Cargo cross workspace cache

[tomassedovic]

Metadata
Point of contact	@ranger-ross
Team(s)	cargo
Goal document	2026/cargo-cross-workspace-cache

Summary

Work towards a build cache that is shared across workspaces to build times and reduce disk usage.

Tasks and status

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Note: we have updated the body to match the 2026 goal. Your original text is preserved below.

Details

Metadata
Point of contact	@ranger-ross
Team(s)	cargo
Goal document	2026/cargo-cross-workspace-cache

Summary

Work towards a build cache that is shared across workspaces to build times and reduce disk usage.

Tasks and status

[tomassedovic]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[ranger-ross]

Update May 28, 2026

The last week for me was spent at Rust Week and the all hands.
There was a lot of good discussion about the possibilities for how the cache may work and usecases.

We are continuing the effort to stabilize the new build-dir layout in Cargo as that is a requirement for the cross workspace cache.
The stabilization PR (rust-lang/cargo#16807) has been posted and FCPed, and it is just waiting on the rust-lang/rust PR (rust-lang/rust#155439) to be merged to add support for the new layout to the compiler. I've been working through the issues and I believe we are quiet close to merging, just need to fix the remaining CI failures.

[ranger-ross]

Update July 28, 2026

The new build-dir layout stabilization was merged in rust-lang/cargo#16807 but later rolled back in rust-lang/cargo#17187 due to build failures discovered in some projects on nightly.

The build failure stemmed from increased arguments (primary -L) being passed to rustc from Cargo hitting OS limits. Also a similar failure mode where the dylib search path (LD_LIBRARY_PATH on Linux and PATH on Windows) would get too long and hit OS limits.
We then merged a series of fixes rust-lang/cargo#17191, rust-lang/cargo#17236, rust-lang/cargo#17168 to reduce the paths being passes to rustc and added to the search.

In addition to the fixes in Cargo, we also added arg file support to sccache in mozilla/sccache#2782 which is a work around for OS command length limits that is already supported in Cargo+rustc.
With the increase in likelihood of a project getting large enough to need an arg file this was a preemptive optimize to minimize breakage on stabilization.

With the fixes above, we are feeling better about the new layout and plan to enable it by default on nightly. (rust-lang/cargo#17258)
Instead of stabilizing the new layout immediately, we are opting to enable it on nightly by default to gather feedback and find the places we can improve before committing to stabilization.

If all goes well, you should see the new build-dir layout on nightly in the near future