Commits on Mar 1, 2021

1. Allow calling UnboundedReceiver::try_next after None …

   Allow calling `UnboundedReceiver::try_next` and `Receiver::try_next`
   after `None`: do not panic.
   
   Not-panicking is equally safe, and does not have negative performance
   implication.
   
   It is irrelevant for `Stream` implementation to panic or not (because
   `Stream` behavior is unspecified after `None`), but panicking in
   `try_next` just complicates the interface: returned `Ok(None)` is
   reasonable assumption to have.
   
   Consider this use case: drain the queue on drop by performing
   app-specific cleanup of queued messages.
   
   The obvious implementation would be:
   
   ```
   impl Drop for MyReceiverWrapper {
       fn drop(&mut self) {
           while let Ok(Some(m)) self.try_next() {
               cleanup(m);
           }
       }
   }
   ```
   
   Without this change, I cannot even say for sure how this code need
   to be implemented to avoid panicking. E. g. is `is_closed` enough
   or some additional checks need to be performed?

   

   stepancheg committed Mar 1, 2021
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