syntax_data: {"ast_node_type":"impl","tokens":[{"end":55,"kind":"keyword","start":0,"text":"def_id:"}]}
source_snippet:         def_id: DefId,              // of impl or trait
context_before:         param_env: ty::ParamEnv<'tcx>,
context_after:         args: GenericArgsRef<'tcx>, // for impl or trait