You are correct in assuming that the assignment would be a type error. Similar to other languages, you'd have to allocate. In rust, it would probably look lik

You are correct in assuming that the assignment would be a type error. Similar to other languages, you'd have to allocate. In rust, it would probably look like: let mut fut: Box<Future<()>> = Box::new(Future<()>::new()); and then fut = Box::new(fut.and_then(|| file.close()));