You can configure Early Access Preview channel in Tools → Kotlin → Configure Kotlin Plugin Updates. Change the update channel to ‘Early Access Preview 1.2.x’ and press Check for updates now.
For Maven or Gradle
Add the https://dl.bintray.com/kotlin/kotlin-eap repository URL to the list of project repositories. Here is how to do this in Gradle:
Inline classes are actually a feature of an upcoming 1.3 (Inline classes · Issue #104 · Kotlin/KEEP · GitHub), which is being developed in the master branch under a separate compiler argument for simplicity. You can enable the feature with the compiler argument -XXLanguage:+InlineClasses. Note that this will mark your compiled code as “pre-release”, so that it’d be impossible to use it as a library when using “release” Kotlin versions (less than 1.3 at the moment).
Gradle crashes with kotlinx.serialization plugin (v0.5.1)
* What went wrong:
A problem occurred configuring project ':caves'.
> Failed to notify project evaluation listener.
> java.lang.AbstractMethodError (no error message)
> java.lang.AbstractMethodError (no error message)