How to concat arrays with differend fixed sizes?

I’m trying Rust and unfortunately I’m already stuck with small tasks like concating u8 arrays.

I am trying to join the following arrays:

let arrayOne: &[u8; 500] = &[0; 500];
let arrayTwo: &[u8; 100] = &[1; 100];
let arrayThree: &[u8] = &[1, 2, 3];

let mergedArray = [arrayOne, arrayTwo, arrayThree].concat();

but im getting the error:

mismatched types
expected an array with a fixed size of 500 elements, found one with 100 elementsrustcClick for full compiler diagnostic
let arrayTwo: &[u8; 100] // size = 8, align = 0x8

I don’t quite understand why this is not allowed and how it is otherwise done.

Especially since the following code seems to be ok:

let arrayOne: &[u8; 500] = &[0; 500];
let arrayThree: &[u8] = &[1, 2, 3];

let mergedArray = [arrayOne, arrayThree].concat();

>Solution :

The problem with the first example is that once the compiler looks at the second item it tries to coerce the &[u8; 500] to a &[u8; 100] and vice versa, neither of which are allowed coercions. It doesn’t see that both could be coerced to the same &[u8] because that would require checking all possible coercion targets every time wich would have a massive performance impact on compilation.

The second example works because once the type of the second element &[u8] is encounterd it sees it can coerce &[u8; 500] to a slice, too and is able to continue.

You can cast the first array reference to a slice to help the compiler figure out that everything should be coerced to slices:

fn main() {
    let array_one: &[u8; 500] = &[0; 500];
    let array_two: &[u8; 100] = &[0; 100];
    let array_three: &[u8] = &[1, 2, 3];

    let merged_array = [array_one as &[_], array_two, array_three].concat();
}