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// check-pass
#![feature(gen_future, generator_trait, negative_impls)]
use std::ops::{Generator, GeneratorState};
use std::task::{Poll, Context};
use std::future::{Future};
use std::ptr::NonNull;
use std::pin::Pin;
fn main() {}
#[derive(Debug, Copy, Clone)]
pub struct ResumeTy(NonNull<Context<'static>>);
unsafe impl Send for ResumeTy {}
unsafe impl Sync for ResumeTy {}
pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
where
T: Generator<ResumeTy, Yield = ()>,
{
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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