But to your original point, its true that a multiplexing server should probably try to stay up even if there is a bug uncovered in handling a request. But that's precisely why rust allows you to catch panics.
https://doc.rust-lang.org/std/panic/fn.catch_unwind.html
Edit. Re. already existing runtime checks, I'd expect a decent compiler to optimize away redundant checks, i.e. explicitly coded checks vs. checks injected automatically by the compiler, as long as their semantics is identical.
> The Result type is more appropriate to use for *functions that can fail on a regular basis.*
Emphasis mine. The DFA::is_match routine can never fail on any input. Yet, you want to use a 'Result' with it.
Seriously. Please please pretty please read the blog post. You are literally caught in precisely the tangled mess that I tried to untangle in the post. If my post didn't do it for you, then please consider responding to specific points you disagree with in the post.
Edit. For technical geeking around:
* The point is about redundant check elimination, not full check elimination. A decent compiler should be able to eliminate redundant checks, though perhaps the language could offer some intrinsics (was: pragmas) to make this 100% reliable.
* For DFA proofs, perhaps something like https://courses.grainger.illinois.edu/cs373/fa2013/Lectures/... could be a start. A great intern may tackle it over a summer ;)