go/analysis/passes/cgocall/cgocall.go - tools - Git at Google

 // Copyright 2015 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 cgocall defines an Analyzer that detects some violations of
 // the cgo pointer passing rules.
 package cgocall

 import (
 	"fmt"
 	"go/ast"
 	"go/format"
 	"go/parser"
 	"go/token"
 	"go/types"
 	"log"
 	"os"
 	"strconv"

 	"golang.org/x/tools/go/analysis"
 	"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
 )

 const debug = false

 const Doc = `detect some violations of the cgo pointer passing rules

 Check for invalid cgo pointer passing.
 This looks for code that uses cgo to call C code passing values
 whose types are almost always invalid according to the cgo pointer
 sharing rules.
 Specifically, it warns about attempts to pass a Go chan, map, func,
 or slice to C, either directly, or via a pointer, array, or struct.`

 var Analyzer = &analysis.Analyzer{
 	Name:             "cgocall",
 	Doc:              Doc,
 	RunDespiteErrors: true,
 	Run:              run,
 }

 func run(pass *analysis.Pass) (interface{}, error) {
 	if !analysisutil.Imports(pass.Pkg, "runtime/cgo") {
 		return nil, nil // doesn't use cgo
 	}

 	cgofiles, info, err := typeCheckCgoSourceFiles(pass.Fset, pass.Pkg, pass.Files, pass.TypesInfo, pass.TypesSizes)
 	if err != nil {
 		return nil, err
 	}
 	for _, f := range cgofiles {
 		checkCgo(pass.Fset, f, info, pass.Reportf)
 	}
 	return nil, nil
 }

 func checkCgo(fset *token.FileSet, f *ast.File, info *types.Info, reportf func(token.Pos, string, ...interface{})) {
 	ast.Inspect(f, func(n ast.Node) bool {
 		call, ok := n.(*ast.CallExpr)
 		if !ok {
 			return true
 		}

 		// Is this a C.f() call?
 		var name string
 		if sel, ok := analysisutil.Unparen(call.Fun).(*ast.SelectorExpr); ok {
 			if id, ok := sel.X.(*ast.Ident); ok && id.Name == "C" {
 				name = sel.Sel.Name
 			}
 		}
 		if name == "" {
 			return true // not a call we need to check
 		}

 		// A call to C.CBytes passes a pointer but is always safe.
 		if name == "CBytes" {
 			return true
 		}

 		if debug {
 			log.Printf("%s: call to C.%s", fset.Position(call.Lparen), name)
 		}

 		for _, arg := range call.Args {
 			if !typeOKForCgoCall(cgoBaseType(info, arg), make(map[types.Type]bool)) {
 				reportf(arg.Pos(), "possibly passing Go type with embedded pointer to C")
 				break
 			}

 			// Check for passing the address of a bad type.
 			if conv, ok := arg.(*ast.CallExpr); ok && len(conv.Args) == 1 &&
 				isUnsafePointer(info, conv.Fun) {
 				arg = conv.Args[0]
 			}
 			if u, ok := arg.(*ast.UnaryExpr); ok && u.Op == token.AND {
 				if !typeOKForCgoCall(cgoBaseType(info, u.X), make(map[types.Type]bool)) {
 					reportf(arg.Pos(), "possibly passing Go type with embedded pointer to C")
 					break
 				}
 			}
 		}
 		return true
 	})
 }

 // typeCheckCgoSourceFiles returns type-checked syntax trees for the raw
 // cgo files of a package (those that import "C"). Such files are not
 // Go, so there may be gaps in type information around C.f references.
 //
 // This checker was initially written in vet to inspect raw cgo source
 // files using partial type information. However, Analyzers in the new
 // analysis API are presented with the type-checked, "cooked" Go ASTs
 // resulting from cgo-processing files, so we must choose between
 // working with the cooked file generated by cgo (which was tried but
 // proved fragile) or locating the raw cgo file (e.g. from //line
 // directives) and working with that, as we now do.
 //
 // Specifically, we must type-check the raw cgo source files (or at
 // least the subtrees needed for this analyzer) in an environment that
 // simulates the rest of the already type-checked package.
 //
 // For example, for each raw cgo source file in the original package,
 // such as this one:
 //
 // 	package p
 // 	import "C"
 //	import "fmt"
 //	type T int
 //	const k = 3
 //	var x, y = fmt.Println()
 //	func f() { ... }
 //	func g() { ... C.malloc(k) ... }
 //	func (T) f(int) string { ... }
 //
 // we synthesize a new ast.File, shown below, that dot-imports the
 // original "cooked" package using a special name ("·this·"), so that all
 // references to package members resolve correctly. (References to
 // unexported names cause an "unexported" error, which we ignore.)
 //
 // To avoid shadowing names imported from the cooked package,
 // package-level declarations in the new source file are modified so
 // that they do not declare any names.
 // (The cgocall analysis is concerned with uses, not declarations.)
 // Specifically, type declarations are discarded;
 // all names in each var and const declaration are blanked out;
 // each method is turned into a regular function by turning
 // the receiver into the first parameter;
 // and all functions are renamed to "_".
 //
 // 	package p
 // 	import . "·this·" // declares T, k, x, y, f, g, T.f
 // 	import "C"
 //	import "fmt"
 //	const _ = 3
 //	var _, _ = fmt.Println()
 //	func _() { ... }
 //	func _() { ... C.malloc(k) ... }
 //	func _(T, int) string { ... }
 //
 // In this way, the raw function bodies and const/var initializer
 // expressions are preserved but refer to the "cooked" objects imported
 // from "·this·", and none of the transformed package-level declarations
 // actually declares anything. In the example above, the reference to k
 // in the argument of the call to C.malloc resolves to "·this·".k, which
 // has an accurate type.
 //
 // This approach could in principle be generalized to more complex
 // analyses on raw cgo files. One could synthesize a "C" package so that
 // C.f would resolve to "·this·"._C_func_f, for example. But we have
 // limited ourselves here to preserving function bodies and initializer
 // expressions since that is all that the cgocall analyzer needs.
 //
 func typeCheckCgoSourceFiles(fset *token.FileSet, pkg *types.Package, files []*ast.File, info *types.Info, sizes types.Sizes) ([]*ast.File, *types.Info, error) {
 	const thispkg = "·this·"

 	// Which files are cgo files?
 	var cgoFiles []*ast.File
 	importMap := map[string]*types.Package{thispkg: pkg}
 	for _, raw := range files {
 		// If f is a cgo-generated file, Position reports
 		// the original file, honoring //line directives.
 		filename := fset.Position(raw.Pos()).Filename
 		f, err := parser.ParseFile(fset, filename, nil, parser.Mode(0))
 		if err != nil {
 			return nil, nil, fmt.Errorf("can't parse raw cgo file: %v", err)
 		}
 		found := false
 		for _, spec := range f.Imports {
 			if spec.Path.Value == `"C"` {
 				found = true
 				break
 			}
 		}
 		if !found {
 			continue // not a cgo file
 		}

 		// Record the original import map.
 		for _, spec := range raw.Imports {
 			path, _ := strconv.Unquote(spec.Path.Value)
 			importMap[path] = imported(info, spec)
 		}

 		// Add special dot-import declaration:
 		//    import . "·this·"
 		var decls []ast.Decl
 		decls = append(decls, &ast.GenDecl{
 			Tok: token.IMPORT,
 			Specs: []ast.Spec{
 				&ast.ImportSpec{
 					Name: &ast.Ident{Name: "."},
 					Path: &ast.BasicLit{
 						Kind:  token.STRING,
 						Value: strconv.Quote(thispkg),
 					},
 				},
 			},
 		})

 		// Transform declarations from the raw cgo file.
 		for _, decl := range f.Decls {
 			switch decl := decl.(type) {
 			case *ast.GenDecl:
 				switch decl.Tok {
 				case token.TYPE:
 					// Discard type declarations.
 					continue
 				case token.IMPORT:
 					// Keep imports.
 				case token.VAR, token.CONST:
 					// Blank the declared var/const names.
 					for _, spec := range decl.Specs {
 						spec := spec.(*ast.ValueSpec)
 						for i := range spec.Names {
 							spec.Names[i].Name = "_"
 						}
 					}
 				}
 			case *ast.FuncDecl:
 				// Blank the declared func name.
 				decl.Name.Name = "_"

 				// Turn a method receiver:  func (T) f(P) R {...}
 				// into regular parameter:  func _(T, P) R {...}
 				if decl.Recv != nil {
 					var params []*ast.Field
 					params = append(params, decl.Recv.List...)
 					params = append(params, decl.Type.Params.List...)
 					decl.Type.Params.List = params
 					decl.Recv = nil
 				}
 			}
 			decls = append(decls, decl)
 		}
 		f.Decls = decls
 		if debug {
 			format.Node(os.Stderr, fset, f) // debugging
 		}
 		cgoFiles = append(cgoFiles, f)
 	}
 	if cgoFiles == nil {
 		return nil, nil, nil // nothing to do (can't happen?)
 	}

 	// Type-check the synthetic files.
 	tc := &types.Config{
 		FakeImportC: true,
 		Importer: importerFunc(func(path string) (*types.Package, error) {
 			return importMap[path], nil
 		}),
 		Sizes: sizes,
 		Error: func(error) {}, // ignore errors (e.g. unused import)
 	}

 	// It's tempting to record the new types in the
 	// existing pass.TypesInfo, but we don't own it.
 	altInfo := &types.Info{
 		Types: make(map[ast.Expr]types.TypeAndValue),
 	}
 	tc.Check(pkg.Path(), fset, cgoFiles, altInfo)

 	return cgoFiles, altInfo, nil
 }

 // cgoBaseType tries to look through type conversions involving
 // unsafe.Pointer to find the real type. It converts:
 //   unsafe.Pointer(x) => x
 //   *(*unsafe.Pointer)(unsafe.Pointer(&x)) => x
 func cgoBaseType(info *types.Info, arg ast.Expr) types.Type {
 	switch arg := arg.(type) {
 	case *ast.CallExpr:
 		if len(arg.Args) == 1 && isUnsafePointer(info, arg.Fun) {
 			return cgoBaseType(info, arg.Args[0])
 		}
 	case *ast.StarExpr:
 		call, ok := arg.X.(*ast.CallExpr)
 		if !ok || len(call.Args) != 1 {
 			break
 		}
 		// Here arg is *f(v).
 		t := info.Types[call.Fun].Type
 		if t == nil {
 			break
 		}
 		ptr, ok := t.Underlying().(*types.Pointer)
 		if !ok {
 			break
 		}
 		// Here arg is *(*p)(v)
 		elem, ok := ptr.Elem().Underlying().(*types.Basic)
 		if !ok || elem.Kind() != types.UnsafePointer {
 			break
 		}
 		// Here arg is *(*unsafe.Pointer)(v)
 		call, ok = call.Args[0].(*ast.CallExpr)
 		if !ok || len(call.Args) != 1 {
 			break
 		}
 		// Here arg is *(*unsafe.Pointer)(f(v))
 		if !isUnsafePointer(info, call.Fun) {
 			break
 		}
 		// Here arg is *(*unsafe.Pointer)(unsafe.Pointer(v))
 		u, ok := call.Args[0].(*ast.UnaryExpr)
 		if !ok || u.Op != token.AND {
 			break
 		}
 		// Here arg is *(*unsafe.Pointer)(unsafe.Pointer(&v))
 		return cgoBaseType(info, u.X)
 	}

 	return info.Types[arg].Type
 }

 // typeOKForCgoCall reports whether the type of arg is OK to pass to a
 // C function using cgo. This is not true for Go types with embedded
 // pointers. m is used to avoid infinite recursion on recursive types.
 func typeOKForCgoCall(t types.Type, m map[types.Type]bool) bool {
 	if t == nil || m[t] {
 		return true
 	}
 	m[t] = true
 	switch t := t.Underlying().(type) {
 	case *types.Chan, *types.Map, *types.Signature, *types.Slice:
 		return false
 	case *types.Pointer:
 		return typeOKForCgoCall(t.Elem(), m)
 	case *types.Array:
 		return typeOKForCgoCall(t.Elem(), m)
 	case *types.Struct:
 		for i := 0; i < t.NumFields(); i++ {
 			if !typeOKForCgoCall(t.Field(i).Type(), m) {
 				return false
 			}
 		}
 	}
 	return true
 }

 func isUnsafePointer(info *types.Info, e ast.Expr) bool {
 	t := info.Types[e].Type
 	return t != nil && t.Underlying() == types.Typ[types.UnsafePointer]
 }

 type importerFunc func(path string) (*types.Package, error)

 func (f importerFunc) Import(path string) (*types.Package, error) { return f(path) }

 // TODO(adonovan): make this a library function or method of Info.
 func imported(info *types.Info, spec *ast.ImportSpec) *types.Package {
 	obj, ok := info.Implicits[spec]
 	if !ok {
 		obj = info.Defs[spec.Name] // renaming import
 	}
 	return obj.(*types.PkgName).Imported()
 }
	// Copyright 2015 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 cgocall defines an Analyzer that detects some violations of
	// the cgo pointer passing rules.
	package cgocall

	import (
	"fmt"
	"go/ast"
	"go/format"
	"go/parser"
	"go/token"
	"go/types"
	"log"
	"os"
	"strconv"

	"golang.org/x/tools/go/analysis"
	"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
	)

	const debug = false

	const Doc = `detect some violations of the cgo pointer passing rules

	Check for invalid cgo pointer passing.
	This looks for code that uses cgo to call C code passing values
	whose types are almost always invalid according to the cgo pointer
	sharing rules.
	Specifically, it warns about attempts to pass a Go chan, map, func,
	or slice to C, either directly, or via a pointer, array, or struct.`

	var Analyzer = &analysis.Analyzer{
	Name: "cgocall",
	Doc: Doc,
	RunDespiteErrors: true,
	Run: run,
	}

	func run(pass *analysis.Pass) (interface{}, error) {
	if !analysisutil.Imports(pass.Pkg, "runtime/cgo") {
	return nil, nil // doesn't use cgo
	}

	cgofiles, info, err := typeCheckCgoSourceFiles(pass.Fset, pass.Pkg, pass.Files, pass.TypesInfo, pass.TypesSizes)
	if err != nil {
	return nil, err
	}
	for _, f := range cgofiles {
	checkCgo(pass.Fset, f, info, pass.Reportf)
	}
	return nil, nil
	}

	func checkCgo(fset token.FileSet, f ast.File, info *types.Info, reportf func(token.Pos, string, ...interface{})) {
	ast.Inspect(f, func(n ast.Node) bool {
	call, ok := n.(*ast.CallExpr)
	if !ok {
	return true
	}

	// Is this a C.f() call?
	var name string
	if sel, ok := analysisutil.Unparen(call.Fun).(*ast.SelectorExpr); ok {
	if id, ok := sel.X.(*ast.Ident); ok && id.Name == "C" {
	name = sel.Sel.Name
	}
	}
	if name == "" {
	return true // not a call we need to check
	}

	// A call to C.CBytes passes a pointer but is always safe.
	if name == "CBytes" {
	return true
	}

	if debug {
	log.Printf("%s: call to C.%s", fset.Position(call.Lparen), name)
	}

	for _, arg := range call.Args {
	if !typeOKForCgoCall(cgoBaseType(info, arg), make(map[types.Type]bool)) {
	reportf(arg.Pos(), "possibly passing Go type with embedded pointer to C")
	break
	}

	// Check for passing the address of a bad type.
	if conv, ok := arg.(*ast.CallExpr); ok && len(conv.Args) == 1 &&
	isUnsafePointer(info, conv.Fun) {
	arg = conv.Args[0]
	}
	if u, ok := arg.(*ast.UnaryExpr); ok && u.Op == token.AND {
	if !typeOKForCgoCall(cgoBaseType(info, u.X), make(map[types.Type]bool)) {
	reportf(arg.Pos(), "possibly passing Go type with embedded pointer to C")
	break
	}
	}
	}
	return true
	})
	}

	// typeCheckCgoSourceFiles returns type-checked syntax trees for the raw
	// cgo files of a package (those that import "C"). Such files are not
	// Go, so there may be gaps in type information around C.f references.
	//
	// This checker was initially written in vet to inspect raw cgo source
	// files using partial type information. However, Analyzers in the new
	// analysis API are presented with the type-checked, "cooked" Go ASTs
	// resulting from cgo-processing files, so we must choose between
	// working with the cooked file generated by cgo (which was tried but
	// proved fragile) or locating the raw cgo file (e.g. from //line
	// directives) and working with that, as we now do.
	//
	// Specifically, we must type-check the raw cgo source files (or at
	// least the subtrees needed for this analyzer) in an environment that
	// simulates the rest of the already type-checked package.
	//
	// For example, for each raw cgo source file in the original package,
	// such as this one:
	//
	// package p
	// import "C"
	// import "fmt"
	// type T int
	// const k = 3
	// var x, y = fmt.Println()
	// func f() { ... }
	// func g() { ... C.malloc(k) ... }
	// func (T) f(int) string { ... }
	//
	// we synthesize a new ast.File, shown below, that dot-imports the
	// original "cooked" package using a special name ("·this·"), so that all
	// references to package members resolve correctly. (References to
	// unexported names cause an "unexported" error, which we ignore.)
	//
	// To avoid shadowing names imported from the cooked package,
	// package-level declarations in the new source file are modified so
	// that they do not declare any names.
	// (The cgocall analysis is concerned with uses, not declarations.)
	// Specifically, type declarations are discarded;
	// all names in each var and const declaration are blanked out;
	// each method is turned into a regular function by turning
	// the receiver into the first parameter;
	// and all functions are renamed to "_".
	//
	// package p
	// import . "·this·" // declares T, k, x, y, f, g, T.f
	// import "C"
	// import "fmt"
	// const _ = 3
	// var _, _ = fmt.Println()
	// func _() { ... }
	// func _() { ... C.malloc(k) ... }
	// func _(T, int) string { ... }
	//
	// In this way, the raw function bodies and const/var initializer
	// expressions are preserved but refer to the "cooked" objects imported
	// from "·this·", and none of the transformed package-level declarations
	// actually declares anything. In the example above, the reference to k
	// in the argument of the call to C.malloc resolves to "·this·".k, which
	// has an accurate type.
	//
	// This approach could in principle be generalized to more complex
	// analyses on raw cgo files. One could synthesize a "C" package so that
	// C.f would resolve to "·this·"._C_func_f, for example. But we have
	// limited ourselves here to preserving function bodies and initializer
	// expressions since that is all that the cgocall analyzer needs.
	//
	func typeCheckCgoSourceFiles(fset token.FileSet, pkg types.Package, files []ast.File, info types.Info, sizes types.Sizes) ([]ast.File, types.Info, error) {
	const thispkg = "·this·"

	// Which files are cgo files?
	var cgoFiles []*ast.File
	importMap := map[string]*types.Package{thispkg: pkg}
	for _, raw := range files {
	// If f is a cgo-generated file, Position reports
	// the original file, honoring //line directives.
	filename := fset.Position(raw.Pos()).Filename
	f, err := parser.ParseFile(fset, filename, nil, parser.Mode(0))
	if err != nil {
	return nil, nil, fmt.Errorf("can't parse raw cgo file: %v", err)
	}
	found := false
	for _, spec := range f.Imports {
	if spec.Path.Value == `"C"` {
	found = true
	break
	}
	}
	if !found {
	continue // not a cgo file
	}

	// Record the original import map.
	for _, spec := range raw.Imports {
	path, _ := strconv.Unquote(spec.Path.Value)
	importMap[path] = imported(info, spec)
	}

	// Add special dot-import declaration:
	// import . "·this·"
	var decls []ast.Decl
	decls = append(decls, &ast.GenDecl{
	Tok: token.IMPORT,
	Specs: []ast.Spec{
	&ast.ImportSpec{
	Name: &ast.Ident{Name: "."},
	Path: &ast.BasicLit{
	Kind: token.STRING,
	Value: strconv.Quote(thispkg),
	},
	},
	},
	})

	// Transform declarations from the raw cgo file.
	for _, decl := range f.Decls {
	switch decl := decl.(type) {
	case *ast.GenDecl:
	switch decl.Tok {
	case token.TYPE:
	// Discard type declarations.
	continue
	case token.IMPORT:
	// Keep imports.
	case token.VAR, token.CONST:
	// Blank the declared var/const names.
	for _, spec := range decl.Specs {
	spec := spec.(*ast.ValueSpec)
	for i := range spec.Names {
	spec.Names[i].Name = "_"
	}
	}
	}
	case *ast.FuncDecl:
	// Blank the declared func name.
	decl.Name.Name = "_"

	// Turn a method receiver: func (T) f(P) R {...}
	// into regular parameter: func _(T, P) R {...}
	if decl.Recv != nil {
	var params []*ast.Field
	params = append(params, decl.Recv.List...)
	params = append(params, decl.Type.Params.List...)
	decl.Type.Params.List = params
	decl.Recv = nil
	}
	}
	decls = append(decls, decl)
	}
	f.Decls = decls
	if debug {
	format.Node(os.Stderr, fset, f) // debugging
	}
	cgoFiles = append(cgoFiles, f)
	}
	if cgoFiles == nil {
	return nil, nil, nil // nothing to do (can't happen?)
	}

	// Type-check the synthetic files.
	tc := &types.Config{
	FakeImportC: true,
	Importer: importerFunc(func(path string) (*types.Package, error) {
	return importMap[path], nil
	}),
	Sizes: sizes,
	Error: func(error) {}, // ignore errors (e.g. unused import)
	}

	// It's tempting to record the new types in the
	// existing pass.TypesInfo, but we don't own it.
	altInfo := &types.Info{
	Types: make(map[ast.Expr]types.TypeAndValue),
	}
	tc.Check(pkg.Path(), fset, cgoFiles, altInfo)

	return cgoFiles, altInfo, nil
	}

	// cgoBaseType tries to look through type conversions involving
	// unsafe.Pointer to find the real type. It converts:
	// unsafe.Pointer(x) => x
	// (unsafe.Pointer)(unsafe.Pointer(&x)) => x
	func cgoBaseType(info *types.Info, arg ast.Expr) types.Type {
	switch arg := arg.(type) {
	case *ast.CallExpr:
	if len(arg.Args) == 1 && isUnsafePointer(info, arg.Fun) {
	return cgoBaseType(info, arg.Args[0])
	}
	case *ast.StarExpr:
	call, ok := arg.X.(*ast.CallExpr)
	if !ok \|\| len(call.Args) != 1 {
	break
	}
	// Here arg is *f(v).
	t := info.Types[call.Fun].Type
	if t == nil {
	break
	}
	ptr, ok := t.Underlying().(*types.Pointer)
	if !ok {
	break
	}
	// Here arg is (p)(v)
	elem, ok := ptr.Elem().Underlying().(*types.Basic)
	if !ok \|\| elem.Kind() != types.UnsafePointer {
	break
	}
	// Here arg is (unsafe.Pointer)(v)
	call, ok = call.Args[0].(*ast.CallExpr)
	if !ok \|\| len(call.Args) != 1 {
	break
	}
	// Here arg is (unsafe.Pointer)(f(v))
	if !isUnsafePointer(info, call.Fun) {
	break
	}
	// Here arg is (unsafe.Pointer)(unsafe.Pointer(v))
	u, ok := call.Args[0].(*ast.UnaryExpr)
	if !ok \|\| u.Op != token.AND {
	break
	}
	// Here arg is (unsafe.Pointer)(unsafe.Pointer(&v))
	return cgoBaseType(info, u.X)
	}

	return info.Types[arg].Type
	}

	// typeOKForCgoCall reports whether the type of arg is OK to pass to a
	// C function using cgo. This is not true for Go types with embedded
	// pointers. m is used to avoid infinite recursion on recursive types.
	func typeOKForCgoCall(t types.Type, m map[types.Type]bool) bool {
	if t == nil \|\| m[t] {
	return true
	}
	m[t] = true
	switch t := t.Underlying().(type) {
	case types.Chan, types.Map, types.Signature, types.Slice:
	return false
	case *types.Pointer:
	return typeOKForCgoCall(t.Elem(), m)
	case *types.Array:
	return typeOKForCgoCall(t.Elem(), m)
	case *types.Struct:
	for i := 0; i < t.NumFields(); i++ {
	if !typeOKForCgoCall(t.Field(i).Type(), m) {
	return false
	}
	}
	}
	return true
	}

	func isUnsafePointer(info *types.Info, e ast.Expr) bool {
	t := info.Types[e].Type
	return t != nil && t.Underlying() == types.Typ[types.UnsafePointer]
	}

	type importerFunc func(path string) (*types.Package, error)

	func (f importerFunc) Import(path string) (*types.Package, error) { return f(path) }

	// TODO(adonovan): make this a library function or method of Info.
	func imported(info types.Info, spec ast.ImportSpec) *types.Package {
	obj, ok := info.Implicits[spec]
	if !ok {
	obj = info.Defs[spec.Name] // renaming import
	}
	return obj.(*types.PkgName).Imported()
	}