[Macros] Set -external-plugin-path when the toolchain is not Xcode

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -275,6 +275,10 @@ extension Driver {
       commandLine.appendPath(localPluginPath)
     }
 
+    if isFrontendArgSupported(.externalPluginPath) {
+      try commandLine.appendAll(.externalPluginPath, from: &parsedOptions)
+    }
+
     if isFrontendArgSupported(.blockListFile) {
       try Driver.findBlocklists(RelativeTo: try toolchain.executableDir).forEach {
         commandLine.appendFlag(.blockListFile)

Sources/SwiftDriver/Toolchains/DarwinToolchain.swift

@@ -373,6 +373,32 @@ public final class DarwinToolchain: Toolchain {
     frontendTargetInfo: FrontendTargetInfo,
     driver: inout Driver
   ) throws {
+    // Pass -external-plugin-path if the current toolchain is not a Xcode
+    // default toolchain.
+    if
+      driver.isFrontendArgSupported(.externalPluginPath),
+      let xcodeDir = try self.findCurrentSelectedXcodeDir(),
+      try !self.executableDir.isDescendant(of: xcodeDir),
+      let xcodeExecutableDir = try self.findXcodeExecutableDir()
+    {
+      let xcodePluginServerPath = xcodeExecutableDir
+        .appending(component: "swift-plugin-server")
+
+      if fileSystem.isExecutableFile(xcodePluginServerPath) {
+        let xcodeToolchainUsrPath = xcodeExecutableDir.parentDirectory
+
+        let xcodePluginPath = xcodeToolchainUsrPath
+          .appending(components: "lib", "swift", "host", "plugins")
+        commandLine.appendFlag(.externalPluginPath)
+        commandLine.appendFlag(xcodePluginPath.pathString + "#" + xcodePluginServerPath.pathString)
+
+        let xcodeLocalPluginPath = xcodeToolchainUsrPath
+          .appending(components: "local", "lib", "swift", "host", "plugins")
+        commandLine.appendFlag(.externalPluginPath)
+        commandLine.appendFlag(xcodeLocalPluginPath.pathString + "#" + xcodePluginServerPath.pathString)
+      }
+    }
+
     guard let sdkPath = frontendTargetInfo.sdkPath?.path,
           let sdkInfo = getTargetSDKInfo(sdkPath: sdkPath) else { return }
 
@@ -441,3 +467,38 @@ private extension Version {
     return self.description
   }
 }
+
+extension DarwinToolchain {
+  func findXcodeExecutableDir() throws -> AbsolutePath? {
+#if os(macOS)
+    let result = try executor.checkNonZeroExit(
+      args: "xcrun", "-toolchain", "default", "-f", "swiftc",
+      environment: env
+    ).trimmingCharacters(in: .whitespacesAndNewlines)
+
+    guard !result.isEmpty else {
+      return nil
+    }
+    return try AbsolutePath(validating: result)
+      .parentDirectory // swiftc
+#else
+    return nil
+#endif
+  }
+
+  func findCurrentSelectedXcodeDir() throws -> AbsolutePath? {
+#if os(macOS)
+    let result = try executor.checkNonZeroExit(
+      args: "xcode-select", "-p",
+      environment: env
+    ).trimmingCharacters(in: .whitespacesAndNewlines)
+
+    guard !result.isEmpty else {
+      return nil
+    }
+    return try AbsolutePath(validating: result)
+#else
+    return nil
+#endif
+  }
+}

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -6739,6 +6739,55 @@ final class SwiftDriverTests: XCTestCase {
     XCTAssertTrue(job.commandLine.contains(.path(.absolute(try driver.toolchain.executableDir.parentDirectory.appending(components: "local", "lib", "swift", "host", "plugins")))))
   }
 
+  func testExternalPluginPaths() throws {
+#if !os(macOS)
+    throw XCTSkip("Supported only in macOS")
+#endif
+
+    try withTemporaryDirectory { tmpDir in
+      var driver = try Driver(args: ["swiftc", "-typecheck", "foo.swift"],
+                              compilerExecutableDir: tmpDir)
+      guard driver.isFrontendArgSupported(.externalPluginPath) else {
+        return
+      }
+
+      let jobs = try driver.planBuild().removingAutolinkExtractJobs()
+      XCTAssertEqual(jobs.count, 1)
+      let job = jobs.first!
+
+      // This happens only Xcode toolchain has 'swift-plugin-server' which we
+      // don't know.
+      let idx1 = job.commandLine.firstIndex(of: .flag("-external-plugin-path"))
+      try XCTSkipIf(idx1 == nil)
+      switch job.commandLine[job.commandLine.index(after: idx1!)] {
+      case .flag(let value):
+        let components = value.split(separator: "#")
+        if components.count == 2 {
+          XCTAssertTrue(components[0].hasSuffix("/usr/lib/swift/host/plugins"))
+          XCTAssertTrue(components[1].hasSuffix("/usr/bin/swift-plugin-server"))
+        } else {
+          XCTFail("# separated count must 2")
+        }
+      default:
+        XCTFail("invalid arg type after '-external-plugin-path'")
+      }
+
+      let idx2 = job.commandLine[job.commandLine.index(after: idx1!)...].firstIndex(of: .flag("-external-plugin-path"))
+      switch job.commandLine[job.commandLine.index(after: try XCTUnwrap(idx2))] {
+      case .flag(let value):
+            let components = value.split(separator: "#")
+            if (components.count == 2) {
+              XCTAssertTrue(components[0].hasSuffix("/usr/local/lib/swift/host/plugins"))
+              XCTAssertTrue(components[1].hasSuffix("/usr/bin/swift-plugin-server"))
+            } else {
+              XCTFail("# separated count must 2")
+            }
+      default:
+        XCTFail("invalid arg type after '-external-plugin-path'")
+      }
+    }
+  }
+
   func testClangModuleValidateOnce() throws {
     let flagTest = try Driver(args: ["swiftc", "-typecheck", "foo.swift"])
     guard flagTest.isFrontendArgSupported(.clangBuildSessionFile),

Tests/TestUtilities/DriverExtensions.swift

@@ -23,7 +23,8 @@ extension Driver {
     env: [String: String] = ProcessEnv.vars,
     diagnosticsEngine: DiagnosticsEngine = DiagnosticsEngine(handlers: [Driver.stderrDiagnosticsHandler]),
     fileSystem: FileSystem = localFileSystem,
-    integratedDriver: Bool = true
+    integratedDriver: Bool = true,
+    compilerExecutableDir: AbsolutePath? = nil
   ) throws {
     let executor = try SwiftDriverExecutor(diagnosticsEngine: diagnosticsEngine,
                                        processSet: ProcessSet(),
@@ -34,7 +35,8 @@ extension Driver {
                   diagnosticsOutput: .engine(diagnosticsEngine),
                   fileSystem: fileSystem,
                   executor: executor,
-                  integratedDriver: integratedDriver)
+                  integratedDriver: integratedDriver,
+                  compilerExecutableDir: compilerExecutableDir)
   }
 
   /// For tests that need to set the sdk path.