Unlike C++, Rust doesn’t allow mutable memory aliasing. That’s because mutable aliasing can never happen in Safe Rust, and not supporting it improves performance. This means that when writing Unsafe Rust, one has to be careful about aliasing.
Note though that it’s perfectly fine to have multiple mutable raw pointers pointing to the same data, as long as there’s no ownership held by any Rust code. The problem only happens if you try to convert them into references.
It seems I misunderstood something important here. I’d take that as proof that Unsafe Rust is rarely needed. 😜 A quick test on the Playground shows that indeed, using raw pointers does not yield the wrong result, while using references does: https://play.rust-lang.org/?version=stable&mode=release&edition=2021&gist=96f80d43d71a73018f23705d74b7e21d
Conclusion: Unsafe Rust is not as difficult as I thought.
I left something important out from my explanation. Your example still holds ownership of the data, so that’s where the rules are violated with those raw pointers. You have to use Box::into_raw or something similar to disassociate the data from the Rust compiler. Then you can alias it using raw pointers.
