{"ast_node_type":"variable","tokens":[{"end":49,"kind":"keyword","start":0,"text":"let"}]}
    let span = tcx.def_span(checker.impl_def_id);
    let self_ty = tcx.impl_trait_ref(checker.impl_def_id).unwrap().instantiate_identity().self_ty();
    if !tcx.is_builtin_derived(checker.impl_def_id.into()) {