Tracking issue for HashMap::extract_if and HashSet::extract_if

[DutchGhost]

The feature gate for the issue is #![feature(hash_extract_if)] (previously hash_drain_filter)

Currently only Vec and LinkedList have a drain_filter method, while other collections such as HashMap and HashSet do not.

This means that currently, removing items from a collection, and getting ownership of those items is fairly...unidiomatic and cumbersome.

For references, see rust-lang/rfcs#2140

pub mod collections {
    pub mod hash_map {
        impl<K, V, S> HashMap<K, V, S> {
            pub fn extract_if<F>(&mut self, pred: F) -> ExtractIf<'_, K, V, F>
            where
                F: FnMut(&K, &mut V) -> bool;
        }

        pub struct ExtractIf<'a, K, V, F>
        where
            F: FnMut(&K, &mut V) -> bool;

        impl<K, V, F> Iterator for ExtractIf<'_, K, V, F>
        where
            F: FnMut(&K, &mut V) -> bool;

        impl<K, V, F> FusedIterator for ExtractIf<'_, K, V, F>
        where
            F: FnMut(&K, &mut V) -> bool;

        impl<K, V, F> Debug for ExtractIf<'_, K, V, F>
        where
            F: FnMut(&K, &mut V) -> bool;
    }

    pub mod hash_set {
        impl<T, S> HashSet<T, S> {
            pub fn extract_if<F>(&mut self, pred: F) -> ExtractIf<'_, T, F>
            where
                F: FnMut(&T) -> bool;
        }

        pub struct ExtractIf<'a, K, F>
        where
            F: FnMut(&K) -> bool;

        impl<K, F> Iterator for ExtractIf<'_, K, F>
        where
            F: FnMut(&K) -> bool;

        impl<K, F> FusedIterator for ExtractIf<'_, K, F>
        where
            F: FnMut(&K) -> bool;

        impl<K, F> Debug for ExtractIf<'_, K, F>
        where
            F: FnMut(&K) -> bool;
    }
}

Implementation History

• Add drain_filter method to HashMap and HashSet #76458
• removed drain-on-drop behavior, renamed to extract_if
  • Don't drain-on-drop in DrainFilter impls of various collections. #104455
  • Remove drain-on-drop behavior from various DrainFilter iterators libs-team#136

[hellow554]

Related #42849

[mbrubeck]

This may be easy to implement since drain_filter was added to the underlying hashbrown types in rust-lang/hashbrown#135 and rust-lang/hashbrown#179. (Requires hashbrown 0.8.1 or later; see #70052.)

[mbrubeck]

I have implemented this on a branch, but it will need to wait for a new release of the hashbrown crate because it depends on rust-lang/hashbrown#185, rust-lang/hashbrown#187, and rust-lang/hashbrown#188:

master...mbrubeck:hash_drain_filter

[mbrubeck]

This is now implemented in nightly behind the unstable hash_drain_filter feature:

• https://doc.rust-lang.org/nightly/std/collections/struct.HashMap.html#method.drain_filter
• https://doc.rust-lang.org/nightly/std/collections/struct.HashSet.html#method.drain_filter

[benesch]

Any reason not to stabilize this @mbrubeck?

[mbrubeck]

There are some open questions about naming and API design; see the summary in #43244 (comment) for some details.

[blagowtf]

@the8472 Nice. How often are these releases (and do they automatically land in std?) Is the vision ultimately to stabilize the extract_if version in std::collections::HashMap.

[Folyd]

The new version of hashbrown will be released soon. 🎉
rust-lang/hashbrown#434

[saiintbrisson]

Is this ready for the FCP? hashbrown released Jun 5.

[the8472]

On the companion Vec issue some people are asking for a mapped version (passing an owned value) instead so if we implemented that it might make sense to do that for hashmap too.

But that could be discussed as part of the FCP.

[esemeniuc]

Would love to use this feature in stable!

[joshtriplett]

Let's see if we have consensus to stabilize these:

@rfcbot merge

[rfcbot]

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.