Support peek() for mpsc::Receiver

[shade]

Is your feature request related to a problem? Please describe.
I have an application that needs to wait for a value to show up in a channel within a certain time-frame before running some business logic.

Describe the solution you'd like
The ideal solution would be to have a async peek() functionality which is ready when a value has been sent but does not consume the value.

let (sender, receiver) = mpsc::channel(10);

timeout(receiver.peek(), TIMEOUT).await?;

// Hand off the receiver to some task to process all the value.
business_logic_task(receiver);

Describe alternatives you've considered
Alternative 1. Is to await on recv() and write our business logic to work with the first channel value as a special case.

Alternative 2. Is to wrap the Receiver in a ReceiverStream, convert it into a Peekable stream and call peek on that.

My main gripe is that after the conversion to Peekable, we cannot get the original receiver back as we have a lot of logic that depends on Receiver type and we will have to change this to be Peekable or worse, a generic Stream.

[shade]

FWIW, I'm happy to write out the PR. But would like to know if there's any strong objection before doing so.

[Darksonn]

My initial reaction is that I don't love this feature request.

It sounds like you don't actually need peek to return the value?

[shade]

It sounds like you don't actually need peek to return the value?

Indeed, we just want a notification that the receiver is ready to be recv()-d or try_recv()-d without consumption of any values. Maybe async fn ready() would be a better?

Also, seems like some other folks have been asking for similar feature as well hence the peek idea:

• https://stackoverflow.com/questions/73405054/wait-for-an-mpsc-channel-to-have-messages-without-receiving-any-message
• Add peek method on Receiver crossbeam-rs/crossbeam-channel#12 (though this is not async rust)

Would be curious to understand any objections you may have :)