To add to that, implementing syntax pre-processors in scalac is not possible. kind-projector runs after parser and before namer. You can’t add a phase before the parser, the compiler implementation forbids it. The closest to making parsing “extensible” has been @Duhemm’s work on parser macros which aren’t sufficient either to implement Scala 3 syntax in Scala 2, so it looks like the only path forward is to implement it in the compiler itself behind a flag.