diff --git a/src/cmd/api/clone.go b/src/cmd/api/clone.go
new file mode 100644
index 00000000000..180215f4b9c
--- /dev/null
+++ b/src/cmd/api/clone.go
@@ -0,0 +1,251 @@
+// Copyright 2012 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 main
+
+import (
+	"fmt"
+	"go/ast"
+	"log"
+	"reflect"
+)
+
+const debugClone = false
+
+// TODO(bradfitz): delete this function (and whole file) once
+// http://golang.org/issue/4380 is fixed.
+func clone(i interface{}) (cloned interface{}) {
+	if debugClone {
+		defer func() {
+			if !reflect.DeepEqual(i, cloned) {
+				log.Printf("cloned %T doesn't match: in=%#v out=%#v", i, i, cloned)
+			}
+		}()
+	}
+	switch v := i.(type) {
+	case nil:
+		return nil
+	case *ast.File:
+		o := &ast.File{
+			Doc:      v.Doc, // shallow
+			Package:  v.Package,
+			Comments: v.Comments, // shallow
+			Name:     v.Name,
+			Scope:    v.Scope,
+		}
+		for _, x := range v.Decls {
+			o.Decls = append(o.Decls, clone(x).(ast.Decl))
+		}
+		for _, x := range v.Imports {
+			o.Imports = append(o.Imports, clone(x).(*ast.ImportSpec))
+		}
+		for _, x := range v.Unresolved {
+			o.Unresolved = append(o.Unresolved, x)
+		}
+		return o
+	case *ast.GenDecl:
+		o := new(ast.GenDecl)
+		*o = *v
+		o.Specs = nil
+		for _, x := range v.Specs {
+			o.Specs = append(o.Specs, clone(x).(ast.Spec))
+		}
+		return o
+	case *ast.TypeSpec:
+		o := new(ast.TypeSpec)
+		*o = *v
+		o.Type = cloneExpr(v.Type)
+		return o
+	case *ast.InterfaceType:
+		o := new(ast.InterfaceType)
+		*o = *v
+		o.Methods = clone(v.Methods).(*ast.FieldList)
+		return o
+	case *ast.FieldList:
+		if v == nil {
+			return v
+		}
+		o := new(ast.FieldList)
+		*o = *v
+		o.List = nil
+		for _, x := range v.List {
+			o.List = append(o.List, clone(x).(*ast.Field))
+		}
+		return o
+	case *ast.Field:
+		o := &ast.Field{
+			Doc:     v.Doc, // shallow
+			Type:    cloneExpr(v.Type),
+			Tag:     clone(v.Tag).(*ast.BasicLit),
+			Comment: v.Comment, // shallow
+		}
+		for _, x := range v.Names {
+			o.Names = append(o.Names, clone(x).(*ast.Ident))
+		}
+		return o
+	case *ast.FuncType:
+		if v == nil {
+			return v
+		}
+		return &ast.FuncType{
+			Func:    v.Func,
+			Params:  clone(v.Params).(*ast.FieldList),
+			Results: clone(v.Results).(*ast.FieldList),
+		}
+	case *ast.FuncDecl:
+		if v == nil {
+			return v
+		}
+		return &ast.FuncDecl{
+			Recv: clone(v.Recv).(*ast.FieldList),
+			Name: v.Name,
+			Type: clone(v.Type).(*ast.FuncType),
+			Body: v.Body, // shallow
+		}
+	case *ast.ValueSpec:
+		if v == nil {
+			return v
+		}
+		o := &ast.ValueSpec{
+			Type: cloneExpr(v.Type),
+		}
+		for _, x := range v.Names {
+			o.Names = append(o.Names, x)
+		}
+		for _, x := range v.Values {
+			o.Values = append(o.Values, cloneExpr(x))
+		}
+		return o
+	case *ast.CallExpr:
+		if v == nil {
+			return v
+		}
+		o := &ast.CallExpr{}
+		*o = *v
+		o.Args = cloneExprs(v.Args)
+		o.Fun = cloneExpr(v.Fun)
+		return o
+	case *ast.SelectorExpr:
+		if v == nil {
+			return nil
+		}
+		return &ast.SelectorExpr{
+			X:   cloneExpr(v.X),
+			Sel: v.Sel,
+		}
+	case *ast.ArrayType:
+		return &ast.ArrayType{
+			Lbrack: v.Lbrack,
+			Len:    cloneExpr(v.Len),
+			Elt:    cloneExpr(v.Elt),
+		}
+	case *ast.StructType:
+		return &ast.StructType{
+			Struct:     v.Struct,
+			Fields:     clone(v.Fields).(*ast.FieldList),
+			Incomplete: v.Incomplete,
+		}
+	case *ast.StarExpr:
+		return &ast.StarExpr{
+			Star: v.Star,
+			X:    cloneExpr(v.X),
+		}
+	case *ast.CompositeLit:
+		return &ast.CompositeLit{
+			Type:   cloneExpr(v.Type),
+			Lbrace: v.Lbrace,
+			Elts:   cloneExprs(v.Elts),
+			Rbrace: v.Rbrace,
+		}
+	case *ast.UnaryExpr:
+		return &ast.UnaryExpr{
+			OpPos: v.OpPos,
+			Op:    v.Op,
+			X:     cloneExpr(v.X),
+		}
+	case *ast.BinaryExpr:
+		return &ast.BinaryExpr{
+			OpPos: v.OpPos,
+			Op:    v.Op,
+			X:     cloneExpr(v.X),
+			Y:     cloneExpr(v.Y),
+		}
+	case *ast.Ellipsis:
+		return &ast.Ellipsis{
+			Ellipsis: v.Ellipsis,
+			Elt:      cloneExpr(v.Elt),
+		}
+	case *ast.KeyValueExpr:
+		return &ast.KeyValueExpr{
+			Key:   cloneExpr(v.Key),
+			Colon: v.Colon,
+			Value: cloneExpr(v.Value),
+		}
+	case *ast.FuncLit:
+		return &ast.FuncLit{
+			Type: clone(v.Type).(*ast.FuncType),
+			Body: v.Body, // shallow
+		}
+	case *ast.MapType:
+		return &ast.MapType{
+			Map:   v.Map,
+			Key:   cloneExpr(v.Key),
+			Value: cloneExpr(v.Value),
+		}
+	case *ast.ParenExpr:
+		return &ast.ParenExpr{
+			Lparen: v.Lparen,
+			X:      cloneExpr(v.X),
+			Rparen: v.Rparen,
+		}
+	case *ast.Ident, *ast.BasicLit:
+		return v
+	case *ast.ImportSpec:
+		return &ast.ImportSpec{
+			Doc:     v.Doc, // shallow
+			Name:    v.Name,
+			Path:    clone(v.Path).(*ast.BasicLit),
+			Comment: v.Comment, // shallow
+			EndPos:  v.EndPos,
+		}
+	case *ast.ChanType:
+		return &ast.ChanType{
+			Begin: v.Begin,
+			Arrow: v.Arrow,
+			Dir:   v.Dir,
+			Value: cloneExpr(v.Value),
+		}
+	case *ast.TypeAssertExpr:
+		return &ast.TypeAssertExpr{
+			X:    cloneExpr(v.X),
+			Type: cloneExpr(v.Type),
+		}
+	case *ast.IndexExpr:
+		return &ast.IndexExpr{
+			X:      cloneExpr(v.X),
+			Index:  cloneExpr(v.Index),
+			Lbrack: v.Lbrack,
+			Rbrack: v.Rbrack,
+		}
+	}
+	panic(fmt.Sprintf("Uncloneable type %T", i))
+}
+
+func cloneExpr(x ast.Expr) ast.Expr {
+	if x == nil {
+		return nil
+	}
+	return clone(x).(ast.Expr)
+}
+
+func cloneExprs(x []ast.Expr) []ast.Expr {
+	if x == nil {
+		return nil
+	}
+	o := make([]ast.Expr, len(x))
+	for i, x := range x {
+		o[i] = cloneExpr(x)
+	}
+	return o
+}
diff --git a/src/cmd/api/goapi.go b/src/cmd/api/goapi.go
index 9b7e0902776..a61497c52dc 100644
--- a/src/cmd/api/goapi.go
+++ b/src/cmd/api/goapi.go
@@ -353,6 +353,21 @@ func fileDeps(f *ast.File) (pkgs []string) {
 	return
 }
 
+var parsedFileCache = make(map[string]*ast.File)
+
+func parseFile(filename string) (*ast.File, error) {
+	f, ok := parsedFileCache[filename]
+	if !ok {
+		var err error
+		f, err = parser.ParseFile(fset, filename, nil, 0)
+		if err != nil {
+			return nil, err
+		}
+		parsedFileCache[filename] = f
+	}
+	return clone(f).(*ast.File), nil
+}
+
 // WalkPackage walks all files in package `name'.
 // WalkPackage does nothing if the package has already been loaded.
 func (w *Walker) WalkPackage(name string) {
@@ -386,7 +401,7 @@ func (w *Walker) WalkPackage(name string) {
 
 	files := append(append([]string{}, info.GoFiles...), info.CgoFiles...)
 	for _, file := range files {
-		f, err := parser.ParseFile(fset, filepath.Join(dir, file), nil, 0)
+		f, err := parseFile(filepath.Join(dir, file))
 		if err != nil {
 			log.Fatalf("error parsing package %s, file %s: %v", name, file, err)
 		}
diff --git a/src/cmd/api/testdata/src/pkg/p1/p1.go b/src/cmd/api/testdata/src/pkg/p1/p1.go
index a98ca1e9115..a18a6418fa4 100644
--- a/src/cmd/api/testdata/src/pkg/p1/p1.go
+++ b/src/cmd/api/testdata/src/pkg/p1/p1.go
@@ -167,3 +167,25 @@ const (
 	foo2  string = "foo2"
 	truth        = foo == "foo" || foo2 == "foo2"
 )
+
+func ellipsis(...string) {}
+
+var x = &S{
+	Public:     nil,
+	private:    nil,
+	publicTime: time.Now(),
+}
+
+var parenExpr = (1 + 5)
+
+var funcLit = func() {}
+
+var m map[string]int
+
+var chanVar chan int
+
+var ifaceVar interface{} = 5
+
+var assertVar = ifaceVar.(int)
+
+var indexVar = m["foo"]