From 6cdb7df44313d395ce582331c9191e278f227422 Mon Sep 17 00:00:00 2001 From: SabrinaJewson Date: Thu, 27 Jul 2023 16:29:13 +0100 Subject: [PATCH] Override `Waker::clone_from` to avoid cloning `Waker`s unnecessarily --- library/core/src/task/wake.rs | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/library/core/src/task/wake.rs b/library/core/src/task/wake.rs index b63fd5c9095fc..817e39942c053 100644 --- a/library/core/src/task/wake.rs +++ b/library/core/src/task/wake.rs @@ -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 @@ -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")] @@ -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")]