ACP: `Iterator::collect_into_owned()`

Proposal

Problem statement

Sometimes we want to "collect" an iterator, except we want to initialize the collection in some way first. For example, we might want to collect into a Vec, but first reserve some capacity. Or we might want to collect into a HashMap, but use a custom BuildHasher that doesn't implement Default.

There exists an unstable Iterator::collect_into method. However, using it for this purpose requires explicitly declaring a variable, then calling collect_into with that, then naming the variable again for the resulting owned collection. This is unergonomic.

Solution sketch

Add a provided method to the Iterator trait.

pub trait Iterator {
    // existing items omitted

    fn collect_into_owned<E: Extend<Self::Item>>(self, collection: E) -> E
    where
        Self: Sized
    { .... }
}

With this method, we can now easily write code like my_iter.collect_into_owned(Vec::with_capacity(n)) or my_iter.collect_into_owned(HashMap::with_hasher(build_hasher)).

Alternatives

• We could name the methods differently. For example, we could rename the existing collect_into to collect_in_mut, and name my proposed method collect_into.
• We could have this method exist only in the itertools crate, and not on the stdlib's Iterator.
• We could change the currently implemented collect_into to instead have the function signature of my proposed method, and have a blanket impl<E: Extend> Extend for &mut E. This way, this one method would work both for collecting into borrowed collections and into owned collections. However, this is a breaking change, since &mut is a fundamental type. Here's a cursory search for code on github that would break if we do this.

Links and related work

I first mentioned this idea at https://github.com/rust-lang/rust/issues/94780#issuecomment-3041901253, where it got positive emote reactions.

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

Start by reading the linked Iterator::collect_into discussion and the libs-api feature lifecycle. This issue is awaiting team review of the API proposal; done means a libs-api decision and, if approved, a follow-up implementation path rather than a self-contained patch.