syntax_data: {"ast_node_type":"variable","tokens":[{"end":104,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:                 let downcast_place = place_builder.downcast(adt_def, variant_index); // `(x as Variant)`
context_before:             PatKind::Variant { adt_def, variant_index, args, ref subpatterns } => {
context_after:                 cx.field_match_pairs(&mut subpairs, extra_data, downcast_place, subpatterns);