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use core::marker::Unpin;
use core::pin::Pin;
use futures_core::future::Future;
use futures_core::task::{LocalWaker, Poll};
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
pub struct Lazy<F> {
f: Option<F>
}
impl<F> Unpin for Lazy<F> {}
pub fn lazy<F, R>(f: F) -> Lazy<F>
where F: FnOnce(&LocalWaker) -> R,
{
Lazy { f: Some(f) }
}
impl<R, F> Future for Lazy<F>
where F: FnOnce(&LocalWaker) -> R,
{
type Output = R;
fn poll(mut self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<R> {
Poll::Ready((self.f.take().unwrap())(lw))
}
}