syntax_data: {"ast_node_type":"function","tokens":[{"end":53,"kind":"keyword","start":0,"text":"pub"}]}
source_snippet:     pub fn gate(&self, feature: Symbol, span: Span) {
context_before:     /// which is same `Symbol` used in `unstable.rs`.
context_after:         self.spans.borrow_mut().entry(feature).or_default().push(span);