Manually Allocating Custom Dynamically Sized Types on the Heap

use std::alloc::{alloc, Layout};
use std::ptr;
 
#[repr(C)]
struct CustomBuffer {
    id: u64,
    data: [u8],
}
 
impl CustomBuffer {
    fn new(id: u64, slice: &[u8]) -> Box<Self> {
        let header_layout = Layout::new::<u64>();
        let array_layout = Layout::array::<u8>(slice.len()).unwrap();
        let (layout, _) = header_layout.extend(array_layout).unwrap();
        let layout = layout.pad_to_align();
        unsafe {
            let raw_ptr = alloc(layout);
            if raw_ptr.is_null() {
                std::alloc::handle_alloc_error(layout);
            }
            let dst_ptr = ptr::slice_from_raw_parts_mut(raw_ptr, slice.len()) as *mut CustomBuffer;
            ptr::write(ptr::addr_of_mut!((*dst_ptr).id), id);
            ptr::copy_nonoverlapping(slice.as_ptr(), ptr::addr_of_mut!((*dst_ptr).data) as *mut u8, slice.len());
            Box::from_raw(dst_ptr)
        }
    }
}