syntax_data: {"ast_node_type":"other","tokens":[{"end":36,"kind":"keyword","start":0,"text":"Mutability::Mut"}]}
source_snippet:                 Mutability::Mut => {
context_before:                 Mutability::Not => (ty::Covariant, VarianceDiagInfo::None),
context_after:                     (ty::Invariant, VarianceDiagInfo::Invariant { ty: a, param_index: 0 })