RcRefCellT

RcRefCellT is a Rust programming language construct that combines reference counting with interior mutability. It is defined in the `std::rc` module and provides a way to have multiple immutable references to a value while still being able to modify that value.

The `Rc` part stands for "Reference Counted." It allows multiple owners of the same data. When the last owner goes out of scope, the data is dropped. This is useful for sharing data across different parts of a program without resorting to cloning.

The `RefCell` part provides "interior mutability." Normally, Rust enforces strict borrowing rules: you can have either one mutable reference or any number of immutable references to a value, but not both simultaneously. `RefCell` bypasses this compile-time check, deferring it to runtime.

When you try to borrow from a `RefCell`, it will either give you a mutable reference (`borrow_mut`) or an immutable reference (`borrow`). If the borrowing rules are violated at runtime (e.g., trying to get a mutable borrow while an immutable borrow is active), the program will panic. This makes `Rc<RefCell<T>>` a powerful tool for scenarios where shared mutable state is necessary, such as implementing data structures that require mutation from multiple points of access. It is commonly used in GUI programming and complex data graph representations.