syntax_data: {"ast_node_type":"function","tokens":[{"end":67,"kind":"keyword","start":0,"text":"fn"}]}
source_snippet:     fn index_mut(&mut self, index: Location) -> &mut Self::Output {
context_before: impl<T> IndexMut<Location> for LocationMap<T> {
context_after:         &mut self.map[index.block][index.statement_index]