A Rust compiler daemon built on rust-analyzer that provides fast incremental cargo builds and live in-process edits to running programs. Saving a source file can update a running program within milliseconds while preserving its state; live editing is demonstrated with a Game of Life demo and integrated via a small library so a program can poll for updates each frame. It installs as a drop-in replacement for cargo commands (build/run/test/check) or can be run directly; installation options include nix or cargo with Rust 1.99.0 and rust-src. The project supports JIT-style codegen (Cranelift), incremental compilation of only changed functions, and is open-source under MIT/Apache-2.0. It started from rustd and is now developed as a separate ra-jit project.
Benchmarks show large speedups: live-update latencies measured on a 96-core Linux machine had medians like 7 ms (single-function edit) versus 169 ms for incremental cargo, and 60 ms when adding 300 functions (vs 282 ms). In a generated 200-crate workspace on 8 CPUs, edits ranged from 4.2x to 10.6x faster for common changes (main-crate function 1.76s→0.42s, dependency-crate function 6.33s→0.60s); widespread signature changes are less sped up (13.87s→10.87s). Live edits are currently macOS Apple silicon only, require Rust 1.99.0, do not provide a borrow-checking live safety guarantee (keep running cargo check), and structural changes to in-use types require a program restart.
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