windows/mkwinsyscall: write source to temp file if formatting fails

This change writes the unformatted Go source code to a temp file if
"format.Source" fails. Print the temp file path to the console to make
it easy to find. The source code is what causes formatting errors, and
it can be difficult to diagnose them without this context.

Fixes golang/go#57925

Change-Id: Ifa4d8a6e8bc5006357b0bc88afce5ba1d6fe0a48
Reviewed-on: https://go-review.googlesource.com/c/sys/+/463216
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

windows/mkwinsyscall/mkwinsyscall.go

@@ -871,6 +871,22 @@ func (src *Source) Generate(w io.Writer) error {
 	return nil
 }
 
+func writeTempSourceFile(data []byte) (string, error) {
+	f, err := os.CreateTemp("", "mkwinsyscall-generated-*.go")
+	if err != nil {
+		return "", err
+	}
+	_, err = f.Write(data)
+	if closeErr := f.Close(); err == nil {
+		err = closeErr
+	}
+	if err != nil {
+		os.Remove(f.Name()) // best effort
+		return "", err
+	}
+	return f.Name(), nil
+}
+
 func usage() {
 	fmt.Fprintf(os.Stderr, "usage: mkwinsyscall [flags] [path ...]\n")
 	flag.PrintDefaults()
@@ -897,7 +913,12 @@ func main() {
 
 	data, err := format.Source(buf.Bytes())
 	if err != nil {
-		log.Fatal(err)
+		log.Printf("failed to format source: %v", err)
+		f, err := writeTempSourceFile(buf.Bytes())
+		if err != nil {
+			log.Fatalf("failed to write unformatted source to file: %v", err)
+		}
+		log.Fatalf("for diagnosis, wrote unformatted source to %v", f)
 	}
 	if *filename == "" {
 		_, err = os.Stdout.Write(data)