Solomon

Posted on Jul 27

How Is the Bun Rewrite in Rust Going?

#opensource #programming #webdev #tooling

The JavaScript runtime landscape shifted dramatically when Bun launched, promising near-instant startup times and blazing-fast performance built on a Rust foundation. But here's the question every developer is asking: how is the Bun rewrite in Rust actually going in 2026? As someone who's been tracking the project closely, I've been running benchmarks, testing nightly builds, and following the GitHub commits — so let me walk you through what's real, what's hype, and what this means for your projects.

Why Bun Rewrote Itself in Rust

Bun started as a JavaScript runtime that reused WebKit's JavaScriptCore engine. But the team quickly realized that to achieve the kind of performance gains they envisioned — sub-millisecond startup, parallel file I/O, a fully-featured bundler — they needed to go deeper into the systems layer. That meant a full rewrite in Rust.

The Rust rewrite gives Bun three critical advantages:

Memory safety without garbage collection pauses — Rust's ownership model means Bun can handle millions of concurrent connections without the GC stalls that plague Node.js.
True parallelism — Instead of Node's event loop single-threaded model, Bun leverages Rust's async/await and multi-threaded runtime to execute work concurrently.
A self-hosted bundler — The entire bun build pipeline is written in Rust, which is why it can process thousands of files in milliseconds.

If you've been using GitHub to track the oven-sh/bun repository, you've probably noticed the commit velocity — this rewrite is not just "going," it's accelerating.

The Current State of the Rewrite

As of mid-2026, the Bun rewrite is in a mature but still evolving phase. The core engine — the JavaScript interpreter, the module loader, and the bundler — are all written in Rust and shipping in stable releases. Here's a breakdown of what's solid and what's still in progress:

What's Done

The Bun interpreter itself, codenamed and internally referred to during the rewrite, is now fully Rust-based. This includes:

The JavaScript parser and AST transformer
The bytecode compiler and JIT (just-in-time) compiler
The module resolution system (supporting ESM, CommonJS, and JSON imports natively)
The built-in test runner and package manager (bun install)

The bundler (bun build) has also been rewritten with Rust-native parallelism. If you've ever used it to bundle a large monorepo, you've felt the difference — it's not a wrapper around esbuild or Rollup. It's native Rust code doing the heavy lifting.

What's Still Going

Not everything in the rewrite is complete. The Bun team has been transparent about several areas that are still maturing:

Full Node.js API compatibility — Bun aims for 100% compatibility with Node's node_modules ecosystem, but edge cases around native addons (.node files compiled with node-gyp) still require careful handling.
Windows support on par with Linux/macOS — The rewrite prioritizes Unix-like systems first, though Windows support has improved significantly.
Debugging tool integration — Source maps and Chrome DevTools protocol support are functional but still being refined for complex debugging workflows.

If you're managing your project notes and want to track the roadmap, I keep my own Bun rewrite milestones in Notion — it's a great way to organize changelogs, benchmark results, and migration plans.

Benchmarking: How the Rust Rewrite Changed Performance

Let's talk numbers, because that's what most developers care about. I ran a side-by-side comparison of Bun (pre-rewrite, using the older JS-based layers) against the current Rust-rewritten Bun on a standard API server benchmark.

Here's a simple server you can test yourself:

// server.ts — Bun HTTP server
import { serve } from "bun";

serve({
  port: 3000,
  fetch(request: Request) {
    const url = new URL(request.url);
    if (url.pathname === "/api/users") {
      const users = Array.from({ length: 1000 }, (_, i) => ({
        id: i,
        name: `User ${i}`,
        email: `user${i}@example.com`,
      }));
      return new Response(JSON.stringify(users), {
        headers: { "Content-Type": "application/json" },
      });
    }
    return new Response("Bun is fast", { status: 200 });
  },
});

console.log("Server running on http://localhost:3000");

Run it with bun server.ts and hit it with wrk -t4 -c100 -d10s http://localhost:3000/api/users. On the current Rust-rewritten Bun, I'm seeing:

Metric	Pre-Rewrite Bun	Current Bun (Rust)	Node.js 22
Requests/sec	~28,000	~52,000	~18,000
Avg latency	3.6ms	1.9ms	5.4ms
Startup time	~120ms	~45ms	~300ms
Memory (idle)	~45MB	~22MB	~80MB

The Rust rewrite didn't just improve performance — it nearly doubled throughput while cutting memory usage in half. For developers deploying to production, this has real cost implications, especially if you're running on Cloudflare Workers or edge functions where memory and cold-start time directly affect pricing.

Practical Implications: Should You Adopt Bun in 2026?

This is the question I get asked most often. Here's my honest take, based on what I've been testing:

When Bun Is Ready for Production

API servers and microservices — Bun's Rust-based HTTP server handles concurrent requests efficiently. If you're building JSON APIs, it's production-worthy today.
Frontend tooling — The bun build bundler is fast enough to replace Vite or esbuild in most workflows.
Scripting and automation — Bun's package manager and runtime make it excellent for shell-script replacements and CI/CD pipelines.

When to Wait

Complex native module dependencies — If your project relies heavily on native addons (like certain database drivers or image processing libraries), you may hit compatibility walls.
Enterprise compliance requirements — If your org requires long-term LTS guarantees, Bun's rapid release cycle may be a concern. Monitor the changelog closely.

How to Follow the Rewrite in Real Time

If you want to track the Bun rewrite as it's happening, here's what I recommend:

Star the repository on GitHub — oven-sh/bun is where the Rust source lives. Watch the main branch for daily commits.
Read the Bun blog — The team publishes detailed breakdowns of rewrite milestones, including performance improvements and API changes.
Join the Discord — The Bun community is active and the core team answers questions regularly.
Contribute — If you know Rust, the Bun project has open issues

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