I'm getting a weird error message related to RPITIT. Rustc complains that the return type is unsized and thus a Self: Sized bound has to be added to the trait method. It also says that it's needed in order to make the impl<D: Distrib> Distrib for &D blanket impl work. Using a concrete return type instead makes the code compile successfully.

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trait Distrib {
    type Sample;

    fn samples(&self) -> impl Iterator<Item = Self::Sample>
    // Why does rustc insist that this is needed?
    // What does it have to do with the size of the return type?
    where
        Self: Sized,
    {
        Samples(self) // self is a reference and thus sized
    }

    // Using a concrete return type works fine without a Sized bound
    fn samples2(&self) -> Samples<&Self> {
        Samples(self)
    }
}

impl<D: Distrib> Distrib for &D {
    type Sample = D::Sample;
}

struct Samples<D>(D);

impl<D: Distrib> Iterator for Samples<D> {
    type Item = D::Sample;
    fn next(&mut self) -> Option<Self::Item> {
        todo!()
    }
}

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