And something like SQLite shows that you can get 100% coverage (as in, every single statement is hit in the tests). That's 100%.
sqlite is bad example, for example features like the FTS tokenizer code (https://research.checkpoint.com/2019/select-code_execution-f...), which were probably paid by commercial requests or sponsors.
That is not to say sqlite does not have 100% coverage, it of course does, but you are building on top of infrastructure which is really really hard to be kept safe.
Rust somehow makes it harder to make certain kinds of bugs, which comes at its own cost.
PS: I don't like or dislike rust, just I have seen the sqlite example being brought a lot, and a lot of people didn't expect sqlite file to be an attack vector :) so nobody is even looking there.