Wake up

It looks bad either way

src/mailbox.rs

@@ -0,0 +1,135 @@
+use std::{
+	future::Future,
+	pin::Pin,
+	sync::{Mutex, MutexGuard},
+	task::{Context, Poll, Waker},
+};
+
+pub struct Mailbox<T>(Mutex<MailboxInner<T>>);
+
+impl<T> Mailbox<T> {
+	pub fn new() -> Self {
+		Mailbox(Mutex::new(MailboxInner {
+			waker: None,
+			value: None,
+		}))
+	}
+}
+
+struct MailboxInner<T> {
+	waker: Option<Waker>,
+	value: Option<T>,
+}
+
+fn wake<T>(mut mb: MutexGuard<MailboxInner<T>>) {
+	let waker = mb.waker.take();
+	drop(mb);
+	if let Some(waker) = waker {
+		waker.wake();
+	}
+}
+
+struct MailboxPut<'a, T>(&'a Mailbox<T>, Option<T>);
+impl<'a, T> Unpin for MailboxPut<'a, T> {}
+impl<'a, T> Future for MailboxPut<'a, T> {
+	type Output = ();
+
+	fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+		let mut mb = self.0 .0.lock().unwrap();
+		let None = mb.value else {
+			mb.waker = Some(cx.waker().clone());
+			return Poll::Pending;
+		};
+		mb.value = self.1.take();
+		wake(mb);
+		Poll::Ready(())
+	}
+}
+
+struct MailboxGet<'a, T>(&'a Mailbox<T>);
+impl<'a, T> Future for MailboxGet<'a, T> {
+	type Output = T;
+
+	fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+		let mut mb = self.0 .0.lock().unwrap();
+		let Some(value) = mb.value.take() else {
+			mb.waker = Some(cx.waker().clone());
+			return Poll::Pending;
+		};
+		wake(mb);
+		Poll::Ready(value)
+	}
+}
+
+impl<T> Mailbox<T> {
+	pub fn put(&self, value: T) -> impl Future<Output = ()> + '_ {
+		MailboxPut(&self, Some(value))
+	}
+
+	pub fn get(&self) -> impl Future<Output = T> + '_ {
+		MailboxGet(&self)
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use std::{pin::pin, sync::Arc, task::Wake};
+
+	use super::*;
+
+	struct NoopWaker;
+	impl Wake for NoopWaker {
+		fn wake(self: std::sync::Arc<Self>) {}
+	}
+
+	fn ready(f: impl Future) -> bool {
+		let f = pin!(f);
+		matches!(
+			f.poll(&mut Context::from_waker(&Arc::new(NoopWaker).into())),
+			Poll::Ready(_)
+		)
+	}
+
+	#[test]
+	fn test_mailbox_empty() {
+		let mb = Mailbox::<i32>::new();
+		assert!(!ready(mb.get()));
+	}
+
+	#[test]
+	fn test_mailbox_once() {
+		let mb = Mailbox::<i32>::new();
+		pollster::block_on(mb.put(42));
+		assert_eq!(pollster::block_on(mb.get()), 42);
+		assert!(!ready(mb.get()));
+	}
+
+	#[test]
+	fn test_mailbox_once_oor() {
+		let mb = Mailbox::<i32>::new();
+		let f = mb.get();
+		pollster::block_on(mb.put(42));
+		assert_eq!(pollster::block_on(f), 42);
+	}
+
+	#[test]
+	fn test_mailbox_twice_no_wait() {
+		let mb = Mailbox::<i32>::new();
+		pollster::block_on(mb.put(42));
+		assert!(!ready(mb.put(13)));
+		assert_eq!(pollster::block_on(mb.get()), 42);
+		assert!(!ready(mb.get()));
+	}
+
+	#[test]
+	fn test_mailbox_twice_in_order() {
+		let mb = Mailbox::<i32>::new();
+		let f = mb.get();
+		pollster::block_on(mb.put(42));
+		assert_eq!(pollster::block_on(f), 42);
+		let f = mb.get();
+		pollster::block_on(mb.put(13));
+		assert_eq!(pollster::block_on(f), 13);
+		assert!(!ready(mb.get()));
+	}
+}

src/main.rs

@@ -6,6 +6,8 @@ use winit::{
 	window::{Window, WindowAttributes},
 };
 
+mod mailbox;
+
 fn main() {
 	let event_loop = EventLoop::new().unwrap();