Use new final element support from AsyncStreaming

Examples/EchoServer/EchoServer.swift

@@ -23,18 +23,9 @@ struct EchoServer {
     }
 
     static func echo<Server: HTTPServer>(server: Server) async throws {
-        try await server.serve { request, requestContext, requestBodyAndTrailers, responseSender in
-            // Needed since we are lacking call-once closures
-            var requestBodyAndTrailers = Optional(requestBodyAndTrailers)
-            let responseBodyAndTrailers = try await responseSender.send(.init(status: .ok))
-
-            try await responseBodyAndTrailers.produceAndConclude { responseBody in
-                // Needed since we are lacking call-once closures
-                var responseBody = responseBody
-                return try await requestBodyAndTrailers.take()!.consumeAndConclude { reader in
-                    try await responseBody.write(reader)
-                }
-            }
+        try await server.serve { request, requestContext, reader, responseSender in
+            let writer = try await responseSender.send(.init(status: .ok))
+            try await reader.pipe(into: writer)
         }
     }
 }

Examples/ExampleMiddleware/ForwardingMiddleware.swift

@@ -26,7 +26,7 @@ public struct ForwardingMiddleware<Input: ~Copyable & ~Escapable>: Middleware {
 }
 
 @available(anyAppleOS 26.0, *)
-extension Middleware {
+extension Middleware where Input: ~Copyable & ~Escapable, NextInput: ~Copyable & ~Escapable {
     public func forwarding() -> ForwardingMiddleware<Input> {
         ForwardingMiddleware()
     }

Examples/ExampleMiddleware/HTTPClientMiddlewareInput.swift

@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift HTTP API Proposal open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift HTTP API Proposal project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+public import AsyncStreaming
+public import HTTPAPIs
+
+/// The input passed through client-side middleware: the request head plus the
+/// request body the user wants to send.
+///
+/// Mirrors ``HTTPServerMiddlewareInput`` on the server side. Wrapping
+/// middlewares can substitute a different `Writer` type for `NextInput` so
+/// the inner stage sees a wrapped body that intercepts the bytes the user
+/// wrote.
+@available(anyAppleOS 26.0, *)
+public struct HTTPClientMiddlewareInput<Writer: CallerAsyncWriter & ~Copyable & SendableMetatype>: ~Copyable
+where Writer.WriteElement == UInt8, Writer.FinalElement == HTTPFields? {
+    public var request: HTTPRequest
+    public var body: HTTPClientRequestBody<Writer>?
+
+    public init(request: HTTPRequest, body: consuming HTTPClientRequestBody<Writer>?) {
+        self.request = request
+        self.body = body
+    }
+}
+
+@available(*, unavailable)
+extension HTTPClientMiddlewareInput: Sendable {}