better placement of /*-style comments interspersed with code on one line

R=rsc
http://go/go-review/1017030

src/pkg/go/printer/printer.go

@@ -256,11 +256,10 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, isFirst, isKeywor
 		return;
 	}
 
-	n := pos.Line - p.last.Line;
-	if n == 0 {
+	if pos.Line == p.last.Line {
 		// comment on the same line as last item:
-		// separate with at least one tab
-		hasTab := false;
+		// separate with at least one separator
+		hasSep := false;
 		if isFirst {
 			j := 0;
 			for i, ch := range p.buffer {
@@ -272,7 +271,7 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, isFirst, isKeywor
 				case vtab:
 					// respect existing tabs - important
 					// for proper formatting of commented structs
-					hasTab = true;
+					hasSep = true;
 					continue;
 				case indent:
 					// apply pending indentation
@@ -283,9 +282,16 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, isFirst, isKeywor
 			}
 			p.writeWhitespace(j);
 		}
-		// make sure there is at least one tab
-		if !hasTab {
-			p.write(htab);
+		// make sure there is at least one separator
+		if !hasSep {
+			if pos.Line == next.Line {
+				// next item is on the same line as the comment
+				// (which must be a /*-style comment): separate
+				// with a blank instead of a tab
+				p.write([]byte{' '});
+			} else {
+				p.write(htab);
+			}
 		}
 
 	} else {
@@ -321,7 +327,7 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, isFirst, isKeywor
 			}
 			p.writeWhitespace(j);
 		}
-		p.writeNewlines(n);
+		p.writeNewlines(pos.Line - p.last.Line);
 	}
 }
 
@@ -560,14 +566,21 @@ func (p *printer) writeCommentSuffix(needsLinebreak bool) {
 func (p *printer) intersperseComments(next token.Position, isKeyword bool) {
 	isFirst := true;
 	needsLinebreak := false;
+	var last *ast.Comment;
 	for ; p.commentBefore(next); p.comment = p.comment.Next {
 		for _, c := range p.comment.List {
 			p.writeCommentPrefix(c.Pos(), next, isFirst, isKeyword);
 			isFirst = false;
 			p.writeComment(c);
 			needsLinebreak = c.Text[1] == '/';
+			last = c;
 		}
 	}
+	if last != nil && !needsLinebreak && last.Pos().Line == next.Line {
+		// the last comment is a /*-style comment and the next item
+		// follows on the same line: separate with an extra blank
+		p.write([]byte{' '});
+	}
 	p.writeCommentSuffix(needsLinebreak);
 }
 

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

@@ -215,6 +215,11 @@ func _() {
 }
 
 
+// Some interesting interspersed comments
+func _(/* this */x/* is *//* an */ int) {
+}
+
+
 // Line comments with tabs
 func _() {
 var	finput		*bufio.Reader;			// input file

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

@@ -215,6 +215,11 @@ func _() {
 }
 
 
+// Some interesting interspersed comments
+func _( /* this */ x /* is */ /* an */ int) {
+}
+
+
 // Line comments with tabs
 func _() {
 	var finput *bufio.Reader;	// input file

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

@@ -294,7 +294,7 @@ func _() {
 // formatting of structs
 type _ struct{}
 
-type _ struct {	/* this comment should be visible */}
+type _ struct { /* this comment should be visible */ }
 
 type _ struct {
 	// this comment should be visible and properly indented