Do not fire unused_crate_dependencies on Kani's injected std extern - #4689
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feliperodri merged 1 commit intoJul 29, 2026
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Kani unconditionally passes '--extern noprelude:std=<kani libstd>' so that user code referencing std gets Kani's overlay. For #[no_std] crates that deny(unused_crate_dependencies) (e.g. zerocopy-style strict-lint crates), that injected extern trips the lint and the build fails through no fault of the crate's own dependency declarations. Add rustc's 'nounused' extern modifier, which exists exactly for such compiler-injected externs, so the lint keeps working for the crate's real dependencies. Co-authored-by: Kiro <kiro-agent@users.noreply.github.com>
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Pull request overview
This PR prevents #[deny(unused_crate_dependencies)] from failing #[no_std] crates under Kani by marking Kani’s injected std extern as nounused, which is intended for compiler-injected externs and preserves the lint’s usefulness for real crate dependencies.
Changes:
- Add the
nounusedmodifier to Kani’s injected--extern ...:std=...argument. - Add a new
cargo-uiregression test exercising ano_stdcrate that deniesunused_crate_dependenciesand verifies successfully.
Reviewed changes
Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
| tests/cargo-ui/no-std-strict-lints/src/lib.rs | New no_std crate that denies unused_crate_dependencies and includes a Kani proof harness. |
| tests/cargo-ui/no-std-strict-lints/expected | Expects successful verification output for the new regression test. |
| tests/cargo-ui/no-std-strict-lints/Cargo.toml | Minimal Cargo manifest enabling cfg(kani) check-cfg linting for the test crate. |
| kani-driver/src/call_single_file.rs | Updates injected std extern wrapper to include nounused alongside noprelude. |
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feliperodri
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Jul 29, 2026
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Description
Kani unconditionally passes
--extern noprelude:std=<kani libstd>so that code referencingstdgets Kani's overlay. A#[no_std]crate never referencesstd, so if it alsodenysunused_crate_dependencies(as strict-lint crates like zerocopy do), the injected extern trips the lint and the crate fails to build under Kani through no fault of its own dependency declarations:Add rustc's
nounusedextern modifier to the injectedstd(alongside the already-usednoprelude); it exists exactly for compiler-injected externs and keeps the lint fully functional for the crate's real dependencies.Found while evaluating
kani autoharnesson the top-100 crates.io crates (and while validating #4682, with which this composes: together, a strict-lintno_stdcrate verifies out of the box).Testing
New cargo-ui test
no-std-strict-lints: ano_stdcrate withdeny(unused_crate_dependencies)and a proof harness now verifies successfully (previously: hard error). Fullcargo-uisuite passes (30 tests).By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.