Add a lint rule for encodeToString/decodeFromString to detect if the value has Serializable annotation

[TimoPtr]

What is your use-case and why do you need this feature?
The Home Assistant project is migrating to KotlinX serialization for JSON mostly. And while migrating we missed some spot where some classes where not annotated with @Serializable.
Describe the solution you'd like
We've made a very basic lint rule to detect if the field value of the method encodeToString or decodeFromString for the Json module. home-assistant/android#5368 it's not complete but it fits our usage. It would be nice to have something better and more robust.

[pdvrieze]

@TimoPtr kotlinx.serialization is designed as a static serializer that does not use reflection (although it can do it to look up the root serializer). As such it should fail to compile (with nowadays a nice error) if elements are not serializable. The main thing you want to do is make sure the classes you call encodeToString on are serializable (use the 2-arg version and explicitly pass in the serializer: encodeToString(MyClass.serializer(), myClassInstance) will ensure that all of this exists.

[TimoPtr]

I'm not just to get your point. Since it's not using reflection (and that's why we choose to use it btw) we thought that it already had everything in place to make sure that at build time you cannot write something like I've wrote in the test in the linked PR.

Something like that shouldn't compile

class Home

fun main() { 
Json.encodeToString(Home())
}

Since it misses the annotation. But today the build succeed and it fails only at runtime which is bad. That's why we've introduce the lint check so it fails earlier than when we release our app, it fails on CI.

[pdvrieze]

@TimoPtr That code should fail (presuming that you have applied the serialization plugin - if you haven't it is valid code and the regular compiler knows nothing about (missing) serializers - the lookup is done by a compiler plugin intrinsic that replaces a best effort dynamic lookup.

[TimoPtr]

IMO this code should fail yes. Yes I did add the serialization plugin you can see it in this PR home-assistant/android#5279 (check the first commits for the Gradle setup). But this code is not failing at build time it indeed fails at runtime that why I've made this lint rules otherwise you only find out too late.

You can see in this small snippet that also the IDE is not warning you of anything without the lint rule. It should be possible to have the feedback from the IDE that the annotation is missing.

[TimoPtr]

I've extend the lint rule to decodeFromString that also has the same issue. home-assistant/android#5375

[pdvrieze]

I had a check in AS and intellij with your project and an independent context. You are right, serializer<MyType> will not signal if the type is not serializable (this function is the intrinsic that will substitute the serializer at compile time if it exists). Instead it defers to runtime lookup. It would be good to have the option in the compiler plugin to warn/fail if the type is known, but has no serializer.

[TimoPtr]

I might take a look to see if I can help on the compiler to check this. But in the meantime the lint rules is the way to go for us.

[pdvrieze]

@TimoPtr You want to search for intrinsic (for the serializer function) in the serialization plugin folder of the kotlin project.

[sandwwraith]

Duplicate of #2522. Summary of the discussion: Such an approach is not feasible.

• Inliner (that intrinsifies serializer<Xxx>()) cannot report diagnostics and is not running in IDE
• json.encodeToString() always performs lookup in the SerializersModule, which may contain missing serializers added dynamically

Maybe one exception here is when the pre-defined Json instance is used, then we can be sure that the module is empty. But given most people tweak json settings to their liking, such a diagnostic would rarely be useful.

[sandwwraith]

There's also #2605, but I doubt this will be beneficial as well.

[TimoPtr]

For now our linter is working great and is able to catch this issues.