[cesarb]

> It's anagolous to `unreachable` in Rust. [...] It doesn't seem like .Net is implementing it correctly either. It is supposed to be optimized away in release code, resulting in UB if the developer was wrong about the condition.

Rust actually has two things called "unreachable". There's the unreachable!() macro (https://doc.rust-lang.org/core/macro.unreachable.html), which is a short-hand for panic!(), and therefore is never UB even if somehow it's reached; and there's the unsafe unreachable_unchecked() compiler hint (https://doc.rust-lang.org/core/hint/fn.unreachable_unchecked...), which is optimized away in release code, and is always UB if somehow it's reached. Most of the time, you should prefer the safer unreachable()! macro; the unsafe hint is for when it makes a performance difference (and as with every use of unsafe in Rust, you really should carefully review the code to make sure it's really unreachable).