Provide a diagnostic when @Serializable annotation is missing on enum.

It is required when enum members contain some special annotations which should be recorded in descriptor.
Due to some recursive/lazy resolve problems, compiler plugin frontend can't infer @Serializable on enum automatically, therefore, we ask a user to explicitly provide it.

Also fix ir tests

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationErrors.java

@@ -20,6 +20,8 @@ public interface SerializationErrors {
     DiagnosticFactory0<PsiElement> INLINE_CLASSES_NOT_SUPPORTED = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<PsiElement> PLUGIN_IS_NOT_ENABLED = DiagnosticFactory0.create(WARNING);
 
+    DiagnosticFactory0<PsiElement> EXPLICIT_SERIALIZABLE_IS_REQUIRED = DiagnosticFactory0.create(WARNING);
+
     DiagnosticFactory0<KtAnnotationEntry> SERIALIZABLE_ANNOTATION_IGNORED = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<KtAnnotationEntry> NON_SERIALIZABLE_PARENT_MUST_HAVE_NOARG_CTOR = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<KtAnnotationEntry> PRIMARY_CONSTRUCTOR_PARAMETER_IS_NOT_A_PROPERTY = DiagnosticFactory0.create(ERROR);

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationPluginDeclarationChecker.kt

@@ -6,10 +6,7 @@
 package org.jetbrains.kotlinx.serialization.compiler.diagnostic
 
 import com.intellij.psi.PsiElement
-import org.jetbrains.kotlin.descriptors.ClassDescriptor
-import org.jetbrains.kotlin.descriptors.DeclarationDescriptor
-import org.jetbrains.kotlin.descriptors.ModuleDescriptor
-import org.jetbrains.kotlin.descriptors.PropertyDescriptor
+import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.diagnostics.DiagnosticFactory0
 import org.jetbrains.kotlin.diagnostics.reportFromPlugin
 import org.jetbrains.kotlin.js.resolve.diagnostics.findPsi
@@ -37,14 +34,23 @@ open class SerializationPluginDeclarationChecker : DeclarationChecker {
     final override fun check(declaration: KtDeclaration, descriptor: DeclarationDescriptor, context: DeclarationCheckerContext) {
         if (descriptor !is ClassDescriptor) return
 
-        if (!canBeSerializedInternally(descriptor, context.trace)) return
+        if (!canBeSerializedInternally(descriptor, declaration, context.trace)) return
         if (declaration !is KtPureClassOrObject) return
         val props = buildSerializableProperties(descriptor, context.trace) ?: return
         checkTransients(declaration, context.trace)
         analyzePropertiesSerializers(context.trace, descriptor, props.serializableProperties)
     }
 
-    private fun canBeSerializedInternally(descriptor: ClassDescriptor, trace: BindingTrace): Boolean {
+    private fun canBeSerializedInternally(descriptor: ClassDescriptor, declaration: KtDeclaration, trace: BindingTrace): Boolean {
+        if (descriptor.isSerializableEnumWithMissingSerializer()) {
+            val declarationToReport = declaration.modifierList ?: declaration
+            trace.reportFromPlugin(
+                SerializationErrors.EXPLICIT_SERIALIZABLE_IS_REQUIRED.on(declarationToReport),
+                SerializationPluginErrorsRendering
+            )
+            return false
+        }
+
         if (!descriptor.annotations.hasAnnotation(SerializationAnnotations.serializableAnnotationFqName)) return false
 
         if (!serializationPluginEnabledOn(descriptor)) {
@@ -74,6 +80,13 @@ open class SerializationPluginDeclarationChecker : DeclarationChecker {
         return true
     }
 
+    private fun ClassDescriptor.isSerializableEnumWithMissingSerializer(): Boolean {
+        if (kind != ClassKind.ENUM_CLASS) return false
+        if (hasSerializableAnnotationWithoutArgs) return false
+        if (annotations.hasAnySerialAnnotation) return true
+        return enumEntries().any { (it.annotations.hasAnySerialAnnotation) }
+    }
+
     open fun serializationPluginEnabledOn(descriptor: ClassDescriptor): Boolean {
         // In the CLI/Gradle compiler, this diagnostic is located in the plugin itself.
         // Therefore, if we are here, plugin is in the compile classpath and enabled.

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationPluginErrorsRendering.kt

@@ -23,6 +23,10 @@ object SerializationPluginErrorsRendering : DefaultErrorMessages.Extension {
             "kotlinx.serialization compiler plugin is not applied to the module, so this annotation would not be processed. " +
                     "Make sure that you've setup your buildscript correctly and re-import project."
         )
+        MAP.put(
+            SerializationErrors.EXPLICIT_SERIALIZABLE_IS_REQUIRED,
+            "Explicit @Serializable annotation on enum class is required when @SerialName or @SerialInfo annotations are used on its members."
+        )
         MAP.put(
             SerializationErrors.SERIALIZABLE_ANNOTATION_IGNORED,
             "@Serializable annotation is ignored because it is impossible to serialize automatically interfaces or enums. " +

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

@@ -119,11 +119,6 @@ internal val ClassDescriptor.isInternalSerializable: Boolean //todo normal check
 internal fun ClassDescriptor.isSerializableEnum(): Boolean {
     if (this.kind != ClassKind.ENUM_CLASS) return false
     if (hasSerializableAnnotationWithoutArgs) return true
-    if (annotations.hasAnySerialAnnotation) return true
-//   todo:   check entries without 'recursive call in lazy class member scope'
-//   for (enumEntry in enumEntries()) {
-//        if (enumEntry.annotations.hasAnySerialAnnotation) return true
-//    }
     return false
 }
 
@@ -138,7 +133,7 @@ internal fun ClassDescriptor.enumEntries(): List<ClassDescriptor> {
         .toList()
 }
 
-private val Annotations.hasAnySerialAnnotation: Boolean
+internal val Annotations.hasAnySerialAnnotation: Boolean
     get() = serialNameValue != null || any { it.annotationClass?.isSerialInfoAnnotation == true }
 
 internal val ClassDescriptor.hasSerializableAnnotationWithoutArgs: Boolean

plugins/kotlin-serialization/kotlin-serialization-compiler/test/org/jetbrains/kotlinx/serialization/SerializationPluginDiagnosticTestGenerated.java

@@ -54,6 +54,11 @@ public class SerializationPluginDiagnosticTestGenerated extends AbstractSerializ
         runTest("plugins/kotlin-serialization/kotlin-serialization-compiler/testData/diagnostics/ParamIsNotProperty.kt");
     }
 
+    @TestMetadata("SerializableEnums.kt")
+    public void testSerializableEnums() throws Exception {
+        runTest("plugins/kotlin-serialization/kotlin-serialization-compiler/testData/diagnostics/SerializableEnums.kt");
+    }
+
     @TestMetadata("SerializableIgnored.kt")
     public void testSerializableIgnored() throws Exception {
         runTest("plugins/kotlin-serialization/kotlin-serialization-compiler/testData/diagnostics/SerializableIgnored.kt");

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/codegen/Basic.ir.txt

@@ -962,4 +962,4 @@ public final class User : java/lang/Object {
     public final java.lang.String getFirstName()
 
     public final java.lang.String getLastName()
-}
+}

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/diagnostics/SerializableEnums.kt

@@ -0,0 +1,13 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
+
+// FILE: test.kt
+import kotlinx.serialization.*
+
+enum class SimpleEnum { A, B }
+
+<!PLUGIN_WARNING("Explicit @Serializable annotation on enum class is required when @SerialName or @SerialInfo annotations are used on its members.")!>enum<!> class MarkedNameEnum { @SerialName("a") A, B}
+
+<!PLUGIN_WARNING("Explicit @Serializable annotation on enum class is required when @SerialName or @SerialInfo annotations are used on its members.")!>enum<!> class MarkedInfoEnum { @SerialId(10) A, B}
+
+@Serializable
+enum class ExplicitlyMarkedEnum { @SerialId(10) A, B}

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/diagnostics/SerializableEnums.txt

@@ -0,0 +1,105 @@
+package
+
+@kotlinx.serialization.Serializable public final enum class ExplicitlyMarkedEnum : kotlin.Enum<ExplicitlyMarkedEnum> {
+    @kotlinx.serialization.SerialId(id = 10) enum entry A
+
+    enum entry B
+
+    private constructor ExplicitlyMarkedEnum()
+    public final override /*1*/ /*fake_override*/ val name: kotlin.String
+    public final override /*1*/ /*fake_override*/ val ordinal: kotlin.Int
+    protected final override /*1*/ /*fake_override*/ fun clone(): kotlin.Any
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: ExplicitlyMarkedEnum): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    protected/*protected and package*/ final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun finalize(): kotlin.Unit
+    public final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun getDeclaringClass(): java.lang.Class<ExplicitlyMarkedEnum!>!
+    public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    @kotlin.Deprecated(level = DeprecationLevel.HIDDEN, message = "This synthesized declaration should not be used directly", replaceWith = kotlin.ReplaceWith(expression = "", imports = {})) public object `$serializer` : kotlinx.serialization.internal.GeneratedSerializer<ExplicitlyMarkedEnum> {
+        private constructor `$serializer`()
+        public open override /*1*/ /*synthesized*/ val descriptor: kotlinx.serialization.SerialDescriptor
+        public open override /*1*/ /*synthesized*/ fun childSerializers(): kotlin.Array<kotlinx.serialization.KSerializer<*>>
+        public open override /*1*/ /*synthesized*/ fun deserialize(/*0*/ decoder: kotlinx.serialization.Decoder): ExplicitlyMarkedEnum
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun patch(/*0*/ decoder: kotlinx.serialization.Decoder, /*1*/ old: ExplicitlyMarkedEnum): ExplicitlyMarkedEnum
+        public open override /*1*/ /*synthesized*/ fun serialize(/*0*/ encoder: kotlinx.serialization.Encoder, /*1*/ obj: ExplicitlyMarkedEnum): kotlin.Unit
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public companion object Companion {
+        private constructor Companion()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final /*synthesized*/ fun serializer(): kotlinx.serialization.KSerializer<ExplicitlyMarkedEnum>
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    // Static members
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): ExplicitlyMarkedEnum
+    public final /*synthesized*/ fun values(): kotlin.Array<ExplicitlyMarkedEnum>
+}
+
+public final enum class MarkedInfoEnum : kotlin.Enum<MarkedInfoEnum> {
+    @kotlinx.serialization.SerialId(id = 10) enum entry A
+
+    enum entry B
+
+    private constructor MarkedInfoEnum()
+    public final override /*1*/ /*fake_override*/ val name: kotlin.String
+    public final override /*1*/ /*fake_override*/ val ordinal: kotlin.Int
+    protected final override /*1*/ /*fake_override*/ fun clone(): kotlin.Any
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: MarkedInfoEnum): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    protected/*protected and package*/ final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun finalize(): kotlin.Unit
+    public final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun getDeclaringClass(): java.lang.Class<MarkedInfoEnum!>!
+    public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): MarkedInfoEnum
+    public final /*synthesized*/ fun values(): kotlin.Array<MarkedInfoEnum>
+}
+
+public final enum class MarkedNameEnum : kotlin.Enum<MarkedNameEnum> {
+    @kotlinx.serialization.SerialName(value = "a") enum entry A
+
+    enum entry B
+
+    private constructor MarkedNameEnum()
+    public final override /*1*/ /*fake_override*/ val name: kotlin.String
+    public final override /*1*/ /*fake_override*/ val ordinal: kotlin.Int
+    protected final override /*1*/ /*fake_override*/ fun clone(): kotlin.Any
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: MarkedNameEnum): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    protected/*protected and package*/ final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun finalize(): kotlin.Unit
+    public final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun getDeclaringClass(): java.lang.Class<MarkedNameEnum!>!
+    public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): MarkedNameEnum
+    public final /*synthesized*/ fun values(): kotlin.Array<MarkedNameEnum>
+}
+
+public final enum class SimpleEnum : kotlin.Enum<SimpleEnum> {
+    enum entry A
+
+    enum entry B
+
+    private constructor SimpleEnum()
+    public final override /*1*/ /*fake_override*/ val name: kotlin.String
+    public final override /*1*/ /*fake_override*/ val ordinal: kotlin.Int
+    protected final override /*1*/ /*fake_override*/ fun clone(): kotlin.Any
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: SimpleEnum): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    protected/*protected and package*/ final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun finalize(): kotlin.Unit
+    public final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun getDeclaringClass(): java.lang.Class<SimpleEnum!>!
+    public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): SimpleEnum
+    public final /*synthesized*/ fun values(): kotlin.Array<SimpleEnum>
+}

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/diagnostics/SerializableIgnored.kt

@@ -5,6 +5,3 @@ import kotlinx.serialization.*
 
 <!PLUGIN_ERROR("@Serializable annotation is ignored because it is impossible to serialize automatically interfaces or enums. Provide serializer manually via e.g. companion object")!>@Serializable<!>
 interface INonSerializable
-
-<!PLUGIN_ERROR("@Serializable annotation is ignored because it is impossible to serialize automatically interfaces or enums. Provide serializer manually via e.g. companion object")!>@Serializable<!>
-enum class ENonSerializable { A, B }

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/diagnostics/SerializableIgnored.txt

@@ -1,26 +1,5 @@
 package
 
-@kotlinx.serialization.Serializable public final enum class ENonSerializable : kotlin.Enum<ENonSerializable> {
-    enum entry A
-
-    enum entry B
-
-    private constructor ENonSerializable()
-    public final override /*1*/ /*fake_override*/ val name: kotlin.String
-    public final override /*1*/ /*fake_override*/ val ordinal: kotlin.Int
-    protected final override /*1*/ /*fake_override*/ fun clone(): kotlin.Any
-    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: ENonSerializable): kotlin.Int
-    public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    protected/*protected and package*/ final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun finalize(): kotlin.Unit
-    public final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun getDeclaringClass(): java.lang.Class<ENonSerializable!>!
-    public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
-    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
-
-    // Static members
-    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): ENonSerializable
-    public final /*synthesized*/ fun values(): kotlin.Array<ENonSerializable>
-}
-
 @kotlinx.serialization.Serializable public interface INonSerializable {
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int