Sorry, I should have been clearer about that. You are completely right that at some point things have to rely on some primitives that are 'magic' (in this case

Sorry, I should have been clearer about that. You are completely right that at some point things have to rely on some primitives that are 'magic' (in this case UnsafeCell). But I meant that RefCell itself uses standard constructs to guarantee the 'single mutable xor multiple immutable' invariant.