TL;DR
Bun has merged a significant rewrite in Rust, passing all pre-existing tests, fixing memory leaks, and improving performance. The update is in the canary stage, with further optimization planned before wider release.
The Bun JavaScript runtime has completed a merge of its codebase rewritten in Rust, passing all existing tests and fixing several memory leaks, according to a recent developer update.
The development team announced that the Rust rewrite of Bun has been merged into the main codebase. This version passes Bun’s pre-existing test suite across all platforms and addresses memory leaks and flaky tests previously identified. The binary size has been reduced by between 3 MB and 8 MB, and benchmark results indicate performance improvements, ranging from neutral to faster speeds. The codebase remains largely unchanged in architecture and data structures, with minimal third-party dependencies and no async Rust components. The update is currently available in the canary channel via the command ‘bun upgrade –canary’. The team emphasizes that additional optimization and cleanup work are ongoing, with further PRs planned before this version is released outside the canary channel.
Why It Matters
This merge marks a major milestone for Bun, as rewriting critical parts of the runtime in Rust aims to improve stability, performance, and developer productivity. The integration of compiler-assisted tools for memory bug detection could reduce debugging time and increase reliability, impacting the broader JavaScript runtime ecosystem by demonstrating the viability of Rust for high-performance, low-level runtime components.
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Background
Bun is a JavaScript runtime built for speed and efficiency, competing with Node.js and Deno. The project announced plans to rewrite parts of its core in Rust to address performance bottlenecks and stability issues. The recent merge follows months of development and testing, including a successful pass of all existing test suites and fixing known memory leak issues. The transition to Rust has been a key focus, as it allows for better memory safety and performance optimizations, with the team maintaining a largely unchanged architecture and minimal dependency footprint.
“The Rust rewrite has passed all our tests, fixed several memory leaks, and improved performance metrics. We are now in the canary phase and plan to optimize further before a stable release.”
— Bun development team
“The integration of compiler-assisted tools for catching memory bugs will significantly reduce debugging time and improve reliability.”
— Lead developer (unnamed)
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What Remains Unclear
It remains unclear when the Rust-based version will be available in the stable release channel, as further optimization and testing are ongoing. Details about the timeline for broader deployment are not yet confirmed.
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What’s Next
The team will continue refining the Rust implementation through additional PRs, with plans to address remaining bugs and performance issues. A non-canary, stable release is expected once testing and optimization are complete, likely within the coming months.
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Key Questions
What are the main benefits of the Rust rewrite for Bun?
The Rust rewrite aims to improve performance, reduce binary size, fix memory leaks, and introduce compiler-assisted tools for better memory safety and debugging.
When will the new Rust version be available outside the canary channel?
The exact timeline is not confirmed, but a stable release is expected after further testing and optimization, likely within the next few months.
Does the Rust rewrite change Bun’s architecture?
No, the core architecture and data structures remain largely the same, with minimal dependency changes.
Are there any known issues with the current canary version?
Yes, ongoing work includes further optimization and cleanup, and users are encouraged to file issues if they encounter problems.
