Macro pin_project

macro_rules! pin_project {
    ($($tt:tt)*) => { ... };
}

A macro that creates a projection type covering all the fields of struct.

This macro creates a projection type according to the following rules:

• For the field that uses #[pin] attribute, makes the pinned reference to the field.
• For the other fields, makes the unpinned reference to the field.

And the following methods are implemented on the original type:

fn project(self: Pin<&mut Self>) -> Projection<'_>;
fn project_ref(self: Pin<&Self>) -> ProjectionRef<'_>;

By passing an attribute with the same name as the method to the macro, you can name the projection type returned from the method. This allows you to use pattern matching on the projected types.

pin_project! {
    #[project = EnumProj]
    enum Enum<T> {
        Variant { #[pin] field: T },
    }
}

impl<T> Enum<T> {
    fn method(self: Pin<&mut Self>) {
        let this: EnumProj<'_, T> = self.project();
        match this {
            EnumProj::Variant { field } => {
                let _: Pin<&mut T> = field;
            }
        }
    }
}

By passing the #[project_replace = MyProjReplace] attribute you may create an additional method which allows the contents of Pin<&mut Self> to be replaced while simultaneously moving out all unpinned fields in Self.

fn project_replace(self: Pin<&mut Self>, replacement: Self) -> MyProjReplace;

Also, note that the projection types returned by project and project_ref have an additional lifetime at the beginning of generics.

let this: EnumProj<'_, T> = self.project();
                   ^^

The visibility of the projected types and projection methods is based on the original type. However, if the visibility of the original type is pub, the visibility of the projected types and the projection methods is downgraded to pub(crate).

Safety

pin_project! macro guarantees safety in much the same way as pin-project crate. Both are completely safe unless you write other unsafe code.

See pin-project crate for more details.

Examples

use std::pin::Pin;

use pin_project_lite::pin_project;

pin_project! {
    struct Struct<T, U> {
        #[pin]
        pinned: T,
        unpinned: U,
    }
}

impl<T, U> Struct<T, U> {
    fn method(self: Pin<&mut Self>) {
        let this = self.project();
        let _: Pin<&mut T> = this.pinned; // Pinned reference to the field
        let _: &mut U = this.unpinned; // Normal reference to the field
    }
}

To use pin_project! on enums, you need to name the projection type returned from the method.

use std::pin::Pin;

use pin_project_lite::pin_project;

pin_project! {
    #[project = EnumProj]
    enum Enum<T> {
        Struct {
            #[pin]
            field: T,
        },
        Unit,
    }
}

impl<T> Enum<T> {
    fn method(self: Pin<&mut Self>) {
        match self.project() {
            EnumProj::Struct { field } => {
                let _: Pin<&mut T> = field;
            }
            EnumProj::Unit => {}
        }
    }
}

If you want to call the project() method multiple times or later use the original Pin type, it needs to use .as_mut() to avoid consuming the Pin.

use std::pin::Pin;

use pin_project_lite::pin_project;

pin_project! {
    struct Struct<T> {
        #[pin]
        field: T,
    }
}

impl<T> Struct<T> {
    fn call_project_twice(mut self: Pin<&mut Self>) {
        // `project` consumes `self`, so reborrow the `Pin<&mut Self>` via `as_mut`.
        self.as_mut().project();
        self.as_mut().project();
    }
}

!Unpin

If you want to make sure Unpin is not implemented, use the #[project(!Unpin)] attribute.

use pin_project_lite::pin_project;

pin_project! {
     #[project(!Unpin)]
     struct Struct<T> {
         #[pin]
         field: T,
     }
}

This is equivalent to using #[pin] attribute for a PhantomPinned field.

use std::marker::PhantomPinned;

use pin_project_lite::pin_project;

pin_project! {
    struct Struct<T> {
        field: T,
        #[pin]
        _pin: PhantomPinned,
    }
}

Note that using PhantomPinned without #[pin] or #[project(!Unpin)] attribute has no effect.

Pinned Drop

In order to correctly implement pin projections, a type’s Drop impl must not move out of any structurally pinned fields. Unfortunately, Drop::drop takes &mut Self, not Pin<&mut Self>.

To implement Drop for type that has pin, add an impl PinnedDrop block at the end of the pin_project macro block. PinnedDrop has the following interface:

trait PinnedDrop {
    fn drop(this: Pin<&mut Self>);
}

Note that the argument to PinnedDrop::drop cannot be named self.

pin_project! implements the actual Drop trait via PinnedDrop you implemented. To explicitly drop a type that implements PinnedDrop, use the drop function just like dropping a type that directly implements Drop.

PinnedDrop::drop will never be called more than once, just like Drop::drop.

use pin_project_lite::pin_project;

pin_project! {
    pub struct Struct<'a> {
        was_dropped: &'a mut bool,
        #[pin]
        field: u8,
    }

    impl PinnedDrop for Struct<'_> {
        fn drop(this: Pin<&mut Self>) { // <----- NOTE: this is not `self`
            **this.project().was_dropped = true;
        }
    }
}

let mut was_dropped = false;
drop(Struct { was_dropped: &mut was_dropped, field: 42 });
assert!(was_dropped);