#![feature(unboxed_closures)]

fn this<F>(mut f: F)
where
    F: FnMut<()>,
    F::Output: Extend<u8>,
{
    f().extend([1,2,3].iter().cloned());
    f().extend([1,2,3].iter().cloned());
}

fn use_that<T: Extend<u8>>(t: &mut T) {
    this(|| with_offset(t, 1))
}

fn with_offset<T: Extend<u8>>(inner: &mut T, offset: u8) -> impl Extend<u8> + '_ {
    ExtendWithOffset { inner, offset }
}

struct ExtendWithOffset<'a, T> {
    inner: &'a mut T,
    offset: u8
}

impl<'a, T: Extend<u8>> Extend<u8> for ExtendWithOffset<'a, T> {
    fn extend<I: IntoIterator<Item=u8>>(&mut self, iter: I) {
        let o = self.offset;
        self.inner.extend(iter.into_iter().map(|c| {
            c + o
        }));
    }
}

fn main() {
    let mut v = vec![];
    use_that(&mut v);
    assert_eq!(vec![2,3,4,2,3,4], v);
}