Hash Prefix collisions

[scottmcm]

@H2CO3

You are right. The problem lies elsewhere, namely in <AHasher as Hasher>::write() :

fn write(&mut self, input: &[u8]) {
    let mut data = input;
    let length = data.len();
    self.add_in_length(length as u64);
    // etc.
}

Hmm, it's not clear to me that this isn't a bug.

It feels to me like the wrong place for this -- it's not Hasher::write that should be doing this, but rather [T]::hash. I wouldn't expect that I'd need to give exactly the same sequence of write calls to the Hasher, so long as I give the same bytes.

And if nothing else, it's doing more work that it needs to. Hash is already adding the length for slices, so doing it again here is a performance loss for no good reason: https://doc.rust-lang.org/1.58.0/src/core/hash/mod.rs.html#754-760

I'll go open a bug to clarify expectations. https://doc.rust-lang.org/std/hash/trait.Hash.html#method.hash_slice does a good job of talking about the "whole unit" behaviour, but https://doc.rust-lang.org/std/hash/trait.Hasher.html#tymethod.write is silent on the question.

EDIT: https://github.com/rust-lang/rust/issues/94026