Comment by Arnavion

Stacked borrows is miri's runtime model. Run it under miri and you will see the error reported for the `*x = 10;` version but not the `write(x);` version - "Undefined Behavior: attempting a write access using [...] but that tag does not exist in the borrow stack for this location".

rustc itself has no reason to reject either version, because y is a *mut and thus has no borrow/lifetime relation to the &mut that x is, from a compile-time/typesystem perspective.