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Auto merge of #96979 - SabrinaJewson:waker-update, r=workingjubilee
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Override `Waker::clone_from` to avoid cloning `Waker`s unnecessarily

This would be very useful for futures — I think it’s pretty much always what they want to do instead of `*waker = cx.waker().clone()`.

Tracking issue: #98287

r? rust-lang/libs-api `@rustbot` label +T-libs-api -T-libs
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bors committed Nov 5, 2023
2 parents c1ccc29 + 6cdb7df commit fee5518
Showing 1 changed file with 14 additions and 1 deletion.
15 changes: 14 additions & 1 deletion library/core/src/task/wake.rs
Original file line number Diff line number Diff line change
Expand Up @@ -231,6 +231,10 @@ impl fmt::Debug for Context<'_> {
/// this might be done to wake a future when a blocking function call completes on another
/// thread.
///
/// Note that it is preferable to use `waker.clone_from(&new_waker)` instead
/// of `*waker = new_waker.clone()`, as the former will avoid cloning the waker
/// unnecessarily if the two wakers [wake the same task](Self::will_wake).
///
/// [`Future::poll()`]: core::future::Future::poll
/// [`Poll::Pending`]: core::task::Poll::Pending
#[cfg_attr(not(doc), repr(transparent))] // work around https://github.com/rust-lang/rust/issues/66401
Expand Down Expand Up @@ -302,7 +306,9 @@ impl Waker {
/// when the `Waker`s would awaken the same task. However, if this function
/// returns `true`, it is guaranteed that the `Waker`s will awaken the same task.
///
/// This function is primarily used for optimization purposes.
/// This function is primarily used for optimization purposes — for example,
/// this type's [`clone_from`](Self::clone_from) implementation uses it to
/// avoid cloning the waker when they would wake the same task anyway.
#[inline]
#[must_use]
#[stable(feature = "futures_api", since = "1.36.0")]
Expand Down Expand Up @@ -382,6 +388,13 @@ impl Clone for Waker {
waker: unsafe { (self.waker.vtable.clone)(self.waker.data) },
}
}

#[inline]
fn clone_from(&mut self, source: &Self) {
if !self.will_wake(source) {
*self = source.clone();
}
}
}

#[stable(feature = "futures_api", since = "1.36.0")]
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