I think you are confusing Rc/Arc with RefCell, since they are often used together. In order to provide interior mutability, the borrow_mut method of RefCell takes &self rather than &mut self. So, it is possible to obtain two mutable references. The following compiles:
let c = RefCell::new(42usize);
let mut mut_ref1 = c.borrow_mut();
let mut mut_ref2 = c.borrow_mut();
*mut_ref1 += 1;
*mut_ref2 += 2;
RefCell uses UnsafeCell to subvert the borrows checker. However, instead RefCell implements run-time borrow checks. So, the code above will panic while attempting to do the second mutable borrow. It is a trade-off. You lose catching errors during compilation, but you can have interior mutability.
This is one of the goals of unsafe, which UnsafeCell requires. Sometimes, you need to do something that safe Rust does not permit, but that you can prove to be safe (such as RefCell).
That said (and it depends a bit on your domain), I have used Rust for probably 1.5 years without using Arc, Rc, and RefCell. The first time I needed these types when was writing a Gtk+ application and later in some multi-threading programs.
These three types are very easy to overuse when you are coming from other languages. IMO it is better to avoid them and try to work with the borrows checker until you have sufficient experience with Rust to identify when interior mutability and reference counting are really necessary.
This is one of the goals of unsafe, which UnsafeCell requires. Sometimes, you need to do something that safe Rust does not permit, but that you can prove to be safe (such as RefCell).
That said (and it depends a bit on your domain), I have used Rust for probably 1.5 years without using Arc, Rc, and RefCell. The first time I needed these types when was writing a Gtk+ application and later in some multi-threading programs.
These three types are very easy to overuse when you are coming from other languages. IMO it is better to avoid them and try to work with the borrows checker until you have sufficient experience with Rust to identify when interior mutability and reference counting are really necessary.