I4221 — CI's Rust cache carries the shared build directory, where cargo actually compiles

○READY wave 0 · p1 · implementation profile · parallel safe

Part of ci-critical-path — The critical path from a change to its verdict, and from a merge to its verified deploy, runs under a minute warm.

Objective

The setup-rust action's cache (Swatinem/rust-cache) also saves and restores ~/.cargo/build-shared, the build-dir .cargo/config.toml names, so a job whose cache key matches does not recompile its dependencies.

Why

Train 6's coverage job (run 40070749630) logged 'Restored from cache key v0-rust-coverage-Linux-x64-ade52a28-aaa1f5f1 full match: true', 'Cache Size: ~50 MB', 179 Compiling/Checking lines, and then 'Finished test profile in 10m 23s'. Its Cache Paths were ~/.cargo/{bin,registry,git,.crates*} and apps/majordomus-cli/target, while every unit ran from the runner's ~/.cargo/build-shared/majordomus/llvm-cov-target. ADR 0139 moved cargo's intermediates to {cargo-cache-home}/build-shared and left only finished artifacts in target/, but the CI cache still saved target/ alone. So 'rust cache hit=true' in every job summary meant nothing compiled was reused. The owner's audit 2 (10-24) raised it as cache-hit=true with a ~38 min coverage job.

Current state

Swatinem/rust-cache caches the registry, git checkouts and apps/majordomus-cli/target (about 50 MB). build-shared, where every dependency is compiled, is rebuilt from nothing in each job.

Desired state

The cache also carries ~/.cargo/build-shared. On an exact key match, the job compiles only the crate's own units. Finished artifacts stay per tree; .cargo/config.toml and the worktree isolation it protects are unchanged.

Scope

• .github/actions/setup-rust/action.yml
• .ai/repo/project/issues/I4221.yaml

Out of scope

• Sharing CARGO_TARGET_DIR between trees, which .cargo/config.toml forbids for a recorded reason
• Changing build-dir locally or the cache keys

Dependencies

None. This issue is a root of the graph.

Acceptance criteria

• A job's Cache Paths list includes the runner's ~/.cargo/build-shared
• A second run of a job at the same cache key compiles markedly fewer units than the first, measured by the count of Compiling lines in two runs' logs, quoted in this record
• Every job that reported a cache hit still passes

Validation

• Read the coverage job log of two consecutive runs at one key: Cache Paths, Cache Size and the Compiling count

Evidence required

• ci

Evidence

None recorded. Every token above needs a command or an artifact behind it before this issue can be completed; narrative is refused.

Risk

The cache grows from ~50 MB to the size of a job's build intermediates (several GB) per key, against GitHub's per-repository cache budget. If eviction then outruns reuse, narrow cache-directories to the jobs that gain, rather than revert.

Timeline

started

—

verified

—

completed

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Those three fields, the evidence above and the state of the dependencies are all the status is made of. There is no status field to disagree with them.

Canonical record: .ai/repo/project/issues/I4221.yaml. Read it back with majordomus plan show I4221.