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use super::local_task;
use core::{
    future::Future,
    marker::Unpin,
    ops::{Generator, GeneratorState},
    pin::Pin,
    task::{Context, Poll},
};

/// Wrap a generator in a future.
#[inline]
pub fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T::Return> {
    GenFuture(x)
}

/// A wrapper around generators used to implement `Future` for `async`/`await`
/// code.
#[must_use = "futures do nothing unless you `.await` or poll them"]
#[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
struct GenFuture<T: Generator<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<Yield = ()>> !Unpin for GenFuture<T> {}

impl<T: Generator<Yield = ()>> Future for GenFuture<T> {
    type Output = T::Return;

    #[inline]
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        // Safe because we're !Unpin + !Drop mapping to a ?Unpin value
        let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) };
        local_task().set_context(cx, || match gen.resume() {
            GeneratorState::Yielded(()) => Poll::Pending,
            GeneratorState::Complete(x) => Poll::Ready(x),
        })
    }
}