Is it possible for a UserData method to return a reference to this (self)?

[drauschenbach]

I'm working on a configuration builder type of UserData, where every method call would ideally return the this object reference. But I'm having trouble finding a way to get my methods to return this instead of some new copy.

My UserData is something like this:

pub struct MessageBuilder {
  pub topic: Mutex<String>,
  pub partition: Mutex<Option<i32>>,
}

impl UserData for MessageBuilder {
fn add_methods<'lua, M: UserDataMethods<'lua, Self>>(methods: &mut M) {
        methods.add_method("partition", |lua, this, partition: i32| {
            let mut locked_partition = this
                .partition
                .lock()
                .map_err(|err| Error::RuntimeError(err.to_string()))?;
            *locked_partition = Some(partition);
            Ok(/* WHAT CAN GO HERE? */)
        });
        methods.add_method("topic", ...
}

And I want to use it like this:

local message = MessageBuilder:new():topic("topic"):partition(23):build()

[khvzak]

You need to use add_function which will receive userdata as first arg in form of AnyUserData to pass back.

impl mlua::UserData for MessageBuilder {
    fn add_methods<'lua, M: LuaUserDataMethods<'lua, Self>>(methods: &mut M) {
        methods.add_function("partition", |_, (ud, partition): (AnyUserData, _)| {
            let mut this = ud.borrow_mut::<Self>()?;
            this.partition = Some(partition);
            Ok(ud)
        });
    }
}

Also you don't really need to mutex to simply change struct fields, as can always borrow underlying data mutably.

[drauschenbach]

Excellent. That allows me to perform a massive cleanup. Thanks!!