cmd/godoc: provide a link from notes to source location

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8122043

lib/godoc/package.html

@@ -169,9 +169,9 @@
 	{{with $.Notes}}
 		{{range $marker, $content := .}}
 			<h2 id="pkg-note-{{$marker}}">{{noteTitle $marker | html}}s</h2>
-			<ul>
+			<ul style="list-style: none; padding: 0;">
 			{{range .}}
-			<li>{{html .Body}}</li>
+			<li><a href="{{posLink_url $ .}}">&#x261e;</a> {{html .Body}}</li>
 			{{end}}
 			</ul>
 		{{end}}

src/cmd/godoc/godoc.go

@@ -481,19 +481,33 @@ func pkgLinkFunc(path string) string {
 	return pkgHandler.pattern[1:] + relpath // remove trailing '/' for relative URL
 }
 
-func posLink_urlFunc(info *PageInfo, node ast.Node) string {
+// n must be an ast.Node or a *doc.Note
+func posLink_urlFunc(info *PageInfo, n interface{}) string {
+	var pos, end token.Pos
+
+	switch n := n.(type) {
+	case ast.Node:
+		pos = n.Pos()
+		end = n.End()
+	case *doc.Note:
+		pos = n.Pos
+		end = n.End
+	default:
+		panic(fmt.Sprintf("wrong type for posLink_url template formatter: %T", n))
+	}
+
 	var relpath string
 	var line int
-	var low, high int // selection
+	var low, high int // selection offset range
 
-	if p := node.Pos(); p.IsValid() {
-		pos := info.FSet.Position(p)
-		relpath = pos.Filename
-		line = pos.Line
-		low = pos.Offset
+	if pos.IsValid() {
+		p := info.FSet.Position(pos)
+		relpath = p.Filename
+		line = p.Line
+		low = p.Offset
 	}
-	if p := node.End(); p.IsValid() {
-		high = info.FSet.Position(p).Offset
+	if end.IsValid() {
+		high = info.FSet.Position(end).Offset
 	}
 
 	var buf bytes.Buffer

src/pkg/go/doc/doc.go

@@ -69,7 +69,7 @@ type Func struct {
 // at least one character is recognized. The ":" following the uid is optional.
 // Notes are collected in the Package.Notes map indexed by the notes marker.
 type Note struct {
-	Pos  token.Pos // position of the comment containing the marker
+	Pos, End token.Pos // position range of the comment containing the marker
 	UID      string    // uid found with the marker
 	Body     string    // note body text
 }

src/pkg/go/doc/reader.go

@@ -419,6 +419,7 @@ func (r *reader) readNote(list []*ast.Comment) {
 			marker := text[m[2]:m[3]]
 			r.notes[marker] = append(r.notes[marker], &Note{
 				Pos:  list[0].Pos(),
+				End:  list[len(list)-1].End(),
 				UID:  text[m[4]:m[5]],
 				Body: body,
 			})