Feature #18481


Porting YJIT to Rust (request for feedback)

Added by maximecb (Maxime Chevalier-Boisvert) 6 months ago. Updated about 2 months ago.

Target version:


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 maintainability. As you know, JIT compilers can get very complex, and C99 doesn't offer many tools to manage this complexity. There are no classes and methods, limited type checking, and it's hard to fully separate code into modules, for instance.

We believe that having access to object oriented programming and a more expressive type system would help us manage growing complexity better and also improve the safety/robustness of YJIT. For instance we would like to add Windows support and a new backend to YJIT. That means we’ll have two separate backends (x86, arm64) and we’ll need to support two different calling conventions (Microsoft, SystemV), but currently, we have limited tools to build the abstractions needed, such as preprocessor macros and if-statements.

We’ve discussed the idea of porting YJIT to Rust with some of the Ruby core developers (@ko1 (Koichi Sasada), @k0kubun (Takashi Kokubun), @mame (Yusuke Endoh)), and it seems they would be open to merging something like this if it works well. I’m opening this ticket so that everyone can have a chance to provide feedback and participate in the discussion. We realize that adding Rust code to the CRuby codebase could be challenging and that there are open questions.

We are planning to make it so that YJIT will only need the Rust compiler and cargo to build. Building YJIT would then require the Rust compiler to be installed, but CRuby could build without YJIT and without the Rust compiler. There would be no new dependencies for the compiled binary. Rust is supported on Mac, Windows, BSDs, and Linux, which covers all the platforms on which we plan to support YJIT. Since Rust is based on LLVM, it has good support for cross-compilation.

We would like to solicit input from Ruby distributors who create .deb and .rpm packages. We will likely remain conservative when updating Rust versions to make OS packaging easier. We believe that in the general, the resulting code should be easier to maintain because it will be better organized, but the YJIT team will help out with YJIT-related backports and will be available to help if needed.

Value proposition:

  • Rust type systems will catch more bugs early, help prevent new bugs
  • Easier to manage growing complexity of YJIT
  • Easier to maintain codebase, fewer “footguns”
  • Easier for newcomers because the compiler catches more bugs
  • Better performance because we can implement more sophisticated optimizations
  • Easier to add support for new platforms (which adds complexity)
  • Rust has mature and easy-to-install tools such as source code formatter and editor plugins
  • Rust as a programming language community has a great deal of enthusiasm behind it. This could translate to more enthusiasm for YJIT and for Ruby as well.


  • YJIT will only depend on the Rust language and the standard library, no other dependencies
  • YJIT will be able to build without an internet connection
  • Rust has good support for cross-compilation
  • Rust is supported on all platforms on which we plan to support with YJIT (Mac, Linux, Windows)
  • The compiled CRuby binary won’t have any new dependencies on shared libraries
  • CRuby will still be able to build without rustc, with YJIT disabled

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