TUN-8456: Update quic-go to 0.45 and collect mtu and congestion control metrics

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

@@ -13,6 +13,8 @@ import (
 	"go/build"
 	"go/parser"
 	"go/token"
+	"go/types"
+	"io/fs"
 	"io/ioutil"
 	"os"
 	"path"
@@ -26,8 +28,10 @@ import (
 	"unicode/utf8"
 
 	"golang.org/x/tools/go/ast/astutil"
+	"golang.org/x/tools/internal/event"
 	"golang.org/x/tools/internal/gocommand"
 	"golang.org/x/tools/internal/gopathwalk"
+	"golang.org/x/tools/internal/stdlib"
 )
 
 // importToGroup is a list of functions which map from an import path to
@@ -106,7 +110,7 @@ func parseOtherFiles(fset *token.FileSet, srcDir, filename string) []*ast.File {
 	considerTests := strings.HasSuffix(filename, "_test.go")
 
 	fileBase := filepath.Base(filename)
-	packageFileInfos, err := ioutil.ReadDir(srcDir)
+	packageFileInfos, err := os.ReadDir(srcDir)
 	if err != nil {
 		return nil
 	}
@@ -252,7 +256,7 @@ type pass struct {
 	otherFiles           []*ast.File    // sibling files.
 
 	// Intermediate state, generated by load.
-	existingImports map[string]*ImportInfo
+	existingImports map[string][]*ImportInfo
 	allRefs         references
 	missingRefs     references
 
@@ -297,6 +301,20 @@ func (p *pass) loadPackageNames(imports []*ImportInfo) error {
 	return nil
 }
 
+// if there is a trailing major version, remove it
+func withoutVersion(nm string) string {
+	if v := path.Base(nm); len(v) > 0 && v[0] == 'v' {
+		if _, err := strconv.Atoi(v[1:]); err == nil {
+			// this is, for instance, called with rand/v2 and returns rand
+			if len(v) < len(nm) {
+				xnm := nm[:len(nm)-len(v)-1]
+				return path.Base(xnm)
+			}
+		}
+	}
+	return nm
+}
+
 // importIdentifier returns the identifier that imp will introduce. It will
 // guess if the package name has not been loaded, e.g. because the source
 // is not available.
@@ -306,7 +324,7 @@ func (p *pass) importIdentifier(imp *ImportInfo) string {
 	}
 	known := p.knownPackages[imp.ImportPath]
 	if known != nil && known.name != "" {
-		return known.name
+		return withoutVersion(known.name)
 	}
 	return ImportPathToAssumedName(imp.ImportPath)
 }
@@ -317,7 +335,7 @@ func (p *pass) importIdentifier(imp *ImportInfo) string {
 func (p *pass) load() ([]*ImportFix, bool) {
 	p.knownPackages = map[string]*packageInfo{}
 	p.missingRefs = references{}
-	p.existingImports = map[string]*ImportInfo{}
+	p.existingImports = map[string][]*ImportInfo{}
 
 	// Load basic information about the file in question.
 	p.allRefs = collectReferences(p.f)
@@ -348,7 +366,7 @@ func (p *pass) load() ([]*ImportFix, bool) {
 		}
 	}
 	for _, imp := range imports {
-		p.existingImports[p.importIdentifier(imp)] = imp
+		p.existingImports[p.importIdentifier(imp)] = append(p.existingImports[p.importIdentifier(imp)], imp)
 	}
 
 	// Find missing references.
@@ -387,31 +405,33 @@ func (p *pass) fix() ([]*ImportFix, bool) {
 
 	// Found everything, or giving up. Add the new imports and remove any unused.
 	var fixes []*ImportFix
-	for _, imp := range p.existingImports {
-		// We deliberately ignore globals here, because we can't be sure
-		// they're in the same package. People do things like put multiple
-		// main packages in the same directory, and we don't want to
-		// remove imports if they happen to have the same name as a var in
-		// a different package.
-		if _, ok := p.allRefs[p.importIdentifier(imp)]; !ok {
-			fixes = append(fixes, &ImportFix{
-				StmtInfo:  *imp,
-				IdentName: p.importIdentifier(imp),
-				FixType:   DeleteImport,
-			})
-			continue
-		}
+	for _, identifierImports := range p.existingImports {
+		for _, imp := range identifierImports {
+			// We deliberately ignore globals here, because we can't be sure
+			// they're in the same package. People do things like put multiple
+			// main packages in the same directory, and we don't want to
+			// remove imports if they happen to have the same name as a var in
+			// a different package.
+			if _, ok := p.allRefs[p.importIdentifier(imp)]; !ok {
+				fixes = append(fixes, &ImportFix{
+					StmtInfo:  *imp,
+					IdentName: p.importIdentifier(imp),
+					FixType:   DeleteImport,
+				})
+				continue
+			}
 
-		// An existing import may need to update its import name to be correct.
-		if name := p.importSpecName(imp); name != imp.Name {
-			fixes = append(fixes, &ImportFix{
-				StmtInfo: ImportInfo{
-					Name:       name,
-					ImportPath: imp.ImportPath,
-				},
-				IdentName: p.importIdentifier(imp),
-				FixType:   SetImportName,
-			})
+			// An existing import may need to update its import name to be correct.
+			if name := p.importSpecName(imp); name != imp.Name {
+				fixes = append(fixes, &ImportFix{
+					StmtInfo: ImportInfo{
+						Name:       name,
+						ImportPath: imp.ImportPath,
+					},
+					IdentName: p.importIdentifier(imp),
+					FixType:   SetImportName,
+				})
+			}
 		}
 	}
 	// Collecting fixes involved map iteration, so sort for stability. See
@@ -506,9 +526,9 @@ func (p *pass) assumeSiblingImportsValid() {
 		}
 		for left, rights := range refs {
 			if imp, ok := importsByName[left]; ok {
-				if m, ok := stdlib[imp.ImportPath]; ok {
+				if m, ok := stdlib.PackageSymbols[imp.ImportPath]; ok {
 					// We have the stdlib in memory; no need to guess.
-					rights = copyExports(m)
+					rights = symbolNameSet(m)
 				}
 				p.addCandidate(imp, &packageInfo{
 					// no name; we already know it.
@@ -543,7 +563,7 @@ func (p *pass) addCandidate(imp *ImportInfo, pkg *packageInfo) {
 var fixImports = fixImportsDefault
 
 func fixImportsDefault(fset *token.FileSet, f *ast.File, filename string, env *ProcessEnv) error {
-	fixes, err := getFixes(fset, f, filename, env)
+	fixes, err := getFixes(context.Background(), fset, f, filename, env)
 	if err != nil {
 		return err
 	}
@@ -553,7 +573,7 @@ func fixImportsDefault(fset *token.FileSet, f *ast.File, filename string, env *P
 
 // getFixes gets the import fixes that need to be made to f in order to fix the imports.
 // It does not modify the ast.
-func getFixes(fset *token.FileSet, f *ast.File, filename string, env *ProcessEnv) ([]*ImportFix, error) {
+func getFixes(ctx context.Context, fset *token.FileSet, f *ast.File, filename string, env *ProcessEnv) ([]*ImportFix, error) {
 	abs, err := filepath.Abs(filename)
 	if err != nil {
 		return nil, err
@@ -607,7 +627,7 @@ func getFixes(fset *token.FileSet, f *ast.File, filename string, env *ProcessEnv
 
 	// Go look for candidates in $GOPATH, etc. We don't necessarily load
 	// the real exports of sibling imports, so keep assuming their contents.
-	if err := addExternalCandidates(p, p.missingRefs, filename); err != nil {
+	if err := addExternalCandidates(ctx, p, p.missingRefs, filename); err != nil {
 		return nil, err
 	}
 
@@ -636,7 +656,7 @@ func getCandidatePkgs(ctx context.Context, wrappedCallback *scanCallback, filena
 	dupCheck := map[string]struct{}{}
 
 	// Start off with the standard library.
-	for importPath, exports := range stdlib {
+	for importPath, symbols := range stdlib.PackageSymbols {
 		p := &pkg{
 			dir:             filepath.Join(goenv["GOROOT"], "src", importPath),
 			importPathShort: importPath,
@@ -645,6 +665,13 @@ func getCandidatePkgs(ctx context.Context, wrappedCallback *scanCallback, filena
 		}
 		dupCheck[importPath] = struct{}{}
 		if notSelf(p) && wrappedCallback.dirFound(p) && wrappedCallback.packageNameLoaded(p) {
+			var exports []stdlib.Symbol
+			for _, sym := range symbols {
+				switch sym.Kind {
+				case stdlib.Func, stdlib.Type, stdlib.Var, stdlib.Const:
+					exports = append(exports, sym)
+				}
+			}
 			wrappedCallback.exportsLoaded(p, exports)
 		}
 	}
@@ -665,7 +692,7 @@ func getCandidatePkgs(ctx context.Context, wrappedCallback *scanCallback, filena
 			dupCheck[pkg.importPathShort] = struct{}{}
 			return notSelf(pkg) && wrappedCallback.packageNameLoaded(pkg)
 		},
-		exportsLoaded: func(pkg *pkg, exports []string) {
+		exportsLoaded: func(pkg *pkg, exports []stdlib.Symbol) {
 			// If we're an x_test, load the package under test's test variant.
 			if strings.HasSuffix(filePkg, "_test") && pkg.dir == filepath.Dir(filename) {
 				var err error
@@ -696,20 +723,21 @@ func ScoreImportPaths(ctx context.Context, env *ProcessEnv, paths []string) (map
 	return result, nil
 }
 
-func PrimeCache(ctx context.Context, env *ProcessEnv) error {
+func PrimeCache(ctx context.Context, resolver Resolver) error {
 	// Fully scan the disk for directories, but don't actually read any Go files.
 	callback := &scanCallback{
-		rootFound: func(gopathwalk.Root) bool {
-			return true
+		rootFound: func(root gopathwalk.Root) bool {
+			// See getCandidatePkgs: walking GOROOT is apparently expensive and
+			// unnecessary.
+			return root.Type != gopathwalk.RootGOROOT
 		},
 		dirFound: func(pkg *pkg) bool {
 			return false
 		},
-		packageNameLoaded: func(pkg *pkg) bool {
-			return false
-		},
+		// packageNameLoaded and exportsLoaded must never be called.
 	}
-	return getCandidatePkgs(ctx, callback, "", "", env)
+
+	return resolver.scan(ctx, callback)
 }
 
 func candidateImportName(pkg *pkg) string {
@@ -789,7 +817,7 @@ func GetImportPaths(ctx context.Context, wrapped func(ImportFix), searchPrefix,
 // A PackageExport is a package and its exports.
 type PackageExport struct {
 	Fix     *ImportFix
-	Exports []string
+	Exports []stdlib.Symbol
 }
 
 // GetPackageExports returns all known packages with name pkg and their exports.
@@ -804,8 +832,8 @@ func GetPackageExports(ctx context.Context, wrapped func(PackageExport), searchP
 		packageNameLoaded: func(pkg *pkg) bool {
 			return pkg.packageName == searchPkg
 		},
-		exportsLoaded: func(pkg *pkg, exports []string) {
-			sort.Strings(exports)
+		exportsLoaded: func(pkg *pkg, exports []stdlib.Symbol) {
+			sortSymbols(exports)
 			wrapped(PackageExport{
 				Fix: &ImportFix{
 					StmtInfo: ImportInfo{
@@ -823,16 +851,45 @@ func GetPackageExports(ctx context.Context, wrapped func(PackageExport), searchP
 	return getCandidatePkgs(ctx, callback, filename, filePkg, env)
 }
 
-var requiredGoEnvVars = []string{"GO111MODULE", "GOFLAGS", "GOINSECURE", "GOMOD", "GOMODCACHE", "GONOPROXY", "GONOSUMDB", "GOPATH", "GOPROXY", "GOROOT", "GOSUMDB", "GOWORK"}
+// TODO(rfindley): we should depend on GOOS and GOARCH, to provide accurate
+// imports when doing cross-platform development.
+var requiredGoEnvVars = []string{
+	"GO111MODULE",
+	"GOFLAGS",
+	"GOINSECURE",
+	"GOMOD",
+	"GOMODCACHE",
+	"GONOPROXY",
+	"GONOSUMDB",
+	"GOPATH",
+	"GOPROXY",
+	"GOROOT",
+	"GOSUMDB",
+	"GOWORK",
+}
 
 // ProcessEnv contains environment variables and settings that affect the use of
 // the go command, the go/build package, etc.
+//
+// ...a ProcessEnv *also* overwrites its Env along with derived state in the
+// form of the resolver. And because it is lazily initialized, an env may just
+// be broken and unusable, but there is no way for the caller to detect that:
+// all queries will just fail.
+//
+// TODO(rfindley): refactor this package so that this type (perhaps renamed to
+// just Env or Config) is an immutable configuration struct, to be exchanged
+// for an initialized object via a constructor that returns an error. Perhaps
+// the signature should be `func NewResolver(*Env) (*Resolver, error)`, where
+// resolver is a concrete type used for resolving imports. Via this
+// refactoring, we can avoid the need to call ProcessEnv.init and
+// ProcessEnv.GoEnv everywhere, and implicitly fix all the places where this
+// these are misused. Also, we'd delegate the caller the decision of how to
+// handle a broken environment.
 type ProcessEnv struct {
 	GocmdRunner *gocommand.Runner
 
 	BuildFlags []string
 	ModFlag    string
-	ModFile    string
 
 	// SkipPathInScan returns true if the path should be skipped from scans of
 	// the RootCurrentModule root type. The function argument is a clean,
@@ -842,7 +899,7 @@ type ProcessEnv struct {
 	// Env overrides the OS environment, and can be used to specify
 	// GOPROXY, GO111MODULE, etc. PATH cannot be set here, because
 	// exec.Command will not honor it.
-	// Specifying all of RequiredGoEnvVars avoids a call to `go env`.
+	// Specifying all of requiredGoEnvVars avoids a call to `go env`.
 	Env map[string]string
 
 	WorkingDir string
@@ -850,9 +907,17 @@ type ProcessEnv struct {
 	// If Logf is non-nil, debug logging is enabled through this function.
 	Logf func(format string, args ...interface{})
 
-	initialized bool
+	// If set, ModCache holds a shared cache of directory info to use across
+	// multiple ProcessEnvs.
+	ModCache *DirInfoCache
 
-	resolver Resolver
+	initialized bool // see TODO above
+
+	// resolver and resolverErr are lazily evaluated (see GetResolver).
+	// This is unclean, but see the big TODO in the docstring for ProcessEnv
+	// above: for now, we can't be sure that the ProcessEnv is fully initialized.
+	resolver    Resolver
+	resolverErr error
 }
 
 func (e *ProcessEnv) goEnv() (map[string]string, error) {
@@ -932,20 +997,33 @@ func (e *ProcessEnv) env() []string {
 }
 
 func (e *ProcessEnv) GetResolver() (Resolver, error) {
-	if e.resolver != nil {
-		return e.resolver, nil
-	}
 	if err := e.init(); err != nil {
 		return nil, err
 	}
-	if len(e.Env["GOMOD"]) == 0 && len(e.Env["GOWORK"]) == 0 {
-		e.resolver = newGopathResolver(e)
-		return e.resolver, nil
+
+	if e.resolver == nil && e.resolverErr == nil {
+		// TODO(rfindley): we should only use a gopathResolver here if the working
+		// directory is actually *in* GOPATH. (I seem to recall an open gopls issue
+		// for this behavior, but I can't find it).
+		//
+		// For gopls, we can optionally explicitly choose a resolver type, since we
+		// already know the view type.
+		if len(e.Env["GOMOD"]) == 0 && len(e.Env["GOWORK"]) == 0 {
+			e.resolver = newGopathResolver(e)
+		} else if r, err := newModuleResolver(e, e.ModCache); err != nil {
+			e.resolverErr = err
+		} else {
+			e.resolver = Resolver(r)
+		}
 	}
-	e.resolver = newModuleResolver(e)
-	return e.resolver, nil
+
+	return e.resolver, e.resolverErr
 }
 
+// buildContext returns the build.Context to use for matching files.
+//
+// TODO(rfindley): support dynamic GOOS, GOARCH here, when doing cross-platform
+// development.
 func (e *ProcessEnv) buildContext() (*build.Context, error) {
 	ctx := build.Default
 	goenv, err := e.goEnv()
@@ -995,24 +1073,40 @@ func addStdlibCandidates(pass *pass, refs references) error {
 	if err != nil {
 		return err
 	}
+	localbase := func(nm string) string {
+		ans := path.Base(nm)
+		if ans[0] == 'v' {
+			// this is called, for instance, with math/rand/v2 and returns rand/v2
+			if _, err := strconv.Atoi(ans[1:]); err == nil {
+				ix := strings.LastIndex(nm, ans)
+				more := path.Base(nm[:ix])
+				ans = path.Join(more, ans)
+			}
+		}
+		return ans
+	}
 	add := func(pkg string) {
 		// Prevent self-imports.
 		if path.Base(pkg) == pass.f.Name.Name && filepath.Join(goenv["GOROOT"], "src", pkg) == pass.srcDir {
 			return
 		}
-		exports := copyExports(stdlib[pkg])
+		exports := symbolNameSet(stdlib.PackageSymbols[pkg])
 		pass.addCandidate(
 			&ImportInfo{ImportPath: pkg},
-			&packageInfo{name: path.Base(pkg), exports: exports})
+			&packageInfo{name: localbase(pkg), exports: exports})
 	}
 	for left := range refs {
 		if left == "rand" {
-			// Make sure we try crypto/rand before math/rand.
+			// Make sure we try crypto/rand before any version of math/rand as both have Int()
+			// and our policy is to recommend crypto
 			add("crypto/rand")
-			add("math/rand")
+			// if the user's no later than go1.21, this should be "math/rand"
+			// but we have no way of figuring out what the user is using
+			// TODO: investigate using the toolchain version to disambiguate in the stdlib
+			add("math/rand/v2")
 			continue
 		}
-		for importPath := range stdlib {
+		for importPath := range stdlib.PackageSymbols {
 			if path.Base(importPath) == left {
 				add(importPath)
 			}
@@ -1025,15 +1119,23 @@ func addStdlibCandidates(pass *pass, refs references) error {
 type Resolver interface {
 	// loadPackageNames loads the package names in importPaths.
 	loadPackageNames(importPaths []string, srcDir string) (map[string]string, error)
+
 	// scan works with callback to search for packages. See scanCallback for details.
 	scan(ctx context.Context, callback *scanCallback) error
+
 	// loadExports returns the set of exported symbols in the package at dir.
 	// loadExports may be called concurrently.
-	loadExports(ctx context.Context, pkg *pkg, includeTest bool) (string, []string, error)
+	loadExports(ctx context.Context, pkg *pkg, includeTest bool) (string, []stdlib.Symbol, error)
+
 	// scoreImportPath returns the relevance for an import path.
 	scoreImportPath(ctx context.Context, path string) float64
 
-	ClearForNewScan()
+	// ClearForNewScan returns a new Resolver based on the receiver that has
+	// cleared its internal caches of directory contents.
+	//
+	// The new resolver should be primed and then set via
+	// [ProcessEnv.UpdateResolver].
+	ClearForNewScan() Resolver
 }
 
 // A scanCallback controls a call to scan and receives its results.
@@ -1052,10 +1154,13 @@ type scanCallback struct {
 	// If it returns true, the package's exports will be loaded.
 	packageNameLoaded func(pkg *pkg) bool
 	// exportsLoaded is called when a package's exports have been loaded.
-	exportsLoaded func(pkg *pkg, exports []string)
+	exportsLoaded func(pkg *pkg, exports []stdlib.Symbol)
 }
 
-func addExternalCandidates(pass *pass, refs references, filename string) error {
+func addExternalCandidates(ctx context.Context, pass *pass, refs references, filename string) error {
+	ctx, done := event.Start(ctx, "imports.addExternalCandidates")
+	defer done()
+
 	var mu sync.Mutex
 	found := make(map[string][]pkgDistance)
 	callback := &scanCallback{
@@ -1113,7 +1218,7 @@ func addExternalCandidates(pass *pass, refs references, filename string) error {
 		go func(pkgName string, symbols map[string]bool) {
 			defer wg.Done()
 
-			found, err := findImport(ctx, pass, found[pkgName], pkgName, symbols, filename)
+			found, err := findImport(ctx, pass, found[pkgName], pkgName, symbols)
 
 			if err != nil {
 				firstErrOnce.Do(func() {
@@ -1144,6 +1249,17 @@ func addExternalCandidates(pass *pass, refs references, filename string) error {
 	}()
 
 	for result := range results {
+		// Don't offer completions that would shadow predeclared
+		// names, such as github.com/coreos/etcd/error.
+		if types.Universe.Lookup(result.pkg.name) != nil { // predeclared
+			// Ideally we would skip this candidate only
+			// if the predeclared name is actually
+			// referenced by the file, but that's a lot
+			// trickier to compute and would still create
+			// an import that is likely to surprise the
+			// user before long.
+			continue
+		}
 		pass.addCandidate(result.imp, result.pkg)
 	}
 	return firstErr
@@ -1186,31 +1302,22 @@ func ImportPathToAssumedName(importPath string) string {
 type gopathResolver struct {
 	env      *ProcessEnv
 	walked   bool
-	cache    *dirInfoCache
+	cache    *DirInfoCache
 	scanSema chan struct{} // scanSema prevents concurrent scans.
 }
 
 func newGopathResolver(env *ProcessEnv) *gopathResolver {
 	r := &gopathResolver{
-		env: env,
-		cache: &dirInfoCache{
-			dirs:      map[string]*directoryPackageInfo{},
-			listeners: map[*int]cacheListener{},
-		},
+		env:      env,
+		cache:    NewDirInfoCache(),
 		scanSema: make(chan struct{}, 1),
 	}
 	r.scanSema <- struct{}{}
 	return r
 }
 
-func (r *gopathResolver) ClearForNewScan() {
-	<-r.scanSema
-	r.cache = &dirInfoCache{
-		dirs:      map[string]*directoryPackageInfo{},
-		listeners: map[*int]cacheListener{},
-	}
-	r.walked = false
-	r.scanSema <- struct{}{}
+func (r *gopathResolver) ClearForNewScan() Resolver {
+	return newGopathResolver(r.env)
 }
 
 func (r *gopathResolver) loadPackageNames(importPaths []string, srcDir string) (map[string]string, error) {
@@ -1228,7 +1335,7 @@ func (r *gopathResolver) loadPackageNames(importPaths []string, srcDir string) (
 // importPathToName finds out the actual package name, as declared in its .go files.
 func importPathToName(bctx *build.Context, importPath, srcDir string) string {
 	// Fast path for standard library without going to disk.
-	if _, ok := stdlib[importPath]; ok {
+	if stdlib.HasPackage(importPath) {
 		return path.Base(importPath) // stdlib packages always match their paths.
 	}
 
@@ -1426,7 +1533,7 @@ func (r *gopathResolver) scan(ctx context.Context, callback *scanCallback) error
 }
 
 func (r *gopathResolver) scoreImportPath(ctx context.Context, path string) float64 {
-	if _, ok := stdlib[path]; ok {
+	if stdlib.HasPackage(path) {
 		return MaxRelevance
 	}
 	return MaxRelevance - 1
@@ -1443,7 +1550,7 @@ func filterRoots(roots []gopathwalk.Root, include func(gopathwalk.Root) bool) []
 	return result
 }
 
-func (r *gopathResolver) loadExports(ctx context.Context, pkg *pkg, includeTest bool) (string, []string, error) {
+func (r *gopathResolver) loadExports(ctx context.Context, pkg *pkg, includeTest bool) (string, []stdlib.Symbol, error) {
 	if info, ok := r.cache.Load(pkg.dir); ok && !includeTest {
 		return r.cache.CacheExports(ctx, r.env, info)
 	}
@@ -1463,13 +1570,13 @@ func VendorlessPath(ipath string) string {
 	return ipath
 }
 
-func loadExportsFromFiles(ctx context.Context, env *ProcessEnv, dir string, includeTest bool) (string, []string, error) {
+func loadExportsFromFiles(ctx context.Context, env *ProcessEnv, dir string, includeTest bool) (string, []stdlib.Symbol, error) {
 	// Look for non-test, buildable .go files which could provide exports.
-	all, err := ioutil.ReadDir(dir)
+	all, err := os.ReadDir(dir)
 	if err != nil {
 		return "", nil, err
 	}
-	var files []os.FileInfo
+	var files []fs.DirEntry
 	for _, fi := range all {
 		name := fi.Name()
 		if !strings.HasSuffix(name, ".go") || (!includeTest && strings.HasSuffix(name, "_test.go")) {
@@ -1487,7 +1594,7 @@ func loadExportsFromFiles(ctx context.Context, env *ProcessEnv, dir string, incl
 	}
 
 	var pkgName string
-	var exports []string
+	var exports []stdlib.Symbol
 	fset := token.NewFileSet()
 	for _, fi := range files {
 		select {
@@ -1514,24 +1621,44 @@ func loadExportsFromFiles(ctx context.Context, env *ProcessEnv, dir string, incl
 			continue
 		}
 		pkgName = f.Name.Name
-		for name := range f.Scope.Objects {
+		for name, obj := range f.Scope.Objects {
 			if ast.IsExported(name) {
-				exports = append(exports, name)
+				var kind stdlib.Kind
+				switch obj.Kind {
+				case ast.Con:
+					kind = stdlib.Const
+				case ast.Typ:
+					kind = stdlib.Type
+				case ast.Var:
+					kind = stdlib.Var
+				case ast.Fun:
+					kind = stdlib.Func
+				}
+				exports = append(exports, stdlib.Symbol{
+					Name:    name,
+					Kind:    kind,
+					Version: 0, // unknown; be permissive
+				})
 			}
 		}
 	}
+	sortSymbols(exports)
 
 	if env.Logf != nil {
-		sortedExports := append([]string(nil), exports...)
-		sort.Strings(sortedExports)
-		env.Logf("loaded exports in dir %v (package %v): %v", dir, pkgName, strings.Join(sortedExports, ", "))
+		env.Logf("loaded exports in dir %v (package %v): %v", dir, pkgName, exports)
 	}
 	return pkgName, exports, nil
 }
 
+func sortSymbols(syms []stdlib.Symbol) {
+	sort.Slice(syms, func(i, j int) bool {
+		return syms[i].Name < syms[j].Name
+	})
+}
+
 // findImport searches for a package with the given symbols.
 // If no package is found, findImport returns ("", false, nil)
-func findImport(ctx context.Context, pass *pass, candidates []pkgDistance, pkgName string, symbols map[string]bool, filename string) (*pkg, error) {
+func findImport(ctx context.Context, pass *pass, candidates []pkgDistance, pkgName string, symbols map[string]bool) (*pkg, error) {
 	// Sort the candidates by their import package length,
 	// assuming that shorter package names are better than long
 	// ones.  Note that this sorts by the de-vendored name, so
@@ -1595,7 +1722,7 @@ func findImport(ctx context.Context, pass *pass, candidates []pkgDistance, pkgNa
 
 				exportsMap := make(map[string]bool, len(exports))
 				for _, sym := range exports {
-					exportsMap[sym] = true
+					exportsMap[sym.Name] = true
 				}
 
 				// If it doesn't have the right
@@ -1753,10 +1880,13 @@ func (fn visitFn) Visit(node ast.Node) ast.Visitor {
 	return fn(node)
 }
 
-func copyExports(pkg []string) map[string]bool {
-	m := make(map[string]bool, len(pkg))
-	for _, v := range pkg {
-		m[v] = true
+func symbolNameSet(symbols []stdlib.Symbol) map[string]bool {
+	names := make(map[string]bool)
+	for _, sym := range symbols {
+		switch sym.Kind {
+		case stdlib.Const, stdlib.Var, stdlib.Type, stdlib.Func:
+			names[sym.Name] = true
+		}
 	}
-	return m
+	return names
 }

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

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:generate go run mkstdlib.go
-
 // Package imports implements a Go pretty-printer (like package "go/format")
 // that also adds or removes import statements as necessary.
 package imports
@@ -11,6 +9,7 @@ package imports
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"fmt"
 	"go/ast"
 	"go/format"
@@ -23,6 +22,7 @@ import (
 	"strings"
 
 	"golang.org/x/tools/go/ast/astutil"
+	"golang.org/x/tools/internal/event"
 )
 
 // Options is golang.org/x/tools/imports.Options with extra internal-only options.
@@ -66,14 +66,17 @@ func Process(filename string, src []byte, opt *Options) (formatted []byte, err e
 //
 // 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) {
+func FixImports(ctx context.Context, filename string, src []byte, opt *Options) (fixes []*ImportFix, err error) {
+	ctx, done := event.Start(ctx, "imports.FixImports")
+	defer done()
+
 	fileSet := token.NewFileSet()
 	file, _, err := parse(fileSet, filename, src, opt)
 	if err != nil {
 		return nil, err
 	}
 
-	return getFixes(fileSet, file, filename, opt.Env)
+	return getFixes(ctx, fileSet, file, filename, opt.Env)
 }
 
 // ApplyFixes applies all of the fixes to the file and formats it. extraMode
@@ -104,7 +107,7 @@ func ApplyFixes(fixes []*ImportFix, filename string, src []byte, opt *Options, e
 }
 
 // formatFile formats the file syntax tree.
-// It may mutate the token.FileSet.
+// It may mutate the token.FileSet and the ast.File.
 //
 // If an adjust function is provided, it is called after formatting
 // with the original source (formatFile's src parameter) and the
@@ -231,7 +234,7 @@ func parse(fset *token.FileSet, filename string, src []byte, opt *Options) (*ast
 			src = src[:len(src)-len("}\n")]
 			// Gofmt has also indented the function body one level.
 			// Remove that indent.
-			src = bytes.Replace(src, []byte("\n\t"), []byte("\n"), -1)
+			src = bytes.ReplaceAll(src, []byte("\n\t"), []byte("\n"))
 			return matchSpace(orig, src)
 		}
 		return file, adjust, nil

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

@@ -9,7 +9,6 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"io/ioutil"
 	"os"
 	"path"
 	"path/filepath"
@@ -19,80 +18,141 @@ import (
 	"strings"
 
 	"golang.org/x/mod/module"
+	"golang.org/x/tools/internal/event"
 	"golang.org/x/tools/internal/gocommand"
 	"golang.org/x/tools/internal/gopathwalk"
+	"golang.org/x/tools/internal/stdlib"
 )
 
-// ModuleResolver implements resolver for modules using the go command as little
-// as feasible.
+// Notes(rfindley): ModuleResolver appears to be heavily optimized for scanning
+// as fast as possible, which is desirable for a call to goimports from the
+// command line, but it doesn't work as well for gopls, where it suffers from
+// slow startup (golang/go#44863) and intermittent hanging (golang/go#59216),
+// both caused by populating the cache, albeit in slightly different ways.
+//
+// A high level list of TODOs:
+//  - Optimize the scan itself, as there is some redundancy statting and
+//    reading go.mod files.
+//  - Invert the relationship between ProcessEnv and Resolver (see the
+//    docstring of ProcessEnv).
+//  - Make it easier to use an external resolver implementation.
+//
+// Smaller TODOs are annotated in the code below.
+
+// ModuleResolver implements the Resolver interface for a workspace using
+// modules.
+//
+// A goal of the ModuleResolver is to invoke the Go command as little as
+// possible. To this end, it runs the Go command only for listing module
+// information (i.e. `go list -m -e -json ...`). Package scanning, the process
+// of loading package information for the modules, is implemented internally
+// via the scan method.
+//
+// It has two types of state: the state derived from the go command, which
+// is populated by init, and the state derived from scans, which is populated
+// via scan. A root is considered scanned if it has been walked to discover
+// directories. However, if the scan did not require additional information
+// from the directory (such as package name or exports), the directory
+// information itself may be partially populated. It will be lazily filled in
+// as needed by scans, using the scanCallback.
 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
+	env *ProcessEnv
 
-	initialized   bool
-	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.
+	// Module state, populated during construction
+	dummyVendorMod *gocommand.ModuleJSON            // if vendoring is enabled, a pseudo-module to represent the /vendor directory
+	moduleCacheDir string                           // GOMODCACHE, inferred from GOPATH if unset
+	roots          []gopathwalk.Root                // roots to scan, in approximate order of importance
+	mains          []*gocommand.ModuleJSON          // main modules
+	mainByDir      map[string]*gocommand.ModuleJSON // module information by dir, to join with roots
+	modsByModPath  []*gocommand.ModuleJSON          // all modules, ordered by # of path components in their module path
+	modsByDir      []*gocommand.ModuleJSON          // ...or by the number of path components in their Dir.
 
-	// moduleCacheCache stores information about the module cache.
-	moduleCacheCache *dirInfoCache
-	otherCache       *dirInfoCache
+	// Scanning state, populated by scan
+
+	// scanSema prevents concurrent scans, and guards scannedRoots and the cache
+	// fields below (though the caches themselves are concurrency safe).
+	// Receive to acquire, send to release.
+	scanSema     chan struct{}
+	scannedRoots map[gopathwalk.Root]bool // if true, root has been walked
+
+	// Caches of directory info, populated by scans and scan callbacks
+	//
+	// moduleCacheCache stores cached information about roots in the module
+	// cache, which are immutable and therefore do not need to be invalidated.
+	//
+	// otherCache stores information about all other roots (even GOROOT), which
+	// may change.
+	moduleCacheCache *DirInfoCache
+	otherCache       *DirInfoCache
 }
 
-func newModuleResolver(e *ProcessEnv) *ModuleResolver {
+// newModuleResolver returns a new module-aware goimports resolver.
+//
+// Note: use caution when modifying this constructor: changes must also be
+// reflected in ModuleResolver.ClearForNewScan.
+func newModuleResolver(e *ProcessEnv, moduleCacheCache *DirInfoCache) (*ModuleResolver, error) {
 	r := &ModuleResolver{
 		env:      e,
 		scanSema: make(chan struct{}, 1),
 	}
-	r.scanSema <- struct{}{}
-	return r
-}
-
-func (r *ModuleResolver) init() error {
-	if r.initialized {
-		return nil
-	}
+	r.scanSema <- struct{}{} // release
 
 	goenv, err := r.env.goEnv()
 	if err != nil {
-		return err
+		return nil, err
 	}
+
+	// TODO(rfindley): can we refactor to share logic with r.env.invokeGo?
 	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
+	var mainModVendor *gocommand.ModuleJSON    // for module vendoring
+	var mainModsVendor []*gocommand.ModuleJSON // for workspace vendoring
 
-	// 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 {
+	goWork := r.env.Env["GOWORK"]
+	if len(goWork) == 0 {
+		// TODO(rfindley): VendorEnabled runs the go command to get GOFLAGS, but
+		// they should be available from the ProcessEnv. Can we avoid the redundant
+		// invocation?
 		vendorEnabled, mainModVendor, err = gocommand.VendorEnabled(context.TODO(), inv, r.env.GocmdRunner)
 		if err != nil {
-			return err
+			return nil, err
+		}
+	} else {
+		vendorEnabled, mainModsVendor, err = gocommand.WorkspaceVendorEnabled(context.Background(), inv, r.env.GocmdRunner)
+		if err != nil {
+			return nil, err
 		}
 	}
 
-	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"),
+	if vendorEnabled {
+		if mainModVendor != nil {
+			// Module 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 {
+			// Workspace vendor mode is on, so all the non-Main modules are irrelevant,
+			// and we need to search /vendor for everything.
+			r.mains = mainModsVendor
+			r.dummyVendorMod = &gocommand.ModuleJSON{
+				Path: "",
+				Dir:  filepath.Join(filepath.Dir(goWork), "vendor"),
+			}
+			r.modsByModPath = append(append([]*gocommand.ModuleJSON{}, mainModsVendor...), r.dummyVendorMod)
+			r.modsByDir = append(append([]*gocommand.ModuleJSON{}, mainModsVendor...), r.dummyVendorMod)
 		}
-		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()
@@ -100,19 +160,14 @@ func (r *ModuleResolver) init() error {
 		// 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
+				return nil, err
 			}
 		}
 	}
 
-	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")
+	r.moduleCacheDir = gomodcacheForEnv(goenv)
+	if r.moduleCacheDir == "" {
+		return nil, fmt.Errorf("cannot resolve GOMODCACHE")
 	}
 
 	sort.Slice(r.modsByModPath, func(i, j int) bool {
@@ -123,13 +178,14 @@ func (r *ModuleResolver) init() error {
 	})
 	sort.Slice(r.modsByDir, func(i, j int) bool {
 		count := func(x int) int {
-			return strings.Count(r.modsByDir[x].Dir, "/")
+			return strings.Count(r.modsByDir[x].Dir, string(filepath.Separator))
 		}
 		return count(j) < count(i) // descending order
 	})
 
-	r.roots = []gopathwalk.Root{
-		{Path: filepath.Join(goenv["GOROOT"], "/src"), Type: gopathwalk.RootGOROOT},
+	r.roots = []gopathwalk.Root{}
+	if goenv["GOROOT"] != "" { // "" happens in tests
+		r.roots = append(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 {
@@ -141,7 +197,11 @@ func (r *ModuleResolver) init() error {
 	} else {
 		addDep := func(mod *gocommand.ModuleJSON) {
 			if mod.Replace == nil {
-				// This is redundant with the cache, but we'll skip it cheaply enough.
+				// This is redundant with the cache, but we'll skip it cheaply enough
+				// when we encounter it in the module cache scan.
+				//
+				// Including it at a lower index in r.roots than the module cache dir
+				// helps prioritize matches from within existing dependencies.
 				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})
@@ -158,24 +218,40 @@ func (r *ModuleResolver) init() error {
 				addDep(mod)
 			}
 		}
+		// If provided, share the moduleCacheCache.
+		//
+		// TODO(rfindley): The module cache is immutable. However, the loaded
+		// exports do depend on GOOS and GOARCH. Fortunately, the
+		// ProcessEnv.buildContext does not adjust these from build.DefaultContext
+		// (even though it should). So for now, this is OK to share, but we need to
+		// add logic for handling GOOS/GOARCH.
+		r.moduleCacheCache = moduleCacheCache
 		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{},
-		}
+		r.moduleCacheCache = NewDirInfoCache()
 	}
-	if r.otherCache == nil {
-		r.otherCache = &dirInfoCache{
-			dirs:      map[string]*directoryPackageInfo{},
-			listeners: map[*int]cacheListener{},
-		}
+	r.otherCache = NewDirInfoCache()
+	return r, nil
+}
+
+// gomodcacheForEnv returns the GOMODCACHE value to use based on the given env
+// map, which must have GOMODCACHE and GOPATH populated.
+//
+// TODO(rfindley): this is defensive refactoring.
+//  1. Is this even relevant anymore? Can't we just read GOMODCACHE.
+//  2. Use this to separate module cache scanning from other scanning.
+func gomodcacheForEnv(goenv map[string]string) string {
+	if gmc := goenv["GOMODCACHE"]; gmc != "" {
+		return gmc
 	}
-	r.initialized = true
-	return nil
+	gopaths := filepath.SplitList(goenv["GOPATH"])
+	if len(gopaths) == 0 {
+		return ""
+	}
+	return filepath.Join(gopaths[0], "/pkg/mod")
 }
 
 func (r *ModuleResolver) initAllMods() error {
@@ -206,30 +282,86 @@ func (r *ModuleResolver) initAllMods() error {
 	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{}{}
-}
+// ClearForNewScan invalidates the last scan.
+//
+// It preserves the set of roots, but forgets about the set of directories.
+// Though it forgets the set of module cache directories, it remembers their
+// contents, since they are assumed to be immutable.
+func (r *ModuleResolver) ClearForNewScan() Resolver {
+	<-r.scanSema // acquire r, to guard scannedRoots
+	r2 := &ModuleResolver{
+		env:            r.env,
+		dummyVendorMod: r.dummyVendorMod,
+		moduleCacheDir: r.moduleCacheDir,
+		roots:          r.roots,
+		mains:          r.mains,
+		mainByDir:      r.mainByDir,
+		modsByModPath:  r.modsByModPath,
 
-func (r *ModuleResolver) ClearForNewMod() {
-	<-r.scanSema
-	*r = ModuleResolver{
-		env:              r.env,
+		scanSema:         make(chan struct{}, 1),
+		scannedRoots:     make(map[gopathwalk.Root]bool),
+		otherCache:       NewDirInfoCache(),
 		moduleCacheCache: r.moduleCacheCache,
-		otherCache:       r.otherCache,
-		scanSema:         r.scanSema,
 	}
-	r.init()
-	r.scanSema <- struct{}{}
+	r2.scanSema <- struct{}{} // r2 must start released
+	// Invalidate root scans. We don't need to invalidate module cache roots,
+	// because they are immutable.
+	// (We don't support a use case where GOMODCACHE is cleaned in the middle of
+	// e.g. a gopls session: the user must restart gopls to get accurate
+	// imports.)
+	//
+	// Scanning for new directories in GOMODCACHE should be handled elsewhere,
+	// via a call to ScanModuleCache.
+	for _, root := range r.roots {
+		if root.Type == gopathwalk.RootModuleCache && r.scannedRoots[root] {
+			r2.scannedRoots[root] = true
+		}
+	}
+	r.scanSema <- struct{}{} // release r
+	return r2
 }
 
-// findPackage returns the module and directory that contains the package at
-// the given import path, or returns nil, "" if no module is in scope.
+// ClearModuleInfo invalidates resolver state that depends on go.mod file
+// contents (essentially, the output of go list -m -json ...).
+//
+// Notably, it does not forget directory contents, which are reset
+// asynchronously via ClearForNewScan.
+//
+// If the ProcessEnv is a GOPATH environment, ClearModuleInfo is a no op.
+//
+// TODO(rfindley): move this to a new env.go, consolidating ProcessEnv methods.
+func (e *ProcessEnv) ClearModuleInfo() {
+	if r, ok := e.resolver.(*ModuleResolver); ok {
+		resolver, err := newModuleResolver(e, e.ModCache)
+		if err != nil {
+			e.resolver = nil
+			e.resolverErr = err
+			return
+		}
+
+		<-r.scanSema // acquire (guards caches)
+		resolver.moduleCacheCache = r.moduleCacheCache
+		resolver.otherCache = r.otherCache
+		r.scanSema <- struct{}{} // release
+
+		e.UpdateResolver(resolver)
+	}
+}
+
+// UpdateResolver sets the resolver for the ProcessEnv to use in imports
+// operations. Only for use with the result of [Resolver.ClearForNewScan].
+//
+// TODO(rfindley): this awkward API is a result of the (arguably) inverted
+// relationship between configuration and state described in the doc comment
+// for [ProcessEnv].
+func (e *ProcessEnv) UpdateResolver(r Resolver) {
+	e.resolver = r
+	e.resolverErr = nil
+}
+
+// findPackage returns the module and directory from within the main modules
+// and their dependencies that contains the package at the given import path,
+// or returns nil, "" if no module is in scope.
 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.
@@ -264,7 +396,7 @@ func (r *ModuleResolver) findPackage(importPath string) (*gocommand.ModuleJSON,
 		}
 
 		// Not cached. Read the filesystem.
-		pkgFiles, err := ioutil.ReadDir(pkgDir)
+		pkgFiles, err := os.ReadDir(pkgDir)
 		if err != nil {
 			continue
 		}
@@ -295,10 +427,6 @@ func (r *ModuleResolver) cacheStore(info directoryPackageInfo) {
 	}
 }
 
-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 {
@@ -307,7 +435,7 @@ func (r *ModuleResolver) cachePackageName(info directoryPackageInfo) (string, er
 	return r.otherCache.CachePackageName(info)
 }
 
-func (r *ModuleResolver) cacheExports(ctx context.Context, env *ProcessEnv, info directoryPackageInfo) (string, []string, error) {
+func (r *ModuleResolver) cacheExports(ctx context.Context, env *ProcessEnv, info directoryPackageInfo) (string, []stdlib.Symbol, error) {
 	if info.rootType == gopathwalk.RootModuleCache {
 		return r.moduleCacheCache.CacheExports(ctx, env, info)
 	}
@@ -327,6 +455,10 @@ func (r *ModuleResolver) findModuleByDir(dir string) *gocommand.ModuleJSON {
 	// - in /vendor/ in -mod=vendor mode.
 	//    - nested module? Dunno.
 	// Rumor has it that replace targets cannot contain other replace targets.
+	//
+	// Note that it is critical here that modsByDir is sorted to have deeper dirs
+	// first. This ensures that findModuleByDir finds the innermost module.
+	// See also golang/go#56291.
 	for _, m := range r.modsByDir {
 		if !strings.HasPrefix(dir, m.Dir) {
 			continue
@@ -363,15 +495,15 @@ func (r *ModuleResolver) dirIsNestedModule(dir string, mod *gocommand.ModuleJSON
 	return modDir != mod.Dir
 }
 
-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)
+func readModName(modFile string) string {
+	modBytes, err := os.ReadFile(modFile)
+	if err != nil {
+		return ""
 	}
+	return modulePath(modBytes)
+}
 
+func (r *ModuleResolver) modInfo(dir string) (modDir, modName string) {
 	if r.dirInModuleCache(dir) {
 		if matches := modCacheRegexp.FindStringSubmatch(dir); len(matches) == 3 {
 			index := strings.Index(dir, matches[1]+"@"+matches[2])
@@ -405,11 +537,9 @@ func (r *ModuleResolver) dirInModuleCache(dir string) bool {
 }
 
 func (r *ModuleResolver) loadPackageNames(importPaths []string, srcDir string) (map[string]string, error) {
-	if err := r.init(); err != nil {
-		return nil, err
-	}
 	names := map[string]string{}
 	for _, path := range importPaths {
+		// TODO(rfindley): shouldn't this use the dirInfoCache?
 		_, packageDir := r.findPackage(path)
 		if packageDir == "" {
 			continue
@@ -424,9 +554,8 @@ func (r *ModuleResolver) loadPackageNames(importPaths []string, srcDir string) (
 }
 
 func (r *ModuleResolver) scan(ctx context.Context, callback *scanCallback) error {
-	if err := r.init(); err != nil {
-		return err
-	}
+	ctx, done := event.Start(ctx, "imports.ModuleResolver.scan")
+	defer done()
 
 	processDir := func(info directoryPackageInfo) {
 		// Skip this directory if we were not able to get the package information successfully.
@@ -437,18 +566,18 @@ func (r *ModuleResolver) scan(ctx context.Context, callback *scanCallback) error
 		if err != nil {
 			return
 		}
-
 		if !callback.dirFound(pkg) {
 			return
 		}
+
 		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
@@ -487,7 +616,6 @@ func (r *ModuleResolver) scan(ctx context.Context, callback *scanCallback) error
 		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))
 	}
@@ -502,9 +630,9 @@ func (r *ModuleResolver) scan(ctx context.Context, callback *scanCallback) error
 		select {
 		case <-ctx.Done():
 			return
-		case <-r.scanSema:
+		case <-r.scanSema: // acquire
 		}
-		defer func() { r.scanSema <- struct{}{} }()
+		defer func() { r.scanSema <- struct{}{} }() // release
 		// We have the lock on r.scannedRoots, and no other scans can run.
 		for _, root := range roots {
 			if ctx.Err() != nil {
@@ -527,7 +655,7 @@ func (r *ModuleResolver) scan(ctx context.Context, callback *scanCallback) error
 }
 
 func (r *ModuleResolver) scoreImportPath(ctx context.Context, path string) float64 {
-	if _, ok := stdlib[path]; ok {
+	if stdlib.HasPackage(path) {
 		return MaxRelevance
 	}
 	mod, _ := r.findPackage(path)
@@ -605,10 +733,7 @@ func (r *ModuleResolver) canonicalize(info directoryPackageInfo) (*pkg, error) {
 	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
-	}
+func (r *ModuleResolver) loadExports(ctx context.Context, pkg *pkg, includeTest bool) (string, []stdlib.Symbol, error) {
 	if info, ok := r.cacheLoad(pkg.dir); ok && !includeTest {
 		return r.cacheExports(ctx, r.env, info)
 	}

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

@@ -7,12 +7,17 @@ package imports
 import (
 	"context"
 	"fmt"
+	"path"
+	"path/filepath"
+	"strings"
 	"sync"
 
+	"golang.org/x/mod/module"
 	"golang.org/x/tools/internal/gopathwalk"
+	"golang.org/x/tools/internal/stdlib"
 )
 
-// To find packages to import, the resolver needs to know about all of the
+// To find packages to import, the resolver needs to know about all of
 // 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
@@ -39,6 +44,8 @@ const (
 	exportsLoaded
 )
 
+// directoryPackageInfo holds (possibly incomplete) information about packages
+// contained in a given directory.
 type directoryPackageInfo struct {
 	// status indicates the extent to which this struct has been filled in.
 	status directoryPackageStatus
@@ -63,8 +70,11 @@ type directoryPackageInfo struct {
 	packageName string // the package name, as declared in the source.
 
 	// Set when status >= exportsLoaded.
-
-	exports []string
+	// TODO(rfindley): it's hard to see this, but exports depend implicitly on
+	// the default build context GOOS and GOARCH.
+	//
+	// We can make this explicit, and key exports by GOOS, GOARCH.
+	exports []stdlib.Symbol
 }
 
 // reachedStatus returns true when info has a status at least target and any error associated with
@@ -79,7 +89,7 @@ func (info *directoryPackageInfo) reachedStatus(target directoryPackageStatus) (
 	return true, nil
 }
 
-// dirInfoCache is a concurrency safe map for storing information about
+// 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.
@@ -92,21 +102,26 @@ func (info *directoryPackageInfo) reachedStatus(target directoryPackageStatus) (
 // 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 {
+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
 }
 
+func NewDirInfoCache() *DirInfoCache {
+	return &DirInfoCache{
+		dirs:      make(map[string]*directoryPackageInfo),
+		listeners: make(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() {
+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
@@ -162,8 +177,10 @@ func (d *dirInfoCache) ScanAndListen(ctx context.Context, listener cacheListener
 }
 
 // Store stores the package info for dir.
-func (d *dirInfoCache) Store(dir string, info directoryPackageInfo) {
+func (d *DirInfoCache) Store(dir string, info directoryPackageInfo) {
 	d.mu.Lock()
+	// TODO(rfindley, golang/go#59216): should we overwrite an existing entry?
+	// That seems incorrect as the cache should be idempotent.
 	_, old := d.dirs[dir]
 	d.dirs[dir] = &info
 	var listeners []cacheListener
@@ -180,7 +197,7 @@ func (d *dirInfoCache) Store(dir string, info directoryPackageInfo) {
 }
 
 // Load returns a copy of the directoryPackageInfo for absolute directory dir.
-func (d *dirInfoCache) Load(dir string) (directoryPackageInfo, bool) {
+func (d *DirInfoCache) Load(dir string) (directoryPackageInfo, bool) {
 	d.mu.Lock()
 	defer d.mu.Unlock()
 	info, ok := d.dirs[dir]
@@ -191,7 +208,7 @@ func (d *dirInfoCache) Load(dir string) (directoryPackageInfo, bool) {
 }
 
 // Keys returns the keys currently present in d.
-func (d *dirInfoCache) Keys() (keys []string) {
+func (d *DirInfoCache) Keys() (keys []string) {
 	d.mu.Lock()
 	defer d.mu.Unlock()
 	for key := range d.dirs {
@@ -200,7 +217,7 @@ func (d *dirInfoCache) Keys() (keys []string) {
 	return keys
 }
 
-func (d *dirInfoCache) CachePackageName(info directoryPackageInfo) (string, error) {
+func (d *DirInfoCache) CachePackageName(info directoryPackageInfo) (string, error) {
 	if loaded, err := info.reachedStatus(nameLoaded); loaded {
 		return info.packageName, err
 	}
@@ -213,7 +230,7 @@ func (d *dirInfoCache) CachePackageName(info directoryPackageInfo) (string, erro
 	return info.packageName, info.err
 }
 
-func (d *dirInfoCache) CacheExports(ctx context.Context, env *ProcessEnv, info directoryPackageInfo) (string, []string, error) {
+func (d *DirInfoCache) CacheExports(ctx context.Context, env *ProcessEnv, info directoryPackageInfo) (string, []stdlib.Symbol, error) {
 	if reached, _ := info.reachedStatus(exportsLoaded); reached {
 		return info.packageName, info.exports, info.err
 	}
@@ -234,3 +251,81 @@ func (d *dirInfoCache) CacheExports(ctx context.Context, env *ProcessEnv, info d
 	d.Store(info.dir, info)
 	return info.packageName, info.exports, info.err
 }
+
+// ScanModuleCache walks the given directory, which must be a GOMODCACHE value,
+// for directory package information, storing the results in cache.
+func ScanModuleCache(dir string, cache *DirInfoCache, logf func(string, ...any)) {
+	// Note(rfindley): it's hard to see, but this function attempts to implement
+	// just the side effects on cache of calling PrimeCache with a ProcessEnv
+	// that has the given dir as its GOMODCACHE.
+	//
+	// Teasing out the control flow, we see that we can avoid any handling of
+	// vendor/ and can infer module info entirely from the path, simplifying the
+	// logic here.
+
+	root := gopathwalk.Root{
+		Path: filepath.Clean(dir),
+		Type: gopathwalk.RootModuleCache,
+	}
+
+	directoryInfo := func(root gopathwalk.Root, dir string) directoryPackageInfo {
+		// This is a copy of ModuleResolver.scanDirForPackage, trimmed down to
+		// logic that applies to a module cache directory.
+
+		subdir := ""
+		if dir != root.Path {
+			subdir = dir[len(root.Path)+len("/"):]
+		}
+
+		matches := modCacheRegexp.FindStringSubmatch(subdir)
+		if len(matches) == 0 {
+			return directoryPackageInfo{
+				status: directoryScanned,
+				err:    fmt.Errorf("invalid module cache path: %v", subdir),
+			}
+		}
+		modPath, err := module.UnescapePath(filepath.ToSlash(matches[1]))
+		if err != nil {
+			if logf != nil {
+				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]))
+		index := strings.Index(dir, matches[1]+"@"+matches[2])
+		modDir := filepath.Join(dir[:index], matches[1]+"@"+matches[2])
+		modName := readModName(filepath.Join(modDir, "go.mod"))
+		return directoryPackageInfo{
+			status:                 directoryScanned,
+			dir:                    dir,
+			rootType:               root.Type,
+			nonCanonicalImportPath: importPath,
+			moduleDir:              modDir,
+			moduleName:             modName,
+		}
+	}
+
+	add := func(root gopathwalk.Root, dir string) {
+		info := directoryInfo(root, dir)
+		cache.Store(info.dir, info)
+	}
+
+	skip := func(_ gopathwalk.Root, dir string) bool {
+		// Skip directories that have already been scanned.
+		//
+		// Note that gopathwalk only adds "package" directories, which must contain
+		// a .go file, and all such package directories in the module cache are
+		// immutable. So if we can load a dir, it can be skipped.
+		info, ok := cache.Load(dir)
+		if !ok {
+			return false
+		}
+		packageScanned, _ := info.reachedStatus(directoryScanned)
+		return packageScanned
+	}
+
+	gopathwalk.WalkSkip([]gopathwalk.Root{root}, add, skip, gopathwalk.Options{Logf: logf, ModulesEnabled: true})
+}

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

@@ -18,7 +18,7 @@ import (
 // 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.
 //
-// It may mutate the token.File.
+// It may mutate the token.File and the ast.File.
 func sortImports(localPrefix string, tokFile *token.File, f *ast.File) {
 	for i, d := range f.Decls {
 		d, ok := d.(*ast.GenDecl)