syntax_data: {"ast_node_type":"function","tokens":[{"end":85,"kind":"keyword","start":0,"text":"fn"}]}
source_snippet:     fn with_let_source(&mut self, let_source: LetSource, f: impl FnOnce(&mut Self)) {
context_before:     #[instrument(level = "trace", skip(self, f))]
context_after:         let old_let_source = self.let_source;