syntax_data: {"ast_node_type":"impl","tokens":[{"end":56,"kind":"keyword","start":0,"text":"args:"}]}
source_snippet:         args: GenericArgsRef<'tcx>, // for impl or trait
context_before:         def_id: DefId,              // of impl or trait
context_after:         parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,