Provide cargo clean --except-locked to keep target/ small in re-used CI caches

[LeoniePhiline]

Problem

When caching target/ in CI, the cache is uploaded and downloaded frequently and must remain manageable in size.

Whenever dependencies have been updated, e.g. by renovate-bot, new versions of crates are compiled and cached in target/deps/ while old versions are never removed.

cargo clean removes all build artifacts, including those of current dependencies - which is not desirable.

Proposed Solution

cargo clean --except-locked would inspect the Cargo.lock file of the project or workspace and keep all build artifacts of currently locked dependency versions. All other artifacts are deleted, as they would be with cargo clean.

Notes

Arguably cargo clean should exactly do that (clean up, not delete everything), while its current behavior can be easily done by hand with a rm -rf ./target.

I assume, however, that it is too late to change the default behavior at this point.

[ehuss]

Thanks for the suggestion! We are currently tracking this in #7150. Cargo currently doesn't have a way of knowing which artifacts exist for which purpose. That issue contains some more information and links to similar issues.

Closing as a duplicate of #7150.