cmd/go: make generate pass correct GOPACKAGE to XTest files

The existing behaviour of go generate is to pass GOPACKAGE=p
to all package files, including XTest files. This however is
incorrect as the package name for the XTest files is p_test.

Fixes #24594

Change-Id: I96b6e5777ec511cdcf1a6267a43f4d8c544c4af3
Reviewed-on: https://go-review.googlesource.com/103415
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -3179,6 +3179,22 @@ func TestGoGenerateEnv(t *testing.T) {
 	}
 }
 
+func TestGoGenerateXTestPkgName(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("skipping because windows has no echo command")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("env_test.go", "package main_test\n\n//go:generate echo $GOPACKAGE")
+	tg.run("generate", tg.path("env_test.go"))
+	want := "main_test"
+	if got := strings.TrimSpace(tg.getStdout()); got != want {
+		t.Errorf("go generate in XTest file got package name %q; want %q", got, want)
+	}
+}
+
 func TestGoGenerateBadImports(t *testing.T) {
 	if runtime.GOOS == "windows" {
 		t.Skip("skipping because windows has no echo command")

src/cmd/go/internal/generate/generate.go

@@ -153,8 +153,18 @@ func runGenerate(cmd *base.Command, args []string) {
 	}
 	// Even if the arguments are .go files, this loop suffices.
 	for _, pkg := range load.Packages(args) {
+		pkgName := pkg.Name
+
 		for _, file := range pkg.InternalGoFiles() {
-			if !generate(pkg.Name, file) {
+			if !generate(pkgName, file) {
+				break
+			}
+		}
+
+		pkgName += "_test"
+
+		for _, file := range pkg.InternalXGoFiles() {
+			if !generate(pkgName, file) {
 				break
 			}
 		}

src/cmd/go/internal/load/pkg.go

@@ -1294,7 +1294,13 @@ func (p *Package) mkAbs(list []string) []string {
 // InternalGoFiles returns the list of Go files being built for the package,
 // using absolute paths.
 func (p *Package) InternalGoFiles() []string {
-	return p.mkAbs(str.StringList(p.GoFiles, p.CgoFiles, p.TestGoFiles, p.XTestGoFiles))
+	return p.mkAbs(str.StringList(p.GoFiles, p.CgoFiles, p.TestGoFiles))
+}
+
+// InternalXGoFiles returns the list of Go files being built for the XTest package,
+// using absolute paths.
+func (p *Package) InternalXGoFiles() []string {
+	return p.mkAbs(p.XTestGoFiles)
 }
 
 // InternalGoFiles returns the list of all Go files possibly relevant for the package,