testing: allow specify temp dir by GOTMPDIR environment variable

src/cmd/go/internal/help/helpdoc.go

@@ -573,8 +573,9 @@ General-purpose environment variables:
 		The name of checksum database to use and optionally its public key and
 		URL. See https://golang.org/ref/mod#authenticating.
 	GOTMPDIR
-		The directory where the go command will write
-		temporary source files, packages, and binaries.
+		The directory where the go command will write temporary source files,
+		packages, and binaries.
+		Also used to specify the prefix of TmpDir function in testing package.
 	GOTOOLCHAIN
 		Controls which Go toolchain is used. See https://go.dev/doc/toolchain.
 	GOVCS

src/testing/testing.go

@@ -1362,7 +1362,7 @@ func (c *common) TempDir() string {
 			return -1
 		}
 		pattern = strings.Map(mapper, pattern)
-		c.tempDir, c.tempDirErr = os.MkdirTemp("", pattern)
+		c.tempDir, c.tempDirErr = os.MkdirTemp(os.Getenv("GOTMPDIR"), pattern)
 		if c.tempDirErr == nil {
 			c.Cleanup(func() {
 				if err := removeAll(c.tempDir); err != nil {

src/testing/testing_test.go

@@ -147,6 +147,40 @@ func testTempDir(t *testing.T) {
 	}
 }
 
+func TestTempDirGOTMPDIR(t *testing.T) {
+	customTmpDir := filepath.Join(os.TempDir(), "custom-gotmpdir-test")
+	if err := os.MkdirAll(customTmpDir, 0777); err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(customTmpDir)
+
+	t.Setenv("GOTMPDIR", customTmpDir)
+
+	dir := t.TempDir()
+	if dir == "" {
+		t.Fatal("expected dir")
+	}
+
+	if !strings.HasPrefix(dir, customTmpDir) {
+		t.Errorf("TempDir did not use GOTMPDIR: got %q, want prefix %q", dir, customTmpDir)
+	}
+
+	fi, err := os.Stat(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !fi.IsDir() {
+		t.Errorf("dir %q is not a dir", dir)
+	}
+
+	t.Setenv("GOTMPDIR", "another-custom-gotmpdir-test")
+
+	dir2 := t.TempDir()
+	if filepath.Dir(dir) != filepath.Dir(dir2) {
+		t.Fatalf("calls to TempDir after changed GOTMPDIR do not share a parent; got %q, %q", dir, dir2)
+	}
+}
+
 func TestSetenv(t *testing.T) {
 	tests := []struct {
 		name               string