x/pkgsite: "// Output:" matcher for test Examples is noncanonical

[adonovan]

The standard logic for extracting "// Output:" comments in example tests is here:
https://cs.opensource.google/go/go/+/master:src/go/doc/example.go;l=113;drc=28f1bf61b7383bd4079d77090e67b3198b75be12
Observe that it has a ^ anchor so that it matches only the start of the comment text.
By contrast, pksite uses this regexp:
https://cs.opensource.google/go/x/pkgsite/+/master:internal/godoc/dochtml/internal/render/render.go;l=24;drc=7431aa5932c48bce425bf3f2dc368c5b1fed6e2c
which lacks the anchor. The consequence is that an example such as this one:

func Example() {
    // A
    // B This Example does not have an "// Output:" comment so it will not run.
    // C
}

is considered executable by pkgsite, and its rendering of the example is abruptly cut off at line C.

[wdhongtw]

This issue can be detected by adding one test case

diff --git a/internal/godoc/dochtml/internal/render/linkify_test.go b/internal/godoc/dochtml/internal/render/linkify_test.go
index d3ea8b83..7b455204 100644
--- a/internal/godoc/dochtml/internal/render/linkify_test.go
+++ b/internal/godoc/dochtml/internal/render/linkify_test.go
@@ -426,6 +426,18 @@ a := 1
 // Output:
 b := 1
 </pre>
+`,
+               },
+               {
+                       "Special \"Output\" comment must on it's own comment block",
+                       `_ = true
+// To test output, use: // output: the output content
+`,
+                       `
+<pre class="Documentation-exampleCode">
+_ = true
+// To test output, use: // output: the output content
+</pre>
 `,
                },
        } {

And can be fixed quickly by ensuring there is only spaces before the // sequence

diff --git a/internal/godoc/dochtml/internal/render/render.go b/internal/godoc/dochtml/internal/render/render.go
index 3875f3ba..0ad66c9e 100644
--- a/internal/godoc/dochtml/internal/render/render.go
+++ b/internal/godoc/dochtml/internal/render/render.go
@@ -21,7 +21,7 @@ import (
 
 var (
        // Regexp for example outputs.
-       exampleOutputRx = regexp.MustCompile(`(?i)//[[:space:]]*(unordered )?output:`)
+       exampleOutputRx = regexp.MustCompile(`(?i)^[[:space:]]*//[[:space:]]*(unordered )?output:`)
 )
 
 type Renderer struct {

Since that there could only be space-like characters before the // Output: block, assuming the the code is formatted.

@findleyr Would you think this issue deserve a fix? If so, maybe I can submit a PR for this. :D

[adonovan]

@wdhongtw a fix would be much appreciated; thanks for offering.

[gopherbot]

Change https://go.dev/cl/600415 mentions this issue: internal/godoc/dochtml: enhance output parsing for example func