Rust PIN思考

Pin

Pin主要用于获取可变引用,不让修改。

pub struct Pin<P> {
    pointer: P,
}

如果Pin的对象实现了Unpin,那么可以通过get_mut,甚至是直接使用(Pin实现了自动解引用)。

pub const fn get_mut(self) -> &'a mut T
where
    T: Unpin,
{
    self.pointer
}

impl<P: DerefMut<Target: Unpin>> DerefMut for Pin<P> {
    fn deref_mut(&mut self) -> &mut P::Target {
        Pin::get_mut(Pin::as_mut(self))
    }
}

实际上,大部分类型都自动实现了Unpin

#[stable(feature = "pin", since = "1.33.0")]
impl<'a, T: ?Sized + 'a> Unpin for &'a T {}

#[stable(feature = "pin", since = "1.33.0")]
impl<'a, T: ?Sized + 'a> Unpin for &'a mut T {}

#[stable(feature = "pin_raw", since = "1.38.0")]
impl<T: ?Sized> Unpin for *const T {}

#[stable(feature = "pin_raw", since = "1.38.0")]
impl<T: ?Sized> Unpin for *mut T {}

除非它们手动实现了 !Unpin,或者在类型中添加了 PhantomPinned这个marker

/// A marker type which does not implement `Unpin`.
///
/// If a type contains a `PhantomPinned`, it will not implement `Unpin` by default.
#[stable(feature = "pin", since = "1.33.0")]
#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
pub struct PhantomPinned;

#[stable(feature = "pin", since = "1.33.0")]
impl !Unpin for PhantomPinned {}

Future

/// 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`].
pub trait Future {
	type Output;
	fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
}
手写

手动实现Future,调用poll 前需要先Pin住。
如果自定义结构没有实现 !Unpin,这样的结构是Pin不住的,此时需要开发者自己注意自引用结构的安全。

Generator

Rust将async代码转换成实现Generator的匿名结构体。
tips: 为了保证在自引用情况下的安全,所以调用resume的时候,self需要Pin住。

pub trait Generator<R = ()> {
	type Yield;
	type Return;
	fn resume(self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return>;
}

然后将匿名结构体包裹到GenFuture,进而实现Future!Unpin

/// Wrap a generator in a future.
pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
where
    T: Generator<ResumeTy, Yield = ()>,
{
    #[rustc_diagnostic_item = "gen_future"]
    struct GenFuture<T: Generator<ResumeTy, Yield = ()>>(T);

    // We rely on the fact that async/await futures are immovable in order to create
    // self-referential borrows in the underlying generator.
    impl<T: Generator<ResumeTy, Yield = ()>> !Unpin for GenFuture<T> {}

    impl<T: Generator<ResumeTy, Yield = ()>> Future for GenFuture<T> {
        type Output = T::Return;
        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            // SAFETY: Safe because we're !Unpin + !Drop, and this is just a field projection.
            let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) };

            // Resume the generator, turning the `&mut Context` into a `NonNull` raw pointer. The
            // `.await` lowering will safely cast that back to a `&mut Context`.
            match gen.resume(ResumeTy(NonNull::from(cx).cast::<Context<'static>>())) {
                GeneratorState::Yielded(()) => Poll::Pending,
                GeneratorState::Complete(x) => Poll::Ready(x),
            }
        }
    }

    GenFuture(gen)
}

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