Code Coverage

[JonathanWoollett-Light]

Recently we have encountered issues with code coverage, specifically cargo-kcov and kcov, this resulted in segmentation faults and inaccurate coverage on #3105 and #3076.
This lead us down 2 different paths:

1. Adjusting our usage of kcov Fix cargo kcov crashing/generating incorrect reports for proc-macros #3207
2. Switching to grcov Update code coverage #3173

Both of these still have issues, notably with support for proc-macros and doctests.

A possible alternative may be cargo-llvm-cov although this has some notable surface level issues:

1. It is a relatively young project.
2. Doc test performance is very slow¹.

A couple other issues have been open for a while about coverage:

• Run rustdoc tests #3206
• [ci] Improved coverage ci needed #3100
• Expose kcov run results as HTML #1301
  This would lead to a discussion worth having about the correct path forward here, be this kcov, grcov, cargo-llvm-cov or some other alternative.

Footnotes

1. In CPUID (2/3): Bit fields #3105 env RUSTFLAGS="-C instrument-coverage" RUSTDOCFLAGS="-C instrument-coverage -Z unstable-options --persist-doctests ../../target/debug/doctestbins" LLVM_PROFILE_FILE="coverage-%p-%m.profraw" cargo +nightly test requires 0.58s while cargo +nightly llvm-cov --html --doctests requires 20.55s. ↩