The rust borrow checker ensures at compile time that references to objects are either non-unique and immutable & or unique &mut. When a type takes on a shared or unique reference, it’s called inherited mutability.
In cases where mutability is desired in spite of shared references, one may use Cell and RefCell. These enable interior mutability.
Cell enables shared mutability for values, RefCell enables it for references.
Cell is a simple wrapper type for the underlying data, and it does not affect memory layout.
pub struct Cell<T: ?Sized> {
value: UnsafeCell<T>,
}
RefCell contains extra metadata to dynamically check borrow constraints and so adds to the memory required to contain the underlying type:
pub struct RefCell<T: ?Sized> {
borrow: Cell<BorrowFlag>,
// ...
value: UnsafeCell<T>,
}