syntax_data: {"ast_node_type":"other","tokens":[{"end":73,"kind":"keyword","start":0,"text":"(ty::GenericArgKind::Lifetime(_),"}]}
source_snippet:                 (ty::GenericArgKind::Lifetime(_), ty::Bivariant) => None,
context_before:                 (_, ty::Invariant) => Some((i, arg)),
context_after:                 _ => panic!("unexpected opaque type arg variance"),