Is there an alternative or way to have Rc<RefCell<X>> that restricts mutability of X?

Question

For example given this code :

use std::rc::Rc;
use std::cell::RefCell;

// Don't want to copy for performance reasons
struct LibraryData {
    // Fields ...
}

// Creates and mutates data field in methods
struct LibraryStruct {
    // Only LibraryStruct should have mutable access to this
    data: Rc<RefCell<LibraryData>>
}

impl LibraryStruct {
    pub fn data(&self) -> Rc<RefCell<LibraryData>> {
        self.data.clone()
    }
}

// Receives data field from LibraryStruct.data()
struct A {
    data: Rc<RefCell<LibraryData>>
}

impl A {
    pub fn do_something(&self) {
        // Do something with self.data immutably

        // I want to prevent this because it can break LibraryStruct
        // Only LibraryStruct should have mutable access 
        let data = self.data.borrow_mut();
        // Manipulate data
    }
}

How can I prevent LibraryData from being mutated outside of LibraryStruct ? LibraryStruct should be the only one able to mutate data in its methods. Is this possible with Rc<RefCell<LibraryData>> or is there an alternative? Note I'm writing the "library" code so I can change it.

Answer 1

If you share a RefCell then it will always be possible to mutate it - that's essentially the whole point of it. Given that you are able to change the implementation of LibraryStruct , you can make sure that data is not public, and control how it is exposed to its users through a getter method:

pub struct LibraryStruct {
    // note: not pub
    data: Rc<RefCell<LibraryData>>
}

impl LibraryStruct {
    // could also have returned `Ref<'a, LibraryData> but this hides your 
    // implementation better
    pub fn data<'a>(&'a self) -> impl Deref<Target = LibraryData> + 'a {
        self.data.borrow()
    }
}

In your other struct, you can keep things simple, by just treating it as a reference:

pub struct A<'a> {
    data: &'a LibraryData,
}

impl<'a> A<'a> {
    pub fn do_something(&self) {
        // self.data is only available immutably here because it's just a reference
    }
}

fn main() { 
    let ld = LibraryData {};
    let ls = LibraryStruct { data: Rc::new(RefCell::new(ld)) };

    let a = A { data: &ls.data() };
}

If you need to hold the reference for longer, during which time the original RefCell needs to be mutably borrowed in the library code, then you need to make a custom wrapper which can manage that. It's possible that there's a standard library type for this, but I don't know of it and it's easy to make something specifically for your use case:

// Wrapper to manage a RC<RefCell> and make it immutably borrowable
pub struct ReadOnly<T> {
    // not public
    inner: Rc<RefCell<T>>,
}

impl<T> ReadOnly<T> {
    pub fn borrow<'a>(&'a self) -> impl Deref<Target = T> + 'a {
        self.inner.borrow()
    }
}

Now return this in your library code:

impl LibraryStruct {
    pub fn data<'a>(&'a self) -> ReadOnly<LibraryData> {
        ReadOnly { inner: self.data.clone() }
    }
}

And when you use it, the inner RefCell will not be directly accessible and the data is only available to borrow immutably:

pub struct A {
    data: ReadOnly<LibraryData>,
}

impl A {
    pub fn do_something(&self) {
        //  data is immutable here
        let data = self.data.borrow();
    }
}

fn main() { 
    let ld = LibraryData {};
    let ls = LibraryStruct { data: Rc::new(RefCell::new(ld)) };

    let a = A { data: ls.data() };
}