`buffer_unordered` footgun: 0 futures

[BatmanAoD]

If n=0 for StreamExt::buffer_unordered(n), then waiting for the stream to complete simply hangs, even if the stream itself is empty. This is a footgun (we ran into it when using len() as the argument to buffer_unordered).

As a breaking change, n could be of type NonZeroUsize instead of usize. As a non-breaking change, would it be possible to make the future complete immediately in the case where the actual underlying stream is empty?

[taiki-e]

Duplicate of #2348, #2740, #2876, etc.

Already fixed in 0.4 (master branch).

AFAIK, the hang here is not an actual hang of the method, but due to the returned future always returning Pending and you only waiting for that future. users of APIs such as select may depend on that behavior, so if we do it in 0.3 as a non-breaking change, it may cause another problem.