fn y(_: std::option::Option<&mut dyn std::iter::ExactSizeIterator<Item = u8>>) {
    dbg!("hello");
}

fn z(ret: impl std::iter::ExactSizeIterator<Item = u8> ) -> impl std::iter::ExactSizeIterator<Item = u8> {
    ret
}

fn main() {
    let x = Some(vec![1u8, 2u8, 3u8]);
    let mut iter_option = x.as_ref().map(|v| (v.iter().cloned()));
    y(iter_option.as_mut().map(|i| i as &mut dyn std::iter::ExactSizeIterator<Item = u8>));
}