Prevent unnecessary allocation in compat03as01 Waker

Author: cramertj

futures-util/src/compat/compat03as01.rs

@@ -115,12 +115,28 @@ impl Current {
         Current(task01::current())
     }
 
-    fn as_waker(&self) -> task03::Waker {
-        // For simplicity reasons wrap the Waker into an Arc.
-        // We can optimize this again later on and reintroduce WakerLt<'a> which
-        // derefs to Waker, and where cloning it through RawWakerVTable returns
-        // an Arc version
-        ArcWake03::into_waker(Arc::new(Current(self.0.clone())))
+    fn as_waker(&self) -> task03::WakerRef<'_> {
+        unsafe fn ptr_to_current(ptr: *const ()) -> &Current {
+            &*(ptr as *const Current)
+        }
+        unsafe fn current_to_ptr(current: &Current) -> *const () {
+            waker as *const Current as *const ()
+        }
+
+        unsafe fn clone(ptr: *const ()) -> RawWaker {
+            // Lazily create the `Arc` only when the waker is actually cloned.
+            Arc::new(ptr_to_current(ptr).clone()).into_waker().into_raw()
+        }
+        unsafe fn drop(_: *const ()) {}
+        unsafe fn wake(ptr: *const ()) {
+            ptr_to_current(ptr).0.notify()
+        }
+
+        let ptr = waker_to_ptr(self);
+        let vtable = &RawWakerVTable { clone, drop, wake };
+        unsafe {
+            task03::WakerRef::new(task03::Waker::new_unchecked(RawWaker::new(ptr, vtable)))
+        }
     }
 }