syntax_data: {"ast_node_type":"other","tokens":[{"end":93,"kind":"keyword","start":0,"text":"cx.field_match_pairs(&mut"}]}
source_snippet:                 cx.field_match_pairs(&mut subpairs, extra_data, downcast_place, subpatterns);
context_before:                 let downcast_place = place_builder.downcast(adt_def, variant_index); // `(x as Variant)`