Intrinsics for serializer() function

[qwwdfsad]

Currently, our format API can be used in two ways: with explicit serializer, and with inferred from the type parameters, for exaple:

// Explicit
val jsonString = Json.decodeToString(MyClass.serializer(), MyClass(...))
val data = Json.encodeFromString(MyClass.serializer(), jsonString)

// Implicit
val jsonString = Json.decodeToString(MyClass(...)) // Inferred type-parameter T
val data = Json.encodeFromString<MyClass>(jsonString) // Or data: MyClass = Json.encodeFromString(sonString)

We always promote usages of the latter, because it has the same meaning, but in the meantime is more expressive and concise. It is based on the experimental typeOf function and we are able to retrieve the serializer using KType.

Unfortunately, it has two downsides:

• To retrieve the serializer, we have to use reflection. It prevents usage of this API with Graal (Kotlinx Serialization with GraalVM Native Images #1125) and also complicates Android usages
• Serializer retrieval is slow and often slower than the serialization itself. The recommended and concise API shouldn't behave this way.

Proposal

Introduce the notion of intrinsic to the serialization plugin. serializer() and serializerOfNull are going to be evaluated in the compile-time. Additionally, this evaluation will happen transitively through all reified inline functions that leverage serializer<T>.

To properly resolve context-specific serializers, more compact contextualOrDefault(defaultSerializer) function will be used instead.

[NorbertSandor]

When may we expect this proposal to be implemented?

[sandwwraith]

@NorbertSandor We can't give any particular timeframe for now, but we aim to provide this before Kotlin 1.6

[sdeleuze]

If possible we would love to get it in Kotlin 1.7 for Spring needs, especially related to GraalVM native images.

[snowe2010]

seems like this has been bumped several times. What work needs to be done here? Is there anything the community can do to help out?

[sandwwraith]

@snowe2010 The work has to be done in the compiler plugin — currently, they can't provide intrinsics, and we have to change that. I'm working at this task at the moment, and I hope I will be able to present results soon

[snowe2010]

@sandwwraith ah, I see. thank you for your work! good luck!

[PedroAlvarado]

It's been some time since the last update on this issue. Is it fair to expect this fixed some time next year?

[qwwdfsad]

It's already implemented in the not-yet-released Kotlin 1.8.0, stay tuned!

[sdeleuze]

Is it expected to be fixed with kotlinx.serialization 1.4.1 and Kotlin 1.8.0, or do we need to wait a new release of kotlinx.serialization?

[sandwwraith]

As there are some runtime functions this functionality depends on, intrinsics should be working once 1.5.0-RC is released

[morki]

I just tested it with Ktor, kontlinx.serialization 1.5.0-RC and plugin version 1.8.10 and it is not still does not work. Serialization fails in native image without reflection and Intrinsics are not used in generated JARs.

@sandwwraith Is there anything else one need to configure to make it work?

[sandwwraith]

@morki Which function do you use? 1.8.0, which is used to compile 1.5.0-RC, has some subtle bug, and as a result, the Json.encodeToString function is not intrinsified. serializer<T>(), however, still should work with 1.8.x + 1.5.0-RC.

[morki]

@sandwwraith thank you for quick response. My code is actually like this:

@Resource("/login")
@Serializable
data class LoginRoute(
    val state: String? = null,
)

fun Route.registerLoginRoutes() {
    get<LoginRoute> {
      // TODO
    }
}

which calls function Route.get which looks like this:

public inline fun <reified T : Any> Route.get(
    noinline body: suspend PipelineContext<Unit, ApplicationCall>.(T) -> Unit
): Route {
    lateinit var builtRoute: Route
    resource<T> {
        builtRoute = method(HttpMethod.Get) {
            handle(body)
        }
    }
    return builtRoute
}

and it calls function Route.resource which looks like this:

public inline fun <reified T : Any> Route.resource(noinline body: Route.() -> Unit): Route {
    val serializer = serializer<T>()
    return resource(serializer, body)
}

and it calls serializer function.

[morki]

@sandwwraith sorry, my fault, it is because Ktor is precompiled with Kotlin 1.7.20, when I compiled locally and pushed to Maven local, all works fine :)

[snowe2010]

Hm. I was seeing the issue still as well, but I think it's because we're using the encode and decode methods.

[sandwwraith]

This feature is fully functional now. Requirements are Kotlin 1.8.10+ and serialization core runtime 1.5.0+.

[sandwwraith]

In an unlikely occasion of problems with replacement, inlining can be disabled with freeCompilerArgs += ["-P", "plugin:org.jetbrains.kotlinx.serialization:disableIntrinsic=true"].

[OliverO2]

Ran a series of tests with ProtoBuf on JVM and Js. Works like a charm!