RefMut

RefMut is a wrapper type in Rust's standard library that represents a temporarily mutably borrowed value stored inside a RefCell<T>. It functions as a guard object returned by RefCell::borrow_mut(), and its lifetime is tied to the RefCell. The purpose is to enable interior mutability with runtime-checked borrow rules.

It enforces at runtime that only one mutable borrow exists at a time, and that there are no outstanding immutable borrows while the mutable borrow is active. If another borrow request conflicts with the current borrow, the borrow attempt will fail or panic, depending on the method used (for example, try_borrow_mut returns an error instead of panicking).

Access to the inner value is provided by dereferencing the RefMut, allowing code to read or modify the inner T while the borrow is active. When the RefMut is dropped, the borrow ends and the value becomes available for other borrows.

RefMut also offers helper operations, such as into_inner to consume the guard and return the inner T, and map or map_split to create a RefMut to a subfield or projection of the inner value without releasing the whole borrow.

Usage considerations: RefCell and RefMut implement interior mutability in single-threaded contexts; they are not suitable for sharing across threads. They provide dynamic borrow checking, which incurs runtime cost and potential panic if misused. For fallible borrow attempts, use try_borrow_mut or try_borrow to handle BorrowMutError or BorrowError gracefully.

See also: RefCell, Ref, BorrowError, BorrowMutError.