mutable references wording

[rust-101.git] / src / part16.rs

// Rust-101, Part 16: Unsafe Rust, Drop
// ====================================

use std::ptr;
use std::mem;
use std::marker::PhantomData;

//@ As we saw, the rules Rust imposes to ensure memory safety can get us pretty far. A large amount of programming patterns
//@ can be written within safe Rust, and, more importantly, library code like iterators or threads can make
//@ use of the type system to ensure some level of correctness beyond basic memory safety.
//@ 
//@ However, there will still be programs that one cannot write in accordance with the borrow checker. And there
//@ will be cases where it may be possible to satisfy the compiler, but only at the cost of some run-time overhead,
//@ as we saw with `RefCell` - overhead which may not be acceptable. In such a situation, it is possible to
//@ use *unsafe* Rust: That's a part of the language that is *known* to open the gate to invalid pointer access
//@ and all other sorts of memory safety. Of course, `unsafe` also means "Here Be Dragons": You are on your own now. <br/>
//@ The goal in these cases is to confine unsafety to the local module. Types like `Rc` and `Vec` are implemented
//@ using unsafe Rust, but *using* them as we did is (believed to be) perfectly safe.
//@ 
//@ ## Unsafe Code
//@ As an example, let us write a doubly-linked list. Clearly, such a data-structure involves aliasing and mutation:
//@ Every node in the list is pointed to by its left and right neighbor, but still we will want to modify the nodes. We
//@ could now try some clever combination of `Rc` and `RefCell`, but this would end up being quite annoying - and it would
//@ incur some overhead. For a low-level data-structure like a doubly-linked list, it makes sense to implement an efficient
//@ version once, that is unsafe internally, but that can be used without any risk by safe client code.

//@ As usually, we start by defining the types. Everything is parameterized by the type `T` of the data stored in the list.
// A node of the list consists of the data, and two node pointers for the predecessor and successor.
struct Node<T> {
    next: NodePtr<T>,
    prev: NodePtr<T>,
    data: T,
}
// A node pointer is a *mutable raw pointer* to a node.
//@ Raw pointers (`*mut T` and `*const T`) are the Rust equivalent of pointers in C. Unlike references, they do not come with
//@ any guarantees: Raw pointers can be null, or they can point to garbage. They don't have a lifetime, either.
type NodePtr<T> = *mut Node<T>;

// The linked list itself stores pointers to the first and the last node. In addition, we tell Rust that this type
// will own data of type `T`.
//@ The type `PhantomData<T>` does not actually store anything in memory - it has size zero. However, logically,
//@ Rust will consider a `T` to be present. In this case, Rust knows that data of type `T` may be dropped
//@ whenever a `LinkedList<T>` is dropped. Dropping has a lot of subtle checks to it, making sure that things can't go
//@ wrong. For this to work, Rust needs to know which types could potentially be dropped. In safe Rust, this can all be inferred
//@ automatically, but here, we just have a `*mut Node<T>`, and we need to tell Rust that we actually own such data and will drop it.
//@ (For more of the glory details, see [this RFC](https://github.com/rust-lang/rfcs/blob/master/text/0769-sound-generic-drop.md).)
pub struct LinkedList<T> {
    first: NodePtr<T>,
    last:  NodePtr<T>,
    _marker: PhantomData<T>,
}

//@ Before we get to the actual linked-list methods, we write two short helper functions converting between mutable raw pointers,
//@ and boxed data. Both employ `mem::transmute`, which can convert anything to anything, by just re-interpreting the bytes.
//@ Clearly, that's an unsafe operation and must only be used with great care - or even better, not at all. Seriously.
//@ If at all possible, you should never use `transmute`. <br/>
//@ We are making the assumption here that a `Box` and a raw pointer have the same representation in memory. In the future,
//@ Rust will [provide](https://doc.rust-lang.org/beta/alloc/boxed/struct.Box.html#method.from_raw) such [operations](https://doc.rust-lang.org/beta/alloc/boxed/struct.Box.html#method.into_raw) in the standard library, but the exact API is still being fleshed out.

//@ We declare `raw_into_box` to be an `unsafe` function, telling Rust that calling this function is not generally safe.
//@ This grants us the unsafe powers for the body of the function: We can dereference raw pointers, and - most importantly - we
//@ can call unsafe functions. (The other unsafe powers won't be relevant here. Go read [The Rustonomicon](https://doc.rust-lang.org/nightly/nomicon/)
//@ if you want to learn all about this, but be warned - That Way Lies Madness.) <br/>
//@ Here, the caller will have to ensure that `r` is a valid pointer, and that nobody else has a pointer to this data.
unsafe fn raw_into_box<T>(r: *mut T) -> Box<T> {
    mem::transmute(r)
}
//@ The case is slightly different for `box_into_raw`: Converting a `Box` to a raw pointer is always safe. It just drops some information.
//@ Hence we keep the function itself safe, and use an *unsafe block* within the function. This is an (unchecked) promise to the Rust
//@ compiler, saying that a safe invocation of `box_into_raw` cannot go wrong. We also have the unsafe powers in the unsafe block.
fn box_into_raw<T>(b: Box<T>) -> *mut T {
    unsafe { mem::transmute(b) }
}

impl<T> LinkedList<T> {
    // A new linked list just contains null pointers. `PhantomData` is how we construct any `PhantomData<T>`.
    pub fn new() -> Self {
        LinkedList { first: ptr::null_mut(), last: ptr::null_mut(), _marker: PhantomData }
    }

    // This function adds a new node to the end of the list.
    pub fn push_back(&mut self, t: T) {
        // Create the new node, and make it a raw pointer.
        //@ Calling `box_into_raw` gives up ownership of the box, which is crucial: We don't want the memory that it points to to be deallocated!
        let new = Box::new( Node { data: t, next: ptr::null_mut(), prev: self.last } );
        let new = box_into_raw(new);
        // Update other pointers to this node.
        if self.last.is_null() {
            debug_assert!(self.first.is_null());
            // The list is currently empty, so we have to update the head pointer.
            self.first = new;                                       /*@*/
        } else {
            debug_assert!(!self.first.is_null());
            // We have to update the `next` pointer of the tail node.
            //@ Since Rust does not know that a raw pointer actually points to anything, dereferencing such a pointer is
            //@ an unsafe operation. So this unsafe block promises that the pointer will actually be valid.
            unsafe { (*self.last).next = new; }                     /*@*/
        }
        // Make this the last node.
        self.last = new;
    }

    // **Exercise 16.1**: Add some more operations to `LinkedList`: `pop_back`, `push_front` and `pop_front`.
    // Add testcases for `push_back` and all of your functions. The `pop` functions should take `&mut self`
    // and return `Option<T>`.

    // Next, we are going to provide an iterator.
    //@ This function just creates an instance of `IterMut`, the iterator type which does the actual work.
    pub fn iter_mut(&mut self) -> IterMut<T> {
        IterMut { next: self.first, _marker: PhantomData  }
    }
}

//@ What does the iterator need to store? Strictly speaking, all it needs is the pointer to the next node
//@ that it is going to visit. However, how do we make sure that this pointer remains valid? We have to
//@ get this right ourselves, as we left the safe realms of borrowing and ownership. Remember that the
//@ key ingredient for iterator safety was to tie the lifetime of the iterator to the lifetime of the
//@ reference used for `iter_mut`. We will thus give `IterMut` two parameters: A type parameter specifying
//@ the type of the data, and a lifetime parameter specifying for how long the list was borrowed to the
//@ iterator.

//@ For Rust to accept the type, we have to add two more annotations. First of all, we have to ensure that
//@ the data in the list lives at least as long as the iterator: If you drop the `T: 'a`, Rust will tell
//@ you to add it back. And secondly, Rust will complain if `'a` is not actually used in the struct.
//@ It doesn't know what it is supposed to do with that lifetime. So we use `PhantomData` again to tell
//@ it that in terms of ownership, this type actually (uniquely) borrows a linked list. This has no
//@ operational effect, but it means that Rust can deduce the intent we had when adding that
//@ seemingly useless lifetime parameter.
pub struct IterMut<'a, T> where T: 'a {
    next: NodePtr<T>,
    _marker: PhantomData<&'a mut LinkedList<T>>,
}

//@ When implementing `Iterator` for `IterMut`, the fact that we have the lifetime `'a` around immediately
//@ pays of: We would not even be able to write down the type `Item` without that lifetime.
impl<'a, T> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;

    fn next(&mut self) -> Option<Self::Item> {
        // The actual iteration is straight-forward: Once we reached a null pointer, we are done.
        if self.next.is_null() {
            None
        } else {
            // Otherwise, we can convert the next pointer to a reference, get a reference to the data
            // and update the iterator.
            let next = unsafe { &mut *self.next };
            let ret = &mut next.data;
            self.next = next.next;                                  /*@*/
            Some(ret)                                               /*@*/
        }
    }
}

//@ In `next` above, we made crucial use of the assumption that `self.next` is either null or a valid pointer.
//@ This only works because if someone tries to delete elements from a list during iteration, we know that the borrow checker
//@ will catch them: If they call `next`, the lifetime `'a` we artificially added to the iterator has to still be
//@ active, which means the mutable reference passed to `iter_mut` is still active, which means nobody can delete
//@ anything from the list. In other words, we make use of the expressive type system of Rust, decorating
//@ our own unsafe implementation with just enough information so that Rust can check *uses* of the linked-list.
//@ If the type system were weaker, we could not write a linked-list like the above with a safe interface!

// **Exercise 16.2**: Add a method `iter` and a type `Iter` providing iteration for shared references.
// Add testcases for both kinds of iterators.

// ## `Drop`
//@ The linked list we wrote is already working quite nicely, but there is one problem: When the list is dropped,
//@ nobody bothers to deallocate the remaining nodes. Even worse, if `T` itself has a destructor that needs to
//@ clean up, it is not called for the element remaining in the list. We need to take care of that ourselves.

//@ In Rust, adding a destructor for a type is done by implementing the `Drop` trait. This is a very special trait.
//@ It can only be implemented for *nominal types*, i.e., you cannot implement `Drop` for `&mut T`. You also cannot
//@ restrict the type and lifetime parameters further than the type does - the `Drop` implementation has to apply to *all* instances
//@ of `LinkedList`.
impl<T> Drop for LinkedList<T> {
    // The destructor itself is a method which takes `self` in mutably borrowed form. It cannot own `self`, because then
    // the destructor of `self` would be called at the end of the function, resulting in endless recursion.
    fn drop(&mut self) {
        let mut cur_ptr = self.first;
        while !cur_ptr.is_null() {
            // In the destructor, we just iterate over the entire list, successively obtaining ownership
            // (`Box`) of every node. When the box is dropped, it will call the destructor on `data` if
            // necessary, and subsequently free the node on the heap.
            //@ We call `drop` explicitly here just for documentation purposes.
            let cur = unsafe { raw_into_box(cur_ptr) };
            cur_ptr = cur.next;
            drop(cur);
        }
    }
}

// ## The End
//@ Congratulations! You completed Rust-101. This was the last part of the course. I hope you enjoyed it.
//@ If you have feedback or want to contribute yourself, please head to the [Rust-101](https://www.ralfj.de/projects/rust-101/) website
//@ fur further information. The entire course is open-source (under [CC-BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/)).
//@ 
//@ If you want to do more, the examples you saw in this course provide lots of playground for coming up with your own little
//@ extensions here and there. The [index](main.html) contains some more links to additional resources you may find useful. 
//@ With that, there's only one thing left to say: Happy Rust Hacking!

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