rust-build

Cross-worktree cache reuse wants a shared target dir; correctness wants isolation. scripts/rust-build.sh resolves it — share by default, isolate automatically only when a worktree diverges a crate other crates link.

Use it

Drop-in for cargo, from any worktree (or the main tree):

scripts/rust-build.sh build -p nub-cli --profile fast
scripts/rust-build.sh test  -p nub-cli --test integration
scripts/rust-build.sh clippy --all-targets --all-features -- -D warnings

It prints which target dir it chose and why, then execs cargo with CARGO_TARGET_DIR set. Two default-on contention controls ride along:

• QoS clamp (darwin only): cargo runs under taskpolicy -c utility, so interactive work preempts fleet builds; an uncontended build still gets all cores. NUB_BUILD_FG=1 opts out.
• Job cap on big hosts (>8 cores): CARGO_BUILD_JOBS = ncpu-4 unless the caller already chose (pre-set CARGO_BUILD_JOBS, NUB_BUILD_JOBS, or an explicit -j/--jobs — cargo's flag outranks the env var).

These cover builds going THROUGH the wrapper (or make). Direct cargo invocations are clamped by a machine-global control: make qos-global installs scripts/rustc-qos.sh as the cargo rustc-wrapper in ~/.cargo/config.toml, so every rustc on the host compiles at utility QoS (install-dev re-runs it, so it self-heals). Same NUB_BUILD_FG=1 opt-out; toggling it does not invalidate fingerprints.

Why one shared target dir

All worktrees default to ~/.cache/nub/shared-target. A fresh worktree reuses the crates.io dependency rlibs a sibling already compiled and recompiles only the ~3 workspace crates — ~5s instead of a ~3-min cold build.

Relocation does NOT defeat reuse. Cargo revalidates a relocated target dir in place: cloning a warm dir to a new path rebuilt 0 crates; an empty dir rebuilt all 13. (The "path is baked into fingerprints" claim is true of sccache, which keys on the rustc command line including absolute --out-dir / -L dependency= paths — not of cargo.) So sharing a live path buys convergence; a CoW clone buys a warm start, which is why the wrapper seeds a fresh private dir from the matching bucket instead of cold-starting.

The hazard sharing creates

Cargo names a crate's output by package id (name + version), not source content. Two worktrees whose source for the same depended-on crate differs — classically vendor/aube on divergent branches — write the same output slot and clobber each other. A dependent crate then links the stale rlib:

error[E0063]: missing field `lockfile_legacy_basenames` in initializer of `aube_util::Embedder`

— a field that exists nowhere in your checkout. Only bites crates that other crates link; a divergent leaf binary (nub-cli) just rebuilds cleanly.

The rule the wrapper enforces

Sharers are grouped by the content of their depended-on crates, hashed into the bucket name (shared-target-<hash>):

• Depended-on crate sources unmodified → share that content's bucket. Everyone in it agrees by construction. The common case: feature work in nub-cli, integration tests, docs, non-Rust files.
• Diverged a depended-on crate → isolate to a private per-worktree target/ (removed with the worktree), CoW-seeded from the matching bucket so you rebuild only what differs.

Content, not merge-base: a merge-base proves only that this worktree made no local changes against its own base, so two worktrees whose bases straddle a nub-core/aube commit both pass while disagreeing on content. The key hashes the git index, which only matters on the shared branch (no local changes by definition), so it moves on rebase, never mid-edit.

Depended-on crates = every workspace/vendored crate except leaf artifacts nothing links: crates/nub-cli (bin), crates/nub-native (cdylib, own workspace), crates/nub-phantom (bin, own workspace). So: crates/nub-core, crates/nub-cache-key, crates/nub-phantom-core, crates/nub-phantom-scan, and all of vendor/aube. nub-phantom-core/nub-phantom-scan are not leaves — nub-cli depends on both — and the pathspec :(exclude)crates/nub-phantom matches only that directory, not those siblings.

Letting the wrapper choose IS the caching strategy

export CARGO_TARGET_DIR overrides the wrapper's resolution — it is how caching is LOST, not gained: you land in a cold dir, or a multi-tenant one you can clobber.

Never put an exported CARGO_TARGET_DIR in a dispatch prompt. A sub-agent cannot know whether the value is the warm bucket, a cold path, or a dir a sibling is mid-build in; the wrapper does, and it prints which it chose and why.

Pin one only when seeding cannot fire — a branch cut before seeding landed, or a filesystem without CoW:

grep -c seed scripts/rust-build.sh      # 0 → this branch has no seeding; pinning may pay

When you must pin for a serial multi-phase epic, point the whole chain at ONE dedicated private target so deps compile cold once:

export CARGO_TARGET_DIR=~/.cache/nub/<epic>-target

• Serial only — cargo's target-dir lock serializes builds; never point two concurrently-building worktrees at it.
• Dedicated, NOT shared-target — the shared dir is multi-tenant.
• A review/verify sub-agent reuses the implementer's already-warm target, never a fresh one.

For a orchestrator run, pinning buys nothing: every lane edits ONE worktree and the orchestrator builds that same worktree.

Trade-offs and edges

• A worktree that edits aube pays a cold build even with no sibling diverging aube concurrently. The invariant is "match origin/main," which doesn't depend on volatile sibling state — that's what makes it robust.
• Concurrent builds in two sharing worktrees serialize on cargo's target-dir lock. A latency cost, never a correctness one. Need two at once? Isolate one.
• NUB_SHARED_TARGET relocates the target dir and the path is used exactly as given — not content-keyed, because a caller naming a path is asking for that path (make verify relies on this to reach $(CURDIR)/target). The value must be private to one checkout; two worktrees on one relocated path recreates the phantom-E0063 clobber. To relocate a cache several worktrees share, leave it unset and move ~/.cache/nub itself.
• Cleanup: git worktree remove <path> --force drops the worktree and its private target/; the shared dir is intentionally left in place.
• A build script can pin an ABSOLUTE PATH into the shared dir — a second contamination shape. A build script resolving inputs through compile-time env!("CARGO_MANIFEST_DIR") bakes the compiling worktree's path into the cached build-script binary, which is cached per package id and survives into every sharing worktree. Symptom: failed to read /…/worktrees/<some-other-tree>/… naming a directory absent from your checkout. Fixed at the source; for a stale one, rm -rf <target-dir>/*/build/<crate>-* and rebuild. Note cargo clean -p <crate> from the repo ROOT is a silent no-op for the aube crates — not root workspace members, so it reports Removed 0 files and exits 0.
• Running the vendored aube test suite from the repo root drops its serial-execution pin. vendor/aube/.cargo/config.toml sets RUST_TEST_THREADS = "1" because some aube-util tests mutate process env, and cargo discovers config from the CWD, not --manifest-path — so cargo test --manifest-path vendor/aube/Cargo.toml … from the root runs them in parallel and produces failures CI never sees. Run it as CI does: (cd vendor/aube && cargo test …).

The gates run on TWO profiles — budget for two dependency builds

fast and dev are different target subdirectories, so clippy artifacts do not serve cargo test.

Do not unify them: moving cargo test onto fast diverges from CI, and dropping fast from clippy drives a second full dependency build under dev (~26 GB of duplicated target/debug + target/fast). fast inherits dev — identical debug-assertions, overflow checks and opt-level, differing only in debuginfo, which no lint reads.

A green gate run leaves NO runnable binary — build one explicitly

So fmt + clippy + test can all be green while target/fast/nub is hours stale or absent — and it fails in the worst direction, because the old behavior usually still works, so the probe reads as a clean pass. Add an explicit build step whenever a gate run is followed by running the thing.

Bind the artifact to the source before trusting a fixture result. An mtime newer than your last edit is necessary and not sufficient — a concurrent build can overwrite the path mid-run. Prove it positively by exercising a behavior only your change produces, and re-hash afterwards (or copy the binary aside and probe the copy).

Long cargo runs go in a BACKGROUND shell — never poll for them

A cold build, --all-targets clippy or a full cargo test outlives the foreground timeout. Start it with the harness's background mechanism and let the completion notification wake you. Do not write a wait loop, and do not sleep on it: the harness already tracks it, so a poller is pure waste and it can be WRONG in both directions.

Measured cost of getting this wrong: a loop polling pgrep -f "cargo|rustc" reported "still building" for ten minutes AFTER the build had finished — pgrep -f matches the full command line INCLUDING the environment, so every process with .cargo/bin in its PATH matched. If you must ask whether a compiler is running, match the executable name exactly:

ps -Ao comm= | grep -cE '^(rustc|cargo)$'      # 0 = idle

ANY cargo command REWRITES that profile's binary — with ITS features, not yours

cargo build -p nub-cli --profile fast --features nub-cli/build-jail-catalog-override then cargo test -p nub-cli --profile fast <filter> leaves target/fast/nub built without the feature. test rebuilt the bin under the default feature set and clobbered it. Nothing warns; the binary just quietly becomes a different one.

So re-arm the binary after ANY bare cargo invocation on the same profile, before running a harness or a fixture against it:

cargo test -p nub-cli --profile fast <filter>          # clobbers target/fast/nub
scripts/rust-build.sh build -p nub-cli --profile fast \
  --features nub-cli/build-jail-catalog-override        # re-arm before probing

Cost when missed: a build-jail catalog probe reported BROKEN-EVEN-WITH-EVERYTHING for a package that installs fine, because the featureless binary refused the override. It was diagnosable only because nub fails LOUD there ("NUB_BUILD_JAIL_CATALOG is set, but this binary was not built with the build-jail-catalog-override feature… Refusing rather than running the compiled-in catalog under an override's name"). A feature that degrades silently would have cost far more.

Related and distinct: --features nub-sandbox/build-jail-catalog-override enables it on the sandbox crate ONLY, so anything gated inside nub-cli compiles out while the sandbox-side banner still prints — the override looks live and half of it is absent. nub-cli forwards the feature (crates/nub-cli/Cargo.toml), so the bare name and nub-cli/… are equivalent; the nub-sandbox/… form is the wrong one.

The gate's exit status must be CARGO's — three ways it silently isn't

• A pipe. cargo … | tail gives you the PIPE's status.
• | head -N additionally closes the pipe and SIGPIPE-kills cargo outright.
• Trailing commands. cargo … > log 2>&1; echo EXIT=$?; tail log redirects correctly and still exits with tail's status, so a harness reports 0 while cargo returned 101.

The habit that holds: redirect to a file, capture RC=$? immediately, and end the command with exit $RC so the shell's status IS cargo's. Read the log with Read/grep, never tail in the same command whose status you care about. Confirm the recorded RC= line before believing a gate passed.

--all-features needs a staged addon

--all-features turns on embed-runtime, whose build.rs bakes an integrity digest of runtime/addons/nub-native.node — gitignored, so absent in a fresh worktree. Without it the gate fails with cannot read entrypoint … for integrity hashing and the lint never runs. Stage a placeholder as CI does:

mkdir -p runtime/addons && printf 'placeholder-addon' > runtime/addons/nub-native.node

The digest only needs bytes to hash — a real addon is not required to lint.

Two crates are their OWN workspaces — -p from the root cannot see them

crates/nub-native (panic-strategy split) and crates/nub-launcher (size-tuned release profile) each have their own [workspace], so cargo build -p nub-launcher from the root fails with package ID specification 'nub-launcher' did not match any packages. Build from inside the crate, or with --manifest-path. CI lints each separately.

Build nub-native with cd crates/nub-native, NEVER --manifest-path

crates/nub-native/.cargo/config.toml sets target-dir = "../../target" so the addon lands in the repo-root target/<profile>/ that the Makefile and CI copy from. Cargo discovers config by walking up from the CWD, not from the --manifest-path directory — so cargo build --manifest-path crates/nub-native/Cargo.toml --release silently writes to crates/nub-native/target/, and the follow-on cp target/release/libnub_native.dylib runtime/addons/nub-native.node copies a stale artifact or fails.

An "unused import" warning does NOT mean the import is unused — check cfg(test) first

A non-test build warns unused import: X for an import only mod tests consumes through use super::*. Deleting it turns one warning into N compile errors in the test target. Before removing a flagged import, grep the symbol in the same file: if the hits are past the #[cfg(test)] line, move the import into mod tests.

Related skills

new-worktree.ts creates the worktree and points you at this wrapper. dev-loop covers the fast-profile incremental loop; this skill owns the target-dir decision. When a build fails with a symbol/field absent from your source, this is almost always the cause — rebuild through rust-build.sh and it isolates you.