pin.rs.html -- source
Description: Source to the Rust file `liballoc/pin.rs`.
Keywords: rust, rustlang, rust-lang

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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)
        }
    }

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