Is Bun Being Rewritten in Rust? A Deep Dive into the Zig vs Rust Technical Decision

Introduction: A Rewrite Topic That Sparked Attention

Recently, a Hacker News thread titled "How is the Bun Rewrite in Rust going?" sparked widespread interest in the developer community, quickly garnering 51 upvotes and nearly 20 comments. As one of the most prominent JavaScript runtimes in recent years, any major change to Bun's technology stack resonates across the entire frontend and backend JavaScript ecosystem.

The notion of "rewriting Bun in Rust" is attention-grabbing enough on its own—it touches on the rivalry between Zig and Rust as systems-level languages and reflects the community's ongoing curiosity about the underlying implementation of high-performance runtimes.

Bun and Its Zig Roots

What Language Is Bun Actually Written In?

To understand this discussion, we first need to clarify a key fact: Bun's core implementation language is not Rust—it's Zig. This is frequently misunderstood. Bun was created by Jarred Sumner with the design goal of providing an "all-in-one" JavaScript runtime that integrates a runtime, bundler, test runner, and package manager into a single tool.

Bun's choice of Zig over Rust was a carefully considered technical decision. Zig is known for its fine-grained control over low-level memory, seamless interoperability with C, and relatively concise syntax. Created by Andrew Kelley in 2015, Zig positions itself as a modern replacement for C rather than a competitor to C++ or Rust. Its core design principles include: no hidden control flow, compile-time computation (comptime) instead of generics and macros, optional safety checks, and most importantly—the ability to directly @cImport C header files and seamlessly call C code without writing any binding layer. The Zig compiler itself can also serve as a C/C++ cross-compiler, making it exceptionally easy to build projects that depend heavily on C code.

For scenarios requiring frequent interaction with C/C++ codebases like JavaScriptCore (the JS engine Bun uses), Zig offers lower cognitive overhead and compilation friction. JavaScriptCore (JSC) is an open-source JavaScript engine developed by Apple, originally designed for Safari's WebKit framework. Unlike Google's V8 engine (used by Node.js and Deno), JSC employs a multi-tier compilation architecture—from a low-latency interpreter (LLInt) to progressively optimizing JIT compilers (Baseline JIT, DFG JIT, FTL JIT)—striking a balance between startup speed and peak performance. Bun chose JSC over V8 precisely for its faster cold-start characteristics and lower memory footprint, which is especially important for command-line tools and short-lived scripts. Zig's near-zero-friction interoperability with JSC's C++ interface enables Bun to efficiently integrate this massive engine codebase.

Why Did "Rewriting Bun in Rust" Become a Thing?

Precisely because Bun's technology choice stands in stark contrast to the mainstream "Rust high-performance toolchain" narrative, the community repeatedly sees discussions about whether Bun should—or is—being rewritten in Rust. These topics fundamentally reflect the ongoing debate among developers about the strengths and weaknesses of both languages in systems programming. Rust boasts a more mature ecosystem, stronger memory safety guarantees, and a larger community, while Zig attracts a dedicated following with its compilation speed and engineering simplicity.

Core Focuses of the Community Discussion

The Zig vs Rust Language Debate

Looking at the actual content of the discussion, the core topic isn't "Bun is really being rewritten" but rather a technical philosophy debate around language choice.

Arguments in favor of Rust typically emphasize:

• Ecosystem maturity: Rust has a comprehensive toolchain with Cargo, crates.io, and a massive library of third-party crates
• Memory safety guarantees: Rust's Ownership system is its most fundamental innovation. Through three compile-time rules—every value has exactly one owner, values are dropped when the owner goes out of scope, and values can be borrowed but cannot be mutated during the borrow (or vice versa)—it eliminates data races, dangling pointers, and double-free bugs without relying on garbage collection. This mechanism is called "zero-cost abstraction" because all checks are completed at compile time with no runtime overhead. The tradeoff is that developers must frequently interact with the borrow checker, especially when dealing with complex data structures and concurrency patterns, resulting in a steep learning curve.
• Community size: A larger contributor pool means better sustainability and easier hiring

Those defending Zig counter with:

• C interoperability: Bun deeply depends on C/C++ components like JavaScriptCore, and Zig creates less friction in this regard
• Compilation performance: Zig's compilation speed is typically faster than Rust's, which is crucial for the development experience on large projects
• Low-level control: For scenarios like runtimes that demand extreme performance optimization, Zig provides direct and transparent control

Mature Projects Can't Easily "Start Over"

You might not realize it, but performing a complete language-level rewrite of a mature project with a large user base comes at an extremely high cost. In software engineering history, large-scale rewrites have often been accompanied by enormous risk. Joel Spolsky called it "the single worst strategic mistake that any software company can make" in his classic 2000 article Things You Should Never Do—Netscape 6's ground-up rewrite leading to the company losing browser market share is a textbook case. In contrast, Mozilla's gradual strategy of replacing C++ components in Firefox with Rust (such as the Stylo style engine and WebRender renderer) is considered a more robust approach.

Bun's codebase has accumulated over years, with extensive low-level infrastructure built around Zig. Even if Rust holds advantages in certain dimensions, the ROI of a "rewrite" is often hard to justify. For Bun, even if some modules adopt Rust in the future, it would more likely take the form of a gradual hybrid approach rather than a complete teardown. This is also why such discussions tend to remain at the level of "hypotheticals" and "exploration" rather than actual engineering roadmaps.

The Engineering Tradeoffs Behind Language Choice

There's No Silver Bullet in Systems Programming

The biggest takeaway from this discussion is: in systems programming, language choice is never black and white.

Rust and Zig each represent different design philosophies—Rust prioritizes safety, even if that means a steeper learning curve and longer compile times; Zig prioritizes simplicity and control, placing more responsibility on the developer.

For a project like Bun, Jarred Sumner's choice of Zig was clearly based on an assessment of specific needs: deep integration with existing C/C++ engines, rapid iteration, and pixel-level control over performance. This pragmatic choice matters more than following language trends.

The Community Mindset Behind "Rewrite Everything in Rust"

"Rewrite X in Rust" has become almost a cultural phenomenon in the tech community, even earning its own abbreviation—"RIIR" (Rewrite It In Rust). From command-line tools to OS components, the Rust rewrite wave keeps coming. Success stories include command-line tools like ripgrep (replacing grep), fd (replacing find), and bat (replacing cat), as well as larger-scale projects like Deno (a Node.js alternative), SWC (a Babel alternative), and Turbopack (a Webpack alternative).

However, it's worth noting that these projects' successes share a common premise: they're either greenfield projects built from scratch or functional replacements of existing tools (rather than rewrites of the same codebase). This is a fundamentally different engineering proposition from "rewriting the internal implementation of an existing mature project." This both proves the vitality of the Rust ecosystem and reminds us to be wary of the technical impulse to "rewrite for the sake of rewriting."

Truly healthy technical decisions should be built on a comprehensive assessment of actual project needs, team capabilities, and long-term maintenance costs—not blind devotion to any particular language. Bun's commitment to Zig is itself a rational response to this impulse.

Conclusion: Focus on Substance, Not Labels

This discussion about "rewriting Bun in Rust" is less about attention to a specific engineering event and more about a collective reflection by the developer community on language choice, performance optimization, and engineering philosophy.

For everyday developers, rather than obsessing over whether Bun is written in Zig or Rust, it's more worthwhile to focus on the actual value it delivers—faster startup times, a more streamlined toolchain, and increasingly comprehensive ecosystem compatibility. The essence of technology lies in solving problems, and language is just one of the means to achieve that goal. Whether it's Rust or Zig, the implementation that reliably and efficiently serves user needs is the best implementation.