[Pitch #2] Structured Concurrency

[ktoso (Konrad 'ktoso' Malawski 🐟🏴‍☠️)]

@michelf

It's confusing that await is used for these APIs:

try await Task.checkCancellation()

await Task.isCancelled()

await Task.currentPriority()

There should be no await unless calling those might actually introduce a suspension point. We should use await to mean "here may be a suspension point", not "here's a function that requires an async context". Otherwise you train people to consider await as likely spurious, like I did at first with group.add .

I've long wanted to add to the proposal an @instantaneous also known as "can not actually suspend", specifically because those APIs.

It would have the effect of:

1. allowing calls to those functions without await
2. forbid the use of await inside the function body of the instantaneous function.

i.e.:

@instantaneous func isCancelled() async -> Bool { ... } 

if Task.isCancelled() { ... } 

The wording I thought of this being instantaneous is because "can not suspend", but totally open to other names.

The same semantics would really help [Pitch] Task Local Values, reading a task local value never suspends, and having to await on them is pretty annoying:

let id = await Task.local(\.traceID) // meh

Perhaps this is the right proposal to include this new capability?