provide a constant for digest length size

[rustcrypto]

provide a constant for digest length size

[burdges]

Right now, the digest crate and these hashes use the generic_array crate because Rust only gained const generics recently. I'd hope that once const generics stabilize the generic_array crate will be removed from all crates including this one, and doing so means adding a constant for the digest length.

[newpavlov]

On stabilization of const generics I'll definitely migrate to them, but right now as @burdges said we are tied to generic-array and typenum. If you want to retrieve digest output size at runtime, you can write the following code:

use generic-array::typenum::Unsigned;
let n = Sha256::OutputSize::to_usize();

We could use associated constant in addition to OutputSize, but right now it's fairly useless and I don't see much benefit in introducing it.

Could you please provide use-case for which you need this constant? Otherwise I am inclined to close this issue.

[cardoe]

Plenty of file formats and other protocols internally include a hash in their data. They typically support multiple hashes in that case and have some form of identifier. When dealing with a binary format you decide which hash is in the data based on the identifier and then read DIGEST_LENGTH of bytes from the data. I honestly expected the Digest trait to provide a length method that was #[inline] that returned the size of the digest. The example provided above by @newpavlov is a bit clunky to say the least.

[newpavlov]

Well, I guess until const generics are introduced we probably could add output_size method to FixedOutput and Digest traits.

[newpavlov]

Fixed with digest v0.8. See output_size method on Digest trait.