go/printer: if comment gets formatted away, don't squash

We were dropping the preceding whitespace.

Fixes #53059

Change-Id: Ide0d6a76c463a7481ec4883a8568f7f7b5e87e9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/408394
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/go/printer/printer.go

@@ -747,6 +747,18 @@ func (p *printer) intersperseComments(next token.Position, tok token.Token) (wro
 			// a top-level doc comment.
 			list = formatDocComment(list)
 			changed = true
+
+			if len(p.comment.List) > 0 && len(list) == 0 {
+				// The doc comment was removed entirely.
+				// Keep preceding whitespace.
+				p.writeCommentPrefix(p.posFor(p.comment.Pos()), next, last, tok)
+				// Change print state to continue at next.
+				p.pos = next
+				p.last = next
+				// There can't be any more comments.
+				p.nextComment()
+				return p.writeCommentSuffix(false)
+			}
 		}
 		for _, c := range list {
 			p.writeCommentPrefix(p.posFor(c.Pos()), next, last, tok)

src/go/printer/testdata/comments.golden

@@ -767,4 +767,8 @@ var _ = []T{	// lone comments
 	// in composite lit
 }
 
+func _()	{}
+
+func _()	{}
+
 /* This comment is the last entry in this file. It must be printed and should be followed by a newline */

src/go/printer/testdata/comments.input

@@ -765,4 +765,9 @@ var _ = []T{// lone comments
 // in composite lit
 }
 
+func _() {}
+
+//
+func _() {}
+
 /* This comment is the last entry in this file. It must be printed and should be followed by a newline */