{"ast_node_type":"function","tokens":[{"end":86,"kind":"keyword","start":0,"text":"fn"}]}
    fn expected_fn_found_fn_mut_call(&self, err: &mut Diag<'_>, sp: Span, act: &str) {
    /// Targeted error when encountering an `FnMut` closure where an `Fn` closure was expected.
        err.span_label(sp, format!("cannot {act}"));