Update dependencies (#5574)

Update vendor

vendor/golang.org/x/tools/internal/imports/imports.go

@@ -18,8 +18,6 @@ import (
 	"go/printer"
 	"go/token"
 	"io"
-	"io/ioutil"
-	"log"
 	"regexp"
 	"strconv"
 	"strings"
@@ -31,6 +29,11 @@ import (
 type Options struct {
 	Env *ProcessEnv // The environment to use. Note: this contains the cached module and filesystem state.
 
+	// LocalPrefix is a comma-separated string of import path prefixes, which, if
+	// set, instructs Process to sort the import paths with the given prefixes
+	// into another group after 3rd-party packages.
+	LocalPrefix string
+
 	Fragment  bool // Accept fragment of a source file (no package statement)
 	AllErrors bool // Report all errors (not just the first 10 on different lines)
 
@@ -41,21 +44,8 @@ type Options struct {
 	FormatOnly bool // Disable the insertion and deletion of imports
 }
 
-// Process implements golang.org/x/tools/imports.Process with explicit context in env.
-func Process(filename string, src []byte, opt *Options) ([]byte, error) {
-	if src == nil {
-		b, err := ioutil.ReadFile(filename)
-		if err != nil {
-			return nil, err
-		}
-		src = b
-	}
-
-	// Set the logger if the user has not provided it.
-	if opt.Env.Logf == nil {
-		opt.Env.Logf = log.Printf
-	}
-
+// Process implements golang.org/x/tools/imports.Process with explicit context in opt.Env.
+func Process(filename string, src []byte, opt *Options) (formatted []byte, err error) {
 	fileSet := token.NewFileSet()
 	file, adjust, err := parse(fileSet, filename, src, opt)
 	if err != nil {
@@ -67,11 +57,63 @@ func Process(filename string, src []byte, opt *Options) ([]byte, error) {
 			return nil, err
 		}
 	}
+	return formatFile(fileSet, file, src, adjust, opt)
+}
 
-	sortImports(opt.Env, fileSet, file)
-	imps := astutil.Imports(fileSet, file)
+// FixImports returns a list of fixes to the imports that, when applied,
+// will leave the imports in the same state as Process. src and opt must
+// be specified.
+//
+// Note that filename's directory influences which imports can be chosen,
+// so it is important that filename be accurate.
+func FixImports(filename string, src []byte, opt *Options) (fixes []*ImportFix, err error) {
+	fileSet := token.NewFileSet()
+	file, _, err := parse(fileSet, filename, src, opt)
+	if err != nil {
+		return nil, err
+	}
+
+	return getFixes(fileSet, file, filename, opt.Env)
+}
+
+// ApplyFixes applies all of the fixes to the file and formats it. extraMode
+// is added in when parsing the file. src and opts must be specified, but no
+// env is needed.
+func ApplyFixes(fixes []*ImportFix, filename string, src []byte, opt *Options, extraMode parser.Mode) (formatted []byte, err error) {
+	// Don't use parse() -- we don't care about fragments or statement lists
+	// here, and we need to work with unparseable files.
+	fileSet := token.NewFileSet()
+	parserMode := parser.Mode(0)
+	if opt.Comments {
+		parserMode |= parser.ParseComments
+	}
+	if opt.AllErrors {
+		parserMode |= parser.AllErrors
+	}
+	parserMode |= extraMode
+
+	file, err := parser.ParseFile(fileSet, filename, src, parserMode)
+	if file == nil {
+		return nil, err
+	}
+
+	// Apply the fixes to the file.
+	apply(fileSet, file, fixes)
+
+	return formatFile(fileSet, file, src, nil, opt)
+}
+
+// formatFile formats the file syntax tree.
+// It may mutate the token.FileSet.
+//
+// If an adjust function is provided, it is called after formatting
+// with the original source (formatFile's src parameter) and the
+// formatted file, and returns the postpocessed result.
+func formatFile(fset *token.FileSet, file *ast.File, src []byte, adjust func(orig []byte, src []byte) []byte, opt *Options) ([]byte, error) {
+	mergeImports(file)
+	sortImports(opt.LocalPrefix, fset.File(file.Pos()), file)
 	var spacesBefore []string // import paths we need spaces before
-	for _, impSection := range imps {
+	for _, impSection := range astutil.Imports(fset, file) {
 		// Within each block of contiguous imports, see if any
 		// import lines are in different group numbers. If so,
 		// we'll need to put a space between them so it's
@@ -79,7 +121,7 @@ func Process(filename string, src []byte, opt *Options) ([]byte, error) {
 		lastGroup := -1
 		for _, importSpec := range impSection {
 			importPath, _ := strconv.Unquote(importSpec.Path.Value)
-			groupNum := importGroup(opt.Env, importPath)
+			groupNum := importGroup(opt.LocalPrefix, importPath)
 			if groupNum != lastGroup && lastGroup != -1 {
 				spacesBefore = append(spacesBefore, importPath)
 			}
@@ -95,7 +137,7 @@ func Process(filename string, src []byte, opt *Options) ([]byte, error) {
 	printConfig := &printer.Config{Mode: printerMode, Tabwidth: opt.TabWidth}
 
 	var buf bytes.Buffer
-	err = printConfig.Fprint(&buf, fileSet, file)
+	err := printConfig.Fprint(&buf, fset, file)
 	if err != nil {
 		return nil, err
 	}
@@ -239,11 +281,11 @@ func cutSpace(b []byte) (before, middle, after []byte) {
 }
 
 // matchSpace reformats src to use the same space context as orig.
-// 1) If orig begins with blank lines, matchSpace inserts them at the beginning of src.
-// 2) matchSpace copies the indentation of the first non-blank line in orig
-//    to every non-blank line in src.
-// 3) matchSpace copies the trailing space from orig and uses it in place
-//   of src's trailing space.
+//  1. If orig begins with blank lines, matchSpace inserts them at the beginning of src.
+//  2. matchSpace copies the indentation of the first non-blank line in orig
+//     to every non-blank line in src.
+//  3. matchSpace copies the trailing space from orig and uses it in place
+//     of src's trailing space.
 func matchSpace(orig []byte, src []byte) []byte {
 	before, _, after := cutSpace(orig)
 	i := bytes.LastIndex(before, []byte{'\n'})
@@ -269,7 +311,7 @@ func matchSpace(orig []byte, src []byte) []byte {
 	return b.Bytes()
 }
 
-var impLine = regexp.MustCompile(`^\s+(?:[\w\.]+\s+)?"(.+)"`)
+var impLine = regexp.MustCompile(`^\s+(?:[\w\.]+\s+)?"(.+?)"`)
 
 func addImportSpaces(r io.Reader, breaks []string) ([]byte, error) {
 	var out bytes.Buffer

vendor/golang.org/x/tools/internal/imports/mod.go

@@ -1,8 +1,14 @@
+// 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 imports
 
 import (
 	"bytes"
+	"context"
 	"encoding/json"
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path"
@@ -11,67 +17,102 @@ import (
 	"sort"
 	"strconv"
 	"strings"
-	"sync"
-	"time"
 
+	"golang.org/x/mod/module"
+	"golang.org/x/tools/internal/gocommand"
 	"golang.org/x/tools/internal/gopathwalk"
-	"golang.org/x/tools/internal/module"
 )
 
-// moduleResolver implements resolver for modules using the go command as little
+// ModuleResolver implements resolver for modules using the go command as little
 // as feasible.
-type moduleResolver struct {
-	env *ProcessEnv
+type ModuleResolver struct {
+	env            *ProcessEnv
+	moduleCacheDir string
+	dummyVendorMod *gocommand.ModuleJSON // If vendoring is enabled, the pseudo-module that represents the /vendor directory.
+	roots          []gopathwalk.Root
+	scanSema       chan struct{} // scanSema prevents concurrent scans and guards scannedRoots.
+	scannedRoots   map[gopathwalk.Root]bool
 
 	initialized   bool
-	main          *moduleJSON
-	modsByModPath []*moduleJSON // All modules, ordered by # of path components in module Path...
-	modsByDir     []*moduleJSON // ...or Dir.
+	mains         []*gocommand.ModuleJSON
+	mainByDir     map[string]*gocommand.ModuleJSON
+	modsByModPath []*gocommand.ModuleJSON // All modules, ordered by # of path components in module Path...
+	modsByDir     []*gocommand.ModuleJSON // ...or Dir.
+
+	// moduleCacheCache stores information about the module cache.
+	moduleCacheCache *dirInfoCache
+	otherCache       *dirInfoCache
 }
 
-type moduleJSON struct {
-	Path     string           // module path
-	Version  string           // module version
-	Versions []string         // available module versions (with -versions)
-	Replace  *moduleJSON      // replaced by this module
-	Time     *time.Time       // time version was created
-	Update   *moduleJSON      // available update, if any (with -u)
-	Main     bool             // is this the main module?
-	Indirect bool             // is this module only an indirect dependency of main module?
-	Dir      string           // directory holding files for this module, if any
-	GoMod    string           // path to go.mod file for this module, if any
-	Error    *moduleErrorJSON // error loading module
+func newModuleResolver(e *ProcessEnv) *ModuleResolver {
+	r := &ModuleResolver{
+		env:      e,
+		scanSema: make(chan struct{}, 1),
+	}
+	r.scanSema <- struct{}{}
+	return r
 }
 
-type moduleErrorJSON struct {
-	Err string // the error itself
-}
-
-func (r *moduleResolver) init() error {
+func (r *ModuleResolver) init() error {
 	if r.initialized {
 		return nil
 	}
-	stdout, err := r.env.invokeGo("list", "-m", "-json", "...")
+
+	goenv, err := r.env.goEnv()
 	if err != nil {
 		return err
 	}
-	for dec := json.NewDecoder(stdout); dec.More(); {
-		mod := &moduleJSON{}
-		if err := dec.Decode(mod); err != nil {
+	inv := gocommand.Invocation{
+		BuildFlags: r.env.BuildFlags,
+		ModFlag:    r.env.ModFlag,
+		ModFile:    r.env.ModFile,
+		Env:        r.env.env(),
+		Logf:       r.env.Logf,
+		WorkingDir: r.env.WorkingDir,
+	}
+
+	vendorEnabled := false
+	var mainModVendor *gocommand.ModuleJSON
+
+	// Module vendor directories are ignored in workspace mode:
+	// https://go.googlesource.com/proposal/+/master/design/45713-workspace.md
+	if len(r.env.Env["GOWORK"]) == 0 {
+		vendorEnabled, mainModVendor, err = gocommand.VendorEnabled(context.TODO(), inv, r.env.GocmdRunner)
+		if err != nil {
 			return err
 		}
-		if mod.Dir == "" {
-			if r.env.Debug {
-				r.env.Logf("module %v has not been downloaded and will be ignored", mod.Path)
+	}
+
+	if mainModVendor != nil && vendorEnabled {
+		// Vendor mode is on, so all the non-Main modules are irrelevant,
+		// and we need to search /vendor for everything.
+		r.mains = []*gocommand.ModuleJSON{mainModVendor}
+		r.dummyVendorMod = &gocommand.ModuleJSON{
+			Path: "",
+			Dir:  filepath.Join(mainModVendor.Dir, "vendor"),
+		}
+		r.modsByModPath = []*gocommand.ModuleJSON{mainModVendor, r.dummyVendorMod}
+		r.modsByDir = []*gocommand.ModuleJSON{mainModVendor, r.dummyVendorMod}
+	} else {
+		// Vendor mode is off, so run go list -m ... to find everything.
+		err := r.initAllMods()
+		// We expect an error when running outside of a module with
+		// GO111MODULE=on. Other errors are fatal.
+		if err != nil {
+			if errMsg := err.Error(); !strings.Contains(errMsg, "working directory is not part of a module") && !strings.Contains(errMsg, "go.mod file not found") {
+				return err
 			}
-			// Can't do anything with a module that's not downloaded.
-			continue
 		}
-		r.modsByModPath = append(r.modsByModPath, mod)
-		r.modsByDir = append(r.modsByDir, mod)
-		if mod.Main {
-			r.main = mod
+	}
+
+	if gmc := r.env.Env["GOMODCACHE"]; gmc != "" {
+		r.moduleCacheDir = gmc
+	} else {
+		gopaths := filepath.SplitList(goenv["GOPATH"])
+		if len(gopaths) == 0 {
+			return fmt.Errorf("empty GOPATH")
 		}
+		r.moduleCacheDir = filepath.Join(gopaths[0], "/pkg/mod")
 	}
 
 	sort.Slice(r.modsByModPath, func(i, j int) bool {
@@ -87,33 +128,151 @@ func (r *moduleResolver) init() error {
 		return count(j) < count(i) // descending order
 	})
 
+	r.roots = []gopathwalk.Root{
+		{Path: filepath.Join(goenv["GOROOT"], "/src"), Type: gopathwalk.RootGOROOT},
+	}
+	r.mainByDir = make(map[string]*gocommand.ModuleJSON)
+	for _, main := range r.mains {
+		r.roots = append(r.roots, gopathwalk.Root{Path: main.Dir, Type: gopathwalk.RootCurrentModule})
+		r.mainByDir[main.Dir] = main
+	}
+	if vendorEnabled {
+		r.roots = append(r.roots, gopathwalk.Root{Path: r.dummyVendorMod.Dir, Type: gopathwalk.RootOther})
+	} else {
+		addDep := func(mod *gocommand.ModuleJSON) {
+			if mod.Replace == nil {
+				// This is redundant with the cache, but we'll skip it cheaply enough.
+				r.roots = append(r.roots, gopathwalk.Root{Path: mod.Dir, Type: gopathwalk.RootModuleCache})
+			} else {
+				r.roots = append(r.roots, gopathwalk.Root{Path: mod.Dir, Type: gopathwalk.RootOther})
+			}
+		}
+		// Walk dependent modules before scanning the full mod cache, direct deps first.
+		for _, mod := range r.modsByModPath {
+			if !mod.Indirect && !mod.Main {
+				addDep(mod)
+			}
+		}
+		for _, mod := range r.modsByModPath {
+			if mod.Indirect && !mod.Main {
+				addDep(mod)
+			}
+		}
+		r.roots = append(r.roots, gopathwalk.Root{Path: r.moduleCacheDir, Type: gopathwalk.RootModuleCache})
+	}
+
+	r.scannedRoots = map[gopathwalk.Root]bool{}
+	if r.moduleCacheCache == nil {
+		r.moduleCacheCache = &dirInfoCache{
+			dirs:      map[string]*directoryPackageInfo{},
+			listeners: map[*int]cacheListener{},
+		}
+	}
+	if r.otherCache == nil {
+		r.otherCache = &dirInfoCache{
+			dirs:      map[string]*directoryPackageInfo{},
+			listeners: map[*int]cacheListener{},
+		}
+	}
 	r.initialized = true
 	return nil
 }
 
+func (r *ModuleResolver) initAllMods() error {
+	stdout, err := r.env.invokeGo(context.TODO(), "list", "-m", "-e", "-json", "...")
+	if err != nil {
+		return err
+	}
+	for dec := json.NewDecoder(stdout); dec.More(); {
+		mod := &gocommand.ModuleJSON{}
+		if err := dec.Decode(mod); err != nil {
+			return err
+		}
+		if mod.Dir == "" {
+			if r.env.Logf != nil {
+				r.env.Logf("module %v has not been downloaded and will be ignored", mod.Path)
+			}
+			// Can't do anything with a module that's not downloaded.
+			continue
+		}
+		// golang/go#36193: the go command doesn't always clean paths.
+		mod.Dir = filepath.Clean(mod.Dir)
+		r.modsByModPath = append(r.modsByModPath, mod)
+		r.modsByDir = append(r.modsByDir, mod)
+		if mod.Main {
+			r.mains = append(r.mains, mod)
+		}
+	}
+	return nil
+}
+
+func (r *ModuleResolver) ClearForNewScan() {
+	<-r.scanSema
+	r.scannedRoots = map[gopathwalk.Root]bool{}
+	r.otherCache = &dirInfoCache{
+		dirs:      map[string]*directoryPackageInfo{},
+		listeners: map[*int]cacheListener{},
+	}
+	r.scanSema <- struct{}{}
+}
+
+func (r *ModuleResolver) ClearForNewMod() {
+	<-r.scanSema
+	*r = ModuleResolver{
+		env:              r.env,
+		moduleCacheCache: r.moduleCacheCache,
+		otherCache:       r.otherCache,
+		scanSema:         r.scanSema,
+	}
+	r.init()
+	r.scanSema <- struct{}{}
+}
+
 // findPackage returns the module and directory that contains the package at
 // the given import path, or returns nil, "" if no module is in scope.
-func (r *moduleResolver) findPackage(importPath string) (*moduleJSON, string) {
+func (r *ModuleResolver) findPackage(importPath string) (*gocommand.ModuleJSON, string) {
+	// This can't find packages in the stdlib, but that's harmless for all
+	// the existing code paths.
 	for _, m := range r.modsByModPath {
 		if !strings.HasPrefix(importPath, m.Path) {
 			continue
 		}
 		pathInModule := importPath[len(m.Path):]
 		pkgDir := filepath.Join(m.Dir, pathInModule)
-		if dirIsNestedModule(pkgDir, m) {
+		if r.dirIsNestedModule(pkgDir, m) {
 			continue
 		}
 
+		if info, ok := r.cacheLoad(pkgDir); ok {
+			if loaded, err := info.reachedStatus(nameLoaded); loaded {
+				if err != nil {
+					continue // No package in this dir.
+				}
+				return m, pkgDir
+			}
+			if scanned, err := info.reachedStatus(directoryScanned); scanned && err != nil {
+				continue // Dir is unreadable, etc.
+			}
+			// This is slightly wrong: a directory doesn't have to have an
+			// importable package to count as a package for package-to-module
+			// resolution. package main or _test files should count but
+			// don't.
+			// TODO(heschi): fix this.
+			if _, err := r.cachePackageName(info); err == nil {
+				return m, pkgDir
+			}
+		}
+
+		// Not cached. Read the filesystem.
 		pkgFiles, err := ioutil.ReadDir(pkgDir)
 		if err != nil {
 			continue
 		}
-
 		// A module only contains a package if it has buildable go
 		// files in that directory. If not, it could be provided by an
 		// outer module. See #29736.
 		for _, fi := range pkgFiles {
-			if ok, _ := r.env.buildContext().MatchFile(pkgDir, fi.Name()); ok {
+			if ok, _ := r.env.matchFile(pkgDir, fi.Name()); ok {
 				return m, pkgDir
 			}
 		}
@@ -121,9 +280,43 @@ func (r *moduleResolver) findPackage(importPath string) (*moduleJSON, string) {
 	return nil, ""
 }
 
+func (r *ModuleResolver) cacheLoad(dir string) (directoryPackageInfo, bool) {
+	if info, ok := r.moduleCacheCache.Load(dir); ok {
+		return info, ok
+	}
+	return r.otherCache.Load(dir)
+}
+
+func (r *ModuleResolver) cacheStore(info directoryPackageInfo) {
+	if info.rootType == gopathwalk.RootModuleCache {
+		r.moduleCacheCache.Store(info.dir, info)
+	} else {
+		r.otherCache.Store(info.dir, info)
+	}
+}
+
+func (r *ModuleResolver) cacheKeys() []string {
+	return append(r.moduleCacheCache.Keys(), r.otherCache.Keys()...)
+}
+
+// cachePackageName caches the package name for a dir already in the cache.
+func (r *ModuleResolver) cachePackageName(info directoryPackageInfo) (string, error) {
+	if info.rootType == gopathwalk.RootModuleCache {
+		return r.moduleCacheCache.CachePackageName(info)
+	}
+	return r.otherCache.CachePackageName(info)
+}
+
+func (r *ModuleResolver) cacheExports(ctx context.Context, env *ProcessEnv, info directoryPackageInfo) (string, []string, error) {
+	if info.rootType == gopathwalk.RootModuleCache {
+		return r.moduleCacheCache.CacheExports(ctx, env, info)
+	}
+	return r.otherCache.CacheExports(ctx, env, info)
+}
+
 // findModuleByDir returns the module that contains dir, or nil if no such
 // module is in scope.
-func (r *moduleResolver) findModuleByDir(dir string) *moduleJSON {
+func (r *ModuleResolver) findModuleByDir(dir string) *gocommand.ModuleJSON {
 	// This is quite tricky and may not be correct. dir could be:
 	// - a package in the main module.
 	// - a replace target underneath the main module's directory.
@@ -139,7 +332,7 @@ func (r *moduleResolver) findModuleByDir(dir string) *moduleJSON {
 			continue
 		}
 
-		if dirIsNestedModule(dir, m) {
+		if r.dirIsNestedModule(dir, m) {
 			continue
 		}
 
@@ -150,33 +343,68 @@ func (r *moduleResolver) findModuleByDir(dir string) *moduleJSON {
 
 // dirIsNestedModule reports if dir is contained in a nested module underneath
 // mod, not actually in mod.
-func dirIsNestedModule(dir string, mod *moduleJSON) bool {
+func (r *ModuleResolver) dirIsNestedModule(dir string, mod *gocommand.ModuleJSON) bool {
 	if !strings.HasPrefix(dir, mod.Dir) {
 		return false
 	}
-	mf := findModFile(dir)
-	if mf == "" {
+	if r.dirInModuleCache(dir) {
+		// Nested modules in the module cache are pruned,
+		// so it cannot be a nested module.
 		return false
 	}
-	return filepath.Dir(mf) != mod.Dir
+	if mod != nil && mod == r.dummyVendorMod {
+		// The /vendor pseudomodule is flattened and doesn't actually count.
+		return false
+	}
+	modDir, _ := r.modInfo(dir)
+	if modDir == "" {
+		return false
+	}
+	return modDir != mod.Dir
 }
 
-func findModFile(dir string) string {
+func (r *ModuleResolver) modInfo(dir string) (modDir string, modName string) {
+	readModName := func(modFile string) string {
+		modBytes, err := ioutil.ReadFile(modFile)
+		if err != nil {
+			return ""
+		}
+		return modulePath(modBytes)
+	}
+
+	if r.dirInModuleCache(dir) {
+		if matches := modCacheRegexp.FindStringSubmatch(dir); len(matches) == 3 {
+			index := strings.Index(dir, matches[1]+"@"+matches[2])
+			modDir := filepath.Join(dir[:index], matches[1]+"@"+matches[2])
+			return modDir, readModName(filepath.Join(modDir, "go.mod"))
+		}
+	}
 	for {
+		if info, ok := r.cacheLoad(dir); ok {
+			return info.moduleDir, info.moduleName
+		}
 		f := filepath.Join(dir, "go.mod")
 		info, err := os.Stat(f)
 		if err == nil && !info.IsDir() {
-			return f
+			return dir, readModName(f)
 		}
+
 		d := filepath.Dir(dir)
 		if len(d) >= len(dir) {
-			return "" // reached top of file system, no go.mod
+			return "", "" // reached top of file system, no go.mod
 		}
 		dir = d
 	}
 }
 
-func (r *moduleResolver) loadPackageNames(importPaths []string, srcDir string) (map[string]string, error) {
+func (r *ModuleResolver) dirInModuleCache(dir string) bool {
+	if r.moduleCacheDir == "" {
+		return false
+	}
+	return strings.HasPrefix(dir, r.moduleCacheDir)
+}
+
+func (r *ModuleResolver) loadPackageNames(importPaths []string, srcDir string) (map[string]string, error) {
 	if err := r.init(); err != nil {
 		return nil, err
 	}
@@ -195,115 +423,249 @@ func (r *moduleResolver) loadPackageNames(importPaths []string, srcDir string) (
 	return names, nil
 }
 
-func (r *moduleResolver) scan(_ references) ([]*pkg, error) {
+func (r *ModuleResolver) scan(ctx context.Context, callback *scanCallback) error {
 	if err := r.init(); err != nil {
-		return nil, err
+		return err
 	}
 
-	// Walk GOROOT, GOPATH/pkg/mod, and the main module.
-	roots := []gopathwalk.Root{
-		{filepath.Join(r.env.GOROOT, "/src"), gopathwalk.RootGOROOT},
-	}
-	if r.main != nil {
-		roots = append(roots, gopathwalk.Root{r.main.Dir, gopathwalk.RootCurrentModule})
-	}
-	for _, p := range filepath.SplitList(r.env.GOPATH) {
-		roots = append(roots, gopathwalk.Root{filepath.Join(p, "/pkg/mod"), gopathwalk.RootModuleCache})
-	}
-
-	// Walk replace targets, just in case they're not in any of the above.
-	for _, mod := range r.modsByModPath {
-		if mod.Replace != nil {
-			roots = append(roots, gopathwalk.Root{mod.Dir, gopathwalk.RootOther})
+	processDir := func(info directoryPackageInfo) {
+		// Skip this directory if we were not able to get the package information successfully.
+		if scanned, err := info.reachedStatus(directoryScanned); !scanned || err != nil {
+			return
 		}
-	}
-
-	var result []*pkg
-	dupCheck := make(map[string]bool)
-	var mu sync.Mutex
-
-	gopathwalk.Walk(roots, func(root gopathwalk.Root, dir string) {
-		mu.Lock()
-		defer mu.Unlock()
-
-		if _, dup := dupCheck[dir]; dup {
+		pkg, err := r.canonicalize(info)
+		if err != nil {
 			return
 		}
 
-		dupCheck[dir] = true
-
-		subdir := ""
-		if dir != root.Path {
-			subdir = dir[len(root.Path)+len("/"):]
-		}
-		importPath := filepath.ToSlash(subdir)
-		if strings.HasPrefix(importPath, "vendor/") {
-			// Ignore vendor dirs. If -mod=vendor is on, then things
-			// should mostly just work, but when it's not vendor/
-			// is a mess. There's no easy way to tell if it's on.
-			// We can still find things in the mod cache and
-			// map them into /vendor when -mod=vendor is on.
+		if !callback.dirFound(pkg) {
 			return
 		}
-		switch root.Type {
-		case gopathwalk.RootCurrentModule:
-			importPath = path.Join(r.main.Path, filepath.ToSlash(subdir))
-		case gopathwalk.RootModuleCache:
-			matches := modCacheRegexp.FindStringSubmatch(subdir)
-			modPath, err := module.DecodePath(filepath.ToSlash(matches[1]))
-			if err != nil {
-				if r.env.Debug {
-					r.env.Logf("decoding module cache path %q: %v", subdir, err)
-				}
+		pkg.packageName, err = r.cachePackageName(info)
+		if err != nil {
+			return
+		}
+
+		if !callback.packageNameLoaded(pkg) {
+			return
+		}
+		_, exports, err := r.loadExports(ctx, pkg, false)
+		if err != nil {
+			return
+		}
+		callback.exportsLoaded(pkg, exports)
+	}
+
+	// Start processing everything in the cache, and listen for the new stuff
+	// we discover in the walk below.
+	stop1 := r.moduleCacheCache.ScanAndListen(ctx, processDir)
+	defer stop1()
+	stop2 := r.otherCache.ScanAndListen(ctx, processDir)
+	defer stop2()
+
+	// We assume cached directories are fully cached, including all their
+	// children, and have not changed. We can skip them.
+	skip := func(root gopathwalk.Root, dir string) bool {
+		if r.env.SkipPathInScan != nil && root.Type == gopathwalk.RootCurrentModule {
+			if root.Path == dir {
+				return false
+			}
+
+			if r.env.SkipPathInScan(filepath.Clean(dir)) {
+				return true
+			}
+		}
+
+		info, ok := r.cacheLoad(dir)
+		if !ok {
+			return false
+		}
+		// This directory can be skipped as long as we have already scanned it.
+		// Packages with errors will continue to have errors, so there is no need
+		// to rescan them.
+		packageScanned, _ := info.reachedStatus(directoryScanned)
+		return packageScanned
+	}
+
+	// Add anything new to the cache, and process it if we're still listening.
+	add := func(root gopathwalk.Root, dir string) {
+		r.cacheStore(r.scanDirForPackage(root, dir))
+	}
+
+	// r.roots and the callback are not necessarily safe to use in the
+	// goroutine below. Process them eagerly.
+	roots := filterRoots(r.roots, callback.rootFound)
+	// We can't cancel walks, because we need them to finish to have a usable
+	// cache. Instead, run them in a separate goroutine and detach.
+	scanDone := make(chan struct{})
+	go func() {
+		select {
+		case <-ctx.Done():
+			return
+		case <-r.scanSema:
+		}
+		defer func() { r.scanSema <- struct{}{} }()
+		// We have the lock on r.scannedRoots, and no other scans can run.
+		for _, root := range roots {
+			if ctx.Err() != nil {
 				return
 			}
-			importPath = path.Join(modPath, filepath.ToSlash(matches[3]))
-		case gopathwalk.RootGOROOT:
-			importPath = subdir
-		}
 
-		// Check if the directory is underneath a module that's in scope.
-		if mod := r.findModuleByDir(dir); mod != nil {
-			// It is. If dir is the target of a replace directive,
-			// our guessed import path is wrong. Use the real one.
-			if mod.Dir == dir {
-				importPath = mod.Path
-			} else {
-				dirInMod := dir[len(mod.Dir)+len("/"):]
-				importPath = path.Join(mod.Path, filepath.ToSlash(dirInMod))
+			if r.scannedRoots[root] {
+				continue
 			}
+			gopathwalk.WalkSkip([]gopathwalk.Root{root}, add, skip, gopathwalk.Options{Logf: r.env.Logf, ModulesEnabled: true})
+			r.scannedRoots[root] = true
+		}
+		close(scanDone)
+	}()
+	select {
+	case <-ctx.Done():
+	case <-scanDone:
+	}
+	return nil
+}
+
+func (r *ModuleResolver) scoreImportPath(ctx context.Context, path string) float64 {
+	if _, ok := stdlib[path]; ok {
+		return MaxRelevance
+	}
+	mod, _ := r.findPackage(path)
+	return modRelevance(mod)
+}
+
+func modRelevance(mod *gocommand.ModuleJSON) float64 {
+	var relevance float64
+	switch {
+	case mod == nil: // out of scope
+		return MaxRelevance - 4
+	case mod.Indirect:
+		relevance = MaxRelevance - 3
+	case !mod.Main:
+		relevance = MaxRelevance - 2
+	default:
+		relevance = MaxRelevance - 1 // main module ties with stdlib
+	}
+
+	_, versionString, ok := module.SplitPathVersion(mod.Path)
+	if ok {
+		index := strings.Index(versionString, "v")
+		if index == -1 {
+			return relevance
+		}
+		if versionNumber, err := strconv.ParseFloat(versionString[index+1:], 64); err == nil {
+			relevance += versionNumber / 1000
+		}
+	}
+
+	return relevance
+}
+
+// canonicalize gets the result of canonicalizing the packages using the results
+// of initializing the resolver from 'go list -m'.
+func (r *ModuleResolver) canonicalize(info directoryPackageInfo) (*pkg, error) {
+	// Packages in GOROOT are already canonical, regardless of the std/cmd modules.
+	if info.rootType == gopathwalk.RootGOROOT {
+		return &pkg{
+			importPathShort: info.nonCanonicalImportPath,
+			dir:             info.dir,
+			packageName:     path.Base(info.nonCanonicalImportPath),
+			relevance:       MaxRelevance,
+		}, nil
+	}
+
+	importPath := info.nonCanonicalImportPath
+	mod := r.findModuleByDir(info.dir)
+	// Check if the directory is underneath a module that's in scope.
+	if mod != nil {
+		// It is. If dir is the target of a replace directive,
+		// our guessed import path is wrong. Use the real one.
+		if mod.Dir == info.dir {
+			importPath = mod.Path
 		} else {
-			// The package is in an unknown module. Check that it's
-			// not obviously impossible to import.
-			var modFile string
-			switch root.Type {
-			case gopathwalk.RootModuleCache:
-				matches := modCacheRegexp.FindStringSubmatch(subdir)
-				modFile = filepath.Join(matches[1], "@", matches[2], "go.mod")
-			default:
-				modFile = findModFile(dir)
-			}
+			dirInMod := info.dir[len(mod.Dir)+len("/"):]
+			importPath = path.Join(mod.Path, filepath.ToSlash(dirInMod))
+		}
+	} else if !strings.HasPrefix(importPath, info.moduleName) {
+		// The module's name doesn't match the package's import path. It
+		// probably needs a replace directive we don't have.
+		return nil, fmt.Errorf("package in %q is not valid without a replace statement", info.dir)
+	}
 
-			modBytes, err := ioutil.ReadFile(modFile)
-			if err == nil && !strings.HasPrefix(importPath, modulePath(modBytes)) {
-				// The module's declared path does not match
-				// its expected path. It probably needs a
-				// replace directive we don't have.
-				return
+	res := &pkg{
+		importPathShort: importPath,
+		dir:             info.dir,
+		relevance:       modRelevance(mod),
+	}
+	// We may have discovered a package that has a different version
+	// in scope already. Canonicalize to that one if possible.
+	if _, canonicalDir := r.findPackage(importPath); canonicalDir != "" {
+		res.dir = canonicalDir
+	}
+	return res, nil
+}
+
+func (r *ModuleResolver) loadExports(ctx context.Context, pkg *pkg, includeTest bool) (string, []string, error) {
+	if err := r.init(); err != nil {
+		return "", nil, err
+	}
+	if info, ok := r.cacheLoad(pkg.dir); ok && !includeTest {
+		return r.cacheExports(ctx, r.env, info)
+	}
+	return loadExportsFromFiles(ctx, r.env, pkg.dir, includeTest)
+}
+
+func (r *ModuleResolver) scanDirForPackage(root gopathwalk.Root, dir string) directoryPackageInfo {
+	subdir := ""
+	if dir != root.Path {
+		subdir = dir[len(root.Path)+len("/"):]
+	}
+	importPath := filepath.ToSlash(subdir)
+	if strings.HasPrefix(importPath, "vendor/") {
+		// Only enter vendor directories if they're explicitly requested as a root.
+		return directoryPackageInfo{
+			status: directoryScanned,
+			err:    fmt.Errorf("unwanted vendor directory"),
+		}
+	}
+	switch root.Type {
+	case gopathwalk.RootCurrentModule:
+		importPath = path.Join(r.mainByDir[root.Path].Path, filepath.ToSlash(subdir))
+	case gopathwalk.RootModuleCache:
+		matches := modCacheRegexp.FindStringSubmatch(subdir)
+		if len(matches) == 0 {
+			return directoryPackageInfo{
+				status: directoryScanned,
+				err:    fmt.Errorf("invalid module cache path: %v", subdir),
 			}
 		}
-		// We may have discovered a package that has a different version
-		// in scope already. Canonicalize to that one if possible.
-		if _, canonicalDir := r.findPackage(importPath); canonicalDir != "" {
-			dir = canonicalDir
+		modPath, err := module.UnescapePath(filepath.ToSlash(matches[1]))
+		if err != nil {
+			if r.env.Logf != nil {
+				r.env.Logf("decoding module cache path %q: %v", subdir, err)
+			}
+			return directoryPackageInfo{
+				status: directoryScanned,
+				err:    fmt.Errorf("decoding module cache path %q: %v", subdir, err),
+			}
 		}
+		importPath = path.Join(modPath, filepath.ToSlash(matches[3]))
+	}
 
-		result = append(result, &pkg{
-			importPathShort: VendorlessPath(importPath),
-			dir:             dir,
-		})
-	}, gopathwalk.Options{Debug: r.env.Debug, ModulesEnabled: true})
-	return result, nil
+	modDir, modName := r.modInfo(dir)
+	result := directoryPackageInfo{
+		status:                 directoryScanned,
+		dir:                    dir,
+		rootType:               root.Type,
+		nonCanonicalImportPath: importPath,
+		moduleDir:              modDir,
+		moduleName:             modName,
+	}
+	if root.Type == gopathwalk.RootGOROOT {
+		// stdlib packages are always in scope, despite the confusing go.mod
+		return result
+	}
+	return result
 }
 
 // modCacheRegexp splits a path in a module cache into module, module version, and package.

vendor/golang.org/x/tools/internal/imports/mod_cache.go

@@ -0,0 +1,236 @@
+// 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 imports
+
+import (
+	"context"
+	"fmt"
+	"sync"
+
+	"golang.org/x/tools/internal/gopathwalk"
+)
+
+// To find packages to import, the resolver needs to know about all of the
+// the packages that could be imported. This includes packages that are
+// already in modules that are in (1) the current module, (2) replace targets,
+// and (3) packages in the module cache. Packages in (1) and (2) may change over
+// time, as the client may edit the current module and locally replaced modules.
+// The module cache (which includes all of the packages in (3)) can only
+// ever be added to.
+//
+// The resolver can thus save state about packages in the module cache
+// and guarantee that this will not change over time. To obtain information
+// about new modules added to the module cache, the module cache should be
+// rescanned.
+//
+// It is OK to serve information about modules that have been deleted,
+// as they do still exist.
+// TODO(suzmue): can we share information with the caller about
+// what module needs to be downloaded to import this package?
+
+type directoryPackageStatus int
+
+const (
+	_ directoryPackageStatus = iota
+	directoryScanned
+	nameLoaded
+	exportsLoaded
+)
+
+type directoryPackageInfo struct {
+	// status indicates the extent to which this struct has been filled in.
+	status directoryPackageStatus
+	// err is non-nil when there was an error trying to reach status.
+	err error
+
+	// Set when status >= directoryScanned.
+
+	// dir is the absolute directory of this package.
+	dir      string
+	rootType gopathwalk.RootType
+	// nonCanonicalImportPath is the package's expected import path. It may
+	// not actually be importable at that path.
+	nonCanonicalImportPath string
+
+	// Module-related information.
+	moduleDir  string // The directory that is the module root of this dir.
+	moduleName string // The module name that contains this dir.
+
+	// Set when status >= nameLoaded.
+
+	packageName string // the package name, as declared in the source.
+
+	// Set when status >= exportsLoaded.
+
+	exports []string
+}
+
+// reachedStatus returns true when info has a status at least target and any error associated with
+// an attempt to reach target.
+func (info *directoryPackageInfo) reachedStatus(target directoryPackageStatus) (bool, error) {
+	if info.err == nil {
+		return info.status >= target, nil
+	}
+	if info.status == target {
+		return true, info.err
+	}
+	return true, nil
+}
+
+// dirInfoCache is a concurrency safe map for storing information about
+// directories that may contain packages.
+//
+// The information in this cache is built incrementally. Entries are initialized in scan.
+// No new keys should be added in any other functions, as all directories containing
+// packages are identified in scan.
+//
+// Other functions, including loadExports and findPackage, may update entries in this cache
+// as they discover new things about the directory.
+//
+// The information in the cache is not expected to change for the cache's
+// lifetime, so there is no protection against competing writes. Users should
+// take care not to hold the cache across changes to the underlying files.
+//
+// TODO(suzmue): consider other concurrency strategies and data structures (RWLocks, sync.Map, etc)
+type dirInfoCache struct {
+	mu sync.Mutex
+	// dirs stores information about packages in directories, keyed by absolute path.
+	dirs      map[string]*directoryPackageInfo
+	listeners map[*int]cacheListener
+}
+
+type cacheListener func(directoryPackageInfo)
+
+// ScanAndListen calls listener on all the items in the cache, and on anything
+// newly added. The returned stop function waits for all in-flight callbacks to
+// finish and blocks new ones.
+func (d *dirInfoCache) ScanAndListen(ctx context.Context, listener cacheListener) func() {
+	ctx, cancel := context.WithCancel(ctx)
+
+	// Flushing out all the callbacks is tricky without knowing how many there
+	// are going to be. Setting an arbitrary limit makes it much easier.
+	const maxInFlight = 10
+	sema := make(chan struct{}, maxInFlight)
+	for i := 0; i < maxInFlight; i++ {
+		sema <- struct{}{}
+	}
+
+	cookie := new(int) // A unique ID we can use for the listener.
+
+	// We can't hold mu while calling the listener.
+	d.mu.Lock()
+	var keys []string
+	for key := range d.dirs {
+		keys = append(keys, key)
+	}
+	d.listeners[cookie] = func(info directoryPackageInfo) {
+		select {
+		case <-ctx.Done():
+			return
+		case <-sema:
+		}
+		listener(info)
+		sema <- struct{}{}
+	}
+	d.mu.Unlock()
+
+	stop := func() {
+		cancel()
+		d.mu.Lock()
+		delete(d.listeners, cookie)
+		d.mu.Unlock()
+		for i := 0; i < maxInFlight; i++ {
+			<-sema
+		}
+	}
+
+	// Process the pre-existing keys.
+	for _, k := range keys {
+		select {
+		case <-ctx.Done():
+			return stop
+		default:
+		}
+		if v, ok := d.Load(k); ok {
+			listener(v)
+		}
+	}
+
+	return stop
+}
+
+// Store stores the package info for dir.
+func (d *dirInfoCache) Store(dir string, info directoryPackageInfo) {
+	d.mu.Lock()
+	_, old := d.dirs[dir]
+	d.dirs[dir] = &info
+	var listeners []cacheListener
+	for _, l := range d.listeners {
+		listeners = append(listeners, l)
+	}
+	d.mu.Unlock()
+
+	if !old {
+		for _, l := range listeners {
+			l(info)
+		}
+	}
+}
+
+// Load returns a copy of the directoryPackageInfo for absolute directory dir.
+func (d *dirInfoCache) Load(dir string) (directoryPackageInfo, bool) {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	info, ok := d.dirs[dir]
+	if !ok {
+		return directoryPackageInfo{}, false
+	}
+	return *info, true
+}
+
+// Keys returns the keys currently present in d.
+func (d *dirInfoCache) Keys() (keys []string) {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	for key := range d.dirs {
+		keys = append(keys, key)
+	}
+	return keys
+}
+
+func (d *dirInfoCache) CachePackageName(info directoryPackageInfo) (string, error) {
+	if loaded, err := info.reachedStatus(nameLoaded); loaded {
+		return info.packageName, err
+	}
+	if scanned, err := info.reachedStatus(directoryScanned); !scanned || err != nil {
+		return "", fmt.Errorf("cannot read package name, scan error: %v", err)
+	}
+	info.packageName, info.err = packageDirToName(info.dir)
+	info.status = nameLoaded
+	d.Store(info.dir, info)
+	return info.packageName, info.err
+}
+
+func (d *dirInfoCache) CacheExports(ctx context.Context, env *ProcessEnv, info directoryPackageInfo) (string, []string, error) {
+	if reached, _ := info.reachedStatus(exportsLoaded); reached {
+		return info.packageName, info.exports, info.err
+	}
+	if reached, err := info.reachedStatus(nameLoaded); reached && err != nil {
+		return "", nil, err
+	}
+	info.packageName, info.exports, info.err = loadExportsFromFiles(ctx, env, info.dir, false)
+	if info.err == context.Canceled || info.err == context.DeadlineExceeded {
+		return info.packageName, info.exports, info.err
+	}
+	// The cache structure wants things to proceed linearly. We can skip a
+	// step here, but only if we succeed.
+	if info.status == nameLoaded || info.err == nil {
+		info.status = exportsLoaded
+	} else {
+		info.status = nameLoaded
+	}
+	d.Store(info.dir, info)
+	return info.packageName, info.exports, info.err
+}

vendor/golang.org/x/tools/internal/imports/sortimports.go

@@ -3,19 +3,23 @@
 // license that can be found in the LICENSE file.
 
 // Hacked up copy of go/ast/import.go
+// Modified to use a single token.File in preference to a FileSet.
 
 package imports
 
 import (
 	"go/ast"
 	"go/token"
+	"log"
 	"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(env *ProcessEnv, fset *token.FileSet, f *ast.File) {
+//
+// It may mutate the token.File.
+func sortImports(localPrefix string, tokFile *token.File, f *ast.File) {
 	for i, d := range f.Decls {
 		d, ok := d.(*ast.GenDecl)
 		if !ok || d.Tok != token.IMPORT {
@@ -38,26 +42,75 @@ func sortImports(env *ProcessEnv, fset *token.FileSet, f *ast.File) {
 		i := 0
 		specs := d.Specs[:0]
 		for j, s := range d.Specs {
-			if j > i && fset.Position(s.Pos()).Line > 1+fset.Position(d.Specs[j-1].End()).Line {
+			if j > i && tokFile.Line(s.Pos()) > 1+tokFile.Line(d.Specs[j-1].End()) {
 				// j begins a new run.  End this one.
-				specs = append(specs, sortSpecs(env, fset, f, d.Specs[i:j])...)
+				specs = append(specs, sortSpecs(localPrefix, tokFile, f, d.Specs[i:j])...)
 				i = j
 			}
 		}
-		specs = append(specs, sortSpecs(env, fset, f, d.Specs[i:])...)
+		specs = append(specs, sortSpecs(localPrefix, tokFile, f, d.Specs[i:])...)
 		d.Specs = specs
 
 		// Deduping can leave a blank line before the rparen; clean that up.
+		// Ignore line directives.
 		if len(d.Specs) > 0 {
 			lastSpec := d.Specs[len(d.Specs)-1]
-			lastLine := fset.Position(lastSpec.Pos()).Line
-			if rParenLine := fset.Position(d.Rparen).Line; rParenLine > lastLine+1 {
-				fset.File(d.Rparen).MergeLine(rParenLine - 1)
+			lastLine := tokFile.PositionFor(lastSpec.Pos(), false).Line
+			if rParenLine := tokFile.PositionFor(d.Rparen, false).Line; rParenLine > lastLine+1 {
+				tokFile.MergeLine(rParenLine - 1) // has side effects!
 			}
 		}
 	}
 }
 
+// mergeImports merges all the import declarations into the first one.
+// Taken from golang.org/x/tools/ast/astutil.
+// This does not adjust line numbers properly
+func mergeImports(f *ast.File) {
+	if len(f.Decls) <= 1 {
+		return
+	}
+
+	// Merge all the import declarations into the first one.
+	var first *ast.GenDecl
+	for i := 0; i < len(f.Decls); i++ {
+		decl := f.Decls[i]
+		gen, ok := decl.(*ast.GenDecl)
+		if !ok || gen.Tok != token.IMPORT || declImports(gen, "C") {
+			continue
+		}
+		if first == nil {
+			first = gen
+			continue // Don't touch the first one.
+		}
+		// We now know there is more than one package in this import
+		// declaration. Ensure that it ends up parenthesized.
+		first.Lparen = first.Pos()
+		// Move the imports of the other import declaration to the first one.
+		for _, spec := range gen.Specs {
+			spec.(*ast.ImportSpec).Path.ValuePos = first.Pos()
+			first.Specs = append(first.Specs, spec)
+		}
+		f.Decls = append(f.Decls[:i], f.Decls[i+1:]...)
+		i--
+	}
+}
+
+// declImports reports whether gen contains an import of path.
+// Taken from golang.org/x/tools/ast/astutil.
+func declImports(gen *ast.GenDecl, path string) bool {
+	if gen.Tok != token.IMPORT {
+		return false
+	}
+	for _, spec := range gen.Specs {
+		impspec := spec.(*ast.ImportSpec)
+		if importPath(impspec) == path {
+			return true
+		}
+	}
+	return false
+}
+
 func importPath(s ast.Spec) string {
 	t, err := strconv.Unquote(s.(*ast.ImportSpec).Path.Value)
 	if err == nil {
@@ -95,7 +148,9 @@ type posSpan struct {
 	End   token.Pos
 }
 
-func sortSpecs(env *ProcessEnv, fset *token.FileSet, f *ast.File, specs []ast.Spec) []ast.Spec {
+// sortSpecs sorts the import specs within each import decl.
+// It may mutate the token.File.
+func sortSpecs(localPrefix string, tokFile *token.File, f *ast.File, specs []ast.Spec) []ast.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.
@@ -111,7 +166,7 @@ func sortSpecs(env *ProcessEnv, fset *token.FileSet, f *ast.File, specs []ast.Sp
 
 	// Identify comments in this range.
 	// Any comment from pos[0].Start to the final line counts.
-	lastLine := fset.Position(pos[len(pos)-1].End).Line
+	lastLine := tokFile.Line(pos[len(pos)-1].End)
 	cstart := len(f.Comments)
 	cend := len(f.Comments)
 	for i, g := range f.Comments {
@@ -121,7 +176,7 @@ func sortSpecs(env *ProcessEnv, fset *token.FileSet, f *ast.File, specs []ast.Sp
 		if i < cstart {
 			cstart = i
 		}
-		if fset.Position(g.End()).Line > lastLine {
+		if tokFile.Line(g.End()) > lastLine {
 			cend = i
 			break
 		}
@@ -144,7 +199,7 @@ func sortSpecs(env *ProcessEnv, fset *token.FileSet, f *ast.File, specs []ast.Sp
 	// Reassign the import paths to have the same position sequence.
 	// Reassign each comment to abut the end of its spec.
 	// Sort the comments by new position.
-	sort.Sort(byImportSpec{env, specs})
+	sort.Sort(byImportSpec{localPrefix, specs})
 
 	// Dedup. Thanks to our sorting, we can just consider
 	// adjacent pairs of imports.
@@ -154,7 +209,7 @@ func sortSpecs(env *ProcessEnv, fset *token.FileSet, f *ast.File, specs []ast.Sp
 			deduped = append(deduped, s)
 		} else {
 			p := s.Pos()
-			fset.File(p).MergeLine(fset.Position(p).Line)
+			tokFile.MergeLine(tokFile.Line(p)) // has side effects!
 		}
 	}
 	specs = deduped
@@ -185,21 +240,30 @@ func sortSpecs(env *ProcessEnv, fset *token.FileSet, f *ast.File, specs []ast.Sp
 
 	// Fixup comments can insert blank lines, because import specs are on different lines.
 	// We remove those blank lines here by merging import spec to the first import spec line.
-	firstSpecLine := fset.Position(specs[0].Pos()).Line
+	firstSpecLine := tokFile.Line(specs[0].Pos())
 	for _, s := range specs[1:] {
 		p := s.Pos()
-		line := fset.File(p).Line(p)
+		line := tokFile.Line(p)
 		for previousLine := line - 1; previousLine >= firstSpecLine; {
-			fset.File(p).MergeLine(previousLine)
-			previousLine--
+			// MergeLine can panic. Avoid the panic at the cost of not removing the blank line
+			// golang/go#50329
+			if previousLine > 0 && previousLine < tokFile.LineCount() {
+				tokFile.MergeLine(previousLine) // has side effects!
+				previousLine--
+			} else {
+				// try to gather some data to diagnose how this could happen
+				req := "Please report what the imports section of your go file looked like."
+				log.Printf("panic avoided: first:%d line:%d previous:%d max:%d. %s",
+					firstSpecLine, line, previousLine, tokFile.LineCount(), req)
+			}
 		}
 	}
 	return specs
 }
 
 type byImportSpec struct {
-	env   *ProcessEnv
-	specs []ast.Spec // slice of *ast.ImportSpec
+	localPrefix string
+	specs       []ast.Spec // slice of *ast.ImportSpec
 }
 
 func (x byImportSpec) Len() int      { return len(x.specs) }
@@ -208,8 +272,8 @@ func (x byImportSpec) Less(i, j int) bool {
 	ipath := importPath(x.specs[i])
 	jpath := importPath(x.specs[j])
 
-	igroup := importGroup(x.env, ipath)
-	jgroup := importGroup(x.env, jpath)
+	igroup := importGroup(x.localPrefix, ipath)
+	jgroup := importGroup(x.localPrefix, jpath)
 	if igroup != jgroup {
 		return igroup < jgroup
 	}