Support hashbrown hash maps and sets with or without use_std
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Add an optional `hashbrown` feature that switches the underlying hash map implementation from `std::collections` to `hashbrown` and makes items that depend on hash maps available without `use_std` (although it still requires and implicitly enables `use_alloc`). This addresses the motivating use case of [rust-itertools#605][] (which is the same use case that I am interested in) but does not implement a generic approach as the issue suggests. [rust-itertools#901][] would be a better solution long term and would make these changes obsolete, but the PR appears to be stalled and I wanted a simple solution that allows using APIs like `unique`/`unique_by` with `no_std` now. It also partially addresses [rust-itertools#322][] because enabling this feature does switch to *a* faster hash algorithm, it just doesn't allow to specify which one exactly (currently the default hasher in `hashbrown` is `foldhash`, so this is what's going to be used until/unless changed upstream in the future). [rust-itertools#605]: rust-itertools#605 [rust-itertools#901]: rust-itertools#901 [rust-itertools#322]: rust-itertools#322
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Add an optional
hashbrownfeature that switches the underlying hash map implementation fromstd::collectionstohashbrownand makes items that depend on hash maps available withoutuse_std(although it still requires and implicitly enablesuse_alloc).This addresses the motivating use case of #605 (which is the same use case that I am interested in) but does not implement a generic approach as the issue suggests. #901 would be a better solution long term and would make these changes obsolete, but the PR appears to be stalled and I wanted a simple solution that allows using APIs like
unique/unique_bywithno_stdnow.It also partially addresses #322 because enabling this feature does switch to a faster hash algorithm, it just doesn't allow to specify which one exactly (currently the default hasher in
hashbrownisfoldhash, so this is what's going to be used until/unless changed upstream in the future).TODO:
--all-features/-F hashbrownuse_stdI'm opening a draft PR early because I'd like to get feedback on whether it's something you want to have.