X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/501b20b712b2e5c176ca581232970e1e74d62ab0..4cac8138d63e49c60d101bee79b9e60644b3de0a:/src/part16.rs?ds=sidebyside diff --git a/src/part16.rs b/src/part16.rs index c02959d..876df27 100644 --- a/src/part16.rs +++ b/src/part16.rs @@ -5,29 +5,32 @@ 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 surprising 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. +//@ 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. It is typically disabled, guarded by the keyword `unsafe`. Of course, -//@ `unsafe` also means "Here Be Dragons": You are on your own now. -//@ -//@ 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. +//@ 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.
+//@ 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/delete 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 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. +//@ 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 { next: NodePtr, @@ -35,44 +38,62 @@ struct Node { 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 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, either. +//@ 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 = *mut Node; -// 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` 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` 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`, 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).) +// 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` 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` 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`, 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 { first: NodePtr, last: NodePtr, _marker: PhantomData, } -//@ 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.
-//@ 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](http://doc.rust-lang.org/beta/alloc/boxed/struct.Box.html#method.from_raw) such [operations](http://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. +//@ 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`.
+//@ 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.)
+//@ 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(r: *mut T) -> Box { 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 a safe invocation of `box_into_raw` cannot go wrong. +//@ 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(b: Box) -> *mut T { unsafe { mem::transmute(b) } } impl LinkedList { - // A new linked list just contains null pointers. `PhantomData` is how we construct any `PhantomData`. + // A new linked list just contains null pointers. `PhantomData` is how we construct any + // `PhantomData`. pub fn new() -> Self { LinkedList { first: ptr::null_mut(), last: ptr::null_mut(), _marker: PhantomData } } @@ -80,11 +101,11 @@ impl LinkedList { // 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. @@ -92,47 +113,50 @@ impl LinkedList { } 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. + //@ 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`. + // **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`. // 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 { + //@ This function just creates an instance of `IterMut`, the iterator type which does the actual + //@ work. + pub fn iter_mut(&mut self) -> IterMut { 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 -//@ 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. -//@ 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 (mutably) 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. +//@ 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, _marker: PhantomData<&'a mut LinkedList>, } -//@ 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. +//@ 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; @@ -141,7 +165,7 @@ impl<'a, T> Iterator for IterMut<'a, T> { if self.next.is_null() { None } else { - // Otherwise, we can convert the next pointer to a borrow, get a borrow to the data + // 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; @@ -151,35 +175,39 @@ impl<'a, T> Iterator for IterMut<'a, T> { } } -//@ 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 borrow 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! +//@ 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 borrows. -// Add testcases for both kinds of iterators. +// **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`. +//@ 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 Drop for LinkedList { - // 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 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. + // 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; @@ -189,12 +217,14 @@ impl Drop for LinkedList { } // ## 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/)). +//@ 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. +//@ 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) | next +//@ [index](main.html) | [previous](part15.html) | [raw source](workspace/src/part16.rs) | next