February 27, 2026 For nearly three decades, CRuby (MRI) has been overwhelmingly a C codebase. That stability has been both a strength and a constraint. Recently, however, something genuinely new appeared inside the official Ruby repository: Parts of Ruby itself are now written in Rust. This is not theoretical, experimental folklore, or third-party tooling. It…

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Parts of CRuby (MRI) are now written in Rust, specifically in the YJIT just-in-time compiler introduced in Ruby 3.1. YJIT compiles Ruby bytecode into native machine code at runtime and is implemented largely in Rust, which provides low-level control with memory safety guarantees C cannot offer. The rest of the Ruby runtime — interpreter, parser, GC, object model, standard library — remains in C. This hybrid C/Rust architecture is a strategic addition, not a rewrite, targeting only the most complex and risk-sensitive subsystem. The move signals that the Ruby core team is open to modern systems languages and prioritizes memory safety in performance-critical components, mirroring similar selective Rust adoption in Linux, Android, and database engines.

🦀 Rust Inside Ruby Core: A New Systems Layer for MRI