Swift 6 warning: No async operations occur

[seanmrich]

I'm auditing an app for Swift 6 support, and I ran into warnings in a TCA dependency while using ActorIsolated. First, this code works as expected:

let isolated = ActorIsolated([Int: String]())
let s = await isolated.value[0]

So far, so good. Now I introduce a simple dependency and initialize it using an inline ActorIsolated value:

struct Client: Sendable {
  var key: @Sendable (Int) async throws -> String
}

let client: Client = {
  let isolated = ActorIsolated([Int: String]())
  return Client { key in
    await isolated.value[key] ?? ""
  }
}()

Again, no problem. But when I add it as a DependencyKey, the warning appears:

extension Client: DependencyKey {
  static let liveValue: Client = {
    let isolated = ActorIsolated([Int: String]())
    return Client { key in
      await isolated.value[key] ?? ""
   // 👆 No 'async' operations occur within 'await' expression
    }
  }()
}

I can't see anything in the declaration of DependencyKey that explains the difference. Is there something I missed? Perhaps a Swift bug?

[stephencelis]

@seanmrich This looks like a Swift bug. I can reproduce outside of swift-dependencies and mainly seems to be the static let assignment:

let f: @Sendable () async -> Int = {
  actor A { var p = 0 }
  return await A().p
}

Want to file a bug with Apple and ask what the expected behavior is?

In your case you can change to a computed static var to fix. But I'd also suggest not using ActorIsolated. We think that LockIsolated should almost always be preferred, and are having enough of a hard time motivating ActorIsolated that we will probably deprecate it soon.

[seanmrich]

Thanks for the help! I started a discussion over at the swift forums to be sure there isn't something new that explains the behavior. Using LockIsolated is good advice for my immediate issue, though. Thanks again!