Use isolated(any) instead of Sendable for withMainSerialExecutor async operation

[jonduenas]

When working with a MainActor isolated type inside the async operation closure for withMainSerialExecutor, the context was not being inferred as MainActor isolated. This can be easily demonstrated like this:

@MainActor
final class Foo {
    func sleep() async {
        try? await Task.sleep(for: .seconds(1))
    }
}

final class MainSerialExecutorActor: XCTestCase {
    @MainActor
    func testWithMainSerialExecutor() async {
        await withMainSerialExecutor {
            let foo = Foo() // ❌ Expression is 'async' but is not marked with 'await'
            await foo.sleep()
        }
    }
}

This only started happening after the change in #28.

@stephencelis pointed out on Slack that this can be resolved by using @isolated(any) instead of @Sendable on withMainSerialExecutor's operation closure. Locally this resolves the issue on my end.

[mercedesbunz]

I am also facing that issue when using https://github.com/pointfreeco/swift-concurrency-extras/releases/tag/1.2.0

Using your example @jonduenas

@MainActor
final class Foo {
	var didSleep: Bool = false

    func sleep() async {
        try? await Task.sleep(for: .seconds(1))
		didSleep = true
    }
}

final class MainSerialExecutorActor: XCTestCase {
    @MainActor
    func testWithMainSerialExecutor() async {
        await withMainSerialExecutor {
            let foo = await Foo() // adding `await`
            await foo.sleep()

			// assuming we don't need to do something to work with `Task.sleep`
			XCTAssertTrue(sut.didSleep, "Expected foo slept") // 🛑 - Main actor-isolated property 'didSleep' can not be referenced from a non-isolated autoclosure
        }
    }
}

I Added await for the construction to work around the issue and that feels ok to me but it starts to "hurt" when asserting.
Although that can be fixed by moving state access into a MainActor.run closure I guess.

[stephencelis]

Thanks!