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Cargo could miss changes to manifest metadata variables such as CARGO_PKG_DESCRIPTION during rebuild checks. This happened when an older value was already set in the host environment, causing Cargo to overlook the updated manifest value and skip recompilation. This fix checks manifest metadata first and falls back to the host environment only when needed. Fixes rust-lang#17635
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Cargo could miss changes to manifest metadata variables such as CARGO_PKG_DESCRIPTION during rebuild checks. This happened when an older value was already set in the host environment, causing Cargo to overlook the updated manifest value and skip recompilation.
This fix checks manifest metadata first and falls back to the host environment only when needed. A regression test has also been added to prevent this issue from happening again.
Fixes #17635
What does this PR try to resolve?
When Cargo checks whether a target needs to be rebuilt, LocalFingerprint::CheckDepInfo goes through the environment variables recorded in the dep-info file.
Previously, when a manifest metadata value such as CARGO_PKG_DESCRIPTION changed in Cargo.toml, Cargo could end up checking the host environment instead of using the updated value. If the host environment still contained the old value, Cargo incorrectly assumed that nothing had changed and skipped recompilation.
This PR fixes the issue by resolving environment variables in step by step order:
CARGO_ENV — the path to the Cargo binary.
Manifest metadata from Cargo.toml, retrieved through pkg.manifest().metadata().env_var(...).
Environment variables defined in Cargo's [env] configuration, accessed through gctx.env_config().
The host environment, used as a fallback through gctx.get_env(...).
With this change, Cargo checks manifest metadata before falling back to the host environment, preventing outdated host values from hiding changes in Cargo.toml.
Notes to reviewers
I ran the regression test locally using cargo test --test testsuite freshness::rebuild_metadata_env_var_overrides_host_env. The test passed, and all freshness tests completed successfully.
🤖 LLM involvement: How an LLM was used in preparing this change.
I used an LLM assistant as a pair-programming mentor to better understand Cargo's fingerprinting system, discuss different implementation approaches, and improve the structure of the regression test. All code changes were reviewed, implemented, and verified manually.