Rusts type system requires that there only ever is one mutable reference to a value or one or more shared references. What happens when you need multiple references to some value, but also need to mutate through them? We use a trick called interor mutability: to the outside world you act like a value is immutable so multiple references are allowed. But internally the type is actually mutable. All types that provide interior mutability have an UnsafeCell at their core. UnsafeCell is the only primitive that allows multiple mutable pointers to its interior, without violating aliasing rules. The only way to use it safely is to only mutate the wrapped value when there are no other readers. No, the garantee has to be even stronger: we can not mutate it and can not create a mutable reference to the wrapped value while there are shared references to its value. Both the book and the std::cell module give a good alternative explanation of interor mutability. What are some patterns that have been developed to use interior mutability safely? How do multithreaded synchronization primitives that provide interior mutability follow similar principles?