You have lots of options for non-borrowing references to instances of T: Indicies, Arc<T>, RefCell<T>, Cell<T>, dictionary keys, paths to files containing seri

You have lots of options for non-borrowing references to instances of T: Indicies, Arc<T>, RefCell<T>, Cell<T>, dictionary keys, paths to files containing serialized Ts... the list is unbounded. This is all perfectly fine: It's not the goal of the borrow checker to prevent simultaneous references to data, that'd be ridiculous and useless. The goal is to prevent unsafe simultaneous access of data. And accessing data through an index still requires you to first acquire e.g. a borrow, and the useful invariants of that borrow still hold: for the duration of that borrow, you have freedom from data races - there will be no other borrows to that instance when you're holding a &mut T, or there will be no other mutable borrows to that instance when you're holding a &T. An index may allow you to reduce the scope and lifetimes of your borrows, but it does not allow you to eliminate your borrows.