main: add go version to tinygo version

This appears to be necessary for Visual Studio Code.

main.go

@@ -896,7 +896,11 @@ func main() {
 	case "help":
 		usage()
 	case "version":
-		fmt.Printf("tinygo version %s %s/%s\n", version, runtime.GOOS, runtime.GOARCH)
+		goversion := "<unknown>"
+		if s, err := getGorootVersionString(goenv.Get("GOROOT")); err == nil {
+			goversion = s
+		}
+		fmt.Printf("tinygo version %s %s/%s (using go version %s)\n", version, runtime.GOOS, runtime.GOARCH, goversion)
 	case "env":
 		if flag.NArg() == 0 {
 			// Show all environment variables.

target.go

@@ -308,25 +308,8 @@ func (spec *TargetSpec) OpenOCDConfiguration() (args []string, err error) {
 // getGorootVersion returns the major and minor version for a given GOROOT path.
 // If the goroot cannot be determined, (0, 0) is returned.
 func getGorootVersion(goroot string) (major, minor int, err error) {
-	var s string
+	s, err := getGorootVersionString(goroot)
-	var n int
+	if err != nil {
-	var trailing string
-
-	if data, err := ioutil.ReadFile(filepath.Join(
-		goroot, "src", "runtime", "internal", "sys", "zversion.go")); err == nil {
-
-		r := regexp.MustCompile("const TheVersion = `(.*)`")
-		matches := r.FindSubmatch(data)
-		if len(matches) != 2 {
-			return 0, 0, errors.New("Invalid go version output:\n" + string(data))
-		}
-
-		s = string(matches[1])
-
-	} else if data, err := ioutil.ReadFile(filepath.Join(goroot, "VERSION")); err == nil {
-		s = string(data)
-
-	} else {
 		return 0, 0, err
 	}
 
@@ -340,7 +323,8 @@ func getGorootVersion(goroot string) (major, minor int, err error) {
 	}
 
 	// Ignore the errors, we don't really handle errors here anyway.
-	n, err = fmt.Sscanf(s, "go%d.%d%s", &major, &minor, &trailing)
+	var trailing string
+	n, err := fmt.Sscanf(s, "go%d.%d%s", &major, &minor, &trailing)
 	if n == 2 && err == io.EOF {
 		// Means there were no trailing characters (i.e., not an alpha/beta)
 		err = nil
@@ -351,6 +335,29 @@ func getGorootVersion(goroot string) (major, minor int, err error) {
 	return
 }
 
+// getGorootVersionString returns the version string as reported by the Go
+// toolchain for the given GOROOT path. It is usually of the form `go1.x.y` but
+// can have some variations (for beta releases, for example).
+func getGorootVersionString(goroot string) (string, error) {
+	if data, err := ioutil.ReadFile(filepath.Join(
+		goroot, "src", "runtime", "internal", "sys", "zversion.go")); err == nil {
+
+		r := regexp.MustCompile("const TheVersion = `(.*)`")
+		matches := r.FindSubmatch(data)
+		if len(matches) != 2 {
+			return "", errors.New("Invalid go version output:\n" + string(data))
+		}
+
+		return string(matches[1]), nil
+
+	} else if data, err := ioutil.ReadFile(filepath.Join(goroot, "VERSION")); err == nil {
+		return string(data), nil
+
+	} else {
+		return "", err
+	}
+}
+
 // getClangHeaderPath returns the path to the built-in Clang headers. It tries
 // multiple locations, which should make it find the directory when installed in
 // various ways.