Hardcoded type Hash = H256 in Preimages traits

[muraca]

QueryPreimage and StorePreimage traits use a hardcoded Hash type set to H256

polkadot-sdk/substrate/frame/support/src/traits/preimages.rs

Line 27 in b18fb35

pub type Hash = H256;

Pallet Preimage itself is bounding to this trait I think the opposite way as it should be

polkadot-sdk/substrate/frame/preimage/src/lib.rs

Line 534 in b18fb35

impl<T: Config<Hash = PreimageHash>> QueryPreimage for Pallet<T> {

As I was working on #1445 I incurred in this issue, and I was forced to do some weird things that include

let proposal_hash: PreimageHash = 
    Decode::decode(&mut &T::Hashing::hash_of(&proposal).encode()[..])
        .expect("Hash length is always 32; qed");

and opposite

if new_hash != PreimageHash::from_slice(hash.as_ref()) {
    // ...	
}

or having to use BlakeTwo256 explicitly instead of T::Hashing in tests/benchmarks.

I think it can be fixed by removing the hardcoded type, and putting a type Hash in the traits, to be used like

impl<T: Config> QueryPreimage for Pallet<T> {
    type Hash = T::Hash;
    // ...
}

Should I proceed? If so, should I handle it in #1445 or is it better to open a new PR on which it will depend?

@kianenigma

[bkchr]

Yeah sounds like something that could be made generic. Please create a separate pr for it.

[kianenigma]

Should I proceed? If so, should I handle it in #1445 or is it better to open a new PR on which it will depend?

Yes looks reasonable to me. I suggest proceeding as a small separate PR 👍

[ggwpez]

Just FYI this was done somewhat intentional though:

https://github.com/paritytech/substrate/pull/11649/files#r905676844
But i guess its fine either way.