Provide a way to detect which field exists.

[iseki0]

What is your use-case and why do you need this feature?
We often use null as a placeholder for the field which doesn't exists. Example:

{
  "foo": 1
}

@Serializable
data class A(val foo: Int, val bar: Int? = null)
println(Json.decodeFromString<A>("...").bar) // null

But some times, the null is a meaningful value, such as deletion.

Passing null value to clear the field in the database.
If you don't passing the field, the field in database will be leaved unchanged.

Describe the solution you'd like
Provide a new annotation, mark an additional field as the "bitmap". Such as:

@Serializable
data class A(
    val foo: Int,
    val bar: Int? = null,
    @FieldsRecord val declaredFields: Set<String> = emptySet(),
)

assert("bar" !in Json.decodeFromString<A>("...").declaredFieds)

To achieve this goal we need to update the plugin.

If that's a huge feature and you don't like it. Could you provide a way to generate the KSerializer even we use a customized KSerializer? (In that case I try to generate my own serializer that will co-work with the plugin generated one.)

[iseki0]

The special Set<String> might be internal implemented using a bitmap, co-works with the SerialDescriptor(which is singleton).

[sandwwraith]

This request seems to be highly-specific, and introducing a whole new annotation to support this narrow use-case is indeed feels like overkill, so I'll close this for now. I remember similar ideas that had been reported before, and they all eventually converged to a custom serializer that treats null specifically.

Could you provide a way to generate the KSerializer even we use a customized KSerializer? (In that case I try to generate my own serializer that will co-work with the plugin generated one.)

It is #1169 and it will be available in 2.0.20

[iseki0]

It is #1169 and it will be available in 2.0.20

So that I can use my own generated serializer to delegate the default generated serializer with a delegated decoder to archieve that. 😎