{"ast_node_type":"variable","tokens":[{"end":56,"kind":"keyword","start":0,"text":"let"}]}
            let new = RegionCtxt::Placeholder(reg_info);
            let mut var_to_origin = self.type_checker.infcx.reg_var_to_origin.borrow_mut();
            let prev = var_to_origin.insert(reg.as_var(), new);