v0.7.0

Add `Lua::unload()` to unload modules

CHANGELOG.md

@@ -1,3 +1,22 @@
+## v0.7.0
+
+- New "application data" api to store arbitrary objects inside Lua
+- New feature flag `luajit52` to build/support LuaJIT with partial compatibility with Lua 5.2
+- Added async meta methods for all Lua (except 5.1)
+- Added `AnyUserData::take()` to take UserData objects from Lua
+- Added `set_nth_user_value`/`get_nth_user_value` to `AnyUserData` for all Lua versions
+- Added `set_named_user_value`/`get_named_user_value` to `AnyUserData` for all Lua versions
+- Added `Lua::inspect_stack()` to get information about the interpreter runtime stack
+- Added `set_warning_function`/`remove_warning_function`/`warning` functions to `Lua` for 5.4
+- Added `TableExt::call()` to call tables with `__call` metamethod as functions
+- Added `Lua::unload()` to unload modules
+- `ToLua` implementation for arrays changed to const generics
+- Added thread (coroutine) cache for async execution (disabled by default and works for Lua 5.4/JIT)
+- LuaOptions and (De)SerializeOptions marked as const
+- Fixed recursive tables serialization when using `serde::Serialize` for Lua Tables
+- Improved errors reporting. Now source included to `fmt::Display` implementation for `Error::CallbackError`
+- Major performance improvements
+
 ## v0.6.6
 
 - Fixed calculating `LUA_REGISTRYINDEX` when cross-compiling for lua51/jit (#82)

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mlua"
-version = "0.6.6" # remember to update html_root_url and mlua_derive
+version = "0.7.0" # remember to update html_root_url and mlua_derive
 authors = ["Aleksandr Orlenko <zxteam@pm.me>", "kyren <catherine@chucklefish.org>"]
 edition = "2018"
 repository = "https://github.com/khvzak/mlua"
@@ -17,7 +17,7 @@ with async/await features and support of writing native Lua modules in Rust.
 """
 
 [package.metadata.docs.rs]
-features = ["lua53", "async", "send", "serialize", "macros"]
+features = ["lua54", "vendored", "async", "send", "serialize", "macros"]
 rustdoc-args = ["--cfg", "docsrs"]
 
 [workspace]
@@ -33,6 +33,7 @@ lua53 = []
 lua52 = []
 lua51 = []
 luajit = []
+luajit52 = ["luajit"]
 vendored = ["lua-src", "luajit-src"]
 module = ["mlua_derive"]
 async = ["futures-core", "futures-task", "futures-util"]
@@ -45,6 +46,7 @@ mlua_derive = { version = "=0.6.0", optional = true, path = "mlua_derive" }
 bstr = { version = "0.2", features = ["std"], default_features = false }
 once_cell = { version = "1.0" }
 num-traits = { version = "0.2.14" }
+rustc-hash = "1.0"
 futures-core = { version = "0.3.5", optional = true }
 futures-task = { version = "0.3.5", optional = true }
 futures-util = { version = "0.3.5", optional = true }
@@ -55,10 +57,10 @@ erased-serde = { version = "0.3", optional = true }
 cc = { version = "1.0" }
 pkg-config = { version = "0.3.17" }
 lua-src = { version = ">= 540.0.0, < 550.0.0", optional = true }
-luajit-src = { version = ">= 210.1.2, < 220.0.0", optional = true }
+luajit-src = { version = ">= 210.3.1, < 220.0.0", optional = true }
 
 [dev-dependencies]
-rustyline = "8.0"
+rustyline = "9.0"
 criterion = { version = "0.3.4", features = ["html_reports", "async_tokio"] }
 trybuild = "1.0"
 futures = "0.3.5"

README.md

@@ -1,5 +1,5 @@
 # mlua
-[![Build Status]][github-actions] [![Latest Version]][crates.io] [![API Documentation]][docs.rs] [![Coverage Status]][codecov.io]
+[![Build Status]][github-actions] [![Latest Version]][crates.io] [![API Documentation]][docs.rs] [![Coverage Status]][codecov.io] ![MSRV]
 
 [Build Status]: https://github.com/khvzak/mlua/workflows/CI/badge.svg
 [github-actions]: https://github.com/khvzak/mlua/actions
@@ -9,17 +9,22 @@
 [docs.rs]: https://docs.rs/mlua
 [Coverage Status]: https://codecov.io/gh/khvzak/mlua/branch/master/graph/badge.svg?token=99339FS1CG
 [codecov.io]: https://codecov.io/gh/khvzak/mlua
+[MSRV]: https://img.shields.io/badge/rust-1.51+-brightgreen.svg?&logo=rust
 
 [Guided Tour](examples/guided_tour.rs)
 
 `mlua` is bindings to [Lua](https://www.lua.org) programming language for Rust with a goal to provide
 _safe_ (as far as it's possible), high level, easy to use, practical and flexible API.
 
-Started as [rlua](https://github.com/amethyst/rlua/tree/0.15.3) fork, `mlua` supports Lua 5.4, 5.3, 5.2 and 5.1 including LuaJIT (2.0.5 and 2.1 beta) and allows to write native Lua modules in Rust as well as use Lua in a standalone mode.
+Started as [rlua] fork, `mlua` supports Lua 5.4, 5.3, 5.2 and 5.1 including LuaJIT (2.0.5 and 2.1 beta) and allows to write native Lua modules in Rust as well as use Lua in a standalone mode.
 
 `mlua` tested on Windows/macOS/Linux including module mode in [GitHub Actions] on `x86_64` platform and cross-compilation to `aarch64` (other targets are also supported).
 
+Check our [benchmark results](https://github.com/khvzak/script-bench-rs) for `mlua`, [rlua] and [hlua].
+
 [GitHub Actions]: https://github.com/khvzak/mlua/actions
+[rlua]: https://github.com/amethyst/rlua
+[hlua]: https://github.com/tomaka/hlua
 
 ## Usage
 
@@ -33,6 +38,7 @@ Below is a list of the available feature flags. By default `mlua` does not enabl
 * `lua52`: activate Lua [5.2] support
 * `lua51`: activate Lua [5.1] support
 * `luajit`: activate [LuaJIT] support
+* `luajit52`: activate [LuaJIT] support with partial compatibility with Lua 5.2
 * `vendored`: build static Lua(JIT) library from sources during `mlua` compilation using [lua-src] or [luajit-src] crates
 * `module`: enable module mode (building loadable `cdylib` library for Lua)
 * `async`: enable async/await support (any executor can be used, eg. [tokio] or [async-std])
@@ -74,7 +80,7 @@ With `serialize` feature flag enabled, `mlua` allows you to serialize/deserializ
 
 ### Compiling
 
-You have to enable one of the features `lua54`, `lua53`, `lua52`, `lua51` or `luajit`, according to the chosen Lua version.
+You have to enable one of the features `lua54`, `lua53`, `lua52`, `lua51` or `luajit(52)`, according to the chosen Lua version.
 
 By default `mlua` uses `pkg-config` tool to find lua includes and libraries for the chosen Lua version.
 In most cases it works as desired, although sometimes could be more preferable to use a custom lua library.
@@ -97,7 +103,7 @@ Add to `Cargo.toml` :
 
 ``` toml
 [dependencies]
-mlua = { version = "0.6", features = ["lua53", "vendored"] }
+mlua = { version = "0.6", features = ["lua54", "vendored"] }
 ```
 
 `main.rs`
@@ -121,7 +127,7 @@ fn main() -> LuaResult<()> {
 ```
 
 ### Module mode
-In a module mode `mlua` allows to create a compiled Lua module that can be loaded from Lua code using [`require`](https://www.lua.org/manual/5.3/manual.html#pdf-require). In this case `mlua` uses an external Lua runtime which could lead to potential unsafety due to unpredictability of the Lua environment and usage of libraries such as [`debug`](https://www.lua.org/manual/5.3/manual.html#6.10).
+In a module mode `mlua` allows to create a compiled Lua module that can be loaded from Lua code using [`require`](https://www.lua.org/manual/5.4/manual.html#pdf-require). In this case `mlua` uses an external Lua runtime which could lead to potential unsafety due to unpredictability of the Lua environment and usage of libraries such as [`debug`](https://www.lua.org/manual/5.4/manual.html#6.10).
 
 [Example](examples/module)
 
@@ -132,7 +138,7 @@ Add to `Cargo.toml` :
 crate-type = ["cdylib"]
 
 [dependencies]
-mlua = { version = "0.6", features = ["lua53", "vendored", "module"] }
+mlua = { version = "0.6", features = ["lua54", "vendored", "module"] }
 ```
 
 `lib.rs` :
@@ -158,7 +164,7 @@ And then (**macOS** example):
 ``` sh
 $ cargo rustc -- -C link-arg=-undefined -C link-arg=dynamic_lookup
 $ ln -s ./target/debug/libmy_module.dylib ./my_module.so
-$ lua5.3 -e 'require("my_module").hello("world")'
+$ lua5.4 -e 'require("my_module").hello("world")'
 hello, world!
 ```
 

benches/benchmark.rs

@@ -120,7 +120,8 @@ fn call_sum_callback(c: &mut Criterion) {
 }
 
 fn call_async_sum_callback(c: &mut Criterion) {
-    let lua = Lua::new();
+    let options = LuaOptions::new().thread_cache_size(1024);
+    let lua = Lua::new_with(LuaStdLib::ALL_SAFE, options).unwrap();
     let callback = lua
         .create_async_function(|_, (a, b, c): (i64, i64, i64)| async move {
             task::yield_now().await;
@@ -244,7 +245,8 @@ fn call_async_userdata_method(c: &mut Criterion) {
         }
     }
 
-    let lua = Lua::new();
+    let options = LuaOptions::new().thread_cache_size(1024);
+    let lua = Lua::new_with(LuaStdLib::ALL_SAFE, options).unwrap();
     lua.globals().set("userdata", UserData(10)).unwrap();
 
     c.bench_function("call async [userdata method] 10", |b| {

build/find_vendored.rs

@@ -10,7 +10,13 @@ pub fn probe_lua() -> PathBuf {
     #[cfg(feature = "lua51")]
     let artifacts = lua_src::Build::new().build(lua_src::Lua51);
     #[cfg(feature = "luajit")]
-    let artifacts = luajit_src::Build::new().build();
+    let artifacts = {
+        let mut builder = luajit_src::Build::new();
+        if cfg!(feature = "luajit52") {
+            builder.lua52compat(true);
+        }
+        builder.build()
+    };
 
     #[cfg(not(feature = "module"))]
     artifacts.print_cargo_metadata();

build/main.rs

@@ -201,7 +201,9 @@ fn main() {
         feature = "lua51",
         feature = "luajit"
     )))]
-    compile_error!("You must enable one of the features: lua54, lua53, lua52, lua51, luajit");
+    compile_error!(
+        "You must enable one of the features: lua54, lua53, lua52, lua51, luajit, luajit52"
+    );
 
     #[cfg(all(
         feature = "lua54",
@@ -212,19 +214,27 @@ fn main() {
             feature = "luajit"
         )
     ))]
-    compile_error!("You can enable only one of the features: lua54, lua53, lua52, lua51, luajit");
+    compile_error!(
+        "You can enable only one of the features: lua54, lua53, lua52, lua51, luajit, luajit52"
+    );
 
     #[cfg(all(
         feature = "lua53",
         any(feature = "lua52", feature = "lua51", feature = "luajit")
     ))]
-    compile_error!("You can enable only one of the features: lua54, lua53, lua52, lua51, luajit");
+    compile_error!(
+        "You can enable only one of the features: lua54, lua53, lua52, lua51, luajit, luajit52"
+    );
 
     #[cfg(all(feature = "lua52", any(feature = "lua51", feature = "luajit")))]
-    compile_error!("You can enable only one of the features: lua54, lua53, lua52, lua51, luajit");
+    compile_error!(
+        "You can enable only one of the features: lua54, lua53, lua52, lua51, luajit, luajit52"
+    );
 
     #[cfg(all(feature = "lua51", feature = "luajit"))]
-    compile_error!("You can enable only one of the features: lua54, lua53, lua52, lua51, luajit");
+    compile_error!(
+        "You can enable only one of the features: lua54, lua53, lua52, lua51, luajit, luajit52"
+    );
 
     // We don't support "vendored module" mode on windows
     #[cfg(all(feature = "vendored", feature = "module", target_os = "windows"))]

src/conversion.rs

@@ -2,6 +2,7 @@
 
 use std::borrow::Cow;
 use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
+use std::convert::TryInto;
 use std::ffi::{CStr, CString};
 use std::hash::{BuildHasher, Hash};
 use std::string::String as StdString;
@@ -348,25 +349,24 @@ macro_rules! lua_convert_int {
     ($x:ty) => {
         impl<'lua> ToLua<'lua> for $x {
             fn to_lua(self, _: &'lua Lua) -> Result<Value<'lua>> {
-                if let Some(i) = cast(self) {
-                    Ok(Value::Integer(i))
-                } else {
-                    // TODO: Remove conversion to Number in v0.7
-                    cast(self)
-                        .ok_or_else(|| Error::ToLuaConversionError {
-                            from: stringify!($x),
-                            to: "number",
-                            message: Some("out of range".to_owned()),
-                        })
-                        .map(Value::Number)
-                }
+                cast(self)
+                    .map(Value::Integer)
+                    .or_else(|| cast(self).map(Value::Number))
+                    // This is impossible error because conversion to Number never fails
+                    .ok_or_else(|| Error::ToLuaConversionError {
+                        from: stringify!($x),
+                        to: "number",
+                        message: Some("out of range".to_owned()),
+                    })
             }
         }
 
         impl<'lua> FromLua<'lua> for $x {
             fn from_lua(value: Value<'lua>, lua: &'lua Lua) -> Result<Self> {
                 let ty = value.type_name();
-                (if let Some(i) = lua.coerce_integer(value.clone())? {
+                (if let Value::Integer(i) = value {
+                    cast(i)
+                } else if let Some(i) = lua.coerce_integer(value.clone())? {
                     cast(i)
                 } else {
                     cast(lua.coerce_number(value)?.ok_or_else(|| {
@@ -451,37 +451,36 @@ where
     }
 }
 
-macro_rules! lua_convert_array {
-    ($($N:literal)+) => {
-        $(
-            impl<'lua, T> ToLua<'lua> for [T; $N]
-            where
-                T: Clone + ToLua<'lua>,
-            {
-                fn to_lua(self, lua: &'lua Lua) -> Result<Value<'lua>> {
-                    (&self).to_lua(lua)
-                }
-            }
-
-            impl<'lua, T> ToLua<'lua> for &[T; $N]
-            where
-                T: Clone + ToLua<'lua>,
-            {
-                fn to_lua(self, lua: &'lua Lua) -> Result<Value<'lua>> {
-                    Ok(Value::Table(
-                        lua.create_sequence_from(self.iter().cloned())?,
-                    ))
-                }
-            }
-        )+
+impl<'lua, T, const N: usize> ToLua<'lua> for [T; N]
+where
+    T: ToLua<'lua>,
+{
+    fn to_lua(self, lua: &'lua Lua) -> Result<Value<'lua>> {
+        Ok(Value::Table(lua.create_sequence_from(self)?))
     }
 }
 
-lua_convert_array! {
-    0  1  2  3  4  5  6  7  8  9
-   10 11 12 13 14 15 16 17 18 19
-   20 21 22 23 24 25 26 27 28 29
-   30 31 32
+impl<'lua, T, const N: usize> FromLua<'lua> for [T; N]
+where
+    T: FromLua<'lua>,
+{
+    fn from_lua(value: Value<'lua>, _: &'lua Lua) -> Result<Self> {
+        if let Value::Table(table) = value {
+            let vec = table.sequence_values().collect::<Result<Vec<_>>>()?;
+            vec.try_into()
+                .map_err(|vec: Vec<T>| Error::FromLuaConversionError {
+                    from: "Table",
+                    to: "Array",
+                    message: Some(format!("expected table of length {}, got {}", N, vec.len())),
+                })
+        } else {
+            Err(Error::FromLuaConversionError {
+                from: value.type_name(),
+                to: "Array",
+                message: Some("expected table".to_string()),
+            })
+        }
+    }
 }
 
 impl<'lua, T: ToLua<'lua>> ToLua<'lua> for Box<[T]> {

src/error.rs

@@ -247,8 +247,28 @@ impl fmt::Display for Error {
             Error::MismatchedRegistryKey => {
                 write!(fmt, "RegistryKey used from different Lua state")
             }
-            Error::CallbackError { ref traceback, .. } => {
-                write!(fmt, "callback error\n{}", traceback)
+            Error::CallbackError { ref cause, ref traceback } => {
+                writeln!(fmt, "callback error")?;
+                // Trace errors down to the root
+                let (mut cause, mut full_traceback) = (cause, None);
+                while let Error::CallbackError { cause: ref cause2, traceback: ref traceback2 } = **cause {
+                    cause = cause2;
+                    full_traceback = Some(traceback2);
+                }
+                if let Some(full_traceback) = full_traceback {
+                    let traceback = traceback.trim_start_matches("stack traceback:");
+                    let traceback = traceback.trim_start().trim_end();
+                    // Try to find local traceback within the full traceback
+                    if let Some(pos) = full_traceback.find(traceback) {
+                        write!(fmt, "{}", &full_traceback[..pos])?;
+                        writeln!(fmt, ">{}", &full_traceback[pos..].trim_end())?;
+                    } else {
+                        writeln!(fmt, "{}", full_traceback.trim_end())?;
+                    }
+                } else {
+                    writeln!(fmt, "{}", traceback.trim_end())?;
+                }
+                write!(fmt, "caused by: {}", cause)
             }
             Error::PreviouslyResumedPanic => {
                 write!(fmt, "previously resumed panic returned again")
@@ -269,7 +289,11 @@ impl fmt::Display for Error {
 impl StdError for Error {
     fn source(&self) -> Option<&(dyn StdError + 'static)> {
         match *self {
-            Error::CallbackError { ref cause, .. } => Some(cause.as_ref()),
+            // An error type with a source error should either return that error via source or
+            // include that source's error message in its own Display output, but never both.
+            // https://blog.rust-lang.org/inside-rust/2021/07/01/What-the-error-handling-project-group-is-working-towards.html
+            // Given that we include source to fmt::Display implementation for `CallbackError`, this call returns nothing.
+            Error::CallbackError { .. } => None,
             Error::ExternalError(ref err) => err.source(),
             _ => None,
         }

src/ffi/lua.rs

@@ -23,6 +23,7 @@
 
 //! Contains definitions from `lua.h`.
 
+use std::marker::{PhantomData, PhantomPinned};
 #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
 use std::os::raw::c_uchar;
 use std::os::raw::{c_char, c_int, c_void};
@@ -63,7 +64,7 @@ pub use super::compat53::lua_getglobal;
 
 #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
 #[inline(always)]
-pub fn lua_upvalueindex(i: c_int) -> c_int {
+pub const fn lua_upvalueindex(i: c_int) -> c_int {
     LUA_REGISTRYINDEX - i
 }
 
@@ -81,7 +82,11 @@ pub const LUA_ERRERR: c_int = 5;
 pub const LUA_ERRERR: c_int = 6;
 
 /// A raw Lua state associated with a thread.
-pub type lua_State = c_void;
+#[repr(C)]
+pub struct lua_State {
+    _data: [u8; 0],
+    _marker: PhantomData<(*mut u8, PhantomPinned)>,
+}
 
 // basic types
 pub const LUA_TNONE: c_int = -1;
@@ -569,7 +574,7 @@ pub unsafe fn lua_resume(
 // warning-related functions
 #[cfg(feature = "lua54")]
 extern "C" {
-    pub fn lua_setwarnf(L: *mut lua_State, f: lua_WarnFunction, ud: *mut c_void);
+    pub fn lua_setwarnf(L: *mut lua_State, f: Option<lua_WarnFunction>, ud: *mut c_void);
     pub fn lua_warning(L: *mut lua_State, msg: *const c_char, tocont: c_int);
 }
 
@@ -621,7 +626,7 @@ extern "C" {
 #[cfg(any(feature = "lua54", feature = "lua53"))]
 #[inline(always)]
 pub unsafe fn lua_getextraspace(L: *mut lua_State) -> *mut c_void {
-    L.offset(-super::glue::LUA_EXTRASPACE as isize) as *mut c_void
+    (L as *mut c_char).offset(-super::glue::LUA_EXTRASPACE as isize) as *mut c_void
 }
 
 #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]

src/function.rs

@@ -6,7 +6,7 @@ use crate::error::{Error, Result};
 use crate::ffi;
 use crate::types::LuaRef;
 use crate::util::{assert_stack, check_stack, error_traceback, pop_error, StackGuard};
-use crate::value::{FromLuaMulti, MultiValue, ToLuaMulti};
+use crate::value::{FromLuaMulti, ToLuaMulti};
 
 #[cfg(feature = "async")]
 use {futures_core::future::LocalBoxFuture, futures_util::future};
@@ -59,7 +59,7 @@ impl<'lua> Function<'lua> {
     pub fn call<A: ToLuaMulti<'lua>, R: FromLuaMulti<'lua>>(&self, args: A) -> Result<R> {
         let lua = self.0.lua;
 
-        let args = args.to_lua_multi(lua)?;
+        let mut args = args.to_lua_multi(lua)?;
         let nargs = args.len() as c_int;
 
         let results = unsafe {
@@ -69,7 +69,7 @@ impl<'lua> Function<'lua> {
             ffi::lua_pushcfunction(lua.state, error_traceback);
             let stack_start = ffi::lua_gettop(lua.state);
             lua.push_ref(&self.0);
-            for arg in args {
+            for arg in args.drain_all() {
                 lua.push_value(arg)?;
             }
             let ret = ffi::lua_pcall(lua.state, nargs, ffi::LUA_MULTRET, stack_start);
@@ -77,7 +77,7 @@ impl<'lua> Function<'lua> {
                 return Err(pop_error(lua.state, ret));
             }
             let nresults = ffi::lua_gettop(lua.state) - stack_start;
-            let mut results = MultiValue::new();
+            let mut results = args; // Reuse MultiValue container
             assert_stack(lua.state, 2);
             for _ in 0..nresults {
                 results.push_front(lua.pop_value());
@@ -126,8 +126,12 @@ impl<'lua> Function<'lua> {
         R: FromLuaMulti<'lua> + 'fut,
     {
         let lua = self.0.lua;
-        match lua.create_thread(self.clone()) {
-            Ok(t) => Box::pin(t.into_async(args)),
+        match lua.create_recycled_thread(self.clone()) {
+            Ok(t) => {
+                let mut t = t.into_async(args);
+                t.set_recyclable(true);
+                Box::pin(t)
+            }
             Err(e) => Box::pin(future::err(e)),
         }
     }

src/hook.rs

@@ -1,29 +1,40 @@
+use std::cell::UnsafeCell;
 use std::ffi::CStr;
-use std::marker::PhantomData;
 use std::ops::{BitOr, BitOrAssign};
 use std::os::raw::{c_char, c_int};
 
-use crate::ffi::{self, lua_Debug, lua_State};
+use crate::ffi::{self, lua_Debug};
 use crate::lua::Lua;
-use crate::util::callback_error;
 
 /// Contains information about currently executing Lua code.
 ///
 /// The `Debug` structure is provided as a parameter to the hook function set with
 /// [`Lua::set_hook`]. You may call the methods on this structure to retrieve information about the
 /// Lua code executing at the time that the hook function was called. Further information can be
-/// found in the [Lua 5.3 documentation][lua_doc].
+/// found in the Lua [documentation][lua_doc].
 ///
-/// [lua_doc]: https://www.lua.org/manual/5.3/manual.html#lua_Debug
+/// [lua_doc]: https://www.lua.org/manual/5.4/manual.html#lua_Debug
 /// [`Lua::set_hook`]: crate::Lua::set_hook
-#[derive(Clone)]
-pub struct Debug<'a> {
-    ar: *mut lua_Debug,
-    state: *mut lua_State,
-    _phantom: PhantomData<&'a ()>,
+pub struct Debug<'lua> {
+    lua: &'lua Lua,
+    ar: ActivationRecord,
 }
 
-impl<'a> Debug<'a> {
+impl<'lua> Debug<'lua> {
+    pub(crate) fn new(lua: &'lua Lua, ar: *mut lua_Debug) -> Self {
+        Debug {
+            lua,
+            ar: ActivationRecord::Borrowed(ar),
+        }
+    }
+
+    pub(crate) fn new_owned(lua: &'lua Lua, ar: lua_Debug) -> Self {
+        Debug {
+            lua,
+            ar: ActivationRecord::Owned(UnsafeCell::new(ar)),
+        }
+    }
+
     /// Returns the specific event that triggered the hook.
     ///
     /// For [Lua 5.1] `DebugEvent::TailCall` is used for return events to indicate a return
@@ -32,44 +43,44 @@ impl<'a> Debug<'a> {
     /// [Lua 5.1]: https://www.lua.org/manual/5.1/manual.html#pdf-LUA_HOOKTAILRET
     pub fn event(&self) -> DebugEvent {
         unsafe {
-            match (*self.ar).event {
+            match (*self.ar.get()).event {
                 ffi::LUA_HOOKCALL => DebugEvent::Call,
                 ffi::LUA_HOOKRET => DebugEvent::Ret,
                 ffi::LUA_HOOKTAILCALL => DebugEvent::TailCall,
                 ffi::LUA_HOOKLINE => DebugEvent::Line,
                 ffi::LUA_HOOKCOUNT => DebugEvent::Count,
-                event => mlua_panic!("Unknown Lua event code: {}", event),
+                event => DebugEvent::Unknown(event),
             }
         }
     }
 
     /// Corresponds to the `n` what mask.
-    pub fn names(&self) -> DebugNames<'a> {
+    pub fn names(&self) -> DebugNames<'lua> {
         unsafe {
             mlua_assert!(
-                ffi::lua_getinfo(self.state, cstr!("n"), self.ar) != 0,
+                ffi::lua_getinfo(self.lua.state, cstr!("n"), self.ar.get()) != 0,
                 "lua_getinfo failed with `n`"
             );
             DebugNames {
-                name: ptr_to_str((*self.ar).name),
-                name_what: ptr_to_str((*self.ar).namewhat),
+                name: ptr_to_str((*self.ar.get()).name),
+                name_what: ptr_to_str((*self.ar.get()).namewhat),
             }
         }
     }
 
     /// Corresponds to the `S` what mask.
-    pub fn source(&self) -> DebugSource<'a> {
+    pub fn source(&self) -> DebugSource<'lua> {
         unsafe {
             mlua_assert!(
-                ffi::lua_getinfo(self.state, cstr!("S"), self.ar) != 0,
+                ffi::lua_getinfo(self.lua.state, cstr!("S"), self.ar.get()) != 0,
                 "lua_getinfo failed with `S`"
             );
             DebugSource {
-                source: ptr_to_str((*self.ar).source),
-                short_src: ptr_to_str((*self.ar).short_src.as_ptr()),
-                line_defined: (*self.ar).linedefined as i32,
-                last_line_defined: (*self.ar).lastlinedefined as i32,
-                what: ptr_to_str((*self.ar).what),
+                source: ptr_to_str((*self.ar.get()).source),
+                short_src: ptr_to_str((*self.ar.get()).short_src.as_ptr()),
+                line_defined: (*self.ar.get()).linedefined as i32,
+                last_line_defined: (*self.ar.get()).lastlinedefined as i32,
+                what: ptr_to_str((*self.ar.get()).what),
             }
         }
     }
@@ -78,10 +89,10 @@ impl<'a> Debug<'a> {
     pub fn curr_line(&self) -> i32 {
         unsafe {
             mlua_assert!(
-                ffi::lua_getinfo(self.state, cstr!("l"), self.ar) != 0,
+                ffi::lua_getinfo(self.lua.state, cstr!("l"), self.ar.get()) != 0,
                 "lua_getinfo failed with `l`"
             );
-            (*self.ar).currentline as i32
+            (*self.ar.get()).currentline as i32
         }
     }
 
@@ -90,10 +101,10 @@ impl<'a> Debug<'a> {
     pub fn is_tail_call(&self) -> bool {
         unsafe {
             mlua_assert!(
-                ffi::lua_getinfo(self.state, cstr!("t"), self.ar) != 0,
+                ffi::lua_getinfo(self.lua.state, cstr!("t"), self.ar.get()) != 0,
                 "lua_getinfo failed with `t`"
             );
-            (*self.ar).currentline != 0
+            (*self.ar.get()).currentline != 0
         }
     }
 
@@ -101,20 +112,35 @@ impl<'a> Debug<'a> {
     pub fn stack(&self) -> DebugStack {
         unsafe {
             mlua_assert!(
-                ffi::lua_getinfo(self.state, cstr!("u"), self.ar) != 0,
+                ffi::lua_getinfo(self.lua.state, cstr!("u"), self.ar.get()) != 0,
                 "lua_getinfo failed with `u`"
             );
             DebugStack {
-                num_ups: (*self.ar).nups as i32,
+                num_ups: (*self.ar.get()).nups as i32,
                 #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
-                num_params: (*self.ar).nparams as i32,
+                num_params: (*self.ar.get()).nparams as i32,
                 #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
-                is_vararg: (*self.ar).isvararg != 0,
+                is_vararg: (*self.ar.get()).isvararg != 0,
             }
         }
     }
 }
 
+enum ActivationRecord {
+    Borrowed(*mut lua_Debug),
+    Owned(UnsafeCell<lua_Debug>),
+}
+
+impl ActivationRecord {
+    #[inline]
+    fn get(&self) -> *mut lua_Debug {
+        match self {
+            ActivationRecord::Borrowed(x) => *x,
+            ActivationRecord::Owned(x) => x.get(),
+        }
+    }
+}
+
 /// Represents a specific event that triggered the hook.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum DebugEvent {
@@ -123,6 +149,7 @@ pub enum DebugEvent {
     TailCall,
     Line,
     Count,
+    Unknown(c_int),
 }
 
 #[derive(Clone, Debug)]
@@ -144,10 +171,10 @@ pub struct DebugSource<'a> {
 pub struct DebugStack {
     pub num_ups: i32,
     /// Requires `feature = "lua54/lua53/lua52"`
-    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
     pub num_params: i32,
     /// Requires `feature = "lua54/lua53/lua52"`
-    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
     pub is_vararg: bool,
 }
 
@@ -255,27 +282,6 @@ impl BitOrAssign for HookTriggers {
     }
 }
 
-pub(crate) unsafe extern "C" fn hook_proc(state: *mut lua_State, ar: *mut lua_Debug) {
-    callback_error(state, |_| {
-        let debug = Debug {
-            ar,
-            state,
-            _phantom: PhantomData,
-        };
-
-        let lua = mlua_expect!(Lua::make_from_ptr(state), "cannot make Lua instance");
-        let hook_cb = mlua_expect!(lua.hook_callback(), "no hook callback set in hook_proc");
-
-        #[allow(clippy::match_wild_err_arm)]
-        match hook_cb.try_borrow_mut() {
-            Ok(mut b) => (&mut *b)(&lua, debug),
-            Err(_) => mlua_panic!("Lua should not allow hooks to be called within another hook"),
-        }?;
-
-        Ok(())
-    })
-}
-
 unsafe fn ptr_to_str<'a>(input: *const c_char) -> Option<&'a [u8]> {
     if input.is_null() {
         None

src/lib.rs

@@ -72,7 +72,7 @@
 //! [`serde::Deserialize`]: https://docs.serde.rs/serde/de/trait.Deserialize.html
 
 // mlua types in rustdoc of other crates get linked to here.
-#![doc(html_root_url = "https://docs.rs/mlua/0.6.6")]
+#![doc(html_root_url = "https://docs.rs/mlua/0.7.0")]
 // Deny warnings inside doc tests / examples. When this isn't present, rustdoc doesn't show *any*
 // warnings at all.
 #![doc(test(attr(deny(warnings))))]
@@ -192,6 +192,23 @@ extern crate mlua_derive;
 #[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
 pub use mlua_derive::chunk;
 
-#[cfg(any(feature = "module"))]
+/// Registers Lua module entrypoint.
+///
+/// You can register multiple entrypoints as required.
+///
+/// ```
+/// use mlua::{Lua, Result, Table};
+///
+/// #[mlua::lua_module]
+/// fn my_module(lua: &Lua) -> Result<Table> {
+///     let exports = lua.create_table()?;
+///     exports.set("hello", "world")?;
+///     Ok(exports)
+/// }
+/// ```
+///
+/// Internally in the code above the compiler defines C function `luaopen_my_module`.
+///
+#[cfg(any(feature = "module", docsrs))]
 #[cfg_attr(docsrs, doc(cfg(feature = "module")))]
 pub use mlua_derive::lua_module;

src/lua.rs

@@ -1,4 +1,4 @@
-use std::any::TypeId;
+use std::any::{Any, TypeId};
 use std::cell::{Ref, RefCell, RefMut, UnsafeCell};
 use std::collections::HashMap;
 use std::ffi::CString;
@@ -9,10 +9,12 @@ use std::panic::{catch_unwind, resume_unwind, AssertUnwindSafe, Location};
 use std::sync::{Arc, Mutex, RwLock};
 use std::{mem, ptr, str};
 
+use rustc_hash::FxHashMap;
+
 use crate::error::{Error, Result};
 use crate::ffi;
 use crate::function::Function;
-use crate::hook::{hook_proc, Debug, HookTriggers};
+use crate::hook::{Debug, HookTriggers};
 use crate::scope::Scope;
 use crate::stdlib::StdLib;
 use crate::string::String;
@@ -29,10 +31,18 @@ use crate::util::{
     self, assert_stack, callback_error, check_stack, get_destructed_userdata_metatable,
     get_gc_metatable, get_gc_userdata, get_main_state, get_userdata, init_error_registry,
     init_gc_metatable, init_userdata_metatable, pop_error, push_gc_userdata, push_string,
-    push_table, push_userdata, rawset_field, safe_pcall, safe_xpcall, StackGuard, WrappedFailure,
+    push_table, rawset_field, safe_pcall, safe_xpcall, StackGuard, WrappedFailure,
 };
 use crate::value::{FromLua, FromLuaMulti, MultiValue, Nil, ToLua, ToLuaMulti, Value};
 
+#[cfg(not(feature = "lua54"))]
+use crate::util::push_userdata;
+#[cfg(feature = "lua54")]
+use {
+    crate::{types::WarnCallback, userdata::USER_VALUE_MAXSLOT, util::push_userdata_uv},
+    std::ffi::CStr,
+};
+
 #[cfg(not(feature = "send"))]
 use std::rc::Rc;
 
@@ -63,10 +73,15 @@ pub struct Lua {
 
 // Data associated with the Lua.
 struct ExtraData {
-    registered_userdata: HashMap<TypeId, c_int>,
-    registered_userdata_mt: HashMap<*const c_void, Option<TypeId>>,
+    registered_userdata: FxHashMap<TypeId, c_int>,
+    registered_userdata_mt: FxHashMap<*const c_void, Option<TypeId>>,
     registry_unref_list: Arc<Mutex<Option<Vec<c_int>>>>,
 
+    #[cfg(not(feature = "send"))]
+    app_data: RefCell<HashMap<TypeId, Box<dyn Any>>>,
+    #[cfg(feature = "send")]
+    app_data: RefCell<HashMap<TypeId, Box<dyn Any + Send>>>,
+
     libs: StdLib,
     mem_info: Option<Box<MemoryInfo>>,
     safe: bool, // Same as in the Lua struct
@@ -76,14 +91,21 @@ struct ExtraData {
     ref_stack_top: c_int,
     ref_free: Vec<c_int>,
 
-    // Pool of preallocated `WrappedFailure` enums on the ref thread
-    wrapped_failures_pool: Vec<c_int>,
+    // Cache of `WrappedFailure` enums on the ref thread (as userdata)
+    wrapped_failures_cache: Vec<c_int>,
+    // Cache of recycled `MultiValue` containers
+    multivalue_cache: Vec<MultiValue<'static>>,
+    // Cache of recycled `Thread`s (coroutines)
+    #[cfg(feature = "async")]
+    recycled_thread_cache: Vec<c_int>,
 
     // Index of `Option<Waker>` userdata on the ref thread
     #[cfg(feature = "async")]
     ref_waker_idx: c_int,
 
     hook_callback: Option<HookCallback>,
+    #[cfg(feature = "lua54")]
+    warn_callback: Option<WarnCallback>,
 }
 
 #[cfg_attr(any(feature = "lua51", feature = "luajit"), allow(dead_code))]
@@ -97,14 +119,14 @@ struct MemoryInfo {
 /// In Lua 5.4 GC can work in two modes: incremental and generational.
 /// Previous Lua versions support only incremental GC.
 ///
-/// More information can be found in the Lua 5.x [documentation].
+/// More information can be found in the Lua [documentation].
 ///
 /// [documentation]: https://www.lua.org/manual/5.4/manual.html#2.5
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum GCMode {
     Incremental,
     /// Requires `feature = "lua54"`
-    #[cfg(any(feature = "lua54", doc))]
+    #[cfg(any(feature = "lua54"))]
     Generational,
 }
 
@@ -122,39 +144,64 @@ pub struct LuaOptions {
     ///
     /// Default: **true**
     ///
-    /// [`pcall`]: https://www.lua.org/manual/5.3/manual.html#pdf-pcall
-    /// [`xpcall`]: https://www.lua.org/manual/5.3/manual.html#pdf-xpcall
+    /// [`pcall`]: https://www.lua.org/manual/5.4/manual.html#pdf-pcall
+    /// [`xpcall`]: https://www.lua.org/manual/5.4/manual.html#pdf-xpcall
     pub catch_rust_panics: bool,
+
+    /// Max size of thread (coroutine) object cache used to execute asynchronous functions.
+    ///
+    /// It works only on Lua 5.4 or LuaJIT (vendored) with [`lua_resetthread`] function,
+    /// and allows to reuse old coroutines with reset state.
+    ///
+    /// Default: **0** (disabled)
+    ///
+    /// [`lua_resetthread`]: https://www.lua.org/manual/5.4/manual.html#lua_resetthread
+    #[cfg(feature = "async")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "async")))]
+    pub thread_cache_size: usize,
 }
 
 impl Default for LuaOptions {
     fn default() -> Self {
-        LuaOptions {
-            catch_rust_panics: true,
-        }
+        LuaOptions::new()
     }
 }
 
 impl LuaOptions {
     /// Returns a new instance of `LuaOptions` with default parameters.
-    pub fn new() -> Self {
-        Self::default()
+    pub const fn new() -> Self {
+        LuaOptions {
+            catch_rust_panics: true,
+            #[cfg(feature = "async")]
+            thread_cache_size: 0,
+        }
     }
 
     /// Sets [`catch_rust_panics`] option.
     ///
     /// [`catch_rust_panics`]: #structfield.catch_rust_panics
-    pub fn catch_rust_panics(mut self, enabled: bool) -> Self {
+    pub const fn catch_rust_panics(mut self, enabled: bool) -> Self {
         self.catch_rust_panics = enabled;
         self
     }
+
+    /// Sets [`thread_cache_size`] option.
+    ///
+    /// [`thread_cache_size`]: #structfield.thread_cache_size
+    #[cfg(feature = "async")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "async")))]
+    pub const fn thread_cache_size(mut self, size: usize) -> Self {
+        self.thread_cache_size = size;
+        self
+    }
 }
 
 #[cfg(feature = "async")]
 pub(crate) static ASYNC_POLL_PENDING: u8 = 0;
 pub(crate) static EXTRA_REGISTRY_KEY: u8 = 0;
 
-const WRAPPED_FAILURES_POOL_SIZE: usize = 16;
+const WRAPPED_FAILURES_CACHE_SIZE: usize = 16;
+const MULTIVALUE_CACHE_SIZE: usize = 16;
 
 /// Requires `feature = "send"`
 #[cfg(feature = "send")]
@@ -166,7 +213,10 @@ impl Drop for Lua {
         unsafe {
             if !self.ephemeral {
                 let extra = &mut *self.extra.get();
-                for index in extra.wrapped_failures_pool.drain(..) {
+                let drain_iter = extra.wrapped_failures_cache.drain(..);
+                #[cfg(feature = "async")]
+                let drain_iter = drain_iter.chain(extra.recycled_thread_cache.drain(..));
+                for index in drain_iter {
                     ffi::lua_pushnil(extra.ref_thread);
                     ffi::lua_replace(extra.ref_thread, index);
                     extra.ref_free.push(index);
@@ -387,6 +437,11 @@ impl Lua {
             )
         }
 
+        #[cfg(feature = "async")]
+        if options.thread_cache_size > 0 {
+            extra.recycled_thread_cache = Vec::with_capacity(options.thread_cache_size);
+        }
+
         lua
     }
 
@@ -456,9 +511,10 @@ impl Lua {
         // Create ExtraData
 
         let extra = Arc::new(UnsafeCell::new(ExtraData {
-            registered_userdata: HashMap::new(),
-            registered_userdata_mt: HashMap::new(),
+            registered_userdata: FxHashMap::default(),
+            registered_userdata_mt: FxHashMap::default(),
             registry_unref_list: Arc::new(Mutex::new(Some(Vec::new()))),
+            app_data: RefCell::new(HashMap::new()),
             ref_thread,
             libs: StdLib::NONE,
             mem_info: None,
@@ -467,10 +523,15 @@ impl Lua {
             ref_stack_size: ffi::LUA_MINSTACK - 1,
             ref_stack_top,
             ref_free: Vec::new(),
-            wrapped_failures_pool: Vec::new(),
+            wrapped_failures_cache: Vec::with_capacity(WRAPPED_FAILURES_CACHE_SIZE),
+            multivalue_cache: Vec::with_capacity(MULTIVALUE_CACHE_SIZE),
+            #[cfg(feature = "async")]
+            recycled_thread_cache: Vec::new(),
             #[cfg(feature = "async")]
             ref_waker_idx,
             hook_callback: None,
+            #[cfg(feature = "lua54")]
+            warn_callback: None,
         }));
 
         mlua_expect!(
@@ -557,7 +618,7 @@ impl Lua {
     ///
     /// Behavior is similar to Lua's [`require`] function.
     ///
-    /// [`require`]: https://www.lua.org/manual/5.3/manual.html#pdf-require
+    /// [`require`]: https://www.lua.org/manual/5.4/manual.html#pdf-require
     pub fn load_from_function<'lua, S, T>(
         &'lua self,
         modname: &S,
@@ -591,6 +652,31 @@ impl Lua {
         T::from_lua(value, self)
     }
 
+    /// Unloads module `modname`.
+    ///
+    /// Removes module from the [`package.loaded`] table which allows to load it again.
+    /// It does not support unloading binary Lua modules since they are internally cached and can be
+    /// unloaded only by closing Lua state.
+    ///
+    /// [`package.loaded`]: https://www.lua.org/manual/5.4/manual.html#pdf-package.loaded
+    pub fn unload<S>(&self, modname: &S) -> Result<()>
+    where
+        S: AsRef<[u8]> + ?Sized,
+    {
+        let loaded = unsafe {
+            let _sg = StackGuard::new(self.state);
+            check_stack(self.state, 2)?;
+            protect_lua!(self.state, 0, 1, fn(state) {
+                ffi::luaL_getsubtable(state, ffi::LUA_REGISTRYINDEX, cstr!("_LOADED"));
+            })?;
+            Table(self.pop_ref())
+        };
+
+        let modname = self.create_string(modname)?;
+        loaded.raw_remove(modname)?;
+        Ok(())
+    }
+
     /// Consumes and leaks `Lua` object, returning a static reference `&'static Lua`.
     ///
     /// This function is useful when the `Lua` object is supposed to live for the remainder
@@ -706,9 +792,32 @@ impl Lua {
     where
         F: 'static + MaybeSend + FnMut(&Lua, Debug) -> Result<()>,
     {
+        unsafe extern "C" fn hook_proc(state: *mut ffi::lua_State, ar: *mut ffi::lua_Debug) {
+            let lua = match Lua::make_from_ptr(state) {
+                Some(lua) => lua,
+                None => return,
+            };
+            let extra = lua.extra.get();
+            callback_error_ext(state, extra, move |_| {
+                let debug = Debug::new(&lua, ar);
+                let hook_cb = (*lua.extra.get()).hook_callback.clone();
+                let hook_cb = mlua_expect!(hook_cb, "no hook callback set in hook_proc");
+
+                #[allow(clippy::match_wild_err_arm)]
+                match hook_cb.try_lock() {
+                    Ok(mut cb) => cb(&lua, debug),
+                    Err(_) => {
+                        mlua_panic!("Lua should not allow hooks to be called within another hook")
+                    }
+                }?;
+
+                Ok(())
+            })
+        }
+
         let state = self.main_state.ok_or(Error::MainThreadNotAvailable)?;
         unsafe {
-            (*self.extra.get()).hook_callback = Some(Arc::new(RefCell::new(callback)));
+            (*self.extra.get()).hook_callback = Some(Arc::new(Mutex::new(callback)));
             ffi::lua_sethook(state, Some(hook_proc), triggers.mask(), triggers.count());
         }
         Ok(())
@@ -728,6 +837,81 @@ impl Lua {
         }
     }
 
+    /// Sets the warning function to be used by Lua to emit warnings.
+    ///
+    /// Requires `feature = "lua54"`
+    #[cfg(feature = "lua54")]
+    pub fn set_warning_function<F>(&self, callback: F)
+    where
+        F: 'static + MaybeSend + Fn(&Lua, &CStr, bool) -> Result<()>,
+    {
+        unsafe extern "C" fn warn_proc(ud: *mut c_void, msg: *const c_char, tocont: c_int) {
+            let state = ud as *mut ffi::lua_State;
+            let lua = match Lua::make_from_ptr(state) {
+                Some(lua) => lua,
+                None => return,
+            };
+            let extra = lua.extra.get();
+            callback_error_ext(state, extra, move |_| {
+                let cb = mlua_expect!(
+                    (*lua.extra.get()).warn_callback.as_ref(),
+                    "no warning callback set in warn_proc"
+                );
+                let msg = CStr::from_ptr(msg);
+                cb(&lua, msg, tocont != 0)
+            });
+        }
+
+        let state = self.main_state.unwrap_or(self.state);
+        unsafe {
+            (*self.extra.get()).warn_callback = Some(Box::new(callback));
+            ffi::lua_setwarnf(state, Some(warn_proc), state as *mut c_void);
+        }
+    }
+
+    /// Removes warning function previously set by `set_warning_function`.
+    ///
+    /// This function has no effect if a warning function was not previously set.
+    ///
+    /// Requires `feature = "lua54"`
+    #[cfg(feature = "lua54")]
+    pub fn remove_warning_function(&self) {
+        let state = self.main_state.unwrap_or(self.state);
+        unsafe {
+            (*self.extra.get()).warn_callback = None;
+            ffi::lua_setwarnf(state, None, ptr::null_mut());
+        }
+    }
+
+    /// Emits a warning with the given message.
+    ///
+    /// A message in a call with `tocont` set to `true` should be continued in another call to this function.
+    ///
+    /// Requires `feature = "lua54"`
+    #[cfg(feature = "lua54")]
+    pub fn warning<S: Into<Vec<u8>>>(&self, msg: S, tocont: bool) -> Result<()> {
+        let msg = CString::new(msg).map_err(|err| Error::RuntimeError(err.to_string()))?;
+        unsafe { ffi::lua_warning(self.state, msg.as_ptr(), if tocont { 1 } else { 0 }) };
+        Ok(())
+    }
+
+    /// Gets information about the interpreter runtime stack.
+    ///
+    /// This function returns [`Debug`] structure that can be used to get information about the function
+    /// executing at a given level. Level `0` is the current running function, whereas level `n+1` is the
+    /// function that has called level `n` (except for tail calls, which do not count in the stack).
+    ///
+    /// [`Debug`]: crate::hook::Debug
+    pub fn inspect_stack(&self, level: usize) -> Option<Debug> {
+        unsafe {
+            let mut ar: ffi::lua_Debug = mem::zeroed();
+            if ffi::lua_getstack(self.state, level as c_int, &mut ar) == 0 {
+                return None;
+            }
+            Some(Debug::new_owned(self, ar))
+        }
+    }
+
     /// Returns the amount of memory (in bytes) currently used inside this Lua state.
     pub fn used_memory(&self) -> usize {
         unsafe {
@@ -753,7 +937,7 @@ impl Lua {
     /// Does not work on module mode where Lua state is managed externally.
     ///
     /// Requires `feature = "lua54/lua53/lua52"`
-    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
     pub fn set_memory_limit(&self, memory_limit: usize) -> Result<usize> {
         unsafe {
             match &mut (*self.extra.get()).mem_info {
@@ -770,7 +954,7 @@ impl Lua {
     /// Returns true if the garbage collector is currently running automatically.
     ///
     /// Requires `feature = "lua54/lua53/lua52"`
-    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
     pub fn gc_is_running(&self) -> bool {
         let state = self.main_state.unwrap_or(self.state);
         unsafe { ffi::lua_gc(state, ffi::LUA_GCISRUNNING, 0) != 0 }
@@ -823,10 +1007,10 @@ impl Lua {
 
     /// Sets the 'pause' value of the collector.
     ///
-    /// Returns the previous value of 'pause'. More information can be found in the [Lua 5.3
-    /// documentation][lua_doc].
+    /// Returns the previous value of 'pause'. More information can be found in the Lua
+    /// [documentation][lua_doc].
     ///
-    /// [lua_doc]: https://www.lua.org/manual/5.3/manual.html#2.5
+    /// [lua_doc]: https://www.lua.org/manual/5.4/manual.html#2.5
     pub fn gc_set_pause(&self, pause: c_int) -> c_int {
         let state = self.main_state.unwrap_or(self.state);
         unsafe { ffi::lua_gc(state, ffi::LUA_GCSETPAUSE, pause) }
@@ -835,9 +1019,9 @@ impl Lua {
     /// Sets the 'step multiplier' value of the collector.
     ///
     /// Returns the previous value of the 'step multiplier'. More information can be found in the
-    /// Lua 5.x [documentation][lua_doc].
+    /// Lua [documentation][lua_doc].
     ///
-    /// [lua_doc]: https://www.lua.org/manual/5.3/manual.html#2.5
+    /// [lua_doc]: https://www.lua.org/manual/5.4/manual.html#2.5
     pub fn gc_set_step_multiplier(&self, step_multiplier: c_int) -> c_int {
         let state = self.main_state.unwrap_or(self.state);
         unsafe { ffi::lua_gc(state, ffi::LUA_GCSETSTEPMUL, step_multiplier) }
@@ -846,7 +1030,7 @@ impl Lua {
     /// Changes the collector to incremental mode with the given parameters.
     ///
     /// Returns the previous mode (always `GCMode::Incremental` in Lua < 5.4).
-    /// More information can be found in the Lua 5.x [documentation][lua_doc].
+    /// More information can be found in the Lua [documentation][lua_doc].
     ///
     /// [lua_doc]: https://www.lua.org/manual/5.4/manual.html#2.5.1
     pub fn gc_inc(&self, pause: c_int, step_multiplier: c_int, step_size: c_int) -> GCMode {
@@ -888,7 +1072,7 @@ impl Lua {
     /// Requires `feature = "lua54"`
     ///
     /// [lua_doc]: https://www.lua.org/manual/5.4/manual.html#2.5.2
-    #[cfg(any(feature = "lua54", doc))]
+    #[cfg(any(feature = "lua54"))]
     pub fn gc_gen(&self, minor_multiplier: c_int, major_multiplier: c_int) -> GCMode {
         let state = self.main_state.unwrap_or(self.state);
         let prev_mode =
@@ -1232,6 +1416,44 @@ impl Lua {
         }
     }
 
+    /// Wraps a Lua function into a new or recycled thread (coroutine).
+    #[cfg(feature = "async")]
+    pub(crate) fn create_recycled_thread<'lua>(
+        &'lua self,
+        func: Function<'lua>,
+    ) -> Result<Thread<'lua>> {
+        #[cfg(any(feature = "lua54", all(feature = "luajit", feature = "vendored")))]
+        unsafe {
+            let _sg = StackGuard::new(self.state);
+            check_stack(self.state, 1)?;
+
+            let extra = &mut *self.extra.get();
+            if let Some(index) = extra.recycled_thread_cache.pop() {
+                let thread_state = ffi::lua_tothread(extra.ref_thread, index);
+                self.push_ref(&func.0);
+                ffi::lua_xmove(self.state, thread_state, 1);
+                return Ok(Thread(LuaRef { lua: self, index }));
+            }
+        };
+        self.create_thread(func)
+    }
+
+    /// Resets thread (coroutine) and returns to the cache for later use.
+    #[cfg(feature = "async")]
+    #[cfg(any(feature = "lua54", all(feature = "luajit", feature = "vendored")))]
+    pub(crate) fn recycle_thread<'lua>(&'lua self, thread: &mut Thread<'lua>) {
+        unsafe {
+            let extra = &mut *self.extra.get();
+            let thread_state = ffi::lua_tothread(extra.ref_thread, thread.0.index);
+            if extra.recycled_thread_cache.len() < extra.recycled_thread_cache.capacity()
+                && ffi::lua_resetthread(self.state, thread_state) == ffi::LUA_OK
+            {
+                extra.recycled_thread_cache.push(thread.0.index);
+                thread.0.index = 0;
+            }
+        }
+    }
+
     /// Create a Lua userdata object from a custom userdata type.
     pub fn create_userdata<T>(&self, data: T) -> Result<AnyUserData>
     where
@@ -1577,6 +1799,74 @@ impl Lua {
         }
     }
 
+    /// Sets or replaces an application data object of type `T`.
+    ///
+    /// Application data could be accessed at any time by using [`Lua::app_data_ref()`] or [`Lua::app_data_mut()`]
+    /// methods where `T` is the data type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use mlua::{Lua, Result};
+    ///
+    /// fn hello(lua: &Lua, _: ()) -> Result<()> {
+    ///     let mut s = lua.app_data_mut::<&str>().unwrap();
+    ///     assert_eq!(*s, "hello");
+    ///     *s = "world";
+    ///     Ok(())
+    /// }
+    ///
+    /// fn main() -> Result<()> {
+    ///     let lua = Lua::new();
+    ///     lua.set_app_data("hello");
+    ///     lua.create_function(hello)?.call(())?;
+    ///     let s = lua.app_data_ref::<&str>().unwrap();
+    ///     assert_eq!(*s, "world");
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn set_app_data<T: 'static + MaybeSend>(&self, data: T) {
+        let extra = unsafe { &mut (*self.extra.get()) };
+        extra
+            .app_data
+            .try_borrow_mut()
+            .expect("cannot borrow mutably app data container")
+            .insert(TypeId::of::<T>(), Box::new(data));
+    }
+
+    /// Gets a reference to an application data object stored by [`Lua::set_app_data()`] of type `T`.
+    pub fn app_data_ref<T: 'static>(&self) -> Option<Ref<T>> {
+        let extra = unsafe { &(*self.extra.get()) };
+        let app_data = extra
+            .app_data
+            .try_borrow()
+            .expect("cannot borrow app data container");
+        let value = app_data.get(&TypeId::of::<T>())?.downcast_ref::<T>()? as *const _;
+        Some(Ref::map(app_data, |_| unsafe { &*value }))
+    }
+
+    /// Gets a mutable reference to an application data object stored by [`Lua::set_app_data()`] of type `T`.
+    pub fn app_data_mut<T: 'static>(&self) -> Option<RefMut<T>> {
+        let extra = unsafe { &(*self.extra.get()) };
+        let mut app_data = extra
+            .app_data
+            .try_borrow_mut()
+            .expect("cannot mutably borrow app data container");
+        let value = app_data.get_mut(&TypeId::of::<T>())?.downcast_mut::<T>()? as *mut _;
+        Some(RefMut::map(app_data, |_| unsafe { &mut *value }))
+    }
+
+    /// Removes an application data of type `T`.
+    pub fn remove_app_data<T: 'static>(&self) -> Option<T> {
+        let extra = unsafe { &mut (*self.extra.get()) };
+        extra
+            .app_data
+            .try_borrow_mut()
+            .expect("cannot mutably borrow app data container")
+            .remove(&TypeId::of::<T>())
+            .and_then(|data| data.downcast().ok().map(|data| *data))
+    }
+
     // Uses 2 stack spaces, does not call checkstack
     pub(crate) unsafe fn push_value(&self, value: Value) -> Result<()> {
         match value {
@@ -1769,11 +2059,18 @@ impl Lua {
 
         // Prepare metatable, add meta methods first and then meta fields
         let metatable_nrec = methods.meta_methods.len() + fields.meta_fields.len();
+        #[cfg(feature = "async")]
+        let metatable_nrec = metatable_nrec + methods.async_meta_methods.len();
         push_table(self.state, 0, metatable_nrec as c_int)?;
         for (k, m) in methods.meta_methods {
             self.push_value(Value::Function(self.create_callback(m)?))?;
             rawset_field(self.state, -2, k.validate()?.name())?;
         }
+        #[cfg(feature = "async")]
+        for (k, m) in methods.async_meta_methods {
+            self.push_value(Value::Function(self.create_async_callback(m)?))?;
+            rawset_field(self.state, -2, k.validate()?.name())?;
+        }
         for (k, f) in fields.meta_fields {
             self.push_value(f(self)?)?;
             rawset_field(self.state, -2, k.validate()?.name())?;
@@ -1807,10 +2104,9 @@ impl Lua {
         }
 
         let mut methods_index = None;
-        #[cfg(feature = "async")]
-        let methods_nrec = methods.methods.len() + methods.async_methods.len();
-        #[cfg(not(feature = "async"))]
         let methods_nrec = methods.methods.len();
+        #[cfg(feature = "async")]
+        let methods_nrec = methods_nrec + methods.async_methods.len();
         if methods_nrec > 0 {
             push_table(self.state, 0, methods_nrec as c_int)?;
             for (k, m) in methods.methods {
@@ -1909,11 +2205,14 @@ impl Lua {
         'lua: 'callback,
     {
         unsafe extern "C" fn call_callback(state: *mut ffi::lua_State) -> c_int {
-            let get_extra = |state| {
-                let upvalue = get_userdata::<CallbackUpvalue>(state, ffi::lua_upvalueindex(1));
-                (*upvalue).lua.extra.get()
+            let extra = match ffi::lua_type(state, ffi::lua_upvalueindex(1)) {
+                ffi::LUA_TUSERDATA => {
+                    let upvalue = get_userdata::<CallbackUpvalue>(state, ffi::lua_upvalueindex(1));
+                    (*upvalue).lua.extra.get()
+                }
+                _ => ptr::null_mut(),
             };
-            callback_error_ext(state, get_extra, |nargs| {
+            callback_error_ext(state, extra, |nargs| {
                 let upvalue_idx = ffi::lua_upvalueindex(1);
                 if ffi::lua_type(state, upvalue_idx) == ffi::LUA_TNIL {
                     return Err(Error::CallbackDestructed);
@@ -1924,22 +2223,23 @@ impl Lua {
                     check_stack(state, ffi::LUA_MINSTACK - nargs)?;
                 }
 
-                let lua = &mut (*upvalue).lua;
+                let mut lua = (*upvalue).lua.clone();
                 lua.state = state;
 
-                let mut args = MultiValue::new();
+                let mut args = MultiValue::new_or_cached(&lua);
                 args.reserve(nargs as usize);
                 for _ in 0..nargs {
                     args.push_front(lua.pop_value());
                 }
 
-                let results = ((*upvalue).func)(lua, args)?;
+                let mut results = ((*upvalue).func)(&lua, args)?;
                 let nresults = results.len() as c_int;
 
                 check_stack(state, nresults)?;
-                for r in results {
+                for r in results.drain_all() {
                     lua.push_value(r)?;
                 }
+                lua.cache_multivalue(results);
 
                 Ok(nresults)
             })
@@ -1977,11 +2277,15 @@ impl Lua {
         }
 
         unsafe extern "C" fn call_callback(state: *mut ffi::lua_State) -> c_int {
-            let get_extra = |state| {
-                let upvalue = get_userdata::<AsyncCallbackUpvalue>(state, ffi::lua_upvalueindex(1));
-                (*upvalue).lua.extra.get()
+            let extra = match ffi::lua_type(state, ffi::lua_upvalueindex(1)) {
+                ffi::LUA_TUSERDATA => {
+                    let upvalue =
+                        get_userdata::<AsyncCallbackUpvalue>(state, ffi::lua_upvalueindex(1));
+                    (*upvalue).lua.extra.get()
+                }
+                _ => ptr::null_mut(),
             };
-            callback_error_ext(state, get_extra, |nargs| {
+            callback_error_ext(state, extra, |nargs| {
                 let upvalue_idx = ffi::lua_upvalueindex(1);
                 if ffi::lua_type(state, upvalue_idx) == ffi::LUA_TNIL {
                     return Err(Error::CallbackDestructed);
@@ -1995,7 +2299,7 @@ impl Lua {
                 let lua = &mut (*upvalue).lua;
                 lua.state = state;
 
-                let mut args = MultiValue::new();
+                let mut args = MultiValue::new_or_cached(lua);
                 args.reserve(nargs as usize);
                 for _ in 0..nargs {
                     args.push_front(lua.pop_value());
@@ -2013,11 +2317,14 @@ impl Lua {
         }
 
         unsafe extern "C" fn poll_future(state: *mut ffi::lua_State) -> c_int {
-            let get_extra = |state| {
-                let upvalue = get_userdata::<AsyncPollUpvalue>(state, ffi::lua_upvalueindex(1));
-                (*upvalue).lua.extra.get()
+            let extra = match ffi::lua_type(state, ffi::lua_upvalueindex(1)) {
+                ffi::LUA_TUSERDATA => {
+                    let upvalue = get_userdata::<AsyncPollUpvalue>(state, ffi::lua_upvalueindex(1));
+                    (*upvalue).lua.extra.get()
+                }
+                _ => ptr::null_mut(),
             };
-            callback_error_ext(state, get_extra, |nargs| {
+            callback_error_ext(state, extra, |nargs| {
                 let upvalue_idx = ffi::lua_upvalueindex(1);
                 if ffi::lua_type(state, upvalue_idx) == ffi::LUA_TNIL {
                     return Err(Error::CallbackDestructed);
@@ -2077,11 +2384,10 @@ impl Lua {
         env.set("yield", coroutine.get::<_, Function>("yield")?)?;
         env.set(
             "unpack",
-            self.create_function(|_, (tbl, len): (Table, Integer)| {
-                Ok(MultiValue::from_vec(
-                    tbl.raw_sequence_values_by_len(Some(len))
-                        .collect::<Result<Vec<Value>>>()?,
-                ))
+            self.create_function(|lua, (tbl, len): (Table, Integer)| {
+                let mut values = MultiValue::new_or_cached(lua);
+                values.refill(tbl.raw_sequence_values_by_len(Some(len)))?;
+                Ok(values)
             })?,
         )?;
         env.set("pending", {
@@ -2108,12 +2414,14 @@ impl Lua {
     }
 
     #[cfg(feature = "async")]
+    #[inline]
     pub(crate) unsafe fn waker(&self) -> Option<Waker> {
         let extra = &*self.extra.get();
         (*get_userdata::<Option<Waker>>(extra.ref_thread, extra.ref_waker_idx)).clone()
     }
 
     #[cfg(feature = "async")]
+    #[inline]
     pub(crate) unsafe fn set_waker(&self, waker: Option<Waker>) -> Option<Waker> {
         let extra = &*self.extra.get();
         let waker_slot = &mut *get_userdata::<Option<Waker>>(extra.ref_thread, extra.ref_waker_idx);
@@ -2133,13 +2441,24 @@ impl Lua {
         // We push metatable first to ensure having correct metatable with `__gc` method
         ffi::lua_pushnil(self.state);
         self.push_userdata_metatable::<T>()?;
+        #[cfg(not(feature = "lua54"))]
         push_userdata(self.state, data)?;
+        #[cfg(feature = "lua54")]
+        push_userdata_uv(self.state, data, USER_VALUE_MAXSLOT as c_int)?;
         ffi::lua_replace(self.state, -3);
         ffi::lua_setmetatable(self.state, -2);
 
+        // Set empty environment for Lua 5.1
+        #[cfg(any(feature = "lua51", feature = "luajit"))]
+        protect_lua!(self.state, 1, 1, fn(state) {
+            ffi::lua_newtable(state);
+            ffi::lua_setuservalue(state, -2);
+        })?;
+
         Ok(AnyUserData(self.pop_ref()))
     }
 
+    #[inline]
     pub(crate) fn clone(&self) -> Self {
         Lua {
             state: self.state,
@@ -2200,8 +2519,23 @@ impl Lua {
         })
     }
 
-    pub(crate) unsafe fn hook_callback(&self) -> Option<HookCallback> {
-        (*self.extra.get()).hook_callback.clone()
+    #[inline]
+    pub(crate) fn new_or_cached_multivalue(&self) -> MultiValue {
+        unsafe {
+            let extra = &mut *self.extra.get();
+            extra.multivalue_cache.pop().unwrap_or_default()
+        }
+    }
+
+    #[inline]
+    pub(crate) fn cache_multivalue(&self, mut multivalue: MultiValue) {
+        unsafe {
+            let extra = &mut *self.extra.get();
+            if extra.multivalue_cache.len() < MULTIVALUE_CACHE_SIZE {
+                multivalue.clear();
+                extra.multivalue_cache.push(mem::transmute(multivalue));
+            }
+        }
     }
 }
 
@@ -2226,7 +2560,7 @@ pub enum ChunkMode {
 
 /// Trait for types [loadable by Lua] and convertible to a [`Chunk`]
 ///
-/// [loadable by Lua]: https://www.lua.org/manual/5.3/manual.html#3.3.2
+/// [loadable by Lua]: https://www.lua.org/manual/5.4/manual.html#3.3.2
 /// [`Chunk`]: crate::Chunk
 pub trait AsChunk<'lua> {
     /// Returns chunk data (can be text or binary)
@@ -2239,7 +2573,7 @@ pub trait AsChunk<'lua> {
 
     /// Returns optional chunk [environment]
     ///
-    /// [environment]: https://www.lua.org/manual/5.3/manual.html#2.2
+    /// [environment]: https://www.lua.org/manual/5.4/manual.html#2.2
     fn env(&self, _lua: &'lua Lua) -> Result<Option<Value<'lua>>> {
         Ok(None)
     }
@@ -2425,7 +2759,7 @@ impl<'lua, T: AsRef<[u8]> + ?Sized> AsChunk<'lua> for T {
 }
 
 // Creates required entries in the metatable cache (see `util::METATABLE_CACHE`)
-pub(crate) fn init_metatable_cache(cache: &mut HashMap<TypeId, u8>) {
+pub(crate) fn init_metatable_cache(cache: &mut FxHashMap<TypeId, u8>) {
     cache.insert(TypeId::of::<Arc<UnsafeCell<ExtraData>>>(), 0);
     cache.insert(TypeId::of::<Callback>(), 0);
     cache.insert(TypeId::of::<CallbackUpvalue>(), 0);
@@ -2442,15 +2776,14 @@ pub(crate) fn init_metatable_cache(cache: &mut HashMap<TypeId, u8>) {
 // An optimized version of `callback_error` that does not allocate `WrappedFailure` userdata
 // and instead reuses unsed and cached values from previous calls (or allocates new).
 // It requires `get_extra` function to return `ExtraData` value.
-unsafe fn callback_error_ext<E, F, R>(state: *mut ffi::lua_State, get_extra: E, f: F) -> R
+unsafe fn callback_error_ext<F, R>(state: *mut ffi::lua_State, extra: *mut ExtraData, f: F) -> R
 where
-    E: Fn(*mut ffi::lua_State) -> *mut ExtraData,
     F: FnOnce(c_int) -> Result<R>,
 {
-    let upvalue_idx = ffi::lua_upvalueindex(1);
-    if ffi::lua_type(state, upvalue_idx) == ffi::LUA_TNIL {
+    if extra.is_null() {
         return callback_error(state, f);
     }
+    let extra = &mut *extra;
 
     let nargs = ffi::lua_gettop(state);
 
@@ -2468,9 +2801,8 @@ where
     }
 
     // We cannot shadow Rust errors with Lua ones, so we need to obtain pre-allocated memory
-    // to store a wrapped error or panic *before* we proceed.
-    let extra = &mut *get_extra(state);
-    let prealloc_failure = match extra.wrapped_failures_pool.pop() {
+    // to store a wrapped failure (error or panic) *before* we proceed.
+    let prealloc_failure = match extra.wrapped_failures_cache.pop() {
         Some(index) => PreallocatedFailure::Cached(index),
         None => {
             let ud = ffi::lua_newuserdata(state, mem::size_of::<WrappedFailure>());
@@ -2500,20 +2832,20 @@ where
             // Return unused WrappedFailure to the cache
             match prealloc_failure {
                 PreallocatedFailure::New(_)
-                    if extra.wrapped_failures_pool.len() < WRAPPED_FAILURES_POOL_SIZE =>
+                    if extra.wrapped_failures_cache.len() < WRAPPED_FAILURES_CACHE_SIZE =>
                 {
                     ffi::lua_rotate(state, 1, -1);
                     ffi::lua_xmove(state, extra.ref_thread, 1);
                     let index = ref_stack_pop(extra);
-                    extra.wrapped_failures_pool.push(index);
+                    extra.wrapped_failures_cache.push(index);
                 }
                 PreallocatedFailure::New(_) => {
                     ffi::lua_remove(state, 1);
                 }
                 PreallocatedFailure::Cached(index)
-                    if extra.wrapped_failures_pool.len() < WRAPPED_FAILURES_POOL_SIZE =>
+                    if extra.wrapped_failures_cache.len() < WRAPPED_FAILURES_CACHE_SIZE =>
                 {
-                    extra.wrapped_failures_pool.push(index);
+                    extra.wrapped_failures_cache.push(index);
                 }
                 PreallocatedFailure::Cached(index) => {
                     ffi::lua_pushnil(extra.ref_thread);
@@ -2525,11 +2857,8 @@ where
         }
         Ok(Err(err)) => {
             let wrapped_error = get_wrapped_failure();
-            ptr::write(wrapped_error, WrappedFailure::Error(err));
-            get_gc_metatable::<WrappedFailure>(state);
-            ffi::lua_setmetatable(state, -2);
 
-            // Convert to CallbackError and attach traceback
+            // Build `CallbackError` with traceback
             let traceback = if ffi::lua_checkstack(state, ffi::LUA_TRACEBACK_STACK) != 0 {
                 ffi::luaL_traceback(state, state, ptr::null(), 0);
                 let traceback = util::to_string(state, -1);
@@ -2538,10 +2867,13 @@ where
             } else {
                 "<not enough stack space for traceback>".to_string()
             };
-            if let WrappedFailure::Error(ref mut err) = *wrapped_error {
-                let cause = Arc::new(err.clone());
-                *err = Error::CallbackError { traceback, cause };
-            }
+            let cause = Arc::new(err);
+            ptr::write(
+                wrapped_error,
+                WrappedFailure::Error(Error::CallbackError { traceback, cause }),
+            );
+            get_gc_metatable::<WrappedFailure>(state);
+            ffi::lua_setmetatable(state, -2);
 
             ffi::lua_error(state)
         }
@@ -2712,6 +3044,8 @@ struct StaticUserDataMethods<'lua, T: 'static + UserData> {
     #[cfg(feature = "async")]
     async_methods: Vec<(Vec<u8>, AsyncCallback<'lua, 'static>)>,
     meta_methods: Vec<(MetaMethod, Callback<'lua, 'static>)>,
+    #[cfg(feature = "async")]
+    async_meta_methods: Vec<(MetaMethod, AsyncCallback<'lua, 'static>)>,
     _type: PhantomData<T>,
 }
 
@@ -2722,6 +3056,8 @@ impl<'lua, T: 'static + UserData> Default for StaticUserDataMethods<'lua, T> {
             #[cfg(feature = "async")]
             async_methods: Vec::new(),
             meta_methods: Vec::new(),
+            #[cfg(feature = "async")]
+            async_meta_methods: Vec::new(),
             _type: PhantomData,
         }
     }
@@ -2821,6 +3157,20 @@ impl<'lua, T: 'static + UserData> UserDataMethods<'lua, T> for StaticUserDataMet
             .push((meta.into(), Self::box_method_mut(method)));
     }
 
+    #[cfg(all(feature = "async", not(feature = "lua51")))]
+    fn add_async_meta_method<S, A, R, M, MR>(&mut self, meta: S, method: M)
+    where
+        T: Clone,
+        S: Into<MetaMethod>,
+        A: FromLuaMulti<'lua>,
+        R: ToLuaMulti<'lua>,
+        M: 'static + MaybeSend + Fn(&'lua Lua, T, A) -> MR,
+        MR: 'lua + Future<Output = Result<R>>,
+    {
+        self.async_meta_methods
+            .push((meta.into(), Self::box_async_method(method)));
+    }
+
     fn add_meta_function<S, A, R, F>(&mut self, meta: S, function: F)
     where
         S: Into<MetaMethod>,
@@ -2843,6 +3193,19 @@ impl<'lua, T: 'static + UserData> UserDataMethods<'lua, T> for StaticUserDataMet
             .push((meta.into(), Self::box_function_mut(function)));
     }
 
+    #[cfg(all(feature = "async", not(feature = "lua51")))]
+    fn add_async_meta_function<S, A, R, F, FR>(&mut self, meta: S, function: F)
+    where
+        S: Into<MetaMethod>,
+        A: FromLuaMulti<'lua>,
+        R: ToLuaMulti<'lua>,
+        F: 'static + MaybeSend + Fn(&'lua Lua, A) -> FR,
+        FR: 'lua + Future<Output = Result<R>>,
+    {
+        self.async_meta_methods
+            .push((meta.into(), Self::box_async_function(function)));
+    }
+
     // Below are internal methods used in generated code
 
     fn add_callback(&mut self, name: Vec<u8>, callback: Callback<'lua, 'static>) {
@@ -2857,6 +3220,15 @@ impl<'lua, T: 'static + UserData> UserDataMethods<'lua, T> for StaticUserDataMet
     fn add_meta_callback(&mut self, meta: MetaMethod, callback: Callback<'lua, 'static>) {
         self.meta_methods.push((meta, callback));
     }
+
+    #[cfg(feature = "async")]
+    fn add_async_meta_callback(
+        &mut self,
+        meta: MetaMethod,
+        callback: AsyncCallback<'lua, 'static>,
+    ) {
+        self.async_meta_methods.push((meta, callback))
+    }
 }
 
 impl<'lua, T: 'static + UserData> StaticUserDataMethods<'lua, T> {
@@ -3202,6 +3574,10 @@ macro_rules! lua_userdata_impl {
                 for (meta, callback) in orig_methods.meta_methods {
                     methods.add_meta_callback(meta, callback);
                 }
+                #[cfg(feature = "async")]
+                for (meta, callback) in orig_methods.async_meta_methods {
+                    methods.add_async_meta_callback(meta, callback);
+                }
             }
         }
     };

src/multi.rs

@@ -12,8 +12,7 @@ use crate::value::{FromLua, FromLuaMulti, MultiValue, Nil, ToLua, ToLuaMulti};
 /// on success, or in the case of an error, returning `nil` and an error message.
 impl<'lua, T: ToLua<'lua>, E: ToLua<'lua>> ToLuaMulti<'lua> for StdResult<T, E> {
     fn to_lua_multi(self, lua: &'lua Lua) -> Result<MultiValue<'lua>> {
-        let mut result = MultiValue::new();
-
+        let mut result = MultiValue::new_or_cached(lua);
         match self {
             Ok(v) => result.push_front(v.to_lua(lua)?),
             Err(e) => {
@@ -21,14 +20,13 @@ impl<'lua, T: ToLua<'lua>, E: ToLua<'lua>> ToLuaMulti<'lua> for StdResult<T, E>
                 result.push_front(Nil);
             }
         }
-
         Ok(result)
     }
 }
 
 impl<'lua, T: ToLua<'lua>> ToLuaMulti<'lua> for T {
     fn to_lua_multi(self, lua: &'lua Lua) -> Result<MultiValue<'lua>> {
-        let mut v = MultiValue::new();
+        let mut v = MultiValue::new_or_cached(lua);
         v.push_front(self.to_lua(lua)?);
         Ok(v)
     }
@@ -36,7 +34,9 @@ impl<'lua, T: ToLua<'lua>> ToLuaMulti<'lua> for T {
 
 impl<'lua, T: FromLua<'lua>> FromLuaMulti<'lua> for T {
     fn from_lua_multi(mut values: MultiValue<'lua>, lua: &'lua Lua) -> Result<Self> {
-        T::from_lua(values.pop_front().unwrap_or(Nil), lua)
+        let res = T::from_lua(values.pop_front().unwrap_or(Nil), lua);
+        lua.cache_multivalue(values);
+        res
     }
 }
 
@@ -125,30 +125,35 @@ impl<T> DerefMut for Variadic<T> {
 
 impl<'lua, T: ToLua<'lua>> ToLuaMulti<'lua> for Variadic<T> {
     fn to_lua_multi(self, lua: &'lua Lua) -> Result<MultiValue<'lua>> {
-        self.0.into_iter().map(|e| e.to_lua(lua)).collect()
+        let mut values = MultiValue::new_or_cached(lua);
+        values.refill(self.0.into_iter().map(|e| e.to_lua(lua)))?;
+        Ok(values)
     }
 }
 
 impl<'lua, T: FromLua<'lua>> FromLuaMulti<'lua> for Variadic<T> {
-    fn from_lua_multi(values: MultiValue<'lua>, lua: &'lua Lua) -> Result<Self> {
-        values
-            .into_iter()
+    fn from_lua_multi(mut values: MultiValue<'lua>, lua: &'lua Lua) -> Result<Self> {
+        let res = values
+            .drain_all()
             .map(|e| T::from_lua(e, lua))
             .collect::<Result<Vec<T>>>()
-            .map(Variadic)
+            .map(Variadic);
+        lua.cache_multivalue(values);
+        res
     }
 }
 
 macro_rules! impl_tuple {
     () => (
         impl<'lua> ToLuaMulti<'lua> for () {
-            fn to_lua_multi(self, _: &'lua Lua) -> Result<MultiValue<'lua>> {
-                Ok(MultiValue::new())
+            fn to_lua_multi(self, lua: &'lua Lua) -> Result<MultiValue<'lua>> {
+                Ok(MultiValue::new_or_cached(lua))
             }
         }
 
         impl<'lua> FromLuaMulti<'lua> for () {
-            fn from_lua_multi(_: MultiValue<'lua>, _: &'lua Lua) -> Result<Self> {
+            fn from_lua_multi(values: MultiValue<'lua>, lua: &'lua Lua) -> Result<Self> {
+                lua.cache_multivalue(values);
                 Ok(())
             }
         }

src/prelude.rs

@@ -7,11 +7,12 @@ pub use crate::{
     Function as LuaFunction, GCMode as LuaGCMode, Integer as LuaInteger,
     LightUserData as LuaLightUserData, Lua, LuaOptions, MetaMethod as LuaMetaMethod,
     MultiValue as LuaMultiValue, Nil as LuaNil, Number as LuaNumber, RegistryKey as LuaRegistryKey,
-    Result as LuaResult, String as LuaString, Table as LuaTable, TableExt as LuaTableExt,
-    TablePairs as LuaTablePairs, TableSequence as LuaTableSequence, Thread as LuaThread,
-    ThreadStatus as LuaThreadStatus, ToLua, ToLuaMulti, UserData as LuaUserData,
-    UserDataFields as LuaUserDataFields, UserDataMetatable as LuaUserDataMetatable,
-    UserDataMethods as LuaUserDataMethods, Value as LuaValue,
+    Result as LuaResult, StdLib as LuaStdLib, String as LuaString, Table as LuaTable,
+    TableExt as LuaTableExt, TablePairs as LuaTablePairs, TableSequence as LuaTableSequence,
+    Thread as LuaThread, ThreadStatus as LuaThreadStatus, ToLua, ToLuaMulti,
+    UserData as LuaUserData, UserDataFields as LuaUserDataFields,
+    UserDataMetatable as LuaUserDataMetatable, UserDataMethods as LuaUserDataMethods,
+    Value as LuaValue,
 };
 
 #[cfg(feature = "async")]

src/scope.rs

@@ -23,6 +23,9 @@ use crate::util::{
 };
 use crate::value::{FromLua, FromLuaMulti, MultiValue, ToLua, ToLuaMulti, Value};
 
+#[cfg(feature = "lua54")]
+use crate::userdata::USER_VALUE_MAXSLOT;
+
 #[cfg(feature = "async")]
 use {
     crate::types::{AsyncCallback, AsyncCallbackUpvalue, AsyncPollUpvalue},
@@ -197,12 +200,22 @@ impl<'lua, 'scope> Scope<'lua, 'scope> {
                     return vec![];
                 }
 
-                // Clear uservalue
-                #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
-                ffi::lua_pushnil(state);
+                // Clear associated user values
+                #[cfg(feature = "lua54")]
+                for i in 1..=USER_VALUE_MAXSLOT {
+                    ffi::lua_pushnil(state);
+                    ffi::lua_setiuservalue(state, -2, i as c_int);
+                }
+                #[cfg(any(feature = "lua53", feature = "lua52"))]
+                {
+                    ffi::lua_pushnil(state);
+                    ffi::lua_setuservalue(state, -2);
+                }
                 #[cfg(any(feature = "lua51", feature = "luajit"))]
-                ud.lua.push_ref(&newtable.0);
-                ffi::lua_setuservalue(state, -2);
+                {
+                    ud.lua.push_ref(&newtable.0);
+                    ffi::lua_setuservalue(state, -2);
+                }
 
                 vec![Box::new(take_userdata::<UserDataCell<T>>(state))]
             });
@@ -323,8 +336,19 @@ impl<'lua, 'scope> Scope<'lua, 'scope> {
             let _sg = StackGuard::new(lua.state);
             check_stack(lua.state, 13)?;
 
+            #[allow(clippy::let_and_return)]
             let data_ptr = protect_lua!(lua.state, 0, 1, |state| {
-                ffi::lua_newuserdata(state, mem::size_of::<UserDataCell<Rc<RefCell<T>>>>())
+                let ud =
+                    ffi::lua_newuserdata(state, mem::size_of::<UserDataCell<Rc<RefCell<T>>>>());
+
+                // Set empty environment for Lua 5.1
+                #[cfg(any(feature = "lua51", feature = "luajit"))]
+                {
+                    ffi::lua_newtable(state);
+                    ffi::lua_setuservalue(state, -2);
+                }
+
+                ud
             })?;
             // Prepare metatable, add meta methods first and then meta fields
             let meta_methods_nrec = ud_methods.meta_methods.len() + ud_fields.meta_fields.len() + 1;
@@ -416,12 +440,22 @@ impl<'lua, 'scope> Scope<'lua, 'scope> {
                 ffi::lua_pop(state, 1);
                 ud.lua.deregister_userdata_metatable(mt_ptr);
 
-                // Clear uservalue
-                #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
-                ffi::lua_pushnil(state);
+                // Clear associated user values
+                #[cfg(feature = "lua54")]
+                for i in 1..=USER_VALUE_MAXSLOT {
+                    ffi::lua_pushnil(state);
+                    ffi::lua_setiuservalue(state, -2, i as c_int);
+                }
+                #[cfg(any(feature = "lua53", feature = "lua52"))]
+                {
+                    ffi::lua_pushnil(state);
+                    ffi::lua_setuservalue(state, -2);
+                }
                 #[cfg(any(feature = "lua51", feature = "luajit"))]
-                ud.lua.push_ref(&newtable.0);
-                ffi::lua_setuservalue(state, -2);
+                {
+                    ud.lua.push_ref(&newtable.0);
+                    ffi::lua_setuservalue(state, -2);
+                }
 
                 // A hack to drop non-static `T`
                 unsafe fn seal<T>(t: T) -> Box<dyn FnOnce() + 'static> {
@@ -693,6 +727,21 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> for NonStaticUserDataMethods<'l
         ));
     }
 
+    #[cfg(all(feature = "async", not(feature = "lua51")))]
+    fn add_async_meta_method<S, A, R, M, MR>(&mut self, _meta: S, _method: M)
+    where
+        T: Clone,
+        S: Into<MetaMethod>,
+        A: FromLuaMulti<'lua>,
+        R: ToLuaMulti<'lua>,
+        M: 'static + MaybeSend + Fn(&'lua Lua, T, A) -> MR,
+        MR: 'lua + Future<Output = Result<R>>,
+    {
+        // The panic should never happen as async non-static code wouldn't compile
+        // Non-static lifetime must be bounded to 'lua lifetime
+        mlua_panic!("asynchronous meta methods are not supported for non-static userdata")
+    }
+
     fn add_meta_function<S, A, R, F>(&mut self, meta: S, function: F)
     where
         S: Into<MetaMethod>,
@@ -722,6 +771,20 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> for NonStaticUserDataMethods<'l
             })),
         ));
     }
+
+    #[cfg(all(feature = "async", not(feature = "lua51")))]
+    fn add_async_meta_function<S, A, R, F, FR>(&mut self, _meta: S, _function: F)
+    where
+        S: Into<MetaMethod>,
+        A: FromLuaMulti<'lua>,
+        R: ToLuaMulti<'lua>,
+        F: 'static + MaybeSend + Fn(&'lua Lua, A) -> FR,
+        FR: 'lua + Future<Output = Result<R>>,
+    {
+        // The panic should never happen as async non-static code wouldn't compile
+        // Non-static lifetime must be bounded to 'lua lifetime
+        mlua_panic!("asynchronous meta functions are not supported for non-static userdata")
+    }
 }
 
 struct NonStaticUserDataFields<'lua, T: UserData> {

src/serde/de.rs

@@ -1,9 +1,9 @@
 use std::cell::RefCell;
-use std::collections::HashSet;
 use std::os::raw::c_void;
 use std::rc::Rc;
 use std::string::String as StdString;
 
+use rustc_hash::FxHashSet;
 use serde::de::{self, IntoDeserializer};
 
 use crate::error::{Error, Result};
@@ -16,7 +16,7 @@ use crate::value::Value;
 pub struct Deserializer<'lua> {
     value: Value<'lua>,
     options: Options,
-    visited: Rc<RefCell<HashSet<*const c_void>>>,
+    visited: Rc<RefCell<FxHashSet<*const c_void>>>,
 }
 
 /// A struct with options to change default deserializer behavior.
@@ -45,23 +45,23 @@ pub struct Options {
 
 impl Default for Options {
     fn default() -> Self {
-        Options {
-            deny_unsupported_types: true,
-            deny_recursive_tables: true,
-        }
+        Self::new()
     }
 }
 
 impl Options {
     /// Returns a new instance of `Options` with default parameters.
-    pub fn new() -> Self {
-        Self::default()
+    pub const fn new() -> Self {
+        Options {
+            deny_unsupported_types: true,
+            deny_recursive_tables: true,
+        }
     }
 
     /// Sets [`deny_unsupported_types`] option.
     ///
     /// [`deny_unsupported_types`]: #structfield.deny_unsupported_types
-    pub fn deny_unsupported_types(mut self, enabled: bool) -> Self {
+    pub const fn deny_unsupported_types(mut self, enabled: bool) -> Self {
         self.deny_unsupported_types = enabled;
         self
     }
@@ -86,14 +86,14 @@ impl<'lua> Deserializer<'lua> {
         Deserializer {
             value,
             options,
-            visited: Rc::new(RefCell::new(HashSet::new())),
+            visited: Rc::new(RefCell::new(FxHashSet::default())),
         }
     }
 
     fn from_parts(
         value: Value<'lua>,
         options: Options,
-        visited: Rc<RefCell<HashSet<*const c_void>>>,
+        visited: Rc<RefCell<FxHashSet<*const c_void>>>,
     ) -> Self {
         Deserializer {
             value,
@@ -313,7 +313,7 @@ impl<'lua, 'de> serde::Deserializer<'de> for Deserializer<'lua> {
 struct SeqDeserializer<'lua> {
     seq: TableSequence<'lua, Value<'lua>>,
     options: Options,
-    visited: Rc<RefCell<HashSet<*const c_void>>>,
+    visited: Rc<RefCell<FxHashSet<*const c_void>>>,
 }
 
 impl<'lua, 'de> de::SeqAccess<'de> for SeqDeserializer<'lua> {
@@ -351,7 +351,7 @@ struct MapDeserializer<'lua> {
     pairs: TablePairs<'lua, Value<'lua>, Value<'lua>>,
     value: Option<Value<'lua>>,
     options: Options,
-    visited: Rc<RefCell<HashSet<*const c_void>>>,
+    visited: Rc<RefCell<FxHashSet<*const c_void>>>,
     processed: usize,
 }
 
@@ -407,7 +407,7 @@ struct EnumDeserializer<'lua> {
     variant: StdString,
     value: Option<Value<'lua>>,
     options: Options,
-    visited: Rc<RefCell<HashSet<*const c_void>>>,
+    visited: Rc<RefCell<FxHashSet<*const c_void>>>,
 }
 
 impl<'lua, 'de> de::EnumAccess<'de> for EnumDeserializer<'lua> {
@@ -431,7 +431,7 @@ impl<'lua, 'de> de::EnumAccess<'de> for EnumDeserializer<'lua> {
 struct VariantDeserializer<'lua> {
     value: Option<Value<'lua>>,
     options: Options,
-    visited: Rc<RefCell<HashSet<*const c_void>>>,
+    visited: Rc<RefCell<FxHashSet<*const c_void>>>,
 }
 
 impl<'lua, 'de> de::VariantAccess<'de> for VariantDeserializer<'lua> {
@@ -499,12 +499,12 @@ impl<'lua, 'de> de::VariantAccess<'de> for VariantDeserializer<'lua> {
 // Used to track recursive tables but allow to traverse same tables multiple times
 struct RecursionGuard {
     ptr: *const c_void,
-    visited: Rc<RefCell<HashSet<*const c_void>>>,
+    visited: Rc<RefCell<FxHashSet<*const c_void>>>,
 }
 
 impl RecursionGuard {
     #[inline]
-    fn new(table: &Table, visited: &Rc<RefCell<HashSet<*const c_void>>>) -> Self {
+    fn new(table: &Table, visited: &Rc<RefCell<FxHashSet<*const c_void>>>) -> Self {
         let visited = Rc::clone(visited);
         let lua = table.0.lua;
         let ptr =
@@ -524,7 +524,7 @@ impl Drop for RecursionGuard {
 fn check_value_if_skip(
     value: &Value,
     options: Options,
-    visited: &RefCell<HashSet<*const c_void>>,
+    visited: &RefCell<FxHashSet<*const c_void>>,
 ) -> Result<bool> {
     match value {
         Value::Table(table) => {

src/serde/ser.rs

@@ -52,24 +52,24 @@ pub struct Options {
 
 impl Default for Options {
     fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Options {
+    /// Returns a new instance of [`Options`] with default parameters.
+    pub const fn new() -> Self {
         Options {
             set_array_metatable: true,
             serialize_none_to_null: true,
             serialize_unit_to_null: true,
         }
     }
-}
-
-impl Options {
-    /// Returns a new instance of [`Options`] with default parameters.
-    pub fn new() -> Self {
-        Self::default()
-    }
 
     /// Sets [`set_array_metatable`] option.
     ///
     /// [`set_array_metatable`]: #structfield.set_array_metatable
-    pub fn set_array_metatable(mut self, enabled: bool) -> Self {
+    pub const fn set_array_metatable(mut self, enabled: bool) -> Self {
         self.set_array_metatable = enabled;
         self
     }
@@ -77,7 +77,7 @@ impl Options {
     /// Sets [`serialize_none_to_null`] option.
     ///
     /// [`serialize_none_to_null`]: #structfield.serialize_none_to_null
-    pub fn serialize_none_to_null(mut self, enabled: bool) -> Self {
+    pub const fn serialize_none_to_null(mut self, enabled: bool) -> Self {
         self.serialize_none_to_null = enabled;
         self
     }
@@ -85,7 +85,7 @@ impl Options {
     /// Sets [`serialize_unit_to_null`] option.
     ///
     /// [`serialize_unit_to_null`]: #structfield.serialize_unit_to_null
-    pub fn serialize_unit_to_null(mut self, enabled: bool) -> Self {
+    pub const fn serialize_unit_to_null(mut self, enabled: bool) -> Self {
         self.serialize_unit_to_null = enabled;
         self
     }

src/stdlib.rs

@@ -6,32 +6,32 @@ use std::u32;
 pub struct StdLib(u32);
 
 impl StdLib {
-    /// [`coroutine`](https://www.lua.org/manual/5.3/manual.html#6.2) library
+    /// [`coroutine`](https://www.lua.org/manual/5.4/manual.html#6.2) library
     ///
     /// Requires `feature = "lua54/lua53/lua52"`
-    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
     pub const COROUTINE: StdLib = StdLib(1);
-    /// [`table`](https://www.lua.org/manual/5.3/manual.html#6.6) library
+    /// [`table`](https://www.lua.org/manual/5.4/manual.html#6.6) library
     pub const TABLE: StdLib = StdLib(1 << 1);
-    /// [`io`](https://www.lua.org/manual/5.3/manual.html#6.8) library
+    /// [`io`](https://www.lua.org/manual/5.4/manual.html#6.8) library
     pub const IO: StdLib = StdLib(1 << 2);
-    /// [`os`](https://www.lua.org/manual/5.3/manual.html#6.9) library
+    /// [`os`](https://www.lua.org/manual/5.4/manual.html#6.9) library
     pub const OS: StdLib = StdLib(1 << 3);
-    /// [`string`](https://www.lua.org/manual/5.3/manual.html#6.4) library
+    /// [`string`](https://www.lua.org/manual/5.4/manual.html#6.4) library
     pub const STRING: StdLib = StdLib(1 << 4);
-    /// [`utf8`](https://www.lua.org/manual/5.3/manual.html#6.5) library
+    /// [`utf8`](https://www.lua.org/manual/5.4/manual.html#6.5) library
     ///
     /// Requires `feature = "lua54/lua53"`
-    #[cfg(any(feature = "lua54", feature = "lua53", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53"))]
     pub const UTF8: StdLib = StdLib(1 << 5);
     /// [`bit`](https://www.lua.org/manual/5.2/manual.html#6.7) library
     ///
     /// Requires `feature = "lua52/luajit"`
     #[cfg(any(feature = "lua52", feature = "luajit", doc))]
     pub const BIT: StdLib = StdLib(1 << 6);
-    /// [`math`](https://www.lua.org/manual/5.3/manual.html#6.7) library
+    /// [`math`](https://www.lua.org/manual/5.4/manual.html#6.7) library
     pub const MATH: StdLib = StdLib(1 << 7);
-    /// [`package`](https://www.lua.org/manual/5.3/manual.html#6.3) library
+    /// [`package`](https://www.lua.org/manual/5.4/manual.html#6.3) library
     pub const PACKAGE: StdLib = StdLib(1 << 8);
     /// [`jit`](http://luajit.org/ext_jit.html) library
     ///
@@ -44,7 +44,7 @@ impl StdLib {
     /// Requires `feature = "luajit"`
     #[cfg(any(feature = "luajit", doc))]
     pub const FFI: StdLib = StdLib(1 << 30);
-    /// (**unsafe**) [`debug`](https://www.lua.org/manual/5.3/manual.html#6.10) library
+    /// (**unsafe**) [`debug`](https://www.lua.org/manual/5.4/manual.html#6.10) library
     pub const DEBUG: StdLib = StdLib(1 << 31);
 
     /// No libraries

src/table.rs

@@ -2,8 +2,9 @@ use std::marker::PhantomData;
 
 #[cfg(feature = "serialize")]
 use {
-    serde::ser::{Serialize, SerializeMap, SerializeSeq, Serializer},
-    std::result::Result as StdResult,
+    rustc_hash::FxHashSet,
+    serde::ser::{self, Serialize, SerializeMap, SerializeSeq, Serializer},
+    std::{cell::RefCell, os::raw::c_void, result::Result as StdResult},
 };
 
 use crate::error::{Error, Result};
@@ -379,7 +380,7 @@ impl<'lua> Table<'lua> {
     /// ```
     ///
     /// [`Result`]: crate::Result
-    /// [Lua manual]: http://www.lua.org/manual/5.3/manual.html#pdf-next
+    /// [Lua manual]: http://www.lua.org/manual/5.4/manual.html#pdf-next
     pub fn pairs<K: FromLua<'lua>, V: FromLua<'lua>>(self) -> TablePairs<'lua, K, V> {
         TablePairs {
             table: self.0,
@@ -428,7 +429,7 @@ impl<'lua> Table<'lua> {
     ///
     /// [`pairs`]: #method.pairs
     /// [`Result`]: crate::Result
-    /// [Lua manual]: http://www.lua.org/manual/5.3/manual.html#pdf-next
+    /// [Lua manual]: http://www.lua.org/manual/5.4/manual.html#pdf-next
     pub fn sequence_values<V: FromLua<'lua>>(self) -> TableSequence<'lua, V> {
         TableSequence {
             table: self.0,
@@ -501,6 +502,25 @@ impl<'lua> AsRef<Table<'lua>> for Table<'lua> {
 
 /// An extension trait for `Table`s that provides a variety of convenient functionality.
 pub trait TableExt<'lua> {
+    /// Calls the table as function assuming it has `__call` metamethod.
+    ///
+    /// The metamethod is called with the table as its first argument, followed by the passed arguments.
+    fn call<A, R>(&self, args: A) -> Result<R>
+    where
+        A: ToLuaMulti<'lua>,
+        R: FromLuaMulti<'lua>;
+
+    /// Asynchronously calls the table as function assuming it has `__call` metamethod.
+    ///
+    /// The metamethod is called with the table as its first argument, followed by the passed arguments.
+    #[cfg(feature = "async")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "async")))]
+    fn call_async<'fut, A, R>(&self, args: A) -> LocalBoxFuture<'fut, Result<R>>
+    where
+        'lua: 'fut,
+        A: ToLuaMulti<'lua>,
+        R: FromLuaMulti<'lua> + 'fut;
+
     /// Gets the function associated to `key` from the table and executes it,
     /// passing the table itself along with `args` as function arguments.
     ///
@@ -563,6 +583,25 @@ pub trait TableExt<'lua> {
 }
 
 impl<'lua> TableExt<'lua> for Table<'lua> {
+    fn call<A, R>(&self, args: A) -> Result<R>
+    where
+        A: ToLuaMulti<'lua>,
+        R: FromLuaMulti<'lua>,
+    {
+        // Convert table to a function and call via pcall that respects the `__call` metamethod.
+        Function(self.0.clone()).call(args)
+    }
+
+    #[cfg(feature = "async")]
+    fn call_async<'fut, A, R>(&self, args: A) -> LocalBoxFuture<'fut, Result<R>>
+    where
+        'lua: 'fut,
+        A: ToLuaMulti<'lua>,
+        R: FromLuaMulti<'lua> + 'fut,
+    {
+        Function(self.0.clone()).call_async(args)
+    }
+
     fn call_method<K, A, R>(&self, key: K, args: A) -> Result<R>
     where
         K: ToLua<'lua>,
@@ -622,22 +661,42 @@ impl<'lua> Serialize for Table<'lua> {
     where
         S: Serializer,
     {
-        let len = self.raw_len() as usize;
-        if len > 0 || self.is_array() {
-            let mut seq = serializer.serialize_seq(Some(len))?;
-            for v in self.clone().raw_sequence_values_by_len::<Value>(None) {
-                let v = v.map_err(serde::ser::Error::custom)?;
-                seq.serialize_element(&v)?;
-            }
-            return seq.end();
+        thread_local! {
+            static VISITED: RefCell<FxHashSet<*const c_void>> = RefCell::new(FxHashSet::default());
         }
 
-        let mut map = serializer.serialize_map(None)?;
-        for kv in self.clone().pairs::<Value, Value>() {
-            let (k, v) = kv.map_err(serde::ser::Error::custom)?;
-            map.serialize_entry(&k, &v)?;
-        }
-        map.end()
+        let lua = self.0.lua;
+        let ptr = unsafe { lua.ref_thread_exec(|refthr| ffi::lua_topointer(refthr, self.0.index)) };
+        let res = VISITED.with(|visited| {
+            {
+                let mut visited = visited.borrow_mut();
+                if visited.contains(&ptr) {
+                    return Err(ser::Error::custom("recursive table detected"));
+                }
+                visited.insert(ptr);
+            }
+
+            let len = self.raw_len() as usize;
+            if len > 0 || self.is_array() {
+                let mut seq = serializer.serialize_seq(Some(len))?;
+                for v in self.clone().raw_sequence_values_by_len::<Value>(None) {
+                    let v = v.map_err(serde::ser::Error::custom)?;
+                    seq.serialize_element(&v)?;
+                }
+                return seq.end();
+            }
+
+            let mut map = serializer.serialize_map(None)?;
+            for kv in self.clone().pairs::<Value, Value>() {
+                let (k, v) = kv.map_err(serde::ser::Error::custom)?;
+                map.serialize_entry(&k, &v)?;
+            }
+            map.end()
+        });
+        VISITED.with(|visited| {
+            visited.borrow_mut().remove(&ptr);
+        });
+        res
     }
 }
 

src/thread.rs

@@ -5,16 +5,16 @@ use crate::error::{Error, Result};
 use crate::ffi;
 use crate::types::LuaRef;
 use crate::util::{check_stack, error_traceback, pop_error, StackGuard};
-use crate::value::{FromLuaMulti, MultiValue, ToLuaMulti};
+use crate::value::{FromLuaMulti, ToLuaMulti};
 
-#[cfg(any(feature = "lua54", all(feature = "luajit", feature = "vendored"), doc))]
+#[cfg(any(feature = "lua54", all(feature = "luajit", feature = "vendored")))]
 use crate::function::Function;
 
 #[cfg(feature = "async")]
 use {
     crate::{
         lua::{Lua, ASYNC_POLL_PENDING},
-        value::Value,
+        value::{MultiValue, Value},
     },
     futures_core::{future::Future, stream::Stream},
     std::{
@@ -58,6 +58,7 @@ pub struct AsyncThread<'lua, R> {
     thread: Thread<'lua>,
     args0: RefCell<Option<Result<MultiValue<'lua>>>>,
     ret: PhantomData<R>,
+    recycle: bool,
 }
 
 impl<'lua> Thread<'lua> {
@@ -108,7 +109,7 @@ impl<'lua> Thread<'lua> {
         R: FromLuaMulti<'lua>,
     {
         let lua = self.0.lua;
-        let args = args.to_lua_multi(lua)?;
+        let mut args = args.to_lua_multi(lua)?;
         let nargs = args.len() as c_int;
         let results = unsafe {
             let _sg = StackGuard::new(lua.state);
@@ -123,7 +124,7 @@ impl<'lua> Thread<'lua> {
             }
 
             check_stack(thread_state, nargs)?;
-            for arg in args {
+            for arg in args.drain_all() {
                 lua.push_value(arg)?;
             }
             ffi::lua_xmove(lua.state, thread_state, nargs);
@@ -136,7 +137,7 @@ impl<'lua> Thread<'lua> {
                 return Err(pop_error(thread_state, ret));
             }
 
-            let mut results = MultiValue::new();
+            let mut results = args; // Reuse MultiValue container
             check_stack(lua.state, nresults + 2)?; // 2 is extra for `lua.pop_value()` below
             ffi::lua_xmove(thread_state, lua.state, nresults);
 
@@ -181,7 +182,7 @@ impl<'lua> Thread<'lua> {
     ///
     /// [Lua 5.4]: https://www.lua.org/manual/5.4/manual.html#lua_resetthread
     /// [LuaJIT]: https://github.com/openresty/luajit2#lua_resetthread
-    #[cfg(any(feature = "lua54", all(feature = "luajit", feature = "vendored"), doc))]
+    #[cfg(any(feature = "lua54", all(feature = "luajit", feature = "vendored")))]
     pub fn reset(&self, func: Function<'lua>) -> Result<()> {
         let lua = self.0.lua;
         unsafe {
@@ -260,6 +261,7 @@ impl<'lua> Thread<'lua> {
             thread: self,
             args0: RefCell::new(Some(args)),
             ret: PhantomData,
+            recycle: false,
         }
     }
 }
@@ -270,6 +272,24 @@ impl<'lua> PartialEq for Thread<'lua> {
     }
 }
 
+#[cfg(feature = "async")]
+impl<'lua, R> AsyncThread<'lua, R> {
+    #[inline]
+    pub(crate) fn set_recyclable(&mut self, recyclable: bool) {
+        self.recycle = recyclable;
+    }
+}
+
+#[cfg(feature = "async")]
+#[cfg(any(feature = "lua54", all(feature = "luajit", feature = "vendored")))]
+impl<'lua, R> Drop for AsyncThread<'lua, R> {
+    fn drop(&mut self) {
+        if self.recycle {
+            self.thread.0.lua.recycle_thread(&mut self.thread);
+        }
+    }
+}
+
 #[cfg(feature = "async")]
 impl<'lua, R> Stream for AsyncThread<'lua, R>
 where

src/types.rs

@@ -1,9 +1,11 @@
-use std::cell::RefCell;
 use std::hash::{Hash, Hasher};
 use std::os::raw::{c_int, c_void};
 use std::sync::{Arc, Mutex};
 use std::{fmt, mem, ptr};
 
+#[cfg(feature = "lua54")]
+use std::ffi::CStr;
+
 #[cfg(feature = "async")]
 use futures_core::future::LocalBoxFuture;
 
@@ -48,10 +50,16 @@ pub(crate) struct AsyncPollUpvalue<'lua> {
 }
 
 #[cfg(feature = "send")]
-pub(crate) type HookCallback = Arc<RefCell<dyn FnMut(&Lua, Debug) -> Result<()> + Send>>;
+pub(crate) type HookCallback = Arc<Mutex<dyn FnMut(&Lua, Debug) -> Result<()> + Send>>;
 
 #[cfg(not(feature = "send"))]
-pub(crate) type HookCallback = Arc<RefCell<dyn FnMut(&Lua, Debug) -> Result<()>>>;
+pub(crate) type HookCallback = Arc<Mutex<dyn FnMut(&Lua, Debug) -> Result<()>>>;
+
+#[cfg(all(feature = "send", feature = "lua54"))]
+pub(crate) type WarnCallback = Box<dyn Fn(&Lua, &CStr, bool) -> Result<()> + Send>;
+
+#[cfg(all(not(feature = "send"), feature = "lua54"))]
+pub(crate) type WarnCallback = Box<dyn Fn(&Lua, &CStr, bool) -> Result<()>>;
 
 #[cfg(feature = "send")]
 pub trait MaybeSend: Send {}
@@ -147,7 +155,9 @@ impl<'lua> Clone for LuaRef<'lua> {
 
 impl<'lua> Drop for LuaRef<'lua> {
     fn drop(&mut self) {
-        self.lua.drop_ref(self)
+        if self.index > 0 {
+            self.lua.drop_ref(self);
+        }
     }
 }
 

src/userdata.rs

@@ -3,6 +3,7 @@ use std::cell::{Ref, RefCell, RefMut};
 use std::fmt;
 use std::hash::{Hash, Hasher};
 use std::ops::{Deref, DerefMut};
+use std::os::raw::{c_char, c_int};
 use std::string::String as StdString;
 
 #[cfg(feature = "async")]
@@ -23,12 +24,12 @@ use crate::types::{Callback, LuaRef, MaybeSend};
 use crate::util::{check_stack, get_userdata, take_userdata, StackGuard};
 use crate::value::{FromLua, FromLuaMulti, ToLua, ToLuaMulti};
 
-#[cfg(any(feature = "lua52", feature = "lua51", feature = "luajit"))]
-use crate::value::Value;
-
 #[cfg(feature = "async")]
 use crate::types::AsyncCallback;
 
+#[cfg(feature = "lua54")]
+pub(crate) const USER_VALUE_MAXSLOT: usize = 8;
+
 /// Kinds of metamethods that can be overridden.
 ///
 /// Currently, this mechanism does not allow overriding the `__gc` metamethod, since there is
@@ -53,29 +54,29 @@ pub enum MetaMethod {
     Unm,
     /// The floor division (//) operator.
     /// Requires `feature = "lua54/lua53"`
-    #[cfg(any(feature = "lua54", feature = "lua53", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53"))]
     IDiv,
     /// The bitwise AND (&) operator.
     /// Requires `feature = "lua54/lua53"`
-    #[cfg(any(feature = "lua54", feature = "lua53", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53"))]
     BAnd,
     /// The bitwise OR (|) operator.
     /// Requires `feature = "lua54/lua53"`
-    #[cfg(any(feature = "lua54", feature = "lua53", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53"))]
     BOr,
     /// The bitwise XOR (binary ~) operator.
     /// Requires `feature = "lua54/lua53"`
-    #[cfg(any(feature = "lua54", feature = "lua53", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53"))]
     BXor,
     /// The bitwise NOT (unary ~) operator.
     /// Requires `feature = "lua54/lua53"`
-    #[cfg(any(feature = "lua54", feature = "lua53", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53"))]
     BNot,
     /// The bitwise left shift (<<) operator.
-    #[cfg(any(feature = "lua54", feature = "lua53", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53"))]
     Shl,
     /// The bitwise right shift (>>) operator.
-    #[cfg(any(feature = "lua54", feature = "lua53", doc))]
+    #[cfg(any(feature = "lua54", feature = "lua53"))]
     Shr,
     /// The string concatenation operator `..`.
     Concat,
@@ -102,7 +103,12 @@ pub enum MetaMethod {
     /// This is not an operator, but it will be called by the built-in `pairs` function.
     ///
     /// Requires `feature = "lua54/lua53/lua52"`
-    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52", doc))]
+    #[cfg(any(
+        feature = "lua54",
+        feature = "lua53",
+        feature = "lua52",
+        feature = "luajit52",
+    ))]
     Pairs,
     /// The `__ipairs` metamethod.
     ///
@@ -111,7 +117,7 @@ pub enum MetaMethod {
     /// Requires `feature = "lua52"`
     ///
     /// [`ipairs`]: https://www.lua.org/manual/5.2/manual.html#pdf-ipairs
-    #[cfg(any(feature = "lua52", doc))]
+    #[cfg(any(feature = "lua52", feature = "luajit52", doc))]
     IPairs,
     /// The `__close` metamethod.
     ///
@@ -123,7 +129,7 @@ pub enum MetaMethod {
     /// Requires `feature = "lua54"`
     ///
     /// [lua_doc]: https://www.lua.org/manual/5.4/manual.html#3.3.8
-    #[cfg(any(feature = "lua54", doc))]
+    #[cfg(any(feature = "lua54"))]
     Close,
     /// A custom metamethod.
     ///
@@ -188,9 +194,14 @@ impl MetaMethod {
             MetaMethod::Call => "__call",
             MetaMethod::ToString => "__tostring",
 
-            #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
+            #[cfg(any(
+                feature = "lua54",
+                feature = "lua53",
+                feature = "lua52",
+                feature = "luajit52"
+            ))]
             MetaMethod::Pairs => "__pairs",
-            #[cfg(feature = "lua52")]
+            #[cfg(any(feature = "lua52", feature = "luajit52"))]
             MetaMethod::IPairs => "__ipairs",
 
             #[cfg(feature = "lua54")]
@@ -250,9 +261,14 @@ impl From<StdString> for MetaMethod {
             "__call" => MetaMethod::Call,
             "__tostring" => MetaMethod::ToString,
 
-            #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
+            #[cfg(any(
+                feature = "lua54",
+                feature = "lua53",
+                feature = "lua52",
+                feature = "luajit52"
+            ))]
             "__pairs" => MetaMethod::Pairs,
-            #[cfg(feature = "lua52")]
+            #[cfg(any(feature = "lua52", feature = "luajit52"))]
             "__ipairs" => MetaMethod::IPairs,
 
             #[cfg(feature = "lua54")]
@@ -395,6 +411,25 @@ pub trait UserDataMethods<'lua, T: UserData> {
         R: ToLuaMulti<'lua>,
         M: 'static + MaybeSend + FnMut(&'lua Lua, &mut T, A) -> Result<R>;
 
+    /// Add an async metamethod which accepts a `T` as the first parameter and returns Future.
+    /// The passed `T` is cloned from the original value.
+    ///
+    /// This is an async version of [`add_meta_method`].
+    ///
+    /// Requires `feature = "async"`
+    ///
+    /// [`add_meta_method`]: #method.add_meta_method
+    #[cfg(all(feature = "async", not(feature = "lua51")))]
+    #[cfg_attr(docsrs, doc(cfg(feature = "async")))]
+    fn add_async_meta_method<S, A, R, M, MR>(&mut self, name: S, method: M)
+    where
+        T: Clone,
+        S: Into<MetaMethod>,
+        A: FromLuaMulti<'lua>,
+        R: ToLuaMulti<'lua>,
+        M: 'static + MaybeSend + Fn(&'lua Lua, T, A) -> MR,
+        MR: 'lua + Future<Output = Result<R>>;
+
     /// Add a metamethod which accepts generic arguments.
     ///
     /// Metamethods for binary operators can be triggered if either the left or right argument to
@@ -419,6 +454,23 @@ pub trait UserDataMethods<'lua, T: UserData> {
         R: ToLuaMulti<'lua>,
         F: 'static + MaybeSend + FnMut(&'lua Lua, A) -> Result<R>;
 
+    /// Add a metamethod which accepts generic arguments and returns Future.
+    ///
+    /// This is an async version of [`add_meta_function`].
+    ///
+    /// Requires `feature = "async"`
+    ///
+    /// [`add_meta_function`]: #method.add_meta_function
+    #[cfg(all(feature = "async", not(feature = "lua51")))]
+    #[cfg_attr(docsrs, doc(cfg(feature = "async")))]
+    fn add_async_meta_function<S, A, R, F, FR>(&mut self, name: S, function: F)
+    where
+        S: Into<MetaMethod>,
+        A: FromLuaMulti<'lua>,
+        R: ToLuaMulti<'lua>,
+        F: 'static + MaybeSend + Fn(&'lua Lua, A) -> FR,
+        FR: 'lua + Future<Output = Result<R>>;
+
     //
     // Below are internal methods used in generated code
     //
@@ -432,6 +484,15 @@ pub trait UserDataMethods<'lua, T: UserData> {
 
     #[doc(hidden)]
     fn add_meta_callback(&mut self, _meta: MetaMethod, _callback: Callback<'lua, 'static>) {}
+
+    #[doc(hidden)]
+    #[cfg(feature = "async")]
+    fn add_async_meta_callback(
+        &mut self,
+        _meta: MetaMethod,
+        _callback: AsyncCallback<'lua, 'static>,
+    ) {
+    }
 }
 
 /// Field registry for [`UserData`] implementors.
@@ -758,12 +819,13 @@ impl<'lua> AnyUserData<'lua> {
 
     /// Takes out the value of `UserData` and sets the special "destructed" metatable that prevents
     /// any further operations with this userdata.
-    #[doc(hidden)]
+    ///
+    /// All associated user values will be also cleared.
     pub fn take<T: 'static + UserData>(&self) -> Result<T> {
         let lua = self.0.lua;
         unsafe {
             let _sg = StackGuard::new(lua.state);
-            check_stack(lua.state, 2)?;
+            check_stack(lua.state, 3)?;
 
             let type_id = lua.push_userdata_ref(&self.0)?;
             match type_id {
@@ -771,12 +833,22 @@ impl<'lua> AnyUserData<'lua> {
                     // Try to borrow userdata exclusively
                     let _ = (*get_userdata::<UserDataCell<T>>(lua.state, -1)).try_borrow_mut()?;
 
-                    // Clear uservalue
-                    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
-                    ffi::lua_pushnil(lua.state);
+                    // Clear associated user values
+                    #[cfg(feature = "lua54")]
+                    for i in 1..=USER_VALUE_MAXSLOT {
+                        ffi::lua_pushnil(lua.state);
+                        ffi::lua_setiuservalue(lua.state, -2, i as c_int);
+                    }
+                    #[cfg(any(feature = "lua53", feature = "lua52"))]
+                    {
+                        ffi::lua_pushnil(lua.state);
+                        ffi::lua_setuservalue(lua.state, -2);
+                    }
                     #[cfg(any(feature = "lua51", feature = "luajit"))]
-                    protect_lua!(lua.state, 0, 1, fn(state) ffi::lua_newtable(state))?;
-                    ffi::lua_setuservalue(lua.state, -2);
+                    protect_lua!(lua.state, 1, 1, fn(state) {
+                        ffi::lua_newtable(state);
+                        ffi::lua_setuservalue(state, -2);
+                    })?;
 
                     Ok(take_userdata::<UserDataCell<T>>(lua.state).into_inner())
                 }
@@ -788,54 +860,196 @@ impl<'lua> AnyUserData<'lua> {
     /// Sets an associated value to this `AnyUserData`.
     ///
     /// The value may be any Lua value whatsoever, and can be retrieved with [`get_user_value`].
-    /// As Lua < 5.3 allows to store only tables, the value will be stored in a table at index 1.
+    ///
+    /// This is the same as calling [`set_nth_user_value`] with `n` set to 1.
     ///
     /// [`get_user_value`]: #method.get_user_value
+    /// [`set_nth_user_value`]: #method.set_nth_user_value
+    #[inline]
     pub fn set_user_value<V: ToLua<'lua>>(&self, v: V) -> Result<()> {
+        self.set_nth_user_value(1, v)
+    }
+
+    /// Returns an associated value set by [`set_user_value`].
+    ///
+    /// This is the same as calling [`get_nth_user_value`] with `n` set to 1.
+    ///
+    /// [`set_user_value`]: #method.set_user_value
+    /// [`get_nth_user_value`]: #method.get_nth_user_value
+    #[inline]
+    pub fn get_user_value<V: FromLua<'lua>>(&self) -> Result<V> {
+        self.get_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 [`get_nth_user_value`].
+    /// `n` starts from 1 and can be up to 65535.
+    ///
+    /// This is supported for all Lua versions.
+    /// In Lua 5.4 first 7 elements are stored in a most efficient way.
+    /// For other Lua versions this functionality is provided using a wrapping table.
+    ///
+    /// [`get_nth_user_value`]: #method.get_nth_user_value
+    pub fn set_nth_user_value<V: ToLua<'lua>>(&self, n: usize, v: V) -> Result<()> {
+        if n < 1 || n > u16::MAX as usize {
+            return Err(Error::RuntimeError(
+                "user value index out of bounds".to_string(),
+            ));
+        }
+
         let lua = self.0.lua;
-        #[cfg(any(feature = "lua52", feature = "lua51", feature = "luajit"))]
-        let v = {
-            // Lua <= 5.2 allows to store only a table. Then we will wrap the value.
-            let t = lua.create_table_with_capacity(1, 0)?;
-            t.raw_set(1, v)?;
-            Value::Table(t)
-        };
-        #[cfg(any(feature = "lua54", feature = "lua53"))]
-        let v = v.to_lua(lua)?;
         unsafe {
             let _sg = StackGuard::new(lua.state);
-            check_stack(lua.state, 3)?;
+            check_stack(lua.state, 5)?;
 
             lua.push_userdata_ref(&self.0)?;
-            lua.push_value(v)?;
-            ffi::lua_setuservalue(lua.state, -2);
+            lua.push_value(v.to_lua(lua)?)?;
+
+            #[cfg(feature = "lua54")]
+            if n < USER_VALUE_MAXSLOT {
+                ffi::lua_setiuservalue(lua.state, -2, n as c_int);
+                return Ok(());
+            }
+
+            // Multiple (extra) user values are emulated by storing them in a table
+            protect_lua!(lua.state, 2, 0, |state| {
+                if getuservalue_table(lua.state, -2) != ffi::LUA_TTABLE {
+                    // Create a new table to use as uservalue
+                    ffi::lua_pop(lua.state, 1);
+                    ffi::lua_newtable(state);
+                    ffi::lua_pushvalue(state, -1);
+
+                    #[cfg(feature = "lua54")]
+                    ffi::lua_setiuservalue(lua.state, -4, USER_VALUE_MAXSLOT as c_int);
+                    #[cfg(not(feature = "lua54"))]
+                    ffi::lua_setuservalue(lua.state, -4);
+                }
+                ffi::lua_pushvalue(state, -2);
+                #[cfg(feature = "lua54")]
+                ffi::lua_rawseti(state, -2, (n - USER_VALUE_MAXSLOT + 1) as ffi::lua_Integer);
+                #[cfg(not(feature = "lua54"))]
+                ffi::lua_rawseti(state, -2, n as ffi::lua_Integer);
+            })?;
 
             Ok(())
         }
     }
 
-    /// Returns an associated value set by [`set_user_value`].
+    /// Returns an associated `n`th value set by [`set_nth_user_value`].
     ///
-    /// For Lua < 5.3 the value will be automatically extracted from the table wrapper from index 1.
+    /// `n` starts from 1 and can be up to 65535.
     ///
-    /// [`set_user_value`]: #method.set_user_value
-    pub fn get_user_value<V: FromLua<'lua>>(&self) -> Result<V> {
+    /// This is supported for all Lua versions.
+    /// In Lua 5.4 first 7 elements are stored in a most efficient way.
+    /// For other Lua versions this functionality is provided using a wrapping table.
+    ///
+    /// [`set_nth_user_value`]: #method.set_nth_user_value
+    pub fn get_nth_user_value<V: FromLua<'lua>>(&self, n: usize) -> Result<V> {
+        if n < 1 || n > u16::MAX as usize {
+            return Err(Error::RuntimeError(
+                "user value index out of bounds".to_string(),
+            ));
+        }
+
         let lua = self.0.lua;
-        let res = unsafe {
+        unsafe {
             let _sg = StackGuard::new(lua.state);
-            check_stack(lua.state, 3)?;
+            check_stack(lua.state, 4)?;
 
             lua.push_userdata_ref(&self.0)?;
-            ffi::lua_getuservalue(lua.state, -1);
-            lua.pop_value()
-        };
-        #[cfg(any(feature = "lua52", feature = "lua51", feature = "luajit"))]
-        return match <Option<Table>>::from_lua(res, lua)? {
-            Some(t) => t.get(1),
-            None => V::from_lua(Value::Nil, lua),
-        };
-        #[cfg(any(feature = "lua54", feature = "lua53"))]
-        V::from_lua(res, lua)
+
+            #[cfg(feature = "lua54")]
+            if n < USER_VALUE_MAXSLOT {
+                ffi::lua_getiuservalue(lua.state, -1, n as c_int);
+                return V::from_lua(lua.pop_value(), lua);
+            }
+
+            // Multiple (extra) user values are emulated by storing them in a table
+            protect_lua!(lua.state, 1, 1, |state| {
+                if getuservalue_table(lua.state, -1) != ffi::LUA_TTABLE {
+                    ffi::lua_pushnil(lua.state);
+                    return;
+                }
+                #[cfg(feature = "lua54")]
+                ffi::lua_rawgeti(state, -1, (n - USER_VALUE_MAXSLOT + 1) as ffi::lua_Integer);
+                #[cfg(not(feature = "lua54"))]
+                ffi::lua_rawgeti(state, -1, n as ffi::lua_Integer);
+            })?;
+
+            V::from_lua(lua.pop_value(), lua)
+        }
+    }
+
+    /// Sets an associated value to this `AnyUserData` by name.
+    ///
+    /// The value can be retrieved with [`get_named_user_value`].
+    ///
+    /// [`get_named_user_value`]: #method.get_named_user_value
+    pub fn set_named_user_value<S, V>(&self, name: &S, v: V) -> Result<()>
+    where
+        S: AsRef<[u8]> + ?Sized,
+        V: ToLua<'lua>,
+    {
+        let lua = self.0.lua;
+        unsafe {
+            let _sg = StackGuard::new(lua.state);
+            check_stack(lua.state, 5)?;
+
+            lua.push_userdata_ref(&self.0)?;
+            lua.push_value(v.to_lua(lua)?)?;
+
+            // Multiple (extra) user values are emulated by storing them in a table
+            let name = name.as_ref();
+            protect_lua!(lua.state, 2, 0, |state| {
+                if getuservalue_table(lua.state, -2) != ffi::LUA_TTABLE {
+                    // Create a new table to use as uservalue
+                    ffi::lua_pop(lua.state, 1);
+                    ffi::lua_newtable(state);
+                    ffi::lua_pushvalue(state, -1);
+
+                    #[cfg(feature = "lua54")]
+                    ffi::lua_setiuservalue(lua.state, -4, USER_VALUE_MAXSLOT as c_int);
+                    #[cfg(not(feature = "lua54"))]
+                    ffi::lua_setuservalue(lua.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`]: #method.set_named_user_value
+    pub fn get_named_user_value<S, V>(&self, name: &S) -> Result<V>
+    where
+        S: AsRef<[u8]> + ?Sized,
+        V: FromLua<'lua>,
+    {
+        let lua = self.0.lua;
+        unsafe {
+            let _sg = StackGuard::new(lua.state);
+            check_stack(lua.state, 4)?;
+
+            lua.push_userdata_ref(&self.0)?;
+
+            // Multiple (extra) user values are emulated by storing them in a table
+            let name = name.as_ref();
+            protect_lua!(lua.state, 1, 1, |state| {
+                if getuservalue_table(lua.state, -1) != ffi::LUA_TTABLE {
+                    ffi::lua_pushnil(lua.state);
+                    return;
+                }
+                ffi::lua_pushlstring(state, name.as_ptr() as *const c_char, name.len());
+                ffi::lua_rawget(state, -2);
+            })?;
+
+            V::from_lua(lua.pop_value(), lua)
+        }
     }
 
     /// Returns a metatable of this `UserData`.
@@ -917,6 +1131,13 @@ impl<'lua> AsRef<AnyUserData<'lua>> for AnyUserData<'lua> {
     }
 }
 
+unsafe fn getuservalue_table(state: *mut ffi::lua_State, idx: c_int) -> c_int {
+    #[cfg(feature = "lua54")]
+    return ffi::lua_getiuservalue(state, idx, USER_VALUE_MAXSLOT as c_int);
+    #[cfg(not(feature = "lua54"))]
+    return ffi::lua_getuservalue(state, idx);
+}
+
 /// Handle to a `UserData` metatable.
 #[derive(Clone, Debug)]
 pub struct UserDataMetatable<'lua>(pub(crate) Table<'lua>);

src/util.rs

@@ -1,6 +1,4 @@
 use std::any::{Any, TypeId};
-use std::collections::HashMap;
-use std::error::Error as StdError;
 use std::fmt::Write;
 use std::os::raw::{c_char, c_int, c_void};
 use std::panic::{catch_unwind, resume_unwind, AssertUnwindSafe};
@@ -8,12 +6,13 @@ use std::sync::Arc;
 use std::{mem, ptr, slice};
 
 use once_cell::sync::Lazy;
+use rustc_hash::FxHashMap;
 
 use crate::error::{Error, Result};
 use crate::ffi;
 
-static METATABLE_CACHE: Lazy<HashMap<TypeId, u8>> = Lazy::new(|| {
-    let mut map = HashMap::with_capacity(32);
+static METATABLE_CACHE: Lazy<FxHashMap<TypeId, u8>> = Lazy::new(|| {
+    let mut map = FxHashMap::with_capacity_and_hasher(32, Default::default());
     crate::lua::init_metatable_cache(&mut map);
     map.insert(TypeId::of::<WrappedFailure>(), 0);
     map.insert(TypeId::of::<String>(), 0);
@@ -288,6 +287,17 @@ pub unsafe fn push_userdata<T>(state: *mut ffi::lua_State, t: T) -> Result<()> {
     Ok(())
 }
 
+// Internally uses 3 stack spaces, does not call checkstack.
+#[cfg(feature = "lua54")]
+#[inline]
+pub unsafe fn push_userdata_uv<T>(state: *mut ffi::lua_State, t: T, nuvalue: c_int) -> Result<()> {
+    let ud = protect_lua!(state, 0, 1, |state| {
+        ffi::lua_newuserdatauv(state, mem::size_of::<T>(), nuvalue) as *mut T
+    })?;
+    ptr::write(ud, t);
+    Ok(())
+}
+
 #[inline]
 pub unsafe fn get_userdata<T>(state: *mut ffi::lua_State, index: c_int) -> *mut T {
     let ud = ffi::lua_touserdata(state, index) as *mut T;
@@ -535,11 +545,8 @@ where
             ffi::lua_settop(state, 1);
 
             let wrapped_error = ud as *mut WrappedFailure;
-            ptr::write(wrapped_error, WrappedFailure::Error(err));
-            get_gc_metatable::<WrappedFailure>(state);
-            ffi::lua_setmetatable(state, -2);
 
-            // Convert to CallbackError and attach traceback
+            // Build `CallbackError` with traceback
             let traceback = if ffi::lua_checkstack(state, ffi::LUA_TRACEBACK_STACK) != 0 {
                 ffi::luaL_traceback(state, state, ptr::null(), 0);
                 let traceback = to_string(state, -1);
@@ -548,10 +555,13 @@ where
             } else {
                 "<not enough stack space for traceback>".to_string()
             };
-            if let WrappedFailure::Error(ref mut err) = *wrapped_error {
-                let cause = Arc::new(err.clone());
-                *err = Error::CallbackError { traceback, cause };
-            }
+            let cause = Arc::new(err);
+            ptr::write(
+                wrapped_error,
+                WrappedFailure::Error(Error::CallbackError { traceback, cause }),
+            );
+            get_gc_metatable::<WrappedFailure>(state);
+            ffi::lua_setmetatable(state, -2);
 
             ffi::lua_error(state)
         }
@@ -736,32 +746,6 @@ pub unsafe fn init_error_registry(state: *mut ffi::lua_State) -> Result<()> {
                     // be possible to make this consume arbitrary amounts of memory (for example, some
                     // kind of recursive error structure?)
                     let _ = write!(&mut (*err_buf), "{}", error);
-                    // Find first two sources that caused the error
-                    let mut source1 = error.source();
-                    let mut source0 = source1.and_then(|s| s.source());
-                    while let Some(source) = source0.and_then(|s| s.source()) {
-                        source1 = source0;
-                        source0 = Some(source);
-                    }
-                    match (source1, source0) {
-                        (_, Some(error0))
-                            if error0.to_string().contains("\nstack traceback:\n") =>
-                        {
-                            let _ = write!(&mut (*err_buf), "\ncaused by: {}", error0);
-                        }
-                        (Some(error1), Some(error0)) => {
-                            let _ = write!(&mut (*err_buf), "\ncaused by: {}", error0);
-                            let s = error1.to_string();
-                            if let Some(traceback) = s.split_once("\nstack traceback:\n") {
-                                let _ =
-                                    write!(&mut (*err_buf), "\nstack traceback:\n{}", traceback.1);
-                            }
-                        }
-                        (Some(error1), None) => {
-                            let _ = write!(&mut (*err_buf), "\ncaused by: {}", error1);
-                        }
-                        _ => {}
-                    }
                     Ok(err_buf)
                 }
                 Some(WrappedFailure::Panic(Some(ref panic))) => {
@@ -805,7 +789,6 @@ pub unsafe fn init_error_registry(state: *mut ffi::lua_State) -> Result<()> {
     // Create destructed userdata metatable
 
     unsafe extern "C" fn destructed_error(state: *mut ffi::lua_State) -> c_int {
-        // TODO: Consider changing error to UserDataDestructed in v0.7
         callback_error(state, |_| Err(Error::CallbackDestructed))
     }
 
@@ -842,9 +825,14 @@ pub unsafe fn init_error_registry(state: *mut ffi::lua_State) -> Result<()> {
         "__newindex",
         "__call",
         "__tostring",
-        #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
+        #[cfg(any(
+            feature = "lua54",
+            feature = "lua53",
+            feature = "lua52",
+            feature = "luajit52"
+        ))]
         "__pairs",
-        #[cfg(any(feature = "lua53", feature = "lua52"))]
+        #[cfg(any(feature = "lua53", feature = "lua52", feature = "luajit52"))]
         "__ipairs",
         #[cfg(feature = "lua54")]
         "__close",

src/value.rs

@@ -53,7 +53,7 @@ pub enum Value<'lua> {
 pub use self::Value::Nil;
 
 impl<'lua> Value<'lua> {
-    pub fn type_name(&self) -> &'static str {
+    pub const fn type_name(&self) -> &'static str {
         match *self {
             Value::Nil => "nil",
             Value::Boolean(_) => "boolean",
@@ -162,6 +162,12 @@ impl<'lua> MultiValue<'lua> {
     pub fn new() -> MultiValue<'lua> {
         MultiValue(Vec::new())
     }
+
+    /// Similar to `new` but can return previously used container with allocated capacity.
+    #[inline]
+    pub(crate) fn new_or_cached(lua: &'lua Lua) -> MultiValue<'lua> {
+        lua.new_or_cached_multivalue()
+    }
 }
 
 impl<'lua> Default for MultiValue<'lua> {
@@ -227,6 +233,11 @@ impl<'lua> MultiValue<'lua> {
         self.0.pop()
     }
 
+    #[inline]
+    pub fn clear(&mut self) {
+        self.0.clear();
+    }
+
     #[inline]
     pub fn len(&self) -> usize {
         self.0.len()
@@ -234,13 +245,31 @@ impl<'lua> MultiValue<'lua> {
 
     #[inline]
     pub fn is_empty(&self) -> bool {
-        self.0.len() == 0
+        self.0.is_empty()
     }
 
     #[inline]
     pub fn iter(&self) -> iter::Rev<slice::Iter<Value<'lua>>> {
         self.0.iter().rev()
     }
+
+    #[inline]
+    pub(crate) fn drain_all(&mut self) -> iter::Rev<vec::Drain<Value<'lua>>> {
+        self.0.drain(..).rev()
+    }
+
+    #[inline]
+    pub(crate) fn refill(
+        &mut self,
+        iter: impl IntoIterator<Item = Result<Value<'lua>>>,
+    ) -> Result<()> {
+        self.0.clear();
+        for value in iter {
+            self.0.push(value?);
+        }
+        self.0.reverse();
+        Ok(())
+    }
 }
 
 /// Trait for types convertible to any number of Lua values.

tests/async.rs

@@ -3,7 +3,7 @@
 use std::cell::Cell;
 use std::rc::Rc;
 use std::sync::{
-    atomic::{AtomicI64, Ordering},
+    atomic::{AtomicI64, AtomicU64, Ordering},
     Arc,
 };
 use std::time::Duration;
@@ -12,7 +12,8 @@ use futures_timer::Delay;
 use futures_util::stream::TryStreamExt;
 
 use mlua::{
-    Error, Function, Lua, Result, Table, TableExt, Thread, UserData, UserDataMethods, Value,
+    Error, Function, Lua, LuaOptions, MetaMethod, Result, StdLib, Table, TableExt, Thread,
+    UserData, UserDataMethods, Value,
 };
 
 #[tokio::test]
@@ -227,7 +228,8 @@ async fn test_async_thread() -> Result<()> {
 
 #[tokio::test]
 async fn test_async_table() -> Result<()> {
-    let lua = Lua::new();
+    let options = LuaOptions::new().thread_cache_size(4);
+    let lua = Lua::new_with(StdLib::ALL_SAFE, options)?;
 
     let table = lua.create_table()?;
     table.set("val", 10)?;
@@ -276,7 +278,7 @@ async fn test_async_table() -> Result<()> {
 #[tokio::test]
 async fn test_async_userdata() -> Result<()> {
     #[derive(Clone)]
-    struct MyUserData(Arc<AtomicI64>);
+    struct MyUserData(Arc<AtomicU64>);
 
     impl UserData for MyUserData {
         fn add_methods<'lua, M: UserDataMethods<'lua, Self>>(methods: &mut M) {
@@ -295,13 +297,20 @@ async fn test_async_userdata() -> Result<()> {
                 Delay::new(Duration::from_millis(n)).await;
                 Ok(format!("elapsed:{}ms", n))
             });
+
+            #[cfg(not(feature = "lua51"))]
+            methods.add_async_meta_method(MetaMethod::Call, |_, data, ()| async move {
+                let n = data.0.load(Ordering::Relaxed);
+                Delay::new(Duration::from_millis(n)).await;
+                Ok(format!("elapsed:{}ms", n))
+            });
         }
     }
 
     let lua = Lua::new();
     let globals = lua.globals();
 
-    let userdata = lua.create_userdata(MyUserData(Arc::new(AtomicI64::new(11))))?;
+    let userdata = lua.create_userdata(MyUserData(Arc::new(AtomicU64::new(11))))?;
     globals.set("userdata", userdata.clone())?;
 
     lua.load(
@@ -315,6 +324,16 @@ async fn test_async_userdata() -> Result<()> {
     .exec_async()
     .await?;
 
+    #[cfg(not(feature = "lua51"))]
+    lua.load(
+        r#"
+        userdata:set_value(15)
+        assert(userdata() == "elapsed:15ms")
+    "#,
+    )
+    .exec_async()
+    .await?;
+
     Ok(())
 }
 

tests/conversion.rs

@@ -3,7 +3,7 @@ use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use std::ffi::{CStr, CString};
 
 use maplit::{btreemap, btreeset, hashmap, hashset};
-use mlua::{Lua, Result};
+use mlua::{Error, Lua, Result};
 
 #[test]
 fn test_conv_vec() -> Result<()> {
@@ -123,3 +123,18 @@ fn test_conv_boxed_slice() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn test_conv_array() -> Result<()> {
+    let lua = Lua::new();
+
+    let v = [1, 2, 3];
+    lua.globals().set("v", v)?;
+    let v2: [i32; 3] = lua.globals().get("v")?;
+    assert_eq!(v, v2);
+
+    let v2 = lua.globals().get::<_, [i32; 4]>("v");
+    assert!(matches!(v2, Err(Error::FromLuaConversionError { .. })));
+
+    Ok(())
+}

tests/serde.rs

@@ -460,6 +460,11 @@ fn test_from_value_with_options() -> Result<(), Box<dyn std::error::Error>> {
         Err(err) => panic!("expected `DeserializeError` error, got {:?}", err),
     };
 
+    // Check recursion when using `Serialize` impl
+    let t = lua.create_table()?;
+    t.set("t", t.clone())?;
+    assert!(serde_json::to_string(&t).is_err());
+
     // Serialize Lua globals table
     #[derive(Debug, Deserialize)]
     struct Globals {

tests/table.rs

@@ -1,4 +1,4 @@
-use mlua::{Lua, Nil, Result, Table, TableExt, Value};
+use mlua::{Error, Lua, Nil, Result, Table, TableExt, Value};
 
 #[test]
 fn test_set_get() -> Result<()> {
@@ -139,14 +139,6 @@ fn test_table_sequence_from() -> Result<()> {
         vec![1, 2, 3]
     );
 
-    assert_eq!(
-        get_table
-            .call::<_, Table>(&[1, 2, 3])?
-            .sequence_values()
-            .collect::<Result<Vec<i64>>>()?,
-        vec![1, 2, 3]
-    );
-
     Ok(())
 }
 
@@ -274,7 +266,12 @@ fn test_table_call() -> Result<()> {
 
     lua.load(
         r#"
-        table = {a = 1}
+        table = {a = 1, b = 2}
+        setmetatable(table, {
+            __call = function(t, key)
+                return "call_"..t[key]
+            end
+        })
 
         function table.func(key)
             return "func_"..key
@@ -289,11 +286,19 @@ fn test_table_call() -> Result<()> {
 
     let table: Table = lua.globals().get("table")?;
 
+    assert_eq!(table.call::<_, String>("b")?, "call_2");
     assert_eq!(table.call_function::<_, _, String>("func", "a")?, "func_a");
     assert_eq!(
         table.call_method::<_, _, String>("method", "a")?,
         "method_1"
     );
 
+    // Test calling non-callable table
+    let table2 = lua.create_table()?;
+    assert!(matches!(
+        table2.call::<_, ()>(()),
+        Err(Error::RuntimeError(_))
+    ));
+
     Ok(())
 }

tests/tests.rs

@@ -847,6 +847,37 @@ fn test_mismatched_registry_key() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_application_data() -> Result<()> {
+    let lua = Lua::new();
+
+    lua.set_app_data("test1");
+    lua.set_app_data(vec!["test2"]);
+
+    let f = lua.create_function(|lua, ()| {
+        {
+            let data1 = lua.app_data_ref::<&str>().unwrap();
+            assert_eq!(*data1, "test1");
+        }
+        let mut data2 = lua.app_data_mut::<Vec<&str>>().unwrap();
+        assert_eq!(*data2, vec!["test2"]);
+        data2.push("test3");
+        Ok(())
+    })?;
+    f.call(())?;
+
+    assert_eq!(*lua.app_data_ref::<&str>().unwrap(), "test1");
+    assert_eq!(
+        *lua.app_data_ref::<Vec<&str>>().unwrap(),
+        vec!["test2", "test3"]
+    );
+
+    lua.remove_app_data::<Vec<&str>>();
+    assert!(matches!(lua.app_data_ref::<Vec<&str>>(), None));
+
+    Ok(())
+}
+
 #[test]
 fn test_recursion() -> Result<()> {
     let lua = Lua::new();
@@ -1046,17 +1077,22 @@ fn test_context_thread() -> Result<()> {
         )
         .into_function()?;
 
-    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
+    #[cfg(any(
+        feature = "lua54",
+        feature = "lua53",
+        feature = "lua52",
+        feature = "luajit52"
+    ))]
     f.call::<_, ()>(lua.current_thread())?;
 
-    #[cfg(any(feature = "lua51", feature = "luajit"))]
+    #[cfg(any(feature = "lua51", all(feature = "luajit", not(feature = "luajit52"))))]
     f.call::<_, ()>(Nil)?;
 
     Ok(())
 }
 
 #[test]
-#[cfg(any(feature = "lua51", feature = "luajit"))]
+#[cfg(any(feature = "lua51", all(feature = "luajit", not(feature = "luajit52"))))]
 fn test_context_thread_51() -> Result<()> {
     let lua = Lua::new();
 
@@ -1105,12 +1141,118 @@ fn test_load_from_function() -> Result<()> {
     assert_eq!(t.get::<_, String>("__name")?, "my_module");
     assert_eq!(i.load(Ordering::Relaxed), 1);
 
-    let _: Value = lua.load_from_function("my_module", func)?;
+    let _: Value = lua.load_from_function("my_module", func.clone())?;
     assert_eq!(i.load(Ordering::Relaxed), 1);
 
     let func_nil = lua.create_function(move |_, _: String| Ok(Value::Nil))?;
     let v: Value = lua.load_from_function("my_module2", func_nil)?;
     assert_eq!(v, Value::Boolean(true));
 
+    // Test unloading and loading again
+    lua.unload("my_module")?;
+    let _: Value = lua.load_from_function("my_module", func)?;
+    assert_eq!(i.load(Ordering::Relaxed), 2);
+
+    // Unloading nonexistent module must not fail
+    lua.unload("my_module2")?;
+
+    Ok(())
+}
+
+#[test]
+fn test_inspect_stack() -> Result<()> {
+    let lua = Lua::new();
+
+    // Not inside any function
+    assert!(lua.inspect_stack(0).is_none());
+
+    let logline = lua.create_function(|lua, msg: StdString| {
+        let debug = lua.inspect_stack(1).unwrap(); // caller
+        let source = debug.source().short_src.map(core::str::from_utf8);
+        let source = source.transpose().unwrap().unwrap_or("?");
+        let line = debug.curr_line();
+        Ok(format!("{}:{} {}", source, line, msg))
+    })?;
+    lua.globals().set("logline", logline)?;
+
+    lua.load(
+        r#"
+        local function foo()
+            local line = logline("hello")
+            return line
+        end
+        local function bar()
+            return foo()
+        end
+
+        assert(foo() == '[string "chunk"]:3 hello')
+        assert(bar() == '[string "chunk"]:3 hello')
+        assert(logline("world") == '[string "chunk"]:12 world')
+    "#,
+    )
+    .set_name("chunk")?
+    .exec()?;
+
+    Ok(())
+}
+
+#[test]
+fn test_multi_states() -> Result<()> {
+    let lua = Lua::new();
+
+    let f = lua.create_function(|_, g: Option<Function>| {
+        if let Some(g) = g {
+            g.call(())?;
+        }
+        Ok(())
+    })?;
+    lua.globals().set("f", f)?;
+
+    lua.load("f(function() coroutine.wrap(function() f() end)() end)")
+        .exec()?;
+
+    Ok(())
+}
+
+#[test]
+#[cfg(feature = "lua54")]
+fn test_warnings() -> Result<()> {
+    let lua = Lua::new();
+    lua.set_app_data::<Vec<(StdString, bool)>>(Vec::new());
+
+    lua.set_warning_function(|lua, msg, tocont| {
+        let msg = msg.to_string_lossy().to_string();
+        lua.app_data_mut::<Vec<(StdString, bool)>>()
+            .unwrap()
+            .push((msg, tocont));
+        Ok(())
+    });
+
+    lua.warning("native warning ...", true)?;
+    lua.warning("finish", false)?;
+    lua.load(r#"warn("lua warning", "continue")"#).exec()?;
+
+    lua.remove_warning_function();
+    lua.warning("one more warning", false)?;
+
+    let messages = lua.app_data_ref::<Vec<(StdString, bool)>>().unwrap();
+    assert_eq!(
+        *messages,
+        vec![
+            ("native warning ...".to_string(), true),
+            ("finish".to_string(), false),
+            ("lua warning".to_string(), true),
+            ("continue".to_string(), false),
+        ]
+    );
+
+    // Trigger error inside warning
+    lua.set_warning_function(|_, _, _| Err(Error::RuntimeError("warning error".to_string())));
+    assert!(matches!(
+        lua.load(r#"warn("test")"#).exec(),
+        Err(Error::CallbackError { cause, .. })
+            if matches!(*cause, Error::RuntimeError(ref err) if err == "warning error")
+    ));
+
     Ok(())
 }

tests/userdata.rs

@@ -111,7 +111,12 @@ fn test_metamethods() -> Result<()> {
                     Err("no such custom index".to_lua_err())
                 }
             });
-            #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
+            #[cfg(any(
+                feature = "lua54",
+                feature = "lua53",
+                feature = "lua52",
+                feature = "luajit52"
+            ))]
             methods.add_meta_method(MetaMethod::Pairs, |lua, data, ()| {
                 use std::iter::FromIterator;
                 let stateless_iter = lua.create_function(|_, (data, i): (MyUserData, i64)| {
@@ -136,11 +141,16 @@ fn test_metamethods() -> Result<()> {
         10
     );
 
-    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
-    let pairs_it = {
-        lua.load(
+    #[cfg(any(
+        feature = "lua54",
+        feature = "lua53",
+        feature = "lua52",
+        feature = "luajit52"
+    ))]
+    let pairs_it = lua
+        .load(
             r#"
-            function pairs_it()
+            function()
                 local r = 0
                 for i, v in pairs(userdata1) do
                     r = r + v
@@ -149,17 +159,21 @@ fn test_metamethods() -> Result<()> {
             end
         "#,
         )
-        .exec()?;
-        globals.get::<_, Function>("pairs_it")?
-    };
+        .eval::<Function>()?;
 
     assert_eq!(lua.load("userdata1 - userdata2").eval::<MyUserData>()?.0, 4);
     assert_eq!(lua.load("userdata1:get()").eval::<i64>()?, 7);
     assert_eq!(lua.load("userdata2.inner").eval::<i64>()?, 3);
-    #[cfg(any(feature = "lua54", feature = "lua53", feature = "lua52"))]
-    assert_eq!(pairs_it.call::<_, i64>(())?, 28);
     assert!(lua.load("userdata2.nonexist_field").eval::<()>().is_err());
 
+    #[cfg(any(
+        feature = "lua54",
+        feature = "lua53",
+        feature = "lua52",
+        feature = "luajit52"
+    ))]
+    assert_eq!(pairs_it.call::<_, i64>(())?, 28);
+
     let userdata2: Value = globals.get("userdata2")?;
     let userdata3: Value = globals.get("userdata3")?;
 
@@ -285,7 +299,7 @@ fn test_userdata_take() -> Result<()> {
 
     fn check_userdata_take(lua: &Lua, userdata: AnyUserData, rc: Arc<i64>) -> Result<()> {
         lua.globals().set("userdata", userdata.clone())?;
-        assert_eq!(Arc::strong_count(&rc), 2);
+        assert_eq!(Arc::strong_count(&rc), 3);
         let userdata_copy = userdata.clone();
         {
             let _value = userdata.borrow::<MyUserdata>()?;
@@ -299,6 +313,7 @@ fn test_userdata_take() -> Result<()> {
         let value = userdata_copy.take::<MyUserdata>()?;
         assert_eq!(*value.0, 18);
         drop(value);
+        lua.gc_collect()?;
         assert_eq!(Arc::strong_count(&rc), 1);
 
         match userdata.borrow::<MyUserdata>() {
@@ -319,6 +334,7 @@ fn test_userdata_take() -> Result<()> {
 
     let rc = Arc::new(18);
     let userdata = lua.create_userdata(MyUserdata(rc.clone()))?;
+    userdata.set_nth_user_value(2, MyUserdata(rc.clone()))?;
     check_userdata_take(&lua, userdata, rc)?;
 
     // Additionally check serializable userdata
@@ -326,6 +342,7 @@ fn test_userdata_take() -> Result<()> {
     {
         let rc = Arc::new(18);
         let userdata = lua.create_ser_userdata(MyUserdata(rc.clone()))?;
+        userdata.set_nth_user_value(2, MyUserdata(rc.clone()))?;
         check_userdata_take(&lua, userdata, rc)?;
     }
 
@@ -355,16 +372,32 @@ fn test_destroy_userdata() -> Result<()> {
 }
 
 #[test]
-fn test_user_value() -> Result<()> {
+fn test_user_values() -> Result<()> {
     struct MyUserData;
 
     impl UserData for MyUserData {}
 
     let lua = Lua::new();
     let ud = lua.create_userdata(MyUserData)?;
-    ud.set_user_value("hello")?;
-    assert_eq!(ud.get_user_value::<String>()?, "hello");
-    assert!(ud.get_user_value::<u32>().is_err());
+
+    ud.set_nth_user_value(1, "hello")?;
+    ud.set_nth_user_value(2, "world")?;
+    ud.set_nth_user_value(65535, 321)?;
+    assert_eq!(ud.get_nth_user_value::<String>(1)?, "hello");
+    assert_eq!(ud.get_nth_user_value::<String>(2)?, "world");
+    assert_eq!(ud.get_nth_user_value::<Value>(3)?, Value::Nil);
+    assert_eq!(ud.get_nth_user_value::<i32>(65535)?, 321);
+
+    assert!(ud.get_nth_user_value::<Value>(0).is_err());
+    assert!(ud.get_nth_user_value::<Value>(65536).is_err());
+
+    // Named user values
+    ud.set_named_user_value("name", "alex")?;
+    ud.set_named_user_value("age", 10)?;
+
+    assert_eq!(ud.get_named_user_value::<_, String>("name")?, "alex");
+    assert_eq!(ud.get_named_user_value::<_, i32>("age")?, 10);
+    assert_eq!(ud.get_named_user_value::<_, Value>("nonexist")?, Value::Nil);
 
     Ok(())
 }