struct Hole<'a, T: 'a> {
    data: &'a mut [T],
    /// `elt` is always `Some` from new until drop.
    elt: Option<T>,
    pos: usize,
}

impl<'a, T> Hole<'a, T> {
    // ...

    unsafe fn get(&self, index: usize) -> &T { &self.data[index] }

    // ...
}