mpsc::Sender should provide a way to check if a send will succeed

[carllerche]

In some cases, it would be very helpful to be able to check if the Sender will be able to accept a new value before "generating" this value. Right now, using Sender always requires an extra "buffer" slot to accept values that are rejected. However, the Sender knows if it can accept another value...

This applies to a number of other Sink implementors as well.

[carllerche]

I propose Sink::poll_ready (in part so that poll_ready shows up when I search the issue tracker!)

[alexcrichton]

We likely want to add this in the form of a try_send method which just returns the message back if it couldn't be sent (e.g. to make it atomic)

[carllerche]

Try_send doesn't satisfy the original issue which wants a way to know if the channel can accept a value before generating it.

[carllerche]

Being able to check before generating will (probably) be an important feature to correctly implement Service backpressure.

[alexcrichton]

Ah sorry yes right, forgot about that!

[carllerche]

This is resolved by Sender::poll_ready()