There is obviously the benefit that this generally only affects unsafe code (as opposed to C/C++ where the scope is less clear), but Rust just passes the buck to LLVM and inherits the partially baked solutions being used in practice for C++. And if Rust ever changes this, it will involve either splitting the language into distinct versions or breaking strict backwards compatibility.
> But there is much, much more to a large programming language like Rust than those things, and Rust is missing documentation (of the sort that intends to be complete and correct) in many important areas that don't have that sort of excuse.
As somebody who has contributed to both rustc and LLVM/Clang (and would probably rather program in Rust than C++ at this point), I think it would be much easier to implement C++ from scratch than Rust for this reason. The C++ spec is incomplete and some portions are internally inconsistent, but at least there's a real spec. When you hit one of these problems you can just look at what other implementations do in practice. Rust doesn't even have anything resembling a spec.