[Returned for Revision] SE-0505: Delayed Enqueuing for Executors

[Jumhyn (Frederick Kellison-Linn)]

Hello Swift community,

The review of SE-0505: Delayed Enqueuing for Executors ran from January 15th to January 27th, 2026. The LSG has decided to return this proposal for revision based on the feedback provided.

Several reviewers raised the point that the proposed API does not provide sufficient surface to support job-level revocation of jobs submitted to an executor. While it is true that this problem exists in the implementation today, as exhibited by the example below:

@johannesweiss

---

If it helps, to make the problem visible today, you could run this

cat > /tmp/repro.swift <<"EOF"
@main
struct Example {
    static func main() async throws {
        while true {
            do {
                async let _ = Task.sleep(for: .seconds(1000000000000000))
                try? await Task.sleep(for: .nanoseconds(0))
            } // EDIT: This is non-obvious but this `}` _will cancel_ all unawaited `async let`s such
              // as the `async let _ = Task.sleep(for: .seconds(1000000000000000))` above.
              // And yet: It accumulates resources --> Structured Concurrency violation
        }
    }
}
EOF

it also the case that it is feasible to day to fix this issue as an implementation detail of the native executors the Swift runtime uses. The LSG is concerned that SE-0505 as written would effectively preclude such job-level revocation from being implemented by executors which choose to adopt SchedulingExecutor. When we eventually got around to solving this problem, this would leave us in the difficult position of having to leave the old, sub-optimal API in place in order to preserve compatibility, and would at best lock us into a long deprecation cycle in order to retire it.

In order to avoid this state of affairs, the LSG would like to see a revision of this proposal which more thoroughly grapples with the issue of job-level revocation.

Thank you to everyone who participated in the review for helping to make Swift a better language.

Freddy Kellison-Linn
Review Manager