{"ast_node_type":"variable","tokens":[{"end":82,"kind":"keyword","start":0,"text":"let"}]}
            let (provided_ty, provided_arg_span) = provided_arg_tys[provided_idx];
            let (formal_ty, expected_ty) = formal_and_expected_inputs[expected_idx];
            let trace = mk_trace(provided_arg_span, (formal_ty, expected_ty), provided_ty);