{"ast_node_type":"variable","tokens":[{"end":95,"kind":"keyword","start":0,"text":"let"}]}
        let msg = if is_arg { "to immutable argument" } else { "twice to immutable variable" };
    ) -> Diag<'infcx> {
        struct_span_code_err!(self.dcx(), span, E0384, "cannot assign {} {}", msg, desc)