Makepad's Relax JIT compiles a Rust 3D game in 11 seconds

Rust is a slow language to build. A medium-sized project takes a minute to compile from scratch. A large project takes several. That cost shows up every time a developer changes a line and runs the code. The Makepad team released Relax to cut that cost. The Hacker News thread about it had 123 points.

What Relax is

Relax is a live compiler and JIT runtime for Rust, written in Rust, hosted at github.com/makepad/relax under an MIT licence. It replaces the standard Rust toolchain for the edit-run loop. It does not use rustc, LLVM or a linker. Relax compiles each function the first time it is called. It optimises the ones that run often while the program runs. And it patches your edits into the running process.

Makepad is a UI framework for Rust apps. Relax is the tool Makepad built for its own development, now released on its own.

The numbers Makepad states

The blog gives four figures, measured on an M4 MacBook. All of them are Makepad's own, on Makepad's own code.

A large 3D game builds from source in 11 seconds, cold. The same build with cargo and rustc takes about three minutes. A warm re-run, with nothing changed, takes 1.15 seconds. A small app builds in 3.7 seconds cold and 0.41 seconds warm. Ninety-two tests re-run in 2.2 seconds after a save.

These are not generic numbers. They were measured on the Makepad code base. A Rust service that leans on procedural macros, build scripts or generics will see a different result. A team interested in Relax should run it on its own project first. Only then is a figure worth trusting.

What Relax is not for

Makepad is clear on the boundary. The docs call Relax experimental. They say Relax is for development iteration. For any release build, use Rust itself and cargo build --release.

So Relax does not replace rustc. It sits beside it. Development runs on Relax. Release builds run on rustc. A team that adopts Relax keeps the cargo build for production binaries. A bug may only show up under one tool and not the other. That is a new class of problem to watch.

Rust at Python iteration speed

Build times are one of the top drains on a team's attention. A three-minute compile makes small changes feel expensive. Developers batch work into bigger changes. An eleven-second compile keeps the edit-run loop tight. If the Makepad numbers hold on your own code, Rust starts to feel like Python or TypeScript during development.

Makepad labels Relax experimental and limits it to development. That is the honest framing. The right next step is a small experiment on a side project. Keep cargo as the release build. Then see whether the figures hold outside Makepad's own code.

Source

• Relax: a live compiler and JIT runtime for Rust, Makepad
• Relax on GitHub, Makepad