Address review comments

Author: Jules-Bertholet

library/core/src/sync/exclusive.rs

@@ -16,8 +16,8 @@ use core::task::{Context, Poll};
 /// While this may seem not very useful, it allows `Exclusive` to _unconditionally_
 /// implement `Sync`. Indeed, the safety requirements of `Sync` state that for `Exclusive`
 /// to be `Sync`, it must be sound to _share_ across threads, that is, it must be sound
-/// for `&Exclusive` to cross thread boundaries. By design, a `&Exclusive` has no API
-/// whatsoever, making it useless, thus harmless, thus memory safe.
+/// for `&Exclusive` to cross thread boundaries. By design, a `&Exclusive<T>` for non-`Sync` T
+/// has no API whatsoever, making it useless, thus harmless, thus memory safe.
 ///
 /// Certain constructs like [`Future`]s can only be used with _exclusive_ access,
 /// and are often `Send` but not `Sync`, so `Exclusive` can be used as hint to the
@@ -201,6 +201,17 @@ where
     }
 }
 
+#[unstable(feature = "exclusive_wrapper", issue = "98407")]
+impl<F, Args> Fn<Args> for Exclusive<F>
+where
+    F: Sync + Fn<Args>,
+    Args: Tuple,
+{
+    extern "rust-call" fn call(&self, args: Args) -> Self::Output {
+        self.as_mut().call(args)
+    }
+}
+
 #[unstable(feature = "exclusive_wrapper", issue = "98407")]
 impl<T> Future for Exclusive<T>
 where