{"ast_node_type":"impl","tokens":[{"end":66,"kind":"keyword","start":0,"text":"f:"}]}
        f: impl FnOnce() -> ty::CanonicalParamEnvCacheEntry<Self>,
        param_env: ty::ParamEnv<'tcx>,
        from_entry: impl FnOnce(&ty::CanonicalParamEnvCacheEntry<Self>) -> R,