[Serialization] Mark decls that can never be cross-referenced

include/swift/AST/Attr.def

@@ -373,6 +373,10 @@ SIMPLE_DECL_ATTR(_frozen, Frozen,
   OnEnum |
   UserInaccessible,
   76)
+SIMPLE_DECL_ATTR(_forbidSerializingReference, ForbidSerializingReference,
+  OnAnyDecl |
+  LongAttribute | RejectByParser | UserInaccessible | NotSerialized,
+  77)
 
 #undef TYPE_ATTR
 #undef DECL_ATTR_ALIAS

lib/ClangImporter/ImportDecl.cpp

@@ -4623,6 +4623,8 @@ namespace {
               "This Objective-C class has only been forward-declared; "
               "import its owning module to use it");
           result->getAttrs().add(attr);
+          result->getAttrs().add(
+              new (Impl.SwiftContext) ForbidSerializingReferenceAttr(true));
           return result;
         }
 

lib/Sema/TypeCheckAttr.cpp

@@ -80,6 +80,7 @@ class AttributeEarlyChecker : public AttributeVisitor<AttributeEarlyChecker> {
   IGNORED_ATTR(Effects)
   IGNORED_ATTR(Exported)
   IGNORED_ATTR(FixedLayout)
+  IGNORED_ATTR(ForbidSerializingReference)
   IGNORED_ATTR(Frozen)
   IGNORED_ATTR(Implements)
   IGNORED_ATTR(ImplicitlyUnwrappedOptional)
@@ -805,6 +806,7 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
     IGNORED_ATTR(Dynamic)
     IGNORED_ATTR(Effects)
     IGNORED_ATTR(Exported)
+    IGNORED_ATTR(ForbidSerializingReference)
     IGNORED_ATTR(GKInspectable)
     IGNORED_ATTR(IBDesignable)
     IGNORED_ATTR(IBInspectable)

lib/Sema/TypeCheckDecl.cpp

@@ -5777,6 +5777,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     UNINTERESTING_ATTR(DynamicMemberLookup)
     UNINTERESTING_ATTR(SILGenName)
     UNINTERESTING_ATTR(Exported)
+    UNINTERESTING_ATTR(ForbidSerializingReference)
     UNINTERESTING_ATTR(GKInspectable)
     UNINTERESTING_ATTR(IBAction)
     UNINTERESTING_ATTR(IBDesignable)

lib/Serialization/Deserialization.cpp

@@ -1226,6 +1226,10 @@ static void filterValues(Type expectedTy, ModuleDecl *expectedModule,
       return true;
     if (value->isStatic() != isStatic)
       return true;
+
+    if (value->getAttrs().hasAttribute<ForbidSerializingReferenceAttr>())
+      return true;
+
     // FIXME: Should be able to move a value from an extension in a derived
     // module to the original definition in a base module.
     if (expectedModule && !value->hasClangNode() &&

lib/Serialization/Serialization.cpp

@@ -622,6 +622,10 @@ DeclID Serializer::addDeclRef(const Decl *D, bool forceSerialization,
           isa<PrecedenceGroupDecl>(D)) &&
          "cannot cross-reference this decl");
 
+  assert((!isDeclXRef(D) ||
+          !D->getAttrs().hasAttribute<ForbidSerializingReferenceAttr>()) &&
+         "cannot cross-reference this decl");
+
   assert((allowTypeAliasXRef || !isa<TypeAliasDecl>(D) ||
           D->getModuleContext() == M) &&
          "cannot cross-reference typealiases directly (use the NameAliasType)");

test/ClangImporter/Inputs/custom-modules/ForwardDeclarationsHelper.h

@@ -0,0 +1,4 @@
+@class Confusing;
+
+@protocol Confusing
+@end

test/ClangImporter/Inputs/custom-modules/module.map

@@ -216,4 +216,8 @@ module Warnings9 { header "Warnings9.h" }
 module ConditionallyFoo {
   header "ConditionallyFoo.h"
   config_macros [exhaustive] WANT_FOO
+}
+
+module ForwardDeclarationsHelper {
+  header "ForwardDeclarationsHelper.h"
 }

test/ClangImporter/objc_forward_declarations.swift

@@ -0,0 +1,9 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -o %t -I %S/Inputs/custom-modules -enable-objc-interop %s
+// RUN: %target-swift-ide-test -print-module -module-to-print objc_forward_declarations -I %t -I %S/Inputs/custom-modules -enable-objc-interop -enable-objc-forward-declarations -source-filename x | %FileCheck %s
+
+// CHECK: class Innocuous : Confusing {
+
+import ForwardDeclarationsHelper
+
+public class Innocuous: Confusing {}