Generate rust coverage data

[torymur]

Closes #33

In order to retain our python coverage and diff-cover tool chain we're:

• generating code coverage file in lcov format with cargo-tarpaulin
• creating python coverage file in lcov instead of xml
• combining these coverage artifacts with lcov into one
• diff-cover seems to be able to handle combined lcov file

genhtml tool is used instead of coverage html to create html from combined file, which will create a bit different UI, but nothing major seems to change.

From something like this:

To this:

Also changed in PR:

• actions-rs action is archived, dtolnay/rust-toolchain now used
• improved caching:
  • for binaries
  • and in general Swatinem/rust-cache currently is the best way to handle rust cache workflows, it's a "smart caching for rust/cargo projects with sensible defaults"

[brandonwillard]

This sounds great! Let's add a temporary commit to make sure that it reports missing Rust and Python coverage.

[torymur]

Alright, I see good cache hits & usage.

And testing commit failed as expected: https://github.com/dottxt-ai/outlines-core/actions/runs/11594946487/job/32282843486

Overall coverage rate:
  lines......: 35.3% (287 of 812 lines)
  functions..: 28.4% (33 of 116 functions)
Failure. Coverage is below 100%.
-------------
Diff Coverage
Diff: origin/main...HEAD, staged and unstaged changes
-------------
src/json_schema/helpers.rs (0.0%): Missing lines [43](https://github.com/dottxt-ai/outlines-core/actions/runs/11594946487/job/32282843486#step:9:44),47,49-53,55-58
-------------
Total:   11 lines
Missing: 11 lines
Coverage: 0%
-------------

Generated html & uploaded it:
Artifact download URL: https://github.com/dottxt-ai/outlines-core/actions/runs/11594946487/artifacts/2123365492

Seems, we're good to go 🚀

The only issue I see with this, is that we're adding a strong necessity to add potentially vain and unnecessary tests (which exist) just for the sake of coverage, which isn't always justified, considering huge compiler support in Rust, which will check so many things and complain on its own. At least tarpaulin allows to exclude some pieces explicitly. But, let's give it a try and see how it's going to be in action.

I would appreciate a review 🙏 @brandonwillard ?

[brandonwillard]

Looks great! @torymur, feel free to merge it after removing the two test commits.

The only issue I see with this, is that we're adding a strong necessity to add potentially vain and unnecessary tests (which exist) just for the sake of coverage, which isn't always justified, considering huge compiler support in Rust, which will check so many things and complain on its own. At least tarpaulin allows to exclude some pieces explicitly. But, let's give it a try and see how it's going to be in action.

Yeah, as you said, we'll need to add some coverage ignore directives; otherwise, we'll leave it up to the admins to ignore those failures and merge anyway.