One of Rust's most criticized problems is that it's difficult to develop an application with shared pointers. As we have seen before, it's true that due to Rust's memory safety guarantees, it might be difficult to develop those kinds of algorithms, but as we will see now, the standard library gives us some types we can use to safely allow that behavior.
The standard library has one interesting module, the std::cell module, that allows us to use objects with interior mutability. This means that we can have an immutable object and still mutate it by getting a mutable borrow to the underlying data. This, of course, would not comply with the mutability rules we saw before, but the cells make sure this works by checking the borrows at runtime or by doing copies of the underlying data.