The Bun JavaScript runtime is undergoing a significant rewrite in Rust, a move aimed at addressing persistent stability issues and memory management complexities. Originally built in Zig, Bun's ambitious scope—encompassing a transpiler, bundler, package manager, test runner, and HTTP server—proved challenging to maintain, leading to numerous memory leaks and crashes documented in recent releases like v1.3.14.

While Zig's low-level control facilitated the initial rapid development, the interplay between Zig's manual memory management and JavaScript's garbage collection created a fertile ground for bugs. The Bun team is now leveraging Rust's memory safety features to systematically prevent these issues. This rewrite will also involve re-integrating components like JavaScriptCore, WebSockets, and BoringSSL, which were previously managed with C++ bindings.

The decision to switch to Rust signals a commitment to long-term stability and developer experience. By adopting a language with stronger compile-time guarantees against memory errors, Bun aims to reduce the bug-fixing burden and provide a more reliable environment for its over 22 million monthly users. Tools like Claude Code and Vercel, which depend on Bun as their runtime, stand to benefit from this enhanced dependability.