task: implement FromIterator for JoinSet

[erikdesjardins]

Motivation

When using JoinSet I usually end up populating it from an iterator, which would be more convenient with collect.

Solution

Implement FromIterator.

(Possibly, spawning each element of the iter is too magical / side-effecty for a FromIterator impl though.)

Closes: #6132

[maminrayej]

Related to issue #6132.

@Darksonn does this have conflicts with #5924?
My understanding is that this would have conflicts if it collected handles. Since this implementation spawns the futures using the JoinSet itself, it's fine.

[hawkw]

@Darksonn does this have conflicts with #5924?
My understanding is that this would have conflicts if it collected handles. Since this implementation spawns the futures using the JoinSet itself, it's fine.

If we were to make the change described in #5924, we could potentially add a FromIterator impl where the iterator's Item is JoinHandle. This would, potentially, be more flexible in terms of how the tasks in the iterator are spawned, because the current FromIterator impl spawns the tasks using JoinSet::spawn, but an iterator of JoinHandles could also contain JoinHandles that were spawned using spawn_local or spawn_blocking. However, because JoinHandle implements Future, the FromIterator implementation for iterators of JoinHandles would conflict with the FromIterator implementation for iterators of F: Futures.

Note that you could pass an iterator of JoinHandles to the current FromIterator implementation, but it would spawn a second task that just awaits the JoinHandle's completion, which introduces some unnecessary overhead (including an additional task allocation). But, it's worth mentioning that JoinHandles can be used with the interface in this PR, even if they probably shouldn't.

However, I think the conclusion from #5924 was that we're probably not going to add an API for adding JoinHandles to a JoinSet, because we'd like to eventually make an internal optimization in JoinSet where the JoinSet linked list nodes are inlined into the task's allocation (see #5924 (comment)). Therefore, I don't really think we should worry too much about the potential addition of a FromIterator implementation for Iterator<Item = JoinSet>, because we're probably not going to do that...

[Darksonn]

Thank you.