Example of the function add that accepts an asynchronous callback

main.rs

use futures::future::BoxFuture;

#[tokio::main]
async fn main() {
    let words = vec!["one", "two"];
    let result = add(words, |word| {
        Box::pin(async {
            let mut kw = word;
            kw.push("three");
            kw
        })
    })
    .await;

    assert_eq!(result.len(), 3);
    assert!(result.contains(&"one"));
    assert!(result.contains(&"two"));
    assert!(result.contains(&"three"));

    dbg!(result);
}

pub async fn add<'input_words, F, Output>(words: Vec<&'input_words str>, callback: F) -> Vec<Output>
where
    F: Fn(Vec<&'input_words str>) -> BoxFuture<'input_words, Vec<Output>>,
{
    let mut result = vec![];
    result.extend(callback(words).await);

    result
}

A little explanation is needed. In the add function, you can't just return Future<'input words, Vec<Output> because its size is not pre-known. That's why it must be wrapped in a pinned Box. Thankfully, the BoxFuture shorthand exists for this. If you look at the code, you will see that BoxFuture is defined as pub type BoxFuture<'a, T> = Pin<alloc::boxed::Box<dyn Future<Output = T> + Send + 'a>>.