go/doc: add // while wrapping a line comment in ToText

Currently, lineWrapper does not detect if it is printing a line comment or not.
Hence, while wrapping a comment, the new line does not get prefixed with a //.

We add logic to lineWrapper to detect this case and add // accordingly. Block
comments do not need any such handling.

Added tests for both cases.

Fixes #20929

Change-Id: I656037c2d865f31dd853cf9195f43ab7c6e6fc53
Reviewed-on: https://go-review.googlesource.com/c/go/+/163578
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/doc/comment.go

@@ -464,6 +464,7 @@ type lineWrapper struct {
 
 var nl = []byte("\n")
 var space = []byte(" ")
+var prefix = []byte("// ")
 
 func (l *lineWrapper) write(text string) {
 	if l.n == 0 && l.printed {
@@ -471,6 +472,8 @@ func (l *lineWrapper) write(text string) {
 	}
 	l.printed = true
 
+	needsPrefix := false
+	isComment := strings.HasPrefix(text, "//")
 	for _, f := range strings.Fields(text) {
 		w := utf8.RuneCountInString(f)
 		// wrap if line is too long
@@ -478,10 +481,15 @@ func (l *lineWrapper) write(text string) {
 			l.out.Write(nl)
 			l.n = 0
 			l.pendSpace = 0
+			needsPrefix = isComment
 		}
 		if l.n == 0 {
 			l.out.Write([]byte(l.indent))
 		}
+		if needsPrefix {
+			l.out.Write(prefix)
+			needsPrefix = false
+		}
 		l.out.Write(space[:l.pendSpace])
 		l.out.Write([]byte(f))
 		l.n += l.pendSpace + w

src/go/doc/comment_test.go

@@ -134,6 +134,26 @@ $	pre2
 		},
 		text: ".   Para.\n\n$	should not be ``escaped''",
 	},
+	{
+		in: "// A very long line of 46 char for line wrapping.",
+		out: []block{
+			{opPara, []string{"// A very long line of 46 char for line wrapping."}},
+		},
+		text: `.   // A very long line of 46 char for line
+.   // wrapping.
+`,
+	},
+	{
+		in: `/* A very long line of 46 char for line wrapping.
+A very long line of 46 char for line wrapping. */`,
+		out: []block{
+			{opPara, []string{"/* A very long line of 46 char for line wrapping.\n", "A very long line of 46 char for line wrapping. */"}},
+		},
+		text: `.   /* A very long line of 46 char for line
+.   wrapping. A very long line of 46 char
+.   for line wrapping. */
+`,
+	},
 }
 
 func TestBlocks(t *testing.T) {