{"ast_node_type":"other","tokens":[{"end":75,"kind":"keyword","start":0,"text":"ProjectionElem::Downcast(_/*adt_def*/,"}]}
                ProjectionElem::Downcast(_/*adt_def*/, _/*variant_idx*/) =>
                // assigning to P[i] requires P to be valid.
                // assigning to (P->variant) is okay if assigning to `P` is okay