[cxx-interop] Emit C++ declarations only when '-clang-header-expose-public-decl' is enabled

This fix also ensures that we only emit C++ functions for now

include/swift/Frontend/FrontendOptions.h

@@ -378,6 +378,10 @@ public:
   /// '.../lib/swift', otherwise '.../lib/swift_static'.
   bool UseSharedResourceFolder = true;
 
+  /// Indicates whether to expose all public declarations in the generated clang
+  /// header.
+  bool ExposePublicDeclsInClangHeader = false;
+
   /// \return true if the given action only parses without doing other compilation steps.
   static bool shouldActionOnlyParse(ActionType);
 

include/swift/Option/FrontendOptions.td

@@ -1031,4 +1031,9 @@ def skip_import_in_public_interface:
   HelpText<"Skip the import statement corresponding to a module name "
            "when printing the public interface.">;
 
+def clang_header_expose_public_decls:
+  Flag<["-"], "clang-header-expose-public-decls">,
+  HelpText<"Expose all public declarations in the generated clang header">,
+  Flags<[FrontendOption, HelpHidden]>;
+
 } // end let Flags = [FrontendOption, NoDriverOption, HelpHidden]

include/swift/PrintAsClang/PrintAsClang.h

@@ -26,13 +26,14 @@ namespace swift {
   /// The Objective-C compatible declarations are printed into a block that
   /// ensures that those declarations are only usable when the header is
   /// compiled in Objective-C mode.
-  ///  The C++ compatible declarations are printed into a block that ensures
+  /// The C++ compatible declarations are printed into a block that ensures
-  ///  that those declarations are only usable when the header is compiled in
+  /// that those declarations are only usable when the header is compiled in
-  ///  C++ mode.
+  /// C++ mode.
   ///
   /// Returns true on error.
   bool printAsClangHeader(raw_ostream &out, ModuleDecl *M,
-                          StringRef bridgingHeader);
+                          StringRef bridgingHeader,
+                          bool ExposePublicDeclsInClangHeader);
 }
 
 #endif

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -274,6 +274,8 @@ bool ArgsToFrontendOptionsConverter::convert(
   Opts.EnableIncrementalDependencyVerifier |= Args.hasArg(OPT_verify_incremental_dependencies);
   Opts.UseSharedResourceFolder = !Args.hasArg(OPT_use_static_resource_dir);
   Opts.DisableBuildingInterface = Args.hasArg(OPT_disable_building_interface);
+  Opts.ExposePublicDeclsInClangHeader =
+      Args.hasArg(OPT_clang_header_expose_public_decls);
 
   computeImportObjCHeaderOptions();
   computeImplicitImportModuleNames(OPT_import_module, /*isTestable=*/false);

lib/FrontendTool/FrontendTool.cpp

@@ -179,13 +179,15 @@ static bool writeSIL(SILModule &SM, const PrimarySpecificPaths &PSPs,
 ///
 /// \see swift::printAsClangHeader
 static bool printAsClangHeaderIfNeeded(StringRef outputPath, ModuleDecl *M,
-                                       StringRef bridgingHeader) {
+                                       StringRef bridgingHeader,
+                                       bool ExposePublicDeclsInClangHeader) {
   if (outputPath.empty())
     return false;
-  return withOutputFile(M->getDiags(), outputPath,
+  return withOutputFile(
-                        [&](raw_ostream &out) -> bool {
+      M->getDiags(), outputPath, [&](raw_ostream &out) -> bool {
-                          return printAsClangHeader(out, M, bridgingHeader);
+        return printAsClangHeader(out, M, bridgingHeader,
-                        });
+                                  ExposePublicDeclsInClangHeader);
+      });
 }
 
 /// Prints the stable module interface for \p M to \p outputPath.
@@ -826,7 +828,8 @@ static bool emitAnyWholeModulePostTypeCheckSupplementaryOutputs(
     }
     hadAnyError |= printAsClangHeaderIfNeeded(
         Invocation.getClangHeaderOutputPathForAtMostOnePrimary(),
-        Instance.getMainModule(), BridgingHeaderPathForPrint);
+        Instance.getMainModule(), BridgingHeaderPathForPrint,
+        opts.ExposePublicDeclsInClangHeader);
   }
 
   // Only want the header if there's been any errors, ie. there's not much

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -124,13 +124,15 @@ class ModuleWriter {
   std::vector<const Decl *> declsToWrite;
   DelayedMemberSet delayedMembers;
   DeclAndTypePrinter printer;
+  OutputLanguageMode outputLangMode;
 
 public:
   ModuleWriter(raw_ostream &os, raw_ostream &prologueOS,
                llvm::SmallPtrSetImpl<ImportModuleTy> &imports, ModuleDecl &mod,
                AccessLevel access, OutputLanguageMode outputLang)
       : os(os), imports(imports), M(mod),
-        printer(M, os, prologueOS, delayedMembers, access, outputLang) {}
+        printer(M, os, prologueOS, delayedMembers, access, outputLang),
+        outputLangMode(outputLang) {}
 
   /// Returns true if we added the decl's module to the import set, false if
   /// the decl is a local decl.
@@ -576,7 +578,11 @@ public:
       const Decl *D = declsToWrite.back();
       bool success = true;
 
-      if (isa<ValueDecl>(D)) {
+      if (outputLangMode == OutputLanguageMode::Cxx) {
+        if (auto FD = dyn_cast<FuncDecl>(D))
+          success = writeFunc(FD);
+        // FIXME: Warn on unsupported exported decl.
+      } else if (isa<ValueDecl>(D)) {
         if (auto CD = dyn_cast<ClassDecl>(D))
           success = writeClass(CD);
         else if (auto PD = dyn_cast<ProtocolDecl>(D))

lib/PrintAsClang/PrintAsClang.cpp

@@ -458,7 +458,8 @@ static std::string getModuleContentsCxxString(ModuleDecl &M) {
 }
 
 bool swift::printAsClangHeader(raw_ostream &os, ModuleDecl *M,
-                               StringRef bridgingHeader) {
+                               StringRef bridgingHeader,
+                               bool ExposePublicDeclsInClangHeader) {
   llvm::PrettyStackTraceString trace("While generating Clang header");
 
   SmallPtrSet<ImportModuleTy, 8> imports;
@@ -472,7 +473,7 @@ bool swift::printAsClangHeader(raw_ostream &os, ModuleDecl *M,
   emitObjCConditional(os, [&] { os << objcModuleContents.str(); });
   emitCxxConditional(os, [&] {
     // FIXME: Expose Swift with @expose by default.
-    if (M->getASTContext().LangOpts.EnableCXXInterop) {
+    if (ExposePublicDeclsInClangHeader) {
       os << getModuleContentsCxxString(*M);
     }
   });

test/Interop/SwiftToCxx/functions/cdecl-execution.cpp

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
 
-// RUN: %target-swift-frontend %S/cdecl.swift -typecheck -module-name CdeclFunctions -enable-cxx-interop -emit-clang-header-path %t/functions.h
+// RUN: %target-swift-frontend %S/cdecl.swift -typecheck -module-name CdeclFunctions -clang-header-expose-public-decls -emit-clang-header-path %t/functions.h
 
 // RUN: %target-interop-build-clangxx -c %s -I %t -o %t/cdecl-execution.o
 // RUN: %target-interop-build-swift %S/cdecl.swift -o %t/cdecl-execution -Xlinker %t/cdecl-execution.o -module-name Functions -Xfrontend -entry-point-function-name -Xfrontend swiftMain

test/Interop/SwiftToCxx/functions/cdecl.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend %s -typecheck -module-name CdeclFunctions -enable-cxx-interop -emit-clang-header-path %t/cdecl.h
+// RUN: %target-swift-frontend %s -typecheck -module-name CdeclFunctions -clang-header-expose-public-decls -emit-clang-header-path %t/cdecl.h
 // RUN: %FileCheck %s < %t/cdecl.h
 
 // RUN: %check-interop-cxx-header-in-clang(%t/cdecl.h)

test/Interop/SwiftToCxx/functions/function-availability.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend %s -typecheck -module-name Functions -enable-cxx-interop -emit-clang-header-path %t/functions.h
+// RUN: %target-swift-frontend %s -typecheck -module-name Functions -clang-header-expose-public-decls -emit-clang-header-path %t/functions.h
 // RUN: %FileCheck %s < %t/functions.h
 
 // RUN: %check-interop-cxx-header-in-clang(%t/functions.h)

test/Interop/SwiftToCxx/functions/swift-functions-execution.cpp

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
 
-// RUN: %target-swift-frontend %S/swift-functions.swift -typecheck -module-name Functions -enable-cxx-interop -emit-clang-header-path %t/functions.h
+// RUN: %target-swift-frontend %S/swift-functions.swift -typecheck -module-name Functions -clang-header-expose-public-decls -emit-clang-header-path %t/functions.h
 
 // RUN: %target-interop-build-clangxx -c %s -I %t -o %t/swift-functions-execution.o
 // RUN: %target-interop-build-swift %S/swift-functions.swift -o %t/swift-functions-execution -Xlinker %t/swift-functions-execution.o -module-name Functions -Xfrontend -entry-point-function-name -Xfrontend swiftMain

test/Interop/SwiftToCxx/functions/swift-functions.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend %s -typecheck -module-name Functions -enable-cxx-interop -emit-clang-header-path %t/functions.h
+// RUN: %target-swift-frontend %s -typecheck -module-name Functions -clang-header-expose-public-decls -emit-clang-header-path %t/functions.h
 // RUN: %FileCheck %s < %t/functions.h
 
 // RUN: %check-interop-cxx-header-in-clang(%t/functions.h)

test/Interop/SwiftToCxx/module/module-to-namespace.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend %s -typecheck -module-name Test -enable-cxx-interop -emit-clang-header-path %t/empty.h
+// RUN: %target-swift-frontend %s -typecheck -module-name Test -clang-header-expose-public-decls -emit-clang-header-path %t/empty.h
 // RUN: %FileCheck %s < %t/empty.h
 
 // RUN: %check-interop-cxx-header-in-clang(%t/empty.h)

test/PrintAsCxx/empty.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend %s -typecheck -enable-cxx-interop -emit-clang-header-path %t/empty.h
+// RUN: %target-swift-frontend %s -typecheck -clang-header-expose-public-decls -emit-clang-header-path %t/empty.h
 // RUN: %FileCheck %s < %t/empty.h
 
 // CHECK-LABEL: #ifndef EMPTY_SWIFT_H