For what it's worth, this alternate implementation has only one line of unsafe code (a call to the libc "memchr" function) and is only 9% slower than the fastest unsafe version:

https://github.com/mbrubeck/benchmarksgame-rs/blob/reverse_c...

It's very easy to write extremely fast safe Rust code. (The safe Rust version above is faster than the fastest C++ submission, on my computer.) Using "unsafe" for optimization is usually only helpful to get a few extra percent speedup in an inner loop. If this were production code rather than the benchmarks game, I'd probably ship the safe version.