Emit a descriptive error when running the generator on iOS, tvOS, or watchOS

Package.swift

@@ -17,7 +17,12 @@ import PackageDescription
 let package = Package(
     name: "swift-openapi-generator",
     platforms: [
-        .macOS(.v10_15)
+        .macOS(.v10_15),
+
+        // The platforms below are not currently supported for running
+        // the generator itself. We include them here to allow the generator
+        // to emit a more descriptive compiler error.
+        .iOS("99"), .tvOS("99"), .watchOS("99"),
     ],
     products: [
         .executable(name: "swift-openapi-generator", targets: ["swift-openapi-generator"]),

Sources/_OpenAPIGeneratorCore/PlatformChecks.swift

@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+// Emit a compiler error if this library is linked with a target in an adopter
+// project.
+#if !(os(macOS) || os(Linux))
+#error("_OpenAPIGeneratorCore is only to be used by swift-openapi-generator itself—your target should not link this library or the command line tool directly.")
+#endif