Add UserData Implementation Proc Macro

[Tired-Fox]

• Update UserData proc macro to implement mlua::UserData automatically based on fields
  • Allows for skipping, readonly, writeonly, and renaming fields for lua
  • Support for tuple struct where the fields are indexed
  • Renaming can push tuple struct values into fields and named struct fields into indexes. ex: data field can be renamed to 1 to make it accessible by indexing the UserData with [1]

se mlua::{IntoLua, FromLua, Lua, StdLib};
use mlua_extras::{UserData, user_data_impl};

#[derive(Clone, UserData)]
struct Data {
    #[field(rename = 1)]
    name: String
}

#[user_data_impl]
impl Data {
    #[method]
    fn get_data(&self) -> mlua::Result<String> {
        Ok(self.name.clone())
    }

    #[metamethod(Index)]
    fn index(&self, lua: &Lua, idx: isize) -> mlua::Result<mlua::Value> {
        match idx {
            -1 => "TESTING".into_lua(lua),
            _ => Ok(mlua::Value::Nil)
        }
    }

    #[metamethod(NewIndex)]
    fn new_index(&mut self, lua: &Lua, idx: isize, value: mlua::Value) -> mlua::Result<()> {
        match idx {
            1 => self.name = String::from_lua(value, lua)?,
            _ => return Err(mlua::Error::runtime(format!("invalid index '{idx}'")))
        }
        Ok(())
    }
}

fn main() -> mlua::Result<()> {
    let lua = unsafe { Lua::unsafe_new_with(StdLib::ALL, Default::default()) };

    lua.globals().set("data", Data { name: "MluaExtras".into() })?;

    lua.load("
    print('Index [1]:', data[1])
    data[1] = 'HelloWorld'
    print('Set data[1] to \\'HelloWorld\\'')
    print('Get Data:', data:get_data())
    print('Index [-1]:', data[-1])
    ").exec()?;

    Ok(())
}

Output:

Index [1]:      MluaExtras
Set data[1] to 'HelloWorld'
Get Data:       HelloWorld
Index [-1]:     TESTING

• Support for Enums and their variants where it fetches the value based on the current variant and sets a value based on the current variant.
  • This throws a runtime error if the user tries to access a value from the wrong variant
  • Adds a _variant field implicitly to fetch the name of the current variant.
  • Variants can be renamed for lua

use mlua_extras::{UserData, user_data_impl};

#[derive(Clone, UserData)]
enum Kind {
   A,
   B(String),
   C {
       name: String,
       age: u8,
   },
   D(u32),
}

#[user_data_impl]
impl Kind {
   #[method]
   fn message(&self) -> String {
       match self {
           Self::A => "Hello, world!".into(),
           Self::B(msg) => msg.clone(),
           Self::C{ name, age } => format!("{name} age {age}"),
           Self::D(count) => count.to_string()
       }
   }
}

fn main() -> mlua::Result<()> {
   let lua = unsafe { Lua::unsafe_new_with(StdLib::ALL, Default::default()) };

   lua.globals().set("kind", Kind::A)?;

   lua.load("
   print('Kind:', kind._variant, kind:message())
   local ok, value = pcall(function() return kind[1] end)
   print('Kind [1]: OK', ok, value)
   ").exec()?;

   Ok(())
}

Output:

Kind:   A       Hello, world!
Kind [1]: OK    false   runtime error: type variant does not contain index '1'