[interop][SwiftToCxx] do not expose Swift APIs with shared FRTs as their ownership is not yet managed at the language boundary

Author: hyp

lib/PrintAsClang/PrintClangFunction.cpp

@@ -30,6 +30,7 @@
 #include "swift/ClangImporter/ClangImporter.h"
 #include "swift/IRGen/IRABIDetailsProvider.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/Attr.h"
 #include "clang/AST/DeclObjC.h"
 #include "llvm/ADT/STLExtras.h"
 
@@ -281,6 +282,25 @@ class CFunctionSignatureTypePrinter
           os << " __strong";
         printInoutTypeModifier();
       }
+      if (isa<clang::CXXRecordDecl>(cd->getClangDecl())) {
+        if (std::find_if(
+                cd->getClangDecl()->getAttrs().begin(),
+                cd->getClangDecl()->getAttrs().end(), [](clang::Attr *attr) {
+                  if (auto *sa = dyn_cast<clang::SwiftAttrAttr>(attr)) {
+                    llvm::StringRef value = sa->getAttribute();
+                    if ((value.startswith("retain:") ||
+                         value.startswith("release:")) &&
+                        !value.endswith(":immortal"))
+                      return true;
+                  }
+                  return false;
+                }) != cd->getClangDecl()->getAttrs().end()) {
+          // This is a shared FRT. Do not bridge it back to
+          // C++ as its ownership is not managed automatically
+          // in C++ yet.
+          return ClangRepresentation::unsupported;
+        }
+      }
       // FIXME: Mark that this is only ObjC representable.
       return ClangRepresentation::representable;
     }

test/Interop/CxxToSwiftToCxx/disallow-shared-frt-back-to-cxx.swift

@@ -0,0 +1,34 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -typecheck %t/use-cxx-types.swift -typecheck -module-name UseCxxTy -emit-clang-header-path %t/UseCxxTy.h -I %t -enable-experimental-cxx-interop -clang-header-expose-decls=all-public -disable-availability-checking
+
+// RUN: %FileCheck %s < %t/UseCxxTy.h
+
+//--- header.h
+
+class SharedFRT {
+public:
+    int x;
+} __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:retainShared")))
+__attribute__((swift_attr("release:releaseShared")));
+
+inline void retainShared(SharedFRT *r) { }
+inline void releaseShared(SharedFRT *r) { }
+
+//--- module.modulemap
+module CxxTest {
+    header "header.h"
+    requires cplusplus
+}
+
+//--- use-cxx-types.swift
+import CxxTest
+
+public func consumeSharedFRT(_ x: consuming SharedFRT) {}
+public func takeSharedFRT(_ x: SharedFRT) {}
+
+// CHECK: Unavailable in C++: Swift global function 'consumeSharedFRT(_:)'.
+
+// CHECK: Unavailable in C++: Swift global function 'takeSharedFRT(_:)'.