syntax_data: {"ast_node_type":"variable","tokens":[{"end":97,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:                 let base_user_ty = self.canonical_user_type_annotations.push(annotation.clone());
context_before:                 // up represented in MIR _somewhere_.
context_after:                 let subpattern_user_tys = user_tys.push_user_type(base_user_ty);