Pinning

To poll futures, they must be pinned using a special type called Pin<T>. If you read the explanation of the Future trait in the previous section "Executing Futures and Tasks", you'll recognize Pin from the self: Pin<&mut Self> in the Future::poll method's definition. But what does it mean, and why do we need it?

Why Pinning

Pin works in tandem with the Unpin marker. Pinning makes it possible to guarantee that an object implementing !Unpin won't ever be moved. To understand why this is necessary, we need to remember how async/.await works. Consider the following code:

let fut_one = /* ... */;
let fut_two = /* ... */;
async move {
    fut_one.await;
    fut_two.await;
}

Under the hood, this creates an anonymous type that implements Future, providing a poll method that looks something like this:

// The `Future` type generated by our `async { ... }` block
struct AsyncFuture {
    fut_one: FutOne,
    fut_two: FutTwo,
    state: State,
}

// List of states our `async` block can be in
enum State {
    AwaitingFutOne,
    AwaitingFutTwo,
    Done,
}

impl Future for AsyncFuture {
    type Output = ();

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
        loop {
            match self.state {
                State::AwaitingFutOne => match self.fut_one.poll(..) {
                    Poll::Ready(()) => self.state = State::AwaitingFutTwo,
                    Poll::Pending => return Poll::Pending,
                }
                State::AwaitingFutTwo => match self.fut_two.poll(..) {
                    Poll::Ready(()) => self.state = State::Done,
                    Poll::Pending => return Poll::Pending,
                }
                State::Done => return Poll::Ready(()),
            }
        }
    }
}

When poll is first called, it will poll fut_one. If fut_one can't complete, AsyncFuture::poll will return. Future calls to poll will pick up where the previous one left off. This process continues until the future is able to successfully complete.

However, what happens if we have an async block that uses references? For example:

async {
    let mut x = [0; 128];
    let read_into_buf_fut = read_into_buf(&mut x);
    read_into_buf_fut.await;
    println!("{:?}", x);
}

What struct does this compile down to?

struct ReadIntoBuf<'a> {
    buf: &'a mut [u8], // points to `x` below
}

struct AsyncFuture {
    x: [u8; 128],
    read_into_buf_fut: ReadIntoBuf<'what_lifetime?>,
}

Here, the ReadIntoBuf future holds a reference into the other field of our structure, x. However, if AsyncFuture is moved, the location of x will move as well, invalidating the pointer stored in read_into_buf_fut.buf.

Pinning futures to a particular spot in memory prevents this problem, making it safe to create references to values inside an async block.

Pinning in Detail

Let's try to understand pinning by using an slightly simpler example. The problem we encounter above is a problem that ultimately boils down to how we handle references in self-referential types in Rust.

For now our example will look like this:

#[derive(Debug)]
struct Test {
    a: String,
    b: *const String,
}

impl Test {
    fn new(txt: &str) -> Self {
        Test {
            a: String::from(txt),
            b: std::ptr::null(),
        }
    }

    fn init(&mut self) {
        let self_ref: *const String = &self.a;
        self.b = self_ref;
    }

    fn a(&self) -> &str {
        &self.a
    }

    fn b(&self) -> &String {
        assert!(!self.b.is_null(), "Test::b called without Test::init being called first");
        unsafe { &*(self.b) }
    }
}

Test provides methods to get a reference to the value of the fields a and b. Since b is a reference to a we store it as a pointer since the borrowing rules of Rust doesn't allow us to define this lifetime. We now have what we call a self-referential struct.

Our example works fine if we don't move any of our data around as you can observe by running this example:

fn main() {
    let mut test1 = Test::new("test1");
    test1.init();
    let mut test2 = Test::new("test2");
    test2.init();

    println!("a: {}, b: {}", test1.a(), test1.b());
    println!("a: {}, b: {}", test2.a(), test2.b());

}
#[derive(Debug)]
struct Test {
    a: String,
    b: *const String,
}

impl Test {
    fn new(txt: &str) -> Self {
        Test {
            a: String::from(txt),
            b: std::ptr::null(),
        }
    }

    // We need an `init` method to actually set our self-reference
    fn init(&mut self) {
        let self_ref: *const String = &self.a;
        self.b = self_ref;
    }

    fn a(&self) -> &str {
        &self.a
    }

    fn b(&self) -> &String {
        assert!(!self.b.is_null(), "Test::b called without Test::init being called first");
        unsafe { &*(self.b) }
    }
}

We get what we'd expect:

a: test1, b: test1
a: test2, b: test2

Let's see what happens if we swap test1 with test2 and thereby move the data:

fn main() {
    let mut test1 = Test::new("test1");
    test1.init();
    let mut test2 = Test::new("test2");
    test2.init();

    println!("a: {}, b: {}", test1.a(), test1.b());
    std::mem::swap(&mut test1, &mut test2);
    println!("a: {}, b: {}", test2.a(), test2.b());

}
#[derive(Debug)]
struct Test {
    a: String,
    b: *const String,
}

impl Test {
    fn new(txt: &str) -> Self {
        Test {
            a: String::from(txt),
            b: std::ptr::null(),
        }
    }

    fn init(&mut self) {
        let self_ref: *const String = &self.a;
        self.b = self_ref;
    }

    fn a(&self) -> &str {
        &self.a
    }

    fn b(&self) -> &String {
        assert!(!self.b.is_null(), "Test::b called without Test::init being called first");
        unsafe { &*(self.b) }
    }
}

Naively, we could think that what we should get a debug print of test1 two times like this:

a: test1, b: test1
a: test1, b: test1

But instead we get:

a: test1, b: test1
a: test1, b: test2

The pointer to test2.b still points to the old location which is inside test1 now. The struct is not self-referential anymore, it holds a pointer to a field in a different object. That means we can't rely on the lifetime of test2.b to be tied to the lifetime of test2 anymore.

If you're still not convinced, this should at least convince you:

fn main() {
    let mut test1 = Test::new("test1");
    test1.init();
    let mut test2 = Test::new("test2");
    test2.init();

    println!("a: {}, b: {}", test1.a(), test1.b());
    std::mem::swap(&mut test1, &mut test2);
    test1.a = "I've totally changed now!".to_string();
    println!("a: {}, b: {}", test2.a(), test2.b());

}
#[derive(Debug)]
struct Test {
    a: String,
    b: *const String,
}

impl Test {
    fn new(txt: &str) -> Self {
        Test {
            a: String::from(txt),
            b: std::ptr::null(),
        }
    }

    fn init(&mut self) {
        let self_ref: *const String = &self.a;
        self.b = self_ref;
    }

    fn a(&self) -> &str {
        &self.a
    }

    fn b(&self) -> &String {
        assert!(!self.b.is_null(), "Test::b called without Test::init being called first");
        unsafe { &*(self.b) }
    }
}

The diagram below can help visualize what's going on:

Fig 1: Before and after swap

It's easy to get this to show undefined behavior and fail in other spectacular ways as well.

Pinning in Practice

Let's see how pinning and the Pin type can help us solve this problem.

The Pin type wraps pointer types, guaranteeing that the values behind the pointer won't be moved if it is not implementing Unpin. For example, Pin<&mut T>, Pin<&T>, Pin<Box<T>> all guarantee that T won't be moved if T: !Unpin.

Most types don't have a problem being moved. These types implement a trait called Unpin. Pointers to Unpin types can be freely placed into or taken out of Pin. For example, u8 is Unpin, so Pin<&mut u8> behaves just like a normal &mut u8.

However, types that can't be moved after they're pinned have a marker called !Unpin. Futures created by async/await is an example of this.

Pinning to the Stack

Back to our example. We can solve our problem by using Pin. Let's take a look at what our example would look like if we required a pinned pointer instead:

use std::pin::Pin;
use std::marker::PhantomPinned;

#[derive(Debug)]
struct Test {
    a: String,
    b: *const String,
    _marker: PhantomPinned,
}


impl Test {
    fn new(txt: &str) -> Self {
        Test {
            a: String::from(txt),
            b: std::ptr::null(),
            _marker: PhantomPinned, // This makes our type `!Unpin`
        }
    }

    fn init(self: Pin<&mut Self>) {
        let self_ptr: *const String = &self.a;
        let this = unsafe { self.get_unchecked_mut() };
        this.b = self_ptr;
    }

    fn a(self: Pin<&Self>) -> &str {
        &self.get_ref().a
    }

    fn b(self: Pin<&Self>) -> &String {
        assert!(!self.b.is_null(), "Test::b called without Test::init being called first");
        unsafe { &*(self.b) }
    }
}

Pinning an object to the stack will always be unsafe if our type implements !Unpin. You can use a crate like pin_utils to avoid writing our own unsafe code when pinning to the stack.

Below, we pin the objects test1 and test2 to the stack:

pub fn main() {
    // test1 is safe to move before we initialize it
    let mut test1 = Test::new("test1");
    // Notice how we shadow `test1` to prevent it from being accessed again
    let mut test1 = unsafe { Pin::new_unchecked(&mut test1) };
    Test::init(test1.as_mut());

    let mut test2 = Test::new("test2");
    let mut test2 = unsafe { Pin::new_unchecked(&mut test2) };
    Test::init(test2.as_mut());

    println!("a: {}, b: {}", Test::a(test1.as_ref()), Test::b(test1.as_ref()));
    println!("a: {}, b: {}", Test::a(test2.as_ref()), Test::b(test2.as_ref()));
}
use std::pin::Pin;
use std::marker::PhantomPinned;

#[derive(Debug)]
struct Test {
    a: String,
    b: *const String,
    _marker: PhantomPinned,
}


impl Test {
    fn new(txt: &str) -> Self {
        Test {
            a: String::from(txt),
            b: std::ptr::null(),
            // This makes our type `!Unpin`
            _marker: PhantomPinned,
        }
    }

    fn init(self: Pin<&mut Self>) {
        let self_ptr: *const String = &self.a;
        let this = unsafe { self.get_unchecked_mut() };
        this.b = self_ptr;
    }

    fn a(self: Pin<&Self>) -> &str {
        &self.get_ref().a
    }

    fn b(self: Pin<&Self>) -> &String {
        assert!(!self.b.is_null(), "Test::b called without Test::init being called first");
        unsafe { &*(self.b) }
    }
}

Now, if we try to move our data now we get a compilation error:

pub fn main() {
    let mut test1 = Test::new("test1");
    let mut test1 = unsafe { Pin::new_unchecked(&mut test1) };
    Test::init(test1.as_mut());

    let mut test2 = Test::new("test2");
    let mut test2 = unsafe { Pin::new_unchecked(&mut test2) };
    Test::init(test2.as_mut());

    println!("a: {}, b: {}", Test::a(test1.as_ref()), Test::b(test1.as_ref()));
    std::mem::swap(test1.get_mut(), test2.get_mut());
    println!("a: {}, b: {}", Test::a(test2.as_ref()), Test::b(test2.as_ref()));
}
use std::pin::Pin;
use std::marker::PhantomPinned;

#[derive(Debug)]
struct Test {
    a: String,
    b: *const String,
    _marker: PhantomPinned,
}


impl Test {
    fn new(txt: &str) -> Self {
        Test {
            a: String::from(txt),
            b: std::ptr::null(),
            _marker: PhantomPinned, // This makes our type `!Unpin`
        }
    }

    fn init(self: Pin<&mut Self>) {
        let self_ptr: *const String = &self.a;
        let this = unsafe { self.get_unchecked_mut() };
        this.b = self_ptr;
    }

    fn a(self: Pin<&Self>) -> &str {
        &self.get_ref().a
    }

    fn b(self: Pin<&Self>) -> &String {
        assert!(!self.b.is_null(), "Test::b called without Test::init being called first");
        unsafe { &*(self.b) }
    }
}

The type system prevents us from moving the data.

It's important to note that stack pinning will always rely on guarantees you give when writing unsafe. While we know that the pointee of &'a mut T is pinned for the lifetime of 'a we can't know if the data &'a mut T points to isn't moved after 'a ends. If it does it will violate the Pin contract.

A mistake that is easy to make is forgetting to shadow the original variable since you could drop the Pin and move the data after &'a mut T like shown below (which violates the Pin contract):

fn main() {
   let mut test1 = Test::new("test1");
   let mut test1_pin = unsafe { Pin::new_unchecked(&mut test1) };
   Test::init(test1_pin.as_mut());

   drop(test1_pin);
   println!(r#"test1.b points to "test1": {:?}..."#, test1.b);

   let mut test2 = Test::new("test2");
   mem::swap(&mut test1, &mut test2);
   println!("... and now it points nowhere: {:?}", test1.b);
}
use std::pin::Pin;
use std::marker::PhantomPinned;
use std::mem;

#[derive(Debug)]
struct Test {
    a: String,
    b: *const String,
    _marker: PhantomPinned,
}


impl Test {
    fn new(txt: &str) -> Self {
        Test {
            a: String::from(txt),
            b: std::ptr::null(),
            // This makes our type `!Unpin`
            _marker: PhantomPinned,
        }
    }

    fn init<'a>(self: Pin<&'a mut Self>) {
        let self_ptr: *const String = &self.a;
        let this = unsafe { self.get_unchecked_mut() };
        this.b = self_ptr;
    }

    fn a<'a>(self: Pin<&'a Self>) -> &'a str {
        &self.get_ref().a
    }

    fn b<'a>(self: Pin<&'a Self>) -> &'a String {
        assert!(!self.b.is_null(), "Test::b called without Test::init being called first");
        unsafe { &*(self.b) }
    }
}

Pinning to the Heap

Pinning an !Unpin type to the heap gives our data a stable address so we know that the data we point to can't move after it's pinned. In contrast to stack pinning, we know that the data will be pinned for the lifetime of the object.

use std::pin::Pin;
use std::marker::PhantomPinned;

#[derive(Debug)]
struct Test {
    a: String,
    b: *const String,
    _marker: PhantomPinned,
}

impl Test {
    fn new(txt: &str) -> Pin<Box<Self>> {
        let t = Test {
            a: String::from(txt),
            b: std::ptr::null(),
            _marker: PhantomPinned,
        };
        let mut boxed = Box::pin(t);
        let self_ptr: *const String = &boxed.a;
        unsafe { boxed.as_mut().get_unchecked_mut().b = self_ptr };

        boxed
    }

    fn a(self: Pin<&Self>) -> &str {
        &self.get_ref().a
    }

    fn b(self: Pin<&Self>) -> &String {
        unsafe { &*(self.b) }
    }
}

pub fn main() {
    let test1 = Test::new("test1");
    let test2 = Test::new("test2");

    println!("a: {}, b: {}",test1.as_ref().a(), test1.as_ref().b());
    println!("a: {}, b: {}",test2.as_ref().a(), test2.as_ref().b());
}

Some functions require the futures they work with to be Unpin. To use a Future or Stream that isn't Unpin with a function that requires Unpin types, you'll first have to pin the value using either Box::pin (to create a Pin<Box<T>>) or the pin_utils::pin_mut! macro (to create a Pin<&mut T>). Pin<Box<Fut>> and Pin<&mut Fut> can both be used as futures, and both implement Unpin.

For example:

use pin_utils::pin_mut; // `pin_utils` is a handy crate available on crates.io

// A function which takes a `Future` that implements `Unpin`.
fn execute_unpin_future(x: impl Future<Output = ()> + Unpin) { /* ... */ }

let fut = async { /* ... */ };
execute_unpin_future(fut); // Error: `fut` does not implement `Unpin` trait

// Pinning with `Box`:
let fut = async { /* ... */ };
let fut = Box::pin(fut);
execute_unpin_future(fut); // OK

// Pinning with `pin_mut!`:
let fut = async { /* ... */ };
pin_mut!(fut);
execute_unpin_future(fut); // OK

Summary

1. If T: Unpin (which is the default), then Pin<'a, T> is entirely equivalent to &'a mut T. in other words: Unpin means it's OK for this type to be moved even when pinned, so Pin will have no effect on such a type.

2. Getting a &mut T to a pinned T requires unsafe if T: !Unpin.

3. Most standard library types implement Unpin. The same goes for most "normal" types you encounter in Rust. A Future generated by async/await is an exception to this rule.

4. You can add a !Unpin bound on a type on nightly with a feature flag, or by adding std::marker::PhantomPinned to your type on stable.

5. You can either pin data to the stack or to the heap.

6. Pinning a !Unpin object to the stack requires unsafe

7. Pinning a !Unpin object to the heap does not require unsafe. There is a shortcut for doing this using Box::pin.

8. For pinned data where T: !Unpin 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. This is an important part of the pin contract.