Document GitLab CI integration

[jaskij]

rendered view

As discussed in #398 , this documents how to integrate cargo-llvm-cov with GitLab CI.

As of now, there's several things missing to be resolved, but I wanted to open the PR as a starting point. For now, this is a draft.

• test coverage rendering
• splitting the Cobertura report into multiple files to obey GitLab limits
• which coverage in the summary?

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Coverage rendering

This I will do soon, but due to the way GitLab renders only diffed files in MRs/PRs, I did not have an opportunity to test.

Splitting

While it's generally out of scope to describe the whole issue, adding a single sentence mention with a link to GitLab's documentation will probably be the right call.

Which coverage in summary

For now, the regex I'm using will use region coverage - I felt it'd be the most accurate, but I can just as easily extract function or line coverage.

[jaskij]

cc @mike-kfed

[jaskij]

Hrmm... CI tells me I made a spelling mistake, but not where. And I'm not seeing it.

I added that note about report size limits

[mike-kfed]

looks like the markdown lint fails:

README.md:596 MD028/no-blanks-blockquote Blank line inside blockquote
README.md:600:2 MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]

seems like tidy.sh runs all checks no matter what and returns an error code if any of them failed.

[jaskij]

Thanks for pointing that out. I fixed the trailing whitespace, but left the empty line as is - it's there on purpose, and I'm explicitly using GitHub Flavored Markdown here. Which, I'm not sure that I should.

In the meantime, I was working on my project using these coverage reports, and can now confirm - they are displayed correctly, although it is only a line-based report.

Overall, I'm fine with how it is now.