Make the echo example async by default (#1427)

Motivation:

The Echo example has an async and a non-async version. We should reduce
it down to just the async version.

Modifications:

- Move the async example to the directory of the existing non-async example
- Add a missing `@main` to the async example

Result:

Echo example is async

Author: glbrntt

Package.swift

@@ -317,23 +317,6 @@ extension Target {
     path: "Sources/Examples/Echo/Runtime"
   )
 
-  static let echoAsync: Target = .executableTarget(
-    name: "AsyncAwaitEcho",
-    dependencies: [
-      .grpc,
-      .echoModel,
-      .echoImplementation,
-      .grpcSampleData,
-      .nioCore,
-      .nioPosix,
-      .logging,
-      .argumentParser,
-    ].appending(
-      .nioSSL, if: includeNIOSSL
-    ),
-    path: "Sources/Examples/Echo/AsyncAwaitRuntime"
-  )
-
   static let helloWorldModel: Target = .target(
     name: "HelloWorldModel",
     dependencies: [
@@ -474,7 +457,6 @@ let package = Package(
     .echoModel,
     .echoImplementation,
     .echo,
-    .echoAsync,
     .helloWorldModel,
     .helloWorldClient,
     .helloWorldServer,

Sources/Examples/Echo/README.md

@@ -8,26 +8,9 @@ There are four subdirectories:
 * `Model/` containing the service and model definitions and generated code,
 * `Implementation/` containing the server implementation of the generated model,
 * `Runtime/` containing a CLI for the server and client using the NIO-based APIs.
-* `AsyncAwaitRuntime/` containing a CLI for the server and client using the
-    async-await–based APIs.
 
 ### CLI implementation
 
-The SwiftPM targets for the NIO-based CLI and the async-await–based CLI are
-`Echo` and `AsyncAwaitEcho` respectively.
-
-The below examples make use the former, with commands of the form:
-
-```sh
-swift run Echo ...
-```
-
-To use the CLI using the async-await APIs, replace these commands with:
-
-```sh
-swift run AsyncAwaitEcho ...
-```
-
 ### Server
 
 To start the server run:

Sources/Examples/Echo/AsyncAwaitRuntime/Echo.swift renamed to Sources/Examples/Echo/Runtime/Echo.swift

@@ -36,6 +36,7 @@ enum RPC: String, ExpressibleByArgument {
   case update
 }
 
+@main
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 struct Echo: AsyncParsableCommand {
   static var configuration = CommandConfiguration(
@@ -247,5 +248,10 @@ func echoUpdate(client: Echo_EchoAsyncClient, message: String) async throws {
 }
 
 #else
-print("ERROR: Concurrency only supported on Swift >= 5.5.")
+@main
+enum Echo {
+  static func main() {
+    print("This example requires Swift >= 5.6")
+  }
+}
 #endif

Sources/Examples/Echo/Runtime/Empty.swift

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2022, gRPC Authors All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// This file exists to workaround https://github.com/apple/swift/issues/55127.