Actor isolation and non-sendable closure (use higher-order function inside Actor)

How to adjust forEachAsync for use within TestActor without a warning? It should stay reusable in other places.

I need TestActor to pass around non-sendable stuff within it. And I want to use higher-order convenience functions like forEachAsync within it. Execution is sequential on purpose.

Build settings -> Strict Concurrency Checking: Complete

class NotSendable { }

actor TestActor {
    func foo() async {
        let nonSendables = [NotSendable(), NotSendable()]

        // No problem
        for notSendable in nonSendables {
            await bar(notSendable)
        }

        // Warning: Sending 'nonSendables' risks causing data races
        await nonSendables.forEachAsync{ notSendable in
            await bar(notSendable)
        }
    }
    
    private func bar(_: NotSendable) async { }
}

extension Sequence {
    func forEachAsync(_ operation: (Element) async -> Void) async {
        for element in self {
            await operation(element)
        }
    }
}

Solution:

class NotSendable { }

actor TestActor {
    func foo() async {
        let nonSendables = [NotSendable(), NotSendable()]
        
        await nonSendables.forEachAsync{ notSendable, isolation in
            await bar(notSendable, isolation)
        }
    }
    
    private func bar(
        _: NotSendable,
        _ isolation: isolated (any Actor)? = #isolation
    ) async {
    }
}

extension Sequence {
    func forEachAsync(
        _ operation: (Element, isolated (any Actor)?) async -> Void,
        isolation: isolated (any Actor)? = #isolation
    ) async {
        for element in self {
            await operation(element, isolation)
        }
    }
}

It's ugly but it works in Xcode 16.2 with Swift Language Version set to Swift 6 in the Build Settings. You can still use forEachAsync outside of an Actor, in which case isolation is nil.

Sources:
SE-0313 – Improved control over actor isolation
SE-0420 – Inheritance of actor isolation

Hopefully this proposal will be accepted and make things easier and prettier:
SE-0461 – Run nonisolated async functions on the caller's actor by default