[Plugin] Add unit tests for plugin support library

Author: rintaro

Package.swift

@@ -201,6 +201,12 @@ let package = Package(
         "SwiftRefactor", "SwiftSyntaxBuilder", "_SwiftSyntaxTestSupport",
       ]
     ),
+    .testTarget(
+      name: "SwiftCompilerPluginTest",
+      dependencies: [
+        "SwiftCompilerPlugin",
+      ]
+    ),
   ]
 )
 

Sources/SwiftCompilerPlugin/Macros.swift

@@ -16,8 +16,8 @@ import SwiftSyntaxMacros
 
 /// Implementation for `CompilerPlugin` macro related request processing.
 extension CompilerPlugin {
-  // @testable
-  public func resolveMacro(moduleName: String, typeName: String) -> Macro.Type? {
+  public // @testable
+  func resolveMacro(moduleName: String, typeName: String) -> Macro.Type? {
     let qualifedName = "\(moduleName).\(typeName)"
 
     for type in self.providingMacros {

Tests/SwiftCompilerPluginTest/CompilerPluginTests.swift

@@ -0,0 +1,66 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import XCTest
+import SwiftCompilerPlugin
+import SwiftSyntax
+import SwiftSyntaxMacros
+
+/// Dummy macro
+struct DummyMacro: ExpressionMacro {
+  static func expansion<
+    Node: FreestandingMacroExpansionSyntax,
+    Context: MacroExpansionContext
+  >(
+    of node: Node,
+    in context: Context
+  ) throws -> ExprSyntax  {
+    fatalError()
+  }
+}
+
+struct RegisteredMacro: ExpressionMacro {
+  static func expansion<
+    Node: FreestandingMacroExpansionSyntax,
+    Context: MacroExpansionContext
+  >(
+    of node: Node,
+    in context: Context
+  ) throws -> ExprSyntax  {
+    fatalError()
+  }
+}
+
+struct MyPlugin: CompilerPlugin {
+  let providingMacros: [Macro.Type] = [
+    RegisteredMacro.self
+  ]
+}
+
+public class CompilerPluginTests: XCTestCase {
+
+  func testResolveMacro() {
+    let plugin = MyPlugin()
+
+    let registeredMacro = plugin.resolveMacro(
+      moduleName: "SwiftCompilerPluginTest", typeName: "RegisteredMacro")
+    XCTAssertNotNil(registeredMacro)
+    XCTAssertTrue(registeredMacro == RegisteredMacro.self)
+
+    /// Test the plugin doesn't provide macros other than ``
+    let dummyMacro = plugin.resolveMacro(
+      moduleName: "SwiftCompilerPluginTest", typeName: "DummyMacro")
+    XCTAssertNil(dummyMacro)
+    XCTAssertFalse(dummyMacro == DummyMacro.self)
+
+  }
+}