Implement convenience factory methods for Async[Throwing]Stream

This is the implementation for swiftlang/swift-evolution#1824

stdlib/public/Concurrency/AsyncStream.swift

@@ -349,6 +349,10 @@ public struct AsyncStream<Element> {
       }
     }
   }
+
+  init(storage: _Storage) {
+    self.context = _Context(storage: storage, produce: storage.next)
+  }
 }
 
 @available(SwiftStdlib 5.1, *)
@@ -428,6 +432,47 @@ extension AsyncStream.Continuation {
   }
 }
 
+@available(SwiftStdlib 5.8, *)
+extension AsyncStream {
+    /// Struct for the return type of ``AsyncStream/makeStream(elementType:limit:)``.
+    ///
+    /// This struct contains two properties:
+    /// 1. The ``continuation`` which should be retained by the producer and is used
+    /// to yield new elements to the stream and finish it.
+    /// 2. The ``stream`` which is the actual ``AsyncStream`` and
+    /// should be passed to the consumer.
+  @available(SwiftStdlib 5.8, *)
+  public struct NewStream: Sendable {
+    /// The continuation of the ``AsyncStream`` used to yield and finish.
+    public let continuation: AsyncStream<Element>.Continuation
+
+    /// The stream which should be passed to the consumer.
+    public let stream: AsyncStream<Element>
+
+    public init(stream: AsyncStream<Element>, continuation: AsyncStream<Element>.Continuation) {
+      self.stream = stream
+      self.continuation = continuation
+    }
+  }
+
+  /// Initializes a new ``AsyncStream`` and an ``AsyncStream/Continuation``.
+  ///
+  /// - Parameters:
+  ///   - elementType: The element type of the stream.
+  ///   - limit: The buffering policy that the stream should use.
+  /// - Returns: A ``NewStream`` struct which contains the stream and its continuation.
+  @available(SwiftStdlib 5.8, *)
+  public static func makeStream(
+      of elementType: Element.Type = Element.self,
+      bufferingPolicy limit: Continuation.BufferingPolicy = .unbounded
+  ) -> NewStream {
+    let storage: _Storage = .create(limit: limit)
+    let stream = AsyncStream<Element>(storage: storage)
+    let continuation = Continuation(storage: storage)
+    return .init(stream: stream, continuation: continuation)
+  }
+}
+
 @available(SwiftStdlib 5.1, *)
 extension AsyncStream: @unchecked Sendable where Element: Sendable { }
 #else

stdlib/public/Concurrency/AsyncThrowingStream.swift

@@ -386,6 +386,10 @@ public struct AsyncThrowingStream<Element, Failure: Error> {
       }
     }
   }
+
+  init(storage: _Storage) {
+    self.context = _Context(storage: storage, produce: storage.next)
+  }
 }
 
 @available(SwiftStdlib 5.1, *)
@@ -473,6 +477,49 @@ extension AsyncThrowingStream.Continuation {
   }
 }
 
+@available(SwiftStdlib 5.8, *)
+extension AsyncThrowingStream {
+  /// Struct for the return type of ``AsyncThrowingStream/makeStream(elementType:limit:)``.
+  ///
+  /// This struct contains two properties:
+  /// 1. The ``continuation`` which should be retained by the producer and is used
+  /// to yield new elements to the stream and finish it.
+  /// 2. The ``stream`` which is the actual ``AsyncThrowingStream`` and
+  /// should be passed to the consumer.
+  @available(SwiftStdlib 5.8, *)
+  public struct NewStream: Sendable {
+    /// The continuation of the ``AsyncThrowingStream`` used to yield and finish.
+    public let continuation: AsyncThrowingStream<Element, Failure>.Continuation
+
+    /// The stream which should be passed to the consumer.
+    public let stream: AsyncThrowingStream<Element, Failure>
+
+    public init(stream: AsyncThrowingStream<Element, Failure>, continuation: AsyncThrowingStream<Element, Failure>.Continuation) {
+      self.stream = stream
+      self.continuation = continuation
+    }
+  }
+
+  /// Initializes a new ``AsyncThrowingStream`` and an ``AsyncThrowingStream/Continuation``.
+  ///
+  /// - Parameters:
+  ///   - elementType: The element type of the stream.
+  ///   - failureType: The failure type of the stream.
+  ///   - limit: The buffering policy that the stream should use.
+  /// - Returns: A ``NewStream`` struct which contains the stream and its continuation.
+  @available(SwiftStdlib 5.8, *)
+  public static func makeStream(
+      of elementType: Element.Type = Element.self,
+      throwing failureType: Failure.Type = Failure.self,
+      bufferingPolicy limit: Continuation.BufferingPolicy = .unbounded
+  ) -> NewStream {
+    let storage: _Storage = .create(limit: limit)
+    let stream = AsyncThrowingStream<Element, Failure>(storage: storage)
+    let continuation = Continuation(storage: storage)
+    return .init(stream: stream, continuation: continuation)
+  }
+}
+
 @available(SwiftStdlib 5.1, *)
 extension AsyncThrowingStream: @unchecked Sendable where Element: Sendable { }
 #else

test/Concurrency/Runtime/async_stream.swift

@@ -7,9 +7,6 @@
 // rdar://78109470
 // UNSUPPORTED: back_deployment_runtime
 
-// Race condition
-// REQUIRES: rdar78033828
-
 import _Concurrency
 import StdlibUnittest
 import Dispatch
@@ -27,6 +24,30 @@ var tests = TestSuite("AsyncStream")
         var fulfilled = false
       }
 
+      tests.test("factory method") {
+        let newStream = AsyncStream.makeStream(of: String.self)
+        newStream.continuation.yield("hello")
+
+        var iterator = newStream.stream.makeAsyncIterator()
+        do {
+          expectEqual(try await iterator.next(), "hello")
+        } catch {
+          expectUnreachable("unexpected error thrown")
+        }
+      }
+
+      tests.test("throwing factory method") {
+        let newStream = AsyncThrowingStream.makeStream(of: String.self, throwing: Error.self)
+        newStream.continuation.yield("hello")
+
+        var iterator = newStream.stream.makeAsyncIterator()
+        do {
+          expectEqual(try await iterator.next(), "hello")
+        } catch {
+          expectUnreachable("unexpected error thrown")
+        }
+      }
+
       tests.test("yield with no awaiting next") {
         let series = AsyncStream(String.self) { continuation in
           continuation.yield("hello")