syntax_data: {"ast_node_type":"variable","tokens":[{"end":50,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:                 let is_union = adt_def.is_union();
context_before:             ExprKind::Adt(box AdtExpr { adt_def, variant_index, args, fields, .. }) => {
context_after:                 let active_field_index = is_union.then(|| fields[0].name);