RefMutT

RefMut<T> is a type in Rust’s standard library used to represent a temporary mutable borrow of the value stored inside a RefCell<T>. It is produced by calling RefCell<T>::borrow_mut(). The mutable borrow rules are enforced at runtime: only one mutable borrow can exist at a time, and no mutable borrow can coexist with any active immutable borrow. If borrow_mut is called when these rules would be violated, the call panics.

A RefMut<T> acts as a smart pointer to the inner value. It implements Deref so you can access the inner T through standard dot syntax, and you can mutate the value via deref coercions when you have a mutable binding to the RefMut. The borrow is tied to the lifetime of the RefMut; as long as the RefMut value exists, the borrow remains active, and once it is dropped, the inner value is once again available for other borrows.

RefMut<T> is part of the runtime borrow checking provided by RefCell and is most useful when interior mutability is required in situations where the compiler cannot guarantee mutability at compile time. It is designed for single-threaded contexts; RefCell and its borrow guards are not thread-safe, so RefMut cannot be safely sent across threads in general.

In typical use, you create a RefMut by borrowing mutably from a RefCell, use the inner value via dereference to read or modify it, and rely on the automatic drop behavior to release the borrow when the RefMut goes out of scope. This pattern provides controlled, temporary mutable access while preserving runtime safety guarantees.