syntax_data: {"ast_node_type":"other","tokens":[{"end":70,"kind":"keyword","start":0,"text":"debug_assert!(_old_hash.is_none()"}]}
source_snippet:         debug_assert!(_old_hash.is_none() || _old_hash == Some(hash));
context_before:         let _old_hash = hygiene_data.foreign_expn_hashes.insert(expn_id, hash);
context_after:         let _old_id = hygiene_data.expn_hash_to_expn_id.insert(hash, expn_id);