Add test for runGenerator function

Package.swift

@@ -114,6 +114,14 @@ let package = Package(
             swiftSettings: swiftSettings
         ),
 
+        // New Test Target for swift-openapi-generator
+        .testTarget(
+            name: "OpenAPIGeneratorTests",
+            dependencies: ["swift-openapi-generator"],
+            resources: [.copy("Resources")],
+            swiftSettings: swiftSettings
+        ),
+
         // Generator CLI
         .executableTarget(
             name: "swift-openapi-generator",

Tests/OpenAPIGeneratorReferenceTests/FileBasedReferenceTests.swift

@@ -31,7 +31,7 @@ extension TestConfig {
 }
 
 /// Tests that the generator produces Swift files that match a reference.
-class FileBasedReferenceTests: XCTestCase {
+final class FileBasedReferenceTests: XCTestCase {
 
     /// Setup method called before the invocation of each test method in the class.
     override func setUp() {

Tests/OpenAPIGeneratorTests/Resources/Docs/malformed-openapi.yaml

@@ -0,0 +1,4 @@
+# openapi: '3.1.0' -> Missing openapi version
+info:
+  title: MalformedDocument
+  version: 1.0.0

Tests/OpenAPIGeneratorTests/Resources/Docs/openapi-generator-config.yaml

@@ -0,0 +1,4 @@
+generate:
+  - types
+  - server
+accessModifier: internal

Tests/OpenAPIGeneratorTests/Test_GenerateOptions.swift

@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+import XCTest
+import _OpenAPIGeneratorCore
+import OpenAPIKit
+import ArgumentParser
+@testable import swift_openapi_generator
+
+final class Test_GenerateOptions: XCTestCase {
+
+    var resourcesDirectory: URL! = nil
+
+    /// Setup method called before the invocation of each test method in the class.
+    override func setUpWithError() throws {
+        resourcesDirectory = try XCTUnwrap(
+            Bundle.module.url(forResource: "Resources", withExtension: nil),
+            "Could not find reference test resources directory."
+        )
+    }
+
+    func testRunGeneratorThrowsErrorDiagnostic() async throws {
+        let outputDirectory = URL(fileURLWithPath: "/invalid/path")
+        let docsDirectory = resourcesDirectory.appendingPathComponent("Docs")
+        let docPath = docsDirectory.appendingPathComponent("malformed-openapi.yaml")
+        let configPath = docsDirectory.appendingPathComponent("openapi-generator-config.yaml")
+
+        let arguments = [docPath.path(), "--config", configPath.path]
+
+        let originalArguments = CommandLine.arguments
+        defer { CommandLine.arguments = originalArguments }
+        CommandLine.arguments = arguments
+
+        let generator = try _GenerateOptions.parse(arguments)
+
+        do {
+            try await generator.runGenerator(outputDirectory: outputDirectory, pluginSource: nil, isDryRun: false)
+            XCTFail("Expected to throw an error, but it did not throw")
+        } catch let diagnostic as Diagnostic {
+            XCTAssertEqual(diagnostic.severity, .error, "Expected diagnostic severity to be `.error`")
+        } catch { XCTFail("Expected to throw a Diagnostic `.error`, but threw a different error: \(error)") }
+    }
+}