Crate indexmap uses RawTable<usize> instead of HashSet<usize> in its core implementation.
Then I checked hashbrown and there is no documentation on when RawTable should be used. I guess it's because the core implementation of indexmap is independent with hash state, as is RawTable, but HashSet is not?
Also RawTable says it with an unsafe API, does that literally mean it includes some extra unsafe methods, or does it include its equivalent to the general HashMap and HashSet methods as well with extra places to look out for?
indexmap does that because the actual keys are stored outside of the table. HashSet<usize> wouldn't work because there's no way to search that for a String key, for example. Using a RawTable<usize> offers callbacks that we can use to lookup the keys in the entries Vec.
There is another question related to hashbrown: std uses (literially) the HashMap implementation of hashbrown. Then std, as a crate, should use hashbrown crate as a dependency. At this point then introduce hashbrown crate, according to the rules of cargo, there should be no additional code added to the final binary. Is this actually the case? I'm actually surprised that std can contain a "third-party" crate itself.
std is prebuilt and distributed as part of the rust toolchain. If you use hashbrown as a dependency it will still be pulled in and built by cargo just like any other crate.
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Cargo doesn't know about those internal-dependency crates, so it resolves your dependency independently and builds that with a different mangling hash. You can actually try to use the hashbrown that's in the sysroot if you declare extern crate hashbrown; without any dependency in your Cargo.toml, but you'll get an error that this is an unstable location. On nightly, you can forge ahead anyway with #![feature(rustc_private)], but there is no support if anything breaks there.
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