fn set_parent_link(&mut self, parent: NonNull<InternalNode<K, V>>, parent_idx: usize) {
        let leaf = Self::as_leaf_ptr(self);
        unsafe { (*leaf).parent = Some(parent) };
        unsafe { (*leaf).parent_idx.write(parent_idx as u16) };
    }
Mutably borrows the owned root node. Unlike `reborrow_mut`, this is safe
because the return value
benchmarks/haystacks/code/rust-library.rs