One example of MIRI not being guaranteed to handle all cases. https://github.com...

ralfj · 2025-07-10T07:00:07 1752130807

Yeah, concurrency bugs that only occur in very specific situations are hard to track down with a pure testing tool. However, we have some ongoing work that should make Miri a lot better at this... we are just not sure yet whether we can get it to have usable performance. ;)

hamcocar · 2025-07-10T07:29:46 1752132586

>Yeah, concurrency bugs that only occur in very specific situations are hard to track down with a pure testing tool.

It would have been better to have prevented it in the first place, and it is inconsistent with "fearless concurrency" when the Rust stdlib has UB.

And there are categories of cases that Miri does not handle either, FFI being a clear example as far as I know.

ralfj · 2025-07-10T09:26:28 1752139588

> it is inconsistent with "fearless concurrency" when the Rust stdlib has UB.

It is not. "fearless X" only applies to safe code. Rust was always very clear about that. It turns out that in practice, if >90% of your code are "fearless", that actually significantly reduces the amount of bugs you have to worry about, even if the remaining 10% can still have UB. (I don't have hard numbers on how much code is unsafe, AFAIK it is much less than 10%.)

But yeah, Miri cannot find all UB bugs. We are also very clear about that. But you don't have to find all UB bugs to make progress in this space compared to the status quo in C/C++.