Test-drive grcov for coverage reports

[garious]

@marco-c advises grcov over llvm-cov for coverage reports. Test-drive it. Compare and contrast.

[garious]

I've not been able to get grcov or llvm-cov to report coverage from the coverage files output by cargo-cov. Rust doesn't ship with llvm-cov and so I manually installed it via llvm-7, which is what Rust depends on. But still it seems only cargo-cov report knows how to read the files cargo-cov test generates.

[marco-c]

That's strange, cargo-cov should simply use -Zprofile, which should be supported by grcov. We are generating coverage reports for grcov itself using -Zprofile (but without cargo-cov).

Mind sending me the gcno/gcda files that you generate, so I can see if grcov is somehow failing to parse them?

[garious]

Got it working now @marco-c, thanks for the push. The run when I used grcov, I mistakenly ran cargo cov test --lib instead of cargo +nightly test --lib. I'm seeing the same output from your released macOS binary as I do when building it from source (on homebrew's latest llvm package). If I run into any trouble, I'm now in position to send you a PR. :)

[garious]

@rob-solana, @mvines, @marco-c, code coverage status:

• cargo +nightly cov test --lib && cargo +nightly cov report works well with on MacOS and Linux with minimal installation dependencies. It's just a little odd that it requires clang's libprofile to be installed. Not sure why that wouldn't be distributed with Rust.
• grcov and lcov works great on MacOS but not on Linux. Their Linux build is using Trusty whereas we run on Stretch, so surprised to see an issue there.
• I upload both the cargo-cov and lcov-via-grcov HTML reports to buildkite and we can see that grcov us unable to generate a report using the same gcno files that cargo-cov is using (and generated).
• codecov.io can consume the lcov.info file that grcov outputs, so as soon as that's working, we should have codecov.io and those nice coverage PR comments back again.

[rob-solana]

so good...

test driving it on WSL

[marco-c]

I upload both the cargo-cov and lcov-via-grcov HTML reports to buildkite and we can see that grcov us unable to generate a report using the same gcno files that cargo-cov is using (and generated).

If you can share the gcnos with me, I can take a look at why grcov is unable to parse them.