cmd/stringer: add a -tags flag that supports build tags

This is reapplying CL121995 after rolling back the change to the importing
methods. There is still a need for a flag to control tags.
The original CL decription:

The feature has been requested but, like build tags in general,
only works in a directory, not when files are specified explicitly.
Unlike the build tools, report when the feature is misused like this
to avoid confusion.

Fixes golang/go#9449

Change-Id: I732627d5f2e6323367e3bdd5de746923868890a9
Reviewed-on: https://go-review.googlesource.com/122537
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

cmd/stringer/endtoend_test.go

@@ -9,6 +9,7 @@
 package main
 
 import (
+	"bytes"
 	"fmt"
 	"go/build"
 	"io"
@@ -26,17 +27,8 @@ import (
 // binary panics if the String method for X is not correct, including for error cases.
 
 func TestEndToEnd(t *testing.T) {
-	dir, err := ioutil.TempDir("", "stringer")
-	if err != nil {
-		t.Fatal(err)
-	}
+	dir, stringer := buildStringer(t)
 	defer os.RemoveAll(dir)
-	// Create stringer in temporary directory.
-	stringer := filepath.Join(dir, "stringer.exe")
-	err = run("go", "build", "-o", stringer)
-	if err != nil {
-		t.Fatalf("building stringer: %s", err)
-	}
 	// Read the testdata directory.
 	fd, err := os.Open("testdata")
 	if err != nil {
@@ -53,6 +45,10 @@ func TestEndToEnd(t *testing.T) {
 			t.Errorf("%s is not a Go file", name)
 			continue
 		}
+		if strings.HasPrefix(name, "tag_") {
+			// This file is used for tag processing in TestTags, below.
+			continue
+		}
 		if name == "cgo.go" && !build.Default.CgoEnabled {
 			t.Logf("cgo is not enabled for %s", name)
 			continue
@@ -63,9 +59,69 @@ func TestEndToEnd(t *testing.T) {
 	}
 }
 
+// TestTags verifies that the -tags flag works as advertised.
+func TestTags(t *testing.T) {
+	dir, stringer := buildStringer(t)
+	defer os.RemoveAll(dir)
+	var (
+		protectedConst = []byte("TagProtected")
+		output         = filepath.Join(dir, "const_string.go")
+	)
+	for _, file := range []string{"tag_main.go", "tag_tag.go"} {
+
+		err := copy(filepath.Join(dir, file), filepath.Join("testdata", file))
+		if err != nil {
+			t.Fatal(err)
+		}
+
+	}
+	err := run(stringer, "-type", "Const", dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	result, err := ioutil.ReadFile(output)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if bytes.Contains(result, protectedConst) {
+		t.Fatal("tagged variable appears in untagged run")
+	}
+	err = os.Remove(output)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = run(stringer, "-type", "Const", "-tags", "tag", dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	result, err = ioutil.ReadFile(output)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Contains(result, protectedConst) {
+		t.Fatal("tagged variable does not appear in tagged run")
+	}
+}
+
+// buildStringer creates a temporary directory and installs stringer there.
+func buildStringer(t *testing.T) (dir string, stringer string) {
+	t.Helper()
+	dir, err := ioutil.TempDir("", "stringer")
+	if err != nil {
+		t.Fatal(err)
+	}
+	stringer = filepath.Join(dir, "stringer.exe")
+	err = run("go", "build", "-o", stringer)
+	if err != nil {
+		t.Fatalf("building stringer: %s", err)
+	}
+	return dir, stringer
+}
+
 // stringerCompileAndRun runs stringer for the named file and compiles and
 // runs the target binary in directory dir. That binary will panic if the String method is incorrect.
 func stringerCompileAndRun(t *testing.T, dir, stringer, typeName, fileName string) {
+	t.Helper()
 	t.Logf("run: %s %s\n", fileName, typeName)
 	source := filepath.Join(dir, fileName)
 	err := copy(source, filepath.Join("testdata", fileName))

cmd/stringer/stringer.go

@@ -82,6 +82,7 @@ var (
 	output      = flag.String("output", "", "output file name; default srcdir/<type>_string.go")
 	trimprefix  = flag.String("trimprefix", "", "trim the `prefix` from the generated constant names")
 	linecomment = flag.Bool("linecomment", false, "use line comment text as printed text when present")
+	buildTags   = flag.String("tags", "", "comma-separated list of build tags to apply")
 )
 
 // Usage is a replacement usage function for the flags package.
@@ -105,6 +106,10 @@ func main() {
 		os.Exit(2)
 	}
 	types := strings.Split(*typeNames, ",")
+	var tags []string
+	if len(*buildTags) > 0 {
+		tags = strings.Split(*buildTags, ",")
+	}
 
 	// We accept either one directory or a list of files. Which do we have?
 	args := flag.Args()
@@ -121,8 +126,11 @@ func main() {
 	}
 	if len(args) == 1 && isDirectory(args[0]) {
 		dir = args[0]
-		g.parsePackageDir(args[0])
+		g.parsePackageDir(args[0], tags)
 	} else {
+		if len(tags) != 0 {
+			log.Fatal("-tags option applies only to directories, not when files are specified")
+		}
 		dir = filepath.Dir(args[0])
 		g.parsePackageFiles(args)
 	}
@@ -197,9 +205,15 @@ type Package struct {
 	typesPkg *types.Package
 }
 
+func buildContext(tags []string) *build.Context {
+	ctx := build.Default
+	ctx.BuildTags = tags
+	return &ctx
+}
+
 // parsePackageDir parses the package residing in the directory.
-func (g *Generator) parsePackageDir(directory string) {
-	pkg, err := build.Default.ImportDir(directory, 0)
+func (g *Generator) parsePackageDir(directory string, tags []string) {
+	pkg, err := buildContext(tags).ImportDir(directory, 0)
 	if err != nil {
 		log.Fatalf("cannot process directory %s: %s", directory, err)
 	}
@@ -261,14 +275,17 @@ func (g *Generator) parsePackage(directory string, names []string, text interfac
 	g.pkg.name = astFiles[0].Name.Name
 	g.pkg.files = files
 	g.pkg.dir = directory
-	// Type check the package.
-	g.pkg.check(fs, astFiles)
+	g.pkg.typeCheck(fs, astFiles)
 }
 
-// check type-checks the package. The package must be OK to proceed.
-func (pkg *Package) check(fs *token.FileSet, astFiles []*ast.File) {
+// check type-checks the package so we can evaluate contants whose values we are printing.
+func (pkg *Package) typeCheck(fs *token.FileSet, astFiles []*ast.File) {
 	pkg.defs = make(map[*ast.Ident]types.Object)
-	config := types.Config{Importer: defaultImporter(), FakeImportC: true}
+	config := types.Config{
+		IgnoreFuncBodies: true, // We only need to evaluate constants.
+		Importer:         defaultImporter(),
+		FakeImportC:      true,
+	}
 	info := &types.Info{
 		Defs: pkg.defs,
 	}

cmd/stringer/testdata/tag_main.go

@@ -0,0 +1,11 @@
+// No build tag in this file.
+
+package main
+
+type Const int
+
+const (
+	A Const = iota
+	B
+	C
+)

cmd/stringer/testdata/tag_tag.go

@@ -0,0 +1,7 @@
+// This file has a build tag "tag"
+
+// +build tag
+
+package main
+
+const TagProtected Const = C + 1