fix for long label names impacting column width of previous lines

R=rsc
http://go/go-review/1013017

src/pkg/go/printer/printer.go

@@ -418,7 +418,12 @@ func (p *printer) writeWhitespace(n int) {
 			// part of the comment whitespace prefix and the comment
 			// will be positioned correctly indented.
 			if i+1 < n && p.buffer[i+1] == unindent {
-				p.buffer[i], p.buffer[i+1] = unindent, ch;
+				// Use a formfeed to terminate the current section.
+				// Otherwise, a long label name on the next line leading
+				// to a wide column may increase the indentation column
+				// of lines before the label; effectively leading to wrong
+				// indentation.
+				p.buffer[i], p.buffer[i+1] = unindent, formfeed;
 				i--;  // do it again
 				continue;
 			}

src/pkg/go/printer/testdata/statements.go

@@ -156,3 +156,20 @@ func _() {
 		_ = 0;
 	}
 }
+
+
+func _() {
+	if {
+		_ = 0;
+	}
+	_ = 0;  // the indentation here should not be affected by the long label name
+AnOverlongLabel:
+	_ = 0;
+	
+	if {
+		_ = 0;
+	}
+	_ = 0;
+
+L:	_ = 0;
+}

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

@@ -174,3 +174,20 @@ func _() {
 		_ = 0;
 	}
 }
+
+
+func _() {
+	if {
+		_ = 0;
+	}
+	_ = 0;	// the indentation here should not be affected by the long label name
+AnOverlongLabel:
+	_ = 0;
+
+	if {
+		_ = 0;
+	}
+	_ = 0;
+
+L:	_ = 0;
+}