{"ast_node_type":"variable","tokens":[{"end":84,"kind":"keyword","start":0,"text":"let"}]}
    let only_region_params = |param: &ty::GenericParamDef, _: &_| match param.kind {
    // it, to validate whether or not the impl is legal to instantiate at all.
        GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),