Move footnote rendering to the renderer

Clean up footnotes part of an AST: don't force HTML-specific pieces
there, just keep a clean list of footnotes. Since some renderers might
want to process footnotes differently, let them know about footnotes by
having a flag on that list.

html.go

@@ -589,10 +589,11 @@ func (r *HTMLRenderer) RenderNode(w io.Writer, node *Node, entering bool) WalkSt
 			tagName = "dl"
 		}
 		if entering {
-			// var start = node.listStart;
-			// if (start !== null && start !== 1) {
-			//     attrs.push(['start', start.toString()]);
-			// }
+			if node.IsFootnotesList {
+				r.out(w, []byte("\n<div class=\"footnotes\">\n\n"))
+				r.out(w, tag("hr", attrs, r.Flags&UseXHTML != 0))
+				r.cr(w)
+			}
 			r.cr(w)
 			if node.Parent.Type == Item && node.Parent.Parent.Tight {
 				r.cr(w)
@@ -611,6 +612,9 @@ func (r *HTMLRenderer) RenderNode(w io.Writer, node *Node, entering bool) WalkSt
 			if node.Parent.Type == Document || node.Parent.Type == BlockQuote {
 				r.cr(w)
 			}
+			if node.IsFootnotesList {
+				r.out(w, []byte("\n</div>\n"))
+			}
 		}
 	case Item:
 		tagName := "li"

inline_test.go

@@ -922,6 +922,12 @@ what happens here
 
 </div>
 `,
+
+	`This text does not reference a footnote.
+
+[^footnote]: But it has a footnote! And it gets omitted.
+`,
+	"<p>This text does not reference a footnote.</p>\n",
 }
 
 func TestFootnotes(t *testing.T) {

markdown.go

@@ -416,9 +416,8 @@ func (p *parser) parseRefsToAST() {
 		return
 	}
 	p.tip = p.doc
-	finalizeHTMLBlock(p.addBlock(HTMLBlock, []byte(`<div class="footnotes">`)))
-	p.addBlock(HorizontalRule, nil)
 	block := p.addBlock(List, nil)
+	block.IsFootnotesList = true
 	block.ListFlags = ListTypeOrdered
 	flags := ListItemBeginningOfList
 	// Note: this loop is intentionally explicit, not range-form. This is
@@ -441,7 +440,6 @@ func (p *parser) parseRefsToAST() {
 	above := block.Parent
 	finalizeList(block)
 	p.tip = above
-	finalizeHTMLBlock(p.addBlock(HTMLBlock, []byte("</div>")))
 	block.Walk(func(node *Node, entering bool) WalkStatus {
 		if node.Type == Paragraph || node.Type == Header {
 			p.inline(node, node.content)

node.go

@@ -69,20 +69,21 @@ func (t NodeType) String() string {
 	return nodeTypeNames[t]
 }
 
-// ListData contains fields relevant to a List node type.
+// ListData contains fields relevant to a List and Item node type.
 type ListData struct {
 	ListFlags       ListType
 	Tight           bool   // Skip <p>s around list item data if true
 	BulletChar      byte   // '*', '+' or '-' in bullet lists
 	Delimiter       byte   // '.' or ')' after the number in ordered lists
 	RefLink         []byte // If not nil, turns this list item into a footnote item and triggers different rendering
+	IsFootnotesList bool   // This is a list of footnotes
 }
 
 // LinkData contains fields relevant to a Link node type.
 type LinkData struct {
-	Destination []byte
-	Title       []byte
-	NoteID      int
+	Destination []byte // Destination is what goes into a href
+	Title       []byte // Title is the tooltip thing that goes in a title attribute
+	NoteID      int    // NoteID contains a serial number of a footnote, zero if it's not a footnote
 }
 
 // CodeBlockData contains fields relevant to a CodeBlock node type.