syntax_data: {"ast_node_type":"other","tokens":[{"end":38,"kind":"keyword","start":0,"text":"b:"}]}
source_snippet:         b: ty::ExistentialTraitRef<I>,
context_before:         a: ty::ExistentialTraitRef<I>,
context_after:     ) -> RelateResult<I, ty::ExistentialTraitRef<I>> {