cmd/fix: Add keys to printer.Config composite literals.

Fixes #4499.

R=rsc
CC=golang-dev
https://golang.org/cl/6931046

src/cmd/fix/printerconfig.go

+// 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 "go/ast"
+
+func init() {
+	register(printerconfigFix)
+}
+
+var printerconfigFix = fix{
+	"printerconfig",
+	"2012-12-11",
+	printerconfig,
+	`Add element keys to Config composite literals.`,
+}
+
+func printerconfig(f *ast.File) bool {
+	if !imports(f, "go/printer") {
+		return false
+	}
+
+	fixed := false
+	walk(f, func(n interface{}) {
+		cl, ok := n.(*ast.CompositeLit)
+		if !ok {
+			return
+		}
+		se, ok := cl.Type.(*ast.SelectorExpr)
+		if !ok {
+			return
+		}
+		if !isTopName(se.X, "printer") || se.Sel == nil {
+			return
+		}
+
+		if ss := se.Sel.String(); ss == "Config" {
+			for i, e := range cl.Elts {
+				if _, ok := e.(*ast.KeyValueExpr); ok {
+					break
+				}
+				switch i {
+				case 0:
+					cl.Elts[i] = &ast.KeyValueExpr{
+						Key:   ast.NewIdent("Mode"),
+						Value: e,
+					}
+				case 1:
+					cl.Elts[i] = &ast.KeyValueExpr{
+						Key:   ast.NewIdent("Tabwidth"),
+						Value: e,
+					}
+				}
+				fixed = true
+			}
+		}
+	})
+	return fixed
+}

src/cmd/fix/printerconfig_test.go

+// 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
+
+func init() {
+	addTestCases(printerconfigTests, printerconfig)
+}
+
+var printerconfigTests = []testCase{
+	{
+		Name: "printerconfig.0",
+		In: `package main
+
+import "go/printer"
+
+func f() printer.Config {
+	b := printer.Config{0, 8}
+	c := &printer.Config{0}
+	d := &printer.Config{Tabwidth: 8, Mode: 0}
+	return printer.Config{0, 8}
+}
+`,
+		Out: `package main
+
+import "go/printer"
+
+func f() printer.Config {
+	b := printer.Config{Mode: 0, Tabwidth: 8}
+	c := &printer.Config{Mode: 0}
+	d := &printer.Config{Tabwidth: 8, Mode: 0}
+	return printer.Config{Mode: 0, Tabwidth: 8}
+}
+`,
+	},
+}