Interior Mutability

r[interior-mut]

r[interior-mut.intro] Sometimes a type needs to be mutated while having multiple aliases. In Rust this is achieved using a pattern called interior mutability.

r[interior-mut.shared-ref] A type has interior mutability if its internal state can be changed through a shared reference to it.

r[interior-mut.no-constraint] This goes against the usual requirement that the value pointed to by a shared reference is not mutated.

r[interior-mut.unsafe-cell] [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.

r[interior-mut.mut-unsafe-cell] As with all other types, it is undefined behavior to have multiple &mut UnsafeCell<T> aliases.

r[interior-mut.abstraction] 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.

r[interior-mut.ref-cell] For example, [std::cell::RefCell<T>] uses run-time borrow checks to ensure the usual rules around multiple references.

r[interior-mut.atomic] 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.