Explicitly completed Future type for Rust, similar to Java's CompletableFuture
// create a new, incomplete ManualFuture
let (future, completer) = ManualFuture::new();
// complete the future with a value
completer.complete(5).await;
// retrieve the value from the future
assert_eq!(future.await, 5);
// you can also create ManualFuture instances that are already completed
assert_eq!(ManualFuture::new_completed(10).await, 10);