syntax_data: {"ast_node_type":"impl","tokens":[{"end":43,"kind":"keyword","start":0,"text":"visit_leaf:"}]}
source_snippet:     visit_leaf: &mut impl FnMut(C, &mut T),
context_before:     // Called when visiting a "leaf" candidate (with no subcandidates).
context_after:     // Called when visiting a "node" candidate (with one or more subcandidates).