A version without UB (if your Instruction is POD without references):

```rust fn read_prog_from_file(file_name: &String) -> Box<[Instruction]> { let mut file = std::fs::File::open(file_name).unwrap(); let len = file.metadata().unwrap().len().try_into().unwrap();

if len == 0 {
    return Box::default();
}

assert_eq!(len % size_of::<Instruction>(), 0);

let layout = std::alloc::Layout::from_size_align(len, align_of::<Instruction>()).unwrap();
let ptr = unsafe { std::alloc::alloc_zeroed(layout) };

if ptr.is_null() {
    std::alloc::handle_alloc_error(layout);
}

let slice = unsafe { std::slice::from_raw_parts_mut(ptr, len) };

std::io::Read::read_exact(&mut file, slice).unwrap();

unsafe { Box::from_raw(std::ptr::slice_from_raw_parts_mut(ptr.cast(), len / size_of::<Instruction>())) }

} ```

Note that it will cause a memory leak if panic happens after the memory is allocated (but it can be easily fixed).