Sometimes a type needs to be mutated while having multiple aliases. In Rust this is achieved using a pattern called interior mutability. A type has interior mutability if its internal state can be changed through a shared reference to it. This goes against the usual requirement that the value pointed to by a shared reference is not mutated.
std::cell::UnsafeCell<T> type is the only allowed way to disable
this requirement. When UnsafeCell<T> is immutably aliased, it is still safe to
mutate, or obtain a mutable reference to, the T it contains. As with all
other types, it is undefined behavior to have multiple &mut UnsafeCell<T>
aliases.
Other types with interior mutability can be created by using UnsafeCell<T> as
a field. The standard library provides a variety of types that provide safe
interior mutability APIs. For example, std::cell::RefCell<T> uses run-time
borrow checks to ensure the usual rules around multiple references. The
std::sync::atomic module contains types that wrap a value that is only
accessed with atomic operations, allowing the value to be shared and mutated
across threads.