Why running KSP 1 requires -language-version to be 1.9?

[wzieba]

hi @ZacSweers 👋 Thank you for maintaining this fork. I have a question regarding PR #196 . You write:

To use KSP, you must set KotlinCompilation.languageVersion to 1.9.

Can you please share why exactly this is required?

[ZacSweers]

KSP1 only works in Kotlin 1.x. It is implemented on top of AnalysisHandlerExtension, which no longer exists in K2.

[ninniuz]

@ZacSweers by reading this thread on Slack it seems to me that KSP1 should be still working with Kotlin 2.1.0 (that is actually what we are seeing in our projects, w/o KSP2 and with Kotlin 2.1.0).

But when using kotlin-compile-testing we don't get KSP to run unless we either set:

• languageVersion = 1.9 or
• configureKsp(useKsp2 = true)

Do you know why this is happening?
We would like to execute our tests in the same env our annotation processing is performed in the app (more or less at least).
Thanks!

[ZacSweers]

Forcing language version to 1.9 forces the K1 compiler (even in K2). Please see the answer I gave above for why that's required #309 (comment). If you are using KSP in a project on kotlin 2.1.0 without KSP2, it is because KSP is forcing it down to languageVersion 1.9.