Can "const fn" in rust concatenate byte slices?

Question

When I compile the following code snippet:

struct Packet([u8; 4]);

impl Packet {
    const fn from(labels: [&[u8; 2]; 2]) -> Packet {
        let mut bytes = [0; 4];
        bytes[..2].copy_from_slice(labels[0]);
        bytes[2..].copy_from_slice(labels[1]);
        Packet(bytes)
    }
}

const AA: &[u8; 2] = b"AA";
const BB: &[u8; 2] = b"BB";
const CC: &[u8; 2] = b"CC";

const AABB: Packet = Packet::from([AA, BB]);
const AACC: Packet = Packet::from([AA, CC]);

I get the following compiler error:

error[E0723]: mutable references in const fn are unstable
 --> src/main.rs:7:9
  |
7 |         bytes[..2].copy_from_slice(labels[0]);
  |         ^^^^^^^^^^
  |
  = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
  = help: add `#![feature(const_fn)]` to the crate attributes to enable

The error is very clear: mutable references in const fn are not yet part of stable Rust. But maybe there's a way to achieve that in stable Rust without using mutable references?

I know I could do this instead:

const AABB: Packet = Packet(*b"AABB");
const AACC: Packet = Packet(*b"AACC");

But in this case, I'm not reusing the "AA" constant which is precisely what I'm trying to achieve.

Thanks for any help on this matter!

Answer 1

You can use an array literal and supply the values like so:

impl Packet {
    const fn from(labels: [&[u8; 2]; 2]) -> Packet {
        let bytes = [labels[0][0], labels[0][1], labels[1][0], labels[1][1]];
        Packet(bytes)
    }
}