Feature request: !Send Ref

[0xdeafbeef]

Can we implement something like this for Ref under async or RefNotSend feature?

Use case:

async fn fast_computation(data: &str) {
    tokio::time::sleep(Duration::from_secs(1)).await;
    drop(data);
}

async fn test(map: &DashMap<String, String>, data: &str) {
    let value = match map.get(data) {
        None => {
            tokio::time::sleep(Duration::from_secs(10)).await;  // some heavy computation
        }
        Some(a) => fast_computation(a.value()).await, // deadlocks here
    };
}

If Ref was not Send, then this won't compile.

[xacrimon]

Closed by #179 on v5. Release containing this will be put out later today.