feat(maitake-sync): add is_closed methods to WaitCell/WaitMap/`…

…WaitQueue` (#480)

This commit adds methods for synchronously testing whether a `WaitCell`,
`WaitQueue`, or `WaitMap` has been closed by a call to the corresponding
`close` method.

maitake-sync/src/wait_cell.rs

@@ -481,13 +481,11 @@ impl WaitCell {
         }
     }
 
-    // TODO(eliza): is this an API we want to have?
-    /*
     /// Returns `true` if this `WaitCell` is [closed](Self::close).
-     pub(crate) fn is_closed(&self) -> bool {
-       self.current_state() == State::CLOSED
+    #[must_use]
+    pub fn is_closed(&self) -> bool {
+        self.current_state() == State::CLOSED
     }
-    */
 
     /// Takes this `WaitCell`'s waker.
     // TODO(eliza): could probably be made a public API...

maitake-sync/src/wait_map.rs

@@ -490,6 +490,12 @@ impl<K: PartialEq, V> WaitMap<K, V> {
         }
     }
 
+    /// Returns `true` if this `WaitMap` is [closed](Self::close).
+    #[must_use]
+    pub fn is_closed(&self) -> bool {
+        self.load() == State::Closed
+    }
+
     /// Close the queue, indicating that it may no longer be used.
     ///
     /// Once a queue is closed, all [`wait`] calls (current or future) will

maitake-sync/src/wait_queue.rs

@@ -875,6 +875,12 @@ impl WaitQueue {
         }
     }
 
+    /// Returns `true` if this `WaitQueue` is [closed](Self::close).
+    #[must_use]
+    pub fn is_closed(&self) -> bool {
+        self.load().get(QueueState::STATE) == State::Closed
+    }
+
     /// Returns a [`Waiter`] entry in this queue.
     ///
     /// This is factored out into a separate function because it's used by both