cmd/vet: add -tags flag

This is needed to control which files to test in the usual manner.
A followup CL on the main repo will add the flag to the go vet command.

Updates golang/go#10228

Change-Id: I820d3c74657b58de5e92276627368dedf4e2096c
Reviewed-on: https://go-review.googlesource.com/10692
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/vet/main.go

@@ -25,10 +25,13 @@ import (
 	"golang.org/x/tools/go/types"
 )
 
-// TODO: Need a flag to set build tags when parsing the package.
+var (
+	verbose  = flag.Bool("v", false, "verbose")
+	testFlag = flag.Bool("test", false, "for testing only: sets -all and -shadow")
+	tags     = flag.String("tags", "", "comma-separated list of build tags to apply when parsing")
+	tagList  = []string{} // exploded version of tags flag; set in main
+)
 
-var verbose = flag.Bool("v", false, "verbose")
-var testFlag = flag.Bool("test", false, "for testing only: sets -all and -shadow")
 var exitCode = 0
 
 // "all" is here only for the appearance of backwards compatibility.
@@ -198,6 +201,8 @@ func main() {
 		}
 	}
 
+	tagList = strings.Split(*tags, ",")
+
 	initPrintFlags()
 	initUnusedFlags()
 
@@ -246,7 +251,13 @@ func prefixDirectory(directory string, names []string) {
 // doPackageDir analyzes the single package found in the directory, if there is one,
 // plus a test package, if there is one.
 func doPackageDir(directory string) {
-	pkg, err := build.Default.ImportDir(directory, 0)
+	context := build.Default
+	if len(context.BuildTags) != 0 {
+		warnf("build tags %s previously set", context.BuildTags)
+	}
+	context.BuildTags = append(tagList, context.BuildTags...)
+
+	pkg, err := context.ImportDir(directory, 0)
 	if err != nil {
 		// If it's just that there are no go source files, that's fine.
 		if _, nogo := err.(*build.NoGoError); nogo {

cmd/vet/testdata/tagtest/file1.go

@@ -0,0 +1,10 @@
+// Copyright 2015 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.
+
+// +build testtag
+
+package main
+
+func main() {
+}

cmd/vet/testdata/tagtest/file2.go

@@ -0,0 +1,10 @@
+// Copyright 2015 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.
+
+// +build !testtag
+
+package main
+
+func ignore() {
+}

cmd/vet/vet_test.go

@@ -72,3 +72,31 @@ func run(c *exec.Cmd, t *testing.T) bool {
 	}
 	return !bytes.Contains(output, []byte("BUG"))
 }
+
+// TestTags verifies that the -tags argument controls which files to check.
+func TestTags(t *testing.T) {
+	// go build
+	cmd := exec.Command("go", "build", "-o", binary)
+	run(cmd, t)
+
+	// defer removal of vet
+	defer os.Remove(binary)
+
+	args := []string{
+		"-tags=testtag",
+		"-v", // We're going to look at the files it examines.
+		"testdata/tagtest",
+	}
+	cmd = exec.Command(filepath.Join(".", binary), args...)
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatal(err)
+	}
+	// file1 has testtag and file2 has !testtag.
+	if !bytes.Contains(output, []byte("tagtest/file1.go")) {
+		t.Error("file1 was excluded, should be included")
+	}
+	if bytes.Contains(output, []byte("tagtest/file2.go")) {
+		t.Error("file2 was included, should be excluded")
+	}
+}