tune explanation of PhantomData ownership annotation

[rust-101.git] / src / part12.rs

// Rust-101, Part 12: Rc, Interior Mutability, Cell, RefCell
// =========================================================

use std::rc::Rc;
use std::cell::{Cell, RefCell};

//@ Our generic callback mechanism is already working quite nicely. However, there's one point we may want to fix:
//@ `Callbacks` does not implement `Clone`. The problem is that closures (or rather, their environment) can never be cloned.
//@ (There's not even an automatic derivation happening for the cases where it would be possible.)
//@ This restriction propagates up to `Callbacks` itself. What could we do about this?

//@ ## `Rc`
//@ The solution is to find some way of cloning `Callbacks` without cloning the environments. This can be achieved with
//@ `Rc<T>`, a *reference-counted* pointer. This is is another example of a smart pointer. You can `clone` an `Rc` as often
//@ as you want, that doesn't affect the data it contains. It only creates more references to the same data. Once all the
//@ references are gone, the data is deleted.
//@ 
//@ Wait a moment, you may say here. Multiple references to the same data? That's aliasing! Indeed:
//@ Once data is stored in an `Rc`, it is read-only and you can only ever get a shared borrow of the data again.

//@ Because of this read-only restriction, we cannot use `FnMut` here: We'd be unable to call the function with a mutable borrow
//@ of it's environment! So we have to go with `Fn`. We wrap that in an `Rc`, and then Rust happily derives `Clone` for us.
#[derive(Clone)]
struct Callbacks {
    callbacks: Vec<Rc<Fn(i32)>>,
}

impl Callbacks {
    pub fn new() -> Self {
        Callbacks { callbacks: Vec::new() }
    }

    // Registration works just like last time, except that we are creating an `Rc` now.
    pub fn register<F: Fn(i32)+'static>(&mut self, callback: F) {
        self.callbacks.push(Rc::new(callback));                     /*@*/
    }

    pub fn call(&self, val: i32) {
        // We only need a shared iterator here. Since `Rc` is a smart pointer, we can directly call the callback.
        for callback in self.callbacks.iter() {
            callback(val);                                          /*@*/
        }
    }
}

// Time for a demo!
fn demo(c: &mut Callbacks) {
    c.register(|val| println!("Callback 1: {}", val));
    c.call(0); c.clone().call(1);
}

pub fn main() {
    let mut c = Callbacks::new();
    demo(&mut c);
}

// ## Interior Mutability
//@ Of course, the counting example from last time does not work anymore: It needs to mutate the environment, which a `Fn`
//@ cannot do. The strict borrowing Rules of Rust are getting into our way. However, when it comes to mutating a mere number
//@ (`usize`), there's not really any chance of problems coming up. Everybody can read and write that variable just as they want.
//@ So it would be rather sad if we were not able to write this program. Lucky enough, Rust's standard library provides a
//@ solution in the form of `Cell<T>`. This type represents a memory cell of some type `T`, providing the two basic operations
//@ `get` and `set`. `get` returns a *copy* of the content of the cell, so all this works only if `T` is `Copy`.
//@ `set`, which overrides the content, only needs a *shared borrow* of the cell. The phenomenon of a type that permits mutation through
//@ shared borrows (i.e., mutation despite the possibility of aliasing) is called *interior mutability*. You can think
//@ of `set` changing only the *contents* of the cell, not its *identity*. In contrast, the kind of mutation we saw so far was
//@ about replacing one piece of data by something else of the same type. This is called *exterior mutability*. <br/>
//@ Notice that it is impossible to *borrow* the contents of the cell, and that is actually the key to why this is safe.

// So, let us put our counter in a `Cell`, and replicate the example from the previous part.
fn demo_cell(c: &mut Callbacks) {
    {
        let count = Cell::new(0);
        // Again, we have to move ownership if the `count` into the environment closure.
        c.register(move |val| {
            // In here, all we have is a shared borrow of our environment. But that's good enough for the `get` and `set` of the cell!
            //@ At run-time, the `Cell` will be almost entirely compiled away, so this becomes pretty much equivalent to the version
            //@ we wrote in the previous part.
            let new_count = count.get()+1;
            count.set(new_count);
            println!("Callback 2: {} ({}. time)", val, new_count);
        } );
    }

    c.call(2); c.clone().call(3);
}

//@ It is worth mentioning that `Rc` itself also has to make use of interior mutability: When you `clone` an `Rc`, all it has available
//@ is a shared borrow. However, it has to increment the reference count! Internally, `Rc` uses `Cell` for the count, such that it
//@ can be updated during `clone`.

// ## `RefCell`
//@ As the next step in the evolution of `Callbacks`, we could try to solve this problem of mutability once and for all, by adding `Cell`
//@ to `Callbacks` such that clients don't have to worry about this. However, that won't end up working: Remember that `Cell` only works
//@ with types that are `Copy`, which the environment of a closure will never be. We need a variant of `Cell` that allows borrowing its
//@ contents, such that we can provide a `FnMut` with its environment. But if `Cell` would allow that, we could write down all those
//@ crashing C++ programs that we wanted to get rid of.
//@ 
//@ 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 borrows,
            //@ and panic if the rules are violated. <br />
            //@ For this check to be performed, `closure` is a *guard*: Rather than a normal borrow, `borrow_mut` returns
            //@ a smart pointer (`RefMut`, in this case) that waits until is goes out of scope, and then
            //@ appropriately updates the number of active borrows.
            //@ 
            //@ Since `call` is the only place that borrows the environments of the closures, we should expect that
            //@ the check will always succeed. However, this function would still typecheck with an immutable borrow of `self` (since we are
            //@ relying on the interior mutability of `RefCell`). Under this condition, it could happen that a callback
            //@ will in turn trigger another round of callbacks, so that `call` would indirectly call itself.
            //@ This is called reentrancy. It would imply that we borrow the closure a second time, and
            //@ panic at run-time. 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 borrow of 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**: Change the type of `call` to ask only for a shared borrow. Then write some piece of code using only the available, public
// interface of `CallbacksMut` such that a reentrant call to `call` is happening, and the program aborts because the `RefCell` refuses to hand
// out a second mutable borrow to its content.

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