That’s right. And forget about the existing rustc compiler implement. If you have something in Rust like

  let a: HashMap = immutable_map.iter().map(…);

then you can infer from the semantics that the ordering doesn’t matter and whether it can be parallelized. C doesn’t have the ability to express what you want to happen, just how to do it. That gives Rust far more opportunity for optimization than C possibly can have.