Switch default hasher to foldhash::quality#126
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BREAKING (0.7.0). `DefaultHashBuilder` was a public type alias for `ahash::RandomState`, leaking the concrete hasher; the `ahash` feature leaked it too. Both are now implementation-agnostic: - `DefaultHashBuilder` is an opaque newtype wrapping a private, cfg-selected inner hasher. The algorithm can change with no public-API churn. - The `ahash` feature is renamed to `custom-hasher` and pulls the hasher crate via `dep:ahash`, so the crate name no longer appears as a feature. Migration: replace feature `ahash` with `custom-hasher`; don't name the concrete hasher behind `DefaultHashBuilder`.
Exercises the DefaultHashBuilder abstraction: swapping the inner hasher touches only the private `DefaultHasherImpl` alias and the optional dependency — the public `DefaultHashBuilder` type and the `custom-hasher` feature are unchanged. foldhash's `quality` variant gives stronger distribution than its `fast` variant while staying far faster than the std SipHash fallback.
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Summary
Makes the default hasher an implementation detail and switches it to foldhash::quality.
Stacked on #124 (base
evolve-lifecycle-with-state) — review/merge #124 first.1. Abstract the default hasher behind an opaque newtype
DefaultHashBuilderwas a public type alias (= ahash::RandomState), which leaked the concrete hasher — naming it or calling ahash-specific methods made the choice part of the public API. It's now an opaque newtype wrapping a private, cfg-selected inner hasher, so the algorithm can change with no public-API churn. Theahashfeature is likewise renamed tocustom-hasherand wired viadep:foldhash, so the crate name no longer appears in the public feature list either.2. Switch the underlying hasher: ahash → foldhash (quality)
With the type/feature opaque, the inner hasher is just
type DefaultHasherImpl = …. This moves it fromahash::RandomStatetofoldhash::quality::RandomState. Thequalityvariant (overfast) is deliberate: the cache reuses one hash for both shard selection and hashbrown's bucket/tag bits, slicing it into independent bit-windows that need good avalanche (see thecompute_shard_indexcomment).RandomStatekeeps per-instance random seeding.3. Doc note on
compute_shard_indexDocuments why the shard-bit rotation is tied to
usize::BITSrather thanu64::BITS: on 32-bit targets a usize-width hasher (e.g. FxHash) leaves the top 32 bits zero, so the low word is where the entropy lives —u64::BITS / 2there would send every key to shard 0 (hashbrown guards against the same thing inh1/Tag::full).Breaking
features = ["ahash"]→["custom-hasher"].DefaultHashBuilder.custom-hasherto fall back to the std SipHash hasher for adversarial-key workloads.Verification
cargo build/cargo test(56 + doctests) /cargo clippy/cargo doc -D warningsall clean;cargo check --no-default-features(std fallback) compiles.