use core::pin::Pin;
use futures_core::future::{FusedFuture, Future};
use futures_core::task::{Context, Poll};
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
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(&mut Context<'_>) -> R,
{
Lazy { f: Some(f) }
}
impl<F, R> FusedFuture for Lazy<F>
where F: FnOnce(&mut Context<'_>) -> R,
{
fn is_terminated(&self) -> bool { self.f.is_none() }
}
impl<F, R> Future for Lazy<F>
where F: FnOnce(&mut Context<'_>) -> R,
{
type Output = R;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<R> {
Poll::Ready((self.f.take().expect("Lazy polled after completion"))(cx))
}
}