Add @SerializableWith on objects too

to be able to retrieve them via getAssociatedObject and serializer<T>()
function. Now object itself is a SerializerFactory.

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializableCompanionIrGenerator.kt

@@ -110,7 +110,7 @@ class SerializableCompanionIrGenerator(
     }
 
     private fun generateSerializerFactoryIfNeeded(getterDescriptor: FunctionDescriptor) {
-        if (serializableDescriptor.declaredTypeParameters.isEmpty()) return
+        if (!companionDescriptor.needSerializerFactory()) return
         val serialFactoryDescriptor = companionDescriptor.unsubstitutedMemberScope.getContributedFunctions(
             SerialEntityNames.SERIALIZER_PROVIDER_NAME,
             NoLookupLocation.FROM_BACKEND

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/resolve/KSerializationUtil.kt

@@ -22,9 +22,11 @@ import org.jetbrains.kotlin.descriptors.annotations.Annotated
 import org.jetbrains.kotlin.descriptors.annotations.Annotations
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
+import org.jetbrains.kotlin.platform.konan.isNative
 import org.jetbrains.kotlin.psi.KtAnnotationEntry
 import org.jetbrains.kotlin.resolve.descriptorUtil.annotationClass
 import org.jetbrains.kotlin.resolve.descriptorUtil.module
+import org.jetbrains.kotlin.resolve.descriptorUtil.platform
 import org.jetbrains.kotlin.resolve.lazy.descriptors.LazyAnnotationDescriptor
 import org.jetbrains.kotlin.resolve.scopes.getDescriptorsFiltered
 import org.jetbrains.kotlin.types.*
@@ -210,6 +212,14 @@ internal fun getSerializableClassDescriptorByCompanion(thisDescriptor: ClassDesc
     return classDescriptor
 }
 
+internal fun ClassDescriptor.needSerializerFactory(): Boolean {
+    if (this.platform?.isNative() != true) return false
+    val serializableClass = getSerializableClassDescriptorByCompanion(this) ?: return false
+    if (serializableClass.isSerializableObject) return true
+    if (serializableClass.declaredTypeParameters.isEmpty()) return false
+    return true
+}
+
 internal fun getSerializableClassDescriptorBySerializer(serializerDescriptor: ClassDescriptor): ClassDescriptor? {
     val serializerForClass = serializerDescriptor.serializerForClass
     if (serializerForClass != null) return serializerForClass.toClassDescriptor

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/resolve/KSerializerDescriptorResolver.kt

@@ -58,13 +58,6 @@ object KSerializerDescriptorResolver {
         }
     }
 
-    private fun ClassDescriptor.needSerializerFactory(): Boolean {
-        if (this.platform?.isNative() != true) return false
-        val serializableClass = getSerializableClassDescriptorByCompanion(this) ?: return false
-        if (serializableClass.declaredTypeParameters.isEmpty()) return false
-        return true
-    }
-
     fun addSerializerFactorySuperType(classDescriptor: ClassDescriptor, supertypes: MutableList<KotlinType>) {
         if (!classDescriptor.needSerializerFactory()) return
         val serializerFactoryClass =