+//@ This is the point where our program got too complex for Rust to guarantee at compile-time that nothing bad will happen. Since we don't
+//@ want to give up the safety guarantee, we are going to need some code that actually checks at run-time that the borrowing rules
+//@ are not violated. Such a check is provided by `RefCell<T>`: Unlike `Cell<T>`, this lets us borrow the contents, and it works for
+//@ non-`Copy` `T`. But, as we will see, it incurs some run-time overhead.
+
+// Our final version of `Callbacks` puts the closure environment into a `RefCell`.
+#[derive(Clone)]
+struct CallbacksMut {
+ callbacks: Vec<Rc<RefCell<FnMut(i32)>>>,
+}
+
+impl CallbacksMut {
+ pub fn new() -> Self {
+ CallbacksMut { callbacks: Vec::new() }
+ }
+
+ pub fn register<F: FnMut(i32)+'static>(&mut self, callback: F) {
+ let cell = Rc::new(RefCell::new(callback)); /*@*/
+ self.callbacks.push(cell); /*@*/
+ }
+
+ pub fn call(&mut self, val: i32) {
+ for callback in self.callbacks.iter() {
+ // We have to *explicitly* borrow the contents of a `RefCell` by calling `borrow` or `borrow_mut`.
+ //@ At run-time, the cell will keep track of the number of outstanding shared and mutable references,
+ //@ and panic if the rules are violated. <br />
+ //@ For this check to be performed, `closure` is a *guard*: Rather than a normal reference, `borrow_mut` returns
+ //@ a smart pointer ([`RefMut`](https://doc.rust-lang.org/stable/std/cell/struct.RefMut.html), in this case) that waits until is goes out of scope, and then
+ //@ appropriately updates the number of active references.
+ //@
+ //@ Since `call` is the only place that borrows the environments of the closures, we should expect that
+ //@ the check will always succeed, as is actually entirely useless. However, this is not actually true. Several different `CallbacksMut` could share
+ //@ a callback (as they were created with `clone`), and calling one callback here could trigger calling
+ //@ all callbacks of the other `CallbacksMut`, which would end up calling the initial callback again. This issue of functions accidentally recursively calling
+ //@ themselves is called *reentrancy*, and it can lead to subtle bugs. Here, it would mean that the closure runs twice, each time thinking it has a
+ //@ unique, mutable reference to its environment - so it may end up dereferencing a dangling pointer. Ouch! Lucky enough,
+ //@ Rust detects this at run-time and panics once we try to borrow the same environment again. I hope this also makes it
+ //@ clear that there's absolutely no hope of Rust performing these checks statically, at compile-time: It would have to detect reentrancy!
+ let mut closure = callback.borrow_mut();
+ // Unfortunately, Rust's auto-dereference of pointers is not clever enough here. We thus have to explicitly
+ // dereference the smart pointer and obtain a mutable reference to the content.
+ (&mut *closure)(val);
+ }
+ }
+}
+
+// Now we can repeat the demo from the previous part - but this time, our `CallbacksMut` type
+// can be cloned.
+fn demo_mut(c: &mut CallbacksMut) {
+ c.register(|val| println!("Callback 1: {}", val));
+ c.call(0);
+
+ {
+ let mut count: usize = 0;
+ c.register(move |val| {
+ count = count+1;
+ println!("Callback 2: {} ({}. time)", val, count);
+ } );
+ }
+ c.call(1); c.clone().call(2);
+}
+
+// **Exercise 12.1**: Write some piece of code using only the available, public interface of `CallbacksMut` such that a reentrant call to a closure
+// is happening, and the program panics because the `RefCell` refuses to hand out a second mutable borrow of the closure's environment.
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