struct Example {
    value: Vec<u8>,
}

struct IteratorExample<'a> {
    value: &'a [u8],
}

impl<'a> Iterator for IteratorExample<'a> {
    type Item = &'a [u8];

    fn next(&mut self) -> Option<Self::Item> {
        // а такой код не работает
        Some(&self.value[..3])

        // None
    }
}

impl<'a> IntoIterator for &'a Example {
    type Item = &'a [u8];
    type IntoIter = IteratorExample<'a>;

    fn into_iter(self) -> Self::IntoIter {
        IteratorExample {
            value: &self.value
        }
    }

}

fn main() {
    let der = Example {
        value: vec![0, 1, 2, 3, 4, 5, 6, 7],
    };
    let mut iter = der.into_iter();
    println!("{:?}", iter.next());
    println!("{:?}", iter.next());
    println!("{:?}", iter.next());

}