There technically is an "unsafe" mode, but not in the way you're thinking. You can drop down to C level by writing code in unsafe blocks and using methods prefixed with "unsafe." This will stop the compiler from doing things like inserting bounds checking and should perform fairly close to Clang.

Generally though, rust strives for the same ideal as C++: you only get what you pay for. You can carve out the subsection of the language you need through things like feature gating without too much trouble right now (and the standard library is still improving in this regard).