go/doc/comment: do not turn ``` into “`

``` is Markdown, not Go doc comment, but some small fraction of users get confused.

In a set of 55M Go doc comments drawn from the latest version of
all public Go modules known to the module proxy in spring 2020,
the current Go 1.19 gofmt reformats about 1.57M of them.
Out of those 1.57M comments, 8k of them (about 0.5%) contain ```.

Instead of rewriting ``` to “`, leave it alone.

For #51082.

Change-Id: I1c8c88aac7ef75ec03e1a396b84ffe711c46f941
Reviewed-on: https://go-review.googlesource.com/c/go/+/410359
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

src/go/doc/comment/parse.go

@@ -840,6 +840,14 @@ func (d *parseDoc) parseText(out []Text, s string, autoLink bool) []Text {
 		}
 		switch {
 		case strings.HasPrefix(t, "``"):
+			if len(t) >= 3 && t[2] == '`' {
+				// Do not convert `` inside ```, in case people are mistakenly writing Markdown.
+				i += 3
+				for i < len(t) && t[i] == '`' {
+					i++
+				}
+				break
+			}
 			writeUntil(i)
 			w.WriteRune('“')
 			i += 2

src/go/doc/comment/testdata/quote.txt

@@ -1,12 +1,15 @@
 -- input --
 Doubled single quotes like `` and '' turn into Unicode double quotes,
 but single quotes ` and ' do not.
+Misplaced markdown fences ``` do not either.
 -- gofmt --
 Doubled single quotes like “ and ” turn into Unicode double quotes,
 but single quotes ` and ' do not.
+Misplaced markdown fences ``` do not either.
 -- text --
 Doubled single quotes like “ and ” turn into Unicode double quotes, but single
-quotes ` and ' do not.
+quotes ` and ' do not. Misplaced markdown fences ``` do not either.
 -- html --
 <p>Doubled single quotes like “ and ” turn into Unicode double quotes,
 but single quotes ` and &apos; do not.
+Misplaced markdown fences ``` do not either.