[josaum]

The bug

build.rs bakes the source revision into the binary so staleness is detectable. Its header says a binary "keeps reporting the commit it was actually built from."

It declared only cargo:rerun-if-changed=build.rs, so Cargo reused the previous run's values whenever that file was unchanged. Any commit that didn't touch build.rs produced a binary stamped with an earlier commit, and the self-contract freshness check then compared against a value that had never been current.

The header described this as deliberate — no .git triggers, so branch switches never force a rebuild. But the consequence isn't limited to branch switches: the stamp refreshes only when build.rs itself is edited, which is almost never.

Observed: cargo install --path . from a freshly merged main compiled the new source and still reported the previous commit, with clean beside it.

The fix

Declare the files that actually determine git rev-parse HEAD:

• HEAD
• the branch ref it names (symbolic-ref --quiet, silent on a detached HEAD)
• packed-refs, since a packed ref has no loose file

Paths come from git rev-parse --git-path, so linked worktrees and submodules — where .git is a file, not a directory — resolve rather than assuming .git/. Outside a git checkout every lookup returns None, nothing is emitted, and the stamp stays unknown.

Cost, accepted deliberately

Moving HEAD now re-runs the script, so switching branches relinks on the next build. An accurate stamp isn't available more cheaply: the commit can't be re-read without re-running the reader. A stamp that is usually wrong seems worse than an occasional relink.

Known limit, documented not fixed

LEIO_BUILD_DIRTY is still only as fresh as the last run. No rerun-if-changed can express "any tracked file differs from HEAD", so editing a source file flips the real dirty state without re-stamping. The header now says clean means "clean when last stamped", rather than implying the stamp is self-maintaining. The commit is the load-bearing half.

Verification

Empty commit touching no files at all, then a rebuild — the case the old code could not handle:

stamp before: leio-code 2.6.8 (951f4dfbe914, ...)
new HEAD:     d8999684fa4c
stamp after:  leio-code 2.6.8 (d8999684fa4c, ...)

Under the old build.rs the stamp would not have moved. cargo fmt and cargo clippy --all-targets clean.

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