reuse.rs

// 🚨 This is UB in current Rust!
fn new_vec_reusing_memory<T, U>(mut old: Vec<U>) -> Vec<T> {
    if size_of::<T>() == 0
        || size_of::<U>() == 0
        || align_of::<T>() > align_of::<U>()
        || old.capacity() == 0
    {
        return vec![];
    }
    old.clear();
    let p = old.as_mut_ptr();
    let cap_bytes = old.capacity() * size_of::<U>();
    let new_cap = cap_bytes / size_of::<T>();
    mem::forget(old);
    Vec::from_raw_parts(p as *mut T, 0, new_cap)
}