gofix: error fix

To make the error fix more useful, expand typecheck to gather
more information about struct fields, typecheck range statements,
typecheck indirect and index of named types, and collect information
about assignment conversions.

Also, change addImport to rename top-level uses of a to-be-imported
identifier to avoid conflicts.  This duplicated some of the code in
the url fix, so that fix is now shorter.

R=iant, r, r
CC=golang-dev
https://golang.org/cl/5305066

13 files changed, 805 insertions, 65 deletions

diff --git a/src/cmd/gofix/Makefile b/src/cmd/gofix/Makefile
index 72d690f58d..fea50cccc5 100644
--- a/src/cmd/gofix/Makefile
+++ b/src/cmd/gofix/Makefile
@@ -6,6 +6,7 @@ include ../../Make.inc
 
 TARG=gofix
 GOFILES=\
+	error.go\
 	filepath.go\
 	fix.go\
 	httpfinalurl.go\
diff --git a/src/cmd/gofix/error.go b/src/cmd/gofix/error.go
new file mode 100644
index 0000000000..e0ced633d9
--- /dev/null
+++ b/src/cmd/gofix/error.go
@@ -0,0 +1,352 @@
+// Copyright 2011 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 (
+	"go/ast"
+	"regexp"
+	"strings"
+)
+
+func init() {
+	fixes = append(fixes, errorFix)
+}
+
+var errorFix = fix{
+	"error",
+	errorFn,
+	`Use error instead of os.Error.
+
+This fix rewrites code using os.Error to use error:
+
+	os.Error -> error
+	os.NewError -> errors.New
+	os.EOF -> io.EOF
+
+Seeing the old names above (os.Error and so on) triggers the following
+heuristic rewrites.  The heuristics can be forced using the -force=error flag.
+
+A top-level function, variable, or constant named error is renamed error_.
+
+Error implementations—those types used as os.Error or named
+XxxError—have their String methods renamed to Error.  Any existing
+Error field or method is renamed to Err.
+
+Error values—those with type os.Error or named e, err, error, err1,
+and so on—have method calls and field references rewritten just
+as the types do (String to Error, Error to Err).  Also, a type assertion
+of the form err.(*os.Waitmsg) becomes err.(*exec.ExitError).
+
+http://codereview.appspot.com/5305066
+`,
+}
+
+// At minimum, this fix applies the following rewrites:
+//
+//	os.Error -> error
+//	os.NewError -> errors.New
+//	os.EOF -> io.EOF
+//
+// However, if can apply any of those rewrites, it assumes that the
+// file predates the error type and tries to update the code to use
+// the new definition for error - an Error method, not a String method.
+// This more heuristic procedure may not be 100% accurate, so it is
+// only run when the file needs updating anyway.  The heuristic can
+// be forced to run using -force=error.
+//
+// First, we must identify the implementations of os.Error.
+// These include the type of any value returned as or assigned to an os.Error.
+// To that set we add any type whose name contains "Error" or "error".
+// The heuristic helps for implementations that are not used as os.Error
+// in the file in which they are defined.
+//
+// In any implementation of os.Error, we rename an existing struct field
+// or method named Error to Err and rename the String method to Error.
+//
+// Second, we must identify the values of type os.Error.
+// These include any value that obviously has type os.Error.
+// To that set we add any variable whose name is e or err or error
+// possibly followed by _ or a numeric or capitalized suffix.
+// The heuristic helps for variables that are initialized using calls
+// to functions in other packages.  The type checker does not have
+// information about those packages available, and in general cannot
+// (because the packages may themselves not compile).
+//
+// For any value of type os.Error, we replace a call to String with a call to Error.
+// We also replace type assertion err.(*os.Waitmsg) with err.(*exec.ExitError).
+
+// Variables matching this regexp are assumed to have type os.Error.
+var errVar = regexp.MustCompile(`^(e|err|error)_?([A-Z0-9].*)?$`)
+
+// Types matching this regexp are assumed to be implementations of os.Error.
+var errType = regexp.MustCompile(`^\*?([Ee]rror|.*Error)$`)
+
+// Type-checking configuration: tell the type-checker this basic
+// information about types, functions, and variables in external packages.
+var errorTypeConfig = &TypeConfig{
+	Type: map[string]*Type{
+		"os.Error": &Type{},
+	},
+	Func: map[string]string{
+		"fmt.Errorf":  "os.Error",
+		"os.NewError": "os.Error",
+	},
+	Var: map[string]string{
+		"os.EPERM":        "os.Error",
+		"os.ENOENT":       "os.Error",
+		"os.ESRCH":        "os.Error",
+		"os.EINTR":        "os.Error",
+		"os.EIO":          "os.Error",
+		"os.ENXIO":        "os.Error",
+		"os.E2BIG":        "os.Error",
+		"os.ENOEXEC":      "os.Error",
+		"os.EBADF":        "os.Error",
+		"os.ECHILD":       "os.Error",
+		"os.EDEADLK":      "os.Error",
+		"os.ENOMEM":       "os.Error",
+		"os.EACCES":       "os.Error",
+		"os.EFAULT":       "os.Error",
+		"os.EBUSY":        "os.Error",
+		"os.EEXIST":       "os.Error",
+		"os.EXDEV":        "os.Error",
+		"os.ENODEV":       "os.Error",
+		"os.ENOTDIR":      "os.Error",
+		"os.EISDIR":       "os.Error",
+		"os.EINVAL":       "os.Error",
+		"os.ENFILE":       "os.Error",
+		"os.EMFILE":       "os.Error",
+		"os.ENOTTY":       "os.Error",
+		"os.EFBIG":        "os.Error",
+		"os.ENOSPC":       "os.Error",
+		"os.ESPIPE":       "os.Error",
+		"os.EROFS":        "os.Error",
+		"os.EMLINK":       "os.Error",
+		"os.EPIPE":        "os.Error",
+		"os.EAGAIN":       "os.Error",
+		"os.EDOM":         "os.Error",
+		"os.ERANGE":       "os.Error",
+		"os.EADDRINUSE":   "os.Error",
+		"os.ECONNREFUSED": "os.Error",
+		"os.ENAMETOOLONG": "os.Error",
+		"os.EAFNOSUPPORT": "os.Error",
+		"os.ETIMEDOUT":    "os.Error",
+		"os.ENOTCONN":     "os.Error",
+	},
+}
+
+func errorFn(f *ast.File) bool {
+	if !imports(f, "os") && !force["error"] {
+		return false
+	}
+
+	// Fix gets called once to run the heuristics described above
+	// when we notice that this file definitely needs fixing
+	// (it mentions os.Error or something similar).
+	var fixed bool
+	var didHeuristic bool
+	heuristic := func() {
+		if didHeuristic {
+			return
+		}
+		didHeuristic = true
+
+		// We have identified a necessary fix (like os.Error -> error)
+		// but have not applied it or any others yet.  Prepare the file
+		// for fixing and apply heuristic fixes.
+
+		// Rename error to error_ to make room for error.
+		fixed = renameTop(f, "error", "error_") || fixed
+
+		// Use type checker to build list of error implementations.
+		typeof, assign := typecheck(errorTypeConfig, f)
+
+		isError := map[string]bool{}
+		for _, val := range assign["os.Error"] {
+			t := typeof[val]
+			if strings.HasPrefix(t, "*") {
+				t = t[1:]
+			}
+			if t != "" && !strings.HasPrefix(t, "func(") {
+				isError[t] = true
+			}
+		}
+
+		// We use both the type check results and the "Error" name heuristic
+		// to identify implementations of os.Error.
+		isErrorImpl := func(typ string) bool {
+			return isError[typ] || errType.MatchString(typ)
+		}
+
+		isErrorVar := func(x ast.Expr) bool {
+			if typ := typeof[x]; typ != "" {
+				return isErrorImpl(typ) || typ == "os.Error"
+			}
+			if sel, ok := x.(*ast.SelectorExpr); ok {
+				return sel.Sel.Name == "Error" || sel.Sel.Name == "Err"
+			}
+			if id, ok := x.(*ast.Ident); ok {
+				return errVar.MatchString(id.Name)
+			}
+			return false
+		}
+
+		walk(f, func(n interface{}) {
+			// In method declaration on error implementation type,
+			// rename String() to Error() and Error() to Err().
+			fn, ok := n.(*ast.FuncDecl)
+			if ok &&
+				fn.Recv != nil &&
+				len(fn.Recv.List) == 1 &&
+				isErrorImpl(typeName(fn.Recv.List[0].Type)) {
+				// Rename.
+				switch fn.Name.Name {
+				case "String":
+					fn.Name.Name = "Error"
+					fixed = true
+				case "Error":
+					fn.Name.Name = "Err"
+					fixed = true
+				}
+				return
+			}
+
+			// In type definition of an error implementation type,
+			// rename Error field to Err to make room for method.
+			// Given type XxxError struct { ... Error T } rename field to Err.
+			d, ok := n.(*ast.GenDecl)
+			if ok {
+				for _, s := range d.Specs {
+					switch s := s.(type) {
+					case *ast.TypeSpec:
+						if isErrorImpl(typeName(s.Name)) {
+							st, ok := s.Type.(*ast.StructType)
+							if ok {
+								for _, f := range st.Fields.List {
+									for _, n := range f.Names {
+										if n.Name == "Error" {
+											n.Name = "Err"
+											fixed = true
+										}
+									}
+								}
+							}
+						}
+					}
+				}
+			}
+
+			// For values that are an error implementation type,
+			// rename .Error to .Err and .String to .Error
+			sel, selok := n.(*ast.SelectorExpr)
+			if selok && isErrorImpl(typeof[sel.X]) {
+				switch sel.Sel.Name {
+				case "Error":
+					sel.Sel.Name = "Err"
+					fixed = true
+				case "String":
+					sel.Sel.Name = "Error"
+					fixed = true
+				}
+			}
+
+			// Assume x.Err is an error value and rename .String to .Error
+			// Children have been processed so the rewrite from Error to Err
+			// has already happened there.
+			if selok {
+				if subsel, ok := sel.X.(*ast.SelectorExpr); ok && subsel.Sel.Name == "Err" && sel.Sel.Name == "String" {
+					sel.Sel.Name = "Error"
+					fixed = true
+				}
+			}
+
+			// For values that are an error variable, rename .String to .Error.
+			if selok && isErrorVar(sel.X) && sel.Sel.Name == "String" {
+				sel.Sel.Name = "Error"
+				fixed = true
+			}
+
+			// Rewrite composite literal of error type to turn Error: into Err:.
+			lit, ok := n.(*ast.CompositeLit)
+			if ok && isErrorImpl(typeof[lit]) {
+				for _, e := range lit.Elts {
+					if kv, ok := e.(*ast.KeyValueExpr); ok && isName(kv.Key, "Error") {
+						kv.Key.(*ast.Ident).Name = "Err"
+						fixed = true
+					}
+				}
+			}
+
+			// Rename os.Waitmsg to exec.ExitError
+			// when used in a type assertion on an error.
+			ta, ok := n.(*ast.TypeAssertExpr)
+			if ok && isErrorVar(ta.X) && isPtrPkgDot(ta.Type, "os", "Waitmsg") {
+				addImport(f, "exec")
+				sel := ta.Type.(*ast.StarExpr).X.(*ast.SelectorExpr)
+				sel.X.(*ast.Ident).Name = "exec"
+				sel.Sel.Name = "ExitError"
+				fixed = true
+			}
+
+		})
+	}
+
+	fix := func() {
+		if fixed {
+			return
+		}
+		fixed = true
+		heuristic()
+	}
+
+	if force["error"] {
+		heuristic()
+	}
+
+	walk(f, func(n interface{}) {
+		p, ok := n.(*ast.Expr)
+		if !ok {
+			return
+		}
+		sel, ok := (*p).(*ast.SelectorExpr)
+		if !ok {
+			return
+		}
+		switch {
+		case isPkgDot(sel, "os", "Error"):
+			fix()
+			*p = &ast.Ident{NamePos: sel.Pos(), Name: "error"}
+		case isPkgDot(sel, "os", "NewError"):
+			fix()
+			addImport(f, "errors")
+			sel.X.(*ast.Ident).Name = "errors"
+			sel.Sel.Name = "New"
+		case isPkgDot(sel, "os", "EOF"):
+			fix()
+			addImport(f, "io")
+			sel.X.(*ast.Ident).Name = "io"
+		}
+	})
+
+	if fixed && !usesImport(f, "os") {
+		deleteImport(f, "os")
+	}
+
+	return fixed
+}
+
+func typeName(typ ast.Expr) string {
+	if p, ok := typ.(*ast.StarExpr); ok {
+		typ = p.X
+	}
+	id, ok := typ.(*ast.Ident)
+	if ok {
+		return id.Name
+	}
+	sel, ok := typ.(*ast.SelectorExpr)
+	if ok {
+		return typeName(sel.X) + "." + sel.Sel.Name
+	}
+	return ""
+}
diff --git a/src/cmd/gofix/error_test.go b/src/cmd/gofix/error_test.go
new file mode 100644
index 0000000000..eeab7e2ee1
--- /dev/null
+++ b/src/cmd/gofix/error_test.go
@@ -0,0 +1,232 @@
+// Copyright 2011 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
+
+func init() {
+	addTestCases(errorTests, errorFn)
+}
+
+var errorTests = []testCase{
+	{
+		Name: "error.0",
+		In: `package main
+
+func error() {}
+
+var error int
+`,
+		Out: `package main
+
+func error() {}
+
+var error int
+`,
+	},
+	{
+		Name: "error.1",
+		In: `package main
+
+import "os"
+
+func f() os.Error {
+	return os.EOF
+}
+
+func error() {}
+
+var error int
+
+func g() {
+	error := 1
+	_ = error
+}
+`,
+		Out: `package main
+
+import "io"
+
+func f() error {
+	return io.EOF
+}
+
+func error_() {}
+
+var error_ int
+
+func g() {
+	error := 1
+	_ = error
+}
+`,
+	},
+	{
+		Name: "error.2",
+		In: `package main
+
+import "os"
+
+func f() os.Error {
+	return os.EOF
+}
+
+func g() string {
+	// these all convert because f is known
+	if err := f(); err != nil {
+		return err.String()
+	}
+	if err1 := f(); err1 != nil {
+		return err1.String()
+	}
+	if e := f(); e != nil {
+		return e.String()
+	}
+	if x := f(); x != nil {
+		return x.String()
+	}
+
+	// only the error names (err, err1, e) convert; u is not known
+	if err := u(); err != nil {
+		return err.String()
+	}
+	if err1 := u(); err1 != nil {
+		return err1.String()
+	}
+	if e := u(); e != nil {
+		return e.String()
+	}
+	if x := u(); x != nil {
+		return x.String()
+	}
+	return ""
+}
+
+type T int
+
+func (t T) String() string { return "t" }
+
+type PT int
+
+func (p *PT) String() string { return "pt" }
+
+type MyError int
+
+func (t MyError) String() string { return "myerror" }
+
+type PMyError int
+
+func (p *PMyError) String() string { return "pmyerror" }
+
+func error() {}
+
+var error int
+`,
+		Out: `package main
+
+import "io"
+
+func f() error {
+	return io.EOF
+}
+
+func g() string {
+	// these all convert because f is known
+	if err := f(); err != nil {
+		return err.Error()
+	}
+	if err1 := f(); err1 != nil {
+		return err1.Error()
+	}
+	if e := f(); e != nil {
+		return e.Error()
+	}
+	if x := f(); x != nil {
+		return x.Error()
+	}
+
+	// only the error names (err, err1, e) convert; u is not known
+	if err := u(); err != nil {
+		return err.Error()
+	}
+	if err1 := u(); err1 != nil {
+		return err1.Error()
+	}
+	if e := u(); e != nil {
+		return e.Error()
+	}
+	if x := u(); x != nil {
+		return x.String()
+	}
+	return ""
+}
+
+type T int
+
+func (t T) String() string { return "t" }
+
+type PT int
+
+func (p *PT) String() string { return "pt" }
+
+type MyError int
+
+func (t MyError) Error() string { return "myerror" }
+
+type PMyError int
+
+func (p *PMyError) Error() string { return "pmyerror" }
+
+func error_() {}
+
+var error_ int
+`,
+	},
+	{
+		Name: "error.3",
+		In: `package main
+
+import "os"
+
+func f() os.Error {
+	return os.EOF
+}
+
+type PathError struct {
+	Name  string
+	Error os.Error
+}
+
+func (p *PathError) String() string {
+	return p.Name + ": " + p.Error.String()
+}
+
+func (p *PathError) Error1() string {
+	p = &PathError{Error: nil}
+	return fmt.Sprint(p.Name, ": ", p.Error)
+}
+`,
+		Out: `package main
+
+import "io"
+
+func f() error {
+	return io.EOF
+}
+
+type PathError struct {
+	Name string
+	Err  error
+}
+
+func (p *PathError) Error() string {
+	return p.Name + ": " + p.Err.Error()
+}
+
+func (p *PathError) Error1() string {
+	p = &PathError{Err: nil}
+	return fmt.Sprint(p.Name, ": ", p.Err)
+}
+`,
+	},
+}
diff --git a/src/cmd/gofix/fix.go b/src/cmd/gofix/fix.go
index 4eaadac2b4..9a51085dd1 100644
--- a/src/cmd/gofix/fix.go
+++ b/src/cmd/gofix/fix.go
@@ -4,11 +4,20 @@
 
 package main
 
+/*
+receiver named error
+function named error
+method on error
+exiterror
+slice of named type (go/scanner)
+*/
+
 import (
 	"fmt"
 	"go/ast"
 	"go/token"
 	"os"
+	"path"
 	"strconv"
 	"strings"
 )
@@ -97,6 +106,8 @@ func walkBeforeAfter(x interface{}, before, after func(interface{})) {
 		walkBeforeAfter(*n, before, after)
 	case **ast.Ident:
 		walkBeforeAfter(*n, before, after)
+	case **ast.BasicLit:
+		walkBeforeAfter(*n, before, after)
 
 	// pointers to slices
 	case *[]ast.Decl:
@@ -114,7 +125,9 @@ func walkBeforeAfter(x interface{}, before, after func(interface{})) {
 
 	// These are ordered and grouped to match ../../pkg/go/ast/ast.go
 	case *ast.Field:
+		walkBeforeAfter(&n.Names, before, after)
 		walkBeforeAfter(&n.Type, before, after)
+		walkBeforeAfter(&n.Tag, before, after)
 	case *ast.FieldList:
 		for _, field := range n.List {
 			walkBeforeAfter(field, before, after)
@@ -484,23 +497,101 @@ func newPkgDot(pos token.Pos, pkg, name string) ast.Expr {
 	}
 }
 
+// renameTop renames all references to the top-level name top.
+// It returns true if it makes any changes.
+func renameTop(f *ast.File, old, new string) bool {
+	var fixed bool
+
+	// Rename any conflicting imports
+	// (assuming package name is last element of path).
+	for _, s := range f.Imports {
+		if s.Name != nil {
+			if s.Name.Name == old {
+				s.Name.Name = new
+				fixed = true
+			}
+		} else {
+			_, thisName := path.Split(importPath(s))
+			if thisName == old {
+				s.Name = ast.NewIdent(new)
+				fixed = true
+			}
+		}
+	}
+
+	// Rename any top-level declarations.
+	for _, d := range f.Decls {
+		switch d := d.(type) {
+		case *ast.FuncDecl:
+			if d.Recv == nil && d.Name.Name == old {
+				d.Name.Name = new
+				d.Name.Obj.Name = new
+				fixed = true
+			}
+		case *ast.GenDecl:
+			for _, s := range d.Specs {
+				switch s := s.(type) {
+				case *ast.TypeSpec:
+					if s.Name.Name == old {
+						s.Name.Name = new
+						s.Name.Obj.Name = new
+						fixed = true
+					}
+				case *ast.ValueSpec:
+					for _, n := range s.Names {
+						if n.Name == old {
+							n.Name = new
+							n.Obj.Name = new
+							fixed = true
+						}
+					}
+				}
+			}
+		}
+	}
+
+	// Rename top-level old to new, both unresolved names
+	// (probably defined in another file) and names that resolve
+	// to a declaration we renamed.
+	walk(f, func(n interface{}) {
+		id, ok := n.(*ast.Ident)
+		if ok && isTopName(id, old) {
+			id.Name = new
+			fixed = true
+		}
+		if ok && id.Obj != nil && id.Name == old && id.Obj.Name == new {
+			id.Name = id.Obj.Name
+			fixed = true
+		}
+	})
+
+	return fixed
+}
+
 // addImport adds the import path to the file f, if absent.
-func addImport(f *ast.File, path string) {
-	if imports(f, path) {
+func addImport(f *ast.File, ipath string) {
+	if imports(f, ipath) {
 		return
 	}
 
+	// Determine name of import.
+	// Assume added imports follow convention of using last element.
+	_, name := path.Split(ipath)
+
+	// Rename any conflicting top-level references from name to name_.
+	renameTop(f, name, name+"_")
+
 	newImport := &ast.ImportSpec{
 		Path: &ast.BasicLit{
 			Kind:  token.STRING,
-			Value: strconv.Quote(path),
+			Value: strconv.Quote(ipath),
 		},
 	}
 
 	var impdecl *ast.GenDecl
 
 	// Find an import decl to add to.
-	var lastImport int = -1
+	var lastImport = -1
 	for i, decl := range f.Decls {
 		gen, ok := decl.(*ast.GenDecl)
 
@@ -535,7 +626,7 @@ func addImport(f *ast.File, path string) {
 	insertAt := len(impdecl.Specs) // default to end of specs
 	for i, spec := range impdecl.Specs {
 		impspec := spec.(*ast.ImportSpec)
-		if importPath(impspec) > path {
+		if importPath(impspec) > ipath {
 			insertAt = i
 			break
 		}
diff --git a/src/cmd/gofix/httpheaders.go b/src/cmd/gofix/httpheaders.go
index 2e906d859c..e9856f5db4 100644
--- a/src/cmd/gofix/httpheaders.go
+++ b/src/cmd/gofix/httpheaders.go
@@ -31,7 +31,7 @@ func httpheaders(f *ast.File) bool {
 	})
 
 	fixed := false
-	typeof := typecheck(headerTypeConfig, f)
+	typeof, _ := typecheck(headerTypeConfig, f)
 	walk(f, func(ni interface{}) {
 		switch n := ni.(type) {
 		case *ast.SelectorExpr:
diff --git a/src/cmd/gofix/imagecolor.go b/src/cmd/gofix/imagecolor.go
index d6171196d9..c7900e4657 100644
--- a/src/cmd/gofix/imagecolor.go
+++ b/src/cmd/gofix/imagecolor.go
@@ -47,8 +47,6 @@ func imagecolor(f *ast.File) (fixed bool) {
 		return
 	}
 
-	importColor := false
-
 	walk(f, func(n interface{}) {
 		s, ok := n.(*ast.SelectorExpr)
 
@@ -66,20 +64,17 @@ func imagecolor(f *ast.File) (fixed bool) {
 		default:
 			for _, rename := range colorRenames {
 				if sel == rename.in {
+					addImport(f, "image/color")
 					s.X.(*ast.Ident).Name = "color"
 					s.Sel.Name = rename.out
 					fixed = true
-					importColor = true
 				}
 			}
 		}
 	})
 
-	if importColor {
-		addImport(f, "image/color")
-		if !usesImport(f, "image") {
-			deleteImport(f, "image")
-		}
+	if fixed && !usesImport(f, "image") {
+		deleteImport(f, "image")
 	}
 	return
 }
diff --git a/src/cmd/gofix/main.go b/src/cmd/gofix/main.go
index e0709fc8ba..56232d708a 100644
--- a/src/cmd/gofix/main.go
+++ b/src/cmd/gofix/main.go
@@ -28,14 +28,18 @@ var (
 var allowedRewrites = flag.String("r", "",
 	"restrict the rewrites to this comma-separated list")
 
-var allowed map[string]bool
+var forceRewrites = flag.String("force", "",
+	"force these fixes to run even if the code looks updated")
+
+var allowed, force map[string]bool
 
 var doDiff = flag.Bool("diff", false, "display diffs instead of rewriting files")
 
 func usage() {
-	fmt.Fprintf(os.Stderr, "usage: gofix [-diff] [-r fixname,...] [path ...]\n")
+	fmt.Fprintf(os.Stderr, "usage: gofix [-diff] [-r fixname,...] [-force fixname,...] [path ...]\n")
 	flag.PrintDefaults()
 	fmt.Fprintf(os.Stderr, "\nAvailable rewrites are:\n")
+	sort.Sort(fixes)
 	for _, f := range fixes {
 		fmt.Fprintf(os.Stderr, "\n%s\n", f.name)
 		desc := strings.TrimSpace(f.desc)
@@ -46,8 +50,6 @@ func usage() {
 }
 
 func main() {
-	sort.Sort(fixes)
-
 	flag.Usage = usage
 	flag.Parse()
 
@@ -58,6 +60,13 @@ func main() {
 		}
 	}
 
+	if *forceRewrites != "" {
+		force = make(map[string]bool)
+		for _, f := range strings.Split(*forceRewrites, ",") {
+			force[f] = true
+		}
+	}
+
 	if flag.NArg() == 0 {
 		if err := processFile("standard input", true); err != nil {
 			report(err)
diff --git a/src/cmd/gofix/math.go b/src/cmd/gofix/math.go
index 8af4e87c7d..a9a11ed615 100644
--- a/src/cmd/gofix/math.go
+++ b/src/cmd/gofix/math.go
@@ -4,14 +4,7 @@
 
 package main
 
-import (
-	"fmt"
-	"os"
-	"go/ast"
-)
-
-var _ fmt.Stringer
-var _ os.Error
+import "go/ast"
 
 var mathFix = fix{
 	"math",
diff --git a/src/cmd/gofix/reflect.go b/src/cmd/gofix/reflect.go
index c292543ab8..2227d69b44 100644
--- a/src/cmd/gofix/reflect.go
+++ b/src/cmd/gofix/reflect.go
@@ -95,7 +95,7 @@ func reflectFn(f *ast.File) bool {
 
 	// Rewrite names in method calls.
 	// Needs basic type information (see above).
-	typeof := typecheck(reflectTypeConfig, f)
+	typeof, _ := typecheck(reflectTypeConfig, f)
 	walk(f, func(n interface{}) {
 		switch n := n.(type) {
 		case *ast.SelectorExpr:
diff --git a/src/cmd/gofix/signal.go b/src/cmd/gofix/signal.go
index aaad348259..9b548bd089 100644
--- a/src/cmd/gofix/signal.go
+++ b/src/cmd/gofix/signal.go
@@ -32,16 +32,14 @@ func signal(f *ast.File) (fixed bool) {
 
 		sel := s.Sel.String()
 		if sel == "Signal" || sel == "UnixSignal" || strings.HasPrefix(sel, "SIG") {
+			addImport(f, "os")
 			s.X = &ast.Ident{Name: "os"}
 			fixed = true
 		}
 	})
 
-	if fixed {
-		addImport(f, "os")
-		if !usesImport(f, "os/signal") {
-			deleteImport(f, "os/signal")
-		}
+	if fixed && !usesImport(f, "os/signal") {
+		deleteImport(f, "os/signal")
 	}
 	return
 }
diff --git a/src/cmd/gofix/typecheck.go b/src/cmd/gofix/typecheck.go
index 2d81b9710e..23fc8bfe96 100644
--- a/src/cmd/gofix/typecheck.go
+++ b/src/cmd/gofix/typecheck.go
@@ -99,6 +99,7 @@ type Type struct {
 	Field  map[string]string // map field name to type
 	Method map[string]string // map method name to comma-separated return types
 	Embed  []string          // list of types this type embeds (for extra methods)
+	Def    string            // definition of named type
 }
 
 // dot returns the type of "typ.name", making its decision
@@ -128,9 +129,15 @@ func (typ *Type) dot(cfg *TypeConfig, name string) string {
 }
 
 // typecheck type checks the AST f assuming the information in cfg.
-// It returns a map from AST nodes to type information in gofmt string form.
-func typecheck(cfg *TypeConfig, f *ast.File) map[interface{}]string {
-	typeof := make(map[interface{}]string)
+// It returns two maps with type information:
+// typeof maps AST nodes to type information in gofmt string form.
+// assign maps type strings to lists of expressions that were assigned
+// to values of another type that were assigned to that type.
+func typecheck(cfg *TypeConfig, f *ast.File) (typeof map[interface{}]string, assign map[string][]interface{}) {
+	typeof = make(map[interface{}]string)
+	assign = make(map[string][]interface{})
+	cfg1 := &TypeConfig{}
+	*cfg1 = *cfg // make copy so we can add locally
 
 	// gather function declarations
 	for _, decl := range f.Decls {
@@ -138,7 +145,7 @@ func typecheck(cfg *TypeConfig, f *ast.File) map[interface{}]string {
 		if !ok {
 			continue
 		}
-		typecheck1(cfg, fn.Type, typeof)
+		typecheck1(cfg, fn.Type, typeof, assign)
 		t := typeof[fn.Type]
 		if fn.Recv != nil {
 			// The receiver must be a type.
@@ -168,8 +175,42 @@ func typecheck(cfg *TypeConfig, f *ast.File) map[interface{}]string {
 		}
 	}
 
-	typecheck1(cfg, f, typeof)
-	return typeof
+	// gather struct declarations
+	for _, decl := range f.Decls {
+		d, ok := decl.(*ast.GenDecl)
+		if ok {
+			for _, s := range d.Specs {
+				switch s := s.(type) {
+				case *ast.TypeSpec:
+					if cfg1.Type[s.Name.Name] != nil {
+						break
+					}
+					if cfg1.Type == cfg.Type || cfg1.Type == nil {
+						// Copy map lazily: it's time.
+						cfg1.Type = make(map[string]*Type)
+						for k, v := range cfg.Type {
+							cfg1.Type[k] = v
+						}
+					}
+					t := &Type{Field: map[string]string{}}
+					cfg1.Type[s.Name.Name] = t
+					switch st := s.Type.(type) {
+					case *ast.StructType:
+						for _, f := range st.Fields.List {
+							for _, n := range f.Names {
+								t.Field[n.Name] = gofmt(f.Type)
+							}
+						}
+					case *ast.ArrayType, *ast.StarExpr, *ast.MapType:
+						t.Def = gofmt(st)
+					}
+				}
+			}
+		}
+	}
+
+	typecheck1(cfg1, f, typeof, assign)
+	return typeof, assign
 }
 
 func makeExprList(a []*ast.Ident) []ast.Expr {
@@ -183,11 +224,14 @@ func makeExprList(a []*ast.Ident) []ast.Expr {
 // Typecheck1 is the recursive form of typecheck.
 // It is like typecheck but adds to the information in typeof
 // instead of allocating a new map.
-func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string) {
+func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string, assign map[string][]interface{}) {
 	// set sets the type of n to typ.
 	// If isDecl is true, n is being declared.
 	set := func(n ast.Expr, typ string, isDecl bool) {
 		if typeof[n] != "" || typ == "" {
+			if typeof[n] != typ {
+				assign[typ] = append(assign[typ], n)
+			}
 			return
 		}
 		typeof[n] = typ
@@ -236,6 +280,14 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string) {
 		}
 	}
 
+	expand := func(s string) string {
+		typ := cfg.Type[s]
+		if typ != nil && typ.Def != "" {
+			return typ.Def
+		}
+		return s
+	}
+
 	// The main type check is a recursive algorithm implemented
 	// by walkBeforeAfter(n, before, after).
 	// Most of it is bottom-up, but in a few places we need
@@ -263,7 +315,7 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string) {
 			defer func() {
 				if t := typeof[n]; t != "" {
 					pos := fset.Position(n.(ast.Node).Pos())
-					fmt.Fprintf(os.Stderr, "%s: typeof[%s] = %s\n", pos.String(), gofmt(n), t)
+					fmt.Fprintf(os.Stderr, "%s: typeof[%s] = %s\n", pos, gofmt(n), t)
 				}
 			}()
 		}
@@ -405,6 +457,8 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string) {
 			// x.(T) has type T.
 			if t := typeof[n.Type]; isType(t) {
 				typeof[n] = getType(t)
+			} else {
+				typeof[n] = gofmt(n.Type)
 			}
 
 		case *ast.SliceExpr:
@@ -413,7 +467,7 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string) {
 
 		case *ast.IndexExpr:
 			// x[i] has key type of x's type.
-			t := typeof[n.X]
+			t := expand(typeof[n.X])
 			if strings.HasPrefix(t, "[") || strings.HasPrefix(t, "map[") {
 				// Lazy: assume there are no nested [] in the array
 				// length or map key type.
@@ -426,7 +480,7 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string) {
 			// *x for x of type *T has type T when x is an expr.
 			// We don't use the result when *x is a type, but
 			// compute it anyway.
-			t := typeof[n.X]
+			t := expand(typeof[n.X])
 			if isType(t) {
 				typeof[n] = "type *" + getType(t)
 			} else if strings.HasPrefix(t, "*") {
@@ -448,6 +502,39 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string) {
 			// (x) has type of x.
 			typeof[n] = typeof[n.X]
 
+		case *ast.RangeStmt:
+			t := expand(typeof[n.X])
+			if t == "" {
+				return
+			}
+			var key, value string
+			if t == "string" {
+				key, value = "int", "rune"
+			} else if strings.HasPrefix(t, "[") {
+				key = "int"
+				if i := strings.Index(t, "]"); i >= 0 {
+					value = t[i+1:]
+				}
+			} else if strings.HasPrefix(t, "map[") {
+				if i := strings.Index(t, "]"); i >= 0 {
+					key, value = t[4:i], t[i+1:]
+				}
+			}
+			changed := false
+			if n.Key != nil && key != "" {
+				changed = true
+				set(n.Key, key, n.Tok == token.DEFINE)
+			}
+			if n.Value != nil && value != "" {
+				changed = true
+				set(n.Value, value, n.Tok == token.DEFINE)
+			}
+			// Ugly failure of vision: already type-checked body.
+			// Do it again now that we have that type info.
+			if changed {
+				typecheck1(cfg, n.Body, typeof, assign)
+			}
+
 		case *ast.TypeSwitchStmt:
 			// Type of variable changes for each case in type switch,
 			// but go/parser generates just one variable.
@@ -471,7 +558,7 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string) {
 						tt = getType(tt)
 						typeof[varx] = tt
 						typeof[varx.Obj] = tt
-						typecheck1(cfg, cas.Body, typeof)
+						typecheck1(cfg, cas.Body, typeof, assign)
 					}
 				}
 			}
diff --git a/src/cmd/gofix/url.go b/src/cmd/gofix/url.go
index 455b544b63..d90f2b0cc1 100644
--- a/src/cmd/gofix/url.go
+++ b/src/cmd/gofix/url.go
@@ -4,14 +4,7 @@
 
 package main
 
-import (
-	"fmt"
-	"os"
-	"go/ast"
-)
-
-var _ fmt.Stringer
-var _ os.Error
+import "go/ast"
 
 var urlFix = fix{
 	"url",
@@ -42,12 +35,7 @@ func url(f *ast.File) bool {
 	fixed := false
 
 	// Update URL code.
-	var skip interface{}
 	urlWalk := func(n interface{}) {
-		if n == skip {
-			skip = nil
-			return
-		}
 		// Is it an identifier?
 		if ident, ok := n.(*ast.Ident); ok && ident.Name == "url" {
 			ident.Name = "url_"
@@ -58,12 +46,6 @@ func url(f *ast.File) bool {
 			fixed = urlDoFields(fn.Params) || fixed
 			fixed = urlDoFields(fn.Results) || fixed
 		}
-		// U{url: ...} is likely a struct field.
-		if kv, ok := n.(*ast.KeyValueExpr); ok {
-			if ident, ok := kv.Key.(*ast.Ident); ok && ident.Name == "url" {
-				skip = ident
-			}
-		}
 	}
 
 	// Fix up URL code and add import, at most once.
@@ -71,8 +53,8 @@ func url(f *ast.File) bool {
 		if fixed {
 			return
 		}
-		walkBeforeAfter(f, urlWalk, nop)
 		addImport(f, "url")
+		walkBeforeAfter(f, urlWalk, nop)
 		fixed = true
 	}
 
diff --git a/src/cmd/gofix/url_test.go b/src/cmd/gofix/url_test.go
index ca886e983e..39827f780e 100644
--- a/src/cmd/gofix/url_test.go
+++ b/src/cmd/gofix/url_test.go
@@ -103,14 +103,14 @@ func h() (url string) {
 
 import "url"
 
-type U struct{ url int }
+type U struct{ url_ int }
 type M map[int]int
 
 func f() {
 	url.Parse(a)
 	var url_ = 23
 	url_, x := 45, y
-	_ = U{url: url_}
+	_ = U{url_: url_}
 	_ = M{url_ + 1: url_}
 }