{"ast_node_type":"other","tokens":[{"end":86,"kind":"keyword","start":0,"text":"err.span_label(span,"}]}
                err.span_label(span, "change this to accept `FnMut` instead of `Fn`");
            if let Some(span) = arg {
                err.span_label(call_span, "expects `Fn` instead of `FnMut`");