[enricopiovesan]

Follow-up to #1801 (fixed in #1803).

Summary

#1803 gave each checkout its own Cargo target directory when CARGO_TARGET_DIR points outside it, so local checks never run binaries another checkout built. It put that directory at ${CARGO_TARGET_DIR}/checkouts/<id>, which nothing ever removes. Each one is a full build of 5–22 GB (debug, release, llvm-cov-target, iOS and wasm targets). Claude Code sessions create many short-lived worktrees, so these folders pile up. On 2026-11-05/06, nine of them filled about 160 GB of disk in roughly 24 hours, and three belonged to worktrees that no longer existed.

checkout_target_dir.sh now points an outside CARGO_TARGET_DIR at <checkout root>/target instead. That folder is git-ignored (target/ in .gitignore), and deleting the worktree deletes it too.

• Local checks can test another checkout when CARGO_TARGET_DIR is shared #1801 still holds: no two checkouts ever share a target directory, because each one builds inside its own root.
• A CARGO_TARGET_DIR that is unset stays unset, so Cargo uses target/ as before.
• A directory already inside the checkout is kept, and so is a relative one.
• Sourcing the helper twice gives the same result.
• The checkout ID hash is gone because nothing uses it now.

What this costs: a checkout no longer keeps its build under the developer's chosen shared location. That folder was never shared between checkouts anyway, so builds are no less incremental than before.

Governing Spec

• 004-spec-alignment-gate
• 075-native-runtime-distribution-contract

Project Item

• Traverse Project 1: Local checks can test another checkout when CARGO_TARGET_DIR is shared #1801

Validation

• scripts/ci/checkout_target_dir_test.sh now asserts that each checkout resolves to its own target/ and that one checkout's target/ is never used by another. It fails against the previous helper (.../shared/checkouts/9c89c4bf1b954c13).
• Sourced under bash in this worktree with CARGO_TARGET_DIR=~/.cargo-target-shared, the helper resolves to <worktree>/target, and sourcing it again gives the same path.
• scripts/ci/repository_checks.sh passes.
• Full local_preflight.sh --pr-body passed with the shared CARGO_TARGET_DIR set. The build (19 GB) went into this worktree's target/. The script skipped native artifact certification because this machine lacks Java, .NET and the Android SDK, and it always skips the hosted-only CodeQL and stress jobs.

🤖 Generated with Claude Code