Adding polyfill back for 5.8 support

Sources/LanguageServerProtocol/Additions/AsyncStreamPolyfill.swift

@@ -0,0 +1,46 @@
+import Foundation
+
+#if compiler(<5.9)
+
+// @available(SwiftStdlib 5.1, *)
+extension AsyncStream {
+	/// 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 tuple containing the stream and its continuation. The continuation should be passed to the
+	/// producer while the stream should be passed to the consumer.
+	// @backDeployed(before: SwiftStdlib 5.9)
+	static func makeStream(
+		of elementType: Element.Type = Element.self,
+		bufferingPolicy limit: Continuation.BufferingPolicy = .unbounded
+	) -> (stream: AsyncStream<Element>, continuation: AsyncStream<Element>.Continuation) {
+		var continuation: AsyncStream<Element>.Continuation!
+		let stream = AsyncStream<Element>(bufferingPolicy: limit) { continuation = $0 }
+		return (stream: stream, continuation: continuation!)
+	}
+}
+
+// @available(SwiftStdlib 5.1, *)
+extension AsyncThrowingStream {
+	/// 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 tuple containing the stream and its continuation. The continuation should be passed to the
+	/// producer while the stream should be passed to the consumer.
+	// @backDeployed(before: SwiftStdlib 5.9)
+	static func makeStream(
+		of elementType: Element.Type = Element.self,
+		throwing failureType: Failure.Type = Failure.self,
+		bufferingPolicy limit: Continuation.BufferingPolicy = .unbounded
+	) -> (stream: AsyncThrowingStream<Element, Failure>, continuation: AsyncThrowingStream<Element, Failure>.Continuation) where Failure == Error {
+		var continuation: AsyncThrowingStream<Element, Failure>.Continuation!
+		let stream = AsyncThrowingStream<Element, Failure>(bufferingPolicy: limit) { continuation = $0 }
+		return (stream: stream, continuation: continuation!)
+	}
+}
+#endif