Use Options instead of a custom enum

It looks bad either way

src/mailbox.rs

@@ -9,13 +9,16 @@ pub struct Mailbox<T>(Mutex<MailboxInner<T>>);
 
 impl<T> Mailbox<T> {
 	pub fn new() -> Self {
-		Mailbox(Mutex::new(MailboxInner::Empty(None)))
+		Mailbox(Mutex::new(MailboxInner {
+			waker: None,
+			value: None,
+		}))
 	}
 }
 
-enum MailboxInner<T> {
-	Empty(Option<Waker>),
-	Full(Option<Waker>, T),
+struct MailboxInner<T> {
+	waker: Option<Waker>,
+	value: Option<T>,
 }
 
 struct MailboxPut<'a, T>(&'a Mailbox<T>, Option<T>);
@@ -25,18 +28,18 @@ impl<'a, T> Future for MailboxPut<'a, T> {
 
 	fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
 		let mut mb = self.0 .0.lock().unwrap();
-		match &mut *mb {
-			MailboxInner::Empty(ref mut waker) => {
-				let waker = waker.take();
-				*mb = MailboxInner::Full(None, self.1.take().expect("Mailbox future overpooled!"));
+		match mb.value {
+			None => {
+				let waker = mb.waker.take();
+				mb.value = self.1.take();
 				drop(mb);
 				if let Some(waker) = waker {
 					waker.wake();
 				}
 				Poll::Ready(())
 			}
-			MailboxInner::Full(ref mut waker, _) => {
-				*waker = Some(cx.waker().clone());
+			Some(_) => {
+				mb.waker = Some(cx.waker().clone());
 				Poll::Pending
 			}
 		}
@@ -49,13 +52,13 @@ impl<'a, T> Future for MailboxGet<'a, T> {
 
 	fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
 		let mut mb = self.0 .0.lock().unwrap();
-		let old = std::mem::replace(&mut *mb, MailboxInner::Empty(None));
-		match old {
-			MailboxInner::Empty(_) => {
-				*mb = MailboxInner::Empty(Some(cx.waker().clone()));
+		match mb.value.take() {
+			None => {
+				mb.waker = Some(cx.waker().clone());
 				Poll::Pending
 			}
-			MailboxInner::Full(waker, value) => {
+			Some(value) => {
+				let waker = mb.waker.take();
 				drop(mb);
 				if let Some(waker) = waker {
 					waker.wake();