The point is that ownership also prevents these errors. If there are never two simultaneous mutable references to something, then you can get the advantages of immutable data structures without paying the performance penalty.

We’re having a semantic debate. Ownership appears to mean something different to Rust folks, which is fine. I agree that Rust’s borrow checker (what you appear to be referring to as ownership or the enforcer thereof) prevents errors and is generally very cool. It’s just not what I understand ownership to mean.