From 17d78c4ef922f81e752feb077d66fbce80630c6c Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Thu, 6 Oct 2022 13:36:07 +0200
Subject: [PATCH] poll_fn and Unpin: fix pinning

---
 library/core/src/future/poll_fn.rs | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/library/core/src/future/poll_fn.rs b/library/core/src/future/poll_fn.rs
index db2a523323b7a..90cb797391a08 100644
--- a/library/core/src/future/poll_fn.rs
+++ b/library/core/src/future/poll_fn.rs
@@ -5,7 +5,9 @@ use crate::task::{Context, Poll};
 
 /// Creates a future that wraps a function returning [`Poll`].
 ///
-/// Polling the future delegates to the wrapped function.
+/// Polling the future delegates to the wrapped function. If the returned future is pinned, then the
+/// captured environment of the wrapped function is also pinned in-place, so as long as the closure
+/// does not move out of its captures it can soundly create pinned references to them.
 ///
 /// # Examples
 ///
@@ -41,7 +43,7 @@ pub struct PollFn<F> {
 }
 
 #[stable(feature = "future_poll_fn", since = "1.64.0")]
-impl<F> Unpin for PollFn<F> {}
+impl<F: Unpin> Unpin for PollFn<F> {}
 
 #[stable(feature = "future_poll_fn", since = "1.64.0")]
 impl<F> fmt::Debug for PollFn<F> {
@@ -57,7 +59,8 @@ where
 {
     type Output = T;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
-        (&mut self.f)(cx)
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
+        // SAFETY: We are not moving out of the pinned field.
+        (unsafe { &mut self.get_unchecked_mut().f })(cx)
     }
 }