std: Add performance warnings to HashMap::get_disjoint_mut

[xizheyin]

Closes #139296

The get_disjoint_mut in HashMap also performs a complexity O(n^2) check. So we need to be reminded of that as well.

https://github.com./rust-lang/hashbrown/blob/b5b0655a37e156f9798ac8dd7e970d4adba9bf90/src/raw/mod.rs#L1216-L1220

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[mattfbacon]

To be honest I don't understand why, why not just sort the list of hashes and check neighbours (n log n)?

[xizheyin]

I'm not sure. I think what you said seems reasonable.We can wait for the members of libs to discuss it. Maybe we can change the implementation.

[mattfbacon]

So, we can't do it with the hashes due to collisions, and T doesn't have PartialOrd.

However, I think it would be possible to do everything up to getting the raw pointer of each element to return, then use this technique there. The performance is worse in the case of an overlap, but I think that is irrelevant since it panics in that case.

But honestly it's probably not worth it, most use cases are probably 2 or 3 keys where the perf would probably even be worse. So I would go ahead with documenting it.

[zirconium-n]

So, we can't do it with the hashes due to collisions, and T doesn't have PartialOrd.

However, I think it would be possible to do everything up to getting the raw pointer of each element to return, then use this technique there. The performance is worse in the case of an overlap, but I think that is irrelevant since it panics in that case.

But honestly it's probably not worth it, most use cases are probably 2 or 3 keys where the perf would probably even be worse. So I would go ahead with documenting it.

Actually std delegates to hashbrown which is indeed already comparing the pointers, so you can sort it. However just like what you said, it's not like we are really going to query 100 keys, and there's unsafe version of API for that anyway.

[xizheyin]

r? libs