Warm-Cache Daemon

The warm-cache daemon (also called cargo-warmup) pre-compiles registry crates once and serves the cached .rlib / .rmeta artefacts on every subsequent build. It is the Rust equivalent of a precompiled-header daemon for C/C++.

The insight

Registry crates do not change between your builds. syn, serde, tokio, proc-macro2 — these are compiled identically every time you run cargo clean && cargo build. Compilation caches like sccache help on the second build, but every fresh environment (new developer, CI machine, Docker container) pays the full cost again.

The warm-cache daemon shifts that cost to a one-time investment. Pre-warm the top-tier registry crates once (takes ~10 minutes). Every cold build afterwards — in any project that depends on those crates — skips their compilation entirely.

Cache key

The cache key is:

SHA256(crate_name + version + rustc_version + edition + features + opt_level)

-C metadata and -C extra-filename are excluded. These differ per project but do not affect the correctness of the compiled artefact. Excluding them is what enables cross-project sharing: the .rlib compiled while building zed is reused directly when building nushell.

Usage

# One-time warm (adds ~10 min, saves that time on every cold build after)
cargo-warmup init --tier=1

# Check cache status
cargo-warmup status

# Use in builds
RUSTC_WRAPPER=$(which cargo_warmup_dispatch) cargo +nightly build --release

cargo-slicer.sh runs cargo-warmup init --tier=1 automatically on first use.

Tiers

Tier	Crates included	Warm time
`--tier=1`	proc-macro2, quote, syn, serde, tokio, + 5 more core crates	~20 s
`--tier=2`	+ 50 most common transitive deps	~3 min
`--tier=3`	All crates.io top-500	~10 min

Tier 1 gives the best return on investment for most projects. Tier 3 is useful in CI environments where build time is money.

How it plugs into cargo

RUSTC_WRAPPER=cargo_warmup_dispatch
    │
    ├─ cache hit?  → copy .rlib to target/, return in < 1 ms
    └─ cache miss? → invoke real rustc, store result in cache

The dispatch binary adds less than 1 ms per crate invocation on a cache hit.

By default the cache lives in ~/.cargo/warmup-cache/. Any project on the same machine with matching crate versions and rustc toolchain automatically benefits from a warm cache built by any other project.

To inspect what is cached:

cargo-warmup status
# or
sqlite3 ~/.cargo/warmup-cache/index.db \
  'SELECT crate, version, cached_at FROM artefacts ORDER BY cached_at DESC LIMIT 20'

cargo-slicer