Remove need for ActorQueueSynchronization (#40)

Author: dfed

Sources/AsyncQueue/ActorQueue.swift

@@ -60,15 +60,26 @@ public final class ActorQueue<ActorType: Actor>: @unchecked Sendable {
         let (taskStream, taskStreamContinuation) = AsyncStream<ActorTask>.makeStream()
         self.taskStreamContinuation = taskStreamContinuation
 
-        Task { @ActorQueueSynchronization in
+        func beginExecuting(
+            _ operation: sending @escaping (isolated ActorType) async -> Void,
+            in context: isolated ActorType
+        ) {
+            // In Swift 6, a `Task` enqueued from an actor begins executing immediately on that actor.
+            // Since we're running on our actor's context already, we can just dispatch a Task to get first-enqueued-first-start task execution.
+            Task {
+                await operation(context)
+            }
+        }
+
+        Task {
+            // In an ideal world, we would isolate this `for await` loop to the `ActorType`.
+            // However, there's no good way to do that without retaining the actor and creating a cycle.
             for await actorTask in taskStream {
-                // In Swift 6, a `Task` enqueued from a global actor begins executing immediately on that global actor.
-                // Since we're running on a global actor already, we can just dispatch a Task to get first-enqueued-first-start
-                // task execution. In an ideal world, we wouldn't need a global actor and would isolate this `for await` loop on
-                // the `ActorType`. However, there's no good way to do that just yet.
-                Task {
-                    await actorTask.task(actorTask.executionContext)
-                }
+                // Await switching to the ActorType context.
+                await beginExecuting(
+                    actorTask.task,
+                    in: actorTask.executionContext
+                )
             }
         }
     }
@@ -152,10 +163,3 @@ public final class ActorQueue<ActorType: Actor>: @unchecked Sendable {
         let task: @Sendable (isolated ActorType) async -> Void
     }
 }
-
-/// A global actor used for synchronizing task execution.
-@globalActor
-private struct ActorQueueSynchronization {
-    fileprivate actor Synchronization {}
-    fileprivate static let shared = Synchronization()
-}