Build layers

Build layers are rusty's project-local answer to "which crates can skip compilation this run?"

Each workspace keeps a layer index under .rusty/ mapping crate identities (name@version) to artifact cache keys for a given profile (debug or release).

Inspect layers

rusty layers list --profile release
rusty layers plan --profile release
rusty layers why serde --profile release

plan is the command you run in CI before a long release build to see whether the cache will actually help.

How keys are formed

Layer keys digest:

• Source tree content (via CAS manifests)
• Dependency graph inputs
• Active rustc version
• Target triple and profile
• Enabled features and relevant flags

rusty layers why prints the resolved key and whether the artifact cache currently holds it.

RUSTC_WRAPPER integration

During builds rusty sets RUSTC_WRAPPER to $RUSTY_HOME/bin/rusty-wrapper. The wrapper intercepts rustc invocations, stores successful outputs in the artifact cache, and updates the layer index.

You do not configure this manually for normal rusty build usage — it is wired through environment preparation.

Profiles

Pass --release or --profile <name> consistently across layers, build, and test so keys align:

rusty layers plan --release
rusty build --release

Mismatching profile flags between plan and build produces confusing miss forecasts.

Relationship to the artifact cache

Layers are the index. The artifact cache (rusty cache *) holds the payload — compiled object files and metadata groups keyed by the layer digest.

See Caching guide for operational patterns.