Allow users to opt-out of implicit Cxx import with -disable-implicit-cxx-module-import

include/swift/Basic/LangOptions.h

@@ -402,6 +402,9 @@ namespace swift {
     bool DisableImplicitBacktracingModuleImport =
         !SWIFT_IMPLICIT_BACKTRACING_IMPORT;
 
+    /// Disable the implicit import of the Cxx module.
+    bool DisableImplicitCxxModuleImport = false;
+
     // Whether to use checked continuations when making an async call from
     // Swift into ObjC. If false, will use unchecked continuations instead.
     bool UseCheckedAsyncObjCBridging = false;

include/swift/DependencyScan/SerializedModuleDependencyCacheFormat.h

@@ -40,7 +40,7 @@ using llvm::BCVBR;
 /// Every .moddepcache file begins with these 4 bytes, for easy identification.
 const unsigned char MODULE_DEPENDENCY_CACHE_FORMAT_SIGNATURE[] = {'I', 'M', 'D','C'};
 const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MAJOR =
-    6; // mappedPCMPath
+    7; // isSystem
 /// Increment this on every change.
 const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MINOR = 1;
 

include/swift/Frontend/Frontend.h

@@ -405,6 +405,9 @@ public:
   /// imported.
   bool shouldImportSwiftBacktracing() const;
 
+  /// Whether the CXX module should be implicitly imported.
+  bool shouldImportCxx() const;
+
   /// Performs input setup common to these tools:
   /// sil-opt, sil-func-extractor, sil-llvm-gen, and sil-nm.
   /// Return value includes the buffer so caller can keep it alive.

include/swift/Option/FrontendOptions.td

@@ -502,6 +502,10 @@ def disable_implicit_backtracing_module_import : Flag<["-"],
   "disable-implicit-backtracing-module-import">,
   HelpText<"Disable the implicit import of the _Backtracing module.">;
 
+def disable_implicit_cxx_module_import : Flag<["-"],
+  "disable-implicit-cxx-module-import">,
+  HelpText<"Disable the implicit import of the C++ Standard Library module.">;
+
 def disable_arc_opts : Flag<["-"], "disable-arc-opts">,
   HelpText<"Don't run SIL ARC optimization passes.">;
 def disable_ossa_opts : Flag<["-"], "disable-ossa-opts">,

lib/Frontend/CompilerInvocation.cpp

@@ -637,6 +637,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.DisableImplicitStringProcessingModuleImport |=
     Args.hasArg(OPT_disable_implicit_string_processing_module_import);
 
+  Opts.DisableImplicitCxxModuleImport |=
+    Args.hasArg(OPT_disable_implicit_cxx_module_import);
+
   Opts.DisableImplicitBacktracingModuleImport =
     Args.hasFlag(OPT_disable_implicit_backtracing_module_import,
                  OPT_enable_implicit_backtracing_module_import,

lib/Frontend/Frontend.cpp

@@ -1094,6 +1094,26 @@ bool CompilerInvocation::shouldImportSwiftBacktracing() const {
       FrontendOptions::ParseInputMode::SwiftModuleInterface;
 }
 
+bool CompilerInvocation::shouldImportCxx() const {
+  // C++ Interop is disabled
+  if (!getLangOptions().EnableCXXInterop)
+    return false;
+  // Avoid C++ stdlib when building Swift stdlib
+  if (getImplicitStdlibKind() == ImplicitStdlibKind::Builtin)
+    return false;
+  // Avoid importing Cxx when building Cxx itself
+  if (getFrontendOptions().ModuleName == CXX_MODULE_NAME)
+    return false;
+  // Cxx cannot be imported when Library evolution is enabled
+  if (getFrontendOptions().EnableLibraryEvolution)
+    return false;
+  // Implicit import of Cxx is disabled
+  if (getLangOptions().DisableImplicitCxxModuleImport)
+    return false;
+
+  return true;
+}
+
 /// Implicitly import the SwiftOnoneSupport module in non-optimized
 /// builds. This allows for use of popular specialized functions
 /// from the standard library, which makes the non-optimized builds
@@ -1286,9 +1306,7 @@ ImplicitImportInfo CompilerInstance::getImplicitImportInfo() const {
   }
 
   if (Invocation.getLangOptions().EnableCXXInterop) {
-    if (imports.StdlibKind != ImplicitStdlibKind::Builtin &&
+    if (Invocation.shouldImportCxx() && canImportCxx())
-        Invocation.getFrontendOptions().ModuleName != CXX_MODULE_NAME &&
-        canImportCxx())
       pushImport(CXX_MODULE_NAME);
     if (canImportCxxShim())
       pushImport(CXX_SHIM_NAME, {ImportFlags::ImplementationOnly});

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1987,6 +1987,8 @@ InterfaceSubContextDelegateImpl::InterfaceSubContextDelegateImpl(
       compatVersion = "default";
     else if (langOpts.cxxInteropCompatVersion[0] == 5)
       compatVersion = "swift-5.9";
+    else if (langOpts.cxxInteropCompatVersion[0] == 6)
+      compatVersion = "swift-6";
     else if (langOpts.cxxInteropCompatVersion[0] ==
              version::getUpcomingCxxInteropCompatVersion())
       compatVersion = "upcoming-swift";

lib/Serialization/SerializedModuleLoader.cpp

@@ -1031,7 +1031,8 @@ LoadedFile *SerializedModuleLoaderBase::loadAST(
   if (M.hasCxxInteroperability() &&
       M.getResilienceStrategy() != ResilienceStrategy::Resilient &&
       !Ctx.LangOpts.EnableCXXInterop &&
-      Ctx.LangOpts.RequireCxxInteropToImportCxxInteropModule) {
+      Ctx.LangOpts.RequireCxxInteropToImportCxxInteropModule &&
+      M.getName().str() != CXX_MODULE_NAME) {
     auto loc = diagLoc.value_or(SourceLoc());
     Ctx.Diags.diagnose(loc, diag::need_cxx_interop_to_import_module,
                        M.getName());

test/Interop/Cxx/library-evolution/prohibit-cxx-api-in-evolving-libraries.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
 // RUN: split-file %s %t
-// RUN: %target-swift-frontend %t/test.swift -I %t/Inputs -typecheck -enable-library-evolution -enable-experimental-cxx-interop -disable-availability-checking -verify
+// RUN: %target-swift-frontend %t/test.swift -I %t/Inputs -typecheck -enable-library-evolution -enable-experimental-cxx-interop -disable-availability-checking -disable-implicit-cxx-module-import -verify
  
 //--- Inputs/module.modulemap
 module CxxModule {

test/Interop/Cxx/library-evolution/prohibit-cxx-calls-in-evolving-inlinable-bodies.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
 // RUN: split-file %s %t
-// RUN: %target-swift-frontend %t/test.swift -I %t/Inputs -typecheck -enable-library-evolution -enable-experimental-cxx-interop -verify
+// RUN: %target-swift-frontend %t/test.swift -I %t/Inputs -typecheck -enable-library-evolution -enable-experimental-cxx-interop -disable-implicit-cxx-module-import -verify
  
 //--- Inputs/module.modulemap
 module CxxModule {