Concurrency Carve-outs

Examples and review guidance for the Swift 6 concurrency rules in ../SKILL.md.

Actor-owned responsibilities

State with a lifecycle, multiple operations, or observers belongs in an actor: process registry, file watcher state, socket session table, retry/idempotency state, provider lifecycle state.

Single-method actor smell

actor ResumeGuard {
    func claim() -> Bool { ... }
}

A synchronous callback needs an immediate compare-and-set, not a Task { await ... } hop. Use the lock carve-out.

Lock carve-out

Canonical case: a process termination handler, a timeout callback, and a spawn failure callback all race to resume exactly one withCheckedContinuation. Keep the guard private to the type, non-blocking, and documented on the declaration. Never for ongoing domain state.

Sendability escape hatches

// Wraps DispatchSourceFileSystemObject; every mutation happens on `queue`.
private final class WatcherAttachment: @unchecked Sendable { ... }

// UserDefaults is Apple-documented thread-safe; OK to read nonisolated.
private nonisolated(unsafe) let defaults: UserDefaults

Narrow the escape hatch to one property rather than marking an entire actor or value type unchecked.

Review reject-list

Reject diffs introducing any of these in new code without a documented carve-out:

• @Published, ObservableObject
• DispatchQueue.main.async, DispatchQueue.asyncAfter
• addObserver(_:forKeyPath:...)
• queue-as-lock synchronization
• a lock guarding ongoing mutable state
• an actor whose only job is guarding a flag
• Task.sleep / Clock.sleep used to poll, settle, or race
• @unchecked Sendable or nonisolated(unsafe) without a safety comment