Dynamically sized struct in Arc

I want to create a dynamically sized struct within an Arc, but I can't find a way to do that.

Here's what I'm trying to achieve

struct UnknownSized(usize, [u8]);

fn main() {
    let attached_data: usize = 42;
    let data: Vec<u8> = vec![1, 2, 3, 4, 5]; 

    let arc_object = Arc::new(UnknownSized(attached_data, data)); // Did not work
}

I was expecting just a single level of indirection for this object, rather than using something like a box around a [u8] or wrapping another Arc around a [u8]. No referencing a slice either. i want an owned slice in an Arc with usize. And i want no dependencies either, if possible.

Implementing this by hand is incredibly tricky. Instead, we can lean on the slice-dst crate to take care of the unsafe heavy lifting for us:

use std::sync::Arc;

use slice_dst::SliceWithHeader;

fn example() -> Arc<SliceWithHeader<usize, u8>> {
    let attached_data: usize = 42;
    let data: Vec<u8> = vec![1, 2, 3, 4, 5];

    SliceWithHeader::new(attached_data, data)
}

This stores the usize along with the u8 data inline.

In this case the additional data stored with the slice is a single usize, but you can replace usize with a tuple or named struct should you need to store multiple values alongside the u8 data.