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