pin.rs.html -- source
Description: Source to the Rust file `libcore/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 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 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) -> 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};

    #[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 furthur 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<P> Unpin for Pin<P> {}

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