cmd/fix: add buildtag fix

Now that Go 1.17 is out and Go 1.15 is unsupported,
removing // +build lines can be done safely: in the worst case,
if code is compiled using Go 1.16 the toolchain will detect
the presence of a //go:build without // +build and fail the build.
(It will not silently choose the wrong files.)

Note that +build lines will continue to work in Go sources forever.
This just provides a mechanism for users who are done with
Go 1.16 to remove them easily, by running "go fix".

Also update for new generics AST.

For #41184.
Fixes #48978.

Change-Id: I11a432c319e5abd05ad68dda9ccd7a7fdcc8bbb8
Reviewed-on: https://go-review.googlesource.com/c/go/+/240611
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/fix/buildtag.go

@@ -0,0 +1,51 @@
+// Copyright 2020 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"
+	"strings"
+)
+
+func init() {
+	register(buildtagFix)
+}
+
+const buildtagGoVersionCutoff = 1_18
+
+var buildtagFix = fix{
+	name: "buildtag",
+	date: "2021-08-25",
+	f:    buildtag,
+	desc: `Remove +build comments from modules using Go 1.18 or later`,
+}
+
+func buildtag(f *ast.File) bool {
+	if goVersion < buildtagGoVersionCutoff {
+		return false
+	}
+
+	// File is already gofmt-ed, so we know that if there are +build lines,
+	// they are in a comment group that starts with a //go:build line followed
+	// by a blank line. While we cannot delete comments from an AST and
+	// expect consistent output in general, this specific case - deleting only
+	// some lines from a comment block - does format correctly.
+	fixed := false
+	for _, g := range f.Comments {
+		sawGoBuild := false
+		for i, c := range g.List {
+			if strings.HasPrefix(c.Text, "//go:build ") {
+				sawGoBuild = true
+			}
+			if sawGoBuild && strings.HasPrefix(c.Text, "// +build ") {
+				g.List = g.List[:i]
+				fixed = true
+				break
+			}
+		}
+	}
+
+	return fixed
+}

src/cmd/fix/buildtag_test.go

@@ -0,0 +1,34 @@
+// Copyright 2020 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(buildtagTests, buildtag)
+}
+
+var buildtagTests = []testCase{
+	{
+		Name:    "buildtag.oldGo",
+		Version: 1_10,
+		In: `//go:build yes
+// +build yes
+
+package main
+`,
+	},
+	{
+		Name:    "buildtag.new",
+		Version: 1_99,
+		In: `//go:build yes
+// +build yes
+
+package main
+`,
+		Out: `//go:build yes
+
+package main
+`,
+	},
+}

src/cmd/fix/fix.go

@@ -125,6 +125,9 @@ func walkBeforeAfter(x interface{}, before, after func(interface{})) {
 	case *ast.IndexExpr:
 		walkBeforeAfter(&n.X, before, after)
 		walkBeforeAfter(&n.Index, before, after)
+	case *ast.IndexListExpr:
+		walkBeforeAfter(&n.X, before, after)
+		walkBeforeAfter(&n.Indices, before, after)
 	case *ast.SliceExpr:
 		walkBeforeAfter(&n.X, before, after)
 		if n.Low != nil {
@@ -156,6 +159,9 @@ func walkBeforeAfter(x interface{}, before, after func(interface{})) {
 	case *ast.StructType:
 		walkBeforeAfter(&n.Fields, before, after)
 	case *ast.FuncType:
+		if n.TypeParams != nil {
+			walkBeforeAfter(&n.TypeParams, before, after)
+		}
 		walkBeforeAfter(&n.Params, before, after)
 		if n.Results != nil {
 			walkBeforeAfter(&n.Results, before, after)
@@ -231,6 +237,9 @@ func walkBeforeAfter(x interface{}, before, after func(interface{})) {
 		walkBeforeAfter(&n.Values, before, after)
 		walkBeforeAfter(&n.Names, before, after)
 	case *ast.TypeSpec:
+		if n.TypeParams != nil {
+			walkBeforeAfter(&n.TypeParams, before, after)
+		}
 		walkBeforeAfter(&n.Type, before, after)
 
 	case *ast.BadDecl:

src/cmd/fix/main.go

@@ -18,6 +18,7 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
+	"strconv"
 	"strings"
 
 	"cmd/internal/diff"
@@ -36,7 +37,12 @@ var forceRewrites = flag.String("force", "",
 
 var allowed, force map[string]bool
 
-var doDiff = flag.Bool("diff", false, "display diffs instead of rewriting files")
+var (
+	doDiff       = flag.Bool("diff", false, "display diffs instead of rewriting files")
+	goVersionStr = flag.String("go", "", "go language version for files")
+
+	goVersion int // 115 for go1.15
+)
 
 // enable for debugging fix failures
 const debug = false // display incorrectly reformatted source and exit
@@ -63,6 +69,26 @@ func main() {
 	flag.Usage = usage
 	flag.Parse()
 
+	if *goVersionStr != "" {
+		if !strings.HasPrefix(*goVersionStr, "go") {
+			report(fmt.Errorf("invalid -go=%s", *goVersionStr))
+			os.Exit(exitCode)
+		}
+		majorStr := (*goVersionStr)[len("go"):]
+		minorStr := "0"
+		if i := strings.Index(majorStr, "."); i >= 0 {
+			majorStr, minorStr = majorStr[:i], majorStr[i+len("."):]
+		}
+		major, err1 := strconv.Atoi(majorStr)
+		minor, err2 := strconv.Atoi(minorStr)
+		if err1 != nil || err2 != nil || major < 0 || major >= 100 || minor < 0 || minor >= 100 {
+			report(fmt.Errorf("invalid -go=%s", *goVersionStr))
+			os.Exit(exitCode)
+		}
+
+		goVersion = major*100 + minor
+	}
+
 	sort.Sort(byDate(fixes))
 
 	if *allowedRewrites != "" {

src/cmd/fix/main_test.go

@@ -14,10 +14,11 @@ import (
 )
 
 type testCase struct {
-	Name string
-	Fn   func(*ast.File) bool
-	In   string
-	Out  string
+	Name    string
+	Fn      func(*ast.File) bool
+	Version int
+	In      string
+	Out     string
 }
 
 var testCases []testCase
@@ -78,7 +79,16 @@ func TestRewrite(t *testing.T) {
 	for _, tt := range testCases {
 		tt := tt
 		t.Run(tt.Name, func(t *testing.T) {
-			t.Parallel()
+			if tt.Version == 0 {
+				t.Parallel()
+			} else {
+				old := goVersion
+				goVersion = tt.Version
+				defer func() {
+					goVersion = old
+				}()
+			}
+
 			// Apply fix: should get tt.Out.
 			out, fixed, ok := parseFixPrint(t, tt.Fn, tt.Name, tt.In, true)
 			if !ok {
@@ -91,6 +101,9 @@ func TestRewrite(t *testing.T) {
 				return
 			}
 
+			if tt.Out == "" {
+				tt.Out = tt.In
+			}
 			if out != tt.Out {
 				t.Errorf("incorrect output.\n")
 				if !strings.HasPrefix(tt.Name, "testdata/") {

src/cmd/go/alldocs.go

@@ -464,14 +464,18 @@
 //
 // Usage:
 //
-// 	go fix [packages]
+// 	go fix [-fix list] [packages]
 //
 // Fix runs the Go fix command on the packages named by the import paths.
 //
+// The -fix flag sets a comma-separated list of fixes to run.
+// The default is all known fixes.
+// (Its value is passed to 'go tool fix -r'.)
+//
 // For more about fix, see 'go doc cmd/fix'.
 // For more about specifying packages, see 'go help packages'.
 //
-// To run fix with specific options, run 'go tool fix'.
+// To run fix with other options, run 'go tool fix'.
 //
 // See also: go fmt, go vet.
 //

src/cmd/go/internal/fix/fix.go

@@ -11,27 +11,39 @@ import (
 	"cmd/go/internal/load"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/str"
+	"cmd/go/internal/work"
 	"context"
 	"fmt"
+	"go/build"
 	"os"
 )
 
 var CmdFix = &base.Command{
-	Run:       runFix,
-	UsageLine: "go fix [packages]",
+	UsageLine: "go fix [-fix list] [packages]",
 	Short:     "update packages to use new APIs",
 	Long: `
 Fix runs the Go fix command on the packages named by the import paths.
 
+The -fix flag sets a comma-separated list of fixes to run.
+The default is all known fixes.
+(Its value is passed to 'go tool fix -r'.)
+
 For more about fix, see 'go doc cmd/fix'.
 For more about specifying packages, see 'go help packages'.
 
-To run fix with specific options, run 'go tool fix'.
+To run fix with other options, run 'go tool fix'.
 
 See also: go fmt, go vet.
 	`,
 }
 
+var fixes = CmdFix.Flag.String("fix", "", "comma-separated list of fixes to apply")
+
+func init() {
+	work.AddBuildFlags(CmdFix, work.DefaultBuildFlags)
+	CmdFix.Run = runFix // fix cycle
+}
+
 func runFix(ctx context.Context, cmd *base.Command, args []string) {
 	pkgs := load.PackagesAndErrors(ctx, load.PackageOpts{}, args)
 	w := 0
@@ -58,6 +70,16 @@ func runFix(ctx context.Context, cmd *base.Command, args []string) {
 		// the command only applies to this package,
 		// not to packages in subdirectories.
 		files := base.RelPaths(pkg.InternalAllGoFiles())
-		base.Run(str.StringList(cfg.BuildToolexec, base.Tool("fix"), files))
+		goVersion := ""
+		if pkg.Module != nil {
+			goVersion = "go" + pkg.Module.GoVersion
+		} else if pkg.Standard {
+			goVersion = build.Default.ReleaseTags[len(build.Default.ReleaseTags)-1]
+		}
+		var fixArg []string
+		if *fixes != "" {
+			fixArg = []string{"-r=" + *fixes}
+		}
+		base.Run(str.StringList(cfg.BuildToolexec, base.Tool("fix"), "-go="+goVersion, fixArg, files))
 	}
 }