcIsOneLine {
+		// possibly a one-line struct/interface
+		if len(list) == 0 {
+			// no blank between keyword and {} in this case
+			p.print(lbrace, token.LBRACE, rbrace, token.RBRACE)
+			return
+		} else if p.isOneLineFieldList(list) {
+			// small enough - print on one line
+			// (don't use identList and ignore source line breaks)
+			p.print(lbrace, token.LBRACE, blank)
+			f := list[0]
+			if isStruct {
+				for i, x := range f.Names {
+					if i > 0 {
+						// no comments so no need for comma position
+						p.print(token.COMMA, blank)
+					}
+					p.expr(x)
+				}
+				if len(f.Names) > 0 {
+					p.print(blank)
+				}
+				p.expr(f.Type)
+			} else { // interface
+				if len(f.Names) > 0 {
+					name := f.Names[0] // method name
+					p.expr(name)
+					p.signature(f.Type.(*ast.FuncType)) // don't print "func"
+				} else {
+					// embedded interface
+					p.expr(f.Type)
+				}
+			}
+			p.print(blank, rbrace, token.RBRACE)
+			return
+		}
+	}
+	// hasComments || !srcIsOneLine
+
+	p.print(blank, lbrace, token.LBRACE, indent)
+	if hasComments || len(list) > 0 {
+		p.print(formfeed)
+	}
+
+	if isStruct {
+
+		sep := vtab
+		if len(list) == 1 {
+			sep = blank
+		}
+		var line int
+		for i, f := range list {
+			if i > 0 {
+				p.linebreak(p.lineFor(f.Pos()), 1, ignore, p.linesFrom(line) > 0)
+			}
+			extraTabs := 0
+			p.setComment(f.Doc)
+			p.recordLine(&line)
+			if len(f.Names) > 0 {
+				// named fields
+				p.identList(f.Names, false)
+				p.print(sep)
+				p.expr(f.Type)
+				extraTabs = 1
+			} else {
+				// anonymous field
+				p.expr(f.Type)
+				extraTabs = 2
+			}
+			if f.Tag != nil {
+				if len(f.Names) > 0 && sep == vtab {
+					p.print(sep)
+				}
+				p.print(sep)
+				p.expr(f.Tag)
+				extraTabs = 0
+			}
+			if f.Comment != nil {
+				for ; extraTabs > 0; extraTabs-- {
+					p.print(sep)
+				}
+				p.setComment(f.Comment)
+			}
+		}
+		if isIncomplete {
+			if len(list) > 0 {
+				p.print(formfeed)
+			}
+			p.flush(p.posFor(rbrace), token.RBRACE) // make sure we don't lose the last line comment
+			p.setLineComment("// " + filteredMsg)
+		}
+
+	} else { // interface
+
+		var line int
+		var prev *ast.Ident // previous "type" identifier
+		for i, f := range list {
+			var name *ast.Ident // first name, or nil
+			if len(f.Names) > 0 {
+				name = f.Names[0]
+			}
+			if i > 0 {
+				// don't do a line break (min == 0) if we are printing a list of types
+				// TODO(gri) this doesn't work quite right if the list of types is
+				//           spread across multiple lines
+				min := 1
+				if prev != nil && name == prev {
+					min = 0
+				}
+				p.linebreak(p.lineFor(f.Pos()), min, ignore, p.linesFrom(line) > 0)
+			}
+			p.setComment(f.Doc)
+			p.recordLine(&line)
+			if name != nil {
+				// method
+				p.expr(name)
+				p.signature(f.Type.(*ast.FuncType)) // don't print "func"
+				prev = nil
+			} else {
+				// embedded interface
+				p.expr(f.Type)
+				prev = nil
+			}
+			p.setComment(f.Comment)
+		}
+		if isIncomplete {
+			if len(list) > 0 {
+				p.print(formfeed)
+			}
+			p.flush(p.posFor(rbrace), token.RBRACE) // make sure we don't lose the last line comment
+			p.setLineComment("// contains filtered or unexported methods")
+		}
+
+	}
+	p.print(unindent, formfeed, rbrace, token.RBRACE)
+}
+
+// ----------------------------------------------------------------------------
+// Expressions
+
+func walkBinary(e *ast.BinaryExpr) (has4, has5 bool, maxProblem int) {
+	switch e.Op.Precedence() {
+	case 4:
+		has4 = true
+	case 5:
+		has5 = true
+	}
+
+	switch l := e.X.(type) {
+	case *ast.BinaryExpr:
+		if l.Op.Precedence() < e.Op.Precedence() {
+			// parens will be inserted.
+			// pretend this is an *ast.ParenExpr and do nothing.
+			break
+		}
+		h4, h5, mp := walkBinary(l)
+		has4 = has4 || h4
+		has5 = has5 || h5
+		if maxProblem < mp {
+			maxProblem = mp
+		}
+	}
+
+	switch r := e.Y.(type) {
+	case *ast.BinaryExpr:
+		if r.Op.Precedence() <= e.Op.Precedence() {
+			// parens will be inserted.
+			// pretend this is an *ast.ParenExpr and do nothing.
+			break
+		}
+		h4, h5, mp := walkBinary(r)
+		has4 = has4 || h4
+		has5 = has5 || h5
+		if maxProblem < mp {
+			maxProblem = mp
+		}
+
+	case *ast.StarExpr:
+		if e.Op == token.QUO { // `*/`
+			maxProblem = 5
+		}
+
+	case *ast.UnaryExpr:
+		switch e.Op.String() + r.Op.String() {
+		case "/*", "&&", "&^":
+			maxProblem = 5
+		case "++", "--":
+			if maxProblem < 4 {
+				maxProblem = 4
+			}
+		}
+	}
+	return
+}
+
+func cutoff(e *ast.BinaryExpr, depth int) int {
+	has4, has5, maxProblem := walkBinary(e)
+	if maxProblem > 0 {
+		return maxProblem + 1
+	}
+	if has4 && has5 {
+		if depth == 1 {
+			return 5
+		}
+		return 4
+	}
+	if depth == 1 {
+		return 6
+	}
+	return 4
+}
+
+func diffPrec(expr ast.Expr, prec int) int {
+	x, ok := expr.(*ast.BinaryExpr)
+	if !ok || prec != x.Op.Precedence() {
+		return 1
+	}
+	return 0
+}
+
+func reduceDepth(depth int) int {
+	depth--
+	if depth < 1 {
+		depth = 1
+	}
+	return depth
+}
+
+// Format the binary expression: decide the cutoff and then format.
+// Let's call depth == 1 Normal mode, and depth > 1 Compact mode.
+// (Algorithm suggestion by Russ Cox.)
+//
+// The precedences are:
+//
+//	5             *  /  %  <<  >>  &  &^
+//	4             +  -  |  ^
+//	3             ==  !=  <  <=  >  >=
+//	2             &&
+//	1             ||
+//
+// The only decision is whether there will be spaces around levels 4 and 5.
+// There are never spaces at level 6 (unary), and always spaces at levels 3 and below.
+//
+// To choose the cutoff, look at the whole expression but excluding primary
+// expressions (function calls, parenthesized exprs), and apply these rules:
+//
+//  1. If there is a binary operator with a right side unary operand
+//     that would clash without a space, the cutoff must be (in order):
+//
+//     /*	6
+//     &&	6
+//     &^	6
+//     ++	5
+//     --	5
+//
+//     (Comparison operators always have spaces around them.)
+//
+//  2. If there is a mix of level 5 and level 4 operators, then the cutoff
+//     is 5 (use spaces to distinguish precedence) in Normal mode
+//     and 4 (never use spaces) in Compact mode.
+//
+//  3. If there are no level 4 operators or no level 5 operators, then the
+//     cutoff is 6 (always use spaces) in Normal mode
+//     and 4 (never use spaces) in Compact mode.
+func (p *printer) binaryExpr(x *ast.BinaryExpr, prec1, cutoff, depth int) {
+	prec := x.Op.Precedence()
+	if prec < prec1 {
+		// parenthesis needed
+		// Note: The parser inserts an ast.ParenExpr node; thus this case
+		//       can only occur if the AST is created in a different way.
+		p.print(token.LPAREN)
+		p.expr0(x, reduceDepth(depth)) // parentheses undo one level of depth
+		p.print(token.RPAREN)
+		return
+	}
+
+	printBlank := prec < cutoff
+
+	ws := indent
+	p.expr1(x.X, prec, depth+diffPrec(x.X, prec))
+	if printBlank {
+		p.print(blank)
+	}
+	xline := p.pos.Line // before the operator (it may be on the next line!)
+	yline := p.lineFor(x.Y.Pos())
+	p.print(x.OpPos, x.Op)
+	if xline != yline && xline > 0 && yline > 0 {
+		// at least one line break, but respect an extra empty line
+		// in the source
+		if p.linebreak(yline, 1, ws, true) > 0 {
+			ws = ignore
+			printBlank = false // no blank after line break
+		}
+	}
+	if printBlank {
+		p.print(blank)
+	}
+	p.expr1(x.Y, prec+1, depth+1)
+	if ws == ignore {
+		p.print(unindent)
+	}
+}
+
+func isBinary(expr ast.Expr) bool {
+	_, ok := expr.(*ast.BinaryExpr)
+	return ok
+}
+
+func (p *printer) expr1(expr ast.Expr, prec1, depth int) {
+	p.print(expr.Pos())
+
+	switch x := expr.(type) {
+	case *ast.BadExpr:
+		p.print("BadExpr")
+
+	case *ast.Ident:
+		p.print(x)
+
+	case *ast.BinaryExpr:
+		if depth < 1 {
+			p.internalError("depth < 1:", depth)
+			depth = 1
+		}
+		p.binaryExpr(x, prec1, cutoff(x, depth), depth)
+
+	case *ast.KeyValueExpr:
+		p.expr(x.Key)
+		p.print(x.Colon, token.COLON, blank)
+		p.expr(x.Value)
+
+	case *ast.StarExpr:
+		const prec = token.UnaryPrec
+		if prec < prec1 {
+			// parenthesis needed
+			p.print(token.LPAREN)
+			p.print(token.MUL)
+			p.expr(x.X)
+			p.print(token.RPAREN)
+		} else {
+			// no parenthesis needed
+			p.print(token.MUL)
+			p.expr(x.X)
+		}
+
+	case *ast.UnaryExpr:
+		const prec = token.UnaryPrec
+		if prec < prec1 {
+			// parenthesis needed
+			p.print(token.LPAREN)
+			p.expr(x)
+			p.print(token.RPAREN)
+		} else {
+			// no parenthesis needed
+			p.print(x.Op)
+			if x.Op == token.RANGE {
+				// TODO(gri) Remove this code if it cannot be reached.
+				p.print(blank)
+			}
+			p.expr1(x.X, prec, depth)
+		}
+
+	case *ast.BasicLit:
+		if p.Config.Mode&normalizeNumbers != 0 {
+			x = normalizedNumber(x)
+		}
+		p.print(x)
+
+	case *ast.FuncLit:
+		p.print(x.Type.Pos(), token.FUNC)
+		// See the comment in funcDecl about how the header size is computed.
+		startCol := p.out.Column - len("func")
+		p.signature(x.Type)
+		p.funcBody(p.distanceFrom(x.Type.Pos(), startCol), blank, x.Body)
+
+	case *ast.ParenExpr:
+		if _, hasParens := x.X.(*ast.ParenExpr); hasParens {
+			// don't print parentheses around an already parenthesized expression
+			// TODO(gri) consider making this more general and incorporate precedence levels
+			p.expr0(x.X, depth)
+		} else {
+			p.print(token.LPAREN)
+			p.expr0(x.X, reduceDepth(depth)) // parentheses undo one level of depth
+			p.print(x.Rparen, token.RPAREN)
+		}
+
+	case *ast.SelectorExpr:
+		p.selectorExpr(x, depth, false)
+
+	case *ast.TypeAssertExpr:
+		p.expr1(x.X, token.HighestPrec, depth)
+		p.print(token.PERIOD, x.Lparen, token.LPAREN)
+		if x.Type != nil {
+			p.expr(x.Type)
+		} else {
+			p.print(token.TYPE)
+		}
+		p.print(x.Rparen, token.RPAREN)
+
+	case *ast.IndexExpr:
+		// TODO(gri): should treat[] like parentheses and undo one level of depth
+		p.expr1(x.X, token.HighestPrec, 1)
+		p.print(x.Lbrack, token.LBRACK)
+		p.expr0(x.Index, depth+1)
+		p.print(x.Rbrack, token.RBRACK)
+
+	case *ast.IndexListExpr:
+		// TODO(gri): as for IndexExpr, should treat [] like parentheses and undo
+		// one level of depth
+		p.expr1(x.X, token.HighestPrec, 1)
+		p.print(x.Lbrack, token.LBRACK)
+		p.exprList(x.Lbrack, x.Indices, depth+1, commaTerm, x.Rbrack, false)
+		p.print(x.Rbrack, token.RBRACK)
+
+	case *ast.SliceExpr:
+		// TODO(gri): should treat[] like parentheses and undo one level of depth
+		p.expr1(x.X, token.HighestPrec, 1)
+		p.print(x.Lbrack, token.LBRACK)
+		indices := []ast.Expr{x.Low, x.High}
+		if x.Max != nil {
+			indices = append(indices, x.Max)
+		}
+		// determine if we need extra blanks around ':'
+		var needsBlanks bool
+		if depth <= 1 {
+			var indexCount int
+			var hasBinaries bool
+			for _, x := range indices {
+				if x != nil {
+					indexCount++
+					if isBinary(x) {
+						hasBinaries = true
+					}
+				}
+			}
+			if indexCount > 1 && hasBinaries {
+				needsBlanks = true
+			}
+		}
+		for i, x := range indices {
+			if i > 0 {
+				if indices[i-1] != nil && needsBlanks {
+					p.print(blank)
+				}
+				p.print(token.COLON)
+				if x != nil && needsBlanks {
+					p.print(blank)
+				}
+			}
+			if x != nil {
+				p.expr0(x, depth+1)
+			}
+		}
+		p.print(x.Rbrack, token.RBRACK)
+
+	case *ast.CallExpr:
+		if len(x.Args) > 1 {
+			depth++
+		}
+		var wasIndented bool
+		if _, ok := x.Fun.(*ast.FuncType); ok {
+			// conversions to literal function types require parentheses around the type
+			p.print(token.LPAREN)
+			wasIndented = p.possibleSelectorExpr(x.Fun, token.HighestPrec, depth)
+			p.print(token.RPAREN)
+		} else {
+			wasIndented = p.possibleSelectorExpr(x.Fun, token.HighestPrec, depth)
+		}
+		p.print(x.Lparen, token.LPAREN)
+		if x.Ellipsis.IsValid() {
+			p.exprList(x.Lparen, x.Args, depth, 0, x.Ellipsis, false)
+			p.print(x.Ellipsis, token.ELLIPSIS)
+			if x.Rparen.IsValid() && p.lineFor(x.Ellipsis) < p.lineFor(x.Rparen) {
+				p.print(token.COMMA, formfeed)
+			}
+		} else {
+			p.exprList(x.Lparen, x.Args, depth, commaTerm, x.Rparen, false)
+		}
+		p.print(x.Rparen, token.RPAREN)
+		if wasIndented {
+			p.print(unindent)
+		}
+
+	case *ast.CompositeLit:
+		// composite literal elements that are composite literals themselves may have the type omitted
+		if x.Type != nil {
+			p.expr1(x.Type, token.HighestPrec, depth)
+		}
+		p.level++
+		p.print(x.Lbrace, token.LBRACE)
+		p.exprList(x.Lbrace, x.Elts, 1, commaTerm, x.Rbrace, x.Incomplete)
+		// do not insert extra line break following a /*-style comment
+		// before the closing '}' as it might break the code if there
+		// is no trailing ','
+		mode := noExtraLinebreak
+		// do not insert extra blank following a /*-style comment
+		// before the closing '}' unless the literal is empty
+		if len(x.Elts) > 0 {
+			mode |= noExtraBlank
+		}
+		// need the initial indent to print lone comments with
+		// the proper level of indentation
+		p.print(indent, unindent, mode, x.Rbrace, token.RBRACE, mode)
+		p.level--
+
+	case *ast.Ellipsis:
+		p.print(token.ELLIPSIS)
+		if x.Elt != nil {
+			p.expr(x.Elt)
+		}
+
+	case *ast.ArrayType:
+		p.print(token.LBRACK)
+		if x.Len != nil {
+			p.expr(x.Len)
+		}
+		p.print(token.RBRACK)
+		p.expr(x.Elt)
+
+	case *ast.StructType:
+		p.print(token.STRUCT)
+		p.fieldList(x.Fields, true, x.Incomplete)
+
+	case *ast.FuncType:
+		p.print(token.FUNC)
+		p.signature(x)
+
+	case *ast.InterfaceType:
+		p.print(token.INTERFACE)
+		p.fieldList(x.Methods, false, x.Incomplete)
+
+	case *ast.MapType:
+		p.print(token.MAP, token.LBRACK)
+		p.expr(x.Key)
+		p.print(token.RBRACK)
+		p.expr(x.Value)
+
+	case *ast.ChanType:
+		switch x.Dir {
+		case ast.SEND | ast.RECV:
+			p.print(token.CHAN)
+		case ast.RECV:
+			p.print(token.ARROW, token.CHAN) // x.Arrow and x.Pos() are the same
+		case ast.SEND:
+			p.print(token.CHAN, x.Arrow, token.ARROW)
+		}
+		p.print(blank)
+		p.expr(x.Value)
+
+	default:
+		panic("unreachable")
+	}
+}
+
+// normalizedNumber rewrites base prefixes and exponents
+// of numbers to use lower-case letters (0X123 to 0x123 and 1.2E3 to 1.2e3),
+// and removes leading 0's from integer imaginary literals (0765i to 765i).
+// It leaves hexadecimal digits alone.
+//
+// normalizedNumber doesn't modify the ast.BasicLit value lit points to.
+// If lit is not a number or a number in canonical format already,
+// lit is returned as is. Otherwise a new ast.BasicLit is created.
+func normalizedNumber(lit *ast.BasicLit) *ast.BasicLit {
+	if lit.Kind != token.INT && lit.Kind != token.FLOAT && lit.Kind != token.IMAG {
+		return lit // not a number - nothing to do
+	}
+	if len(lit.Value) < 2 {
+		return lit // only one digit (common case) - nothing to do
+	}
+	// len(lit.Value) >= 2
+
+	// We ignore lit.Kind because for lit.Kind == token.IMAG the literal may be an integer
+	// or floating-point value, decimal or not. Instead, just consider the literal pattern.
+	x := lit.Value
+	switch x[:2] {
+	default:
+		// 0-prefix octal, decimal int, or float (possibly with 'i' suffix)
+		if i := strings.LastIndexByte(x, 'E'); i >= 0 {
+			x = x[:i] + "e" + x[i+1:]
+			break
+		}
+		// remove leading 0's from integer (but not floating-point) imaginary literals
+		if x[len(x)-1] == 'i' && !strings.ContainsAny(x, ".e") {
+			x = strings.TrimLeft(x, "0_")
+			if x == "i" {
+				x = "0i"
+			}
+		}
+	case "0X":
+		x = "0x" + x[2:]
+		// possibly a hexadecimal float
+		if i := strings.LastIndexByte(x, 'P'); i >= 0 {
+			x = x[:i] + "p" + x[i+1:]
+		}
+	case "0x":
+		// possibly a hexadecimal float
+		i := strings.LastIndexByte(x, 'P')
+		if i == -1 {
+			return lit // nothing to do
+		}
+		x = x[:i] + "p" + x[i+1:]
+	case "0O":
+		x = "0o" + x[2:]
+	case "0o":
+		return lit // nothing to do
+	case "0B":
+		x = "0b" + x[2:]
+	case "0b":
+		return lit // nothing to do
+	}
+
+	return &ast.BasicLit{ValuePos: lit.ValuePos, Kind: lit.Kind, Value: x}
+}
+
+func (p *printer) possibleSelectorExpr(expr ast.Expr, prec1, depth int) bool {
+	if x, ok := expr.(*ast.SelectorExpr); ok {
+		return p.selectorExpr(x, depth, true)
+	}
+	p.expr1(expr, prec1, depth)
+	return false
+}
+
+// selectorExpr handles an *ast.SelectorExpr node and reports whether x spans
+// multiple lines.
+func (p *printer) selectorExpr(x *ast.SelectorExpr, depth int, isMethod bool) bool {
+	p.expr1(x.X, token.HighestPrec, depth)
+	p.print(token.PERIOD)
+	if line := p.lineFor(x.Sel.Pos()); p.pos.IsValid() && p.pos.Line < line {
+		p.print(indent, newline, x.Sel.Pos(), x.Sel)
+		if !isMethod {
+			p.print(unindent)
+		}
+		return true
+	}
+	p.print(x.Sel.Pos(), x.Sel)
+	return false
+}
+
+func (p *printer) expr0(x ast.Expr, depth int) {
+	p.expr1(x, token.LowestPrec, depth)
+}
+
+func (p *printer) expr(x ast.Expr) {
+	const depth = 1
+	p.expr1(x, token.LowestPrec, depth)
+}
+
+// ----------------------------------------------------------------------------
+// Statements
+
+// Print the statement list indented, but without a newline after the last statement.
+// Extra line breaks between statements in the source are respected but at most one
+// empty line is printed between statements.
+func (p *printer) stmtList(list []ast.Stmt, nindent int, nextIsRBrace bool) {
+	if nindent > 0 {
+		p.print(indent)
+	}
+	var line int
+	i := 0
+	for _, s := range list {
+		// ignore empty statements (was issue 3466)
+		if _, isEmpty := s.(*ast.EmptyStmt); !isEmpty {
+			// nindent == 0 only for lists of switch/select case clauses;
+			// in those cases each clause is a new section
+			if len(p.output) > 0 {
+				// only print line break if we are not at the beginning of the output
+				// (i.e., we are not printing only a partial program)
+				p.linebreak(p.lineFor(s.Pos()), 1, ignore, i == 0 || nindent == 0 || p.linesFrom(line) > 0)
+			}
+			p.recordLine(&line)
+			p.stmt(s, nextIsRBrace && i == len(list)-1)
+			// labeled statements put labels on a separate line, but here
+			// we only care about the start line of the actual statement
+			// without label - correct line for each label
+			for t := s; ; {
+				lt, _ := t.(*ast.LabeledStmt)
+				if lt == nil {
+					break
+				}
+				line++
+				t = lt.Stmt
+			}
+			i++
+		}
+	}
+	if nindent > 0 {
+		p.print(unindent)
+	}
+}
+
+// block prints an *ast.BlockStmt; it always spans at least two lines.
+func (p *printer) block(b *ast.BlockStmt, nindent int) {
+	p.print(b.Lbrace, token.LBRACE)
+	p.stmtList(b.List, nindent, true)
+	p.linebreak(p.lineFor(b.Rbrace), 1, ignore, true)
+	p.print(b.Rbrace, token.RBRACE)
+}
+
+func isTypeName(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.Ident:
+		return true
+	case *ast.SelectorExpr:
+		return isTypeName(t.X)
+	}
+	return false
+}
+
+func stripParens(x ast.Expr) ast.Expr {
+	if px, strip := x.(*ast.ParenExpr); strip {
+		// parentheses must not be stripped if there are any
+		// unparenthesized composite literals starting with
+		// a type name
+		ast.Inspect(px.X, func(node ast.Node) bool {
+			switch x := node.(type) {
+			case *ast.ParenExpr:
+				// parentheses protect enclosed composite literals
+				return false
+			case *ast.CompositeLit:
+				if isTypeName(x.Type) {
+					strip = false // do not strip parentheses
+				}
+				return false
+			}
+			// in all other cases, keep inspecting
+			return true
+		})
+		if strip {
+			return stripParens(px.X)
+		}
+	}
+	return x
+}
+
+func stripParensAlways(x ast.Expr) ast.Expr {
+	if x, ok := x.(*ast.ParenExpr); ok {
+		return stripParensAlways(x.X)
+	}
+	return x
+}
+
+func (p *printer) controlClause(isForStmt bool, init ast.Stmt, expr ast.Expr, post ast.Stmt) {
+	p.print(blank)
+	needsBlank := false
+	if init == nil && post == nil {
+		// no semicolons required
+		if expr != nil {
+			p.expr(stripParens(expr))
+			needsBlank = true
+		}
+	} else {
+		// all semicolons required
+		// (they are not separators, print them explicitly)
+		if init != nil {
+			p.stmt(init, false)
+		}
+		p.print(token.SEMICOLON, blank)
+		if expr != nil {
+			p.expr(stripParens(expr))
+			needsBlank = true
+		}
+		if isForStmt {
+			p.print(token.SEMICOLON, blank)
+			needsBlank = false
+			if post != nil {
+				p.stmt(post, false)
+				needsBlank = true
+			}
+		}
+	}
+	if needsBlank {
+		p.print(blank)
+	}
+}
+
+// indentList reports whether an expression list would look better if it
+// were indented wholesale (starting with the very first element, rather
+// than starting at the first line break).
+func (p *printer) indentList(list []ast.Expr) bool {
+	// Heuristic: indentList reports whether there are more than one multi-
+	// line element in the list, or if there is any element that is not
+	// starting on the same line as the previous one ends.
+	if len(list) >= 2 {
+		var b = p.lineFor(list[0].Pos())
+		var e = p.lineFor(list[len(list)-1].End())
+		if 0 < b && b < e {
+			// list spans multiple lines
+			n := 0 // multi-line element count
+			line := b
+			for _, x := range list {
+				xb := p.lineFor(x.Pos())
+				xe := p.lineFor(x.End())
+				if line < xb {
+					// x is not starting on the same
+					// line as the previous one ended
+					return true
+				}
+				if xb < xe {
+					// x is a multi-line element
+					n++
+				}
+				line = xe
+			}
+			return n > 1
+		}
+	}
+	return false
+}
+
+func (p *printer) stmt(stmt ast.Stmt, nextIsRBrace bool) {
+	p.print(stmt.Pos())
+
+	switch s := stmt.(type) {
+	case *ast.BadStmt:
+		p.print("BadStmt")
+
+	case *ast.DeclStmt:
+		p.decl(s.Decl)
+
+	case *ast.EmptyStmt:
+		// nothing to do
+
+	case *ast.LabeledStmt:
+		// a "correcting" unindent immediately following a line break
+		// is applied before the line break if there is no comment
+		// between (see writeWhitespace)
+		p.print(unindent)
+		p.expr(s.Label)
+		p.print(s.Colon, token.COLON, indent)
+		if e, isEmpty := s.Stmt.(*ast.EmptyStmt); isEmpty {
+			if !nextIsRBrace {
+				p.print(newline, e.Pos(), token.SEMICOLON)
+				break
+			}
+		} else {
+			p.linebreak(p.lineFor(s.Stmt.Pos()), 1, ignore, true)
+		}
+		p.stmt(s.Stmt, nextIsRBrace)
+
+	case *ast.ExprStmt:
+		const depth = 1
+		p.expr0(s.X, depth)
+
+	case *ast.SendStmt:
+		const depth = 1
+		p.expr0(s.Chan, depth)
+		p.print(blank, s.Arrow, token.ARROW, blank)
+		p.expr0(s.Value, depth)
+
+	case *ast.IncDecStmt:
+		const depth = 1
+		p.expr0(s.X, depth+1)
+		p.print(s.TokPos, s.Tok)
+
+	case *ast.AssignStmt:
+		var depth = 1
+		if len(s.Lhs) > 1 && len(s.Rhs) > 1 {
+			depth++
+		}
+		p.exprList(s.Pos(), s.Lhs, depth, 0, s.TokPos, false)
+		p.print(blank, s.TokPos, s.Tok, blank)
+		p.exprList(s.TokPos, s.Rhs, depth, 0, token.NoPos, false)
+
+	case *ast.GoStmt:
+		p.print(token.GO, blank)
+		p.expr(s.Call)
+
+	case *ast.DeferStmt:
+		p.print(token.DEFER, blank)
+		p.expr(s.Call)
+
+	case *ast.ReturnStmt:
+		p.print(token.RETURN)
+		if s.Results != nil {
+			p.print(blank)
+			// Use indentList heuristic to make corner cases look
+			// better (issue 1207). A more systematic approach would
+			// always indent, but this would cause significant
+			// reformatting of the code base and not necessarily
+			// lead to more nicely formatted code in general.
+			if p.indentList(s.Results) {
+				p.print(indent)
+				// Use NoPos so that a newline never goes before
+				// the results (see issue #32854).
+				p.exprList(token.NoPos, s.Results, 1, noIndent, token.NoPos, false)
+				p.print(unindent)
+			} else {
+				p.exprList(token.NoPos, s.Results, 1, 0, token.NoPos, false)
+			}
+		}
+
+	case *ast.BranchStmt:
+		p.print(s.Tok)
+		if s.Label != nil {
+			p.print(blank)
+			p.expr(s.Label)
+		}
+
+	case *ast.BlockStmt:
+		p.block(s, 1)
+
+	case *ast.IfStmt:
+		p.print(token.IF)
+		p.controlClause(false, s.Init, s.Cond, nil)
+		p.block(s.Body, 1)
+		if s.Else != nil {
+			p.print(blank, token.ELSE, blank)
+			switch s.Else.(type) {
+			case *ast.BlockStmt, *ast.IfStmt:
+				p.stmt(s.Else, nextIsRBrace)
+			default:
+				// This can only happen with an incorrectly
+				// constructed AST. Permit it but print so
+				// that it can be parsed without errors.
+				p.print(token.LBRACE, indent, formfeed)
+				p.stmt(s.Else, true)
+				p.print(unindent, formfeed, token.RBRACE)
+			}
+		}
+
+	case *ast.CaseClause:
+		if s.List != nil {
+			p.print(token.CASE, blank)
+			p.exprList(s.Pos(), s.List, 1, 0, s.Colon, false)
+		} else {
+			p.print(token.DEFAULT)
+		}
+		p.print(s.Colon, token.COLON)
+		p.stmtList(s.Body, 1, nextIsRBrace)
+
+	case *ast.SwitchStmt:
+		p.print(token.SWITCH)
+		p.controlClause(false, s.Init, s.Tag, nil)
+		p.block(s.Body, 0)
+
+	case *ast.TypeSwitchStmt:
+		p.print(token.SWITCH)
+		if s.Init != nil {
+			p.print(blank)
+			p.stmt(s.Init, false)
+			p.print(token.SEMICOLON)
+		}
+		p.print(blank)
+		p.stmt(s.Assign, false)
+		p.print(blank)
+		p.block(s.Body, 0)
+
+	case *ast.CommClause:
+		if s.Comm != nil {
+			p.print(token.CASE, blank)
+			p.stmt(s.Comm, false)
+		} else {
+			p.print(token.DEFAULT)
+		}
+		p.print(s.Colon, token.COLON)
+		p.stmtList(s.Body, 1, nextIsRBrace)
+
+	case *ast.SelectStmt:
+		p.print(token.SELECT, blank)
+		body := s.Body
+		if len(body.List) == 0 && !p.commentBefore(p.posFor(body.Rbrace)) {
+			// print empty select statement w/o comments on one line
+			p.print(body.Lbrace, token.LBRACE, body.Rbrace, token.RBRACE)
+		} else {
+			p.block(body, 0)
+		}
+
+	case *ast.ForStmt:
+		p.print(token.FOR)
+		p.controlClause(true, s.Init, s.Cond, s.Post)
+		p.block(s.Body, 1)
+
+	case *ast.RangeStmt:
+		p.print(token.FOR, blank)
+		if s.Key != nil {
+			p.expr(s.Key)
+			if s.Value != nil {
+				// use position of value following the comma as
+				// comma position for correct comment placement
+				p.print(s.Value.Pos(), token.COMMA, blank)
+				p.expr(s.Value)
+			}
+			p.print(blank, s.TokPos, s.Tok, blank)
+		}
+		p.print(token.RANGE, blank)
+		p.expr(stripParens(s.X))
+		p.print(blank)
+		p.block(s.Body, 1)
+
+	default:
+		panic("unreachable")
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Declarations
+
+// The keepTypeColumn function determines if the type column of a series of
+// consecutive const or var declarations must be kept, or if initialization
+// values (V) can be placed in the type column (T) instead. The i'th entry
+// in the result slice is true if the type column in spec[i] must be kept.
+//
+// For example, the declaration:
+//
+//		const (
+//			foobar int = 42 // comment
+//			x          = 7  // comment
+//			foo
+//	             bar = 991
+//		)
+//
+// leads to the type/values matrix below. A run of value columns (V) can
+// be moved into the type column if there is no type for any of the values
+// in that column (we only move entire columns so that they align properly).
+//
+//		matrix        formatted     result
+//	                   matrix
+//		T  V    ->    T  V     ->   true      there is a T and so the type
+//		-  V          -  V          true      column must be kept
+//		-  -          -  -          false
+//		-  V          V  -          false     V is moved into T column
+func keepTypeColumn(specs []ast.Spec) []bool {
+	m := make([]bool, len(specs))
+
+	populate := func(i, j int, keepType bool) {
+		if keepType {
+			for ; i < j; i++ {
+				m[i] = true
+			}
+		}
+	}
+
+	i0 := -1 // if i0 >= 0 we are in a run and i0 is the start of the run
+	var keepType bool
+	for i, s := range specs {
+		t := s.(*ast.ValueSpec)
+		if t.Values != nil {
+			if i0 < 0 {
+				// start of a run of ValueSpecs with non-nil Values
+				i0 = i
+				keepType = false
+			}
+		} else {
+			if i0 >= 0 {
+				// end of a run
+				populate(i0, i, keepType)
+				i0 = -1
+			}
+		}
+		if t.Type != nil {
+			keepType = true
+		}
+	}
+	if i0 >= 0 {
+		// end of a run
+		populate(i0, len(specs), keepType)
+	}
+
+	return m
+}
+
+func (p *printer) valueSpec(s *ast.ValueSpec, keepType bool) {
+	p.setComment(s.Doc)
+	p.identList(s.Names, false) // always present
+	extraTabs := 3
+	if s.Type != nil || keepType {
+		p.print(vtab)
+		extraTabs--
+	}
+	if s.Type != nil {
+		p.expr(s.Type)
+	}
+	if s.Values != nil {
+		p.print(vtab, token.ASSIGN, blank)
+		p.exprList(token.NoPos, s.Values, 1, 0, token.NoPos, false)
+		extraTabs--
+	}
+	if s.Comment != nil {
+		for ; extraTabs > 0; extraTabs-- {
+			p.print(vtab)
+		}
+		p.setComment(s.Comment)
+	}
+}
+
+func sanitizeImportPath(lit *ast.BasicLit) *ast.BasicLit {
+	// Note: An unmodified AST generated by go/parser will already
+	// contain a backward- or double-quoted path string that does
+	// not contain any invalid characters, and most of the work
+	// here is not needed. However, a modified or generated AST
+	// may possibly contain non-canonical paths. Do the work in
+	// all cases since it's not too hard and not speed-critical.
+
+	// if we don't have a proper string, be conservative and return whatever we have
+	if lit.Kind != token.STRING {
+		return lit
+	}
+	s, err := strconv.Unquote(lit.Value)
+	if err != nil {
+		return lit
+	}
+
+	// if the string is an invalid path, return whatever we have
+	//
+	// spec: "Implementation restriction: A compiler may restrict
+	// ImportPaths to non-empty strings using only characters belonging
+	// to Unicode's L, M, N, P, and S general categories (the Graphic
+	// characters without spaces) and may also exclude the characters
+	// !"#$%&'()*,:;<=>?[\]^`{|} and the Unicode replacement character
+	// U+FFFD."
+	if s == "" {
+		return lit
+	}
+	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
+	for _, r := range s {
+		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
+			return lit
+		}
+	}
+
+	// otherwise, return the double-quoted path
+	s = strconv.Quote(s)
+	if s == lit.Value {
+		return lit // nothing wrong with lit
+	}
+	return &ast.BasicLit{ValuePos: lit.ValuePos, Kind: token.STRING, Value: s}
+}
+
+// The parameter n is the number of specs in the group. If doIndent is set,
+// multi-line identifier lists in the spec are indented when the first
+// linebreak is encountered.
+func (p *printer) spec(spec ast.Spec, n int, doIndent bool) {
+	switch s := spec.(type) {
+	case *ast.ImportSpec:
+		p.setComment(s.Doc)
+		if s.Name != nil {
+			p.expr(s.Name)
+			p.print(blank)
+		}
+		p.expr(sanitizeImportPath(s.Path))
+		p.setComment(s.Comment)
+		p.print(s.EndPos)
+
+	case *ast.ValueSpec:
+		if n != 1 {
+			p.internalError("expected n = 1; got", n)
+		}
+		p.setComment(s.Doc)
+		p.identList(s.Names, doIndent) // always present
+		if s.Type != nil {
+			p.print(blank)
+			p.expr(s.Type)
+		}
+		if s.Values != nil {
+			p.print(blank, token.ASSIGN, blank)
+			p.exprList(token.NoPos, s.Values, 1, 0, token.NoPos, false)
+		}
+		p.setComment(s.Comment)
+
+	case *ast.TypeSpec:
+		p.setComment(s.Doc)
+		p.expr(s.Name)
+		if s.TypeParams != nil {
+			p.parameters(s.TypeParams, typeTParam)
+		}
+		if n == 1 {
+			p.print(blank)
+		} else {
+			p.print(vtab)
+		}
+		if s.Assign.IsValid() {
+			p.print(token.ASSIGN, blank)
+		}
+		p.expr(s.Type)
+		p.setComment(s.Comment)
+
+	default:
+		panic("unreachable")
+	}
+}
+
+func (p *printer) genDecl(d *ast.GenDecl) {
+	p.setComment(d.Doc)
+	p.print(d.Pos(), d.Tok, blank)
+
+	if d.Lparen.IsValid() || len(d.Specs) > 1 {
+		// group of parenthesized declarations
+		p.print(d.Lparen, token.LPAREN)
+		if n := len(d.Specs); n > 0 {
+			p.print(indent, formfeed)
+			if n > 1 && (d.Tok == token.CONST || d.Tok == token.VAR) {
+				// two or more grouped const/var declarations:
+				// determine if the type column must be kept
+				keepType := keepTypeColumn(d.Specs)
+				var line int
+				for i, s := range d.Specs {
+					if i > 0 {
+						p.linebreak(p.lineFor(s.Pos()), 1, ignore, p.linesFrom(line) > 0)
+					}
+					p.recordLine(&line)
+					p.valueSpec(s.(*ast.ValueSpec), keepType[i])
+				}
+			} else {
+				var line int
+				for i, s := range d.Specs {
+					if i > 0 {
+						p.linebreak(p.lineFor(s.Pos()), 1, ignore, p.linesFrom(line) > 0)
+					}
+					p.recordLine(&line)
+					p.spec(s, n, false)
+				}
+			}
+			p.print(unindent, formfeed)
+		}
+		p.print(d.Rparen, token.RPAREN)
+
+	} else if len(d.Specs) > 0 {
+		// single declaration
+		p.spec(d.Specs[0], 1, true)
+	}
+}
+
+// nodeSize determines the size of n in chars after formatting.
+// The result is <= maxSize if the node fits on one line with at
+// most maxSize chars and the formatted output doesn't contain
+// any control chars. Otherwise, the result is > maxSize.
+func (p *printer) nodeSize(n ast.Node, maxSize int) (size int) {
+	// nodeSize invokes the printer, which may invoke nodeSize
+	// recursively. For deep composite literal nests, this can
+	// lead to an exponential algorithm. Remember previous
+	// results to prune the recursion (was issue 1628).
+	if size, found := p.nodeSizes[n]; found {
+		return size
+	}
+
+	size = maxSize + 1 // assume n doesn't fit
+	p.nodeSizes[n] = size
+
+	// nodeSize computation must be independent of particular
+	// style so that we always get the same decision; print
+	// in RawFormat
+	cfg := Config{Mode: RawFormat}
+	var buf bytes.Buffer
+	if err := cfg.fprint(&buf, p.fset, n, p.nodeSizes); err != nil {
+		return
+	}
+	if buf.Len() <= maxSize {
+		for _, ch := range buf.Bytes() {
+			if ch < ' ' {
+				return
+			}
+		}
+		size = buf.Len() // n fits
+		p.nodeSizes[n] = size
+	}
+	return
+}
+
+// numLines returns the number of lines spanned by node n in the original source.
+func (p *printer) numLines(n ast.Node) int {
+	if from := n.Pos(); from.IsValid() {
+		if to := n.End(); to.IsValid() {
+			return p.lineFor(to) - p.lineFor(from) + 1
+		}
+	}
+	return infinity
+}
+
+// bodySize is like nodeSize but it is specialized for *ast.BlockStmt's.
+func (p *printer) bodySize(b *ast.BlockStmt, maxSize int) int {
+	pos1 := b.Pos()
+	pos2 := b.Rbrace
+	if pos1.IsValid() && pos2.IsValid() && p.lineFor(pos1) != p.lineFor(pos2) {
+		// opening and closing brace are on different lines - don't make it a one-liner
+		return maxSize + 1
+	}
+	if len(b.List) > 5 {
+		// too many statements - don't make it a one-liner
+		return maxSize + 1
+	}
+	// otherwise, estimate body size
+	bodySize := p.commentSizeBefore(p.posFor(pos2))
+	for i, s := range b.List {
+		if bodySize > maxSize {
+			break // no need to continue
+		}
+		if i > 0 {
+			bodySize += 2 // space for a semicolon and blank
+		}
+		bodySize += p.nodeSize(s, maxSize)
+	}
+	return bodySize
+}
+
+// funcBody prints a function body following a function header of given headerSize.
+// If the header's and block's size are "small enough" and the block is "simple enough",
+// the block is printed on the current line, without line breaks, spaced from the header
+// by sep. Otherwise the block's opening "{" is printed on the current line, followed by
+// lines for the block's statements and its closing "}".
+func (p *printer) funcBody(headerSize int, sep whiteSpace, b *ast.BlockStmt) {
+	if b == nil {
+		return
+	}
+
+	// save/restore composite literal nesting level
+	defer func(level int) {
+		p.level = level
+	}(p.level)
+	p.level = 0
+
+	const maxSize = 100
+	if headerSize+p.bodySize(b, maxSize) <= maxSize {
+		p.print(sep, b.Lbrace, token.LBRACE)
+		if len(b.List) > 0 {
+			p.print(blank)
+			for i, s := range b.List {
+				if i > 0 {
+					p.print(token.SEMICOLON, blank)
+				}
+				p.stmt(s, i == len(b.List)-1)
+			}
+			p.print(blank)
+		}
+		p.print(noExtraLinebreak, b.Rbrace, token.RBRACE, noExtraLinebreak)
+		return
+	}
+
+	if sep != ignore {
+		p.print(blank) // always use blank
+	}
+	p.block(b, 1)
+}
+
+// distanceFrom returns the column difference between p.out (the current output
+// position) and startOutCol. If the start position is on a different line from
+// the current position (or either is unknown), the result is infinity.
+func (p *printer) distanceFrom(startPos token.Pos, startOutCol int) int {
+	if startPos.IsValid() && p.pos.IsValid() && p.posFor(startPos).Line == p.pos.Line {
+		return p.out.Column - startOutCol
+	}
+	return infinity
+}
+
+func (p *printer) funcDecl(d *ast.FuncDecl) {
+	p.setComment(d.Doc)
+	p.print(d.Pos(), token.FUNC, blank)
+	// We have to save startCol only after emitting FUNC; otherwise it can be on a
+	// different line (all whitespace preceding the FUNC is emitted only when the
+	// FUNC is emitted).
+	startCol := p.out.Column - len("func ")
+	if d.Recv != nil {
+		p.parameters(d.Recv, funcParam) // method: print receiver
+		p.print(blank)
+	}
+	p.expr(d.Name)
+	p.signature(d.Type)
+	p.funcBody(p.distanceFrom(d.Pos(), startCol), vtab, d.Body)
+}
+
+func (p *printer) decl(decl ast.Decl) {
+	switch d := decl.(type) {
+	case *ast.BadDecl:
+		p.print(d.Pos(), "BadDecl")
+	case *ast.GenDecl:
+		p.genDecl(d)
+	case *ast.FuncDecl:
+		p.funcDecl(d)
+	default:
+		panic("unreachable")
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Files
+
+func declToken(decl ast.Decl) (tok token.Token) {
+	tok = token.ILLEGAL
+	switch d := decl.(type) {
+	case *ast.GenDecl:
+		tok = d.Tok
+	case *ast.FuncDecl:
+		tok = token.FUNC
+	}
+	return
+}
+
+func (p *printer) declList(list []ast.Decl) {
+	tok := token.ILLEGAL
+	for _, d := range list {
+		prev := tok
+		tok = declToken(d)
+		// If the declaration token changed (e.g., from CONST to TYPE)
+		// or the next declaration has documentation associated with it,
+		// print an empty line between top-level declarations.
+		// (because p.linebreak is called with the position of d, which
+		// is past any documentation, the minimum requirement is satisfied
+		// even w/o the extra getDoc(d) nil-check - leave it in case the
+		// linebreak logic improves - there's already a TODO).
+		if len(p.output) > 0 {
+			// only print line break if we are not at the beginning of the output
+			// (i.e., we are not printing only a partial program)
+			min := 1
+			if prev != tok || getDoc(d) != nil {
+				min = 2
+			}
+			// start a new section if the next declaration is a function
+			// that spans multiple lines (see also issue #19544)
+			p.linebreak(p.lineFor(d.Pos()), min, ignore, tok == token.FUNC && p.numLines(d) > 1)
+		}
+		p.decl(d)
+	}
+}
+
+func (p *printer) file(src *ast.File) {
+	p.setComment(src.Doc)
+	p.print(src.Pos(), token.PACKAGE, blank)
+	p.expr(src.Name)
+	p.declList(src.Decls)
+	p.print(newline)
+}
diff --git a/internal/backport/go/printer/performance_test.go b/internal/backport/go/printer/performance_test.go
new file mode 100644
index 0000000..ea3eac4
--- /dev/null
+++ b/internal/backport/go/printer/performance_test.go
@@ -0,0 +1,58 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements a simple printer performance benchmark:
+// go test -bench=BenchmarkPrint
+
+package printer
+
+import (
+	"bytes"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+	"io"
+	"log"
+	"os"
+	"testing"
+)
+
+var testfile *ast.File
+
+func testprint(out io.Writer, file *ast.File) {
+	if err := (&Config{TabIndent | UseSpaces | normalizeNumbers, 8, 0}).Fprint(out, fset, file); err != nil {
+		log.Fatalf("print error: %s", err)
+	}
+}
+
+// cannot initialize in init because (printer) Fprint launches goroutines.
+func initialize() {
+	const filename = "testdata/parser.go"
+
+	src, err := os.ReadFile(filename)
+	if err != nil {
+		log.Fatalf("%s", err)
+	}
+
+	file, err := parser.ParseFile(fset, filename, src, parser.ParseComments)
+	if err != nil {
+		log.Fatalf("%s", err)
+	}
+
+	var buf bytes.Buffer
+	testprint(&buf, file)
+	if !bytes.Equal(buf.Bytes(), src) {
+		log.Fatalf("print error: %s not idempotent", filename)
+	}
+
+	testfile = file
+}
+
+func BenchmarkPrint(b *testing.B) {
+	if testfile == nil {
+		initialize()
+	}
+	for i := 0; i < b.N; i++ {
+		testprint(io.Discard, testfile)
+	}
+}
diff --git a/internal/backport/go/printer/printer.go b/internal/backport/go/printer/printer.go
new file mode 100644
index 0000000..a8aa7cf
--- /dev/null
+++ b/internal/backport/go/printer/printer.go
@@ -0,0 +1,1380 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package printer implements printing of AST nodes.
+package printer
+
+import (
+	"fmt"
+	"go/build/constraint"
+	"io"
+	"os"
+	"strings"
+	"text/tabwriter"
+	"unicode"
+
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+const (
+	maxNewlines = 2     // max. number of newlines between source text
+	debug       = false // enable for debugging
+	infinity    = 1 << 30
+)
+
+type whiteSpace byte
+
+const (
+	ignore   = whiteSpace(0)
+	blank    = whiteSpace(' ')
+	vtab     = whiteSpace('\v')
+	newline  = whiteSpace('\n')
+	formfeed = whiteSpace('\f')
+	indent   = whiteSpace('>')
+	unindent = whiteSpace('<')
+)
+
+// A pmode value represents the current printer mode.
+type pmode int
+
+const (
+	noExtraBlank     pmode = 1 << iota // disables extra blank after /*-style comment
+	noExtraLinebreak                   // disables extra line break after /*-style comment
+)
+
+type commentInfo struct {
+	cindex         int               // current comment index
+	comment        *ast.CommentGroup // = printer.comments[cindex]; or nil
+	commentOffset  int               // = printer.posFor(printer.comments[cindex].List[0].Pos()).Offset; or infinity
+	commentNewline bool              // true if the comment group contains newlines
+}
+
+type printer struct {
+	// Configuration (does not change after initialization)
+	Config
+	fset *token.FileSet
+
+	// Current state
+	output       []byte       // raw printer result
+	indent       int          // current indentation
+	level        int          // level == 0: outside composite literal; level > 0: inside composite literal
+	mode         pmode        // current printer mode
+	endAlignment bool         // if set, terminate alignment immediately
+	impliedSemi  bool         // if set, a linebreak implies a semicolon
+	lastTok      token.Token  // last token printed (token.ILLEGAL if it's whitespace)
+	prevOpen     token.Token  // previous non-brace "open" token (, [, or token.ILLEGAL
+	wsbuf        []whiteSpace // delayed white space
+	goBuild      []int        // start index of all //go:build comments in output
+	plusBuild    []int        // start index of all // +build comments in output
+
+	// Positions
+	// The out position differs from the pos position when the result
+	// formatting differs from the source formatting (in the amount of
+	// white space). If there's a difference and SourcePos is set in
+	// ConfigMode, //line directives are used in the output to restore
+	// original source positions for a reader.
+	pos     token.Position // current position in AST (source) space
+	out     token.Position // current position in output space
+	last    token.Position // value of pos after calling writeString
+	linePtr *int           // if set, record out.Line for the next token in *linePtr
+
+	// The list of all source comments, in order of appearance.
+	comments        []*ast.CommentGroup // may be nil
+	useNodeComments bool                // if not set, ignore lead and line comments of nodes
+
+	// Information about p.comments[p.cindex]; set up by nextComment.
+	commentInfo
+
+	// Cache of already computed node sizes.
+	nodeSizes map[ast.Node]int
+
+	// Cache of most recently computed line position.
+	cachedPos  token.Pos
+	cachedLine int // line corresponding to cachedPos
+}
+
+func (p *printer) init(cfg *Config, fset *token.FileSet, nodeSizes map[ast.Node]int) {
+	p.Config = *cfg
+	p.fset = fset
+	p.pos = token.Position{Line: 1, Column: 1}
+	p.out = token.Position{Line: 1, Column: 1}
+	p.wsbuf = make([]whiteSpace, 0, 16) // whitespace sequences are short
+	p.nodeSizes = nodeSizes
+	p.cachedPos = -1
+}
+
+func (p *printer) internalError(msg ...interface{}) {
+	if debug {
+		fmt.Print(p.pos.String() + ": ")
+		fmt.Println(msg...)
+		panic("golang.org/x/website/internal/backport/go/printer")
+	}
+}
+
+// commentsHaveNewline reports whether a list of comments belonging to
+// an *ast.CommentGroup contains newlines. Because the position information
+// may only be partially correct, we also have to read the comment text.
+func (p *printer) commentsHaveNewline(list []*ast.Comment) bool {
+	// len(list) > 0
+	line := p.lineFor(list[0].Pos())
+	for i, c := range list {
+		if i > 0 && p.lineFor(list[i].Pos()) != line {
+			// not all comments on the same line
+			return true
+		}
+		if t := c.Text; len(t) >= 2 && (t[1] == '/' || strings.Contains(t, "\n")) {
+			return true
+		}
+	}
+	_ = line
+	return false
+}
+
+func (p *printer) nextComment() {
+	for p.cindex < len(p.comments) {
+		c := p.comments[p.cindex]
+		p.cindex++
+		if list := c.List; len(list) > 0 {
+			p.comment = c
+			p.commentOffset = p.posFor(list[0].Pos()).Offset
+			p.commentNewline = p.commentsHaveNewline(list)
+			return
+		}
+		// we should not reach here (correct ASTs don't have empty
+		// ast.CommentGroup nodes), but be conservative and try again
+	}
+	// no more comments
+	p.commentOffset = infinity
+}
+
+// commentBefore reports whether the current comment group occurs
+// before the next position in the source code and printing it does
+// not introduce implicit semicolons.
+func (p *printer) commentBefore(next token.Position) bool {
+	return p.commentOffset < next.Offset && (!p.impliedSemi || !p.commentNewline)
+}
+
+// commentSizeBefore returns the estimated size of the
+// comments on the same line before the next position.
+func (p *printer) commentSizeBefore(next token.Position) int {
+	// save/restore current p.commentInfo (p.nextComment() modifies it)
+	defer func(info commentInfo) {
+		p.commentInfo = info
+	}(p.commentInfo)
+
+	size := 0
+	for p.commentBefore(next) {
+		for _, c := range p.comment.List {
+			size += len(c.Text)
+		}
+		p.nextComment()
+	}
+	return size
+}
+
+// recordLine records the output line number for the next non-whitespace
+// token in *linePtr. It is used to compute an accurate line number for a
+// formatted construct, independent of pending (not yet emitted) whitespace
+// or comments.
+func (p *printer) recordLine(linePtr *int) {
+	p.linePtr = linePtr
+}
+
+// linesFrom returns the number of output lines between the current
+// output line and the line argument, ignoring any pending (not yet
+// emitted) whitespace or comments. It is used to compute an accurate
+// size (in number of lines) for a formatted construct.
+func (p *printer) linesFrom(line int) int {
+	return p.out.Line - line
+}
+
+func (p *printer) posFor(pos token.Pos) token.Position {
+	// not used frequently enough to cache entire token.Position
+	return p.fset.PositionFor(pos, false /* absolute position */)
+}
+
+func (p *printer) lineFor(pos token.Pos) int {
+	if pos != p.cachedPos {
+		p.cachedPos = pos
+		p.cachedLine = p.fset.PositionFor(pos, false /* absolute position */).Line
+	}
+	return p.cachedLine
+}
+
+// writeLineDirective writes a //line directive if necessary.
+func (p *printer) writeLineDirective(pos token.Position) {
+	if pos.IsValid() && (p.out.Line != pos.Line || p.out.Filename != pos.Filename) {
+		p.output = append(p.output, tabwriter.Escape) // protect '\n' in //line from tabwriter interpretation
+		p.output = append(p.output, fmt.Sprintf("//line %s:%d\n", pos.Filename, pos.Line)...)
+		p.output = append(p.output, tabwriter.Escape)
+		// p.out must match the //line directive
+		p.out.Filename = pos.Filename
+		p.out.Line = pos.Line
+	}
+}
+
+// writeIndent writes indentation.
+func (p *printer) writeIndent() {
+	// use "hard" htabs - indentation columns
+	// must not be discarded by the tabwriter
+	n := p.Config.Indent + p.indent // include base indentation
+	for i := 0; i < n; i++ {
+		p.output = append(p.output, '\t')
+	}
+
+	// update positions
+	p.pos.Offset += n
+	p.pos.Column += n
+	p.out.Column += n
+}
+
+// writeByte writes ch n times to p.output and updates p.pos.
+// Only used to write formatting (white space) characters.
+func (p *printer) writeByte(ch byte, n int) {
+	if p.endAlignment {
+		// Ignore any alignment control character;
+		// and at the end of the line, break with
+		// a formfeed to indicate termination of
+		// existing columns.
+		switch ch {
+		case '\t', '\v':
+			ch = ' '
+		case '\n', '\f':
+			ch = '\f'
+			p.endAlignment = false
+		}
+	}
+
+	if p.out.Column == 1 {
+		// no need to write line directives before white space
+		p.writeIndent()
+	}
+
+	for i := 0; i < n; i++ {
+		p.output = append(p.output, ch)
+	}
+
+	// update positions
+	p.pos.Offset += n
+	if ch == '\n' || ch == '\f' {
+		p.pos.Line += n
+		p.out.Line += n
+		p.pos.Column = 1
+		p.out.Column = 1
+		return
+	}
+	p.pos.Column += n
+	p.out.Column += n
+}
+
+// writeString writes the string s to p.output and updates p.pos, p.out,
+// and p.last. If isLit is set, s is escaped w/ tabwriter.Escape characters
+// to protect s from being interpreted by the tabwriter.
+//
+// Note: writeString is only used to write Go tokens, literals, and
+// comments, all of which must be written literally. Thus, it is correct
+// to always set isLit = true. However, setting it explicitly only when
+// needed (i.e., when we don't know that s contains no tabs or line breaks)
+// avoids processing extra escape characters and reduces run time of the
+// printer benchmark by up to 10%.
+func (p *printer) writeString(pos token.Position, s string, isLit bool) {
+	if p.out.Column == 1 {
+		if p.Config.Mode&SourcePos != 0 {
+			p.writeLineDirective(pos)
+		}
+		p.writeIndent()
+	}
+
+	if pos.IsValid() {
+		// update p.pos (if pos is invalid, continue with existing p.pos)
+		// Note: Must do this after handling line beginnings because
+		// writeIndent updates p.pos if there's indentation, but p.pos
+		// is the position of s.
+		p.pos = pos
+	}
+
+	if isLit {
+		// Protect s such that is passes through the tabwriter
+		// unchanged. Note that valid Go programs cannot contain
+		// tabwriter.Escape bytes since they do not appear in legal
+		// UTF-8 sequences.
+		p.output = append(p.output, tabwriter.Escape)
+	}
+
+	if debug {
+		p.output = append(p.output, fmt.Sprintf("/*%s*/", pos)...) // do not update p.pos!
+	}
+	p.output = append(p.output, s...)
+
+	// update positions
+	nlines := 0
+	var li int // index of last newline; valid if nlines > 0
+	for i := 0; i < len(s); i++ {
+		// Raw string literals may contain any character except back quote (`).
+		if ch := s[i]; ch == '\n' || ch == '\f' {
+			// account for line break
+			nlines++
+			li = i
+			// A line break inside a literal will break whatever column
+			// formatting is in place; ignore any further alignment through
+			// the end of the line.
+			p.endAlignment = true
+		}
+	}
+	p.pos.Offset += len(s)
+	if nlines > 0 {
+		p.pos.Line += nlines
+		p.out.Line += nlines
+		c := len(s) - li
+		p.pos.Column = c
+		p.out.Column = c
+	} else {
+		p.pos.Column += len(s)
+		p.out.Column += len(s)
+	}
+
+	if isLit {
+		p.output = append(p.output, tabwriter.Escape)
+	}
+
+	p.last = p.pos
+}
+
+// writeCommentPrefix writes the whitespace before a comment.
+// If there is any pending whitespace, it consumes as much of
+// it as is likely to help position the comment nicely.
+// pos is the comment position, next the position of the item
+// after all pending comments, prev is the previous comment in
+// a group of comments (or nil), and tok is the next token.
+func (p *printer) writeCommentPrefix(pos, next token.Position, prev *ast.Comment, tok token.Token) {
+	if len(p.output) == 0 {
+		// the comment is the first item to be printed - don't write any whitespace
+		return
+	}
+
+	if pos.IsValid() && pos.Filename != p.last.Filename {
+		// comment in a different file - separate with newlines
+		p.writeByte('\f', maxNewlines)
+		return
+	}
+
+	if pos.Line == p.last.Line && (prev == nil || prev.Text[1] != '/') {
+		// comment on the same line as last item:
+		// separate with at least one separator
+		hasSep := false
+		if prev == nil {
+			// first comment of a comment group
+			j := 0
+			for i, ch := range p.wsbuf {
+				switch ch {
+				case blank:
+					// ignore any blanks before a comment
+					p.wsbuf[i] = ignore
+					continue
+				case vtab:
+					// respect existing tabs - important
+					// for proper formatting of commented structs
+					hasSep = true
+					continue
+				case indent:
+					// apply pending indentation
+					continue
+				}
+				j = i
+				break
+			}
+			p.writeWhitespace(j)
+		}
+		// make sure there is at least one separator
+		if !hasSep {
+			sep := byte('\t')
+			if pos.Line == next.Line {
+				// next item is on the same line as the comment
+				// (which must be a /*-style comment): separate
+				// with a blank instead of a tab
+				sep = ' '
+			}
+			p.writeByte(sep, 1)
+		}
+
+	} else {
+		// comment on a different line:
+		// separate with at least one line break
+		droppedLinebreak := false
+		j := 0
+		for i, ch := range p.wsbuf {
+			switch ch {
+			case blank, vtab:
+				// ignore any horizontal whitespace before line breaks
+				p.wsbuf[i] = ignore
+				continue
+			case indent:
+				// apply pending indentation
+				continue
+			case unindent:
+				// if this is not the last unindent, apply it
+				// as it is (likely) belonging to the last
+				// construct (e.g., a multi-line expression list)
+				// and is not part of closing a block
+				if i+1 < len(p.wsbuf) && p.wsbuf[i+1] == unindent {
+					continue
+				}
+				// if the next token is not a closing }, apply the unindent
+				// if it appears that the comment is aligned with the
+				// token; otherwise assume the unindent is part of a
+				// closing block and stop (this scenario appears with
+				// comments before a case label where the comments
+				// apply to the next case instead of the current one)
+				if tok != token.RBRACE && pos.Column == next.Column {
+					continue
+				}
+			case newline, formfeed:
+				p.wsbuf[i] = ignore
+				droppedLinebreak = prev == nil // record only if first comment of a group
+			}
+			j = i
+			break
+		}
+		p.writeWhitespace(j)
+
+		// determine number of linebreaks before the comment
+		n := 0
+		if pos.IsValid() && p.last.IsValid() {
+			n = pos.Line - p.last.Line
+			if n < 0 { // should never happen
+				n = 0
+			}
+		}
+
+		// at the package scope level only (p.indent == 0),
+		// add an extra newline if we dropped one before:
+		// this preserves a blank line before documentation
+		// comments at the package scope level (issue 2570)
+		if p.indent == 0 && droppedLinebreak {
+			n++
+		}
+
+		// make sure there is at least one line break
+		// if the previous comment was a line comment
+		if n == 0 && prev != nil && prev.Text[1] == '/' {
+			n = 1
+		}
+
+		if n > 0 {
+			// use formfeeds to break columns before a comment;
+			// this is analogous to using formfeeds to separate
+			// individual lines of /*-style comments
+			p.writeByte('\f', nlimit(n))
+		}
+	}
+}
+
+// Returns true if s contains only white space
+// (only tabs and blanks can appear in the printer's context).
+func isBlank(s string) bool {
+	for i := 0; i < len(s); i++ {
+		if s[i] > ' ' {
+			return false
+		}
+	}
+	return true
+}
+
+// commonPrefix returns the common prefix of a and b.
+func commonPrefix(a, b string) string {
+	i := 0
+	for i < len(a) && i < len(b) && a[i] == b[i] && (a[i] <= ' ' || a[i] == '*') {
+		i++
+	}
+	return a[0:i]
+}
+
+// trimRight returns s with trailing whitespace removed.
+func trimRight(s string) string {
+	return strings.TrimRightFunc(s, unicode.IsSpace)
+}
+
+// stripCommonPrefix removes a common prefix from /*-style comment lines (unless no
+// comment line is indented, all but the first line have some form of space prefix).
+// The prefix is computed using heuristics such that is likely that the comment
+// contents are nicely laid out after re-printing each line using the printer's
+// current indentation.
+func stripCommonPrefix(lines []string) {
+	if len(lines) <= 1 {
+		return // at most one line - nothing to do
+	}
+	// len(lines) > 1
+
+	// The heuristic in this function tries to handle a few
+	// common patterns of /*-style comments: Comments where
+	// the opening /* and closing */ are aligned and the
+	// rest of the comment text is aligned and indented with
+	// blanks or tabs, cases with a vertical "line of stars"
+	// on the left, and cases where the closing */ is on the
+	// same line as the last comment text.
+
+	// Compute maximum common white prefix of all but the first,
+	// last, and blank lines, and replace blank lines with empty
+	// lines (the first line starts with /* and has no prefix).
+	// In cases where only the first and last lines are not blank,
+	// such as two-line comments, or comments where all inner lines
+	// are blank, consider the last line for the prefix computation
+	// since otherwise the prefix would be empty.
+	//
+	// Note that the first and last line are never empty (they
+	// contain the opening /* and closing */ respectively) and
+	// thus they can be ignored by the blank line check.
+	prefix := ""
+	prefixSet := false
+	if len(lines) > 2 {
+		for i, line := range lines[1 : len(lines)-1] {
+			if isBlank(line) {
+				lines[1+i] = "" // range starts with lines[1]
+			} else {
+				if !prefixSet {
+					prefix = line
+					prefixSet = true
+				}
+				prefix = commonPrefix(prefix, line)
+			}
+
+		}
+	}
+	// If we don't have a prefix yet, consider the last line.
+	if !prefixSet {
+		line := lines[len(lines)-1]
+		prefix = commonPrefix(line, line)
+	}
+
+	/*
+	 * Check for vertical "line of stars" and correct prefix accordingly.
+	 */
+	lineOfStars := false
+	if p, _, ok := stringsCut(prefix, "*"); ok {
+		// remove trailing blank from prefix so stars remain aligned
+		prefix = strings.TrimSuffix(p, " ")
+		lineOfStars = true
+	} else {
+		// No line of stars present.
+		// Determine the white space on the first line after the /*
+		// and before the beginning of the comment text, assume two
+		// blanks instead of the /* unless the first character after
+		// the /* is a tab. If the first comment line is empty but
+		// for the opening /*, assume up to 3 blanks or a tab. This
+		// whitespace may be found as suffix in the common prefix.
+		first := lines[0]
+		if isBlank(first[2:]) {
+			// no comment text on the first line:
+			// reduce prefix by up to 3 blanks or a tab
+			// if present - this keeps comment text indented
+			// relative to the /* and */'s if it was indented
+			// in the first place
+			i := len(prefix)
+			for n := 0; n < 3 && i > 0 && prefix[i-1] == ' '; n++ {
+				i--
+			}
+			if i == len(prefix) && i > 0 && prefix[i-1] == '\t' {
+				i--
+			}
+			prefix = prefix[0:i]
+		} else {
+			// comment text on the first line
+			suffix := make([]byte, len(first))
+			n := 2 // start after opening /*
+			for n < len(first) && first[n] <= ' ' {
+				suffix[n] = first[n]
+				n++
+			}
+			if n > 2 && suffix[2] == '\t' {
+				// assume the '\t' compensates for the /*
+				suffix = suffix[2:n]
+			} else {
+				// otherwise assume two blanks
+				suffix[0], suffix[1] = ' ', ' '
+				suffix = suffix[0:n]
+			}
+			// Shorten the computed common prefix by the length of
+			// suffix, if it is found as suffix of the prefix.
+			prefix = strings.TrimSuffix(prefix, string(suffix))
+		}
+	}
+
+	// Handle last line: If it only contains a closing */, align it
+	// with the opening /*, otherwise align the text with the other
+	// lines.
+	last := lines[len(lines)-1]
+	closing := "*/"
+	before, _, _ := stringsCut(last, closing) // closing always present
+	if isBlank(before) {
+		// last line only contains closing */
+		if lineOfStars {
+			closing = " */" // add blank to align final star
+		}
+		lines[len(lines)-1] = prefix + closing
+	} else {
+		// last line contains more comment text - assume
+		// it is aligned like the other lines and include
+		// in prefix computation
+		prefix = commonPrefix(prefix, last)
+	}
+
+	// Remove the common prefix from all but the first and empty lines.
+	for i, line := range lines {
+		if i > 0 && line != "" {
+			lines[i] = line[len(prefix):]
+		}
+	}
+}
+
+func (p *printer) writeComment(comment *ast.Comment) {
+	text := comment.Text
+	pos := p.posFor(comment.Pos())
+
+	const linePrefix = "//line "
+	if strings.HasPrefix(text, linePrefix) && (!pos.IsValid() || pos.Column == 1) {
+		// Possibly a //-style line directive.
+		// Suspend indentation temporarily to keep line directive valid.
+		defer func(indent int) { p.indent = indent }(p.indent)
+		p.indent = 0
+	}
+
+	// shortcut common case of //-style comments
+	if text[1] == '/' {
+		if constraint.IsGoBuild(text) {
+			p.goBuild = append(p.goBuild, len(p.output))
+		} else if constraint.IsPlusBuild(text) {
+			p.plusBuild = append(p.plusBuild, len(p.output))
+		}
+		p.writeString(pos, trimRight(text), true)
+		return
+	}
+
+	// for /*-style comments, print line by line and let the
+	// write function take care of the proper indentation
+	lines := strings.Split(text, "\n")
+
+	// The comment started in the first column but is going
+	// to be indented. For an idempotent result, add indentation
+	// to all lines such that they look like they were indented
+	// before - this will make sure the common prefix computation
+	// is the same independent of how many times formatting is
+	// applied (was issue 1835).
+	if pos.IsValid() && pos.Column == 1 && p.indent > 0 {
+		for i, line := range lines[1:] {
+			lines[1+i] = "   " + line
+		}
+	}
+
+	stripCommonPrefix(lines)
+
+	// write comment lines, separated by formfeed,
+	// without a line break after the last line
+	for i, line := range lines {
+		if i > 0 {
+			p.writeByte('\f', 1)
+			pos = p.pos
+		}
+		if len(line) > 0 {
+			p.writeString(pos, trimRight(line), true)
+		}
+	}
+}
+
+// writeCommentSuffix writes a line break after a comment if indicated
+// and processes any leftover indentation information. If a line break
+// is needed, the kind of break (newline vs formfeed) depends on the
+// pending whitespace. The writeCommentSuffix result indicates if a
+// newline was written or if a formfeed was dropped from the whitespace
+// buffer.
+func (p *printer) writeCommentSuffix(needsLinebreak bool) (wroteNewline, droppedFF bool) {
+	for i, ch := range p.wsbuf {
+		switch ch {
+		case blank, vtab:
+			// ignore trailing whitespace
+			p.wsbuf[i] = ignore
+		case indent, unindent:
+			// don't lose indentation information
+		case newline, formfeed:
+			// if we need a line break, keep exactly one
+			// but remember if we dropped any formfeeds
+			if needsLinebreak {
+				needsLinebreak = false
+				wroteNewline = true
+			} else {
+				if ch == formfeed {
+					droppedFF = true
+				}
+				p.wsbuf[i] = ignore
+			}
+		}
+	}
+	p.writeWhitespace(len(p.wsbuf))
+
+	// make sure we have a line break
+	if needsLinebreak {
+		p.writeByte('\n', 1)
+		wroteNewline = true
+	}
+
+	return
+}
+
+// containsLinebreak reports whether the whitespace buffer contains any line breaks.
+func (p *printer) containsLinebreak() bool {
+	for _, ch := range p.wsbuf {
+		if ch == newline || ch == formfeed {
+			return true
+		}
+	}
+	return false
+}
+
+// intersperseComments consumes all comments that appear before the next token
+// tok and prints it together with the buffered whitespace (i.e., the whitespace
+// that needs to be written before the next token). A heuristic is used to mix
+// the comments and whitespace. The intersperseComments result indicates if a
+// newline was written or if a formfeed was dropped from the whitespace buffer.
+func (p *printer) intersperseComments(next token.Position, tok token.Token) (wroteNewline, droppedFF bool) {
+	var last *ast.Comment
+	for p.commentBefore(next) {
+		for _, c := range p.comment.List {
+			p.writeCommentPrefix(p.posFor(c.Pos()), next, last, tok)
+			p.writeComment(c)
+			last = c
+		}
+		p.nextComment()
+	}
+
+	if last != nil {
+		// If the last comment is a /*-style comment and the next item
+		// follows on the same line but is not a comma, and not a "closing"
+		// token immediately following its corresponding "opening" token,
+		// add an extra separator unless explicitly disabled. Use a blank
+		// as separator unless we have pending linebreaks, they are not
+		// disabled, and we are outside a composite literal, in which case
+		// we want a linebreak (issue 15137).
+		// TODO(gri) This has become overly complicated. We should be able
+		// to track whether we're inside an expression or statement and
+		// use that information to decide more directly.
+		needsLinebreak := false
+		if p.mode&noExtraBlank == 0 &&
+			last.Text[1] == '*' && p.lineFor(last.Pos()) == next.Line &&
+			tok != token.COMMA &&
+			(tok != token.RPAREN || p.prevOpen == token.LPAREN) &&
+			(tok != token.RBRACK || p.prevOpen == token.LBRACK) {
+			if p.containsLinebreak() && p.mode&noExtraLinebreak == 0 && p.level == 0 {
+				needsLinebreak = true
+			} else {
+				p.writeByte(' ', 1)
+			}
+		}
+		// Ensure that there is a line break after a //-style comment,
+		// before EOF, and before a closing '}' unless explicitly disabled.
+		if last.Text[1] == '/' ||
+			tok == token.EOF ||
+			tok == token.RBRACE && p.mode&noExtraLinebreak == 0 {
+			needsLinebreak = true
+		}
+		return p.writeCommentSuffix(needsLinebreak)
+	}
+
+	// no comment was written - we should never reach here since
+	// intersperseComments should not be called in that case
+	p.internalError("intersperseComments called without pending comments")
+	return
+}
+
+// whiteWhitespace writes the first n whitespace entries.
+func (p *printer) writeWhitespace(n int) {
+	// write entries
+	for i := 0; i < n; i++ {
+		switch ch := p.wsbuf[i]; ch {
+		case ignore:
+			// ignore!
+		case indent:
+			p.indent++
+		case unindent:
+			p.indent--
+			if p.indent < 0 {
+				p.internalError("negative indentation:", p.indent)
+				p.indent = 0
+			}
+		case newline, formfeed:
+			// A line break immediately followed by a "correcting"
+			// unindent is swapped with the unindent - this permits
+			// proper label positioning. If a comment is between
+			// the line break and the label, the unindent is not
+			// part of the comment whitespace prefix and the comment
+			// will be positioned correctly indented.
+			if i+1 < n && p.wsbuf[i+1] == unindent {
+				// Use a formfeed to terminate the current section.
+				// Otherwise, a long label name on the next line leading
+				// to a wide column may increase the indentation column
+				// of lines before the label; effectively leading to wrong
+				// indentation.
+				p.wsbuf[i], p.wsbuf[i+1] = unindent, formfeed
+				i-- // do it again
+				continue
+			}
+			fallthrough
+		default:
+			p.writeByte(byte(ch), 1)
+		}
+	}
+
+	// shift remaining entries down
+	l := copy(p.wsbuf, p.wsbuf[n:])
+	p.wsbuf = p.wsbuf[:l]
+}
+
+// ----------------------------------------------------------------------------
+// Printing interface
+
+// nlimit limits n to maxNewlines.
+func nlimit(n int) int {
+	if n > maxNewlines {
+		n = maxNewlines
+	}
+	return n
+}
+
+func mayCombine(prev token.Token, next byte) (b bool) {
+	switch prev {
+	case token.INT:
+		b = next == '.' // 1.
+	case token.ADD:
+		b = next == '+' // ++
+	case token.SUB:
+		b = next == '-' // --
+	case token.QUO:
+		b = next == '*' // /*
+	case token.LSS:
+		b = next == '-' || next == '<' // <- or <<
+	case token.AND:
+		b = next == '&' || next == '^' // && or &^
+	}
+	return
+}
+
+// print prints a list of "items" (roughly corresponding to syntactic
+// tokens, but also including whitespace and formatting information).
+// It is the only print function that should be called directly from
+// any of the AST printing functions in nodes.go.
+//
+// Whitespace is accumulated until a non-whitespace token appears. Any
+// comments that need to appear before that token are printed first,
+// taking into account the amount and structure of any pending white-
+// space for best comment placement. Then, any leftover whitespace is
+// printed, followed by the actual token.
+func (p *printer) print(args ...interface{}) {
+	for _, arg := range args {
+		// information about the current arg
+		var data string
+		var isLit bool
+		var impliedSemi bool // value for p.impliedSemi after this arg
+
+		// record previous opening token, if any
+		switch p.lastTok {
+		case token.ILLEGAL:
+			// ignore (white space)
+		case token.LPAREN, token.LBRACK:
+			p.prevOpen = p.lastTok
+		default:
+			// other tokens followed any opening token
+			p.prevOpen = token.ILLEGAL
+		}
+
+		switch x := arg.(type) {
+		case pmode:
+			// toggle printer mode
+			p.mode ^= x
+			continue
+
+		case whiteSpace:
+			if x == ignore {
+				// don't add ignore's to the buffer; they
+				// may screw up "correcting" unindents (see
+				// LabeledStmt)
+				continue
+			}
+			i := len(p.wsbuf)
+			if i == cap(p.wsbuf) {
+				// Whitespace sequences are very short so this should
+				// never happen. Handle gracefully (but possibly with
+				// bad comment placement) if it does happen.
+				p.writeWhitespace(i)
+				i = 0
+			}
+			p.wsbuf = p.wsbuf[0 : i+1]
+			p.wsbuf[i] = x
+			if x == newline || x == formfeed {
+				// newlines affect the current state (p.impliedSemi)
+				// and not the state after printing arg (impliedSemi)
+				// because comments can be interspersed before the arg
+				// in this case
+				p.impliedSemi = false
+			}
+			p.lastTok = token.ILLEGAL
+			continue
+
+		case *ast.Ident:
+			data = x.Name
+			impliedSemi = true
+			p.lastTok = token.IDENT
+
+		case *ast.BasicLit:
+			data = x.Value
+			isLit = true
+			impliedSemi = true
+			p.lastTok = x.Kind
+
+		case token.Token:
+			s := x.String()
+			if mayCombine(p.lastTok, s[0]) {
+				// the previous and the current token must be
+				// separated by a blank otherwise they combine
+				// into a different incorrect token sequence
+				// (except for token.INT followed by a '.' this
+				// should never happen because it is taken care
+				// of via binary expression formatting)
+				if len(p.wsbuf) != 0 {
+					p.internalError("whitespace buffer not empty")
+				}
+				p.wsbuf = p.wsbuf[0:1]
+				p.wsbuf[0] = ' '
+			}
+			data = s
+			// some keywords followed by a newline imply a semicolon
+			switch x {
+			case token.BREAK, token.CONTINUE, token.FALLTHROUGH, token.RETURN,
+				token.INC, token.DEC, token.RPAREN, token.RBRACK, token.RBRACE:
+				impliedSemi = true
+			}
+			p.lastTok = x
+
+		case token.Pos:
+			if x.IsValid() {
+				p.pos = p.posFor(x) // accurate position of next item
+			}
+			continue
+
+		case string:
+			// incorrect AST - print error message
+			data = x
+			isLit = true
+			impliedSemi = true
+			p.lastTok = token.STRING
+
+		default:
+			fmt.Fprintf(os.Stderr, "print: unsupported argument %v (%T)\n", arg, arg)
+			panic("golang.org/x/website/internal/backport/go/printer type")
+		}
+		// data != ""
+
+		next := p.pos // estimated/accurate position of next item
+		wroteNewline, droppedFF := p.flush(next, p.lastTok)
+
+		// intersperse extra newlines if present in the source and
+		// if they don't cause extra semicolons (don't do this in
+		// flush as it will cause extra newlines at the end of a file)
+		if !p.impliedSemi {
+			n := nlimit(next.Line - p.pos.Line)
+			// don't exceed maxNewlines if we already wrote one
+			if wroteNewline && n == maxNewlines {
+				n = maxNewlines - 1
+			}
+			if n > 0 {
+				ch := byte('\n')
+				if droppedFF {
+					ch = '\f' // use formfeed since we dropped one before
+				}
+				p.writeByte(ch, n)
+				impliedSemi = false
+			}
+		}
+
+		// the next token starts now - record its line number if requested
+		if p.linePtr != nil {
+			*p.linePtr = p.out.Line
+			p.linePtr = nil
+		}
+
+		p.writeString(next, data, isLit)
+		p.impliedSemi = impliedSemi
+	}
+}
+
+// flush prints any pending comments and whitespace occurring textually
+// before the position of the next token tok. The flush result indicates
+// if a newline was written or if a formfeed was dropped from the whitespace
+// buffer.
+func (p *printer) flush(next token.Position, tok token.Token) (wroteNewline, droppedFF bool) {
+	if p.commentBefore(next) {
+		// if there are comments before the next item, intersperse them
+		wroteNewline, droppedFF = p.intersperseComments(next, tok)
+	} else {
+		// otherwise, write any leftover whitespace
+		p.writeWhitespace(len(p.wsbuf))
+	}
+	return
+}
+
+// getNode returns the ast.CommentGroup associated with n, if any.
+func getDoc(n ast.Node) *ast.CommentGroup {
+	switch n := n.(type) {
+	case *ast.Field:
+		return n.Doc
+	case *ast.ImportSpec:
+		return n.Doc
+	case *ast.ValueSpec:
+		return n.Doc
+	case *ast.TypeSpec:
+		return n.Doc
+	case *ast.GenDecl:
+		return n.Doc
+	case *ast.FuncDecl:
+		return n.Doc
+	case *ast.File:
+		return n.Doc
+	}
+	return nil
+}
+
+func getLastComment(n ast.Node) *ast.CommentGroup {
+	switch n := n.(type) {
+	case *ast.Field:
+		return n.Comment
+	case *ast.ImportSpec:
+		return n.Comment
+	case *ast.ValueSpec:
+		return n.Comment
+	case *ast.TypeSpec:
+		return n.Comment
+	case *ast.GenDecl:
+		if len(n.Specs) > 0 {
+			return getLastComment(n.Specs[len(n.Specs)-1])
+		}
+	case *ast.File:
+		if len(n.Comments) > 0 {
+			return n.Comments[len(n.Comments)-1]
+		}
+	}
+	return nil
+}
+
+func (p *printer) printNode(node interface{}) error {
+	// unpack *CommentedNode, if any
+	var comments []*ast.CommentGroup
+	if cnode, ok := node.(*CommentedNode); ok {
+		node = cnode.Node
+		comments = cnode.Comments
+	}
+
+	if comments != nil {
+		// commented node - restrict comment list to relevant range
+		n, ok := node.(ast.Node)
+		if !ok {
+			goto unsupported
+		}
+		beg := n.Pos()
+		end := n.End()
+		// if the node has associated documentation,
+		// include that commentgroup in the range
+		// (the comment list is sorted in the order
+		// of the comment appearance in the source code)
+		if doc := getDoc(n); doc != nil {
+			beg = doc.Pos()
+		}
+		if com := getLastComment(n); com != nil {
+			if e := com.End(); e > end {
+				end = e
+			}
+		}
+		// token.Pos values are global offsets, we can
+		// compare them directly
+		i := 0
+		for i < len(comments) && comments[i].End() < beg {
+			i++
+		}
+		j := i
+		for j < len(comments) && comments[j].Pos() < end {
+			j++
+		}
+		if i < j {
+			p.comments = comments[i:j]
+		}
+	} else if n, ok := node.(*ast.File); ok {
+		// use ast.File comments, if any
+		p.comments = n.Comments
+	}
+
+	// if there are no comments, use node comments
+	p.useNodeComments = p.comments == nil
+
+	// get comments ready for use
+	p.nextComment()
+
+	p.print(pmode(0))
+
+	// format node
+	switch n := node.(type) {
+	case ast.Expr:
+		p.expr(n)
+	case ast.Stmt:
+		// A labeled statement will un-indent to position the label.
+		// Set p.indent to 1 so we don't get indent "underflow".
+		if _, ok := n.(*ast.LabeledStmt); ok {
+			p.indent = 1
+		}
+		p.stmt(n, false)
+	case ast.Decl:
+		p.decl(n)
+	case ast.Spec:
+		p.spec(n, 1, false)
+	case []ast.Stmt:
+		// A labeled statement will un-indent to position the label.
+		// Set p.indent to 1 so we don't get indent "underflow".
+		for _, s := range n {
+			if _, ok := s.(*ast.LabeledStmt); ok {
+				p.indent = 1
+			}
+		}
+		p.stmtList(n, 0, false)
+	case []ast.Decl:
+		p.declList(n)
+	case *ast.File:
+		p.file(n)
+	default:
+		goto unsupported
+	}
+
+	return nil
+
+unsupported:
+	return fmt.Errorf("golang.org/x/website/internal/backport/go/printer: unsupported node type %T", node)
+}
+
+// ----------------------------------------------------------------------------
+// Trimmer
+
+// A trimmer is an io.Writer filter for stripping tabwriter.Escape
+// characters, trailing blanks and tabs, and for converting formfeed
+// and vtab characters into newlines and htabs (in case no tabwriter
+// is used). Text bracketed by tabwriter.Escape characters is passed
+// through unchanged.
+type trimmer struct {
+	output io.Writer
+	state  int
+	space  []byte
+}
+
+// trimmer is implemented as a state machine.
+// It can be in one of the following states:
+const (
+	inSpace  = iota // inside space
+	inEscape        // inside text bracketed by tabwriter.Escapes
+	inText          // inside text
+)
+
+func (p *trimmer) resetSpace() {
+	p.state = inSpace
+	p.space = p.space[0:0]
+}
+
+// Design note: It is tempting to eliminate extra blanks occurring in
+//              whitespace in this function as it could simplify some
+//              of the blanks logic in the node printing functions.
+//              However, this would mess up any formatting done by
+//              the tabwriter.
+
+var aNewline = []byte("\n")
+
+func (p *trimmer) Write(data []byte) (n int, err error) {
+	// invariants:
+	// p.state == inSpace:
+	//	p.space is unwritten
+	// p.state == inEscape, inText:
+	//	data[m:n] is unwritten
+	m := 0
+	var b byte
+	for n, b = range data {
+		if b == '\v' {
+			b = '\t' // convert to htab
+		}
+		switch p.state {
+		case inSpace:
+			switch b {
+			case '\t', ' ':
+				p.space = append(p.space, b)
+			case '\n', '\f':
+				p.resetSpace() // discard trailing space
+				_, err = p.output.Write(aNewline)
+			case tabwriter.Escape:
+				_, err = p.output.Write(p.space)
+				p.state = inEscape
+				m = n + 1 // +1: skip tabwriter.Escape
+			default:
+				_, err = p.output.Write(p.space)
+				p.state = inText
+				m = n
+			}
+		case inEscape:
+			if b == tabwriter.Escape {
+				_, err = p.output.Write(data[m:n])
+				p.resetSpace()
+			}
+		case inText:
+			switch b {
+			case '\t', ' ':
+				_, err = p.output.Write(data[m:n])
+				p.resetSpace()
+				p.space = append(p.space, b)
+			case '\n', '\f':
+				_, err = p.output.Write(data[m:n])
+				p.resetSpace()
+				if err == nil {
+					_, err = p.output.Write(aNewline)
+				}
+			case tabwriter.Escape:
+				_, err = p.output.Write(data[m:n])
+				p.state = inEscape
+				m = n + 1 // +1: skip tabwriter.Escape
+			}
+		default:
+			panic("unreachable")
+		}
+		if err != nil {
+			return
+		}
+	}
+	n = len(data)
+
+	switch p.state {
+	case inEscape, inText:
+		_, err = p.output.Write(data[m:n])
+		p.resetSpace()
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Public interface
+
+// A Mode value is a set of flags (or 0). They control printing.
+type Mode uint
+
+const (
+	RawFormat Mode = 1 << iota // do not use a tabwriter; if set, UseSpaces is ignored
+	TabIndent                  // use tabs for indentation independent of UseSpaces
+	UseSpaces                  // use spaces instead of tabs for alignment
+	SourcePos                  // emit //line directives to preserve original source positions
+)
+
+// The mode below is not included in printer's public API because
+// editing code text is deemed out of scope. Because this mode is
+// unexported, it's also possible to modify or remove it based on
+// the evolving needs of go/format and cmd/gofmt without breaking
+// users. See discussion in CL 240683.
+const (
+	// normalizeNumbers means to canonicalize number
+	// literal prefixes and exponents while printing.
+	//
+	// This value is known in and used by go/format and cmd/gofmt.
+	// It is currently more convenient and performant for those
+	// packages to apply number normalization during printing,
+	// rather than by modifying the AST in advance.
+	normalizeNumbers Mode = 1 << 30
+)
+
+// A Config node controls the output of Fprint.
+type Config struct {
+	Mode     Mode // default: 0
+	Tabwidth int  // default: 8
+	Indent   int  // default: 0 (all code is indented at least by this much)
+}
+
+// fprint implements Fprint and takes a nodesSizes map for setting up the printer state.
+func (cfg *Config) fprint(output io.Writer, fset *token.FileSet, node interface{}, nodeSizes map[ast.Node]int) (err error) {
+	// print node
+	var p printer
+	p.init(cfg, fset, nodeSizes)
+	if err = p.printNode(node); err != nil {
+		return
+	}
+	// print outstanding comments
+	p.impliedSemi = false // EOF acts like a newline
+	p.flush(token.Position{Offset: infinity, Line: infinity}, token.EOF)
+
+	// output is buffered in p.output now.
+	// fix //go:build and // +build comments if needed.
+	p.fixGoBuildLines()
+
+	// redirect output through a trimmer to eliminate trailing whitespace
+	// (Input to a tabwriter must be untrimmed since trailing tabs provide
+	// formatting information. The tabwriter could provide trimming
+	// functionality but no tabwriter is used when RawFormat is set.)
+	output = &trimmer{output: output}
+
+	// redirect output through a tabwriter if necessary
+	if cfg.Mode&RawFormat == 0 {
+		minwidth := cfg.Tabwidth
+
+		padchar := byte('\t')
+		if cfg.Mode&UseSpaces != 0 {
+			padchar = ' '
+		}
+
+		twmode := tabwriter.DiscardEmptyColumns
+		if cfg.Mode&TabIndent != 0 {
+			minwidth = 0
+			twmode |= tabwriter.TabIndent
+		}
+
+		output = tabwriter.NewWriter(output, minwidth, cfg.Tabwidth, 1, padchar, twmode)
+	}
+
+	// write printer result via tabwriter/trimmer to output
+	if _, err = output.Write(p.output); err != nil {
+		return
+	}
+
+	// flush tabwriter, if any
+	if tw, _ := output.(*tabwriter.Writer); tw != nil {
+		err = tw.Flush()
+	}
+
+	return
+}
+
+// A CommentedNode bundles an AST node and corresponding comments.
+// It may be provided as argument to any of the Fprint functions.
+type CommentedNode struct {
+	Node     interface{} // *ast.File, or ast.Expr, ast.Decl, ast.Spec, or ast.Stmt
+	Comments []*ast.CommentGroup
+}
+
+// Fprint "pretty-prints" an AST node to output for a given configuration cfg.
+// Position information is interpreted relative to the file set fset.
+// The node type must be *ast.File, *CommentedNode, []ast.Decl, []ast.Stmt,
+// or assignment-compatible to ast.Expr, ast.Decl, ast.Spec, or ast.Stmt.
+func (cfg *Config) Fprint(output io.Writer, fset *token.FileSet, node interface{}) error {
+	return cfg.fprint(output, fset, node, make(map[ast.Node]int))
+}
+
+// Fprint "pretty-prints" an AST node to output.
+// It calls Config.Fprint with default settings.
+// Note that gofmt uses tabs for indentation but spaces for alignment;
+// use format.Node (package go/format) for output that matches gofmt.
+func Fprint(output io.Writer, fset *token.FileSet, node interface{}) error {
+	return (&Config{Tabwidth: 8}).Fprint(output, fset, node)
+}
+
+func stringsCut(s, sep string) (before, after string, found bool) {
+	if i := strings.Index(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
+	}
+	return s, "", false
+}
diff --git a/internal/backport/go/printer/printer_test.go b/internal/backport/go/printer/printer_test.go
new file mode 100644
index 0000000..aed4ee6
--- /dev/null
+++ b/internal/backport/go/printer/printer_test.go
@@ -0,0 +1,823 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package printer
+
+import (
+	"bytes"
+	"errors"
+	"flag"
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
+	"io"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+const (
+	dataDir  = "testdata"
+	tabwidth = 8
+)
+
+var update = flag.Bool("update", false, "update golden files")
+
+var fset = token.NewFileSet()
+
+type checkMode uint
+
+const (
+	export checkMode = 1 << iota
+	rawFormat
+	normNumber
+	idempotent
+	allowTypeParams
+)
+
+// format parses src, prints the corresponding AST, verifies the resulting
+// src is syntactically correct, and returns the resulting src or an error
+// if any.
+func format(src []byte, mode checkMode) ([]byte, error) {
+	// parse src
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		return nil, fmt.Errorf("parse: %s\n%s", err, src)
+	}
+
+	// filter exports if necessary
+	if mode&export != 0 {
+		ast.FileExports(f) // ignore result
+		f.Comments = nil   // don't print comments that are not in AST
+	}
+
+	// determine printer configuration
+	cfg := Config{Tabwidth: tabwidth}
+	if mode&rawFormat != 0 {
+		cfg.Mode |= RawFormat
+	}
+	if mode&normNumber != 0 {
+		cfg.Mode |= normalizeNumbers
+	}
+
+	// print AST
+	var buf bytes.Buffer
+	if err := cfg.Fprint(&buf, fset, f); err != nil {
+		return nil, fmt.Errorf("print: %s", err)
+	}
+
+	// make sure formatted output is syntactically correct
+	res := buf.Bytes()
+	if _, err := parser.ParseFile(fset, "", res, parser.ParseComments); err != nil {
+		return nil, fmt.Errorf("re-parse: %s\n%s", err, buf.Bytes())
+	}
+
+	return res, nil
+}
+
+// lineAt returns the line in text starting at offset offs.
+func lineAt(text []byte, offs int) []byte {
+	i := offs
+	for i < len(text) && text[i] != '\n' {
+		i++
+	}
+	return text[offs:i]
+}
+
+// diff compares a and b.
+func diff(aname, bname string, a, b []byte) error {
+	if bytes.Equal(a, b) {
+		return nil
+	}
+
+	var buf bytes.Buffer // holding long error message
+	// compare lengths
+	if len(a) != len(b) {
+		fmt.Fprintf(&buf, "\nlength changed: len(%s) = %d, len(%s) = %d", aname, len(a), bname, len(b))
+	}
+
+	// compare contents
+	line := 1
+	offs := 0
+	for i := 0; i < len(a) && i < len(b); i++ {
+		ch := a[i]
+		if ch != b[i] {
+			fmt.Fprintf(&buf, "\n%s:%d:%d: %s", aname, line, i-offs+1, lineAt(a, offs))
+			fmt.Fprintf(&buf, "\n%s:%d:%d: %s", bname, line, i-offs+1, lineAt(b, offs))
+			fmt.Fprintf(&buf, "\n\n")
+			break
+		}
+		if ch == '\n' {
+			line++
+			offs = i + 1
+		}
+	}
+
+	fmt.Fprintf(&buf, "\n%s:\n%s\n%s:\n%s", aname, a, bname, b)
+	return errors.New(buf.String())
+}
+
+func runcheck(t *testing.T, source, golden string, mode checkMode) {
+	src, err := os.ReadFile(source)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	res, err := format(src, mode)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	// update golden files if necessary
+	if *update {
+		if err := os.WriteFile(golden, res, 0644); err != nil {
+			t.Error(err)
+		}
+		return
+	}
+
+	// get golden
+	gld, err := os.ReadFile(golden)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	// formatted source and golden must be the same
+	if err := diff(source, golden, res, gld); err != nil {
+		t.Error(err)
+		return
+	}
+
+	if mode&idempotent != 0 {
+		// formatting golden must be idempotent
+		// (This is very difficult to achieve in general and for now
+		// it is only checked for files explicitly marked as such.)
+		res, err = format(gld, mode)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		if err := diff(golden, fmt.Sprintf("format(%s)", golden), gld, res); err != nil {
+			t.Errorf("golden is not idempotent: %s", err)
+		}
+	}
+}
+
+func check(t *testing.T, source, golden string, mode checkMode) {
+	// run the test
+	cc := make(chan int, 1)
+	go func() {
+		runcheck(t, source, golden, mode)
+		cc <- 0
+	}()
+
+	// wait with timeout
+	select {
+	case <-time.After(10 * time.Second): // plenty of a safety margin, even for very slow machines
+		// test running past time out
+		t.Errorf("%s: running too slowly", source)
+	case <-cc:
+		// test finished within allotted time margin
+	}
+}
+
+type entry struct {
+	source, golden string
+	mode           checkMode
+}
+
+// Use go test -update to create/update the respective golden files.
+var data = []entry{
+	{"empty.input", "empty.golden", idempotent},
+	{"comments.input", "comments.golden", 0},
+	{"comments.input", "comments.x", export},
+	{"comments2.input", "comments2.golden", idempotent},
+	{"alignment.input", "alignment.golden", idempotent},
+	{"linebreaks.input", "linebreaks.golden", idempotent},
+	{"expressions.input", "expressions.golden", idempotent},
+	{"expressions.input", "expressions.raw", rawFormat | idempotent},
+	{"declarations.input", "declarations.golden", 0},
+	{"statements.input", "statements.golden", 0},
+	{"slow.input", "slow.golden", idempotent},
+	{"complit.input", "complit.x", export},
+	{"go2numbers.input", "go2numbers.golden", idempotent},
+	{"go2numbers.input", "go2numbers.norm", normNumber | idempotent},
+	{"generics.input", "generics.golden", idempotent | allowTypeParams},
+	{"gobuild1.input", "gobuild1.golden", idempotent},
+	{"gobuild2.input", "gobuild2.golden", idempotent},
+	{"gobuild3.input", "gobuild3.golden", idempotent},
+	{"gobuild4.input", "gobuild4.golden", idempotent},
+	{"gobuild5.input", "gobuild5.golden", idempotent},
+	{"gobuild6.input", "gobuild6.golden", idempotent},
+	{"gobuild7.input", "gobuild7.golden", idempotent},
+}
+
+func TestFiles(t *testing.T) {
+	t.Parallel()
+	for _, e := range data {
+		source := filepath.Join(dataDir, e.source)
+		golden := filepath.Join(dataDir, e.golden)
+		mode := e.mode
+		t.Run(e.source, func(t *testing.T) {
+			t.Parallel()
+			check(t, source, golden, mode)
+			// TODO(gri) check that golden is idempotent
+			//check(t, golden, golden, e.mode)
+		})
+	}
+}
+
+// TestLineComments, using a simple test case, checks that consecutive line
+// comments are properly terminated with a newline even if the AST position
+// information is incorrect.
+func TestLineComments(t *testing.T) {
+	const src = `// comment 1
+	// comment 2
+	// comment 3
+	package main
+	`
+
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		panic(err) // error in test
+	}
+
+	var buf bytes.Buffer
+	fset = token.NewFileSet() // use the wrong file set
+	Fprint(&buf, fset, f)
+
+	nlines := 0
+	for _, ch := range buf.Bytes() {
+		if ch == '\n' {
+			nlines++
+		}
+	}
+
+	const expected = 3
+	if nlines < expected {
+		t.Errorf("got %d, expected %d\n", nlines, expected)
+		t.Errorf("result:\n%s", buf.Bytes())
+	}
+}
+
+// Verify that the printer can be invoked during initialization.
+func init() {
+	const name = "foobar"
+	var buf bytes.Buffer
+	if err := Fprint(&buf, fset, &ast.Ident{Name: name}); err != nil {
+		panic(err) // error in test
+	}
+	// in debug mode, the result contains additional information;
+	// ignore it
+	if s := buf.String(); !debug && s != name {
+		panic("got " + s + ", want " + name)
+	}
+}
+
+// Verify that the printer doesn't crash if the AST contains BadXXX nodes.
+func TestBadNodes(t *testing.T) {
+	const src = "package p\n("
+	const res = "package p\nBadDecl\n"
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err == nil {
+		t.Error("expected illegal program") // error in test
+	}
+	var buf bytes.Buffer
+	Fprint(&buf, fset, f)
+	if buf.String() != res {
+		t.Errorf("got %q, expected %q", buf.String(), res)
+	}
+}
+
+// testComment verifies that f can be parsed again after printing it
+// with its first comment set to comment at any possible source offset.
+func testComment(t *testing.T, f *ast.File, srclen int, comment *ast.Comment) {
+	f.Comments[0].List[0] = comment
+	var buf bytes.Buffer
+	for offs := 0; offs <= srclen; offs++ {
+		buf.Reset()
+		// Printing f should result in a correct program no
+		// matter what the (incorrect) comment position is.
+		if err := Fprint(&buf, fset, f); err != nil {
+			t.Error(err)
+		}
+		if _, err := parser.ParseFile(fset, "", buf.Bytes(), 0); err != nil {
+			t.Fatalf("incorrect program for pos = %d:\n%s", comment.Slash, buf.String())
+		}
+		// Position information is just an offset.
+		// Move comment one byte down in the source.
+		comment.Slash++
+	}
+}
+
+// Verify that the printer produces a correct program
+// even if the position information of comments introducing newlines
+// is incorrect.
+func TestBadComments(t *testing.T) {
+	t.Parallel()
+	const src = `
+// first comment - text and position changed by test
+package p
+import "fmt"
+const pi = 3.14 // rough circle
+var (
+	x, y, z int = 1, 2, 3
+	u, v float64
+)
+func fibo(n int) {
+	if n < 2 {
+		return n /* seed values */
+	}
+	return fibo(n-1) + fibo(n-2)
+}
+`
+
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Error(err) // error in test
+	}
+
+	comment := f.Comments[0].List[0]
+	pos := comment.Pos()
+	if fset.PositionFor(pos, false /* absolute position */).Offset != 1 {
+		t.Error("expected offset 1") // error in test
+	}
+
+	testComment(t, f, len(src), &ast.Comment{Slash: pos, Text: "//-style comment"})
+	testComment(t, f, len(src), &ast.Comment{Slash: pos, Text: "/*-style comment */"})
+	testComment(t, f, len(src), &ast.Comment{Slash: pos, Text: "/*-style \n comment */"})
+	testComment(t, f, len(src), &ast.Comment{Slash: pos, Text: "/*-style comment \n\n\n */"})
+}
+
+type visitor chan *ast.Ident
+
+func (v visitor) Visit(n ast.Node) (w ast.Visitor) {
+	if ident, ok := n.(*ast.Ident); ok {
+		v <- ident
+	}
+	return v
+}
+
+// idents is an iterator that returns all idents in f via the result channel.
+func idents(f *ast.File) <-chan *ast.Ident {
+	v := make(visitor)
+	go func() {
+		ast.Walk(v, f)
+		close(v)
+	}()
+	return v
+}
+
+// identCount returns the number of identifiers found in f.
+func identCount(f *ast.File) int {
+	n := 0
+	for range idents(f) {
+		n++
+	}
+	return n
+}
+
+// Verify that the SourcePos mode emits correct //line directives
+// by testing that position information for matching identifiers
+// is maintained.
+func TestSourcePos(t *testing.T) {
+	const src = `
+package p
+import ( "golang.org/x/website/internal/backport/go/printer"; "math" )
+const pi = 3.14; var x = 0
+type t struct{ x, y, z int; u, v, w float32 }
+func (t *t) foo(a, b, c int) int {
+	return a*t.x + b*t.y +
+		// two extra lines here
+		// ...
+		c*t.z
+}
+`
+
+	// parse original
+	f1, err := parser.ParseFile(fset, "src", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// pretty-print original
+	var buf bytes.Buffer
+	err = (&Config{Mode: UseSpaces | SourcePos, Tabwidth: 8}).Fprint(&buf, fset, f1)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// parse pretty printed original
+	// (//line directives must be interpreted even w/o parser.ParseComments set)
+	f2, err := parser.ParseFile(fset, "", buf.Bytes(), 0)
+	if err != nil {
+		t.Fatalf("%s\n%s", err, buf.Bytes())
+	}
+
+	// At this point the position information of identifiers in f2 should
+	// match the position information of corresponding identifiers in f1.
+
+	// number of identifiers must be > 0 (test should run) and must match
+	n1 := identCount(f1)
+	n2 := identCount(f2)
+	if n1 == 0 {
+		t.Fatal("got no idents")
+	}
+	if n2 != n1 {
+		t.Errorf("got %d idents; want %d", n2, n1)
+	}
+
+	// verify that all identifiers have correct line information
+	i2range := idents(f2)
+	for i1 := range idents(f1) {
+		i2 := <-i2range
+
+		if i2.Name != i1.Name {
+			t.Errorf("got ident %s; want %s", i2.Name, i1.Name)
+		}
+
+		// here we care about the relative (line-directive adjusted) positions
+		l1 := fset.Position(i1.Pos()).Line
+		l2 := fset.Position(i2.Pos()).Line
+		if l2 != l1 {
+			t.Errorf("got line %d; want %d for %s", l2, l1, i1.Name)
+		}
+	}
+
+	if t.Failed() {
+		t.Logf("\n%s", buf.Bytes())
+	}
+}
+
+// Verify that the SourcePos mode doesn't emit unnecessary //line directives
+// before empty lines.
+func TestIssue5945(t *testing.T) {
+	const orig = `
+package p   // line 2
+func f() {} // line 3
+
+var x, y, z int
+
+
+func g() { // line 8
+}
+`
+
+	const want = `//line src.go:2
+package p
+
+//line src.go:3
+func f() {}
+
+var x, y, z int
+
+//line src.go:8
+func g() {
+}
+`
+
+	// parse original
+	f1, err := parser.ParseFile(fset, "src.go", orig, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// pretty-print original
+	var buf bytes.Buffer
+	err = (&Config{Mode: UseSpaces | SourcePos, Tabwidth: 8}).Fprint(&buf, fset, f1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := buf.String()
+
+	// compare original with desired output
+	if got != want {
+		t.Errorf("got:\n%s\nwant:\n%s\n", got, want)
+	}
+}
+
+var decls = []string{
+	`import "fmt"`,
+	"const pi = 3.1415\nconst e = 2.71828\n\nvar x = pi",
+	"func sum(x, y int) int\t{ return x + y }",
+}
+
+func TestDeclLists(t *testing.T) {
+	for _, src := range decls {
+		file, err := parser.ParseFile(fset, "", "package p;"+src, parser.ParseComments)
+		if err != nil {
+			panic(err) // error in test
+		}
+
+		var buf bytes.Buffer
+		err = Fprint(&buf, fset, file.Decls) // only print declarations
+		if err != nil {
+			panic(err) // error in test
+		}
+
+		out := buf.String()
+		if out != src {
+			t.Errorf("\ngot : %q\nwant: %q\n", out, src)
+		}
+	}
+}
+
+var stmts = []string{
+	"i := 0",
+	"select {}\nvar a, b = 1, 2\nreturn a + b",
+	"go f()\ndefer func() {}()",
+}
+
+func TestStmtLists(t *testing.T) {
+	for _, src := range stmts {
+		file, err := parser.ParseFile(fset, "", "package p; func _() {"+src+"}", parser.ParseComments)
+		if err != nil {
+			panic(err) // error in test
+		}
+
+		var buf bytes.Buffer
+		err = Fprint(&buf, fset, file.Decls[0].(*ast.FuncDecl).Body.List) // only print statements
+		if err != nil {
+			panic(err) // error in test
+		}
+
+		out := buf.String()
+		if out != src {
+			t.Errorf("\ngot : %q\nwant: %q\n", out, src)
+		}
+	}
+}
+
+func TestBaseIndent(t *testing.T) {
+	t.Parallel()
+	// The testfile must not contain multi-line raw strings since those
+	// are not indented (because their values must not change) and make
+	// this test fail.
+	const filename = "printer.go"
+	src, err := os.ReadFile(filename)
+	if err != nil {
+		panic(err) // error in test
+	}
+
+	file, err := parser.ParseFile(fset, filename, src, 0)
+	if err != nil {
+		panic(err) // error in test
+	}
+
+	for indent := 0; indent < 4; indent++ {
+		indent := indent
+		t.Run(fmt.Sprint(indent), func(t *testing.T) {
+			t.Parallel()
+			var buf bytes.Buffer
+			(&Config{Tabwidth: tabwidth, Indent: indent}).Fprint(&buf, fset, file)
+			// all code must be indented by at least 'indent' tabs
+			lines := bytes.Split(buf.Bytes(), []byte{'\n'})
+			for i, line := range lines {
+				if len(line) == 0 {
+					continue // empty lines don't have indentation
+				}
+				n := 0
+				for j, b := range line {
+					if b != '\t' {
+						// end of indentation
+						n = j
+						break
+					}
+				}
+				if n < indent {
+					t.Errorf("line %d: got only %d tabs; want at least %d: %q", i, n, indent, line)
+				}
+			}
+		})
+	}
+}
+
+// TestFuncType tests that an ast.FuncType with a nil Params field
+// can be printed (per go/ast specification). Test case for issue 3870.
+func TestFuncType(t *testing.T) {
+	src := &ast.File{
+		Name: &ast.Ident{Name: "p"},
+		Decls: []ast.Decl{
+			&ast.FuncDecl{
+				Name: &ast.Ident{Name: "f"},
+				Type: &ast.FuncType{},
+			},
+		},
+	}
+
+	var buf bytes.Buffer
+	if err := Fprint(&buf, fset, src); err != nil {
+		t.Fatal(err)
+	}
+	got := buf.String()
+
+	const want = `package p
+
+func f()
+`
+
+	if got != want {
+		t.Fatalf("got:\n%s\nwant:\n%s\n", got, want)
+	}
+}
+
+type limitWriter struct {
+	remaining int
+	errCount  int
+}
+
+func (l *limitWriter) Write(buf []byte) (n int, err error) {
+	n = len(buf)
+	if n >= l.remaining {
+		n = l.remaining
+		err = io.EOF
+		l.errCount++
+	}
+	l.remaining -= n
+	return n, err
+}
+
+// Test whether the printer stops writing after the first error
+func TestWriteErrors(t *testing.T) {
+	t.Parallel()
+	const filename = "printer.go"
+	src, err := os.ReadFile(filename)
+	if err != nil {
+		panic(err) // error in test
+	}
+	file, err := parser.ParseFile(fset, filename, src, 0)
+	if err != nil {
+		panic(err) // error in test
+	}
+	for i := 0; i < 20; i++ {
+		lw := &limitWriter{remaining: i}
+		err := (&Config{Mode: RawFormat}).Fprint(lw, fset, file)
+		if lw.errCount > 1 {
+			t.Fatal("Writes continued after first error returned")
+		}
+		// We expect errCount be 1 iff err is set
+		if (lw.errCount != 0) != (err != nil) {
+			t.Fatal("Expected err when errCount != 0")
+		}
+	}
+}
+
+// TextX is a skeleton test that can be filled in for debugging one-off cases.
+// Do not remove.
+func TestX(t *testing.T) {
+	const src = `
+package p
+func _() {}
+`
+	_, err := format([]byte(src), 0)
+	if err != nil {
+		t.Error(err)
+	}
+}
+
+func TestCommentedNode(t *testing.T) {
+	const (
+		input = `package main
+
+func foo() {
+	// comment inside func
+}
+
+// leading comment
+type bar int // comment2
+
+`
+
+		foo = `func foo() {
+	// comment inside func
+}`
+
+		bar = `// leading comment
+type bar int	// comment2
+`
+	)
+
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "input.go", input, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+
+	err = Fprint(&buf, fset, &CommentedNode{Node: f.Decls[0], Comments: f.Comments})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if buf.String() != foo {
+		t.Errorf("got %q, want %q", buf.String(), foo)
+	}
+
+	buf.Reset()
+
+	err = Fprint(&buf, fset, &CommentedNode{Node: f.Decls[1], Comments: f.Comments})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if buf.String() != bar {
+		t.Errorf("got %q, want %q", buf.String(), bar)
+	}
+}
+
+func TestIssue11151(t *testing.T) {
+	const src = "package p\t/*\r/1\r*\r/2*\r\r\r\r/3*\r\r+\r\r/4*/\n"
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+	Fprint(&buf, fset, f)
+	got := buf.String()
+	const want = "package p\t/*/1*\r/2*\r/3*+/4*/\n" // \r following opening /* should be stripped
+	if got != want {
+		t.Errorf("\ngot : %q\nwant: %q", got, want)
+	}
+
+	// the resulting program must be valid
+	_, err = parser.ParseFile(fset, "", got, 0)
+	if err != nil {
+		t.Errorf("%v\norig: %q\ngot : %q", err, src, got)
+	}
+}
+
+// If a declaration has multiple specifications, a parenthesized
+// declaration must be printed even if Lparen is token.NoPos.
+func TestParenthesizedDecl(t *testing.T) {
+	// a package with multiple specs in a single declaration
+	const src = "package p; var ( a float64; b int )"
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// print the original package
+	var buf bytes.Buffer
+	err = Fprint(&buf, fset, f)
+	if err != nil {
+		t.Fatal(err)
+	}
+	original := buf.String()
+
+	// now remove parentheses from the declaration
+	for i := 0; i != len(f.Decls); i++ {
+		f.Decls[i].(*ast.GenDecl).Lparen = token.NoPos
+	}
+	buf.Reset()
+	err = Fprint(&buf, fset, f)
+	if err != nil {
+		t.Fatal(err)
+	}
+	noparen := buf.String()
+
+	if noparen != original {
+		t.Errorf("got %q, want %q", noparen, original)
+	}
+}
+
+// Verify that we don't print a newline between "return" and its results, as
+// that would incorrectly cause a naked return.
+func TestIssue32854(t *testing.T) {
+	src := `package foo
+
+func f() {
+        return Composite{
+                call(),
+        }
+}`
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	// Replace the result with call(), which is on the next line.
+	fd := file.Decls[0].(*ast.FuncDecl)
+	ret := fd.Body.List[0].(*ast.ReturnStmt)
+	ret.Results[0] = ret.Results[0].(*ast.CompositeLit).Elts[0]
+
+	var buf bytes.Buffer
+	if err := Fprint(&buf, fset, ret); err != nil {
+		t.Fatal(err)
+	}
+	want := "return call()"
+	if got := buf.String(); got != want {
+		t.Fatalf("got %q, want %q", got, want)
+	}
+}
diff --git a/internal/backport/go/printer/testdata/alignment.golden b/internal/backport/go/printer/testdata/alignment.golden
new file mode 100644
index 0000000..96086ed
--- /dev/null
+++ b/internal/backport/go/printer/testdata/alignment.golden
@@ -0,0 +1,172 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package alignment
+
+// ----------------------------------------------------------------------------
+// Examples from issue #7335.
+
+func main() {
+	z := MyStruct{
+		Foo:		"foo",
+		Bar:		"bar",
+		Name:		"name",
+		LongName:	"longname",
+		Baz:		"baz",
+	}
+	y := MyStruct{
+		Foo:			"foo",
+		Bar:			"bar",
+		NameXX:			"name",
+		LongNameXXXXXXXXXXXXX:	"longname",
+		Baz:			"baz",
+	}
+	z := MyStruct{
+		Foo:			"foo",
+		Bar:			"bar",
+		Name:			"name",
+		LongNameXXXXXXXXXXXXX:	"longname",
+		Baz:			"baz",
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #10392.
+
+var kcfg = KubeletConfig{
+	Address:			s.Address,
+	AllowPrivileged:		s.AllowPrivileged,
+	HostNetworkSources:		hostNetworkSources,
+	HostnameOverride:		s.HostnameOverride,
+	RootDirectory:			s.RootDirectory,
+	ConfigFile:			s.Config,
+	ManifestURL:			s.ManifestURL,
+	FileCheckFrequency:		s.FileCheckFrequency,
+	HTTPCheckFrequency:		s.HTTPCheckFrequency,
+	PodInfraContainerImage:		s.PodInfraContainerImage,
+	SyncFrequency:			s.SyncFrequency,
+	RegistryPullQPS:		s.RegistryPullQPS,
+	RegistryBurst:			s.RegistryBurst,
+	MinimumGCAge:			s.MinimumGCAge,
+	MaxPerPodContainerCount:	s.MaxPerPodContainerCount,
+	MaxContainerCount:		s.MaxContainerCount,
+	ClusterDomain:			s.ClusterDomain,
+	ClusterDNS:			s.ClusterDNS,
+	Runonce:			s.RunOnce,
+	Port:				s.Port,
+	ReadOnlyPort:			s.ReadOnlyPort,
+	CadvisorInterface:		cadvisorInterface,
+	EnableServer:			s.EnableServer,
+	EnableDebuggingHandlers:	s.EnableDebuggingHandlers,
+	DockerClient:			dockertools.ConnectToDockerOrDie(s.DockerEndpoint),
+	KubeClient:			client,
+	MasterServiceNamespace:		s.MasterServiceNamespace,
+	VolumePlugins:			ProbeVolumePlugins(),
+	NetworkPlugins:			ProbeNetworkPlugins(),
+	NetworkPluginName:		s.NetworkPluginName,
+	StreamingConnectionIdleTimeout:	s.StreamingConnectionIdleTimeout,
+	TLSOptions:			tlsOptions,
+	ImageGCPolicy:			imageGCPolicy, imageGCPolicy,
+	Cloud:				cloud,
+	NodeStatusUpdateFrequency:	s.NodeStatusUpdateFrequency,
+}
+
+var a = A{
+	Long:					1,
+	LongLong:				1,
+	LongLongLong:				1,
+	LongLongLongLong:			1,
+	LongLongLongLongLong:			1,
+	LongLongLongLongLongLong:		1,
+	LongLongLongLongLongLongLong:		1,
+	LongLongLongLongLongLongLongLong:	1,
+	Short:					1,
+	LongLongLongLongLongLongLongLongLong:	3,
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #22852.
+
+var fmtMap = map[string]string{
+	"1":					"123",
+	"12":					"123",
+	"123":					"123",
+	"1234":					"123",
+	"12345":				"123",
+	"123456":				"123",
+	"12345678901234567890123456789":	"123",
+	"abcde":				"123",
+	"123456789012345678901234567890":	"123",
+	"1234567":				"123",
+	"abcdefghijklmnopqrstuvwxyzabcd":	"123",
+	"abcd":					"123",
+}
+
+type Fmt struct {
+	abcdefghijklmnopqrstuvwx	string
+	abcdefghijklmnopqrstuvwxy	string
+	abcdefghijklmnopqrstuvwxyz	string
+	abcdefghijklmnopqrstuvwxyza	string
+	abcdefghijklmnopqrstuvwxyzab	string
+	abcdefghijklmnopqrstuvwxyzabc	string
+	abcde				string
+	abcdefghijklmnopqrstuvwxyzabcde	string
+	abcdefg				string
+}
+
+func main() {
+	_ := Fmt{
+		abcdefghijklmnopqrstuvwx:		"foo",
+		abcdefghijklmnopqrstuvwxyza:		"foo",
+		abcdefghijklmnopqrstuvwxyzab:		"foo",
+		abcdefghijklmnopqrstuvwxyzabc:		"foo",
+		abcde:					"foo",
+		abcdefghijklmnopqrstuvwxyzabcde:	"foo",
+		abcdefg:				"foo",
+		abcdefghijklmnopqrstuvwxy:		"foo",
+		abcdefghijklmnopqrstuvwxyz:		"foo",
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #26352.
+
+var _ = map[int]string{
+	1:	"",
+
+	12345678901234567890123456789:		"",
+	12345678901234567890123456789012345678:	"",
+}
+
+func f() {
+	_ = map[int]string{
+		1:	"",
+
+		12345678901234567:				"",
+		12345678901234567890123456789012345678901:	"",
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #26930.
+
+var _ = S{
+	F1:	[]string{},
+	F2____:	[]string{},
+}
+
+var _ = S{
+	F1:	[]string{},
+	F2____:	[]string{},
+}
+
+var _ = S{
+	F1____:	[]string{},
+	F2:	[]string{},
+}
+
+var _ = S{
+	F1____:	[]string{},
+	F2:	[]string{},
+}
diff --git a/internal/backport/go/printer/testdata/alignment.input b/internal/backport/go/printer/testdata/alignment.input
new file mode 100644
index 0000000..323d268
--- /dev/null
+++ b/internal/backport/go/printer/testdata/alignment.input
@@ -0,0 +1,179 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package alignment
+
+// ----------------------------------------------------------------------------
+// Examples from issue #7335.
+
+func main() {
+    z := MyStruct{
+        Foo:      "foo",
+        Bar:      "bar",
+        Name:     "name",
+        LongName: "longname",
+        Baz:      "baz",
+    }
+    y := MyStruct{
+        Foo:                   "foo",
+        Bar:                   "bar",
+        NameXX:                "name",
+        LongNameXXXXXXXXXXXXX: "longname",
+        Baz: "baz",
+    }
+    z := MyStruct{
+        Foo:  "foo",
+        Bar:  "bar",
+        Name: "name",
+        LongNameXXXXXXXXXXXXX: "longname",
+        Baz: "baz",
+    }
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #10392.
+
+var kcfg = KubeletConfig{
+    Address:                        s.Address,
+    AllowPrivileged:                s.AllowPrivileged,
+    HostNetworkSources:             hostNetworkSources,
+    HostnameOverride:               s.HostnameOverride,
+    RootDirectory:                  s.RootDirectory,
+    ConfigFile:                     s.Config,
+    ManifestURL:                    s.ManifestURL,
+    FileCheckFrequency:             s.FileCheckFrequency,
+    HTTPCheckFrequency:             s.HTTPCheckFrequency,
+    PodInfraContainerImage:         s.PodInfraContainerImage,
+    SyncFrequency:                  s.SyncFrequency,
+    RegistryPullQPS:                s.RegistryPullQPS,
+    RegistryBurst:                  s.RegistryBurst,
+    MinimumGCAge:                   s.MinimumGCAge,
+    MaxPerPodContainerCount:        s.MaxPerPodContainerCount,
+    MaxContainerCount:              s.MaxContainerCount,
+    ClusterDomain:                  s.ClusterDomain,
+    ClusterDNS:                     s.ClusterDNS,
+    Runonce:                        s.RunOnce,
+    Port:                           s.Port,
+    ReadOnlyPort:                   s.ReadOnlyPort,
+    CadvisorInterface:              cadvisorInterface,
+    EnableServer:                   s.EnableServer,
+    EnableDebuggingHandlers:        s.EnableDebuggingHandlers,
+    DockerClient:                   dockertools.ConnectToDockerOrDie(s.DockerEndpoint),
+    KubeClient:                     client,
+    MasterServiceNamespace:         s.MasterServiceNamespace,
+    VolumePlugins:                  ProbeVolumePlugins(),
+    NetworkPlugins:                 ProbeNetworkPlugins(),
+    NetworkPluginName:              s.NetworkPluginName,
+    StreamingConnectionIdleTimeout: s.StreamingConnectionIdleTimeout,
+    TLSOptions:                     tlsOptions,
+    ImageGCPolicy:                  imageGCPolicy,imageGCPolicy,
+    Cloud:                          cloud,
+    NodeStatusUpdateFrequency: s.NodeStatusUpdateFrequency,
+}
+
+var a = A{
+    Long:                             1,
+    LongLong:                         1,
+    LongLongLong:                     1,
+    LongLongLongLong:                 1,
+    LongLongLongLongLong:             1,
+    LongLongLongLongLongLong:         1,
+    LongLongLongLongLongLongLong:     1,
+    LongLongLongLongLongLongLongLong: 1,
+    Short: 1,
+    LongLongLongLongLongLongLongLongLong: 3,
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #22852.
+
+var fmtMap = map[string]string{
+	"1": "123",
+	"12": "123",
+	"123": "123",
+	"1234": "123",
+	"12345": "123",
+	"123456": "123",
+	"12345678901234567890123456789": "123",
+	"abcde": "123",
+	"123456789012345678901234567890": "123",
+	"1234567": "123",
+	"abcdefghijklmnopqrstuvwxyzabcd": "123",
+	"abcd": "123",
+}
+
+type Fmt struct {
+	abcdefghijklmnopqrstuvwx string
+	abcdefghijklmnopqrstuvwxy string
+	abcdefghijklmnopqrstuvwxyz string
+	abcdefghijklmnopqrstuvwxyza string
+	abcdefghijklmnopqrstuvwxyzab string
+	abcdefghijklmnopqrstuvwxyzabc string
+	abcde string
+	abcdefghijklmnopqrstuvwxyzabcde string
+	abcdefg string
+}
+
+func main() {
+	_ := Fmt{
+		abcdefghijklmnopqrstuvwx: "foo",
+		abcdefghijklmnopqrstuvwxyza: "foo",
+		abcdefghijklmnopqrstuvwxyzab: "foo",
+		abcdefghijklmnopqrstuvwxyzabc: "foo",
+		abcde: "foo",
+		abcdefghijklmnopqrstuvwxyzabcde: "foo",
+		abcdefg: "foo",
+		abcdefghijklmnopqrstuvwxy: "foo",
+		abcdefghijklmnopqrstuvwxyz: "foo",
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #26352.
+
+var _ = map[int]string{
+	1: "",
+
+	12345678901234567890123456789: "",
+	12345678901234567890123456789012345678: "",
+}
+
+func f() {
+	_ = map[int]string{
+		1: "",
+
+		12345678901234567: "",
+		12345678901234567890123456789012345678901: "",
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #26930.
+
+var _ = S{
+	F1: []string{
+	},
+	F2____: []string{},
+}
+
+var _ = S{
+	F1: []string{
+
+
+	},
+	F2____: []string{},
+}
+
+var _ = S{
+	F1____: []string{
+	},
+	F2: []string{},
+}
+
+var _ = S{
+	F1____: []string{
+
+	},
+	F2: []string{},
+}
diff --git a/internal/backport/go/printer/testdata/comments.golden b/internal/backport/go/printer/testdata/comments.golden
new file mode 100644
index 0000000..1a21fff
--- /dev/null
+++ b/internal/backport/go/printer/testdata/comments.golden
@@ -0,0 +1,759 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+import "fmt"	// fmt
+
+const c0 = 0	// zero
+const (
+	c1	= iota	// c1
+	c2		// c2
+)
+
+// Alignment of comments in declarations>
+const (
+	_	T	= iota	// comment
+	_			// comment
+	_			// comment
+	_	= iota + 10
+	_	// comments
+
+	_		= 10	// comment
+	_	T	= 20	// comment
+)
+
+const (
+	_____	= iota	// foo
+	_		// bar
+	_	= 0	// bal
+	_		// bat
+)
+
+const (
+	_	T	= iota	// comment
+	_			// comment
+	_			// comment
+	_	= iota + 10
+	_		// comment
+	_		= 10
+	_		= 20	// comment
+	_	T	= 0	// comment
+)
+
+// The SZ struct; it is empty.
+type SZ struct{}
+
+// The S0 struct; no field is exported.
+type S0 struct {
+	int
+	x, y, z	int	// 3 unexported fields
+}
+
+// The S1 struct; some fields are not exported.
+type S1 struct {
+	S0
+	A, B, C	float	// 3 exported fields
+	D, b, c	int	// 2 unexported fields
+}
+
+// The S2 struct; all fields are exported.
+type S2 struct {
+	S1
+	A, B, C	float	// 3 exported fields
+}
+
+// The IZ interface; it is empty.
+type SZ interface{}
+
+// The I0 interface; no method is exported.
+type I0 interface {
+	f(x int) int	// unexported method
+}
+
+// The I1 interface; some methods are not exported.
+type I1 interface {
+	I0
+	F(x float) float	// exported methods
+	g(x int) int		// unexported method
+}
+
+// The I2 interface; all methods are exported.
+type I2 interface {
+	I0
+	F(x float) float	// exported method
+	G(x float) float	// exported method
+}
+
+// The S3 struct; all comments except for the last one must appear in the export.
+type S3 struct {
+	// lead comment for F1
+	F1	int	// line comment for F1
+	// lead comment for F2
+	F2	int	// line comment for F2
+	f3	int	// f3 is not exported
+}
+
+// This comment group should be separated
+// with a newline from the next comment
+// group.
+
+// This comment should NOT be associated with the next declaration.
+
+var x int	// x
+var ()
+
+// This comment SHOULD be associated with f0.
+func f0() {
+	const pi = 3.14	// pi
+	var s1 struct{}	/* an empty struct */	/* foo */
+	// a struct constructor
+	// --------------------
+	var s2 struct{} = struct{}{}
+	x := pi
+}
+
+//
+// This comment should be associated with f1, with one blank line before the comment.
+//
+func f1() {
+	f0()
+	/* 1 */
+	// 2
+	/* 3 */
+	/* 4 */
+	f0()
+}
+
+func _() {
+	// this comment should be properly indented
+}
+
+func _(x int) int {
+	if x < 0 {	// the tab printed before this comment's // must not affect the remaining lines
+		return -x	// this statement should be properly indented
+	}
+	if x < 0 {	/* the tab printed before this comment's /* must not affect the remaining lines */
+		return -x	// this statement should be properly indented
+	}
+	return x
+}
+
+func typeswitch(x interface{}) {
+	switch v := x.(type) {
+	case bool, int, float:
+	case string:
+	default:
+	}
+
+	switch x.(type) {
+	}
+
+	switch v0, ok := x.(int); v := x.(type) {
+	}
+
+	switch v0, ok := x.(int); x.(type) {
+	case byte:	// this comment should be on the same line as the keyword
+		// this comment should be normally indented
+		_ = 0
+	case bool, int, float:
+		// this comment should be indented
+	case string:
+	default:
+		// this comment should be indented
+	}
+	// this comment should not be indented
+}
+
+//
+// Indentation of comments after possibly indented multi-line constructs
+// (test cases for issue 3147).
+//
+
+func _() {
+	s := 1 +
+		2
+	// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2	// comment
+	// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2	// comment
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+
+	// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2	// comment
+
+	// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2	// comment
+
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+
+	// should be indented like s
+	_ = 0
+}
+
+// Test case from issue 3147.
+func f() {
+	templateText := "a" +	// A
+		"b" +	// B
+		"c"	// C
+
+	// should be aligned with f()
+	f()
+}
+
+// Modified test case from issue 3147.
+func f() {
+	templateText := "a" +	// A
+		"b" +	// B
+		"c"	// C
+
+		// may not be aligned with f() (source is not aligned)
+	f()
+}
+
+//
+// Test cases for alignment of lines in general comments.
+//
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line */
+}
+
+// Issue 9751.
+func _() {
+	/*a string
+
+	b string*/
+
+	/*A string
+
+
+
+	Z string*/
+
+	/*a string
+
+	b string
+
+	c string*/
+
+	{
+		/*a string
+		b string*/
+
+		/*a string
+
+		b string*/
+
+		/*a string
+
+		b string
+
+		c string*/
+	}
+
+	{
+		/*a string
+		b string*/
+
+		/*a string
+
+		b string*/
+
+		/*a string
+
+		b string
+
+		c string*/
+	}
+
+	/*
+	 */
+
+	/*
+
+	 */
+
+	/*
+
+	 * line
+
+	 */
+}
+
+/*
+ * line
+ * of
+ * stars
+ */
+
+/* another line
+ * of
+ * stars */
+
+/*	and another line
+ *	of
+ *	stars */
+
+/* a line of
+ * stars */
+
+/*	and another line of
+ *	stars */
+
+/* a line of stars
+ */
+
+/*	and another line of
+ */
+
+/* a line of stars
+ */
+
+/*	and another line of
+ */
+
+/*
+aligned in middle
+here
+        not here
+*/
+
+/*
+blank line in middle:
+
+with no leading spaces on blank line.
+*/
+
+/*
+   aligned in middle
+   here
+           not here
+*/
+
+/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+*/
+
+func _() {
+	/*
+	 * line
+	 * of
+	 * stars
+	 */
+
+	/*
+		aligned in middle
+		here
+			not here
+	*/
+
+	/*
+		blank line in middle:
+
+		with no leading spaces on blank line.
+	*/
+}
+
+// Some interesting interspersed comments.
+// See below for more common cases.
+func _( /* this */ x /* is */ /* an */ int) {
+}
+
+func _( /* no params - extra blank before and after comment */ )	{}
+func _(a, b int /* params - no extra blank after comment */)		{}
+
+func _()	{ f( /* no args - extra blank before and after comment */ ) }
+func _()	{ f(a, b /* args - no extra blank after comment */) }
+
+func _() {
+	f( /* no args - extra blank before and after comment */ )
+	f(a, b /* args - no extra blank after comment */)
+}
+
+func ( /* comment1 */ T /* comment2 */) _()	{}
+
+func _()	{ /* "short-ish one-line functions with comments are formatted as multi-line functions */ }
+func _()	{ x := 0; /* comment */ y = x /* comment */ }
+
+func _() {
+	_ = 0
+	/* closing curly brace should be on new line */
+}
+
+func _() {
+	_ = []int{0, 1 /* don't introduce a newline after this comment - was issue 1365 */}
+}
+
+// Test cases from issue 1542:
+// Comments must not be placed before commas and cause invalid programs.
+func _() {
+	var a = []int{1, 2	/*jasldf*/}
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2}/*jasldf
+	 */
+
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2}// jasldf
+
+	_ = a
+}
+
+// Test cases from issues 11274, 15137:
+// Semicolon must not be lost when multiple statements are on the same line with a comment.
+func _() {
+	x := 0 /**/
+	y := 1
+}
+
+func _() {
+	f()
+	f()
+	f() /* comment */
+	f()
+	f() /* comment */
+	f()
+	f() /* a */ /* b */
+	f()
+	f() /* a */ /* b */
+	f()
+	f() /* a */ /* b */
+	f()
+}
+
+func _() {
+	f() /* a */ /* b */
+}
+
+// Comments immediately adjacent to punctuation followed by a newline
+// remain after the punctuation (looks better and permits alignment of
+// comments).
+func _() {
+	_ = T{
+		1,	// comment after comma
+		2,	/* comment after comma */
+		3,	// comment after comma
+	}
+	_ = T{
+		1,	// comment after comma
+		2,	/* comment after comma */
+		3,	// comment after comma
+	}
+	_ = T{
+		/* comment before literal */ 1,
+		2,	/* comment before comma - ok to move after comma */
+		3,	/* comment before comma - ok to move after comma */
+	}
+
+	for i = 0;	// comment after semicolon
+	i < 9;		/* comment after semicolon */
+	i++ {		// comment after opening curly brace
+	}
+
+	// TODO(gri) the last comment in this example should be aligned */
+	for i = 0;	// comment after semicolon
+	i < 9;		/* comment before semicolon - ok to move after semicolon */
+	i++ /* comment before opening curly brace */ {
+	}
+}
+
+// If there is no newline following punctuation, commas move before the punctuation.
+// This way, commas interspersed in lists stay with the respective expression.
+func f(x /* comment */, y int, z int /* comment */, u, v, w int /* comment */) {
+	f(x /* comment */, y)
+	f(x,	/* comment */
+		y)
+	f(
+		x,	/* comment */
+	)
+}
+
+func g(
+	x int,	/* comment */
+) {
+}
+
+type _ struct {
+	a, b /* comment */, c int
+}
+
+type _ struct {
+	a, b /* comment */, c int
+}
+
+func _() {
+	for a /* comment */, b := range x {
+	}
+}
+
+// Print line directives correctly.
+
+// The following is a legal line directive.
+//line foo:1
+func _() {
+	_ = 0
+	// The following is a legal line directive. It must not be indented:
+//line foo:2
+	_ = 1
+
+	// The following is not a legal line directive (it doesn't start in column 1):
+	//line foo:2
+	_ = 2
+
+	// The following is not a legal line directive (missing colon):
+//line foo -3
+	_ = 3
+}
+
+// Line comments with tabs
+func _() {
+	var finput *bufio.Reader	// input file
+	var stderr *bufio.Writer
+	var ftable *bufio.Writer	// y.go file
+	var foutput *bufio.Writer	// y.output file
+
+	var oflag string	// -o [y.go]		- y.go file
+	var vflag string	// -v [y.output]	- y.output file
+	var lflag bool		// -l			- disable line directives
+}
+
+// Trailing white space in comments should be trimmed
+func _() {
+	// This comment has 4 blanks following that should be trimmed:
+	/* Each line of this comment has blanks or tabs following that should be trimmed:
+	   line 2:
+	   line 3:
+	*/
+}
+
+var _ = []T{ /* lone comment */ }
+
+var _ = []T{
+	/* lone comment */
+}
+
+var _ = []T{
+	// lone comments
+	// in composite lit
+}
+
+var _ = [][]T{
+	{
+		// lone comments
+		// in composite lit
+	},
+}
+
+// TODO: gofmt doesn't add these tabs; make it so that these golden
+// tests run the printer in a way that it's exactly like gofmt.
+
+var _ = []T{	// lone comment
+}
+
+var _ = []T{	// lone comments
+	// in composite lit
+}
+
+/* This comment is the last entry in this file. It must be printed and should be followed by a newline */
diff --git a/internal/backport/go/printer/testdata/comments.input b/internal/backport/go/printer/testdata/comments.input
new file mode 100644
index 0000000..aa428a2
--- /dev/null
+++ b/internal/backport/go/printer/testdata/comments.input
@@ -0,0 +1,756 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+import "fmt"  // fmt
+
+const c0 = 0  // zero
+const (
+	c1 = iota  // c1
+	c2  // c2
+)
+
+// Alignment of comments in declarations>
+const (
+	_ T = iota  // comment
+	_  // comment
+	_  // comment
+	_ = iota+10
+	_  // comments
+
+	_ = 10  // comment
+	_ T = 20  // comment
+)
+
+const (
+	_____ = iota // foo
+	_ // bar
+	_  = 0    // bal
+	_ // bat
+)
+
+const (
+	_ T = iota // comment
+	_ // comment
+	_ // comment
+	_ = iota + 10
+	_ // comment
+	_ = 10
+	_ = 20 // comment
+	_ T = 0 // comment
+)
+
+// The SZ struct; it is empty.
+type SZ struct {}
+
+// The S0 struct; no field is exported.
+type S0 struct {
+	int
+	x, y, z int  // 3 unexported fields
+}
+
+// The S1 struct; some fields are not exported.
+type S1 struct {
+	S0
+	A, B, C float  // 3 exported fields
+	D, b, c int  // 2 unexported fields
+}
+
+// The S2 struct; all fields are exported.
+type S2 struct {
+	S1
+	A, B, C float  // 3 exported fields
+}
+
+// The IZ interface; it is empty.
+type SZ interface {}
+
+// The I0 interface; no method is exported.
+type I0 interface {
+	f(x int) int  // unexported method
+}
+
+// The I1 interface; some methods are not exported.
+type I1 interface {
+	I0
+	F(x float) float  // exported methods
+	g(x int) int  // unexported method
+}
+
+// The I2 interface; all methods are exported.
+type I2 interface {
+	I0
+	F(x float) float  // exported method
+	G(x float) float  // exported method
+}
+
+// The S3 struct; all comments except for the last one must appear in the export.
+type S3 struct {
+	// lead comment for F1
+	F1 int // line comment for F1
+	// lead comment for F2
+	F2 int // line comment for F2
+	f3 int // f3 is not exported
+}
+
+// This comment group should be separated
+// with a newline from the next comment
+// group.
+
+// This comment should NOT be associated with the next declaration.
+
+var x int  // x
+var ()
+
+
+// This comment SHOULD be associated with f0.
+func f0() {
+	const pi = 3.14  // pi
+	var s1 struct {}  /* an empty struct */ /* foo */
+	// a struct constructor
+	// --------------------
+	var s2 struct {} = struct {}{}
+	x := pi
+}
+//
+// This comment should be associated with f1, with one blank line before the comment.
+//
+func f1() {
+	f0()
+	/* 1 */
+	// 2
+	/* 3 */
+	/* 4 */
+	f0()
+}
+
+
+func _() {
+// this comment should be properly indented
+}
+
+
+func _(x int) int {
+	if x < 0 {  // the tab printed before this comment's // must not affect the remaining lines
+		return -x  // this statement should be properly indented
+	}
+	if x < 0 {  /* the tab printed before this comment's /* must not affect the remaining lines */
+		return -x  // this statement should be properly indented
+	}
+	return x
+}
+
+
+func typeswitch(x interface{}) {
+	switch v := x.(type) {
+	case bool, int, float:
+	case string:
+	default:
+	}
+
+	switch x.(type) {
+	}
+
+	switch v0, ok := x.(int); v := x.(type) {
+	}
+
+	switch v0, ok := x.(int); x.(type) {
+	case byte:  // this comment should be on the same line as the keyword
+		// this comment should be normally indented
+		_ = 0
+	case bool, int, float:
+		// this comment should be indented
+	case string:
+	default:
+		// this comment should be indented
+	}
+	// this comment should not be indented
+}
+
+//
+// Indentation of comments after possibly indented multi-line constructs
+// (test cases for issue 3147).
+//
+
+func _() {
+	s := 1 +
+		2
+// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2 // comment
+		// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2 // comment
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+
+// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2 // comment
+
+		// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2 // comment
+
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+
+	// should be indented like s
+	_ = 0
+}
+
+// Test case from issue 3147.
+func f() {
+	templateText := "a" + // A
+		"b" + // B
+		"c" // C
+
+	// should be aligned with f()
+	f()
+}
+
+// Modified test case from issue 3147.
+func f() {
+	templateText := "a" + // A
+		"b" + // B
+		"c" // C
+
+		// may not be aligned with f() (source is not aligned)
+	f()
+}
+
+//
+// Test cases for alignment of lines in general comments.
+//
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	   */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+		*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line */
+}
+
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	   */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+		*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	   */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+		*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line */
+}
+
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	   */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+		*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line */
+}
+
+// Issue 9751.
+func _() {
+	/*a string
+
+	b string*/
+
+	/*A string
+
+
+
+	Z string*/
+
+	/*a string
+
+	b string
+
+	c string*/
+
+	{
+		/*a string
+b string*/
+
+		/*a string
+
+b string*/
+
+		/*a string
+
+b string
+
+c string*/
+	}
+
+	{
+		/*a string
+				b string*/
+
+		/*a string
+
+				b string*/
+
+		/*a string
+
+				b string
+
+				c string*/
+	}
+
+	/*
+	*/
+
+	/*
+
+	*/
+
+	/*
+
+	 * line
+
+	*/
+}
+
+/*
+ * line
+ * of
+ * stars
+ */
+
+/* another line
+ * of
+ * stars */
+
+/*	and another line
+ *	of
+ *	stars */
+
+/* a line of
+ * stars */
+
+/*	and another line of
+ *	stars */
+
+/* a line of stars
+*/
+
+/*	and another line of
+*/
+
+/* a line of stars
+ */
+
+/*	and another line of
+ */
+
+/*
+aligned in middle
+here
+        not here
+*/
+
+/*
+blank line in middle:
+
+with no leading spaces on blank line.
+*/
+
+/*
+   aligned in middle
+   here
+           not here
+*/
+
+/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+*/
+
+func _() {
+	/*
+	 * line
+	 * of
+	 * stars
+	 */
+
+	/*
+	aligned in middle
+	here
+		not here
+	*/
+
+	/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+*/
+}
+
+
+// Some interesting interspersed comments.
+// See below for more common cases.
+func _(/* this */x/* is *//* an */ int) {
+}
+
+func _(/* no params - extra blank before and after comment */) {}
+func _(a, b int /* params - no extra blank after comment */) {}
+
+func _() { f(/* no args - extra blank before and after comment */) }
+func _() { f(a, b /* args - no extra blank after comment */) }
+
+func _() {
+	f(/* no args - extra blank before and after comment */)
+	f(a, b /* args - no extra blank after comment */)
+}
+
+func (/* comment1 */ T /* comment2 */) _() {}
+
+func _() { /* "short-ish one-line functions with comments are formatted as multi-line functions */ }
+func _() { x := 0; /* comment */ y = x /* comment */ }
+
+func _() {
+	_ = 0
+	/* closing curly brace should be on new line */ }
+
+func _() {
+	_ = []int{0, 1 /* don't introduce a newline after this comment - was issue 1365 */}
+}
+
+// Test cases from issue 1542:
+// Comments must not be placed before commas and cause invalid programs.
+func _() {
+	var a = []int{1, 2, /*jasldf*/
+	}
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2, /*jasldf
+						*/
+	}
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2, // jasldf
+	}
+	_ = a
+}
+
+// Test cases from issues 11274, 15137:
+// Semicolon must not be lost when multiple statements are on the same line with a comment.
+func _() {
+    x := 0 /**/; y := 1
+}
+
+func _() {
+	f(); f()
+	f(); /* comment */ f()
+	f() /* comment */; f()	
+	f(); /* a */ /* b */ f()
+	f() /* a */ /* b */; f()
+	f() /* a */; /* b */ f()
+}
+
+func _() {
+	f() /* a */ /* b */ }
+
+// Comments immediately adjacent to punctuation followed by a newline
+// remain after the punctuation (looks better and permits alignment of
+// comments).
+func _() {
+	_ = T{
+		1,    // comment after comma
+		2,    /* comment after comma */
+		3  ,  // comment after comma
+	}
+	_ = T{
+		1  ,// comment after comma
+		2  ,/* comment after comma */
+		3,// comment after comma
+	}
+	_ = T{
+		/* comment before literal */1,
+		2/* comment before comma - ok to move after comma */,
+		3  /* comment before comma - ok to move after comma */  ,
+	}
+
+	for
+		i=0;// comment after semicolon
+		i<9;/* comment after semicolon */
+		i++{// comment after opening curly brace
+	}
+
+	// TODO(gri) the last comment in this example should be aligned */
+	for
+		i=0;// comment after semicolon
+		i<9/* comment before semicolon - ok to move after semicolon */;
+		i++ /* comment before opening curly brace */ {
+	}
+}
+
+// If there is no newline following punctuation, commas move before the punctuation.
+// This way, commas interspersed in lists stay with the respective expression.
+func f(x/* comment */, y int, z int /* comment */, u, v, w int /* comment */) {
+	f(x /* comment */, y)
+	f(x /* comment */, 
+	y)
+	f(
+		x /* comment */,
+	)
+}
+
+func g(
+	x int /* comment */,
+) {}
+
+type _ struct {
+	a, b /* comment */, c int
+}
+
+type _ struct { a, b /* comment */, c int }
+
+func _() {
+	for a /* comment */, b := range x {
+	}
+}
+
+// Print line directives correctly.
+
+// The following is a legal line directive.
+//line foo:1
+func _() {
+	_ = 0
+// The following is a legal line directive. It must not be indented:
+//line foo:2
+	_ = 1
+
+// The following is not a legal line directive (it doesn't start in column 1):
+	//line foo:2
+	_ = 2
+
+// The following is not a legal line directive (missing colon):
+//line foo -3
+	_ = 3
+}
+
+// Line comments with tabs
+func _() {
+var	finput		*bufio.Reader			// input file
+var	stderr		*bufio.Writer
+var	ftable		*bufio.Writer			// y.go file
+var	foutput		*bufio.Writer			// y.output file
+
+var	oflag		string				// -o [y.go]		- y.go file
+var	vflag		string				// -v [y.output]	- y.output file
+var	lflag		bool				// -l			- disable line directives
+}
+
+// Trailing white space in comments should be trimmed
+func _() {
+// This comment has 4 blanks following that should be trimmed:    
+/* Each line of this comment has blanks or tabs following that should be trimmed:	
+   line 2:    
+   line 3:    			
+*/
+}
+
+var _ = []T{/* lone comment */}
+
+var _ = []T{
+/* lone comment */
+}
+
+var _ = []T{
+// lone comments
+// in composite lit
+}
+
+var _ = [][]T{
+	{
+		// lone comments
+		// in composite lit
+	},
+}
+
+// TODO: gofmt doesn't add these tabs; make it so that these golden
+// tests run the printer in a way that it's exactly like gofmt.
+
+var _ = []T{// lone comment
+}
+
+var _ = []T{// lone comments
+// in composite lit
+}
+
+/* This comment is the last entry in this file. It must be printed and should be followed by a newline */
diff --git a/internal/backport/go/printer/testdata/comments.x b/internal/backport/go/printer/testdata/comments.x
new file mode 100644
index 0000000..ae77292
--- /dev/null
+++ b/internal/backport/go/printer/testdata/comments.x
@@ -0,0 +1,56 @@
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+// The SZ struct; it is empty.
+type SZ struct{}
+
+// The S0 struct; no field is exported.
+type S0 struct {
+	// contains filtered or unexported fields
+}
+
+// The S1 struct; some fields are not exported.
+type S1 struct {
+	S0
+	A, B, C	float	// 3 exported fields
+	D	int	// 2 unexported fields
+	// contains filtered or unexported fields
+}
+
+// The S2 struct; all fields are exported.
+type S2 struct {
+	S1
+	A, B, C	float	// 3 exported fields
+}
+
+// The IZ interface; it is empty.
+type SZ interface{}
+
+// The I0 interface; no method is exported.
+type I0 interface {
+	// contains filtered or unexported methods
+}
+
+// The I1 interface; some methods are not exported.
+type I1 interface {
+	I0
+	F(x float) float	// exported methods
+	// contains filtered or unexported methods
+}
+
+// The I2 interface; all methods are exported.
+type I2 interface {
+	I0
+	F(x float) float	// exported method
+	G(x float) float	// exported method
+}
+
+// The S3 struct; all comments except for the last one must appear in the export.
+type S3 struct {
+	// lead comment for F1
+	F1	int	// line comment for F1
+	// lead comment for F2
+	F2	int	// line comment for F2
+	// contains filtered or unexported fields
+}
diff --git a/internal/backport/go/printer/testdata/comments2.golden b/internal/backport/go/printer/testdata/comments2.golden
new file mode 100644
index 0000000..8b3a94d
--- /dev/null
+++ b/internal/backport/go/printer/testdata/comments2.golden
@@ -0,0 +1,164 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+// Test cases for idempotent comment formatting (was issue 1835).
+/*
+c1a
+*/
+/*
+   c1b
+*/
+/* foo
+c1c
+*/
+/* foo
+   c1d
+*/
+/*
+c1e
+foo */
+/*
+   c1f
+   foo */
+
+func f() {
+	/*
+	   c2a
+	*/
+	/*
+	   c2b
+	*/
+	/* foo
+	   c2c
+	*/
+	/* foo
+	   c2d
+	*/
+	/*
+	   c2e
+	   foo */
+	/*
+	   c2f
+	   foo */
+}
+
+func g() {
+	/*
+	   c3a
+	*/
+	/*
+	   c3b
+	*/
+	/* foo
+	   c3c
+	*/
+	/* foo
+	   c3d
+	*/
+	/*
+	   c3e
+	   foo */
+	/*
+	   c3f
+	   foo */
+}
+
+// Test case taken literally from issue 1835.
+func main() {
+	/*
+	   prints test 5 times
+	*/
+	for i := 0; i < 5; i++ {
+		println("test")
+	}
+}
+
+func issue5623() {
+L:
+	_ = yyyyyyyyyyyyyyyy			// comment - should be aligned
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	/* comment */
+
+	_ = yyyyyyyyyyyyyyyy			/* comment - should be aligned */
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
+
+LLLLLLL:
+	_ = yyyyyyyyyyyyyyyy			// comment - should be aligned
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
+
+LL:
+LLLLL:
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	/* comment */
+	_ = yyyyyyyyyyyyyyyy			/* comment - should be aligned */
+
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
+	_ = yyyyyyyyyyyyyyyy			// comment - should be aligned
+
+	// test case from issue
+label:
+	mask := uint64(1)<<c - 1		// Allocation mask
+	used := atomic.LoadUint64(&h.used)	// Current allocations
+}
+
+// Test cases for issue 18782
+var _ = [][]int{
+	/*       a, b, c, d, e */
+	/* a */ {0, 0, 0, 0, 0},
+	/* b */ {0, 5, 4, 4, 4},
+	/* c */ {0, 4, 5, 4, 4},
+	/* d */ {0, 4, 4, 5, 4},
+	/* e */ {0, 4, 4, 4, 5},
+}
+
+var _ = T{ /* a */ 0}
+
+var _ = T{ /* a */ /* b */ 0}
+
+var _ = T{	/* a */	/* b */
+	/* c */ 0,
+}
+
+var _ = T{	/* a */	/* b */
+	/* c */
+	/* d */ 0,
+}
+
+var _ = T{
+	/* a */
+	/* b */ 0,
+}
+
+var _ = T{ /* a */ {}}
+
+var _ = T{ /* a */ /* b */ {}}
+
+var _ = T{	/* a */	/* b */
+	/* c */ {},
+}
+
+var _ = T{	/* a */	/* b */
+	/* c */
+	/* d */ {},
+}
+
+var _ = T{
+	/* a */
+	/* b */ {},
+}
+
+var _ = []T{
+	func() {
+		var _ = [][]int{
+			/*       a, b, c, d, e */
+			/* a */ {0, 0, 0, 0, 0},
+			/* b */ {0, 5, 4, 4, 4},
+			/* c */ {0, 4, 5, 4, 4},
+			/* d */ {0, 4, 4, 5, 4},
+			/* e */ {0, 4, 4, 4, 5},
+		}
+	},
+}
diff --git a/internal/backport/go/printer/testdata/comments2.input b/internal/backport/go/printer/testdata/comments2.input
new file mode 100644
index 0000000..8d38c41
--- /dev/null
+++ b/internal/backport/go/printer/testdata/comments2.input
@@ -0,0 +1,168 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+// Test cases for idempotent comment formatting (was issue 1835).
+/*
+c1a
+*/
+/*
+   c1b
+*/
+/* foo
+c1c
+*/
+/* foo
+   c1d
+*/
+/*
+c1e
+foo */
+/*
+   c1f
+   foo */
+
+func f() {
+/*
+c2a
+*/
+/*
+   c2b
+*/
+/* foo
+c2c
+*/
+/* foo
+   c2d
+*/
+/*
+c2e
+foo */
+/*
+   c2f
+   foo */
+}
+
+func g() {
+/*
+c3a
+*/
+/*
+   c3b
+*/
+/* foo
+c3c
+*/
+/* foo
+   c3d
+*/
+/*
+c3e
+foo */
+/*
+   c3f
+   foo */
+}
+
+// Test case taken literally from issue 1835.
+func main() {
+/*
+prints test 5 times
+*/
+   for i := 0; i < 5; i++ {
+      println("test")
+   }
+}
+
+func issue5623() {
+L:
+   _ = yyyyyyyyyyyyyyyy // comment - should be aligned
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx /* comment */
+
+   _ = yyyyyyyyyyyyyyyy /* comment - should be aligned */
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
+
+LLLLLLL:
+   _ = yyyyyyyyyyyyyyyy // comment - should be aligned
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
+
+LL:
+LLLLL:
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx /* comment */
+   _ = yyyyyyyyyyyyyyyy /* comment - should be aligned */
+
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
+   _ = yyyyyyyyyyyyyyyy // comment - should be aligned
+
+// test case from issue
+label:
+   mask := uint64(1)<<c - 1 // Allocation mask
+   used := atomic.LoadUint64(&h.used) // Current allocations
+}
+
+// Test cases for issue 18782
+var _ = [][]int{
+   /*       a, b, c, d, e */
+   /* a */ {0, 0, 0, 0, 0},
+   /* b */ {0, 5, 4, 4, 4},
+   /* c */ {0, 4, 5, 4, 4},
+   /* d */ {0, 4, 4, 5, 4},
+   /* e */ {0, 4, 4, 4, 5},
+}
+
+var _ = T{ /* a */ 0,
+}
+
+var _ = T{ /* a */ /* b */ 0,
+}
+
+var _ = T{ /* a */ /* b */
+   /* c */ 0,
+}
+
+var _ = T{ /* a */ /* b */
+   /* c */
+   /* d */ 0,
+}
+
+var _ = T{
+   /* a */
+   /* b */ 0,
+}
+
+var _ = T{ /* a */ {},
+}
+
+var _ = T{ /* a */ /* b */ {},
+}
+
+var _ = T{ /* a */ /* b */
+   /* c */ {},
+}
+
+var _ = T{ /* a */ /* b */
+   /* c */
+   /* d */ {},
+}
+
+var _ = T{
+   /* a */
+   /* b */ {},
+}
+
+var _ = []T{
+   func() {
+      var _ = [][]int{
+         /*       a, b, c, d, e */
+         /* a */ {0, 0, 0, 0, 0},
+         /* b */ {0, 5, 4, 4, 4},
+         /* c */ {0, 4, 5, 4, 4},
+         /* d */ {0, 4, 4, 5, 4},
+         /* e */ {0, 4, 4, 4, 5},
+      }
+   },
+}
diff --git a/internal/backport/go/printer/testdata/complit.input b/internal/backport/go/printer/testdata/complit.input
new file mode 100644
index 0000000..82806a4
--- /dev/null
+++ b/internal/backport/go/printer/testdata/complit.input
@@ -0,0 +1,65 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package complit
+
+var (
+	// Multi-line declarations
+	V1 = T{
+		F1: "hello",
+		f2: 1,
+	}
+	V2 = T{
+		f2: 1,
+		F1: "hello",
+	}
+	V3 = T{
+		F1: "hello",
+		F2: T2{
+			A: "world",
+			b: "hidden",
+		},
+		f3: T2{
+			A: "world",
+		},
+	}
+	V4 = T{
+		f2: 1,
+	}
+
+	// Single-line declarations
+	V5 = T{F1: "hello", f2: 1}
+	V6 = T{f2: 1, F1: "hello"}
+	V7 = T{f2: 1}
+
+	// Mixed-mode declarations
+	V8 = T{
+		F1: "hello", f2: 1,
+		F3: "world",
+		f4: 2}
+	V9 = T{
+	f2: 1, F1: "hello",}
+	V10 = T{
+		F1: "hello", f2: 1,
+		f3: 2,
+		F4: "world", f5: 3,
+	}
+
+	// Other miscellaneous declarations
+	V11 = T{
+		t{
+			A: "world",
+			b: "hidden",
+		},
+		f2: t{
+			A: "world",
+			b: "hidden",
+		},
+	}
+	V12 = T{
+		F1: make(chan int),
+		f2: []int{},
+		F3: make(map[int]string), f4: 1,
+	}
+)
\ No newline at end of file
diff --git a/internal/backport/go/printer/testdata/complit.x b/internal/backport/go/printer/testdata/complit.x
new file mode 100644
index 0000000..458ac61
--- /dev/null
+++ b/internal/backport/go/printer/testdata/complit.x
@@ -0,0 +1,62 @@
+package complit
+
+var (
+	// Multi-line declarations
+	V1	= T{
+		F1: "hello",
+		// contains filtered or unexported fields
+	}
+	V2	= T{
+
+		F1: "hello",
+		// contains filtered or unexported fields
+	}
+	V3	= T{
+		F1:	"hello",
+		F2: T2{
+			A: "world",
+			// contains filtered or unexported fields
+		},
+		// contains filtered or unexported fields
+	}
+	V4	= T{
+		// contains filtered or unexported fields
+	}
+
+	// Single-line declarations
+	V5	= T{F1: "hello", /* contains filtered or unexported fields */}
+	V6	= T{F1: "hello", /* contains filtered or unexported fields */}
+	V7	= T{/* contains filtered or unexported fields */}
+
+	// Mixed-mode declarations
+	V8	= T{
+		F1:	"hello",
+		F3:	"world",
+		// contains filtered or unexported fields
+	}
+	V9	= T{
+		F1: "hello",
+		// contains filtered or unexported fields
+	}
+	V10	= T{
+		F1:	"hello",
+
+		F4:	"world",
+		// contains filtered or unexported fields
+	}
+
+	// Other miscellaneous declarations
+	V11	= T{
+		t{
+			A: "world",
+			// contains filtered or unexported fields
+		},
+		// contains filtered or unexported fields
+	}
+	V12	= T{
+		F1:	make(chan int),
+
+		F3:	make(map[int]string),
+		// contains filtered or unexported fields
+	}
+)
diff --git a/internal/backport/go/printer/testdata/declarations.golden b/internal/backport/go/printer/testdata/declarations.golden
new file mode 100644
index 0000000..fe0f783
--- /dev/null
+++ b/internal/backport/go/printer/testdata/declarations.golden
@@ -0,0 +1,1008 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package imports
+
+import "io"
+
+import (
+	_ "io"
+)
+
+import _ "io"
+
+import (
+	"io"
+	"io"
+	"io"
+)
+
+import (
+	"io"
+	aLongRename "io"
+
+	b "io"
+)
+
+import (
+	"unrenamed"
+	renamed "renameMe"
+	. "io"
+	_ "io"
+	"io"
+	. "os"
+)
+
+// no newlines between consecutive single imports, but
+// respect extra line breaks in the source (at most one empty line)
+import _ "io"
+import _ "io"
+import _ "io"
+
+import _ "os"
+import _ "os"
+import _ "os"
+
+import _ "fmt"
+import _ "fmt"
+import _ "fmt"
+
+import "foo"	// a comment
+import "bar"	// a comment
+
+import (
+	_ "foo"
+	// a comment
+	"bar"
+	"foo"	// a comment
+	"bar"	// a comment
+)
+
+// comments + renames
+import (
+	"unrenamed"	// a comment
+	renamed "renameMe"
+	. "io"		/* a comment */
+	_ "io/ioutil"	// a comment
+	"io"		// testing alignment
+	. "os"
+	// a comment
+)
+
+// a case that caused problems in the past (comment placement)
+import (
+	. "fmt"
+	"io"
+	"malloc"	// for the malloc count test only
+	"math"
+	"strings"
+	"testing"
+)
+
+// more import examples
+import (
+	"xxx"
+	"much_longer_name"	// comment
+	"short_name"		// comment
+)
+
+import (
+	_ "xxx"
+	"much_longer_name"	// comment
+)
+
+import (
+	mymath "math"
+	"/foo/bar/long_package_path"	// a comment
+)
+
+import (
+	"package_a"	// comment
+	"package_b"
+	my_better_c "package_c"	// comment
+	"package_d"		// comment
+	my_e "package_e"	// comment
+
+	"package_a"	// comment
+	"package_bb"
+	"package_ccc"	// comment
+	"package_dddd"	// comment
+)
+
+// print import paths as double-quoted strings
+// (we would like more test cases but the go/parser
+// already excludes most incorrect paths, and we don't
+// bother setting up test-ASTs manually)
+import (
+	"fmt"
+	"math"
+)
+
+// at least one empty line between declarations of different kind
+import _ "io"
+
+var _ int
+
+// at least one empty line between declarations of the same kind
+// if there is associated documentation (was issue 2570)
+type T1 struct{}
+
+// T2 comment
+type T2 struct {
+}	// should be a two-line struct
+
+// T3 comment
+type T2 struct {
+}	// should be a two-line struct
+
+// printing of constant literals
+const (
+	_	= "foobar"
+	_	= "a۰۱۸"
+	_	= "foo६४"
+	_	= "bar９８７６"
+	_	= 0
+	_	= 1
+	_	= 123456789012345678890
+	_	= 01234567
+	_	= 0xcafebabe
+	_	= 0.
+	_	= .0
+	_	= 3.14159265
+	_	= 1e0
+	_	= 1e+100
+	_	= 1e-100
+	_	= 2.71828e-1000
+	_	= 0i
+	_	= 1i
+	_	= 012345678901234567889i
+	_	= 123456789012345678890i
+	_	= 0.i
+	_	= .0i
+	_	= 3.14159265i
+	_	= 1e0i
+	_	= 1e+100i
+	_	= 1e-100i
+	_	= 2.71828e-1000i
+	_	= 'a'
+	_	= '\000'
+	_	= '\xFF'
+	_	= '\uff16'
+	_	= '\U0000ff16'
+	_	= `foobar`
+	_	= `foo
+---
+---
+bar`
+)
+
+func _() {
+	type _ int
+	type _ *int
+	type _ []int
+	type _ map[string]int
+	type _ chan int
+	type _ func() int
+
+	var _ int
+	var _ *int
+	var _ []int
+	var _ map[string]int
+	var _ chan int
+	var _ func() int
+
+	type _ struct{}
+	type _ *struct{}
+	type _ []struct{}
+	type _ map[string]struct{}
+	type _ chan struct{}
+	type _ func() struct{}
+
+	type _ interface{}
+	type _ *interface{}
+	type _ []interface{}
+	type _ map[string]interface{}
+	type _ chan interface{}
+	type _ func() interface{}
+
+	var _ struct{}
+	var _ *struct{}
+	var _ []struct{}
+	var _ map[string]struct{}
+	var _ chan struct{}
+	var _ func() struct{}
+
+	var _ interface{}
+	var _ *interface{}
+	var _ []interface{}
+	var _ map[string]interface{}
+	var _ chan interface{}
+	var _ func() interface{}
+}
+
+// don't lose blank lines in grouped declarations
+const (
+	_	int	= 0
+	_	float	= 1
+
+	_	string	= "foo"
+
+	_	= iota
+	_
+
+	// a comment
+	_
+
+	_
+)
+
+type (
+	_	int
+	_	struct{}
+
+	_	interface{}
+
+	// a comment
+	_	map[string]int
+)
+
+var (
+	_	int	= 0
+	_	float	= 1
+
+	_	string	= "foo"
+
+	_	bool
+
+	// a comment
+	_	bool
+)
+
+// don't lose blank lines in this struct
+type _ struct {
+	String	struct {
+		Str, Len int
+	}
+	Slice	struct {
+		Array, Len, Cap int
+	}
+	Eface	struct {
+		Typ, Ptr int
+	}
+
+	UncommonType	struct {
+		Name, PkgPath int
+	}
+	CommonType	struct {
+		Size, Hash, Alg, Align, FieldAlign, String, UncommonType int
+	}
+	Type	struct {
+		Typ, Ptr int
+	}
+	StructField	struct {
+		Name, PkgPath, Typ, Tag, Offset int
+	}
+	StructType	struct {
+		Fields int
+	}
+	PtrType	struct {
+		Elem int
+	}
+	SliceType	struct {
+		Elem int
+	}
+	ArrayType	struct {
+		Elem, Len int
+	}
+
+	Stktop	struct {
+		Stackguard, Stackbase, Gobuf int
+	}
+	Gobuf	struct {
+		Sp, Pc, G int
+	}
+	G	struct {
+		Stackbase, Sched, Status, Alllink int
+	}
+}
+
+// no blank lines in empty structs and interfaces, but leave 1- or 2-line layout alone
+type _ struct{}
+type _ struct {
+}
+
+type _ interface{}
+type _ interface {
+}
+
+// no tabs for single or ungrouped decls
+func _() {
+	const xxxxxx = 0
+	type x int
+	var xxx int
+	var yyyy float = 3.14
+	var zzzzz = "bar"
+
+	const (
+		xxxxxx = 0
+	)
+	type (
+		x int
+	)
+	var (
+		xxx int
+	)
+	var (
+		yyyy float = 3.14
+	)
+	var (
+		zzzzz = "bar"
+	)
+}
+
+// tabs for multiple or grouped decls
+func _() {
+	// no entry has a type
+	const (
+		zzzzzz	= 1
+		z	= 2
+		zzz	= 3
+	)
+	// some entries have a type
+	const (
+		xxxxxx			= 1
+		x			= 2
+		xxx			= 3
+		yyyyyyyy	float	= iota
+		yyyy			= "bar"
+		yyy
+		yy	= 2
+	)
+}
+
+func _() {
+	// no entry has a type
+	var (
+		zzzzzz	= 1
+		z	= 2
+		zzz	= 3
+	)
+	// no entry has a value
+	var (
+		_	int
+		_	float
+		_	string
+
+		_	int	// comment
+		_	float	// comment
+		_	string	// comment
+	)
+	// some entries have a type
+	var (
+		xxxxxx		int
+		x		float
+		xxx		string
+		yyyyyyyy	int	= 1234
+		y		float	= 3.14
+		yyyy			= "bar"
+		yyy		string	= "foo"
+	)
+	// mixed entries - all comments should be aligned
+	var (
+		a, b, c			int
+		x			= 10
+		d			int			// comment
+		y				= 20		// comment
+		f, ff, fff, ffff	int	= 0, 1, 2, 3	// comment
+	)
+	// respect original line breaks
+	var _ = []T{
+		T{0x20, "Telugu"},
+	}
+	var _ = []T{
+		// respect original line breaks
+		T{0x20, "Telugu"},
+	}
+}
+
+// use the formatted output rather than the input to decide when to align
+// (was issue 4505)
+const (
+	short		= 2 * (1 + 2)
+	aMuchLongerName	= 3
+)
+
+var (
+	short		= X{}
+	aMuchLongerName	= X{}
+
+	x1	= X{}	// foo
+	x2	= X{}	// foo
+)
+
+func _() {
+	type (
+		xxxxxx	int
+		x	float
+		xxx	string
+		xxxxx	[]x
+		xx	struct{}
+		xxxxxxx	struct {
+			_, _	int
+			_	float
+		}
+		xxxx	chan<- string
+	)
+}
+
+// alignment of "=" in consecutive lines (extended example from issue 1414)
+const (
+	umax	uint	= ^uint(0)		// maximum value for a uint
+	bpu		= 1 << (5 + umax>>63)	// bits per uint
+	foo
+	bar	= -1
+)
+
+// typical enum
+const (
+	a	MyType	= iota
+	abcd
+	b
+	c
+	def
+)
+
+// excerpt from godoc.go
+var (
+	goroot		= flag.String("goroot", runtime.GOROOT(), "Go root directory")
+	testDir		= flag.String("testdir", "", "Go root subdirectory - for testing only (faster startups)")
+	pkgPath		= flag.String("path", "", "additional package directories (colon-separated)")
+	filter		= flag.String("filter", "", "filter file containing permitted package directory paths")
+	filterMin	= flag.Int("filter_minutes", 0, "filter file update interval in minutes; disabled if <= 0")
+	filterDelay	delayTime	// actual filter update interval in minutes; usually filterDelay == filterMin, but filterDelay may back off exponentially
+)
+
+// formatting of structs
+type _ struct{}
+
+type _ struct { /* this comment should be visible */
+}
+
+type _ struct {
+	// this comment should be visible and properly indented
+}
+
+type _ struct {	// this comment must not change indentation
+	f			int
+	f, ff, fff, ffff	int
+}
+
+type _ struct {
+	string
+}
+
+type _ struct {
+	string	// comment
+}
+
+type _ struct {
+	string "tag"
+}
+
+type _ struct {
+	string "tag"	// comment
+}
+
+type _ struct {
+	f int
+}
+
+type _ struct {
+	f int	// comment
+}
+
+type _ struct {
+	f int "tag"
+}
+
+type _ struct {
+	f int "tag"	// comment
+}
+
+type _ struct {
+	bool
+	a, b, c			int
+	int			"tag"
+	ES				// comment
+	float			"tag"	// comment
+	f			int	// comment
+	f, ff, fff, ffff	int	// comment
+	g			float	"tag"
+	h			float	"tag"	// comment
+}
+
+type _ struct {
+	a, b,
+	c, d	int	// this line should be indented
+	u, v, w, x	float	// this line should be indented
+	p, q,
+	r, s	float	// this line should be indented
+}
+
+// difficult cases
+type _ struct {
+	bool		// comment
+	text	[]byte	// comment
+}
+
+// formatting of interfaces
+type EI interface{}
+
+type _ interface {
+	EI
+}
+
+type _ interface {
+	f()
+	fffff()
+}
+
+type _ interface {
+	EI
+	f()
+	fffffg()
+}
+
+type _ interface {	// this comment must not change indentation
+	EI				// here's a comment
+	f()				// no blank between identifier and ()
+	fffff()				// no blank between identifier and ()
+	gggggggggggg(x, y, z int)	// hurray
+}
+
+// formatting of variable declarations
+func _() {
+	type day struct {
+		n		int
+		short, long	string
+	}
+	var (
+		Sunday		= day{0, "SUN", "Sunday"}
+		Monday		= day{1, "MON", "Monday"}
+		Tuesday		= day{2, "TUE", "Tuesday"}
+		Wednesday	= day{3, "WED", "Wednesday"}
+		Thursday	= day{4, "THU", "Thursday"}
+		Friday		= day{5, "FRI", "Friday"}
+		Saturday	= day{6, "SAT", "Saturday"}
+	)
+}
+
+// formatting of multi-line variable declarations
+var a1, b1, c1 int	// all on one line
+
+var a2, b2,
+	c2 int	// this line should be indented
+
+var (
+	a3, b3,
+	c3, d3	int	// this line should be indented
+	a4, b4, c4	int	// this line should be indented
+)
+
+// Test case from issue 3304: multi-line declarations must end
+// a formatting section and not influence indentation of the
+// next line.
+var (
+	minRefreshTimeSec	= flag.Int64("min_refresh_time_sec", 604800,
+		"minimum time window between two refreshes for a given user.")
+	x	= flag.Int64("refresh_user_rollout_percent", 100,
+		"temporary flag to ramp up the refresh user rpc")
+	aVeryLongVariableName	= stats.GetVarInt("refresh-user-count")
+)
+
+func _() {
+	var privateKey2 = &Block{Type: "RSA PRIVATE KEY",
+		Headers:	map[string]string{},
+		Bytes: []uint8{0x30, 0x82, 0x1, 0x3a, 0x2, 0x1, 0x0, 0x2,
+			0x41, 0x0, 0xb2, 0x99, 0xf, 0x49, 0xc4, 0x7d, 0xfa, 0x8c,
+			0xd4, 0x0, 0xae, 0x6a, 0x4d, 0x1b, 0x8a, 0x3b, 0x6a, 0x13,
+			0x64, 0x2b, 0x23, 0xf2, 0x8b, 0x0, 0x3b, 0xfb, 0x97, 0x79,
+		},
+	}
+}
+
+func _() {
+	var Universe = Scope{
+		Names: map[string]*Ident{
+			// basic types
+			"bool":		nil,
+			"byte":		nil,
+			"int8":		nil,
+			"int16":	nil,
+			"int32":	nil,
+			"int64":	nil,
+			"uint8":	nil,
+			"uint16":	nil,
+			"uint32":	nil,
+			"uint64":	nil,
+			"float32":	nil,
+			"float64":	nil,
+			"string":	nil,
+
+			// convenience types
+			"int":		nil,
+			"uint":		nil,
+			"uintptr":	nil,
+			"float":	nil,
+
+			// constants
+			"false":	nil,
+			"true":		nil,
+			"iota":		nil,
+			"nil":		nil,
+
+			// functions
+			"cap":		nil,
+			"len":		nil,
+			"new":		nil,
+			"make":		nil,
+			"panic":	nil,
+			"panicln":	nil,
+			"print":	nil,
+			"println":	nil,
+		},
+	}
+}
+
+// alignment of map composite entries
+var _ = map[int]int{
+	// small key sizes: always align even if size ratios are large
+	a:			a,
+	abcdefghabcdefgh:	a,
+	ab:			a,
+	abc:			a,
+	abcdefgabcdefg:		a,
+	abcd:			a,
+	abcde:			a,
+	abcdef:			a,
+
+	// mixed key sizes: align when key sizes change within accepted ratio
+	abcdefgh:		a,
+	abcdefghabcdefg:	a,
+	abcdefghij:		a,
+	abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij:	a,	// outlier - do not align with previous line
+	abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij:		a,	// align with previous line
+
+	ab:	a,	// do not align with previous line
+	abcde:	a,	// align with previous line
+}
+
+// alignment of map composite entries: test cases from issue 3965
+// aligned
+var _ = T1{
+	a:			x,
+	b:			y,
+	cccccccccccccccccccc:	z,
+}
+
+// not aligned
+var _ = T2{
+	a:			x,
+	b:			y,
+	ccccccccccccccccccccc:	z,
+}
+
+// aligned
+var _ = T3{
+	aaaaaaaaaaaaaaaaaaaa:	x,
+	b:			y,
+	c:			z,
+}
+
+// not aligned
+var _ = T4{
+	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa:	x,
+	b:						y,
+	c:						z,
+}
+
+// no alignment of map composite entries if they are not the first entry on a line
+var _ = T{0: 0}	// not aligned
+var _ = T{0: 0,	// not aligned
+	1:	1,				// aligned
+	22:	22,				// aligned
+	333:	333, 1234: 12, 12345: 0,	// first on line aligned
+}
+
+// test cases form issue 8685
+// not aligned
+var _ = map[int]string{1: "spring", 2: "summer",
+	3:	"autumn", 4: "winter"}
+
+// not aligned
+var _ = map[string]string{"a": "spring", "b": "summer",
+	"c":	"autumn", "d": "winter"}
+
+// aligned
+var _ = map[string]string{"a": "spring",
+	"b":	"summer",
+	"c":	"autumn",
+	"d":	"winter"}
+
+func _() {
+	var _ = T{
+		a,	// must introduce trailing comma
+	}
+}
+
+// formatting of function results
+func _() func()				{}
+func _() func(int)			{ return nil }
+func _() func(int) int			{ return nil }
+func _() func(int) func(int) func()	{ return nil }
+
+// formatting of consecutive single-line functions
+func _()	{}
+func _()	{}
+func _()	{}
+
+func _()	{}	// an empty line before this function
+func _()	{}
+func _()	{}
+
+func _()		{ f(1, 2, 3) }
+func _(x int) int	{ y := x; return y + 1 }
+func _() int		{ type T struct{}; var x T; return x }
+
+// these must remain multi-line since they are multi-line in the source
+func _() {
+	f(1, 2, 3)
+}
+func _(x int) int {
+	y := x
+	return y + 1
+}
+func _() int {
+	type T struct{}
+	var x T
+	return x
+}
+
+// making function declarations safe for new semicolon rules
+func _()	{ /* single-line function because of "short-ish" comment */ }
+func _() { /* multi-line function because of "long-ish" comment - much more comment text is following here */ /* and more */
+}
+
+func _() {
+	/* multi-line func because block is on multiple lines */
+}
+
+// test case for issue #19544
+func _()	{}
+func _longer_name_() {	// this comment must not force the {} from above to alignment
+	// multiple lines
+}
+
+// ellipsis parameters
+func _(...int)
+func _(...*int)
+func _(...[]int)
+func _(...struct{})
+func _(bool, ...interface{})
+func _(bool, ...func())
+func _(bool, ...func(...int))
+func _(bool, ...map[string]int)
+func _(bool, ...chan int)
+
+func _(b bool, x ...int)
+func _(b bool, x ...*int)
+func _(b bool, x ...[]int)
+func _(b bool, x ...struct{})
+func _(x ...interface{})
+func _(x ...func())
+func _(x ...func(...int))
+func _(x ...map[string]int)
+func _(x ...chan int)
+
+// these parameter lists must remain multi-line since they are multi-line in the source
+func _(bool,
+	int) {
+}
+func _(x bool,
+	y int) {
+}
+func _(x,
+	y bool) {
+}
+func _(bool,	// comment
+	int) {
+}
+func _(x bool,	// comment
+	y int) {
+}
+func _(x,	// comment
+	y bool) {
+}
+func _(bool,	// comment
+	// comment
+	int) {
+}
+func _(x bool,	// comment
+	// comment
+	y int) {
+}
+func _(x,	// comment
+	// comment
+	y bool) {
+}
+func _(bool,
+	// comment
+	int) {
+}
+func _(x bool,
+	// comment
+	y int) {
+}
+func _(x,
+	// comment
+	y bool) {
+}
+func _(x,	// comment
+	y,	// comment
+	z bool) {
+}
+func _(x,	// comment
+	y,	// comment
+	z bool) {
+}
+func _(x int,	// comment
+	y float,	// comment
+	z bool) {
+}
+
+// properly indent multi-line signatures
+func ManageStatus(in <-chan *Status, req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int) {
+}
+
+func MultiLineSignature0(
+	a, b, c int,
+) {
+}
+
+func MultiLineSignature1(
+	a, b, c int,
+	u, v, w float,
+) {
+}
+
+func MultiLineSignature2(
+	a, b,
+	c int,
+) {
+}
+
+func MultiLineSignature3(
+	a, b,
+	c int, u, v,
+	w float,
+	x ...int) {
+}
+
+func MultiLineSignature4(
+	a, b, c int,
+	u, v,
+	w float,
+	x ...int) {
+}
+
+func MultiLineSignature5(
+	a, b, c int,
+	u, v, w float,
+	p, q,
+	r string,
+	x ...int) {
+}
+
+// make sure it also works for methods in interfaces
+type _ interface {
+	MultiLineSignature0(
+		a, b, c int,
+	)
+
+	MultiLineSignature1(
+		a, b, c int,
+		u, v, w float,
+	)
+
+	MultiLineSignature2(
+		a, b,
+		c int,
+	)
+
+	MultiLineSignature3(
+		a, b,
+		c int, u, v,
+		w float,
+		x ...int)
+
+	MultiLineSignature4(
+		a, b, c int,
+		u, v,
+		w float,
+		x ...int)
+
+	MultiLineSignature5(
+		a, b, c int,
+		u, v, w float,
+		p, q,
+		r string,
+		x ...int)
+}
+
+// omit superfluous parentheses in parameter lists
+func _(int)
+func _(int)
+func _(x int)
+func _(x int)
+func _(x, y int)
+func _(x, y int)
+
+func _() int
+func _() int
+func _() int
+
+func _() (x int)
+func _() (x int)
+func _() (x int)
+
+// special cases: some channel types require parentheses
+func _(x chan (<-chan int))
+func _(x chan (<-chan int))
+func _(x chan (<-chan int))
+
+func _(x chan<- (chan int))
+func _(x chan<- (chan int))
+func _(x chan<- (chan int))
+
+// don't introduce comma after last parameter if the closing ) is on the same line
+// even if the parameter type itself is multi-line (test cases from issue 4533)
+func _(...interface{})
+func _(...interface {
+	m()
+	n()
+})	// no extra comma between } and )
+
+func (t *T) _(...interface{})
+func (t *T) _(...interface {
+	m()
+	n()
+})	// no extra comma between } and )
+
+func _(interface{})
+func _(interface {
+	m()
+})	// no extra comma between } and )
+
+func _(struct{})
+func _(struct {
+	x	int
+	y	int
+})	// no extra comma between } and )
+
+// alias declarations
+
+type c0 struct{}
+type c1 = C
+type c2 = struct{ x int }
+type c3 = p.C
+type (
+	s	struct{}
+	a	= A
+	b	= A
+	c	= foo
+	d	= interface{}
+	ddd	= p.Foo
+)
diff --git a/internal/backport/go/printer/testdata/declarations.input b/internal/backport/go/printer/testdata/declarations.input
new file mode 100644
index 0000000..f34395b
--- /dev/null
+++ b/internal/backport/go/printer/testdata/declarations.input
@@ -0,0 +1,1021 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package imports
+
+import "io"
+
+import (
+	_ "io"
+)
+
+import _ "io"
+
+import (
+	"io"
+	"io"
+	"io"
+)
+
+import (
+	"io"
+	aLongRename "io"
+
+	b "io"
+)
+
+import (
+       "unrenamed"
+       renamed "renameMe"
+       . "io"
+       _ "io"
+       "io"
+       . "os"
+)
+
+// no newlines between consecutive single imports, but
+// respect extra line breaks in the source (at most one empty line)
+import _ "io"
+import _ "io"
+import _ "io"
+
+import _ "os"
+import _ "os"
+import _ "os"
+
+
+import _ "fmt"
+import _ "fmt"
+import _ "fmt"
+
+import "foo"  // a comment
+import "bar"  // a comment
+
+import (
+	_ "foo"
+	// a comment
+	"bar"
+	"foo"  // a comment
+	"bar"  // a comment
+)
+
+// comments + renames
+import (
+       "unrenamed" // a comment
+       renamed "renameMe"
+       . "io" /* a comment */
+       _ "io/ioutil" // a comment
+       "io" // testing alignment
+       . "os"
+       // a comment
+)
+
+// a case that caused problems in the past (comment placement)
+import (
+	. "fmt"
+	"io"
+	"malloc"	// for the malloc count test only
+	"math"
+	"strings"
+	"testing"
+)
+
+// more import examples
+import (
+	"xxx"
+	"much_longer_name" // comment
+	"short_name" // comment
+)
+
+import (
+	_ "xxx"
+	"much_longer_name" // comment
+)
+
+import (
+	mymath "math"
+	"/foo/bar/long_package_path" // a comment
+)
+
+import (
+	"package_a" // comment
+	"package_b"
+	my_better_c "package_c" // comment
+	"package_d" // comment
+	my_e "package_e" // comment
+
+	"package_a"    // comment
+	"package_bb"
+	"package_ccc"  // comment
+	"package_dddd" // comment
+)
+
+// print import paths as double-quoted strings
+// (we would like more test cases but the go/parser
+// already excludes most incorrect paths, and we don't
+// bother setting up test-ASTs manually)
+import (
+	`fmt`
+	"math"
+)
+
+// at least one empty line between declarations of different kind
+import _ "io"
+var _ int
+
+// at least one empty line between declarations of the same kind
+// if there is associated documentation (was issue 2570)
+type T1 struct{}
+// T2 comment
+type T2 struct {
+} // should be a two-line struct
+
+
+// T3 comment
+type T2 struct {
+
+
+} // should be a two-line struct
+
+
+// printing of constant literals
+const (
+	_ = "foobar"
+	_ = "a۰۱۸"
+	_ = "foo६४"
+	_ = "bar９８７６"
+	_ = 0
+	_ = 1
+	_ = 123456789012345678890
+	_ = 01234567
+	_ = 0xcafebabe
+	_ = 0.
+	_ = .0
+	_ = 3.14159265
+	_ = 1e0
+	_ = 1e+100
+	_ = 1e-100
+	_ = 2.71828e-1000
+	_ = 0i
+	_ = 1i
+	_ = 012345678901234567889i
+	_ = 123456789012345678890i
+	_ = 0.i
+	_ = .0i
+	_ = 3.14159265i
+	_ = 1e0i
+	_ = 1e+100i
+	_ = 1e-100i
+	_ = 2.71828e-1000i
+	_ = 'a'
+	_ = '\000'
+	_ = '\xFF'
+	_ = '\uff16'
+	_ = '\U0000ff16'
+	_ = `foobar`
+	_ = `foo
+---
+---
+bar`
+)
+
+
+func _() {
+	type _ int
+	type _ *int
+	type _ []int
+	type _ map[string]int
+	type _ chan int
+	type _ func() int
+
+	var _ int
+	var _ *int
+	var _ []int
+	var _ map[string]int
+	var _ chan int
+	var _ func() int
+
+	type _ struct{}
+	type _ *struct{}
+	type _ []struct{}
+	type _ map[string]struct{}
+	type _ chan struct{}
+	type _ func() struct{}
+
+	type _ interface{}
+	type _ *interface{}
+	type _ []interface{}
+	type _ map[string]interface{}
+	type _ chan interface{}
+	type _ func() interface{}
+
+	var _ struct{}
+	var _ *struct{}
+	var _ []struct{}
+	var _ map[string]struct{}
+	var _ chan struct{}
+	var _ func() struct{}
+
+	var _ interface{}
+	var _ *interface{}
+	var _ []interface{}
+	var _ map[string]interface{}
+	var _ chan interface{}
+	var _ func() interface{}
+}
+
+
+// don't lose blank lines in grouped declarations
+const (
+	_ int = 0
+	_ float = 1
+
+	_ string = "foo"
+
+	_ = iota
+	_
+	
+	// a comment
+	_
+
+	_
+)
+
+
+type (
+	_ int
+	_ struct {}
+	
+	_ interface{}
+	
+	// a comment
+	_ map[string]int
+)
+
+
+var (
+	_ int = 0
+	_ float = 1
+
+	_ string = "foo"
+
+	_ bool
+	
+	// a comment
+	_ bool
+)
+
+
+// don't lose blank lines in this struct
+type _ struct {
+	String struct {
+		Str, Len int
+	}
+	Slice struct {
+		Array, Len, Cap int
+	}
+	Eface struct {
+		Typ, Ptr int
+	}
+
+	UncommonType struct {
+		Name, PkgPath int
+	}
+	CommonType struct {
+		Size, Hash, Alg, Align, FieldAlign, String, UncommonType int
+	}
+	Type struct {
+		Typ, Ptr int
+	}
+	StructField struct {
+		Name, PkgPath, Typ, Tag, Offset int
+	}
+	StructType struct {
+		Fields int
+	}
+	PtrType struct {
+		Elem int
+	}
+	SliceType struct {
+		Elem int
+	}
+	ArrayType struct {
+		Elem, Len int
+	}
+
+	Stktop struct {
+		Stackguard, Stackbase, Gobuf int
+	}
+	Gobuf struct {
+		Sp, Pc, G int
+	}
+	G struct {
+		Stackbase, Sched, Status, Alllink int
+	}
+}
+
+
+// no blank lines in empty structs and interfaces, but leave 1- or 2-line layout alone
+type _ struct{            }
+type _ struct {
+
+}
+
+type _ interface{            }
+type _ interface {
+
+}
+
+
+// no tabs for single or ungrouped decls
+func _() {
+	const xxxxxx = 0
+	type x int
+	var xxx int
+	var yyyy float = 3.14
+	var zzzzz = "bar"
+
+	const (
+		xxxxxx = 0
+	)
+	type (
+		x int
+	)
+	var (
+		xxx int
+	)
+	var (
+		yyyy float = 3.14
+	)
+	var (
+		zzzzz = "bar"
+	)
+}
+
+// tabs for multiple or grouped decls
+func _() {
+	// no entry has a type
+	const (
+		zzzzzz = 1
+		z = 2
+		zzz = 3
+	)
+	// some entries have a type
+	const (
+		xxxxxx = 1
+		x = 2
+		xxx = 3
+		yyyyyyyy float = iota
+		yyyy = "bar"
+		yyy
+		yy = 2
+	)
+}
+
+func _() {
+	// no entry has a type
+	var (
+		zzzzzz = 1
+		z = 2
+		zzz = 3
+	)
+	// no entry has a value
+	var (
+		_ int
+		_ float
+		_ string
+
+		_ int  // comment
+		_ float  // comment
+		_ string  // comment
+	)
+	// some entries have a type
+	var (
+		xxxxxx int
+		x float
+		xxx string
+		yyyyyyyy int = 1234
+		y float = 3.14
+		yyyy = "bar"
+		yyy string = "foo"
+	)
+	// mixed entries - all comments should be aligned
+	var (
+		a, b, c int
+		x = 10
+		d int  // comment
+		y = 20  // comment
+		f, ff, fff, ffff int = 0, 1, 2, 3  // comment
+	)
+	// respect original line breaks
+	var _ = []T {
+		T{0x20,	"Telugu"},
+	}
+	var _ = []T {
+		// respect original line breaks
+		T{0x20,	"Telugu"},
+	}
+}
+
+// use the formatted output rather than the input to decide when to align
+// (was issue 4505)
+const (
+	short = 2 * (
+	1 + 2)
+	aMuchLongerName = 3
+)
+
+var (
+	short = X{
+	}
+	aMuchLongerName = X{}
+
+	x1 = X{} // foo
+	x2 = X{
+	} // foo
+)
+
+func _() {
+	type (
+		xxxxxx int
+		x float
+		xxx string
+		xxxxx []x
+		xx struct{}
+		xxxxxxx struct {
+			_, _ int
+			_ float
+		}
+		xxxx chan<- string
+	)
+}
+
+// alignment of "=" in consecutive lines (extended example from issue 1414)
+const (
+	umax uint                  = ^uint(0) // maximum value for a uint
+	bpu  = 1 << (5 + umax>>63)            // bits per uint
+	foo
+	bar  = -1
+)
+
+// typical enum
+const (
+	a MyType = iota
+	abcd
+	b
+	c
+	def
+)
+
+// excerpt from godoc.go
+var (
+	goroot = flag.String("goroot", runtime.GOROOT(), "Go root directory")
+	testDir = flag.String("testdir", "", "Go root subdirectory - for testing only (faster startups)")
+	pkgPath = flag.String("path", "", "additional package directories (colon-separated)")
+	filter = flag.String("filter", "", "filter file containing permitted package directory paths")
+	filterMin = flag.Int("filter_minutes", 0, "filter file update interval in minutes; disabled if <= 0")
+	filterDelay delayTime // actual filter update interval in minutes; usually filterDelay == filterMin, but filterDelay may back off exponentially
+)
+
+
+// formatting of structs
+type _ struct{}
+
+type _ struct{ /* this comment should be visible */ }
+
+type _ struct{
+	// this comment should be visible and properly indented
+}
+
+type _ struct {  // this comment must not change indentation
+	f int
+	f, ff, fff, ffff int
+}
+
+type _ struct {
+	string
+}
+
+type _ struct {
+	string  // comment
+}
+
+type _ struct {
+	string "tag"
+}
+
+type _ struct {
+	string "tag"  // comment
+}
+
+type _ struct {
+	f int
+}
+
+type _ struct {
+	f int  // comment
+}
+
+type _ struct {
+	f int "tag"
+}
+
+type _ struct {
+	f int "tag"  // comment
+}
+
+type _ struct {
+	bool
+	a, b, c int
+	int "tag"
+	ES // comment
+	float "tag"  // comment
+	f int  // comment
+	f, ff, fff, ffff int  // comment
+	g float "tag"
+	h float "tag"  // comment
+}
+
+type _ struct { a, b,
+c, d int  // this line should be indented
+u, v, w, x float // this line should be indented
+p, q,
+r, s float // this line should be indented
+}
+
+
+// difficult cases
+type _ struct {
+	bool  // comment
+	text []byte  // comment
+}
+
+
+// formatting of interfaces
+type EI interface{}
+
+type _ interface {
+	EI
+}
+
+type _ interface {
+	f()
+	fffff()
+}
+
+type _ interface {
+	EI
+	f()
+	fffffg()
+}
+
+type _ interface {  // this comment must not change indentation
+	EI  // here's a comment
+	f()  // no blank between identifier and ()
+	fffff()  // no blank between identifier and ()
+	gggggggggggg(x, y, z int) ()  // hurray
+}
+
+
+// formatting of variable declarations
+func _() {
+	type day struct { n int; short, long string }
+	var (
+		Sunday = day{ 0, "SUN", "Sunday" }
+		Monday = day{ 1, "MON", "Monday" }
+		Tuesday = day{ 2, "TUE", "Tuesday" }
+		Wednesday = day{ 3, "WED", "Wednesday" }
+		Thursday = day{ 4, "THU", "Thursday" }
+		Friday = day{ 5, "FRI", "Friday" }
+		Saturday = day{ 6, "SAT", "Saturday" }
+	)
+}
+
+
+// formatting of multi-line variable declarations
+var a1, b1, c1 int  // all on one line
+
+var a2, b2,
+c2 int  // this line should be indented
+
+var (a3, b3,
+c3, d3 int  // this line should be indented
+a4, b4, c4 int  // this line should be indented
+)
+
+// Test case from issue 3304: multi-line declarations must end
+// a formatting section and not influence indentation of the
+// next line.
+var (
+	minRefreshTimeSec = flag.Int64("min_refresh_time_sec", 604800,
+		"minimum time window between two refreshes for a given user.")
+	x = flag.Int64("refresh_user_rollout_percent", 100,
+		"temporary flag to ramp up the refresh user rpc")
+	aVeryLongVariableName = stats.GetVarInt("refresh-user-count")
+)
+
+func _() {
+	var privateKey2 = &Block{Type: "RSA PRIVATE KEY",
+					Headers: map[string]string{},
+					Bytes: []uint8{0x30, 0x82, 0x1, 0x3a, 0x2, 0x1, 0x0, 0x2,
+			0x41, 0x0, 0xb2, 0x99, 0xf, 0x49, 0xc4, 0x7d, 0xfa, 0x8c,
+			0xd4, 0x0, 0xae, 0x6a, 0x4d, 0x1b, 0x8a, 0x3b, 0x6a, 0x13,
+			0x64, 0x2b, 0x23, 0xf2, 0x8b, 0x0, 0x3b, 0xfb, 0x97, 0x79,
+		},
+	}
+}
+
+
+func _() {
+	var Universe = Scope {
+		Names: map[string]*Ident {
+			// basic types
+			"bool": nil,
+			"byte": nil,
+			"int8": nil,
+			"int16": nil,
+			"int32": nil,
+			"int64": nil,
+			"uint8": nil,
+			"uint16": nil,
+			"uint32": nil,
+			"uint64": nil,
+			"float32": nil,
+			"float64": nil,
+			"string": nil,
+
+			// convenience types
+			"int": nil,
+			"uint": nil,
+			"uintptr": nil,
+			"float": nil,
+
+			// constants
+			"false": nil,
+			"true": nil,
+			"iota": nil,
+			"nil": nil,
+
+			// functions
+			"cap": nil,
+			"len": nil,
+			"new": nil,
+			"make": nil,
+			"panic": nil,
+			"panicln": nil,
+			"print": nil,
+			"println": nil,
+		},
+	}
+}
+
+
+// alignment of map composite entries
+var _ = map[int]int{
+	// small key sizes: always align even if size ratios are large
+	a: a,
+	abcdefghabcdefgh: a,
+	ab: a,
+	abc: a,
+	abcdefgabcdefg: a,
+	abcd: a,
+	abcde: a,
+	abcdef: a,
+
+	// mixed key sizes: align when key sizes change within accepted ratio
+	abcdefgh: a,
+	abcdefghabcdefg: a,
+	abcdefghij: a,
+	abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij: a, // outlier - do not align with previous line
+	abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij: a, // align with previous line
+
+	ab: a, // do not align with previous line
+	abcde: a, // align with previous line
+}
+
+// alignment of map composite entries: test cases from issue 3965
+// aligned
+var _ = T1{
+	a:                    x,
+	b:                    y,
+	cccccccccccccccccccc: z,
+}
+
+// not aligned
+var _ = T2{
+	a: x,
+	b: y,
+	ccccccccccccccccccccc: z,
+}
+
+// aligned
+var _ = T3{
+	aaaaaaaaaaaaaaaaaaaa: x,
+	b:                    y,
+	c:                    z,
+}
+
+// not aligned
+var _ = T4{
+	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa: x,
+	b:                                       y,
+	c:                                       z,
+}
+
+
+// no alignment of map composite entries if they are not the first entry on a line
+var _ = T{0: 0} // not aligned
+var _ = T{0: 0, // not aligned
+	1: 1, // aligned
+	22: 22, // aligned
+	333: 333, 1234: 12, 12345: 0, // first on line aligned
+}
+
+
+// test cases form issue 8685
+// not aligned
+var _ = map[int]string{1: "spring", 2: "summer",
+					3:             "autumn", 4: "winter"}
+
+// not aligned
+var _ = map[string]string{"a": "spring", "b": "summer",
+	"c": "autumn", "d": "winter"}
+
+// aligned
+var _ = map[string]string{"a": "spring",
+"b": "summer",
+	"c": "autumn",
+"d": "winter"}
+
+
+func _() {
+	var _ = T{
+		a,	// must introduce trailing comma
+	}
+}
+
+
+// formatting of function results
+func _() func() {}
+func _() func(int) { return nil }
+func _() func(int) int { return nil }
+func _() func(int) func(int) func() { return nil }
+
+
+// formatting of consecutive single-line functions
+func _() {}
+func _() {}
+func _() {}
+
+func _() {}  // an empty line before this function
+func _() {}
+func _() {}
+
+func _() { f(1, 2, 3) }
+func _(x int) int { y := x; return y+1 }
+func _() int { type T struct{}; var x T; return x }
+
+// these must remain multi-line since they are multi-line in the source
+func _() {
+	f(1, 2, 3)
+}
+func _(x int) int {
+	y := x; return y+1
+}
+func _() int {
+	type T struct{}; var x T; return x
+}
+
+
+// making function declarations safe for new semicolon rules
+func _() { /* single-line function because of "short-ish" comment */ }
+func _() { /* multi-line function because of "long-ish" comment - much more comment text is following here */ /* and more */ }
+
+func _() {
+/* multi-line func because block is on multiple lines */ }
+
+// test case for issue #19544
+func _() {}
+func _longer_name_() { // this comment must not force the {} from above to alignment
+	// multiple lines
+}
+
+// ellipsis parameters
+func _(...int)
+func _(...*int)
+func _(...[]int)
+func _(...struct{})
+func _(bool, ...interface{})
+func _(bool, ...func())
+func _(bool, ...func(...int))
+func _(bool, ...map[string]int)
+func _(bool, ...chan int)
+
+func _(b bool, x ...int)
+func _(b bool, x ...*int)
+func _(b bool, x ...[]int)
+func _(b bool, x ...struct{})
+func _(x ...interface{})
+func _(x ...func())
+func _(x ...func(...int))
+func _(x ...map[string]int)
+func _(x ...chan int)
+
+
+// these parameter lists must remain multi-line since they are multi-line in the source
+func _(bool,
+int) {
+}
+func _(x bool,
+y int) {
+}
+func _(x,
+y bool) {
+}
+func _(bool, // comment
+int) {
+}
+func _(x bool, // comment
+y int) {
+}
+func _(x, // comment
+y bool) {
+}
+func _(bool, // comment
+// comment
+int) {
+}
+func _(x bool, // comment
+// comment
+y int) {
+}
+func _(x, // comment
+// comment
+y bool) {
+}
+func _(bool,
+// comment
+int) {
+}
+func _(x bool,
+// comment
+y int) {
+}
+func _(x,
+// comment
+y bool) {
+}
+func _(x, // comment
+y,// comment
+z bool) {
+}
+func _(x, // comment
+	y,// comment
+	z bool) {
+}
+func _(x int,	// comment
+	y float,	// comment
+	z bool) {
+}
+
+
+// properly indent multi-line signatures
+func ManageStatus(in <-chan *Status, req <-chan Request,
+stat chan<- *TargetInfo,
+TargetHistorySize int) {
+}
+
+func MultiLineSignature0(
+a, b, c int,
+) {}
+
+func MultiLineSignature1(
+a, b, c int,
+u, v, w float,
+) {}
+
+func MultiLineSignature2(
+a, b,
+c int,
+) {}
+
+func MultiLineSignature3(
+a, b,
+c int, u, v,
+w float,
+		x ...int) {}
+
+func MultiLineSignature4(
+a, b, c int,
+u, v,
+w float,
+		x ...int) {}
+
+func MultiLineSignature5(
+a, b, c int,
+u, v, w float,
+p, q,
+r string,
+		x ...int) {}
+
+// make sure it also works for methods in interfaces
+type _ interface {
+MultiLineSignature0(
+a, b, c int,
+)
+
+MultiLineSignature1(
+a, b, c int,
+u, v, w float,
+)
+
+MultiLineSignature2(
+a, b,
+c int,
+)
+
+MultiLineSignature3(
+a, b,
+c int, u, v,
+w float,
+		x ...int)
+
+MultiLineSignature4(
+a, b, c int,
+u, v,
+w float,
+		x ...int)
+
+MultiLineSignature5(
+a, b, c int,
+u, v, w float,
+p, q,
+r string,
+		x ...int)
+}
+
+// omit superfluous parentheses in parameter lists
+func _((int))
+func _((((((int))))))
+func _(x (int))
+func _(x (((((int))))))
+func _(x, y (int))
+func _(x, y (((((int))))))
+
+func _() (int)
+func _() ((int))
+func _() ((((((int))))))
+
+func _() (x int)
+func _() (x (int))
+func _() (x (((((int))))))
+
+// special cases: some channel types require parentheses
+func _(x chan(<-chan int))
+func _(x (chan(<-chan int)))
+func _(x ((((chan(<-chan int))))))
+
+func _(x chan<-(chan int))
+func _(x (chan<-(chan int)))
+func _(x ((((chan<-(chan int))))))
+
+// don't introduce comma after last parameter if the closing ) is on the same line
+// even if the parameter type itself is multi-line (test cases from issue 4533)
+func _(...interface{})
+func _(...interface {
+	m()
+	n()
+}) // no extra comma between } and )
+
+func (t *T) _(...interface{})
+func (t *T) _(...interface {
+	m()
+	n()
+}) // no extra comma between } and )
+
+func _(interface{})
+func _(interface {
+	m()
+}) // no extra comma between } and )
+
+func _(struct{})
+func _(struct {
+	x int
+	y int
+}) // no extra comma between } and )
+
+// alias declarations
+
+type c0 struct{}
+type c1 = C
+type c2 = struct{ x int}
+type c3 = p.C
+type (
+	s struct{}
+	a = A
+	b = A
+	c = foo
+	d = interface{}
+	ddd = p.Foo
+)
diff --git a/internal/backport/go/printer/testdata/empty.golden b/internal/backport/go/printer/testdata/empty.golden
new file mode 100644
index 0000000..a055f47
--- /dev/null
+++ b/internal/backport/go/printer/testdata/empty.golden
@@ -0,0 +1,5 @@
+// a comment at the beginning of the file
+
+package empty
+
+// a comment at the end of the file
diff --git a/internal/backport/go/printer/testdata/empty.input b/internal/backport/go/printer/testdata/empty.input
new file mode 100644
index 0000000..a055f47
--- /dev/null
+++ b/internal/backport/go/printer/testdata/empty.input
@@ -0,0 +1,5 @@
+// a comment at the beginning of the file
+
+package empty
+
+// a comment at the end of the file
diff --git a/internal/backport/go/printer/testdata/expressions.golden b/internal/backport/go/printer/testdata/expressions.golden
new file mode 100644
index 0000000..16a68c7
--- /dev/null
+++ b/internal/backport/go/printer/testdata/expressions.golden
@@ -0,0 +1,743 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package expressions
+
+type T struct {
+	x, y, z int
+}
+
+var (
+	a, b, c, d, e						int
+	under_bar						int
+	longIdentifier1, longIdentifier2, longIdentifier3	int
+	t0, t1, t2						T
+	s							string
+	p							*int
+)
+
+func _() {
+	// no spaces around simple or parenthesized expressions
+	_ = (a + 0)
+	_ = a + b
+	_ = a + b + c
+	_ = a + b - c
+	_ = a - b - c
+	_ = a + (b * c)
+	_ = a + (b / c)
+	_ = a - (b % c)
+	_ = 1 + a
+	_ = a + 1
+	_ = a + b + 1
+	_ = s[a]
+	_ = s[a:]
+	_ = s[:b]
+	_ = s[1:2]
+	_ = s[a:b]
+	_ = s[0:len(s)]
+	_ = s[0] << 1
+	_ = (s[0] << 1) & 0xf
+	_ = s[0]<<2 | s[1]>>4
+	_ = "foo" + s
+	_ = s + "foo"
+	_ = 'a' + 'b'
+	_ = len(s) / 2
+	_ = len(t0.x) / a
+
+	// spaces around expressions of different precedence or expressions containing spaces
+	_ = a + -b
+	_ = a - ^b
+	_ = a / *p
+	_ = a + b*c
+	_ = 1 + b*c
+	_ = a + 2*c
+	_ = a + c*2
+	_ = 1 + 2*3
+	_ = s[1 : 2*3]
+	_ = s[a : b-c]
+	_ = s[0:]
+	_ = s[a+b]
+	_ = s[:b-c]
+	_ = s[a+b:]
+	_ = a[a<<b+1]
+	_ = a[a<<b+1:]
+	_ = s[a+b : len(s)]
+	_ = s[len(s):-a]
+	_ = s[a : len(s)+1]
+	_ = s[a:len(s)+1] + s
+
+	// spaces around operators with equal or lower precedence than comparisons
+	_ = a == b
+	_ = a != b
+	_ = a > b
+	_ = a >= b
+	_ = a < b
+	_ = a <= b
+	_ = a < b && c > d
+	_ = a < b || c > d
+
+	// spaces around "long" operands
+	_ = a + longIdentifier1
+	_ = longIdentifier1 + a
+	_ = longIdentifier1 + longIdentifier2*longIdentifier3
+	_ = s + "a longer string"
+
+	// some selected cases
+	_ = a + t0.x
+	_ = a + t0.x + t1.x*t2.x
+	_ = a + b + c + d + e + 2*3
+	_ = a + b + c + 2*3 + d + e
+	_ = (a + b + c) * 2
+	_ = a - b + c - d + (a + b + c) + d&e
+	_ = under_bar - 1
+	_ = Open(dpath+"/file", O_WRONLY|O_CREAT, 0666)
+	_ = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
+
+	// test case for issue 8021
+	// want:
+	//  ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+	_ = ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+
+	// the parser does not restrict expressions that may appear as statements
+	true
+	42
+	"foo"
+	x
+	(x)
+	a + b
+	a + b + c
+	a + (b * c)
+	a + (b / c)
+	1 + a
+	a + 1
+	s[a]
+	x << 1
+	(s[0] << 1) & 0xf
+	"foo" + s
+	x == y
+	x < y || z > 42
+}
+
+// slice expressions with cap
+func _() {
+	_ = x[a:b:c]
+	_ = x[a : b : c+d]
+	_ = x[a : b+d : c]
+	_ = x[a : b+d : c+d]
+	_ = x[a+d : b : c]
+	_ = x[a+d : b : c+d]
+	_ = x[a+d : b+d : c]
+	_ = x[a+d : b+d : c+d]
+
+	_ = x[:b:c]
+	_ = x[: b : c+d]
+	_ = x[: b+d : c]
+	_ = x[: b+d : c+d]
+}
+
+func issue22111() {
+	_ = x[:]
+
+	_ = x[:b]
+	_ = x[:b+1]
+
+	_ = x[a:]
+	_ = x[a+1:]
+
+	_ = x[a:b]
+	_ = x[a+1 : b]
+	_ = x[a : b+1]
+	_ = x[a+1 : b+1]
+
+	_ = x[:b:c]
+	_ = x[: b+1 : c]
+	_ = x[: b : c+1]
+	_ = x[: b+1 : c+1]
+
+	_ = x[a:b:c]
+	_ = x[a+1 : b : c]
+	_ = x[a : b+1 : c]
+	_ = x[a+1 : b+1 : c]
+	_ = x[a : b : c+1]
+	_ = x[a+1 : b : c+1]
+	_ = x[a : b+1 : c+1]
+	_ = x[a+1 : b+1 : c+1]
+}
+
+func _() {
+	_ = a + b
+	_ = a + b + c
+	_ = a + b*c
+	_ = a + (b * c)
+	_ = (a + b) * c
+	_ = a + (b * c * d)
+	_ = a + (b*c + d)
+
+	_ = 1 << x
+	_ = -1 << x
+	_ = 1<<x - 1
+	_ = -1<<x - 1
+
+	_ = f(a + b)
+	_ = f(a + b + c)
+	_ = f(a + b*c)
+	_ = f(a + (b * c))
+	_ = f(1<<x-1, 1<<x-2)
+
+	_ = 1<<d.logWindowSize - 1
+
+	buf = make(x, 2*cap(b.buf)+n)
+
+	dst[i*3+2] = dbuf[0] << 2
+	dst[i*3+2] = dbuf[0]<<2 | dbuf[1]>>4
+
+	b.buf = b.buf[0 : b.off+m+n]
+	b.buf = b.buf[0 : b.off+m*n]
+	f(b.buf[0 : b.off+m+n])
+
+	signed += ' ' * 8
+	tw.octal(header[148:155], chksum)
+
+	_ = x > 0 && i >= 0
+
+	x1, x0 := x>>w2, x&m2
+	z0 = t1<<w2 + t0
+	z1 = (t1 + t0>>w2) >> w2
+	q1, r1 := x1/d1, x1%d1
+	r1 = r1*b2 | x0>>w2
+	x1 = (x1 << z) | (x0 >> (uint(w) - z))
+	x1 = x1<<z | x0>>(uint(w)-z)
+
+	_ = buf[0 : len(buf)+1]
+	_ = buf[0 : n+1]
+
+	a, b = b, a
+	a = b + c
+	a = b*c + d
+	_ = a*b + c
+	_ = a - b - c
+	_ = a - (b - c)
+	_ = a - b*c
+	_ = a - (b * c)
+	_ = a * b / c
+	_ = a / *b
+	_ = x[a|^b]
+	_ = x[a / *b]
+	_ = a & ^b
+	_ = a + +b
+	_ = a - -b
+	_ = x[a*-b]
+	_ = x[a + +b]
+	_ = x ^ y ^ z
+	_ = b[a>>24] ^ b[(a>>16)&0xFF] ^ b[(a>>8)&0xFF] ^ b[a&0xFF]
+	_ = len(longVariableName) * 2
+
+	_ = token(matchType + xlength<<lengthShift + xoffset)
+}
+
+func f(x int, args ...int) {
+	f(0, args...)
+	f(1, args)
+	f(2, args[0])
+
+	// make sure syntactically legal code remains syntactically legal
+	f(3, 42 ...)	// a blank must remain between 42 and ...
+	f(4, 42....)
+	f(5, 42....)
+	f(6, 42.0...)
+	f(7, 42.0...)
+	f(8, .42...)
+	f(9, .42...)
+	f(10, 42e0...)
+	f(11, 42e0...)
+
+	_ = 42 .x	// a blank must remain between 42 and .x
+	_ = 42..x
+	_ = 42..x
+	_ = 42.0.x
+	_ = 42.0.x
+	_ = .42.x
+	_ = .42.x
+	_ = 42e0.x
+	_ = 42e0.x
+
+	// a blank must remain between the binary operator and the 2nd operand
+	_ = x / *y
+	_ = x < -1
+	_ = x < <-1
+	_ = x + +1
+	_ = x - -1
+	_ = x & &x
+	_ = x & ^x
+
+	_ = f(x / *y, x < -1, x < <-1, x + +1, x - -1, x & &x, x & ^x)
+}
+
+func _() {
+	_ = T{}
+	_ = struct{}{}
+	_ = [10]T{}
+	_ = [...]T{}
+	_ = []T{}
+	_ = map[int]T{}
+}
+
+// one-line structs/interfaces in composite literals (up to a threshold)
+func _() {
+	_ = struct{}{}
+	_ = struct{ x int }{0}
+	_ = struct{ x, y, z int }{0, 1, 2}
+	_ = struct{ int }{0}
+	_ = struct{ s struct{ int } }{struct{ int }{0}}
+
+	_ = (interface{})(nil)
+	_ = (interface{ String() string })(nil)
+	_ = (interface {
+		String() string
+	})(nil)
+	_ = (interface{ fmt.Stringer })(nil)
+	_ = (interface {
+		fmt.Stringer
+	})(nil)
+}
+
+func _() {
+	// do not modify literals
+	_ = "tab1	tab2	tab3	end"	// string contains 3 tabs
+	_ = "tab1 tab2 tab3 end"	// same string with 3 blanks - may be unaligned because editors see tabs in strings
+	_ = ""				// this comment should be aligned with the one on the previous line
+	_ = ``
+	_ = `
+`
+	_ = `foo
+		bar`
+	_ = `three spaces before the end of the line starting here:   
+they must not be removed`
+}
+
+func _() {
+	// smart handling of indentation for multi-line raw strings
+	var _ = ``
+	var _ = `foo`
+	var _ = `foo
+bar`
+
+	var _ = ``
+	var _ = `foo`
+	var _ =
+	// the next line should remain indented
+	`foo
+bar`
+
+	var _ =	// comment
+	``
+	var _ =	// comment
+	`foo`
+	var _ =	// comment
+	// the next line should remain indented
+	`foo
+bar`
+
+	var _ = /* comment */ ``
+	var _ = /* comment */ `foo`
+	var _ = /* comment */ `foo
+bar`
+
+	var _ =	/* comment */
+	``
+	var _ =	/* comment */
+	`foo`
+	var _ =	/* comment */
+	// the next line should remain indented
+	`foo
+bar`
+
+	var board = []int(
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`)
+
+	var state = S{
+		"foo",
+		// the next line should remain indented
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`,
+		"bar",
+	}
+}
+
+func _() {
+	// one-line function literals (body is on a single line)
+	_ = func() {}
+	_ = func() int { return 0 }
+	_ = func(x, y int) bool { m := (x + y) / 2; return m < 0 }
+
+	// multi-line function literals (body is not on one line)
+	_ = func() {
+	}
+	_ = func() int {
+		return 0
+	}
+	_ = func(x, y int) bool {
+		m := (x + y) / 2
+		return x < y
+	}
+
+	f(func() {
+	})
+	f(func() int {
+		return 0
+	})
+	f(func(x, y int) bool {
+		m := (x + y) / 2
+		return x < y
+	})
+}
+
+func _() {
+	_ = [][]int{
+		[]int{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int{
+		{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int{
+		{1},
+		{1, 2},
+		{1, 2, 3},
+	}
+	_ = [][]int{{1}, {1, 2}, {1, 2, 3}}
+}
+
+// various multi-line expressions
+func _() {
+	// do not add extra indentation to multi-line string lists
+	_ = "foo" + "bar"
+	_ = "foo" +
+		"bar" +
+		"bah"
+	_ = []string{
+		"abc" +
+			"def",
+		"foo" +
+			"bar",
+	}
+}
+
+const _ = F1 +
+	`string = "%s";` +
+	`ptr = *;` +
+	`datafmt.T2 = s ["-" p "-"];`
+
+const _ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+const _ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+func _() {
+	_ = F1 +
+		`string = "%s";` +
+		`ptr = *;` +
+		`datafmt.T2 = s ["-" p "-"];`
+
+	_ =
+		`datafmt "datafmt";` +
+			`default = "%v";` +
+			`array = *;` +
+			`datafmt.T3 = s  {" " a a / ","};`
+
+	_ = `datafmt "datafmt";` +
+		`default = "%v";` +
+		`array = *;` +
+		`datafmt.T3 = s  {" " a a / ","};`
+}
+
+func _() {
+	// respect source lines in multi-line expressions
+	_ = a +
+		b +
+		c
+	_ = a < b ||
+		b < a
+	_ = "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000"	// 100!
+	_ = "170141183460469231731687303715884105727"	// prime
+}
+
+// Alignment after overlong lines
+const (
+	_	= "991"
+	_	= "2432902008176640000"	// 20!
+	_	= "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000"	// 100!
+	_	= "170141183460469231731687303715884105727"	// prime
+)
+
+// Correct placement of operators and comments in multi-line expressions
+func _() {
+	_ = a +	// comment
+		b +	// comment
+		c
+	_ = "a" +
+		"b" +	// comment
+		"c"
+	_ = "ba0408" + "7265717569726564"	// field 71, encoding 2, string "required"
+}
+
+// Correct placement of terminating comma/closing parentheses in multi-line calls.
+func _() {
+	f(1,
+		2,
+		3)
+	f(1,
+		2,
+		3,
+	)
+	f(1,
+		2,
+		3)	// comment
+	f(1,
+		2,
+		3,	// comment
+	)
+	f(1,
+		2,
+		3)	// comment
+	f(1,
+		2,
+		3,	// comment
+	)
+}
+
+// Align comments in multi-line lists of single-line expressions.
+var txpix = [NCOL]draw.Color{
+	draw.Yellow,		// yellow
+	draw.Cyan,		// cyan
+	draw.Green,		// lime green
+	draw.GreyBlue,		// slate
+	draw.Red,		/* red */
+	draw.GreyGreen,		/* olive green */
+	draw.Blue,		/* blue */
+	draw.Color(0xFF55AAFF),	/* pink */
+	draw.Color(0xFFAAFFFF),	/* lavender */
+	draw.Color(0xBB005DFF),	/* maroon */
+}
+
+func same(t, u *Time) bool {
+	// respect source lines in multi-line expressions
+	return t.Year == u.Year &&
+		t.Month == u.Month &&
+		t.Day == u.Day &&
+		t.Hour == u.Hour &&
+		t.Minute == u.Minute &&
+		t.Second == u.Second &&
+		t.Weekday == u.Weekday &&
+		t.ZoneOffset == u.ZoneOffset &&
+		t.Zone == u.Zone
+}
+
+func (p *parser) charClass() {
+	// respect source lines in multi-line expressions
+	if cc.negate && len(cc.ranges) == 2 &&
+		cc.ranges[0] == '\n' && cc.ranges[1] == '\n' {
+		nl := new(_NotNl)
+		p.re.add(nl)
+	}
+}
+
+func addState(s []state, inst instr, match []int) {
+	// handle comments correctly in multi-line expressions
+	for i := 0; i < l; i++ {
+		if s[i].inst.index() == index &&	// same instruction
+			s[i].match[0] < pos {	// earlier match already going; leftmost wins
+			return s
+		}
+	}
+}
+
+func (self *T) foo(x int) *T	{ return self }
+
+func _()	{ module.Func1().Func2() }
+
+func _() {
+	_ = new(T).
+		foo(1).
+		foo(2).
+		foo(3)
+
+	_ = new(T).
+		foo(1).
+		foo(2).	// inline comments
+		foo(3)
+
+	_ = new(T).foo(1).foo(2).foo(3)
+
+	// handle multiline argument list correctly
+	_ = new(T).
+		foo(
+			1).
+		foo(2)
+
+	_ = new(T).foo(
+		1).foo(2)
+
+	_ = Array[3+
+		4]
+
+	_ = Method(1, 2,
+		3)
+
+	_ = new(T).
+		foo().
+		bar().(*Type)
+
+	_ = new(T).
+		foo().
+		bar().(*Type).
+		baz()
+
+	_ = new(T).
+		foo().
+		bar()["idx"]
+
+	_ = new(T).
+		foo().
+		bar()["idx"].
+		baz()
+
+	_ = new(T).
+		foo().
+		bar()[1:2]
+
+	_ = new(T).
+		foo().
+		bar()[1:2].
+		baz()
+
+	_ = new(T).
+		Field.
+		Array[3+
+		4].
+		Table["foo"].
+		Blob.(*Type).
+		Slices[1:4].
+		Method(1, 2,
+			3).
+		Thingy
+
+	_ = a.b.c
+	_ = a.
+		b.
+		c
+	_ = a.b().c
+	_ = a.
+		b().
+		c
+	_ = a.b[0].c
+	_ = a.
+		b[0].
+		c
+	_ = a.b[0:].c
+	_ = a.
+		b[0:].
+		c
+	_ = a.b.(T).c
+	_ = a.
+		b.(T).
+		c
+}
+
+// Don't introduce extra newlines in strangely formatted expression lists.
+func f() {
+	// os.Open parameters should remain on two lines
+	if writer, err = os.Open(outfile, s.O_WRONLY|os.O_CREATE|
+		os.O_TRUNC, 0666); err != nil {
+		log.Fatal(err)
+	}
+}
+
+// Handle multi-line argument lists ending in ... correctly.
+// Was issue 3130.
+func _() {
+	_ = append(s, a...)
+	_ = append(
+		s, a...)
+	_ = append(s,
+		a...)
+	_ = append(
+		s,
+		a...)
+	_ = append(s, a...,
+	)
+	_ = append(s,
+		a...,
+	)
+	_ = append(
+		s,
+		a...,
+	)
+}
+
+// Literal function types in conversions must be parenthesized;
+// for now go/parser accepts the unparenthesized form where it
+// is non-ambiguous.
+func _() {
+	// these conversions should be rewritten to look
+	// the same as the parenthesized conversions below
+	_ = (func())(nil)
+	_ = (func(x int) float)(nil)
+	_ = (func() func() func())(nil)
+
+	_ = (func())(nil)
+	_ = (func(x int) float)(nil)
+	_ = (func() func() func())(nil)
+}
+
+func _() {
+	_ = f().
+		f(func() {
+			f()
+		}).
+		f(map[int]int{
+			1:	2,
+			3:	4,
+		})
+
+	_ = f().
+		f(
+			func() {
+				f()
+			},
+		)
+}
diff --git a/internal/backport/go/printer/testdata/expressions.input b/internal/backport/go/printer/testdata/expressions.input
new file mode 100644
index 0000000..8c523b6
--- /dev/null
+++ b/internal/backport/go/printer/testdata/expressions.input
@@ -0,0 +1,771 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package expressions
+
+type T struct {
+	x, y, z int
+}
+
+var (
+	a, b, c, d, e int
+	under_bar int
+	longIdentifier1, longIdentifier2, longIdentifier3 int
+	t0, t1, t2 T
+	s string
+	p *int
+)
+
+
+func _() {
+	// no spaces around simple or parenthesized expressions
+	_ = (a+0)
+	_ = a+b
+	_ = a+b+c
+	_ = a+b-c
+	_ = a-b-c
+	_ = a+(b*c)
+	_ = a+(b/c)
+	_ = a-(b%c)
+	_ = 1+a
+	_ = a+1
+	_ = a+b+1
+	_ = s[a]
+	_ = s[a:]
+	_ = s[:b]
+	_ = s[1:2]
+	_ = s[a:b]
+	_ = s[0:len(s)]
+	_ = s[0]<<1
+	_ = (s[0]<<1)&0xf
+	_ = s[0] << 2 | s[1] >> 4
+	_ = "foo"+s
+	_ = s+"foo"
+	_ = 'a'+'b'
+	_ = len(s)/2
+	_ = len(t0.x)/a
+
+	// spaces around expressions of different precedence or expressions containing spaces
+	_ = a + -b
+	_ = a - ^b
+	_ = a / *p
+	_ = a + b*c
+	_ = 1 + b*c
+	_ = a + 2*c
+	_ = a + c*2
+	_ = 1 + 2*3
+	_ = s[1 : 2*3]
+	_ = s[a : b-c]
+	_ = s[0:]
+	_ = s[a+b]
+	_ = s[: b-c]
+	_ = s[a+b :]
+	_ = a[a<<b+1]
+	_ = a[a<<b+1 :]
+	_ = s[a+b : len(s)]
+	_ = s[len(s) : -a]
+	_ = s[a : len(s)+1]
+	_ = s[a : len(s)+1]+s
+
+	// spaces around operators with equal or lower precedence than comparisons
+	_ = a == b
+	_ = a != b
+	_ = a > b
+	_ = a >= b
+	_ = a < b
+	_ = a <= b
+	_ = a < b && c > d
+	_ = a < b || c > d
+
+	// spaces around "long" operands
+	_ = a + longIdentifier1
+	_ = longIdentifier1 + a
+	_ = longIdentifier1 + longIdentifier2 * longIdentifier3
+	_ = s + "a longer string"
+
+	// some selected cases
+	_ = a + t0.x
+	_ = a + t0.x + t1.x * t2.x
+	_ = a + b + c + d + e + 2*3
+	_ = a + b + c + 2*3 + d + e
+	_ = (a+b+c)*2
+	_ = a - b + c - d + (a+b+c) + d&e
+	_ = under_bar-1
+	_ = Open(dpath + "/file", O_WRONLY | O_CREAT, 0666)
+	_ = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
+
+	// test case for issue 8021
+	// want:
+	//  ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+	_ = ([]bool{})[([]int{})[((1) + (((((1) + (((((((1) * (((((1) + (1))) + (1))))) + (1))) * (1))))) + (1))))]]
+
+	// the parser does not restrict expressions that may appear as statements
+	true
+	42
+	"foo"
+	x
+	(x)
+	a+b
+	a+b+c
+	a+(b*c)
+	a+(b/c)
+	1+a
+	a+1
+	s[a]
+	x<<1
+	(s[0]<<1)&0xf
+	"foo"+s
+	x == y
+	x < y || z > 42
+}
+
+
+// slice expressions with cap
+func _() {
+	_ = x[a:b:c]
+	_ = x[a:b:c+d]
+	_ = x[a:b+d:c]
+	_ = x[a:b+d:c+d]
+	_ = x[a+d:b:c]
+	_ = x[a+d:b:c+d]
+	_ = x[a+d:b+d:c]
+	_ = x[a+d:b+d:c+d]
+
+	_ = x[:b:c]
+	_ = x[:b:c+d]
+	_ = x[:b+d:c]
+	_ = x[:b+d:c+d]
+}
+
+func issue22111() {
+	_ = x[:]
+
+	_ = x[:b]
+	_ = x[:b+1]
+
+	_ = x[a:]
+	_ = x[a+1:]
+
+	_ = x[a:b]
+	_ = x[a+1:b]
+	_ = x[a:b+1]
+	_ = x[a+1:b+1]
+
+	_ = x[:b:c]
+	_ = x[:b+1:c]
+	_ = x[:b:c+1]
+	_ = x[:b+1:c+1]
+
+	_ = x[a:b:c]
+	_ = x[a+1:b:c]
+	_ = x[a:b+1:c]
+	_ = x[a+1:b+1:c]
+	_ = x[a:b:c+1]
+	_ = x[a+1:b:c+1]
+	_ = x[a:b+1:c+1]
+	_ = x[a+1:b+1:c+1]
+}
+
+func _() {
+	_ = a+b
+	_ = a+b+c
+	_ = a+b*c
+	_ = a+(b*c)
+	_ = (a+b)*c
+	_ = a+(b*c*d)
+	_ = a+(b*c+d)
+
+	_ = 1<<x
+	_ = -1<<x
+	_ = 1<<x-1
+	_ = -1<<x-1
+
+	_ = f(a+b)
+	_ = f(a+b+c)
+	_ = f(a+b*c)
+	_ = f(a+(b*c))
+	_ = f(1<<x-1, 1<<x-2)
+
+	_ = 1<<d.logWindowSize-1
+
+	buf = make(x, 2*cap(b.buf) + n)
+
+	dst[i*3+2] = dbuf[0]<<2
+	dst[i*3+2] = dbuf[0]<<2 | dbuf[1]>>4
+
+	b.buf = b.buf[0:b.off+m+n]
+	b.buf = b.buf[0:b.off+m*n]
+	f(b.buf[0:b.off+m+n])
+
+	signed += ' '*8
+	tw.octal(header[148:155], chksum)
+
+	_ = x > 0 && i >= 0
+
+	x1, x0 := x>>w2, x&m2
+	z0 = t1<<w2+t0
+	z1 = (t1+t0>>w2)>>w2
+	q1, r1 := x1/d1, x1%d1
+	r1 = r1*b2 | x0>>w2
+	x1 = (x1<<z)|(x0>>(uint(w)-z))
+	x1 = x1<<z | x0>>(uint(w)-z)
+
+	_ = buf[0:len(buf)+1]
+	_ = buf[0:n+1]
+
+	a,b = b,a
+	a = b+c
+	a = b*c+d
+	_ = a*b+c
+	_ = a-b-c
+	_ = a-(b-c)
+	_ = a-b*c
+	_ = a-(b*c)
+	_ = a*b/c
+	_ = a/ *b
+	_ = x[a|^b]
+	_ = x[a/ *b]
+	_ = a& ^b
+	_ = a+ +b
+	_ = a- -b
+	_ = x[a*-b]
+	_ = x[a+ +b]
+	_ = x^y^z
+	_ = b[a>>24] ^ b[(a>>16)&0xFF] ^ b[(a>>8)&0xFF] ^ b[a&0xFF]
+	_ = len(longVariableName)*2
+
+	_ = token(matchType + xlength<<lengthShift + xoffset)
+}
+
+
+func f(x int, args ...int) {
+	f(0, args...)
+	f(1, args)
+	f(2, args[0])
+
+	// make sure syntactically legal code remains syntactically legal
+	f(3, 42 ...) // a blank must remain between 42 and ...
+	f(4, 42. ...)
+	f(5, 42....)
+	f(6, 42.0 ...)
+	f(7, 42.0...)
+	f(8, .42 ...)
+	f(9, .42...)
+	f(10, 42e0 ...)
+	f(11, 42e0...)
+
+	_ = 42 .x // a blank must remain between 42 and .x
+	_ = 42. .x
+	_ = 42..x
+	_ = 42.0 .x
+	_ = 42.0.x
+	_ = .42 .x
+	_ = .42.x
+	_ = 42e0 .x
+	_ = 42e0.x
+
+	// a blank must remain between the binary operator and the 2nd operand
+	_ = x/ *y
+	_ = x< -1
+	_ = x< <-1
+	_ = x+ +1
+	_ = x- -1
+	_ = x& &x
+	_ = x& ^x
+
+	_ = f(x/ *y, x< -1, x< <-1, x+ +1, x- -1, x& &x, x& ^x)
+}
+
+
+func _() {
+	_ = T{}
+	_ = struct{}{}
+	_ = [10]T{}
+	_ = [...]T{}
+	_ = []T{}
+	_ = map[int]T{}
+}
+
+
+// one-line structs/interfaces in composite literals (up to a threshold)
+func _() {
+	_ = struct{}{}
+	_ = struct{ x int }{0}
+	_ = struct{ x, y, z int }{0, 1, 2}
+	_ = struct{ int }{0}
+	_ = struct{ s struct { int } }{struct{ int}{0} }
+
+	_ = (interface{})(nil)
+	_ = (interface{String() string})(nil)
+	_ = (interface{
+		String()    string
+	})(nil)
+	_ = (interface{fmt.Stringer})(nil)
+	_ = (interface{
+		    fmt.Stringer
+	})(nil)
+}
+
+func _() {
+	// do not modify literals
+	_ = "tab1	tab2	tab3	end"  // string contains 3 tabs
+	_ = "tab1 tab2 tab3 end"  // same string with 3 blanks - may be unaligned because editors see tabs in strings
+	_ = ""  // this comment should be aligned with the one on the previous line
+	_ = ``
+	_ = `
+`
+_ = `foo
+		bar`
+	_ = `three spaces before the end of the line starting here:   
+they must not be removed`
+}
+
+
+func _() {
+	// smart handling of indentation for multi-line raw strings
+	var _ = ``
+	var _ = `foo`
+	var _ = `foo
+bar`
+
+
+var _ =
+	``
+var _ =
+	`foo`
+var _ =
+	// the next line should remain indented
+	`foo
+bar`
+
+
+	var _ = // comment
+		``
+	var _ = // comment
+		`foo`
+	var _ = // comment
+		// the next line should remain indented
+		`foo
+bar`
+
+
+var _ = /* comment */ ``
+var _ = /* comment */ `foo`
+var _ = /* comment */ `foo
+bar`
+
+
+	var _ = /* comment */
+		``
+	var _ = /* comment */
+		`foo`
+	var _ = /* comment */
+		// the next line should remain indented
+		`foo
+bar`
+
+
+var board = []int(
+	`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`)
+
+
+	var state = S{
+		"foo",
+		// the next line should remain indented
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`,
+		"bar",
+	}
+}
+
+
+func _() {
+	// one-line function literals (body is on a single line)
+	_ = func() {}
+	_ = func() int { return 0 }
+	_ = func(x, y int) bool { m := (x+y)/2; return m < 0 }
+
+	// multi-line function literals (body is not on one line)
+	_ = func() {
+	}
+	_ = func() int {
+		return 0
+	}
+	_ = func(x, y int) bool {
+		m := (x+y)/2; return x < y }
+
+	f(func() {
+	})
+	f(func() int {
+		return 0
+	})
+	f(func(x, y int) bool {
+		m := (x+y)/2; return x < y })
+}
+
+
+func _() {
+	_ = [][]int {
+		[]int{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int {
+		{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int {
+		{1},
+		{1, 2},
+		{1, 2, 3},
+	}
+	_ = [][]int {{1}, {1, 2}, {1, 2, 3}}
+}
+
+
+// various multi-line expressions
+func _() {
+	// do not add extra indentation to multi-line string lists
+	_ = "foo" + "bar"
+	_ = "foo" +
+	"bar" +
+	"bah"
+	_ = []string {
+		"abc" +
+		"def",
+		"foo" +
+		"bar",
+	}
+}
+
+
+const _ = F1 +
+	`string = "%s";` +
+	`ptr = *;` +
+	`datafmt.T2 = s ["-" p "-"];`
+
+
+const _ =
+	`datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+
+const _ = `datafmt "datafmt";` +
+`default = "%v";` +
+`array = *;` +
+`datafmt.T3 = s  {" " a a / ","};`
+
+
+func _() {
+	_ = F1 +
+		`string = "%s";` +
+		`ptr = *;` +
+		`datafmt.T2 = s ["-" p "-"];`
+
+	_ =
+		`datafmt "datafmt";` +
+		`default = "%v";` +
+		`array = *;` +
+		`datafmt.T3 = s  {" " a a / ","};`
+
+	_ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+}
+
+
+func _() {
+	// respect source lines in multi-line expressions
+	_ = a+
+	b+
+	c
+	_ = a < b ||
+		b < a
+	_ = "933262154439441526816992388562667004907159682643816214685929" +
+	"638952175999932299156089414639761565182862536979208272237582" +
+	"51185210916864000000000000000000000000"  // 100!
+	_ = "170141183460469231731687303715884105727"  // prime
+}
+
+
+// Alignment after overlong lines
+const (
+	_ = "991"
+	_ = "2432902008176640000"  // 20!
+	_ = "933262154439441526816992388562667004907159682643816214685929" +
+	"638952175999932299156089414639761565182862536979208272237582" +
+	"51185210916864000000000000000000000000"  // 100!
+	_ = "170141183460469231731687303715884105727"  // prime
+)
+
+
+// Correct placement of operators and comments in multi-line expressions
+func _() {
+	_ = a +  // comment
+		b +  // comment
+		c
+	_ = "a"	+
+		"b" +	// comment
+		"c"
+	_ = "ba0408" + "7265717569726564"     // field 71, encoding 2, string "required"
+}
+
+
+// Correct placement of terminating comma/closing parentheses in multi-line calls.
+func _() {
+	f(1,
+		2,
+		3)
+	f(1,
+		2,
+		3,
+	)
+	f(1,
+		2,
+		3)  // comment
+	f(1,
+		2,
+		3,  // comment
+	)
+	f(1,
+		2,
+		3)// comment
+	f(1,
+		2,
+		3,// comment
+	)
+}
+
+
+// Align comments in multi-line lists of single-line expressions.
+var txpix = [NCOL]draw.Color{
+	draw.Yellow, // yellow
+	draw.Cyan, // cyan
+	draw.Green, // lime green
+	draw.GreyBlue, // slate
+	draw.Red, /* red */
+	draw.GreyGreen, /* olive green */
+	draw.Blue, /* blue */
+	draw.Color(0xFF55AAFF), /* pink */
+	draw.Color(0xFFAAFFFF), /* lavender */
+	draw.Color(0xBB005DFF), /* maroon */
+}
+
+
+func same(t, u *Time) bool {
+	// respect source lines in multi-line expressions
+	return t.Year == u.Year &&
+		t.Month == u.Month &&
+		t.Day == u.Day &&
+		t.Hour == u.Hour &&
+		t.Minute == u.Minute &&
+		t.Second == u.Second &&
+		t.Weekday == u.Weekday &&
+		t.ZoneOffset == u.ZoneOffset &&
+		t.Zone == u.Zone
+}
+
+
+func (p *parser) charClass() {
+	// respect source lines in multi-line expressions
+	if cc.negate && len(cc.ranges) == 2 &&
+		cc.ranges[0] == '\n' && cc.ranges[1] == '\n' {
+		nl := new(_NotNl)
+		p.re.add(nl)
+	}
+}
+
+
+func addState(s []state, inst instr, match []int) {
+	// handle comments correctly in multi-line expressions
+	for i := 0; i < l; i++ {
+		if s[i].inst.index() == index && // same instruction
+		   s[i].match[0] < pos {	// earlier match already going; leftmost wins
+		   	return s
+		 }
+	}
+}
+
+func (self *T) foo(x int) *T { return self }
+
+func _() { module.Func1().Func2() }
+
+func _() {
+	_ = new(T).
+		foo(1).
+			foo(2).
+		foo(3)
+
+	_ = new(T).
+	foo(1).
+	foo(2). // inline comments
+	foo(3)
+
+	_ = new(T).foo(1).foo(2).foo(3)
+
+	// handle multiline argument list correctly
+	_ = new(T).
+	foo(
+		1).
+		foo(2)
+
+	_ = new(T).foo(
+		1).foo(2)
+
+	_ = Array[3 +
+4]
+
+	_ = Method(1, 2,
+		3)
+
+	_ = new(T).
+   foo().
+   bar() . (*Type)
+
+	_ = new(T).
+foo().
+bar().(*Type).
+baz()
+
+	_ = new(T).
+	foo().
+	bar()["idx"]
+
+	_ = new(T).
+	foo().
+	bar()["idx"]	.
+	baz()
+
+	_ = new(T).
+	foo().
+	bar()[1:2]
+
+	_ = new(T).
+	foo().
+	bar()[1:2].
+	baz()
+
+	_ = new(T).
+		Field.
+		Array[3+
+       		4].
+		Table ["foo"].
+		Blob. (*Type).
+	Slices[1:4].
+	Method(1, 2,
+	3).
+		Thingy
+
+	_ = a.b.c
+	_ = a.
+	b.
+	c
+	_ = a.b().c
+	_ = a.
+	b().
+	c
+	_ = a.b[0].c
+	_ = a.
+	b[0].
+	c
+	_ = a.b[0:].c
+	_ = a.
+	b[0:].
+	c
+	_ = a.b.(T).c
+	_ = a.
+	b.
+	(T).
+	c
+}
+
+
+// Don't introduce extra newlines in strangely formatted expression lists.
+func f() {
+	// os.Open parameters should remain on two lines
+	if writer, err = os.Open(outfile, s.O_WRONLY|os.O_CREATE|
+		os.O_TRUNC, 0666); err != nil {
+	    log.Fatal(err)
+	}
+}
+
+// Handle multi-line argument lists ending in ... correctly.
+// Was issue 3130.
+func _() {
+	_ = append(s, a...)
+	_ = append(
+		s, a...)
+	_ = append(s,
+		a...)
+	_ = append(
+		s,
+		a...)
+	_ = append(s, a...,
+	)
+	_ = append(s,
+		a...,
+	)
+	_ = append(
+		s,
+		a...,
+	)
+}
+
+// Literal function types in conversions must be parenthesized;
+// for now go/parser accepts the unparenthesized form where it
+// is non-ambiguous.
+func _() {
+	// these conversions should be rewritten to look
+	// the same as the parenthesized conversions below
+	_ = func()()(nil)
+	_ = func(x int)(float)(nil)
+	_ = func() func() func()()(nil)
+
+	_ = (func()())(nil)
+	_ = (func(x int)(float))(nil)
+	_ = (func() func() func()())(nil)
+}
+
+func _() {
+	_ = f().
+	f(func() {
+		f()
+	}).
+	f(map[int]int{
+	1: 2,
+	3: 4,
+})
+
+	_ = f().
+	f(
+	func() {
+		f()
+	},
+	)
+}
diff --git a/internal/backport/go/printer/testdata/expressions.raw b/internal/backport/go/printer/testdata/expressions.raw
new file mode 100644
index 0000000..058fded
--- /dev/null
+++ b/internal/backport/go/printer/testdata/expressions.raw
@@ -0,0 +1,743 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package expressions
+
+type T struct {
+	x, y, z int
+}
+
+var (
+	a, b, c, d, e	int
+	under_bar	int
+	longIdentifier1, longIdentifier2, longIdentifier3	int
+	t0, t1, t2	T
+	s	string
+	p	*int
+)
+
+func _() {
+	// no spaces around simple or parenthesized expressions
+	_ = (a + 0)
+	_ = a + b
+	_ = a + b + c
+	_ = a + b - c
+	_ = a - b - c
+	_ = a + (b * c)
+	_ = a + (b / c)
+	_ = a - (b % c)
+	_ = 1 + a
+	_ = a + 1
+	_ = a + b + 1
+	_ = s[a]
+	_ = s[a:]
+	_ = s[:b]
+	_ = s[1:2]
+	_ = s[a:b]
+	_ = s[0:len(s)]
+	_ = s[0] << 1
+	_ = (s[0] << 1) & 0xf
+	_ = s[0]<<2 | s[1]>>4
+	_ = "foo" + s
+	_ = s + "foo"
+	_ = 'a' + 'b'
+	_ = len(s) / 2
+	_ = len(t0.x) / a
+
+	// spaces around expressions of different precedence or expressions containing spaces
+	_ = a + -b
+	_ = a - ^b
+	_ = a / *p
+	_ = a + b*c
+	_ = 1 + b*c
+	_ = a + 2*c
+	_ = a + c*2
+	_ = 1 + 2*3
+	_ = s[1 : 2*3]
+	_ = s[a : b-c]
+	_ = s[0:]
+	_ = s[a+b]
+	_ = s[:b-c]
+	_ = s[a+b:]
+	_ = a[a<<b+1]
+	_ = a[a<<b+1:]
+	_ = s[a+b : len(s)]
+	_ = s[len(s):-a]
+	_ = s[a : len(s)+1]
+	_ = s[a:len(s)+1] + s
+
+	// spaces around operators with equal or lower precedence than comparisons
+	_ = a == b
+	_ = a != b
+	_ = a > b
+	_ = a >= b
+	_ = a < b
+	_ = a <= b
+	_ = a < b && c > d
+	_ = a < b || c > d
+
+	// spaces around "long" operands
+	_ = a + longIdentifier1
+	_ = longIdentifier1 + a
+	_ = longIdentifier1 + longIdentifier2*longIdentifier3
+	_ = s + "a longer string"
+
+	// some selected cases
+	_ = a + t0.x
+	_ = a + t0.x + t1.x*t2.x
+	_ = a + b + c + d + e + 2*3
+	_ = a + b + c + 2*3 + d + e
+	_ = (a + b + c) * 2
+	_ = a - b + c - d + (a + b + c) + d&e
+	_ = under_bar - 1
+	_ = Open(dpath+"/file", O_WRONLY|O_CREAT, 0666)
+	_ = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
+
+	// test case for issue 8021
+	// want:
+	//  ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+	_ = ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+
+	// the parser does not restrict expressions that may appear as statements
+	true
+	42
+	"foo"
+	x
+	(x)
+	a + b
+	a + b + c
+	a + (b * c)
+	a + (b / c)
+	1 + a
+	a + 1
+	s[a]
+	x << 1
+	(s[0] << 1) & 0xf
+	"foo" + s
+	x == y
+	x < y || z > 42
+}
+
+// slice expressions with cap
+func _() {
+	_ = x[a:b:c]
+	_ = x[a : b : c+d]
+	_ = x[a : b+d : c]
+	_ = x[a : b+d : c+d]
+	_ = x[a+d : b : c]
+	_ = x[a+d : b : c+d]
+	_ = x[a+d : b+d : c]
+	_ = x[a+d : b+d : c+d]
+
+	_ = x[:b:c]
+	_ = x[: b : c+d]
+	_ = x[: b+d : c]
+	_ = x[: b+d : c+d]
+}
+
+func issue22111() {
+	_ = x[:]
+
+	_ = x[:b]
+	_ = x[:b+1]
+
+	_ = x[a:]
+	_ = x[a+1:]
+
+	_ = x[a:b]
+	_ = x[a+1 : b]
+	_ = x[a : b+1]
+	_ = x[a+1 : b+1]
+
+	_ = x[:b:c]
+	_ = x[: b+1 : c]
+	_ = x[: b : c+1]
+	_ = x[: b+1 : c+1]
+
+	_ = x[a:b:c]
+	_ = x[a+1 : b : c]
+	_ = x[a : b+1 : c]
+	_ = x[a+1 : b+1 : c]
+	_ = x[a : b : c+1]
+	_ = x[a+1 : b : c+1]
+	_ = x[a : b+1 : c+1]
+	_ = x[a+1 : b+1 : c+1]
+}
+
+func _() {
+	_ = a + b
+	_ = a + b + c
+	_ = a + b*c
+	_ = a + (b * c)
+	_ = (a + b) * c
+	_ = a + (b * c * d)
+	_ = a + (b*c + d)
+
+	_ = 1 << x
+	_ = -1 << x
+	_ = 1<<x - 1
+	_ = -1<<x - 1
+
+	_ = f(a + b)
+	_ = f(a + b + c)
+	_ = f(a + b*c)
+	_ = f(a + (b * c))
+	_ = f(1<<x-1, 1<<x-2)
+
+	_ = 1<<d.logWindowSize - 1
+
+	buf = make(x, 2*cap(b.buf)+n)
+
+	dst[i*3+2] = dbuf[0] << 2
+	dst[i*3+2] = dbuf[0]<<2 | dbuf[1]>>4
+
+	b.buf = b.buf[0 : b.off+m+n]
+	b.buf = b.buf[0 : b.off+m*n]
+	f(b.buf[0 : b.off+m+n])
+
+	signed += ' ' * 8
+	tw.octal(header[148:155], chksum)
+
+	_ = x > 0 && i >= 0
+
+	x1, x0 := x>>w2, x&m2
+	z0 = t1<<w2 + t0
+	z1 = (t1 + t0>>w2) >> w2
+	q1, r1 := x1/d1, x1%d1
+	r1 = r1*b2 | x0>>w2
+	x1 = (x1 << z) | (x0 >> (uint(w) - z))
+	x1 = x1<<z | x0>>(uint(w)-z)
+
+	_ = buf[0 : len(buf)+1]
+	_ = buf[0 : n+1]
+
+	a, b = b, a
+	a = b + c
+	a = b*c + d
+	_ = a*b + c
+	_ = a - b - c
+	_ = a - (b - c)
+	_ = a - b*c
+	_ = a - (b * c)
+	_ = a * b / c
+	_ = a / *b
+	_ = x[a|^b]
+	_ = x[a / *b]
+	_ = a & ^b
+	_ = a + +b
+	_ = a - -b
+	_ = x[a*-b]
+	_ = x[a + +b]
+	_ = x ^ y ^ z
+	_ = b[a>>24] ^ b[(a>>16)&0xFF] ^ b[(a>>8)&0xFF] ^ b[a&0xFF]
+	_ = len(longVariableName) * 2
+
+	_ = token(matchType + xlength<<lengthShift + xoffset)
+}
+
+func f(x int, args ...int) {
+	f(0, args...)
+	f(1, args)
+	f(2, args[0])
+
+	// make sure syntactically legal code remains syntactically legal
+	f(3, 42 ...)	// a blank must remain between 42 and ...
+	f(4, 42....)
+	f(5, 42....)
+	f(6, 42.0...)
+	f(7, 42.0...)
+	f(8, .42...)
+	f(9, .42...)
+	f(10, 42e0...)
+	f(11, 42e0...)
+
+	_ = 42 .x	// a blank must remain between 42 and .x
+	_ = 42..x
+	_ = 42..x
+	_ = 42.0.x
+	_ = 42.0.x
+	_ = .42.x
+	_ = .42.x
+	_ = 42e0.x
+	_ = 42e0.x
+
+	// a blank must remain between the binary operator and the 2nd operand
+	_ = x / *y
+	_ = x < -1
+	_ = x < <-1
+	_ = x + +1
+	_ = x - -1
+	_ = x & &x
+	_ = x & ^x
+
+	_ = f(x / *y, x < -1, x < <-1, x + +1, x - -1, x & &x, x & ^x)
+}
+
+func _() {
+	_ = T{}
+	_ = struct{}{}
+	_ = [10]T{}
+	_ = [...]T{}
+	_ = []T{}
+	_ = map[int]T{}
+}
+
+// one-line structs/interfaces in composite literals (up to a threshold)
+func _() {
+	_ = struct{}{}
+	_ = struct{ x int }{0}
+	_ = struct{ x, y, z int }{0, 1, 2}
+	_ = struct{ int }{0}
+	_ = struct{ s struct{ int } }{struct{ int }{0}}
+
+	_ = (interface{})(nil)
+	_ = (interface{ String() string })(nil)
+	_ = (interface {
+		String() string
+	})(nil)
+	_ = (interface{ fmt.Stringer })(nil)
+	_ = (interface {
+		fmt.Stringer
+	})(nil)
+}
+
+func _() {
+	// do not modify literals
+	_ = "tab1	tab2	tab3	end"	// string contains 3 tabs
+	_ = "tab1 tab2 tab3 end"	// same string with 3 blanks - may be unaligned because editors see tabs in strings
+	_ = ""	// this comment should be aligned with the one on the previous line
+	_ = ``
+	_ = `
+`
+	_ = `foo
+		bar`
+	_ = `three spaces before the end of the line starting here:   
+they must not be removed`
+}
+
+func _() {
+	// smart handling of indentation for multi-line raw strings
+	var _ = ``
+	var _ = `foo`
+	var _ = `foo
+bar`
+
+	var _ = ``
+	var _ = `foo`
+	var _ =
+	// the next line should remain indented
+	`foo
+bar`
+
+	var _ =	// comment
+	``
+	var _ =	// comment
+	`foo`
+	var _ =	// comment
+	// the next line should remain indented
+	`foo
+bar`
+
+	var _ = /* comment */ ``
+	var _ = /* comment */ `foo`
+	var _ = /* comment */ `foo
+bar`
+
+	var _ =	/* comment */
+	``
+	var _ =	/* comment */
+	`foo`
+	var _ =	/* comment */
+	// the next line should remain indented
+	`foo
+bar`
+
+	var board = []int(
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`)
+
+	var state = S{
+		"foo",
+		// the next line should remain indented
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`,
+		"bar",
+	}
+}
+
+func _() {
+	// one-line function literals (body is on a single line)
+	_ = func() {}
+	_ = func() int { return 0 }
+	_ = func(x, y int) bool { m := (x + y) / 2; return m < 0 }
+
+	// multi-line function literals (body is not on one line)
+	_ = func() {
+	}
+	_ = func() int {
+		return 0
+	}
+	_ = func(x, y int) bool {
+		m := (x + y) / 2
+		return x < y
+	}
+
+	f(func() {
+	})
+	f(func() int {
+		return 0
+	})
+	f(func(x, y int) bool {
+		m := (x + y) / 2
+		return x < y
+	})
+}
+
+func _() {
+	_ = [][]int{
+		[]int{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int{
+		{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int{
+		{1},
+		{1, 2},
+		{1, 2, 3},
+	}
+	_ = [][]int{{1}, {1, 2}, {1, 2, 3}}
+}
+
+// various multi-line expressions
+func _() {
+	// do not add extra indentation to multi-line string lists
+	_ = "foo" + "bar"
+	_ = "foo" +
+		"bar" +
+		"bah"
+	_ = []string{
+		"abc" +
+			"def",
+		"foo" +
+			"bar",
+	}
+}
+
+const _ = F1 +
+	`string = "%s";` +
+	`ptr = *;` +
+	`datafmt.T2 = s ["-" p "-"];`
+
+const _ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+const _ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+func _() {
+	_ = F1 +
+		`string = "%s";` +
+		`ptr = *;` +
+		`datafmt.T2 = s ["-" p "-"];`
+
+	_ =
+		`datafmt "datafmt";` +
+			`default = "%v";` +
+			`array = *;` +
+			`datafmt.T3 = s  {" " a a / ","};`
+
+	_ = `datafmt "datafmt";` +
+		`default = "%v";` +
+		`array = *;` +
+		`datafmt.T3 = s  {" " a a / ","};`
+}
+
+func _() {
+	// respect source lines in multi-line expressions
+	_ = a +
+		b +
+		c
+	_ = a < b ||
+		b < a
+	_ = "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000"	// 100!
+	_ = "170141183460469231731687303715884105727"	// prime
+}
+
+// Alignment after overlong lines
+const (
+	_	= "991"
+	_	= "2432902008176640000"		// 20!
+	_	= "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000"	// 100!
+	_	= "170141183460469231731687303715884105727"		// prime
+)
+
+// Correct placement of operators and comments in multi-line expressions
+func _() {
+	_ = a +	// comment
+		b +	// comment
+		c
+	_ = "a" +
+		"b" +	// comment
+		"c"
+	_ = "ba0408" + "7265717569726564"	// field 71, encoding 2, string "required"
+}
+
+// Correct placement of terminating comma/closing parentheses in multi-line calls.
+func _() {
+	f(1,
+		2,
+		3)
+	f(1,
+		2,
+		3,
+	)
+	f(1,
+		2,
+		3)	// comment
+	f(1,
+		2,
+		3,	// comment
+	)
+	f(1,
+		2,
+		3)	// comment
+	f(1,
+		2,
+		3,	// comment
+	)
+}
+
+// Align comments in multi-line lists of single-line expressions.
+var txpix = [NCOL]draw.Color{
+	draw.Yellow,	// yellow
+	draw.Cyan,	// cyan
+	draw.Green,	// lime green
+	draw.GreyBlue,	// slate
+	draw.Red,	/* red */
+	draw.GreyGreen,	/* olive green */
+	draw.Blue,	/* blue */
+	draw.Color(0xFF55AAFF),	/* pink */
+	draw.Color(0xFFAAFFFF),	/* lavender */
+	draw.Color(0xBB005DFF),	/* maroon */
+}
+
+func same(t, u *Time) bool {
+	// respect source lines in multi-line expressions
+	return t.Year == u.Year &&
+		t.Month == u.Month &&
+		t.Day == u.Day &&
+		t.Hour == u.Hour &&
+		t.Minute == u.Minute &&
+		t.Second == u.Second &&
+		t.Weekday == u.Weekday &&
+		t.ZoneOffset == u.ZoneOffset &&
+		t.Zone == u.Zone
+}
+
+func (p *parser) charClass() {
+	// respect source lines in multi-line expressions
+	if cc.negate && len(cc.ranges) == 2 &&
+		cc.ranges[0] == '\n' && cc.ranges[1] == '\n' {
+		nl := new(_NotNl)
+		p.re.add(nl)
+	}
+}
+
+func addState(s []state, inst instr, match []int) {
+	// handle comments correctly in multi-line expressions
+	for i := 0; i < l; i++ {
+		if s[i].inst.index() == index &&	// same instruction
+			s[i].match[0] < pos {	// earlier match already going; leftmost wins
+			return s
+		}
+	}
+}
+
+func (self *T) foo(x int) *T	{ return self }
+
+func _()	{ module.Func1().Func2() }
+
+func _() {
+	_ = new(T).
+		foo(1).
+		foo(2).
+		foo(3)
+
+	_ = new(T).
+		foo(1).
+		foo(2).	// inline comments
+		foo(3)
+
+	_ = new(T).foo(1).foo(2).foo(3)
+
+	// handle multiline argument list correctly
+	_ = new(T).
+		foo(
+			1).
+		foo(2)
+
+	_ = new(T).foo(
+		1).foo(2)
+
+	_ = Array[3+
+		4]
+
+	_ = Method(1, 2,
+		3)
+
+	_ = new(T).
+		foo().
+		bar().(*Type)
+
+	_ = new(T).
+		foo().
+		bar().(*Type).
+		baz()
+
+	_ = new(T).
+		foo().
+		bar()["idx"]
+
+	_ = new(T).
+		foo().
+		bar()["idx"].
+		baz()
+
+	_ = new(T).
+		foo().
+		bar()[1:2]
+
+	_ = new(T).
+		foo().
+		bar()[1:2].
+		baz()
+
+	_ = new(T).
+		Field.
+		Array[3+
+		4].
+		Table["foo"].
+		Blob.(*Type).
+		Slices[1:4].
+		Method(1, 2,
+			3).
+		Thingy
+
+	_ = a.b.c
+	_ = a.
+		b.
+		c
+	_ = a.b().c
+	_ = a.
+		b().
+		c
+	_ = a.b[0].c
+	_ = a.
+		b[0].
+		c
+	_ = a.b[0:].c
+	_ = a.
+		b[0:].
+		c
+	_ = a.b.(T).c
+	_ = a.
+		b.(T).
+		c
+}
+
+// Don't introduce extra newlines in strangely formatted expression lists.
+func f() {
+	// os.Open parameters should remain on two lines
+	if writer, err = os.Open(outfile, s.O_WRONLY|os.O_CREATE|
+		os.O_TRUNC, 0666); err != nil {
+		log.Fatal(err)
+	}
+}
+
+// Handle multi-line argument lists ending in ... correctly.
+// Was issue 3130.
+func _() {
+	_ = append(s, a...)
+	_ = append(
+		s, a...)
+	_ = append(s,
+		a...)
+	_ = append(
+		s,
+		a...)
+	_ = append(s, a...,
+	)
+	_ = append(s,
+		a...,
+	)
+	_ = append(
+		s,
+		a...,
+	)
+}
+
+// Literal function types in conversions must be parenthesized;
+// for now go/parser accepts the unparenthesized form where it
+// is non-ambiguous.
+func _() {
+	// these conversions should be rewritten to look
+	// the same as the parenthesized conversions below
+	_ = (func())(nil)
+	_ = (func(x int) float)(nil)
+	_ = (func() func() func())(nil)
+
+	_ = (func())(nil)
+	_ = (func(x int) float)(nil)
+	_ = (func() func() func())(nil)
+}
+
+func _() {
+	_ = f().
+		f(func() {
+			f()
+		}).
+		f(map[int]int{
+			1:	2,
+			3:	4,
+		})
+
+	_ = f().
+		f(
+			func() {
+				f()
+			},
+		)
+}
diff --git a/internal/backport/go/printer/testdata/generics.golden b/internal/backport/go/printer/testdata/generics.golden
new file mode 100644
index 0000000..c3a7df8
--- /dev/null
+++ b/internal/backport/go/printer/testdata/generics.golden
@@ -0,0 +1,92 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package generics
+
+func _[A, B any](a A, b B) int	{}
+func _[T any](x, y T) T
+
+type T[P any] struct{}
+type T[P1, P2, P3 any] struct{}
+
+type T[P C] struct{}
+type T[P1, P2, P3 C] struct{}
+
+type T[P C[P]] struct{}
+type T[P1, P2, P3 C[P1, P2, P3]] struct{}
+
+func f[P any](x P)
+func f[P1, P2, P3 any](x1 P1, x2 P2, x3 P3) struct{}
+
+func f[P interface{}](x P)
+func f[P1, P2, P3 interface {
+	m1(P1)
+	~P2 | ~P3
+}](x1 P1, x2 P2, x3 P3) struct{}
+func f[P any](T1[P], T2[P]) T3[P]
+
+func (x T[P]) m()
+func (T[P]) m(x T[P]) P
+
+func _() {
+	type _ []T[P]
+	var _ []T[P]
+	_ = []T[P]{}
+}
+
+// type constraint literals with elided interfaces
+func _[P ~int, Q int | string]()	{}
+func _[P struct{ f int }, Q *P]()	{}
+
+// various potentially ambiguous type parameter lists (issue #49482)
+type _[P *T,] struct{}
+type _[P *T, _ any] struct{}
+type _[P *T,] struct{}
+type _[P *T, _ any] struct{}
+type _[P T] struct{}
+type _[P T, _ any] struct{}
+
+type _[P *struct{}] struct{}
+type _[P *struct{}] struct{}
+type _[P []int] struct{}
+
+// array type declarations
+type _ [P(T)]struct{}
+type _ [P((T))]struct{}
+type _ [P * *T]struct{}
+type _ [P * T]struct{}
+type _ [P(*T)]struct{}
+type _ [P(**T)]struct{}
+type _ [P * T]struct{}
+type _ [P*T - T]struct{}
+
+type _[
+	P *T,
+] struct{}
+
+// equivalent test cases for potentially ambiguous type parameter lists, except
+// for function declarations there is no ambiguity (issue #51548)
+func _[P *T]()		{}
+func _[P *T, _ any]()	{}
+func _[P *T]()		{}
+func _[P *T, _ any]()	{}
+func _[P T]()		{}
+func _[P T, _ any]()	{}
+
+func _[P *struct{}]()	{}
+func _[P *struct{}]()	{}
+func _[P []int]()	{}
+
+func _[P T]()	{}
+func _[P T]()	{}
+func _[P **T]()	{}
+func _[P *T]()	{}
+func _[P *T]()	{}
+func _[P **T]()	{}
+func _[P *T]()	{}
+
+func _[
+	P *T,
+]() {
+}
diff --git a/internal/backport/go/printer/testdata/generics.input b/internal/backport/go/printer/testdata/generics.input
new file mode 100644
index 0000000..66e1554
--- /dev/null
+++ b/internal/backport/go/printer/testdata/generics.input
@@ -0,0 +1,88 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package generics
+
+func _[A, B any](a A, b B) int {}
+func _[T any](x, y T) T
+
+type T[P any] struct{}
+type T[P1, P2, P3 any] struct{}
+
+type T[P C] struct{}
+type T[P1, P2, P3 C] struct{}
+
+type T[P C[P]] struct{}
+type T[P1, P2, P3 C[P1, P2, P3]] struct{}
+
+func f[P any](x P)
+func f[P1, P2, P3 any](x1 P1, x2 P2, x3 P3) struct{}
+
+func f[P interface{}](x P)
+func f[P1, P2, P3 interface{ m1(P1); ~P2|~P3 }](x1 P1, x2 P2, x3 P3) struct{}
+func f[P any](T1[P], T2[P]) T3[P]
+
+func (x T[P]) m()
+func ((T[P])) m(x T[P]) P
+
+func _() {
+	type _ []T[P]
+	var _ []T[P]
+	_ = []T[P]{}
+}
+
+// type constraint literals with elided interfaces
+func _[P ~int, Q int | string]() {}
+func _[P struct{f int}, Q *P]() {}
+
+// various potentially ambiguous type parameter lists (issue #49482)
+type _[P *T,] struct{}
+type _[P *T, _ any] struct{}
+type _[P (*T),] struct{}
+type _[P (*T), _ any] struct{}
+type _[P (T),] struct{}
+type _[P (T), _ any] struct{}
+
+type _[P *struct{}] struct{}
+type _[P (*struct{})] struct{}
+type _[P ([]int)] struct{}
+
+// array type declarations
+type _ [P(T)]struct{}
+type _ [P((T))]struct{}
+type _ [P * *T]struct{}
+type _ [P * T]struct{}
+type _ [P(*T)]struct{}
+type _ [P(**T)]struct{}
+type _ [P * T]struct{}
+type _ [P * T - T]struct{}
+
+type _[
+	P *T,
+] struct{}
+
+// equivalent test cases for potentially ambiguous type parameter lists, except
+// for function declarations there is no ambiguity (issue #51548)
+func _[P *T,]() {}
+func _[P *T, _ any]() {}
+func _[P (*T),]() {}
+func _[P (*T), _ any]() {}
+func _[P (T),]() {}
+func _[P (T), _ any]() {}
+
+func _[P *struct{}] () {}
+func _[P (*struct{})] () {}
+func _[P ([]int)] () {}
+
+func _ [P(T)]() {}
+func _ [P((T))]() {}
+func _ [P * *T]() {}
+func _ [P * T]() {}
+func _ [P(*T)]() {}
+func _ [P(**T)]() {}
+func _ [P * T]() {}
+
+func _[
+	P *T,
+]() {}
diff --git a/internal/backport/go/printer/testdata/go2numbers.golden b/internal/backport/go/printer/testdata/go2numbers.golden
new file mode 100644
index 0000000..3c12049
--- /dev/null
+++ b/internal/backport/go/printer/testdata/go2numbers.golden
@@ -0,0 +1,186 @@
+package p
+
+const (
+	// 0-octals
+	_	= 0
+	_	= 0123
+	_	= 0123456
+
+	_	= 0_123
+	_	= 0123_456
+
+	// decimals
+	_	= 1
+	_	= 1234
+	_	= 1234567
+
+	_	= 1_234
+	_	= 1_234_567
+
+	// hexadecimals
+	_	= 0x0
+	_	= 0x1234
+	_	= 0xcafef00d
+
+	_	= 0X0
+	_	= 0X1234
+	_	= 0XCAFEf00d
+
+	_	= 0X_0
+	_	= 0X_1234
+	_	= 0X_CAFE_f00d
+
+	// octals
+	_	= 0o0
+	_	= 0o1234
+	_	= 0o01234567
+
+	_	= 0O0
+	_	= 0O1234
+	_	= 0O01234567
+
+	_	= 0o_0
+	_	= 0o_1234
+	_	= 0o0123_4567
+
+	_	= 0O_0
+	_	= 0O_1234
+	_	= 0O0123_4567
+
+	// binaries
+	_	= 0b0
+	_	= 0b1011
+	_	= 0b00101101
+
+	_	= 0B0
+	_	= 0B1011
+	_	= 0B00101101
+
+	_	= 0b_0
+	_	= 0b10_11
+	_	= 0b_0010_1101
+
+	// decimal floats
+	_	= 0.
+	_	= 123.
+	_	= 0123.
+
+	_	= .0
+	_	= .123
+	_	= .0123
+
+	_	= 0e0
+	_	= 123e+0
+	_	= 0123E-1
+
+	_	= 0e-0
+	_	= 123E+0
+	_	= 0123E123
+
+	_	= 0.e+1
+	_	= 123.E-10
+	_	= 0123.e123
+
+	_	= .0e-1
+	_	= .123E+10
+	_	= .0123E123
+
+	_	= 0.0
+	_	= 123.123
+	_	= 0123.0123
+
+	_	= 0.0e1
+	_	= 123.123E-10
+	_	= 0123.0123e+456
+
+	_	= 1_2_3.
+	_	= 0_123.
+
+	_	= 0_0e0
+	_	= 1_2_3e0
+	_	= 0_123e0
+
+	_	= 0e-0_0
+	_	= 1_2_3E+0
+	_	= 0123E1_2_3
+
+	_	= 0.e+1
+	_	= 123.E-1_0
+	_	= 01_23.e123
+
+	_	= .0e-1
+	_	= .123E+10
+	_	= .0123E123
+
+	_	= 1_2_3.123
+	_	= 0123.01_23
+
+	// hexadecimal floats
+	_	= 0x0.p+0
+	_	= 0Xdeadcafe.p-10
+	_	= 0x1234.P123
+
+	_	= 0x.1p-0
+	_	= 0X.deadcafep2
+	_	= 0x.1234P+10
+
+	_	= 0x0p0
+	_	= 0Xdeadcafep+1
+	_	= 0x1234P-10
+
+	_	= 0x0.0p0
+	_	= 0Xdead.cafep+1
+	_	= 0x12.34P-10
+
+	_	= 0Xdead_cafep+1
+	_	= 0x_1234P-10
+
+	_	= 0X_dead_cafe.p-10
+	_	= 0x12_34.P1_2_3
+	_	= 0X1_2_3_4.P-1_2_3
+
+	// imaginaries
+	_	= 0i
+	_	= 00i
+	_	= 08i
+	_	= 0000000000i
+	_	= 0123i
+	_	= 0000000123i
+	_	= 0000056789i
+	_	= 1234i
+	_	= 1234567i
+
+	_	= 0i
+	_	= 0_0i
+	_	= 0_8i
+	_	= 0_000_000_000i
+	_	= 0_123i
+	_	= 0_000_000_123i
+	_	= 0_000_056_789i
+	_	= 1_234i
+	_	= 1_234_567i
+
+	_	= 0.i
+	_	= 123.i
+	_	= 0123.i
+	_	= 000123.i
+
+	_	= 0e0i
+	_	= 123e0i
+	_	= 0123E0i
+	_	= 000123E0i
+
+	_	= 0.e+1i
+	_	= 123.E-1_0i
+	_	= 01_23.e123i
+	_	= 00_01_23.e123i
+
+	_	= 0b1010i
+	_	= 0B1010i
+	_	= 0o660i
+	_	= 0O660i
+	_	= 0xabcDEFi
+	_	= 0XabcDEFi
+	_	= 0xabcDEFP0i
+	_	= 0XabcDEFp0i
+)
diff --git a/internal/backport/go/printer/testdata/go2numbers.input b/internal/backport/go/printer/testdata/go2numbers.input
new file mode 100644
index 0000000..f3e7828
--- /dev/null
+++ b/internal/backport/go/printer/testdata/go2numbers.input
@@ -0,0 +1,186 @@
+package p
+
+const (
+	// 0-octals
+	_ = 0
+	_ = 0123
+	_ = 0123456
+
+	_ = 0_123
+	_ = 0123_456
+
+	// decimals
+	_ = 1
+	_ = 1234
+	_ = 1234567
+
+	_ = 1_234
+	_ = 1_234_567
+
+	// hexadecimals
+	_ = 0x0
+	_ = 0x1234
+	_ = 0xcafef00d
+
+	_ = 0X0
+	_ = 0X1234
+	_ = 0XCAFEf00d
+
+	_ = 0X_0
+	_ = 0X_1234
+	_ = 0X_CAFE_f00d
+
+	// octals
+	_ = 0o0
+	_ = 0o1234
+	_ = 0o01234567
+
+	_ = 0O0
+	_ = 0O1234
+	_ = 0O01234567
+
+	_ = 0o_0
+	_ = 0o_1234
+	_ = 0o0123_4567
+
+	_ = 0O_0
+	_ = 0O_1234
+	_ = 0O0123_4567
+
+	// binaries
+	_ = 0b0
+	_ = 0b1011
+	_ = 0b00101101
+
+	_ = 0B0
+	_ = 0B1011
+	_ = 0B00101101
+
+	_ = 0b_0
+	_ = 0b10_11
+	_ = 0b_0010_1101
+
+	// decimal floats
+	_ = 0.
+	_ = 123.
+	_ = 0123.
+
+	_ = .0
+	_ = .123
+	_ = .0123
+
+	_ = 0e0
+	_ = 123e+0
+	_ = 0123E-1
+
+	_ = 0e-0
+	_ = 123E+0
+	_ = 0123E123
+
+	_ = 0.e+1
+	_ = 123.E-10
+	_ = 0123.e123
+
+	_ = .0e-1
+	_ = .123E+10
+	_ = .0123E123
+
+	_ = 0.0
+	_ = 123.123
+	_ = 0123.0123
+
+	_ = 0.0e1
+	_ = 123.123E-10
+	_ = 0123.0123e+456
+
+	_ = 1_2_3.
+	_ = 0_123.
+
+	_ = 0_0e0
+	_ = 1_2_3e0
+	_ = 0_123e0
+
+	_ = 0e-0_0
+	_ = 1_2_3E+0
+	_ = 0123E1_2_3
+
+	_ = 0.e+1
+	_ = 123.E-1_0
+	_ = 01_23.e123
+
+	_ = .0e-1
+	_ = .123E+10
+	_ = .0123E123
+
+	_ = 1_2_3.123
+	_ = 0123.01_23
+
+	// hexadecimal floats
+	_ = 0x0.p+0
+	_ = 0Xdeadcafe.p-10
+	_ = 0x1234.P123
+
+	_ = 0x.1p-0
+	_ = 0X.deadcafep2
+	_ = 0x.1234P+10
+
+	_ = 0x0p0
+	_ = 0Xdeadcafep+1
+	_ = 0x1234P-10
+
+	_ = 0x0.0p0
+	_ = 0Xdead.cafep+1
+	_ = 0x12.34P-10
+
+	_ = 0Xdead_cafep+1
+	_ = 0x_1234P-10
+
+	_ = 0X_dead_cafe.p-10
+	_ = 0x12_34.P1_2_3
+	_ = 0X1_2_3_4.P-1_2_3
+
+	// imaginaries
+	_ = 0i
+	_ = 00i
+	_ = 08i
+	_ = 0000000000i
+	_ = 0123i
+	_ = 0000000123i
+	_ = 0000056789i
+	_ = 1234i
+	_ = 1234567i
+
+	_ = 0i
+	_ = 0_0i
+	_ = 0_8i
+	_ = 0_000_000_000i
+	_ = 0_123i
+	_ = 0_000_000_123i
+	_ = 0_000_056_789i
+	_ = 1_234i
+	_ = 1_234_567i
+
+	_ = 0.i
+	_ = 123.i
+	_ = 0123.i
+	_ = 000123.i
+
+	_ = 0e0i
+	_ = 123e0i
+	_ = 0123E0i
+	_ = 000123E0i
+
+	_ = 0.e+1i
+	_ = 123.E-1_0i
+	_ = 01_23.e123i
+	_ = 00_01_23.e123i
+
+	_ = 0b1010i
+	_ = 0B1010i
+	_ = 0o660i
+	_ = 0O660i
+	_ = 0xabcDEFi
+	_ = 0XabcDEFi
+	_ = 0xabcDEFP0i
+	_ = 0XabcDEFp0i
+)
diff --git a/internal/backport/go/printer/testdata/go2numbers.norm b/internal/backport/go/printer/testdata/go2numbers.norm
new file mode 100644
index 0000000..855f0fc
--- /dev/null
+++ b/internal/backport/go/printer/testdata/go2numbers.norm
@@ -0,0 +1,186 @@
+package p
+
+const (
+	// 0-octals
+	_	= 0
+	_	= 0123
+	_	= 0123456
+
+	_	= 0_123
+	_	= 0123_456
+
+	// decimals
+	_	= 1
+	_	= 1234
+	_	= 1234567
+
+	_	= 1_234
+	_	= 1_234_567
+
+	// hexadecimals
+	_	= 0x0
+	_	= 0x1234
+	_	= 0xcafef00d
+
+	_	= 0x0
+	_	= 0x1234
+	_	= 0xCAFEf00d
+
+	_	= 0x_0
+	_	= 0x_1234
+	_	= 0x_CAFE_f00d
+
+	// octals
+	_	= 0o0
+	_	= 0o1234
+	_	= 0o01234567
+
+	_	= 0o0
+	_	= 0o1234
+	_	= 0o01234567
+
+	_	= 0o_0
+	_	= 0o_1234
+	_	= 0o0123_4567
+
+	_	= 0o_0
+	_	= 0o_1234
+	_	= 0o0123_4567
+
+	// binaries
+	_	= 0b0
+	_	= 0b1011
+	_	= 0b00101101
+
+	_	= 0b0
+	_	= 0b1011
+	_	= 0b00101101
+
+	_	= 0b_0
+	_	= 0b10_11
+	_	= 0b_0010_1101
+
+	// decimal floats
+	_	= 0.
+	_	= 123.
+	_	= 0123.
+
+	_	= .0
+	_	= .123
+	_	= .0123
+
+	_	= 0e0
+	_	= 123e+0
+	_	= 0123e-1
+
+	_	= 0e-0
+	_	= 123e+0
+	_	= 0123e123
+
+	_	= 0.e+1
+	_	= 123.e-10
+	_	= 0123.e123
+
+	_	= .0e-1
+	_	= .123e+10
+	_	= .0123e123
+
+	_	= 0.0
+	_	= 123.123
+	_	= 0123.0123
+
+	_	= 0.0e1
+	_	= 123.123e-10
+	_	= 0123.0123e+456
+
+	_	= 1_2_3.
+	_	= 0_123.
+
+	_	= 0_0e0
+	_	= 1_2_3e0
+	_	= 0_123e0
+
+	_	= 0e-0_0
+	_	= 1_2_3e+0
+	_	= 0123e1_2_3
+
+	_	= 0.e+1
+	_	= 123.e-1_0
+	_	= 01_23.e123
+
+	_	= .0e-1
+	_	= .123e+10
+	_	= .0123e123
+
+	_	= 1_2_3.123
+	_	= 0123.01_23
+
+	// hexadecimal floats
+	_	= 0x0.p+0
+	_	= 0xdeadcafe.p-10
+	_	= 0x1234.p123
+
+	_	= 0x.1p-0
+	_	= 0x.deadcafep2
+	_	= 0x.1234p+10
+
+	_	= 0x0p0
+	_	= 0xdeadcafep+1
+	_	= 0x1234p-10
+
+	_	= 0x0.0p0
+	_	= 0xdead.cafep+1
+	_	= 0x12.34p-10
+
+	_	= 0xdead_cafep+1
+	_	= 0x_1234p-10
+
+	_	= 0x_dead_cafe.p-10
+	_	= 0x12_34.p1_2_3
+	_	= 0x1_2_3_4.p-1_2_3
+
+	// imaginaries
+	_	= 0i
+	_	= 0i
+	_	= 8i
+	_	= 0i
+	_	= 123i
+	_	= 123i
+	_	= 56789i
+	_	= 1234i
+	_	= 1234567i
+
+	_	= 0i
+	_	= 0i
+	_	= 8i
+	_	= 0i
+	_	= 123i
+	_	= 123i
+	_	= 56_789i
+	_	= 1_234i
+	_	= 1_234_567i
+
+	_	= 0.i
+	_	= 123.i
+	_	= 0123.i
+	_	= 000123.i
+
+	_	= 0e0i
+	_	= 123e0i
+	_	= 0123e0i
+	_	= 000123e0i
+
+	_	= 0.e+1i
+	_	= 123.e-1_0i
+	_	= 01_23.e123i
+	_	= 00_01_23.e123i
+
+	_	= 0b1010i
+	_	= 0b1010i
+	_	= 0o660i
+	_	= 0o660i
+	_	= 0xabcDEFi
+	_	= 0xabcDEFi
+	_	= 0xabcDEFp0i
+	_	= 0xabcDEFp0i
+)
diff --git a/internal/backport/go/printer/testdata/gobuild1.golden b/internal/backport/go/printer/testdata/gobuild1.golden
new file mode 100644
index 0000000..649da40
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild1.golden
@@ -0,0 +1,6 @@
+//go:build x
+// +build x
+
+package p
+
+func f()
diff --git a/internal/backport/go/printer/testdata/gobuild1.input b/internal/backport/go/printer/testdata/gobuild1.input
new file mode 100644
index 0000000..6538ee6
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild1.input
@@ -0,0 +1,7 @@
+package p
+
+//go:build x
+
+func f()
+
+// +build y
diff --git a/internal/backport/go/printer/testdata/gobuild2.golden b/internal/backport/go/printer/testdata/gobuild2.golden
new file mode 100644
index 0000000..c46fd34
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild2.golden
@@ -0,0 +1,8 @@
+//go:build x
+// +build x
+
+// other comment
+
+package p
+
+func f()
diff --git a/internal/backport/go/printer/testdata/gobuild2.input b/internal/backport/go/printer/testdata/gobuild2.input
new file mode 100644
index 0000000..f0f772a
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild2.input
@@ -0,0 +1,9 @@
+// +build y
+
+// other comment
+
+package p
+
+func f()
+
+//go:build x
diff --git a/internal/backport/go/printer/testdata/gobuild3.golden b/internal/backport/go/printer/testdata/gobuild3.golden
new file mode 100644
index 0000000..db92c57
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild3.golden
@@ -0,0 +1,10 @@
+// other comment
+
+//go:build x
+// +build x
+
+// yet another comment
+
+package p
+
+func f()
diff --git a/internal/backport/go/printer/testdata/gobuild3.input b/internal/backport/go/printer/testdata/gobuild3.input
new file mode 100644
index 0000000..d0c97b2
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild3.input
@@ -0,0 +1,11 @@
+// other comment
+
+// +build y
+
+// yet another comment
+
+package p
+
+//go:build x
+
+func f()
diff --git a/internal/backport/go/printer/testdata/gobuild4.golden b/internal/backport/go/printer/testdata/gobuild4.golden
new file mode 100644
index 0000000..b16477f
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild4.golden
@@ -0,0 +1,6 @@
+//go:build (x || y) && z
+// +build x y
+// +build z
+
+// doc comment
+package p
diff --git a/internal/backport/go/printer/testdata/gobuild4.input b/internal/backport/go/printer/testdata/gobuild4.input
new file mode 100644
index 0000000..29d5a0a
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild4.input
@@ -0,0 +1,5 @@
+// doc comment
+package p
+
+// +build x y
+// +build z
diff --git a/internal/backport/go/printer/testdata/gobuild5.golden b/internal/backport/go/printer/testdata/gobuild5.golden
new file mode 100644
index 0000000..2808a53
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild5.golden
@@ -0,0 +1,4 @@
+//go:build !(x || y) && z
+// +build !x,!y,z
+
+package p
diff --git a/internal/backport/go/printer/testdata/gobuild5.input b/internal/backport/go/printer/testdata/gobuild5.input
new file mode 100644
index 0000000..ec5815c
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild5.input
@@ -0,0 +1,4 @@
+//go:build !(x || y) && z
+// +build something else
+
+package p
diff --git a/internal/backport/go/printer/testdata/gobuild6.golden b/internal/backport/go/printer/testdata/gobuild6.golden
new file mode 100644
index 0000000..abb1e2a
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild6.golden
@@ -0,0 +1,5 @@
+//go:build !(x || y) && z
+
+// no +build line
+
+package p
diff --git a/internal/backport/go/printer/testdata/gobuild6.input b/internal/backport/go/printer/testdata/gobuild6.input
new file mode 100644
index 0000000..1621897
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild6.input
@@ -0,0 +1,4 @@
+//go:build !(x || y) && z
+// no +build line
+
+package p
diff --git a/internal/backport/go/printer/testdata/gobuild7.golden b/internal/backport/go/printer/testdata/gobuild7.golden
new file mode 100644
index 0000000..bf41dd4
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild7.golden
@@ -0,0 +1,11 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// TODO(rsc): Delete this file once Go 1.17 comes out and we can retire Go 1.15 support.
+
+//go:build !go1.16
+// +build !go1.16
+
+// Package buildtag defines an Analyzer that checks build tags.
+package buildtag
diff --git a/internal/backport/go/printer/testdata/gobuild7.input b/internal/backport/go/printer/testdata/gobuild7.input
new file mode 100644
index 0000000..bf41dd4
--- /dev/null
+++ b/internal/backport/go/printer/testdata/gobuild7.input
@@ -0,0 +1,11 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// TODO(rsc): Delete this file once Go 1.17 comes out and we can retire Go 1.15 support.
+
+//go:build !go1.16
+// +build !go1.16
+
+// Package buildtag defines an Analyzer that checks build tags.
+package buildtag
diff --git a/internal/backport/go/printer/testdata/linebreaks.golden b/internal/backport/go/printer/testdata/linebreaks.golden
new file mode 100644
index 0000000..17d2b5c
--- /dev/null
+++ b/internal/backport/go/printer/testdata/linebreaks.golden
@@ -0,0 +1,295 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package linebreaks
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+type writerTestEntry struct {
+	header		*Header
+	contents	string
+}
+
+type writerTest struct {
+	file	string	// filename of expected output
+	entries	[]*writerTestEntry
+}
+
+var writerTests = []*writerTest{
+	&writerTest{
+		file:	"testdata/writer.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name:		"small.txt",
+					Mode:		0640,
+					Uid:		73025,
+					Gid:		5000,
+					Size:		5,
+					Mtime:		1246508266,
+					Typeflag:	'0',
+					Uname:		"dsymonds",
+					Gname:		"eng",
+				},
+				contents:	"Kilts",
+			},
+			&writerTestEntry{
+				header: &Header{
+					Name:		"small2.txt",
+					Mode:		0640,
+					Uid:		73025,
+					Gid:		5000,
+					Size:		11,
+					Mtime:		1245217492,
+					Typeflag:	'0',
+					Uname:		"dsymonds",
+					Gname:		"eng",
+				},
+				contents:	"Google.com\n",
+			},
+		},
+	},
+	// The truncated test file was produced using these commands:
+	//   dd if=/dev/zero bs=1048576 count=16384 > /tmp/16gig.txt
+	//   tar -b 1 -c -f- /tmp/16gig.txt | dd bs=512 count=8 > writer-big.tar
+	&writerTest{
+		file:	"testdata/writer-big.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name:		"tmp/16gig.txt",
+					Mode:		0640,
+					Uid:		73025,
+					Gid:		5000,
+					Size:		16 << 30,
+					Mtime:		1254699560,
+					Typeflag:	'0',
+					Uname:		"dsymonds",
+					Gname:		"eng",
+				},
+				// no contents
+			},
+		},
+	},
+}
+
+type untarTest struct {
+	file	string
+	headers	[]*Header
+}
+
+var untarTests = []*untarTest{
+	&untarTest{
+		file:	"testdata/gnu.tar",
+		headers: []*Header{
+			&Header{
+				Name:		"small.txt",
+				Mode:		0640,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		5,
+				Mtime:		1244428340,
+				Typeflag:	'0',
+				Uname:		"dsymonds",
+				Gname:		"eng",
+			},
+			&Header{
+				Name:		"small2.txt",
+				Mode:		0640,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		11,
+				Mtime:		1244436044,
+				Typeflag:	'0',
+				Uname:		"dsymonds",
+				Gname:		"eng",
+			},
+		},
+	},
+	&untarTest{
+		file:	"testdata/star.tar",
+		headers: []*Header{
+			&Header{
+				Name:		"small.txt",
+				Mode:		0640,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		5,
+				Mtime:		1244592783,
+				Typeflag:	'0',
+				Uname:		"dsymonds",
+				Gname:		"eng",
+				Atime:		1244592783,
+				Ctime:		1244592783,
+			},
+			&Header{
+				Name:		"small2.txt",
+				Mode:		0640,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		11,
+				Mtime:		1244592783,
+				Typeflag:	'0',
+				Uname:		"dsymonds",
+				Gname:		"eng",
+				Atime:		1244592783,
+				Ctime:		1244592783,
+			},
+		},
+	},
+	&untarTest{
+		file:	"testdata/v7.tar",
+		headers: []*Header{
+			&Header{
+				Name:		"small.txt",
+				Mode:		0444,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		5,
+				Mtime:		1244593104,
+				Typeflag:	'\x00',
+			},
+			&Header{
+				Name:		"small2.txt",
+				Mode:		0444,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		11,
+				Mtime:		1244593104,
+				Typeflag:	'\x00',
+			},
+		},
+	},
+}
+
+var facts = map[int]string{
+	0:	"1",
+	1:	"1",
+	2:	"2",
+	10:	"3628800",
+	20:	"2432902008176640000",
+	100: "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000",
+}
+
+func usage() {
+	fmt.Fprintf(os.Stderr,
+		// TODO(gri): the 2nd string of this string list should not be indented
+		"usage: godoc package [name ...]\n"+
+			"	godoc -http=:6060\n")
+	flag.PrintDefaults()
+	os.Exit(2)
+}
+
+func TestReader(t *testing.T) {
+testLoop:
+	for i, test := range untarTests {
+		f, err := os.Open(test.file, os.O_RDONLY, 0444)
+		if err != nil {
+			t.Errorf("test %d: Unexpected error: %v", i, err)
+			continue
+		}
+		tr := NewReader(f)
+		for j, header := range test.headers {
+			hdr, err := tr.Next()
+			if err != nil || hdr == nil {
+				t.Errorf("test %d, entry %d: Didn't get entry: %v", i, j, err)
+				f.Close()
+				continue testLoop
+			}
+			if !reflect.DeepEqual(hdr, header) {
+				t.Errorf("test %d, entry %d: Incorrect header:\nhave %+v\nwant %+v",
+					i, j, *hdr, *header)
+			}
+		}
+		hdr, err := tr.Next()
+		if hdr != nil || err != nil {
+			t.Errorf("test %d: Unexpected entry or error: hdr=%v err=%v", i, err)
+		}
+		f.Close()
+	}
+}
+
+// Respect line breaks in function calls.
+func _() {
+	f(x)
+	f(x,
+		x)
+	f(x,
+		x,
+	)
+	f(
+		x,
+		x)
+	f(
+		x,
+		x,
+	)
+}
+
+// Respect line breaks in function declarations.
+func _(x T)	{}
+func _(x T,
+	y T) {
+}
+func _(x T,
+	y T,
+) {
+}
+func _(
+	x T,
+	y T) {
+}
+func _(
+	x T,
+	y T,
+) {
+}
+
+// Example from issue #2597.
+func ManageStatus0(
+	in <-chan *Status,
+	req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int) {
+}
+
+func ManageStatus1(
+	in <-chan *Status,
+	req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int,
+) {
+}
+
+// Example from issue #9064.
+func (y *y) xerrors() error {
+	_ = "xerror.test"	//TODO-
+	_ = []byte(`
+foo bar foo bar foo bar
+`) //TODO-
+}
+
+func _() {
+	_ = "abc"		// foo
+	_ = `abc_0123456789_`	// foo
+}
+
+func _() {
+	_ = "abc"	// foo
+	_ = `abc
+0123456789
+` // foo
+}
+
+// There should be exactly one linebreak after this comment.
diff --git a/internal/backport/go/printer/testdata/linebreaks.input b/internal/backport/go/printer/testdata/linebreaks.input
new file mode 100644
index 0000000..9e714f3
--- /dev/null
+++ b/internal/backport/go/printer/testdata/linebreaks.input
@@ -0,0 +1,291 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package linebreaks
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+type writerTestEntry struct {
+	header *Header
+	contents string
+}
+
+type writerTest struct {
+	file string  // filename of expected output
+	entries []*writerTestEntry
+}
+
+var writerTests = []*writerTest{
+	&writerTest{
+		file: "testdata/writer.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name: "small.txt",
+					Mode: 0640,
+					Uid: 73025,
+					Gid: 5000,
+					Size: 5,
+					Mtime: 1246508266,
+					Typeflag: '0',
+					Uname: "dsymonds",
+					Gname: "eng",
+				},
+				contents: "Kilts",
+			},
+			&writerTestEntry{
+				header: &Header{
+					Name: "small2.txt",
+					Mode: 0640,
+					Uid: 73025,
+					Gid: 5000,
+					Size: 11,
+					Mtime: 1245217492,
+					Typeflag: '0',
+					Uname: "dsymonds",
+					Gname: "eng",
+				},
+				contents: "Google.com\n",
+			},
+		},
+	},
+	// The truncated test file was produced using these commands:
+	//   dd if=/dev/zero bs=1048576 count=16384 > /tmp/16gig.txt
+	//   tar -b 1 -c -f- /tmp/16gig.txt | dd bs=512 count=8 > writer-big.tar
+	&writerTest{
+		file: "testdata/writer-big.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name: "tmp/16gig.txt",
+					Mode: 0640,
+					Uid: 73025,
+					Gid: 5000,
+					Size: 16 << 30,
+					Mtime: 1254699560,
+					Typeflag: '0',
+					Uname: "dsymonds",
+					Gname: "eng",
+				},
+				// no contents
+			},
+		},
+	},
+}
+
+type untarTest struct {
+	file string
+	headers []*Header
+}
+
+var untarTests = []*untarTest{
+	&untarTest{
+		file: "testdata/gnu.tar",
+		headers: []*Header{
+			&Header{
+				Name: "small.txt",
+				Mode: 0640,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 5,
+				Mtime: 1244428340,
+				Typeflag: '0',
+				Uname: "dsymonds",
+				Gname: "eng",
+			},
+			&Header{
+				Name: "small2.txt",
+				Mode: 0640,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 11,
+				Mtime: 1244436044,
+				Typeflag: '0',
+				Uname: "dsymonds",
+				Gname: "eng",
+			},
+		},
+	},
+	&untarTest{
+		file: "testdata/star.tar",
+		headers: []*Header{
+			&Header{
+				Name: "small.txt",
+				Mode: 0640,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 5,
+				Mtime: 1244592783,
+				Typeflag: '0',
+				Uname: "dsymonds",
+				Gname: "eng",
+				Atime: 1244592783,
+				Ctime: 1244592783,
+			},
+			&Header{
+				Name: "small2.txt",
+				Mode: 0640,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 11,
+				Mtime: 1244592783,
+				Typeflag: '0',
+				Uname: "dsymonds",
+				Gname: "eng",
+				Atime: 1244592783,
+				Ctime: 1244592783,
+			},
+		},
+	},
+	&untarTest{
+		file: "testdata/v7.tar",
+		headers: []*Header{
+			&Header{
+				Name: "small.txt",
+				Mode: 0444,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 5,
+				Mtime: 1244593104,
+				Typeflag: '\x00',
+			},
+			&Header{
+				Name: "small2.txt",
+				Mode: 0444,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 11,
+				Mtime: 1244593104,
+				Typeflag: '\x00',
+			},
+		},
+	},
+}
+
+var facts = map[int] string {
+	0: "1",
+	1: "1",
+	2: "2",
+	10: "3628800",
+	20: "2432902008176640000",
+	100: "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000",
+}
+
+func usage() {
+	fmt.Fprintf(os.Stderr,
+		// TODO(gri): the 2nd string of this string list should not be indented
+		"usage: godoc package [name ...]\n" +
+		"	godoc -http=:6060\n")
+	flag.PrintDefaults()
+	os.Exit(2)
+}
+
+func TestReader(t *testing.T) {
+testLoop:
+	for i, test := range untarTests {
+		f, err := os.Open(test.file, os.O_RDONLY, 0444)
+		if err != nil {
+			t.Errorf("test %d: Unexpected error: %v", i, err)
+			continue
+		}
+		tr := NewReader(f)
+		for j, header := range test.headers {
+			hdr, err := tr.Next()
+			if err != nil || hdr == nil {
+				t.Errorf("test %d, entry %d: Didn't get entry: %v", i, j, err)
+				f.Close()
+				continue testLoop
+			}
+			if !reflect.DeepEqual(hdr, header) {
+				t.Errorf("test %d, entry %d: Incorrect header:\nhave %+v\nwant %+v",
+					 i, j, *hdr, *header)
+			}
+		}
+		hdr, err := tr.Next()
+		if hdr != nil || err != nil {
+			t.Errorf("test %d: Unexpected entry or error: hdr=%v err=%v", i, err)
+		}
+		f.Close()
+	}
+}
+
+// Respect line breaks in function calls.
+func _() {
+	f(x)
+	f(x,
+	  x)
+	f(x,
+	  x,
+	)
+	f(
+	  x,
+	  x)
+	f(
+	  x,
+	  x,
+	)
+}
+
+// Respect line breaks in function declarations.
+func _(x T) {}
+func _(x T,
+       y T) {}
+func _(x T,
+       y T,
+) {}
+func _(
+       x T,
+       y T) {}
+func _(
+       x T,
+       y T,
+) {}
+
+// Example from issue #2597.
+func ManageStatus0(
+	in <-chan *Status,
+	req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int) {
+}
+    
+func ManageStatus1(
+	in <-chan *Status,
+	req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int,
+) {
+}
+
+// Example from issue #9064.
+func (y *y) xerrors() error {
+	_ = "xerror.test" //TODO-
+	_ = []byte(`
+foo bar foo bar foo bar
+`) //TODO-
+}
+
+func _() {
+	_ = "abc" // foo
+	_ = `abc_0123456789_` // foo
+}
+
+func _() {
+	_ = "abc" // foo
+	_ = `abc
+0123456789
+` // foo
+}
+
+// There should be exactly one linebreak after this comment.
diff --git a/internal/backport/go/printer/testdata/parser.go b/internal/backport/go/printer/testdata/parser.go
new file mode 100644
index 0000000..762f8ff
--- /dev/null
+++ b/internal/backport/go/printer/testdata/parser.go
@@ -0,0 +1,2149 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package parser implements a parser for Go source files. Input may be
+// provided in a variety of forms (see the various Parse* functions); the
+// output is an abstract syntax tree (AST) representing the Go source. The
+// parser is invoked through one of the Parse* functions.
+
+package parser
+
+import (
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/scanner"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+// The mode parameter to the Parse* functions is a set of flags (or 0).
+// They control the amount of source code parsed and other optional
+// parser functionality.
+const (
+	PackageClauseOnly uint = 1 << iota // parsing stops after package clause
+	ImportsOnly                        // parsing stops after import declarations
+	ParseComments                      // parse comments and add them to AST
+	Trace                              // print a trace of parsed productions
+	DeclarationErrors                  // report declaration errors
+)
+
+// The parser structure holds the parser's internal state.
+type parser struct {
+	file *token.File
+	scanner.ErrorVector
+	scanner scanner.Scanner
+
+	// Tracing/debugging
+	mode   uint // parsing mode
+	trace  bool // == (mode & Trace != 0)
+	indent uint // indentation used for tracing output
+
+	// Comments
+	comments    []*ast.CommentGroup
+	leadComment *ast.CommentGroup // last lead comment
+	lineComment *ast.CommentGroup // last line comment
+
+	// Next token
+	pos token.Pos   // token position
+	tok token.Token // one token look-ahead
+	lit string      // token literal
+
+	// Non-syntactic parser control
+	exprLev int // < 0: in control clause, >= 0: in expression
+
+	// Ordinary identifier scopes
+	pkgScope   *ast.Scope        // pkgScope.Outer == nil
+	topScope   *ast.Scope        // top-most scope; may be pkgScope
+	unresolved []*ast.Ident      // unresolved identifiers
+	imports    []*ast.ImportSpec // list of imports
+
+	// Label scope
+	// (maintained by open/close LabelScope)
+	labelScope  *ast.Scope     // label scope for current function
+	targetStack [][]*ast.Ident // stack of unresolved labels
+}
+
+// scannerMode returns the scanner mode bits given the parser's mode bits.
+func scannerMode(mode uint) uint {
+	var m uint = scanner.InsertSemis
+	if mode&ParseComments != 0 {
+		m |= scanner.ScanComments
+	}
+	return m
+}
+
+func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode uint) {
+	p.file = fset.AddFile(filename, fset.Base(), len(src))
+	p.scanner.Init(p.file, src, p, scannerMode(mode))
+
+	p.mode = mode
+	p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)
+
+	p.next()
+
+	// set up the pkgScope here (as opposed to in parseFile) because
+	// there are other parser entry points (ParseExpr, etc.)
+	p.openScope()
+	p.pkgScope = p.topScope
+
+	// for the same reason, set up a label scope
+	p.openLabelScope()
+}
+
+// ----------------------------------------------------------------------------
+// Scoping support
+
+func (p *parser) openScope() {
+	p.topScope = ast.NewScope(p.topScope)
+}
+
+func (p *parser) closeScope() {
+	p.topScope = p.topScope.Outer
+}
+
+func (p *parser) openLabelScope() {
+	p.labelScope = ast.NewScope(p.labelScope)
+	p.targetStack = append(p.targetStack, nil)
+}
+
+func (p *parser) closeLabelScope() {
+	// resolve labels
+	n := len(p.targetStack) - 1
+	scope := p.labelScope
+	for _, ident := range p.targetStack[n] {
+		ident.Obj = scope.Lookup(ident.Name)
+		if ident.Obj == nil && p.mode&DeclarationErrors != 0 {
+			p.error(ident.Pos(), fmt.Sprintf("label %s undefined", ident.Name))
+		}
+	}
+	// pop label scope
+	p.targetStack = p.targetStack[0:n]
+	p.labelScope = p.labelScope.Outer
+}
+
+func (p *parser) declare(decl interface{}, scope *ast.Scope, kind ast.ObjKind, idents ...*ast.Ident) {
+	for _, ident := range idents {
+		assert(ident.Obj == nil, "identifier already declared or resolved")
+		if ident.Name != "_" {
+			obj := ast.NewObj(kind, ident.Name)
+			// remember the corresponding declaration for redeclaration
+			// errors and global variable resolution/typechecking phase
+			obj.Decl = decl
+			if alt := scope.Insert(obj); alt != nil && p.mode&DeclarationErrors != 0 {
+				prevDecl := ""
+				if pos := alt.Pos(); pos.IsValid() {
+					prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", p.file.Position(pos))
+				}
+				p.error(ident.Pos(), fmt.Sprintf("%s redeclared in this block%s", ident.Name, prevDecl))
+			}
+			ident.Obj = obj
+		}
+	}
+}
+
+func (p *parser) shortVarDecl(idents []*ast.Ident) {
+	// Go spec: A short variable declaration may redeclare variables
+	// provided they were originally declared in the same block with
+	// the same type, and at least one of the non-blank variables is new.
+	n := 0 // number of new variables
+	for _, ident := range idents {
+		assert(ident.Obj == nil, "identifier already declared or resolved")
+		if ident.Name != "_" {
+			obj := ast.NewObj(ast.Var, ident.Name)
+			// short var declarations cannot have redeclaration errors
+			// and are not global => no need to remember the respective
+			// declaration
+			alt := p.topScope.Insert(obj)
+			if alt == nil {
+				n++ // new declaration
+				alt = obj
+			}
+			ident.Obj = alt
+		}
+	}
+	if n == 0 && p.mode&DeclarationErrors != 0 {
+		p.error(idents[0].Pos(), "no new variables on left side of :=")
+	}
+}
+
+// The unresolved object is a sentinel to mark identifiers that have been added
+// to the list of unresolved identifiers. The sentinel is only used for verifying
+// internal consistency.
+var unresolved = new(ast.Object)
+
+func (p *parser) resolve(x ast.Expr) {
+	// nothing to do if x is not an identifier or the blank identifier
+	ident, _ := x.(*ast.Ident)
+	if ident == nil {
+		return
+	}
+	assert(ident.Obj == nil, "identifier already declared or resolved")
+	if ident.Name == "_" {
+		return
+	}
+	// try to resolve the identifier
+	for s := p.topScope; s != nil; s = s.Outer {
+		if obj := s.Lookup(ident.Name); obj != nil {
+			ident.Obj = obj
+			return
+		}
+	}
+	// all local scopes are known, so any unresolved identifier
+	// must be found either in the file scope, package scope
+	// (perhaps in another file), or universe scope --- collect
+	// them so that they can be resolved later
+	ident.Obj = unresolved
+	p.unresolved = append(p.unresolved, ident)
+}
+
+// ----------------------------------------------------------------------------
+// Parsing support
+
+func (p *parser) printTrace(a ...interface{}) {
+	const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . " +
+		". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
+	const n = uint(len(dots))
+	pos := p.file.Position(p.pos)
+	fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
+	i := 2 * p.indent
+	for ; i > n; i -= n {
+		fmt.Print(dots)
+	}
+	fmt.Print(dots[0:i])
+	fmt.Println(a...)
+}
+
+func trace(p *parser, msg string) *parser {
+	p.printTrace(msg, "(")
+	p.indent++
+	return p
+}
+
+// Usage pattern: defer un(trace(p, "..."));
+func un(p *parser) {
+	p.indent--
+	p.printTrace(")")
+}
+
+// Advance to the next token.
+func (p *parser) next0() {
+	// Because of one-token look-ahead, print the previous token
+	// when tracing as it provides a more readable output. The
+	// very first token (!p.pos.IsValid()) is not initialized
+	// (it is token.ILLEGAL), so don't print it.
+	if p.trace && p.pos.IsValid() {
+		s := p.tok.String()
+		switch {
+		case p.tok.IsLiteral():
+			p.printTrace(s, p.lit)
+		case p.tok.IsOperator(), p.tok.IsKeyword():
+			p.printTrace("\"" + s + "\"")
+		default:
+			p.printTrace(s)
+		}
+	}
+
+	p.pos, p.tok, p.lit = p.scanner.Scan()
+}
+
+// Consume a comment and return it and the line on which it ends.
+func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
+	// /*-style comments may end on a different line than where they start.
+	// Scan the comment for '\n' chars and adjust endline accordingly.
+	endline = p.file.Line(p.pos)
+	if p.lit[1] == '*' {
+		// don't use range here - no need to decode Unicode code points
+		for i := 0; i < len(p.lit); i++ {
+			if p.lit[i] == '\n' {
+				endline++
+			}
+		}
+	}
+
+	comment = &ast.Comment{p.pos, p.lit}
+	p.next0()
+
+	return
+}
+
+// Consume a group of adjacent comments, add it to the parser's
+// comments list, and return it together with the line at which
+// the last comment in the group ends. An empty line or non-comment
+// token terminates a comment group.
+func (p *parser) consumeCommentGroup() (comments *ast.CommentGroup, endline int) {
+	var list []*ast.Comment
+	endline = p.file.Line(p.pos)
+	for p.tok == token.COMMENT && endline+1 >= p.file.Line(p.pos) {
+		var comment *ast.Comment
+		comment, endline = p.consumeComment()
+		list = append(list, comment)
+	}
+
+	// add comment group to the comments list
+	comments = &ast.CommentGroup{list}
+	p.comments = append(p.comments, comments)
+
+	return
+}
+
+// Advance to the next non-comment token. In the process, collect
+// any comment groups encountered, and remember the last lead and
+// line comments.
+//
+// A lead comment is a comment group that starts and ends in a
+// line without any other tokens and that is followed by a non-comment
+// token on the line immediately after the comment group.
+//
+// A line comment is a comment group that follows a non-comment
+// token on the same line, and that has no tokens after it on the line
+// where it ends.
+//
+// Lead and line comments may be considered documentation that is
+// stored in the AST.
+func (p *parser) next() {
+	p.leadComment = nil
+	p.lineComment = nil
+	line := p.file.Line(p.pos) // current line
+	p.next0()
+
+	if p.tok == token.COMMENT {
+		var comment *ast.CommentGroup
+		var endline int
+
+		if p.file.Line(p.pos) == line {
+			// The comment is on same line as the previous token; it
+			// cannot be a lead comment but may be a line comment.
+			comment, endline = p.consumeCommentGroup()
+			if p.file.Line(p.pos) != endline {
+				// The next token is on a different line, thus
+				// the last comment group is a line comment.
+				p.lineComment = comment
+			}
+		}
+
+		// consume successor comments, if any
+		endline = -1
+		for p.tok == token.COMMENT {
+			comment, endline = p.consumeCommentGroup()
+		}
+
+		if endline+1 == p.file.Line(p.pos) {
+			// The next token is following on the line immediately after the
+			// comment group, thus the last comment group is a lead comment.
+			p.leadComment = comment
+		}
+	}
+}
+
+func (p *parser) error(pos token.Pos, msg string) {
+	p.Error(p.file.Position(pos), msg)
+}
+
+func (p *parser) errorExpected(pos token.Pos, msg string) {
+	msg = "expected " + msg
+	if pos == p.pos {
+		// the error happened at the current position;
+		// make the error message more specific
+		if p.tok == token.SEMICOLON && p.lit[0] == '\n' {
+			msg += ", found newline"
+		} else {
+			msg += ", found '" + p.tok.String() + "'"
+			if p.tok.IsLiteral() {
+				msg += " " + p.lit
+			}
+		}
+	}
+	p.error(pos, msg)
+}
+
+func (p *parser) expect(tok token.Token) token.Pos {
+	pos := p.pos
+	if p.tok != tok {
+		p.errorExpected(pos, "'"+tok.String()+"'")
+	}
+	p.next() // make progress
+	return pos
+}
+
+func (p *parser) expectSemi() {
+	if p.tok != token.RPAREN && p.tok != token.RBRACE {
+		p.expect(token.SEMICOLON)
+	}
+}
+
+func assert(cond bool, msg string) {
+	if !cond {
+		panic("go/parser internal error: " + msg)
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Identifiers
+
+func (p *parser) parseIdent() *ast.Ident {
+	pos := p.pos
+	name := "_"
+	if p.tok == token.IDENT {
+		name = p.lit
+		p.next()
+	} else {
+		p.expect(token.IDENT) // use expect() error handling
+	}
+	return &ast.Ident{pos, name, nil}
+}
+
+func (p *parser) parseIdentList() (list []*ast.Ident) {
+	if p.trace {
+		defer un(trace(p, "IdentList"))
+	}
+
+	list = append(list, p.parseIdent())
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseIdent())
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Common productions
+
+// If lhs is set, result list elements which are identifiers are not resolved.
+func (p *parser) parseExprList(lhs bool) (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ExpressionList"))
+	}
+
+	list = append(list, p.parseExpr(lhs))
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseExpr(lhs))
+	}
+
+	return
+}
+
+func (p *parser) parseLhsList() []ast.Expr {
+	list := p.parseExprList(true)
+	switch p.tok {
+	case token.DEFINE:
+		// lhs of a short variable declaration
+		p.shortVarDecl(p.makeIdentList(list))
+	case token.COLON:
+		// lhs of a label declaration or a communication clause of a select
+		// statement (parseLhsList is not called when parsing the case clause
+		// of a switch statement):
+		// - labels are declared by the caller of parseLhsList
+		// - for communication clauses, if there is a stand-alone identifier
+		//   followed by a colon, we have a syntax error; there is no need
+		//   to resolve the identifier in that case
+	default:
+		// identifiers must be declared elsewhere
+		for _, x := range list {
+			p.resolve(x)
+		}
+	}
+	return list
+}
+
+func (p *parser) parseRhsList() []ast.Expr {
+	return p.parseExprList(false)
+}
+
+// ----------------------------------------------------------------------------
+// Types
+
+func (p *parser) parseType() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Type"))
+	}
+
+	typ := p.tryType()
+
+	if typ == nil {
+		pos := p.pos
+		p.errorExpected(pos, "type")
+		p.next() // make progress
+		return &ast.BadExpr{pos, p.pos}
+	}
+
+	return typ
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) parseTypeName() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "TypeName"))
+	}
+
+	ident := p.parseIdent()
+	// don't resolve ident yet - it may be a parameter or field name
+
+	if p.tok == token.PERIOD {
+		// ident is a package name
+		p.next()
+		p.resolve(ident)
+		sel := p.parseIdent()
+		return &ast.SelectorExpr{ident, sel}
+	}
+
+	return ident
+}
+
+func (p *parser) parseArrayType(ellipsisOk bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "ArrayType"))
+	}
+
+	lbrack := p.expect(token.LBRACK)
+	var len ast.Expr
+	if ellipsisOk && p.tok == token.ELLIPSIS {
+		len = &ast.Ellipsis{p.pos, nil}
+		p.next()
+	} else if p.tok != token.RBRACK {
+		len = p.parseRhs()
+	}
+	p.expect(token.RBRACK)
+	elt := p.parseType()
+
+	return &ast.ArrayType{lbrack, len, elt}
+}
+
+func (p *parser) makeIdentList(list []ast.Expr) []*ast.Ident {
+	idents := make([]*ast.Ident, len(list))
+	for i, x := range list {
+		ident, isIdent := x.(*ast.Ident)
+		if !isIdent {
+			pos := x.(ast.Expr).Pos()
+			p.errorExpected(pos, "identifier")
+			ident = &ast.Ident{pos, "_", nil}
+		}
+		idents[i] = ident
+	}
+	return idents
+}
+
+func (p *parser) parseFieldDecl(scope *ast.Scope) *ast.Field {
+	if p.trace {
+		defer un(trace(p, "FieldDecl"))
+	}
+
+	doc := p.leadComment
+
+	// fields
+	list, typ := p.parseVarList(false)
+
+	// optional tag
+	var tag *ast.BasicLit
+	if p.tok == token.STRING {
+		tag = &ast.BasicLit{p.pos, p.tok, p.lit}
+		p.next()
+	}
+
+	// analyze case
+	var idents []*ast.Ident
+	if typ != nil {
+		// IdentifierList Type
+		idents = p.makeIdentList(list)
+	} else {
+		// ["*"] TypeName (AnonymousField)
+		typ = list[0] // we always have at least one element
+		p.resolve(typ)
+		if n := len(list); n > 1 || !isTypeName(deref(typ)) {
+			pos := typ.Pos()
+			p.errorExpected(pos, "anonymous field")
+			typ = &ast.BadExpr{pos, list[n-1].End()}
+		}
+	}
+
+	p.expectSemi() // call before accessing p.linecomment
+
+	field := &ast.Field{doc, idents, typ, tag, p.lineComment}
+	p.declare(field, scope, ast.Var, idents...)
+
+	return field
+}
+
+func (p *parser) parseStructType() *ast.StructType {
+	if p.trace {
+		defer un(trace(p, "StructType"))
+	}
+
+	pos := p.expect(token.STRUCT)
+	lbrace := p.expect(token.LBRACE)
+	scope := ast.NewScope(nil) // struct scope
+	var list []*ast.Field
+	for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
+		// a field declaration cannot start with a '(' but we accept
+		// it here for more robust parsing and better error messages
+		// (parseFieldDecl will check and complain if necessary)
+		list = append(list, p.parseFieldDecl(scope))
+	}
+	rbrace := p.expect(token.RBRACE)
+
+	// TODO(gri): store struct scope in AST
+	return &ast.StructType{pos, &ast.FieldList{lbrace, list, rbrace}, false}
+}
+
+func (p *parser) parsePointerType() *ast.StarExpr {
+	if p.trace {
+		defer un(trace(p, "PointerType"))
+	}
+
+	star := p.expect(token.MUL)
+	base := p.parseType()
+
+	return &ast.StarExpr{star, base}
+}
+
+func (p *parser) tryVarType(isParam bool) ast.Expr {
+	if isParam && p.tok == token.ELLIPSIS {
+		pos := p.pos
+		p.next()
+		typ := p.tryIdentOrType(isParam) // don't use parseType so we can provide better error message
+		if typ == nil {
+			p.error(pos, "'...' parameter is missing type")
+			typ = &ast.BadExpr{pos, p.pos}
+		}
+		if p.tok != token.RPAREN {
+			p.error(pos, "can use '...' with last parameter type only")
+		}
+		return &ast.Ellipsis{pos, typ}
+	}
+	return p.tryIdentOrType(false)
+}
+
+func (p *parser) parseVarType(isParam bool) ast.Expr {
+	typ := p.tryVarType(isParam)
+	if typ == nil {
+		pos := p.pos
+		p.errorExpected(pos, "type")
+		p.next() // make progress
+		typ = &ast.BadExpr{pos, p.pos}
+	}
+	return typ
+}
+
+func (p *parser) parseVarList(isParam bool) (list []ast.Expr, typ ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "VarList"))
+	}
+
+	// a list of identifiers looks like a list of type names
+	for {
+		// parseVarType accepts any type (including parenthesized ones)
+		// even though the syntax does not permit them here: we
+		// accept them all for more robust parsing and complain
+		// afterwards
+		list = append(list, p.parseVarType(isParam))
+		if p.tok != token.COMMA {
+			break
+		}
+		p.next()
+	}
+
+	// if we had a list of identifiers, it must be followed by a type
+	typ = p.tryVarType(isParam)
+	if typ != nil {
+		p.resolve(typ)
+	}
+
+	return
+}
+
+func (p *parser) parseParameterList(scope *ast.Scope, ellipsisOk bool) (params []*ast.Field) {
+	if p.trace {
+		defer un(trace(p, "ParameterList"))
+	}
+
+	list, typ := p.parseVarList(ellipsisOk)
+	if typ != nil {
+		// IdentifierList Type
+		idents := p.makeIdentList(list)
+		field := &ast.Field{nil, idents, typ, nil, nil}
+		params = append(params, field)
+		// Go spec: The scope of an identifier denoting a function
+		// parameter or result variable is the function body.
+		p.declare(field, scope, ast.Var, idents...)
+		if p.tok == token.COMMA {
+			p.next()
+		}
+
+		for p.tok != token.RPAREN && p.tok != token.EOF {
+			idents := p.parseIdentList()
+			typ := p.parseVarType(ellipsisOk)
+			field := &ast.Field{nil, idents, typ, nil, nil}
+			params = append(params, field)
+			// Go spec: The scope of an identifier denoting a function
+			// parameter or result variable is the function body.
+			p.declare(field, scope, ast.Var, idents...)
+			if p.tok != token.COMMA {
+				break
+			}
+			p.next()
+		}
+
+	} else {
+		// Type { "," Type } (anonymous parameters)
+		params = make([]*ast.Field, len(list))
+		for i, x := range list {
+			p.resolve(x)
+			params[i] = &ast.Field{Type: x}
+		}
+	}
+
+	return
+}
+
+func (p *parser) parseParameters(scope *ast.Scope, ellipsisOk bool) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Parameters"))
+	}
+
+	var params []*ast.Field
+	lparen := p.expect(token.LPAREN)
+	if p.tok != token.RPAREN {
+		params = p.parseParameterList(scope, ellipsisOk)
+	}
+	rparen := p.expect(token.RPAREN)
+
+	return &ast.FieldList{lparen, params, rparen}
+}
+
+func (p *parser) parseResult(scope *ast.Scope) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Result"))
+	}
+
+	if p.tok == token.LPAREN {
+		return p.parseParameters(scope, false)
+	}
+
+	typ := p.tryType()
+	if typ != nil {
+		list := make([]*ast.Field, 1)
+		list[0] = &ast.Field{Type: typ}
+		return &ast.FieldList{List: list}
+	}
+
+	return nil
+}
+
+func (p *parser) parseSignature(scope *ast.Scope) (params, results *ast.FieldList) {
+	if p.trace {
+		defer un(trace(p, "Signature"))
+	}
+
+	params = p.parseParameters(scope, true)
+	results = p.parseResult(scope)
+
+	return
+}
+
+func (p *parser) parseFuncType() (*ast.FuncType, *ast.Scope) {
+	if p.trace {
+		defer un(trace(p, "FuncType"))
+	}
+
+	pos := p.expect(token.FUNC)
+	scope := ast.NewScope(p.topScope) // function scope
+	params, results := p.parseSignature(scope)
+
+	return &ast.FuncType{pos, params, results}, scope
+}
+
+func (p *parser) parseMethodSpec(scope *ast.Scope) *ast.Field {
+	if p.trace {
+		defer un(trace(p, "MethodSpec"))
+	}
+
+	doc := p.leadComment
+	var idents []*ast.Ident
+	var typ ast.Expr
+	x := p.parseTypeName()
+	if ident, isIdent := x.(*ast.Ident); isIdent && p.tok == token.LPAREN {
+		// method
+		idents = []*ast.Ident{ident}
+		scope := ast.NewScope(nil) // method scope
+		params, results := p.parseSignature(scope)
+		typ = &ast.FuncType{token.NoPos, params, results}
+	} else {
+		// embedded interface
+		typ = x
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	spec := &ast.Field{doc, idents, typ, nil, p.lineComment}
+	p.declare(spec, scope, ast.Fun, idents...)
+
+	return spec
+}
+
+func (p *parser) parseInterfaceType() *ast.InterfaceType {
+	if p.trace {
+		defer un(trace(p, "InterfaceType"))
+	}
+
+	pos := p.expect(token.INTERFACE)
+	lbrace := p.expect(token.LBRACE)
+	scope := ast.NewScope(nil) // interface scope
+	var list []*ast.Field
+	for p.tok == token.IDENT {
+		list = append(list, p.parseMethodSpec(scope))
+	}
+	rbrace := p.expect(token.RBRACE)
+
+	// TODO(gri): store interface scope in AST
+	return &ast.InterfaceType{pos, &ast.FieldList{lbrace, list, rbrace}, false}
+}
+
+func (p *parser) parseMapType() *ast.MapType {
+	if p.trace {
+		defer un(trace(p, "MapType"))
+	}
+
+	pos := p.expect(token.MAP)
+	p.expect(token.LBRACK)
+	key := p.parseType()
+	p.expect(token.RBRACK)
+	value := p.parseType()
+
+	return &ast.MapType{pos, key, value}
+}
+
+func (p *parser) parseChanType() *ast.ChanType {
+	if p.trace {
+		defer un(trace(p, "ChanType"))
+	}
+
+	pos := p.pos
+	dir := ast.SEND | ast.RECV
+	if p.tok == token.CHAN {
+		p.next()
+		if p.tok == token.ARROW {
+			p.next()
+			dir = ast.SEND
+		}
+	} else {
+		p.expect(token.ARROW)
+		p.expect(token.CHAN)
+		dir = ast.RECV
+	}
+	value := p.parseType()
+
+	return &ast.ChanType{pos, dir, value}
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) tryIdentOrType(ellipsisOk bool) ast.Expr {
+	switch p.tok {
+	case token.IDENT:
+		return p.parseTypeName()
+	case token.LBRACK:
+		return p.parseArrayType(ellipsisOk)
+	case token.STRUCT:
+		return p.parseStructType()
+	case token.MUL:
+		return p.parsePointerType()
+	case token.FUNC:
+		typ, _ := p.parseFuncType()
+		return typ
+	case token.INTERFACE:
+		return p.parseInterfaceType()
+	case token.MAP:
+		return p.parseMapType()
+	case token.CHAN, token.ARROW:
+		return p.parseChanType()
+	case token.LPAREN:
+		lparen := p.pos
+		p.next()
+		typ := p.parseType()
+		rparen := p.expect(token.RPAREN)
+		return &ast.ParenExpr{lparen, typ, rparen}
+	}
+
+	// no type found
+	return nil
+}
+
+func (p *parser) tryType() ast.Expr {
+	typ := p.tryIdentOrType(false)
+	if typ != nil {
+		p.resolve(typ)
+	}
+	return typ
+}
+
+// ----------------------------------------------------------------------------
+// Blocks
+
+func (p *parser) parseStmtList() (list []ast.Stmt) {
+	if p.trace {
+		defer un(trace(p, "StatementList"))
+	}
+
+	for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
+		list = append(list, p.parseStmt())
+	}
+
+	return
+}
+
+func (p *parser) parseBody(scope *ast.Scope) *ast.BlockStmt {
+	if p.trace {
+		defer un(trace(p, "Body"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	p.topScope = scope // open function scope
+	p.openLabelScope()
+	list := p.parseStmtList()
+	p.closeLabelScope()
+	p.closeScope()
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.BlockStmt{lbrace, list, rbrace}
+}
+
+func (p *parser) parseBlockStmt() *ast.BlockStmt {
+	if p.trace {
+		defer un(trace(p, "BlockStmt"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	p.openScope()
+	list := p.parseStmtList()
+	p.closeScope()
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.BlockStmt{lbrace, list, rbrace}
+}
+
+// ----------------------------------------------------------------------------
+// Expressions
+
+func (p *parser) parseFuncTypeOrLit() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "FuncTypeOrLit"))
+	}
+
+	typ, scope := p.parseFuncType()
+	if p.tok != token.LBRACE {
+		// function type only
+		return typ
+	}
+
+	p.exprLev++
+	body := p.parseBody(scope)
+	p.exprLev--
+
+	return &ast.FuncLit{typ, body}
+}
+
+// parseOperand may return an expression or a raw type (incl. array
+// types of the form [...]T. Callers must verify the result.
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parseOperand(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Operand"))
+	}
+
+	switch p.tok {
+	case token.IDENT:
+		x := p.parseIdent()
+		if !lhs {
+			p.resolve(x)
+		}
+		return x
+
+	case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
+		x := &ast.BasicLit{p.pos, p.tok, p.lit}
+		p.next()
+		return x
+
+	case token.LPAREN:
+		lparen := p.pos
+		p.next()
+		p.exprLev++
+		x := p.parseRhs()
+		p.exprLev--
+		rparen := p.expect(token.RPAREN)
+		return &ast.ParenExpr{lparen, x, rparen}
+
+	case token.FUNC:
+		return p.parseFuncTypeOrLit()
+
+	default:
+		if typ := p.tryIdentOrType(true); typ != nil {
+			// could be type for composite literal or conversion
+			_, isIdent := typ.(*ast.Ident)
+			assert(!isIdent, "type cannot be identifier")
+			return typ
+		}
+	}
+
+	pos := p.pos
+	p.errorExpected(pos, "operand")
+	p.next() // make progress
+	return &ast.BadExpr{pos, p.pos}
+}
+
+func (p *parser) parseSelector(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Selector"))
+	}
+
+	sel := p.parseIdent()
+
+	return &ast.SelectorExpr{x, sel}
+}
+
+func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "TypeAssertion"))
+	}
+
+	p.expect(token.LPAREN)
+	var typ ast.Expr
+	if p.tok == token.TYPE {
+		// type switch: typ == nil
+		p.next()
+	} else {
+		typ = p.parseType()
+	}
+	p.expect(token.RPAREN)
+
+	return &ast.TypeAssertExpr{x, typ}
+}
+
+func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "IndexOrSlice"))
+	}
+
+	lbrack := p.expect(token.LBRACK)
+	p.exprLev++
+	var low, high ast.Expr
+	isSlice := false
+	if p.tok != token.COLON {
+		low = p.parseRhs()
+	}
+	if p.tok == token.COLON {
+		isSlice = true
+		p.next()
+		if p.tok != token.RBRACK {
+			high = p.parseRhs()
+		}
+	}
+	p.exprLev--
+	rbrack := p.expect(token.RBRACK)
+
+	if isSlice {
+		return &ast.SliceExpr{x, lbrack, low, high, rbrack}
+	}
+	return &ast.IndexExpr{x, lbrack, low, rbrack}
+}
+
+func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
+	if p.trace {
+		defer un(trace(p, "CallOrConversion"))
+	}
+
+	lparen := p.expect(token.LPAREN)
+	p.exprLev++
+	var list []ast.Expr
+	var ellipsis token.Pos
+	for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
+		list = append(list, p.parseRhs())
+		if p.tok == token.ELLIPSIS {
+			ellipsis = p.pos
+			p.next()
+		}
+		if p.tok != token.COMMA {
+			break
+		}
+		p.next()
+	}
+	p.exprLev--
+	rparen := p.expect(token.RPAREN)
+
+	return &ast.CallExpr{fun, lparen, list, ellipsis, rparen}
+}
+
+func (p *parser) parseElement(keyOk bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Element"))
+	}
+
+	if p.tok == token.LBRACE {
+		return p.parseLiteralValue(nil)
+	}
+
+	x := p.parseExpr(keyOk) // don't resolve if map key
+	if keyOk {
+		if p.tok == token.COLON {
+			colon := p.pos
+			p.next()
+			return &ast.KeyValueExpr{x, colon, p.parseElement(false)}
+		}
+		p.resolve(x) // not a map key
+	}
+
+	return x
+}
+
+func (p *parser) parseElementList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ElementList"))
+	}
+
+	for p.tok != token.RBRACE && p.tok != token.EOF {
+		list = append(list, p.parseElement(true))
+		if p.tok != token.COMMA {
+			break
+		}
+		p.next()
+	}
+
+	return
+}
+
+func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "LiteralValue"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	var elts []ast.Expr
+	p.exprLev++
+	if p.tok != token.RBRACE {
+		elts = p.parseElementList()
+	}
+	p.exprLev--
+	rbrace := p.expect(token.RBRACE)
+	return &ast.CompositeLit{typ, lbrace, elts, rbrace}
+}
+
+// checkExpr checks that x is an expression (and not a type).
+func (p *parser) checkExpr(x ast.Expr) ast.Expr {
+	switch t := unparen(x).(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.BasicLit:
+	case *ast.FuncLit:
+	case *ast.CompositeLit:
+	case *ast.ParenExpr:
+		panic("unreachable")
+	case *ast.SelectorExpr:
+	case *ast.IndexExpr:
+	case *ast.SliceExpr:
+	case *ast.TypeAssertExpr:
+		if t.Type == nil {
+			// the form X.(type) is only allowed in type switch expressions
+			p.errorExpected(x.Pos(), "expression")
+			x = &ast.BadExpr{x.Pos(), x.End()}
+		}
+	case *ast.CallExpr:
+	case *ast.StarExpr:
+	case *ast.UnaryExpr:
+		if t.Op == token.RANGE {
+			// the range operator is only allowed at the top of a for statement
+			p.errorExpected(x.Pos(), "expression")
+			x = &ast.BadExpr{x.Pos(), x.End()}
+		}
+	case *ast.BinaryExpr:
+	default:
+		// all other nodes are not proper expressions
+		p.errorExpected(x.Pos(), "expression")
+		x = &ast.BadExpr{x.Pos(), x.End()}
+	}
+	return x
+}
+
+// isTypeName reports whether x is a (qualified) TypeName.
+func isTypeName(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.SelectorExpr:
+		_, isIdent := t.X.(*ast.Ident)
+		return isIdent
+	default:
+		return false // all other nodes are not type names
+	}
+	return true
+}
+
+// isLiteralType reports whether x is a legal composite literal type.
+func isLiteralType(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.SelectorExpr:
+		_, isIdent := t.X.(*ast.Ident)
+		return isIdent
+	case *ast.ArrayType:
+	case *ast.StructType:
+	case *ast.MapType:
+	default:
+		return false // all other nodes are not legal composite literal types
+	}
+	return true
+}
+
+// If x is of the form *T, deref returns T, otherwise it returns x.
+func deref(x ast.Expr) ast.Expr {
+	if p, isPtr := x.(*ast.StarExpr); isPtr {
+		x = p.X
+	}
+	return x
+}
+
+// If x is of the form (T), unparen returns unparen(T), otherwise it returns x.
+func unparen(x ast.Expr) ast.Expr {
+	if p, isParen := x.(*ast.ParenExpr); isParen {
+		x = unparen(p.X)
+	}
+	return x
+}
+
+// checkExprOrType checks that x is an expression or a type
+// (and not a raw type such as [...]T).
+func (p *parser) checkExprOrType(x ast.Expr) ast.Expr {
+	switch t := unparen(x).(type) {
+	case *ast.ParenExpr:
+		panic("unreachable")
+	case *ast.UnaryExpr:
+		if t.Op == token.RANGE {
+			// the range operator is only allowed at the top of a for statement
+			p.errorExpected(x.Pos(), "expression")
+			x = &ast.BadExpr{x.Pos(), x.End()}
+		}
+	case *ast.ArrayType:
+		if len, isEllipsis := t.Len.(*ast.Ellipsis); isEllipsis {
+			p.error(len.Pos(), "expected array length, found '...'")
+			x = &ast.BadExpr{x.Pos(), x.End()}
+		}
+	}
+
+	// all other nodes are expressions or types
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parsePrimaryExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "PrimaryExpr"))
+	}
+
+	x := p.parseOperand(lhs)
+L:
+	for {
+		switch p.tok {
+		case token.PERIOD:
+			p.next()
+			if lhs {
+				p.resolve(x)
+			}
+			switch p.tok {
+			case token.IDENT:
+				x = p.parseSelector(p.checkExpr(x))
+			case token.LPAREN:
+				x = p.parseTypeAssertion(p.checkExpr(x))
+			default:
+				pos := p.pos
+				p.next() // make progress
+				p.errorExpected(pos, "selector or type assertion")
+				x = &ast.BadExpr{pos, p.pos}
+			}
+		case token.LBRACK:
+			if lhs {
+				p.resolve(x)
+			}
+			x = p.parseIndexOrSlice(p.checkExpr(x))
+		case token.LPAREN:
+			if lhs {
+				p.resolve(x)
+			}
+			x = p.parseCallOrConversion(p.checkExprOrType(x))
+		case token.LBRACE:
+			if isLiteralType(x) && (p.exprLev >= 0 || !isTypeName(x)) {
+				if lhs {
+					p.resolve(x)
+				}
+				x = p.parseLiteralValue(x)
+			} else {
+				break L
+			}
+		default:
+			break L
+		}
+		lhs = false // no need to try to resolve again
+	}
+
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parseUnaryExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "UnaryExpr"))
+	}
+
+	switch p.tok {
+	case token.ADD, token.SUB, token.NOT, token.XOR, token.AND, token.RANGE:
+		pos, op := p.pos, p.tok
+		p.next()
+		x := p.parseUnaryExpr(false)
+		return &ast.UnaryExpr{pos, op, p.checkExpr(x)}
+
+	case token.ARROW:
+		// channel type or receive expression
+		pos := p.pos
+		p.next()
+		if p.tok == token.CHAN {
+			p.next()
+			value := p.parseType()
+			return &ast.ChanType{pos, ast.RECV, value}
+		}
+
+		x := p.parseUnaryExpr(false)
+		return &ast.UnaryExpr{pos, token.ARROW, p.checkExpr(x)}
+
+	case token.MUL:
+		// pointer type or unary "*" expression
+		pos := p.pos
+		p.next()
+		x := p.parseUnaryExpr(false)
+		return &ast.StarExpr{pos, p.checkExprOrType(x)}
+	}
+
+	return p.parsePrimaryExpr(lhs)
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parseBinaryExpr(lhs bool, prec1 int) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "BinaryExpr"))
+	}
+
+	x := p.parseUnaryExpr(lhs)
+	for prec := p.tok.Precedence(); prec >= prec1; prec-- {
+		for p.tok.Precedence() == prec {
+			pos, op := p.pos, p.tok
+			p.next()
+			if lhs {
+				p.resolve(x)
+				lhs = false
+			}
+			y := p.parseBinaryExpr(false, prec+1)
+			x = &ast.BinaryExpr{p.checkExpr(x), pos, op, p.checkExpr(y)}
+		}
+	}
+
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+// TODO(gri): parseExpr may return a type or even a raw type ([..]int) -
+//
+//	should reject when a type/raw type is obviously not allowed
+func (p *parser) parseExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Expression"))
+	}
+
+	return p.parseBinaryExpr(lhs, token.LowestPrec+1)
+}
+
+func (p *parser) parseRhs() ast.Expr {
+	return p.parseExpr(false)
+}
+
+// ----------------------------------------------------------------------------
+// Statements
+
+func (p *parser) parseSimpleStmt(labelOk bool) ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "SimpleStmt"))
+	}
+
+	x := p.parseLhsList()
+
+	switch p.tok {
+	case
+		token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
+		token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
+		token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,
+		token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
+		// assignment statement
+		pos, tok := p.pos, p.tok
+		p.next()
+		y := p.parseRhsList()
+		return &ast.AssignStmt{x, pos, tok, y}
+	}
+
+	if len(x) > 1 {
+		p.errorExpected(x[0].Pos(), "1 expression")
+		// continue with first expression
+	}
+
+	switch p.tok {
+	case token.COLON:
+		// labeled statement
+		colon := p.pos
+		p.next()
+		if label, isIdent := x[0].(*ast.Ident); labelOk && isIdent {
+			// Go spec: The scope of a label is the body of the function
+			// in which it is declared and excludes the body of any nested
+			// function.
+			stmt := &ast.LabeledStmt{label, colon, p.parseStmt()}
+			p.declare(stmt, p.labelScope, ast.Lbl, label)
+			return stmt
+		}
+		p.error(x[0].Pos(), "illegal label declaration")
+		return &ast.BadStmt{x[0].Pos(), colon + 1}
+
+	case token.ARROW:
+		// send statement
+		arrow := p.pos
+		p.next() // consume "<-"
+		y := p.parseRhs()
+		return &ast.SendStmt{x[0], arrow, y}
+
+	case token.INC, token.DEC:
+		// increment or decrement
+		s := &ast.IncDecStmt{x[0], p.pos, p.tok}
+		p.next() // consume "++" or "--"
+		return s
+	}
+
+	// expression
+	return &ast.ExprStmt{x[0]}
+}
+
+func (p *parser) parseCallExpr() *ast.CallExpr {
+	x := p.parseRhs()
+	if call, isCall := x.(*ast.CallExpr); isCall {
+		return call
+	}
+	p.errorExpected(x.Pos(), "function/method call")
+	return nil
+}
+
+func (p *parser) parseGoStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "GoStmt"))
+	}
+
+	pos := p.expect(token.GO)
+	call := p.parseCallExpr()
+	p.expectSemi()
+	if call == nil {
+		return &ast.BadStmt{pos, pos + 2} // len("go")
+	}
+
+	return &ast.GoStmt{pos, call}
+}
+
+func (p *parser) parseDeferStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "DeferStmt"))
+	}
+
+	pos := p.expect(token.DEFER)
+	call := p.parseCallExpr()
+	p.expectSemi()
+	if call == nil {
+		return &ast.BadStmt{pos, pos + 5} // len("defer")
+	}
+
+	return &ast.DeferStmt{pos, call}
+}
+
+func (p *parser) parseReturnStmt() *ast.ReturnStmt {
+	if p.trace {
+		defer un(trace(p, "ReturnStmt"))
+	}
+
+	pos := p.pos
+	p.expect(token.RETURN)
+	var x []ast.Expr
+	if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
+		x = p.parseRhsList()
+	}
+	p.expectSemi()
+
+	return &ast.ReturnStmt{pos, x}
+}
+
+func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
+	if p.trace {
+		defer un(trace(p, "BranchStmt"))
+	}
+
+	pos := p.expect(tok)
+	var label *ast.Ident
+	if tok != token.FALLTHROUGH && p.tok == token.IDENT {
+		label = p.parseIdent()
+		// add to list of unresolved targets
+		n := len(p.targetStack) - 1
+		p.targetStack[n] = append(p.targetStack[n], label)
+	}
+	p.expectSemi()
+
+	return &ast.BranchStmt{pos, tok, label}
+}
+
+func (p *parser) makeExpr(s ast.Stmt) ast.Expr {
+	if s == nil {
+		return nil
+	}
+	if es, isExpr := s.(*ast.ExprStmt); isExpr {
+		return p.checkExpr(es.X)
+	}
+	p.error(s.Pos(), "expected condition, found simple statement")
+	return &ast.BadExpr{s.Pos(), s.End()}
+}
+
+func (p *parser) parseIfStmt() *ast.IfStmt {
+	if p.trace {
+		defer un(trace(p, "IfStmt"))
+	}
+
+	pos := p.expect(token.IF)
+	p.openScope()
+	defer p.closeScope()
+
+	var s ast.Stmt
+	var x ast.Expr
+	{
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok == token.SEMICOLON {
+			p.next()
+			x = p.parseRhs()
+		} else {
+			s = p.parseSimpleStmt(false)
+			if p.tok == token.SEMICOLON {
+				p.next()
+				x = p.parseRhs()
+			} else {
+				x = p.makeExpr(s)
+				s = nil
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	body := p.parseBlockStmt()
+	var else_ ast.Stmt
+	if p.tok == token.ELSE {
+		p.next()
+		else_ = p.parseStmt()
+	} else {
+		p.expectSemi()
+	}
+
+	return &ast.IfStmt{pos, s, x, body, else_}
+}
+
+func (p *parser) parseTypeList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "TypeList"))
+	}
+
+	list = append(list, p.parseType())
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseType())
+	}
+
+	return
+}
+
+func (p *parser) parseCaseClause(exprSwitch bool) *ast.CaseClause {
+	if p.trace {
+		defer un(trace(p, "CaseClause"))
+	}
+
+	pos := p.pos
+	var list []ast.Expr
+	if p.tok == token.CASE {
+		p.next()
+		if exprSwitch {
+			list = p.parseRhsList()
+		} else {
+			list = p.parseTypeList()
+		}
+	} else {
+		p.expect(token.DEFAULT)
+	}
+
+	colon := p.expect(token.COLON)
+	p.openScope()
+	body := p.parseStmtList()
+	p.closeScope()
+
+	return &ast.CaseClause{pos, list, colon, body}
+}
+
+func isExprSwitch(s ast.Stmt) bool {
+	if s == nil {
+		return true
+	}
+	if e, ok := s.(*ast.ExprStmt); ok {
+		if a, ok := e.X.(*ast.TypeAssertExpr); ok {
+			return a.Type != nil // regular type assertion
+		}
+		return true
+	}
+	return false
+}
+
+func (p *parser) parseSwitchStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "SwitchStmt"))
+	}
+
+	pos := p.expect(token.SWITCH)
+	p.openScope()
+	defer p.closeScope()
+
+	var s1, s2 ast.Stmt
+	if p.tok != token.LBRACE {
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok != token.SEMICOLON {
+			s2 = p.parseSimpleStmt(false)
+		}
+		if p.tok == token.SEMICOLON {
+			p.next()
+			s1 = s2
+			s2 = nil
+			if p.tok != token.LBRACE {
+				s2 = p.parseSimpleStmt(false)
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	exprSwitch := isExprSwitch(s2)
+	lbrace := p.expect(token.LBRACE)
+	var list []ast.Stmt
+	for p.tok == token.CASE || p.tok == token.DEFAULT {
+		list = append(list, p.parseCaseClause(exprSwitch))
+	}
+	rbrace := p.expect(token.RBRACE)
+	p.expectSemi()
+	body := &ast.BlockStmt{lbrace, list, rbrace}
+
+	if exprSwitch {
+		return &ast.SwitchStmt{pos, s1, p.makeExpr(s2), body}
+	}
+	// type switch
+	// TODO(gri): do all the checks!
+	return &ast.TypeSwitchStmt{pos, s1, s2, body}
+}
+
+func (p *parser) parseCommClause() *ast.CommClause {
+	if p.trace {
+		defer un(trace(p, "CommClause"))
+	}
+
+	p.openScope()
+	pos := p.pos
+	var comm ast.Stmt
+	if p.tok == token.CASE {
+		p.next()
+		lhs := p.parseLhsList()
+		if p.tok == token.ARROW {
+			// SendStmt
+			if len(lhs) > 1 {
+				p.errorExpected(lhs[0].Pos(), "1 expression")
+				// continue with first expression
+			}
+			arrow := p.pos
+			p.next()
+			rhs := p.parseRhs()
+			comm = &ast.SendStmt{lhs[0], arrow, rhs}
+		} else {
+			// RecvStmt
+			pos := p.pos
+			tok := p.tok
+			var rhs ast.Expr
+			if tok == token.ASSIGN || tok == token.DEFINE {
+				// RecvStmt with assignment
+				if len(lhs) > 2 {
+					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
+					// continue with first two expressions
+					lhs = lhs[0:2]
+				}
+				p.next()
+				rhs = p.parseRhs()
+			} else {
+				// rhs must be single receive operation
+				if len(lhs) > 1 {
+					p.errorExpected(lhs[0].Pos(), "1 expression")
+					// continue with first expression
+				}
+				rhs = lhs[0]
+				lhs = nil // there is no lhs
+			}
+			if x, isUnary := rhs.(*ast.UnaryExpr); !isUnary || x.Op != token.ARROW {
+				p.errorExpected(rhs.Pos(), "send or receive operation")
+				rhs = &ast.BadExpr{rhs.Pos(), rhs.End()}
+			}
+			if lhs != nil {
+				comm = &ast.AssignStmt{lhs, pos, tok, []ast.Expr{rhs}}
+			} else {
+				comm = &ast.ExprStmt{rhs}
+			}
+		}
+	} else {
+		p.expect(token.DEFAULT)
+	}
+
+	colon := p.expect(token.COLON)
+	body := p.parseStmtList()
+	p.closeScope()
+
+	return &ast.CommClause{pos, comm, colon, body}
+}
+
+func (p *parser) parseSelectStmt() *ast.SelectStmt {
+	if p.trace {
+		defer un(trace(p, "SelectStmt"))
+	}
+
+	pos := p.expect(token.SELECT)
+	lbrace := p.expect(token.LBRACE)
+	var list []ast.Stmt
+	for p.tok == token.CASE || p.tok == token.DEFAULT {
+		list = append(list, p.parseCommClause())
+	}
+	rbrace := p.expect(token.RBRACE)
+	p.expectSemi()
+	body := &ast.BlockStmt{lbrace, list, rbrace}
+
+	return &ast.SelectStmt{pos, body}
+}
+
+func (p *parser) parseForStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "ForStmt"))
+	}
+
+	pos := p.expect(token.FOR)
+	p.openScope()
+	defer p.closeScope()
+
+	var s1, s2, s3 ast.Stmt
+	if p.tok != token.LBRACE {
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok != token.SEMICOLON {
+			s2 = p.parseSimpleStmt(false)
+		}
+		if p.tok == token.SEMICOLON {
+			p.next()
+			s1 = s2
+			s2 = nil
+			if p.tok != token.SEMICOLON {
+				s2 = p.parseSimpleStmt(false)
+			}
+			p.expectSemi()
+			if p.tok != token.LBRACE {
+				s3 = p.parseSimpleStmt(false)
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	body := p.parseBlockStmt()
+	p.expectSemi()
+
+	if as, isAssign := s2.(*ast.AssignStmt); isAssign {
+		// possibly a for statement with a range clause; check assignment operator
+		if as.Tok != token.ASSIGN && as.Tok != token.DEFINE {
+			p.errorExpected(as.TokPos, "'=' or ':='")
+			return &ast.BadStmt{pos, body.End()}
+		}
+		// check lhs
+		var key, value ast.Expr
+		switch len(as.Lhs) {
+		case 2:
+			key, value = as.Lhs[0], as.Lhs[1]
+		case 1:
+			key = as.Lhs[0]
+		default:
+			p.errorExpected(as.Lhs[0].Pos(), "1 or 2 expressions")
+			return &ast.BadStmt{pos, body.End()}
+		}
+		// check rhs
+		if len(as.Rhs) != 1 {
+			p.errorExpected(as.Rhs[0].Pos(), "1 expression")
+			return &ast.BadStmt{pos, body.End()}
+		}
+		if rhs, isUnary := as.Rhs[0].(*ast.UnaryExpr); isUnary && rhs.Op == token.RANGE {
+			// rhs is range expression
+			// (any short variable declaration was handled by parseSimpleStat above)
+			return &ast.RangeStmt{pos, key, value, as.TokPos, as.Tok, rhs.X, body}
+		}
+		p.errorExpected(s2.Pos(), "range clause")
+		return &ast.BadStmt{pos, body.End()}
+	}
+
+	// regular for statement
+	return &ast.ForStmt{pos, s1, p.makeExpr(s2), s3, body}
+}
+
+func (p *parser) parseStmt() (s ast.Stmt) {
+	if p.trace {
+		defer un(trace(p, "Statement"))
+	}
+
+	switch p.tok {
+	case token.CONST, token.TYPE, token.VAR:
+		s = &ast.DeclStmt{p.parseDecl()}
+	case
+		// tokens that may start a top-level expression
+		token.IDENT, token.INT, token.FLOAT, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operand
+		token.LBRACK, token.STRUCT, // composite type
+		token.MUL, token.AND, token.ARROW, token.ADD, token.SUB, token.XOR: // unary operators
+		s = p.parseSimpleStmt(true)
+		// because of the required look-ahead, labeled statements are
+		// parsed by parseSimpleStmt - don't expect a semicolon after
+		// them
+		if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
+			p.expectSemi()
+		}
+	case token.GO:
+		s = p.parseGoStmt()
+	case token.DEFER:
+		s = p.parseDeferStmt()
+	case token.RETURN:
+		s = p.parseReturnStmt()
+	case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
+		s = p.parseBranchStmt(p.tok)
+	case token.LBRACE:
+		s = p.parseBlockStmt()
+		p.expectSemi()
+	case token.IF:
+		s = p.parseIfStmt()
+	case token.SWITCH:
+		s = p.parseSwitchStmt()
+	case token.SELECT:
+		s = p.parseSelectStmt()
+	case token.FOR:
+		s = p.parseForStmt()
+	case token.SEMICOLON:
+		s = &ast.EmptyStmt{p.pos}
+		p.next()
+	case token.RBRACE:
+		// a semicolon may be omitted before a closing "}"
+		s = &ast.EmptyStmt{p.pos}
+	default:
+		// no statement found
+		pos := p.pos
+		p.errorExpected(pos, "statement")
+		p.next() // make progress
+		s = &ast.BadStmt{pos, p.pos}
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Declarations
+
+type parseSpecFunction func(p *parser, doc *ast.CommentGroup, iota int) ast.Spec
+
+func parseImportSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "ImportSpec"))
+	}
+
+	var ident *ast.Ident
+	switch p.tok {
+	case token.PERIOD:
+		ident = &ast.Ident{p.pos, ".", nil}
+		p.next()
+	case token.IDENT:
+		ident = p.parseIdent()
+	}
+
+	var path *ast.BasicLit
+	if p.tok == token.STRING {
+		path = &ast.BasicLit{p.pos, p.tok, p.lit}
+		p.next()
+	} else {
+		p.expect(token.STRING) // use expect() error handling
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// collect imports
+	spec := &ast.ImportSpec{doc, ident, path, p.lineComment}
+	p.imports = append(p.imports, spec)
+
+	return spec
+}
+
+func parseConstSpec(p *parser, doc *ast.CommentGroup, iota int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "ConstSpec"))
+	}
+
+	idents := p.parseIdentList()
+	typ := p.tryType()
+	var values []ast.Expr
+	if typ != nil || p.tok == token.ASSIGN || iota == 0 {
+		p.expect(token.ASSIGN)
+		values = p.parseRhsList()
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// Go spec: The scope of a constant or variable identifier declared inside
+	// a function begins at the end of the ConstSpec or VarSpec and ends at
+	// the end of the innermost containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.ValueSpec{doc, idents, typ, values, p.lineComment}
+	p.declare(spec, p.topScope, ast.Con, idents...)
+
+	return spec
+}
+
+func parseTypeSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "TypeSpec"))
+	}
+
+	ident := p.parseIdent()
+
+	// Go spec: The scope of a type identifier declared inside a function begins
+	// at the identifier in the TypeSpec and ends at the end of the innermost
+	// containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.TypeSpec{doc, ident, nil, nil}
+	p.declare(spec, p.topScope, ast.Typ, ident)
+
+	spec.Type = p.parseType()
+	p.expectSemi() // call before accessing p.linecomment
+	spec.Comment = p.lineComment
+
+	return spec
+}
+
+func parseVarSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "VarSpec"))
+	}
+
+	idents := p.parseIdentList()
+	typ := p.tryType()
+	var values []ast.Expr
+	if typ == nil || p.tok == token.ASSIGN {
+		p.expect(token.ASSIGN)
+		values = p.parseRhsList()
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// Go spec: The scope of a constant or variable identifier declared inside
+	// a function begins at the end of the ConstSpec or VarSpec and ends at
+	// the end of the innermost containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.ValueSpec{doc, idents, typ, values, p.lineComment}
+	p.declare(spec, p.topScope, ast.Var, idents...)
+
+	return spec
+}
+
+func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
+	if p.trace {
+		defer un(trace(p, "GenDecl("+keyword.String()+")"))
+	}
+
+	doc := p.leadComment
+	pos := p.expect(keyword)
+	var lparen, rparen token.Pos
+	var list []ast.Spec
+	if p.tok == token.LPAREN {
+		lparen = p.pos
+		p.next()
+		for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
+			list = append(list, f(p, p.leadComment, iota))
+		}
+		rparen = p.expect(token.RPAREN)
+		p.expectSemi()
+	} else {
+		list = append(list, f(p, nil, 0))
+	}
+
+	return &ast.GenDecl{doc, pos, keyword, lparen, list, rparen}
+}
+
+func (p *parser) parseReceiver(scope *ast.Scope) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Receiver"))
+	}
+
+	pos := p.pos
+	par := p.parseParameters(scope, false)
+
+	// must have exactly one receiver
+	if par.NumFields() != 1 {
+		p.errorExpected(pos, "exactly one receiver")
+		// TODO determine a better range for BadExpr below
+		par.List = []*ast.Field{{Type: &ast.BadExpr{pos, pos}}}
+		return par
+	}
+
+	// recv type must be of the form ["*"] identifier
+	recv := par.List[0]
+	base := deref(recv.Type)
+	if _, isIdent := base.(*ast.Ident); !isIdent {
+		p.errorExpected(base.Pos(), "(unqualified) identifier")
+		par.List = []*ast.Field{{Type: &ast.BadExpr{recv.Pos(), recv.End()}}}
+	}
+
+	return par
+}
+
+func (p *parser) parseFuncDecl() *ast.FuncDecl {
+	if p.trace {
+		defer un(trace(p, "FunctionDecl"))
+	}
+
+	doc := p.leadComment
+	pos := p.expect(token.FUNC)
+	scope := ast.NewScope(p.topScope) // function scope
+
+	var recv *ast.FieldList
+	if p.tok == token.LPAREN {
+		recv = p.parseReceiver(scope)
+	}
+
+	ident := p.parseIdent()
+
+	params, results := p.parseSignature(scope)
+
+	var body *ast.BlockStmt
+	if p.tok == token.LBRACE {
+		body = p.parseBody(scope)
+	}
+	p.expectSemi()
+
+	decl := &ast.FuncDecl{doc, recv, ident, &ast.FuncType{pos, params, results}, body}
+	if recv == nil {
+		// Go spec: The scope of an identifier denoting a constant, type,
+		// variable, or function (but not method) declared at top level
+		// (outside any function) is the package block.
+		//
+		// init() functions cannot be referred to and there may
+		// be more than one - don't put them in the pkgScope
+		if ident.Name != "init" {
+			p.declare(decl, p.pkgScope, ast.Fun, ident)
+		}
+	}
+
+	return decl
+}
+
+func (p *parser) parseDecl() ast.Decl {
+	if p.trace {
+		defer un(trace(p, "Declaration"))
+	}
+
+	var f parseSpecFunction
+	switch p.tok {
+	case token.CONST:
+		f = parseConstSpec
+
+	case token.TYPE:
+		f = parseTypeSpec
+
+	case token.VAR:
+		f = parseVarSpec
+
+	case token.FUNC:
+		return p.parseFuncDecl()
+
+	default:
+		pos := p.pos
+		p.errorExpected(pos, "declaration")
+		p.next() // make progress
+		decl := &ast.BadDecl{pos, p.pos}
+		return decl
+	}
+
+	return p.parseGenDecl(p.tok, f)
+}
+
+func (p *parser) parseDeclList() (list []ast.Decl) {
+	if p.trace {
+		defer un(trace(p, "DeclList"))
+	}
+
+	for p.tok != token.EOF {
+		list = append(list, p.parseDecl())
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Source files
+
+func (p *parser) parseFile() *ast.File {
+	if p.trace {
+		defer un(trace(p, "File"))
+	}
+
+	// package clause
+	doc := p.leadComment
+	pos := p.expect(token.PACKAGE)
+	// Go spec: The package clause is not a declaration;
+	// the package name does not appear in any scope.
+	ident := p.parseIdent()
+	if ident.Name == "_" {
+		p.error(p.pos, "invalid package name _")
+	}
+	p.expectSemi()
+
+	var decls []ast.Decl
+
+	// Don't bother parsing the rest if we had errors already.
+	// Likely not a Go source file at all.
+
+	if p.ErrorCount() == 0 && p.mode&PackageClauseOnly == 0 {
+		// import decls
+		for p.tok == token.IMPORT {
+			decls = append(decls, p.parseGenDecl(token.IMPORT, parseImportSpec))
+		}
+
+		if p.mode&ImportsOnly == 0 {
+			// rest of package body
+			for p.tok != token.EOF {
+				decls = append(decls, p.parseDecl())
+			}
+		}
+	}
+
+	assert(p.topScope == p.pkgScope, "imbalanced scopes")
+
+	// resolve global identifiers within the same file
+	i := 0
+	for _, ident := range p.unresolved {
+		// i <= index for current ident
+		assert(ident.Obj == unresolved, "object already resolved")
+		ident.Obj = p.pkgScope.Lookup(ident.Name) // also removes unresolved sentinel
+		if ident.Obj == nil {
+			p.unresolved[i] = ident
+			i++
+		}
+	}
+
+	// TODO(gri): store p.imports in AST
+	return &ast.File{doc, pos, ident, decls, p.pkgScope, p.imports, p.unresolved[0:i], p.comments}
+}
diff --git a/internal/backport/go/printer/testdata/slow.golden b/internal/backport/go/printer/testdata/slow.golden
new file mode 100644
index 0000000..43a15cb
--- /dev/null
+++ b/internal/backport/go/printer/testdata/slow.golden
@@ -0,0 +1,85 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package deepequal_test
+
+import (
+	"testing"
+	"google3/spam/archer/frontend/deepequal"
+)
+
+func TestTwoNilValues(t *testing.T) {
+	if err := deepequal.Check(nil, nil); err != nil {
+		t.Errorf("expected nil, saw %v", err)
+	}
+}
+
+type Foo struct {
+	bar	*Bar
+	bang	*Bar
+}
+
+type Bar struct {
+	baz	*Baz
+	foo	[]*Foo
+}
+
+type Baz struct {
+	entries		map[int]interface{}
+	whatever	string
+}
+
+func newFoo() *Foo {
+	return &Foo{bar: &Bar{baz: &Baz{
+		entries: map[int]interface{}{
+			42:	&Foo{},
+			21:	&Bar{},
+			11:	&Baz{whatever: "it's just a test"}}}},
+		bang: &Bar{foo: []*Foo{
+			&Foo{bar: &Bar{baz: &Baz{
+				entries: map[int]interface{}{
+					43:	&Foo{},
+					22:	&Bar{},
+					13:	&Baz{whatever: "this is nuts"}}}},
+				bang: &Bar{foo: []*Foo{
+					&Foo{bar: &Bar{baz: &Baz{
+						entries: map[int]interface{}{
+							61:	&Foo{},
+							71:	&Bar{},
+							11:	&Baz{whatever: "no, it's Go"}}}},
+						bang: &Bar{foo: []*Foo{
+							&Foo{bar: &Bar{baz: &Baz{
+								entries: map[int]interface{}{
+									0:	&Foo{},
+									-2:	&Bar{},
+									-11:	&Baz{whatever: "we need to go deeper"}}}},
+								bang: &Bar{foo: []*Foo{
+									&Foo{bar: &Bar{baz: &Baz{
+										entries: map[int]interface{}{
+											-2:	&Foo{},
+											-5:	&Bar{},
+											-7:	&Baz{whatever: "are you serious?"}}}},
+										bang:	&Bar{foo: []*Foo{}}},
+									&Foo{bar: &Bar{baz: &Baz{
+										entries: map[int]interface{}{
+											-100:	&Foo{},
+											50:	&Bar{},
+											20:	&Baz{whatever: "na, not really ..."}}}},
+										bang:	&Bar{foo: []*Foo{}}}}}}}}},
+					&Foo{bar: &Bar{baz: &Baz{
+						entries: map[int]interface{}{
+							2:	&Foo{},
+							1:	&Bar{},
+							-1:	&Baz{whatever: "... it's just a test."}}}},
+						bang:	&Bar{foo: []*Foo{}}}}}}}}}
+}
+
+func TestElaborate(t *testing.T) {
+	a := newFoo()
+	b := newFoo()
+
+	if err := deepequal.Check(a, b); err != nil {
+		t.Errorf("expected nil, saw %v", err)
+	}
+}
diff --git a/internal/backport/go/printer/testdata/slow.input b/internal/backport/go/printer/testdata/slow.input
new file mode 100644
index 0000000..0e5a23d
--- /dev/null
+++ b/internal/backport/go/printer/testdata/slow.input
@@ -0,0 +1,85 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package deepequal_test
+
+import (
+        "testing"
+        "google3/spam/archer/frontend/deepequal"
+)
+
+func TestTwoNilValues(t *testing.T) {
+        if err := deepequal.Check(nil, nil); err != nil {
+                t.Errorf("expected nil, saw %v", err)
+        }
+}
+
+type Foo struct {
+        bar *Bar
+        bang *Bar
+}
+
+type Bar struct {
+        baz *Baz
+        foo []*Foo
+}
+
+type Baz struct {
+        entries  map[int]interface{}
+        whatever string
+}
+
+func newFoo() (*Foo) {
+return &Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+42: &Foo{},
+21: &Bar{},
+11: &Baz{ whatever: "it's just a test" }}}},
+        bang: &Bar{foo: []*Foo{
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+43: &Foo{},
+22: &Bar{},
+13: &Baz{ whatever: "this is nuts" }}}},
+        bang: &Bar{foo: []*Foo{
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+61: &Foo{},
+71: &Bar{},
+11: &Baz{ whatever: "no, it's Go" }}}},
+        bang: &Bar{foo: []*Foo{
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+0: &Foo{},
+-2: &Bar{},
+-11: &Baz{ whatever: "we need to go deeper" }}}},
+        bang: &Bar{foo: []*Foo{
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+-2: &Foo{},
+-5: &Bar{},
+-7: &Baz{ whatever: "are you serious?" }}}},
+        bang: &Bar{foo: []*Foo{}}},
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+-100: &Foo{},
+50: &Bar{},
+20: &Baz{ whatever: "na, not really ..." }}}},
+        bang: &Bar{foo: []*Foo{}}}}}}}}},
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+2: &Foo{},
+1: &Bar{},
+-1: &Baz{ whatever: "... it's just a test." }}}},
+        bang: &Bar{foo: []*Foo{}}}}}}}}}
+}
+
+func TestElaborate(t *testing.T) {
+        a := newFoo()
+        b := newFoo()
+
+        if err := deepequal.Check(a, b); err != nil {
+                t.Errorf("expected nil, saw %v", err)
+        }
+}
diff --git a/internal/backport/go/printer/testdata/statements.golden b/internal/backport/go/printer/testdata/statements.golden
new file mode 100644
index 0000000..4b13460
--- /dev/null
+++ b/internal/backport/go/printer/testdata/statements.golden
@@ -0,0 +1,644 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package statements
+
+var expr bool
+
+func use(x interface{})	{}
+
+// Formatting of multi-line return statements.
+func _f() {
+	return
+	return x, y, z
+	return T{}
+	return T{1, 2, 3},
+		x, y, z
+	return T{1, 2, 3},
+		x, y,
+		z
+	return T{1,
+		2,
+		3}
+	return T{1,
+		2,
+		3,
+	}
+	return T{
+		1,
+		2,
+		3}
+	return T{
+		1,
+		2,
+		3,
+	}
+	return T{
+		1,
+		T{1, 2, 3},
+		3,
+	}
+	return T{
+		1,
+		T{1,
+			2, 3},
+		3,
+	}
+	return T{
+		1,
+		T{1,
+			2,
+			3},
+		3,
+	}
+	return T{
+		1,
+		2,
+	}, nil
+	return T{
+			1,
+			2,
+		},
+		T{
+			x:	3,
+			y:	4,
+		}, nil
+	return T{
+			1,
+			2,
+		},
+		nil
+	return T{
+			1,
+			2,
+		},
+		T{
+			x:	3,
+			y:	4,
+		},
+		nil
+	return x + y +
+		z
+	return func() {}
+	return func() {
+			_ = 0
+		}, T{
+			1, 2,
+		}
+	return func() {
+		_ = 0
+	}
+	return func() T {
+		return T{
+			1, 2,
+		}
+	}
+}
+
+// Formatting of multi-line returns: test cases from issue 1207.
+func F() (*T, os.Error) {
+	return &T{
+			X:	1,
+			Y:	2,
+		},
+		nil
+}
+
+func G() (*T, *T, os.Error) {
+	return &T{
+			X:	1,
+			Y:	2,
+		},
+		&T{
+			X:	3,
+			Y:	4,
+		},
+		nil
+}
+
+func _() interface{} {
+	return &fileStat{
+		name:		basename(file.name),
+		size:		mkSize(d.FileSizeHigh, d.FileSizeLow),
+		modTime:	mkModTime(d.LastWriteTime),
+		mode:		mkMode(d.FileAttributes),
+		sys:		mkSysFromFI(&d),
+	}, nil
+}
+
+// Formatting of if-statement headers.
+func _() {
+	if true {
+	}
+	if true {
+	}	// no semicolon printed
+	if expr {
+	}
+	if expr {
+	}	// no semicolon printed
+	if expr {
+	}	// no parens printed
+	if expr {
+	}	// no semicolon and parens printed
+	if x := expr; true {
+		use(x)
+	}
+	if x := expr; expr {
+		use(x)
+	}
+}
+
+// Formatting of switch-statement headers.
+func _() {
+	switch {
+	}
+	switch {
+	}	// no semicolon printed
+	switch expr {
+	}
+	switch expr {
+	}	// no semicolon printed
+	switch expr {
+	}	// no parens printed
+	switch expr {
+	}	// no semicolon and parens printed
+	switch x := expr; {
+	default:
+		use(
+			x)
+	}
+	switch x := expr; expr {
+	default:
+		use(x)
+	}
+}
+
+// Formatting of switch statement bodies.
+func _() {
+	switch {
+	}
+
+	switch x := 0; x {
+	case 1:
+		use(x)
+		use(x)	// followed by an empty line
+
+	case 2:	// followed by an empty line
+
+		use(x)	// followed by an empty line
+
+	case 3:	// no empty lines
+		use(x)
+		use(x)
+	}
+
+	switch x {
+	case 0:
+		use(x)
+	case 1:	// this comment should have no effect on the previous or next line
+		use(x)
+	}
+
+	switch x := 0; x {
+	case 1:
+		x = 0
+		// this comment should be indented
+	case 2:
+		x = 0
+	// this comment should not be indented, it is aligned with the next case
+	case 3:
+		x = 0
+		/* indented comment
+		   aligned
+		   aligned
+		*/
+		// bla
+		/* and more */
+	case 4:
+		x = 0
+	/* not indented comment
+	   aligned
+	   aligned
+	*/
+	// bla
+	/* and more */
+	case 5:
+	}
+}
+
+// Formatting of selected select statements.
+func _() {
+	select {}
+	select { /* this comment should not be tab-aligned because the closing } is on the same line */
+	}
+	select {	/* this comment should be tab-aligned */
+	}
+	select {	// this comment should be tab-aligned
+	}
+	select {
+	case <-c:
+	}
+}
+
+// Formatting of for-statement headers for single-line for-loops.
+func _() {
+	for {
+	}
+	for expr {
+	}
+	for expr {
+	}	// no parens printed
+	for {
+	}	// no semicolons printed
+	for x := expr; ; {
+		use(x)
+	}
+	for expr {
+	}	// no semicolons printed
+	for expr {
+	}	// no semicolons and parens printed
+	for ; ; expr = false {
+	}
+	for x := expr; expr; {
+		use(x)
+	}
+	for x := expr; ; expr = false {
+		use(x)
+	}
+	for ; expr; expr = false {
+	}
+	for x := expr; expr; expr = false {
+		use(x)
+	}
+	for x := range []int{} {
+		use(x)
+	}
+	for x := range []int{} {
+		use(x)
+	}	// no parens printed
+}
+
+// Formatting of for-statement headers for multi-line for-loops.
+func _() {
+	for {
+	}
+	for expr {
+	}
+	for expr {
+	}	// no parens printed
+	for {
+	}	// no semicolons printed
+	for x := expr; ; {
+		use(x)
+	}
+	for expr {
+	}	// no semicolons printed
+	for expr {
+	}	// no semicolons and parens printed
+	for ; ; expr = false {
+	}
+	for x := expr; expr; {
+		use(x)
+	}
+	for x := expr; ; expr = false {
+		use(x)
+	}
+	for ; expr; expr = false {
+	}
+	for x := expr; expr; expr = false {
+		use(x)
+	}
+	for range []int{} {
+		println("foo")
+	}
+	for x := range []int{} {
+		use(x)
+	}
+	for x := range []int{} {
+		use(x)
+	}	// no parens printed
+}
+
+// Formatting of selected short single- and multi-line statements.
+func _() {
+	if cond {
+	}
+	if cond {
+	}	// multiple lines
+	if cond {
+	} else {
+	}	// else clause always requires multiple lines
+
+	for {
+	}
+	for i := 0; i < len(a); 1++ {
+	}
+	for i := 0; i < len(a); 1++ {
+		a[i] = i
+	}
+	for i := 0; i < len(a); 1++ {
+		a[i] = i
+	}	// multiple lines
+
+	for range a {
+	}
+	for _ = range a {
+	}
+	for _, _ = range a {
+	}
+	for i := range a {
+	}
+	for i := range a {
+		a[i] = i
+	}
+	for i := range a {
+		a[i] = i
+	}	// multiple lines
+
+	go func() {
+		for {
+			a <- <-b
+		}
+	}()
+	defer func() {
+		if x := recover(); x != nil {
+			err = fmt.Sprintf("error: %s", x.msg)
+		}
+	}()
+}
+
+// Don't remove mandatory parentheses around composite literals in control clauses.
+func _() {
+	// strip parentheses - no composite literals or composite literals don't start with a type name
+	if x {
+	}
+	if x {
+	}
+	if []T{} {
+	}
+	if []T{} {
+	}
+	if []T{} {
+	}
+
+	for x {
+	}
+	for x {
+	}
+	for []T{} {
+	}
+	for []T{} {
+	}
+	for []T{} {
+	}
+
+	switch x {
+	}
+	switch x {
+	}
+	switch []T{} {
+	}
+	switch []T{} {
+	}
+
+	for _ = range []T{T{42}} {
+	}
+
+	// leave parentheses - composite literals start with a type name
+	if (T{}) {
+	}
+	if (T{}) {
+	}
+	if (T{}) {
+	}
+
+	for (T{}) {
+	}
+	for (T{}) {
+	}
+	for (T{}) {
+	}
+
+	switch (T{}) {
+	}
+	switch (T{}) {
+	}
+
+	for _ = range (T1{T{42}}) {
+	}
+
+	if x == (T{42}[0]) {
+	}
+	if (x == T{42}[0]) {
+	}
+	if x == (T{42}[0]) {
+	}
+	if x == (T{42}[0]) {
+	}
+	if x == (T{42}[0]) {
+	}
+	if x == a+b*(T{42}[0]) {
+	}
+	if (x == a+b*T{42}[0]) {
+	}
+	if x == a+b*(T{42}[0]) {
+	}
+	if x == a+(b*(T{42}[0])) {
+	}
+	if x == a+b*(T{42}[0]) {
+	}
+	if (a + b*(T{42}[0])) == x {
+	}
+	if (a + b*(T{42}[0])) == x {
+	}
+
+	if struct{ x bool }{false}.x {
+	}
+	if (struct{ x bool }{false}.x) == false {
+	}
+	if struct{ x bool }{false}.x == false {
+	}
+}
+
+// Extra empty lines inside functions. Do respect source code line
+// breaks between statement boundaries but print at most one empty
+// line at a time.
+func _() {
+
+	const _ = 0
+
+	const _ = 1
+	type _ int
+	type _ float
+
+	var _ = 0
+	var x = 1
+
+	// Each use(x) call below should have at most one empty line before and after.
+	// Known bug: The first use call may have more than one empty line before
+	//            (see go/printer/nodes.go, func linebreak).
+
+	use(x)
+
+	if x < x {
+
+		use(x)
+
+	} else {
+
+		use(x)
+
+	}
+}
+
+// Formatting around labels.
+func _() {
+L:
+}
+
+func _() {
+	// this comment should be indented
+L:	// no semicolon needed
+}
+
+func _() {
+	switch 0 {
+	case 0:
+	L0:
+		;	// semicolon required
+	case 1:
+	L1:
+		;	// semicolon required
+	default:
+	L2:	// no semicolon needed
+	}
+}
+
+func _() {
+	f()
+L1:
+	f()
+L2:
+	;
+L3:
+}
+
+func _() {
+	// this comment should be indented
+L:
+}
+
+func _() {
+L:
+	_ = 0
+}
+
+func _() {
+	// this comment should be indented
+L:
+	_ = 0
+}
+
+func _() {
+	for {
+	L1:
+		_ = 0
+	L2:
+		_ = 0
+	}
+}
+
+func _() {
+	// this comment should be indented
+	for {
+	L1:
+		_ = 0
+	L2:
+		_ = 0
+	}
+}
+
+func _() {
+	if true {
+		_ = 0
+	}
+	_ = 0	// the indentation here should not be affected by the long label name
+AnOverlongLabel:
+	_ = 0
+
+	if true {
+		_ = 0
+	}
+	_ = 0
+
+L:
+	_ = 0
+}
+
+func _() {
+	for {
+		goto L
+	}
+L:
+
+	MoreCode()
+}
+
+func _() {
+	for {
+		goto L
+	}
+L:	// A comment on the same line as the label, followed by a single empty line.
+	// Known bug: There may be more than one empty line before MoreCode()
+	//            (see go/printer/nodes.go, func linebreak).
+
+	MoreCode()
+}
+
+func _() {
+	for {
+		goto L
+	}
+L:
+
+	// There should be a single empty line before this comment.
+	MoreCode()
+}
+
+func _() {
+	for {
+		goto AVeryLongLabelThatShouldNotAffectFormatting
+	}
+AVeryLongLabelThatShouldNotAffectFormatting:
+	// There should be a single empty line after this comment.
+
+	// There should be a single empty line before this comment.
+	MoreCode()
+}
+
+// Formatting of empty statements.
+func _() {
+
+}
+
+func _() {
+}
+
+func _() {
+}
+
+func _() {
+	f()
+}
+
+func _() {
+L:
+	;
+}
+
+func _() {
+L:
+	;
+	f()
+}
diff --git a/internal/backport/go/printer/testdata/statements.input b/internal/backport/go/printer/testdata/statements.input
new file mode 100644
index 0000000..cade157
--- /dev/null
+++ b/internal/backport/go/printer/testdata/statements.input
@@ -0,0 +1,555 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package statements
+
+var expr bool
+
+func use(x interface{}) {}
+
+// Formatting of multi-line return statements.
+func _f() {
+	return
+	return x, y, z
+	return T{}
+	return T{1, 2, 3},
+		x, y, z
+	return T{1, 2, 3},
+		x, y,
+		z
+	return T{1,
+		2,
+		3}
+	return T{1,
+		2,
+		3,
+	}
+	return T{
+		1,
+		2,
+		3}
+	return T{
+		1,
+		2,
+		3,
+	}
+	return T{
+		1,
+		T{1, 2, 3},
+		3,
+	}
+	return T{
+		1,
+		T{1,
+			2, 3},
+		3,
+	}
+	return T{
+		1,
+		T{1,
+			2,
+			3},
+		3,
+	}
+	return T{
+			1,
+			2,
+		}, nil
+	return T{
+			1,
+			2,
+		},
+		T{
+			x: 3,
+			y: 4,
+		}, nil
+	return T{
+			1,
+			2,
+		},
+		nil
+	return T{
+			1,
+			2,
+		},
+		T{
+			x: 3,
+			y: 4,
+		},
+		nil
+	return x + y +
+		z
+	return func() {}
+	return func() {
+		_ = 0
+	}, T{
+		1, 2,
+	}
+	return func() {
+		_ = 0
+	}
+	return func() T {
+		return T {
+			1, 2,
+		}
+	}
+}
+
+// Formatting of multi-line returns: test cases from issue 1207.
+func F() (*T, os.Error) {
+       return &T{
+               X: 1,
+               Y: 2,
+       },
+               nil
+}
+
+func G() (*T, *T, os.Error) {
+       return &T{
+               X: 1,
+               Y: 2,
+       },
+               &T{
+                       X: 3,
+                       Y: 4,
+               },
+               nil
+}
+
+func _() interface{} {
+	return &fileStat{
+			name:    basename(file.name),
+			size:    mkSize(d.FileSizeHigh, d.FileSizeLow),
+			modTime: mkModTime(d.LastWriteTime),
+			mode:    mkMode(d.FileAttributes),
+			sys:     mkSysFromFI(&d),
+		}, nil
+}
+
+// Formatting of if-statement headers.
+func _() {
+	if true {}
+	if; true {}  // no semicolon printed
+	if expr{}
+	if;expr{}  // no semicolon printed
+	if (expr){}  // no parens printed
+	if;((expr)){}  // no semicolon and parens printed
+	if x:=expr;true{
+	use(x)}
+	if x:=expr; expr {use(x)}
+}
+
+
+// Formatting of switch-statement headers.
+func _() {
+	switch {}
+	switch;{}  // no semicolon printed
+	switch expr {}
+	switch;expr{}  // no semicolon printed
+	switch (expr) {}  // no parens printed
+	switch;((expr)){}  // no semicolon and parens printed
+	switch x := expr; { default:use(
+x)
+	}
+	switch x := expr; expr {default:use(x)}
+}
+
+
+// Formatting of switch statement bodies.
+func _() {
+	switch {
+	}
+
+	switch x := 0; x {
+	case 1:
+		use(x)
+		use(x)  // followed by an empty line
+
+	case 2:  // followed by an empty line
+
+		use(x)  // followed by an empty line
+
+	case 3:  // no empty lines
+		use(x)
+		use(x)
+	}
+
+	switch x {
+	case 0:
+		use(x)
+	case 1:  // this comment should have no effect on the previous or next line
+		use(x)
+	}
+
+	switch x := 0; x {
+	case 1:
+		x = 0
+		// this comment should be indented
+	case 2:
+		x = 0
+	// this comment should not be indented, it is aligned with the next case
+	case 3:
+		x = 0
+		/* indented comment
+		   aligned
+		   aligned
+		*/
+		// bla
+		/* and more */
+	case 4:
+		x = 0
+	/* not indented comment
+	   aligned
+	   aligned
+	*/
+	// bla
+	/* and more */
+	case 5:
+	}
+}
+
+
+// Formatting of selected select statements.
+func _() {
+	select {
+	}
+	select { /* this comment should not be tab-aligned because the closing } is on the same line */ }
+	select { /* this comment should be tab-aligned */
+	}
+	select { // this comment should be tab-aligned
+	}
+	select { case <-c: }
+}
+
+
+// Formatting of for-statement headers for single-line for-loops.
+func _() {
+	for{}
+	for expr {}
+	for (expr) {}  // no parens printed
+	for;;{}  // no semicolons printed
+	for x :=expr;; {use( x)}
+	for; expr;{}  // no semicolons printed
+	for; ((expr));{}  // no semicolons and parens printed
+	for; ; expr = false {}
+	for x :=expr; expr; {use(x)}
+	for x := expr;; expr=false {use(x)}
+	for;expr;expr =false {}
+	for x := expr;expr;expr = false { use(x) }
+	for x := range []int{} { use(x) }
+	for x := range (([]int{})) { use(x) }  // no parens printed
+}
+
+
+// Formatting of for-statement headers for multi-line for-loops.
+func _() {
+	for{
+	}
+	for expr {
+	}
+	for (expr) {
+	}  // no parens printed
+	for;;{
+	}  // no semicolons printed
+	for x :=expr;; {use( x)
+	}
+	for; expr;{
+	}  // no semicolons printed
+	for; ((expr));{
+	}  // no semicolons and parens printed
+	for; ; expr = false {
+	}
+	for x :=expr; expr; {use(x)
+	}
+	for x := expr;; expr=false {use(x)
+	}
+	for;expr;expr =false {
+	}
+	for x := expr;expr;expr = false {
+	use(x)
+	}
+	for range []int{} {
+	println("foo")}
+	for x := range []int{} {
+	use(x) }
+	for x := range (([]int{})) {
+	use(x) }  // no parens printed
+}
+
+
+// Formatting of selected short single- and multi-line statements.
+func _() {
+	if cond {}
+	if cond {
+	} // multiple lines
+	if cond {} else {} // else clause always requires multiple lines
+
+	for {}
+	for i := 0; i < len(a); 1++ {}
+	for i := 0; i < len(a); 1++ { a[i] = i }
+	for i := 0; i < len(a); 1++ { a[i] = i
+	} // multiple lines
+
+	for range a{}
+	for _ = range a{}
+	for _, _ = range a{}
+	for i := range a {}
+	for i := range a { a[i] = i }
+	for i := range a { a[i] = i
+	} // multiple lines
+
+	go func() { for { a <- <-b } }()
+	defer func() { if x := recover(); x != nil { err = fmt.Sprintf("error: %s", x.msg) } }()
+}
+
+
+// Don't remove mandatory parentheses around composite literals in control clauses.
+func _() {
+	// strip parentheses - no composite literals or composite literals don't start with a type name
+	if (x) {}
+	if (((x))) {}
+	if ([]T{}) {}
+	if (([]T{})) {}
+	if ; (((([]T{})))) {}
+
+	for (x) {}
+	for (((x))) {}
+	for ([]T{}) {}
+	for (([]T{})) {}
+	for ; (((([]T{})))) ; {}
+
+	switch (x) {}
+	switch (((x))) {}
+	switch ([]T{}) {}
+	switch ; (((([]T{})))) {}
+
+	for _ = range ((([]T{T{42}}))) {}
+
+	// leave parentheses - composite literals start with a type name
+	if (T{}) {}
+	if ((T{})) {}
+	if ; ((((T{})))) {}
+
+	for (T{}) {}
+	for ((T{})) {}
+	for ; ((((T{})))) ; {}
+
+	switch (T{}) {}
+	switch ; ((((T{})))) {}
+
+	for _ = range (((T1{T{42}}))) {}
+
+	if x == (T{42}[0]) {}
+	if (x == T{42}[0]) {}
+	if (x == (T{42}[0])) {}
+	if (x == (((T{42}[0])))) {}
+	if (((x == (T{42}[0])))) {}
+	if x == a + b*(T{42}[0]) {}
+	if (x == a + b*T{42}[0]) {}
+	if (x == a + b*(T{42}[0])) {}
+	if (x == a + ((b * (T{42}[0])))) {}
+	if (((x == a + b * (T{42}[0])))) {}
+	if (((a + b * (T{42}[0])) == x)) {}
+	if (((a + b * (T{42}[0])))) == x {}
+
+	if (struct{x bool}{false}.x) {}
+	if (struct{x bool}{false}.x) == false {}
+	if (struct{x bool}{false}.x == false) {}
+}
+
+
+// Extra empty lines inside functions. Do respect source code line
+// breaks between statement boundaries but print at most one empty
+// line at a time.
+func _() {
+
+	const _ = 0
+
+	const _ = 1
+	type _ int
+	type _ float
+
+	var _ = 0
+	var x = 1
+
+	// Each use(x) call below should have at most one empty line before and after.
+	// Known bug: The first use call may have more than one empty line before
+	//            (see go/printer/nodes.go, func linebreak).
+
+
+
+	use(x)
+
+	if x < x {
+
+		use(x)
+
+	} else {
+
+		use(x)
+
+	}
+}
+
+
+// Formatting around labels.
+func _() {
+	L:
+}
+
+
+func _() {
+	// this comment should be indented
+	L: ;  // no semicolon needed
+}
+
+
+func _() {
+	switch 0 {
+	case 0:
+		L0: ;  // semicolon required
+	case 1:
+		L1: ;  // semicolon required
+	default:
+		L2: ;  // no semicolon needed
+	}
+}
+
+
+func _() {
+	f()
+L1:
+	f()
+L2:
+	;
+L3:
+}
+
+
+func _() {
+	// this comment should be indented
+	L:
+}
+
+
+func _() {
+	L: _ = 0
+}
+
+
+func _() {
+	// this comment should be indented
+	L: _ = 0
+}
+
+
+func _() {
+	for {
+	L1: _ = 0
+	L2:
+		_ = 0
+	}
+}
+
+
+func _() {
+		// this comment should be indented
+	for {
+	L1: _ = 0
+	L2:
+		_ = 0
+	}
+}
+
+
+func _() {
+	if true {
+		_ = 0
+	}
+	_ = 0  // the indentation here should not be affected by the long label name
+AnOverlongLabel:
+	_ = 0
+	
+	if true {
+		_ = 0
+	}
+	_ = 0
+
+L:	_ = 0
+}
+
+
+func _() {
+	for {
+		goto L
+	}
+L:
+
+	MoreCode()
+}
+
+
+func _() {
+	for {
+		goto L
+	}
+L:	// A comment on the same line as the label, followed by a single empty line.
+	// Known bug: There may be more than one empty line before MoreCode()
+	//            (see go/printer/nodes.go, func linebreak).
+
+
+
+
+	MoreCode()
+}
+
+
+func _() {
+	for {
+		goto L
+	}
+L:
+
+
+
+
+	// There should be a single empty line before this comment.
+	MoreCode()
+}
+
+
+func _() {
+	for {
+		goto AVeryLongLabelThatShouldNotAffectFormatting
+	}
+AVeryLongLabelThatShouldNotAffectFormatting:
+	// There should be a single empty line after this comment.
+
+	// There should be a single empty line before this comment.
+	MoreCode()
+}
+
+
+// Formatting of empty statements.
+func _() {
+	;;;;;;;;;;;;;;;;;;;;;;;;;
+}
+
+func _() {;;;;;;;;;;;;;;;;;;;;;;;;;
+}
+
+func _() {;;;;;;;;;;;;;;;;;;;;;;;;;}
+
+func _() {
+f();;;;;;;;;;;;;;;;;;;;;;;;;
+}
+
+func _() {
+L:;;;;;;;;;;;;
+}
+
+func _() {
+L:;;;;;;;;;;;;
+	f()
+}
diff --git a/internal/backport/go/scanner/errors.go b/internal/backport/go/scanner/errors.go
new file mode 100644
index 0000000..539bbfd
--- /dev/null
+++ b/internal/backport/go/scanner/errors.go
@@ -0,0 +1,120 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner
+
+import (
+	"fmt"
+	"golang.org/x/website/internal/backport/go/token"
+	"io"
+	"sort"
+)
+
+// In an ErrorList, an error is represented by an *Error.
+// The position Pos, if valid, points to the beginning of
+// the offending token, and the error condition is described
+// by Msg.
+type Error struct {
+	Pos token.Position
+	Msg string
+}
+
+// Error implements the error interface.
+func (e Error) Error() string {
+	if e.Pos.Filename != "" || e.Pos.IsValid() {
+		// don't print "<unknown position>"
+		// TODO(gri) reconsider the semantics of Position.IsValid
+		return e.Pos.String() + ": " + e.Msg
+	}
+	return e.Msg
+}
+
+// ErrorList is a list of *Errors.
+// The zero value for an ErrorList is an empty ErrorList ready to use.
+type ErrorList []*Error
+
+// Add adds an Error with given position and error message to an ErrorList.
+func (p *ErrorList) Add(pos token.Position, msg string) {
+	*p = append(*p, &Error{pos, msg})
+}
+
+// Reset resets an ErrorList to no errors.
+func (p *ErrorList) Reset() { *p = (*p)[0:0] }
+
+// ErrorList implements the sort Interface.
+func (p ErrorList) Len() int      { return len(p) }
+func (p ErrorList) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
+
+func (p ErrorList) Less(i, j int) bool {
+	e := &p[i].Pos
+	f := &p[j].Pos
+	// Note that it is not sufficient to simply compare file offsets because
+	// the offsets do not reflect modified line information (through //line
+	// comments).
+	if e.Filename != f.Filename {
+		return e.Filename < f.Filename
+	}
+	if e.Line != f.Line {
+		return e.Line < f.Line
+	}
+	if e.Column != f.Column {
+		return e.Column < f.Column
+	}
+	return p[i].Msg < p[j].Msg
+}
+
+// Sort sorts an ErrorList. *Error entries are sorted by position,
+// other errors are sorted by error message, and before any *Error
+// entry.
+func (p ErrorList) Sort() {
+	sort.Sort(p)
+}
+
+// RemoveMultiples sorts an ErrorList and removes all but the first error per line.
+func (p *ErrorList) RemoveMultiples() {
+	sort.Sort(p)
+	var last token.Position // initial last.Line is != any legal error line
+	i := 0
+	for _, e := range *p {
+		if e.Pos.Filename != last.Filename || e.Pos.Line != last.Line {
+			last = e.Pos
+			(*p)[i] = e
+			i++
+		}
+	}
+	*p = (*p)[0:i]
+}
+
+// An ErrorList implements the error interface.
+func (p ErrorList) Error() string {
+	switch len(p) {
+	case 0:
+		return "no errors"
+	case 1:
+		return p[0].Error()
+	}
+	return fmt.Sprintf("%s (and %d more errors)", p[0], len(p)-1)
+}
+
+// Err returns an error equivalent to this error list.
+// If the list is empty, Err returns nil.
+func (p ErrorList) Err() error {
+	if len(p) == 0 {
+		return nil
+	}
+	return p
+}
+
+// PrintError is a utility function that prints a list of errors to w,
+// one error per line, if the err parameter is an ErrorList. Otherwise
+// it prints the err string.
+func PrintError(w io.Writer, err error) {
+	if list, ok := err.(ErrorList); ok {
+		for _, e := range list {
+			fmt.Fprintf(w, "%s\n", e)
+		}
+	} else if err != nil {
+		fmt.Fprintf(w, "%s\n", err)
+	}
+}
diff --git a/internal/backport/go/scanner/example_test.go b/internal/backport/go/scanner/example_test.go
new file mode 100644
index 0000000..e909237
--- /dev/null
+++ b/internal/backport/go/scanner/example_test.go
@@ -0,0 +1,46 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner_test
+
+import (
+	"fmt"
+	"golang.org/x/website/internal/backport/go/scanner"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+func ExampleScanner_Scan() {
+	// src is the input that we want to tokenize.
+	src := []byte("cos(x) + 1i*sin(x) // Euler")
+
+	// Initialize the scanner.
+	var s scanner.Scanner
+	fset := token.NewFileSet()                      // positions are relative to fset
+	file := fset.AddFile("", fset.Base(), len(src)) // register input "file"
+	s.Init(file, src, nil /* no error handler */, scanner.ScanComments)
+
+	// Repeated calls to Scan yield the token sequence found in the input.
+	for {
+		pos, tok, lit := s.Scan()
+		if tok == token.EOF {
+			break
+		}
+		fmt.Printf("%s\t%s\t%q\n", fset.Position(pos), tok, lit)
+	}
+
+	// output:
+	// 1:1	IDENT	"cos"
+	// 1:4	(	""
+	// 1:5	IDENT	"x"
+	// 1:6	)	""
+	// 1:8	+	""
+	// 1:10	IMAG	"1i"
+	// 1:12	*	""
+	// 1:13	IDENT	"sin"
+	// 1:16	(	""
+	// 1:17	IDENT	"x"
+	// 1:18	)	""
+	// 1:20	;	"\n"
+	// 1:20	COMMENT	"// Euler"
+}
diff --git a/internal/backport/go/scanner/scanner.go b/internal/backport/go/scanner/scanner.go
new file mode 100644
index 0000000..f96569e
--- /dev/null
+++ b/internal/backport/go/scanner/scanner.go
@@ -0,0 +1,983 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package scanner implements a scanner for Go source text.
+// It takes a []byte as source which can then be tokenized
+// through repeated calls to the Scan method.
+package scanner
+
+import (
+	"bytes"
+	"fmt"
+	"golang.org/x/website/internal/backport/go/token"
+	"path/filepath"
+	"strconv"
+	"unicode"
+	"unicode/utf8"
+)
+
+// An ErrorHandler may be provided to Scanner.Init. If a syntax error is
+// encountered and a handler was installed, the handler is called with a
+// position and an error message. The position points to the beginning of
+// the offending token.
+type ErrorHandler func(pos token.Position, msg string)
+
+// A Scanner holds the scanner's internal state while processing
+// a given text. It can be allocated as part of another data
+// structure but must be initialized via Init before use.
+type Scanner struct {
+	// immutable state
+	file *token.File  // source file handle
+	dir  string       // directory portion of file.Name()
+	src  []byte       // source
+	err  ErrorHandler // error reporting; or nil
+	mode Mode         // scanning mode
+
+	// scanning state
+	ch         rune // current character
+	offset     int  // character offset
+	rdOffset   int  // reading offset (position after current character)
+	lineOffset int  // current line offset
+	insertSemi bool // insert a semicolon before next newline
+
+	// public state - ok to modify
+	ErrorCount int // number of errors encountered
+}
+
+const (
+	bom = 0xFEFF // byte order mark, only permitted as very first character
+	eof = -1     // end of file
+)
+
+// Read the next Unicode char into s.ch.
+// s.ch < 0 means end-of-file.
+//
+// For optimization, there is some overlap between this method and
+// s.scanIdentifier.
+func (s *Scanner) next() {
+	if s.rdOffset < len(s.src) {
+		s.offset = s.rdOffset
+		if s.ch == '\n' {
+			s.lineOffset = s.offset
+			s.file.AddLine(s.offset)
+		}
+		r, w := rune(s.src[s.rdOffset]), 1
+		switch {
+		case r == 0:
+			s.error(s.offset, "illegal character NUL")
+		case r >= utf8.RuneSelf:
+			// not ASCII
+			r, w = utf8.DecodeRune(s.src[s.rdOffset:])
+			if r == utf8.RuneError && w == 1 {
+				s.error(s.offset, "illegal UTF-8 encoding")
+			} else if r == bom && s.offset > 0 {
+				s.error(s.offset, "illegal byte order mark")
+			}
+		}
+		s.rdOffset += w
+		s.ch = r
+	} else {
+		s.offset = len(s.src)
+		if s.ch == '\n' {
+			s.lineOffset = s.offset
+			s.file.AddLine(s.offset)
+		}
+		s.ch = eof
+	}
+}
+
+// peek returns the byte following the most recently read character without
+// advancing the scanner. If the scanner is at EOF, peek returns 0.
+func (s *Scanner) peek() byte {
+	if s.rdOffset < len(s.src) {
+		return s.src[s.rdOffset]
+	}
+	return 0
+}
+
+// A mode value is a set of flags (or 0).
+// They control scanner behavior.
+type Mode uint
+
+const (
+	ScanComments    Mode = 1 << iota // return comments as COMMENT tokens
+	dontInsertSemis                  // do not automatically insert semicolons - for testing only
+)
+
+// Init prepares the scanner s to tokenize the text src by setting the
+// scanner at the beginning of src. The scanner uses the file set file
+// for position information and it adds line information for each line.
+// It is ok to re-use the same file when re-scanning the same file as
+// line information which is already present is ignored. Init causes a
+// panic if the file size does not match the src size.
+//
+// Calls to Scan will invoke the error handler err if they encounter a
+// syntax error and err is not nil. Also, for each error encountered,
+// the Scanner field ErrorCount is incremented by one. The mode parameter
+// determines how comments are handled.
+//
+// Note that Init may call err if there is an error in the first character
+// of the file.
+func (s *Scanner) Init(file *token.File, src []byte, err ErrorHandler, mode Mode) {
+	// Explicitly initialize all fields since a scanner may be reused.
+	if file.Size() != len(src) {
+		panic(fmt.Sprintf("file size (%d) does not match src len (%d)", file.Size(), len(src)))
+	}
+	s.file = file
+	s.dir, _ = filepath.Split(file.Name())
+	s.src = src
+	s.err = err
+	s.mode = mode
+
+	s.ch = ' '
+	s.offset = 0
+	s.rdOffset = 0
+	s.lineOffset = 0
+	s.insertSemi = false
+	s.ErrorCount = 0
+
+	s.next()
+	if s.ch == bom {
+		s.next() // ignore BOM at file beginning
+	}
+}
+
+func (s *Scanner) error(offs int, msg string) {
+	if s.err != nil {
+		s.err(s.file.Position(s.file.Pos(offs)), msg)
+	}
+	s.ErrorCount++
+}
+
+func (s *Scanner) errorf(offs int, format string, args ...interface{}) {
+	s.error(offs, fmt.Sprintf(format, args...))
+}
+
+func (s *Scanner) scanComment() string {
+	// initial '/' already consumed; s.ch == '/' || s.ch == '*'
+	offs := s.offset - 1 // position of initial '/'
+	next := -1           // position immediately following the comment; < 0 means invalid comment
+	numCR := 0
+
+	if s.ch == '/' {
+		//-style comment
+		// (the final '\n' is not considered part of the comment)
+		s.next()
+		for s.ch != '\n' && s.ch >= 0 {
+			if s.ch == '\r' {
+				numCR++
+			}
+			s.next()
+		}
+		// if we are at '\n', the position following the comment is afterwards
+		next = s.offset
+		if s.ch == '\n' {
+			next++
+		}
+		goto exit
+	}
+
+	/*-style comment */
+	s.next()
+	for s.ch >= 0 {
+		ch := s.ch
+		if ch == '\r' {
+			numCR++
+		}
+		s.next()
+		if ch == '*' && s.ch == '/' {
+			s.next()
+			next = s.offset
+			goto exit
+		}
+	}
+
+	s.error(offs, "comment not terminated")
+
+exit:
+	lit := s.src[offs:s.offset]
+
+	// On Windows, a (//-comment) line may end in "\r\n".
+	// Remove the final '\r' before analyzing the text for
+	// line directives (matching the compiler). Remove any
+	// other '\r' afterwards (matching the pre-existing be-
+	// havior of the scanner).
+	if numCR > 0 && len(lit) >= 2 && lit[1] == '/' && lit[len(lit)-1] == '\r' {
+		lit = lit[:len(lit)-1]
+		numCR--
+	}
+
+	// interpret line directives
+	// (//line directives must start at the beginning of the current line)
+	if next >= 0 /* implies valid comment */ && (lit[1] == '*' || offs == s.lineOffset) && bytes.HasPrefix(lit[2:], prefix) {
+		s.updateLineInfo(next, offs, lit)
+	}
+
+	if numCR > 0 {
+		lit = stripCR(lit, lit[1] == '*')
+	}
+
+	return string(lit)
+}
+
+var prefix = []byte("line ")
+
+// updateLineInfo parses the incoming comment text at offset offs
+// as a line directive. If successful, it updates the line info table
+// for the position next per the line directive.
+func (s *Scanner) updateLineInfo(next, offs int, text []byte) {
+	// extract comment text
+	if text[1] == '*' {
+		text = text[:len(text)-2] // lop off trailing "*/"
+	}
+	text = text[7:] // lop off leading "//line " or "/*line "
+	offs += 7
+
+	i, n, ok := trailingDigits(text)
+	if i == 0 {
+		return // ignore (not a line directive)
+	}
+	// i > 0
+
+	if !ok {
+		// text has a suffix :xxx but xxx is not a number
+		s.error(offs+i, "invalid line number: "+string(text[i:]))
+		return
+	}
+
+	var line, col int
+	i2, n2, ok2 := trailingDigits(text[:i-1])
+	if ok2 {
+		//line filename:line:col
+		i, i2 = i2, i
+		line, col = n2, n
+		if col == 0 {
+			s.error(offs+i2, "invalid column number: "+string(text[i2:]))
+			return
+		}
+		text = text[:i2-1] // lop off ":col"
+	} else {
+		//line filename:line
+		line = n
+	}
+
+	if line == 0 {
+		s.error(offs+i, "invalid line number: "+string(text[i:]))
+		return
+	}
+
+	// If we have a column (//line filename:line:col form),
+	// an empty filename means to use the previous filename.
+	filename := string(text[:i-1]) // lop off ":line", and trim white space
+	if filename == "" && ok2 {
+		filename = s.file.Position(s.file.Pos(offs)).Filename
+	} else if filename != "" {
+		// Put a relative filename in the current directory.
+		// This is for compatibility with earlier releases.
+		// See issue 26671.
+		filename = filepath.Clean(filename)
+		if !filepath.IsAbs(filename) {
+			filename = filepath.Join(s.dir, filename)
+		}
+	}
+
+	s.file.AddLineColumnInfo(next, filename, line, col)
+}
+
+func trailingDigits(text []byte) (int, int, bool) {
+	i := bytes.LastIndexByte(text, ':') // look from right (Windows filenames may contain ':')
+	if i < 0 {
+		return 0, 0, false // no ":"
+	}
+	// i >= 0
+	n, err := strconv.ParseUint(string(text[i+1:]), 10, 0)
+	return i + 1, int(n), err == nil
+}
+
+func (s *Scanner) findLineEnd() bool {
+	// initial '/' already consumed
+
+	defer func(offs int) {
+		// reset scanner state to where it was upon calling findLineEnd
+		s.ch = '/'
+		s.offset = offs
+		s.rdOffset = offs + 1
+		s.next() // consume initial '/' again
+	}(s.offset - 1)
+
+	// read ahead until a newline, EOF, or non-comment token is found
+	for s.ch == '/' || s.ch == '*' {
+		if s.ch == '/' {
+			//-style comment always contains a newline
+			return true
+		}
+		/*-style comment: look for newline */
+		s.next()
+		for s.ch >= 0 {
+			ch := s.ch
+			if ch == '\n' {
+				return true
+			}
+			s.next()
+			if ch == '*' && s.ch == '/' {
+				s.next()
+				break
+			}
+		}
+		s.skipWhitespace() // s.insertSemi is set
+		if s.ch < 0 || s.ch == '\n' {
+			return true
+		}
+		if s.ch != '/' {
+			// non-comment token
+			return false
+		}
+		s.next() // consume '/'
+	}
+
+	return false
+}
+
+func isLetter(ch rune) bool {
+	return 'a' <= lower(ch) && lower(ch) <= 'z' || ch == '_' || ch >= utf8.RuneSelf && unicode.IsLetter(ch)
+}
+
+func isDigit(ch rune) bool {
+	return isDecimal(ch) || ch >= utf8.RuneSelf && unicode.IsDigit(ch)
+}
+
+// scanIdentifier reads the string of valid identifier characters at s.offset.
+// It must only be called when s.ch is known to be a valid letter.
+//
+// Be careful when making changes to this function: it is optimized and affects
+// scanning performance significantly.
+func (s *Scanner) scanIdentifier() string {
+	offs := s.offset
+
+	// Optimize for the common case of an ASCII identifier.
+	//
+	// Ranging over s.src[s.rdOffset:] lets us avoid some bounds checks, and
+	// avoids conversions to runes.
+	//
+	// In case we encounter a non-ASCII character, fall back on the slower path
+	// of calling into s.next().
+	for rdOffset, b := range s.src[s.rdOffset:] {
+		if 'a' <= b && b <= 'z' || 'A' <= b && b <= 'Z' || b == '_' || '0' <= b && b <= '9' {
+			// Avoid assigning a rune for the common case of an ascii character.
+			continue
+		}
+		s.rdOffset += rdOffset
+		if 0 < b && b < utf8.RuneSelf {
+			// Optimization: we've encountered an ASCII character that's not a letter
+			// or number. Avoid the call into s.next() and corresponding set up.
+			//
+			// Note that s.next() does some line accounting if s.ch is '\n', so this
+			// shortcut is only possible because we know that the preceding character
+			// is not '\n'.
+			s.ch = rune(b)
+			s.offset = s.rdOffset
+			s.rdOffset++
+			goto exit
+		}
+		// We know that the preceding character is valid for an identifier because
+		// scanIdentifier is only called when s.ch is a letter, so calling s.next()
+		// at s.rdOffset resets the scanner state.
+		s.next()
+		for isLetter(s.ch) || isDigit(s.ch) {
+			s.next()
+		}
+		goto exit
+	}
+	s.offset = len(s.src)
+	s.rdOffset = len(s.src)
+	s.ch = eof
+
+exit:
+	return string(s.src[offs:s.offset])
+}
+
+func digitVal(ch rune) int {
+	switch {
+	case '0' <= ch && ch <= '9':
+		return int(ch - '0')
+	case 'a' <= lower(ch) && lower(ch) <= 'f':
+		return int(lower(ch) - 'a' + 10)
+	}
+	return 16 // larger than any legal digit val
+}
+
+func lower(ch rune) rune     { return ('a' - 'A') | ch } // returns lower-case ch iff ch is ASCII letter
+func isDecimal(ch rune) bool { return '0' <= ch && ch <= '9' }
+func isHex(ch rune) bool     { return '0' <= ch && ch <= '9' || 'a' <= lower(ch) && lower(ch) <= 'f' }
+
+// digits accepts the sequence { digit | '_' }.
+// If base <= 10, digits accepts any decimal digit but records
+// the offset (relative to the source start) of a digit >= base
+// in *invalid, if *invalid < 0.
+// digits returns a bitset describing whether the sequence contained
+// digits (bit 0 is set), or separators '_' (bit 1 is set).
+func (s *Scanner) digits(base int, invalid *int) (digsep int) {
+	if base <= 10 {
+		max := rune('0' + base)
+		for isDecimal(s.ch) || s.ch == '_' {
+			ds := 1
+			if s.ch == '_' {
+				ds = 2
+			} else if s.ch >= max && *invalid < 0 {
+				*invalid = s.offset // record invalid rune offset
+			}
+			digsep |= ds
+			s.next()
+		}
+	} else {
+		for isHex(s.ch) || s.ch == '_' {
+			ds := 1
+			if s.ch == '_' {
+				ds = 2
+			}
+			digsep |= ds
+			s.next()
+		}
+	}
+	return
+}
+
+func (s *Scanner) scanNumber() (token.Token, string) {
+	offs := s.offset
+	tok := token.ILLEGAL
+
+	base := 10        // number base
+	prefix := rune(0) // one of 0 (decimal), '0' (0-octal), 'x', 'o', or 'b'
+	digsep := 0       // bit 0: digit present, bit 1: '_' present
+	invalid := -1     // index of invalid digit in literal, or < 0
+
+	// integer part
+	if s.ch != '.' {
+		tok = token.INT
+		if s.ch == '0' {
+			s.next()
+			switch lower(s.ch) {
+			case 'x':
+				s.next()
+				base, prefix = 16, 'x'
+			case 'o':
+				s.next()
+				base, prefix = 8, 'o'
+			case 'b':
+				s.next()
+				base, prefix = 2, 'b'
+			default:
+				base, prefix = 8, '0'
+				digsep = 1 // leading 0
+			}
+		}
+		digsep |= s.digits(base, &invalid)
+	}
+
+	// fractional part
+	if s.ch == '.' {
+		tok = token.FLOAT
+		if prefix == 'o' || prefix == 'b' {
+			s.error(s.offset, "invalid radix point in "+litname(prefix))
+		}
+		s.next()
+		digsep |= s.digits(base, &invalid)
+	}
+
+	if digsep&1 == 0 {
+		s.error(s.offset, litname(prefix)+" has no digits")
+	}
+
+	// exponent
+	if e := lower(s.ch); e == 'e' || e == 'p' {
+		switch {
+		case e == 'e' && prefix != 0 && prefix != '0':
+			s.errorf(s.offset, "%q exponent requires decimal mantissa", s.ch)
+		case e == 'p' && prefix != 'x':
+			s.errorf(s.offset, "%q exponent requires hexadecimal mantissa", s.ch)
+		}
+		s.next()
+		tok = token.FLOAT
+		if s.ch == '+' || s.ch == '-' {
+			s.next()
+		}
+		ds := s.digits(10, nil)
+		digsep |= ds
+		if ds&1 == 0 {
+			s.error(s.offset, "exponent has no digits")
+		}
+	} else if prefix == 'x' && tok == token.FLOAT {
+		s.error(s.offset, "hexadecimal mantissa requires a 'p' exponent")
+	}
+
+	// suffix 'i'
+	if s.ch == 'i' {
+		tok = token.IMAG
+		s.next()
+	}
+
+	lit := string(s.src[offs:s.offset])
+	if tok == token.INT && invalid >= 0 {
+		s.errorf(invalid, "invalid digit %q in %s", lit[invalid-offs], litname(prefix))
+	}
+	if digsep&2 != 0 {
+		if i := invalidSep(lit); i >= 0 {
+			s.error(offs+i, "'_' must separate successive digits")
+		}
+	}
+
+	return tok, lit
+}
+
+func litname(prefix rune) string {
+	switch prefix {
+	case 'x':
+		return "hexadecimal literal"
+	case 'o', '0':
+		return "octal literal"
+	case 'b':
+		return "binary literal"
+	}
+	return "decimal literal"
+}
+
+// invalidSep returns the index of the first invalid separator in x, or -1.
+func invalidSep(x string) int {
+	x1 := ' ' // prefix char, we only care if it's 'x'
+	d := '.'  // digit, one of '_', '0' (a digit), or '.' (anything else)
+	i := 0
+
+	// a prefix counts as a digit
+	if len(x) >= 2 && x[0] == '0' {
+		x1 = lower(rune(x[1]))
+		if x1 == 'x' || x1 == 'o' || x1 == 'b' {
+			d = '0'
+			i = 2
+		}
+	}
+
+	// mantissa and exponent
+	for ; i < len(x); i++ {
+		p := d // previous digit
+		d = rune(x[i])
+		switch {
+		case d == '_':
+			if p != '0' {
+				return i
+			}
+		case isDecimal(d) || x1 == 'x' && isHex(d):
+			d = '0'
+		default:
+			if p == '_' {
+				return i - 1
+			}
+			d = '.'
+		}
+	}
+	if d == '_' {
+		return len(x) - 1
+	}
+
+	return -1
+}
+
+// scanEscape parses an escape sequence where rune is the accepted
+// escaped quote. In case of a syntax error, it stops at the offending
+// character (without consuming it) and returns false. Otherwise
+// it returns true.
+func (s *Scanner) scanEscape(quote rune) bool {
+	offs := s.offset
+
+	var n int
+	var base, max uint32
+	switch s.ch {
+	case 'a', 'b', 'f', 'n', 'r', 't', 'v', '\\', quote:
+		s.next()
+		return true
+	case '0', '1', '2', '3', '4', '5', '6', '7':
+		n, base, max = 3, 8, 255
+	case 'x':
+		s.next()
+		n, base, max = 2, 16, 255
+	case 'u':
+		s.next()
+		n, base, max = 4, 16, unicode.MaxRune
+	case 'U':
+		s.next()
+		n, base, max = 8, 16, unicode.MaxRune
+	default:
+		msg := "unknown escape sequence"
+		if s.ch < 0 {
+			msg = "escape sequence not terminated"
+		}
+		s.error(offs, msg)
+		return false
+	}
+
+	var x uint32
+	for n > 0 {
+		d := uint32(digitVal(s.ch))
+		if d >= base {
+			msg := fmt.Sprintf("illegal character %#U in escape sequence", s.ch)
+			if s.ch < 0 {
+				msg = "escape sequence not terminated"
+			}
+			s.error(s.offset, msg)
+			return false
+		}
+		x = x*base + d
+		s.next()
+		n--
+	}
+
+	if x > max || 0xD800 <= x && x < 0xE000 {
+		s.error(offs, "escape sequence is invalid Unicode code point")
+		return false
+	}
+
+	return true
+}
+
+func (s *Scanner) scanRune() string {
+	// '\'' opening already consumed
+	offs := s.offset - 1
+
+	valid := true
+	n := 0
+	for {
+		ch := s.ch
+		if ch == '\n' || ch < 0 {
+			// only report error if we don't have one already
+			if valid {
+				s.error(offs, "rune literal not terminated")
+				valid = false
+			}
+			break
+		}
+		s.next()
+		if ch == '\'' {
+			break
+		}
+		n++
+		if ch == '\\' {
+			if !s.scanEscape('\'') {
+				valid = false
+			}
+			// continue to read to closing quote
+		}
+	}
+
+	if valid && n != 1 {
+		s.error(offs, "illegal rune literal")
+	}
+
+	return string(s.src[offs:s.offset])
+}
+
+func (s *Scanner) scanString() string {
+	// '"' opening already consumed
+	offs := s.offset - 1
+
+	for {
+		ch := s.ch
+		if ch == '\n' || ch < 0 {
+			s.error(offs, "string literal not terminated")
+			break
+		}
+		s.next()
+		if ch == '"' {
+			break
+		}
+		if ch == '\\' {
+			s.scanEscape('"')
+		}
+	}
+
+	return string(s.src[offs:s.offset])
+}
+
+func stripCR(b []byte, comment bool) []byte {
+	c := make([]byte, len(b))
+	i := 0
+	for j, ch := range b {
+		// In a /*-style comment, don't strip \r from *\r/ (incl.
+		// sequences of \r from *\r\r...\r/) since the resulting
+		// */ would terminate the comment too early unless the \r
+		// is immediately following the opening /* in which case
+		// it's ok because /*/ is not closed yet (issue #11151).
+		if ch != '\r' || comment && i > len("/*") && c[i-1] == '*' && j+1 < len(b) && b[j+1] == '/' {
+			c[i] = ch
+			i++
+		}
+	}
+	return c[:i]
+}
+
+func (s *Scanner) scanRawString() string {
+	// '`' opening already consumed
+	offs := s.offset - 1
+
+	hasCR := false
+	for {
+		ch := s.ch
+		if ch < 0 {
+			s.error(offs, "raw string literal not terminated")
+			break
+		}
+		s.next()
+		if ch == '`' {
+			break
+		}
+		if ch == '\r' {
+			hasCR = true
+		}
+	}
+
+	lit := s.src[offs:s.offset]
+	if hasCR {
+		lit = stripCR(lit, false)
+	}
+
+	return string(lit)
+}
+
+func (s *Scanner) skipWhitespace() {
+	for s.ch == ' ' || s.ch == '\t' || s.ch == '\n' && !s.insertSemi || s.ch == '\r' {
+		s.next()
+	}
+}
+
+// Helper functions for scanning multi-byte tokens such as >> += >>= .
+// Different routines recognize different length tok_i based on matches
+// of ch_i. If a token ends in '=', the result is tok1 or tok3
+// respectively. Otherwise, the result is tok0 if there was no other
+// matching character, or tok2 if the matching character was ch2.
+
+func (s *Scanner) switch2(tok0, tok1 token.Token) token.Token {
+	if s.ch == '=' {
+		s.next()
+		return tok1
+	}
+	return tok0
+}
+
+func (s *Scanner) switch3(tok0, tok1 token.Token, ch2 rune, tok2 token.Token) token.Token {
+	if s.ch == '=' {
+		s.next()
+		return tok1
+	}
+	if s.ch == ch2 {
+		s.next()
+		return tok2
+	}
+	return tok0
+}
+
+func (s *Scanner) switch4(tok0, tok1 token.Token, ch2 rune, tok2, tok3 token.Token) token.Token {
+	if s.ch == '=' {
+		s.next()
+		return tok1
+	}
+	if s.ch == ch2 {
+		s.next()
+		if s.ch == '=' {
+			s.next()
+			return tok3
+		}
+		return tok2
+	}
+	return tok0
+}
+
+// Scan scans the next token and returns the token position, the token,
+// and its literal string if applicable. The source end is indicated by
+// token.EOF.
+//
+// If the returned token is a literal (token.IDENT, token.INT, token.FLOAT,
+// token.IMAG, token.CHAR, token.STRING) or token.COMMENT, the literal string
+// has the corresponding value.
+//
+// If the returned token is a keyword, the literal string is the keyword.
+//
+// If the returned token is token.SEMICOLON, the corresponding
+// literal string is ";" if the semicolon was present in the source,
+// and "\n" if the semicolon was inserted because of a newline or
+// at EOF.
+//
+// If the returned token is token.ILLEGAL, the literal string is the
+// offending character.
+//
+// In all other cases, Scan returns an empty literal string.
+//
+// For more tolerant parsing, Scan will return a valid token if
+// possible even if a syntax error was encountered. Thus, even
+// if the resulting token sequence contains no illegal tokens,
+// a client may not assume that no error occurred. Instead it
+// must check the scanner's ErrorCount or the number of calls
+// of the error handler, if there was one installed.
+//
+// Scan adds line information to the file added to the file
+// set with Init. Token positions are relative to that file
+// and thus relative to the file set.
+func (s *Scanner) Scan() (pos token.Pos, tok token.Token, lit string) {
+scanAgain:
+	s.skipWhitespace()
+
+	// current token start
+	pos = s.file.Pos(s.offset)
+
+	// determine token value
+	insertSemi := false
+	switch ch := s.ch; {
+	case isLetter(ch):
+		lit = s.scanIdentifier()
+		if len(lit) > 1 {
+			// keywords are longer than one letter - avoid lookup otherwise
+			tok = token.Lookup(lit)
+			switch tok {
+			case token.IDENT, token.BREAK, token.CONTINUE, token.FALLTHROUGH, token.RETURN:
+				insertSemi = true
+			}
+		} else {
+			insertSemi = true
+			tok = token.IDENT
+		}
+	case isDecimal(ch) || ch == '.' && isDecimal(rune(s.peek())):
+		insertSemi = true
+		tok, lit = s.scanNumber()
+	default:
+		s.next() // always make progress
+		switch ch {
+		case -1:
+			if s.insertSemi {
+				s.insertSemi = false // EOF consumed
+				return pos, token.SEMICOLON, "\n"
+			}
+			tok = token.EOF
+		case '\n':
+			// we only reach here if s.insertSemi was
+			// set in the first place and exited early
+			// from s.skipWhitespace()
+			s.insertSemi = false // newline consumed
+			return pos, token.SEMICOLON, "\n"
+		case '"':
+			insertSemi = true
+			tok = token.STRING
+			lit = s.scanString()
+		case '\'':
+			insertSemi = true
+			tok = token.CHAR
+			lit = s.scanRune()
+		case '`':
+			insertSemi = true
+			tok = token.STRING
+			lit = s.scanRawString()
+		case ':':
+			tok = s.switch2(token.COLON, token.DEFINE)
+		case '.':
+			// fractions starting with a '.' are handled by outer switch
+			tok = token.PERIOD
+			if s.ch == '.' && s.peek() == '.' {
+				s.next()
+				s.next() // consume last '.'
+				tok = token.ELLIPSIS
+			}
+		case ',':
+			tok = token.COMMA
+		case ';':
+			tok = token.SEMICOLON
+			lit = ";"
+		case '(':
+			tok = token.LPAREN
+		case ')':
+			insertSemi = true
+			tok = token.RPAREN
+		case '[':
+			tok = token.LBRACK
+		case ']':
+			insertSemi = true
+			tok = token.RBRACK
+		case '{':
+			tok = token.LBRACE
+		case '}':
+			insertSemi = true
+			tok = token.RBRACE
+		case '+':
+			tok = s.switch3(token.ADD, token.ADD_ASSIGN, '+', token.INC)
+			if tok == token.INC {
+				insertSemi = true
+			}
+		case '-':
+			tok = s.switch3(token.SUB, token.SUB_ASSIGN, '-', token.DEC)
+			if tok == token.DEC {
+				insertSemi = true
+			}
+		case '*':
+			tok = s.switch2(token.MUL, token.MUL_ASSIGN)
+		case '/':
+			if s.ch == '/' || s.ch == '*' {
+				// comment
+				if s.insertSemi && s.findLineEnd() {
+					// reset position to the beginning of the comment
+					s.ch = '/'
+					s.offset = s.file.Offset(pos)
+					s.rdOffset = s.offset + 1
+					s.insertSemi = false // newline consumed
+					return pos, token.SEMICOLON, "\n"
+				}
+				comment := s.scanComment()
+				if s.mode&ScanComments == 0 {
+					// skip comment
+					s.insertSemi = false // newline consumed
+					goto scanAgain
+				}
+				tok = token.COMMENT
+				lit = comment
+			} else {
+				tok = s.switch2(token.QUO, token.QUO_ASSIGN)
+			}
+		case '%':
+			tok = s.switch2(token.REM, token.REM_ASSIGN)
+		case '^':
+			tok = s.switch2(token.XOR, token.XOR_ASSIGN)
+		case '<':
+			if s.ch == '-' {
+				s.next()
+				tok = token.ARROW
+			} else {
+				tok = s.switch4(token.LSS, token.LEQ, '<', token.SHL, token.SHL_ASSIGN)
+			}
+		case '>':
+			tok = s.switch4(token.GTR, token.GEQ, '>', token.SHR, token.SHR_ASSIGN)
+		case '=':
+			tok = s.switch2(token.ASSIGN, token.EQL)
+		case '!':
+			tok = s.switch2(token.NOT, token.NEQ)
+		case '&':
+			if s.ch == '^' {
+				s.next()
+				tok = s.switch2(token.AND_NOT, token.AND_NOT_ASSIGN)
+			} else {
+				tok = s.switch3(token.AND, token.AND_ASSIGN, '&', token.LAND)
+			}
+		case '|':
+			tok = s.switch3(token.OR, token.OR_ASSIGN, '|', token.LOR)
+		case '~':
+			tok = token.TILDE
+		default:
+			// next reports unexpected BOMs - don't repeat
+			if ch != bom {
+				s.errorf(s.file.Offset(pos), "illegal character %#U", ch)
+			}
+			insertSemi = s.insertSemi // preserve insertSemi info
+			tok = token.ILLEGAL
+			lit = string(ch)
+		}
+	}
+	if s.mode&dontInsertSemis == 0 {
+		s.insertSemi = insertSemi
+	}
+
+	return
+}
diff --git a/internal/backport/go/scanner/scanner_test.go b/internal/backport/go/scanner/scanner_test.go
new file mode 100644
index 0000000..d56abcb
--- /dev/null
+++ b/internal/backport/go/scanner/scanner_test.go
@@ -0,0 +1,1127 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner
+
+import (
+	"golang.org/x/website/internal/backport/go/token"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+var fset = token.NewFileSet()
+
+const /* class */ (
+	special = iota
+	literal
+	operator
+	keyword
+)
+
+func tokenclass(tok token.Token) int {
+	switch {
+	case tok.IsLiteral():
+		return literal
+	case tok.IsOperator():
+		return operator
+	case tok.IsKeyword():
+		return keyword
+	}
+	return special
+}
+
+type elt struct {
+	tok   token.Token
+	lit   string
+	class int
+}
+
+var tokens = []elt{
+	// Special tokens
+	{token.COMMENT, "/* a comment */", special},
+	{token.COMMENT, "// a comment \n", special},
+	{token.COMMENT, "/*\r*/", special},
+	{token.COMMENT, "/**\r/*/", special}, // issue 11151
+	{token.COMMENT, "/**\r\r/*/", special},
+	{token.COMMENT, "//\r\n", special},
+
+	// Identifiers and basic type literals
+	{token.IDENT, "foobar", literal},
+	{token.IDENT, "a۰۱۸", literal},
+	{token.IDENT, "foo६४", literal},
+	{token.IDENT, "bar９８７６", literal},
+	{token.IDENT, "ŝ", literal},    // was bug (issue 4000)
+	{token.IDENT, "ŝfoo", literal}, // was bug (issue 4000)
+	{token.INT, "0", literal},
+	{token.INT, "1", literal},
+	{token.INT, "123456789012345678890", literal},
+	{token.INT, "01234567", literal},
+	{token.INT, "0xcafebabe", literal},
+	{token.FLOAT, "0.", literal},
+	{token.FLOAT, ".0", literal},
+	{token.FLOAT, "3.14159265", literal},
+	{token.FLOAT, "1e0", literal},
+	{token.FLOAT, "1e+100", literal},
+	{token.FLOAT, "1e-100", literal},
+	{token.FLOAT, "2.71828e-1000", literal},
+	{token.IMAG, "0i", literal},
+	{token.IMAG, "1i", literal},
+	{token.IMAG, "012345678901234567889i", literal},
+	{token.IMAG, "123456789012345678890i", literal},
+	{token.IMAG, "0.i", literal},
+	{token.IMAG, ".0i", literal},
+	{token.IMAG, "3.14159265i", literal},
+	{token.IMAG, "1e0i", literal},
+	{token.IMAG, "1e+100i", literal},
+	{token.IMAG, "1e-100i", literal},
+	{token.IMAG, "2.71828e-1000i", literal},
+	{token.CHAR, "'a'", literal},
+	{token.CHAR, "'\\000'", literal},
+	{token.CHAR, "'\\xFF'", literal},
+	{token.CHAR, "'\\uff16'", literal},
+	{token.CHAR, "'\\U0000ff16'", literal},
+	{token.STRING, "`foobar`", literal},
+	{token.STRING, "`" + `foo
+	                        bar` +
+		"`",
+		literal,
+	},
+	{token.STRING, "`\r`", literal},
+	{token.STRING, "`foo\r\nbar`", literal},
+
+	// Operators and delimiters
+	{token.ADD, "+", operator},
+	{token.SUB, "-", operator},
+	{token.MUL, "*", operator},
+	{token.QUO, "/", operator},
+	{token.REM, "%", operator},
+
+	{token.AND, "&", operator},
+	{token.OR, "|", operator},
+	{token.XOR, "^", operator},
+	{token.SHL, "<<", operator},
+	{token.SHR, ">>", operator},
+	{token.AND_NOT, "&^", operator},
+
+	{token.ADD_ASSIGN, "+=", operator},
+	{token.SUB_ASSIGN, "-=", operator},
+	{token.MUL_ASSIGN, "*=", operator},
+	{token.QUO_ASSIGN, "/=", operator},
+	{token.REM_ASSIGN, "%=", operator},
+
+	{token.AND_ASSIGN, "&=", operator},
+	{token.OR_ASSIGN, "|=", operator},
+	{token.XOR_ASSIGN, "^=", operator},
+	{token.SHL_ASSIGN, "<<=", operator},
+	{token.SHR_ASSIGN, ">>=", operator},
+	{token.AND_NOT_ASSIGN, "&^=", operator},
+
+	{token.LAND, "&&", operator},
+	{token.LOR, "||", operator},
+	{token.ARROW, "<-", operator},
+	{token.INC, "++", operator},
+	{token.DEC, "--", operator},
+
+	{token.EQL, "==", operator},
+	{token.LSS, "<", operator},
+	{token.GTR, ">", operator},
+	{token.ASSIGN, "=", operator},
+	{token.NOT, "!", operator},
+
+	{token.NEQ, "!=", operator},
+	{token.LEQ, "<=", operator},
+	{token.GEQ, ">=", operator},
+	{token.DEFINE, ":=", operator},
+	{token.ELLIPSIS, "...", operator},
+
+	{token.LPAREN, "(", operator},
+	{token.LBRACK, "[", operator},
+	{token.LBRACE, "{", operator},
+	{token.COMMA, ",", operator},
+	{token.PERIOD, ".", operator},
+
+	{token.RPAREN, ")", operator},
+	{token.RBRACK, "]", operator},
+	{token.RBRACE, "}", operator},
+	{token.SEMICOLON, ";", operator},
+	{token.COLON, ":", operator},
+	{token.TILDE, "~", operator},
+
+	// Keywords
+	{token.BREAK, "break", keyword},
+	{token.CASE, "case", keyword},
+	{token.CHAN, "chan", keyword},
+	{token.CONST, "const", keyword},
+	{token.CONTINUE, "continue", keyword},
+
+	{token.DEFAULT, "default", keyword},
+	{token.DEFER, "defer", keyword},
+	{token.ELSE, "else", keyword},
+	{token.FALLTHROUGH, "fallthrough", keyword},
+	{token.FOR, "for", keyword},
+
+	{token.FUNC, "func", keyword},
+	{token.GO, "go", keyword},
+	{token.GOTO, "goto", keyword},
+	{token.IF, "if", keyword},
+	{token.IMPORT, "import", keyword},
+
+	{token.INTERFACE, "interface", keyword},
+	{token.MAP, "map", keyword},
+	{token.PACKAGE, "package", keyword},
+	{token.RANGE, "range", keyword},
+	{token.RETURN, "return", keyword},
+
+	{token.SELECT, "select", keyword},
+	{token.STRUCT, "struct", keyword},
+	{token.SWITCH, "switch", keyword},
+	{token.TYPE, "type", keyword},
+	{token.VAR, "var", keyword},
+}
+
+const whitespace = "  \t  \n\n\n" // to separate tokens
+
+var source = func() []byte {
+	var src []byte
+	for _, t := range tokens {
+		src = append(src, t.lit...)
+		src = append(src, whitespace...)
+	}
+	return src
+}()
+
+func newlineCount(s string) int {
+	n := 0
+	for i := 0; i < len(s); i++ {
+		if s[i] == '\n' {
+			n++
+		}
+	}
+	return n
+}
+
+func checkPos(t *testing.T, lit string, p token.Pos, expected token.Position) {
+	pos := fset.Position(p)
+	// Check cleaned filenames so that we don't have to worry about
+	// different os.PathSeparator values.
+	if pos.Filename != expected.Filename && filepath.Clean(pos.Filename) != filepath.Clean(expected.Filename) {
+		t.Errorf("bad filename for %q: got %s, expected %s", lit, pos.Filename, expected.Filename)
+	}
+	if pos.Offset != expected.Offset {
+		t.Errorf("bad position for %q: got %d, expected %d", lit, pos.Offset, expected.Offset)
+	}
+	if pos.Line != expected.Line {
+		t.Errorf("bad line for %q: got %d, expected %d", lit, pos.Line, expected.Line)
+	}
+	if pos.Column != expected.Column {
+		t.Errorf("bad column for %q: got %d, expected %d", lit, pos.Column, expected.Column)
+	}
+}
+
+// Verify that calling Scan() provides the correct results.
+func TestScan(t *testing.T) {
+	whitespace_linecount := newlineCount(whitespace)
+
+	// error handler
+	eh := func(_ token.Position, msg string) {
+		t.Errorf("error handler called (msg = %s)", msg)
+	}
+
+	// verify scan
+	var s Scanner
+	s.Init(fset.AddFile("", fset.Base(), len(source)), source, eh, ScanComments|dontInsertSemis)
+
+	// set up expected position
+	epos := token.Position{
+		Filename: "",
+		Offset:   0,
+		Line:     1,
+		Column:   1,
+	}
+
+	index := 0
+	for {
+		pos, tok, lit := s.Scan()
+
+		// check position
+		if tok == token.EOF {
+			// correction for EOF
+			epos.Line = newlineCount(string(source))
+			epos.Column = 2
+		}
+		checkPos(t, lit, pos, epos)
+
+		// check token
+		e := elt{token.EOF, "", special}
+		if index < len(tokens) {
+			e = tokens[index]
+			index++
+		}
+		if tok != e.tok {
+			t.Errorf("bad token for %q: got %s, expected %s", lit, tok, e.tok)
+		}
+
+		// check token class
+		if tokenclass(tok) != e.class {
+			t.Errorf("bad class for %q: got %d, expected %d", lit, tokenclass(tok), e.class)
+		}
+
+		// check literal
+		elit := ""
+		switch e.tok {
+		case token.COMMENT:
+			// no CRs in comments
+			elit = string(stripCR([]byte(e.lit), e.lit[1] == '*'))
+			//-style comment literal doesn't contain newline
+			if elit[1] == '/' {
+				elit = elit[0 : len(elit)-1]
+			}
+		case token.IDENT:
+			elit = e.lit
+		case token.SEMICOLON:
+			elit = ";"
+		default:
+			if e.tok.IsLiteral() {
+				// no CRs in raw string literals
+				elit = e.lit
+				if elit[0] == '`' {
+					elit = string(stripCR([]byte(elit), false))
+				}
+			} else if e.tok.IsKeyword() {
+				elit = e.lit
+			}
+		}
+		if lit != elit {
+			t.Errorf("bad literal for %q: got %q, expected %q", lit, lit, elit)
+		}
+
+		if tok == token.EOF {
+			break
+		}
+
+		// update position
+		epos.Offset += len(e.lit) + len(whitespace)
+		epos.Line += newlineCount(e.lit) + whitespace_linecount
+
+	}
+
+	if s.ErrorCount != 0 {
+		t.Errorf("found %d errors", s.ErrorCount)
+	}
+}
+
+func TestStripCR(t *testing.T) {
+	for _, test := range []struct{ have, want string }{
+		{"//\n", "//\n"},
+		{"//\r\n", "//\n"},
+		{"//\r\r\r\n", "//\n"},
+		{"//\r*\r/\r\n", "//*/\n"},
+		{"/**/", "/**/"},
+		{"/*\r/*/", "/*/*/"},
+		{"/*\r*/", "/**/"},
+		{"/**\r/*/", "/**\r/*/"},
+		{"/*\r/\r*\r/*/", "/*/*\r/*/"},
+		{"/*\r\r\r\r*/", "/**/"},
+	} {
+		got := string(stripCR([]byte(test.have), len(test.have) >= 2 && test.have[1] == '*'))
+		if got != test.want {
+			t.Errorf("stripCR(%q) = %q; want %q", test.have, got, test.want)
+		}
+	}
+}
+
+func checkSemi(t *testing.T, line string, mode Mode) {
+	var S Scanner
+	file := fset.AddFile("TestSemis", fset.Base(), len(line))
+	S.Init(file, []byte(line), nil, mode)
+	pos, tok, lit := S.Scan()
+	for tok != token.EOF {
+		if tok == token.ILLEGAL {
+			// the illegal token literal indicates what
+			// kind of semicolon literal to expect
+			semiLit := "\n"
+			if lit[0] == '#' {
+				semiLit = ";"
+			}
+			// next token must be a semicolon
+			semiPos := file.Position(pos)
+			semiPos.Offset++
+			semiPos.Column++
+			pos, tok, lit = S.Scan()
+			if tok == token.SEMICOLON {
+				if lit != semiLit {
+					t.Errorf(`bad literal for %q: got %q, expected %q`, line, lit, semiLit)
+				}
+				checkPos(t, line, pos, semiPos)
+			} else {
+				t.Errorf("bad token for %q: got %s, expected ;", line, tok)
+			}
+		} else if tok == token.SEMICOLON {
+			t.Errorf("bad token for %q: got ;, expected no ;", line)
+		}
+		pos, tok, lit = S.Scan()
+	}
+}
+
+var lines = []string{
+	// # indicates a semicolon present in the source
+	// $ indicates an automatically inserted semicolon
+	"",
+	"\ufeff#;", // first BOM is ignored
+	"#;",
+	"foo$\n",
+	"123$\n",
+	"1.2$\n",
+	"'x'$\n",
+	`"x"` + "$\n",
+	"`x`$\n",
+
+	"+\n",
+	"-\n",
+	"*\n",
+	"/\n",
+	"%\n",
+
+	"&\n",
+	"|\n",
+	"^\n",
+	"<<\n",
+	">>\n",
+	"&^\n",
+
+	"+=\n",
+	"-=\n",
+	"*=\n",
+	"/=\n",
+	"%=\n",
+
+	"&=\n",
+	"|=\n",
+	"^=\n",
+	"<<=\n",
+	">>=\n",
+	"&^=\n",
+
+	"&&\n",
+	"||\n",
+	"<-\n",
+	"++$\n",
+	"--$\n",
+
+	"==\n",
+	"<\n",
+	">\n",
+	"=\n",
+	"!\n",
+
+	"!=\n",
+	"<=\n",
+	">=\n",
+	":=\n",
+	"...\n",
+
+	"(\n",
+	"[\n",
+	"{\n",
+	",\n",
+	".\n",
+
+	")$\n",
+	"]$\n",
+	"}$\n",
+	"#;\n",
+	":\n",
+
+	"break$\n",
+	"case\n",
+	"chan\n",
+	"const\n",
+	"continue$\n",
+
+	"default\n",
+	"defer\n",
+	"else\n",
+	"fallthrough$\n",
+	"for\n",
+
+	"func\n",
+	"go\n",
+	"goto\n",
+	"if\n",
+	"import\n",
+
+	"interface\n",
+	"map\n",
+	"package\n",
+	"range\n",
+	"return$\n",
+
+	"select\n",
+	"struct\n",
+	"switch\n",
+	"type\n",
+	"var\n",
+
+	"foo$//comment\n",
+	"foo$//comment",
+	"foo$/*comment*/\n",
+	"foo$/*\n*/",
+	"foo$/*comment*/    \n",
+	"foo$/*\n*/    ",
+
+	"foo    $// comment\n",
+	"foo    $// comment",
+	"foo    $/*comment*/\n",
+	"foo    $/*\n*/",
+	"foo    $/*  */ /* \n */ bar$/**/\n",
+	"foo    $/*0*/ /*1*/ /*2*/\n",
+
+	"foo    $/*comment*/    \n",
+	"foo    $/*0*/ /*1*/ /*2*/    \n",
+	"foo	$/**/ /*-------------*/       /*----\n*/bar       $/*  \n*/baa$\n",
+	"foo    $/* an EOF terminates a line */",
+	"foo    $/* an EOF terminates a line */ /*",
+	"foo    $/* an EOF terminates a line */ //",
+
+	"package main$\n\nfunc main() {\n\tif {\n\t\treturn /* */ }$\n}$\n",
+	"package main$",
+}
+
+func TestSemis(t *testing.T) {
+	for _, line := range lines {
+		checkSemi(t, line, 0)
+		checkSemi(t, line, ScanComments)
+
+		// if the input ended in newlines, the input must tokenize the
+		// same with or without those newlines
+		for i := len(line) - 1; i >= 0 && line[i] == '\n'; i-- {
+			checkSemi(t, line[0:i], 0)
+			checkSemi(t, line[0:i], ScanComments)
+		}
+	}
+}
+
+type segment struct {
+	srcline      string // a line of source text
+	filename     string // filename for current token; error message for invalid line directives
+	line, column int    // line and column for current token; error position for invalid line directives
+}
+
+var segments = []segment{
+	// exactly one token per line since the test consumes one token per segment
+	{"  line1", "TestLineDirectives", 1, 3},
+	{"\nline2", "TestLineDirectives", 2, 1},
+	{"\nline3  //line File1.go:100", "TestLineDirectives", 3, 1}, // bad line comment, ignored
+	{"\nline4", "TestLineDirectives", 4, 1},
+	{"\n//line File1.go:100\n  line100", "File1.go", 100, 0},
+	{"\n//line  \t :42\n  line1", " \t ", 42, 0},
+	{"\n//line File2.go:200\n  line200", "File2.go", 200, 0},
+	{"\n//line foo\t:42\n  line42", "foo\t", 42, 0},
+	{"\n //line foo:42\n  line43", "foo\t", 44, 0}, // bad line comment, ignored (use existing, prior filename)
+	{"\n//line foo 42\n  line44", "foo\t", 46, 0},  // bad line comment, ignored (use existing, prior filename)
+	{"\n//line /bar:42\n  line45", "/bar", 42, 0},
+	{"\n//line ./foo:42\n  line46", "foo", 42, 0},
+	{"\n//line a/b/c/File1.go:100\n  line100", "a/b/c/File1.go", 100, 0},
+	{"\n//line c:\\bar:42\n  line200", "c:\\bar", 42, 0},
+	{"\n//line c:\\dir\\File1.go:100\n  line201", "c:\\dir\\File1.go", 100, 0},
+
+	// tests for new line directive syntax
+	{"\n//line :100\na1", "", 100, 0}, // missing filename means empty filename
+	{"\n//line bar:100\nb1", "bar", 100, 0},
+	{"\n//line :100:10\nc1", "bar", 100, 10}, // missing filename means current filename
+	{"\n//line foo:100:10\nd1", "foo", 100, 10},
+
+	{"\n/*line :100*/a2", "", 100, 0}, // missing filename means empty filename
+	{"\n/*line bar:100*/b2", "bar", 100, 0},
+	{"\n/*line :100:10*/c2", "bar", 100, 10}, // missing filename means current filename
+	{"\n/*line foo:100:10*/d2", "foo", 100, 10},
+	{"\n/*line foo:100:10*/    e2", "foo", 100, 14}, // line-directive relative column
+	{"\n/*line foo:100:10*/\n\nf2", "foo", 102, 1},  // absolute column since on new line
+}
+
+var dirsegments = []segment{
+	// exactly one token per line since the test consumes one token per segment
+	{"  line1", "TestLineDir/TestLineDirectives", 1, 3},
+	{"\n//line File1.go:100\n  line100", "TestLineDir/File1.go", 100, 0},
+}
+
+var dirUnixSegments = []segment{
+	{"\n//line /bar:42\n  line42", "/bar", 42, 0},
+}
+
+var dirWindowsSegments = []segment{
+	{"\n//line c:\\bar:42\n  line42", "c:\\bar", 42, 0},
+}
+
+// Verify that line directives are interpreted correctly.
+func TestLineDirectives(t *testing.T) {
+	testSegments(t, segments, "TestLineDirectives")
+	testSegments(t, dirsegments, "TestLineDir/TestLineDirectives")
+	if runtime.GOOS == "windows" {
+		testSegments(t, dirWindowsSegments, "TestLineDir/TestLineDirectives")
+	} else {
+		testSegments(t, dirUnixSegments, "TestLineDir/TestLineDirectives")
+	}
+}
+
+func testSegments(t *testing.T, segments []segment, filename string) {
+	var src string
+	for _, e := range segments {
+		src += e.srcline
+	}
+
+	// verify scan
+	var S Scanner
+	file := fset.AddFile(filename, fset.Base(), len(src))
+	S.Init(file, []byte(src), func(pos token.Position, msg string) { t.Error(Error{pos, msg}) }, dontInsertSemis)
+	for _, s := range segments {
+		p, _, lit := S.Scan()
+		pos := file.Position(p)
+		checkPos(t, lit, p, token.Position{
+			Filename: s.filename,
+			Offset:   pos.Offset,
+			Line:     s.line,
+			Column:   s.column,
+		})
+	}
+
+	if S.ErrorCount != 0 {
+		t.Errorf("got %d errors", S.ErrorCount)
+	}
+}
+
+// The filename is used for the error message in these test cases.
+// The first line directive is valid and used to control the expected error line.
+var invalidSegments = []segment{
+	{"\n//line :1:1\n//line foo:42 extra text\ndummy", "invalid line number: 42 extra text", 1, 12},
+	{"\n//line :2:1\n//line foobar:\ndummy", "invalid line number: ", 2, 15},
+	{"\n//line :5:1\n//line :0\ndummy", "invalid line number: 0", 5, 9},
+	{"\n//line :10:1\n//line :1:0\ndummy", "invalid column number: 0", 10, 11},
+	{"\n//line :1:1\n//line :foo:0\ndummy", "invalid line number: 0", 1, 13}, // foo is considered part of the filename
+}
+
+// Verify that invalid line directives get the correct error message.
+func TestInvalidLineDirectives(t *testing.T) {
+	// make source
+	var src string
+	for _, e := range invalidSegments {
+		src += e.srcline
+	}
+
+	// verify scan
+	var S Scanner
+	var s segment // current segment
+	file := fset.AddFile(filepath.Join("dir", "TestInvalidLineDirectives"), fset.Base(), len(src))
+	S.Init(file, []byte(src), func(pos token.Position, msg string) {
+		if msg != s.filename {
+			t.Errorf("got error %q; want %q", msg, s.filename)
+		}
+		if pos.Line != s.line || pos.Column != s.column {
+			t.Errorf("got position %d:%d; want %d:%d", pos.Line, pos.Column, s.line, s.column)
+		}
+	}, dontInsertSemis)
+	for _, s = range invalidSegments {
+		S.Scan()
+	}
+
+	if S.ErrorCount != len(invalidSegments) {
+		t.Errorf("got %d errors; want %d", S.ErrorCount, len(invalidSegments))
+	}
+}
+
+// Verify that initializing the same scanner more than once works correctly.
+func TestInit(t *testing.T) {
+	var s Scanner
+
+	// 1st init
+	src1 := "if true { }"
+	f1 := fset.AddFile("src1", fset.Base(), len(src1))
+	s.Init(f1, []byte(src1), nil, dontInsertSemis)
+	if f1.Size() != len(src1) {
+		t.Errorf("bad file size: got %d, expected %d", f1.Size(), len(src1))
+	}
+	s.Scan()              // if
+	s.Scan()              // true
+	_, tok, _ := s.Scan() // {
+	if tok != token.LBRACE {
+		t.Errorf("bad token: got %s, expected %s", tok, token.LBRACE)
+	}
+
+	// 2nd init
+	src2 := "go true { ]"
+	f2 := fset.AddFile("src2", fset.Base(), len(src2))
+	s.Init(f2, []byte(src2), nil, dontInsertSemis)
+	if f2.Size() != len(src2) {
+		t.Errorf("bad file size: got %d, expected %d", f2.Size(), len(src2))
+	}
+	_, tok, _ = s.Scan() // go
+	if tok != token.GO {
+		t.Errorf("bad token: got %s, expected %s", tok, token.GO)
+	}
+
+	if s.ErrorCount != 0 {
+		t.Errorf("found %d errors", s.ErrorCount)
+	}
+}
+
+func TestStdErrorHander(t *testing.T) {
+	const src = "@\n" + // illegal character, cause an error
+		"@ @\n" + // two errors on the same line
+		"//line File2:20\n" +
+		"@\n" + // different file, but same line
+		"//line File2:1\n" +
+		"@ @\n" + // same file, decreasing line number
+		"//line File1:1\n" +
+		"@ @ @" // original file, line 1 again
+
+	var list ErrorList
+	eh := func(pos token.Position, msg string) { list.Add(pos, msg) }
+
+	var s Scanner
+	s.Init(fset.AddFile("File1", fset.Base(), len(src)), []byte(src), eh, dontInsertSemis)
+	for {
+		if _, tok, _ := s.Scan(); tok == token.EOF {
+			break
+		}
+	}
+
+	if len(list) != s.ErrorCount {
+		t.Errorf("found %d errors, expected %d", len(list), s.ErrorCount)
+	}
+
+	if len(list) != 9 {
+		t.Errorf("found %d raw errors, expected 9", len(list))
+		PrintError(os.Stderr, list)
+	}
+
+	list.Sort()
+	if len(list) != 9 {
+		t.Errorf("found %d sorted errors, expected 9", len(list))
+		PrintError(os.Stderr, list)
+	}
+
+	list.RemoveMultiples()
+	if len(list) != 4 {
+		t.Errorf("found %d one-per-line errors, expected 4", len(list))
+		PrintError(os.Stderr, list)
+	}
+}
+
+type errorCollector struct {
+	cnt int            // number of errors encountered
+	msg string         // last error message encountered
+	pos token.Position // last error position encountered
+}
+
+func checkError(t *testing.T, src string, tok token.Token, pos int, lit, err string) {
+	var s Scanner
+	var h errorCollector
+	eh := func(pos token.Position, msg string) {
+		h.cnt++
+		h.msg = msg
+		h.pos = pos
+	}
+	s.Init(fset.AddFile("", fset.Base(), len(src)), []byte(src), eh, ScanComments|dontInsertSemis)
+	_, tok0, lit0 := s.Scan()
+	if tok0 != tok {
+		t.Errorf("%q: got %s, expected %s", src, tok0, tok)
+	}
+	if tok0 != token.ILLEGAL && lit0 != lit {
+		t.Errorf("%q: got literal %q, expected %q", src, lit0, lit)
+	}
+	cnt := 0
+	if err != "" {
+		cnt = 1
+	}
+	if h.cnt != cnt {
+		t.Errorf("%q: got cnt %d, expected %d", src, h.cnt, cnt)
+	}
+	if h.msg != err {
+		t.Errorf("%q: got msg %q, expected %q", src, h.msg, err)
+	}
+	if h.pos.Offset != pos {
+		t.Errorf("%q: got offset %d, expected %d", src, h.pos.Offset, pos)
+	}
+}
+
+var errors = []struct {
+	src string
+	tok token.Token
+	pos int
+	lit string
+	err string
+}{
+	{"\a", token.ILLEGAL, 0, "", "illegal character U+0007"},
+	{`#`, token.ILLEGAL, 0, "", "illegal character U+0023 '#'"},
+	{`…`, token.ILLEGAL, 0, "", "illegal character U+2026 '…'"},
+	{"..", token.PERIOD, 0, "", ""}, // two periods, not invalid token (issue #28112)
+	{`' '`, token.CHAR, 0, `' '`, ""},
+	{`''`, token.CHAR, 0, `''`, "illegal rune literal"},
+	{`'12'`, token.CHAR, 0, `'12'`, "illegal rune literal"},
+	{`'123'`, token.CHAR, 0, `'123'`, "illegal rune literal"},
+	{`'\0'`, token.CHAR, 3, `'\0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\07'`, token.CHAR, 4, `'\07'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\8'`, token.CHAR, 2, `'\8'`, "unknown escape sequence"},
+	{`'\08'`, token.CHAR, 3, `'\08'`, "illegal character U+0038 '8' in escape sequence"},
+	{`'\x'`, token.CHAR, 3, `'\x'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\x0'`, token.CHAR, 4, `'\x0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\x0g'`, token.CHAR, 4, `'\x0g'`, "illegal character U+0067 'g' in escape sequence"},
+	{`'\u'`, token.CHAR, 3, `'\u'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u0'`, token.CHAR, 4, `'\u0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u00'`, token.CHAR, 5, `'\u00'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u000'`, token.CHAR, 6, `'\u000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u000`, token.CHAR, 6, `'\u000`, "escape sequence not terminated"},
+	{`'\u0000'`, token.CHAR, 0, `'\u0000'`, ""},
+	{`'\U'`, token.CHAR, 3, `'\U'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0'`, token.CHAR, 4, `'\U0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U00'`, token.CHAR, 5, `'\U00'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U000'`, token.CHAR, 6, `'\U000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0000'`, token.CHAR, 7, `'\U0000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U00000'`, token.CHAR, 8, `'\U00000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U000000'`, token.CHAR, 9, `'\U000000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0000000'`, token.CHAR, 10, `'\U0000000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0000000`, token.CHAR, 10, `'\U0000000`, "escape sequence not terminated"},
+	{`'\U00000000'`, token.CHAR, 0, `'\U00000000'`, ""},
+	{`'\Uffffffff'`, token.CHAR, 2, `'\Uffffffff'`, "escape sequence is invalid Unicode code point"},
+	{`'`, token.CHAR, 0, `'`, "rune literal not terminated"},
+	{`'\`, token.CHAR, 2, `'\`, "escape sequence not terminated"},
+	{"'\n", token.CHAR, 0, "'", "rune literal not terminated"},
+	{"'\n   ", token.CHAR, 0, "'", "rune literal not terminated"},
+	{`""`, token.STRING, 0, `""`, ""},
+	{`"abc`, token.STRING, 0, `"abc`, "string literal not terminated"},
+	{"\"abc\n", token.STRING, 0, `"abc`, "string literal not terminated"},
+	{"\"abc\n   ", token.STRING, 0, `"abc`, "string literal not terminated"},
+	{"``", token.STRING, 0, "``", ""},
+	{"`", token.STRING, 0, "`", "raw string literal not terminated"},
+	{"/**/", token.COMMENT, 0, "/**/", ""},
+	{"/*", token.COMMENT, 0, "/*", "comment not terminated"},
+	{"077", token.INT, 0, "077", ""},
+	{"078.", token.FLOAT, 0, "078.", ""},
+	{"07801234567.", token.FLOAT, 0, "07801234567.", ""},
+	{"078e0", token.FLOAT, 0, "078e0", ""},
+	{"0E", token.FLOAT, 2, "0E", "exponent has no digits"}, // issue 17621
+	{"078", token.INT, 2, "078", "invalid digit '8' in octal literal"},
+	{"07090000008", token.INT, 3, "07090000008", "invalid digit '9' in octal literal"},
+	{"0x", token.INT, 2, "0x", "hexadecimal literal has no digits"},
+	{"\"abc\x00def\"", token.STRING, 4, "\"abc\x00def\"", "illegal character NUL"},
+	{"\"abc\x80def\"", token.STRING, 4, "\"abc\x80def\"", "illegal UTF-8 encoding"},
+	{"\ufeff\ufeff", token.ILLEGAL, 3, "\ufeff\ufeff", "illegal byte order mark"},                        // only first BOM is ignored
+	{"//\ufeff", token.COMMENT, 2, "//\ufeff", "illegal byte order mark"},                                // only first BOM is ignored
+	{"'\ufeff" + `'`, token.CHAR, 1, "'\ufeff" + `'`, "illegal byte order mark"},                         // only first BOM is ignored
+	{`"` + "abc\ufeffdef" + `"`, token.STRING, 4, `"` + "abc\ufeffdef" + `"`, "illegal byte order mark"}, // only first BOM is ignored
+	{"abc\x00def", token.IDENT, 3, "abc", "illegal character NUL"},
+	{"abc\x00", token.IDENT, 3, "abc", "illegal character NUL"},
+}
+
+func TestScanErrors(t *testing.T) {
+	for _, e := range errors {
+		checkError(t, e.src, e.tok, e.pos, e.lit, e.err)
+	}
+}
+
+// Verify that no comments show up as literal values when skipping comments.
+func TestIssue10213(t *testing.T) {
+	const src = `
+		var (
+			A = 1 // foo
+		)
+
+		var (
+			B = 2
+			// foo
+		)
+
+		var C = 3 // foo
+
+		var D = 4
+		// foo
+
+		func anycode() {
+		// foo
+		}
+	`
+	var s Scanner
+	s.Init(fset.AddFile("", fset.Base(), len(src)), []byte(src), nil, 0)
+	for {
+		pos, tok, lit := s.Scan()
+		class := tokenclass(tok)
+		if lit != "" && class != keyword && class != literal && tok != token.SEMICOLON {
+			t.Errorf("%s: tok = %s, lit = %q", fset.Position(pos), tok, lit)
+		}
+		if tok <= token.EOF {
+			break
+		}
+	}
+}
+
+func TestIssue28112(t *testing.T) {
+	const src = "... .. 0.. .." // make sure to have stand-alone ".." immediately before EOF to test EOF behavior
+	tokens := []token.Token{token.ELLIPSIS, token.PERIOD, token.PERIOD, token.FLOAT, token.PERIOD, token.PERIOD, token.PERIOD, token.EOF}
+	var s Scanner
+	s.Init(fset.AddFile("", fset.Base(), len(src)), []byte(src), nil, 0)
+	for _, want := range tokens {
+		pos, got, lit := s.Scan()
+		if got != want {
+			t.Errorf("%s: got %s, want %s", fset.Position(pos), got, want)
+		}
+		// literals expect to have a (non-empty) literal string and we don't care about other tokens for this test
+		if tokenclass(got) == literal && lit == "" {
+			t.Errorf("%s: for %s got empty literal string", fset.Position(pos), got)
+		}
+	}
+}
+
+func BenchmarkScan(b *testing.B) {
+	b.StopTimer()
+	fset := token.NewFileSet()
+	file := fset.AddFile("", fset.Base(), len(source))
+	var s Scanner
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		s.Init(file, source, nil, ScanComments)
+		for {
+			_, tok, _ := s.Scan()
+			if tok == token.EOF {
+				break
+			}
+		}
+	}
+}
+
+func BenchmarkScanFiles(b *testing.B) {
+	// Scan a few arbitrary large files, and one small one, to provide some
+	// variety in benchmarks.
+	for _, p := range []string{
+		"golang.org/x/website/internal/backport/go/types/expr.go",
+		"golang.org/x/website/internal/backport/go/parser/parser.go",
+		"net/http/server.go",
+		"golang.org/x/website/internal/backport/go/scanner/errors.go",
+	} {
+		b.Run(p, func(b *testing.B) {
+			b.StopTimer()
+			filename := filepath.Join("..", "..", filepath.FromSlash(p))
+			src, err := os.ReadFile(filename)
+			if err != nil {
+				b.Fatal(err)
+			}
+			fset := token.NewFileSet()
+			file := fset.AddFile(filename, fset.Base(), len(src))
+			b.SetBytes(int64(len(src)))
+			var s Scanner
+			b.StartTimer()
+			for i := 0; i < b.N; i++ {
+				s.Init(file, src, nil, ScanComments)
+				for {
+					_, tok, _ := s.Scan()
+					if tok == token.EOF {
+						break
+					}
+				}
+			}
+		})
+	}
+}
+
+func TestNumbers(t *testing.T) {
+	for _, test := range []struct {
+		tok              token.Token
+		src, tokens, err string
+	}{
+		// binaries
+		{token.INT, "0b0", "0b0", ""},
+		{token.INT, "0b1010", "0b1010", ""},
+		{token.INT, "0B1110", "0B1110", ""},
+
+		{token.INT, "0b", "0b", "binary literal has no digits"},
+		{token.INT, "0b0190", "0b0190", "invalid digit '9' in binary literal"},
+		{token.INT, "0b01a0", "0b01 a0", ""}, // only accept 0-9
+
+		{token.FLOAT, "0b.", "0b.", "invalid radix point in binary literal"},
+		{token.FLOAT, "0b.1", "0b.1", "invalid radix point in binary literal"},
+		{token.FLOAT, "0b1.0", "0b1.0", "invalid radix point in binary literal"},
+		{token.FLOAT, "0b1e10", "0b1e10", "'e' exponent requires decimal mantissa"},
+		{token.FLOAT, "0b1P-1", "0b1P-1", "'P' exponent requires hexadecimal mantissa"},
+
+		{token.IMAG, "0b10i", "0b10i", ""},
+		{token.IMAG, "0b10.0i", "0b10.0i", "invalid radix point in binary literal"},
+
+		// octals
+		{token.INT, "0o0", "0o0", ""},
+		{token.INT, "0o1234", "0o1234", ""},
+		{token.INT, "0O1234", "0O1234", ""},
+
+		{token.INT, "0o", "0o", "octal literal has no digits"},
+		{token.INT, "0o8123", "0o8123", "invalid digit '8' in octal literal"},
+		{token.INT, "0o1293", "0o1293", "invalid digit '9' in octal literal"},
+		{token.INT, "0o12a3", "0o12 a3", ""}, // only accept 0-9
+
+		{token.FLOAT, "0o.", "0o.", "invalid radix point in octal literal"},
+		{token.FLOAT, "0o.2", "0o.2", "invalid radix point in octal literal"},
+		{token.FLOAT, "0o1.2", "0o1.2", "invalid radix point in octal literal"},
+		{token.FLOAT, "0o1E+2", "0o1E+2", "'E' exponent requires decimal mantissa"},
+		{token.FLOAT, "0o1p10", "0o1p10", "'p' exponent requires hexadecimal mantissa"},
+
+		{token.IMAG, "0o10i", "0o10i", ""},
+		{token.IMAG, "0o10e0i", "0o10e0i", "'e' exponent requires decimal mantissa"},
+
+		// 0-octals
+		{token.INT, "0", "0", ""},
+		{token.INT, "0123", "0123", ""},
+
+		{token.INT, "08123", "08123", "invalid digit '8' in octal literal"},
+		{token.INT, "01293", "01293", "invalid digit '9' in octal literal"},
+		{token.INT, "0F.", "0 F .", ""}, // only accept 0-9
+		{token.INT, "0123F.", "0123 F .", ""},
+		{token.INT, "0123456x", "0123456 x", ""},
+
+		// decimals
+		{token.INT, "1", "1", ""},
+		{token.INT, "1234", "1234", ""},
+
+		{token.INT, "1f", "1 f", ""}, // only accept 0-9
+
+		{token.IMAG, "0i", "0i", ""},
+		{token.IMAG, "0678i", "0678i", ""},
+
+		// decimal floats
+		{token.FLOAT, "0.", "0.", ""},
+		{token.FLOAT, "123.", "123.", ""},
+		{token.FLOAT, "0123.", "0123.", ""},
+
+		{token.FLOAT, ".0", ".0", ""},
+		{token.FLOAT, ".123", ".123", ""},
+		{token.FLOAT, ".0123", ".0123", ""},
+
+		{token.FLOAT, "0.0", "0.0", ""},
+		{token.FLOAT, "123.123", "123.123", ""},
+		{token.FLOAT, "0123.0123", "0123.0123", ""},
+
+		{token.FLOAT, "0e0", "0e0", ""},
+		{token.FLOAT, "123e+0", "123e+0", ""},
+		{token.FLOAT, "0123E-1", "0123E-1", ""},
+
+		{token.FLOAT, "0.e+1", "0.e+1", ""},
+		{token.FLOAT, "123.E-10", "123.E-10", ""},
+		{token.FLOAT, "0123.e123", "0123.e123", ""},
+
+		{token.FLOAT, ".0e-1", ".0e-1", ""},
+		{token.FLOAT, ".123E+10", ".123E+10", ""},
+		{token.FLOAT, ".0123E123", ".0123E123", ""},
+
+		{token.FLOAT, "0.0e1", "0.0e1", ""},
+		{token.FLOAT, "123.123E-10", "123.123E-10", ""},
+		{token.FLOAT, "0123.0123e+456", "0123.0123e+456", ""},
+
+		{token.FLOAT, "0e", "0e", "exponent has no digits"},
+		{token.FLOAT, "0E+", "0E+", "exponent has no digits"},
+		{token.FLOAT, "1e+f", "1e+ f", "exponent has no digits"},
+		{token.FLOAT, "0p0", "0p0", "'p' exponent requires hexadecimal mantissa"},
+		{token.FLOAT, "1.0P-1", "1.0P-1", "'P' exponent requires hexadecimal mantissa"},
+
+		{token.IMAG, "0.i", "0.i", ""},
+		{token.IMAG, ".123i", ".123i", ""},
+		{token.IMAG, "123.123i", "123.123i", ""},
+		{token.IMAG, "123e+0i", "123e+0i", ""},
+		{token.IMAG, "123.E-10i", "123.E-10i", ""},
+		{token.IMAG, ".123E+10i", ".123E+10i", ""},
+
+		// hexadecimals
+		{token.INT, "0x0", "0x0", ""},
+		{token.INT, "0x1234", "0x1234", ""},
+		{token.INT, "0xcafef00d", "0xcafef00d", ""},
+		{token.INT, "0XCAFEF00D", "0XCAFEF00D", ""},
+
+		{token.INT, "0x", "0x", "hexadecimal literal has no digits"},
+		{token.INT, "0x1g", "0x1 g", ""},
+
+		{token.IMAG, "0xf00i", "0xf00i", ""},
+
+		// hexadecimal floats
+		{token.FLOAT, "0x0p0", "0x0p0", ""},
+		{token.FLOAT, "0x12efp-123", "0x12efp-123", ""},
+		{token.FLOAT, "0xABCD.p+0", "0xABCD.p+0", ""},
+		{token.FLOAT, "0x.0189P-0", "0x.0189P-0", ""},
+		{token.FLOAT, "0x1.ffffp+1023", "0x1.ffffp+1023", ""},
+
+		{token.FLOAT, "0x.", "0x.", "hexadecimal literal has no digits"},
+		{token.FLOAT, "0x0.", "0x0.", "hexadecimal mantissa requires a 'p' exponent"},
+		{token.FLOAT, "0x.0", "0x.0", "hexadecimal mantissa requires a 'p' exponent"},
+		{token.FLOAT, "0x1.1", "0x1.1", "hexadecimal mantissa requires a 'p' exponent"},
+		{token.FLOAT, "0x1.1e0", "0x1.1e0", "hexadecimal mantissa requires a 'p' exponent"},
+		{token.FLOAT, "0x1.2gp1a", "0x1.2 gp1a", "hexadecimal mantissa requires a 'p' exponent"},
+		{token.FLOAT, "0x0p", "0x0p", "exponent has no digits"},
+		{token.FLOAT, "0xeP-", "0xeP-", "exponent has no digits"},
+		{token.FLOAT, "0x1234PAB", "0x1234P AB", "exponent has no digits"},
+		{token.FLOAT, "0x1.2p1a", "0x1.2p1 a", ""},
+
+		{token.IMAG, "0xf00.bap+12i", "0xf00.bap+12i", ""},
+
+		// separators
+		{token.INT, "0b_1000_0001", "0b_1000_0001", ""},
+		{token.INT, "0o_600", "0o_600", ""},
+		{token.INT, "0_466", "0_466", ""},
+		{token.INT, "1_000", "1_000", ""},
+		{token.FLOAT, "1_000.000_1", "1_000.000_1", ""},
+		{token.IMAG, "10e+1_2_3i", "10e+1_2_3i", ""},
+		{token.INT, "0x_f00d", "0x_f00d", ""},
+		{token.FLOAT, "0x_f00d.0p1_2", "0x_f00d.0p1_2", ""},
+
+		{token.INT, "0b__1000", "0b__1000", "'_' must separate successive digits"},
+		{token.INT, "0o60___0", "0o60___0", "'_' must separate successive digits"},
+		{token.INT, "0466_", "0466_", "'_' must separate successive digits"},
+		{token.FLOAT, "1_.", "1_.", "'_' must separate successive digits"},
+		{token.FLOAT, "0._1", "0._1", "'_' must separate successive digits"},
+		{token.FLOAT, "2.7_e0", "2.7_e0", "'_' must separate successive digits"},
+		{token.IMAG, "10e+12_i", "10e+12_i", "'_' must separate successive digits"},
+		{token.INT, "0x___0", "0x___0", "'_' must separate successive digits"},
+		{token.FLOAT, "0x1.0_p0", "0x1.0_p0", "'_' must separate successive digits"},
+	} {
+		var s Scanner
+		var err string
+		s.Init(fset.AddFile("", fset.Base(), len(test.src)), []byte(test.src), func(_ token.Position, msg string) {
+			if err == "" {
+				err = msg
+			}
+		}, 0)
+		for i, want := range strings.Split(test.tokens, " ") {
+			err = ""
+			_, tok, lit := s.Scan()
+
+			// compute lit where for tokens where lit is not defined
+			switch tok {
+			case token.PERIOD:
+				lit = "."
+			case token.ADD:
+				lit = "+"
+			case token.SUB:
+				lit = "-"
+			}
+
+			if i == 0 {
+				if tok != test.tok {
+					t.Errorf("%q: got token %s; want %s", test.src, tok, test.tok)
+				}
+				if err != test.err {
+					t.Errorf("%q: got error %q; want %q", test.src, err, test.err)
+				}
+			}
+
+			if lit != want {
+				t.Errorf("%q: got literal %q (%s); want %s", test.src, lit, tok, want)
+			}
+		}
+
+		// make sure we read all
+		_, tok, _ := s.Scan()
+		if tok == token.SEMICOLON {
+			_, tok, _ = s.Scan()
+		}
+		if tok != token.EOF {
+			t.Errorf("%q: got %s; want EOF", test.src, tok)
+		}
+	}
+}
diff --git a/internal/backport/go/token/example_test.go b/internal/backport/go/token/example_test.go
new file mode 100644
index 0000000..e41a958
--- /dev/null
+++ b/internal/backport/go/token/example_test.go
@@ -0,0 +1,77 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token_test
+
+import (
+	"fmt"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
+)
+
+func Example_retrievePositionInfo() {
+	fset := token.NewFileSet()
+
+	const src = `package main
+
+import "fmt"
+
+import "go/token"
+
+//line :1:5
+type p = token.Pos
+
+const bad = token.NoPos
+
+//line fake.go:42:11
+func ok(pos p) bool {
+	return pos != bad
+}
+
+/*line :7:9*/func main() {
+	fmt.Println(ok(bad) == bad.IsValid())
+}
+`
+
+	f, err := parser.ParseFile(fset, "main.go", src, 0)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	// Print the location and kind of each declaration in f.
+	for _, decl := range f.Decls {
+		// Get the filename, line, and column back via the file set.
+		// We get both the relative and absolute position.
+		// The relative position is relative to the last line directive.
+		// The absolute position is the exact position in the source.
+		pos := decl.Pos()
+		relPosition := fset.Position(pos)
+		absPosition := fset.PositionFor(pos, false)
+
+		// Either a FuncDecl or GenDecl, since we exit on error.
+		kind := "func"
+		if gen, ok := decl.(*ast.GenDecl); ok {
+			kind = gen.Tok.String()
+		}
+
+		// If the relative and absolute positions differ, show both.
+		fmtPosition := relPosition.String()
+		if relPosition != absPosition {
+			fmtPosition += "[" + absPosition.String() + "]"
+		}
+
+		fmt.Printf("%s: %s\n", fmtPosition, kind)
+	}
+
+	//Output:
+	//
+	// main.go:3:1: import
+	// main.go:5:1: import
+	// main.go:1:5[main.go:8:1]: type
+	// main.go:3:1[main.go:10:1]: const
+	// fake.go:42:11[main.go:13:1]: func
+	// fake.go:7:9[main.go:17:14]: func
+}
diff --git a/internal/backport/go/token/position.go b/internal/backport/go/token/position.go
new file mode 100644
index 0000000..00f2453
--- /dev/null
+++ b/internal/backport/go/token/position.go
@@ -0,0 +1,529 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+import (
+	"fmt"
+	"sort"
+	"sync"
+)
+
+// -----------------------------------------------------------------------------
+// Positions
+
+// Position describes an arbitrary source position
+// including the file, line, and column location.
+// A Position is valid if the line number is > 0.
+type Position struct {
+	Filename string // filename, if any
+	Offset   int    // offset, starting at 0
+	Line     int    // line number, starting at 1
+	Column   int    // column number, starting at 1 (byte count)
+}
+
+// IsValid reports whether the position is valid.
+func (pos *Position) IsValid() bool { return pos.Line > 0 }
+
+// String returns a string in one of several forms:
+//
+//	file:line:column    valid position with file name
+//	file:line           valid position with file name but no column (column == 0)
+//	line:column         valid position without file name
+//	line                valid position without file name and no column (column == 0)
+//	file                invalid position with file name
+//	-                   invalid position without file name
+func (pos Position) String() string {
+	s := pos.Filename
+	if pos.IsValid() {
+		if s != "" {
+			s += ":"
+		}
+		s += fmt.Sprintf("%d", pos.Line)
+		if pos.Column != 0 {
+			s += fmt.Sprintf(":%d", pos.Column)
+		}
+	}
+	if s == "" {
+		s = "-"
+	}
+	return s
+}
+
+// Pos is a compact encoding of a source position within a file set.
+// It can be converted into a Position for a more convenient, but much
+// larger, representation.
+//
+// The Pos value for a given file is a number in the range [base, base+size],
+// where base and size are specified when a file is added to the file set.
+// The difference between a Pos value and the corresponding file base
+// corresponds to the byte offset of that position (represented by the Pos value)
+// from the beginning of the file. Thus, the file base offset is the Pos value
+// representing the first byte in the file.
+//
+// To create the Pos value for a specific source offset (measured in bytes),
+// first add the respective file to the current file set using FileSet.AddFile
+// and then call File.Pos(offset) for that file. Given a Pos value p
+// for a specific file set fset, the corresponding Position value is
+// obtained by calling fset.Position(p).
+//
+// Pos values can be compared directly with the usual comparison operators:
+// If two Pos values p and q are in the same file, comparing p and q is
+// equivalent to comparing the respective source file offsets. If p and q
+// are in different files, p < q is true if the file implied by p was added
+// to the respective file set before the file implied by q.
+type Pos int
+
+// The zero value for Pos is NoPos; there is no file and line information
+// associated with it, and NoPos.IsValid() is false. NoPos is always
+// smaller than any other Pos value. The corresponding Position value
+// for NoPos is the zero value for Position.
+const NoPos Pos = 0
+
+// IsValid reports whether the position is valid.
+func (p Pos) IsValid() bool {
+	return p != NoPos
+}
+
+// -----------------------------------------------------------------------------
+// File
+
+// A File is a handle for a file belonging to a FileSet.
+// A File has a name, size, and line offset table.
+type File struct {
+	set  *FileSet
+	name string // file name as provided to AddFile
+	base int    // Pos value range for this file is [base...base+size]
+	size int    // file size as provided to AddFile
+
+	// lines and infos are protected by mutex
+	mutex sync.Mutex
+	lines []int // lines contains the offset of the first character for each line (the first entry is always 0)
+	infos []lineInfo
+}
+
+// Name returns the file name of file f as registered with AddFile.
+func (f *File) Name() string {
+	return f.name
+}
+
+// Base returns the base offset of file f as registered with AddFile.
+func (f *File) Base() int {
+	return f.base
+}
+
+// Size returns the size of file f as registered with AddFile.
+func (f *File) Size() int {
+	return f.size
+}
+
+// LineCount returns the number of lines in file f.
+func (f *File) LineCount() int {
+	f.mutex.Lock()
+	n := len(f.lines)
+	f.mutex.Unlock()
+	return n
+}
+
+// AddLine adds the line offset for a new line.
+// The line offset must be larger than the offset for the previous line
+// and smaller than the file size; otherwise the line offset is ignored.
+func (f *File) AddLine(offset int) {
+	f.mutex.Lock()
+	if i := len(f.lines); (i == 0 || f.lines[i-1] < offset) && offset < f.size {
+		f.lines = append(f.lines, offset)
+	}
+	f.mutex.Unlock()
+}
+
+// MergeLine merges a line with the following line. It is akin to replacing
+// the newline character at the end of the line with a space (to not change the
+// remaining offsets). To obtain the line number, consult e.g. Position.Line.
+// MergeLine will panic if given an invalid line number.
+func (f *File) MergeLine(line int) {
+	if line < 1 {
+		panic(fmt.Sprintf("invalid line number %d (should be >= 1)", line))
+	}
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+	if line >= len(f.lines) {
+		panic(fmt.Sprintf("invalid line number %d (should be < %d)", line, len(f.lines)))
+	}
+	// To merge the line numbered <line> with the line numbered <line+1>,
+	// we need to remove the entry in lines corresponding to the line
+	// numbered <line+1>. The entry in lines corresponding to the line
+	// numbered <line+1> is located at index <line>, since indices in lines
+	// are 0-based and line numbers are 1-based.
+	copy(f.lines[line:], f.lines[line+1:])
+	f.lines = f.lines[:len(f.lines)-1]
+}
+
+// SetLines sets the line offsets for a file and reports whether it succeeded.
+// The line offsets are the offsets of the first character of each line;
+// for instance for the content "ab\nc\n" the line offsets are {0, 3}.
+// An empty file has an empty line offset table.
+// Each line offset must be larger than the offset for the previous line
+// and smaller than the file size; otherwise SetLines fails and returns
+// false.
+// Callers must not mutate the provided slice after SetLines returns.
+func (f *File) SetLines(lines []int) bool {
+	// verify validity of lines table
+	size := f.size
+	for i, offset := range lines {
+		if i > 0 && offset <= lines[i-1] || size <= offset {
+			return false
+		}
+	}
+
+	// set lines table
+	f.mutex.Lock()
+	f.lines = lines
+	f.mutex.Unlock()
+	return true
+}
+
+// SetLinesForContent sets the line offsets for the given file content.
+// It ignores position-altering //line comments.
+func (f *File) SetLinesForContent(content []byte) {
+	var lines []int
+	line := 0
+	for offset, b := range content {
+		if line >= 0 {
+			lines = append(lines, line)
+		}
+		line = -1
+		if b == '\n' {
+			line = offset + 1
+		}
+	}
+
+	// set lines table
+	f.mutex.Lock()
+	f.lines = lines
+	f.mutex.Unlock()
+}
+
+// LineStart returns the Pos value of the start of the specified line.
+// It ignores any alternative positions set using AddLineColumnInfo.
+// LineStart panics if the 1-based line number is invalid.
+func (f *File) LineStart(line int) Pos {
+	if line < 1 {
+		panic(fmt.Sprintf("invalid line number %d (should be >= 1)", line))
+	}
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+	if line > len(f.lines) {
+		panic(fmt.Sprintf("invalid line number %d (should be < %d)", line, len(f.lines)))
+	}
+	return Pos(f.base + f.lines[line-1])
+}
+
+// A lineInfo object describes alternative file, line, and column
+// number information (such as provided via a //line directive)
+// for a given file offset.
+type lineInfo struct {
+	// fields are exported to make them accessible to gob
+	Offset       int
+	Filename     string
+	Line, Column int
+}
+
+// AddLineInfo is like AddLineColumnInfo with a column = 1 argument.
+// It is here for backward-compatibility for code prior to Go 1.11.
+func (f *File) AddLineInfo(offset int, filename string, line int) {
+	f.AddLineColumnInfo(offset, filename, line, 1)
+}
+
+// AddLineColumnInfo adds alternative file, line, and column number
+// information for a given file offset. The offset must be larger
+// than the offset for the previously added alternative line info
+// and smaller than the file size; otherwise the information is
+// ignored.
+//
+// AddLineColumnInfo is typically used to register alternative position
+// information for line directives such as //line filename:line:column.
+func (f *File) AddLineColumnInfo(offset int, filename string, line, column int) {
+	f.mutex.Lock()
+	if i := len(f.infos); i == 0 || f.infos[i-1].Offset < offset && offset < f.size {
+		f.infos = append(f.infos, lineInfo{offset, filename, line, column})
+	}
+	f.mutex.Unlock()
+}
+
+// Pos returns the Pos value for the given file offset;
+// the offset must be <= f.Size().
+// f.Pos(f.Offset(p)) == p.
+func (f *File) Pos(offset int) Pos {
+	if offset > f.size {
+		panic(fmt.Sprintf("invalid file offset %d (should be <= %d)", offset, f.size))
+	}
+	return Pos(f.base + offset)
+}
+
+// Offset returns the offset for the given file position p;
+// p must be a valid Pos value in that file.
+// f.Offset(f.Pos(offset)) == offset.
+func (f *File) Offset(p Pos) int {
+	if int(p) < f.base || int(p) > f.base+f.size {
+		panic(fmt.Sprintf("invalid Pos value %d (should be in [%d, %d])", p, f.base, f.base+f.size))
+	}
+	return int(p) - f.base
+}
+
+// Line returns the line number for the given file position p;
+// p must be a Pos value in that file or NoPos.
+func (f *File) Line(p Pos) int {
+	return f.Position(p).Line
+}
+
+func searchLineInfos(a []lineInfo, x int) int {
+	return sort.Search(len(a), func(i int) bool { return a[i].Offset > x }) - 1
+}
+
+// unpack returns the filename and line and column number for a file offset.
+// If adjusted is set, unpack will return the filename and line information
+// possibly adjusted by //line comments; otherwise those comments are ignored.
+func (f *File) unpack(offset int, adjusted bool) (filename string, line, column int) {
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+	filename = f.name
+	if i := searchInts(f.lines, offset); i >= 0 {
+		line, column = i+1, offset-f.lines[i]+1
+	}
+	if adjusted && len(f.infos) > 0 {
+		// few files have extra line infos
+		if i := searchLineInfos(f.infos, offset); i >= 0 {
+			alt := &f.infos[i]
+			filename = alt.Filename
+			if i := searchInts(f.lines, alt.Offset); i >= 0 {
+				// i+1 is the line at which the alternative position was recorded
+				d := line - (i + 1) // line distance from alternative position base
+				line = alt.Line + d
+				if alt.Column == 0 {
+					// alternative column is unknown => relative column is unknown
+					// (the current specification for line directives requires
+					// this to apply until the next PosBase/line directive,
+					// not just until the new newline)
+					column = 0
+				} else if d == 0 {
+					// the alternative position base is on the current line
+					// => column is relative to alternative column
+					column = alt.Column + (offset - alt.Offset)
+				}
+			}
+		}
+	}
+	return
+}
+
+func (f *File) position(p Pos, adjusted bool) (pos Position) {
+	offset := int(p) - f.base
+	pos.Offset = offset
+	pos.Filename, pos.Line, pos.Column = f.unpack(offset, adjusted)
+	return
+}
+
+// PositionFor returns the Position value for the given file position p.
+// If adjusted is set, the position may be adjusted by position-altering
+// //line comments; otherwise those comments are ignored.
+// p must be a Pos value in f or NoPos.
+func (f *File) PositionFor(p Pos, adjusted bool) (pos Position) {
+	if p != NoPos {
+		if int(p) < f.base || int(p) > f.base+f.size {
+			panic(fmt.Sprintf("invalid Pos value %d (should be in [%d, %d])", p, f.base, f.base+f.size))
+		}
+		pos = f.position(p, adjusted)
+	}
+	return
+}
+
+// Position returns the Position value for the given file position p.
+// Calling f.Position(p) is equivalent to calling f.PositionFor(p, true).
+func (f *File) Position(p Pos) (pos Position) {
+	return f.PositionFor(p, true)
+}
+
+// -----------------------------------------------------------------------------
+// FileSet
+
+// A FileSet represents a set of source files.
+// Methods of file sets are synchronized; multiple goroutines
+// may invoke them concurrently.
+//
+// The byte offsets for each file in a file set are mapped into
+// distinct (integer) intervals, one interval [base, base+size]
+// per file. Base represents the first byte in the file, and size
+// is the corresponding file size. A Pos value is a value in such
+// an interval. By determining the interval a Pos value belongs
+// to, the file, its file base, and thus the byte offset (position)
+// the Pos value is representing can be computed.
+//
+// When adding a new file, a file base must be provided. That can
+// be any integer value that is past the end of any interval of any
+// file already in the file set. For convenience, FileSet.Base provides
+// such a value, which is simply the end of the Pos interval of the most
+// recently added file, plus one. Unless there is a need to extend an
+// interval later, using the FileSet.Base should be used as argument
+// for FileSet.AddFile.
+type FileSet struct {
+	mutex sync.RWMutex // protects the file set
+	base  int          // base offset for the next file
+	files []*File      // list of files in the order added to the set
+	last  *File        // cache of last file looked up
+}
+
+// NewFileSet creates a new file set.
+func NewFileSet() *FileSet {
+	return &FileSet{
+		base: 1, // 0 == NoPos
+	}
+}
+
+// Base returns the minimum base offset that must be provided to
+// AddFile when adding the next file.
+func (s *FileSet) Base() int {
+	s.mutex.RLock()
+	b := s.base
+	s.mutex.RUnlock()
+	return b
+
+}
+
+// AddFile adds a new file with a given filename, base offset, and file size
+// to the file set s and returns the file. Multiple files may have the same
+// name. The base offset must not be smaller than the FileSet's Base(), and
+// size must not be negative. As a special case, if a negative base is provided,
+// the current value of the FileSet's Base() is used instead.
+//
+// Adding the file will set the file set's Base() value to base + size + 1
+// as the minimum base value for the next file. The following relationship
+// exists between a Pos value p for a given file offset offs:
+//
+//	int(p) = base + offs
+//
+// with offs in the range [0, size] and thus p in the range [base, base+size].
+// For convenience, File.Pos may be used to create file-specific position
+// values from a file offset.
+func (s *FileSet) AddFile(filename string, base, size int) *File {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	if base < 0 {
+		base = s.base
+	}
+	if base < s.base {
+		panic(fmt.Sprintf("invalid base %d (should be >= %d)", base, s.base))
+	}
+	if size < 0 {
+		panic(fmt.Sprintf("invalid size %d (should be >= 0)", size))
+	}
+	// base >= s.base && size >= 0
+	f := &File{set: s, name: filename, base: base, size: size, lines: []int{0}}
+	base += size + 1 // +1 because EOF also has a position
+	if base < 0 {
+		panic("token.Pos offset overflow (> 2G of source code in file set)")
+	}
+	// add the file to the file set
+	s.base = base
+	s.files = append(s.files, f)
+	s.last = f
+	return f
+}
+
+// Iterate calls f for the files in the file set in the order they were added
+// until f returns false.
+func (s *FileSet) Iterate(f func(*File) bool) {
+	for i := 0; ; i++ {
+		var file *File
+		s.mutex.RLock()
+		if i < len(s.files) {
+			file = s.files[i]
+		}
+		s.mutex.RUnlock()
+		if file == nil || !f(file) {
+			break
+		}
+	}
+}
+
+func searchFiles(a []*File, x int) int {
+	return sort.Search(len(a), func(i int) bool { return a[i].base > x }) - 1
+}
+
+func (s *FileSet) file(p Pos) *File {
+	s.mutex.RLock()
+	// common case: p is in last file
+	if f := s.last; f != nil && f.base <= int(p) && int(p) <= f.base+f.size {
+		s.mutex.RUnlock()
+		return f
+	}
+	// p is not in last file - search all files
+	if i := searchFiles(s.files, int(p)); i >= 0 {
+		f := s.files[i]
+		// f.base <= int(p) by definition of searchFiles
+		if int(p) <= f.base+f.size {
+			s.mutex.RUnlock()
+			s.mutex.Lock()
+			s.last = f // race is ok - s.last is only a cache
+			s.mutex.Unlock()
+			return f
+		}
+	}
+	s.mutex.RUnlock()
+	return nil
+}
+
+// File returns the file that contains the position p.
+// If no such file is found (for instance for p == NoPos),
+// the result is nil.
+func (s *FileSet) File(p Pos) (f *File) {
+	if p != NoPos {
+		f = s.file(p)
+	}
+	return
+}
+
+// PositionFor converts a Pos p in the fileset into a Position value.
+// If adjusted is set, the position may be adjusted by position-altering
+// //line comments; otherwise those comments are ignored.
+// p must be a Pos value in s or NoPos.
+func (s *FileSet) PositionFor(p Pos, adjusted bool) (pos Position) {
+	if p != NoPos {
+		if f := s.file(p); f != nil {
+			return f.position(p, adjusted)
+		}
+	}
+	return
+}
+
+// Position converts a Pos p in the fileset into a Position value.
+// Calling s.Position(p) is equivalent to calling s.PositionFor(p, true).
+func (s *FileSet) Position(p Pos) (pos Position) {
+	return s.PositionFor(p, true)
+}
+
+// -----------------------------------------------------------------------------
+// Helper functions
+
+func searchInts(a []int, x int) int {
+	// This function body is a manually inlined version of:
+	//
+	//   return sort.Search(len(a), func(i int) bool { return a[i] > x }) - 1
+	//
+	// With better compiler optimizations, this may not be needed in the
+	// future, but at the moment this change improves the go/printer
+	// benchmark performance by ~30%. This has a direct impact on the
+	// speed of gofmt and thus seems worthwhile (2011-04-29).
+	// TODO(gri): Remove this when compilers have caught up.
+	i, j := 0, len(a)
+	for i < j {
+		h := int(uint(i+j) >> 1) // avoid overflow when computing h
+		// i ≤ h < j
+		if a[h] <= x {
+			i = h + 1
+		} else {
+			j = h
+		}
+	}
+	return i - 1
+}
diff --git a/internal/backport/go/token/position_bench_test.go b/internal/backport/go/token/position_bench_test.go
new file mode 100644
index 0000000..41be728
--- /dev/null
+++ b/internal/backport/go/token/position_bench_test.go
@@ -0,0 +1,24 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+import (
+	"testing"
+)
+
+func BenchmarkSearchInts(b *testing.B) {
+	data := make([]int, 10000)
+	for i := 0; i < 10000; i++ {
+		data[i] = i
+	}
+	const x = 8
+	if r := searchInts(data, x); r != x {
+		b.Errorf("got index = %d; want %d", r, x)
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		searchInts(data, x)
+	}
+}
diff --git a/internal/backport/go/token/position_test.go b/internal/backport/go/token/position_test.go
new file mode 100644
index 0000000..7d465df
--- /dev/null
+++ b/internal/backport/go/token/position_test.go
@@ -0,0 +1,341 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+import (
+	"fmt"
+	"math/rand"
+	"sync"
+	"testing"
+)
+
+func checkPos(t *testing.T, msg string, got, want Position) {
+	if got.Filename != want.Filename {
+		t.Errorf("%s: got filename = %q; want %q", msg, got.Filename, want.Filename)
+	}
+	if got.Offset != want.Offset {
+		t.Errorf("%s: got offset = %d; want %d", msg, got.Offset, want.Offset)
+	}
+	if got.Line != want.Line {
+		t.Errorf("%s: got line = %d; want %d", msg, got.Line, want.Line)
+	}
+	if got.Column != want.Column {
+		t.Errorf("%s: got column = %d; want %d", msg, got.Column, want.Column)
+	}
+}
+
+func TestNoPos(t *testing.T) {
+	if NoPos.IsValid() {
+		t.Errorf("NoPos should not be valid")
+	}
+	var fset *FileSet
+	checkPos(t, "nil NoPos", fset.Position(NoPos), Position{})
+	fset = NewFileSet()
+	checkPos(t, "fset NoPos", fset.Position(NoPos), Position{})
+}
+
+var tests = []struct {
+	filename string
+	source   []byte // may be nil
+	size     int
+	lines    []int
+}{
+	{"a", []byte{}, 0, []int{}},
+	{"b", []byte("01234"), 5, []int{0}},
+	{"c", []byte("\n\n\n\n\n\n\n\n\n"), 9, []int{0, 1, 2, 3, 4, 5, 6, 7, 8}},
+	{"d", nil, 100, []int{0, 5, 10, 20, 30, 70, 71, 72, 80, 85, 90, 99}},
+	{"e", nil, 777, []int{0, 80, 100, 120, 130, 180, 267, 455, 500, 567, 620}},
+	{"f", []byte("package p\n\nimport \"fmt\""), 23, []int{0, 10, 11}},
+	{"g", []byte("package p\n\nimport \"fmt\"\n"), 24, []int{0, 10, 11}},
+	{"h", []byte("package p\n\nimport \"fmt\"\n "), 25, []int{0, 10, 11, 24}},
+}
+
+func linecol(lines []int, offs int) (int, int) {
+	prevLineOffs := 0
+	for line, lineOffs := range lines {
+		if offs < lineOffs {
+			return line, offs - prevLineOffs + 1
+		}
+		prevLineOffs = lineOffs
+	}
+	return len(lines), offs - prevLineOffs + 1
+}
+
+func verifyPositions(t *testing.T, fset *FileSet, f *File, lines []int) {
+	for offs := 0; offs < f.Size(); offs++ {
+		p := f.Pos(offs)
+		offs2 := f.Offset(p)
+		if offs2 != offs {
+			t.Errorf("%s, Offset: got offset %d; want %d", f.Name(), offs2, offs)
+		}
+		line, col := linecol(lines, offs)
+		msg := fmt.Sprintf("%s (offs = %d, p = %d)", f.Name(), offs, p)
+		checkPos(t, msg, f.Position(f.Pos(offs)), Position{f.Name(), offs, line, col})
+		checkPos(t, msg, fset.Position(p), Position{f.Name(), offs, line, col})
+	}
+}
+
+func makeTestSource(size int, lines []int) []byte {
+	src := make([]byte, size)
+	for _, offs := range lines {
+		if offs > 0 {
+			src[offs-1] = '\n'
+		}
+	}
+	return src
+}
+
+func TestPositions(t *testing.T) {
+	const delta = 7 // a non-zero base offset increment
+	fset := NewFileSet()
+	for _, test := range tests {
+		// verify consistency of test case
+		if test.source != nil && len(test.source) != test.size {
+			t.Errorf("%s: inconsistent test case: got file size %d; want %d", test.filename, len(test.source), test.size)
+		}
+
+		// add file and verify name and size
+		f := fset.AddFile(test.filename, fset.Base()+delta, test.size)
+		if f.Name() != test.filename {
+			t.Errorf("got filename %q; want %q", f.Name(), test.filename)
+		}
+		if f.Size() != test.size {
+			t.Errorf("%s: got file size %d; want %d", f.Name(), f.Size(), test.size)
+		}
+		if fset.File(f.Pos(0)) != f {
+			t.Errorf("%s: f.Pos(0) was not found in f", f.Name())
+		}
+
+		// add lines individually and verify all positions
+		for i, offset := range test.lines {
+			f.AddLine(offset)
+			if f.LineCount() != i+1 {
+				t.Errorf("%s, AddLine: got line count %d; want %d", f.Name(), f.LineCount(), i+1)
+			}
+			// adding the same offset again should be ignored
+			f.AddLine(offset)
+			if f.LineCount() != i+1 {
+				t.Errorf("%s, AddLine: got unchanged line count %d; want %d", f.Name(), f.LineCount(), i+1)
+			}
+			verifyPositions(t, fset, f, test.lines[0:i+1])
+		}
+
+		// add lines with SetLines and verify all positions
+		if ok := f.SetLines(test.lines); !ok {
+			t.Errorf("%s: SetLines failed", f.Name())
+		}
+		if f.LineCount() != len(test.lines) {
+			t.Errorf("%s, SetLines: got line count %d; want %d", f.Name(), f.LineCount(), len(test.lines))
+		}
+		verifyPositions(t, fset, f, test.lines)
+
+		// add lines with SetLinesForContent and verify all positions
+		src := test.source
+		if src == nil {
+			// no test source available - create one from scratch
+			src = makeTestSource(test.size, test.lines)
+		}
+		f.SetLinesForContent(src)
+		if f.LineCount() != len(test.lines) {
+			t.Errorf("%s, SetLinesForContent: got line count %d; want %d", f.Name(), f.LineCount(), len(test.lines))
+		}
+		verifyPositions(t, fset, f, test.lines)
+	}
+}
+
+func TestLineInfo(t *testing.T) {
+	fset := NewFileSet()
+	f := fset.AddFile("foo", fset.Base(), 500)
+	lines := []int{0, 42, 77, 100, 210, 220, 277, 300, 333, 401}
+	// add lines individually and provide alternative line information
+	for _, offs := range lines {
+		f.AddLine(offs)
+		f.AddLineInfo(offs, "bar", 42)
+	}
+	// verify positions for all offsets
+	for offs := 0; offs <= f.Size(); offs++ {
+		p := f.Pos(offs)
+		_, col := linecol(lines, offs)
+		msg := fmt.Sprintf("%s (offs = %d, p = %d)", f.Name(), offs, p)
+		checkPos(t, msg, f.Position(f.Pos(offs)), Position{"bar", offs, 42, col})
+		checkPos(t, msg, fset.Position(p), Position{"bar", offs, 42, col})
+	}
+}
+
+func TestFiles(t *testing.T) {
+	fset := NewFileSet()
+	for i, test := range tests {
+		base := fset.Base()
+		if i%2 == 1 {
+			// Setting a negative base is equivalent to
+			// fset.Base(), so test some of each.
+			base = -1
+		}
+		fset.AddFile(test.filename, base, test.size)
+		j := 0
+		fset.Iterate(func(f *File) bool {
+			if f.Name() != tests[j].filename {
+				t.Errorf("got filename = %s; want %s", f.Name(), tests[j].filename)
+			}
+			j++
+			return true
+		})
+		if j != i+1 {
+			t.Errorf("got %d files; want %d", j, i+1)
+		}
+	}
+}
+
+// FileSet.File should return nil if Pos is past the end of the FileSet.
+func TestFileSetPastEnd(t *testing.T) {
+	fset := NewFileSet()
+	for _, test := range tests {
+		fset.AddFile(test.filename, fset.Base(), test.size)
+	}
+	if f := fset.File(Pos(fset.Base())); f != nil {
+		t.Errorf("got %v, want nil", f)
+	}
+}
+
+func TestFileSetCacheUnlikely(t *testing.T) {
+	fset := NewFileSet()
+	offsets := make(map[string]int)
+	for _, test := range tests {
+		offsets[test.filename] = fset.Base()
+		fset.AddFile(test.filename, fset.Base(), test.size)
+	}
+	for file, pos := range offsets {
+		f := fset.File(Pos(pos))
+		if f.Name() != file {
+			t.Errorf("got %q at position %d, want %q", f.Name(), pos, file)
+		}
+	}
+}
+
+// issue 4345. Test that concurrent use of FileSet.Pos does not trigger a
+// race in the FileSet position cache.
+func TestFileSetRace(t *testing.T) {
+	fset := NewFileSet()
+	for i := 0; i < 100; i++ {
+		fset.AddFile(fmt.Sprintf("file-%d", i), fset.Base(), 1031)
+	}
+	max := int32(fset.Base())
+	var stop sync.WaitGroup
+	r := rand.New(rand.NewSource(7))
+	for i := 0; i < 2; i++ {
+		r := rand.New(rand.NewSource(r.Int63()))
+		stop.Add(1)
+		go func() {
+			for i := 0; i < 1000; i++ {
+				fset.Position(Pos(r.Int31n(max)))
+			}
+			stop.Done()
+		}()
+	}
+	stop.Wait()
+}
+
+// issue 16548. Test that concurrent use of File.AddLine and FileSet.PositionFor
+// does not trigger a race in the FileSet position cache.
+func TestFileSetRace2(t *testing.T) {
+	const N = 1e3
+	var (
+		fset = NewFileSet()
+		file = fset.AddFile("", -1, N)
+		ch   = make(chan int, 2)
+	)
+
+	go func() {
+		for i := 0; i < N; i++ {
+			file.AddLine(i)
+		}
+		ch <- 1
+	}()
+
+	go func() {
+		pos := file.Pos(0)
+		for i := 0; i < N; i++ {
+			fset.PositionFor(pos, false)
+		}
+		ch <- 1
+	}()
+
+	<-ch
+	<-ch
+}
+
+func TestPositionFor(t *testing.T) {
+	src := []byte(`
+foo
+b
+ar
+//line :100
+foobar
+//line bar:3
+done
+`)
+
+	const filename = "foo"
+	fset := NewFileSet()
+	f := fset.AddFile(filename, fset.Base(), len(src))
+	f.SetLinesForContent(src)
+
+	// verify position info
+	for i, offs := range f.lines {
+		got1 := f.PositionFor(f.Pos(offs), false)
+		got2 := f.PositionFor(f.Pos(offs), true)
+		got3 := f.Position(f.Pos(offs))
+		want := Position{filename, offs, i + 1, 1}
+		checkPos(t, "1. PositionFor unadjusted", got1, want)
+		checkPos(t, "1. PositionFor adjusted", got2, want)
+		checkPos(t, "1. Position", got3, want)
+	}
+
+	// manually add //line info on lines l1, l2
+	const l1, l2 = 5, 7
+	f.AddLineInfo(f.lines[l1-1], "", 100)
+	f.AddLineInfo(f.lines[l2-1], "bar", 3)
+
+	// unadjusted position info must remain unchanged
+	for i, offs := range f.lines {
+		got1 := f.PositionFor(f.Pos(offs), false)
+		want := Position{filename, offs, i + 1, 1}
+		checkPos(t, "2. PositionFor unadjusted", got1, want)
+	}
+
+	// adjusted position info should have changed
+	for i, offs := range f.lines {
+		got2 := f.PositionFor(f.Pos(offs), true)
+		got3 := f.Position(f.Pos(offs))
+		want := Position{filename, offs, i + 1, 1}
+		// manually compute wanted filename and line
+		line := want.Line
+		if i+1 >= l1 {
+			want.Filename = ""
+			want.Line = line - l1 + 100
+		}
+		if i+1 >= l2 {
+			want.Filename = "bar"
+			want.Line = line - l2 + 3
+		}
+		checkPos(t, "3. PositionFor adjusted", got2, want)
+		checkPos(t, "3. Position", got3, want)
+	}
+}
+
+func TestLineStart(t *testing.T) {
+	const src = "one\ntwo\nthree\n"
+	fset := NewFileSet()
+	f := fset.AddFile("input", -1, len(src))
+	f.SetLinesForContent([]byte(src))
+
+	for line := 1; line <= 3; line++ {
+		pos := f.LineStart(line)
+		position := fset.Position(pos)
+		if position.Line != line || position.Column != 1 {
+			t.Errorf("LineStart(%d) returned wrong pos %d: %s", line, pos, position)
+		}
+	}
+}
diff --git a/internal/backport/go/token/serialize.go b/internal/backport/go/token/serialize.go
new file mode 100644
index 0000000..d0ea345
--- /dev/null
+++ b/internal/backport/go/token/serialize.go
@@ -0,0 +1,71 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+type serializedFile struct {
+	// fields correspond 1:1 to fields with same (lower-case) name in File
+	Name  string
+	Base  int
+	Size  int
+	Lines []int
+	Infos []lineInfo
+}
+
+type serializedFileSet struct {
+	Base  int
+	Files []serializedFile
+}
+
+// Read calls decode to deserialize a file set into s; s must not be nil.
+func (s *FileSet) Read(decode func(interface{}) error) error {
+	var ss serializedFileSet
+	if err := decode(&ss); err != nil {
+		return err
+	}
+
+	s.mutex.Lock()
+	s.base = ss.Base
+	files := make([]*File, len(ss.Files))
+	for i := 0; i < len(ss.Files); i++ {
+		f := &ss.Files[i]
+		files[i] = &File{
+			set:   s,
+			name:  f.Name,
+			base:  f.Base,
+			size:  f.Size,
+			lines: f.Lines,
+			infos: f.Infos,
+		}
+	}
+	s.files = files
+	s.last = nil
+	s.mutex.Unlock()
+
+	return nil
+}
+
+// Write calls encode to serialize the file set s.
+func (s *FileSet) Write(encode func(interface{}) error) error {
+	var ss serializedFileSet
+
+	s.mutex.Lock()
+	ss.Base = s.base
+	files := make([]serializedFile, len(s.files))
+	for i, f := range s.files {
+		f.mutex.Lock()
+		files[i] = serializedFile{
+			Name:  f.name,
+			Base:  f.base,
+			Size:  f.size,
+			Lines: append([]int(nil), f.lines...),
+			Infos: append([]lineInfo(nil), f.infos...),
+		}
+		f.mutex.Unlock()
+	}
+	ss.Files = files
+	s.mutex.Unlock()
+
+	return encode(ss)
+}
diff --git a/internal/backport/go/token/serialize_test.go b/internal/backport/go/token/serialize_test.go
new file mode 100644
index 0000000..4e925ad
--- /dev/null
+++ b/internal/backport/go/token/serialize_test.go
@@ -0,0 +1,111 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+import (
+	"bytes"
+	"encoding/gob"
+	"fmt"
+	"testing"
+)
+
+// equal returns nil if p and q describe the same file set;
+// otherwise it returns an error describing the discrepancy.
+func equal(p, q *FileSet) error {
+	if p == q {
+		// avoid deadlock if p == q
+		return nil
+	}
+
+	// not strictly needed for the test
+	p.mutex.Lock()
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+	defer p.mutex.Unlock()
+
+	if p.base != q.base {
+		return fmt.Errorf("different bases: %d != %d", p.base, q.base)
+	}
+
+	if len(p.files) != len(q.files) {
+		return fmt.Errorf("different number of files: %d != %d", len(p.files), len(q.files))
+	}
+
+	for i, f := range p.files {
+		g := q.files[i]
+		if f.set != p {
+			return fmt.Errorf("wrong fileset for %q", f.name)
+		}
+		if g.set != q {
+			return fmt.Errorf("wrong fileset for %q", g.name)
+		}
+		if f.name != g.name {
+			return fmt.Errorf("different filenames: %q != %q", f.name, g.name)
+		}
+		if f.base != g.base {
+			return fmt.Errorf("different base for %q: %d != %d", f.name, f.base, g.base)
+		}
+		if f.size != g.size {
+			return fmt.Errorf("different size for %q: %d != %d", f.name, f.size, g.size)
+		}
+		for j, l := range f.lines {
+			m := g.lines[j]
+			if l != m {
+				return fmt.Errorf("different offsets for %q", f.name)
+			}
+		}
+		for j, l := range f.infos {
+			m := g.infos[j]
+			if l.Offset != m.Offset || l.Filename != m.Filename || l.Line != m.Line {
+				return fmt.Errorf("different infos for %q", f.name)
+			}
+		}
+	}
+
+	// we don't care about .last - it's just a cache
+	return nil
+}
+
+func checkSerialize(t *testing.T, p *FileSet) {
+	var buf bytes.Buffer
+	encode := func(x interface{}) error {
+		return gob.NewEncoder(&buf).Encode(x)
+	}
+	if err := p.Write(encode); err != nil {
+		t.Errorf("writing fileset failed: %s", err)
+		return
+	}
+	q := NewFileSet()
+	decode := func(x interface{}) error {
+		return gob.NewDecoder(&buf).Decode(x)
+	}
+	if err := q.Read(decode); err != nil {
+		t.Errorf("reading fileset failed: %s", err)
+		return
+	}
+	if err := equal(p, q); err != nil {
+		t.Errorf("filesets not identical: %s", err)
+	}
+}
+
+func TestSerialization(t *testing.T) {
+	p := NewFileSet()
+	checkSerialize(t, p)
+	// add some files
+	for i := 0; i < 10; i++ {
+		f := p.AddFile(fmt.Sprintf("file%d", i), p.Base()+i, i*100)
+		checkSerialize(t, p)
+		// add some lines and alternative file infos
+		line := 1000
+		for offs := 0; offs < f.Size(); offs += 40 + i {
+			f.AddLine(offs)
+			if offs%7 == 0 {
+				f.AddLineInfo(offs, fmt.Sprintf("file%d", offs), line)
+				line += 33
+			}
+		}
+		checkSerialize(t, p)
+	}
+}
diff --git a/internal/backport/go/token/token.go b/internal/backport/go/token/token.go
new file mode 100644
index 0000000..db4e955
--- /dev/null
+++ b/internal/backport/go/token/token.go
@@ -0,0 +1,338 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package token defines constants representing the lexical tokens of the Go
+// programming language and basic operations on tokens (printing, predicates).
+package token
+
+import (
+	"strconv"
+	"unicode"
+	"unicode/utf8"
+)
+
+// Token is the set of lexical tokens of the Go programming language.
+type Token int
+
+// The list of tokens.
+const (
+	// Special tokens
+	ILLEGAL Token = iota
+	EOF
+	COMMENT
+
+	literal_beg
+	// Identifiers and basic type literals
+	// (these tokens stand for classes of literals)
+	IDENT  // main
+	INT    // 12345
+	FLOAT  // 123.45
+	IMAG   // 123.45i
+	CHAR   // 'a'
+	STRING // "abc"
+	literal_end
+
+	operator_beg
+	// Operators and delimiters
+	ADD // +
+	SUB // -
+	MUL // *
+	QUO // /
+	REM // %
+
+	AND     // &
+	OR      // |
+	XOR     // ^
+	SHL     // <<
+	SHR     // >>
+	AND_NOT // &^
+
+	ADD_ASSIGN // +=
+	SUB_ASSIGN // -=
+	MUL_ASSIGN // *=
+	QUO_ASSIGN // /=
+	REM_ASSIGN // %=
+
+	AND_ASSIGN     // &=
+	OR_ASSIGN      // |=
+	XOR_ASSIGN     // ^=
+	SHL_ASSIGN     // <<=
+	SHR_ASSIGN     // >>=
+	AND_NOT_ASSIGN // &^=
+
+	LAND  // &&
+	LOR   // ||
+	ARROW // <-
+	INC   // ++
+	DEC   // --
+
+	EQL    // ==
+	LSS    // <
+	GTR    // >
+	ASSIGN // =
+	NOT    // !
+
+	NEQ      // !=
+	LEQ      // <=
+	GEQ      // >=
+	DEFINE   // :=
+	ELLIPSIS // ...
+
+	LPAREN // (
+	LBRACK // [
+	LBRACE // {
+	COMMA  // ,
+	PERIOD // .
+
+	RPAREN    // )
+	RBRACK    // ]
+	RBRACE    // }
+	SEMICOLON // ;
+	COLON     // :
+	operator_end
+
+	keyword_beg
+	// Keywords
+	BREAK
+	CASE
+	CHAN
+	CONST
+	CONTINUE
+
+	DEFAULT
+	DEFER
+	ELSE
+	FALLTHROUGH
+	FOR
+
+	FUNC
+	GO
+	GOTO
+	IF
+	IMPORT
+
+	INTERFACE
+	MAP
+	PACKAGE
+	RANGE
+	RETURN
+
+	SELECT
+	STRUCT
+	SWITCH
+	TYPE
+	VAR
+	keyword_end
+
+	additional_beg
+	// additional tokens, handled in an ad-hoc manner
+	TILDE
+	additional_end
+)
+
+var tokens = [...]string{
+	ILLEGAL: "ILLEGAL",
+
+	EOF:     "EOF",
+	COMMENT: "COMMENT",
+
+	IDENT:  "IDENT",
+	INT:    "INT",
+	FLOAT:  "FLOAT",
+	IMAG:   "IMAG",
+	CHAR:   "CHAR",
+	STRING: "STRING",
+
+	ADD: "+",
+	SUB: "-",
+	MUL: "*",
+	QUO: "/",
+	REM: "%",
+
+	AND:     "&",
+	OR:      "|",
+	XOR:     "^",
+	SHL:     "<<",
+	SHR:     ">>",
+	AND_NOT: "&^",
+
+	ADD_ASSIGN: "+=",
+	SUB_ASSIGN: "-=",
+	MUL_ASSIGN: "*=",
+	QUO_ASSIGN: "/=",
+	REM_ASSIGN: "%=",
+
+	AND_ASSIGN:     "&=",
+	OR_ASSIGN:      "|=",
+	XOR_ASSIGN:     "^=",
+	SHL_ASSIGN:     "<<=",
+	SHR_ASSIGN:     ">>=",
+	AND_NOT_ASSIGN: "&^=",
+
+	LAND:  "&&",
+	LOR:   "||",
+	ARROW: "<-",
+	INC:   "++",
+	DEC:   "--",
+
+	EQL:    "==",
+	LSS:    "<",
+	GTR:    ">",
+	ASSIGN: "=",
+	NOT:    "!",
+
+	NEQ:      "!=",
+	LEQ:      "<=",
+	GEQ:      ">=",
+	DEFINE:   ":=",
+	ELLIPSIS: "...",
+
+	LPAREN: "(",
+	LBRACK: "[",
+	LBRACE: "{",
+	COMMA:  ",",
+	PERIOD: ".",
+
+	RPAREN:    ")",
+	RBRACK:    "]",
+	RBRACE:    "}",
+	SEMICOLON: ";",
+	COLON:     ":",
+
+	BREAK:    "break",
+	CASE:     "case",
+	CHAN:     "chan",
+	CONST:    "const",
+	CONTINUE: "continue",
+
+	DEFAULT:     "default",
+	DEFER:       "defer",
+	ELSE:        "else",
+	FALLTHROUGH: "fallthrough",
+	FOR:         "for",
+
+	FUNC:   "func",
+	GO:     "go",
+	GOTO:   "goto",
+	IF:     "if",
+	IMPORT: "import",
+
+	INTERFACE: "interface",
+	MAP:       "map",
+	PACKAGE:   "package",
+	RANGE:     "range",
+	RETURN:    "return",
+
+	SELECT: "select",
+	STRUCT: "struct",
+	SWITCH: "switch",
+	TYPE:   "type",
+	VAR:    "var",
+
+	TILDE: "~",
+}
+
+// String returns the string corresponding to the token tok.
+// For operators, delimiters, and keywords the string is the actual
+// token character sequence (e.g., for the token ADD, the string is
+// "+"). For all other tokens the string corresponds to the token
+// constant name (e.g. for the token IDENT, the string is "IDENT").
+func (tok Token) String() string {
+	s := ""
+	if 0 <= tok && tok < Token(len(tokens)) {
+		s = tokens[tok]
+	}
+	if s == "" {
+		s = "token(" + strconv.Itoa(int(tok)) + ")"
+	}
+	return s
+}
+
+// A set of constants for precedence-based expression parsing.
+// Non-operators have lowest precedence, followed by operators
+// starting with precedence 1 up to unary operators. The highest
+// precedence serves as "catch-all" precedence for selector,
+// indexing, and other operator and delimiter tokens.
+const (
+	LowestPrec  = 0 // non-operators
+	UnaryPrec   = 6
+	HighestPrec = 7
+)
+
+// Precedence returns the operator precedence of the binary
+// operator op. If op is not a binary operator, the result
+// is LowestPrecedence.
+func (op Token) Precedence() int {
+	switch op {
+	case LOR:
+		return 1
+	case LAND:
+		return 2
+	case EQL, NEQ, LSS, LEQ, GTR, GEQ:
+		return 3
+	case ADD, SUB, OR, XOR:
+		return 4
+	case MUL, QUO, REM, SHL, SHR, AND, AND_NOT:
+		return 5
+	}
+	return LowestPrec
+}
+
+var keywords map[string]Token
+
+func init() {
+	keywords = make(map[string]Token)
+	for i := keyword_beg + 1; i < keyword_end; i++ {
+		keywords[tokens[i]] = i
+	}
+}
+
+// Lookup maps an identifier to its keyword token or IDENT (if not a keyword).
+func Lookup(ident string) Token {
+	if tok, is_keyword := keywords[ident]; is_keyword {
+		return tok
+	}
+	return IDENT
+}
+
+// Predicates
+
+// IsLiteral returns true for tokens corresponding to identifiers
+// and basic type literals; it returns false otherwise.
+func (tok Token) IsLiteral() bool { return literal_beg < tok && tok < literal_end }
+
+// IsOperator returns true for tokens corresponding to operators and
+// delimiters; it returns false otherwise.
+func (tok Token) IsOperator() bool {
+	return (operator_beg < tok && tok < operator_end) || tok == TILDE
+}
+
+// IsKeyword returns true for tokens corresponding to keywords;
+// it returns false otherwise.
+func (tok Token) IsKeyword() bool { return keyword_beg < tok && tok < keyword_end }
+
+// IsExported reports whether name starts with an upper-case letter.
+func IsExported(name string) bool {
+	ch, _ := utf8.DecodeRuneInString(name)
+	return unicode.IsUpper(ch)
+}
+
+// IsKeyword reports whether name is a Go keyword, such as "func" or "return".
+func IsKeyword(name string) bool {
+	// TODO: opt: use a perfect hash function instead of a global map.
+	_, ok := keywords[name]
+	return ok
+}
+
+// IsIdentifier reports whether name is a Go identifier, that is, a non-empty
+// string made up of letters, digits, and underscores, where the first character
+// is not a digit. Keywords are not identifiers.
+func IsIdentifier(name string) bool {
+	for i, c := range name {
+		if !unicode.IsLetter(c) && c != '_' && (i == 0 || !unicode.IsDigit(c)) {
+			return false
+		}
+	}
+	return name != "" && !IsKeyword(name)
+}
diff --git a/internal/backport/go/token/token_test.go b/internal/backport/go/token/token_test.go
new file mode 100644
index 0000000..eff38cc
--- /dev/null
+++ b/internal/backport/go/token/token_test.go
@@ -0,0 +1,33 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+import "testing"
+
+func TestIsIdentifier(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want bool
+	}{
+		{"Empty", "", false},
+		{"Space", " ", false},
+		{"SpaceSuffix", "foo ", false},
+		{"Number", "123", false},
+		{"Keyword", "func", false},
+
+		{"LettersASCII", "foo", true},
+		{"MixedASCII", "_bar123", true},
+		{"UppercaseKeyword", "Func", true},
+		{"LettersUnicode", "fóö", true},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if got := IsIdentifier(test.in); got != test.want {
+				t.Fatalf("IsIdentifier(%q) = %t, want %v", test.in, got, test.want)
+			}
+		})
+	}
+}
diff --git a/internal/pkgdoc/dir.go b/internal/pkgdoc/dir.go
index 2d71230..5e75e16 100644
--- a/internal/pkgdoc/dir.go
+++ b/internal/pkgdoc/dir.go
@@ -8,10 +8,10 @@
 
 import (
 	"bytes"
-	"go/ast"
-	"go/doc"
-	"go/parser"
-	"go/token"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/doc"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
 	"io/fs"
 	"log"
 	"path"
diff --git a/internal/pkgdoc/dir_test.go b/internal/pkgdoc/dir_test.go
index 1b110ff..09e0cfd 100644
--- a/internal/pkgdoc/dir_test.go
+++ b/internal/pkgdoc/dir_test.go
@@ -5,7 +5,7 @@
 package pkgdoc
 
 import (
-	"go/token"
+	"golang.org/x/website/internal/backport/go/token"
 	"os"
 	"runtime"
 	"sort"
diff --git a/internal/pkgdoc/doc.go b/internal/pkgdoc/doc.go
index 97bfd21..9058ffe 100644
--- a/internal/pkgdoc/doc.go
+++ b/internal/pkgdoc/doc.go
@@ -11,10 +11,9 @@
 
 import (
 	"bytes"
-	"go/ast"
-	"go/build"
-	"go/doc"
-	"go/token"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/doc"
+	"golang.org/x/website/internal/backport/go/token"
 	"io"
 	"io/fs"
 	"io/ioutil"
@@ -30,6 +29,7 @@
 	"unicode/utf8"
 
 	"golang.org/x/website/internal/api"
+	"golang.org/x/website/internal/backport/go/build"
 	"golang.org/x/website/internal/web"
 )
 
@@ -294,7 +294,6 @@
 // (as is the convention for packages). This is sufficient
 // to resolve package identifiers without doing an actual
 // import. It never returns an error.
-//
 func simpleImporter(imports map[string]*ast.Object, path string) (*ast.Object, error) {
 	pkg := imports[path]
 	if pkg == nil {
@@ -310,7 +309,6 @@
 // which correctly updates each package file's comment list.
 // (The ast.PackageExports signature is frozen, hence the local
 // implementation).
-//
 func packageExports(fset *token.FileSet, pkg *ast.Package) {
 	for _, src := range pkg.Files {
 		cmap := ast.NewCommentMap(fset, src, src.Comments)
diff --git a/internal/pkgdoc/funcs.go b/internal/pkgdoc/funcs.go
index 01305d0..e5b8571 100644
--- a/internal/pkgdoc/funcs.go
+++ b/internal/pkgdoc/funcs.go
@@ -8,11 +8,11 @@
 	"bufio"
 	"bytes"
 	"fmt"
-	"go/ast"
-	"go/doc"
-	"go/format"
-	"go/printer"
-	"go/token"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/doc"
+	"golang.org/x/website/internal/backport/go/format"
+	"golang.org/x/website/internal/backport/go/printer"
+	"golang.org/x/website/internal/backport/go/token"
 	"io"
 	"log"
 	"path"
diff --git a/internal/pkgdoc/html_test.go b/internal/pkgdoc/html_test.go
index af540ed..1a98420 100644
--- a/internal/pkgdoc/html_test.go
+++ b/internal/pkgdoc/html_test.go
@@ -7,8 +7,8 @@
 import (
 	"bytes"
 	"fmt"
-	"go/parser"
-	"go/token"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/token"
 	"strings"
 	"testing"
 
diff --git a/internal/texthtml/ast.go b/internal/texthtml/ast.go
index c7e0560..7a9bb32 100644
--- a/internal/texthtml/ast.go
+++ b/internal/texthtml/ast.go
@@ -7,9 +7,9 @@
 import (
 	"bytes"
 	"fmt"
-	"go/ast"
-	"go/doc"
-	"go/token"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/doc"
+	"golang.org/x/website/internal/backport/go/token"
 	"strconv"
 	"unicode"
 	"unicode/utf8"
diff --git a/internal/texthtml/texthtml.go b/internal/texthtml/texthtml.go
index d3b1f16..90cc448 100644
--- a/internal/texthtml/texthtml.go
+++ b/internal/texthtml/texthtml.go
@@ -8,9 +8,9 @@
 import (
 	"bytes"
 	"fmt"
-	"go/ast"
-	"go/scanner"
-	"go/token"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/scanner"
+	"golang.org/x/website/internal/backport/go/token"
 	"io"
 	"regexp"
 	"text/template"
@@ -367,7 +367,6 @@
 // bit 0: comments
 // bit 1: highlights
 // bit 2: selections
-//
 var startTags = [][]byte{
 	/* 000 */ []byte(``),
 	/* 001 */ []byte(`<span class="comment">`),
diff --git a/internal/tour/fmt.go b/internal/tour/fmt.go
index 3010895..e126fb4 100644
--- a/internal/tour/fmt.go
+++ b/internal/tour/fmt.go
@@ -7,10 +7,10 @@
 import (
 	"bytes"
 	"encoding/json"
-	"go/ast"
-	"go/parser"
-	"go/printer"
-	"go/token"
+	"golang.org/x/website/internal/backport/go/ast"
+	"golang.org/x/website/internal/backport/go/parser"
+	"golang.org/x/website/internal/backport/go/printer"
+	"golang.org/x/website/internal/backport/go/token"
 	"net/http"
 
 	"golang.org/x/tools/imports"