`const fn` hashing

[rubdos]

Hi!

I'm using Shake256 in a setting where all input data is known at compile-time, a case where the hash value could be computed at compile-time too.

I've been using tiny-keccak before (I'm not sure why anymore, exactly), and I started toying with it to support const fn hashing. I've bounced on some hurdles, which is why I made that issue.

This week I realized I could probably start the same discussion here: how painful would const fn-hashing be to implement? I feel like it would probably not work due to the Digest family of traits (trait fns cannot be const afaik), but I'd nevertheless would want to discuss!

Best regards,

Ruben

[newpavlov]

It will depend on progress of impl const, which, unfortunately, does not even has an accepted RFC. In theory we may add inherent const methods, but I would prefer not to go this direction. Also before constifying implementations I would prefer to have const_eval_select. Without it we would be unable to add intrinsics or assembly based implementations without breaking backward compatibility.

As a stopgap solution I would use build scripts to compute hashes over data. Maybe procedural macros could work as well?

[rubdos]

It will depend on progress of impl const, which, unfortunately, does not even has an accepted RFC. In theory we may add inherent const methods, but I would prefer not to go this direction. Also before constifying implementations I would prefer to have const_eval_select. Without it we would be unable to add intrinsics or assembly based implementations without breaking backward compatibility.

Thanks for documenting what needs to be in place!

As a stopgap solution I would use build scripts to compute hashes over data. Maybe procedural macros could work as well?

I already have a build script in place, so it might make sense to do it in a build script indeed. It's generated code. It would be a bit cleaner without it though.

What do you say, leave this issue open for now? I might get my hands dirty and make a tiny-keccak fork (say, const-keccak) to have a base implementation without any traits.

[tarcieri]

There are already a few const fn implementations of hash functions, FWIW:

• https://crates.io/crates/const-sha1
• https://crates.io/crates/sha2-const

[rubdos]

I was tempted to use sha2-const, but I'm kind-of in love with Keccak and sponges... :'-)