async_recursion and mlua: Future created by async block is not Send

[elipp]

I can somewhat reason what the compiler is trying to say, but I haven't been able to figure out how to fix this - or if what I'm trying to do is possible in the first place 😹

Consider the following (contrived) example:

use std::{time::Duration};

use mlua::{Lua, Result as LuaResult, UserData};
use async_recursion::async_recursion;
use tokio::{time::sleep};

#[derive(Debug, Clone)]
struct Sleeper;

impl UserData for Sleeper {
    fn add_methods<'lua, M: mlua::UserDataMethods<'lua, Self>>(methods: &mut M) {
        methods.add_async_function("sleep", |lua, (reader, millis): (Self, u64) | async move {
            println!("sleeping for {millis} ms");
            sleep(Duration::from_millis(millis)).await;
            println!("woke up :)");
            Ok(millis)
        });
    }
}

#[async_recursion]
async fn recursive_fn(lua: Lua) -> LuaResult<u64> {
    let recurse: bool = lua.globals().get("recurse")?;
    if recurse {
        lua.globals().set("recurse", false)?;
        let res: LuaResult<u64> = lua.load("SLEEPER:sleep(1500)").eval_async().await;
        recursive_fn(lua).await
    }
    else {
        Ok(0)
    }
}


#[tokio::main]
async fn main() -> LuaResult<()> {
    let lua = Lua::new();
    lua.globals().set("recurse", true)?;
    lua.globals().set("SLEEPER", Sleeper)?;
    let res = recursive_fn(lua).await?;
    Ok(())
}

The compiler has the following to say about this piece of code:

21 | #[async_recursion]
   | ^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
   |
   = help: the trait `Sync` is not implemented for `(dyn for<'a, 'b> Fn(&'a Lua, mlua::Debug<'b>) -> Result<(), LuaError> + Send + 'static)`
note: future is not `Send` as this value is used across an await
  --> src/main.rs:26:79
   |
26 |         let res: LuaResult<u64> = lua.load("SLEEPER:sleep(1500)").eval_async().await;
   |                                   --- has type `&Lua` which is not `Send`     ^^^^^^ await occurs here, with `lua` maybe used later
note: `lua` is later dropped here
  --> src/main.rs:26:85
   |
26 |         let res: LuaResult<u64> = lua.load("SLEEPER:sleep(1500)").eval_async().await;
   |                                                                                     ^
help: consider moving this into a `let` binding to create a shorter lived borrow

Is there anything one can do to make this work, or is the concept fundamentally flawed? I have enabled the "sync" feature.

Thanks!

[elipp]

I cross-posted this question on Stack Overflow:

https://stackoverflow.com/questions/75535427/rust-mlua-async-and-recursion

[elipp]

Oh, seems like there already was a discussion (and an answer) on basically this exact matter. Delete the thread if you like =)