Reconsider Choice of HashMap Hasher

[brody4hire]

As discussed for changes proposed in #6938 - #6938 (comment): this project has been using some kind of "fast" hasher from rustc-hash v1, which we are now planning to use with hashbrown, and I have identified some other possible hashers to recommend evaluating:

• ahash - used by egui
• nohash-hasher - also used by egui
• consider using hasher from rustc-hash v2 - ??? - I found some resources worth checking out:
  • Replace hash with faster and better finalized hash rust-lang/rustc-hash#37
  • https://nnethercote.github.io/2021/12/08/a-brutally-effective-hash-function-in-rust.html
  • https://www.reddit.com/r/rust/comments/rbe3vn/a_brutally_effective_hash_function_in_rust/ - contains some very interesting discussion & comparisons with other hashers
  • https://www.reddit.com/r/rust/comments/1div5vh/rustchash_20/
• I noticed more hashers listed in: https://github.com/RustCrypto/hashes

This kind of decision should be backed up with some benchmark testing.

Context: PR #6938 is part of some preparation I am contributing for no-std support, as requested in: #6826

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P.S. I found some older context here, which I would like to comment on:

• Both fxhash and rustc-hash are in the dependency tree #3369 / PR Replace fxhash with rustc-hash #3502 - it looks to me like the old fxhash crate was some kind of copy of the hasher from rustc-hash. I think it was 100% 200% correct to drop use of the old fxhash in favor of rustc-hash. My one issue with rustc-hash is that it seems to contain both FxHasher, which can be used with hashbrown, and FxHashMap / FxHashSet typedefs which are hard-coded to using std::collections. So moving forward, we should use rustc_hash::FxHasher but not rustc_hash::FxHashMap or rustc_hash::FxHashSet.

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P.S. 2: Considering that this could affect API, as Fitzgerald noted with the label, I start to wonder if this could have serious impact on any external users?

[cwfitzgerald]

I think I just misclassified this, this isn't exposed to the user.

[a1phyr]

foldhash is the new default hasher from hashbrown, it may also be worth considering.

[cwfitzgerald]

I think the default hashbrown hasher is a very sensible default.

[brody4hire]

https://github.com/xacrimon/dashmap - found this in my GitHub recommendations FWIW