Interesting timing, we were discussing it yesterday in Matrix https://matrix.to/#/!BHcierreUuwCMxVqOf:matrix.org/$e8f4Ojv7DjdxQcGhRarquhTGY88GPgchLVjG93ZPjxE?via=matrix.org&via=catircservices.org&via=mozilla.org

IMO it's a bit niche. For example, the digest crate itself doesn't provide a similar adapter for std::io. What is your use case for this? Do you have something that writes data to a Write and you want to just hash it and not keep the data? (if you keep the data you can just hash the &[u8] afterwards)

Also the rust-crypto ecosystem isn't known for its stability. Latest digest is 0.10, but there's already 0.11 release candidates in the pipeline. If we do this we should name the features digest-010 and digest-011.

thanks for taking a look. i find these traits far superior to those in std::io, so i tend to prefer using these, even for projects that have the standard library available.

long story short, i am currently working on a JSON Web Token library, which often needs to write data after encoding it & authenticating it. it ends up being intuitive to chain APIs that implement embedded_io::Write for this use case. i never care about keeping the data around because i have the original version.

e.g. writing HS256 signed data encoded as base64url becomes incredibly simple (can be done directly to a TCP stream)

fn main() {
 let mut stdout = FromStd::new(std::io::stdout());
 let mut authenticator = AuthenticatedWriter::new(&mut stdout, Sha256::new());
 let mut encoder = Base64BlockEncoder::new(&mut authenticator);
 encoder.write_all(b"hello world").unwrap();
 let (_, _) = encoder.finalize(false).unwrap();
 let hash = authenticator.finalize(b"some secret for HS256");
 stdout.write_all(b"\nsignature: ").unwrap();
 stdout.write_all(hex::encode(hash).as_bytes()).unwrap();
}

output:

aGVsbG8gd29ybGQ
signature: b0d79a7623c6be2bc62685176eefe28b2b8f320ba2fb6093391b66204827f2f1

as for the digest crate not providing an adapter, i think there are 2 reasons:

1. std::io doesn't provide a way to specify the error type, meaning there's no way to use Infallible as the return type
2. digest & all the other RustCrypto traits haven't been updated in ages. they have not updated to use const generics because presumably it is a lot of work to migrate everything across multiple crates.