modfile/rule.go

// 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 modfile implements a parser and formatter for go.mod files.
//
// The go.mod syntax is described in
// https://pkg.go.dev/cmd/go/#hdr-The_go_mod_file.
//
// The [Parse] and [ParseLax] functions both parse a go.mod file and return an
// abstract syntax tree. ParseLax ignores unknown statements and may be used to
// parse go.mod files that may have been developed with newer versions of Go.
//
// The [File] struct returned by Parse and ParseLax represent an abstract
// go.mod file. File has several methods like [File.AddNewRequire] and
// [File.DropReplace] that can be used to programmatically edit a file.
//
// The [Format] function formats a File back to a byte slice which can be
// written to a file.
package modfile

import (
	"errors"
	"fmt"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"unicode"

	"golang.org/x/mod/internal/lazyregexp"
	"golang.org/x/mod/module"
	"golang.org/x/mod/semver"
)

// A File is the parsed, interpreted form of a go.mod file.
type File struct {
	Module    *Module
	Go        *Go
	Toolchain *Toolchain
	Godebug   []*Godebug
	Require   []*Require
	Exclude   []*Exclude
	Replace   []*Replace
	Retract   []*Retract
	Tool      []*Tool

	Syntax *FileSyntax
}

// A Module is the module statement.
type Module struct {
	Mod        module.Version
	Deprecated string
	Syntax     *Line
}

// A Go is the go statement.
type Go struct {
	Version string // "1.23"
	Syntax  *Line
}

// A Toolchain is the toolchain statement.
type Toolchain struct {
	Name   string // "go1.21rc1"
	Syntax *Line
}

// A Godebug is a single godebug key=value statement.
type Godebug struct {
	Key    string
	Value  string
	Syntax *Line
}

// An Exclude is a single exclude statement.
type Exclude struct {
	Mod    module.Version
	Syntax *Line
}

// A Replace is a single replace statement.
type Replace struct {
	Old    module.Version
	New    module.Version
	Syntax *Line
}

// A Retract is a single retract statement.
type Retract struct {
	VersionInterval
	Rationale string
	Syntax    *Line
}

// A Tool is a single tool statement.
type Tool struct {
	Path   string
	Syntax *Line
}

// A VersionInterval represents a range of versions with upper and lower bounds.
// Intervals are closed: both bounds are included. When Low is equal to High,
// the interval may refer to a single version ('v1.2.3') or an interval
// ('[v1.2.3, v1.2.3]'); both have the same representation.
type VersionInterval struct {
	Low, High string
}

// A Require is a single require statement.
type Require struct {
	Mod      module.Version
	Indirect bool // has "// indirect" comment
	Syntax   *Line
}

func (r *Require) markRemoved() {
	r.Syntax.markRemoved()
	*r = Require{}
}

func (r *Require) setVersion(v string) {
	r.Mod.Version = v

	if line := r.Syntax; len(line.Token) > 0 {
		if line.InBlock {
			// If the line is preceded by an empty line, remove it; see
			// https://golang.org/issue/33779.
			if len(line.Comments.Before) == 1 && len(line.Comments.Before[0].Token) == 0 {
				line.Comments.Before = line.Comments.Before[:0]
			}
			if len(line.Token) >= 2 { // example.com v1.2.3
				line.Token[1] = v
			}
		} else {
			if len(line.Token) >= 3 { // require example.com v1.2.3
				line.Token[2] = v
			}
		}
	}
}

// setIndirect sets line to have (or not have) a "// indirect" comment.
func (r *Require) setIndirect(indirect bool) {
	r.Indirect = indirect
	line := r.Syntax
	if isIndirect(line) == indirect {
		return
	}
	if indirect {
		// Adding comment.
		if len(line.Suffix) == 0 {
			// New comment.
			line.Suffix = []Comment{{Token: "// indirect", Suffix: true}}
			return
		}

		com := &line.Suffix[0]
		text := strings.TrimSpace(strings.TrimPrefix(com.Token, string(slashSlash)))
		if text == "" {
			// Empty comment.
			com.Token = "// indirect"
			return
		}

		// Insert at beginning of existing comment.
		com.Token = "// indirect; " + text
		return
	}

	// Removing comment.
	f := strings.TrimSpace(strings.TrimPrefix(line.Suffix[0].Token, string(slashSlash)))
	if f == "indirect" {
		// Remove whole comment.
		line.Suffix = nil
		return
	}

	// Remove comment prefix.
	com := &line.Suffix[0]
	i := strings.Index(com.Token, "indirect;")
	com.Token = "//" + com.Token[i+len("indirect;"):]
}

// isIndirect reports whether line has a "// indirect" comment,
// meaning it is in go.mod only for its effect on indirect dependencies,
// so that it can be dropped entirely once the effective version of the
// indirect dependency reaches the given minimum version.
func isIndirect(line *Line) bool {
	if len(line.Suffix) == 0 {
		return false
	}
	f := strings.Fields(strings.TrimPrefix(line.Suffix[0].Token, string(slashSlash)))
	return (len(f) == 1 && f[0] == "indirect" || len(f) > 1 && f[0] == "indirect;")
}

func (f *File) AddModuleStmt(path string) error {
	if f.Syntax == nil {
		f.Syntax = new(FileSyntax)
	}
	if f.Module == nil {
		f.Module = &Module{
			Mod:    module.Version{Path: path},
			Syntax: f.Syntax.addLine(nil, "module", AutoQuote(path)),
		}
	} else {
		f.Module.Mod.Path = path
		f.Syntax.updateLine(f.Module.Syntax, "module", AutoQuote(path))
	}
	return nil
}

func (f *File) AddComment(text string) {
	if f.Syntax == nil {
		f.Syntax = new(FileSyntax)
	}
	f.Syntax.Stmt = append(f.Syntax.Stmt, &CommentBlock{
		Comments: Comments{
			Before: []Comment{
				{
					Token: text,
				},
			},
		},
	})
}

type VersionFixer func(path, version string) (string, error)

// errDontFix is returned by a VersionFixer to indicate the version should be
// left alone, even if it's not canonical.
var dontFixRetract VersionFixer = func(_, vers string) (string, error) {
	return vers, nil
}

// Parse parses and returns a go.mod file.
//
// file is the name of the file, used in positions and errors.
//
// data is the content of the file.
//
// fix is an optional function that canonicalizes module versions.
// If fix is nil, all module versions must be canonical ([module.CanonicalVersion]
// must return the same string).
func Parse(file string, data []byte, fix VersionFixer) (*File, error) {
	return parseToFile(file, data, fix, true)
}

// ParseLax is like Parse but ignores unknown statements.
// It is used when parsing go.mod files other than the main module,
// under the theory that most statement types we add in the future will
// only apply in the main module, like exclude and replace,
// and so we get better gradual deployments if old go commands
// simply ignore those statements when found in go.mod files
// in dependencies.
func ParseLax(file string, data []byte, fix VersionFixer) (*File, error) {
	return parseToFile(file, data, fix, false)
}

func parseToFile(file string, data []byte, fix VersionFixer, strict bool) (parsed *File, err error) {
	fs, err := parse(file, data)
	if err != nil {
		return nil, err
	}
	f := &File{
		Syntax: fs,
	}
	var errs ErrorList

	// fix versions in retract directives after the file is parsed.
	// We need the module path to fix versions, and it might be at the end.
	defer func() {
		oldLen := len(errs)
		f.fixRetract(fix, &errs)
		if len(errs) > oldLen {
			parsed, err = nil, errs
		}
	}()

	for _, x := range fs.Stmt {
		switch x := x.(type) {
		case *Line:
			f.add(&errs, nil, x, x.Token[0], x.Token[1:], fix, strict)

		case *LineBlock:
			if len(x.Token) > 1 {
				if strict {
					errs = append(errs, Error{
						Filename: file,
						Pos:      x.Start,
						Err:      fmt.Errorf("unknown block type: %s", strings.Join(x.Token, " ")),
					})
				}
				continue
			}
			switch x.Token[0] {
			default:
				if strict {
					errs = append(errs, Error{
						Filename: file,
						Pos:      x.Start,
						Err:      fmt.Errorf("unknown block type: %s", strings.Join(x.Token, " ")),
					})
				}
				continue
			case "module", "godebug", "require", "exclude", "replace", "retract", "tool":
				for _, l := range x.Line {
					f.add(&errs, x, l, x.Token[0], l.Token, fix, strict)
				}
			}
		}
	}

	if len(errs) > 0 {
		return nil, errs
	}
	return f, nil
}

var GoVersionRE = lazyregexp.New(`^([1-9][0-9]*)\.(0|[1-9][0-9]*)(\.(0|[1-9][0-9]*))?([a-z]+[0-9]+)?$`)
var laxGoVersionRE = lazyregexp.New(`^v?(([1-9][0-9]*)\.(0|[1-9][0-9]*))([^0-9].*)$`)

// Toolchains must be named beginning with `go1`,
// like "go1.20.3" or "go1.20.3-gccgo". As a special case, "default" is also permitted.
// Note that this regexp is a much looser condition than go/version.IsValid,
// for forward compatibility.
// (This code has to be work to identify new toolchains even if we tweak the syntax in the future.)
var ToolchainRE = lazyregexp.New(`^default$|^go1($|\.)`)

func (f *File) add(errs *ErrorList, block *LineBlock, line *Line, verb string, args []string, fix VersionFixer, strict bool) {
	// If strict is false, this module is a dependency.
	// We ignore all unknown directives as well as main-module-only
	// directives like replace and exclude. It will work better for
	// forward compatibility if we can depend on modules that have unknown
	// statements (presumed relevant only when acting as the main module)
	// and simply ignore those statements.
	if !strict {
		switch verb {
		case "go", "module", "retract", "require":
			// want these even for dependency go.mods
		default:
			return
		}
	}

	wrapModPathError := func(modPath string, err error) {
		*errs = append(*errs, Error{
			Filename: f.Syntax.Name,
			Pos:      line.Start,
			ModPath:  modPath,
			Verb:     verb,
			Err:      err,
		})
	}
	wrapError := func(err error) {
		*errs = append(*errs, Error{
			Filename: f.Syntax.Name,
			Pos:      line.Start,
			Err:      err,
		})
	}
	errorf := func(format string, args ...interface{}) {
		wrapError(fmt.Errorf(format, args...))
	}

	switch verb {
	default:
		errorf("unknown directive: %s", verb)

	case "go":
		if f.Go != nil {
			errorf("repeated go statement")
			return
		}
		if len(args) != 1 {
			errorf("go directive expects exactly one argument")
			return
		} else if !GoVersionRE.MatchString(args[0]) {
			fixed := false
			if !strict {
				if m := laxGoVersionRE.FindStringSubmatch(args[0]); m != nil {
					args[0] = m[1]
					fixed = true
				}
			}
			if !fixed {
				errorf("invalid go version '%s': must match format 1.23.0", args[0])
				return
			}
		}

		f.Go = &Go{Syntax: line}
		f.Go.Version = args[0]

	case "toolchain":
		if f.Toolchain != nil {
			errorf("repeated toolchain statement")
			return
		}
		if len(args) != 1 {
			errorf("toolchain directive expects exactly one argument")
			return
		} else if !ToolchainRE.MatchString(args[0]) {
			errorf("invalid toolchain version '%s': must match format go1.23.0 or default", args[0])
			return
		}
		f.Toolchain = &Toolchain{Syntax: line}
		f.Toolchain.Name = args[0]

	case "module":
		if f.Module != nil {
			errorf("repeated module statement")
			return
		}
		deprecated := parseDeprecation(block, line)
		f.Module = &Module{
			Syntax:     line,
			Deprecated: deprecated,
		}
		if len(args) != 1 {
			errorf("usage: module module/path")
			return
		}
		s, err := parseString(&args[0])
		if err != nil {
			errorf("invalid quoted string: %v", err)
			return
		}
		f.Module.Mod = module.Version{Path: s}

	case "godebug":
		if len(args) != 1 || strings.ContainsAny(args[0], "\"`',") {
			errorf("usage: godebug key=value")
			return
		}
		key, value, ok := strings.Cut(args[0], "=")
		if !ok {
			errorf("usage: godebug key=value")
			return
		}
		f.Godebug = append(f.Godebug, &Godebug{
			Key:    key,
			Value:  value,
			Syntax: line,
		})

	case "require", "exclude":
		if len(args) != 2 {
			errorf("usage: %s module/path v1.2.3", verb)
			return
		}
		s, err := parseString(&args[0])
		if err != nil {
			errorf("invalid quoted string: %v", err)
			return
		}
		v, err := parseVersion(verb, s, &args[1], fix)
		if err != nil {
			wrapError(err)
			return
		}
		pathMajor, err := modulePathMajor(s)
		if err != nil {
			wrapError(err)
			return
		}
		if err := module.CheckPathMajor(v, pathMajor); err != nil {
			wrapModPathError(s, err)
			return
		}
		if verb == "require" {
			f.Require = append(f.Require, &Require{
				Mod:      module.Version{Path: s, Version: v},
				Syntax:   line,
				Indirect: isIndirect(line),
			})
		} else {
			f.Exclude = append(f.Exclude, &Exclude{
				Mod:    module.Version{Path: s, Version: v},
				Syntax: line,
			})
		}

	case "replace":
		replace, wrappederr := parseReplace(f.Syntax.Name, line, verb, args, fix)
		if wrappederr != nil {
			*errs = append(*errs, *wrappederr)
			return
		}
		f.Replace = append(f.Replace, replace)

	case "retract":
		rationale := parseDirectiveComment(block, line)
		vi, err := parseVersionInterval(verb, "", &args, dontFixRetract)
		if err != nil {
			if strict {
				wrapError(err)
				return
			} else {
				// Only report errors parsing intervals in the main module. We may
				// support additional syntax in the future, such as open and half-open
				// intervals. Those can't be supported now, because they break the
				// go.mod parser, even in lax mode.
				return
			}
		}
		if len(args) > 0 && strict {
			// In the future, there may be additional information after the version.
			errorf("unexpected token after version: %q", args[0])
			return
		}
		retract := &Retract{
			VersionInterval: vi,
			Rationale:       rationale,
			Syntax:          line,
		}
		f.Retract = append(f.Retract, retract)

	case "tool":
		if len(args) != 1 {
			errorf("tool directive expects exactly one argument")
			return
		}
		s, err := parseString(&args[0])
		if err != nil {
			errorf("invalid quoted string: %v", err)
			return
		}
		f.Tool = append(f.Tool, &Tool{
			Path:   s,
			Syntax: line,
		})
	}
}

func parseReplace(filename string, line *Line, verb string, args []string, fix VersionFixer) (*Replace, *Error) {
	wrapModPathError := func(modPath string, err error) *Error {
		return &Error{
			Filename: filename,
			Pos:      line.Start,
			ModPath:  modPath,
			Verb:     verb,
			Err:      err,
		}
	}
	wrapError := func(err error) *Error {
		return &Error{
			Filename: filename,
			Pos:      line.Start,
			Err:      err,
		}
	}
	errorf := func(format string, args ...interface{}) *Error {
		return wrapError(fmt.Errorf(format, args...))
	}

	arrow := 2
	if len(args) >= 2 && args[1] == "=>" {
		arrow = 1
	}
	if len(args) < arrow+2 || len(args) > arrow+3 || args[arrow] != "=>" {
		return nil, errorf("usage: %s module/path [v1.2.3] => other/module v1.4\n\t or %s module/path [v1.2.3] => ../local/directory", verb, verb)
	}
	s, err := parseString(&args[0])
	if err != nil {
		return nil, errorf("invalid quoted string: %v", err)
	}
	pathMajor, err := modulePathMajor(s)
	if err != nil {
		return nil, wrapModPathError(s, err)

	}
	var v string
	if arrow == 2 {
		v, err = parseVersion(verb, s, &args[1], fix)
		if err != nil {
			return nil, wrapError(err)
		}
		if err := module.CheckPathMajor(v, pathMajor); err != nil {
			return nil, wrapModPathError(s, err)
		}
	}
	ns, err := parseString(&args[arrow+1])
	if err != nil {
		return nil, errorf("invalid quoted string: %v", err)
	}
	nv := ""
	if len(args) == arrow+2 {
		if !IsDirectoryPath(ns) {
			if strings.Contains(ns, "@") {
				return nil, errorf("replacement module must match format 'path version', not 'path@version'")
			}
			return nil, errorf("replacement module without version must be directory path (rooted or starting with . or ..)")
		}
		if filepath.Separator == '/' && strings.Contains(ns, `\`) {
			return nil, errorf("replacement directory appears to be Windows path (on a non-windows system)")
		}
	}
	if len(args) == arrow+3 {
		nv, err = parseVersion(verb, ns, &args[arrow+2], fix)
		if err != nil {
			return nil, wrapError(err)
		}
		if IsDirectoryPath(ns) {
			return nil, errorf("replacement module directory path %q cannot have version", ns)
		}
	}
	return &Replace{
		Old:    module.Version{Path: s, Version: v},
		New:    module.Version{Path: ns, Version: nv},
		Syntax: line,
	}, nil
}

// fixRetract applies fix to each retract directive in f, appending any errors
// to errs.
//
// Most versions are fixed as we parse the file, but for retract directives,
// the relevant module path is the one specified with the module directive,
// and that might appear at the end of the file (or not at all).
func (f *File) fixRetract(fix VersionFixer, errs *ErrorList) {
	if fix == nil {
		return
	}
	path := ""
	if f.Module != nil {
		path = f.Module.Mod.Path
	}
	var r *Retract
	wrapError := func(err error) {
		*errs = append(*errs, Error{
			Filename: f.Syntax.Name,
			Pos:      r.Syntax.Start,
			Err:      err,
		})
	}

	for _, r = range f.Retract {
		if path == "" {
			wrapError(errors.New("no module directive found, so retract cannot be used"))
			return // only print the first one of these
		}

		args := r.Syntax.Token
		if args[0] == "retract" {
			args = args[1:]
		}
		vi, err := parseVersionInterval("retract", path, &args, fix)
		if err != nil {
			wrapError(err)
		}
		r.VersionInterval = vi
	}
}

func (f *WorkFile) add(errs *ErrorList, line *Line, verb string, args []string, fix VersionFixer) {
	wrapError := func(err error) {
		*errs = append(*errs, Error{
			Filename: f.Syntax.Name,
			Pos:      line.Start,
			Err:      err,
		})
	}
	errorf := func(format string, args ...interface{}) {
		wrapError(fmt.Errorf(format, args...))
	}

	switch verb {
	default:
		errorf("unknown directive: %s", verb)

	case "go":
		if f.Go != nil {
			errorf("repeated go statement")
			return
		}
		if len(args) != 1 {
			errorf("go directive expects exactly one argument")
			return
		} else if !GoVersionRE.MatchString(args[0]) {
			errorf("invalid go version '%s': must match format 1.23.0", args[0])
			return
		}

		f.Go = &Go{Syntax: line}
		f.Go.Version = args[0]

	case "toolchain":
		if f.Toolchain != nil {
			errorf("repeated toolchain statement")
			return
		}
		if len(args) != 1 {
			errorf("toolchain directive expects exactly one argument")
			return
		} else if !ToolchainRE.MatchString(args[0]) {
			errorf("invalid toolchain version '%s': must match format go1.23.0 or default", args[0])
			return
		}

		f.Toolchain = &Toolchain{Syntax: line}
		f.Toolchain.Name = args[0]

	case "godebug":
		if len(args) != 1 || strings.ContainsAny(args[0], "\"`',") {
			errorf("usage: godebug key=value")
			return
		}
		key, value, ok := strings.Cut(args[0], "=")
		if !ok {
			errorf("usage: godebug key=value")
			return
		}
		f.Godebug = append(f.Godebug, &Godebug{
			Key:    key,
			Value:  value,
			Syntax: line,
		})

	case "use":
		if len(args) != 1 {
			errorf("usage: %s local/dir", verb)
			return
		}
		s, err := parseString(&args[0])
		if err != nil {
			errorf("invalid quoted string: %v", err)
			return
		}
		f.Use = append(f.Use, &Use{
			Path:   s,
			Syntax: line,
		})

	case "replace":
		replace, wrappederr := parseReplace(f.Syntax.Name, line, verb, args, fix)
		if wrappederr != nil {
			*errs = append(*errs, *wrappederr)
			return
		}
		f.Replace = append(f.Replace, replace)
	}
}

// IsDirectoryPath reports whether the given path should be interpreted as a directory path.
// Just like on the go command line, relative paths starting with a '.' or '..' path component
// and rooted paths are directory paths; the rest are module paths.
func IsDirectoryPath(ns string) bool {
	// Because go.mod files can move from one system to another,
	// we check all known path syntaxes, both Unix and Windows.
	return ns == "." || strings.HasPrefix(ns, "./") || strings.HasPrefix(ns, `.\`) ||
		ns == ".." || strings.HasPrefix(ns, "../") || strings.HasPrefix(ns, `..\`) ||
		strings.HasPrefix(ns, "/") || strings.HasPrefix(ns, `\`) ||
		len(ns) >= 2 && ('A' <= ns[0] && ns[0] <= 'Z' || 'a' <= ns[0] && ns[0] <= 'z') && ns[1] == ':'
}

// MustQuote reports whether s must be quoted in order to appear as
// a single token in a go.mod line.
func MustQuote(s string) bool {
	for _, r := range s {
		switch r {
		case ' ', '"', '\'', '`':
			return true

		case '(', ')', '[', ']', '{', '}', ',':
			if len(s) > 1 {
				return true
			}

		default:
			if !unicode.IsPrint(r) {
				return true
			}
		}
	}
	return s == "" || strings.Contains(s, "//") || strings.Contains(s, "/*")
}

// AutoQuote returns s or, if quoting is required for s to appear in a go.mod,
// the quotation of s.
func AutoQuote(s string) string {
	if MustQuote(s) {
		return strconv.Quote(s)
	}
	return s
}

func parseVersionInterval(verb string, path string, args *[]string, fix VersionFixer) (VersionInterval, error) {
	toks := *args
	if len(toks) == 0 || toks[0] == "(" {
		return VersionInterval{}, fmt.Errorf("expected '[' or version")
	}
	if toks[0] != "[" {
		v, err := parseVersion(verb, path, &toks[0], fix)
		if err != nil {
			return VersionInterval{}, err
		}
		*args = toks[1:]
		return VersionInterval{Low: v, High: v}, nil
	}
	toks = toks[1:]

	if len(toks) == 0 {
		return VersionInterval{}, fmt.Errorf("expected version after '['")
	}
	low, err := parseVersion(verb, path, &toks[0], fix)
	if err != nil {
		return VersionInterval{}, err
	}
	toks = toks[1:]

	if len(toks) == 0 || toks[0] != "," {
		return VersionInterval{}, fmt.Errorf("expected ',' after version")
	}
	toks = toks[1:]

	if len(toks) == 0 {
		return VersionInterval{}, fmt.Errorf("expected version after ','")
	}
	high, err := parseVersion(verb, path, &toks[0], fix)
	if err != nil {
		return VersionInterval{}, err
	}
	toks = toks[1:]

	if len(toks) == 0 || toks[0] != "]" {
		return VersionInterval{}, fmt.Errorf("expected ']' after version")
	}
	toks = toks[1:]

	*args = toks
	return VersionInterval{Low: low, High: high}, nil
}

func parseString(s *string) (string, error) {
	t := *s
	if strings.HasPrefix(t, `"`) {
		var err error
		if t, err = strconv.Unquote(t); err != nil {
			return "", err
		}
	} else if strings.ContainsAny(t, "\"'`") {
		// Other quotes are reserved both for possible future expansion
		// and to avoid confusion. For example if someone types 'x'
		// we want that to be a syntax error and not a literal x in literal quotation marks.
		return "", fmt.Errorf("unquoted string cannot contain quote")
	}
	*s = AutoQuote(t)
	return t, nil
}

var deprecatedRE = lazyregexp.New(`(?s)(?:^|\n\n)Deprecated: *(.*?)(?:$|\n\n)`)

// parseDeprecation extracts the text of comments on a "module" directive and
// extracts a deprecation message from that.
//
// A deprecation message is contained in a paragraph within a block of comments
// that starts with "Deprecated:" (case sensitive). The message runs until the
// end of the paragraph and does not include the "Deprecated:" prefix. If the
// comment block has multiple paragraphs that start with "Deprecated:",
// parseDeprecation returns the message from the first.
func parseDeprecation(block *LineBlock, line *Line) string {
	text := parseDirectiveComment(block, line)
	m := deprecatedRE.FindStringSubmatch(text)
	if m == nil {
		return ""
	}
	return m[1]
}

// parseDirectiveComment extracts the text of comments on a directive.
// If the directive's line does not have comments and is part of a block that
// does have comments, the block's comments are used.
func parseDirectiveComment(block *LineBlock, line *Line) string {
	comments := line.Comment()
	if block != nil && len(comments.Before) == 0 && len(comments.Suffix) == 0 {
		comments = block.Comment()
	}
	groups := [][]Comment{comments.Before, comments.Suffix}
	var lines []string
	for _, g := range groups {
		for _, c := range g {
			if !strings.HasPrefix(c.Token, "//") {
				continue // blank line
			}
			lines = append(lines, strings.TrimSpace(strings.TrimPrefix(c.Token, "//")))
		}
	}
	return strings.Join(lines, "\n")
}

type ErrorList []Error

func (e ErrorList) Error() string {
	errStrs := make([]string, len(e))
	for i, err := range e {
		errStrs[i] = err.Error()
	}
	return strings.Join(errStrs, "\n")
}

type Error struct {
	Filename string
	Pos      Position
	Verb     string
	ModPath  string
	Err      error
}

func (e *Error) Error() string {
	var pos string
	if e.Pos.LineRune > 1 {
		// Don't print LineRune if it's 1 (beginning of line).
		// It's always 1 except in scanner errors, which are rare.
		pos = fmt.Sprintf("%s:%d:%d: ", e.Filename, e.Pos.Line, e.Pos.LineRune)
	} else if e.Pos.Line > 0 {
		pos = fmt.Sprintf("%s:%d: ", e.Filename, e.Pos.Line)
	} else if e.Filename != "" {
		pos = fmt.Sprintf("%s: ", e.Filename)
	}

	var directive string
	if e.ModPath != "" {
		directive = fmt.Sprintf("%s %s: ", e.Verb, e.ModPath)
	} else if e.Verb != "" {
		directive = fmt.Sprintf("%s: ", e.Verb)
	}

	return pos + directive + e.Err.Error()
}

func (e *Error) Unwrap() error { return e.Err }

func parseVersion(verb string, path string, s *string, fix VersionFixer) (string, error) {
	t, err := parseString(s)
	if err != nil {
		return "", &Error{
			Verb:    verb,
			ModPath: path,
			Err: &module.InvalidVersionError{
				Version: *s,
				Err:     err,
			},
		}
	}
	if fix != nil {
		fixed, err := fix(path, t)
		if err != nil {
			if err, ok := err.(*module.ModuleError); ok {
				return "", &Error{
					Verb:    verb,
					ModPath: path,
					Err:     err.Err,
				}
			}
			return "", err
		}
		t = fixed
	} else {
		cv := module.CanonicalVersion(t)
		if cv == "" {
			return "", &Error{
				Verb:    verb,
				ModPath: path,
				Err: &module.InvalidVersionError{
					Version: t,
					Err:     errors.New("must be of the form v1.2.3"),
				},
			}
		}
		t = cv
	}
	*s = t
	return *s, nil
}

func modulePathMajor(path string) (string, error) {
	_, major, ok := module.SplitPathVersion(path)
	if !ok {
		return "", fmt.Errorf("invalid module path")
	}
	return major, nil
}

func (f *File) Format() ([]byte, error) {
	return Format(f.Syntax), nil
}

// Cleanup cleans up the file f after any edit operations.
// To avoid quadratic behavior, modifications like [File.DropRequire]
// clear the entry but do not remove it from the slice.
// Cleanup cleans out all the cleared entries.
func (f *File) Cleanup() {
	w := 0
	for _, g := range f.Godebug {
		if g.Key != "" {
			f.Godebug[w] = g
			w++
		}
	}
	f.Godebug = f.Godebug[:w]

	w = 0
	for _, r := range f.Require {
		if r.Mod.Path != "" {
			f.Require[w] = r
			w++
		}
	}
	f.Require = f.Require[:w]

	w = 0
	for _, x := range f.Exclude {
		if x.Mod.Path != "" {
			f.Exclude[w] = x
			w++
		}
	}
	f.Exclude = f.Exclude[:w]

	w = 0
	for _, r := range f.Replace {
		if r.Old.Path != "" {
			f.Replace[w] = r
			w++
		}
	}
	f.Replace = f.Replace[:w]

	w = 0
	for _, r := range f.Retract {
		if r.Low != "" || r.High != "" {
			f.Retract[w] = r
			w++
		}
	}
	f.Retract = f.Retract[:w]

	f.Syntax.Cleanup()
}

func (f *File) AddGoStmt(version string) error {
	if !GoVersionRE.MatchString(version) {
		return fmt.Errorf("invalid language version %q", version)
	}
	if f.Go == nil {
		var hint Expr
		if f.Module != nil && f.Module.Syntax != nil {
			hint = f.Module.Syntax
		} else if f.Syntax == nil {
			f.Syntax = new(FileSyntax)
		}
		f.Go = &Go{
			Version: version,
			Syntax:  f.Syntax.addLine(hint, "go", version),
		}
	} else {
		f.Go.Version = version
		f.Syntax.updateLine(f.Go.Syntax, "go", version)
	}
	return nil
}

// DropGoStmt deletes the go statement from the file.
func (f *File) DropGoStmt() {
	if f.Go != nil {
		f.Go.Syntax.markRemoved()
		f.Go = nil
	}
}

// DropToolchainStmt deletes the toolchain statement from the file.
func (f *File) DropToolchainStmt() {
	if f.Toolchain != nil {
		f.Toolchain.Syntax.markRemoved()
		f.Toolchain = nil
	}
}

func (f *File) AddToolchainStmt(name string) error {
	if !ToolchainRE.MatchString(name) {
		return fmt.Errorf("invalid toolchain name %q", name)
	}
	if f.Toolchain == nil {
		var hint Expr
		if f.Go != nil && f.Go.Syntax != nil {
			hint = f.Go.Syntax
		} else if f.Module != nil && f.Module.Syntax != nil {
			hint = f.Module.Syntax
		}
		f.Toolchain = &Toolchain{
			Name:   name,
			Syntax: f.Syntax.addLine(hint, "toolchain", name),
		}
	} else {
		f.Toolchain.Name = name
		f.Syntax.updateLine(f.Toolchain.Syntax, "toolchain", name)
	}
	return nil
}

// AddGodebug sets the first godebug line for key to value,
// preserving any existing comments for that line and removing all
// other godebug lines for key.
//
// If no line currently exists for key, AddGodebug adds a new line
// at the end of the last godebug block.
func (f *File) AddGodebug(key, value string) error {
	need := true
	for _, g := range f.Godebug {
		if g.Key == key {
			if need {
				g.Value = value
				f.Syntax.updateLine(g.Syntax, "godebug", key+"="+value)
				need = false
			} else {
				g.Syntax.markRemoved()
				*g = Godebug{}
			}
		}
	}

	if need {
		f.addNewGodebug(key, value)
	}
	return nil
}

// addNewGodebug adds a new godebug key=value line at the end
// of the last godebug block, regardless of any existing godebug lines for key.
func (f *File) addNewGodebug(key, value string) {
	line := f.Syntax.addLine(nil, "godebug", key+"="+value)
	g := &Godebug{
		Key:    key,
		Value:  value,
		Syntax: line,
	}
	f.Godebug = append(f.Godebug, g)
}

// AddRequire sets the first require line for path to version vers,
// preserving any existing comments for that line and removing all
// other lines for path.
//
// If no line currently exists for path, AddRequire adds a new line
// at the end of the last require block.
func (f *File) AddRequire(path, vers string) error {
	need := true
	for _, r := range f.Require {
		if r.Mod.Path == path {
			if need {
				r.Mod.Version = vers
				f.Syntax.updateLine(r.Syntax, "require", AutoQuote(path), vers)
				need = false
			} else {
				r.Syntax.markRemoved()
				*r = Require{}
			}
		}
	}

	if need {
		f.AddNewRequire(path, vers, false)
	}
	return nil
}

// AddNewRequire adds a new require line for path at version vers at the end of
// the last require block, regardless of any existing require lines for path.
func (f *File) AddNewRequire(path, vers string, indirect bool) {
	line := f.Syntax.addLine(nil, "require", AutoQuote(path), vers)
	r := &Require{
		Mod:    module.Version{Path: path, Version: vers},
		Syntax: line,
	}
	r.setIndirect(indirect)
	f.Require = append(f.Require, r)
}

// SetRequire updates the requirements of f to contain exactly req, preserving
// the existing block structure and line comment contents (except for 'indirect'
// markings) for the first requirement on each named module path.
//
// The Syntax field is ignored for the requirements in req.
//
// Any requirements not already present in the file are added to the block
// containing the last require line.
//
// The requirements in req must specify at most one distinct version for each
// module path.
//
// If any existing requirements may be removed, the caller should call
// [File.Cleanup] after all edits are complete.
func (f *File) SetRequire(req []*Require) {
	type elem struct {
		version  string
		indirect bool
	}
	need := make(map[string]elem)
	for _, r := range req {
		if prev, dup := need[r.Mod.Path]; dup && prev.version != r.Mod.Version {
			panic(fmt.Errorf("SetRequire called with conflicting versions for path %s (%s and %s)", r.Mod.Path, prev.version, r.Mod.Version))
		}
		need[r.Mod.Path] = elem{r.Mod.Version, r.Indirect}
	}

	// Update or delete the existing Require entries to preserve
	// only the first for each module path in req.
	for _, r := range f.Require {
		e, ok := need[r.Mod.Path]
		if ok {
			r.setVersion(e.version)
			r.setIndirect(e.indirect)
		} else {
			r.markRemoved()
		}
		delete(need, r.Mod.Path)
	}

	// Add new entries in the last block of the file for any paths that weren't
	// already present.
	//
	// This step is nondeterministic, but the final result will be deterministic
	// because we will sort the block.
	for path, e := range need {
		f.AddNewRequire(path, e.version, e.indirect)
	}

	f.SortBlocks()
}

// SetRequireSeparateIndirect updates the requirements of f to contain the given
// requirements. Comment contents (except for 'indirect' markings) are retained
// from the first existing requirement for each module path. Like SetRequire,
// SetRequireSeparateIndirect adds requirements for new paths in req,
// updates the version and "// indirect" comment on existing requirements,
// and deletes requirements on paths not in req. Existing duplicate requirements
// are deleted.
//
// As its name suggests, SetRequireSeparateIndirect puts direct and indirect
// requirements into two separate blocks, one containing only direct
// requirements, and the other containing only indirect requirements.
// SetRequireSeparateIndirect may move requirements between these two blocks
// when their indirect markings change. However, SetRequireSeparateIndirect
// won't move requirements from other blocks, especially blocks with comments.
//
// If the file initially has one uncommented block of requirements,
// SetRequireSeparateIndirect will split it into a direct-only and indirect-only
// block. This aids in the transition to separate blocks.
func (f *File) SetRequireSeparateIndirect(req []*Require) {
	// hasComments returns whether a line or block has comments
	// other than "indirect".
	hasComments := func(c Comments) bool {
		return len(c.Before) > 0 || len(c.After) > 0 || len(c.Suffix) > 1 ||
			(len(c.Suffix) == 1 &&
				strings.TrimSpace(strings.TrimPrefix(c.Suffix[0].Token, string(slashSlash))) != "indirect")
	}

	// moveReq adds r to block. If r was in another block, moveReq deletes
	// it from that block and transfers its comments.
	moveReq := func(r *Require, block *LineBlock) {
		var line *Line
		if r.Syntax == nil {
			line = &Line{Token: []string{AutoQuote(r.Mod.Path), r.Mod.Version}}
			r.Syntax = line
			if r.Indirect {
				r.setIndirect(true)
			}
		} else {
			line = new(Line)
			*line = *r.Syntax
			if !line.InBlock && len(line.Token) > 0 && line.Token[0] == "require" {
				line.Token = line.Token[1:]
			}
			r.Syntax.Token = nil // Cleanup will delete the old line.
			r.Syntax = line
		}
		line.InBlock = true
		block.Line = append(block.Line, line)
	}

	// Examine existing require lines and blocks.
	var (
		// We may insert new requirements into the last uncommented
		// direct-only and indirect-only blocks. We may also move requirements
		// to the opposite block if their indirect markings change.
		lastDirectIndex   = -1
		lastIndirectIndex = -1

		// If there are no direct-only or indirect-only blocks, a new block may
		// be inserted after the last require line or block.
		lastRequireIndex = -1

		// If there's only one require line or block, and it's uncommented,
		// we'll move its requirements to the direct-only or indirect-only blocks.
		requireLineOrBlockCount = 0

		// Track the block each requirement belongs to (if any) so we can
		// move them later.
		lineToBlock = make(map[*Line]*LineBlock)
	)
	for i, stmt := range f.Syntax.Stmt {
		switch stmt := stmt.(type) {
		case *Line:
			if len(stmt.Token) == 0 || stmt.Token[0] != "require" {
				continue
			}
			lastRequireIndex = i
			requireLineOrBlockCount++
			if !hasComments(stmt.Comments) {
				if isIndirect(stmt) {
					lastIndirectIndex = i
				} else {
					lastDirectIndex = i
				}
			}

		case *LineBlock:
			if len(stmt.Token) == 0 || stmt.Token[0] != "require" {
				continue
			}
			lastRequireIndex = i
			requireLineOrBlockCount++
			allDirect := len(stmt.Line) > 0 && !hasComments(stmt.Comments)
			allIndirect := len(stmt.Line) > 0 && !hasComments(stmt.Comments)
			for _, line := range stmt.Line {
				lineToBlock[line] = stmt
				if hasComments(line.Comments) {
					allDirect = false
					allIndirect = false
				} else if isIndirect(line) {
					allDirect = false
				} else {
					allIndirect = false
				}
			}
			if allDirect {
				lastDirectIndex = i
			}
			if allIndirect {
				lastIndirectIndex = i
			}
		}
	}

	oneFlatUncommentedBlock := requireLineOrBlockCount == 1 &&
		!hasComments(*f.Syntax.Stmt[lastRequireIndex].Comment())

	// Create direct and indirect blocks if needed. Convert lines into blocks
	// if needed. If we end up with an empty block or a one-line block,
	// Cleanup will delete it or convert it to a line later.
	insertBlock := func(i int) *LineBlock {
		block := &LineBlock{Token: []string{"require"}}
		f.Syntax.Stmt = append(f.Syntax.Stmt, nil)
		copy(f.Syntax.Stmt[i+1:], f.Syntax.Stmt[i:])
		f.Syntax.Stmt[i] = block
		return block
	}

	ensureBlock := func(i int) *LineBlock {
		switch stmt := f.Syntax.Stmt[i].(type) {
		case *LineBlock:
			return stmt
		case *Line:
			block := &LineBlock{
				Token: []string{"require"},
				Line:  []*Line{stmt},
			}
			stmt.Token = stmt.Token[1:] // remove "require"
			stmt.InBlock = true
			f.Syntax.Stmt[i] = block
			return block
		default:
			panic(fmt.Sprintf("unexpected statement: %v", stmt))
		}
	}

	var lastDirectBlock *LineBlock
	if lastDirectIndex < 0 {
		if lastIndirectIndex >= 0 {
			lastDirectIndex = lastIndirectIndex
			lastIndirectIndex++
		} else if lastRequireIndex >= 0 {
			lastDirectIndex = lastRequireIndex + 1
		} else {
			lastDirectIndex = len(f.Syntax.Stmt)
		}
		lastDirectBlock = insertBlock(lastDirectIndex)
	} else {
		lastDirectBlock = ensureBlock(lastDirectIndex)
	}

	var lastIndirectBlock *LineBlock
	if lastIndirectIndex < 0 {
		lastIndirectIndex = lastDirectIndex + 1
		lastIndirectBlock = insertBlock(lastIndirectIndex)
	} else {
		lastIndirectBlock = ensureBlock(lastIndirectIndex)
	}

	// Delete requirements we don't want anymore.
	// Update versions and indirect comments on requirements we want to keep.
	// If a requirement is in last{Direct,Indirect}Block with the wrong
	// indirect marking after this, or if the requirement is in an single
	// uncommented mixed block (oneFlatUncommentedBlock), move it to the
	// correct block.
	//
	// Some blocks may be empty after this. Cleanup will remove them.
	need := make(map[string]*Require)
	for _, r := range req {
		need[r.Mod.Path] = r
	}
	have := make(map[string]*Require)
	for _, r := range f.Require {
		path := r.Mod.Path
		if need[path] == nil || have[path] != nil {
			// Requirement not needed, or duplicate requirement. Delete.
			r.markRemoved()
			continue
		}
		have[r.Mod.Path] = r
		r.setVersion(need[path].Mod.Version)
		r.setIndirect(need[path].Indirect)
		if need[path].Indirect &&
			(oneFlatUncommentedBlock || lineToBlock[r.Syntax] == lastDirectBlock) {
			moveReq(r, lastIndirectBlock)
		} else if !need[path].Indirect &&
			(oneFlatUncommentedBlock || lineToBlock[r.Syntax] == lastIndirectBlock) {
			moveReq(r, lastDirectBlock)
		}
	}

	// Add new requirements.
	for path, r := range need {
		if have[path] == nil {
			if r.Indirect {
				moveReq(r, lastIndirectBlock)
			} else {
				moveReq(r, lastDirectBlock)
			}
			f.Require = append(f.Require, r)
		}
	}

	f.SortBlocks()
}

func (f *File) DropGodebug(key string) error {
	for _, g := range f.Godebug {
		if g.Key == key {
			g.Syntax.markRemoved()
			*g = Godebug{}
		}
	}
	return nil
}

func (f *File) DropRequire(path string) error {
	for _, r := range f.Require {
		if r.Mod.Path == path {
			r.Syntax.markRemoved()
			*r = Require{}
		}
	}
	return nil
}

// AddExclude adds a exclude statement to the mod file. Errors if the provided
// version is not a canonical version string
func (f *File) AddExclude(path, vers string) error {
	if err := checkCanonicalVersion(path, vers); err != nil {
		return err
	}

	var hint *Line
	for _, x := range f.Exclude {
		if x.Mod.Path == path && x.Mod.Version == vers {
			return nil
		}
		if x.Mod.Path == path {
			hint = x.Syntax
		}
	}

	f.Exclude = append(f.Exclude, &Exclude{Mod: module.Version{Path: path, Version: vers}, Syntax: f.Syntax.addLine(hint, "exclude", AutoQuote(path), vers)})
	return nil
}

func (f *File) DropExclude(path, vers string) error {
	for _, x := range f.Exclude {
		if x.Mod.Path == path && x.Mod.Version == vers {
			x.Syntax.markRemoved()
			*x = Exclude{}
		}
	}
	return nil
}

func (f *File) AddReplace(oldPath, oldVers, newPath, newVers string) error {
	return addReplace(f.Syntax, &f.Replace, oldPath, oldVers, newPath, newVers)
}

func addReplace(syntax *FileSyntax, replace *[]*Replace, oldPath, oldVers, newPath, newVers string) error {
	need := true
	old := module.Version{Path: oldPath, Version: oldVers}
	new := module.Version{Path: newPath, Version: newVers}
	tokens := []string{"replace", AutoQuote(oldPath)}
	if oldVers != "" {
		tokens = append(tokens, oldVers)
	}
	tokens = append(tokens, "=>", AutoQuote(newPath))
	if newVers != "" {
		tokens = append(tokens, newVers)
	}

	var hint *Line
	for _, r := range *replace {
		if r.Old.Path == oldPath && (oldVers == "" || r.Old.Version == oldVers) {
			if need {
				// Found replacement for old; update to use new.
				r.New = new
				syntax.updateLine(r.Syntax, tokens...)
				need = false
				continue
			}
			// Already added; delete other replacements for same.
			r.Syntax.markRemoved()
			*r = Replace{}
		}
		if r.Old.Path == oldPath {
			hint = r.Syntax
		}
	}
	if need {
		*replace = append(*replace, &Replace{Old: old, New: new, Syntax: syntax.addLine(hint, tokens...)})
	}
	return nil
}

func (f *File) DropReplace(oldPath, oldVers string) error {
	for _, r := range f.Replace {
		if r.Old.Path == oldPath && r.Old.Version == oldVers {
			r.Syntax.markRemoved()
			*r = Replace{}
		}
	}
	return nil
}

// AddRetract adds a retract statement to the mod file. Errors if the provided
// version interval does not consist of canonical version strings
func (f *File) AddRetract(vi VersionInterval, rationale string) error {
	var path string
	if f.Module != nil {
		path = f.Module.Mod.Path
	}
	if err := checkCanonicalVersion(path, vi.High); err != nil {
		return err
	}
	if err := checkCanonicalVersion(path, vi.Low); err != nil {
		return err
	}

	r := &Retract{
		VersionInterval: vi,
	}
	if vi.Low == vi.High {
		r.Syntax = f.Syntax.addLine(nil, "retract", AutoQuote(vi.Low))
	} else {
		r.Syntax = f.Syntax.addLine(nil, "retract", "[", AutoQuote(vi.Low), ",", AutoQuote(vi.High), "]")
	}
	if rationale != "" {
		for _, line := range strings.Split(rationale, "\n") {
			com := Comment{Token: "// " + line}
			r.Syntax.Comment().Before = append(r.Syntax.Comment().Before, com)
		}
	}
	return nil
}

func (f *File) DropRetract(vi VersionInterval) error {
	for _, r := range f.Retract {
		if r.VersionInterval == vi {
			r.Syntax.markRemoved()
			*r = Retract{}
		}
	}
	return nil
}

// AddTool adds a new tool directive with the given path.
// It does nothing if the tool line already exists.
func (f *File) AddTool(path string) error {
	for _, t := range f.Tool {
		if t.Path == path {
			return nil
		}
	}

	f.Tool = append(f.Tool, &Tool{
		Path:   path,
		Syntax: f.Syntax.addLine(nil, "tool", path),
	})

	f.SortBlocks()
	return nil
}

// RemoveTool removes a tool directive with the given path.
// It does nothing if no such tool directive exists.
func (f *File) DropTool(path string) error {
	for _, t := range f.Tool {
		if t.Path == path {
			t.Syntax.markRemoved()
			*t = Tool{}
		}
	}
	return nil
}

func (f *File) SortBlocks() {
	f.removeDups() // otherwise sorting is unsafe

	// semanticSortForExcludeVersionV is the Go version (plus leading "v") at which
	// lines in exclude blocks start to use semantic sort instead of lexicographic sort.
	// See go.dev/issue/60028.
	const semanticSortForExcludeVersionV = "v1.21"
	useSemanticSortForExclude := f.Go != nil && semver.Compare("v"+f.Go.Version, semanticSortForExcludeVersionV) >= 0

	for _, stmt := range f.Syntax.Stmt {
		block, ok := stmt.(*LineBlock)
		if !ok {
			continue
		}
		less := lineLess
		if block.Token[0] == "exclude" && useSemanticSortForExclude {
			less = lineExcludeLess
		} else if block.Token[0] == "retract" {
			less = lineRetractLess
		}
		sort.SliceStable(block.Line, func(i, j int) bool {
			return less(block.Line[i], block.Line[j])
		})
	}
}

// removeDups removes duplicate exclude, replace and tool directives.
//
// Earlier exclude and tool directives take priority.
//
// Later replace directives take priority.
//
// require directives are not de-duplicated. That's left up to higher-level
// logic (MVS).
//
// retract directives are not de-duplicated since comments are
// meaningful, and versions may be retracted multiple times.
func (f *File) removeDups() {
	removeDups(f.Syntax, &f.Exclude, &f.Replace, &f.Tool)
}

func removeDups(syntax *FileSyntax, exclude *[]*Exclude, replace *[]*Replace, tool *[]*Tool) {
	kill := make(map[*Line]bool)

	// Remove duplicate excludes.
	if exclude != nil {
		haveExclude := make(map[module.Version]bool)
		for _, x := range *exclude {
			if haveExclude[x.Mod] {
				kill[x.Syntax] = true
				continue
			}
			haveExclude[x.Mod] = true
		}
		var excl []*Exclude
		for _, x := range *exclude {
			if !kill[x.Syntax] {
				excl = append(excl, x)
			}
		}
		*exclude = excl
	}

	// Remove duplicate replacements.
	// Later replacements take priority over earlier ones.
	haveReplace := make(map[module.Version]bool)
	for i := len(*replace) - 1; i >= 0; i-- {
		x := (*replace)[i]
		if haveReplace[x.Old] {
			kill[x.Syntax] = true
			continue
		}
		haveReplace[x.Old] = true
	}
	var repl []*Replace
	for _, x := range *replace {
		if !kill[x.Syntax] {
			repl = append(repl, x)
		}
	}
	*replace = repl

	if tool != nil {
		haveTool := make(map[string]bool)
		for _, t := range *tool {
			if haveTool[t.Path] {
				kill[t.Syntax] = true
				continue
			}
			haveTool[t.Path] = true
		}
		var newTool []*Tool
		for _, t := range *tool {
			if !kill[t.Syntax] {
				newTool = append(newTool, t)
			}
		}
		*tool = newTool
	}

	// Duplicate require and retract directives are not removed.

	// Drop killed statements from the syntax tree.
	var stmts []Expr
	for _, stmt := range syntax.Stmt {
		switch stmt := stmt.(type) {
		case *Line:
			if kill[stmt] {
				continue
			}
		case *LineBlock:
			var lines []*Line
			for _, line := range stmt.Line {
				if !kill[line] {
					lines = append(lines, line)
				}
			}
			stmt.Line = lines
			if len(lines) == 0 {
				continue
			}
		}
		stmts = append(stmts, stmt)
	}
	syntax.Stmt = stmts
}

// lineLess returns whether li should be sorted before lj. It sorts
// lexicographically without assigning any special meaning to tokens.
func lineLess(li, lj *Line) bool {
	for k := 0; k < len(li.Token) && k < len(lj.Token); k++ {
		if li.Token[k] != lj.Token[k] {
			return li.Token[k] < lj.Token[k]
		}
	}
	return len(li.Token) < len(lj.Token)
}

// lineExcludeLess reports whether li should be sorted before lj for lines in
// an "exclude" block.
func lineExcludeLess(li, lj *Line) bool {
	if len(li.Token) != 2 || len(lj.Token) != 2 {
		// Not a known exclude specification.
		// Fall back to sorting lexicographically.
		return lineLess(li, lj)
	}
	// An exclude specification has two tokens: ModulePath and Version.
	// Compare module path by string order and version by semver rules.
	if pi, pj := li.Token[0], lj.Token[0]; pi != pj {
		return pi < pj
	}
	return semver.Compare(li.Token[1], lj.Token[1]) < 0
}

// lineRetractLess returns whether li should be sorted before lj for lines in
// a "retract" block. It treats each line as a version interval. Single versions
// are compared as if they were intervals with the same low and high version.
// Intervals are sorted in descending order, first by low version, then by
// high version, using semver.Compare.
func lineRetractLess(li, lj *Line) bool {
	interval := func(l *Line) VersionInterval {
		if len(l.Token) == 1 {
			return VersionInterval{Low: l.Token[0], High: l.Token[0]}
		} else if len(l.Token) == 5 && l.Token[0] == "[" && l.Token[2] == "," && l.Token[4] == "]" {
			return VersionInterval{Low: l.Token[1], High: l.Token[3]}
		} else {
			// Line in unknown format. Treat as an invalid version.
			return VersionInterval{}
		}
	}
	vii := interval(li)
	vij := interval(lj)
	if cmp := semver.Compare(vii.Low, vij.Low); cmp != 0 {
		return cmp > 0
	}
	return semver.Compare(vii.High, vij.High) > 0
}

// checkCanonicalVersion returns a non-nil error if vers is not a canonical
// version string or does not match the major version of path.
//
// If path is non-empty, the error text suggests a format with a major version
// corresponding to the path.
func checkCanonicalVersion(path, vers string) error {
	_, pathMajor, pathMajorOk := module.SplitPathVersion(path)

	if vers == "" || vers != module.CanonicalVersion(vers) {
		if pathMajor == "" {
			return &module.InvalidVersionError{
				Version: vers,
				Err:     fmt.Errorf("must be of the form v1.2.3"),
			}
		}
		return &module.InvalidVersionError{
			Version: vers,
			Err:     fmt.Errorf("must be of the form %s.2.3", module.PathMajorPrefix(pathMajor)),
		}
	}

	if pathMajorOk {
		if err := module.CheckPathMajor(vers, pathMajor); err != nil {
			if pathMajor == "" {
				// In this context, the user probably wrote "v2.3.4" when they meant
				// "v2.3.4+incompatible". Suggest that instead of "v0 or v1".
				return &module.InvalidVersionError{
					Version: vers,
					Err:     fmt.Errorf("should be %s+incompatible (or module %s/%v)", vers, path, semver.Major(vers)),
				}
			}
			return err
		}
	}

	return nil
}