Question about yielding from lua to rust

[se7kn8]

In mlua it is possible to yield from an AsyncThread back to rust. I want to run multiple coroutines in parallel, demonstrated in the following example.

use std::{time::{Duration, Instant}};

use futures_util::TryStreamExt;
use mlua::{Lua, UserData, chunk};

#[derive(Default)]
struct MyUserData {}

impl UserData for MyUserData {
    fn add_methods<M: mlua::UserDataMethods<Self>>(methods: &mut M) {
        methods.add_async_method("wait_ms", async |lua, _, milliseconds: u64| {
            let end_time = Instant::now() + Duration::from_millis(milliseconds);

            while Instant::now() < end_time {
                lua.yield_with::<()>(()).await?;
            }

            Ok(())
        });
    }
}

#[tokio::main]
async fn main() {
    let lua = Lua::new();
    lua.globals()
        .set("custom_data", MyUserData::default())
        .unwrap();

    print!("Starting Lua code execution...\n");
    let function = lua
        .load(chunk! {
            print("Lua code started.")
            custom_data:wait_ms(2000)
            print("Lua code finished after waiting.")
        })
        .into_function()
        .unwrap();

    let mut threads = vec![];

    for _ in 0..100 {
        let thread = lua
            .create_thread(function.clone())
            .unwrap()
            .into_async::<()>(())
            .unwrap();

        threads.push(thread);
    }

    loop {
        let mut all_done = true;

        for thread in &mut threads {
            if thread.try_next().await.unwrap().is_some() {
                all_done = false;
            }
        }

        if all_done {
            break;
        }

    }

}

At the first try, this code works without any problems. However as soon as i enable the send feature on the mlua-crate it crashes with the following error (UserDataBorrowError):

called `Result::unwrap()` on an `Err` value: CallbackError { traceback: "stack traceback:\n\t[C]: in ?\n\t__mlua_async_poll:4: in function <__mlua_async_poll:1>\n\tmlua-test/src/main.rs:35:1: in function <mlua-test/src/main.rs:35:1>", cause: BadArgument { to: Some("MyUserData.wait_ms"), pos: 1, name: Some("self"), cause: UserDataBorrowError } }

In this example, the yield is not necessary, however this is only for demonstration purposes.

I've read in the docs: Please note that you cannot cross Thread boundries. Is this meant by the note? Why is it than possible without the Send feature.