[cxx-interop] Implement foreign reference types.

This is an expiremental feature to allow an attribute, `import_as_ref`, to import a C++ record as a non-reference-counted reference type in Swift.

Author: zoecarver

include/swift/ABI/Metadata.h

@@ -1518,6 +1518,11 @@ struct TargetStructMetadata : public TargetValueMetadata<Runtime> {
   }
 
   bool isCanonicalStaticallySpecializedGenericMetadata() const {
+    // Struct metadata is used for foreign reference types, in which case the
+    // descriptor is not a struct descriptor.
+    if (!llvm::isa<TargetStructDescriptor<Runtime>>(this->Description))
+      return false;
+
     auto *description = getDescription();
     if (!description->isGeneric())
       return false;

include/swift/AST/Decl.h

@@ -28,6 +28,7 @@
 #include "swift/AST/GenericParamKey.h"
 #include "swift/AST/IfConfigClause.h"
 #include "swift/AST/LayoutConstraint.h"
+#include "swift/AST/ReferenceCounting.h"
 #include "swift/AST/StorageImpl.h"
 #include "swift/AST/TypeAlignments.h"
 #include "swift/AST/TypeWalker.h"
@@ -37,10 +38,10 @@
 #include "swift/Basic/Compiler.h"
 #include "swift/Basic/Debug.h"
 #include "swift/Basic/InlineBitfield.h"
+#include "swift/Basic/Located.h"
 #include "swift/Basic/NullablePtr.h"
 #include "swift/Basic/OptionalEnum.h"
 #include "swift/Basic/Range.h"
-#include "swift/Basic/Located.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/Support/TrailingObjects.h"
 #include <type_traits>
@@ -3900,7 +3901,8 @@ class ClassDecl final : public NominalTypeDecl {
   ///
   /// \see getForeignClassKind
   bool isForeign() const {
-    return getForeignClassKind() != ForeignKind::Normal;
+    return getForeignClassKind() != ForeignKind::Normal ||
+      const_cast<ClassDecl *>(this)->isForeignReferenceType();
   }
 
   /// Whether the class is (known to be) a default actor.
@@ -3935,9 +3937,22 @@ class ClassDecl final : public NominalTypeDecl {
   bool isNativeNSObjectSubclass() const;
 
   /// Whether the class uses the ObjC object model (reference counting,
-  /// allocation, etc.) instead of the Swift model.
-  bool usesObjCObjectModel() const {
-    return checkAncestry(AncestryFlags::ObjCObjectModel);
+  /// allocation, etc.), the Swift model, or has no reference counting at all.
+  ReferenceCounting getObjectModel() {
+    if (isForeignReferenceType())
+      return ReferenceCounting::None;
+
+    if (checkAncestry(AncestryFlags::ObjCObjectModel))
+      return ReferenceCounting::ObjC;
+
+    return ReferenceCounting::Native;
+  }
+
+  LayoutConstraintKind getLayoutConstraintKind() {
+    if (getObjectModel() == ReferenceCounting::ObjC)
+      return LayoutConstraintKind::Class;
+
+    return LayoutConstraintKind::NativeClass;
   }
 
   /// Returns true if the class has designated initializers that are not listed
@@ -4059,6 +4074,11 @@ class ClassDecl final : public NominalTypeDecl {
   bool hasKnownSwiftImplementation() const {
     return !hasClangNode();
   }
+
+  /// Used to determine if this class decl is a foriegn reference type. I.e., a
+  /// non-reference-counted swift reference type that was imported from a C++
+  /// record.
+  bool isForeignReferenceType();
 };
 
 /// A convenience wrapper around the \c SelfReferencePosition::Kind enum.

include/swift/AST/ReferenceCounting.h

@@ -29,6 +29,10 @@ enum class ReferenceCounting : uint8_t {
   /// Blocks are always ObjC reference counting compatible.
   ObjC,
 
+  /// The object has no reference counting. This is used by foreign reference
+  /// types.
+  None,
+
   /// The object uses _Block_copy/_Block_release reference counting.
   ///
   /// This is a strict subset of ObjC; all blocks are also ObjC reference

include/swift/AST/Type.h

@@ -488,6 +488,8 @@ class CanType : public Type {
   NominalTypeDecl *getAnyNominal() const;
   GenericTypeDecl *getAnyGeneric() const;
 
+  bool isForeignReferenceType(); // in Types.h
+
   CanType getOptionalObjectType() const {
     return getOptionalObjectTypeImpl(*this);
   }

include/swift/AST/Types.h

@@ -811,16 +811,19 @@ class alignas(1 << TypeAlignInBits) TypeBase
 
   /// If this is a class type or a bound generic class type, returns the
   /// (possibly generic) class.
-  ClassDecl *getClassOrBoundGenericClass();
-  
+  ClassDecl *getClassOrBoundGenericClass() const;
+
   /// If this is a struct type or a bound generic struct type, returns
   /// the (possibly generic) class.
   StructDecl *getStructOrBoundGenericStruct();
 
   /// If this is an enum or a bound generic enum type, returns the
   /// (possibly generic) enum.
   EnumDecl *getEnumOrBoundGenericEnum();
-  
+
+  /// If this is a class, check if this class is a foreign reference type.
+  bool isForeignReferenceType();
+
   /// Determine whether this type may have a superclass, which holds for
   /// classes, bound generic classes, and archetypes that are only instantiable
   /// with a class type.
@@ -6175,7 +6178,7 @@ inline bool TypeBase::canDynamicallyBeOptionalType(bool includeExistential) {
   return isArchetypeOrExistential && !T.isAnyClassReferenceType();
 }
 
-inline ClassDecl *TypeBase::getClassOrBoundGenericClass() {
+inline ClassDecl *TypeBase::getClassOrBoundGenericClass() const {
   return getCanonicalType().getClassOrBoundGenericClass();
 }
 
@@ -6239,8 +6242,6 @@ inline GenericTypeDecl *TypeBase::getAnyGeneric() {
   return getCanonicalType().getAnyGeneric();
 }
 
-  
-  
 inline bool TypeBase::isBuiltinIntegerType(unsigned n) {
   if (auto intTy = dyn_cast<BuiltinIntegerType>(getCanonicalType()))
     return intTy->getWidth().isFixedWidth()

include/swift/ClangImporter/ClangImporterRequests.h

@@ -132,10 +132,10 @@ class CXXNamespaceMemberLookup
 
 /// The input type for a record member lookup request.
 struct ClangRecordMemberLookupDescriptor final {
-  StructDecl *recordDecl;
+  NominalTypeDecl *recordDecl;
   DeclName name;
 
-  ClangRecordMemberLookupDescriptor(StructDecl *recordDecl, DeclName name)
+  ClangRecordMemberLookupDescriptor(NominalTypeDecl *recordDecl, DeclName name)
       : recordDecl(recordDecl), name(name) {
     assert(isa<clang::RecordDecl>(recordDecl->getClangDecl()));
   }

include/swift/SIL/SILType.h

@@ -236,7 +236,13 @@ class SILType {
   NominalTypeDecl *getNominalOrBoundGenericNominal() const {
     return getASTType().getNominalOrBoundGenericNominal();
   }
-  
+
+  /// If this type maps to a Swift class, check if that class is a foreign
+  /// reference type.
+  bool isForeignReferenceType() const {
+    return getASTType().isForeignReferenceType();
+  }
+
   /// True if the type is an address type.
   bool isAddress() const { return getCategory() == SILValueCategory::Address; }
 

lib/AST/ClangTypeConverter.cpp

@@ -545,7 +545,8 @@ ClangTypeConverter::visitBoundGenericType(BoundGenericType *type) {
     auto args = type->getGenericArgs();
     assert((args.size() == 1) && "Optional should have 1 generic argument.");
     clang::QualType innerTy = convert(args[0]);
-    if (swift::canImportAsOptional(innerTy.getTypePtrOrNull()))
+    if (swift::canImportAsOptional(innerTy.getTypePtrOrNull()) ||
+        args[0]->isForeignReferenceType())
       return innerTy;
     return clang::QualType();
   }

lib/AST/Decl.cpp

@@ -1940,7 +1940,11 @@ static bool isPolymorphic(const AbstractStorageDecl *storage) {
     return true;
 
   if (auto *classDecl = dyn_cast<ClassDecl>(storage->getDeclContext())) {
-    if (storage->isFinal() || classDecl->isFinal())
+    // Accesses to members of foreign reference types should be made directly
+    // to storage as these are references to clang records which are not allowed
+    // to have dynamic dispatch.
+    if (storage->isFinal() || classDecl->isFinal() ||
+        classDecl->isForeignReferenceType())
       return false;
 
     return true;
@@ -4720,6 +4724,10 @@ bool ClassDecl::walkSuperclasses(
   return false;
 }
 
+bool ClassDecl::isForeignReferenceType() {
+  return getClangDecl() && isa<clang::RecordDecl>(getClangDecl());
+}
+
 EnumCaseDecl *EnumCaseDecl::create(SourceLoc CaseLoc,
                                    ArrayRef<EnumElementDecl *> Elements,
                                    DeclContext *DC) {

lib/AST/GenericSignatureBuilder.cpp

@@ -4443,11 +4443,8 @@ bool GenericSignatureBuilder::updateSuperclass(
     auto layoutReqSource =
       source.getSource(*this, type)->viaLayout(*this, superclass);
 
-    auto layout =
-      LayoutConstraint::getLayoutConstraint(
-        superclass->getClassOrBoundGenericClass()->usesObjCObjectModel()
-          ? LayoutConstraintKind::Class
-          : LayoutConstraintKind::NativeClass,
+    auto layout = LayoutConstraint::getLayoutConstraint(
+        superclass->getClassOrBoundGenericClass()->getLayoutConstraintKind(),
         getASTContext());
     addLayoutRequirementDirect(type, layout, layoutReqSource);
     return true;