Function format_fn_root_binder_with 

fn format_fn_root_binder_with<T: TypeVisitable>(
    binder: &Binder<T>,
    cx: &PrettyCx<'_, '_>,
    fn_root_layer_type: FnRootLayerType,
    binder_name: &str,
    f: &mut Formatter<'_>,
    fmt_body: impl FnOnce(&PrettyCx<'_, '_>) -> String,
) -> Result

Like format_fn_root_binder but takes a closure to produce the body string, allowing the caller to format multiple pieces (e.g. return type, requires, ensures) together under the same binder context so that bound variables get the correct # decorator.