A modern programming language that makes developers happier.

kotlinlang

I'm in a situation where Gradle is too slow for me, I'd like to write a custom build tool. Are there any documentations about how to use the Kotlin compiler's API?

No, as far as I know (and this is not a stable API).
You can ask me anytime in DM some specific questions and I will try to answer them.

Well, there are a few resources you can start with:

<https://github.com/demiurg906/kotlin-compiler-plugin-template.git>

<https://medium.com/google-developer-experts/crash-course-on-the-kotlin-compiler-1-frontend-parsing-phase-9898490d922b>

<https://bnorm.medium.com/writing-your-second-kotlin-compiler-plugin-part-1-project-setup-7b05c7d93f6c>

This last one is a bit outdated but still a good starting point.