sync: breaking changes to watch channel.

[carllerche]

Tracks breaking changes to sync::watch

Now that broadcast exists, watch should be specialized to track a single value. The API should be made more consistent with other channels.

• "sender shutdown" is out of scope and Receiver::recv() can just return T.
• Sender should have send fn and not broadcast.

[cdmistman]

Are you suggesting that watch::Sender::send should be consuming like oneshot::Sender::send? In one of my projects, I'm using watch over broadcast because I only need the most recent value (I'm sending time-sensitive but loss-permissive signals from a 'master' task to several other tasks), but I can't use oneshot because I need to send multiple values.

I'd like to propose an alternative that won't break current uses of either channel type, but still allows each channel type to fill its niche: making oneshot be single-producer multi-consumer. This can be accomplished by adding oneshot::Sender::subscribe and/or impl<T> Clone for oneshot::Receiver<T>.

[Diggsey]

@donnellycolton I think you may have got the wrong idea?

One thing that would be useful is if there were convenience methods that made it act more like a Cell. For example, methods to access the value from the Sender side so you don't have to store an extra copy.