syntax_data: {"ast_node_type":"other","tokens":[{"end":61,"kind":"keyword","start":0,"text":"used_to_be_allowed:"}]}
source_snippet:     used_to_be_allowed: Option<FutureCompatOverlapErrorKind>,
context_before:     impl_def_id: LocalDefId,
context_after: ) -> Result<(), ErrorGuaranteed> {