Struct Pin

#[repr(transparent)]
pub struct Pin<Ptr> { /* private fields */ }

A pointer which pins its pointee in place.

Pin is a wrapper around some kind of pointer Ptr which makes that pointer “pin” its pointee value in place, thus preventing the value referenced by that pointer from being moved or otherwise invalidated at that place in memory unless it implements Unpin.

See the pin module documentation for a more thorough exploration of pinning.

Pinning values with Pin<Ptr>

In order to pin a value, we wrap a pointer to that value (of some type Ptr) in a Pin<Ptr>. Pin<Ptr> can wrap any pointer type, forming a promise that the pointee will not be moved or otherwise invalidated. If the pointee value’s type implements Unpin, we are free to disregard these requirements entirely and can wrap any pointer to that value in Pin directly via Pin::new. If the pointee value’s type does not implement Unpin, then Rust will not let us use the Pin::new function directly and we’ll need to construct a Pin-wrapped pointer in one of the more specialized manners discussed below.

We call such a Pin-wrapped pointer a pinning pointer (or pinning ref, or pinning Box, etc.) because its existence is the thing that is pinning the underlying pointee in place: it is the metaphorical “pin” securing the data in place on the pinboard (in memory).

It is important to stress that the thing in the Pin is not the value which we want to pin itself, but rather a pointer to that value! A Pin<Ptr> does not pin the Ptr but rather the pointer’s pointee value.

The most common set of types which require pinning related guarantees for soundness are the compiler-generated state machines that implement Future for the return value of async fns. These compiler-generated Futures may contain self-referential pointers, one of the most common use cases for Pin. More details on this point are provided in the pin module docs, but suffice it to say they require the guarantees provided by pinning to be implemented soundly.

This requirement for the implementation of async fns means that the Future trait requires all calls to poll to use a self: Pin<&mut Self> parameter instead of the usual &mut self. Therefore, when manually polling a future, you will need to pin it first.

You may notice that async fn-sourced Futures are only a small percentage of all Futures that exist, yet we had to modify the signature of poll for all Futures to accommodate them. This is unfortunate, but there is a way that the language attempts to alleviate the extra friction that this API choice incurs: the Unpin trait.

The vast majority of Rust types have no reason to ever care about being pinned. These types implement the Unpin trait, which entirely opts all values of that type out of pinning-related guarantees. For values of these types, pinning a value by pointing to it with a Pin<Ptr> will have no actual effect.

The reason this distinction exists is exactly to allow APIs like Future::poll to take a Pin<Ptr> as an argument for all types while only forcing Future types that actually care about pinning guarantees pay the ergonomics cost. For the majority of Future types that don’t have a reason to care about being pinned and therefore implement Unpin, the Pin<&mut Self> will act exactly like a regular &mut Self, allowing direct access to the underlying value. Only types that don’t implement Unpin will be restricted.

Pinning a value of a type that implements Unpin

If the type of the value you need to “pin” implements Unpin, you can trivially wrap any pointer to that value in a Pin by calling Pin::new.

use std::pin::Pin;

// Create a value of a type that implements `Unpin`
let mut unpin_future = std::future::ready(5);

// Pin it by creating a pinning mutable reference to it (ready to be `poll`ed!)
let my_pinned_unpin_future: Pin<&mut _> = Pin::new(&mut unpin_future);

Pinning a value inside a Box

The simplest and most flexible way to pin a value that does not implement Unpin is to put that value inside a Box and then turn that Box into a “pinning Box” by wrapping it in a Pin. You can do both of these in a single step using Box::pin. Let’s see an example of using this flow to pin a Future returned from calling an async fn, a common use case as described above.

use std::pin::Pin;

async fn add_one(x: u32) -> u32 {
    x + 1
}

// Call the async function to get a future back
let fut = add_one(42);

// Pin the future inside a pinning box
let pinned_fut: Pin<Box<_>> = Box::pin(fut);

If you have a value which is already boxed, for example a Box<dyn Future>, you can pin that value in-place at its current memory address using Box::into_pin.

use std::pin::Pin;
use std::future::Future;

async fn add_one(x: u32) -> u32 {
    x + 1
}

fn boxed_add_one(x: u32) -> Box<dyn Future<Output = u32>> {
    Box::new(add_one(x))
}

let boxed_fut = boxed_add_one(42);

// Pin the future inside the existing box
let pinned_fut: Pin<Box<_>> = Box::into_pin(boxed_fut);

There are similar pinning methods offered on the other standard library smart pointer types as well, like Rc and Arc.

Pinning a value on the stack using pin!

There are some situations where it is desirable or even required (for example, in a #[no_std] context where you don’t have access to the standard library or allocation in general) to pin a value which does not implement Unpin to its location on the stack. Doing so is possible using the pin! macro. See its documentation for more.

Layout and ABI

Pin<Ptr> is guaranteed to have the same memory layout and ABI¹ as Ptr.

---

1. There is a bit of nuance here that is still being decided about whether the aliasing semantics of Pin<&mut T> should be different than &mut T, but this is true as of today. ↩

Implementations

impl<Ptr> Pin<Ptr>

where Ptr: Deref, <Ptr as Deref>::Target: Unpin,

pub const fn new(pointer: Ptr) -> Pin<Ptr>

Constructs a new Pin<Ptr> around a pointer to some data of a type that implements Unpin.

Unlike Pin::new_unchecked, this method is safe because the pointer Ptr dereferences to an Unpin type, which cancels the pinning guarantees.

Examples

use std::pin::Pin;

let mut val: u8 = 5;

// Since `val` doesn't care about being moved, we can safely create a "facade" `Pin`
// which will allow `val` to participate in `Pin`-bound apis  without checking that
// pinning guarantees are actually upheld.
let mut pinned: Pin<&mut u8> = Pin::new(&mut val);

pub fn into_inner(pin: Pin<Ptr>) -> Ptr

Unwraps this Pin<Ptr>, returning the underlying pointer.

Doing this operation safely requires that the data pointed at by this pinning pointer implements Unpin so that we can ignore the pinning invariants when unwrapping it.

Examples

use std::pin::Pin;

let mut val: u8 = 5;
let pinned: Pin<&mut u8> = Pin::new(&mut val);

// Unwrap the pin to get the underlying mutable reference to the value. We can do
// this because `val` doesn't care about being moved, so the `Pin` was just
// a "facade" anyway.
let r = Pin::into_inner(pinned);
assert_eq!(*r, 5);

impl<Ptr> Pin<Ptr>

where Ptr: Deref,

pub const unsafe fn new_unchecked(pointer: Ptr) -> Pin<Ptr>

Constructs a new Pin<Ptr> 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. At its core, pinning a value means making the guarantee that the value’s data will not be moved nor have its storage invalidated until it gets dropped. For a more thorough explanation of pinning, see the pin module docs.

If the caller that is constructing this Pin<Ptr> does not ensure that the data Ptr points to is pinned, that is a violation of the API contract and may lead to undefined behavior in later (even safe) operations.

By using this method, you are also making a promise about the Deref and DerefMut implementations of Ptr, 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 pointer type Ptr and expect these methods to uphold the pinning invariants. Moreover, by calling this method you promise that the reference Ptr dereferences to will not be moved out of again; in particular, it must not be possible to obtain a &mut Ptr::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, and therefore cannot uphold the guarantee that a value, once pinned, remains pinned until it is dropped:

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); // Potential UB down the road ⚠️
    // 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 until it is dropped (unless its type implements Unpin). Because Pin<&mut T> does not own the value, dropping the Pin will not drop the value and will not end the pinning contract. So moving the value after dropping the Pin<&mut T> is still a violation of the API contract.

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>) {
    // This should mean the pointee can never move again.
    let pin = unsafe { Pin::new_unchecked(Rc::clone(&x)) };
    {
        let p: Pin<&T> = pin.as_ref();
        // ...
    }
    drop(pin);

    let content = Rc::get_mut(&mut x).unwrap(); // Potential UB down the road ⚠️
    // 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.
}

Pinning of closure captures

Particular care is required when using Pin::new_unchecked in a closure: Pin::new_unchecked(&mut var) where var is a by-value (moved) closure capture implicitly makes the promise that the closure itself is pinned, and that all uses of this closure capture respect that pinning.

use std::pin::Pin;
use std::task::Context;
use std::future::Future;

fn move_pinned_closure(mut x: impl Future, cx: &mut Context<'_>) {
    // Create a closure that moves `x`, and then internally uses it in a pinned way.
    let mut closure = move || unsafe {
        let _ignore = Pin::new_unchecked(&mut x).poll(cx);
    };
    // Call the closure, so the future can assume it has been pinned.
    closure();
    // Move the closure somewhere else. This also moves `x`!
    let mut moved = closure;
    // Calling it again means we polled the future from two different locations,
    // violating the pinning API contract.
    moved(); // Potential UB ⚠️
}

When passing a closure to another API, it might be moving the closure any time, so Pin::new_unchecked on closure captures may only be used if the API explicitly documents that the closure is pinned.

The better alternative is to avoid all that trouble and do the pinning in the outer function instead (here using the pin! macro):

use std::pin::pin;
use std::task::Context;
use std::future::Future;

fn move_pinned_closure(mut x: impl Future, cx: &mut Context<'_>) {
    let mut x = pin!(x);
    // Create a closure that captures `x: Pin<&mut _>`, which is safe to move.
    let mut closure = move || {
        let _ignore = x.as_mut().poll(cx);
    };
    // Call the closure, so the future can assume it has been pinned.
    closure();
    // Move the closure somewhere else.
    let mut moved = closure;
    // Calling it again here is fine (except that we might be polling a future that already
    // returned `Poll::Ready`, but that is a separate problem).
    moved();
}

pub fn as_ref(&self) -> Pin<&<Ptr as Deref>::Target>

Gets a shared reference to the pinned value this Pin points to.

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.

impl<Ptr> Pin<Ptr>

where Ptr: DerefMut,

pub fn as_mut(&mut self) -> Pin<&mut <Ptr as Deref>::Target>

Gets a mutable reference to the pinned value this Pin<Ptr> points to.

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 pinning pointer.

Example

use std::pin::Pin;

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();
    }
}

pub fn as_deref_mut( self: Pin<&mut Pin<Ptr>>, ) -> Pin<&mut <Ptr as Deref>::Target>

Gets Pin<&mut T> to the underlying pinned value from this nested Pin-pointer.

This is a generic method to go from Pin<&mut Pin<Pointer<T>>> to Pin<&mut T>. It is safe because the existence of a Pin<Pointer<T>> ensures that the pointee, T, cannot move in the future, and this method does not enable the pointee to move. “Malicious” implementations of Ptr::DerefMut are likewise ruled out by the contract of Pin::new_unchecked.

pub fn set(&mut self, value: <Ptr as Deref>::Target)

where <Ptr as Deref>::Target: Sized,

Assigns a new value to the memory location pointed to by the Pin<Ptr>.

This overwrites pinned data, but that is okay: the original pinned value’s destructor gets run before being overwritten and the new value is also a valid value of the same type, so no pinning invariant is violated. See the pin module documentation for more information on how this upholds the pinning invariants.

Example

use std::pin::Pin;

let mut val: u8 = 5;
let mut pinned: Pin<&mut u8> = Pin::new(&mut val);
println!("{}", pinned); // 5
pinned.set(10);
println!("{}", pinned); // 10

impl<Ptr> Pin<Ptr>

where Ptr: Deref,

pub unsafe fn into_inner_unchecked(pin: Pin<Ptr>) -> Ptr

Unwraps this Pin<Ptr>, returning the underlying Ptr.

Safety

This function is unsafe. You must guarantee that you will continue to treat the pointer Ptr as pinned after you call this function, so that the invariants on the Pin type can be upheld. If the code using the resulting Ptr 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.

Note that you must be able to guarantee that the data pointed to by Ptr will be treated as pinned all the way until its drop handler is complete!

For more information, see the pin module docs

If the underlying data is Unpin, Pin::into_inner should be used instead.

impl<'a, T> Pin<&'a T>

where T: ?Sized,

pub unsafe fn map_unchecked<U, F>(self, func: F) -> Pin<&'a U>

where F: FnOnce(&T) -> &U, U: ?Sized,

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.

pub const fn get_ref(self) -> &'a T

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 inner T inside the RefCell, and RefCell<T> does not let you get a Pin<&T> pointer 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 reference contained in the Pin. This method allows turning the Pin into a reference with the same lifetime as the reference it wraps.

impl<'a, T> Pin<&'a mut T>

where T: ?Sized,

pub const fn into_ref(self) -> Pin<&'a T>

Converts this Pin<&mut T> into a Pin<&T> with the same lifetime.

pub const fn get_mut(self) -> &'a mut T

where T: Unpin,

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.

pub const unsafe fn get_unchecked_mut(self) -> &'a mut T

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.

pub unsafe fn map_unchecked_mut<U, F>(self, func: F) -> Pin<&'a mut U>

where F: FnOnce(&mut T) -> &mut U, U: ?Sized,

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.

impl<T> Pin<&'static T>

where T: ?Sized,

pub const fn static_ref(r: &'static T) -> Pin<&'static T>

Gets a pinning reference from a &'static reference.

This is safe because T is borrowed immutably for the 'static lifetime, which never ends.

impl<T> Pin<&'static mut T>

where T: ?Sized,

pub const fn static_mut(r: &'static mut T) -> Pin<&'static mut T>

Gets a pinning mutable reference from a static mutable reference.

This is safe because T is borrowed for the 'static lifetime, which never ends.

Trait Implementations

impl<P> AsyncIterator for Pin<P>

where P: DerefMut, <P as Deref>::Target: AsyncIterator,

type Item = <<P as Deref>::Target as AsyncIterator>::Item

🔬This is a nightly-only experimental API. (async_iterator)

The type of items yielded by the async iterator.

fn poll_next( self: Pin<&mut Pin<P>>, cx: &mut Context<'_>, ) -> Poll<Option<<Pin<P> as AsyncIterator>::Item>>

🔬This is a nightly-only experimental API. (async_iterator)

Attempts to pull out the next value of this async iterator, registering the current task for wakeup if the value is not yet available, and returning None if the async iterator is exhausted.

fn size_hint(&self) -> (usize, Option<usize>)

🔬This is a nightly-only experimental API. (async_iterator)

Returns the bounds on the remaining length of the async iterator.

impl<Ptr> Clone for Pin<Ptr>

where Ptr: Clone,

fn clone(&self) -> Pin<Ptr>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<G, R> Coroutine<R> for Pin<&mut G>

where G: Coroutine<R> + ?Sized,

type Yield = <G as Coroutine<R>>::Yield

🔬This is a nightly-only experimental API. (coroutine_trait)

The type of value this coroutine yields.

type Return = <G as Coroutine<R>>::Return

🔬This is a nightly-only experimental API. (coroutine_trait)

The type of value this coroutine returns.

fn resume( self: Pin<&mut Pin<&mut G>>, arg: R, ) -> CoroutineState<<Pin<&mut G> as Coroutine<R>>::Yield, <Pin<&mut G> as Coroutine<R>>::Return>

🔬This is a nightly-only experimental API. (coroutine_trait)

Resumes the execution of this coroutine.

impl<Ptr> Debug for Pin<Ptr>

where Ptr: Debug,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<Ptr> Deref for Pin<Ptr>

where Ptr: Deref,

type Target = <Ptr as Deref>::Target

The resulting type after dereferencing.

fn deref(&self) -> &<Ptr as Deref>::Target

Dereferences the value.

impl<Ptr> DerefMut for Pin<Ptr>

where Ptr: DerefMut, <Ptr as Deref>::Target: Unpin,

fn deref_mut(&mut self) -> &mut <Ptr as Deref>::Target

Mutably dereferences the value.

impl<Ptr> Display for Pin<Ptr>

where Ptr: Display,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<P> Future for Pin<P>

where P: DerefMut, <P as Deref>::Target: Future,

type Output = <<P as Deref>::Target as Future>::Output

The type of value produced on completion.

fn poll( self: Pin<&mut Pin<P>>, cx: &mut Context<'_>, ) -> Poll<<Pin<P> as Future>::Output>

Attempts to resolve the future to a final value, registering the current task for wakeup if the value is not yet available.

impl<Ptr> Hash for Pin<Ptr>

where Ptr: Deref, <Ptr as Deref>::Target: Hash,

fn hash<H>(&self, state: &mut H)

where H: Hasher,

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<Ptr> Ord for Pin<Ptr>

where Ptr: Deref, <Ptr as Deref>::Target: Ord,

fn cmp(&self, other: &Pin<Ptr>) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl<Ptr, Q> PartialEq<Pin<Q>> for Pin<Ptr>

where Ptr: Deref, Q: Deref, <Ptr as Deref>::Target: PartialEq<<Q as Deref>::Target>,

fn eq(&self, other: &Pin<Q>) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Pin<Q>) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<Ptr, Q> PartialOrd<Pin<Q>> for Pin<Ptr>

where Ptr: Deref, Q: Deref, <Ptr as Deref>::Target: PartialOrd<<Q as Deref>::Target>,

fn partial_cmp(&self, other: &Pin<Q>) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Pin<Q>) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Pin<Q>) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Pin<Q>) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Pin<Q>) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl<Ptr> Pointer for Pin<Ptr>

where Ptr: Pointer,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<Ptr, U> CoerceUnsized<Pin<U>> for Pin<Ptr>

where Ptr: CoerceUnsized<U> + PinCoerceUnsized, U: PinCoerceUnsized,

impl<Ptr> Copy for Pin<Ptr>

where Ptr: Copy,

impl<Ptr> DerefPure for Pin<Ptr>

where Ptr: DerefPure,

impl<Ptr, U> DispatchFromDyn<Pin<U>> for Pin<Ptr>

where Ptr: DispatchFromDyn<U> + PinCoerceUnsized, U: PinCoerceUnsized,

impl<Ptr> Eq for Pin<Ptr>

where Ptr: Deref, <Ptr as Deref>::Target: Eq,

impl<T> PinCoerceUnsized for Pin<T>

where T: PinCoerceUnsized,