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