#![allow(dead_code)]

use std::cell::RefCell;
use std::marker::PhantomData;

// Lua is Send but !Sync
struct Lua(RefCell<()>);

struct LuaValue<'a>(&'a Lua);

trait ToLua<'a> {
    fn to_lua(self, &'a Lua) -> LuaValue<'a>;
}

trait FromLua<'a> {
    fn from_lua(&'a Lua, LuaValue<'a>) -> Self;
}

struct LuaTable<'a>(&'a Lua);

impl<'a> ToLua<'a> for LuaTable<'a> {
    fn to_lua(self, lua: &'a Lua) -> LuaValue<'a> {
        LuaValue(lua)
    }
}

impl<'a> FromLua<'a> for LuaTable<'a> {
    fn from_lua(lua: &'a Lua, _: LuaValue<'a>) -> Self {
        LuaTable(lua)
    }
}

type LuaCallback = Box<for<'a> Fn(&'a Lua, LuaValue<'a>) -> LuaValue<'a> + Send>;

fn example_function<'a>(_lua: &'a Lua, arg: LuaTable<'a>) -> LuaTable<'a> {
    arg
}

trait CallbackTrait<'lua> {
    fn to_callback(self) -> Box<Fn(&'lua Lua, LuaValue<'lua>) -> LuaValue<'lua> + Send>;
}

struct Wrapper<A, R, F>(F, PhantomData<(A, R)>);

impl<'lua, A, R, F> CallbackTrait<'lua> for Wrapper<A, R, F>
where
    A: FromLua<'lua>,
    R: ToLua<'lua>,
    F: Fn(&'lua Lua, A) -> R + 'static + Send,
{
    fn to_callback(self) -> Box<Fn(&'lua Lua, LuaValue<'lua>) -> LuaValue<'lua> + Send> {
        let f = self.0;
        Box::new(move |lua, arg| f(lua, FromLua::from_lua(lua, arg)).to_lua(lua))
    }
}

fn wrap_callback<A, R, F>(f: F) -> LuaCallback
where
    Wrapper<A, R, F>: for<'lua> CallbackTrait<'lua>,
{
    Wrapper(f, PhantomData).to_callback()
}

fn main() {}