Provide a way to introspect NothingSerialDescriptor

[BenWoodworth]

What is your use-case and why do you need this feature?

I support a format where Nothing in its schema is meaningful, e.g. List<Nothing> is encoded differently than an empty List<Int>. So far I have been getting by with descriptor == NothingSerializer().descriptor, but I'm finding issues with this now that I'm supporting new features of the format in my library.

Basically, if NothingSerialDescriptor is decorated at all, e.g.:

• NothingSerialDescriptor.nullable
• SerialDescriptor("renamed", NothingSerialDescriptor) like with serializer delegation
• Or any other SerialDescriptor delegating its interface to it

Specifically though, it's been useful for serializing list element types right away, like when encodeCollection is called, instead of holding off until the first element is serialized.

In addition to that, I have another feature planned with dynamic element names for serial types that would similarly decorate a serial descriptor by adding an annotation, and that would also be affected by this.

Describe the solution you'd like

It feels like Nothing is a special enough type that formats that could support it should be able to tell it apart from any other StructureKind.OBJECT (which is how it appears now), and making that transparent through decorated/delegated SerialDescriptors would be beneficial.

I don't know the best way to support this, but I have a couple ideas.

Adding another serial kind for Nothing would work, though it feels too late at this point. If it's not though, that feels like a good option.

Otherwise, a solution that would work for me is adding an (internal) annotation that kxs can use to identify a Nothing serial descriptor, even when wrapping serial descriptors. Something like this is what I have in mind:

internal object NothingSerialDescriptor : SerialDescriptor {
    public override val kind: SerialKind = StructureKind.OBJECT
    public override val annotations: List<Annotation> = listOf(NothingSerialKind()) // Added line

    public override val serialName: String = "kotlin.Nothing"
    // ...
}

internal annotation class NothingSerialKind

// In non-internal package:

@ExperimentalSerializationApi
public val SerialDescriptor.isNothing: Boolean
    get() = annotations.any { it is NothingSerialKind }

This way the Nothing-ness is passed down when delegating (assuming the outer descriptor doesn't remove annotations). And I don't think this should be a problem with annotations being experimental since the implementation for isNothing implemented can be changed if needed.

[BenWoodworth]

I am curious if this is a proper design concern, or if maybe I'm just approaching this the wrong way. Recommendations welcome, and I'm up for posting a PR if this is a good solution :)