Always add an implicit import of 'Cxx' module when C++ Interop is enabled.

We cannot always rely on being able to do so only as an overlay query upon loading 'requires cplusplus' modulemap modules. The 'requires' statement only applies to submodules, and we may not be able to query language feature modulemap attributes in dependency scanning context.

Author: artemcm

include/swift/Frontend/Frontend.h

@@ -660,6 +660,9 @@ class CompilerInstance {
   /// i.e. if it can be found.
   bool canImportSwiftBacktracing() const;
 
+  /// Whether the Cxx library can be imported
+  bool canImportCxx() const;
+
   /// Whether the CxxShim library can be imported
   /// i.e. if it can be found.
   bool canImportCxxShim() const;

include/swift/Serialization/SerializedModuleLoader.h

@@ -172,7 +172,7 @@ class SerializedModuleLoaderBase : public ModuleLoader {
   };
 
   llvm::ErrorOr<BinaryModuleImports>
-  getImportsOfModule(Twine modulePath,
+  getImportsOfModule(const ModuleFileSharedCore &loadedModuleFile,
                      ModuleLoadingBehavior transitiveBehavior,
                      StringRef packageName, bool isTestableImport);
 

include/swift/Strings.h

@@ -39,6 +39,8 @@ constexpr static const StringLiteral SWIFT_BACKTRACING_NAME = "_Backtracing";
 constexpr static const StringLiteral SWIFT_SHIMS_NAME = "SwiftShims";
 /// The name of the CxxShim module, which contains a cxx casting utility.
 constexpr static const StringLiteral CXX_SHIM_NAME = "CxxShim";
+/// The name of the Cxx module, which contains C++ interop helper protocols.
+constexpr static const StringLiteral CXX_MODULE_NAME = "Cxx";
 /// The name of the Builtin module, which contains Builtin functions.
 constexpr static const StringLiteral BUILTIN_NAME = "Builtin";
 /// The name of the clang imported header module.

lib/Frontend/Frontend.cpp

@@ -1165,6 +1165,13 @@ bool CompilerInstance::canImportSwiftBacktracing() const {
   return getASTContext().testImportModule(modulePath);
 }
 
+bool CompilerInstance::canImportCxx() const {
+  ImportPath::Module::Builder builder(
+      getASTContext().getIdentifier(CXX_MODULE_NAME));
+  auto modulePath = builder.get();
+  return getASTContext().testImportModule(modulePath);
+}
+
 bool CompilerInstance::canImportCxxShim() const {
   ImportPath::Module::Builder builder(
       getASTContext().getIdentifier(CXX_SHIM_NAME));
@@ -1268,8 +1275,13 @@ ImplicitImportInfo CompilerInstance::getImplicitImportInfo() const {
     }
   }
 
-  if (Invocation.getLangOptions().EnableCXXInterop && canImportCxxShim()) {
-    pushImport(CXX_SHIM_NAME, {ImportFlags::ImplementationOnly});
+  if (Invocation.getLangOptions().EnableCXXInterop) {
+    if (imports.StdlibKind != ImplicitStdlibKind::Builtin &&
+        Invocation.getFrontendOptions().ModuleName != CXX_MODULE_NAME &&
+        canImportCxx())
+      pushImport(CXX_MODULE_NAME);
+    if (canImportCxxShim())
+      pushImport(CXX_SHIM_NAME, {ImportFlags::ImplementationOnly});
   }
 
   imports.ShouldImportUnderlyingModule = frontendOpts.ImportUnderlyingModule;

lib/Serialization/SerializedModuleLoader.cpp

@@ -392,7 +392,7 @@ std::error_code SerializedModuleLoaderBase::openModuleFile(
   return std::error_code();
 }
 
-SerializedModuleLoaderBase::BinaryModuleImports
+llvm::ErrorOr<SerializedModuleLoaderBase::BinaryModuleImports>
 SerializedModuleLoaderBase::getImportsOfModule(
     const ModuleFileSharedCore &loadedModuleFile,
     ModuleLoadingBehavior transitiveBehavior, StringRef packageName,
@@ -478,7 +478,7 @@ SerializedModuleLoaderBase::scanModuleFile(Twine modulePath, bool isFramework,
       getImportsOfModule(*loadedModuleFile, ModuleLoadingBehavior::Optional,
                          Ctx.LangOpts.PackageName, isTestableImport);
 
-  auto importedModuleSet = binaryModuleImports.moduleImports;
+  auto importedModuleSet = binaryModuleImports->moduleImports;
   std::vector<std::string> importedModuleNames;
   importedModuleNames.reserve(importedModuleSet.size());
   llvm::transform(importedModuleSet.keys(),
@@ -487,8 +487,8 @@ SerializedModuleLoaderBase::scanModuleFile(Twine modulePath, bool isFramework,
                      return N.str();
                   });
 
-  auto importedHeader = binaryModuleImports.headerImport;
-  auto &importedOptionalModuleSet = binaryModuleOptionalImports.moduleImports;
+  auto importedHeader = binaryModuleImports->headerImport;
+  auto &importedOptionalModuleSet = binaryModuleOptionalImports->moduleImports;
   std::vector<std::string> importedOptionalModuleNames;
   for (const auto optionalImportedModule : importedOptionalModuleSet.keys())
     if (!importedModuleSet.contains(optionalImportedModule))