Cannot Borrow As Mutable Because It Is Also Borrowed As Immutable

Transforming Non-Cloneable Types into Cloneable Values

Reference-Counted Values and Interior Mutability

If your type is not cloneable, you can transform it into a reference-counted value such as Rc or Arc, which can then be cloned. You may or may not also need to use interior mutability.

Interior Mutability

Interior mutability allows you to mutate a field of a struct while it is borrowed. This is useful when you need to modify the state of an object without taking ownership of it.

The Mutex and RefCell types are two common ways to achieve interior mutability. Mutex provides mutual exclusion, while RefCell is non-blocking.

Example

 struct Foo {     a: Vec,     b: i32, }  let mut foo = Foo { a: vec![1, 2, 3], b: 4 };  // Borrow foo's vec let a = &mut foo.a;  // Modify the vec a.push(4);  // Now we can mutate foo's b field foo.b = 5; 

In this example, we have a struct Foo with two fields: a mutable Vec and an immutable i32. We borrow the Vec and modify it, which is allowed because we have interior mutability. We can then mutate the i32 field without any problems.