That point would be stronger if Rust were showing up as faster than C compilers on this axis, and it isn't. In point of fact aliasing rules (which is what C calls the problem you're talking about) are very well travelled ground and the benefits and risks of -fno-strict-aliasing are well understood. Static analysis tools are in fact extremely good at this particular area of the problem. And of course at the level of emerging technologies, LTO makes this largely a non-issue because the compiler is able to see the aliasing opportunities at the level of global analysis. It doesn't need the programmer or the compiler to promise it an access is unaliased, it can just check and see.
>>ajross+(OP)
It’s not a problem, because compilers simply declare it as UB and don’t care about it. If you by accident did depend on that, your program will just get a nice, hard to debug bug.