feat: allow downcasting pinned Sleep

This commit allows downcasting pinned `Sleep` object to support
implementations of `Timer::reset`.

One example where this is useful is when using `TokioSleep`, i.e.
`Sleep` provided by tokio.

benches/support/tokiort.rs

@@ -43,8 +43,8 @@ impl Timer for TokioTimer {
     }
 
     fn reset(&self, sleep: &mut Pin<Box<dyn Sleep>>, new_deadline: Instant) {
-        if sleep.downcast_ref::<TokioSleep>().is_some() {
-            *sleep = self.sleep_until(new_deadline);
+        if let Some(sleep) = sleep.as_mut().downcast_mut_pin::<TokioSleep>() {
+            sleep.reset(new_deadline.into())
         }
     }
 }
@@ -81,7 +81,10 @@ impl Future for TokioSleep {
     }
 }
 
-// Use HasSleep to get tokio::time::Sleep to implement Unpin.
-// see https://docs.rs/tokio/latest/tokio/time/struct.Sleep.html
-
 impl Sleep for TokioSleep {}
+
+impl TokioSleep {
+    pub fn reset(self: Pin<&mut Self>, deadline: Instant) {
+        self.project().inner.as_mut().reset(deadline.into());
+    }
+}

src/rt/timer.rs

@@ -29,8 +29,8 @@
 //!     }
 //!
 //!     fn reset(&self, sleep: &mut Pin<Box<dyn Sleep>>, new_deadline: Instant) {
-//!         if sleep.downcast_ref::<TokioSleep>().is_some() {
-//!             *sleep = self.sleep_until(new_deadline);
+//!         if let Some(sleep) = sleep.as_mut().downcast_mut_pin::<TokioSleep>() {
+//!             sleep.reset(new_deadline.into())
 //!         }
 //!     }
 //! }
@@ -51,6 +51,12 @@
 //! }
 //!
 //! impl Sleep for TokioSleep {}
+//!
+//! impl TokioSleep {
+//!     pub fn reset(self: Pin<&mut Self>, deadline: Instant) {
+//!         self.project().inner.as_mut().reset(deadline.into());
+//!     }
+//! }
 //! ````
 
 use std::{
@@ -120,6 +126,23 @@ impl dyn Sleep {
             None
         }
     }
+
+    /// Downcast a pinned &mut Sleep object to its original type
+    pub fn downcast_mut_pin<T>(self: Pin<&mut Self>) -> Option<Pin<&'static mut T>>
+    where
+        T: Sleep + 'static,
+    {
+        if self.is::<T>() {
+            unsafe {
+                let inner = Pin::into_inner_unchecked(self);
+                Some(Pin::new_unchecked(
+                    &mut *(&mut *inner as *mut dyn Sleep as *mut T),
+                ))
+            }
+        } else {
+            None
+        }
+    }
 }
 
 mod private {