Cobertura coverage gutter

[dustinlagoy]

This adds a neat feature to display a coverage gutter in the current document, similar to the look of the diff gutter. It works by reading cobertura coverage information from a file set by the HELIX_COVERAGE_FILE environment variable, checking if the current document was contained in the coverage file and checking that the coverage file was modified more recently than the document. The coverage is only displayed if all these checks pass.

A screenshot showing some valid coverage, generated with RUSTFLAGS="-C instrument-coverage", LLVM, and lcov_cobertura:

If there is any interest in merging this, here are a few things that could be improved:

• Cache the loaded coverage information so it is not re-read every time the gutter is updated
• Add support for methods and branches (need to think about how to display this)
• Use a coverage-specific style instead of borrowing from the diff gutter style
• Better path handling if the coverage file has relative paths that are not relative to the current directory when running helix
• Support more coverage formats than just cobertura

[the-mikedavis]

Putting coverage in the gutter is a cool idea but this seems to me like something that should live as a plugin rather than a core feature. I believe there are a bunch of different file formats for coverage information and I wouldn't want to support multiple in core.

[dustinlagoy]

@the-mikedavis I agree this seems better as a plugin overall. If you would rather not have any part of it implemented in the core that's fine by me and I can write the plugin once that is an option. Do you think any of the proposed plugin options could handle custom gutters like this?

[kirawi]

#8675 should be able to, yeah

[CraigglesO]

I LOVE this. I hope you can find time to make it a plugin when that system becomes available. I use cobertura all the time, so this would help a lot.