{"ast_node_type":"variable","tokens":[{"end":72,"kind":"keyword","start":0,"text":"let"}]}
        let index = generics.param_def_id_to_index[&def_id.to_def_id()];
        let generics = tcx.generics_of(item_def_id);
        // HACK(eddyb) should get the original `Span`.