Add support for caching target json specs for Rust compilation.

[ColinFinck]

This has been implemented in a backward-compatible way:

• --target arguments specifying target names are still passed as arguments just like before.
• --target arguments specifying a json file path have previously not been cached at all. The json file is now hashed like other external file dependencies.

Fixes #2174

[sylvestre]

Could you please add a test here to make sure we don't regress?
https://github.com/mozilla/sccache/blob/main/.github/workflows/integration-tests.yml

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (0cc0c62) to head (c61187f).
Report is 129 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #2269       +/-   ##
==========================================
- Coverage   30.91%       0   -30.92%     
==========================================
  Files          53       0       -53     
  Lines       20112       0    -20112     
  Branches     9755       0     -9755     
==========================================
- Hits         6217       0     -6217     
+ Misses       7922       0     -7922     
+ Partials     5973       0     -5973     

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[ColinFinck]

Hey @sylvestre, thanks a lot for your quick response! I've checked the integration-tests.yml file you gave me, but it only contains a single test for each platform/compiler, not for individual flags of the Rust compiler. I'm willing to add a test for the newly supported flag, but I need a good example on how sccache currently does per-flag tests.

[sylvestre]

see https://github.com/mozilla/sccache/blob/main/tests/system.rs for example

[ColinFinck]

@sylvestre I've rebased to latest main and added a test in 616dd8c

[ColinFinck]

Bump

[sylvestre]

sorry, i missed your comment. CI started!

[ColinFinck]

@sylvestre Thanks for getting back to this PR. However, I'm still seeing "2 workflows awaiting approval" in GitHub.

[sylvestre]

done
could you please add a test in
https://github.com/mozilla/sccache/blob/main/tests/system.rs
too ?

[ColinFinck]

could you please add a test in
https://github.com/mozilla/sccache/blob/main/tests/system.rs
too ?

@sylvestre There is currently not a single test in that file which exercises the Rust compiler. This is only done in src/compiler/rust.rs. Which is why I added my test there.
Multiple C compilers are tested in tests/system.rs, but not rustc. Adding a test framework for the Rust compiler to that file exceeds the scope of this PR and goes beyond my limited knowledge of sccache.

I hope we're able to merge the few lines of this PR without requiring an entirely new test framework for the Rust compiler first.

[sylvestre]

@sylvestre There is currently not a single test in that file which exercises the Rust compiler.

@ColinFinck we have proper Rustc testing:

See:
https://github.com/mozilla/sccache/blob/main/tests/sccache_cargo.rs
and
https://github.com/mozilla/sccache/blob/main/tests/sccache_rustc.rs

[ColinFinck]

@sylvestre There is currently not a single test in that file which exercises the Rust compiler.

@ColinFinck we have proper Rustc testing:

See: https://github.com/mozilla/sccache/blob/main/tests/sccache_cargo.rs and https://github.com/mozilla/sccache/blob/main/tests/sccache_rustc.rs

@sylvestre These are different files. I already had a look at them, but the first one just contains very high-level tests of running cargo and checking for cache hits and misses. The second file tests against a mock rustc and not the real one. None of these files are prepared for adding a low-level test of validating a single parameter (--target) passed to a real rustc compiler.

Which is why I added a test for --target to src/compiler/rust.rs. This is as much as I can do given the current state of the sccache testing framework.