trait Stream {
    type Output;

    fn next(&self) -> Option<Self::Output>;
}

struct Foo<'a, T: 'a>(&'a T);

impl<'a, T: 'a> Stream for Foo<'a, T> {
    type Output = &'a T;

    fn next(&self) -> Option<Self::Output> {
        Some(&self.0)
    }
}

fn main() {
    let i = 123;
    let foo = Foo(&i);

    assert_eq!(foo.next(), Some(&123));
}