[SE-0306] Disallow actor inheritance from NSObject, but allow @objc actor.

Allow an actor to be exposed to Objective-C via `@objc` without
inheriting from `NSObject`, and remove the loophole that allowed
actors to inherit from `NSObject`.

Fixes rdar://78333614

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -4351,6 +4351,8 @@ ERROR(async_objc_dynamic_self,none,
 ERROR(actor_inheritance,none,
       "%select{actor|distributed actor}0 types do not support inheritance",
       (bool))
+NOTE(actor_inheritance_nsobject,none,
+      "use '@objc' to expose actor %0 to Objective-C", (DeclName))
 
 ERROR(actor_protocol_illegal_inheritance,none,
       "non-actor type %0 cannot conform to the 'Actor' protocol",

lib/AST/Decl.cpp

@@ -8289,15 +8289,14 @@ bool ClassDecl::isRootDefaultActor(ModuleDecl *M,
 }
 
 bool ClassDecl::isNativeNSObjectSubclass() const {
-  // Only if we inherit from NSObject.
-  auto superclass = getSuperclassDecl();
-  if (!superclass || !superclass->isNSObject())
-    return false;
+  // @objc actors implicitly inherit from NSObject.
+  if (isActor() && getAttrs().hasAttribute<ObjCAttr>())
+    return true;
 
-  // For now, only actors (regardless of whether they're default actors).
-  // Eventually we should roll this out to more classes, but we have to
-  // do it with ABI compatibility.
-  return isActor();
+  // For now, non-actor classes cannot use the native NSObject subclass.
+  // Eventually we should roll this out to more classes that directly
+  // inherit NSObject, but we have to do it with ABI compatibility.
+  return false;
 }
 
 bool ClassDecl::isNSObject() const {

lib/Sema/TypeCheckDeclObjC.cpp

@@ -1271,10 +1271,11 @@ static Optional<ObjCReason> shouldMarkClassAsObjC(const ClassDecl *CD) {
       reason.describe(CD);
     }
 
-    // Only allow ObjC-rooted classes to be @objc.
+    // Only allow actors and ObjC-rooted classes to be @objc.
     // (Leave a hole for test cases.)
     if (ancestry.contains(AncestryFlags::ObjC) &&
-        !ancestry.contains(AncestryFlags::ClangImported)) {
+        !ancestry.contains(AncestryFlags::ClangImported) &&
+        !CD->isActor()) {
       if (ctx.LangOpts.EnableObjCAttrRequiresFoundation) {
         swift::diagnoseAndRemoveAttr(CD, attr,
                                      diag::invalid_objc_swift_rooted_class)

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -2398,12 +2398,18 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
 
     if (auto superclass = CD->getSuperclassDecl()) {
       // Actors cannot have superclasses, nor can they be superclasses.
-      if (CD->isActor() && !superclass->isNSObject())
+      if (CD->isActor()) {
         CD->diagnose(diag::actor_inheritance,
                      /*distributed=*/CD->isDistributedActor());
-      else if (superclass->isActor())
+        if (superclass->isNSObject() && !CD->isDistributedActor()) {
+          CD->diagnose(diag::actor_inheritance_nsobject, CD->getName())
+            .fixItInsert(CD->getAttributeInsertionLoc(/*forModifier=*/false),
+                         "@objc ");
+        }
+      } else if (superclass->isActor()) {
         CD->diagnose(diag::actor_inheritance,
                      /*distributed=*/CD->isDistributedActor());
+      }
     }
 
     // Force lowering of stored properties.

test/IRGen/actor_class_objc.swift

@@ -9,8 +9,6 @@ import Foundation
 
 // CHECK-LABEL: @"OBJC_METACLASS_$__TtC16actor_class_objc7MyClass" = global
 //   Metaclass is an instance of the root class.
-// CHECK-SAME: %objc_class* {{.*}}@"OBJC_METACLASS_$_NSObject{{(.ptrauth)?}}"
-//   Metaclass superclass is the metaclass of the superclass.
 // CHECK-SAME: %objc_class* {{.*}}@"OBJC_METACLASS_$_SwiftNativeNSObject{{(.ptrauth)?}}"
 
 // CHECK: @"$s16actor_class_objc7MyClassCMf" = internal global
@@ -29,9 +27,9 @@ import Foundation
 // CHECK-64-SAME: i64 112,
 // CHECK-32-SAME: i32 56,
 
-public actor MyClass: NSObject {
+@objc public actor MyClass {
   public var x: Int
-  public override init() { self.x = 0 }
+  public init() { self.x = 0 }
 }
 
 // CHECK-LABEL: define {{.*}} @"$s16actor_class_objc7MyClassC1xSivg"

test/Interpreter/actor_class_forbid_objc_assoc_objects.swift

@@ -10,6 +10,7 @@
 // UNSUPPORTED: back_deployment_runtime
 
 import ObjectiveC
+import Foundation
 import _Concurrency
 import StdlibUnittest
 
@@ -70,7 +71,7 @@ if #available(macOS 10.4.4, iOS 12.2, watchOS 5.2, tvOS 12.2, *) {
 }
 
 @available(macOS 10.4.4, iOS 12.2, watchOS 5.2, tvOS 12.2, *)
-actor ActorNSObjectSubKlass : NSObject {}
+@objc actor ActorNSObjectSubKlass {}
 
 if #available(macOS 10.4.4, iOS 12.2, watchOS 5.2, tvOS 12.2, *) {
   Tests.test("no crash when inherit from nsobject")
@@ -79,17 +80,3 @@ if #available(macOS 10.4.4, iOS 12.2, watchOS 5.2, tvOS 12.2, *) {
     objc_setAssociatedObject(x, "myKey", "myValue", .OBJC_ASSOCIATION_RETAIN)
   }
 }
-
-@available(macOS 10.4.4, iOS 12.2, watchOS 5.2, tvOS 12.2, *)
-actor ActorNSObjectSubKlassGeneric<T> : NSObject {
-  var state: T
-  init(state: T) { self.state = state }
-}
-
-if #available(macOS 10.4.4, iOS 12.2, watchOS 5.2, tvOS 12.2, *) {
-  Tests.test("no crash when generic inherit from nsobject")
-  .code {
-    let x = ActorNSObjectSubKlassGeneric(state: 5)
-    objc_setAssociatedObject(x, "myKey", "myValue", .OBJC_ASSOCIATION_RETAIN)
-  }
-}

test/ModuleInterface/actor_objc.swift

@@ -12,7 +12,7 @@
 
 import Foundation
 
-// CHECK-LABEL: @objc @_inheritsConvenienceInitializers public actor SomeActor : ObjectiveC.NSObject {
-// CHECK: @objc override public init()
-public actor SomeActor: NSObject {
+// CHECK-LABEL: @objc public actor SomeActor {
+// CHECK-NOT: @objc override public init()
+@objc public actor SomeActor {
 }

test/attr/attr_objc_async.swift

@@ -51,7 +51,10 @@ actor class MyActor2 { }
 // expected-error@-1 {{keyword 'class' cannot be used as an identifier here}}
 
 // CHECK: @objc actor MyObjCActor
-@objc actor MyObjCActor: NSObject { }
+@objc actor MyObjCActor { }
 
-@objc actor class MyObjCActor2: NSObject {}
+@objc actor class MyObjCActor2 {}
 // expected-error@-1 {{keyword 'class' cannot be used as an identifier here}}
+
+actor MyObjCActor3: NSObject { } // expected-error{{actor types do not support inheritance}}
+// expected-note@-1{{use '@objc' to expose actor 'MyObjCActor3' to Objective-C}}