Maybe sqlite3: https://www.sqlite.org/testing.html.

I considered mentioning it, but to my previous understanding sqlite measures coverage at the assembly level and not at the C level. Skimming the link, it seems I was wrong, so thanks, TIL. My previous conception doesn't make much sense, now that I think of it.

Ironically, that's a fantastic example of just how unachievable full test coverage actually is in most projects.

> By comparison, the project has 590 times as much test code [as program code].