[KxSerialization] Fix SERIALIZER_NOT_FOUND diagnostic for enums

Fixes #KT-56738

Merge-request: KT-MR-8938
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

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

@@ -63,7 +63,17 @@ internal fun IrClass.findPluginGeneratedMethod(name: String, afterK2: Boolean):
     }
 }
 
-internal fun IrClass.isEnumWithLegacyGeneratedSerializer(context: SerializationBaseContext): Boolean = isInternallySerializableEnum() && !context.runtimeHasEnumSerializerFactoryFunctions
+internal fun IrClass.isEnumWithLegacyGeneratedSerializer(): Boolean {
+    return findEnumLegacySerializer() != null
+}
+
+internal fun IrClass.findEnumLegacySerializer(): IrClass? {
+    return if (kind == ClassKind.ENUM_CLASS) {
+        declarations.filterIsInstance<IrClass>().singleOrNull { it.name == SerialEntityNames.SERIALIZER_CLASS_NAME }
+    } else {
+        null
+    }
+}
 
 internal val IrClass.isSealedSerializableInterface: Boolean
     get() = kind == ClassKind.INTERFACE && modality == Modality.SEALED && hasSerializableOrMetaAnnotation()
@@ -102,9 +112,9 @@ internal val IrClass.isSerialInfoAnnotation: Boolean
 internal val IrClass.isInheritableSerialInfoAnnotation: Boolean
     get() = annotations.hasAnnotation(SerializationAnnotations.inheritableSerialInfoFqName)
 
-internal fun IrClass.shouldHaveGeneratedSerializer(context: SerializationBaseContext): Boolean
-    = (isInternalSerializable && (modality == Modality.FINAL || modality == Modality.OPEN))
-            || isEnumWithLegacyGeneratedSerializer(context)
+internal fun IrClass.shouldHaveGeneratedSerializer(): Boolean =
+    (isInternalSerializable && (modality == Modality.FINAL || modality == Modality.OPEN))
+            || isEnumWithLegacyGeneratedSerializer()
 
 internal val IrClass.shouldHaveGeneratedMethodsInCompanion: Boolean
     get() = this.isSerializableObject || this.isSerializableEnum() || (this.kind == ClassKind.CLASS && hasSerializableOrMetaAnnotation()) || this.isSealedSerializableInterface

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

@@ -630,7 +630,7 @@ open class SerializerIrGenerator(
         ) {
             val serializableDesc = context.getSerializableClassDescriptorBySerializer(irClass) ?: return
             val generator = when {
-                serializableDesc.isEnumWithLegacyGeneratedSerializer(context) -> SerializerForEnumsGenerator(
+                serializableDesc.isEnumWithLegacyGeneratedSerializer() -> SerializerForEnumsGenerator(
                     irClass,
                     context
                 )

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

@@ -127,9 +127,13 @@ fun findTypeSerializer(context: SerializationBaseContext, type: IrType): IrClass
 
 fun findEnumTypeSerializer(context: SerializationBaseContext, type: IrType): IrClassSymbol? {
     val classSymbol = type.classOrNull?.owner ?: return null
-    return if (classSymbol.kind == ClassKind.ENUM_CLASS && !classSymbol.isEnumWithLegacyGeneratedSerializer(context))
-        context.referenceClassId(enumSerializerId)
-    else null
+
+    // in any case, the function returns the serializer for the enum
+    if (classSymbol.kind != ClassKind.ENUM_CLASS) return null
+
+    val legacySerializer = classSymbol.findEnumLegacySerializer()
+    // $serializer for legacy compiled enums, or EnumSerializer for factories
+    return legacySerializer?.symbol ?: context.referenceClassId(enumSerializerId)
 }
 
 internal fun IrClass?.classSerializer(context: SerializationBaseContext): IrClassSymbol? = this?.let {
@@ -140,7 +144,7 @@ internal fun IrClass?.classSerializer(context: SerializationBaseContext): IrClas
     // can infer @Poly?
     polymorphicSerializerIfApplicableAutomatically(context)?.let { return it }
     // default serializable?
-    if (shouldHaveGeneratedSerializer(context)) {
+    if (shouldHaveGeneratedSerializer()) {
         // $serializer nested class
         return this.declarations
             .filterIsInstance<IrClass>()
@@ -237,7 +241,7 @@ internal fun SerializationBaseContext.getSerializableClassDescriptorBySerializer
         )
     ) return null
     val classDescriptor = (serializer.parent as? IrClass) ?: return null
-    if (!classDescriptor.shouldHaveGeneratedSerializer(this)) return null
+    if (!classDescriptor.shouldHaveGeneratedSerializer()) return null
     return classDescriptor
 }