{"ast_node_type":"function","tokens":[{"end":64,"kind":"keyword","start":0,"text":"fn"}]}
    fn new(dcx: DiagCtxtHandle<'dcx>, root_span: Span) -> Self {
impl<'dcx> CollectTrackerAndEmitter<'dcx, '_> {
        Self { dcx, remaining_matcher: None, best_failure: None, root_span, result: None }