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>

5 files changed, 139 insertions, 6 deletions

diff --git a/src/cmd/fix/buildtag.go b/src/cmd/fix/buildtag.go
new file mode 100644
index 0000000000..5f4fbfef16
--- /dev/null
+++ b/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
+}
diff --git a/src/cmd/fix/buildtag_test.go b/src/cmd/fix/buildtag_test.go
new file mode 100644
index 0000000000..1c6efbe9e0
--- /dev/null
+++ b/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
+`,
+	},
+}
diff --git a/src/cmd/fix/fix.go b/src/cmd/fix/fix.go
index b49db37571..b9980c17b9 100644
--- a/src/cmd/fix/fix.go
+++ b/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:
diff --git a/src/cmd/fix/main.go b/src/cmd/fix/main.go
index d055929aac..b5f7b901d6 100644
--- a/src/cmd/fix/main.go
+++ b/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 != "" {
diff --git a/src/cmd/fix/main_test.go b/src/cmd/fix/main_test.go
index af16bcaa31..1baa95c545 100644
--- a/src/cmd/fix/main_test.go
+++ b/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/") {