more comment formatting:

- preserve (some) indentation of comment text for /*-style comments
  even if the first comment line does not contain any text that might
  suggest the "correct" indentation
- enabled because otherwise existing larger comments get re-formatted
  (this will not introduce a lot of changes since comments of this
  kind - until now - were not changed with respect to indentation)

R=rsc
http://go/go-review/1016047

src/pkg/go/printer/printer.go

@@ -443,9 +443,26 @@ func stripCommonPrefix(lines [][]byte) {
 		// Determine the white space on the first line after the /*
 		// and before the beginning of the comment text, assume two
 		// blanks instead of the /* unless the first character after
-		// the /* is a tab. This whitespace may be found as suffix
-		// in the common prefix.
+		// the /* is a tab. If the first comment line is empty but
+		// for the opening /*, assume up to 3 blanks or a tab. This
+		// whitespace may be found as suffix in the common prefix.
 		first := lines[0];
+		if isBlank(first[2 : len(first)]) {
+			// no comment text on the first line:
+			// reduce prefix by up to 3 blanks or a tab
+			// if present - this keeps comment text indented
+			// relative to the /* and */'s if it was indented
+			// in the first place
+			i := len(prefix);
+			for n := 0; n < 3 && i > 0 && prefix[i-1] == ' '; n++ {
+				i--;
+			}
+			if i == len(prefix) && i > 0 && prefix[i-1] == '\t' {
+				i--;
+			}
+			prefix = prefix[0:i];
+		} else {
+			// comment text on the first line
 			suffix := make([]byte, len(first));
 			n := 2;
 			for n < len(first) && first[n] <= ' ' {
@@ -466,6 +483,7 @@ func stripCommonPrefix(lines [][]byte) {
 				prefix = prefix[0 : len(prefix) - len(suffix)];
 			}
 		}
+	}
 
 	// Handle last line: If it only contains a closing */, align it
 	// with the opening /*, otherwise align the text with the other

src/pkg/go/printer/testdata/comments.golden

@@ -168,6 +168,53 @@ func _() {
 }
 
 
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line */
+}
+
+
 /*
  * line
  * of
@@ -194,6 +241,18 @@ blank line in middle:
 with no leading spaces on blank line.
 */
 
+/*
+   aligned in middle
+   here
+           not here
+*/
+
+/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+*/
+
 func _() {
 	/*
 	 * line

src/pkg/go/printer/testdata/comments.input

@@ -168,6 +168,53 @@ func _() {
 }
 
 
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	   */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+		*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line */
+}
+
+
 /*
  * line
  * of
@@ -194,6 +241,18 @@ blank line in middle:
 with no leading spaces on blank line.
 */
 
+/*
+   aligned in middle
+   here
+           not here
+*/
+
+/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+*/
+
 func _() {
 	/*
 	 * line