I would guess all the aliasing stuff will get you. In C it's very difficult for the compiler to know whether two pointers are aliased, if we change X maybe Y changes too (because actually X and Y were the same). In Rust if we can write to it then it isn't aliased, and if we can't write to it then nobody can change it, thus changing X definitely can't change Y and the emitted machine code is sometimes simpler as a result, doing what you naively expected rather than what the C needs to do just in case you're crazy and there is an alias.

Now, in modern C you can say you don't have aliasing, but you're probably wrong and so there's a high risk when you do that you now get "impossible" bugs because you swore to the optimiser that if X changes, Y is unaffected, then created a situation where that wasn't true and now your program has no defined meaning, which is going to be tricky to debug. So, on the whole C programmers do not use this, indeed in places like the Linux kernel they even turn off the C standard's very minimal aliasing rules (which forbid aliasing objects of different types), they just don't trust themselves.