-// Rust-101, Part 16: Unsafe, Drop (WIP)
-// ===============================
+// Rust-101, Part 16: Unsafe Rust, Drop
+// ====================================
use std::ptr;
use std::mem;
use std::marker::PhantomData;
-//@ As we saw, the rules Rust imposes can get us pretty far: A surprising amount of programming patterns
+//@ 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.
//@
//@ 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. It is typically disabled, guarded by the keyword `unsafe`. Of course,
-//@ `unsafe` also means "Here Be Dragons": You are on your own now. Types like `Rc` and `Vec` are implemented
-//@ `using unsafe Rust.
+//@ 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
-//@ (either to change the value at that place, or to insert new 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 over-head. 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 taht can be used without any risk by safe client code.
+//@ 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.
prev: NodePtr<T>,
data: T,
}
-// A node pointer is a *mutable raw point* to a node.
+// 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 borrows, they do not come with
-//@ any guarantees: Raw pointers can be null, or they can point to garbage. They don't have a lifetime.
+//@ 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. The checks involving destructors are pretty subtle, so it's always
-//@ a good idea to provide such extra information. In safe Rust, this can all be done automatically, but here,
-//@ we just have a `*mut Node<T>`, which Rust does not consider as actually owning the data it points to.
+//@ 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 owned pointers (aka `Box`). Both employ `mem::transmute`, which is Rust's
-//@ `reinterpret_cast`: It can convert anything to anything, by just re-interpreting the bytes. Clearly,
-//@ that's an unsafe operation.
+//@ 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.
-//@ The caller will have to ensure that `r` is a valid pointer, and that nobody else has a pointer to this data.
+//@ 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. I 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 even though the code inside that block *could* go wrong, we actually know that
-//@ it will not.
+//@ 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) }
}
LinkedList { first: ptr::null_mut(), last: ptr::null_mut(), _marker: PhantomData }
}
- // Add a new node to the end of the list.
+ // 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!
+ //@ 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 points to this node.
+ // 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.
} 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 to anything, dereferencing such a pointer is
+ //@ 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; } /*@*/
}
// Add testcases for `push_back` and all of your functions. The `pop` functions should take `&mut self`
// and return `Option<T>`.
- // Of course, we will also want to provide an iterator.
+ // 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(&self) -> IterMut<T> {
+ pub fn iter_mut(&mut self) -> IterMut<T> {
IterMut { next: self.first, _marker: PhantomData }
}
}
//@ borrow 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.
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
+ None
} else {
// Otherwise, we can convert the next pointer to a borrow, get a borrow to the data
// and update the iterator.
let next = unsafe { &mut *self.next };
let ret = &mut next.data;
- self.next = next.next;
- Some(ret)
+ self.next = next.next; /*@*/
+ Some(ret) /*@*/
}
}
}
// 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 removed,
+//@ 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 - the `Drop` implementation has to apply to *all* instances
+//@ 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 pf the function, resulting in endless recursion...
+ // 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() {
}
}
-//@ ## The End
-//@ Congratulations! You complete Rust-101. This was the last example of the last part. I hope you enjoyed it.
-//@ If you have feedback, please head to the [Rust-101](https://www.ralfj.de/projects/rust-101/) website
-//@ and let me know how you liked it. The entire course is open-source (under CC-BY-SA 4.0), and contributions are welcome!
+// ## 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/)).
//@
-//@ 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!
+//@ 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!
-//@ [index](main.html) | [previous](part15.html)
+//@ [index](main.html) | [previous](part15.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part16.rs) | next
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