syntax_data: {"ast_node_type":"variable","tokens":[{"end":78,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:         let _old_id = hygiene_data.expn_hash_to_expn_id.insert(hash, expn_id);
context_before:         debug_assert!(_old_hash.is_none() || _old_hash == Some(hash));
context_after:         debug_assert!(_old_id.is_none() || _old_id == Some(expn_id));