syntax_data: {"ast_node_type":"other","tokens":[{"end":45,"kind":"keyword","start":0,"text":".map(|(i,"}]}
source_snippet:             .map(|(i, subpattern)| FieldPat {
context_before:             .enumerate_and_adjust(expected_len, gap_pos)
context_after:                 field: FieldIdx::new(i),