{"ast_node_type":"other","tokens":[{"end":69,"kind":"keyword","start":0,"text":"impl<'tcx>"}]}
impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ExternalConstraints<'tcx> {
// `region_constraints` here or something.
    fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(