Allocators are not the only source of memory-safety bugs, and RAII can't fix all problems. In particular, aliasing probably caused most of the safety problems Chrome has had to deal with (I admit that I haven't gone and counted them). This is especially true for use-after-free bugs, where some part of the code has a pointer to some memory that has been deallocated; there's not much way for that to happen without aliasing. In Rust, memory can only be deallocated if there are no outstanding references to it. That can be determined at compile time (specifically by the borrow checker), or at run time using reference counter.

Unit tests, assertions, and sanitizers are nice, but they demonstrably don't work; Chrome and Firefox use all three. They have hundreds of thousands or millions of tests, assertions on every other line, and all kinds of compile-time sanitizers but they still have hundreds of memory safety problems a year.