[cxx-interop] compilations that do not enable C++ interoperability should not be able to import modules that do enable C++ interoperability by default

A supplemental hidden frontend option allows advanced users to opt-out of this requirement.

Fixes #65833
Fixes #65832

Author: hyp

include/swift/AST/Decl.h

@@ -659,7 +659,7 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl> {
     HasAnyUnavailableValues : 1
   );
 
-  SWIFT_INLINE_BITFIELD(ModuleDecl, TypeDecl, 1+1+1+1+1+1+1+1+1+1+1+1+1+1+1,
+  SWIFT_INLINE_BITFIELD(ModuleDecl, TypeDecl, 1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1,
     /// If the module is compiled as static library.
     StaticLibrary : 1,
 
@@ -709,7 +709,10 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl> {
 
     /// If the map from @objc provided name to top level swift::Decl in this
     /// module is populated
-    ObjCNameLookupCachePopulated : 1
+    ObjCNameLookupCachePopulated : 1,
+
+    /// Whether this module has been built with C++ interoperability enabled.
+    HasCxxInteroperability : 1
   );
 
   SWIFT_INLINE_BITFIELD(PrecedenceGroupDecl, Decl, 1+2,

include/swift/AST/DiagnosticsSema.def

@@ -874,6 +874,12 @@ ERROR(need_hermetic_seal_to_import_module,none,
       "module %0 was built with -experimental-hermetic-seal-at-link, but "
       "current compilation does not have -experimental-hermetic-seal-at-link",
       (Identifier))
+ERROR(need_cxx_interop_to_import_module,none,
+      "module %0 was built with C++ interoperability enabled, but "
+      "current compilation does not enable C++ interoperability",
+      (Identifier))
+NOTE(enable_cxx_interop_docs,none,
+     "visit https://www.swift.org/documentation/cxx-interop/project-build-setup to learn how to enable C++ interoperability", ())
 
 ERROR(modularization_issue_decl_moved,Fatal,
       "reference to %select{top-level declaration|type}0 %1 broken by a context change; "

include/swift/AST/Module.h

@@ -654,6 +654,14 @@ class ModuleDecl
     Bits.ModuleDecl.HasHermeticSealAtLink = enabled;
   }
 
+  /// Returns true if this module was built with C++ interoperability enabled.
+  bool hasCxxInteroperability() const {
+    return Bits.ModuleDecl.HasCxxInteroperability;
+  }
+  void setHasCxxInteroperability(bool enabled = true) {
+    Bits.ModuleDecl.HasCxxInteroperability = enabled;
+  }
+
   /// \returns true if this module is a system module; note that the StdLib is
   /// considered a system module.
   bool isSystemModule() const {

include/swift/Basic/LangOptions.h

@@ -313,6 +313,10 @@ namespace swift {
     /// Imports getters and setters as computed properties.
     bool CxxInteropGettersSettersAsProperties = false;
 
+    /// Should the compiler require C++ interoperability to be enabled
+    /// when importing Swift modules that enable C++ interoperability.
+    bool RequireCxxInteropToImportCxxInteropModule = true;
+
     /// On Darwin platforms, use the pre-stable ABI's mark bit for Swift
     /// classes instead of the stable ABI's bit. This is needed when
     /// targeting OSes prior to macOS 10.14.4 and iOS 12.2, where

include/swift/Option/FrontendOptions.td

@@ -865,6 +865,11 @@ def cxx_interop_getters_setters_as_properties :
   HelpText<"Import getters and setters as computed properties in Swift">,
   Flags<[FrontendOption, HelpHidden]>;
 
+def cxx_interop_disable_requirement_at_import :
+  Flag<["-"], "disable-cxx-interop-requirement-at-import">,
+  HelpText<"Do not require C++ interoperability to be enabled when importing a Swift module that enables C++ interoperability">,
+  Flags<[FrontendOption, HelpHidden]>;
+
 def use_malloc : Flag<["-"], "use-malloc">,
   HelpText<"Allocate internal data structures using malloc "
            "(for memory debugging)">;

include/swift/Serialization/Validation.h

@@ -131,6 +131,7 @@ class ExtendedValidationInfo {
     unsigned IsBuiltFromInterface : 1;
     unsigned IsAllowModuleWithCompilerErrorsEnabled : 1;
     unsigned IsConcurrencyChecked : 1;
+    unsigned HasCxxInteroperability : 1;
   } Bits;
 public:
   ExtendedValidationInfo() : Bits() {}
@@ -232,6 +233,10 @@ class ExtendedValidationInfo {
   void setIsConcurrencyChecked(bool val = true) {
     Bits.IsConcurrencyChecked = val;
   }
+  bool hasCxxInteroperability() const { return Bits.HasCxxInteroperability; }
+  void setHasCxxInteroperability(bool val) {
+    Bits.HasCxxInteroperability = val;
+  }
 };
 
 struct SearchPath {

lib/AST/Module.cpp

@@ -646,6 +646,7 @@ ModuleDecl::ModuleDecl(Identifier name, ASTContext &ctx,
   Bits.ModuleDecl.HasHermeticSealAtLink = 0;
   Bits.ModuleDecl.IsConcurrencyChecked = 0;
   Bits.ModuleDecl.ObjCNameLookupCachePopulated = 0;
+  Bits.ModuleDecl.HasCxxInteroperability = 0;
 }
 
 void ModuleDecl::setIsSystemModule(bool flag) {

lib/Frontend/CompilerInvocation.cpp

@@ -1043,6 +1043,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
       Args.hasArg(OPT_experimental_c_foreign_reference_types);
 
   Opts.CxxInteropGettersSettersAsProperties = Args.hasArg(OPT_cxx_interop_getters_setters_as_properties);
+  Opts.RequireCxxInteropToImportCxxInteropModule =
+      !Args.hasArg(OPT_cxx_interop_disable_requirement_at_import);
 
   Opts.VerifyAllSubstitutionMaps |= Args.hasArg(OPT_verify_all_substitution_maps);
 

lib/Frontend/Frontend.cpp

@@ -1319,6 +1319,9 @@ ModuleDecl *CompilerInstance::getMainModule() const {
       MainModule->setResilienceStrategy(ResilienceStrategy::Resilient);
     if (Invocation.getLangOptions().isSwiftVersionAtLeast(6))
       MainModule->setIsConcurrencyChecked(true);
+    if (Invocation.getLangOptions().EnableCXXInterop &&
+        Invocation.getLangOptions().RequireCxxInteropToImportCxxInteropModule)
+      MainModule->setHasCxxInteroperability();
 
     // Register the main module with the AST context.
     Context->addLoadedModule(MainModule);

lib/Serialization/ModuleFile.h

@@ -621,6 +621,11 @@ class ModuleFile
     return Core->Bits.HasHermeticSealAtLink;
   }
 
+  /// Whether this module was built with C++ interoperability enabled.
+  bool hasCxxInteroperability() const {
+    return Core->Bits.HasCxxInteroperability;
+  }
+
   /// Whether the module is resilient. ('-enable-library-evolution')
   ResilienceStrategy getResilienceStrategy() const {
     return ResilienceStrategy(Core->Bits.ResilienceStrategy);