The Most Controversial Codebase Rewrite in History: How Bun Migrated from Zig to Rust in 11 Days

When it comes to software development, full codebase rewrites are widely considered one of the riskiest, most time-consuming tasks a team can take on. Most projects that attempt a complete rewrite spend months or even years migrating code, testing for regressions, and onboarding contributors to new languages. That’s what makes the recent migration of the popular JavaScript runtime Bun so notable: the team completely refactored their entire codebase from Zig to Rust in just 11 days, a timeline that has sparked intense debate across the developer community.

The Unprecedented Speed of Bun’s Rust Migration

Bun launched in 2022 as a high-performance alternative to Node.js, built from the ground up in the Zig programming language. Zig is a low-level systems language designed for maximum control, minimal runtime overhead, and cross-platform compatibility, making it a popular choice for performance-critical tools like runtimes and compilers. The initial choice of Zig helped Bun deliver industry-leading speed for JavaScript and TypeScript execution, quickly earning a loyal user base of developers looking for faster tooling.

But the team’s decision to switch to Rust came after months of internal evaluation. Rust has seen explosive growth in systems programming over the past half-decade, thanks to its memory safety guarantees without garbage collection, robust standard library, and massive ecosystem of third-party crates. According to the 2024 Stack Overflow Developer Survey, Rust has been the most loved programming language for 8 consecutive years, with 83% of respondents who use it expressing interest in continuing to work with it. For Bun’s maintainers, the switch promised easier onboarding for new contributors, better cross-platform support, and fewer hard-to-debug memory safety issues that can plague low-level C-adjacent languages like Zig.

The 11-day migration timeline shocked many onlookers, who assumed a full rewrite of a production-grade runtime with millions of lines of code would take far longer. The team achieved the speed by breaking the migration into small, testable modules, running parallel test suites for Zig and Rust builds during the transition, and leveraging automated code translation tools to jumpstart the process.

Why the Switch Sparked Widespread Debate

Not all developers have welcomed the change. A vocal subset of the Bun user base, many of whom are Zig enthusiasts, expressed frustration that the team was abandoning a language that was specifically chosen for its simplicity and low-level control. Critics argued that Zig’s minimal design, with no hidden runtime or implicit allocations, made it a better fit for a runtime that prioritizes predictable performance. Some also raised concerns that the rushed migration could introduce subtle bugs or performance regressions that might not show up until months after the release.

Supporters of the move countered that Rust’s strong type system and memory safety guarantees would lead to a more stable, maintainable codebase in the long run, and that the larger pool of Rust developers would make it easier for outside contributors to submit bug fixes and new features. Many noted that Bun’s core priority is speed and reliability for end users, not loyalty to a specific programming language, and that the team’s track record of shipping high-performance updates justified the risk of a fast rewrite.

Key Takeaways for Development Teams Considering Large-Scale Rewrites

While most teams will never face a rewrite as high-stakes as Bun’s, the migration offers valuable lessons for any group considering a major codebase overhaul:

• Anchor rewrites to clear, user-focused goals: Bun didn’t switch languages on a whim. The team had specific, measurable objectives: reduce memory safety bugs, expand cross-platform support, and lower the barrier to entry for new contributors. Any rewrite that isn’t tied to concrete user or business benefits is likely to waste resources.
• Prioritize testing and incremental migration: Even with a tight 11-day deadline, the Bun team ran parallel test suites for both the old Zig code and new Rust code throughout the migration, ensuring no regressions slipped through. Breaking a large rewrite into small, testable chunks reduces risk and makes it easier to roll back if issues arise.
• Communicate transparently with your community: High-profile projects like Bun have large user bases that rely on their stability. The team shared regular updates about the migration progress, addressed community concerns publicly, and provided clear timelines for when users could expect the updated runtime. This transparency helped reduce backlash even among users who were skeptical of the change.

What Comes Next for Bun?

As of the July 2026 release of the Rust-based Bun runtime, early benchmarks show performance on par with or better than the previous Zig version, with a 20% reduction in reported memory safety bugs in the first month post-launch. The team has also reported a 40% increase in outside contribution requests since the switch, suggesting the move to Rust has lowered the barrier for new developers to get involved.

The migration has set a new precedent for how fast high-profile open source projects can execute large-scale rewrites when they have clear goals, strong testing practices, and a motivated team. While the controversy over the switch may fade as Bun continues to ship updates, the project’s success could encourage other teams to rethink whether slow, multi-year rewrites are always necessary.

The Bun team’s 11-day Zig-to-Rust rewrite is a reminder that even the most daunting software engineering tasks can be completed quickly with the right strategy and clear priorities. For developers interested in the finer technical details of the migration, the full deep dive into the process, including performance benchmarks and lessons learned, is available to watch.