internal/godoc/dochtml: enhance output parsing for example func

Current logic for detecting "concluding line comment" is not strict
enough that it may cause false-positive, causing some normal comment
(and code following it) to be discarded in the rendered web page.

Ensure that "concluding line comment" can only be prefixed by spaces.
Also add corresponding test case.

Fixes golang/go#65450

Change-Id: Ief9ec5326aa94965ca02d3278398cc5663ba395f
GitHub-Last-Rev: 7312ff3
GitHub-Pull-Request: #93
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/600415
kokoro-CI: kokoro <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: wdhongtw

internal/godoc/dochtml/internal/render/linkify_test.go

@@ -296,6 +296,18 @@ b := 1
 	line comment.
 */
 </pre>
+`,
+		},
+		{
+			"An Output comment must appear at the start of the line.",
+			`_ = true
+// This comment containing "// Output:" is not treated specially.
+`,
+			`
+<pre class="Documentation-exampleCode">
+_ = true
+// This comment containing &#34;// Output:&#34; is not treated specially.
+</pre>
 `,
 		},
 	} {

internal/godoc/dochtml/internal/render/render.go

@@ -21,7 +21,8 @@ import (
 
 var (
 	// Regexp for example outputs.
-	exampleOutputRx = regexp.MustCompile(`(?i)//[[:space:]]*(unordered )?output:`)
+	// Keep consistent with outputPrefix in GOROOT/src/go/doc/example.go.
+	exampleOutputRx = regexp.MustCompile(`(?i)^[[:space:]]*//[[:space:]]*(unordered )?output:`)
 )
 
 type Renderer struct {