The nightlies are built with optimizations. And trust me, nobody's going to be distributing Rust binaries that weren't built with optimizations because Rust code built without optimizations has performance on par with Ruby rather than C++ (LLVM's optimization passes do a hell of a good job).
Ah sorry, my mistake. I think I had optimized builds confused with "ndebug" builds. But it sounds like only ndebug builds disable overflow checks, and Rust nightlies don't use ndebug.
