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I'm a PhD student at EPFL working in pushing the boundaries of software and systems security
It may be true that this holds for issues related with out-of-bounds accesses or temporal safety errors, but nothing eludes your application to contain logic bugs. In fact, whenever the application is big enough and you spot a logic error, the debugging burden is still there. Good part is that the debugging utilities for C/C++ are still helpful for Rust programs.
Simple, testing won't prevent you from all the bugs bun only the ones you run over when testing (either fuzzing or unit testing). While enforcing some safety semantics like they do here through capabilities, does. This means, at least for using CHERI you have to go through a custom compilation stack, the CHERI team has already been working on this tooling for long.
Birdtray extension (https://github.com/gyunaev/birdtray) does that work quite well, I'm so happy with it. It also lets you keep Thunderbird as a background process.