[Explicit Modules] Have -clang-target on Darwin platforms set to the SDK version by-default

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -74,17 +74,6 @@ extension Driver {
       break
     }
 
-    // Pass down -clang-target.
-    // If not specified otherwise, we should use the same triple as -target
-    // TODO: enable -clang-target for implicit module build as well.
-    if !parsedOptions.hasArgument(.disableClangTarget) &&
-        isFrontendArgSupported(.clangTarget) &&
-        parsedOptions.contains(.driverExplicitModuleBuild) {
-      let clangTriple = parsedOptions.getLastArgument(.clangTarget)?.asSingle ?? targetTriple.triple
-      commandLine.appendFlag(.clangTarget)
-      commandLine.appendFlag(clangTriple)
-    }
-
     // If in ExplicitModuleBuild mode and the dependency graph has been computed, add module
     // dependencies.
     // May also be used for generation of the dependency graph itself in ExplicitModuleBuild mode.
@@ -387,7 +376,7 @@ extension Driver {
     try toolchain.addPlatformSpecificCommonFrontendOptions(commandLine: &commandLine,
                                                            inputs: &inputs,
                                                            frontendTargetInfo: frontendTargetInfo,
-                                                           driver: self)
+                                                           driver: &self)
   }
 
   mutating func addFrontendSupplementaryOutputArguments(commandLine: inout [Job.ArgTemplate],

Sources/SwiftDriver/Toolchains/DarwinToolchain.swift

@@ -371,7 +371,7 @@ public final class DarwinToolchain: Toolchain {
     commandLine: inout [Job.ArgTemplate],
     inputs: inout [TypedVirtualPath],
     frontendTargetInfo: FrontendTargetInfo,
-    driver: Driver
+    driver: inout Driver
   ) throws {
     guard let sdkPath = frontendTargetInfo.sdkPath?.path,
           let sdkInfo = getTargetSDKInfo(sdkPath: sdkPath) else { return }
@@ -401,6 +401,26 @@ public final class DarwinToolchain: Toolchain {
         .appending(component: "macosx").appending(component: "prebuilt-modules")
         .appending(component: sdkInfo.versionString))
     }
+
+    // Pass down -clang-target.
+    // If not specified otherwise, we should use the same triple as -target
+    if !driver.parsedOptions.hasArgument(.disableClangTarget) &&
+        driver.isFrontendArgSupported(.clangTarget) &&
+        driver.parsedOptions.contains(.driverExplicitModuleBuild) {
+      // The common target triple for all Clang dependencies of this compilation,
+      // both direct and transitive is computed as:
+      // 1. An explicitly-specified `-clang-target` argument to this driver invocation
+      // 2. (On Darwin) The target triple of the selected SDK
+      let clangTargetTriple: String
+      if let explicitClangTripleArg = driver.parsedOptions.getLastArgument(.clangTarget)?.asSingle {
+        clangTargetTriple = explicitClangTripleArg
+      } else {
+        clangTargetTriple = frontendTargetInfo.target.unversionedTriple.triple + sdkInfo.versionString
+      }
+
+      commandLine.appendFlag(.clangTarget)
+      commandLine.appendFlag(clangTargetTriple)
+    }
   }
 }
 

Sources/SwiftDriver/Toolchains/Toolchain.swift

@@ -151,7 +151,7 @@ public protocol Toolchain {
     commandLine: inout [Job.ArgTemplate],
     inputs: inout [TypedVirtualPath],
     frontendTargetInfo: FrontendTargetInfo,
-    driver: Driver
+    driver: inout Driver
   ) throws
 
   var dummyForTestingObjectFormat: Triple.ObjectFormat {get}
@@ -306,7 +306,7 @@ extension Toolchain {
     commandLine: inout [Job.ArgTemplate],
     inputs: inout [TypedVirtualPath],
     frontendTargetInfo: FrontendTargetInfo,
-    driver: Driver
+    driver: inout Driver
   ) throws {}
 
   /// Resolves the path to the given tool and whether or not it supports response files so that it

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -3626,20 +3626,24 @@ final class SwiftDriverTests: XCTestCase {
 
   func testClangTargetForExplicitModule() throws {
     #if os(macOS)
+    let sdkRoot = testInputsPath.appending(component: "SDKChecks").appending(component: "iPhoneOS.sdk")
+
     // Check -clang-target is on by default when explicit module is on.
     try withTemporaryDirectory { path in
       let main = path.appending(component: "Foo.swift")
       try localFileSystem.writeFileContents(main) {
         $0 <<< "import Swift"
       }
-      var driver = try Driver(args: ["swiftc", "-explicit-module-build", "-target",
-                                     "x86_64-apple-macosx10.14", main.pathString])
+      var driver = try Driver(args: ["swiftc", "-explicit-module-build",
+                                     "-target", "arm64-apple-ios14.0",
+                                     "-sdk", sdkRoot.pathString,
+                                     main.pathString])
       guard driver.isFrontendArgSupported(.clangTarget) else {
         throw XCTSkip("Skipping: compiler does not support '-clang-target'")
       }
       let plannedJobs = try driver.planBuild()
       XCTAssertTrue(plannedJobs.contains { job in
-        job.commandLine.contains(.flag("-clang-target"))
+        job.commandLine.contains(subsequence: [.flag("-clang-target"), .flag("arm64-apple-ios13.0")])
       })
     }
     // Check -disable-clang-target works
@@ -3649,8 +3653,10 @@ final class SwiftDriverTests: XCTestCase {
         $0 <<< "import Swift"
       }
       var driver = try Driver(args: ["swiftc", "-disable-clang-target",
-                                     "-explicit-module-build", "-target",
-                                     "x86_64-apple-macosx10.14", main.pathString])
+                                     "-explicit-module-build",
+                                     "-target", "arm64-apple-ios14.0",
+                                     "-sdk", sdkRoot.pathString,
+                                     main.pathString])
       guard driver.isFrontendArgSupported(.clangTarget) else {
         throw XCTSkip("Skipping: compiler does not support '-clang-target'")
       }
@@ -3663,16 +3669,20 @@ final class SwiftDriverTests: XCTestCase {
   }
 
   func testDisableClangTargetForImplicitModule() throws {
+#if os(macOS)
     var envVars = ProcessEnv.vars
     envVars["SWIFT_DRIVER_LD_EXEC"] = ld.nativePathString(escaped: false)
 
+    let sdkRoot = testInputsPath.appending(component: "SDKChecks").appending(component: "iPhoneOS.sdk")
     var driver = try Driver(args: ["swiftc", "-target",
-                                   "x86_64-apple-macosx10.14", "foo.swift"],
+                                   "arm64-apple-ios12.0", "foo.swift",
+                                   "-sdk", sdkRoot.pathString],
                             env: envVars)
     let plannedJobs = try driver.planBuild()
     XCTAssertEqual(plannedJobs.count, 2)
-    XCTAssert(plannedJobs[0].commandLine.contains(.flag("-target")))
+    XCTAssertTrue(plannedJobs[0].commandLine.contains(.flag("-target")))
     XCTAssertFalse(plannedJobs[0].commandLine.contains(.flag("-clang-target")))
+#endif
   }
 
   func testPCHasCompileInput() throws {