async closure does not implement Fn because it captures state from its environment

[FelicitusNeko]

Hi,

I'm relatively new to Rust, and I'm trying to make a program that allows for Lua scripting to interact with the Twitch API. However, I'm running into a problem with trying to use the shared state object inside of functions created within create_async_function:

let state = Arc::new(SMState::new(&twitch.token, &ttv_client, &twitch.userid));
let lua = Lua::new();
{
    let lglobal = lua.globals();
    let ltwitch = lua.create_table()?;
    ltwitch.set("me", state.my_user_id.as_str())?;
    let ltwitch_chat = lua.create_table()?;
    {
        let state_c = state.clone();
        ltwitch_chat.set(
            "send",
            lua.create_async_function(async move |lua, (to_user, msg): (String, String)| { // line 130
                let broadcaster = BroadcasterId::from(to_user);
                let me = UserId::from(lua.globals().get_path::<String>("twitch.me")?);
                if let Err(e) = state_c
                    .api
                    .send_chat_message(&broadcaster, &me, &msg)
                    .json()
                    .await
                {
                    println!("Message failed to send: {}", e);
                }
                Ok(())
            })?,
        )?;
    }

    ltwitch.set("chat", ltwitch_chat)?;
    lglobal.set("twitch", ltwitch)?;
}

async closure does not implement `Fn` because it captures state from its environment [Ln 130, Col 51]

Maybe MLua provides a solution for this that would work for me, but I'm not familiar enough with all this to really know how to do this. My main concern is that I want to avoid exposing the Twitch access token within the Lua environment. If it matters, twitch_highway is being used for Twitch API access.

[khvzak]

Try regular closure with async block |...| async move {} instead of async closure async move || {}

See https://blog.rust-lang.org/2025/02/20/Rust-1.85.0/#async-closures also https://www.reddit.com/r/rust/comments/1br6e6i/difference_between_async_and_async/

[FelicitusNeko]

Thank you for your help. I've now tried doing this, and I get this error and suggestion instead:

cannot move out of `state_c`, a captured variable in an `Fn` closure
consider cloning the value before moving it into the closure: `{
                            let value = state_c.clone();
                            `, `value`, `
                            }`

If I follow the suggestion and move the clone into the closure, I get:

closure may outlive the current function, but it borrows `state`, which is owned by the current function
may outlive borrowed value `state`

So it suggests to add move to the closure, but, of course, that moves state out of the main function, thus can't be used later.
If I add another clone outside of the lot, it's no longer reporting an error, but that seems messy. Then I'm working with a clone of a clone inside the actual function, and I effectively have to make two clones for each of the dozens of Twitch API functions I want my application to use. As a visual reference, it looks like this now:

let state = Arc::new(SMState::new(&twitch.token, &ttv_client, &twitch.userid));
let lua = Lua::new();
{
    let lglobal = lua.globals();
    let ltwitch = lua.create_table()?;
    ltwitch.set("me", state.my_user_id.as_str())?;
    let ltwitch_chat = lua.create_table()?;
    {
        let state_c = state.clone();
        ltwitch_chat.set(
            "send",
            lua.create_async_function(move |lua, (to_user, msg): (String, String)| {
                let state_cc = state_c.clone();
                async move {
                    let broadcaster = BroadcasterId::from(to_user);
                    let me =
                        UserId::from(lua.globals().get_path::<String>("twitch.me")?);
                    if let Err(e) = state_cc
                        .api
                        .send_chat_message(&broadcaster, &me, &msg)
                        .json()
                        .await
                    {
                        println!("Message failed to send: {}", e);
                    }
                    Ok(())
                }
            })?,
        )?;
    }

    ltwitch.set("chat", ltwitch_chat)?;
    lglobal.set("twitch", ltwitch)?;
}

Is there a cleaner way to do this?

[khvzak]

Looks okay to me, state is Arc so clones are very cheap (just increasing number of references).

Another option is saving state in Lua application data Lua::set_app_data which can be retrieved anywhere inside Lua function using Lua::app_data_ref

[FelicitusNeko]

Oh, yo! set_app_data is exactly what I was hoping to find in the first place. I guess I glazed over it. Thanks!