Source-based coverage doesn't support branch coverage

[ErikBjare]

Hi there, nice job with grcov!

We've been having some issues with coverage for our project (probably not grcov's fault), and when I was working on fixing it I discovered the new source-based coverage, which looked like the way forward (given you use it youself in your CI). So I adopted it, but noticed branch coverage was 0/0. (the PR in question: ActivityWatch/aw-server-rust#192)

Is this planned/intended? From what I've read gcov's source-based coverage supports branches just fine. (https://clang.llvm.org/docs/SourceBasedCodeCoverage.html)

Thanks for your work! ❤️

Edit: After reading the rust unstable book on source-based code coverage it seems "branch coverage" is superseded by "region coverage". Is this a correct interpretation? Is "branch coverage" redundant in a "region coverage" paradigm?

[marco-c]

Support for branch coverage in LLVM and Clang is hopefully going to be added soon, https://reviews.llvm.org/D84467. I think it will need porting to Rust before it can be used.

In the meantime, it might be possible to extrapolate branch coverage from region coverage, even though branch coverage would offer additional information (e.g. how often a given subcondition in an expression was hit) that region coverage can't offer by itself.
I'm not sure if it's worth working on that though, maybe it's better to just wait on proper support.

[marco-c]

rust-lang/rust#79649