How to infallibly read into a `Box<[T]>`?

[adriandelgado]

I wrote the function box_from_prefix that is similar to read_from_prefix except that it is infallible (or potentially just emit alloc errors). This is my first try at implementing it with unsafe:

fn box_from_prefix<T: FromBytes>(src: &[u8]) -> (Box<[T]>, &[u8]) {
    let dst_count = src.len() / size_of::<T>();
    let dst_size = dst_count * size_of::<T>();
    let mut dst: Box<[MaybeUninit<T>]> = Box::new_uninit_slice(dst_count);
    unsafe {
        std::ptr::copy_nonoverlapping(src.as_ptr(), dst.as_mut_ptr() as *mut u8, dst_size);
        (dst.assume_init(), &src[dst_size..])
    }
}

And this is my safe implementation that unnecessarily needs an extra IntoBytes trait bound and also requieres to zero initialize the memory.

fn box_from_prefix2<T: FromBytes + IntoBytes>(src: &[u8]) -> (Box<[T]>, &[u8]) {
    let dst_count = src.len() / size_of::<T>();
    let dst_size = dst_count * size_of::<T>();
    let mut dst: Box<[T]> = FromZeros::new_box_zeroed_with_elems(dst_count).unwrap();

    let dst_bytes = dst.as_mut_bytes();
    let (src_bytes, rest) = src.split_at(dst_size);
    dst_bytes.copy_from_slice(src_bytes);
    (dst, rest)
}

Maybe there's another way to read directly into a box or vec that I haven't thought about.

[adriandelgado]

I think it is similar to #2243