drain_filter for all collections

[Havvy]

For all collections, provide a method drain_filter. For some discussion of the method name and signature, see rust-lang/rust#43244, especially this comment.

Status

Implemented, awaiting stabilization

• Vec, LinkedList: Tracking issue for Vec::extract_if and LinkedList::extract_if rust#43244
• BTreeMap, BTreeSet: Tracking Issue for {BTreeMap,BTreeSet}::extract_if rust#70530
• HashMap, HashSet: Tracking issue for HashMap::extract_if and HashSet::extract_if rust#59618

Unimplemented

• BinaryHeap: Add drain_filter for BinaryHeap rust#42849
• VecDeque
• String

Description

This method mutably borrows the collection and takes a predicate on the elements of the collection (e.g. values for lists and key-value pairs for maps). It then creates an iterator over the elements of the collection where the predicate returned truthfully and accessing that element in the iterator removes it from the collection. For ordered collections, it should return them in order.

Example usage

let v = vec![1, 2, 3, 4, 5, 6, 7, 8];
let primes = v.drain_filter(|n| n.is_prime()).collect::Vec<_>();
assert!(v == vec![1, 4, 6, 8]);
assert!(primes == vec![2, 3, 5, 7]);

Related ideas

Also of use would be a method that finds and removes the first element matching a predicate, as it would not have to do things like pre-poop its pants in case the Drain Where iterator leaks nor would it have to traverse the entire collection if e.g. the first element checked matches the predicate.

[eddyb]

For Vec see rust-lang/rust#43245 - although I believe all collections can eventually have this.

[mark-i-m]

I can't say how many times I've needed something like this!

[WaffleLapkin]

VecDeque doesn't have drain_filter either. Shouldn't it be included in the unimplemented list?

[Boscop]

@WaffleLapkin VecDeque doesn't have splice either (or something similar that allows extending at the front), but it should!
(SliceDeque supports splice and drain_filter but doesn't work on wasm.)

[mbrubeck]

Added VecDeque to the checklist above.

[Boscop]

@mbrubeck What about String::drain_filter? It has retain whose closure gets a char (not &mut char), so we need drain_filter as well for String.