Repository: rust
Issue: Tracking Issue for `Hasher::{write_str, write_length_prefix}` (`feature(hasher_prefixfree_extras)`)
Labels: T-libs-api,C-tracking-issue

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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

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```rust
// core::hash

pub trait Hasher {
    // ... existing stuff ...
    fn write_length_prefix(&mut self, len: usize);
    fn write_str(&mut self, s: &str);
}
```

### Steps / History

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- [x] Implementation: #94598
- [ ] Final comment period (FCP)
- [ ] Stabilization PR

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### Unresolved Questions

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- 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.