Use CanIgnore atom kind

src/diff/changes.rs

@@ -47,6 +47,7 @@ pub(crate) fn insert_deep_unchanged<'a>(
             },
         ) => {
             for (child, opposite_child) in node_children.iter().zip(opposite_children) {
+                // TODO: one side may now be CanIgnore
                 insert_deep_unchanged(child, opposite_child, change_map);
             }
         }

src/display/json.rs

@@ -267,6 +267,7 @@ impl Highlight {
                 AtomKind::Comment => Highlight::Comment,
                 AtomKind::Type => Highlight::Type,
                 AtomKind::Normal => Highlight::Normal,
+                AtomKind::CanIgnore => Highlight::Normal,
                 AtomKind::TreeSitterError => Highlight::TreeSitterError,
             },
         }

src/display/style.rs

@@ -359,7 +359,7 @@ pub(crate) fn color_positions(
                                 style = style.bold();
                             }
                             AtomKind::TreeSitterError => style = style.purple(),
-                            AtomKind::Normal => {}
+                            AtomKind::Normal | AtomKind::CanIgnore => {}
                         }
                     }
                 }

src/parse/syntax.rs

@@ -392,8 +392,14 @@ fn set_content_id(nodes: &[&Syntax], existing: &mut DftHashMap<ContentKey, u32>)
                 // Recurse first, so children all have their content_id set.
                 set_content_id(children, existing);
 
-                let children_content_ids: Vec<_> =
-                    children.iter().map(|c| c.info().content_id.get()).collect();
+                let children_content_ids: Vec<_> = children
+                    .iter()
+                    .filter(|c| match c {
+                        List { .. } => true,
+                        Atom { kind, .. } => *kind != AtomKind::CanIgnore,
+                    })
+                    .map(|c| c.info().content_id.get())
+                    .collect();
 
                 (
                     Some(open_content.clone()),
@@ -579,6 +585,11 @@ pub(crate) enum AtomKind {
     Comment,
     Keyword,
     TreeSitterError,
+    /// Trailing commas can be ignored in some positions, such as the
+    /// last comma in `[1, 2,]` in JS. However, it's not obligatory,
+    /// and it's useful when diffing `[1,]` against `[1, 2]` to be
+    /// able to match up the commas.
+    CanIgnore,
 }
 
 /// Unlike atoms, tokens can be delimiters like `{`.

src/parse/tree_sitter_parser.rs

@@ -1875,15 +1875,12 @@ fn list_from_cursor<'a>(
     if should_ignore_last_child(config, &root_node, &between_delim) {
         if let Some(last_child) = between_delim.pop() {
             if let Syntax::Atom {
-                position,
-                content,
-                kind,
-                ..
+                position, content, ..
             } = last_child
             {
                 let position = position.clone();
-                // TODO: New unused kind for atoms.
-                let new_last_child = Syntax::new_atom(arena, position, content, *kind);
+                let new_last_child =
+                    Syntax::new_atom(arena, position, content, AtomKind::CanIgnore);
                 between_delim.push(new_last_child);
             }
         }