[codeandsolder]

Summary

Rust distributed toolchain archives preserve the sysroot's filesystem layout, including absolute paths, but the Rust weak toolchain key currently only incorporates compiler shared-library digests.

As a result, two otherwise-identical Rust toolchains installed at different sysroot paths can share the same weak key even though their packaged toolchain archives are path-sensitive. A persisted dist-client "weak_map" can then select an archive produced from the wrong installation layout.

Include the resolved Rust sysroot path in the distributed-toolchain weak key.

This intentionally does not change the normal compilation cache key, so equivalent toolchains installed at different locations can continue sharing ordinary object-cache entries.

A version/domain marker is also included in the new weak-key derivation so previously persisted mappings are not reused after upgrading.

Why this belongs in sccache

rustup already resolves the active compiler correctly before this point. Cargo also does not own sccache's distributed-toolchain packaging or "weak_map".

The collision happens specifically in sccache's dist-client toolchain identity: the weak key chooses a previously packaged archive, while that archive contains path-sensitive Rust sysroot contents.

Testing

• Added a regression verifying that:
  • the same toolchain contents and sysroot produce the same weak key;
  • identical toolchain contents at different sysroot paths produce different weak keys.
• "cargo +1.99.0 test --locked --no-default-features --lib rust_dist_toolchain_weak_key_tracks_sysroot_path"
• Rust 1.99 through sccache

AI Disclosure: AI developed, human directed and reviewed

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.27%. Comparing base (49a4f5d) to head (9312728).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2900      +/-   ##
==========================================
+ Coverage   78.22%   78.27%   +0.05%     
==========================================
  Files          72       72              
  Lines       42007    42123     +116     
==========================================
+ Hits        32858    32972     +114     
- Misses       9149     9151       +2     

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[sylvestre]

Thanks for your PR