61dfb1f8d0
Make async generators fused by default I actually changed my mind about this since the implementation PR landed. I think it's beneficial for `async gen` blocks to be "fused" by default -- i.e., for them to repeatedly return `Poll::Ready(None)` -- rather than panic. We have [`FusedStream`](https://docs.rs/futures/latest/futures/stream/trait.FusedStream.html) in futures-rs to represent streams with this capability already anyways. r? eholk cc ```@rust-lang/wg-async,``` would like to know if anyone else has opinions about this.