Relax: JIT compiler for Rust to reduce build times
Relax is a novel JIT compiler and runtime written in Rust, designed to drastically reduce development iteration times by replacing the entire rustc/LLVM toolchain. It compiles functions on demand, hot-patches live code, and aims to provide an interactive development experience often missed in Rust, despite trade-offs in final execution speed and memory usage. Its ambitious scope highlights a growing trend of AI-assisted engineering for complex software projects, resonating with developers frustrated by Rust's notoriously slow build times.
The Lowdown
Relax introduces a new paradigm for Rust development, acting as a live compiler and JIT runtime that completely bypasses rustc, LLVM, and the linker. Written entirely in Rust, it aims to eliminate the lengthy build-test cycles common in the ecosystem by compiling code on demand, optimizing hot functions dynamically, and patching live edits into a running program without losing state.
- Live Compilation & Hot-Patching: Functions are compiled when first called, hot ones are optimized in the background (interpreter → O1 → O3), and code changes can be patched directly into the running application, preserving its state.
- Rapid Iteration: Demonstrates dramatic speedups for development, turning multi-minute cold builds into seconds (e.g., a large 3D game goes from 3 minutes to 11 seconds for the first frame on an M4 Mac). Subsequent runs are near-instant.
- Enhanced Debugging & Tooling: Offers capabilities like pausing on panic to inspect state, breakpoints without a separate debugger, hang explanations, and integrated profiling and memory analysis tools.
- Full Toolchain Replacement: Relax implements its own lexer, parser, type checker, borrow checker, and code generator (for x86-64 and AArch64), eliminating
crates.iodependencies in its core. - Tradeoffs: Optimized code run by Relax is generally slower (1.12x to 1.25x) and uses more memory than
rustc's--releasebuilds. Some structural code changes still require a restart, and itscheckcommand isn't yet faster thancargo check. - AI-Driven Development: The project originated from Makepad and is specifically designed to accelerate development for AI agents, allowing for rapid iteration and testing in scenarios where traditional build times are a significant bottleneck.
Ultimately, Relax is positioned as a powerful tool for the development iteration loop, not for shipping production binaries, which should still leverage cargo build --release for maximum performance.
The Gossip
The Lingering Lisp Loop
Commenters observe that Relax's live-reloading and interactive features echo the "Lisp machine" or REPL-driven development paradigms championed decades ago. This recurring theme suggests a fundamental desire for tighter feedback loops in software development. While acknowledging that Rust's strong type system and borrow checker make such an environment challenging, many express enthusiasm for this "rediscovery" of rapid iteration, comparing it to the continuous evolution of development environments seen across various languages.
Rust's Runtime Race
A significant portion of the discussion revolves around Rust's notoriously slow compile times, a pain point for many developers. Relax is lauded as a potential solution, joining a growing list of projects (rustd, blaze, PLIRON, mr boxington, kache) aiming to speed up Rust development. Debates arise about the root cause of Rust's compilation overhead—some point to the aggressive LLVM optimizations and complex trait solver, others to the heavy dependency trees necessitated by Rust's minimal standard library, rather than the borrow checker itself. The trade-off of potentially moving some compile-time errors to runtime for faster iteration is also a key point of discussion.
AI's Ambitious Accomplices
A surprising revelation from the author — that AI tools like Claude and Codex were instrumental in building Relax — sparked considerable interest. Commenters noted this as a "textbook example" of how AI agents can commoditize conceptually understood but labor-intensive engineering problems. The sentiment is that while the underlying concepts of JIT compilation are not new, AI significantly lowers the barrier to executing such large-scale, ambitious projects, making a complete Rust toolchain re-implementation feasible in a timeframe previously unimaginable.
Pragmatic Project Particulars
Users delve into the practical implications of Relax's design choices. The "0 crates.io dependencies" is praised for its self-sufficiency, but also raises questions about the maintenance burden of 438k lines of code. The acknowledged trade-off of slower runtime performance in exchange for faster development iterations is generally accepted, with the understanding that Relax is for the `edit-run` loop, not release builds. The approach of running the borrow checker asynchronously, jokingly called 'Gandalf' for its potential to halt the JIT, is seen as an innovative way to balance safety with speed in development.