Async version of keywords to identify await points on async drop

language design
1

From async drop - async fn fundamentals initiative:

  • Able to identify the await points that result from async cleanup if needed

This proposal focuses on that point.

Async control flow

There could be async version of control flow keywords namely areturn, abreak, and acontinue. When one or more values must be asynchronously dropped then one of these keywords must be at the end of scope to identify the drop point.

Below is rewritten example from linked above document:

async fn foo(dbc: DatabaseConnection) -> io::Result<()> {
    let data = match socket().read().await {
        Ok(x) => x,
        e => areturn e
    };
    match dbc.write(data).await {
        Ok(_) => areturn Ok(()),
        e => areturn e
    };
}

Perhaps there could be also aawait to identify that async drop may be performed on task cancelation:

async fn foo(dbc: DatabaseConnection) -> io::Result<()> {
    let data = match socket().read().aawait {
        Ok(x) => x,
        e => areturn e
    };
    match dbc.write(data).aawait {
        Ok(_) => areturn Ok(()),
        e => areturn e
    };
}

Notice that because areturn must be explicit there's no support for ? operator when AsyncDrop value is moved in scope. If async drop wouldn't be used often that should be an acceptable compromise.

Async assignment

To asynchronously drop on assignment there could be aassign keyword:

async fn assign_stream(target: &mut TlsStream, source: TlsStream) {
    *target aassign source;
    return // no more `AsyncDrop` implementing value in scope
}

Async move

To explicitly asynchronously drop value we could have the following syntax:

drop(amove value)

Where drop is generic over async keyword.

Perhaps it shouldn't be possible to move AsyncDrop values in a different way, so:

let a = returns_async_drop_value();
let b = amove a;
vec.push(amove b);
drop(amove vec);

let c = returns_async_drop_value();
let d = amove || c;

let e = returns_async_drop_value();
e amove
    .into_method_chain()
    .f().g().h();

Summary

This all of course is very chaotic, based on my very naive understanding of status quo of async drop, and I have no idea how it might work under the hood if it might. I only wanted to demonstrate the idea of prepending "a" to various keywords to identify that they support async drop — the rest is made up and up to implementation.

2

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