stream: add stream extend function

[blasrodri]

Allows extending a Vec with a Stream<Item=T>.

Ref: #2842

Motivation

Solution

[MikailBag]

Shouldn't it be generic over C: std::iter::Extend (and then using coll.extend(std::iter::once(...))?
That way HashSet, String and other types will also be supported.

[blasrodri]

Shouldn't it be generic over C: std::iter::Extend (and then using coll.extend(std::iter::once(...))?
That way HashSet, String and other types will also be supported.

It makes sense. However, in order to call .await() I need extend_one() to be stable. Or do you have another idea?

[MikailBag]

coll.extend_one(x) can be replaced with coll.extend(std::iter::once(x)) IIUC

[Darksonn]

I haven't decided if we want this yet, but I noticed that it could be simplified.

[Darksonn]

It's unfortunate that we are not able to reserve space before-hand using the Stream::size_hint method when using the Extend trait.

[blasrodri]

@Darksonn shall we close this PR?

[Darksonn]

I've decided to close this because I feel that this is not the best possible API for this function. I want an API that:

1. Is not restricted to Vec.
2. Is able to use the size_hint of the stream.

This PR only satisfies the first. You should feel free to open an issue proposing a different API if you have an implementation idea that satisfies these.

[blasrodri]

@taiki-e based on your comment do you think we should wait until rust-lang/rust#72631 in order to implement this properly?

[Darksonn]

That would be one solution, but it's unclear when and if it will be merged, which is why I didn't just merge this now to wait for that.