Build & Run

The rx build and rx run commands compile your project with automatic fast linker detection, artifact caching, and cross-compilation support.

Basic usage

rx build                        # debug build
rx build --release              # release build
rx run                          # build and run the binary
rx run -- --port 8080           # pass arguments to the binary
rx check                        # type-check only (no codegen)

Fast linker

rx auto-detects the fastest available linker on your system:

1. mold – fastest, preferred when available
2. lld – fast, widely available
3. system – default fallback (cc/ld)

The detected linker is cached at ~/.rx/env.lock so detection only runs once. Override it in rx.toml:

[build]
linker = "mold"    # or "lld", "system", "auto"

Cross-compilation

rx build --target aarch64-unknown-linux-gnu
rx build --target wasm32-unknown-unknown
rx build --target x86_64-pc-windows-msvc

rx passes the --target flag through to cargo. Ensure the target is installed:

rustup target add aarch64-unknown-linux-gnu

Note: cross-compiled builds always invoke cargo directly — the artifact cache never answers --target or --package builds.

Caching

When cache = true (opt-in, off by default — see PRODUCT.md), rx maintains a global content-addressed cache at ~/.rx/cache:

1. Mtime check – if no source file has changed, skip everything
2. Fingerprint – compute xxh3-128 hash of sources, Cargo.toml, Cargo.lock, profile, and RUSTFLAGS
3. Cache hit – restore artifacts via reflink/hardlink into target/
4. Cache miss – build normally, then store artifacts for next time

Disable caching per-command or in config:

rx build --no-cache             # skip cache for this build

[build]
cache = false

Incremental linking

When incremental_link = true (the default), rx enables:

• split-debuginfo – keeps debug info separate from the binary
• --as-needed – only links referenced libraries

This significantly reduces link times for iterative debug builds.

RUSTFLAGS

Add extra RUSTFLAGS in rx.toml:

[build]
rustflags = ["-C", "target-cpu=native"]

These are appended to any existing RUSTFLAGS environment variable.

Parallel jobs

[build]
jobs = 0    # auto-detect (default)
jobs = 4    # use exactly 4 parallel jobs

Flags reference

Related commands

• rx check – type-check without building (faster feedback loop)
• rx clean – remove build artifacts
• rx clean --gc – clean and garbage-collect the global cache