[ghoulspa]

The Rust implementation of the hasher doesn't respect SCCACHE_BASEDIRS currently. This PR adds support for the basedirs during the hashing process, by stripping the basedir from the working directory, the arguments, the CARGO_* variables and the env-dep values.

Not respecting basedirs causes issues when using crates that use OUT_DIR, such as serde, which miss the cache in every new checkout. Path dependency crates also miss the cache because of CARGO_MANIFEST_DIR and the working directory, and every crate that depends on them misses as well, so this fixes that too. This also fixes env-dep values by unescaping them, since rustc doubles their backslashes and the strip wouldn't match otherwise.

This change to the env-dep parser may cause new misses on Windows, once per crate, for anyone whose env-deps contain backslashes, such as serde's OUT_DIR. For all other cache keys there will be no changes, except where you use the new basedirs support.

We also added unit tests which check that the hash keys match when basedirs are in use and differ when they aren't, plus a few edge cases, and an integration test that builds with cargo from two checkouts.

Fixes #2870

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.86099% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.22%. Comparing base (78c418b) to head (994db6a).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/sccache_cargo.rs	92.94%	6 Missing ⚠️
src/compiler/rust.rs	99.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2918      +/-   ##
==========================================
+ Coverage   78.15%   78.22%   +0.07%     
==========================================
  Files          74       74              
  Lines       43324    43775     +451     
==========================================
+ Hits        33861    34245     +384     
- Misses       9463     9530      +67     

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