This is insightful. That is exactly what is happening, but our sense is that this language will be no more complicated than C itself, and will to a large extent parallel it.
The question is really what is more effective, working under one set of constraints imposed universally (like the ownership semantics in Rust), or taking upon yourself the responsibility of defining such constraints within each program. In this way what we're doing is an application of the "trust the programmer" design philosophy; trust him, not only to know what he's doing, but to be able to express it (in the interface specification).
> Sure, in a three-line function, it's kind of obvious (if you know to be aware of this possibility!), but in a function where the pointer and the use are separated by hundreds of lines of code, it's easy to miss that there is a requirement that the hash table not be resized between the two calls. With Rust, the fact that &mut for V means that I can't touch the hash table until I'm finished with V makes it a compiler error.
It can seem as though the example we provide is contrived (because of course it is), but it is not mere contrivance. The point we stress the most is not that the bug is obvious if there are only three lines, but that the presence or absence of a bug is impossible to see even with a single function call whose interface is informal. So what is remarkable is not that we can write three lines of code that have or do not have an obvious error, but that by the mere fact of the interfaces being informal, a three-lined function can be impossible to analyse by itself.
> The ability to mutate an object necessarily implies the ability to mutate the state of an object to break guarantees. While there are definitely patterns where multiple mutable references to the same object can be safe, it is not inherently safe, and proving safety is far more challenging because you can't as easily rely on pre/postconditions for lemmas.
This doesn't answer the question. "it is not inherently safe" does not mean "it is inherently unsafe", it only means "it may be unsafe". So our point is that Rust precludes many kinds of programming that can be safe in relevant contexts, and this is what is so limiting.