imports: redesign fixImports

Redesign fixImports to have a clearer workflow, and hopefully create
clear places to plug in go/packages. This change is mostly
performance/functionality neutral, but does clean up some corner cases.

The new flow centers around the pass type, which encapsulates the
process of loading information about the current code, adding possible
new imports, and trying to apply them. I'm hoping that it's easy to
understand what's happening just by reading fixImports, and that new
sources of information (e.g. a network service) fit well into that flow.

Where possible, I left the functions near where they were extracted in
hopes of making review easier, but it's probably not going to be easy.
Sorry. I might move them into a more reasonable order in a followup CL.

Notable modifications:
- The stdlib cache is restructured to match pass' internal storage.
- Sibling imports with conflicting names are considered.
- Package name lookups are batched, hopefully making it easier to plug
in go/packages.

Questions that might be worth answering:
- Should findImportGoPath really scan $GOROOT? Unless the user is working
on a development copy, it's totally redundant with the cache.
- What is the best way to combine candidates from multiple sources?
Right now the first one wins, and findStdlibCandidates relies on that
to get crypto/rand ahead of math/rand.
- In the third pass, should it assume sibling imports or should it
actually go load the exports? It didn't load them before, but that seems
arbitrary.

Change-Id: Ie4ad0b69bfbe9b16883f2b0517b1278575c9f540
Reviewed-on: https://go-review.googlesource.com/c/150339
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

imports/fix.go

@@ -74,42 +74,30 @@ func importGroup(importPath string) int {
 	return 0
 }
 
-// importInfo is a summary of information about one import.
+// An importInfo represents a single import statement.
 type importInfo struct {
-	Path  string // full import path (e.g. "crypto/rand")
-	Alias string // import alias, if present (e.g. "crand")
+	importPath string // import path, e.g. "crypto/rand".
+	name       string // import name, e.g. "crand", or "" if none.
 }
 
-// packageInfo is a summary of features found in a package.
+// A packageInfo represents what's known about a package.
 type packageInfo struct {
-	Globals map[string]bool       // symbol => true
-	Imports map[string]importInfo // pkg base name or alias => info
-	// refs are a set of package references currently satisfied by imports.
-	// first key: either base package (e.g. "fmt") or renamed package
-	// second key: referenced package symbol (e.g. "Println")
-	Refs map[string]map[string]bool
+	name    string          // discovered package name.
+	exports map[string]bool // known exports.
 }
 
-// dirPackageInfo exposes the dirPackageInfoFile function so that it can be overridden.
-var dirPackageInfo = dirPackageInfoFile
-
-// dirPackageInfoFile gets information from other files in the package.
-func dirPackageInfoFile(pkgName, srcDir, filename string) (*packageInfo, error) {
+// parseOtherFiles parses all the Go files in srcDir except filename, including
+// test files if filename looks like a test.
+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)
 	if err != nil {
-		return nil, err
+		return nil
 	}
 
-	info := &packageInfo{
-		Globals: make(map[string]bool),
-		Imports: make(map[string]importInfo),
-		Refs:    make(map[string]map[string]bool),
-	}
-
-	visitor := collectReferences(info.Refs)
+	var files []*ast.File
 	for _, fi := range packageFileInfos {
 		if fi.Name() == fileBase || !strings.HasSuffix(fi.Name(), ".go") {
 			continue
@@ -118,13 +106,20 @@ func dirPackageInfoFile(pkgName, srcDir, filename string) (*packageInfo, error)
 			continue
 		}
 
-		fileSet := token.NewFileSet()
-		root, err := parser.ParseFile(fileSet, filepath.Join(srcDir, fi.Name()), nil, 0)
+		f, err := parser.ParseFile(fset, filepath.Join(srcDir, fi.Name()), nil, 0)
 		if err != nil {
 			continue
 		}
 
-		for _, decl := range root.Decls {
+		files = append(files, f)
+	}
+
+	return files
+}
+
+// addGlobals puts the names of package vars into the provided map.
+func addGlobals(f *ast.File, globals map[string]bool) {
+	for _, decl := range f.Decls {
 		genDecl, ok := decl.(*ast.GenDecl)
 		if !ok {
 			continue
@@ -135,28 +130,16 @@ func dirPackageInfoFile(pkgName, srcDir, filename string) (*packageInfo, error)
 			if !ok {
 				continue
 			}
-				info.Globals[valueSpec.Names[0].Name] = true
+			globals[valueSpec.Names[0].Name] = true
+		}
 	}
 }
 
-		for _, imp := range root.Imports {
-			impInfo := importInfo{Path: strings.Trim(imp.Path.Value, `"`)}
-			name := path.Base(impInfo.Path)
-			if imp.Name != nil {
-				name = strings.Trim(imp.Name.Name, `"`)
-				impInfo.Alias = name
-			}
-			info.Imports[name] = impInfo
-		}
+// collectReferences builds a map of selector expressions, from
+// left hand side (X) to a set of right hand sides (Sel).
+func collectReferences(f *ast.File) map[string]map[string]bool {
+	refs := map[string]map[string]bool{}
 
-		ast.Walk(visitor, root)
-	}
-	return info, nil
-}
-
-// collectReferences returns a visitor that collects all exported package
-// references
-func collectReferences(refs map[string]map[string]bool) visitFn {
 	var visitor visitFn
 	visitor = func(node ast.Node) ast.Visitor {
 		if node == nil {
@@ -184,113 +167,308 @@ func collectReferences(refs map[string]map[string]bool) visitFn {
 		}
 		return visitor
 	}
-	return visitor
+	ast.Walk(visitor, f)
+	return refs
 }
 
-func fixImports(fset *token.FileSet, f *ast.File, filename string) (added []string, err error) {
-	// refs are a set of possible package references currently unsatisfied by imports.
-	// first key: either base package (e.g. "fmt") or renamed package
-	// second key: referenced package symbol (e.g. "Println")
-	refs := make(map[string]map[string]bool)
+// collectImports returns all the imports in f, keyed by their package name as
+// determined by pathToName. Unnamed imports (., _) and "C" are ignored.
+func collectImports(f *ast.File) []*importInfo {
+	var imports []*importInfo
+	for _, imp := range f.Imports {
+		var name string
+		if imp.Name != nil {
+			name = imp.Name.Name
+		}
+		if imp.Path.Value == `"C"` || name == "_" || name == "." {
+			continue
+		}
+		path := strings.Trim(imp.Path.Value, `"`)
+		imports = append(imports, &importInfo{
+			name:       name,
+			importPath: path,
+		})
+	}
+	return imports
+}
 
-	// decls are the current package imports. key is base package or renamed package.
-	decls := make(map[string]*ast.ImportSpec)
+// findMissingImport searches pass's candidates for an import that provides
+// pkg, containing all of syms.
+func (p *pass) findMissingImport(pkg string, syms map[string]bool) *importInfo {
+	for _, candidate := range p.candidates {
+		pkgInfo, ok := p.knownPackages[candidate.importPath]
+		if !ok {
+			continue
+		}
+		if candidate.name != "" && candidate.name != pkg {
+			continue
+		}
+		if pkgInfo.name != pkg {
+			continue
+		}
 
+		allFound := true
+		for right := range syms {
+			if !pkgInfo.exports[right] {
+				allFound = false
+				break
+			}
+		}
+
+		if allFound {
+			return candidate
+		}
+	}
+	return nil
+}
+
+// A pass contains all the inputs and state necessary to fix a file's imports.
+// It can be modified in some ways during use; see comments below.
+type pass struct {
+	// Inputs. These must be set before a call to load, and not modified after.
+	fset       *token.FileSet                          // fset used to parse f and its siblings.
+	f          *ast.File                               // the file being fixed.
+	otherFiles []*ast.File                             // sibling files.
+	srcDir     string                                  // the directory containing f.
+	pathToName func(path string, srcDir string) string // the function to use to resolve an import path to a package name.
+
+	// Intermediate state, generated by load.
+	existingImports map[string]*importInfo
+	missing         map[string]map[string]bool
+	used            map[*importInfo]bool
+
+	// Inputs to fix. These can be augmented between successive fix calls.
+	lastTry       bool                    // indicates that this is the last call and fix should clean up as best it can.
+	candidates    []*importInfo           // candidate imports in priority order.
+	knownPackages map[string]*packageInfo // information about all known packages.
+}
+
+// loadPackageNames saves the package names for everything referenced by imports.
+func (p *pass) loadPackageNames(imports []*importInfo) {
+	for _, imp := range imports {
+		if _, ok := p.knownPackages[imp.importPath]; ok {
+			continue
+		}
+		p.knownPackages[imp.importPath] = &packageInfo{
+			name:    p.pathToName(imp.importPath, p.srcDir),
+			exports: map[string]bool{},
+		}
+	}
+}
+
+// importIdentifier returns the indentifier that imp will introduce.
+func (p *pass) importIdentifier(imp *importInfo) string {
+	if imp.name != "" {
+		return imp.name
+	}
+	return p.knownPackages[imp.importPath].name
+}
+
+// load reads in everything necessary to run a pass, and reports whether the
+// file already has all the imports it needs. It fills in p.missing with the
+// file's missing symbols, if any, or removes unused imports if not.
+func (p *pass) load() bool {
+	p.knownPackages = map[string]*packageInfo{}
+	p.missing = map[string]map[string]bool{}
+	p.used = map[*importInfo]bool{}
+	p.existingImports = map[string]*importInfo{}
+
+	// Load basic information about the file in question.
+	allReferences := collectReferences(p.f)
+
+	// Load stuff from other files in the same package:
+	// global variables so we know they don't need resolving, and imports
+	// that we might want to mimic.
+	globals := map[string]bool{}
+	for _, otherFile := range p.otherFiles {
+		addGlobals(otherFile, globals)
+		p.candidates = append(p.candidates, collectImports(otherFile)...)
+	}
+
+	// Resolve all the import paths we've seen to package names, and store
+	// f's imports by the identifier they introduce.
+	imports := collectImports(p.f)
+	p.loadPackageNames(append(imports, p.candidates...))
+	for _, imp := range imports {
+		p.existingImports[p.importIdentifier(imp)] = imp
+	}
+
+	// Find missing references and mark used imports used.
+	for left, rights := range allReferences {
+		if globals[left] {
+			continue
+		}
+		imp, ok := p.existingImports[left]
+		if !ok {
+			p.missing[left] = rights
+			continue
+		}
+		p.used[imp] = true
+	}
+	if len(p.missing) != 0 {
+		return false
+	}
+
+	return p.fix()
+}
+
+// fix attempts to satisfy missing imports using p.candidates. If it finds
+// everything, or if p.lastTry is true, it adds the imports it found,
+// removes anything unused, and returns true.
+func (p *pass) fix() bool {
+	// Find missing imports.
+	var selected []*importInfo
+	for left, rights := range p.missing {
+		if imp := p.findMissingImport(left, rights); imp != nil {
+			selected = append(selected, imp)
+		}
+	}
+
+	if !p.lastTry && len(selected) != len(p.missing) {
+		return false
+	}
+
+	// Found everything, or giving up. Add the new imports and remove any unused.
+	for _, imp := range p.existingImports {
+		if !p.used[imp] {
+			astutil.DeleteNamedImport(p.fset, p.f, imp.name, imp.importPath)
+		}
+	}
+	for _, imp := range selected {
+		astutil.AddNamedImport(p.fset, p.f, imp.name, imp.importPath)
+	}
+
+	return true
+}
+
+// assumeSiblingImportsValid assumes that siblings' use of packages is valid,
+// adding the exports they use.
+func (p *pass) assumeSiblingImportsValid() {
+	for _, f := range p.otherFiles {
+		refs := collectReferences(f)
+		imports := collectImports(f)
+		importsByName := map[string]*importInfo{}
+		for _, imp := range imports {
+			importsByName[p.importIdentifier(imp)] = imp
+		}
+		for left, rights := range refs {
+			if imp, ok := importsByName[left]; ok {
+				if _, ok := stdlib[imp.importPath]; ok {
+					// We have the stdlib in memory; no need to guess.
+					continue
+				}
+				p.addCandidate(imp, &packageInfo{
+					// no name; we already know it.
+					exports: rights,
+				})
+			}
+		}
+	}
+}
+
+// addCandidate adds a candidate import to p, and merges in the information
+// in pkg.
+func (p *pass) addCandidate(imp *importInfo, pkg *packageInfo) {
+	p.candidates = append(p.candidates, imp)
+	if existing, ok := p.knownPackages[imp.importPath]; ok {
+		for export := range pkg.exports {
+			existing.exports[export] = true
+		}
+	} else {
+		p.knownPackages[imp.importPath] = pkg
+	}
+}
+
+func fixImports(fset *token.FileSet, f *ast.File, filename string) error {
 	abs, err := filepath.Abs(filename)
 	if err != nil {
-		return nil, err
+		return err
 	}
 	srcDir := filepath.Dir(abs)
 	if Debug {
 		log.Printf("fixImports(filename=%q), abs=%q, srcDir=%q ...", filename, abs, srcDir)
 	}
 
-	var packageInfo *packageInfo
-	var loadedPackageInfo bool
+	// First pass: looking only at f, and using the naive algorithm to
+	// derive package names from import paths, see if the file is already
+	// complete. We can't add any imports yet, because we don't know
+	// if missing references are actually package vars.
+	p := &pass{fset: fset, f: f, srcDir: srcDir}
+	p.pathToName = importPathToNameBasic
+	if p.load() {
+		return nil
+	}
 
-	// collect potential uses of packages.
-	var visitor visitFn
-	visitor = visitFn(func(node ast.Node) ast.Visitor {
-		if node == nil {
-			return visitor
-		}
-		switch v := node.(type) {
-		case *ast.ImportSpec:
-			if v.Name != nil {
-				decls[v.Name.Name] = v
-				break
-			}
-			ipath := strings.Trim(v.Path.Value, `"`)
-			if ipath == "C" {
-				break
-			}
-			local := importPathToName(ipath, srcDir)
-			decls[local] = v
-		case *ast.SelectorExpr:
-			xident, ok := v.X.(*ast.Ident)
-			if !ok {
-				break
-			}
-			if xident.Obj != nil {
-				// if the parser can resolve it, it's not a package ref
-				break
-			}
-			pkgName := xident.Name
-			if refs[pkgName] == nil {
-				refs[pkgName] = make(map[string]bool)
-			}
-			if !loadedPackageInfo {
-				loadedPackageInfo = true
-				packageInfo, _ = dirPackageInfo(f.Name.Name, srcDir, filename)
-			}
-			if decls[pkgName] == nil && (packageInfo == nil || !packageInfo.Globals[pkgName]) {
-				refs[pkgName][v.Sel.Name] = true
-			}
-		}
-		return visitor
-	})
-	ast.Walk(visitor, f)
+	otherFiles := parseOtherFiles(fset, srcDir, filename)
 
-	// Nil out any unused ImportSpecs, to be removed in following passes
-	unusedImport := map[string]string{}
-	for pkg, is := range decls {
-		if refs[pkg] == nil && pkg != "_" && pkg != "." {
-			name := ""
-			if is.Name != nil {
-				name = is.Name.Name
+	// Second pass: add information from other files in the same package,
+	// like their package vars and imports.
+	p = &pass{fset: fset, f: f, srcDir: srcDir}
+	p.pathToName = importPathToNameBasic
+	p.otherFiles = otherFiles
+	if p.load() {
+		return nil
 	}
-			unusedImport[strings.Trim(is.Path.Value, `"`)] = name
+
+	// Now we can try adding imports from the stdlib.
+	p.assumeSiblingImportsValid()
+	addStdlibCandidates(p, p.missing)
+	if p.fix() {
+		return nil
 	}
+
+	// Third pass: get real package names where we had previously used
+	// the naive algorithm.
+	p = &pass{fset: fset, f: f, srcDir: srcDir}
+	p.pathToName = importPathToName
+	p.otherFiles = otherFiles
+	if p.load() {
+		return nil
 	}
-	for ipath, name := range unusedImport {
-		if ipath == "C" {
-			// Don't remove cgo stuff.
+
+	addStdlibCandidates(p, p.missing)
+	p.assumeSiblingImportsValid()
+	if p.fix() {
+		return nil
+	}
+
+	// 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 := addGoPathCandidates(p, p.missing, filename); err != nil {
+		return err
+	}
+
+	p.lastTry = true
+	p.fix()
+	return nil
+}
+
+func addStdlibCandidates(pass *pass, refs map[string]map[string]bool) {
+	add := func(pkg string) {
+		pass.addCandidate(
+			&importInfo{importPath: pkg},
+			&packageInfo{name: path.Base(pkg), exports: stdlib[pkg]})
+	}
+	for left := range refs {
+		if left == "rand" {
+			// Make sure we try crypto/rand before math/rand.
+			add("crypto/rand")
+			add("math/rand")
 			continue
 		}
-		astutil.DeleteNamedImport(fset, f, name, ipath)
+		for importPath := range stdlib {
+			if path.Base(importPath) == left {
+				add(importPath)
+			}
 		}
-
-	for pkgName, symbols := range refs {
-		if len(symbols) == 0 {
-			// skip over packages already imported
-			delete(refs, pkgName)
 	}
 }
 
-	// Fast path, all references already imported.
-	if len(refs) == 0 {
-		return nil, nil
-	}
-
-	// Can assume this will be necessary in all cases now.
-	if !loadedPackageInfo {
-		packageInfo, _ = dirPackageInfo(f.Name.Name, srcDir, filename)
-	}
-
+func addGoPathCandidates(pass *pass, refs map[string]map[string]bool, filename string) error {
 	// Search for imports matching potential package references.
 	type result struct {
-		ipath string // import path
-		name  string // optional name to rename import as
+		imp *importInfo
+		pkg *packageInfo
 	}
 	results := make(chan result, len(refs))
 
@@ -309,20 +487,8 @@ func fixImports(fset *token.FileSet, f *ast.File, filename string) (added []stri
 		go func(pkgName string, symbols map[string]bool) {
 			defer wg.Done()
 
-			if packageInfo != nil {
-				sibling := packageInfo.Imports[pkgName]
-				if sibling.Path != "" {
-					refs := packageInfo.Refs[pkgName]
-					for symbol := range symbols {
-						if refs[symbol] {
-							results <- result{ipath: sibling.Path, name: sibling.Alias}
-							return
-						}
-					}
-				}
-			}
+			ipath, _, err := findImport(ctx, pkgName, symbols, filename)
 
-			ipath, rename, err := findImport(ctx, pkgName, symbols, filename)
 			if err != nil {
 				firstErrOnce.Do(func() {
 					firstErr = err
@@ -335,12 +501,20 @@ func fixImports(fset *token.FileSet, f *ast.File, filename string) (added []stri
 				return // No matching package.
 			}
 
-			r := result{ipath: ipath}
-			if rename {
-				r.name = pkgName
+			imp := &importInfo{
+				importPath: ipath,
 			}
-			results <- r
-			return
+			// If the package name isn't what you'd expect looking
+			// at the import path, add an explicit name.
+			if path.Base(ipath) != pkgName {
+				imp.name = pkgName
+			}
+
+			pkg := &packageInfo{
+				name:    pkgName,
+				exports: symbols,
+			}
+			results <- result{imp, pkg}
 		}(pkgName, symbols)
 	}
 	go func() {
@@ -349,18 +523,9 @@ func fixImports(fset *token.FileSet, f *ast.File, filename string) (added []stri
 	}()
 
 	for result := range results {
-		if result.name != "" {
-			astutil.AddNamedImport(fset, f, result.name, result.ipath)
-		} else {
-			astutil.AddImport(fset, f, result.ipath)
+		pass.addCandidate(result.imp, result.pkg)
 	}
-		added = append(added, result.ipath)
-	}
-
-	if firstErr != nil {
-		return nil, firstErr
-	}
-	return added, nil
+	return firstErr
 }
 
 // importPathToName returns the package name for the given import path.
@@ -385,8 +550,8 @@ func importPathToNameBasic(importPath, srcDir string) (packageName string) {
 // If there's a problem, it falls back to using importPathToNameBasic.
 func importPathToNameGoPath(importPath, srcDir string) (packageName string) {
 	// Fast path for standard library without going to disk.
-	if pkg, ok := stdImportPackage[importPath]; ok {
-		return pkg
+	if _, ok := stdlib[importPath]; ok {
+		return path.Base(importPath) // stdlib packages always match their paths.
 	}
 
 	pkgName, err := importPathToNameGoPathParse(importPath, srcDir)
@@ -455,18 +620,6 @@ func importPathToNameGoPathParse(importPath, srcDir string) (packageName string,
 	return "", fmt.Errorf("no importable package found in %d Go files", nfile)
 }
 
-var stdImportPackage = map[string]string{} // "net/http" => "http"
-
-func init() {
-	// Nothing in the standard library has a package name not
-	// matching its import base name.
-	for _, pkg := range stdlib {
-		if _, ok := stdImportPackage[pkg]; !ok {
-			stdImportPackage[pkg] = path.Base(pkg)
-		}
-	}
-}
-
 // Directory-scanning state.
 var (
 	// scanOnce guards calling scanGoDirs and assigning dirScan
@@ -651,12 +804,7 @@ func loadExportsGoPath(ctx context.Context, expectPackage, dir string) (map[stri
 // This is declared as a variable rather than a function so goimports
 // can be easily extended by adding a file with an init function.
 //
-// The rename value tells goimports whether to use the package name as
-// a local qualifier in an import. For example, if findImports("pkg",
-// "X") returns ("foo/bar", rename=true), then goimports adds the
-// import line:
-// 	import pkg "foo/bar"
-// to satisfy uses of pkg.X in the file.
+// The rename value is ignored.
 var findImport func(ctx context.Context, pkgName string, symbols map[string]bool, filename string) (foundPkg string, rename bool, err error) = findImportGoPath
 
 // findImportGoPath is the normal implementation of findImport.
@@ -667,29 +815,6 @@ func findImportGoPath(ctx context.Context, pkgName string, symbols map[string]bo
 		return "", false, err
 	}
 	pkgDir = filepath.Dir(pkgDir)
-
-	// Fast path for the standard library.
-	// In the common case we hopefully never have to scan the GOPATH, which can
-	// be slow with moving disks.
-	if pkg, ok := findImportStdlib(pkgName, symbols); ok {
-		return pkg, false, nil
-	}
-	if pkgName == "rand" && symbols["Read"] {
-		// Special-case rand.Read.
-		//
-		// If findImportStdlib didn't find it above, don't go
-		// searching for it, lest it find and pick math/rand
-		// in GOROOT (new as of Go 1.6)
-		//
-		// crypto/rand is the safer choice.
-		return "", false, nil
-	}
-
-	// TODO(sameer): look at the import lines for other Go files in the
-	// local directory, since the user is likely to import the same packages
-	// in the current Go file.  Return rename=true when the other Go files
-	// use a renamed package that's also used in the current file.
-
 	// Scan $GOROOT and each $GOPATH.
 	scanOnce.Do(func() { dirScan = scanGoDirs() })
 
@@ -772,10 +897,7 @@ func findImportGoPath(ctx context.Context, pkgName string, symbols map[string]bo
 		if pkg == nil {
 			continue
 		}
-		// If the package name in the source doesn't match the import path's base,
-		// return true so the rewriter adds a name (import foo "github.com/bar/go-foo")
-		needsRename := path.Base(pkg.importPath) != pkgName
-		return pkg.importPathShort, needsRename, nil
+		return pkg.importPathShort, false, nil
 	}
 	return "", false, nil
 }
@@ -910,25 +1032,3 @@ type visitFn func(node ast.Node) ast.Visitor
 func (fn visitFn) Visit(node ast.Node) ast.Visitor {
 	return fn(node)
 }
-
-func findImportStdlib(shortPkg string, symbols map[string]bool) (importPath string, ok bool) {
-	for symbol := range symbols {
-		key := shortPkg + "." + symbol
-		path := stdlib[key]
-		if path == "" {
-			if key == "rand.Read" {
-				continue
-			}
-			return "", false
-		}
-		if importPath != "" && importPath != path {
-			// Ambiguous. Symbols pointed to different things.
-			return "", false
-		}
-		importPath = path
-	}
-	if importPath == "" && shortPkg == "rand" && symbols["Read"] {
-		return "crypto/rand", true
-	}
-	return importPath, importPath != ""
-}

imports/imports.go

@@ -63,8 +63,7 @@ func Process(filename string, src []byte, opt *Options) ([]byte, error) {
 	}
 
 	if !opt.FormatOnly {
-		_, err = fixImports(fileSet, file, filename)
-		if err != nil {
+		if err := fixImports(fileSet, file, filename); err != nil {
 			return nil, err
 		}
 	}

imports/mkstdlib.go

@@ -14,7 +14,6 @@ import (
 	"io/ioutil"
 	"log"
 	"os"
-	"path"
 	"path/filepath"
 	"regexp"
 	"runtime"
@@ -36,6 +35,8 @@ func api(base string) string {
 
 var sym = regexp.MustCompile(`^pkg (\S+).*?, (?:var|func|type|const) ([A-Z]\w*)`)
 
+var unsafeSyms = map[string]bool{"Alignof": true, "ArbitraryType": true, "Offsetof": true, "Pointer": true, "Sizeof": true}
+
 func main() {
 	var buf bytes.Buffer
 	outf := func(format string, args ...interface{}) {
@@ -43,7 +44,7 @@ func main() {
 	}
 	outf("// Code generated by mkstdlib.go. DO NOT EDIT.\n\n")
 	outf("package imports\n")
-	outf("var stdlib = map[string]string{\n")
+	outf("var stdlib = map[string]map[string]bool{\n")
 	f := io.MultiReader(
 		mustOpen(api("go1.txt")),
 		mustOpen(api("go1.1.txt")),
@@ -59,9 +60,12 @@ func main() {
 		mustOpen(api("go1.11.txt")),
 	)
 	sc := bufio.NewScanner(f)
-	fullImport := map[string]string{} // "zip.NewReader" => "archive/zip"
-	ambiguous := map[string]bool{}
-	var keys []string
+
+	pkgs := map[string]map[string]bool{
+		"unsafe": unsafeSyms,
+	}
+	paths := []string{"unsafe"}
+
 	for sc.Scan() {
 		l := sc.Text()
 		has := func(v string) bool { return strings.Contains(l, v) }
@@ -69,32 +73,31 @@ func main() {
 			continue
 		}
 		if m := sym.FindStringSubmatch(l); m != nil {
-			full := m[1]
-			key := path.Base(full) + "." + m[2]
-			if exist, ok := fullImport[key]; ok {
-				if exist != full {
-					ambiguous[key] = true
-				}
-			} else {
-				fullImport[key] = full
-				keys = append(keys, key)
+			path, sym := m[1], m[2]
+
+			if _, ok := pkgs[path]; !ok {
+				pkgs[path] = map[string]bool{}
+				paths = append(paths, path)
 			}
+			pkgs[path][sym] = true
 		}
 	}
 	if err := sc.Err(); err != nil {
 		log.Fatal(err)
 	}
-	sort.Strings(keys)
-	for _, key := range keys {
-		if ambiguous[key] {
-			outf("\t// %q is ambiguous\n", key)
-		} else {
-			outf("\t%q: %q,\n", key, fullImport[key])
+	sort.Strings(paths)
+	for _, path := range paths {
+		outf("\t%q: map[string]bool{\n", path)
+		pkg := pkgs[path]
+		var syms []string
+		for sym := range pkg {
+			syms = append(syms, sym)
 		}
+		sort.Strings(syms)
+		for _, sym := range syms {
+			outf("\t\t%q: true,\n", sym)
 		}
-	outf("\n")
-	for _, sym := range [...]string{"Alignof", "ArbitraryType", "Offsetof", "Pointer", "Sizeof"} {
-		outf("\t%q: %q,\n", "unsafe."+sym, "unsafe")
+		outf("},\n")
 	}
 	outf("}\n")
 	fmtbuf, err := format.Source(buf.Bytes())