Hard-code the 'Darwin' module as having been built without C++ interop

Textual interfaces for 'Darwin' built with recent compilers specify that it is built witout C++ interop enabled. However, to ensure compatibility with versions of the 'Darwin' module built with older compilers, we hard-code this fact. This is required to break the module cycle that occurs when building the 'Darwin' module with C++ interop enabled, where the underlying 'Darwin' clang module depends on C++ standard library for which the compiler brings in the 'CxxStdlib' Swift overlay, which depends on 'Darwin'.

Author: artemcm

include/swift/Basic/LangOptions.h

@@ -45,6 +45,7 @@ namespace swift {
 
   struct DiagnosticBehavior;
   class DiagnosticEngine;
+  class FrontendOptions;
 
   /// Kind of implicit platform conditions.
   enum class PlatformConditionKind {
@@ -339,7 +340,8 @@ namespace swift {
     std::optional<version::Version> FormalCxxInteropMode;
 
     void setCxxInteropFromArgs(llvm::opt::ArgList &Args,
-                               swift::DiagnosticEngine &Diags);
+                               swift::DiagnosticEngine &Diags,
+                               const FrontendOptions &FrontendOpts);
 
     /// The C++ standard library used for the current build. This can differ
     /// from the default C++ stdlib on a particular platform when `-Xcc

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -1478,7 +1478,8 @@ void ModuleDependencyScanner::resolveSwiftOverlayDependenciesForModule(
 
     // If the textual interface was built without C++ interop, do not query
     // the C++ Standard Library Swift overlay for its compilation.
-    if (llvm::find(commandLine, "-formal-cxx-interoperability-mode=off") ==
+    if (moduleID.ModuleName != "Darwin" &&
+        llvm::find(commandLine, "-formal-cxx-interoperability-mode=off") ==
         commandLine.end()) {
       for (const auto &clangDepName : allClangDependencies) {
         // If this Clang module is a part of the C++ stdlib, and we haven't

lib/DriverTool/swift_symbolgraph_extract_main.cpp

@@ -214,7 +214,8 @@ int swift_symbolgraph_extract_main(ArrayRef<const char *> Args,
     Options.AvailabilityIsBlockList = A->getOption().matches(OPT_block_availability_platforms);
   }
 
-  Invocation.getLangOptions().setCxxInteropFromArgs(ParsedArgs, Diags);
+  Invocation.getLangOptions().setCxxInteropFromArgs(ParsedArgs, Diags,
+                                                    Invocation.getFrontendOptions());
 
   std::string InstanceSetupError;
   if (CI.setup(Invocation, InstanceSetupError)) {

lib/DriverTool/swift_synthesize_interface_main.cpp

@@ -133,7 +133,8 @@ int swift_synthesize_interface_main(ArrayRef<const char *> Args,
   Invocation.setImportSearchPaths(ImportSearchPaths);
 
   Invocation.getLangOptions().EnableObjCInterop = Target.isOSDarwin();
-  Invocation.getLangOptions().setCxxInteropFromArgs(ParsedArgs, Diags);
+  Invocation.getLangOptions().setCxxInteropFromArgs(ParsedArgs, Diags,
+                                                    Invocation.getFrontendOptions());
 
   std::string ModuleCachePath = "";
   if (auto *A = ParsedArgs.getLastArg(OPT_module_cache_path)) {

lib/Frontend/CompilerInvocation.cpp

@@ -682,7 +682,8 @@ static void diagnoseCxxInteropCompatMode(Arg *verArg, ArgList &Args,
 }
 
 void LangOptions::setCxxInteropFromArgs(ArgList &Args,
-                                        swift::DiagnosticEngine &Diags) {
+                                        swift::DiagnosticEngine &Diags,
+                                        const FrontendOptions &FrontendOpts) {
   if (Arg *A = Args.getLastArg(options::OPT_cxx_interoperability_mode)) {
     if (Args.hasArg(options::OPT_enable_experimental_cxx_interop)) {
       Diags.diagnose(SourceLoc(), diag::dont_enable_interop_and_compat);
@@ -737,7 +738,10 @@ void LangOptions::setCxxInteropFromArgs(ArgList &Args,
     // version, and is either 4, 5, 6, or 7 (even though only 5.9 and 6.* make
     // any sense). For now, we don't actually care about the version, so we'll
     // just use version 6 (i.e., 'swift-6') to mean that C++ interop mode is on.
-    if (EnableCXXInterop)
+    //
+    // FIXME: Always declare the 'Darwin' module as formally having been built
+    // without C++Interop, for compatibility with prior versions.
+    if (EnableCXXInterop && (FrontendOpts.ModuleName.compare("Darwin") != 0))
       FormalCxxInteropMode = {6};
     else
       FormalCxxInteropMode = std::nullopt;
@@ -1560,7 +1564,7 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   if (const Arg *A = Args.getLastArg(OPT_clang_target_variant))
     Opts.ClangTargetVariant = llvm::Triple(A->getValue());
 
-  Opts.setCxxInteropFromArgs(Args, Diags);
+  Opts.setCxxInteropFromArgs(Args, Diags, FrontendOpts);
   if (!Args.hasArg(options::OPT_formal_cxx_interoperability_mode))
     ModuleInterfaceOpts.PublicFlags.IgnorableFlags +=
         " " + printFormalCxxInteropVersion(Opts);