[6.2] Include search path for Swift Testing's macro plugin from toolchain if present

Sources/PackageModel/UserToolchain.swift

@@ -636,26 +636,28 @@ public final class UserToolchain: Toolchain {
         )
 
         if triple.isMacOSX, let swiftTestingPath {
-            // swift-testing in CommandLineTools, needs extra frameworks search path
+            // Swift Testing is a framework (e.g. from CommandLineTools) so use -F.
             if swiftTestingPath.extension == "framework" {
                 swiftCompilerFlags += ["-F", swiftTestingPath.pathString]
-            }
 
-            // Otherwise we must have a custom toolchain, add overrides to find its swift-testing ahead of any in the
-            // SDK. We expect the library to be in `lib/swift/macosx/testing` and the plugin in
-            // `lib/swift/host/plugins/testing`
-            if let pluginsPath = try? AbsolutePath(
-                validating: "../../host/plugins/testing",
-                relativeTo: swiftTestingPath
-            ) {
+            // Otherwise Swift Testing is assumed to be a swiftmodule + library, so use -I and -L.
+            } else {
                 swiftCompilerFlags += [
                     "-I", swiftTestingPath.pathString,
                     "-L", swiftTestingPath.pathString,
-                    "-plugin-path", pluginsPath.pathString,
                 ]
             }
         }
 
+        // Specify the plugin path for Swift Testing's macro plugin if such a
+        // path exists in this toolchain.
+        if let swiftTestingPluginPath = Self.deriveSwiftTestingPluginPath(
+            derivedSwiftCompiler: swiftCompilers.compile,
+            fileSystem: fileSystem
+        ) {
+            swiftCompilerFlags += ["-plugin-path", swiftTestingPluginPath.pathString]
+        }
+
         swiftCompilerFlags += try Self.deriveSwiftCFlags(
             triple: triple,
             swiftSDK: swiftSDK,
@@ -1037,6 +1039,35 @@ public final class UserToolchain: Toolchain {
         return nil
     }
 
+    /// Derive the plugin path needed to locate the Swift Testing macro plugin,
+    /// if such a path exists in the toolchain of the specified compiler.
+    ///
+    /// - Parameters:
+    ///   - derivedSwiftCompiler: The derived path of the Swift compiler to use
+    ///       when deriving the Swift Testing plugin path.
+    ///   - fileSystem: The file system instance to use when validating the path
+    ///       to return.
+    ///
+    /// - Returns: A path to the directory containing Swift Testing's macro
+    ///     plugin, or `nil` if the path does not exist or cannot be determined.
+    ///
+    /// The path returned is a directory containing a library, suitable for
+    /// passing to a client compiler via the `-plugin-path` flag.
+    private static func deriveSwiftTestingPluginPath(
+        derivedSwiftCompiler: Basics.AbsolutePath,
+        fileSystem: any FileSystem
+    ) -> AbsolutePath? {
+        guard let toolchainLibDir = try? toolchainLibDir(swiftCompilerPath: derivedSwiftCompiler) else {
+            return nil
+        }
+
+        if let pluginsPath = try? AbsolutePath(validating: "swift/host/plugins/testing", relativeTo: toolchainLibDir), fileSystem.exists(pluginsPath) {
+            return pluginsPath
+        }
+
+        return nil
+    }
+
     public var sdkRootPath: AbsolutePath? {
         configuration.sdkRootPath
     }

Tests/BuildTests/BuildPlanTests.swift

@@ -4997,6 +4997,122 @@ class BuildPlanTestCase: BuildSystemProviderTestCase {
         ])
     }
 
+    func testSwiftTestingFlagsOnMacOSWithoutCustomToolchain() async throws {
+        #if !os(macOS)
+        // This is testing swift-testing in a toolchain which is macOS only feature.
+        try XCTSkipIf(true, "test is only supported on macOS")
+        #endif
+
+        let fs = InMemoryFileSystem(
+            emptyFiles:
+            "/fake/path/lib/swift/host/plugins/testing/libTestingMacros.dylib",
+            "/Pkg/Sources/Lib/main.swift",
+            "/Pkg/Tests/LibTest/test.swift"
+        )
+        try fs.createMockToolchain()
+
+        let userSwiftSDK = SwiftSDK(
+            hostTriple: .x86_64MacOS,
+            targetTriple: .x86_64MacOS,
+            toolset: .init(
+                knownTools: [
+                    .cCompiler: .init(extraCLIOptions: []),
+                    .swiftCompiler: .init(extraCLIOptions: []),
+                ],
+                rootPaths: ["/fake/path/to"]
+            ),
+            pathsConfiguration: .init(
+                sdkRootPath: "/fake/sdk",
+                swiftResourcesPath: "/fake/lib/swift",
+                swiftStaticResourcesPath: "/fake/lib/swift_static"
+            )
+        )
+
+        let env = Environment.mockEnvironment
+        let mockToolchain = try UserToolchain(
+            swiftSDK: userSwiftSDK,
+            environment: env,
+            searchStrategy: .custom(
+                searchPaths: getEnvSearchPaths(
+                    pathString: env[.path],
+                    currentWorkingDirectory: fs.currentWorkingDirectory
+                ),
+                useXcrun: true
+            ),
+            fileSystem: fs
+        )
+
+        XCTAssertEqual(
+            mockToolchain.extraFlags.swiftCompilerFlags,
+            [
+                "-plugin-path", "/fake/path/lib/swift/host/plugins/testing",
+                "-sdk", "/fake/sdk",
+            ]
+        )
+        XCTAssertNoMatch(mockToolchain.extraFlags.linkerFlags, ["-rpath"])
+        XCTAssertNoMatch(mockToolchain.extraFlags.swiftCompilerFlags, [
+            "-I", "/fake/path/lib/swift/macosx/testing",
+            "-L", "/fake/path/lib/swift/macosx/testing",
+        ])
+
+        let observability = ObservabilitySystem.makeForTesting()
+        let graph = try loadModulesGraph(
+            fileSystem: fs,
+            manifests: [
+                Manifest.createRootManifest(
+                    displayName: "Pkg",
+                    path: "/Pkg",
+                    targets: [
+                        TargetDescription(name: "Lib", dependencies: []),
+                        TargetDescription(
+                            name: "LibTest",
+                            dependencies: ["Lib"],
+                            type: .test
+                        ),
+                    ]
+                ),
+            ],
+            observabilityScope: observability.topScope
+        )
+        XCTAssertNoDiagnostics(observability.diagnostics)
+
+        let result = try await BuildPlanResult(plan: mockBuildPlan(
+            toolchain: mockToolchain,
+            graph: graph,
+            commonFlags: .init(),
+            fileSystem: fs,
+            observabilityScope: observability.topScope
+        ))
+        result.checkProductsCount(2)
+        result.checkTargetsCount(3)
+
+        let testProductLinkArgs = try result.buildProduct(for: "Lib").linkArguments()
+        XCTAssertNoMatch(testProductLinkArgs, [
+            "-I", "/fake/path/lib/swift/macosx/testing",
+            "-L", "/fake/path/lib/swift/macosx/testing",
+        ])
+
+        let libModuleArgs = try result.moduleBuildDescription(for: "Lib").swift().compileArguments()
+        XCTAssertMatch(libModuleArgs, [
+            "-plugin-path", "/fake/path/lib/swift/host/plugins/testing",
+        ])
+        XCTAssertNoMatch(libModuleArgs, ["-Xlinker"])
+        XCTAssertNoMatch(libModuleArgs, [
+            "-I", "/fake/path/lib/swift/macosx/testing",
+            "-L", "/fake/path/lib/swift/macosx/testing",
+        ])
+
+        let testModuleArgs = try result.moduleBuildDescription(for: "LibTest").swift().compileArguments()
+        XCTAssertMatch(testModuleArgs, [
+            "-plugin-path", "/fake/path/lib/swift/host/plugins/testing",
+        ])
+        XCTAssertNoMatch(testModuleArgs, ["-Xlinker"])
+        XCTAssertNoMatch(testModuleArgs, [
+            "-I", "/fake/path/lib/swift/macosx/testing",
+            "-L", "/fake/path/lib/swift/macosx/testing",
+        ])
+    }
+
     func testSwiftTestingFlagsOnMacOSWithCustomToolchain() async throws {
         #if !os(macOS)
         // This is testing swift-testing in a toolchain which is macOS only feature.
@@ -5006,7 +5122,7 @@ class BuildPlanTestCase: BuildSystemProviderTestCase {
         let fs = InMemoryFileSystem(
             emptyFiles:
             "/fake/path/lib/swift/macosx/testing/Testing.swiftmodule",
-            "/fake/path/lib/swift/host/plugins/testing/libTesting.dylib",
+            "/fake/path/lib/swift/host/plugins/testing/libTestingMacros.dylib",
             "/Pkg/Sources/Lib/main.swift",
             "/Pkg/Tests/LibTest/test.swift"
         )