# Struct std::pin::Pin

1.33.0 · source ·

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

Expand description

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`.

`Pin<P>` is guaranteed to have the same memory layout and ABI as `P`.

See the `pin` module documentation for an explanation of pinning.

## Implementations§

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### impl<P> Pin<P>where P: Deref, <P as Deref>::Target: Unpin,

const: unstable · source

#### pub fn new(pointer: P) -> 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.

##### Examples

    use std::pin::Pin;

    let mut val: u8 = 5;
    // We can pin the value, since it doesn't care about being moved
    let mut pinned: Pin<&mut u8> = Pin::new(&mut val);

Run
1.39.0 (const: unstable) · source

#### pub fn into_inner(pin: Pin<P>) -> P

Unwraps this `Pin<P>` returning the underlying pointer.

This requires that the data inside this `Pin` 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 a reference to the value
    let r = Pin::into_inner(pinned);
    assert_eq!(*r, 5);

Run
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### impl<P> Pin<P>where P: Deref,

const: unstable · source

#### pub unsafe fn new_unchecked(pointer: P) -> 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); // 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.
    }

Run

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>) {
        let pinned = unsafe { Pin::new_unchecked(Rc::clone(&x)) };
        {
            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(); // 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.
     }

Run

###### 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 ⚠️
    }

Run

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();
    }

Run
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#### pub fn as_ref(&self) -> Pin<&<P as Deref>::Target>

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`.

1.39.0 (const: unstable) · source

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

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.

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### impl<P> Pin<P>where P: DerefMut,

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

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;

    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();
        }
    }

Run
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#### pub fn set(&mut self, value: <P as Deref>::Target)where <P as Deref>::Target: Sized,

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.

##### Example

    use std::pin::Pin;

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

Run
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### impl<'a, T> Pin<&'a T>where T: ?Sized,

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#### 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.

const: unstable · source

#### pub 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 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`.

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### impl<'a, T> Pin<&'a mut T>where T: ?Sized,

const: unstable · source

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

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

const: unstable · source

#### pub 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`.

const: unstable · source

#### pub 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.

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#### pub unsafe fn map_unchecked_mut<U, F>(self, func: F) -> Pin<&'a mut U>where F: FnOnce(&mut T) -> &mut U, U: ?Sized,

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.

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### impl<T> Pin<&'static T>where T: ?Sized,

1.61.0 (const: unstable) · source

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

Get a pinned reference from a static reference.

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

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### impl<'a, P> Pin<&'a mut Pin<P>>where P: DerefMut,

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#### pub fn as_deref_mut(self) -> Pin<&'a mut <P as Deref>::Target>

🔬This is a nightly-only experimental API. (`pin_deref_mut` #86918)

Gets a pinned mutable reference from this nested pinned 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 `P::DerefMut` are likewise ruled out by the contract of `Pin::new_unchecked`.

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### impl<T> Pin<&'static mut T>where T: ?Sized,

1.61.0 (const: unstable) · source

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

Get a pinned mutable reference from a static mutable reference.

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

## Trait Implementations§

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### impl<P> AsyncIterator for Pin<P>where P: DerefMut, <P as Deref>::Target: AsyncIterator,

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

🔬This is a nightly-only experimental API. (`async_iterator` #79024)
The type of items yielded by the async iterator.
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#### 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` #79024)
Attempt 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. Read more
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#### fn size_hint(&self) -> (usize, Option<usize>)

🔬This is a nightly-only experimental API. (`async_iterator` #79024)
Returns the bounds on the remaining length of the async iterator. Read more
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### impl<P> Clone for Pin<P>where P: Clone,

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#### fn clone(&self) -> Pin<P>

Returns a copy of the value. Read more
1.0.0 · source§

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

Performs copy-assignment from `source`. Read more
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### impl<G, R> Coroutine<R> for Pin<&mut G>where G: Coroutine<R> + ?Sized,

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

🔬This is a nightly-only experimental API. (`coroutine_trait` #43122)
The type of value this coroutine yields. Read more
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#### type Return = <G as Coroutine<R>>::Return

🔬This is a nightly-only experimental API. (`coroutine_trait` #43122)
The type of value this coroutine returns. Read more
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#### 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` #43122)
Resumes the execution of this coroutine. Read more
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### impl<G, R, A> Coroutine<R> for Pin<Box<G, A>>where G: Coroutine<R> + ?Sized, A: Allocator + 'static,

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

🔬This is a nightly-only experimental API. (`coroutine_trait` #43122)
The type of value this coroutine yields. Read more
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#### type Return = <G as Coroutine<R>>::Return

🔬This is a nightly-only experimental API. (`coroutine_trait` #43122)
The type of value this coroutine returns. Read more
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#### fn resume( self: Pin<&mut Pin<Box<G, A>>>, arg: R ) -> CoroutineState<<Pin<Box<G, A>> as Coroutine<R>>::Yield, <Pin<Box<G, A>> as Coroutine<R>>::Return>

🔬This is a nightly-only experimental API. (`coroutine_trait` #43122)
Resumes the execution of this coroutine. Read more
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### impl<P> Debug for Pin<P>where P: Debug,

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

Formats the value using the given formatter. Read more
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### impl<P> Deref for Pin<P>where P: Deref,

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#### type Target = <P as Deref>::Target

The resulting type after dereferencing.
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#### fn deref(&self) -> &<P as Deref>::Target

Dereferences the value.
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### impl<P> DerefMut for Pin<P>where P: DerefMut, <P as Deref>::Target: Unpin,

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

Mutably dereferences the value.
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### impl<P> Display for Pin<P>where P: Display,

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

Formats the value using the given formatter. Read more
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### impl<T, A> From<Box<T, A>> for Pin<Box<T, A>>where A: Allocator + 'static, T: ?Sized,

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#### fn from(boxed: Box<T, A>) -> Pin<Box<T, A>>

Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement `Unpin`, then `*boxed` will be pinned in memory and unable to be moved.

This conversion does not allocate on the heap and happens in place.

This is also available via `Box::into_pin`.

Constructing and pinning a `Box` with `<Pin<Box<T>>>::from(Box::new(x))` can also be written more concisely using `Box::pin(x)`. This `From` implementation is useful if you already have a `Box<T>`, or you are constructing a (pinned) `Box` in a different way than with `Box::new`.

1.36.0 · source§

### impl<P> Future for Pin<P>where P: DerefMut, <P as Deref>::Target: Future,

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

The type of value produced on completion.
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#### fn poll( self: Pin<&mut Pin<P>>, cx: &mut Context<'_> ) -> Poll<<Pin<P> as Future>::Output>

Attempt to resolve the future to a final value, registering the current task for wakeup if the value is not yet available. Read more
1.41.0 · source§

### impl<P> Hash for Pin<P>where P: Deref, <P as Deref>::Target: Hash,

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

Feeds this value into the given `Hasher`. Read more
1.3.0 · source§

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

Feeds a slice of this type into the given `Hasher`. Read more
1.41.0 · source§

### impl<P> Ord for Pin<P>where P: Deref, <P as Deref>::Target: Ord,

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

This method returns an `Ordering` between `self` and `other`. Read more
1.21.0 · source§

#### fn max(self, other: Self) -> Selfwhere Self: Sized,

Compares and returns the maximum of two values. Read more
1.21.0 · source§

#### fn min(self, other: Self) -> Selfwhere Self: Sized,

Compares and returns the minimum of two values. Read more
1.50.0 · source§

#### fn clamp(self, min: Self, max: Self) -> Selfwhere Self: Sized + PartialOrd,

Restrict a value to a certain interval. Read more
1.41.0 · source§

### impl<P, Q> PartialEq<Pin<Q>> for Pin<P>where P: Deref, Q: Deref, <P as Deref>::Target: PartialEq<<Q as Deref>::Target>,

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

This method tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &Pin<Q>) -> bool

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.41.0 · source§

### impl<P, Q> PartialOrd<Pin<Q>> for Pin<P>where P: Deref, Q: Deref, <P as Deref>::Target: PartialOrd<<Q as Deref>::Target>,

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

This method returns an ordering between `self` and `other` values if one exists. Read more
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#### fn lt(&self, other: &Pin<Q>) -> bool

This method tests less than (for `self` and `other`) and is used by the `<` operator. Read more
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#### fn le(&self, other: &Pin<Q>) -> bool

This method tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more
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#### fn gt(&self, other: &Pin<Q>) -> bool

This method tests greater than (for `self` and `other`) and is used by the `>` operator. Read more
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#### fn ge(&self, other: &Pin<Q>) -> bool

This method tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more
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### impl<P> Pointer for Pin<P>where P: Pointer,

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

Formats the value using the given formatter.
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### impl<P, U> CoerceUnsized<Pin<U>> for Pin<P>where P: CoerceUnsized<U>,

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### impl<P> Copy for Pin<P>where P: Copy,

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### impl<P, U> DispatchFromDyn<Pin<U>> for Pin<P>where P: DispatchFromDyn<U>,

1.41.0 · source§

### impl<P> Eq for Pin<P>where P: Deref, <P as Deref>::Target: Eq,

## Auto Trait Implementations§

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### impl<P> RefUnwindSafe for Pin<P>where P: RefUnwindSafe,

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### impl<P> Send for Pin<P>where P: Send,

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### impl<P> Sync for Pin<P>where P: Sync,

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### impl<P> Unpin for Pin<P>where P: Unpin,

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### impl<P> UnwindSafe for Pin<P>where P: UnwindSafe,

## Blanket Implementations§

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### impl<T> Any for Twhere T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for Twhere T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for Twhere T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for Twhere U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<F> IntoFuture for Fwhere F: Future,

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#### type Output = <F as Future>::Output

The output that the future will produce on completion.
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#### type IntoFuture = F

Which kind of future are we turning this into?
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#### fn into_future(self) -> <F as IntoFuture>::IntoFuture

Creates a future from a value. Read more
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### impl<T> ToOwned for Twhere T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T> ToString for Twhere T: Display + ?Sized,

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#### default fn to_string(&self) -> String

Converts the given value to a `String`. Read more
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### impl<T, U> TryFrom<U> for Twhere U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for Twhere U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.