From 665f2babd8b9a8d78e8239b4808d832100f83454 Mon Sep 17 00:00:00 2001
From: Frank Rehwinkel <frankrehwinkel@gmail.com>
Date: Sun, 30 Oct 2022 12:49:02 -0400
Subject: [PATCH] poll_fn and Unpin: fix pinning

Keeping this crate's poll_fn in sync with that of rust's
library/core/src/future/poll_fn.rs.

Their merge provides all the details.

    https://github.com/rust-lang/rust/pull/102737

None of this crate's uses of poll_fn are affected. It protects against
misuse in the future which could have led to UB as the above mentioned
PR points out with an example of a double free becoming possible.
---
 src/future.rs | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/future.rs b/src/future.rs
index f27df0eb..76fba131 100644
--- a/src/future.rs
+++ b/src/future.rs
@@ -16,7 +16,7 @@ pub(crate) struct PollFn<F> {
     f: F,
 }
 
-impl<F> Unpin for PollFn<F> {}
+impl<F: Unpin> Unpin for PollFn<F> {}
 
 pub(crate) fn poll_fn<T, F>(f: F) -> PollFn<F>
 where
@@ -31,7 +31,8 @@ where
 {
     type Output = T;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
-        (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)
     }
 }