Are you literally me 2 months ago? I was trying to implement the same HashFunction trait as yours and I finally settled with:

    pub trait HashFunction<const OUTPUT_SIZE: usize> {
        fn hash(data: &[u8]) -> [u8; OUTPUT_SIZE];
    }

I believe this approach has some shortcomings in my case, but it has been a while and I didn't remember anything.

Also for the Rust crowd, is it possible to do something like this?

    pub trait HashFunction<const OUTPUT_SIZE: usize> {
        // I want Output to always be [u8; OUTPUT_SIZE], but using associated type means
        // whatever implements HashFunction can redefine Output.
        type Output;

        fn hash(data: &[u8]) -> Output;
    }