mlua/

userdata.rs

//! Lua userdata handling.
//!
//! This module provides types for creating and working with Lua userdata from Rust.
//!
//! # Main Types
//!
//! - [`AnyUserData`] - A handle to a Lua userdata value of any Rust type.
//! - [`UserData`] - Trait to implement for types that should be exposed to Lua as userdata.
//! - [`UserDataFields`] - Trait for registering fields on userdata types.
//! - [`UserDataMethods`] - Trait for registering methods on userdata types.
//! - [`UserDataRegistry`] - Registry for userdata methods and fields.
//! - [`UserDataMetatable`] - A handle to the metatable of a userdata type.
//! - [`UserDataRef`] - A borrowed reference to a userdata value.
//! - [`UserDataRefMut`] - A mutably borrowed reference to a userdata value.
//! - [`MetaMethod`] - Metamethod names for customizing Lua operators.

use std::any::TypeId;
use std::ffi::CStr;
use std::fmt;
use std::hash::Hash;
use std::os::raw::{c_char, c_void};

use crate::error::{Error, Result};
use crate::function::Function;
use crate::state::Lua;
use crate::string::LuaString;
use crate::table::{Table, TablePairs};
use crate::traits::{FromLua, FromLuaMulti, IntoLua, IntoLuaMulti};
use crate::types::{MaybeSend, MaybeSync, ValueRef};
use crate::util::{StackGuard, check_stack, get_userdata, push_string, short_type_name, take_userdata};
use crate::value::Value;

#[cfg(feature = "async")]
use std::future::Future;

#[cfg(feature = "serde")]
use {
    serde::ser::{self, Serialize, Serializer},
    std::result::Result as StdResult,
};

// Re-export for convenience
pub(crate) use cell::UserDataStorage;
pub use r#ref::{UserDataRef, UserDataRefMut};
pub use registry::UserDataRegistry;
pub(crate) use registry::{RawUserDataRegistry, UserDataProxy};
pub(crate) use util::{
    TypeIdHints, borrow_userdata_scoped, borrow_userdata_scoped_mut, collect_userdata,
    init_userdata_metatable,
};

/// Kinds of metamethods that can be overridden.
///
/// Currently, this mechanism does not allow overriding the `__gc` metamethod, since there is
/// generally no need to do so: [`UserData`] implementors can instead just implement `Drop`.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum MetaMethod {
    /// The `+` operator.
    Add,
    /// The `-` operator.
    Sub,
    /// The `*` operator.
    Mul,
    /// The `/` operator.
    Div,
    /// The `%` operator.
    Mod,
    /// The `^` operator.
    Pow,
    /// The unary minus (`-`) operator.
    Unm,
    /// The floor division (//) operator.
    #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53", feature = "luau"))]
    #[cfg_attr(
        docsrs,
        doc(cfg(any(feature = "lua55", feature = "lua54", feature = "lua53", feature = "luau")))
    )]
    IDiv,
    /// The bitwise AND (&) operator.
    #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53"))]
    #[cfg_attr(
        docsrs,
        doc(cfg(any(feature = "lua55", feature = "lua54", feature = "lua53")))
    )]
    BAnd,
    /// The bitwise OR (|) operator.
    #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53"))]
    #[cfg_attr(
        docsrs,
        doc(cfg(any(feature = "lua55", feature = "lua54", feature = "lua53")))
    )]
    BOr,
    /// The bitwise XOR (binary ~) operator.
    #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53"))]
    #[cfg_attr(
        docsrs,
        doc(cfg(any(feature = "lua55", feature = "lua54", feature = "lua53")))
    )]
    BXor,
    /// The bitwise NOT (unary ~) operator.
    #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53"))]
    #[cfg_attr(
        docsrs,
        doc(cfg(any(feature = "lua55", feature = "lua54", feature = "lua53")))
    )]
    BNot,
    /// The bitwise left shift (<<) operator.
    #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53"))]
    #[cfg_attr(
        docsrs,
        doc(cfg(any(feature = "lua55", feature = "lua54", feature = "lua53")))
    )]
    Shl,
    /// The bitwise right shift (>>) operator.
    #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53"))]
    #[cfg_attr(
        docsrs,
        doc(cfg(any(feature = "lua55", feature = "lua54", feature = "lua53")))
    )]
    Shr,
    /// The string concatenation operator `..`.
    Concat,
    /// The length operator `#`.
    Len,
    /// The `==` operator.
    Eq,
    /// The `<` operator.
    Lt,
    /// The `<=` operator.
    Le,
    /// Index access `obj[key]`.
    Index,
    /// Index write access `obj[key] = value`.
    NewIndex,
    /// The call "operator" `obj(arg1, args2, ...)`.
    Call,
    /// The `__tostring` metamethod.
    ///
    /// This is not an operator, but will be called by methods such as `tostring` and `print`.
    ToString,
    /// The `__todebugstring` metamethod for debug purposes.
    ///
    /// This is an mlua-specific metamethod that can be used to provide debug representation for
    /// userdata.
    ToDebugString,
    /// The `__pairs` metamethod.
    ///
    /// This is not an operator, but it will be called by the built-in `pairs` function.
    #[cfg(any(
        feature = "lua55",
        feature = "lua54",
        feature = "lua53",
        feature = "lua52",
        feature = "luajit52"
    ))]
    #[cfg_attr(
        docsrs,
        doc(cfg(any(
            feature = "lua55",
            feature = "lua54",
            feature = "lua53",
            feature = "lua52",
            feature = "luajit52"
        )))
    )]
    Pairs,
    /// The `__ipairs` metamethod.
    ///
    /// This is not an operator, but it will be called by the built-in [`ipairs`] function.
    ///
    /// [`ipairs`]: https://www.lua.org/manual/5.2/manual.html#pdf-ipairs
    #[cfg(any(feature = "lua52", feature = "luajit52", doc))]
    #[cfg_attr(docsrs, doc(cfg(any(feature = "lua52", feature = "luajit52"))))]
    IPairs,
    /// The `__iter` metamethod.
    ///
    /// Executed before the iteration begins, and should return an iterator function like `next`
    /// (or a custom one).
    #[cfg(any(feature = "luau", doc))]
    #[cfg_attr(docsrs, doc(cfg(feature = "luau")))]
    Iter,
    /// The `__close` metamethod.
    ///
    /// Executed when a variable, that marked as to-be-closed, goes out of scope.
    ///
    /// More information about to-be-closed variables can be found in the Lua 5.4
    /// [documentation][lua_doc].
    ///
    /// [lua_doc]: https://www.lua.org/manual/5.4/manual.html#3.3.8
    #[cfg(any(feature = "lua55", feature = "lua54"))]
    #[cfg_attr(docsrs, doc(cfg(any(feature = "lua55", feature = "lua54"))))]
    Close,
    /// The `__name`/`__type` metafield.
    ///
    /// This is not a function, but it's value can be used by `tostring` and `typeof` built-in
    /// functions.
    #[doc(hidden)]
    Type,
}

impl PartialEq<MetaMethod> for &str {
    fn eq(&self, other: &MetaMethod) -> bool {
        *self == other.name()
    }
}

impl PartialEq<MetaMethod> for String {
    fn eq(&self, other: &MetaMethod) -> bool {
        self == other.name()
    }
}

impl fmt::Display for MetaMethod {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(fmt, "{}", self.name())
    }
}

impl MetaMethod {
    /// Returns Lua metamethod name, usually prefixed by two underscores.
    pub const fn name(self) -> &'static str {
        match self {
            MetaMethod::Add => "__add",
            MetaMethod::Sub => "__sub",
            MetaMethod::Mul => "__mul",
            MetaMethod::Div => "__div",
            MetaMethod::Mod => "__mod",
            MetaMethod::Pow => "__pow",
            MetaMethod::Unm => "__unm",

            #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53", feature = "luau"))]
            MetaMethod::IDiv => "__idiv",
            #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53"))]
            MetaMethod::BAnd => "__band",
            #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53"))]
            MetaMethod::BOr => "__bor",
            #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53"))]
            MetaMethod::BXor => "__bxor",
            #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53"))]
            MetaMethod::BNot => "__bnot",
            #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53"))]
            MetaMethod::Shl => "__shl",
            #[cfg(any(feature = "lua55", feature = "lua54", feature = "lua53"))]
            MetaMethod::Shr => "__shr",

            MetaMethod::Concat => "__concat",
            MetaMethod::Len => "__len",
            MetaMethod::Eq => "__eq",
            MetaMethod::Lt => "__lt",
            MetaMethod::Le => "__le",
            MetaMethod::Index => "__index",
            MetaMethod::NewIndex => "__newindex",
            MetaMethod::Call => "__call",
            MetaMethod::ToString => "__tostring",
            MetaMethod::ToDebugString => "__todebugstring",

            #[cfg(any(
                feature = "lua55",
                feature = "lua54",
                feature = "lua53",
                feature = "lua52",
                feature = "luajit52"
            ))]
            MetaMethod::Pairs => "__pairs",
            #[cfg(any(feature = "lua52", feature = "luajit52"))]
            MetaMethod::IPairs => "__ipairs",
            #[cfg(feature = "luau")]
            MetaMethod::Iter => "__iter",

            #[cfg(any(feature = "lua55", feature = "lua54"))]
            MetaMethod::Close => "__close",

            #[rustfmt::skip]
            MetaMethod::Type => if cfg!(feature = "luau") { "__type" } else { "__name" },
        }
    }

    pub(crate) const fn as_cstr(self) -> &'static CStr {
        match self {
            #[rustfmt::skip]
            MetaMethod::Type => if cfg!(feature = "luau") { c"__type" } else { c"__name" },
            _ => unreachable!(),
        }
    }

    pub(crate) fn validate(name: &str) -> Result<&str> {
        match name {
            "__gc" => Err(Error::MetaMethodRestricted(name.to_string())),
            "__metatable" => Err(Error::MetaMethodRestricted(name.to_string())),
            _ if name.starts_with("__mlua") => Err(Error::MetaMethodRestricted(name.to_string())),
            name => Ok(name),
        }
    }
}

impl AsRef<str> for MetaMethod {
    fn as_ref(&self) -> &str {
        self.name()
    }
}

impl From<MetaMethod> for String {
    #[inline]
    fn from(method: MetaMethod) -> Self {
        method.name().to_owned()
    }
}

/// Method registry for [`UserData`] implementors.
pub trait UserDataMethods<T> {
    /// Add a regular method which accepts a `&T` as the first parameter.
    ///
    /// Regular methods are implemented by overriding the `__index` metamethod and returning the
    /// accessed method. This allows them to be used with the expected `userdata:method()` syntax.
    ///
    /// If `add_meta_method` is used to set the `__index` metamethod, the `__index` metamethod will
    /// be used as a fall-back if no regular method is found.
    fn add_method<M, A, R>(&mut self, name: impl Into<String>, method: M)
    where
        M: Fn(&Lua, &T, A) -> Result<R> + MaybeSend + 'static,
        A: FromLuaMulti,
        R: IntoLuaMulti;

    /// Add a regular method which accepts a `&mut T` as the first parameter.
    ///
    /// Refer to [`add_method`] for more information about the implementation.
    ///
    /// [`add_method`]: UserDataMethods::add_method
    fn add_method_mut<M, A, R>(&mut self, name: impl Into<String>, method: M)
    where
        M: FnMut(&Lua, &mut T, A) -> Result<R> + MaybeSend + 'static,
        A: FromLuaMulti,
        R: IntoLuaMulti;

    /// Add a method which accepts `T` as the first parameter.
    ///
    /// The userdata `T` will be moved out of the userdata container. This is useful for
    /// methods that need to consume the userdata.
    ///
    /// The method can be called only once per userdata instance, subsequent calls will result in a
    /// [`Error::UserDataDestructed`] error.
    fn add_method_once<M, A, R>(&mut self, name: impl Into<String>, method: M)
    where
        T: 'static,
        M: Fn(&Lua, T, A) -> Result<R> + MaybeSend + 'static,
        A: FromLuaMulti,
        R: IntoLuaMulti,
    {
        let name = name.into();
        let method_name = format!("{}.{name}", short_type_name::<T>());
        self.add_function(name, move |lua, (ud, args): (AnyUserData, A)| {
            let this = (ud.take()).map_err(|err| Error::bad_self_argument(&method_name, err))?;
            method(lua, this, args)
        });
    }

    /// Add an async method which accepts a `&T` as the first parameter and returns [`Future`].
    ///
    /// Refer to [`add_method`] for more information about the implementation.
    ///
    /// [`add_method`]: UserDataMethods::add_method
    #[cfg(feature = "async")]
    #[cfg_attr(docsrs, doc(cfg(feature = "async")))]
    fn add_async_method<M, A, MR, R>(&mut self, name: impl Into<String>, method: M)
    where
        T: 'static,
        M: Fn(Lua, UserDataRef<T>, A) -> MR + MaybeSend + 'static,
        A: FromLuaMulti,
        MR: Future<Output = Result<R>> + MaybeSend + 'static,
        R: IntoLuaMulti;

    /// Add an async method which accepts a `&mut T` as the first parameter and returns [`Future`].
    ///
    /// Refer to [`add_method`] for more information about the implementation.
    ///
    /// [`add_method`]: UserDataMethods::add_method
    #[cfg(feature = "async")]
    #[cfg_attr(docsrs, doc(cfg(feature = "async")))]
    fn add_async_method_mut<M, A, MR, R>(&mut self, name: impl Into<String>, method: M)
    where
        T: 'static,
        M: Fn(Lua, UserDataRefMut<T>, A) -> MR + MaybeSend + 'static,
        A: FromLuaMulti,
        MR: Future<Output = Result<R>> + MaybeSend + 'static,
        R: IntoLuaMulti;

    /// Add an async method which accepts a `T` as the first parameter and returns [`Future`].
    ///
    /// The userdata `T` will be moved out of the userdata container. This is useful for
    /// methods that need to consume the userdata.
    ///
    /// The method can be called only once per userdata instance, subsequent calls will result in a
    /// [`Error::UserDataDestructed`] error.
    #[cfg(feature = "async")]
    #[cfg_attr(docsrs, doc(cfg(feature = "async")))]
    fn add_async_method_once<M, A, MR, R>(&mut self, name: impl Into<String>, method: M)
    where
        T: 'static,
        M: Fn(Lua, T, A) -> MR + MaybeSend + 'static,
        A: FromLuaMulti,
        MR: Future<Output = Result<R>> + MaybeSend + 'static,
        R: IntoLuaMulti,
    {
        let name = name.into();
        let method_name = format!("{}.{name}", short_type_name::<T>());
        self.add_async_function(name, move |lua, (ud, args): (AnyUserData, A)| {
            match (ud.take()).map_err(|err| Error::bad_self_argument(&method_name, err)) {
                Ok(this) => either::Either::Left(method(lua, this, args)),
                Err(err) => either::Either::Right(async move { Err(err) }),
            }
        });
    }

    /// Add a regular method as a function which accepts generic arguments.
    ///
    /// The first argument will be a [`AnyUserData`] of type `T` if the method is called with Lua
    /// method syntax: `my_userdata:my_method(arg1, arg2)`, or it is passed in as the first
    /// argument: `my_userdata.my_method(my_userdata, arg1, arg2)`.
    fn add_function<F, A, R>(&mut self, name: impl Into<String>, function: F)
    where
        F: Fn(&Lua, A) -> Result<R> + MaybeSend + 'static,
        A: FromLuaMulti,
        R: IntoLuaMulti;

    /// Add a regular method as a mutable function which accepts generic arguments.
    ///
    /// This is a version of [`add_function`] that accepts a `FnMut` argument.
    ///
    /// [`add_function`]: UserDataMethods::add_function
    fn add_function_mut<F, A, R>(&mut self, name: impl Into<String>, function: F)
    where
        F: FnMut(&Lua, A) -> Result<R> + MaybeSend + 'static,
        A: FromLuaMulti,
        R: IntoLuaMulti;

    /// Add a regular method as an async function which accepts generic arguments and returns
    /// [`Future`].
    ///
    /// This is an async version of [`add_function`].
    ///
    /// [`add_function`]: UserDataMethods::add_function
    #[cfg(feature = "async")]
    #[cfg_attr(docsrs, doc(cfg(feature = "async")))]
    fn add_async_function<F, A, FR, R>(&mut self, name: impl Into<String>, function: F)
    where
        F: Fn(Lua, A) -> FR + MaybeSend + 'static,
        A: FromLuaMulti,
        FR: Future<Output = Result<R>> + MaybeSend + 'static,
        R: IntoLuaMulti;

    /// Add a metamethod which accepts a `&T` as the first parameter.
    ///
    /// # Note
    ///
    /// This can cause an error with certain binary metamethods that can trigger if only the right
    /// side has a metatable. To prevent this, use [`add_meta_function`].
    ///
    /// [`add_meta_function`]: UserDataMethods::add_meta_function
    fn add_meta_method<M, A, R>(&mut self, name: impl Into<String>, method: M)
    where
        M: Fn(&Lua, &T, A) -> Result<R> + MaybeSend + 'static,
        A: FromLuaMulti,
        R: IntoLuaMulti;

    /// Add a metamethod as a function which accepts a `&mut T` as the first parameter.
    ///
    /// # Note
    ///
    /// This can cause an error with certain binary metamethods that can trigger if only the right
    /// side has a metatable. To prevent this, use [`add_meta_function`].
    ///
    /// [`add_meta_function`]: UserDataMethods::add_meta_function
    fn add_meta_method_mut<M, A, R>(&mut self, name: impl Into<String>, method: M)
    where
        M: FnMut(&Lua, &mut T, A) -> Result<R> + MaybeSend + 'static,
        A: FromLuaMulti,
        R: IntoLuaMulti;

    /// Add an async metamethod which accepts a `&T` as the first parameter and returns [`Future`].
    ///
    /// This is an async version of [`add_meta_method`].
    ///
    /// [`add_meta_method`]: UserDataMethods::add_meta_method
    #[cfg(all(feature = "async", not(any(feature = "lua51", feature = "luau"))))]
    #[cfg_attr(
        docsrs,
        doc(cfg(all(feature = "async", not(any(feature = "lua51", feature = "luau")))))
    )]
    fn add_async_meta_method<M, A, MR, R>(&mut self, name: impl Into<String>, method: M)
    where
        T: 'static,
        M: Fn(Lua, UserDataRef<T>, A) -> MR + MaybeSend + 'static,
        A: FromLuaMulti,
        MR: Future<Output = Result<R>> + MaybeSend + 'static,
        R: IntoLuaMulti;

    /// Add an async metamethod which accepts a `&mut T` as the first parameter and returns
    /// [`Future`].
    ///
    /// This is an async version of [`add_meta_method_mut`].
    ///
    /// [`add_meta_method_mut`]: UserDataMethods::add_meta_method_mut
    #[cfg(all(feature = "async", not(any(feature = "lua51", feature = "luau"))))]
    #[cfg_attr(docsrs, doc(cfg(feature = "async")))]
    fn add_async_meta_method_mut<M, A, MR, R>(&mut self, name: impl Into<String>, method: M)
    where
        T: 'static,
        M: Fn(Lua, UserDataRefMut<T>, A) -> MR + MaybeSend + 'static,
        A: FromLuaMulti,
        MR: Future<Output = Result<R>> + MaybeSend + 'static,
        R: IntoLuaMulti;

    /// Add a metamethod which accepts generic arguments.
    ///
    /// Metamethods for binary operators can be triggered if either the left or right argument to
    /// the binary operator has a metatable, so the first argument here is not necessarily a
    /// userdata of type `T`.
    fn add_meta_function<F, A, R>(&mut self, name: impl Into<String>, function: F)
    where
        F: Fn(&Lua, A) -> Result<R> + MaybeSend + 'static,
        A: FromLuaMulti,
        R: IntoLuaMulti;

    /// Add a metamethod as a mutable function which accepts generic arguments.
    ///
    /// This is a version of [`add_meta_function`] that accepts a `FnMut` argument.
    ///
    /// [`add_meta_function`]: UserDataMethods::add_meta_function
    fn add_meta_function_mut<F, A, R>(&mut self, name: impl Into<String>, function: F)
    where
        F: FnMut(&Lua, A) -> Result<R> + MaybeSend + 'static,
        A: FromLuaMulti,
        R: IntoLuaMulti;

    /// Add a metamethod which accepts generic arguments and returns [`Future`].
    ///
    /// This is an async version of [`add_meta_function`].
    ///
    /// [`add_meta_function`]: UserDataMethods::add_meta_function
    #[cfg(all(feature = "async", not(any(feature = "lua51", feature = "luau"))))]
    #[cfg_attr(
        docsrs,
        doc(cfg(all(feature = "async", not(any(feature = "lua51", feature = "luau")))))
    )]
    fn add_async_meta_function<F, A, FR, R>(&mut self, name: impl Into<String>, function: F)
    where
        F: Fn(Lua, A) -> FR + MaybeSend + 'static,
        A: FromLuaMulti,
        FR: Future<Output = Result<R>> + MaybeSend + 'static,
        R: IntoLuaMulti;
}

/// Field registry for [`UserData`] implementors.
pub trait UserDataFields<T> {
    /// Add a static field to the [`UserData`].
    ///
    /// Static fields are implemented by updating the `__index` metamethod and returning the
    /// accessed field. This allows them to be used with the expected `userdata.field` syntax.
    ///
    /// Static fields are usually shared between all instances of the [`UserData`] of the same type.
    ///
    /// If `add_meta_method` is used to set the `__index` metamethod, it will
    /// be used as a fall-back if no regular field or method are found.
    fn add_field<V>(&mut self, name: impl Into<String>, value: V)
    where
        V: IntoLua + 'static;

    /// Add a regular field getter as a method which accepts a `&T` as the parameter.
    ///
    /// Regular field getters are implemented by overriding the `__index` metamethod and returning
    /// the accessed field. This allows them to be used with the expected `userdata.field` syntax.
    ///
    /// If `add_meta_method` is used to set the `__index` metamethod, the `__index` metamethod will
    /// be used as a fall-back if no regular field or method are found.
    fn add_field_method_get<M, R>(&mut self, name: impl Into<String>, method: M)
    where
        M: Fn(&Lua, &T) -> Result<R> + MaybeSend + 'static,
        R: IntoLua;

    /// Add a regular field setter as a method which accepts a `&mut T` as the first parameter.
    ///
    /// Regular field setters are implemented by overriding the `__newindex` metamethod and setting
    /// the accessed field. This allows them to be used with the expected `userdata.field = value`
    /// syntax.
    ///
    /// If `add_meta_method` is used to set the `__newindex` metamethod, the `__newindex` metamethod
    /// will be used as a fall-back if no regular field is found.
    fn add_field_method_set<M, A>(&mut self, name: impl Into<String>, method: M)
    where
        M: FnMut(&Lua, &mut T, A) -> Result<()> + MaybeSend + 'static,
        A: FromLua;

    /// Add a regular field getter as a function which accepts a generic [`AnyUserData`] of type `T`
    /// argument.
    fn add_field_function_get<F, R>(&mut self, name: impl Into<String>, function: F)
    where
        F: Fn(&Lua, AnyUserData) -> Result<R> + MaybeSend + 'static,
        R: IntoLua;

    /// Add a regular field setter as a function which accepts a generic [`AnyUserData`] of type `T`
    /// first argument.
    fn add_field_function_set<F, A>(&mut self, name: impl Into<String>, function: F)
    where
        F: FnMut(&Lua, AnyUserData, A) -> Result<()> + MaybeSend + 'static,
        A: FromLua;

    /// Add a metatable field.
    ///
    /// This will initialize the metatable field with `value` on [`UserData`] creation.
    ///
    /// # Note
    ///
    /// `mlua` will trigger an error on an attempt to define a protected metamethod,
    /// like `__gc` or `__metatable`.
    fn add_meta_field<V>(&mut self, name: impl Into<String>, value: V)
    where
        V: IntoLua + 'static;

    /// Add a metatable field computed from `f`.
    ///
    /// This will initialize the metatable field from `f` on [`UserData`] creation.
    ///
    /// # Note
    ///
    /// `mlua` will trigger an error on an attempt to define a protected metamethod,
    /// like `__gc` or `__metatable`.
    fn add_meta_field_with<F, R>(&mut self, name: impl Into<String>, f: F)
    where
        F: FnOnce(&Lua) -> Result<R> + 'static,
        R: IntoLua;
}

/// Trait for custom userdata types.
///
/// By implementing this trait, a struct becomes eligible for use inside Lua code.
///
/// Implementation of [`IntoLua`] is automatically provided, [`FromLua`] needs to be implemented
/// manually.
///
///
/// # Examples
///
/// ```
/// # use mlua::{Lua, Result, UserData};
/// # fn main() -> Result<()> {
/// # let lua = Lua::new();
/// struct MyUserData;
///
/// impl UserData for MyUserData {}
///
/// // `MyUserData` now implements `IntoLua`:
/// lua.globals().set("myobject", MyUserData)?;
///
/// lua.load("assert(type(myobject) == 'userdata')").exec()?;
/// # Ok(())
/// # }
/// ```
///
/// Custom fields, methods and operators can be provided by implementing `add_fields` or
/// `add_methods` (refer to [`UserDataFields`] and [`UserDataMethods`] for more information):
///
/// ```
/// # use mlua::{Lua, MetaMethod, Result, UserData, UserDataFields, UserDataMethods};
/// # fn main() -> Result<()> {
/// # let lua = Lua::new();
/// struct MyUserData(i32);
///
/// impl UserData for MyUserData {
///     fn add_fields<F: UserDataFields<Self>>(fields: &mut F) {
///         fields.add_field_method_get("val", |_, this| Ok(this.0));
///     }
///
///     fn add_methods<M: UserDataMethods<Self>>(methods: &mut M) {
///         methods.add_method_mut("add", |_, mut this, value: i32| {
///             this.0 += value;
///             Ok(())
///         });
///
///         methods.add_meta_method(MetaMethod::Add, |_, this, value: i32| {
///             Ok(this.0 + value)
///         });
///     }
/// }
///
/// lua.globals().set("myobject", MyUserData(123))?;
///
/// lua.load(r#"
///     assert(myobject.val == 123)
///     myobject:add(7)
///     assert(myobject.val == 130)
///     assert(myobject + 10 == 140)
/// "#).exec()?;
/// # Ok(())
/// # }
/// ```
pub trait UserData: Sized {
    /// Adds custom fields specific to this userdata.
    #[allow(unused_variables)]
    fn add_fields<F: UserDataFields<Self>>(fields: &mut F) {}

    /// Adds custom methods and operators specific to this userdata.
    #[allow(unused_variables)]
    fn add_methods<M: UserDataMethods<Self>>(methods: &mut M) {}

    /// Registers this type for use in Lua.
    ///
    /// This method is responsible for calling `add_fields` and `add_methods` on the provided
    /// [`UserDataRegistry`].
    fn register(registry: &mut UserDataRegistry<Self>) {
        Self::add_fields(registry);
        Self::add_methods(registry);
    }
}

/// Handle to an internal Lua userdata for any type that implements [`UserData`].
///
/// Similar to [`std::any::Any`], this provides an interface for dynamic type checking via the
/// [`is`] and [`borrow`] methods.
///
/// # Note
///
/// This API should only be used when necessary. Implementing [`UserData`] already allows defining
/// methods which check the type and acquire a borrow behind the scenes.
///
/// [`is`]: crate::AnyUserData::is
/// [`borrow`]: crate::AnyUserData::borrow
#[derive(Clone, PartialEq)]
pub struct AnyUserData(pub(crate) ValueRef);

impl AnyUserData {
    /// Checks whether the type of this userdata is `T`.
    #[inline]
    pub fn is<T: 'static>(&self) -> bool {
        let type_id = self.type_id();
        // We do not use wrapped types here, rather prefer to check the "real" type of the userdata
        matches!(type_id, Some(type_id) if type_id == TypeId::of::<T>())
    }

    /// Checks whether the type of this userdata is a [proxy object] for `T`.
    ///
    /// [proxy object]: crate::Lua::create_proxy
    #[inline]
    pub fn is_proxy<T: 'static>(&self) -> bool {
        self.is::<UserDataProxy<T>>()
    }

    /// Borrow this userdata immutably if it is of type `T`.
    ///
    /// # Errors
    ///
    /// Returns a [`UserDataBorrowError`] if the userdata is already mutably borrowed.
    /// Returns a [`DataTypeMismatch`] if the userdata is not of type `T` or if it's
    /// scoped.
    ///
    /// [`UserDataBorrowError`]: crate::Error::UserDataBorrowError
    /// [`DataTypeMismatch`]: crate::Error::UserDataTypeMismatch
    #[inline]
    pub fn borrow<T: 'static>(&self) -> Result<UserDataRef<T>> {
        let lua = self.0.lua.lock();
        unsafe { UserDataRef::borrow_from_stack(&lua, lua.ref_thread(), self.0.index) }
    }

    /// Borrow this userdata immutably if it is of type `T`, passing the borrowed value
    /// to the closure.
    ///
    /// This method is the only way to borrow scoped userdata (created inside [`Lua::scope`]).
    pub fn borrow_scoped<T: 'static, R>(&self, f: impl FnOnce(&T) -> R) -> Result<R> {
        let lua = self.0.lua.lock();
        let type_id = lua.get_userdata_ref_type_id(&self.0)?;
        let type_hints = TypeIdHints::new::<T>();
        unsafe { borrow_userdata_scoped(lua.ref_thread(), self.0.index, type_id, type_hints, f) }
    }

    /// Borrow this userdata mutably if it is of type `T`.
    ///
    /// # Errors
    ///
    /// Returns a [`UserDataBorrowMutError`] if the userdata cannot be mutably borrowed.
    /// Returns a [`UserDataTypeMismatch`] if the userdata is not of type `T` or if it's
    /// scoped.
    ///
    /// [`UserDataBorrowMutError`]: crate::Error::UserDataBorrowMutError
    /// [`UserDataTypeMismatch`]: crate::Error::UserDataTypeMismatch
    #[inline]
    pub fn borrow_mut<T: 'static>(&self) -> Result<UserDataRefMut<T>> {
        let lua = self.0.lua.lock();
        unsafe { UserDataRefMut::borrow_from_stack(&lua, lua.ref_thread(), self.0.index) }
    }

    /// Borrow this userdata mutably if it is of type `T`, passing the borrowed value
    /// to the closure.
    ///
    /// This method is the only way to borrow scoped userdata (created inside [`Lua::scope`]).
    pub fn borrow_mut_scoped<T: 'static, R>(&self, f: impl FnOnce(&mut T) -> R) -> Result<R> {
        let lua = self.0.lua.lock();
        let type_id = lua.get_userdata_ref_type_id(&self.0)?;
        let type_hints = TypeIdHints::new::<T>();
        unsafe { borrow_userdata_scoped_mut(lua.ref_thread(), self.0.index, type_id, type_hints, f) }
    }

    /// Takes the value out of this userdata.
    ///
    /// Sets the special "destructed" metatable that prevents any further operations with this
    /// userdata.
    ///
    /// Keeps associated user values unchanged (they will be collected by Lua's GC).
    pub fn take<T: 'static>(&self) -> Result<T> {
        let lua = self.0.lua.lock();
        match lua.get_userdata_ref_type_id(&self.0)? {
            Some(type_id) if type_id == TypeId::of::<T>() => unsafe {
                let ref_thread = lua.ref_thread();
                if (*get_userdata::<UserDataStorage<T>>(ref_thread, self.0.index)).has_exclusive_access() {
                    take_userdata::<UserDataStorage<T>>(ref_thread, self.0.index).into_inner()
                } else {
                    Err(Error::UserDataBorrowMutError)
                }
            },
            _ => Err(Error::UserDataTypeMismatch),
        }
    }

    /// Destroys this userdata.
    ///
    /// This is similar to [`AnyUserData::take`], but it doesn't require a type.
    ///
    /// This method works for non-scoped userdata only.
    pub fn destroy(&self) -> Result<()> {
        let lua = self.0.lua.lock();
        let state = lua.state();
        unsafe {
            let _sg = StackGuard::new(state);
            check_stack(state, 3)?;

            lua.push_userdata_ref(&self.0)?;
            protect_lua!(state, 1, 1, fn(state) {
                if ffi::luaL_callmeta(state, -1, cstr!("__gc")) == 0 {
                    ffi::lua_pushboolean(state, 0);
                }
            })?;
            if ffi::lua_isboolean(state, -1) != 0 && ffi::lua_toboolean(state, -1) != 0 {
                return Ok(());
            }
            Err(Error::UserDataBorrowMutError)
        }
    }

    /// Sets an associated value to this [`AnyUserData`].
    ///
    /// The value may be any Lua value whatsoever, and can be retrieved with [`user_value`].
    ///
    /// This is the same as calling [`set_nth_user_value`] with `n` set to 1.
    ///
    /// [`user_value`]: AnyUserData::user_value
    /// [`set_nth_user_value`]: AnyUserData::set_nth_user_value
    #[inline]
    pub fn set_user_value(&self, v: impl IntoLua) -> Result<()> {
        self.set_nth_user_value(1, v)
    }

    /// Returns an associated value set by [`set_user_value`].
    ///
    /// This is the same as calling [`nth_user_value`] with `n` set to 1.
    ///
    /// [`set_user_value`]: AnyUserData::set_user_value
    /// [`nth_user_value`]: AnyUserData::nth_user_value
    #[inline]
    pub fn user_value<V: FromLua>(&self) -> Result<V> {
        self.nth_user_value(1)
    }

    /// Sets an associated `n`th value to this [`AnyUserData`].
    ///
    /// The value may be any Lua value whatsoever, and can be retrieved with [`nth_user_value`].
    /// `n` starts from 1 and can be up to 65535.
    ///
    /// This is supported for all Lua versions using a wrapping table.
    ///
    /// [`nth_user_value`]: AnyUserData::nth_user_value
    pub fn set_nth_user_value(&self, n: usize, v: impl IntoLua) -> Result<()> {
        if n < 1 || n > u16::MAX as usize {
            return Err(Error::runtime("user value index out of bounds"));
        }

        let lua = self.0.lua.lock();
        let state = lua.state();
        unsafe {
            let _sg = StackGuard::new(state);
            check_stack(state, 5)?;

            lua.push_userdata_ref(&self.0)?;
            lua.push(v)?;

            // Multiple (extra) user values are emulated by storing them in a table
            protect_lua!(state, 2, 0, |state| {
                if ffi::lua_getuservalue(state, -2) != ffi::LUA_TTABLE {
                    // Create a new table to use as uservalue
                    ffi::lua_pop(state, 1);
                    ffi::lua_newtable(state);
                    ffi::lua_pushvalue(state, -1);
                    ffi::lua_setuservalue(state, -4);
                }
                ffi::lua_pushvalue(state, -2);
                ffi::lua_rawseti(state, -2, n as ffi::lua_Integer);
            })?;

            Ok(())
        }
    }

    /// Returns an associated `n`th value set by [`set_nth_user_value`].
    ///
    /// `n` starts from 1 and can be up to 65535.
    ///
    /// This is supported for all Lua versions using a wrapping table.
    ///
    /// [`set_nth_user_value`]: AnyUserData::set_nth_user_value
    pub fn nth_user_value<V: FromLua>(&self, n: usize) -> Result<V> {
        if n < 1 || n > u16::MAX as usize {
            return Err(Error::runtime("user value index out of bounds"));
        }

        let lua = self.0.lua.lock();
        let state = lua.state();
        unsafe {
            let _sg = StackGuard::new(state);
            check_stack(state, 4)?;

            lua.push_userdata_ref(&self.0)?;

            // Multiple (extra) user values are emulated by storing them in a table
            if ffi::lua_getuservalue(state, -1) != ffi::LUA_TTABLE {
                return V::from_lua(Value::Nil, lua.lua());
            }
            ffi::lua_rawgeti(state, -1, n as ffi::lua_Integer);

            V::from_lua(lua.pop_value(), lua.lua())
        }
    }

    /// Sets an associated value to this [`AnyUserData`] by name.
    ///
    /// The value can be retrieved with [`named_user_value`].
    ///
    /// [`named_user_value`]: AnyUserData::named_user_value
    pub fn set_named_user_value(&self, name: &str, v: impl IntoLua) -> Result<()> {
        let lua = self.0.lua.lock();
        let state = lua.state();
        unsafe {
            let _sg = StackGuard::new(state);
            check_stack(state, 5)?;

            lua.push_userdata_ref(&self.0)?;
            lua.push(v)?;

            // Multiple (extra) user values are emulated by storing them in a table
            protect_lua!(state, 2, 0, |state| {
                if ffi::lua_getuservalue(state, -2) != ffi::LUA_TTABLE {
                    // Create a new table to use as uservalue
                    ffi::lua_pop(state, 1);
                    ffi::lua_newtable(state);
                    ffi::lua_pushvalue(state, -1);
                    ffi::lua_setuservalue(state, -4);
                }
                ffi::lua_pushlstring(state, name.as_ptr() as *const c_char, name.len());
                ffi::lua_pushvalue(state, -3);
                ffi::lua_rawset(state, -3);
            })?;

            Ok(())
        }
    }

    /// Returns an associated value by name set by [`set_named_user_value`].
    ///
    /// [`set_named_user_value`]: AnyUserData::set_named_user_value
    pub fn named_user_value<V: FromLua>(&self, name: &str) -> Result<V> {
        let lua = self.0.lua.lock();
        let state = lua.state();
        unsafe {
            let _sg = StackGuard::new(state);
            check_stack(state, 4)?;

            lua.push_userdata_ref(&self.0)?;

            // Multiple (extra) user values are emulated by storing them in a table
            if ffi::lua_getuservalue(state, -1) != ffi::LUA_TTABLE {
                return V::from_lua(Value::Nil, lua.lua());
            }
            push_string(state, name.as_bytes(), !lua.unlikely_memory_error())?;
            ffi::lua_rawget(state, -2);

            V::from_stack(-1, &lua)
        }
    }

    /// Returns a metatable of this [`AnyUserData`].
    ///
    /// Returned [`UserDataMetatable`] object wraps the original metatable and
    /// provides safe access to its methods.
    ///
    /// For `T: 'static` returned metatable is shared among all instances of type `T`.
    #[inline]
    pub fn metatable(&self) -> Result<UserDataMetatable> {
        self.raw_metatable().map(UserDataMetatable)
    }

    /// Returns a raw metatable of this [`AnyUserData`].
    fn raw_metatable(&self) -> Result<Table> {
        let lua = self.0.lua.lock();
        let ref_thread = lua.ref_thread();
        unsafe {
            // Check that userdata is registered and not destructed
            // All registered userdata types have a non-empty metatable
            let _type_id = lua.get_userdata_ref_type_id(&self.0)?;

            ffi::lua_getmetatable(ref_thread, self.0.index);
            Ok(Table(lua.pop_ref_thread()))
        }
    }

    /// Converts this userdata to a generic C pointer.
    ///
    /// There is no way to convert the pointer back to its original value.
    ///
    /// Typically this function is used only for hashing and debug information.
    #[inline]
    pub fn to_pointer(&self) -> *const c_void {
        self.0.to_pointer()
    }

    /// Returns [`TypeId`] of this userdata if it is registered and `'static`.
    ///
    /// This method is not available for scoped userdata.
    #[inline]
    pub fn type_id(&self) -> Option<TypeId> {
        let lua = self.0.lua.lock();
        lua.get_userdata_ref_type_id(&self.0).ok().flatten()
    }

    /// Returns a type name of this userdata (from a metatable field).
    ///
    /// If no type name is set, returns `None`.
    pub fn type_name(&self) -> Result<Option<String>> {
        let lua = self.0.lua.lock();
        let state = lua.state();
        unsafe {
            let _sg = StackGuard::new(state);
            check_stack(state, 3)?;

            lua.push_userdata_ref(&self.0)?;
            let protect = !lua.unlikely_memory_error();
            let name_type = if protect {
                protect_lua!(state, 1, 1, |state| {
                    ffi::luaL_getmetafield(state, -1, MetaMethod::Type.as_cstr().as_ptr())
                })?
            } else {
                ffi::luaL_getmetafield(state, -1, MetaMethod::Type.as_cstr().as_ptr())
            };
            match name_type {
                ffi::LUA_TSTRING => Ok(Some(LuaString(lua.pop_ref()).to_str()?.to_owned())),
                _ => Ok(None),
            }
        }
    }

    pub(crate) fn equals(&self, other: &Self) -> Result<bool> {
        // Uses lua_rawequal() under the hood
        if self == other {
            return Ok(true);
        }

        let mt = self.raw_metatable()?;
        if mt != other.raw_metatable()? {
            return Ok(false);
        }

        if mt.contains_key("__eq")? {
            return mt.get::<Function>("__eq")?.call((self, other));
        }

        Ok(false)
    }

    /// Returns `true` if this [`AnyUserData`] is serializable (e.g. was created using
    /// [`Lua::create_ser_userdata`]).
    #[cfg(feature = "serde")]
    pub(crate) fn is_serializable(&self) -> bool {
        let lua = self.0.lua.lock();
        let is_serializable = || unsafe {
            // Userdata must be registered and not destructed
            let _ = lua.get_userdata_ref_type_id(&self.0)?;
            let ud = &*get_userdata::<UserDataStorage<()>>(lua.ref_thread(), self.0.index);
            Ok::<_, Error>((*ud).is_serializable())
        };
        is_serializable().unwrap_or(false)
    }

    unsafe fn invoke_tostring_dbg(&self) -> Result<Option<String>> {
        let lua = self.0.lua.lock();
        let state = lua.state();
        let _guard = StackGuard::new(state);
        check_stack(state, 3)?;

        lua.push_ref(&self.0);
        protect_lua!(state, 1, 1, fn(state) {
            // Try `__todebugstring` metamethod first, then `__tostring`
            #[allow(clippy::collapsible_if)]
            if ffi::luaL_callmeta(state, -1, cstr!("__todebugstring")) == 0 {
                if ffi::luaL_callmeta(state, -1, cstr!("__tostring")) == 0 {
                    ffi::lua_pushnil(state);
                }
            }
        })?;
        Ok(lua.pop_value().as_string().map(|s| s.to_string_lossy()))
    }

    pub(crate) fn fmt_pretty(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        // Try converting to a (debug) string first, with fallback to `__name/__type`
        match unsafe { self.invoke_tostring_dbg() } {
            Ok(Some(s)) => write!(fmt, "{s}"),
            _ => {
                let name = self.type_name().ok().flatten();
                let name = name.as_deref().unwrap_or("userdata");
                write!(fmt, "{name}: {:?}", self.to_pointer())
            }
        }
    }
}

impl fmt::Debug for AnyUserData {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        if fmt.alternate() {
            return self.fmt_pretty(fmt);
        }
        fmt.debug_tuple("AnyUserData").field(&self.0).finish()
    }
}

/// Handle to a [`AnyUserData`] metatable.
#[derive(Clone, Debug)]
pub struct UserDataMetatable(pub(crate) Table);

impl UserDataMetatable {
    /// Gets the value associated to `key` from the metatable.
    ///
    /// If no value is associated to `key`, returns the `Nil` value.
    /// Access to restricted metamethods such as `__gc` or `__metatable` will cause an error.
    pub fn get<V: FromLua>(&self, key: impl AsRef<str>) -> Result<V> {
        self.0.raw_get(MetaMethod::validate(key.as_ref())?)
    }

    /// Sets a key-value pair in the metatable.
    ///
    /// If the value is `Nil`, this will effectively remove the `key`.
    /// Access to restricted metamethods such as `__gc` or `__metatable` will cause an error.
    /// Setting `__index` or `__newindex` metamethods is also restricted because their values are
    /// cached for `mlua` internal usage.
    pub fn set(&self, key: impl AsRef<str>, value: impl IntoLua) -> Result<()> {
        let key = MetaMethod::validate(key.as_ref())?;
        // `__index` and `__newindex` cannot be changed in runtime, because values are cached
        if key == MetaMethod::Index || key == MetaMethod::NewIndex {
            return Err(Error::MetaMethodRestricted(key.to_string()));
        }
        self.0.raw_set(key, value)
    }

    /// Checks whether the metatable contains a non-nil value for `key`.
    pub fn contains(&self, key: impl AsRef<str>) -> Result<bool> {
        self.0.contains_key(MetaMethod::validate(key.as_ref())?)
    }

    /// Returns an iterator over the pairs of the metatable.
    ///
    /// The pairs are wrapped in a [`Result`], since they are lazily converted to `V` type.
    ///
    /// [`Result`]: crate::Result
    pub fn pairs<V: FromLua>(&self) -> UserDataMetatablePairs<'_, V> {
        UserDataMetatablePairs(self.0.pairs())
    }
}

/// An iterator over the pairs of a [`AnyUserData`] metatable.
///
/// It skips restricted metamethods, such as `__gc` or `__metatable`.
///
/// This struct is created by the [`UserDataMetatable::pairs`] method.
pub struct UserDataMetatablePairs<'a, V>(TablePairs<'a, String, V>);

impl<V> Iterator for UserDataMetatablePairs<'_, V>
where
    V: FromLua,
{
    type Item = Result<(String, V)>;

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            match self.0.next()? {
                Ok((key, value)) => {
                    // Skip restricted metamethods
                    if MetaMethod::validate(&key).is_ok() {
                        break Some(Ok((key, value)));
                    }
                }
                Err(e) => break Some(Err(e)),
            }
        }
    }
}

#[cfg(feature = "serde")]
impl Serialize for AnyUserData {
    fn serialize<S>(&self, serializer: S) -> StdResult<S::Ok, S::Error>
    where
        S: Serializer,
    {
        let lua = self.0.lua.lock();
        unsafe {
            let _ = lua
                .get_userdata_ref_type_id(&self.0)
                .map_err(ser::Error::custom)?;
            let ud = &*get_userdata::<UserDataStorage<()>>(lua.ref_thread(), self.0.index);
            ud.serialize(serializer)
        }
    }
}

struct WrappedUserdata<F: FnOnce(&Lua) -> Result<AnyUserData>>(F);

impl AnyUserData {
    /// Wraps any Rust type, returning an opaque type that implements [`IntoLua`] trait.
    ///
    /// This function uses [`Lua::create_any_userdata`] under the hood.
    pub fn wrap<T: MaybeSend + MaybeSync + 'static>(data: T) -> impl IntoLua {
        WrappedUserdata(move |lua| lua.create_any_userdata(data))
    }

    /// Wraps any Rust type that implements [`Serialize`], returning an opaque type that implements
    /// [`IntoLua`] trait.
    ///
    /// This function uses [`Lua::create_ser_any_userdata`] under the hood.
    #[cfg(feature = "serde")]
    #[cfg_attr(docsrs, doc(cfg(feature = "serde")))]
    pub fn wrap_ser<T: Serialize + MaybeSend + MaybeSync + 'static>(data: T) -> impl IntoLua {
        WrappedUserdata(move |lua| lua.create_ser_any_userdata(data))
    }
}

impl<F> IntoLua for WrappedUserdata<F>
where
    F: for<'l> FnOnce(&'l Lua) -> Result<AnyUserData>,
{
    fn into_lua(self, lua: &Lua) -> Result<Value> {
        (self.0)(lua).map(Value::UserData)
    }
}

mod cell;
mod lock;
mod object;
mod r#ref;
mod registry;
mod util;

#[cfg(test)]
mod assertions {
    use super::*;

    #[cfg(not(feature = "send"))]
    static_assertions::assert_not_impl_any!(AnyUserData: Send);
    #[cfg(feature = "send")]
    static_assertions::assert_impl_all!(AnyUserData: Send, Sync);
}