possible to remove tags representing class wrapper (flatten)?

[2x2xplz]

I have a nested structure such that a property of my main class is a sub-class. By default, during serialization the sub-class creates a tag even when it has no attributes. My goal is to flatten this structure by eliminating the <MessageDate> tags and just have <Year> <Month> and <Day> be immediate children of <MyMessage> (and siblings of <MessageText>)

Can this be done with an annotation (best option) or policy, or a custom serializer? I will also want to be able to de-serialize XML strings with a flatter structure than my class hierarchy.

Thank you very much!

@Serializable
data class MyMessage (
    val SenderID: String,
    val SendDate: MessageDate,
    @XmlElement(true) val MessageText: String
)

@Serializable
data class MessageDate (  // has no attributes, goal is to eliminate the <MessageDate> tag
    @XmlElement(true) val Year : Int,
    @XmlElement(true) val Month : Int,
    @XmlElement(true) val Day : Int,
)

val hal9000 = MyMessage("HAL9000", MessageDate(1992, 1, 12), "I am completely operational, and all my circuits are functioning perfectly.")
fun makeHal() = xml.encodeToString(MyMessage.serializer(), hal9000).also { println(it) }

default output:

<MyMessage SenderID="HAL9000">
	<MessageDate>
		<Year>1992</Year>
		<Month>1</Month>
		<Day>12</Day>
	</MessageDate>
	<MessageText>I am completely operational, and all my circuits are functioning perfectly.</MessageText>
</MyMessage>

desired output:

<MyMessage SenderID="HAL9000">
        <Year>1992</Year>
        <Month>1</Month>
        <Day>12</Day>
        <MessageText>I am completely operational, and all my circuits are functioning perfectly.</MessageText>
</MyMessage>

[pdvrieze]

Unfortunately this isn't possible at this point without a custom serializer for MyMessage that serializes the fields rather than MessageDate directly.