Sometimes a type needs 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 in Rust 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.
© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.