77 lines
1.7 KiB
Rust
77 lines
1.7 KiB
Rust
use pin_project_lite::pin_project;
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use super::{task, Future, Pin, Poll};
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pub(crate) trait Started: Future {
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fn started(&self) -> bool;
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}
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pub(crate) fn lazy<F, R>(func: F) -> Lazy<F, R>
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where
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F: FnOnce() -> R,
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R: Future + Unpin,
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{
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Lazy {
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inner: Inner::Init { func },
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}
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}
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// FIXME: allow() required due to `impl Trait` leaking types to this lint
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pin_project! {
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#[allow(missing_debug_implementations)]
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pub(crate) struct Lazy<F, R> {
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#[pin]
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inner: Inner<F, R>,
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}
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}
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pin_project! {
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#[project = InnerProj]
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#[project_replace = InnerProjReplace]
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enum Inner<F, R> {
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Init { func: F },
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Fut { #[pin] fut: R },
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Empty,
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}
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}
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impl<F, R> Started for Lazy<F, R>
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where
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F: FnOnce() -> R,
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R: Future,
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{
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fn started(&self) -> bool {
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match self.inner {
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Inner::Init { .. } => false,
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Inner::Fut { .. } | Inner::Empty => true,
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}
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}
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}
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impl<F, R> Future for Lazy<F, R>
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where
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F: FnOnce() -> R,
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R: Future,
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{
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type Output = R::Output;
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fn poll(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
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let mut this = self.project();
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if let InnerProj::Fut { fut } = this.inner.as_mut().project() {
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return fut.poll(cx);
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}
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match this.inner.as_mut().project_replace(Inner::Empty) {
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InnerProjReplace::Init { func } => {
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this.inner.set(Inner::Fut { fut: func() });
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if let InnerProj::Fut { fut } = this.inner.project() {
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return fut.poll(cx);
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}
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unreachable!()
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}
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_ => unreachable!("lazy state wrong"),
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}
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}
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}
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