Drain/retain for HashSet?

I am wondering whether there is a nicely efficient way to remove and collect a subset of elements from a HashSet according to a predicate. My current best guess is to drain the entire set, pipe that through partition and add back in the subset that I want to retain. This seems terribly inefficient for an operation that seems like it could be done in place. Does anyone have a better suggestion?

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There is an open issue for adding these methods to HashSet BTreeMap and BTreeSet, and a tracking issue for some of the implementation work involved.

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Sorry, I got confused between HashSet and BTreeSet. Currently we have the following methods:

What you really want is HashSet::drain_filter, which does not exist yet. This is issue #59618.


Sorry, @mbrubeck, seems you were still a little confused. There is no BTreeSet::drain, because it's not very useful without range, and you can already do such a full drain on virtually any container with:

std::mem::take(&mut container).into_iter()


Thanks, that's exactly what I was looking for. It turns out that the hashbrown HashMap does have drain_filter which is a good second best for me at the moment. I love that the hashbrown crate seems to double as a way to access unstable features of the standard library.


A warning for anyone using the drain_filter method in the hashbrown crate: It was accidentally implemented with opposite semantics from the ones in std. Instead of removing items that match the predicate, it removes items that do not match.

The next major release of hashbrown will fix this, by changing the behavior of drain_filter to match the standard library. Unfortunately, this is a silent breaking change (i.e., it changes behavior but does not trigger any compile-time errors or warnings). Existing crates that uses these methods will become incorrect if they upgrade to hashbrown > 0.8 without also changing their own code.


How did this get through testing? Weren't any of the std::collections::HashMap::drain() tests applied to hashbrown::HashMap::drain_filter()?

drain doesn't take a predicate, so none of its tests could catch this.

It might have helped to have shared tests between HashMap::drain_filter and BTreeMap::drain_filter, but the former was added to hashbrown before the latter was implemented in libstd, so this wasn't an option at the time.

My fix for the hashbrown issue added test cases adapted from std::collections.


hashbrown 0.9.0 is now released, and all users of its drain_filter methods will need to update their code when upgrading to this version.

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