internal/lsp/cache/parse.go - tools - Git at Google

 // 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 cache

 import (
 	"bytes"
 	"context"
 	"go/ast"
 	"go/parser"
 	"go/scanner"
 	"go/token"
 	"reflect"

 	"golang.org/x/tools/internal/lsp/protocol"
 	"golang.org/x/tools/internal/lsp/source"
 	"golang.org/x/tools/internal/lsp/telemetry"
 	"golang.org/x/tools/internal/memoize"
 	"golang.org/x/tools/internal/span"
 	"golang.org/x/tools/internal/telemetry/log"
 	"golang.org/x/tools/internal/telemetry/trace"
 	errors "golang.org/x/xerrors"
 )

 // Limits the number of parallel parser calls per process.
 var parseLimit = make(chan struct{}, 20)

 // parseKey uniquely identifies a parsed Go file.
 type parseKey struct {
 	file source.FileIdentity
 	mode source.ParseMode
 }

 type parseGoHandle struct {
 	handle *memoize.Handle
 	file   source.FileHandle
 	mode   source.ParseMode
 }

 type parseGoData struct {
 	memoize.NoCopy

 	ast        *ast.File
 	parseError error // errors associated with parsing the file
 	mapper     *protocol.ColumnMapper
 	err        error
 }

 func (c *cache) ParseGoHandle(fh source.FileHandle, mode source.ParseMode) source.ParseGoHandle {
 	key := parseKey{
 		file: fh.Identity(),
 		mode: mode,
 	}
 	h := c.store.Bind(key, func(ctx context.Context) interface{} {
 		data := &parseGoData{}
 		data.ast, data.mapper, data.parseError, data.err = parseGo(ctx, c, fh, mode)
 		return data
 	})
 	return &parseGoHandle{
 		handle: h,
 		file:   fh,
 		mode:   mode,
 	}
 }

 func (h *parseGoHandle) File() source.FileHandle {
 	return h.file
 }

 func (h *parseGoHandle) Mode() source.ParseMode {
 	return h.mode
 }

 func (h *parseGoHandle) Parse(ctx context.Context) (*ast.File, *protocol.ColumnMapper, error, error) {
 	v := h.handle.Get(ctx)
 	if v == nil {
 		return nil, nil, nil, ctx.Err()
 	}
 	data := v.(*parseGoData)
 	return data.ast, data.mapper, data.parseError, data.err
 }

 func (h *parseGoHandle) Cached(ctx context.Context) (*ast.File, *protocol.ColumnMapper, error, error) {
 	v := h.handle.Cached()
 	if v == nil {
 		return nil, nil, nil, errors.Errorf("no cached AST for %s", h.file.Identity().URI)
 	}
 	data := v.(*parseGoData)
 	return data.ast, data.mapper, data.parseError, data.err
 }

 func hashParseKey(ph source.ParseGoHandle) string {
 	b := bytes.NewBuffer(nil)
 	b.WriteString(ph.File().Identity().String())
 	b.WriteString(string(ph.Mode()))
 	return hashContents(b.Bytes())
 }

 func hashParseKeys(phs []source.ParseGoHandle) string {
 	b := bytes.NewBuffer(nil)
 	for _, ph := range phs {
 		b.WriteString(hashParseKey(ph))
 	}
 	return hashContents(b.Bytes())
 }

 func parseGo(ctx context.Context, c *cache, fh source.FileHandle, mode source.ParseMode) (file *ast.File, mapper *protocol.ColumnMapper, parseError error, err error) {
 	ctx, done := trace.StartSpan(ctx, "cache.parseGo", telemetry.File.Of(fh.Identity().URI.Filename()))
 	defer done()

 	buf, _, err := fh.Read(ctx)
 	if err != nil {
 		return nil, nil, nil, err
 	}
 	parseLimit <- struct{}{}
 	defer func() { <-parseLimit }()
 	parserMode := parser.AllErrors | parser.ParseComments
 	if mode == source.ParseHeader {
 		parserMode = parser.ImportsOnly | parser.ParseComments
 	}
 	file, parseError = parser.ParseFile(c.fset, fh.Identity().URI.Filename(), buf, parserMode)
 	if file != nil {
 		if mode == source.ParseExported {
 			trimAST(file)
 		}
 		// Fix any badly parsed parts of the AST.
 		tok := c.fset.File(file.Pos())
 		if err := fix(ctx, file, tok, buf); err != nil {
 			log.Error(ctx, "failed to fix AST", err)
 		}
 	}

 	if file == nil {
 		// If the file is nil only due to parse errors,
 		// the parse errors are the actual errors.
 		err := parseError
 		if err == nil {
 			err = errors.Errorf("no AST for %s", fh.Identity().URI)
 		}
 		return nil, nil, parseError, err
 	}
 	tok := c.FileSet().File(file.Pos())
 	if tok == nil {
 		return nil, nil, parseError, errors.Errorf("no token.File for %s", fh.Identity().URI)
 	}
 	uri := fh.Identity().URI
 	content, _, err := fh.Read(ctx)
 	if err != nil {
 		return nil, nil, parseError, err
 	}
 	m := &protocol.ColumnMapper{
 		URI:       uri,
 		Converter: span.NewTokenConverter(c.FileSet(), tok),
 		Content:   content,
 	}
 	return file, m, parseError, nil
 }

 // trimAST clears any part of the AST not relevant to type checking
 // expressions at pos.
 func trimAST(file *ast.File) {
 	ast.Inspect(file, func(n ast.Node) bool {
 		if n == nil {
 			return false
 		}
 		switch n := n.(type) {
 		case *ast.FuncDecl:
 			n.Body = nil
 		case *ast.BlockStmt:
 			n.List = nil
 		case *ast.CaseClause:
 			n.Body = nil
 		case *ast.CommClause:
 			n.Body = nil
 		case *ast.CompositeLit:
 			// Leave elts in place for [...]T
 			// array literals, because they can
 			// affect the expression's type.
 			if !isEllipsisArray(n.Type) {
 				n.Elts = nil
 			}
 		}
 		return true
 	})
 }

 func isEllipsisArray(n ast.Expr) bool {
 	at, ok := n.(*ast.ArrayType)
 	if !ok {
 		return false
 	}
 	_, ok = at.Len.(*ast.Ellipsis)
 	return ok
 }

 // fix inspects the AST and potentially modifies any *ast.BadStmts so that it can be
 // type-checked more effectively.
 func fix(ctx context.Context, n ast.Node, tok *token.File, src []byte) error {
 	var (
 		ancestors []ast.Node
 		err       error
 	)
 	ast.Inspect(n, func(n ast.Node) bool {
 		if n == nil {
 			if len(ancestors) > 0 {
 				ancestors = ancestors[:len(ancestors)-1]
 			}
 			return false
 		}
 		switch n := n.(type) {
 		case *ast.BadStmt:
 			var parent ast.Node
 			if len(ancestors) > 0 {
 				parent = ancestors[len(ancestors)-1]
 			}
 			err = parseDeferOrGoStmt(n, parent, tok, src) // don't shadow err
 			if err == nil {
 				// Recursively fix any BadStmts in our fixed node.
 				err = fix(ctx, parent, tok, src)
 			} else {
 				err = errors.Errorf("unable to parse defer or go from *ast.BadStmt: %v", err)
 			}
 			return false
 		default:
 			ancestors = append(ancestors, n)
 			return true
 		}
 	})
 	return err
 }

 // parseDeferOrGoStmt tries to parse an *ast.BadStmt into a defer or a go statement.
 //
 // go/parser packages a statement of the form "defer x." as an *ast.BadStmt because
 // it does not include a call expression. This means that go/types skips type-checking
 // this statement entirely, and we can't use the type information when completing.
 // Here, we try to generate a fake *ast.DeferStmt or *ast.GoStmt to put into the AST,
 // instead of the *ast.BadStmt.
 func parseDeferOrGoStmt(bad *ast.BadStmt, parent ast.Node, tok *token.File, src []byte) error {
 	// Check if we have a bad statement containing either a "go" or "defer".
 	s := &scanner.Scanner{}
 	s.Init(tok, src, nil, 0)

 	var (
 		pos token.Pos
 		tkn token.Token
 	)
 	for {
 		if tkn == token.EOF {
 			return errors.Errorf("reached the end of the file")
 		}
 		if pos >= bad.From {
 			break
 		}
 		pos, tkn, _ = s.Scan()
 	}

 	var stmt ast.Stmt
 	switch tkn {
 	case token.DEFER:
 		stmt = &ast.DeferStmt{
 			Defer: pos,
 		}
 	case token.GO:
 		stmt = &ast.GoStmt{
 			Go: pos,
 		}
 	default:
 		return errors.Errorf("no defer or go statement found")
 	}

 	var (
 		from, to, last   token.Pos
 		lastToken        token.Token
 		braceDepth       int
 		phantomSelectors []token.Pos
 	)
 FindTo:
 	for {
 		to, tkn, _ = s.Scan()

 		if from == token.NoPos {
 			from = to
 		}

 		switch tkn {
 		case token.EOF:
 			break FindTo
 		case token.SEMICOLON:
 			// If we aren't in nested braces, end of statement means
 			// end of expression.
 			if braceDepth == 0 {
 				break FindTo
 			}
 		case token.LBRACE:
 			braceDepth++
 		}

 		// This handles the common dangling selector case. For example in
 		//
 		// defer fmt.
 		// y := 1
 		//
 		// we notice the dangling period and end our expression.
 		//
 		// If the previous token was a "." and we are looking at a "}",
 		// the period is likely a dangling selector and needs a phantom
 		// "_". Likewise if the current token is on a different line than
 		// the period, the period is likely a dangling selector.
 		if lastToken == token.PERIOD && (tkn == token.RBRACE || tok.Line(to) > tok.Line(last)) {
 			// Insert phantom "_" selector after the dangling ".".
 			phantomSelectors = append(phantomSelectors, last+1)
 			// If we aren't in a block then end the expression after the ".".
 			if braceDepth == 0 {
 				to = last + 1
 				break
 			}
 		}

 		lastToken = tkn
 		last = to

 		switch tkn {
 		case token.RBRACE:
 			braceDepth--
 			if braceDepth <= 0 {
 				if braceDepth == 0 {
 					// +1 to include the "}" itself.
 					to += 1
 				}
 				break FindTo
 			}
 		}
 	}

 	if !from.IsValid() || tok.Offset(from) >= len(src) {
 		return errors.Errorf("invalid from position")
 	}

 	if !to.IsValid() || tok.Offset(to) >= len(src) {
 		return errors.Errorf("invalid to position %d", to)
 	}

 	// Insert any phantom selectors needed to prevent dangling "." from messing
 	// up the AST.
 	exprBytes := make([]byte, 0, int(to-from)+len(phantomSelectors))
 	for i, b := range src[tok.Offset(from):tok.Offset(to)] {
 		if len(phantomSelectors) > 0 && from+token.Pos(i) == phantomSelectors[0] {
 			exprBytes = append(exprBytes, '_')
 			phantomSelectors = phantomSelectors[1:]
 		}
 		exprBytes = append(exprBytes, b)
 	}

 	if len(phantomSelectors) > 0 {
 		exprBytes = append(exprBytes, '_')
 	}

 	// Wrap our expression to make it a valid Go file we can pass to ParseFile.
 	fileSrc := bytes.Join([][]byte{
 		[]byte("package fake;func _(){"),
 		exprBytes,
 		[]byte("}"),
 	}, nil)

 	// Use ParseFile instead of ParseExpr because ParseFile has
 	// best-effort behavior, whereas ParseExpr fails hard on any error.
 	fakeFile, err := parser.ParseFile(token.NewFileSet(), "", fileSrc, 0)
 	if fakeFile == nil {
 		return errors.Errorf("error reading fake file source: %v", err)
 	}

 	// Extract our expression node from inside the fake file.
 	if len(fakeFile.Decls) == 0 {
 		return errors.Errorf("error parsing fake file: %v", err)
 	}
 	fakeDecl, _ := fakeFile.Decls[0].(*ast.FuncDecl)
 	if fakeDecl == nil || len(fakeDecl.Body.List) == 0 {
 		return errors.Errorf("no statement in %s: %v", exprBytes, err)
 	}
 	exprStmt, ok := fakeDecl.Body.List[0].(*ast.ExprStmt)
 	if !ok {
 		return errors.Errorf("no expr in %s: %v", exprBytes, err)
 	}
 	expr := exprStmt.X

 	// parser.ParseExpr returns undefined positions.
 	// Adjust them for the current file.
 	offsetPositions(expr, from-1-(expr.Pos()-1))

 	// Package the expression into a fake *ast.CallExpr and re-insert
 	// into the function.
 	call := &ast.CallExpr{
 		Fun:    expr,
 		Lparen: to,
 		Rparen: to,
 	}
 	switch stmt := stmt.(type) {
 	case *ast.DeferStmt:
 		stmt.Call = call
 	case *ast.GoStmt:
 		stmt.Call = call
 	}

 	if !replaceNode(parent, bad, stmt) {
 		return errors.Errorf("couldn't replace %T in %T", stmt, parent)
 	}

 	return nil
 }

 var tokenPosType = reflect.TypeOf(token.NoPos)

 // offsetPositions applies an offset to the positions in an ast.Node.
 func offsetPositions(expr ast.Expr, offset token.Pos) {
 	ast.Inspect(expr, func(n ast.Node) bool {
 		if n == nil {
 			return false
 		}

 		v := reflect.ValueOf(n).Elem()

 		switch v.Kind() {
 		case reflect.Struct:
 			for i := 0; i < v.NumField(); i++ {
 				f := v.Field(i)
 				if f.Type() != tokenPosType {
 					continue
 				}

 				if !f.CanSet() {
 					continue
 				}

 				f.SetInt(int64(f.Interface().(token.Pos) + offset))
 			}
 		}

 		return true
 	})
 }

 // replaceNode updates parent's child oldChild to be newChild. It
 // retuns whether it replaced successfully.
 func replaceNode(parent, oldChild, newChild ast.Node) bool {
 	if parent == nil || oldChild == nil || newChild == nil {
 		return false
 	}

 	parentVal := reflect.ValueOf(parent).Elem()
 	if parentVal.Kind() != reflect.Struct {
 		return false
 	}

 	newChildVal := reflect.ValueOf(newChild)

 	tryReplace := func(v reflect.Value) bool {
 		if !v.CanSet() || !v.CanInterface() {
 			return false
 		}

 		// If the existing value is oldChild, we found our child. Make
 		// sure our newChild is assignable and then make the swap.
 		if v.Interface() == oldChild && newChildVal.Type().AssignableTo(v.Type()) {
 			v.Set(newChildVal)
 			return true
 		}

 		return false
 	}

 	// Loop over parent's struct fields.
 	for i := 0; i < parentVal.NumField(); i++ {
 		f := parentVal.Field(i)

 		switch f.Kind() {
 		// Check interface and pointer fields.
 		case reflect.Interface, reflect.Ptr:
 			if tryReplace(f) {
 				return true
 			}

 		// Search through any slice fields.
 		case reflect.Slice:
 			for i := 0; i < f.Len(); i++ {
 				if tryReplace(f.Index(i)) {
 					return true
 				}
 			}
 		}
 	}

 	return false
 }
	// 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 cache

	import (
	"bytes"
	"context"
	"go/ast"
	"go/parser"
	"go/scanner"
	"go/token"
	"reflect"

	"golang.org/x/tools/internal/lsp/protocol"
	"golang.org/x/tools/internal/lsp/source"
	"golang.org/x/tools/internal/lsp/telemetry"
	"golang.org/x/tools/internal/memoize"
	"golang.org/x/tools/internal/span"
	"golang.org/x/tools/internal/telemetry/log"
	"golang.org/x/tools/internal/telemetry/trace"
	errors "golang.org/x/xerrors"
	)

	// Limits the number of parallel parser calls per process.
	var parseLimit = make(chan struct{}, 20)

	// parseKey uniquely identifies a parsed Go file.
	type parseKey struct {
	file source.FileIdentity
	mode source.ParseMode
	}

	type parseGoHandle struct {
	handle *memoize.Handle
	file source.FileHandle
	mode source.ParseMode
	}

	type parseGoData struct {
	memoize.NoCopy

	ast *ast.File
	parseError error // errors associated with parsing the file
	mapper *protocol.ColumnMapper
	err error
	}

	func (c *cache) ParseGoHandle(fh source.FileHandle, mode source.ParseMode) source.ParseGoHandle {
	key := parseKey{
	file: fh.Identity(),
	mode: mode,
	}
	h := c.store.Bind(key, func(ctx context.Context) interface{} {
	data := &parseGoData{}
	data.ast, data.mapper, data.parseError, data.err = parseGo(ctx, c, fh, mode)
	return data
	})
	return &parseGoHandle{
	handle: h,
	file: fh,
	mode: mode,
	}
	}

	func (h *parseGoHandle) File() source.FileHandle {
	return h.file
	}

	func (h *parseGoHandle) Mode() source.ParseMode {
	return h.mode
	}

	func (h parseGoHandle) Parse(ctx context.Context) (ast.File, *protocol.ColumnMapper, error, error) {
	v := h.handle.Get(ctx)
	if v == nil {
	return nil, nil, nil, ctx.Err()
	}
	data := v.(*parseGoData)
	return data.ast, data.mapper, data.parseError, data.err
	}

	func (h parseGoHandle) Cached(ctx context.Context) (ast.File, *protocol.ColumnMapper, error, error) {
	v := h.handle.Cached()
	if v == nil {
	return nil, nil, nil, errors.Errorf("no cached AST for %s", h.file.Identity().URI)
	}
	data := v.(*parseGoData)
	return data.ast, data.mapper, data.parseError, data.err
	}

	func hashParseKey(ph source.ParseGoHandle) string {
	b := bytes.NewBuffer(nil)
	b.WriteString(ph.File().Identity().String())
	b.WriteString(string(ph.Mode()))
	return hashContents(b.Bytes())
	}

	func hashParseKeys(phs []source.ParseGoHandle) string {
	b := bytes.NewBuffer(nil)
	for _, ph := range phs {
	b.WriteString(hashParseKey(ph))
	}
	return hashContents(b.Bytes())
	}

	func parseGo(ctx context.Context, c cache, fh source.FileHandle, mode source.ParseMode) (file ast.File, mapper *protocol.ColumnMapper, parseError error, err error) {
	ctx, done := trace.StartSpan(ctx, "cache.parseGo", telemetry.File.Of(fh.Identity().URI.Filename()))
	defer done()

	buf, _, err := fh.Read(ctx)
	if err != nil {
	return nil, nil, nil, err
	}
	parseLimit <- struct{}{}
	defer func() { <-parseLimit }()
	parserMode := parser.AllErrors \| parser.ParseComments
	if mode == source.ParseHeader {
	parserMode = parser.ImportsOnly \| parser.ParseComments
	}
	file, parseError = parser.ParseFile(c.fset, fh.Identity().URI.Filename(), buf, parserMode)
	if file != nil {
	if mode == source.ParseExported {
	trimAST(file)
	}
	// Fix any badly parsed parts of the AST.
	tok := c.fset.File(file.Pos())
	if err := fix(ctx, file, tok, buf); err != nil {
	log.Error(ctx, "failed to fix AST", err)
	}
	}

	if file == nil {
	// If the file is nil only due to parse errors,
	// the parse errors are the actual errors.
	err := parseError
	if err == nil {
	err = errors.Errorf("no AST for %s", fh.Identity().URI)
	}
	return nil, nil, parseError, err
	}
	tok := c.FileSet().File(file.Pos())
	if tok == nil {
	return nil, nil, parseError, errors.Errorf("no token.File for %s", fh.Identity().URI)
	}
	uri := fh.Identity().URI
	content, _, err := fh.Read(ctx)
	if err != nil {
	return nil, nil, parseError, err
	}
	m := &protocol.ColumnMapper{
	URI: uri,
	Converter: span.NewTokenConverter(c.FileSet(), tok),
	Content: content,
	}
	return file, m, parseError, nil
	}

	// trimAST clears any part of the AST not relevant to type checking
	// expressions at pos.
	func trimAST(file *ast.File) {
	ast.Inspect(file, func(n ast.Node) bool {
	if n == nil {
	return false
	}
	switch n := n.(type) {
	case *ast.FuncDecl:
	n.Body = nil
	case *ast.BlockStmt:
	n.List = nil
	case *ast.CaseClause:
	n.Body = nil
	case *ast.CommClause:
	n.Body = nil
	case *ast.CompositeLit:
	// Leave elts in place for [...]T
	// array literals, because they can
	// affect the expression's type.
	if !isEllipsisArray(n.Type) {
	n.Elts = nil
	}
	}
	return true
	})
	}

	func isEllipsisArray(n ast.Expr) bool {
	at, ok := n.(*ast.ArrayType)
	if !ok {
	return false
	}
	_, ok = at.Len.(*ast.Ellipsis)
	return ok
	}

	// fix inspects the AST and potentially modifies any *ast.BadStmts so that it can be
	// type-checked more effectively.
	func fix(ctx context.Context, n ast.Node, tok *token.File, src []byte) error {
	var (
	ancestors []ast.Node
	err error
	)
	ast.Inspect(n, func(n ast.Node) bool {
	if n == nil {
	if len(ancestors) > 0 {
	ancestors = ancestors[:len(ancestors)-1]
	}
	return false
	}
	switch n := n.(type) {
	case *ast.BadStmt:
	var parent ast.Node
	if len(ancestors) > 0 {
	parent = ancestors[len(ancestors)-1]
	}
	err = parseDeferOrGoStmt(n, parent, tok, src) // don't shadow err
	if err == nil {
	// Recursively fix any BadStmts in our fixed node.
	err = fix(ctx, parent, tok, src)
	} else {
	err = errors.Errorf("unable to parse defer or go from *ast.BadStmt: %v", err)
	}
	return false
	default:
	ancestors = append(ancestors, n)
	return true
	}
	})
	return err
	}

	// parseDeferOrGoStmt tries to parse an *ast.BadStmt into a defer or a go statement.
	//
	// go/parser packages a statement of the form "defer x." as an *ast.BadStmt because
	// it does not include a call expression. This means that go/types skips type-checking
	// this statement entirely, and we can't use the type information when completing.
	// Here, we try to generate a fake ast.DeferStmt or ast.GoStmt to put into the AST,
	// instead of the *ast.BadStmt.
	func parseDeferOrGoStmt(bad ast.BadStmt, parent ast.Node, tok token.File, src []byte) error {
	// Check if we have a bad statement containing either a "go" or "defer".
	s := &scanner.Scanner{}
	s.Init(tok, src, nil, 0)

	var (
	pos token.Pos
	tkn token.Token
	)
	for {
	if tkn == token.EOF {
	return errors.Errorf("reached the end of the file")
	}
	if pos >= bad.From {
	break
	}
	pos, tkn, _ = s.Scan()
	}

	var stmt ast.Stmt
	switch tkn {
	case token.DEFER:
	stmt = &ast.DeferStmt{
	Defer: pos,
	}
	case token.GO:
	stmt = &ast.GoStmt{
	Go: pos,
	}
	default:
	return errors.Errorf("no defer or go statement found")
	}

	var (
	from, to, last token.Pos
	lastToken token.Token
	braceDepth int
	phantomSelectors []token.Pos
	)
	FindTo:
	for {
	to, tkn, _ = s.Scan()

	if from == token.NoPos {
	from = to
	}

	switch tkn {
	case token.EOF:
	break FindTo
	case token.SEMICOLON:
	// If we aren't in nested braces, end of statement means
	// end of expression.
	if braceDepth == 0 {
	break FindTo
	}
	case token.LBRACE:
	braceDepth++
	}

	// This handles the common dangling selector case. For example in
	//
	// defer fmt.
	// y := 1
	//
	// we notice the dangling period and end our expression.
	//
	// If the previous token was a "." and we are looking at a "}",
	// the period is likely a dangling selector and needs a phantom
	// "_". Likewise if the current token is on a different line than
	// the period, the period is likely a dangling selector.
	if lastToken == token.PERIOD && (tkn == token.RBRACE \|\| tok.Line(to) > tok.Line(last)) {
	// Insert phantom "_" selector after the dangling ".".
	phantomSelectors = append(phantomSelectors, last+1)
	// If we aren't in a block then end the expression after the ".".
	if braceDepth == 0 {
	to = last + 1
	break
	}
	}

	lastToken = tkn
	last = to

	switch tkn {
	case token.RBRACE:
	braceDepth--
	if braceDepth <= 0 {
	if braceDepth == 0 {
	// +1 to include the "}" itself.
	to += 1
	}
	break FindTo
	}
	}
	}

	if !from.IsValid() \|\| tok.Offset(from) >= len(src) {
	return errors.Errorf("invalid from position")
	}

	if !to.IsValid() \|\| tok.Offset(to) >= len(src) {
	return errors.Errorf("invalid to position %d", to)
	}

	// Insert any phantom selectors needed to prevent dangling "." from messing
	// up the AST.
	exprBytes := make([]byte, 0, int(to-from)+len(phantomSelectors))
	for i, b := range src[tok.Offset(from):tok.Offset(to)] {
	if len(phantomSelectors) > 0 && from+token.Pos(i) == phantomSelectors[0] {
	exprBytes = append(exprBytes, '_')
	phantomSelectors = phantomSelectors[1:]
	}
	exprBytes = append(exprBytes, b)
	}

	if len(phantomSelectors) > 0 {
	exprBytes = append(exprBytes, '_')
	}

	// Wrap our expression to make it a valid Go file we can pass to ParseFile.
	fileSrc := bytes.Join([][]byte{
	[]byte("package fake;func _(){"),
	exprBytes,
	[]byte("}"),
	}, nil)

	// Use ParseFile instead of ParseExpr because ParseFile has
	// best-effort behavior, whereas ParseExpr fails hard on any error.
	fakeFile, err := parser.ParseFile(token.NewFileSet(), "", fileSrc, 0)
	if fakeFile == nil {
	return errors.Errorf("error reading fake file source: %v", err)
	}

	// Extract our expression node from inside the fake file.
	if len(fakeFile.Decls) == 0 {
	return errors.Errorf("error parsing fake file: %v", err)
	}
	fakeDecl, _ := fakeFile.Decls[0].(*ast.FuncDecl)
	if fakeDecl == nil \|\| len(fakeDecl.Body.List) == 0 {
	return errors.Errorf("no statement in %s: %v", exprBytes, err)
	}
	exprStmt, ok := fakeDecl.Body.List[0].(*ast.ExprStmt)
	if !ok {
	return errors.Errorf("no expr in %s: %v", exprBytes, err)
	}
	expr := exprStmt.X

	// parser.ParseExpr returns undefined positions.
	// Adjust them for the current file.
	offsetPositions(expr, from-1-(expr.Pos()-1))

	// Package the expression into a fake *ast.CallExpr and re-insert
	// into the function.
	call := &ast.CallExpr{
	Fun: expr,
	Lparen: to,
	Rparen: to,
	}
	switch stmt := stmt.(type) {
	case *ast.DeferStmt:
	stmt.Call = call
	case *ast.GoStmt:
	stmt.Call = call
	}

	if !replaceNode(parent, bad, stmt) {
	return errors.Errorf("couldn't replace %T in %T", stmt, parent)
	}

	return nil
	}

	var tokenPosType = reflect.TypeOf(token.NoPos)

	// offsetPositions applies an offset to the positions in an ast.Node.
	func offsetPositions(expr ast.Expr, offset token.Pos) {
	ast.Inspect(expr, func(n ast.Node) bool {
	if n == nil {
	return false
	}

	v := reflect.ValueOf(n).Elem()

	switch v.Kind() {
	case reflect.Struct:
	for i := 0; i < v.NumField(); i++ {
	f := v.Field(i)
	if f.Type() != tokenPosType {
	continue
	}

	if !f.CanSet() {
	continue
	}

	f.SetInt(int64(f.Interface().(token.Pos) + offset))
	}
	}

	return true
	})
	}

	// replaceNode updates parent's child oldChild to be newChild. It
	// retuns whether it replaced successfully.
	func replaceNode(parent, oldChild, newChild ast.Node) bool {
	if parent == nil \|\| oldChild == nil \|\| newChild == nil {
	return false
	}

	parentVal := reflect.ValueOf(parent).Elem()
	if parentVal.Kind() != reflect.Struct {
	return false
	}

	newChildVal := reflect.ValueOf(newChild)

	tryReplace := func(v reflect.Value) bool {
	if !v.CanSet() \|\| !v.CanInterface() {
	return false
	}

	// If the existing value is oldChild, we found our child. Make
	// sure our newChild is assignable and then make the swap.
	if v.Interface() == oldChild && newChildVal.Type().AssignableTo(v.Type()) {
	v.Set(newChildVal)
	return true
	}

	return false
	}

	// Loop over parent's struct fields.
	for i := 0; i < parentVal.NumField(); i++ {
	f := parentVal.Field(i)

	switch f.Kind() {
	// Check interface and pointer fields.
	case reflect.Interface, reflect.Ptr:
	if tryReplace(f) {
	return true
	}

	// Search through any slice fields.
	case reflect.Slice:
	for i := 0; i < f.Len(); i++ {
	if tryReplace(f.Index(i)) {
	return true
	}
	}
	}
	}

	return false
	}