Rust nightly experiment disables embedded metadata to reduce target directory size by up to 33%

The Cargo team is testing a change that stops embedding duplicate crate metadata in .rlib files on Rust nightly. This metadata is already present in .rmeta files, so omitting it from .rlib files reduces redundancy. The experiment enables the -Zembed-metadata=no flag by default, which was previously opt-in. No user migration is required, but engineers must monitor for tooling or workflow issues.

Disk space savings vary by build configuration. In release builds without incremental compilation or debug info, the target directory shrinks by nearly 30%. Debug builds with incremental compilation see smaller gains, around 5-8%, because debug symbols and incremental artifacts dominate the directory size. The largest absolute reduction observed was nearly 300 MiB for Cargo itself, a 33% decrease.

The change is experimental and not guaranteed to stabilize. Nightly users are expected to report issues or opt out if needed. Tools or scripts that rely on embedded metadata in .rlib files may break, though no such cases have been identified yet. The experiment’s goal is to assess real-world viability before considering stabilization.

Engineers using nightly Rust should expect no performance impact, as the metadata is still available in .rmeta files. The trade-off is purely disk space versus compatibility. If the experiment succeeds, the change could reduce storage overhead for all Rust users, particularly in CI environments where target directories are frequently rebuilt.