Update to Kotlin 2.0

[translatenix]

Pkl currently uses Kotlin 1.7.
Kotlin 2.0 was released in May 2024.
2.0 offers many internal and user-facing improvements over 1.7.
Updating to 2.0 means that Pkl's Kotlin libraries will require Kotlin 2.0 or higher.

I'd love to take this on if I get the green light from the Pkl team.

[bioball]

So far, we've been capping Kotlin's version to that used by the minimum version of Gradle supported by pkl-gradle.
I'm not totally sure if we need to be doing, this, though. pkl-gradle uses a fat jar (pkl-tools) that includes the kotlin stdlib, so it might be okay to untether these two.

In general, I'm also not sure if we want to be on the bleeding edge of Kotlin, because it can be painful for our users to migrate the newer versions of our libraries. We don't actually have a policy in place here, but something like "we are compatible with Kotlin released two years ago" sounds like a good goal to have.

In both of these cases, it should be pretty safe to upgrade to Kotlin 1.8, but not totally sure about 2.0.

[translatenix]

pkl-gradle shouldn’t impose any limitations (compatibility tests will be the judge).

I’d prefer to update to a Kotlin version that is actively maintained.
Kotlin 1.8 is EOL. Kotlin 1.9 is EOL to the best of my knowledge.
Kotlin 2.0 and 2.1 (currently in beta) are actively maintained.

Minimum Kotlin version supported by latest version of popular Kotlin libraries:

• clikt (*): 2.0
• kotest: 1.9 (master: 2.0)
• kotlinpoet (*): 2.0
• kotlinx-html (*): 1.9 (master: 2.0)
• kotlinx-serialization (*): 2.0
• ktor: 2.0

(*) used by Pkl

[bioball]

That's compelling. If Kotlin 1.9 EOL, that's a good enough reason to upgrade.

By the way, I don't see any materials from JetBrains themselves about this. But endoflife.date claims that 1.9 is EOL.

@jamesward do you have context, and can you advise here?

[translatenix]

I couldn’t find official information either. Apparently such “formalities” aren’t JetBrain’s strength.
1.9 had some bugfix releases in early 2024, before 2.0 was released. 1.8 is clearly EOL.

[jamesward]

Yeah, I don't think JetBrains has a concept of EOL for Kotlin. But older versions definitely don't get active updates. The latest 1.9 release was July 19, 2024 so not too long ago, and it was after the 2.0.0 release. So likely still some active critical fixes on 1.9.

[translatenix]

The latest 1.9 release was July 19, 2024

FWIW I didn’t count 1.9.25 because it says “This is a technical release for IntelliJ IDEA.”
The last time JetBrains shipped a .30 release was Kotlin 1.5.30.
A 1.9.30 branch exists but looks abandoned.
I guess we’ll have to wait and see.

[StefMa]

Please note that Jetbrains "only" changed the compiler backend in version 2.0, rather than shipping tons of new features or removing deprecated items (which is what the name might suggest). I guess there wouldn't be much difference in choosing version 2.0 over 1.9...

[translatenix]

I'll send a PR after the 0.27 release.