# Insights from Bun's AI-Assisted Rust Rewrite

How AI played a critical role in migrating from Zig to Rust

- **Source:** [The Pragmatic Engineer](https://blog.pragmaticengineer.com)
- **Published:** 2026-07-16
- **Topic:** [Data & AI](https://brifing.si/c/data-ai)
- **Original article:** <https://blog.pragmaticengineer.com/the-pulse-what-can-we-learn-from-buns-rapid-rust-rewrite-with-ai/>
- **On Brifing:** https://brifing.si/s/1d0c781d-e239-4df7-a60b-a448d5904f09

## Introduction: A Rewrite Story

JavaScript runtime Bun underwent a major technical transformation by migrating from Zig to Rust, with AI tool Fable playing a crucial role. This decision for Bun, which has 22 million monthly downloads, was driven by serious technical requirements.

## Memory Issues Forced the Change

Zig's lack of memory safety caused constant crashes and leaks in Bun. Developer Jarred Sumner admits he even dreamed about these issues. Rust's memory management features promised a definitive solution to these persistent problems.

## Traditional Rewrite Risks

Rewriting 535k lines of code with traditional methods was estimated to take 1.5-2 years. Adding new features to the original project during this time could extend it further. Jarred stated they couldn't afford this time loss.

## Enter Fable

Anthropic's AI model Fable was used to accelerate the process. But it wasn't as simple as a 'translate to Rust' command. Jarred conducted a 3-hour intensive prep session with Claude, creating a 600-line PORTING.md guide.

## Porting Guide Details

The guide contained specific Zig-to-Rust conversion rules: 'no tokio', 'no async functions', 'std::fs banned' etc. These constraints were necessary to preserve Bun's unique event loop structure. Each rule was technically detailed for Rust experts.

## AI's Code Conversion Process

Fable analyzed code line-by-line for conversion, but this wasn't automatic. Developers had to verify each step. AI's biggest contribution was quickly implementing repetitive patterns to save time.

## Borrow Checker Struggle

Adapting to Rust's famous borrow checker was among the toughest challenges. AI creatively adapted Zig's memory access patterns to Rust's rules, sometimes using temporary variables to reorganize borrows.

## Performance Comparison

Initial tests showed the Rust version ran 5-10% faster than Zig - an unexpected advantage since the main goal was stability, not performance. Memory safety errors were almost completely eliminated.

## Future of AI-Assisted Development

This project proves large codebases can now undergo language migrations with AI help. However, success is unlikely without expert supervision and detailed rules. AI currently remains a 'copilot'.

## Industry Impact

Bun's experience may accelerate Rust adoption, especially for memory-critical systems. AI-assisted code conversion could reduce development costs, giving tech companies strategic flexibility.

## Conclusion: Dawn of a New Era

Bun's Rust migration marked a turning point in software engineering, demonstrating AI's key role in complex engineering problems. This approach may enable larger-scale language migrations in the future.

## Sources covering this story

- https://blog.pragmaticengineer.com/the-pulse-what-can-we-learn-from-buns-rapid-rust-rewrite-with-ai/ — 

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_Summary written by Brifing from the original reporting. Copyright in the original article belongs to its publisher; read the full piece at the link above._