UncheckedSendable: AsyncSequence (#44)

* `UncheckedSendable: AsyncSequence` Currently, a non-sendable sequence cannot be erased to `AsyncStream` using this library's initializers even if one takes care to traffic it through via `nonisolated(unsafe)`: ```swift nonisolated(unsafe) let nonSendable = nonSendable AsyncStream(nonSendable) // 🛑 ``` This conditional conformance acts as a workaround: ```swift AsyncStream(UncheckedSendable(nonSendable)) ``` Ideally folks can stop using our concrete async sequence eraser in favor of `any AsyncSequence<Element, Failure>`, but this requires a minimum deployment target of iOS 18, so it won't be an option for many people for some time. * Xcode 15 support * add deprecation * fix wasm * Update AsyncStream.swift * Update AsyncThrowingStream.swift --------- Co-authored-by: Brandon Williams <[email protected]>
pointfreeco · Oct 14, 2024 · e284ff5 · e284ff5
1 parent 4fee5ec
commit e284ff5
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Showing 4 changed files with 66 additions and 16 deletions.
diff --git a/Sources/ConcurrencyExtras/AsyncStream.swift b/Sources/ConcurrencyExtras/AsyncStream.swift
@@ -71,6 +71,12 @@ extension AsyncStream {
     }
   }
 
+  @available(*, deprecated, message: "Explicitly wrap the given async sequence with 'UncheckedSendable' first.")
+  @_disfavoredOverload
+  public init<S: AsyncSequence>(_ sequence: S) where S.Element == Element {
+    self.init(UncheckedSendable(sequence))
+  }
+
   /// An `AsyncStream` that never emits and never completes unless cancelled.
   public static var never: Self {
     Self { _ in }
@@ -94,4 +100,9 @@ extension AsyncSequence {
   public func eraseToStream() -> AsyncStream<Element> where Self: Sendable {
     AsyncStream(self)
   }
+
+  @available(*, deprecated, message: "Explicitly wrap this async sequence with 'UncheckedSendable' before erasing to stream.")
+  public func eraseToStream() -> AsyncStream<Element> {
+    AsyncStream(UncheckedSendable(self))
+  }
 }
diff --git a/Sources/ConcurrencyExtras/AsyncThrowingStream.swift b/Sources/ConcurrencyExtras/AsyncThrowingStream.swift
@@ -26,6 +26,12 @@ extension AsyncThrowingStream where Failure == Error {
     }
   }
 
+  @available(*, deprecated, message: "Explicitly wrap the given async sequence with 'UncheckedSendable' first.")
+  @_disfavoredOverload
+  public init<S: AsyncSequence>(_ sequence: S) where S.Element == Element {
+    self.init(UncheckedSendable(sequence))
+  }
+
   /// An `AsyncThrowingStream` that never emits and never completes unless cancelled.
   public static var never: Self {
     Self { _ in }
@@ -53,4 +59,9 @@ extension AsyncSequence {
   public func eraseToThrowingStream() -> AsyncThrowingStream<Element, Error> where Self: Sendable {
     AsyncThrowingStream(self)
   }
+
+  @available(*, deprecated, message: "Explicitly wrap this async sequence with 'UncheckedSendable' before erasing to throwing stream.")
+  public func eraseToThrowingStream() -> AsyncThrowingStream<Element, Error> {
+    AsyncThrowingStream(UncheckedSendable(self))
+  }
 }
diff --git a/Sources/ConcurrencyExtras/UncheckedSendable.swift b/Sources/ConcurrencyExtras/UncheckedSendable.swift
@@ -55,6 +55,15 @@ public struct UncheckedSendable<Value>: @unchecked Sendable {
   }
 }
 
+extension UncheckedSendable: AsyncSequence where Value: AsyncSequence {
+  public typealias AsyncIterator = Value.AsyncIterator
+  public typealias Element = Value.Element
+
+  public func makeAsyncIterator() -> AsyncIterator {
+    value.makeAsyncIterator()
+  }
+}
+
 #if swift(>=5.10)
   @available(iOS, deprecated: 9999, message: "Use 'nonisolated(unsafe) let', instead.")@available(
     macOS, deprecated: 9999, message: "Use 'nonisolated(unsafe) let', instead."

diff --git a/Tests/ConcurrencyExtrasTests/AsyncStreamTests.swift b/Tests/ConcurrencyExtrasTests/AsyncStreamTests.swift
@@ -4,33 +4,52 @@
 
   @available(iOS 15, *)
   private let sendable: @Sendable () async -> AsyncStream<Void> = {
-    NotificationCenter.default
-      .notifications(named: UIApplication.userDidTakeScreenshotNotification)
-      .map { _ in }
-      .eraseToStream()
+    UncheckedSendable(
+      NotificationCenter.default
+        .notifications(named: UIApplication.userDidTakeScreenshotNotification)
+        .map { _ in }
+    )
+    .eraseToStream()
+  }
+
+  @available(iOS 15, *)
+  private let sendableInitializer: @Sendable () async -> AsyncStream<Void> = {
+    AsyncStream(
+      UncheckedSendable(
+        NotificationCenter.default
+          .notifications(named: UIApplication.userDidTakeScreenshotNotification)
+          .map { _ in }
+      )
+    )
   }
 
   @available(iOS 15, *)
   private let mainActor: @MainActor () -> AsyncStream<Void> = {
-    NotificationCenter.default
-      .notifications(named: UIApplication.userDidTakeScreenshotNotification)
-      .map { _ in }
-      .eraseToStream()
+    UncheckedSendable(
+      NotificationCenter.default
+        .notifications(named: UIApplication.userDidTakeScreenshotNotification)
+        .map { _ in }
+    )
+    .eraseToStream()
   }
 
   @available(iOS 15, *)
   private let sendableThrowing: @Sendable () async -> AsyncThrowingStream<Void, Error> = {
-    NotificationCenter.default
-      .notifications(named: UIApplication.userDidTakeScreenshotNotification)
-      .map { _ in }
-      .eraseToThrowingStream()
+    UncheckedSendable(
+      NotificationCenter.default
+        .notifications(named: UIApplication.userDidTakeScreenshotNotification)
+        .map { _ in }
+    )
+    .eraseToThrowingStream()
   }
 
   @available(iOS 15, *)
   private let mainActorThrowing: @MainActor () -> AsyncThrowingStream<Void, Error> = {
-    NotificationCenter.default
-      .notifications(named: UIApplication.userDidTakeScreenshotNotification)
-      .map { _ in }
-      .eraseToThrowingStream()
+    UncheckedSendable(
+      NotificationCenter.default
+        .notifications(named: UIApplication.userDidTakeScreenshotNotification)
+        .map { _ in }
+    )
+    .eraseToThrowingStream()
   }
 #endif