syntax_data: {"ast_node_type":"other","tokens":[{"end":52,"kind":"keyword","start":0,"text":"#[allow(rustc::potential_query_instability)]"}]}
source_snippet:         #[allow(rustc::potential_query_instability)]
context_before:         let head = self.stack.next_index();
context_after:         self.provisional_cache.retain(|&input, entries| {