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//! Types which pin data to its location in memory //! //! It is sometimes useful to have objects that are guaranteed to not move, //! in the sense that their placement in memory does not change, and can thus be relied upon. //! //! A prime example of such a scenario would be building self-referential structs, //! since moving an object with pointers to itself will invalidate them, //! which could cause undefined behavior. //! //! In order to prevent objects from moving, they must be pinned //! by wrapping a pointer to the data in the [`Pin`] type. A pointer wrapped //! in a `Pin` is otherwise equivalent to its normal version, e.g. `Pin<Box<T>>` //! and `Box<T>` work the same way except that the first is pinning the value //! of `T` in place. //! //! First of all, these are pointer types because pinned data mustn't be passed around by value //! (that would change its location in memory). //! Secondly, since data can be moved out of `&mut` and `Box` with functions such as [`swap`], //! which causes their contents to swap places in memory, //! we need dedicated types that prohibit such operations. //! //! However, these restrictions are usually not necessary, //! so most types implement the [`Unpin`] auto-trait, //! which indicates that the type can be moved out safely. //! Doing so removes the limitations of pinning types, //! making them the same as their non-pinning counterparts. //! //! [`Pin`]: struct.Pin.html //! [`Unpin`]: trait.Unpin.html //! [`swap`]: ../../std/mem/fn.swap.html //! [`Box`]: ../../std/boxed/struct.Box.html //! //! # Examples //! //! ```rust //! #![feature(pin)] //! //! use std::pin::Pin; //! use std::marker::Pinned; //! use std::ptr::NonNull; //! //! // This is a self-referential struct since the slice field points to the data field. //! // We cannot inform the compiler about that with a normal reference, //! // since this pattern cannot be described with the usual borrowing rules. //! // Instead we use a raw pointer, though one which is known to not be null, //! // since we know it's pointing at the string. //! struct Unmovable { //! data: String, //! slice: NonNull<String>, //! _pin: Pinned, //! } //! //! impl Unmovable { //! // To ensure the data doesn't move when the function returns, //! // we place it in the heap where it will stay for the lifetime of the object, //! // and the only way to access it would be through a pointer to it. //! fn new(data: String) -> Pin<Box<Self>> { //! let res = Unmovable { //! data, //! // we only create the pointer once the data is in place //! // otherwise it will have already moved before we even started //! slice: NonNull::dangling(), //! _pin: Pinned, //! }; //! let mut boxed = Box::pinned(res); //! //! let slice = NonNull::from(&boxed.data); //! // we know this is safe because modifying a field doesn't move the whole struct //! unsafe { //! let mut_ref: Pin<&mut Self> = Pin::as_mut(&mut boxed); //! Pin::get_mut_unchecked(mut_ref).slice = slice; //! } //! boxed //! } //! } //! //! let unmoved = Unmovable::new("hello".to_string()); //! // The pointer should point to the correct location, //! // so long as the struct hasn't moved. //! // Meanwhile, we are free to move the pointer around. //! # #[allow(unused_mut)] //! let mut still_unmoved = unmoved; //! assert_eq!(still_unmoved.slice, NonNull::from(&still_unmoved.data)); //! //! // Since our type doesn't implement Unpin, this will fail to compile: //! // let new_unmoved = Unmovable::new("world".to_string()); //! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved); //! ``` #![unstable(feature = "pin", issue = "49150")] use fmt; use marker::Sized; use ops::{Deref, DerefMut, CoerceUnsized, DispatchFromDyn}; #[doc(inline)] pub use marker::Unpin; /// A pinned pointer. /// /// This is a wrapper around a kind of pointer which makes that pointer "pin" its /// value in place, preventing the value referenced by that pointer from being moved /// unless it implements [`Unpin`]. /// /// See the [`pin` module] documentation for further explanation on pinning. /// /// [`Unpin`]: ../../std/marker/trait.Unpin.html /// [`pin` module]: ../../std/pin/index.html // // Note: the derives below are allowed because they all only use `&P`, so they // cannot move the value behind `pointer`. #[unstable(feature = "pin", issue = "49150")] #[fundamental] #[derive(Copy, Clone, Hash, Eq, PartialEq, Ord, PartialOrd)] pub struct Pin<P> { pointer: P, } impl<P: Deref> Pin<P> where P::Target: Unpin, { /// Construct a new `Pin` around a pointer to some data of a type that /// implements `Unpin`. #[unstable(feature = "pin", issue = "49150")] #[inline(always)] pub fn new(pointer: P) -> Pin<P> { // Safety: the value pointed to is `Unpin`, and so has no requirements // around pinning. unsafe { Pin::new_unchecked(pointer) } } } impl<P: Deref> Pin<P> { /// Construct a new `Pin` around a reference to some data of a type that /// may or may not implement `Unpin`. /// /// # Safety /// /// This constructor is unsafe because we cannot guarantee that the data /// pointed to by `pointer` is pinned. If the constructed `Pin<P>` does /// not guarantee that the data `P` points to is pinned, constructing a /// `Pin<P>` is undefined behavior. /// /// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used /// instead. #[unstable(feature = "pin", issue = "49150")] #[inline(always)] pub unsafe fn new_unchecked(pointer: P) -> Pin<P> { Pin { pointer } } /// Get a pinned shared reference from this pinned pointer. #[unstable(feature = "pin", issue = "49150")] #[inline(always)] pub fn as_ref(self: &Pin<P>) -> Pin<&P::Target> { unsafe { Pin::new_unchecked(&*self.pointer) } } } impl<P: DerefMut> Pin<P> { /// Get a pinned mutable reference from this pinned pointer. #[unstable(feature = "pin", issue = "49150")] #[inline(always)] pub fn as_mut(self: &mut Pin<P>) -> Pin<&mut P::Target> { unsafe { Pin::new_unchecked(&mut *self.pointer) } } /// Assign a new value to the memory behind the pinned reference. #[unstable(feature = "pin", issue = "49150")] #[inline(always)] pub fn set(mut self: Pin<P>, value: P::Target) where P::Target: Sized, { *self.pointer = value; } } impl<'a, T: ?Sized> Pin<&'a T> { /// Construct a new pin by mapping the interior value. /// /// For example, if you wanted to get a `Pin` of a field of something, /// you could use this to get access to that field in one line of code. /// /// # Safety /// /// This function is unsafe. You must guarantee that the data you return /// will not move so long as the argument value does not move (for example, /// because it is one of the fields of that value), and also that you do /// not move out of the argument you receive to the interior function. #[unstable(feature = "pin", issue = "49150")] pub unsafe fn map_unchecked<U, F>(this: Pin<&'a T>, func: F) -> Pin<&'a U> where F: FnOnce(&T) -> &U, { let pointer = &*this.pointer; let new_pointer = func(pointer); Pin::new_unchecked(new_pointer) } /// Get a shared reference out of a pin. /// /// Note: `Pin` also implements `Deref` to the target, which can be used /// to access the inner value. However, `Deref` only provides a reference /// that lives for as long as the borrow of the `Pin`, not the lifetime of /// the `Pin` itself. This method allows turning the `Pin` into a reference /// with the same lifetime as the original `Pin`. #[unstable(feature = "pin", issue = "49150")] #[inline(always)] pub fn get_ref(this: Pin<&'a T>) -> &'a T { this.pointer } } impl<'a, T: ?Sized> Pin<&'a mut T> { /// Convert this `Pin<&mut T>` into a `Pin<&T>` with the same lifetime. #[unstable(feature = "pin", issue = "49150")] #[inline(always)] pub fn into_ref(this: Pin<&'a mut T>) -> Pin<&'a T> { Pin { pointer: this.pointer } } /// Get a mutable reference to the data inside of this `Pin`. /// /// This requires that the data inside this `Pin` is `Unpin`. /// /// Note: `Pin` also implements `DerefMut` to the data, which can be used /// to access the inner value. However, `DerefMut` only provides a reference /// that lives for as long as the borrow of the `Pin`, not the lifetime of /// the `Pin` itself. This method allows turning the `Pin` into a reference /// with the same lifetime as the original `Pin`. #[unstable(feature = "pin", issue = "49150")] #[inline(always)] pub fn get_mut(this: Pin<&'a mut T>) -> &'a mut T where T: Unpin, { this.pointer } /// Get a mutable reference to the data inside of this `Pin`. /// /// # Safety /// /// This function is unsafe. You must guarantee that you will never move /// the data out of the mutable reference you receive when you call this /// function, so that the invariants on the `Pin` type can be upheld. /// /// If the underlying data is `Unpin`, `Pin::get_mut` should be used /// instead. #[unstable(feature = "pin", issue = "49150")] #[inline(always)] pub unsafe fn get_mut_unchecked(this: Pin<&'a mut T>) -> &'a mut T { this.pointer } /// Construct a new pin by mapping the interior value. /// /// For example, if you wanted to get a `Pin` of a field of something, /// you could use this to get access to that field in one line of code. /// /// # Safety /// /// This function is unsafe. You must guarantee that the data you return /// will not move so long as the argument value does not move (for example, /// because it is one of the fields of that value), and also that you do /// not move out of the argument you receive to the interior function. #[unstable(feature = "pin", issue = "49150")] pub unsafe fn map_unchecked_mut<U, F>(this: Pin<&'a mut T>, func: F) -> Pin<&'a mut U> where F: FnOnce(&mut T) -> &mut U, { let pointer = Pin::get_mut_unchecked(this); let new_pointer = func(pointer); Pin::new_unchecked(new_pointer) } } #[unstable(feature = "pin", issue = "49150")] impl<P: Deref> Deref for Pin<P> { type Target = P::Target; fn deref(&self) -> &P::Target { Pin::get_ref(Pin::as_ref(self)) } } #[unstable(feature = "pin", issue = "49150")] impl<P: DerefMut> DerefMut for Pin<P> where P::Target: Unpin { fn deref_mut(&mut self) -> &mut P::Target { Pin::get_mut(Pin::as_mut(self)) } } #[unstable(feature = "pin", issue = "49150")] impl<P: fmt::Debug> fmt::Debug for Pin<P> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Debug::fmt(&self.pointer, f) } } #[unstable(feature = "pin", issue = "49150")] impl<P: fmt::Display> fmt::Display for Pin<P> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Display::fmt(&self.pointer, f) } } #[unstable(feature = "pin", issue = "49150")] impl<P: fmt::Pointer> fmt::Pointer for Pin<P> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Pointer::fmt(&self.pointer, f) } } // Note: this means that any impl of `CoerceUnsized` that allows coercing from // a type that impls `Deref<Target=impl !Unpin>` to a type that impls // `Deref<Target=Unpin>` is unsound. Any such impl would probably be unsound // for other reasons, though, so we just need to take care not to allow such // impls to land in std. #[unstable(feature = "pin", issue = "49150")] impl<P, U> CoerceUnsized<Pin<U>> for Pin<P> where P: CoerceUnsized<U>, {} #[unstable(feature = "pin", issue = "49150")] impl<'a, P, U> DispatchFromDyn<Pin<U>> for Pin<P> where P: DispatchFromDyn<U>, {} #[unstable(feature = "pin", issue = "49150")] impl<P> Unpin for Pin<P> {}