PR testing: add a coverage report for Rust tests

[jrose-signal]

This is using the still-experimental -Zinstrument-coverage, so it may not be perfect, but hopefully it gives us a starting point.

[jrose-signal]

…well, okay, it would have worked if we had a working llvm-profdata (which you can get through rustup) and llvm-cov (which you currently can't). I suppose we could pin to a particular LLVM version?

[jack-signal]

Do the llvm-cov and Rust's LLVM version need to strictly match? I thought llvm-cov was just using gcov format which is pretty standard iirc. If not, then we can just use llvm-cov from the Ubuntu llvm package.

[jrose-signal]

llvm-cov is needed to go from profdata to gcov; the way I've been setting it up has been skipping the gcov step. As far as I know profdata is a fairly stable format but the LLVM in Ubuntu's default package repository is super old.

[jack-signal]

LLVM in Ubuntu's default package repository is super old.

I was confused by this statement since Ubuntu 20.04 has LLVM 10, and LLVM 11 was only released this past summer. But then I see that "ubuntu-latest" image is ... not the latest (https://github.com/actions/virtual-environments) and we need to request "ubuntu-20.04" instead.

[jrose-signal]

Hm, I'll try Ubuntu 20.04, but maybe I'm wrong about the cause instead—maybe profdata isn't as stable as I hoped. It's just funny cause this all works on my machine, using Xcode's llvm-profdata and llvm-cov and Rust's -Zinstrument-coverage. That's what had me assuming it was a format problem.