Serialization: Option to skip writing decls marked @_implementationOnly

include/swift/Serialization/SerializationOptions.h

@@ -163,6 +163,7 @@ public:
   bool HermeticSealAtLink = false;
   bool EmbeddedSwiftModule = false;
   bool SkipNonExportableDecls = false;
+  bool SkipImplementationOnlyDecls = false;
   bool ExplicitModuleBuild = false;
   bool EnableSerializationRemarks = false;
   bool IsInterfaceSDKRelative = false;

lib/Frontend/Frontend.cpp

@@ -284,6 +284,9 @@ SerializationOptions CompilerInvocation::computeSerializationOptions(
   serializationOpts.SkipNonExportableDecls =
       getLangOptions().SkipNonExportableDecls;
 
+  serializationOpts.SkipImplementationOnlyDecls =
+      getLangOptions().hasFeature(Feature::CheckImplementationOnly);
+
   serializationOpts.ExplicitModuleBuild = FrontendOpts.DisableImplicitModules;
 
   serializationOpts.EnableSerializationRemarks =

lib/Serialization/Serialization.cpp

@@ -5390,6 +5390,20 @@ bool Serializer::shouldSkipDecl(const Decl *D) const {
 void Serializer::writeASTBlockEntity(const Decl *D) {
   using namespace decls_block;
 
+  if (Options.SkipImplementationOnlyDecls) {
+    // Skip @_implementationOnly types.
+    if (D->getAttrs().hasAttribute<ImplementationOnlyAttr>())
+      return;
+
+    // Skip non-public @export(interface) functions.
+    auto FD = dyn_cast<AbstractFunctionDecl>(D);
+    if (FD &&
+        FD->getAttrs().hasAttribute<NeverEmitIntoClientAttr>() &&
+        !FD->getFormalAccessScope(/*useDC*/nullptr,
+          /*treatUsableFromInlineAsPublic*/true).isPublicOrPackage())
+      return;
+  }
+
   PrettyStackTraceDecl trace("serializing", D);
   assert(DeclsToSerialize.hasRef(D));