sleep_until: add reason to allow(unused)

Co-authored-by: Josh Stone <cuviper@gmail.com>

Author: dvdsk

library/std/src/sys/pal/unix/time.rs

@@ -292,9 +292,7 @@ impl Instant {
         Some(Instant { t: self.t.checked_sub_duration(other)? })
     }
 
-    // reason for allow(unused): this is needed by the `sleep_until` implementation
-    // for some unix platforms not all.
-    #[allow(unused)]
+    #[allow(unused, reason = "needed by the `sleep_until` on some unix platforms")]
     pub(crate) fn into_timespec(self) -> Timespec {
         self.t
     }

library/std/src/time.rs

@@ -409,8 +409,7 @@ impl Instant {
     }
 
     // used by platform specific `sleep_until` implementations.
-    // reason for #[allow(unused)]: not every platform has a specific `sleep_until`.
-    #[allow(unused)]
+    #[allow(unused, reason = "not every platform has a specific `sleep_until`")]
     pub(crate) fn into_inner(self) -> time::Instant {
         self.0
     }