Deprecate SingleMessage.Serializer for compatibility issue with array polymorphism

mirai-core-api/src/commonMain/kotlin/message/MessageSerializers.kt

@@ -11,6 +11,7 @@ package net.mamoe.mirai.message
 
 import kotlinx.serialization.ContextualSerializer
 import kotlinx.serialization.KSerializer
+import kotlinx.serialization.PolymorphicSerializer
 import kotlinx.serialization.json.Json
 import kotlinx.serialization.modules.PolymorphicModuleBuilder
 import kotlinx.serialization.modules.SerializersModule
@@ -26,14 +27,13 @@ import kotlin.reflect.KClass
 /**
  * 消息序列化器.
  *
- * [MessageSerializers] 存放 [SerializersModule], 用于协助 [SingleMessage.Serializer] 的多态序列化.
+ * [MessageSerializers] 存放 [SerializersModule], 用于协助 [SingleMessage] [PolymorphicSerializer] 的多态序列化.
  *
  * 要序列化一个 [MessageChain], 请使用内建的 [MessageChain.serializeToJsonString]
  *
  * @see serializersModule
  *
  *
- * @see SingleMessage.Serializer
  * @see MessageChain.Serializer
  *
  * @see MessageSerializers.INSTANCE

mirai-core-api/src/commonMain/kotlin/message/data/MessageChain.kt

@@ -245,7 +245,7 @@ public interface MessageChain :
      */
     public object Serializer : KSerializer<MessageChain> {
         @Suppress("DEPRECATION_ERROR")
-        private val delegate = ListSerializer(SingleMessage.Serializer)
+        private val delegate = ListSerializer(PolymorphicSerializer(SingleMessage::class))
         override val descriptor: SerialDescriptor = delegate.descriptor
         override fun deserialize(decoder: Decoder): MessageChain = delegate.deserialize(decoder).toMessageChain()
         override fun serialize(encoder: Encoder, value: MessageChain): Unit = delegate.serialize(encoder, value)

mirai-core-api/src/commonMain/kotlin/message/data/SingleMessage.kt

@@ -19,19 +19,30 @@ package net.mamoe.mirai.message.data
 
 import kotlinx.serialization.KSerializer
 import kotlinx.serialization.PolymorphicSerializer
-import kotlinx.serialization.Serializable
 import net.mamoe.mirai.utils.safeCast
 
 /**
  * 单个消息元素. 与之相对的是 [MessageChain], 是多个 [SingleMessage] 的集合.
  */
-@Serializable(SingleMessage.Serializer::class)
+// @Serializable(SingleMessage.Serializer::class)
 public interface SingleMessage : Message { // TODO: 2021/1/10 Make sealed interface in Kotlin 1.5
+
+    /**
+     * @suppress deprecated since 2.4.0
+     */
+    @Deprecated(
+        "Please create PolymorphicSerializer(SingleMessage::class) on your own.",
+        ReplaceWith(
+            "PolymorphicSerializer(SingleMessage::class)",
+            "kotlinx.serialization.PolymorphicSerializer",
+            "net.mamoe.mirai.message.data.SingleMessage",
+        ),
+        level = DeprecationLevel.WARNING
+    )
     public object Serializer : KSerializer<SingleMessage> by PolymorphicSerializer(SingleMessage::class)
 }
 
 
-
 /**
  * 消息元数据, 即不含内容的元素.
  *