Obviously, there are some cases where it makes sense to use an unsafe block. However, I think there might be fewer cases then people might think.
As an example, both the popular generic self-referencing crates ouroboros and self_cell have had memory safety bugs in them in the past. (links at the end) Both of them were carefully reviewed by experienced rust developers before their first public release, and yet they still ended up with such bugs. Admittedly, part of the issue is that both crates are trying to be more generic, so they have to be correct over a larger range of circumstances.
But still, these crates have one job, are both less than 1500 LOC, and they were carefully reviewed to ensure they did that one job before their public releases, and they still ended up having issues that were not caught. They might still have issues.
Thus, while it might be fine to use unsafe to state that your array of zeros is a valid utf-8 string without a runtime check, it's probably a good idea to twist yourself into a pretzel if the invariants are not trivial to prove and the overhead to maintenance/runtime isn't too high.
As an example, both the popular generic self-referencing crates ouroboros and self_cell have had memory safety bugs in them in the past. (links at the end) Both of them were carefully reviewed by experienced rust developers before their first public release, and yet they still ended up with such bugs. Admittedly, part of the issue is that both crates are trying to be more generic, so they have to be correct over a larger range of circumstances.
But still, these crates have one job, are both less than 1500 LOC, and they were carefully reviewed to ensure they did that one job before their public releases, and they still ended up having issues that were not caught. They might still have issues.
Thus, while it might be fine to use unsafe to state that your array of zeros is a valid utf-8 string without a runtime check, it's probably a good idea to twist yourself into a pretzel if the invariants are not trivial to prove and the overhead to maintenance/runtime isn't too high.
[0]: https://rustsec.org/advisories/RUSTSEC-2023-0042.html [1]: https://rustsec.org/advisories/RUSTSEC-2023-0070.html