Specify -clang-target only when explicit module is on

We should turn clang target on for implicit module when we are confident
the feature works nicely in explicit module builds.

Author: nkcsgexi

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -68,8 +68,10 @@ extension Driver {
 
     // 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) {
+        isFrontendArgSupported(.clangTarget) &&
+        parsedOptions.contains(.driverExplicitModuleBuild) {
       let clangTriple = parsedOptions.getLastArgument(.clangTarget)?.asSingle ?? targetTriple.triple
       commandLine.appendFlag(.clangTarget)
       commandLine.appendFlag(clangTriple)

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -2490,23 +2490,45 @@ final class SwiftDriverTests: XCTestCase {
     }
   }
 
-  func testClangTarget() throws {
-    var driver = try Driver(args: ["swiftc", "-target",
-                                   "x86_64-apple-macosx10.14", "foo.swift", "bar.swift"])
-    guard driver.isFrontendArgSupported(.clangTarget) else {
-      throw XCTSkip("Skipping: compiler does not support '-clang-target'")
+  func testClangTargetForExplicitModule() throws {
+    // Check -clang-target is on by defualt 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", "-experimental-explicit-module-build", "-target",
+                                     "x86_64-apple-macosx10.14", 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"))
+      })
+    }
+    // Check -disable-clang-target works
+    try withTemporaryDirectory { path in
+      let main = path.appending(component: "Foo.swift")
+      try localFileSystem.writeFileContents(main) {
+        $0 <<< "import Swift"
+      }
+      var driver = try Driver(args: ["swiftc", "-disable-clang-target",
+                                     "-experimental-explicit-module-build", "-target",
+                                     "x86_64-apple-macosx10.14", main.pathString])
+      guard driver.isFrontendArgSupported(.clangTarget) else {
+        throw XCTSkip("Skipping: compiler does not support '-clang-target'")
+      }
+      let plannedJobs = try driver.planBuild()
+      XCTAssertFalse(plannedJobs.contains { job in
+        job.commandLine.contains(.flag("-clang-target"))
+      })
     }
-    let plannedJobs = try driver.planBuild()
-    XCTAssertEqual(plannedJobs.count, 3)
-    XCTAssert(plannedJobs[0].commandLine.contains(.flag("-target")))
-    XCTAssert(plannedJobs[0].commandLine.contains(.flag("-clang-target")))
-    XCTAssert(plannedJobs[1].commandLine.contains(.flag("-target")))
-    XCTAssert(plannedJobs[1].commandLine.contains(.flag("-clang-target")))
   }
 
-  func testDisableClangTarget() throws {
+  func testDisableClangTargetForImplicitModule() throws {
     var driver = try Driver(args: ["swiftc", "-target",
-                                   "x86_64-apple-macosx10.14", "foo.swift", "-disable-clang-target"])
+                                   "x86_64-apple-macosx10.14", "foo.swift"])
     let plannedJobs = try driver.planBuild()
     XCTAssertEqual(plannedJobs.count, 2)
     XCTAssert(plannedJobs[0].commandLine.contains(.flag("-target")))