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kotlinlang

Once the compiler plugin API is stabilized, will the plugins that were added to the Kotlin repo get moved out again (e.g. Compose)?

Doubtful. We're focused on stabilizing specifically the _*frontend*_ plugin API right now (FIR stuff) and not the backend (IR stuff). This should allow third-party compiler plugins to be enabled in the IDE by default. This is the biggest pain-point for users of compiler plugins. But this also means that plugins which use IR will still require compatibility with the matching version of Kotlin.

We also don't plan to remove the current FIR extensions. So I also doubt that the existing plugins would even migrate to the stable API.

Are there plans to stabilize the backend at some point?

It would be nice to decouple Compose again, so iteration on fixing bugs could be faster.