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Remove GenFuture from core #107445

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Jan 31, 2023
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Remove GenFuture from core #107445

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45 changes: 0 additions & 45 deletions library/core/src/future/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -56,51 +56,6 @@ unsafe impl Send for ResumeTy {}
#[unstable(feature = "gen_future", issue = "50547")]
unsafe impl Sync for ResumeTy {}

/// Wrap a generator in a future.
///
/// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
/// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
// This is `const` to avoid extra errors after we recover from `const async fn`
#[doc(hidden)]
#[unstable(feature = "gen_future", issue = "50547")]
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Is there some code that uses this feature gate? Possibly in ast lowering? Probably should remove that too.

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The feature gate is still being used for the other implementation details of async lowering in this file.

#[rustc_const_unstable(feature = "gen_future", issue = "50547")]
#[inline]
pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
where
T: crate::ops::Generator<ResumeTy, Yield = ()>,
{
use crate::{
ops::{Generator, GeneratorState},
pin::Pin,
task::Poll,
};

#[rustc_diagnostic_item = "gen_future"]
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Is there any code that uses this diagnosics item?

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I don’t think so, grepping through the whole codebase only yields leftover uses of the feature gate.

struct GenFuture<T: Generator<ResumeTy, Yield = ()>>(T);

// We rely on the fact that async/await futures are immovable in order to create
// self-referential borrows in the underlying generator.
impl<T: Generator<ResumeTy, Yield = ()>> !Unpin for GenFuture<T> {}

impl<T: Generator<ResumeTy, Yield = ()>> Future for GenFuture<T> {
type Output = T::Return;
#[track_caller]
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
// SAFETY: Safe because we're !Unpin + !Drop, and this is just a field projection.
let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) };

// Resume the generator, turning the `&mut Context` into a `NonNull` raw pointer. The
// `.await` lowering will safely cast that back to a `&mut Context`.
match gen.resume(ResumeTy(NonNull::from(cx).cast::<Context<'static>>())) {
GeneratorState::Yielded(()) => Poll::Pending,
GeneratorState::Complete(x) => Poll::Ready(x),
}
}
}

GenFuture(gen)
}

#[lang = "get_context"]
#[doc(hidden)]
#[unstable(feature = "gen_future", issue = "50547")]
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