Support retrieving an arbitrary element about hashbrown HOT 6 CLOSED

As you said, this is a very niche use case and I don't think that it is appropriate to support this API in hashbrown. Have you considered using IndexMap which does support your use case?

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cc @ms705

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Another option is to keep a long lived fork of hashbrown, which I think would be much less painful than a fork of the standard library. The standard hashmap uses a ton of unstable features, including a couple compiler internal features. My fork could thus only run on nightly and even then frequently breaks without warning because of some compiler change. The only way I've stood a chance at keeping it working was to signup for notification emails any time there was a change to the hash_map directory in the main Rust repository.

To me this points to a broader point that the standard library is too tightly coupled to the compiler. hashbrown gets better performance despite not relying on these internal features.

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@Amanieu unfortunately it's a little complicated by the fact that we are accessing the map through evmap, and I don't think evmap will want to switch from hashbrown to IndexMap... That said, maybe the trick will be to somehow make evmap generic over the underlying map (if that doesn't require higher-kinded trait bounds that is)... Thanks anyway!

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An alternative here I suppose is to emulate Python's dict::popitem which just removes and returns an arbitrary (key, value). This has the added benefit that you can do things like:

while let Some((key, value)) = map.popitem() {
    // ...
    map.insert(key2, value2);
}

The downside though is that if it removes, say, the first item by hash value, it will always remove the same key if you re-insert it, which makes it a poor choice for eviction. One workaround for that might be to remove the first element following a random starting position.

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Just for completeness and for others looking at this, this ties in to rust-lang/rust#27804 and rust-lang/rfcs#1800

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