why

slice-lifetime-weirdness.rs

fn slice_of_slice<'pool, 'slice, 'ret>(
    pool: &'pool [usize],
    slice: &'slice [usize],
) -> &'ret [usize]
where
    'pool: 'slice, // `pool` outlives `slice`
    'slice: 'ret,  // `slice` outlives the return value
{
    // Most of this is bullshit so that both `slice` and `pool` are used.
    let pool_idx = slice[0];
    let slice_idx = pool[pool_idx];
    &slice[slice_idx..slice_idx + 1]
}

#[test]
fn test_slice_of_slice() {
    let pool = vec![1, 2, 3, 1, 2, 99, 3, 1, 1, 1, 2, 3, 4, 2];
    let ret = {
        let slice = &pool[3..6]; // == &[1, 99, 2]
        slice_of_slice(&pool, slice)
    }; // `slice` is dropped here.
    // `ret` is still alive.
    assert_eq!(ret, &[99]); // I thought `slice` had to outlive `ret`!
}