fix: keep a label on its added equation

src/render.rs

@@ -18,12 +18,29 @@ pub fn render(results: &[DiffResult]) -> String {
     out.push_str(PREAMBLE);
     out.push('\n');
 
-    for result in results {
+    let mut results = results.iter().peekable();
+    while let Some(result) = results.next() {
         match result {
             DiffResult::Unchanged(block) => {
                 render_block(block, &mut out);
                 out.push('\n');
             }
+            // A label after the wrapper would label the styled wrapper, which
+            // cannot be referenced. Keep it inside, next to the equation.
+            DiffResult::Added(Block::Equation { content, .. })
+                if results
+                    .peek()
+                    .is_some_and(|next| next_label(next).is_some()) =>
+            {
+                let label = results.next().and_then(next_label).unwrap();
+                write!(
+                    out,
+                    "#diff-added[{} {label}]",
+                    escape_content(content, false)
+                )
+                .unwrap();
+                out.push('\n');
+            }
             DiffResult::Added(block) => {
                 render_added_or_deleted(block, DiffTag::Added, &mut out);
                 out.push('\n');
@@ -95,6 +112,19 @@ fn is_label_only(text: &str) -> bool {
     TypstLabel::is_only(text)
 }
 
+/// Returns the label if this result is a label that will be output bare.
+fn next_label(result: &DiffResult) -> Option<&str> {
+    match result {
+        DiffResult::Unchanged(Block::Paragraph { source_text })
+        | DiffResult::Added(Block::Paragraph { source_text })
+            if is_label_only(source_text) =>
+        {
+            Some(source_text.trim())
+        }
+        _ => None,
+    }
+}
+
 /// Render a block wrapped in #diff-added[...] or #diff-deleted[...].
 fn render_added_or_deleted(block: &Block, tag: DiffTag, out: &mut String) {
     let escape_refs = tag == DiffTag::Deleted;

tests/integration.rs

@@ -209,3 +209,15 @@ fn test_indented_line_comment_does_not_split_paragraph() {
     // A whitespace-only line would be treated as a paragraph break by Typst.
     assert!(!output.contains("\n  \n"));
 }
+
+#[test]
+fn test_label_after_changed_equation_stays_on_equation() {
+    let old = "#set math.equation(numbering: \"(1)\")\n\n$ x = 1 $ <eq-main>\n\nSee @eq-main.\n";
+    let new = "#set math.equation(numbering: \"(1)\")\n\n$ x = 2 $ <eq-main>\n\nSee @eq-main.\n";
+    let output = run_diff(old, new);
+
+    // Outside the wrapper the label would attach to the styled wrapper, and
+    // `@eq-main` would fail with "cannot reference styled".
+    assert!(output.contains("#diff-added[$ x = 2 $ <eq-main>]"));
+    assert!(!output.contains("\n<eq-main>\n"));
+}