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

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    //! Types that pin data to its location in memory.
    //!
    //! It is sometimes useful to have objects that are guaranteed not to 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,
    //! as moving an object with pointers to itself will invalidate them, which could cause undefined
    //! behavior.
    //!
    //! A [`Pin<P>`] ensures that the pointee of any pointer type `P` has a stable location in memory,
    //! meaning it cannot be moved elsewhere and its memory cannot be deallocated
    //! until it gets dropped. We say that the pointee is "pinned".
    //!
    //! By default, all types in Rust are movable. Rust allows passing all types by-value,
    //! and common smart-pointer types such as [`Box<T>`] and `&mut T` allow replacing and
    //! moving the values they contain: you can move out of a [`Box<T>`], or you can use [`mem::swap`].
    //! [`Pin<P>`] wraps a pointer type `P`, so [`Pin`]`<`[`Box`]`<T>>` functions much like a regular
    //! [`Box<T>`]: when a [`Pin`]`<`[`Box`]`<T>>` gets dropped, so do its contents, and the memory gets
    //! deallocated. Similarly, [`Pin`]`<&mut T>` is a lot like `&mut T`. However, [`Pin<P>`] does
    //! not let clients actually obtain a [`Box<T>`] or `&mut T` to pinned data, which implies that you
    //! cannot use operations such as [`mem::swap`]:
    //!
    //! ```
    //! use std::pin::Pin;
    //! fn swap_pins<T>(x: Pin<&mut T>, y: Pin<&mut T>) {
    //!     // `mem::swap` needs `&mut T`, but we cannot get it.
    //!     // We are stuck, we cannot swap the contents of these references.
    //!     // We could use `Pin::get_unchecked_mut`, but that is unsafe for a reason:
    //!     // we are not allowed to use it for moving things out of the `Pin`.
    //! }
    //! ```
    //!
    //! It is worth reiterating that [`Pin<P>`] does *not* change the fact that a Rust compiler
    //! considers all types movable. [`mem::swap`] remains callable for any `T`. Instead, [`Pin<P>`]
    //! prevents certain *values* (pointed to by pointers wrapped in [`Pin<P>`]) from being
    //! moved by making it impossible to call methods that require `&mut T` on them
    //! (like [`mem::swap`]).
    //!
    //! [`Pin<P>`] can be used to wrap any pointer type `P`, and as such it interacts with
    //! [`Deref`] and [`DerefMut`]. A [`Pin<P>`] where `P: Deref` should be considered
    //! as a "`P`-style pointer" to a pinned `P::Target` -- so, a [`Pin`]`<`[`Box`]`<T>>` is
    //! an owned pointer to a pinned `T`, and a [`Pin`]`<`[`Rc`]`<T>>` is a reference-counted
    //! pointer to a pinned `T`.
    //! For correctness, [`Pin<P>`] relies on the implementations of [`Deref`] and
    //! [`DerefMut`] not to move out of their `self` parameter, and only ever to
    //! return a pointer to pinned data when they are called on a pinned pointer.
    //!
    //! # `Unpin`
    //!
    //! Many types are always freely movable, even when pinned, because they do not
    //! rely on having a stable address. This includes all the basic types (like
    //! [`bool`], [`i32`], and references) as well as types consisting solely of these
    //! types. Types that do not care about pinning implement the [`Unpin`]
    //! auto-trait, which cancels the effect of [`Pin<P>`]. For `T: Unpin`,
    //! [`Pin`]`<`[`Box`]`<T>>` and [`Box<T>`] function identically, as do [`Pin`]`<&mut T>` and
    //! `&mut T`.
    //!
    //! Note that pinning and [`Unpin`] only affect the pointed-to type `P::Target`, not the pointer
    //! type `P` itself that got wrapped in [`Pin<P>`]. For example, whether or not [`Box<T>`] is
    //! [`Unpin`] has no effect on the behavior of [`Pin`]`<`[`Box`]`<T>>` (here, `T` is the
    //! pointed-to type).
    //!
    //! # Example: self-referential struct
    //!
    //! ```rust
    //! use std::pin::Pin;
    //! use std::marker::PhantomPinned;
    //! use std::ptr::NonNull;
    //!
    //! // This is a self-referential struct because the slice field points to the data field.
    //! // We cannot inform the compiler about that with a normal reference,
    //! // as this pattern cannot be described with the usual borrowing rules.
    //! // Instead we use a raw pointer, though one which is known not to be null,
    //! // as we know it's pointing at the string.
    //! struct Unmovable {
    //!     data: String,
    //!     slice: NonNull<String>,
    //!     _pin: PhantomPinned,
    //! }
    //!
    //! 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: PhantomPinned,
    //!         };
    //!         let mut boxed = Box::pin(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_unchecked_mut(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 mut new_unmoved = Unmovable::new("world".to_string());
    //! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved);
    //! ```
    //!
    //! # Example: intrusive doubly-linked list
    //!
    //! In an intrusive doubly-linked list, the collection does not actually allocate
    //! the memory for the elements itself. Allocation is controlled by the clients,
    //! and elements can live on a stack frame that lives shorter than the collection does.
    //!
    //! To make this work, every element has pointers to its predecessor and successor in
    //! the list. Elements can only be added when they are pinned, because moving the elements
    //! around would invalidate the pointers. Moreover, the [`Drop`] implementation of a linked
    //! list element will patch the pointers of its predecessor and successor to remove itself
    //! from the list.
    //!
    //! Crucially, we have to be able to rely on [`drop`] being called. If an element
    //! could be deallocated or otherwise invalidated without calling [`drop`], the pointers into it
    //! from its neighbouring elements would become invalid, which would break the data structure.
    //!
    //! Therefore, pinning also comes with a [`drop`]-related guarantee.
    //!
    //! # `Drop` guarantee
    //!
    //! The purpose of pinning is to be able to rely on the placement of some data in memory.
    //! To make this work, not just moving the data is restricted; deallocating, repurposing, or
    //! otherwise invalidating the memory used to store the data is restricted, too.
    //! Concretely, for pinned data you have to maintain the invariant
    //! that *its memory will not get invalidated or repurposed from the moment it gets pinned until
    //! when [`drop`] is called*.  Only once [`drop`] returns or panics, the memory may be reused.
    //!
    //! Memory can be "invalidated" by deallocation, but also by
    //! replacing a [`Some(v)`] by [`None`], or calling [`Vec::set_len`] to "kill" some elements
    //! off of a vector. It can be repurposed by using [`ptr::write`] to overwrite it without
    //! calling the destructor first. None of this is allowed for pinned data without calling [`drop`].
    //!
    //! This is exactly the kind of guarantee that the intrusive linked list from the previous
    //! section needs to function correctly.
    //!
    //! Notice that this guarantee does *not* mean that memory does not leak! It is still
    //! completely okay not ever to call [`drop`] on a pinned element (e.g., you can still
    //! call [`mem::forget`] on a [`Pin`]`<`[`Box`]`<T>>`). In the example of the doubly-linked
    //! list, that element would just stay in the list. However you may not free or reuse the storage
    //! *without calling [`drop`]*.
    //!
    //! # `Drop` implementation
    //!
    //! If your type uses pinning (such as the two examples above), you have to be careful
    //! when implementing [`Drop`]. The [`drop`] function takes `&mut self`, but this
    //! is called *even if your type was previously pinned*! It is as if the
    //! compiler automatically called [`Pin::get_unchecked_mut`].
    //!
    //! This can never cause a problem in safe code because implementing a type that
    //! relies on pinning requires unsafe code, but be aware that deciding to make
    //! use of pinning in your type (for example by implementing some operation on
    //! [`Pin`]`<&Self>` or [`Pin`]`<&mut Self>`) has consequences for your [`Drop`]
    //! implementation as well: if an element of your type could have been pinned,
    //! you must treat [`Drop`] as implicitly taking [`Pin`]`<&mut Self>`.
    //!
    //! For example, you could implement `Drop` as follows:
    //!
    //! ```rust,no_run
    //! # use std::pin::Pin;
    //! # struct Type { }
    //! impl Drop for Type {
    //!     fn drop(&mut self) {
    //!         // `new_unchecked` is okay because we know this value is never used
    //!         // again after being dropped.
    //!         inner_drop(unsafe { Pin::new_unchecked(self)});
    //!         fn inner_drop(this: Pin<&mut Type>) {
    //!             // Actual drop code goes here.
    //!         }
    //!     }
    //! }
    //! ```
    //!
    //! The function `inner_drop` has the type that [`drop`] *should* have, so this makes sure that
    //! you do not accidentally use `self`/`this` in a way that is in conflict with pinning.
    //!
    //! Moreover, if your type is `#[repr(packed)]`, the compiler will automatically
    //! move fields around to be able to drop them. It might even do
    //! that for fields that happen to be sufficiently aligned. As a consequence, you cannot use
    //! pinning with a `#[repr(packed)]` type.
    //!
    //! # Projections and Structural Pinning
    //!
    //! When working with pinned structs, the question arises how one can access the
    //! fields of that struct in a method that takes just [`Pin`]`<&mut Struct>`.
    //! The usual approach is to write helper methods (so called *projections*)
    //! that turn [`Pin`]`<&mut Struct>` into a reference to the field, but what
    //! type should that reference have? Is it [`Pin`]`<&mut Field>` or `&mut Field`?
    //! The same question arises with the fields of an `enum`, and also when considering
    //! container/wrapper types such as [`Vec<T>`], [`Box<T>`], or [`RefCell<T>`].
    //! (This question applies to both mutable and shared references, we just
    //! use the more common case of mutable references here for illustration.)
    //!
    //! It turns out that it is actually up to the author of the data structure
    //! to decide whether the pinned projection for a particular field turns
    //! [`Pin`]`<&mut Struct>` into [`Pin`]`<&mut Field>` or `&mut Field`. There are some
    //! constraints though, and the most important constraint is *consistency*:
    //! every field can be *either* projected to a pinned reference, *or* have
    //! pinning removed as part of the projection. If both are done for the same field,
    //! that will likely be unsound!
    //!
    //! As the author of a data structure you get to decide for each field whether pinning
    //! "propagates" to this field or not. Pinning that propagates is also called "structural",
    //! because it follows the structure of the type.
    //! In the following subsections, we describe the considerations that have to be made
    //! for either choice.
    //!
    //! ## Pinning *is not* structural for `field`
    //!
    //! It may seem counter-intuitive that the field of a pinned struct might not be pinned,
    //! but that is actually the easiest choice: if a [`Pin`]`<&mut Field>` is never created,
    //! nothing can go wrong! So, if you decide that some field does not have structural pinning,
    //! all you have to ensure is that you never create a pinned reference to that field.
    //!
    //! Fields without structural pinning may have a projection method that turns
    //! [`Pin`]`<&mut Struct>` into `&mut Field`:
    //!
    //! ```rust,no_run
    //! # use std::pin::Pin;
    //! # type Field = i32;
    //! # struct Struct { field: Field }
    //! impl Struct {
    //!     fn pin_get_field(self: Pin<&mut Self>) -> &mut Field {
    //!         // This is okay because `field` is never considered pinned.
    //!         unsafe { &mut self.get_unchecked_mut().field }
    //!     }
    //! }
    //! ```
    //!
    //! You may also `impl Unpin for Struct` *even if* the type of `field`
    //! is not [`Unpin`]. What that type thinks about pinning is not relevant
    //! when no [`Pin`]`<&mut Field>` is ever created.
    //!
    //! ## Pinning *is* structural for `field`
    //!
    //! The other option is to decide that pinning is "structural" for `field`,
    //! meaning that if the struct is pinned then so is the field.
    //!
    //! This allows writing a projection that creates a [`Pin`]`<&mut Field>`, thus
    //! witnessing that the field is pinned:
    //!
    //! ```rust,no_run
    //! # use std::pin::Pin;
    //! # type Field = i32;
    //! # struct Struct { field: Field }
    //! impl Struct {
    //!     fn pin_get_field(self: Pin<&mut Self>) -> Pin<&mut Field> {
    //!         // This is okay because `field` is pinned when `self` is.
    //!         unsafe { self.map_unchecked_mut(|s| &mut s.field) }
    //!     }
    //! }
    //! ```
    //!
    //! However, structural pinning comes with a few extra requirements:
    //!
    //! 1.  The struct must only be [`Unpin`] if all the structural fields are
    //!     [`Unpin`]. This is the default, but [`Unpin`] is a safe trait, so as the author of
    //!     the struct it is your responsibility *not* to add something like
    //!     `impl<T> Unpin for Struct<T>`. (Notice that adding a projection operation
    //!     requires unsafe code, so the fact that [`Unpin`] is a safe trait does not break
    //!     the principle that you only have to worry about any of this if you use `unsafe`.)
    //! 2.  The destructor of the struct must not move structural fields out of its argument. This
    //!     is the exact point that was raised in the [previous section][drop-impl]: `drop` takes
    //!     `&mut self`, but the struct (and hence its fields) might have been pinned before.
    //!     You have to guarantee that you do not move a field inside your [`Drop`] implementation.
    //!     In particular, as explained previously, this means that your struct must *not*
    //!     be `#[repr(packed)]`.
    //!     See that section for how to write [`drop`] in a way that the compiler can help you
    //!     not accidentally break pinning.
    //! 3.  You must make sure that you uphold the [`Drop` guarantee][drop-guarantee]:
    //!     once your struct is pinned, the memory that contains the
    //!     content is not overwritten or deallocated without calling the content's destructors.
    //!     This can be tricky, as witnessed by [`VecDeque<T>`]: the destructor of [`VecDeque<T>`]
    //!     can fail to call [`drop`] on all elements if one of the destructors panics. This violates
    //!     the [`Drop`] guarantee, because it can lead to elements being deallocated without
    //!     their destructor being called. ([`VecDeque<T>`] has no pinning projections, so this
    //!     does not cause unsoundness.)
    //! 4.  You must not offer any other operations that could lead to data being moved out of
    //!     the structural fields when your type is pinned. For example, if the struct contains an
    //!     [`Option<T>`] and there is a `take`-like operation with type
    //!     `fn(Pin<&mut Struct<T>>) -> Option<T>`,
    //!     that operation can be used to move a `T` out of a pinned `Struct<T>` -- which means
    //!     pinning cannot be structural for the field holding this data.
    //!
    //!     For a more complex example of moving data out of a pinned type, imagine if [`RefCell<T>`]
    //!     had a method `fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T>`.
    //!     Then we could do the following:
    //!     ```compile_fail
    //!     fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>>) {
    //!         { let p = rc.as_mut().get_pin_mut(); } // Here we get pinned access to the `T`.
    //!         let rc_shr: &RefCell<T> = rc.into_ref().get_ref();
    //!         let b = rc_shr.borrow_mut();
    //!         let content = &mut *b; // And here we have `&mut T` to the same data.
    //!     }
    //!     ```
    //!     This is catastrophic, it means we can first pin the content of the [`RefCell<T>`]
    //!     (using `RefCell::get_pin_mut`) and then move that content using the mutable
    //!     reference we got later.
    //!
    //! ## Examples
    //!
    //! For a type like [`Vec<T>`], both possibilities (structural pinning or not) make sense.
    //! A [`Vec<T>`] with structural pinning could have `get_pin`/`get_pin_mut` methods to get
    //! pinned references to elements. However, it could *not* allow calling
    //! [`pop`][Vec::pop] on a pinned [`Vec<T>`] because that would move the (structurally pinned)
    //! contents! Nor could it allow [`push`][Vec::push], which might reallocate and thus also move the
    //! contents.
    //!
    //! A [`Vec<T>`] without structural pinning could `impl<T> Unpin for Vec<T>`, because the contents
    //! are never pinned and the [`Vec<T>`] itself is fine with being moved as well.
    //! At that point pinning just has no effect on the vector at all.
    //!
    //! In the standard library, pointer types generally do not have structural pinning,
    //! and thus they do not offer pinning projections. This is why `Box<T>: Unpin` holds for all `T`.
    //! It makes sense to do this for pointer types, because moving the `Box<T>`
    //! does not actually move the `T`: the [`Box<T>`] can be freely movable (aka `Unpin`) even if
    //! the `T` is not. In fact, even [`Pin`]`<`[`Box`]`<T>>` and [`Pin`]`<&mut T>` are always
    //! [`Unpin`] themselves, for the same reason: their contents (the `T`) are pinned, but the
    //! pointers themselves can be moved without moving the pinned data. For both [`Box<T>`] and
    //! [`Pin`]`<`[`Box`]`<T>>`, whether the content is pinned is entirely independent of whether the
    //! pointer is pinned, meaning pinning is *not* structural.
    //!
    //! When implementing a [`Future`] combinator, you will usually need structural pinning
    //! for the nested futures, as you need to get pinned references to them to call [`poll`].
    //! But if your combinator contains any other data that does not need to be pinned,
    //! you can make those fields not structural and hence freely access them with a
    //! mutable reference even when you just have [`Pin`]`<&mut Self>` (such as in your own
    //! [`poll`] implementation).
    //!
    //! [`Pin<P>`]: struct.Pin.html
    //! [`Unpin`]: ../marker/trait.Unpin.html
    //! [`Deref`]: ../ops/trait.Deref.html
    //! [`DerefMut`]: ../ops/trait.DerefMut.html
    //! [`mem::swap`]: ../mem/fn.swap.html
    //! [`mem::forget`]: ../mem/fn.forget.html
    //! [`Box<T>`]: ../../std/boxed/struct.Box.html
    //! [`Vec<T>`]: ../../std/vec/struct.Vec.html
    //! [`Vec::set_len`]: ../../std/vec/struct.Vec.html#method.set_len
    //! [`Pin`]: struct.Pin.html
    //! [`Box`]: ../../std/boxed/struct.Box.html
    //! [Vec::pop]: ../../std/vec/struct.Vec.html#method.pop
    //! [Vec::push]: ../../std/vec/struct.Vec.html#method.push
    //! [`Rc`]: ../../std/rc/struct.Rc.html
    //! [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
    //! [`Drop`]: ../../std/ops/trait.Drop.html
    //! [`drop`]: ../../std/ops/trait.Drop.html#tymethod.drop
    //! [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
    //! [`Option<T>`]: ../../std/option/enum.Option.html
    //! [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
    //! [`RefCell<T>`]: ../cell/struct.RefCell.html
    //! [`None`]: ../option/enum.Option.html#variant.None
    //! [`Some(v)`]: ../option/enum.Option.html#variant.Some
    //! [`ptr::write`]: ../ptr/fn.write.html
    //! [`Future`]: ../future/trait.Future.html
    //! [drop-impl]: #drop-implementation
    //! [drop-guarantee]: #drop-guarantee
    //! [`poll`]: ../../std/future/trait.Future.html#tymethod.poll
    //! [`Pin::get_unchecked_mut`]: struct.Pin.html#method.get_unchecked_mut
    //! [`bool`]: ../../std/primitive.bool.html
    //! [`i32`]: ../../std/primitive.i32.html

    #![stable(feature = "pin", since = "1.33.0")]

    use crate::cmp::{self, PartialEq, PartialOrd};
    use crate::fmt;
    use crate::hash::{Hash, Hasher};
    use crate::marker::{Sized, Unpin};
    use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver};

    /// 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 an explanation of pinning.*
    ///
    /// [`Unpin`]: ../../std/marker/trait.Unpin.html
    /// [`pin` module]: ../../std/pin/index.html
    //
    // Note: the `Clone` derive below causes unsoundness as it's possible to implement
    // `Clone` for mutable references.
    // See <https://internals.rust-lang.org/t/unsoundness-in-pin/11311> for more details.
    #[stable(feature = "pin", since = "1.33.0")]
    #[lang = "pin"]
    #[fundamental]
    #[repr(transparent)]
    #[derive(Copy, Clone)]
    pub struct Pin<P> {
        pointer: P,
    }

    // The following implementations aren't derived in order to avoid soundness
    // issues. `&self.pointer` should not be accessible to untrusted trait
    // implementations.
    //
    // See <https://internals.rust-lang.org/t/unsoundness-in-pin/11311/73> for more details.

    #[stable(feature = "pin_trait_impls", since = "1.41.0")]
    impl<P: Deref, Q: Deref> PartialEq<Pin<Q>> for Pin<P>
    where
        P::Target: PartialEq<Q::Target>,
    {
        fn eq(&self, other: &Pin<Q>) -> bool {
            P::Target::eq(self, other)
        }

        fn ne(&self, other: &Pin<Q>) -> bool {
            P::Target::ne(self, other)
        }
    }

    #[stable(feature = "pin_trait_impls", since = "1.41.0")]
    impl<P: Deref<Target: Eq>> Eq for Pin<P> {}

    #[stable(feature = "pin_trait_impls", since = "1.41.0")]
    impl<P: Deref, Q: Deref> PartialOrd<Pin<Q>> for Pin<P>
    where
        P::Target: PartialOrd<Q::Target>,
    {
        fn partial_cmp(&self, other: &Pin<Q>) -> Option<cmp::Ordering> {
            P::Target::partial_cmp(self, other)
        }

        fn lt(&self, other: &Pin<Q>) -> bool {
            P::Target::lt(self, other)
        }

        fn le(&self, other: &Pin<Q>) -> bool {
            P::Target::le(self, other)
        }

        fn gt(&self, other: &Pin<Q>) -> bool {
            P::Target::gt(self, other)
        }

        fn ge(&self, other: &Pin<Q>) -> bool {
            P::Target::ge(self, other)
        }
    }

    #[stable(feature = "pin_trait_impls", since = "1.41.0")]
    impl<P: Deref<Target: Ord>> Ord for Pin<P> {
        fn cmp(&self, other: &Self) -> cmp::Ordering {
            P::Target::cmp(self, other)
        }
    }

    #[stable(feature = "pin_trait_impls", since = "1.41.0")]
    impl<P: Deref<Target: Hash>> Hash for Pin<P> {
        fn hash<H: Hasher>(&self, state: &mut H) {
            P::Target::hash(self, state);
        }
    }

    impl<P: Deref<Target: Unpin>> Pin<P> {
        /// Construct a new `Pin<P>` around a pointer to some data of a type that
        /// implements [`Unpin`].
        ///
        /// Unlike `Pin::new_unchecked`, this method is safe because the pointer
        /// `P` dereferences to an [`Unpin`] type, which cancels the pinning guarantees.
        ///
        /// [`Unpin`]: ../../std/marker/trait.Unpin.html
        #[stable(feature = "pin", since = "1.33.0")]
        #[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) }
        }

        /// Unwraps this `Pin<P>` returning the underlying pointer.
        ///
        /// This requires that the data inside this `Pin` is [`Unpin`] so that we
        /// can ignore the pinning invariants when unwrapping it.
        ///
        /// [`Unpin`]: ../../std/marker/trait.Unpin.html
        #[stable(feature = "pin_into_inner", since = "1.39.0")]
        #[inline(always)]
        pub fn into_inner(pin: Pin<P>) -> P {
            pin.pointer
        }
    }

    impl<P: Deref> Pin<P> {
        /// Construct a new `Pin<P>` around a reference to some data of a type that
        /// may or may not implement `Unpin`.
        ///
        /// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used
        /// instead.
        ///
        /// # Safety
        ///
        /// This constructor is unsafe because we cannot guarantee that the data
        /// pointed to by `pointer` is pinned, meaning that the data will not be moved or
        /// its storage invalidated until it gets dropped. If the constructed `Pin<P>` does
        /// not guarantee that the data `P` points to is pinned, that is a violation of
        /// the API contract and may lead to undefined behavior in later (safe) operations.
        ///
        /// By using this method, you are making a promise about the `P::Deref` and
        /// `P::DerefMut` implementations, if they exist. Most importantly, they
        /// must not move out of their `self` arguments: `Pin::as_mut` and `Pin::as_ref`
        /// will call `DerefMut::deref_mut` and `Deref::deref` *on the pinned pointer*
        /// and expect these methods to uphold the pinning invariants.
        /// Moreover, by calling this method you promise that the reference `P`
        /// dereferences to will not be moved out of again; in particular, it
        /// must not be possible to obtain a `&mut P::Target` and then
        /// move out of that reference (using, for example [`mem::swap`]).
        ///
        /// For example, calling `Pin::new_unchecked` on an `&'a mut T` is unsafe because
        /// while you are able to pin it for the given lifetime `'a`, you have no control
        /// over whether it is kept pinned once `'a` ends:
        /// ```
        /// use std::mem;
        /// use std::pin::Pin;
        ///
        /// fn move_pinned_ref<T>(mut a: T, mut b: T) {
        ///     unsafe {
        ///         let p: Pin<&mut T> = Pin::new_unchecked(&mut a);
        ///         // This should mean the pointee `a` can never move again.
        ///     }
        ///     mem::swap(&mut a, &mut b);
        ///     // The address of `a` changed to `b`'s stack slot, so `a` got moved even
        ///     // though we have previously pinned it! We have violated the pinning API contract.
        /// }
        /// ```
        /// A value, once pinned, must remain pinned forever (unless its type implements `Unpin`).
        ///
        /// Similarly, calling `Pin::new_unchecked` on an `Rc<T>` is unsafe because there could be
        /// aliases to the same data that are not subject to the pinning restrictions:
        /// ```
        /// use std::rc::Rc;
        /// use std::pin::Pin;
        ///
        /// fn move_pinned_rc<T>(mut x: Rc<T>) {
        ///     let pinned = unsafe { Pin::new_unchecked(x.clone()) };
        ///     {
        ///         let p: Pin<&T> = pinned.as_ref();
        ///         // This should mean the pointee can never move again.
        ///     }
        ///     drop(pinned);
        ///     let content = Rc::get_mut(&mut x).unwrap();
        ///     // Now, if `x` was the only reference, we have a mutable reference to
        ///     // data that we pinned above, which we could use to move it as we have
        ///     // seen in the previous example. We have violated the pinning API contract.
        ///  }
        ///  ```
        ///
        /// [`mem::swap`]: ../../std/mem/fn.swap.html
        #[stable(feature = "pin", since = "1.33.0")]
        #[inline(always)]
        pub unsafe fn new_unchecked(pointer: P) -> Pin<P> {
            Pin { pointer }
        }

        /// Gets a pinned shared reference from this pinned pointer.
        ///
        /// This is a generic method to go from `&Pin<Pointer<T>>` to `Pin<&T>`.
        /// It is safe because, as part of the contract of `Pin::new_unchecked`,
        /// the pointee cannot move after `Pin<Pointer<T>>` got created.
        /// "Malicious" implementations of `Pointer::Deref` are likewise
        /// ruled out by the contract of `Pin::new_unchecked`.
        #[stable(feature = "pin", since = "1.33.0")]
        #[inline(always)]
        pub fn as_ref(&self) -> Pin<&P::Target> {
            // SAFETY: see documentation on this function
            unsafe { Pin::new_unchecked(&*self.pointer) }
        }

        /// Unwraps this `Pin<P>` returning the underlying pointer.
        ///
        /// # Safety
        ///
        /// This function is unsafe. You must guarantee that you will continue to
        /// treat the pointer `P` as pinned after you call this function, so that
        /// the invariants on the `Pin` type can be upheld. If the code using the
        /// resulting `P` does not continue to maintain the pinning invariants that
        /// is a violation of the API contract and may lead to undefined behavior in
        /// later (safe) operations.
        ///
        /// If the underlying data is [`Unpin`], [`Pin::into_inner`] should be used
        /// instead.
        ///
        /// [`Unpin`]: ../../std/marker/trait.Unpin.html
        /// [`Pin::into_inner`]: #method.into_inner
        #[stable(feature = "pin_into_inner", since = "1.39.0")]
        #[inline(always)]
        pub unsafe fn into_inner_unchecked(pin: Pin<P>) -> P {
            pin.pointer
        }
    }

    impl<P: DerefMut> Pin<P> {
        /// Gets a pinned mutable reference from this pinned pointer.
        ///
        /// This is a generic method to go from `&mut Pin<Pointer<T>>` to `Pin<&mut T>`.
        /// It is safe because, as part of the contract of `Pin::new_unchecked`,
        /// the pointee cannot move after `Pin<Pointer<T>>` got created.
        /// "Malicious" implementations of `Pointer::DerefMut` are likewise
        /// ruled out by the contract of `Pin::new_unchecked`.
        ///
        /// This method is useful when doing multiple calls to functions that consume the pinned type.
        ///
        /// # Example
        ///
        /// ```
        /// use std::pin::Pin;
        ///
        /// # struct Type {}
        /// impl Type {
        ///     fn method(self: Pin<&mut Self>) {
        ///         // do something
        ///     }
        ///
        ///     fn call_method_twice(mut self: Pin<&mut Self>) {
        ///         // `method` consumes `self`, so reborrow the `Pin<&mut Self>` via `as_mut`.
        ///         self.as_mut().method();
        ///         self.as_mut().method();
        ///     }
        /// }
        /// ```
        #[stable(feature = "pin", since = "1.33.0")]
        #[inline(always)]
        pub fn as_mut(&mut self) -> Pin<&mut P::Target> {
            // SAFETY: see documentation on this function
            unsafe { Pin::new_unchecked(&mut *self.pointer) }
        }

        /// Assigns a new value to the memory behind the pinned reference.
        ///
        /// This overwrites pinned data, but that is okay: its destructor gets
        /// run before being overwritten, so no pinning guarantee is violated.
        #[stable(feature = "pin", since = "1.33.0")]
        #[inline(always)]
        pub fn set(&mut self, value: P::Target)
        where
            P::Target: Sized,
        {
            *(self.pointer) = value;
        }
    }

    impl<'a, T: ?Sized> Pin<&'a T> {
        /// Constructs 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.
        /// However, there are several gotchas with these "pinning projections";
        /// see the [`pin` module] documentation for further details on that topic.
        ///
        /// # 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.
        ///
        /// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
        #[stable(feature = "pin", since = "1.33.0")]
        pub unsafe fn map_unchecked<U, F>(self, func: F) -> Pin<&'a U>
        where
            U: ?Sized,
            F: FnOnce(&T) -> &U,
        {
            let pointer = &*self.pointer;
            let new_pointer = func(pointer);

            // SAFETY: the safety contract for `new_unchecked` must be
            // upheld by the caller.
            unsafe { Pin::new_unchecked(new_pointer) }
        }

        /// Gets a shared reference out of a pin.
        ///
        /// This is safe because it is not possible to move out of a shared reference.
        /// It may seem like there is an issue here with interior mutability: in fact,
        /// it *is* possible to move a `T` out of a `&RefCell<T>`. However, this is
        /// not a problem as long as there does not also exist a `Pin<&T>` pointing
        /// to the same data, and `RefCell<T>` does not let you create a pinned reference
        /// to its contents. See the discussion on ["pinning projections"] for further
        /// details.
        ///
        /// 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`.
        ///
        /// ["pinning projections"]: ../../std/pin/index.html#projections-and-structural-pinning
        #[stable(feature = "pin", since = "1.33.0")]
        #[inline(always)]
        pub fn get_ref(self) -> &'a T {
            self.pointer
        }
    }

    impl<'a, T: ?Sized> Pin<&'a mut T> {
        /// Converts this `Pin<&mut T>` into a `Pin<&T>` with the same lifetime.
        #[stable(feature = "pin", since = "1.33.0")]
        #[inline(always)]
        pub fn into_ref(self) -> Pin<&'a T> {
            Pin { pointer: self.pointer }
        }

        /// Gets 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`.
        #[stable(feature = "pin", since = "1.33.0")]
        #[inline(always)]
        pub fn get_mut(self) -> &'a mut T
        where
            T: Unpin,
        {
            self.pointer
        }

        /// Gets 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.
        #[stable(feature = "pin", since = "1.33.0")]
        #[inline(always)]
        pub unsafe fn get_unchecked_mut(self) -> &'a mut T {
            self.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.
        /// However, there are several gotchas with these "pinning projections";
        /// see the [`pin` module] documentation for further details on that topic.
        ///
        /// # 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.
        ///
        /// [`pin` module]: ../../std/pin/index.html#projections-and-structural-pinning
        #[stable(feature = "pin", since = "1.33.0")]
        pub unsafe fn map_unchecked_mut<U, F>(self, func: F) -> Pin<&'a mut U>
        where
            U: ?Sized,
            F: FnOnce(&mut T) -> &mut U,
        {
            // SAFETY: the caller is responsible for not moving the
            // value out of this reference.
            let pointer = unsafe { Pin::get_unchecked_mut(self) };
            let new_pointer = func(pointer);
            // SAFETY: as the value of `this` is guaranteed to not have
            // been moved out, this call to `new_unchecked` is safe.
            unsafe { Pin::new_unchecked(new_pointer) }
        }
    }

    #[stable(feature = "pin", since = "1.33.0")]
    impl<P: Deref> Deref for Pin<P> {
        type Target = P::Target;
        fn deref(&self) -> &P::Target {
            Pin::get_ref(Pin::as_ref(self))
        }
    }

    #[stable(feature = "pin", since = "1.33.0")]
    impl<P: DerefMut<Target: Unpin>> DerefMut for Pin<P> {
        fn deref_mut(&mut self) -> &mut P::Target {
            Pin::get_mut(Pin::as_mut(self))
        }
    }

    #[unstable(feature = "receiver_trait", issue = "none")]
    impl<P: Receiver> Receiver for Pin<P> {}

    #[stable(feature = "pin", since = "1.33.0")]
    impl<P: fmt::Debug> fmt::Debug for Pin<P> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Debug::fmt(&self.pointer, f)
        }
    }

    #[stable(feature = "pin", since = "1.33.0")]
    impl<P: fmt::Display> fmt::Display for Pin<P> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt(&self.pointer, f)
        }
    }

    #[stable(feature = "pin", since = "1.33.0")]
    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.
    #[stable(feature = "pin", since = "1.33.0")]
    impl<P, U> CoerceUnsized<Pin<U>> for Pin<P> where P: CoerceUnsized<U> {}

    #[stable(feature = "pin", since = "1.33.0")]
    impl<P, U> DispatchFromDyn<Pin<U>> for Pin<P> where P: DispatchFromDyn<U> {}