Macro timeout 

macro_rules! timeout {
    ($duration:expr, $future:expr) => { ... };
    ($cx:expr, $duration:expr, $future:expr) => { ... };
}

Runs a future with a timeout.

This macro races the provided future against a sleep, returning the result if it completes in time, or an error if it times out.

Semantics

let result = timeout!(Duration::from_secs(5), operation).await;

match result {
    Ok(value) => println!("Completed: {:?}", value),
    Err(Elapsed) => println!("Timed out"),
}

Cancellation Behavior

When timeout fires:

1. Main future is cancelled
2. Cancellation follows standard protocol (drain + finalize)
3. timeout! returns after main future is fully drained

When main future completes:

1. Sleep is cancelled
2. timeout! returns immediately (sleep cleanup is fast)