gofmt: leave nil nodes of the AST unchanged.

Without this check, gofmt panics when trying to apply
the identity transformation on "item.field" expressions.
Fixes #2410.

R=rsc, gri
CC=golang-dev, remy
https://golang.org/cl/5376061

src/cmd/gofmt/gofmt_test.go

@@ -76,6 +76,7 @@ var tests = []struct {
 	{"testdata/old.input", ""},
 	{"testdata/rewrite1.input", "-r=Foo->Bar"},
 	{"testdata/rewrite2.input", "-r=int->bool"},
+	{"testdata/rewrite3.input", "-r=x->x"},
 	{"testdata/stdin*.input", "-stdin"},
 	{"testdata/comments.input", ""},
 	{"testdata/import.input", ""},

src/cmd/gofmt/rewrite.go

@@ -159,8 +159,8 @@ func match(m map[string]reflect.Value, pattern, val reflect.Value) bool {
 	if m != nil && pattern.IsValid() && pattern.Type() == identType {
 		name := pattern.Interface().(*ast.Ident).Name
 		if isWildcard(name) && val.IsValid() {
-			// wildcards only match expressions
-			if _, ok := val.Interface().(ast.Expr); ok {
+			// wildcards only match valid (non-nil) expressions.
+			if _, ok := val.Interface().(ast.Expr); ok && !val.IsNil() {
 				if old, ok := m[name]; ok {
 					return match(nil, old, val)
 				}

src/cmd/gofmt/testdata/rewrite3.golden

+// 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
+
+// Field tags are *ast.BasicLit nodes that are nil when the tag is
+// absent. These nil nodes must not be mistaken for expressions,
+// the rewriter should not try to dereference them. Was issue 2410.
+type Foo struct {
+	Field int
+}

src/cmd/gofmt/testdata/rewrite3.input

+// 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
+
+// Field tags are *ast.BasicLit nodes that are nil when the tag is
+// absent. These nil nodes must not be mistaken for expressions,
+// the rewriter should not try to dereference them. Was issue 2410.
+type Foo struct {
+	Field int
+}