>That said, these overheads stem from Rust's safety guarantees rather than its strong type system: you can have a language with a strong type system that does not do these checks.
The difference in semantics between a `&mut T` and a `*mut T` is a type system one. `&mut T` requires that two do not exist at the same time, regardless of whether they are used at the same time or not; this is the contract of the type.
>Furthermore, there are of course ways to avoid this overhead within safe Rust: if you can use the type system to prove that the cell cannot be borrowed at the same time, then you don't need to do the checks, and in that sense a strong type system can actually help avoid overheads that were introduced by being a safe language.
Correct, which is why I made the effort of pointing out that rustc is not smart enough to do it, not that it's impossible to do it.
This may be splitting hairs a bit, because we all agree that this is a good example where using Rust in this straightforward manner leads to suboptimal performance. But I agree with the grand parent that this is mainly an issue with safety, not with the type system itself.
To show why, consider two alternative languages.
“Weak Rust”: an equally safe Rust with a weaker type system. It might not distinguish & and &mut, but it would still need those checks, because you might use those shared references to break a data structure invariant. It would have to detect such unsafe usage at runtime and raise the equivalent of Java’s ConcurrentModificationException.
“Unsafe Rust”: a less safe Rust with an equally strong type system. It wouldn’t need to do those checks. In fact, that’s basically C++.
The difference in semantics between a `&mut T` and a `*mut T` is a type system one. `&mut T` requires that two do not exist at the same time, regardless of whether they are used at the same time or not; this is the contract of the type.
>Furthermore, there are of course ways to avoid this overhead within safe Rust: if you can use the type system to prove that the cell cannot be borrowed at the same time, then you don't need to do the checks, and in that sense a strong type system can actually help avoid overheads that were introduced by being a safe language.
Correct, which is why I made the effort of pointing out that rustc is not smart enough to do it, not that it's impossible to do it.