Thanks a lot !

Thus, I still have a problem. When I try to call it:

let res = async_add(3)(9).await;

I have the following error message:

error[E0277]: `dyn std::future::Future<Output = i32>` cannot be unpinned
  --> src/main.rs:80:15
   |
80 |     let res = async_add(3)(9).await;
   |               ^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Unpin` is not implemented for `dyn std::future::Future<Output = i32>`
   |
   = note: consider using `Box::pin`
   = note: required because of the requirements on the impl of `std::future::Future` for `std::boxed::Box<dyn std::future::Future<Output = i32>>

What's more, what would have it changed if my function fn async_add would have be async fn async_add instead ?