TL;DR: The YJIT team wants to explore using Rust to help develop YJIT. The rest of CRuby will continue to build without Rust tools and building YJIT will remain optional. We’re currently exploring the possibility of porting YJIT to Rust and working on a small proof of concept that should be ready next month. The motivation behind this is that we are facing challenges in terms of code maintainabili