`std::hint::unreachable_unchecked`: docs note about compiler optimizations being sufficient?

[sharnoff]

I don't know whether this is something that the docs team already has a particular policy on, so perhaps this is out of scope, but:

The docs for std::hint::unreachable_unchecked give an example that -- with optimizations -- compiles to the same assembly using plain unwrap instead of unwrap_or_else(|| unsafe { unreachable_unchecked }). (See: https://godbolt.org/z/4ha9K1rGK)

Given that this is likely often the case for simple inputs (particularly for arithemtic operations like the example), I was wondering if it would be good to have some kind of note in the documentation to advising that this function should only be used if it's actually necessary. In particular, this could note that rustc with -C opt-level=1 will produce the same assembly for the example as a version that uses unwrap instead.

Of course several caveats apply (it's not ideal to rely on compiler optimizations in docs, etc) - but IMO it could be worthwhile for the purpose of discouraging unecessary unsafe.

[leonardo-m]

Also the example could be replaced with a better one where unreachable_unchecked gives an advantage.