Kotlinx.serialization: @Flat option annotation

[InsanusMokrassar]

I offer to include @Flat (or @Flatten, for example) annotation which will be used in cases when value object of field must be included directly into parent object.

@Serializable
data class Parent(
    val stringField: String = "stringValue",
    @Flat
    val flatField: Child = Child()
)

@Serializable
class Child {
    val childString: String = "childValue"
}

fun main(args: Array<String>) {
    println(JSON.stringify(Parent.serializer(), Parent()))
}

Will give result:

{"stringField":"stringValue","childString":"childValue"}

Here I see a few problems:

• String and other primitives must be resolved like common fields (with their keys) or those fields must lead to exception like IllegalArgumentException
• Collections in this case, I think, must be interpreted as associated map: with keys numbers and values - values if list

[InsanusMokrassar]

So, next code will give the same effect:

@Serializable(ParentFlatChildSerializer::class)
data class Parent(
    val stringField: String = "stringValue",
    val flatField: Child = Child()
)

@Serializable
class Child {
    @Optional
    val flatFieldValue: String = "flatFieldValue"
}

@Serializer(Parent::class)
object ParentFlatChildSerializer : KSerializer<Parent> {
    override fun serialize(output: Encoder, obj: Parent) {
        output.beginStructure(
            descriptor
        ).apply {
            encodeStringElement(
                descriptor, 0, obj.stringField
            )
            encodeStringElement(Child.serializer().descriptor, 0, obj.flatField.flatFieldValue)
        }.endStructure(
            descriptor
        )
    }
}

fun main(args: Array<String>) {
    println(JSON.stringify(ParentFlatChildSerializer, Parent()))
}

But it is much longer and is not useful to write serializer like this for each object/class in projects.

[Coneys]

Any information about this? Is there some workaround, or will it be implemented in near future?