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use core::mem;
use futures_core::{Future, IntoFuture, Poll};
use futures_core::task;
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
pub struct Lazy<R: IntoFuture, F> {
inner: _Lazy<R::Future, F>,
}
#[derive(Debug)]
enum _Lazy<R, F> {
First(F),
Second(R),
Moved,
}
pub fn lazy<R, F>(f: F) -> Lazy<R, F>
where F: FnOnce(&mut task::Context) -> R,
R: IntoFuture
{
Lazy {
inner: _Lazy::First(f),
}
}
impl<R, F> Lazy<R, F>
where F: FnOnce(&mut task::Context) -> R,
R: IntoFuture,
{
fn get(&mut self, cx: &mut task::Context) -> &mut R::Future {
match self.inner {
_Lazy::First(_) => {}
_Lazy::Second(ref mut f) => return f,
_Lazy::Moved => panic!(),
}
match mem::replace(&mut self.inner, _Lazy::Moved) {
_Lazy::First(f) => self.inner = _Lazy::Second(f(cx).into_future()),
_ => panic!(),
}
match self.inner {
_Lazy::Second(ref mut f) => f,
_ => panic!(),
}
}
}
impl<R, F> Future for Lazy<R, F>
where F: FnOnce(&mut task::Context) -> R,
R: IntoFuture,
{
type Item = R::Item;
type Error = R::Error;
fn poll(&mut self, cx: &mut task::Context) -> Poll<R::Item, R::Error> {
self.get(cx).poll(cx)
}
}