Error code E0708

async non-move closures with parameters are currently not supported.

Erroneous code example:

#![feature(async_closure)]

fn main() {
    let add_one = async |num: u8| { // error!
        num + 1
    };
}

async with non-move is currently not supported with the current version, you can use successfully by using move:

#![feature(async_closure)]

fn main() {
    let add_one = async move |num: u8| { // ok!
        num + 1
    };
}