[interop][SwiftToC++] print C++ interface for top-level Swift functions

include/swift/AST/SwiftNameTranslation.h

@@ -54,6 +54,16 @@ namespace objc_translation {
                        bool checkParent = true);
 
 } // end namespace objc_translation
+
+namespace cxx_translation {
+
+/// Returns true if the given value decl D is visible to C++ of its
+/// own accord (i.e. without considering its context)
+bool isVisibleToCxx(const ValueDecl *VD, AccessLevel minRequiredAccess,
+                    bool checkParent = true);
+
+} // end namespace cxx_translation
+
 } // end namespace swift
 
 #endif

lib/AST/SwiftNameTranslation.cpp

@@ -150,3 +150,20 @@ isVisibleToObjC(const ValueDecl *VD, AccessLevel minRequiredAccess,
   }
   return false;
 }
+
+bool swift::cxx_translation::isVisibleToCxx(const ValueDecl *VD,
+                                            AccessLevel minRequiredAccess,
+                                            bool checkParent) {
+  if (VD->isObjC())
+    return false;
+  if (VD->getFormalAccess() >= minRequiredAccess) {
+    return true;
+  } else if (checkParent) {
+    if (auto ctor = dyn_cast<ConstructorDecl>(VD)) {
+      // Check if we're overriding an initializer that is visible to obj-c
+      if (auto parent = ctor->getOverriddenDecl())
+        return isVisibleToCxx(parent, minRequiredAccess, false);
+    }
+  }
+  return false;
+}

lib/PrintAsClang/CMakeLists.txt

@@ -1,5 +1,6 @@
 
 add_swift_host_library(swiftPrintAsClang STATIC
+  CxxSynthesis.cpp
   DeclAndTypePrinter.cpp
   ModuleContentsWriter.cpp
   PrintAsClang.cpp)

lib/PrintAsClang/CxxSynthesis.cpp

@@ -0,0 +1,37 @@
+//===--- CxxSynthesis.cpp - Rules for synthesizing C++ code -----*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#include "CxxSynthesis.h"
+
+using namespace swift;
+using namespace cxx_synthesis;
+
+StringRef cxx_synthesis::getCxxImplNamespaceName() { return "_impl"; }
+
+/// Print a C++ namespace declaration with the give name and body.
+void CxxPrinter::printNamespace(
+    llvm::function_ref<void(raw_ostream &OS)> namePrinter,
+    llvm::function_ref<void(raw_ostream &OS)> bodyPrinter) const {
+  os << "namespace ";
+  namePrinter(os);
+  os << " {\n\n";
+  bodyPrinter(os);
+  os << "\n} // namespace ";
+  namePrinter(os);
+  os << "\n\n";
+}
+
+void CxxPrinter::printNamespace(
+    StringRef name,
+    llvm::function_ref<void(raw_ostream &OS)> bodyPrinter) const {
+  printNamespace([&](raw_ostream &os) { os << name; }, bodyPrinter);
+}

lib/PrintAsClang/CxxSynthesis.h

@@ -0,0 +1,49 @@
+//===--- CxxSynthesis.h - Rules for synthesizing C++ code -------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_PRINTASCLANG_CXXSYNTHESIS_H
+#define SWIFT_PRINTASCLANG_CXXSYNTHESIS_H
+
+#include "swift/Basic/LLVM.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace swift {
+
+namespace cxx_synthesis {
+
+/// Return the name of the implementation namespace that is used to hide
+/// declarations from the namespace that corresponds to the imported Swift
+/// module in C++.
+StringRef getCxxImplNamespaceName();
+
+class CxxPrinter {
+public:
+  CxxPrinter(raw_ostream &os) : os(os) {}
+
+  /// Print a C++ namespace declaration with the give name and body.
+  void
+  printNamespace(llvm::function_ref<void(raw_ostream &OS)> namePrinter,
+                 llvm::function_ref<void(raw_ostream &OS)> bodyPrinter) const;
+
+  void
+  printNamespace(StringRef name,
+                 llvm::function_ref<void(raw_ostream &OS)> bodyPrinter) const;
+
+private:
+  raw_ostream &os;
+};
+
+} // end namespace cxx_synthesis
+} // end namespace swift
+
+#endif