fix(markdown): slug a heading line break as the space it renders

src/render/markdown/anchors.rs

@@ -50,7 +50,10 @@ pub(crate) fn resolve_anchors(doc: &Document) -> AnchorMap {
     // inside the heading content) to those slugs.
     walk_blocks(&doc.blocks, &mut |block| {
         if let Block::Heading { anchor, content, .. } = block {
-            let Some(slug) = ids.claim(gfm_slug(&inlines_to_plain_text(content))) else {
+            // A line break renders as a space in a heading, and the slug is
+            // taken from the rendered text.
+            let text = inlines_to_plain_text(content).replace('\n', " ");
+            let Some(slug) = ids.claim(gfm_slug(&text)) else {
                 return;
             };
             let mut bind = |id: &str| {

src/render/markdown/tests.rs

@@ -742,6 +742,24 @@ fn heading_coincident_anchor_uses_slug() {
     assert_eq!(md, "## Section Two\n\n[go](#section-two)\n");
 }
 
+#[test]
+fn heading_line_break_slugs_as_the_space_it_renders() {
+    // A heading renders a line break as a space, so the slug a link to it
+    // uses must separate the words the same way.
+    let md = doc(vec![
+        Block::Paragraph(vec![Inline::Link {
+            content: vec![Inline::plain("go")],
+            target: LinkTarget::Anchor("toc1".into()),
+        }]),
+        Block::Heading {
+            level: 1,
+            anchor: Some("toc1".into()),
+            content: vec![Inline::plain("Part One"), Inline::LineBreak, Inline::plain("Overview")],
+        },
+    ]);
+    assert_eq!(md, "[go](#part-one-overview)\n\n# Part One Overview\n");
+}
+
 #[test]
 fn duplicate_heading_slugs_deduped() {
     let md = doc(vec![