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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-referencial 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 the data in pinning pointer types, such as [`PinMut`] and [`PinBox`], //! which are otherwise equivalent to `& mut` and [`Box`], respectively. //! //! 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. //! //! [`PinMut`]: struct.PinMut.html //! [`PinBox`]: struct.PinBox.html //! [`Unpin`]: trait.Unpin.html //! [`swap`]: ../../std/mem/fn.swap.html //! [`Box`]: ../boxed/struct.Box.html //! //! # Examples //! //! ```rust //! #![feature(pin)] //! //! use std::pin::PinBox; //! use std::marker::Pinned; //! use std::ptr::NonNull; //! //! // This is a self referencial 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) -> PinBox<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 = PinBox::new(res); //! //! let slice = NonNull::from(&boxed.data); //! // we know this is safe because modifying a field doesn't move the whole struct //! unsafe { PinBox::get_mut(&mut boxed).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")] pub use core::pin::*; pub use core::marker::Unpin; use core::convert::From; use core::fmt; use core::future::{Future, FutureObj, LocalFutureObj, UnsafeFutureObj}; use core::marker::Unsize; use core::ops::{CoerceUnsized, Deref, DerefMut}; use core::task::{Context, Poll}; use boxed::Box; /// A pinned, heap allocated reference. /// /// This type is similar to [`Box`], except that it pins its value, /// which prevents it from moving out of the reference, unless it implements [`Unpin`]. /// /// See the [module documentation] for furthur explaination on pinning. /// /// [`Box`]: ../boxed/struct.Box.html /// [`Unpin`]: ../../std/marker/trait.Unpin.html /// [module documentation]: index.html #[unstable(feature = "pin", issue = "49150")] #[fundamental] #[repr(transparent)] pub struct PinBox<T: ?Sized> { inner: Box<T>, } #[unstable(feature = "pin", issue = "49150")] impl<T> PinBox<T> { /// Allocate memory on the heap, move the data into it and pin it. #[unstable(feature = "pin", issue = "49150")] pub fn new(data: T) -> PinBox<T> { PinBox { inner: Box::new(data) } } } #[unstable(feature = "pin", issue = "49150")] impl<T: ?Sized> PinBox<T> { /// Get a pinned reference to the data in this PinBox. #[inline] pub fn as_pin_mut<'a>(&'a mut self) -> PinMut<'a, T> { unsafe { PinMut::new_unchecked(&mut *self.inner) } } /// Constructs a `PinBox` from a raw pointer. /// /// After calling this function, the raw pointer is owned by the /// resulting `PinBox`. Specifically, the `PinBox` destructor will call /// the destructor of `T` and free the allocated memory. Since the /// way `PinBox` allocates and releases memory is unspecified, the /// only valid pointer to pass to this function is the one taken /// from another `PinBox` via the [`PinBox::into_raw`] function. /// /// This function is unsafe because improper use may lead to /// memory problems. For example, a double-free may occur if the /// function is called twice on the same raw pointer. /// /// [`PinBox::into_raw`]: struct.PinBox.html#method.into_raw /// /// # Examples /// /// ``` /// #![feature(pin)] /// use std::pin::PinBox; /// let x = PinBox::new(5); /// let ptr = PinBox::into_raw(x); /// let x = unsafe { PinBox::from_raw(ptr) }; /// ``` #[inline] pub unsafe fn from_raw(raw: *mut T) -> Self { PinBox { inner: Box::from_raw(raw) } } /// Consumes the `PinBox`, returning the wrapped raw pointer. /// /// After calling this function, the caller is responsible for the /// memory previously managed by the `PinBox`. In particular, the /// caller should properly destroy `T` and release the memory. The /// proper way to do so is to convert the raw pointer back into a /// `PinBox` with the [`PinBox::from_raw`] function. /// /// Note: this is an associated function, which means that you have /// to call it as `PinBox::into_raw(b)` instead of `b.into_raw()`. This /// is so that there is no conflict with a method on the inner type. /// /// [`PinBox::from_raw`]: struct.PinBox.html#method.from_raw /// /// # Examples /// /// ``` /// #![feature(pin)] /// use std::pin::PinBox; /// let x = PinBox::new(5); /// let ptr = PinBox::into_raw(x); /// ``` #[inline] pub fn into_raw(b: PinBox<T>) -> *mut T { Box::into_raw(b.inner) } /// Get a mutable reference to the data inside this PinBox. /// /// This function is unsafe. Users must guarantee that the data is never /// moved out of this reference. #[inline] pub unsafe fn get_mut<'a>(this: &'a mut PinBox<T>) -> &'a mut T { &mut *this.inner } /// Convert this PinBox into an unpinned Box. /// /// This function is unsafe. Users must guarantee that the data is never /// moved out of the box. #[inline] pub unsafe fn unpin(this: PinBox<T>) -> Box<T> { this.inner } } #[unstable(feature = "pin", issue = "49150")] impl<T: ?Sized> From<Box<T>> for PinBox<T> { fn from(boxed: Box<T>) -> PinBox<T> { PinBox { inner: boxed } } } #[unstable(feature = "pin", issue = "49150")] impl<T: ?Sized> Deref for PinBox<T> { type Target = T; fn deref(&self) -> &T { &*self.inner } } #[unstable(feature = "pin", issue = "49150")] impl<T: Unpin + ?Sized> DerefMut for PinBox<T> { fn deref_mut(&mut self) -> &mut T { &mut *self.inner } } #[unstable(feature = "pin", issue = "49150")] impl<T: fmt::Display + ?Sized> fmt::Display for PinBox<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Display::fmt(&*self.inner, f) } } #[unstable(feature = "pin", issue = "49150")] impl<T: fmt::Debug + ?Sized> fmt::Debug for PinBox<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Debug::fmt(&*self.inner, f) } } #[unstable(feature = "pin", issue = "49150")] impl<T: ?Sized> fmt::Pointer for PinBox<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { // It's not possible to extract the inner Uniq directly from the Box, // instead we cast it to a *const which aliases the Unique let ptr: *const T = &*self.inner; fmt::Pointer::fmt(&ptr, f) } } #[unstable(feature = "pin", issue = "49150")] impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<PinBox<U>> for PinBox<T> {} #[unstable(feature = "pin", issue = "49150")] impl<T: ?Sized> Unpin for PinBox<T> {} #[unstable(feature = "futures_api", issue = "50547")] impl<F: ?Sized + Future> Future for PinBox<F> { type Output = F::Output; fn poll(mut self: PinMut<Self>, cx: &mut Context) -> Poll<Self::Output> { self.as_pin_mut().poll(cx) } } #[unstable(feature = "futures_api", issue = "50547")] unsafe impl<'a, T, F> UnsafeFutureObj<'a, T> for PinBox<F> where F: Future<Output = T> + 'a { fn into_raw(self) -> *mut () { PinBox::into_raw(self) as *mut () } unsafe fn poll(ptr: *mut (), cx: &mut Context) -> Poll<T> { let ptr = ptr as *mut F; let pin: PinMut<F> = PinMut::new_unchecked(&mut *ptr); pin.poll(cx) } unsafe fn drop(ptr: *mut ()) { drop(PinBox::from_raw(ptr as *mut F)) } } #[unstable(feature = "futures_api", issue = "50547")] impl<'a, F: Future<Output = ()> + Send + 'a> From<PinBox<F>> for FutureObj<'a, ()> { fn from(boxed: PinBox<F>) -> Self { FutureObj::new(boxed) } } #[unstable(feature = "futures_api", issue = "50547")] impl<'a, F: Future<Output = ()> + 'a> From<PinBox<F>> for LocalFutureObj<'a, ()> { fn from(boxed: PinBox<F>) -> Self { LocalFutureObj::new(boxed) } }