{"ast_node_type":"other","tokens":[{"end":55,"kind":"keyword","start":0,"text":"///"}]}
    /// active field index would identity the field `c`
    /// -- e.g., for a union expression `SomeUnion { c: .. }`, the
    Adt(DefId, VariantIdx, GenericArgsRef<'tcx>, Option<UserTypeAnnotationIndex>, Option<FieldIdx>),