1use pin_project_lite::pin_project;
2
3use std::future::Future;
4use std::pin::Pin;
5use std::task::{self, Poll};
6
7pub(crate) trait Started: Future {
8 fn started(&self) -> bool;
9}
10
11pub(crate) fn lazy<F, R>(func: F) -> Lazy<F, R>
12where
13 F: FnOnce() -> R,
14 R: Future + Unpin,
15{
16 Lazy {
17 inner: Inner::Init { func },
18 }
19}
20
21// FIXME: allow() required due to `impl Trait` leaking types to this lint
22pin_project! {
23 #[allow(missing_debug_implementations)]
24 pub(crate) struct Lazy<F, R> {
25 #[pin]
26 inner: Inner<F, R>,
27 }
28}
29
30pin_project! {
31 #[project = InnerProj]
32 #[project_replace = InnerProjReplace]
33 enum Inner<F, R> {
34 Init { func: F },
35 Fut { #[pin] fut: R },
36 Empty,
37 }
38}
39
40impl<F, R> Started for Lazy<F, R>
41where
42 F: FnOnce() -> R,
43 R: Future,
44{
45 fn started(&self) -> bool {
46 match self.inner {
47 Inner::Init { .. } => false,
48 Inner::Fut { .. } | Inner::Empty => true,
49 }
50 }
51}
52
53impl<F, R> Future for Lazy<F, R>
54where
55 F: FnOnce() -> R,
56 R: Future,
57{
58 type Output = R::Output;
59
60 fn poll(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
61 let mut this: Projection<'_, F, R> = self.project();
62
63 if let InnerProj::Fut { fut: Pin<&mut R> } = this.inner.as_mut().project() {
64 return fut.poll(cx);
65 }
66
67 match this.inner.as_mut().project_replace(replacement:Inner::Empty) {
68 InnerProjReplace::Init { func: F } => {
69 this.inner.set(Inner::Fut { fut: func() });
70 if let InnerProj::Fut { fut: Pin<&mut R> } = this.inner.project() {
71 return fut.poll(cx);
72 }
73 unreachable!()
74 }
75 _ => unreachable!("lazy state wrong"),
76 }
77 }
78}
79