for the following defination:

    struct AsyncWrapper<F, R, T, U>(F, PhantomData<T>)
    where
        F: Fn(&mut T) -> R + Send + Sync,
        U: Message + 'static,
        T: Message + Default + 'static,
        R: Future<Output = U> + Send + 'static;

only T is considered as unused, thus requires the PhantomData, why?

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