I was wondering if I can enable the new K2 compiler from some ConfigurationKeys instead of K2JVMCompilerArguments().useK2. I found out that it actually enables FIR. I don’t think FIR uses the K2 compiler automatically. But if it does, I think it should be renamed as earlier it was USE_NEW_COMPILER which was pretty much self explanatory. Any help would be greatly appreciated 
K2 is basically FIR frontend + IR backend. IR backend is enabled by default, and in fact the support for old backend has been removed (at least for JVM, idk about JS), and so IR is almost the only option. Therefore, enabling K2 is synonymous with enabling FIR, and yes, FIR uses K2 automatically, because it’s not even compatible with the old backend, and never was.