How It Works

Rust builds are slow for a structural reason: rustc compiles each crate in isolation. When it compiles a library crate it cannot know which of its public functions will be called by downstream code, so it compiles all of them. In a large workspace most of that work is wasted.

cargo-slicer attacks the problem from two angles simultaneously.

The two techniques

Virtual Slicer

The virtual slicer inserts itself as a RUSTC_WRAPPER. Before each crate is compiled it performs a reachability analysis — a BFS starting from the entry point of the binary — and replaces every unreachable function body with an abort() stub. LLVM never sees those functions, so it never optimises, inlines, or emits machine code for them.

For a crate like image, which exposes hundreds of format decoders and pixel converters, a typical application uses one or two formats. The slicer stubs the rest. LLVM's work drops by 97%.

→ Virtual Slicer details

Warm-Cache Daemon

Registry crates (crates.io dependencies) do not change between builds. The warm-cache daemon pre-compiles them once and stores the resulting .rlib and .rmeta artefacts. On every subsequent build, instead of re-running rustc, the RUSTC_WRAPPER copies the cached artefact into target/ in milliseconds.

The cache key is SHA256(crate + version + rustc_version + features + opt_level), so the cached artefact is safe to share across projects and across git branches. A warmed cache built while compiling zed is reused immediately when compiling nushell.

→ Warm-Cache Daemon details

The three-layer pipeline

When both techniques run together, three wrappers are chained:

cargo build --release
    │
    ▼  RUSTC_WRAPPER = cargo_warmup_dispatch
    │  ├─ registry crate? → serve from cache, return immediately
    │  └─ local crate? → pass to next wrapper
    │
    ▼  CARGO_WARMUP_INNER_WRAPPER = cargo_slicer_dispatch
    │  ├─ no unreachable fns (cache hit)? → pass to real rustc directly
    │  └─ has unreachable fns? → pass to driver
    │
    ▼  CARGO_SLICER_DRIVER = cargo-slicer-rustc
       └─ MIR analysis → stub unreachable fns → LLVM codegen on minimum set

The cargo-slicer.sh script sets up this chain automatically.

→ The All-in-One Script