1 files changed, 80 insertions, 41 deletions

diff --git a/src/cmd/gofmt/gofmt_test.go b/src/cmd/gofmt/gofmt_test.go
index b9335b8f3..d1edb7bcc 100644
--- a/src/cmd/gofmt/gofmt_test.go
+++ b/src/cmd/gofmt/gofmt_test.go
@@ -6,18 +6,60 @@ package main
 
 import (
 	"bytes"
+	"flag"
 	"io/ioutil"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
+	"text/scanner"
 )
 
-func runTest(t *testing.T, in, out, flags string) {
+var update = flag.Bool("update", false, "update .golden files")
+
+// gofmtFlags looks for a comment of the form
+//
+//	//gofmt flags
+//
+// within the first maxLines lines of the given file,
+// and returns the flags string, if any. Otherwise it
+// returns the empty string.
+func gofmtFlags(filename string, maxLines int) string {
+	f, err := os.Open(filename)
+	if err != nil {
+		return "" // ignore errors - they will be found later
+	}
+	defer f.Close()
+
+	// initialize scanner
+	var s scanner.Scanner
+	s.Init(f)
+	s.Error = func(*scanner.Scanner, string) {}       // ignore errors
+	s.Mode = scanner.GoTokens &^ scanner.SkipComments // want comments
+
+	// look for //gofmt comment
+	for s.Line <= maxLines {
+		switch s.Scan() {
+		case scanner.Comment:
+			const prefix = "//gofmt "
+			if t := s.TokenText(); strings.HasPrefix(t, prefix) {
+				return strings.TrimSpace(t[len(prefix):])
+			}
+		case scanner.EOF:
+			return ""
+		}
+
+	}
+
+	return ""
+}
+
+func runTest(t *testing.T, in, out string) {
 	// process flags
 	*simplifyAST = false
 	*rewriteRule = ""
 	stdin := false
-	for _, flag := range strings.Split(flags, " ") {
+	for _, flag := range strings.Split(gofmtFlags(in, 20), " ") {
 		elts := strings.SplitN(flag, "=", 2)
 		name := elts[0]
 		value := ""
@@ -56,6 +98,17 @@ func runTest(t *testing.T, in, out, flags string) {
 	}
 
 	if got := buf.Bytes(); !bytes.Equal(got, expected) {
+		if *update {
+			if in != out {
+				if err := ioutil.WriteFile(out, got, 0666); err != nil {
+					t.Error(err)
+				}
+				return
+			}
+			// in == out: don't accidentally destroy input
+			t.Errorf("WARNING: -update did not rewrite input file %s", in)
+		}
+
 		t.Errorf("(gofmt %s) != %s (see %s.gofmt)", in, out, in)
 		d, err := diff(expected, got)
 		if err == nil {
@@ -67,51 +120,37 @@ func runTest(t *testing.T, in, out, flags string) {
 	}
 }
 
-var tests = []struct {
-	in, flags string
-}{
-	{"gofmt.go", ""},
-	{"gofmt_test.go", ""},
-	{"testdata/composites.input", "-s"},
-	{"testdata/slices1.input", "-s"},
-	{"testdata/slices2.input", "-s"},
-	{"testdata/old.input", ""},
-	{"testdata/rewrite1.input", "-r=Foo->Bar"},
-	{"testdata/rewrite2.input", "-r=int->bool"},
-	{"testdata/rewrite3.input", "-r=x->x"},
-	{"testdata/rewrite4.input", "-r=(x)->x"},
-	{"testdata/rewrite5.input", "-r=x+x->2*x"},
-	{"testdata/rewrite6.input", "-r=fun(x)->Fun(x)"},
-	{"testdata/rewrite7.input", "-r=fun(x...)->Fun(x)"},
-	{"testdata/rewrite8.input", "-r=interface{}->int"},
-	{"testdata/stdin*.input", "-stdin"},
-	{"testdata/comments.input", ""},
-	{"testdata/import.input", ""},
-	{"testdata/crlf.input", ""},       // test case for issue 3961; see also TestCRLF
-	{"testdata/typeswitch.input", ""}, // test case for issue 4470
-}
-
+// TestRewrite processes testdata/*.input files and compares them to the
+// corresponding testdata/*.golden files. The gofmt flags used to process
+// a file must be provided via a comment of the form
+//
+//	//gofmt flags
+//
+// in the processed file within the first 20 lines, if any.
 func TestRewrite(t *testing.T) {
-	for _, test := range tests {
-		match, err := filepath.Glob(test.in)
-		if err != nil {
-			t.Error(err)
-			continue
+	// determine input files
+	match, err := filepath.Glob("testdata/*.input")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// add larger examples
+	match = append(match, "gofmt.go", "gofmt_test.go")
+
+	for _, in := range match {
+		out := in // for files where input and output are identical
+		if strings.HasSuffix(in, ".input") {
+			out = in[:len(in)-len(".input")] + ".golden"
 		}
-		for _, in := range match {
-			out := in
-			if strings.HasSuffix(in, ".input") {
-				out = in[:len(in)-len(".input")] + ".golden"
-			}
-			runTest(t, in, out, test.flags)
-			if in != out {
-				// Check idempotence.
-				runTest(t, out, out, test.flags)
-			}
+		runTest(t, in, out)
+		if in != out {
+			// Check idempotence.
+			runTest(t, out, out)
 		}
 	}
 }
 
+// Test case for issue 3961.
 func TestCRLF(t *testing.T) {
 	const input = "testdata/crlf.input"   // must contain CR/LF's
 	const golden = "testdata/crlf.golden" // must not contain any CR's