Why is there no `FusedStream` implementation for `FuturesOrdered`?

[phil-opp]

While FuturesUnordered has a FusedStream implementation, the FuturesOrdered type does not. Is there any reason for this missing implementation or is this just an oversight?

[taiki-e]

I think this is an oversight. (Basically, any stream that can see that itself is empty can implement FusedStream).

[phil-opp]

Good to hear that. A FusedStream implementation would make it much easier to work with FuturesOrdered in a select block. (You can't use the fuse() method when you're adding futures to the set later.)

Would a simple check for FuturesOrdered::len() == 0 suffice for a FusedStream implementation? If so, I'll be happy to send in a PR for this.

[taiki-e]

Would a simple check for FuturesOrdered::len() == 0 suffice for a FusedStream implementation?

I think the correct implementation is self.in_progress_queue.is_terminated() && self.queued_outputs.is_empty().

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(Basically, any stream that can see that itself is empty can implement FusedStream).

This (my comment) is not correct (sorry!).
If I understand correctly, it's probably actually something like:

• Even if it is empty, if it may return Ready(Some) in the future, it should be considered as "terminated" only after Ready(None) is returned once. (e.g., FuturesUnordered that elements may be added)
  (However, FusedStream implementations may be provided that considers is_empty as is_terminated for specific use cases)
• If there is no possibility of returning Ready(Some) in the future, it can be considered as "terminated" regardless of whether Ready(None) was returned once. (e.g., stream::once, stream::empty, stream::pending, StreamExt::take)

(cc @cramertj: my understanding is correct?)