> Well you are saying that even in C you can use the restrict keyword to tell the compiler that 2 memory locations can overlap. Of course is in the hand of the programmer to tell the compiler to do so.

It says that they can't overlap, but yes, you can get the compiler to optimize based on this if you provide the aliasing information and remember to keep it accurate. You probably won't do that, though, for anything except the most performance-critical of inner loops. A compiler that can infer more about aliasing can provide more optimization, safely, in the >99% of code that doesn't have explicit aliasing annotations, and that's probably worth some decent speedups in practice.

There are two main things you might be talking about when you call a programming language fast:

1. Given some really performance-critical code and enough time to hand-optimize it, how close can you get the compiler's output to the optimal machine code?

2. If you write code normally, not making any effort to micro-optimize, how fast will it usually be?

Both of these matter. #1 matters when you've got some bottlenecks that you really care about, and #2 matters when you've got a flatter profile -- and both situations are common.

Another illustrative example of the #2 type of performance with Rust is the default collections compared to the ones in C++. Rust's default HashMap is faster than C++'s std::unordered_map because of some ill-advised API constraints in the C++ STL. You can get similar performance by using a custom C++ hash table implementation, and in fact Rust's HashMap is a port of a C++ hash table that Google wrote for that purpose, but most people probably won't bother.

So, a semantic question: if you can get the same speed in one language as you can in another, but in practice usually don't, is one language faster than the other?