[cxx-interop] Make reverse interop header compile in embedded mode

These are the minimal changes to make the reverse interop header compile
when embedded Swift is used. Previously, the _SwiftStdlibCxxOverlay.h
and the generated interop header disagreed on the mangling of a symbol.

This is not sufficient yet to use the embedded Swift standard library from
reverse interop, there are some missing symbols while linking. But this
PR enables doing reverse interop without using the stdlib types like
swift::String in C++. Follow-up PRs will fix the rest of the issues.

rdar://154740225

Author: Gabor Horvath

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -29,6 +29,7 @@
 #include "swift/AST/SwiftNameTranslation.h"
 #include "swift/AST/TypeDeclFinder.h"
 #include "swift/Basic/Assertions.h"
+#include "swift/Basic/Feature.h"
 #include "swift/Basic/SourceManager.h"
 #include "swift/ClangImporter/ClangImporter.h"
 #include "swift/SIL/SILInstruction.h"
@@ -1183,13 +1184,14 @@ EmittedClangHeaderDependencyInfo swift::printModuleContentsAsCxx(
   std::string modulePrologueBuf;
   llvm::raw_string_ostream prologueOS{modulePrologueBuf};
   EmittedClangHeaderDependencyInfo info;
+  auto &context = M.getASTContext();
 
   // Define the `SWIFT_SYMBOL` macro.
   os << "#ifdef SWIFT_SYMBOL\n";
   os << "#undef SWIFT_SYMBOL\n";
   os << "#endif\n";
   os << "#define SWIFT_SYMBOL(usrValue) SWIFT_SYMBOL_MODULE_USR(\"";
-  ClangSyntaxPrinter(M.getASTContext(), os).printBaseName(&M);
+  ClangSyntaxPrinter(context, os).printBaseName(&M);
   os << "\", usrValue)\n";
 
   // FIXME: Use getRequiredAccess once @expose is supported.
@@ -1204,17 +1206,19 @@ EmittedClangHeaderDependencyInfo swift::printModuleContentsAsCxx(
     os << "#include <string>\n";
     os << "#endif\n";
     os << "#include <new>\n";
+    if (context.LangOpts.hasFeature(Feature::Embedded))
+      os << "#define __EmbeddedSwift__\n";
     // Embed an overlay for the standard library.
-    ClangSyntaxPrinter(M.getASTContext(), moduleOS).printIncludeForShimHeader(
-        "_SwiftStdlibCxxOverlay.h");
+    ClangSyntaxPrinter(context, moduleOS)
+        .printIncludeForShimHeader("_SwiftStdlibCxxOverlay.h");
     // Ignore typos in Swift stdlib doc comments.
     os << "#pragma clang diagnostic push\n";
     os << "#pragma clang diagnostic ignored \"-Wdocumentation\"\n";
   }
 
   os << "#ifndef SWIFT_PRINTED_CORE\n";
   os << "#define SWIFT_PRINTED_CORE\n";
-  printSwiftToClangCoreScaffold(interopContext, M.getASTContext(),
+  printSwiftToClangCoreScaffold(interopContext, context,
                                 writer.getTypeMapping(), os);
   os << "#endif\n";
 
@@ -1226,9 +1230,9 @@ EmittedClangHeaderDependencyInfo swift::printModuleContentsAsCxx(
       os << "#endif\n";
     os << "#ifdef __cplusplus\n";
     os << "namespace ";
-    ClangSyntaxPrinter(M.getASTContext(), os).printBaseName(&M);
+    ClangSyntaxPrinter(context, os).printBaseName(&M);
     os << " SWIFT_PRIVATE_ATTR";
-    ClangSyntaxPrinter(M.getASTContext(), os).printSymbolUSRAttribute(&M);
+    ClangSyntaxPrinter(context, os).printSymbolUSRAttribute(&M);
     os << " {\n";
     os << "namespace " << cxx_synthesis::getCxxImplNamespaceName() << " {\n";
     os << "extern \"C\" {\n";
@@ -1246,10 +1250,13 @@ EmittedClangHeaderDependencyInfo swift::printModuleContentsAsCxx(
   os << "#pragma clang diagnostic push\n";
   os << "#pragma clang diagnostic ignored \"-Wreserved-identifier\"\n";
   // Construct a C++ namespace for the module.
-  ClangSyntaxPrinter(M.getASTContext(), os).printNamespace(
-      [&](raw_ostream &os) { ClangSyntaxPrinter(M.getASTContext(), os).printBaseName(&M); },
-      [&](raw_ostream &os) { os << moduleOS.str(); },
-      ClangSyntaxPrinter::NamespaceTrivia::AttributeSwiftPrivate, &M);
+  ClangSyntaxPrinter(context, os)
+      .printNamespace(
+          [&](raw_ostream &os) {
+            ClangSyntaxPrinter(context, os).printBaseName(&M);
+          },
+          [&](raw_ostream &os) { os << moduleOS.str(); },
+          ClangSyntaxPrinter::NamespaceTrivia::AttributeSwiftPrivate, &M);
   os << "#pragma clang diagnostic pop\n";
 
   if (M.isStdlibModule()) {

lib/PrintAsClang/_SwiftStdlibCxxOverlay.h

@@ -38,7 +38,11 @@ SWIFT_INLINE_THUNK T_0_0 get() const
 /// Constructs a Swift string from a C string.
 SWIFT_INLINE_THUNK String(const char *cString) noexcept {
   if (!cString) {
+#ifdef __EmbeddedSwift__
+    auto res = _impl::$eS2SycfC();
+#else
     auto res = _impl::$sS2SycfC();
+#endif
     memcpy(_getOpaquePointer(), &res, sizeof(res));
     return;
   }

test/Interop/SwiftToCxx/core/embedded-swift-execution.cpp

@@ -0,0 +1,23 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend %S/embedded-swift.swift -target arm64-apple-macosx15.0 -module-name Core -swift-version 6 -enable-experimental-feature Embedded -clang-header-expose-decls=all-public -typecheck -verify -emit-clang-header-path %t/core.h
+
+// RUN: %target-interop-build-clangxx -c %s -I %t -o %t/swift-embedded-execution.o -fno-exceptions -fno-rtti
+// RUN: %target-interop-build-swift -target arm64-apple-macosx15.0 -wmo %S/embedded-swift.swift -o %t/swift-embedded-execution -Xlinker %t/swift-embedded-execution.o -module-name Core -Xfrontend -entry-point-function-name -Xfrontend swiftMain -enable-experimental-feature Embedded -Xcc -fno-rtti -Xcc -fno-exceptions -Xlinker %swift_obj_root/lib/swift/embedded/%target-cpu-apple-macos/libswiftUnicodeDataTables.a
+
+// RUN: %target-codesign %t/swift-embedded-execution
+// RUN: %target-run %t/swift-embedded-execution
+
+// REQUIRES: OS=macosx
+// REQUIRES: embedded_stdlib
+// REQUIRES: swift_feature_Embedded
+// REQUIRES: executable_test
+// REQUIRES: CPU=arm64e || CPU=arm64
+
+#include <assert.h>
+#include "core.h"
+
+int main() {
+  assert(Core::id(5) == 5);
+  return 0;
+}