Make enums and interfaces not serializable internally. However, they

still can be serialized using custom companion object.
Fixes https://github.com/Kotlin/kotlinx.serialization/issues/138 and https://github.com/Kotlin/kotlinx.serialization/issues/304

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

@@ -3,6 +3,7 @@ package org.jetbrains.kotlinx.serialization.compiler.backend.ir
 import org.jetbrains.kotlin.backend.common.BackendContext
 import org.jetbrains.kotlin.backend.common.lower.irThrow
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
+import org.jetbrains.kotlin.codegen.CompilationException
 import org.jetbrains.kotlin.descriptors.ClassConstructorDescriptor
 import org.jetbrains.kotlin.descriptors.FunctionDescriptor
 import org.jetbrains.kotlin.ir.builders.*
@@ -12,6 +13,7 @@ import org.jetbrains.kotlin.ir.declarations.IrProperty
 import org.jetbrains.kotlin.ir.expressions.IrExpression
 import org.jetbrains.kotlin.ir.util.SymbolTable
 import org.jetbrains.kotlin.ir.util.TypeTranslator
+import org.jetbrains.kotlin.js.resolve.diagnostics.findPsi
 import org.jetbrains.kotlin.resolve.BindingContext
 import org.jetbrains.kotlin.resolve.descriptorUtil.getSuperClassOrAny
 import org.jetbrains.kotlin.resolve.descriptorUtil.module
@@ -19,6 +21,8 @@ import org.jetbrains.kotlin.util.OperatorNameConventions
 import org.jetbrains.kotlinx.serialization.compiler.backend.common.SerializableCodegen
 import org.jetbrains.kotlinx.serialization.compiler.resolve.SerialEntityNames.MISSING_FIELD_EXC
 import org.jetbrains.kotlinx.serialization.compiler.resolve.getClassFromSerializationPackage
+import org.jetbrains.kotlinx.serialization.compiler.resolve.hasCompanionObjectAsSerializer
+import org.jetbrains.kotlinx.serialization.compiler.resolve.hasSerializableAnnotationWithoutArgs
 import org.jetbrains.kotlinx.serialization.compiler.resolve.isInternalSerializable
 
 class SerializableIrGenerator(
@@ -98,8 +102,16 @@ class SerializableIrGenerator(
             context: BackendContext,
             bindingContext: BindingContext
         ) {
-            if (irClass.descriptor.isInternalSerializable)
+            val serializableClass = irClass.descriptor
+
+            if (serializableClass.isInternalSerializable)
                 SerializableIrGenerator(irClass, context, bindingContext).generate()
+            else if (serializableClass.hasSerializableAnnotationWithoutArgs && !serializableClass.hasCompanionObjectAsSerializer) {
+                throw CompilationException(
+                    "@Serializable annotation on $serializableClass would be ignored because it is impossible to serialize it automatically. " +
+                            "Provide serializer manually via e.g. companion object", null, serializableClass.findPsi()
+                )
+            }
         }
     }
 }

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/js/SerializableJsTranslator.kt

@@ -16,11 +16,13 @@
 
 package org.jetbrains.kotlinx.serialization.compiler.backend.js
 
+import org.jetbrains.kotlin.codegen.CompilationException
 import org.jetbrains.kotlin.descriptors.ClassConstructorDescriptor
 import org.jetbrains.kotlin.descriptors.ClassDescriptor
 import org.jetbrains.kotlin.descriptors.FunctionDescriptor
 import org.jetbrains.kotlin.descriptors.PropertyDescriptor
 import org.jetbrains.kotlin.js.backend.ast.*
+import org.jetbrains.kotlin.js.resolve.diagnostics.findPsi
 import org.jetbrains.kotlin.js.translate.context.Namer
 import org.jetbrains.kotlin.js.translate.context.TranslationContext
 import org.jetbrains.kotlin.js.translate.declaration.DeclarationBodyVisitor
@@ -30,10 +32,10 @@ import org.jetbrains.kotlin.psi.KtExpression
 import org.jetbrains.kotlin.psi.KtPureClassOrObject
 import org.jetbrains.kotlinx.serialization.compiler.backend.common.SerializableCodegen
 import org.jetbrains.kotlinx.serialization.compiler.backend.common.anonymousInitializers
-import org.jetbrains.kotlinx.serialization.compiler.backend.common.bodyPropertiesDescriptorsMap
-import org.jetbrains.kotlinx.serialization.compiler.backend.common.primaryPropertiesDescriptorsMap
 import org.jetbrains.kotlinx.serialization.compiler.resolve.SerialEntityNames.MISSING_FIELD_EXC
 import org.jetbrains.kotlinx.serialization.compiler.resolve.getClassFromSerializationPackage
+import org.jetbrains.kotlinx.serialization.compiler.resolve.hasCompanionObjectAsSerializer
+import org.jetbrains.kotlinx.serialization.compiler.resolve.hasSerializableAnnotationWithoutArgs
 import org.jetbrains.kotlinx.serialization.compiler.resolve.isInternalSerializable
 
 class SerializableJsTranslator(
@@ -104,12 +106,18 @@ class SerializableJsTranslator(
     companion object {
         fun translate(
             declaration: KtPureClassOrObject,
-            descriptor: ClassDescriptor,
+            serializableClass: ClassDescriptor,
             translator: DeclarationBodyVisitor,
             context: TranslationContext
         ) {
-            if (descriptor.isInternalSerializable)
-                SerializableJsTranslator(declaration, descriptor, translator, context).generate()
+            if (serializableClass.isInternalSerializable)
+                SerializableJsTranslator(declaration, serializableClass, translator, context).generate()
+            else if (serializableClass.hasSerializableAnnotationWithoutArgs && !serializableClass.hasCompanionObjectAsSerializer) {
+                throw CompilationException(
+                    "@Serializable annotation on $serializableClass would be ignored because it is impossible to serialize it automatically. " +
+                            "Provide serializer manually via e.g. companion object", null, serializableClass.findPsi()
+                )
+            }
         }
     }
 }

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/jvm/SerializableCodegenImpl.kt

@@ -21,6 +21,7 @@ import org.jetbrains.kotlin.descriptors.ClassConstructorDescriptor
 import org.jetbrains.kotlin.descriptors.ClassDescriptor
 import org.jetbrains.kotlin.descriptors.FunctionDescriptor
 import org.jetbrains.kotlin.descriptors.PropertyDescriptor
+import org.jetbrains.kotlin.js.resolve.diagnostics.findPsi
 import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.psi.KtExpression
 import org.jetbrains.kotlin.psi.KtParameter
@@ -46,6 +47,12 @@ class SerializableCodegenImpl(
             val serializableClass = codegen.descriptor
             if (serializableClass.isInternalSerializable)
                 SerializableCodegenImpl(codegen, serializableClass).generate()
+            else if (serializableClass.hasSerializableAnnotationWithoutArgs && !serializableClass.hasCompanionObjectAsSerializer) {
+                throw CompilationException(
+                    "@Serializable annotation on $serializableClass would be ignored because it is impossible to serialize it automatically. " +
+                            "Provide serializer manually via e.g. companion object", null, serializableClass.findPsi()
+                )
+            }
         }
     }
 

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

@@ -35,7 +35,9 @@ import org.jetbrains.kotlinx.serialization.compiler.resolve.SerialEntityNames.KS
 import org.jetbrains.kotlinx.serialization.compiler.resolve.SerializationAnnotations.serialInfoFqName
 import org.jetbrains.kotlinx.serialization.compiler.resolve.SerializationPackages.packageFqName
 
-fun isAllowedToHaveAutoGeneratedSerializerMethods(classDescriptor: ClassDescriptor): Boolean {
+fun isAllowedToHaveAutoGeneratedSerializerMethods(classDescriptor: ClassDescriptor, serializableClassDescriptor: ClassDescriptor): Boolean {
+    // don't generate automatically anything for enums or interfaces or other strange things
+    if (serializableClassDescriptor.kind != ClassKind.CLASS) return false
     // it is either GeneratedSerializer implementation
     // or user implementation which does not have type parameters (to be able to correctly initialize descriptor)
     return classDescriptor.typeConstructor.supertypes.any(::isGeneratedKSerializer) ||
@@ -97,6 +99,12 @@ val KotlinType?.toClassDescriptor: ClassDescriptor?
 
 
 val ClassDescriptor.isInternalSerializable: Boolean //todo normal checking
+    get() {
+        if (kind != ClassKind.CLASS) return false
+        return hasSerializableAnnotationWithoutArgs
+    }
+
+internal val ClassDescriptor.hasSerializableAnnotationWithoutArgs: Boolean
     get() {
         if (!annotations.hasAnnotation(SerializationAnnotations.serializableAnnotationFqName)) return false
         // If provided descriptor is lazy, carefully look at psi in order not to trigger full resolve which may be recursive.
@@ -112,10 +120,10 @@ internal val ClassDescriptor?.classSerializer: KotlinType?
     get() = this?.let {
         // serializer annotation on class?
         serializableWith?.let { return it }
+        // companion object serializer?
+        if (hasCompanionObjectAsSerializer) return companionObjectDescriptor?.defaultType
         // default serializable?
         if (isInternalSerializable) {
-            // companion object serializer?
-            if (hasCompanionObjectAsSerializer) return companionObjectDescriptor?.defaultType
             // $serializer nested class
             return this.unsubstitutedMemberScope
                     .getDescriptorsFiltered(nameFilter = {it == SerialEntityNames.SERIALIZER_CLASS_NAME})

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

@@ -136,10 +136,11 @@ object KSerializerDescriptorResolver {
         name: Name,
         result: MutableSet<PropertyDescriptor>
     ) {
-        // Do not auto-generate anything for user serializers
-        if (!isAllowedToHaveAutoGeneratedSerializerMethods(thisDescriptor)) return
-
         val classDescriptor = getSerializableClassDescriptorBySerializer(thisDescriptor) ?: return
+
+        // Do not auto-generate anything for user serializers
+        if (!isAllowedToHaveAutoGeneratedSerializerMethods(thisDescriptor, classDescriptor)) return
+
         if (name == SerialEntityNames.SERIAL_DESC_FIELD_NAME && result.none(thisDescriptor::checkSerializableClassPropertyResult) &&
             fromSupertypes.none { thisDescriptor.checkSerializableClassPropertyResult(it) && it.modality == Modality.FINAL }
         ) {
@@ -171,11 +172,11 @@ object KSerializerDescriptorResolver {
         name: Name,
         result: MutableCollection<SimpleFunctionDescriptor>
     ) {
-        // Do not auto-generate anything for user serializers
-        if (!isAllowedToHaveAutoGeneratedSerializerMethods(thisDescriptor)) return
-
         val classDescriptor = getSerializableClassDescriptorBySerializer(thisDescriptor) ?: return
 
+        // Do not auto-generate anything for user serializers
+        if (!isAllowedToHaveAutoGeneratedSerializerMethods(thisDescriptor, classDescriptor)) return
+
         fun shouldAddSerializerFunction(checkParameters: (FunctionDescriptor) -> Boolean): Boolean {
             // Add 'save' / 'load' iff there is no such declared member AND there is no such final member in supertypes
             return result.none(checkParameters) &&