Tracking Issue for Hasher::{write_str, write_length_prefix} (feature(hasher_prefixfree_extras))

[scottmcm]

Feature gate: #![feature(hasher_prefixfree_extras)]

This is a tracking issue for the new provided methods on Hasher added to fix #94026

• write_str lets the hasher customize how it works with str, so it can use the b'\xFF' trick if it's byte-wise, or a different approach if it does chunked rounds.
• write_length_prefix gives an obvious choice when implementing Hash for collections (like VecDeque) which can't just use the slice hash, and allows the hasher to optimize how best to represent the length.

Public API

// core::hash

pub trait Hasher {
    // ... existing stuff ...
    fn write_length_prefix(&mut self, len: usize);
    fn write_str(&mut self, s: &str);
}

Steps / History

• Implementation: Add a dedicated length-prefixing method to Hasher #94598
• Final comment period (FCP)
• Stabilization PR

Unresolved Questions

• What should the write_str provided implementation be? It was added matching the previous impl Hash for str behaviour to get in without breaking hash checks (like the one in cargo), but that's not always prefix-free (it depends on the round strategy), so there's an argument that a different implementation would be better.

[tgross35]

Possible bikeshed: it was indirectly mentioned in passing here that this is the only std API that comes up when you search length. The rest all seem to use len