Does Kotlin support Checker Framework?
I'd prefer not to use anything that implies a different runtime or compiler, which rules out Android & Eclipse respectively. JSR-305 causes module issues and that seems like a bad idea. FindBugs isn't maintained, and Lombok brings a large pile of other stuff that I don't think is a good idea to have.
So I guess that leaves the JetBrains ones, which have the benefit of being a single library, but I still feel a bit uncomfortable depending on a particular tooling vendor.
I wish JSpecify would hurry up, but I've not seen anything from them since the JVMLS 2019 talk. Though at least they finally put the slides for that talk up.
That said, I'd still prefer to have no dependencies in the core library, but I understand that causes problems for Kotlin.