`Rc`

Rc is a reference-counted shared pointer. Use this when you need to refer to the same data from multiple places:

use std::rc::Rc;

fn main() {
    let mut a = Rc::new(10);
    let mut b = Rc::clone(&a);

    println!("a: {a}");
    println!("b: {b}");
}

See Arc and Mutex if you are in a multi-threaded context.
You can downgrade a shared pointer into a Weak pointer to create cycles that will get dropped.

Rc's count ensures that its contained value is valid for as long as there are references.
Rc in Rust is like std::shared_ptr in C++.
Rc::clone is cheap: it creates a pointer to the same allocation and increases the reference count. Does not make a deep clone and can generally be ignored when looking for performance issues in code.
make_mut actually clones the inner value if necessary ("clone-on-write") and returns a mutable reference.
Use Rc::strong_count to check the reference count.
Rc::downgrade gives you a weakly reference-counted object to create cycles that will be dropped properly (likely in combination with RefCell, on the next slide).

Rc