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/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