Async apis for NonBlockingFileIO

Sources/NIOPosix/NIOThreadPool.swift

@@ -360,4 +360,4 @@ extension NIOThreadPool {
             }
         }
     }
-}
+}

Sources/NIOPosix/UnsafeTransfer.swift

@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftNIO open source project
+//
+// Copyright (c) 2021-2022 Apple Inc. and the SwiftNIO project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+/// ``UnsafeTransfer`` can be used to make non-`Sendable` values `Sendable`.
+/// As the name implies, the usage of this is unsafe because it disables the sendable checking of the compiler.
+/// It can be used similar to `@unsafe Sendable` but for values instead of types.
+@usableFromInline
+struct UnsafeTransfer<Wrapped> {
+    @usableFromInline
+    var wrappedValue: Wrapped
+
+    @inlinable
+    init(_ wrappedValue: Wrapped) {
+        self.wrappedValue = wrappedValue
+    }
+}
+
+extension UnsafeTransfer: @unchecked Sendable {}
+
+extension UnsafeTransfer: Equatable where Wrapped: Equatable {}
+extension UnsafeTransfer: Hashable where Wrapped: Hashable {}
+