v0.9.1

This causes Lua API correctness check assertion in debug mode.
Fixes #311.

CHANGELOG.md

@@ -1,3 +1,27 @@
+## v0.9.1
+
+- impl Default for Lua
+- impl IntoLuaMulti for `std::result::Result<(), E>`
+- Fix using wrong userdata index after processing Variadic args (#311)
+
+## v0.9.0
+
+Changes since v0.9.0-rc.3
+
+- Improved non-static (scoped) userdata support
+- Added `Scope::create_any_userdata()` method
+- Added `Lua::set_vector_metatable()` method (`unstable` feature flag)
+- Added `OwnedThread` type (`unstable` feature flag)
+- Minimal Luau updated to 0.590
+- Added new option `sort_keys` to `DeserializeOptions` (`Lua::from_value()` method)
+- Changed `Table::raw_len()` output type to `usize`
+- Helper functions for `Value` (eg: `Value::as_number()`/`Value::as_string`/etc)
+- Performance improvements
+
+## v0.9.0-rc.3
+
+- Minimal Luau updated to 0.588
+
 ## v0.9.0-rc.2
 
 - Added `#[derive(FromLua)]` macro to opt-in into `FromLua<T> where T: 'static + Clone` (userdata type).
@@ -81,6 +105,18 @@ Other:
 - Support setting module name in `#[lua_module(name = "...")]` macro
 - Minor fixes and improvements
 
+## v0.8.10
+
+- Update to Luau 0.590 (luau0-src to 0.7.x)
+- Fix loading luau code starting with \t
+- Pin lua-src and luajit-src versions
+
+## v0.8.9
+
+- Update minimal (vendored) Lua 5.4 to 5.4.6
+- Use `lua_closethread` instead of `lua_resetthread` in vendored mode (Lua 5.4.6)
+- Allow deserializing Lua null into unit (`()`) or unit struct.
+
 ## v0.8.8
 
 - Fix potential deadlock when trying to reuse dropped registry keys.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mlua"
-version = "0.9.0-rc.2" # remember to update mlua_derive
+version = "0.9.1" # remember to update mlua_derive
 authors = ["Aleksandr Orlenko <zxteam@pm.me>", "kyren <catherine@chucklefish.org>"]
 rust-version = "1.71"
 edition = "2021"
@@ -44,7 +44,7 @@ macros = ["mlua_derive/macros"]
 unstable = []
 
 [dependencies]
-mlua_derive = { version = "=0.9.0-rc.2", optional = true, path = "mlua_derive" }
+mlua_derive = { version = "=0.9.0", optional = true, path = "mlua_derive" }
 bstr = { version = "1.0", features = ["std"], default_features = false }
 once_cell = { version = "1.0" }
 num-traits = { version = "0.2.14" }
@@ -55,7 +55,7 @@ erased-serde = { version = "0.3", optional = true }
 serde-value = { version = "0.7", optional = true }
 parking_lot = { version = "0.12", optional = true }
 
-ffi = { package = "mlua-sys", version = "0.3.0", path = "mlua-sys" }
+ffi = { package = "mlua-sys", version = "0.3.2", path = "mlua-sys" }
 
 [dev-dependencies]
 rustyline = "12.0"

README.md

@@ -19,9 +19,7 @@
 
 > **Note**
 >
-> Please see the [v0.8](https://github.com/khvzak/mlua/tree/v0.8) branch for the stable versions of `mlua` released to crates.io.
->
-> v0.9 release notes can be found [here](docs/release_notes/v0.9.md).
+> See v0.9 [release notes](https://github.com/khvzak/mlua/blob/master/docs/release_notes/v0.9.md).
 
 `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.
@@ -47,7 +45,7 @@ Below is a list of the available feature flags. By default `mlua` does not enabl
 * `luajit`: activate [LuaJIT] support
 * `luajit52`: activate [LuaJIT] support with partial compatibility with Lua 5.2
 * `luau`: activate [Luau] support (auto vendored mode)
-* `luau-jit`: activate [Luau] support with experimental jit backend. This is unstable feature and not recommended to use.
+* `luau-jit`: activate [Luau] support with JIT backend.
 * `luau-vector4`: activate [Luau] support with 4-dimensional vector.
 * `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)
@@ -119,7 +117,7 @@ Add to `Cargo.toml` :
 
 ``` toml
 [dependencies]
-mlua = { version = "0.9.0-rc.2", features = ["lua54", "vendored"] }
+mlua = { version = "0.9.1", features = ["lua54", "vendored"] }
 ```
 
 `main.rs`
@@ -154,7 +152,7 @@ Add to `Cargo.toml` :
 crate-type = ["cdylib"]
 
 [dependencies]
-mlua = { version = "0.9.0-rc.2", features = ["lua54", "vendored", "module"] }
+mlua = { version = "0.9.1", features = ["lua54", "module"] }
 ```
 
 `lib.rs` :

benches/benchmark.rs

@@ -59,6 +59,31 @@ fn create_string_table(c: &mut Criterion) {
     });
 }
 
+fn table_get_set(c: &mut Criterion) {
+    let lua = Lua::new();
+
+    let table = lua.create_table().unwrap();
+
+    c.bench_function("table raw_get and raw_set [10]", |b| {
+        b.iter_batched(
+            || {
+                collect_gc_twice(&lua);
+                table.clear().unwrap();
+            },
+            |_| {
+                for (i, &s) in ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"]
+                    .iter()
+                    .enumerate()
+                {
+                    table.raw_set(s, i).unwrap();
+                    assert_eq!(table.raw_get::<_, usize>(s).unwrap(), i);
+                }
+            },
+            BatchSize::SmallInput,
+        );
+    });
+}
+
 fn create_function(c: &mut Criterion) {
     let lua = Lua::new();
 
@@ -305,6 +330,7 @@ criterion_group! {
         create_table,
         create_array,
         create_string_table,
+        table_get_set,
         create_function,
         call_lua_function,
         call_sum_callback,

docs/release_notes/v0.9.md

@@ -83,7 +83,7 @@ println!("{s}!");
 
 One of the common questions was how to embed a Lua type into Rust struct to use it later. It was non-trivial to do because of the `'lua` lifetime attached to every Lua value.
 
-In v0.9 mlua introduces "owned" types `OwnedTable`/`OwnedFunction`/`OwnedString`/`OwnedAnyUserData` that are `'static` (no lifetime attached).
+In v0.9 mlua introduces "owned" types `OwnedTable`/`OwnedFunction`/`OwnedString`/`OwnedAnyUserData`/ `OwnedThread`that are `'static` (no lifetime attached).
 
 ```rust
 let lua = Lua::new();
@@ -141,11 +141,10 @@ unsafe {
 
 #### Luau JIT support
 
-mlua brings support for the new experimental [Luau] JIT backend under the `luau-jit` feature flag. This backend is still under development and not yet ready for production use.
+mlua brings support for the new [Luau] JIT backend under the `luau-jit` feature flag.
 
-To enable it, just call `lua.enable_jit(true)` before loading Lua code. mlua will automatically trigger JIT compilation for new Lua chunks.
-
-When calling this function with `false` argument, mlua will disable JIT compilation but any previously compiled chunks will remain JIT-compiled.
+It will automatically trigger JIT compilation for new Lua chunks. To disable it, just call `lua.enable_jit(false)` before loading Lua code
+(but any previously compiled chunks will remain JIT-compiled).
 
 [Luau]: https://luau-lang.org
 
@@ -197,16 +196,14 @@ let read = lua.create_function(|lua, path: String| {
 
 lua.load(chunk! {
     local ok, err = pcall($read, "/nonexistent")
+    /// Prints:
+    /// Failed to open /nonexistent
+    /// No such file or directory (os error 2)
+    /// stack traceback:
+    /// ...
     print(err)
 })
 .exec()?;
-
-Prints:
-```text
-Failed to open /nonexistent
-No such file or directory (os error 2)
-stack traceback:
-...
 ```
 
 [`anyhow`]: https://crates.io/crates/anyhow

mlua-sys/Cargo.toml

@@ -1,7 +1,8 @@
 [package]
 name = "mlua-sys"
-version = "0.3.0"
+version = "0.3.2"
 authors = ["Aleksandr Orlenko <zxteam@pm.me>"]
+rust-version = "1.71"
 edition = "2021"
 repository = "https://github.com/khvzak/mlua"
 documentation = "https://docs.rs/mlua-sys"
@@ -38,5 +39,5 @@ cc = "1.0"
 cfg-if = "1.0"
 pkg-config = "0.3.17"
 lua-src = { version = ">= 546.0.0, < 546.1.0", optional = true }
-luajit-src = { version = ">= 210.4.6, < 210.6.0", optional = true }
-luau0-src = { version = "0.5.11", optional = true }
+luajit-src = { version = ">= 210.4.6, < 210.5.0", optional = true }
+luau0-src = { version = "0.7.0", optional = true }

mlua-sys/src/luau/luacode.rs

@@ -10,7 +10,8 @@ pub struct lua_CompileOptions {
     pub coverageLevel: c_int,
     pub vectorLib: *const c_char,
     pub vectorCtor: *const c_char,
-    pub mutableGlobals: *mut *const c_char,
+    pub vectorType: *const c_char,
+    pub mutableGlobals: *const *const c_char,
 }
 
 extern "C-unwind" {

mlua_derive/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mlua_derive"
-version = "0.9.0-rc.2"
+version = "0.9.0"
 authors = ["Aleksandr Orlenko <zxteam@pm.me>"]
 edition = "2021"
 description = "Procedural macros for the mlua crate."

mlua_derive/src/lib.rs

@@ -61,12 +61,8 @@ pub fn lua_module(attr: TokenStream, item: TokenStream) -> TokenStream {
         #[no_mangle]
         unsafe extern "C-unwind" fn #ext_entrypoint_name(state: *mut ::mlua::lua_State) -> ::std::os::raw::c_int {
             let lua = ::mlua::Lua::init_from_ptr(state);
-            if #skip_memory_check {
-                lua.skip_memory_check(true);
-            }
-            lua
-                .entrypoint1(#func_name)
-                .expect("cannot initialize module")
+            lua.skip_memory_check(#skip_memory_check);
+            lua.entrypoint1(#func_name)
         }
     };
 

src/chunk.rs

@@ -125,6 +125,7 @@ pub struct Compiler {
     coverage_level: u8,
     vector_lib: Option<String>,
     vector_ctor: Option<String>,
+    vector_type: Option<String>,
     mutable_globals: Vec<String>,
 }
 
@@ -146,6 +147,7 @@ impl Compiler {
             coverage_level: 0,
             vector_lib: None,
             vector_ctor: None,
+            vector_type: None,
             mutable_globals: Vec::new(),
         }
     }
@@ -188,15 +190,22 @@ impl Compiler {
 
     #[doc(hidden)]
     #[must_use]
-    pub fn set_vector_lib(mut self, lib: Option<String>) -> Self {
-        self.vector_lib = lib;
+    pub fn set_vector_lib(mut self, lib: impl Into<String>) -> Self {
+        self.vector_lib = Some(lib.into());
         self
     }
 
     #[doc(hidden)]
     #[must_use]
-    pub fn set_vector_ctor(mut self, ctor: Option<String>) -> Self {
-        self.vector_ctor = ctor;
+    pub fn set_vector_ctor(mut self, ctor: impl Into<String>) -> Self {
+        self.vector_ctor = Some(ctor.into());
+        self
+    }
+
+    #[doc(hidden)]
+    #[must_use]
+    pub fn set_vector_type(mut self, r#type: impl Into<String>) -> Self {
+        self.vector_type = Some(r#type.into());
         self
     }
 
@@ -220,6 +229,9 @@ impl Compiler {
         let vector_ctor = self.vector_ctor.clone();
         let vector_ctor = vector_ctor.and_then(|ctor| CString::new(ctor).ok());
         let vector_ctor = vector_ctor.as_ref();
+        let vector_type = self.vector_type.clone();
+        let vector_type = vector_type.and_then(|t| CString::new(t).ok());
+        let vector_type = vector_type.as_ref();
 
         let mutable_globals = self
             .mutable_globals
@@ -231,10 +243,10 @@ impl Compiler {
             .iter()
             .map(|s| s.as_ptr())
             .collect::<Vec<_>>();
-        let mut mutable_globals_ptr = ptr::null_mut();
+        let mut mutable_globals_ptr = ptr::null();
         if !mutable_globals.is_empty() {
             mutable_globals.push(ptr::null());
-            mutable_globals_ptr = mutable_globals.as_mut_ptr();
+            mutable_globals_ptr = mutable_globals.as_ptr();
         }
 
         unsafe {
@@ -244,6 +256,7 @@ impl Compiler {
                 coverageLevel: self.coverage_level as c_int,
                 vectorLib: vector_lib.map_or(ptr::null(), |s| s.as_ptr()),
                 vectorCtor: vector_ctor.map_or(ptr::null(), |s| s.as_ptr()),
+                vectorType: vector_type.map_or(ptr::null(), |s| s.as_ptr()),
                 mutableGlobals: mutable_globals_ptr,
             };
             ffi::luau_compile(source.as_ref(), options)

src/conversion.rs

@@ -3,7 +3,9 @@ use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use std::convert::TryInto;
 use std::ffi::{CStr, CString};
 use std::hash::{BuildHasher, Hash};
+use std::os::raw::c_int;
 use std::string::String as StdString;
+use std::{slice, str};
 
 use bstr::{BStr, BString};
 use num_traits::cast;
@@ -302,6 +304,11 @@ impl<'lua> IntoLua<'lua> for StdString {
     fn into_lua(self, lua: &'lua Lua) -> Result<Value<'lua>> {
         Ok(Value::String(lua.create_string(&self)?))
     }
+
+    #[inline]
+    unsafe fn push_into_stack(self, lua: &'lua Lua) -> Result<()> {
+        push_bytes_into_stack(self, lua)
+    }
 }
 
 impl<'lua> FromLua<'lua> for StdString {
@@ -318,6 +325,25 @@ impl<'lua> FromLua<'lua> for StdString {
             .to_str()?
             .to_owned())
     }
+
+    #[inline]
+    unsafe fn from_stack(idx: c_int, lua: &'lua Lua) -> Result<Self> {
+        let state = lua.state();
+        if ffi::lua_type(state, idx) == ffi::LUA_TSTRING {
+            let mut size = 0;
+            let data = ffi::lua_tolstring(state, idx, &mut size);
+            let bytes = slice::from_raw_parts(data as *const u8, size);
+            return str::from_utf8(bytes).map(|s| s.to_owned()).map_err(|e| {
+                Error::FromLuaConversionError {
+                    from: "string",
+                    to: "String",
+                    message: Some(e.to_string()),
+                }
+            });
+        }
+        // Fallback to default
+        Self::from_lua(lua.stack_value(idx), lua)
+    }
 }
 
 impl<'lua> IntoLua<'lua> for &str {
@@ -325,6 +351,11 @@ impl<'lua> IntoLua<'lua> for &str {
     fn into_lua(self, lua: &'lua Lua) -> Result<Value<'lua>> {
         Ok(Value::String(lua.create_string(self)?))
     }
+
+    #[inline]
+    unsafe fn push_into_stack(self, lua: &'lua Lua) -> Result<()> {
+        push_bytes_into_stack(self, lua)
+    }
 }
 
 impl<'lua> IntoLua<'lua> for Cow<'_, str> {
@@ -433,6 +464,21 @@ impl<'lua> IntoLua<'lua> for &BStr {
     }
 }
 
+#[inline]
+unsafe fn push_bytes_into_stack<'lua, T>(this: T, lua: &'lua Lua) -> Result<()>
+where
+    T: IntoLua<'lua> + AsRef<[u8]>,
+{
+    let bytes = this.as_ref();
+    if lua.unlikely_memory_error() && bytes.len() < (1 << 30) {
+        // Fast path: push directly into the Lua stack.
+        ffi::lua_pushlstring(lua.state(), bytes.as_ptr() as *const _, bytes.len());
+        return Ok(());
+    }
+    // Fallback to default
+    lua.push_value(T::into_lua(this, lua)?)
+}
+
 macro_rules! lua_convert_int {
     ($x:ty) => {
         impl<'lua> IntoLua<'lua> for $x {

src/function.rs

@@ -126,34 +126,25 @@ impl<'lua> Function<'lua> {
     pub fn call<A: IntoLuaMulti<'lua>, R: FromLuaMulti<'lua>>(&self, args: A) -> Result<R> {
         let lua = self.0.lua;
         let state = lua.state();
-
-        let mut args = args.into_lua_multi(lua)?;
-        let nargs = args.len() as c_int;
-
-        let results = unsafe {
+        unsafe {
             let _sg = StackGuard::new(state);
-            check_stack(state, nargs + 3)?;
+            check_stack(state, 2)?;
 
+            // Push error handler
             MemoryState::relax_limit_with(state, || ffi::lua_pushcfunction(state, error_traceback));
             let stack_start = ffi::lua_gettop(state);
+            // Push function and the arguments
             lua.push_ref(&self.0);
-            for arg in args.drain_all() {
-                lua.push_value(arg)?;
-            }
+            let nargs = args.push_into_stack_multi(lua)?;
+            // Call the function
             let ret = ffi::lua_pcall(state, nargs, ffi::LUA_MULTRET, stack_start);
             if ret != ffi::LUA_OK {
                 return Err(pop_error(state, ret));
             }
+            // Get the results
             let nresults = ffi::lua_gettop(state) - stack_start;
-            let mut results = args; // Reuse MultiValue container
-            assert_stack(state, 2);
-            for _ in 0..nresults {
-                results.push_front(lua.pop_value());
-            }
-            ffi::lua_pop(state, 1);
-            results
-        };
-        R::from_lua_multi(results, lua)
+            R::from_stack_multi(nresults, lua)
+        }
     }
 
     /// Returns a future that, when polled, calls `self`, passing `args` as function arguments,
@@ -564,8 +555,9 @@ impl<'lua> Function<'lua> {
         R: IntoLuaMulti<'lua>,
         F: Fn(&'lua Lua, A) -> Result<R> + MaybeSend + 'static,
     {
-        WrappedFunction(Box::new(move |lua, args| {
-            func(lua, A::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
+        WrappedFunction(Box::new(move |lua, nargs| unsafe {
+            let args = A::from_stack_args(nargs, 1, None, lua)?;
+            func(lua, args)?.push_into_stack_multi(lua)
         }))
     }
 
@@ -578,11 +570,12 @@ impl<'lua> Function<'lua> {
         F: FnMut(&'lua Lua, A) -> Result<R> + MaybeSend + 'static,
     {
         let func = RefCell::new(func);
-        WrappedFunction(Box::new(move |lua, args| {
+        WrappedFunction(Box::new(move |lua, nargs| unsafe {
             let mut func = func
                 .try_borrow_mut()
                 .map_err(|_| Error::RecursiveMutCallback)?;
-            func(lua, A::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
+            let args = A::from_stack_args(nargs, 1, None, lua)?;
+            func(lua, args)?.push_into_stack_multi(lua)
         }))
     }
 
@@ -596,13 +589,13 @@ impl<'lua> Function<'lua> {
         F: Fn(&'lua Lua, A) -> FR + MaybeSend + 'static,
         FR: Future<Output = Result<R>> + 'lua,
     {
-        WrappedAsyncFunction(Box::new(move |lua, args| {
-            let args = match A::from_lua_multi(args, lua) {
+        WrappedAsyncFunction(Box::new(move |lua, args| unsafe {
+            let args = match A::from_lua_args(args, 1, None, lua) {
                 Ok(args) => args,
                 Err(e) => return Box::pin(future::err(e)),
             };
             let fut = func(lua, args);
-            Box::pin(async move { fut.await?.into_lua_multi(lua) })
+            Box::pin(async move { fut.await?.push_into_stack_multi(lua) })
         }))
     }
 }

src/lib.rs

@@ -157,7 +157,8 @@ extern crate mlua_derive;
 // Unstable features
 #[cfg(feature = "unstable")]
 pub use crate::{
-    function::OwnedFunction, string::OwnedString, table::OwnedTable, userdata::OwnedAnyUserData,
+    function::OwnedFunction, string::OwnedString, table::OwnedTable, thread::OwnedThread,
+    userdata::OwnedAnyUserData,
 };
 
 /// Create a type that implements [`AsChunk`] and can capture Rust variables.

src/lua.rs

@@ -107,7 +107,7 @@ pub(crate) struct ExtraData {
     // Pool of `WrappedFailure` enums in the ref thread (as userdata)
     wrapped_failure_pool: Vec<c_int>,
     // Pool of `MultiValue` containers
-    multivalue_pool: Vec<MultiValue<'static>>,
+    multivalue_pool: Vec<Vec<Value<'static>>>,
     // Pool of `Thread`s (coroutines) for async execution
     #[cfg(feature = "async")]
     thread_pool: Vec<c_int>,
@@ -282,6 +282,13 @@ impl Deref for Lua {
     }
 }
 
+impl Default for Lua {
+    #[inline]
+    fn default() -> Self {
+        Lua::new()
+    }
+}
+
 impl Lua {
     /// Creates a new Lua state and loads the **safe** subset of the standard libraries.
     ///
@@ -292,7 +299,6 @@ impl Lua {
     /// See [`StdLib`] documentation for a list of unsafe modules that cannot be loaded.
     ///
     /// [`StdLib`]: crate::StdLib
-    #[allow(clippy::new_without_default)]
     pub fn new() -> Lua {
         mlua_expect!(
             Self::new_with(StdLib::ALL_SAFE, LuaOptions::default()),
@@ -718,50 +724,28 @@ impl Lua {
     // The returned value then pushed onto the stack.
     #[doc(hidden)]
     #[cfg(not(tarpaulin_include))]
-    pub unsafe fn entrypoint<'lua, A, R, F>(self, func: F) -> Result<c_int>
+    pub unsafe fn entrypoint<'lua, A, R, F>(self, func: F) -> c_int
     where
         A: FromLuaMulti<'lua>,
         R: IntoLua<'lua>,
         F: Fn(&'lua Lua, A) -> Result<R> + MaybeSend + 'static,
     {
-        let entrypoint_inner = |lua: &'lua Lua, func: F| {
-            let state = lua.state();
-            let nargs = ffi::lua_gettop(state);
-            check_stack(state, 3)?;
+        let (state, extra) = (self.state(), self.extra.get());
+        // It must be safe to drop `self` as in the module mode we keep strong reference to `Lua` in the registry
+        drop(self);
 
-            let mut args = MultiValue::new();
-            args.reserve(nargs as usize);
-            for _ in 0..nargs {
-                args.push_front(lua.pop_value());
-            }
-
-            // We create callback rather than call `func` directly to catch errors
-            // with attached stacktrace.
-            let callback = lua.create_callback(Box::new(move |lua, args| {
-                func(lua, A::from_lua_multi_args(args, 1, None, lua)?)?.into_lua_multi(lua)
-            }))?;
-            callback.call(args)
-        };
-
-        match entrypoint_inner(mem::transmute(&self), func) {
-            Ok(res) => {
-                self.push_value(res)?;
-                Ok(1)
-            }
-            Err(err) => {
-                self.push_value(Value::Error(err))?;
-                let state = self.state();
-                // Lua (self) must be dropped before triggering longjmp
-                drop(self);
-                ffi::lua_error(state)
-            }
-        }
+        callback_error_ext(state, extra, move |nargs| {
+            let lua: &Lua = mem::transmute((*extra).inner.assume_init_ref());
+            let args = A::from_stack_args(nargs, 1, None, lua)?;
+            func(lua, args)?.push_into_stack(lua)?;
+            Ok(1)
+        })
     }
 
     // A simple module entrypoint without arguments
     #[doc(hidden)]
     #[cfg(not(tarpaulin_include))]
-    pub unsafe fn entrypoint1<'lua, R, F>(self, func: F) -> Result<c_int>
+    pub unsafe fn entrypoint1<'lua, R, F>(self, func: F) -> c_int
     where
         R: IntoLua<'lua>,
         F: Fn(&'lua Lua) -> Result<R> + MaybeSend + 'static,
@@ -1537,8 +1521,9 @@ impl Lua {
         R: IntoLuaMulti<'lua>,
         F: Fn(&'lua Lua, A) -> Result<R> + MaybeSend + 'static,
     {
-        self.create_callback(Box::new(move |lua, args| {
-            func(lua, A::from_lua_multi_args(args, 1, None, lua)?)?.into_lua_multi(lua)
+        self.create_callback(Box::new(move |lua, nargs| unsafe {
+            let args = A::from_stack_args(nargs, 1, None, lua)?;
+            func(lua, args)?.push_into_stack_multi(lua)
         }))
     }
 
@@ -1621,13 +1606,13 @@ impl Lua {
         F: Fn(&'lua Lua, A) -> FR + MaybeSend + 'static,
         FR: Future<Output = Result<R>> + 'lua,
     {
-        self.create_async_callback(Box::new(move |lua, args| {
-            let args = match A::from_lua_multi_args(args, 1, None, lua) {
+        self.create_async_callback(Box::new(move |lua, args| unsafe {
+            let args = match A::from_lua_args(args, 1, None, lua) {
                 Ok(args) => args,
                 Err(e) => return Box::pin(future::err(e)),
             };
             let fut = func(lua, args);
-            Box::pin(async move { fut.await?.into_lua_multi(lua) })
+            Box::pin(async move { fut.await?.push_into_stack_multi(lua) })
         }))
     }
 
@@ -1655,7 +1640,7 @@ impl Lua {
             self.push_ref(&func.0);
             ffi::lua_xmove(state, thread_state, 1);
 
-            Ok(Thread(self.pop_ref()))
+            Ok(Thread::new(self.pop_ref()))
         }
     }
 
@@ -1683,7 +1668,7 @@ impl Lua {
                     ffi::lua_replace(thread_state, ffi::LUA_GLOBALSINDEX);
                 }
 
-                return Ok(Thread(LuaRef::new(self, index)));
+                return Ok(Thread::new(LuaRef::new(self, index)));
             }
         };
         self.create_thread_inner(func)
@@ -1816,6 +1801,27 @@ impl Lua {
         unsafe { self.make_userdata(UserDataCell::new(UserDataProxy::<T>(PhantomData))) }
     }
 
+    /// Sets the metatable for a Luau builtin vector type.
+    #[cfg(any(all(feature = "luau", feature = "unstable"), doc))]
+    #[cfg_attr(docsrs, doc(cfg(all(feature = "luau", feature = "unstable"))))]
+    pub fn set_vector_metatable(&self, metatable: Option<Table>) {
+        unsafe {
+            let state = self.state();
+            let _sg = StackGuard::new(state);
+            assert_stack(state, 2);
+
+            #[cfg(not(feature = "luau-vector4"))]
+            ffi::lua_pushvector(state, 0., 0., 0.);
+            #[cfg(feature = "luau-vector4")]
+            ffi::lua_pushvector(state, 0., 0., 0., 0.);
+            match metatable {
+                Some(metatable) => self.push_ref(&metatable.0),
+                None => ffi::lua_pushnil(state),
+            };
+            ffi::lua_setmetatable(state, -2);
+        }
+    }
+
     /// Returns a handle to the global environment.
     pub fn globals(&self) -> Table {
         let state = self.state();
@@ -1838,7 +1844,7 @@ impl Lua {
             let _sg = StackGuard::new(state);
             assert_stack(state, 1);
             ffi::lua_pushthread(state);
-            Thread(self.pop_ref())
+            Thread::new(self.pop_ref())
         }
     }
 
@@ -2433,7 +2439,7 @@ impl Lua {
                 }
             }
 
-            ffi::LUA_TTHREAD => Value::Thread(Thread(self.pop_ref())),
+            ffi::LUA_TTHREAD => Value::Thread(Thread::new(self.pop_ref())),
 
             #[cfg(feature = "luajit")]
             ffi::LUA_TCDATA => {
@@ -2446,6 +2452,104 @@ impl Lua {
         }
     }
 
+    /// Returns value at given stack index without popping it.
+    ///
+    /// Uses 2 stack spaces, does not call checkstack.
+    pub(crate) unsafe fn stack_value(&self, idx: c_int) -> Value {
+        let state = self.state();
+        match ffi::lua_type(state, idx) {
+            ffi::LUA_TNIL => Nil,
+
+            ffi::LUA_TBOOLEAN => Value::Boolean(ffi::lua_toboolean(state, idx) != 0),
+
+            ffi::LUA_TLIGHTUSERDATA => {
+                Value::LightUserData(LightUserData(ffi::lua_touserdata(state, idx)))
+            }
+
+            #[cfg(any(feature = "lua54", feature = "lua53"))]
+            ffi::LUA_TNUMBER => {
+                if ffi::lua_isinteger(state, idx) != 0 {
+                    Value::Integer(ffi::lua_tointeger(state, idx))
+                } else {
+                    Value::Number(ffi::lua_tonumber(state, idx))
+                }
+            }
+
+            #[cfg(any(
+                feature = "lua52",
+                feature = "lua51",
+                feature = "luajit",
+                feature = "luau"
+            ))]
+            ffi::LUA_TNUMBER => {
+                let n = ffi::lua_tonumber(state, idx);
+                match num_traits::cast(n) {
+                    Some(i) if (n - (i as Number)).abs() < Number::EPSILON => Value::Integer(i),
+                    _ => Value::Number(n),
+                }
+            }
+
+            #[cfg(feature = "luau")]
+            ffi::LUA_TVECTOR => {
+                let v = ffi::lua_tovector(state, idx);
+                mlua_debug_assert!(!v.is_null(), "vector is null");
+                #[cfg(not(feature = "luau-vector4"))]
+                return Value::Vector(Vector([*v, *v.add(1), *v.add(2)]));
+                #[cfg(feature = "luau-vector4")]
+                return Value::Vector(Vector([*v, *v.add(1), *v.add(2), *v.add(3)]));
+            }
+
+            ffi::LUA_TSTRING => {
+                ffi::lua_xpush(state, self.ref_thread(), idx);
+                Value::String(String(self.pop_ref_thread()))
+            }
+
+            ffi::LUA_TTABLE => {
+                ffi::lua_xpush(state, self.ref_thread(), idx);
+                Value::Table(Table(self.pop_ref_thread()))
+            }
+
+            ffi::LUA_TFUNCTION => {
+                ffi::lua_xpush(state, self.ref_thread(), idx);
+                Value::Function(Function(self.pop_ref_thread()))
+            }
+
+            ffi::LUA_TUSERDATA => {
+                let wrapped_failure_mt_ptr = (*self.extra.get()).wrapped_failure_mt_ptr;
+                // We must prevent interaction with userdata types other than UserData OR a WrappedError.
+                // WrappedPanics are automatically resumed.
+                match get_gc_userdata::<WrappedFailure>(state, idx, wrapped_failure_mt_ptr).as_mut()
+                {
+                    Some(WrappedFailure::Error(err)) => Value::Error(err.clone()),
+                    Some(WrappedFailure::Panic(panic)) => {
+                        if let Some(panic) = panic.take() {
+                            resume_unwind(panic);
+                        }
+                        // Previously resumed panic?
+                        Value::Nil
+                    }
+                    _ => {
+                        ffi::lua_xpush(state, self.ref_thread(), idx);
+                        Value::UserData(AnyUserData(self.pop_ref_thread()))
+                    }
+                }
+            }
+
+            ffi::LUA_TTHREAD => {
+                ffi::lua_xpush(state, self.ref_thread(), idx);
+                Value::Thread(Thread::new(self.pop_ref_thread()))
+            }
+
+            #[cfg(feature = "luajit")]
+            ffi::LUA_TCDATA => {
+                // TODO: Fix this in a next major release
+                panic!("cdata objects cannot be handled by mlua yet");
+            }
+
+            _ => mlua_panic!("LUA_TNONE in pop_value"),
+        }
+    }
+
     // Pushes a LuaRef value onto the stack, uses 1 stack space, does not call checkstack
     pub(crate) unsafe fn push_ref(&self, lref: &LuaRef) {
         assert!(
@@ -2532,7 +2636,7 @@ impl Lua {
         let mut has_name = false;
         for (k, f) in registry.meta_fields {
             has_name = has_name || k == MetaMethod::Type;
-            self.push_value(f(self, MultiValue::new())?.pop_front().unwrap())?;
+            mlua_assert!(f(self, 0)? == 1, "field function must return one value");
             rawset_field(state, -2, MetaMethod::validate(&k)?)?;
         }
         // Set `__name/__type` if not provided
@@ -2557,7 +2661,7 @@ impl Lua {
                         push_table(state, 0, fields_nrec, true)?;
                     }
                     for (k, f) in registry.fields {
-                        self.push_value(f(self, MultiValue::new())?.pop_front().unwrap())?;
+                        mlua_assert!(f(self, 0)? == 1, "field function must return one value");
                         rawset_field(state, -2, &k)?;
                     }
                     rawset_field(state, metatable_index, "__index")?;
@@ -2686,17 +2790,28 @@ impl Lua {
         }
     }
 
-    // Returns `TypeId` for the LuaRef, checking that it's a registered
-    // and not destructed UserData.
+    // Returns `TypeId` for the `lref` userdata, checking that it's registered and not destructed.
     //
     // Returns `None` if the userdata is registered but non-static.
-    pub(crate) unsafe fn get_userdata_type_id(&self, lref: &LuaRef) -> Result<Option<TypeId>> {
-        let ref_thread = self.ref_thread();
-        if ffi::lua_getmetatable(ref_thread, lref.index) == 0 {
+    pub(crate) unsafe fn get_userdata_ref_type_id(&self, lref: &LuaRef) -> Result<Option<TypeId>> {
+        self.get_userdata_type_id_inner(self.ref_thread(), lref.index)
+    }
+
+    // Same as `get_userdata_ref_type_id` but assumes the userdata is already on the stack.
+    pub(crate) unsafe fn get_userdata_type_id(&self, idx: c_int) -> Result<Option<TypeId>> {
+        self.get_userdata_type_id_inner(self.state(), idx)
+    }
+
+    unsafe fn get_userdata_type_id_inner(
+        &self,
+        state: *mut ffi::lua_State,
+        idx: c_int,
+    ) -> Result<Option<TypeId>> {
+        if ffi::lua_getmetatable(state, idx) == 0 {
             return Err(Error::UserDataTypeMismatch);
         }
-        let mt_ptr = ffi::lua_topointer(ref_thread, -1);
-        ffi::lua_pop(ref_thread, 1);
+        let mt_ptr = ffi::lua_topointer(state, -1);
+        ffi::lua_pop(state, 1);
 
         // Fast path to skip looking up the metatable in the map
         let (last_mt, last_type_id) = (*self.extra.get()).last_checked_userdata_mt;
@@ -2719,7 +2834,7 @@ impl Lua {
     // Pushes a LuaRef (userdata) value onto the stack, returning their `TypeId`.
     // Uses 1 stack space, does not call checkstack.
     pub(crate) unsafe fn push_userdata_ref(&self, lref: &LuaRef) -> Result<Option<TypeId>> {
-        let type_id = self.get_userdata_type_id(lref)?;
+        let type_id = self.get_userdata_type_id_inner(self.ref_thread(), lref.index)?;
         self.push_ref(lref);
         Ok(type_id)
     }
@@ -2754,24 +2869,9 @@ impl Lua {
 
                 let lua: &Lua = mem::transmute((*extra).inner.assume_init_ref());
                 let _guard = StateGuard::new(&lua.0, state);
-
-                let mut args = MultiValue::new_or_pooled(lua);
-                args.reserve(nargs as usize);
-                for _ in 0..nargs {
-                    args.push_front(lua.pop_value());
-                }
-
                 let func = &*(*upvalue).data;
-                let mut results = func(lua, args)?;
-                let nresults = results.len() as c_int;
 
-                check_stack(state, nresults)?;
-                for r in results.drain_all() {
-                    lua.push_value(r)?;
-                }
-                MultiValue::return_to_pool(results, lua);
-
-                Ok(nresults)
+                func(lua, nargs)
             })
         }
 
@@ -2824,12 +2924,7 @@ impl Lua {
                 let lua: &Lua = mem::transmute((*extra).inner.assume_init_ref());
                 let _guard = StateGuard::new(&lua.0, state);
 
-                let mut args = MultiValue::new_or_pooled(lua);
-                args.reserve(nargs as usize);
-                for _ in 0..nargs {
-                    args.push_front(lua.pop_value());
-                }
-
+                let args = MultiValue::from_stack_multi(nargs, lua)?;
                 let func = &*(*upvalue).data;
                 let fut = func(lua, args);
                 let extra = Arc::clone(&(*upvalue).extra);
@@ -2851,6 +2946,7 @@ impl Lua {
             let upvalue = get_userdata::<AsyncPollUpvalue>(state, ffi::lua_upvalueindex(1));
             let extra = (*upvalue).extra.get();
             callback_error_ext(state, extra, |_| {
+                // Lua ensures that `LUA_MINSTACK` stack spaces are available (after pushing arguments)
                 let lua: &Lua = mem::transmute((*extra).inner.assume_init_ref());
                 let _guard = StateGuard::new(&lua.0, state);
 
@@ -2858,21 +2954,20 @@ impl Lua {
                 let mut ctx = Context::from_waker(lua.waker());
                 match fut.as_mut().poll(&mut ctx) {
                     Poll::Pending => Ok(0),
-                    Poll::Ready(results) => {
-                        let mut results = results?;
-                        let nresults = results.len();
-                        lua.push_value(Value::Integer(nresults as _))?;
+                    Poll::Ready(nresults) => {
+                        let nresults = nresults?;
                         match nresults {
-                            0 => Ok(1),
-                            1 | 2 => {
-                                // Fast path for 1 or 2 results without creating a table
-                                for r in results.drain_all() {
-                                    lua.push_value(r)?;
+                            0..=2 => {
+                                // Fast path for up to 2 results without creating a table
+                                ffi::lua_pushinteger(state, nresults as _);
+                                if nresults > 0 {
+                                    ffi::lua_insert(state, -nresults - 1);
                                 }
-                                MultiValue::return_to_pool(results, lua);
-                                Ok(nresults as c_int + 1)
+                                Ok(nresults + 1)
                             }
                             _ => {
+                                let results = MultiValue::from_stack_multi(nresults, lua)?;
+                                ffi::lua_pushinteger(state, nresults as _);
                                 lua.push_value(Value::Table(lua.create_sequence_from(results)?))?;
                                 Ok(2)
                             }
@@ -3110,13 +3205,13 @@ impl LuaInner {
     }
 
     #[inline]
-    pub(crate) fn new_multivalue_from_pool(&self) -> MultiValue {
+    pub(crate) fn pop_multivalue_from_pool(&self) -> Option<Vec<Value>> {
         let extra = unsafe { &mut *self.extra.get() };
-        extra.multivalue_pool.pop().unwrap_or_default()
+        extra.multivalue_pool.pop()
     }
 
     #[inline]
-    pub(crate) fn return_multivalue_to_pool(&self, mut multivalue: MultiValue) {
+    pub(crate) fn push_multivalue_to_pool(&self, mut multivalue: Vec<Value>) {
         let extra = unsafe { &mut *self.extra.get() };
         if extra.multivalue_pool.len() < MULTIVALUE_POOL_SIZE {
             multivalue.clear();

src/multi.rs

@@ -1,9 +1,11 @@
 use std::iter::FromIterator;
 use std::ops::{Deref, DerefMut};
+use std::os::raw::c_int;
 use std::result::Result as StdResult;
 
 use crate::error::Result;
 use crate::lua::Lua;
+use crate::util::check_stack;
 use crate::value::{FromLua, FromLuaMulti, IntoLua, IntoLuaMulti, MultiValue, Nil};
 
 /// Result is convertible to `MultiValue` following the common Lua idiom of returning the result
@@ -11,7 +13,7 @@ use crate::value::{FromLua, FromLuaMulti, IntoLua, IntoLuaMulti, MultiValue, Nil
 impl<'lua, T: IntoLua<'lua>, E: IntoLua<'lua>> IntoLuaMulti<'lua> for StdResult<T, E> {
     #[inline]
     fn into_lua_multi(self, lua: &'lua Lua) -> Result<MultiValue<'lua>> {
-        let mut result = MultiValue::new_or_pooled(lua);
+        let mut result = MultiValue::with_lua_and_capacity(lua, 2);
         match self {
             Ok(v) => result.push_front(v.into_lua(lua)?),
             Err(e) => {
@@ -23,33 +25,71 @@ impl<'lua, T: IntoLua<'lua>, E: IntoLua<'lua>> IntoLuaMulti<'lua> for StdResult<
     }
 }
 
+impl<'lua, E: IntoLua<'lua>> IntoLuaMulti<'lua> for StdResult<(), E> {
+    #[inline]
+    fn into_lua_multi(self, lua: &'lua Lua) -> Result<MultiValue<'lua>> {
+        match self {
+            Ok(_) => return Ok(MultiValue::new()),
+            Err(e) => {
+                let mut result = MultiValue::with_lua_and_capacity(lua, 2);
+                result.push_front(e.into_lua(lua)?);
+                result.push_front(Nil);
+                Ok(result)
+            }
+        }
+    }
+}
+
 impl<'lua, T: IntoLua<'lua>> IntoLuaMulti<'lua> for T {
     #[inline]
     fn into_lua_multi(self, lua: &'lua Lua) -> Result<MultiValue<'lua>> {
-        let mut v = MultiValue::new_or_pooled(lua);
+        let mut v = MultiValue::with_lua_and_capacity(lua, 1);
         v.push_front(self.into_lua(lua)?);
         Ok(v)
     }
+
+    #[inline]
+    unsafe fn push_into_stack_multi(self, lua: &'lua Lua) -> Result<c_int> {
+        self.push_into_stack(lua)?;
+        Ok(1)
+    }
 }
 
 impl<'lua, T: FromLua<'lua>> FromLuaMulti<'lua> for T {
     #[inline]
     fn from_lua_multi(mut values: MultiValue<'lua>, lua: &'lua Lua) -> Result<Self> {
-        let res = T::from_lua(values.pop_front().unwrap_or(Nil), lua);
-        MultiValue::return_to_pool(values, lua);
-        res
+        T::from_lua(values.pop_front().unwrap_or(Nil), lua)
     }
 
     #[inline]
-    fn from_lua_multi_args(
-        mut values: MultiValue<'lua>,
+    fn from_lua_args(
+        mut args: MultiValue<'lua>,
         i: usize,
         to: Option<&str>,
         lua: &'lua Lua,
     ) -> Result<Self> {
-        let res = T::from_lua_arg(values.pop_front().unwrap_or(Nil), i, to, lua);
-        MultiValue::return_to_pool(values, lua);
-        res
+        T::from_lua_arg(args.pop_front().unwrap_or(Nil), i, to, lua)
+    }
+
+    #[inline]
+    unsafe fn from_stack_multi(nvals: c_int, lua: &'lua Lua) -> Result<Self> {
+        if nvals == 0 {
+            return T::from_lua(Nil, lua);
+        }
+        T::from_stack(-nvals, lua)
+    }
+
+    #[inline]
+    unsafe fn from_stack_args(
+        nargs: c_int,
+        i: usize,
+        to: Option<&str>,
+        lua: &'lua Lua,
+    ) -> Result<Self> {
+        if nargs == 0 {
+            return T::from_lua_arg(Nil, i, to, lua);
+        }
+        T::from_stack_arg(-nargs, i, to, lua)
     }
 }
 
@@ -141,7 +181,7 @@ impl<T> DerefMut for Variadic<T> {
 impl<'lua, T: IntoLua<'lua>> IntoLuaMulti<'lua> for Variadic<T> {
     #[inline]
     fn into_lua_multi(self, lua: &'lua Lua) -> Result<MultiValue<'lua>> {
-        let mut values = MultiValue::new_or_pooled(lua);
+        let mut values = MultiValue::with_lua_and_capacity(lua, self.0.len());
         values.refill(self.0.into_iter().map(|e| e.into_lua(lua)))?;
         Ok(values)
     }
@@ -150,13 +190,11 @@ impl<'lua, T: IntoLua<'lua>> IntoLuaMulti<'lua> for Variadic<T> {
 impl<'lua, T: FromLua<'lua>> FromLuaMulti<'lua> for Variadic<T> {
     #[inline]
     fn from_lua_multi(mut values: MultiValue<'lua>, lua: &'lua Lua) -> Result<Self> {
-        let res = values
+        values
             .drain_all()
             .map(|e| T::from_lua(e, lua))
             .collect::<Result<Vec<T>>>()
-            .map(Variadic);
-        MultiValue::return_to_pool(values, lua);
-        res
+            .map(Variadic)
     }
 }
 
@@ -165,14 +203,26 @@ macro_rules! impl_tuple {
         impl<'lua> IntoLuaMulti<'lua> for () {
             #[inline]
             fn into_lua_multi(self, lua: &'lua Lua) -> Result<MultiValue<'lua>> {
-                Ok(MultiValue::new_or_pooled(lua))
+                Ok(MultiValue::with_lua_and_capacity(lua, 0))
+            }
+
+            #[inline]
+            unsafe fn push_into_stack_multi(self, _lua: &'lua Lua) -> Result<c_int> {
+                Ok(0)
             }
         }
 
         impl<'lua> FromLuaMulti<'lua> for () {
             #[inline]
-            fn from_lua_multi(values: MultiValue<'lua>, lua: &'lua Lua) -> Result<Self> {
-                MultiValue::return_to_pool(values, lua);
+            fn from_lua_multi(_values: MultiValue<'lua>, _lua: &'lua Lua) -> Result<Self> {
+                Ok(())
+            }
+
+            #[inline]
+            unsafe fn from_stack_multi(nvals: c_int, lua: &'lua Lua) -> Result<Self> {
+                if nvals > 0 {
+                    ffi::lua_pop(lua.state(), nvals);
+                }
                 Ok(())
             }
         }
@@ -183,8 +233,7 @@ macro_rules! impl_tuple {
             where $($name: IntoLua<'lua>,)*
                   $last: IntoLuaMulti<'lua>
         {
-            #[allow(unused_mut)]
-            #[allow(non_snake_case)]
+            #[allow(unused_mut, non_snake_case)]
             #[inline]
             fn into_lua_multi(self, lua: &'lua Lua) -> Result<MultiValue<'lua>> {
                 let ($($name,)* $last,) = self;
@@ -193,14 +242,30 @@ macro_rules! impl_tuple {
                 push_reverse!(results, $($name.into_lua(lua)?,)*);
                 Ok(results)
             }
+
+            #[allow(non_snake_case)]
+            #[inline]
+            unsafe fn push_into_stack_multi(self, lua: &'lua Lua) -> Result<c_int> {
+                let ($($name,)* $last,) = self;
+                let mut nresults = 0;
+                $(
+                    _ = $name;
+                    nresults += 1;
+                )*
+                check_stack(lua.state(), nresults + 1)?;
+                $(
+                    $name.push_into_stack(lua)?;
+                )*
+                nresults += $last.push_into_stack_multi(lua)?;
+                Ok(nresults)
+            }
         }
 
         impl<'lua, $($name,)* $last> FromLuaMulti<'lua> for ($($name,)* $last,)
             where $($name: FromLua<'lua>,)*
                   $last: FromLuaMulti<'lua>
         {
-            #[allow(unused_mut)]
-            #[allow(non_snake_case)]
+            #[allow(unused_mut, non_snake_case)]
             #[inline]
             fn from_lua_multi(mut values: MultiValue<'lua>, lua: &'lua Lua) -> Result<Self> {
                 $(let $name = FromLua::from_lua(values.pop_front().unwrap_or(Nil), lua)?;)*
@@ -208,15 +273,45 @@ macro_rules! impl_tuple {
                 Ok(($($name,)* $last,))
             }
 
-            #[allow(unused_mut)]
-            #[allow(non_snake_case)]
+            #[allow(unused_mut, non_snake_case)]
             #[inline]
-            fn from_lua_multi_args(mut values: MultiValue<'lua>, mut i: usize, to: Option<&str>, lua: &'lua Lua) -> Result<Self> {
+            fn from_lua_args(mut args: MultiValue<'lua>, mut i: usize, to: Option<&str>, lua: &'lua Lua) -> Result<Self> {
                 $(
-                    let $name = FromLua::from_lua_arg(values.pop_front().unwrap_or(Nil), i, to, lua)?;
+                    let $name = FromLua::from_lua_arg(args.pop_front().unwrap_or(Nil), i, to, lua)?;
                     i += 1;
                 )*
-                let $last = FromLuaMulti::from_lua_multi_args(values, i, to, lua)?;
+                let $last = FromLuaMulti::from_lua_args(args, i, to, lua)?;
+                Ok(($($name,)* $last,))
+            }
+
+            #[allow(unused_mut, non_snake_case)]
+            #[inline]
+            unsafe fn from_stack_multi(mut nvals: c_int, lua: &'lua Lua) -> Result<Self> {
+                $(
+                    let $name = if nvals > 0 {
+                        nvals -= 1;
+                        FromLua::from_stack(-(nvals + 1), lua)
+                    } else {
+                        FromLua::from_lua(Nil, lua)
+                    }?;
+                )*
+                let $last = FromLuaMulti::from_stack_multi(nvals, lua)?;
+                Ok(($($name,)* $last,))
+            }
+
+            #[allow(unused_mut, non_snake_case)]
+            #[inline]
+            unsafe fn from_stack_args(mut nargs: c_int, mut i: usize, to: Option<&str>, lua: &'lua Lua) -> Result<Self> {
+                $(
+                    let $name = if nargs > 0 {
+                        nargs -= 1;
+                        FromLua::from_stack_arg(-(nargs + 1), i, to, lua)
+                    } else {
+                        FromLua::from_lua_arg(Nil, i, to, lua)
+                    }?;
+                    i += 1;
+                )*
+                let $last = FromLuaMulti::from_stack_args(nargs, i, to, lua)?;
                 Ok(($($name,)* $last,))
             }
         }

src/prelude.rs

@@ -40,5 +40,5 @@ pub use crate::{
 #[doc(no_inline)]
 pub use crate::{
     OwnedAnyUserData as LuaOwnedAnyUserData, OwnedFunction as LuaOwnedFunction,
-    OwnedString as LuaOwnedString, OwnedTable as LuaOwnedTable,
+    OwnedString as LuaOwnedString, OwnedTable as LuaOwnedTable, OwnedThread as LuaOwnedThread,
 };

src/scope.rs

@@ -2,6 +2,7 @@ use std::any::Any;
 use std::cell::{Cell, RefCell};
 use std::marker::PhantomData;
 use std::mem;
+use std::os::raw::c_int;
 
 #[cfg(feature = "serialize")]
 use serde::Serialize;
@@ -15,10 +16,10 @@ use crate::userdata::{
 };
 use crate::userdata_impl::UserDataRegistry;
 use crate::util::{
-    self, assert_stack, check_stack, get_userdata, init_userdata_metatable, push_table,
-    rawset_field, take_userdata, StackGuard,
+    self, assert_stack, check_stack, init_userdata_metatable, push_string, push_table,
+    rawset_field, short_type_name, take_userdata, StackGuard,
 };
-use crate::value::{FromLua, FromLuaMulti, IntoLua, IntoLuaMulti, MultiValue, Value};
+use crate::value::{FromLua, FromLuaMulti, IntoLua, IntoLuaMulti, Value};
 
 #[cfg(feature = "lua54")]
 use crate::userdata::USER_VALUE_MAXSLOT;
@@ -75,8 +76,9 @@ impl<'lua, 'scope> Scope<'lua, 'scope> {
         // to make sure callbacks can't capture handles with lifetime outside the scope, inside the
         // scope, and owned inside the callback itself.
         unsafe {
-            self.create_callback(Box::new(move |lua, args| {
-                func(lua, A::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
+            self.create_callback(Box::new(move |lua, nargs| {
+                let args = A::from_stack_args(nargs, 1, None, lua)?;
+                func(lua, args)?.push_into_stack_multi(lua)
             }))
         }
     }
@@ -186,6 +188,23 @@ impl<'lua, 'scope> Scope<'lua, 'scope> {
         }
     }
 
+    /// Creates a Lua userdata object from a custom Rust type.
+    ///
+    /// This is a version of [`Lua::create_any_userdata`] that creates a userdata which expires on
+    /// scope drop and does not require that the userdata type be Send (but still requires that the
+    /// UserData be 'static). See [`Lua::scope`] for more details.
+    #[inline]
+    pub fn create_any_userdata<T>(&self, data: T) -> Result<AnyUserData<'lua>>
+    where
+        T: 'static,
+    {
+        unsafe {
+            let ud = self.lua.make_any_userdata(UserDataCell::new(data))?;
+            self.seal_userdata::<T>(&ud)?;
+            Ok(ud)
+        }
+    }
+
     /// Creates a Lua userdata object from a reference to custom Rust type.
     ///
     /// This is a version of [`Lua::create_any_userdata`] that creates a userdata which expires on
@@ -293,9 +312,10 @@ impl<'lua, 'scope> Scope<'lua, 'scope> {
         // 'lua. This is safe though, because `UserData::add_methods` does not get to pick the 'lua
         // lifetime, so none of the static methods UserData types can add can possibly capture
         // parameters.
-        fn wrap_method<'scope, 'lua, 'callback: 'scope, T: 'scope>(
+        unsafe fn wrap_method<'scope, 'lua, 'callback: 'scope, T: 'scope>(
             scope: &Scope<'lua, 'scope>,
             ud_ptr: *const UserDataCell<T>,
+            name: &str,
             method: NonStaticMethod<'callback, T>,
         ) -> Result<Function<'lua>> {
             // On methods that actually receive the userdata, we fake a type check on the passed in
@@ -305,61 +325,56 @@ impl<'lua, 'scope> Scope<'lua, 'scope> {
             // with a type mismatch, but here without this check would proceed as though you had
             // called the method on the original value (since we otherwise completely ignore the
             // first argument).
-            let check_ud_type = move |lua: &Lua, value| -> Result<&UserDataCell<T>> {
-                if let Some(Value::UserData(ud)) = value {
-                    let state = lua.state();
-                    unsafe {
-                        let _sg = StackGuard::new(state);
-                        check_stack(state, 2)?;
-                        lua.push_userdata_ref(&ud.0)?;
-                        if get_userdata(state, -1) as *const _ == ud_ptr {
-                            return Ok(&*ud_ptr);
-                        }
-                    }
-                };
-                Err(Error::UserDataTypeMismatch)
+            let func_name = format!("{}.{name}", short_type_name::<T>());
+            let check_self_type = move |lua: &Lua, nargs: c_int| -> Result<&UserDataCell<T>> {
+                let state = lua.state();
+                if nargs > 0 && ffi::lua_touserdata(state, -nargs) as *const _ == ud_ptr {
+                    return Ok(&*ud_ptr);
+                }
+                Err(Error::bad_self_argument(
+                    &func_name,
+                    Error::UserDataTypeMismatch,
+                ))
             };
 
             match method {
                 NonStaticMethod::Method(method) => {
-                    let f = Box::new(move |lua, mut args: MultiValue<'callback>| {
-                        let data = check_ud_type(lua, args.pop_front())?;
+                    let f = Box::new(move |lua, nargs| {
+                        let data = check_self_type(lua, nargs)?;
                         let data = data.try_borrow()?;
-                        method(lua, &*data, args)
+                        method(lua, &*data, nargs - 1)
                     });
-                    unsafe { scope.create_callback(f) }
+                    scope.create_callback(f)
                 }
                 NonStaticMethod::MethodMut(method) => {
                     let method = RefCell::new(method);
-                    let f = Box::new(move |lua, mut args: MultiValue<'callback>| {
-                        let data = check_ud_type(lua, args.pop_front())?;
+                    let f = Box::new(move |lua, nargs| {
+                        let data = check_self_type(lua, nargs)?;
                         let mut method = method
                             .try_borrow_mut()
                             .map_err(|_| Error::RecursiveMutCallback)?;
                         let mut data = data.try_borrow_mut()?;
-                        (*method)(lua, &mut *data, args)
+                        (*method)(lua, &mut *data, nargs - 1)
                     });
-                    unsafe { scope.create_callback(f) }
+                    scope.create_callback(f)
                 }
-                NonStaticMethod::Function(function) => unsafe { scope.create_callback(function) },
+                NonStaticMethod::Function(function) => scope.create_callback(function),
                 NonStaticMethod::FunctionMut(function) => {
                     let function = RefCell::new(function);
-                    let f = Box::new(move |lua, args| {
-                        (*function
+                    let f = Box::new(move |lua, nargs| {
+                        let mut func = function
                             .try_borrow_mut()
-                            .map_err(|_| Error::RecursiveMutCallback)?)(
-                            lua, args
-                        )
+                            .map_err(|_| Error::RecursiveMutCallback)?;
+                        func(lua, nargs)
                     });
-                    unsafe { scope.create_callback(f) }
+                    scope.create_callback(f)
                 }
             }
         }
 
-        let mut ud_fields = NonStaticUserDataFields::default();
-        let mut ud_methods = NonStaticUserDataMethods::default();
-        T::add_fields(&mut ud_fields);
-        T::add_methods(&mut ud_methods);
+        let mut registry = NonStaticUserDataRegistry::new();
+        T::add_fields(&mut registry);
+        T::add_methods(&mut registry);
 
         let lua = self.lua;
         let state = lua.state();
@@ -387,48 +402,82 @@ impl<'lua, 'scope> Scope<'lua, 'scope> {
             };
 
             // 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;
+            let meta_methods_nrec = registry.meta_methods.len() + registry.meta_fields.len() + 1;
             push_table(state, 0, meta_methods_nrec, true)?;
-
-            for (k, m) in ud_methods.meta_methods {
-                lua.push_value(Value::Function(wrap_method(self, ud_ptr, m)?))?;
+            for (k, m) in registry.meta_methods {
+                lua.push_value(Value::Function(wrap_method(self, ud_ptr, &k, m)?))?;
                 rawset_field(state, -2, MetaMethod::validate(&k)?)?;
             }
-            for (k, f) in ud_fields.meta_fields {
-                lua.push_value(f(lua, MultiValue::new())?.pop_front().unwrap())?;
+            let mut has_name = false;
+            for (k, f) in registry.meta_fields {
+                has_name = has_name || k == MetaMethod::Type;
+                mlua_assert!(f(lua, 0)? == 1, "field function must return one value");
                 rawset_field(state, -2, MetaMethod::validate(&k)?)?;
             }
+            // Set `__name/__type` if not provided
+            if !has_name {
+                let type_name = short_type_name::<T>();
+                push_string(state, type_name.as_bytes(), !lua.unlikely_memory_error())?;
+                rawset_field(state, -2, MetaMethod::Type.name())?;
+            }
             let metatable_index = ffi::lua_absindex(state, -1);
 
+            let fields_nrec = registry.fields.len();
+            if fields_nrec > 0 {
+                // If __index is a table then update it inplace
+                let index_type = ffi::lua_getfield(state, metatable_index, cstr!("__index"));
+                match index_type {
+                    ffi::LUA_TNIL | ffi::LUA_TTABLE => {
+                        if index_type == ffi::LUA_TNIL {
+                            // Create a new table
+                            ffi::lua_pop(state, 1);
+                            push_table(state, 0, fields_nrec, true)?;
+                        }
+                        for (k, f) in registry.fields {
+                            let NonStaticMethod::Function(f) = f else { unreachable!() };
+                            mlua_assert!(f(lua, 0)? == 1, "field function must return one value");
+                            rawset_field(state, -2, &k)?;
+                        }
+                        rawset_field(state, metatable_index, "__index")?;
+                    }
+                    _ => {
+                        // Propagate fields to the field getters
+                        for (k, f) in registry.fields {
+                            registry.field_getters.push((k, f))
+                        }
+                    }
+                }
+            }
+
             let mut field_getters_index = None;
-            let field_getters_nrec = ud_fields.field_getters.len();
+            let field_getters_nrec = registry.field_getters.len();
             if field_getters_nrec > 0 {
                 push_table(state, 0, field_getters_nrec, true)?;
-                for (k, m) in ud_fields.field_getters {
-                    lua.push_value(Value::Function(wrap_method(self, ud_ptr, m)?))?;
+                for (k, m) in registry.field_getters {
+                    lua.push_value(Value::Function(wrap_method(self, ud_ptr, &k, m)?))?;
                     rawset_field(state, -2, &k)?;
                 }
                 field_getters_index = Some(ffi::lua_absindex(state, -1));
             }
 
             let mut field_setters_index = None;
-            let field_setters_nrec = ud_fields.field_setters.len();
+            let field_setters_nrec = registry.field_setters.len();
             if field_setters_nrec > 0 {
                 push_table(state, 0, field_setters_nrec, true)?;
-                for (k, m) in ud_fields.field_setters {
-                    lua.push_value(Value::Function(wrap_method(self, ud_ptr, m)?))?;
+                for (k, m) in registry.field_setters {
+                    lua.push_value(Value::Function(wrap_method(self, ud_ptr, &k, m)?))?;
                     rawset_field(state, -2, &k)?;
                 }
                 field_setters_index = Some(ffi::lua_absindex(state, -1));
             }
 
             let mut methods_index = None;
-            let methods_nrec = ud_methods.methods.len();
+            let methods_nrec = registry.methods.len();
             if methods_nrec > 0 {
                 // Create table used for methods lookup
                 push_table(state, 0, methods_nrec, true)?;
-                for (k, m) in ud_methods.methods {
-                    lua.push_value(Value::Function(wrap_method(self, ud_ptr, m)?))?;
+                for (k, m) in registry.methods {
+                    lua.push_value(Value::Function(wrap_method(self, ud_ptr, &k, m)?))?;
                     rawset_field(state, -2, &k)?;
                 }
                 methods_index = Some(ffi::lua_absindex(state, -1));
@@ -572,35 +621,144 @@ impl<'lua, 'scope> Drop for Scope<'lua, 'scope> {
 
 #[allow(clippy::type_complexity)]
 enum NonStaticMethod<'lua, T> {
-    Method(Box<dyn Fn(&'lua Lua, &T, MultiValue<'lua>) -> Result<MultiValue<'lua>>>),
-    MethodMut(Box<dyn FnMut(&'lua Lua, &mut T, MultiValue<'lua>) -> Result<MultiValue<'lua>>>),
-    Function(Box<dyn Fn(&'lua Lua, MultiValue<'lua>) -> Result<MultiValue<'lua>>>),
-    FunctionMut(Box<dyn FnMut(&'lua Lua, MultiValue<'lua>) -> Result<MultiValue<'lua>>>),
+    Method(Box<dyn Fn(&'lua Lua, &T, c_int) -> Result<c_int>>),
+    MethodMut(Box<dyn FnMut(&'lua Lua, &mut T, c_int) -> Result<c_int>>),
+    Function(Box<dyn Fn(&'lua Lua, c_int) -> Result<c_int>>),
+    FunctionMut(Box<dyn FnMut(&'lua Lua, c_int) -> Result<c_int>>),
 }
 
-struct NonStaticUserDataMethods<'lua, T: UserData> {
+struct NonStaticUserDataRegistry<'lua, T> {
+    // Fields
+    fields: Vec<(String, NonStaticMethod<'lua, T>)>,
+    field_getters: Vec<(String, NonStaticMethod<'lua, T>)>,
+    field_setters: Vec<(String, NonStaticMethod<'lua, T>)>,
+    meta_fields: Vec<(String, Callback<'lua, 'static>)>,
+
+    // Methods
     methods: Vec<(String, NonStaticMethod<'lua, T>)>,
     meta_methods: Vec<(String, NonStaticMethod<'lua, T>)>,
 }
 
-impl<'lua, T: UserData> Default for NonStaticUserDataMethods<'lua, T> {
-    fn default() -> NonStaticUserDataMethods<'lua, T> {
-        NonStaticUserDataMethods {
+impl<'lua, T> NonStaticUserDataRegistry<'lua, T> {
+    const fn new() -> NonStaticUserDataRegistry<'lua, T> {
+        NonStaticUserDataRegistry {
+            fields: Vec::new(),
+            field_getters: Vec::new(),
+            field_setters: Vec::new(),
+            meta_fields: Vec::new(),
             methods: Vec::new(),
             meta_methods: Vec::new(),
         }
     }
 }
 
-impl<'lua, T: UserData> UserDataMethods<'lua, T> for NonStaticUserDataMethods<'lua, T> {
+impl<'lua, T> UserDataFields<'lua, T> for NonStaticUserDataRegistry<'lua, T> {
+    fn add_field<V>(&mut self, name: impl AsRef<str>, value: V)
+    where
+        V: IntoLua<'lua> + Clone + 'static,
+    {
+        let name = name.as_ref().to_string();
+        self.fields.push((
+            name,
+            NonStaticMethod::Function(Box::new(move |lua, _| unsafe {
+                value.clone().push_into_stack_multi(lua)
+            })),
+        ));
+    }
+
+    fn add_field_method_get<M, R>(&mut self, name: impl AsRef<str>, method: M)
+    where
+        M: Fn(&'lua Lua, &T) -> Result<R> + MaybeSend + 'static,
+        R: IntoLua<'lua>,
+    {
+        let method = NonStaticMethod::Method(Box::new(move |lua, ud, _| unsafe {
+            method(lua, ud)?.push_into_stack_multi(lua)
+        }));
+        self.field_getters.push((name.as_ref().into(), method));
+    }
+
+    fn add_field_method_set<M, A>(&mut self, name: impl AsRef<str>, mut method: M)
+    where
+        M: FnMut(&'lua Lua, &mut T, A) -> Result<()> + MaybeSend + 'static,
+        A: FromLua<'lua>,
+    {
+        let func_name = format!("{}.{}", short_type_name::<T>(), name.as_ref());
+        let method = NonStaticMethod::MethodMut(Box::new(move |lua, ud, nargs| unsafe {
+            let val = A::from_stack_args(nargs, 2, Some(&func_name), lua)?;
+            method(lua, ud, val)?.push_into_stack_multi(lua)
+        }));
+        self.field_setters.push((name.as_ref().into(), method));
+    }
+
+    fn add_field_function_get<F, R>(&mut self, name: impl AsRef<str>, function: F)
+    where
+        F: Fn(&'lua Lua, AnyUserData<'lua>) -> Result<R> + MaybeSend + 'static,
+        R: IntoLua<'lua>,
+    {
+        let func_name = format!("{}.{}", short_type_name::<T>(), name.as_ref());
+        let func = NonStaticMethod::Function(Box::new(move |lua, nargs| unsafe {
+            let ud = AnyUserData::from_stack_args(nargs, 1, Some(&func_name), lua)?;
+            function(lua, ud)?.push_into_stack_multi(lua)
+        }));
+        self.field_getters.push((name.as_ref().into(), func));
+    }
+
+    fn add_field_function_set<F, A>(&mut self, name: impl AsRef<str>, mut function: F)
+    where
+        F: FnMut(&'lua Lua, AnyUserData<'lua>, A) -> Result<()> + MaybeSend + 'static,
+        A: FromLua<'lua>,
+    {
+        let func_name = format!("{}.{}", short_type_name::<T>(), name.as_ref());
+        let func = NonStaticMethod::FunctionMut(Box::new(move |lua, nargs| unsafe {
+            let (ud, val) = <_>::from_stack_args(nargs, 1, Some(&func_name), lua)?;
+            function(lua, ud, val)?.push_into_stack_multi(lua)
+        }));
+        self.field_setters.push((name.as_ref().into(), func));
+    }
+
+    fn add_meta_field<V>(&mut self, name: impl AsRef<str>, value: V)
+    where
+        V: IntoLua<'lua> + Clone + 'static,
+    {
+        let name = name.as_ref().to_string();
+        let name2 = name.clone();
+        self.meta_fields.push((
+            name,
+            Box::new(move |lua, _| unsafe {
+                UserDataRegistry::<()>::check_meta_field(lua, &name2, value.clone())?
+                    .push_into_stack_multi(lua)
+            }),
+        ));
+    }
+
+    fn add_meta_field_with<F, R>(&mut self, name: impl AsRef<str>, f: F)
+    where
+        F: Fn(&'lua Lua) -> Result<R> + MaybeSend + 'static,
+        R: IntoLua<'lua>,
+    {
+        let name = name.as_ref().to_string();
+        let name2 = name.clone();
+        self.meta_fields.push((
+            name,
+            Box::new(move |lua, _| unsafe {
+                UserDataRegistry::<()>::check_meta_field(lua, &name2, f(lua)?)?
+                    .push_into_stack_multi(lua)
+            }),
+        ));
+    }
+}
+
+impl<'lua, T> UserDataMethods<'lua, T> for NonStaticUserDataRegistry<'lua, T> {
     fn add_method<M, A, R>(&mut self, name: impl AsRef<str>, method: M)
     where
         M: Fn(&'lua Lua, &T, A) -> Result<R> + MaybeSend + 'static,
         A: FromLuaMulti<'lua>,
         R: IntoLuaMulti<'lua>,
     {
-        let method = NonStaticMethod::Method(Box::new(move |lua, ud, args| {
-            method(lua, ud, A::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
+        let func_name = format!("{}.{}", short_type_name::<T>(), name.as_ref());
+        let method = NonStaticMethod::Method(Box::new(move |lua, ud, nargs| unsafe {
+            let args = A::from_stack_args(nargs, 2, Some(&func_name), lua)?;
+            method(lua, ud, args)?.push_into_stack_multi(lua)
         }));
         self.methods.push((name.as_ref().into(), method));
     }
@@ -611,8 +769,10 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> for NonStaticUserDataMethods<'l
         A: FromLuaMulti<'lua>,
         R: IntoLuaMulti<'lua>,
     {
-        let method = NonStaticMethod::MethodMut(Box::new(move |lua, ud, args| {
-            method(lua, ud, A::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
+        let func_name = format!("{}.{}", short_type_name::<T>(), name.as_ref());
+        let method = NonStaticMethod::MethodMut(Box::new(move |lua, ud, nargs| unsafe {
+            let args = A::from_stack_args(nargs, 2, Some(&func_name), lua)?;
+            method(lua, ud, args)?.push_into_stack_multi(lua)
         }));
         self.methods.push((name.as_ref().into(), method));
     }
@@ -653,8 +813,10 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> for NonStaticUserDataMethods<'l
         A: FromLuaMulti<'lua>,
         R: IntoLuaMulti<'lua>,
     {
-        let func = NonStaticMethod::Function(Box::new(move |lua, args| {
-            function(lua, A::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
+        let func_name = format!("{}.{}", short_type_name::<T>(), name.as_ref());
+        let func = NonStaticMethod::Function(Box::new(move |lua, nargs| unsafe {
+            let args = A::from_stack_args(nargs, 1, Some(&func_name), lua)?;
+            function(lua, args)?.push_into_stack_multi(lua)
         }));
         self.methods.push((name.as_ref().into(), func));
     }
@@ -665,8 +827,10 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> for NonStaticUserDataMethods<'l
         A: FromLuaMulti<'lua>,
         R: IntoLuaMulti<'lua>,
     {
-        let func = NonStaticMethod::FunctionMut(Box::new(move |lua, args| {
-            function(lua, A::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
+        let func_name = format!("{}.{}", short_type_name::<T>(), name.as_ref());
+        let func = NonStaticMethod::FunctionMut(Box::new(move |lua, nargs| unsafe {
+            let args = A::from_stack_args(nargs, 1, Some(&func_name), lua)?;
+            function(lua, args)?.push_into_stack_multi(lua)
         }));
         self.methods.push((name.as_ref().into(), func));
     }
@@ -690,8 +854,10 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> for NonStaticUserDataMethods<'l
         A: FromLuaMulti<'lua>,
         R: IntoLuaMulti<'lua>,
     {
-        let method = NonStaticMethod::Method(Box::new(move |lua, ud, args| {
-            method(lua, ud, A::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
+        let func_name = format!("{}.{}", short_type_name::<T>(), name.as_ref());
+        let method = NonStaticMethod::Method(Box::new(move |lua, ud, nargs| unsafe {
+            let args = A::from_stack_args(nargs, 2, Some(&func_name), lua)?;
+            method(lua, ud, args)?.push_into_stack_multi(lua)
         }));
         self.meta_methods.push((name.as_ref().into(), method));
     }
@@ -702,8 +868,10 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> for NonStaticUserDataMethods<'l
         A: FromLuaMulti<'lua>,
         R: IntoLuaMulti<'lua>,
     {
-        let method = NonStaticMethod::MethodMut(Box::new(move |lua, ud, args| {
-            method(lua, ud, A::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
+        let func_name = format!("{}.{}", short_type_name::<T>(), name.as_ref());
+        let method = NonStaticMethod::MethodMut(Box::new(move |lua, ud, nargs| unsafe {
+            let args = A::from_stack_args(nargs, 2, Some(&func_name), lua)?;
+            method(lua, ud, args)?.push_into_stack_multi(lua)
         }));
         self.meta_methods.push((name.as_ref().into(), method));
     }
@@ -744,8 +912,10 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> for NonStaticUserDataMethods<'l
         A: FromLuaMulti<'lua>,
         R: IntoLuaMulti<'lua>,
     {
-        let func = NonStaticMethod::Function(Box::new(move |lua, args| {
-            function(lua, A::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
+        let func_name = format!("{}.{}", short_type_name::<T>(), name.as_ref());
+        let func = NonStaticMethod::Function(Box::new(move |lua, nargs| unsafe {
+            let args = A::from_stack_args(nargs, 1, Some(&func_name), lua)?;
+            function(lua, args)?.push_into_stack_multi(lua)
         }));
         self.meta_methods.push((name.as_ref().into(), func));
     }
@@ -756,8 +926,10 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> for NonStaticUserDataMethods<'l
         A: FromLuaMulti<'lua>,
         R: IntoLuaMulti<'lua>,
     {
-        let func = NonStaticMethod::FunctionMut(Box::new(move |lua, args| {
-            function(lua, A::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
+        let func_name = format!("{}.{}", short_type_name::<T>(), name.as_ref());
+        let func = NonStaticMethod::FunctionMut(Box::new(move |lua, nargs| unsafe {
+            let args = A::from_stack_args(nargs, 1, Some(&func_name), lua)?;
+            function(lua, args)?.push_into_stack_multi(lua)
         }));
         self.meta_methods.push((name.as_ref().into(), func));
     }
@@ -775,106 +947,3 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> for NonStaticUserDataMethods<'l
         panic!("asynchronous meta functions are not supported for non-static userdata")
     }
 }
-
-struct NonStaticUserDataFields<'lua, T: UserData> {
-    fields: Vec<(String, Callback<'lua, 'static>)>,
-    field_getters: Vec<(String, NonStaticMethod<'lua, T>)>,
-    field_setters: Vec<(String, NonStaticMethod<'lua, T>)>,
-    meta_fields: Vec<(String, Callback<'lua, 'static>)>,
-}
-
-impl<'lua, T: UserData> Default for NonStaticUserDataFields<'lua, T> {
-    fn default() -> NonStaticUserDataFields<'lua, T> {
-        NonStaticUserDataFields {
-            fields: Vec::new(),
-            field_getters: Vec::new(),
-            field_setters: Vec::new(),
-            meta_fields: Vec::new(),
-        }
-    }
-}
-
-impl<'lua, T: UserData> UserDataFields<'lua, T> for NonStaticUserDataFields<'lua, T> {
-    fn add_field<V>(&mut self, name: impl AsRef<str>, value: V)
-    where
-        V: IntoLua<'lua> + Clone + 'static,
-    {
-        let name = name.as_ref().to_string();
-        self.fields.push((
-            name,
-            Box::new(move |lua, _| value.clone().into_lua_multi(lua)),
-        ));
-    }
-
-    fn add_field_method_get<M, R>(&mut self, name: impl AsRef<str>, method: M)
-    where
-        M: Fn(&'lua Lua, &T) -> Result<R> + MaybeSend + 'static,
-        R: IntoLua<'lua>,
-    {
-        let method = NonStaticMethod::Method(Box::new(move |lua, ud, _| {
-            method(lua, ud)?.into_lua_multi(lua)
-        }));
-        self.field_getters.push((name.as_ref().into(), method));
-    }
-
-    fn add_field_method_set<M, A>(&mut self, name: impl AsRef<str>, mut method: M)
-    where
-        M: FnMut(&'lua Lua, &mut T, A) -> Result<()> + MaybeSend + 'static,
-        A: FromLua<'lua>,
-    {
-        let method = NonStaticMethod::MethodMut(Box::new(move |lua, ud, args| {
-            method(lua, ud, A::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
-        }));
-        self.field_setters.push((name.as_ref().into(), method));
-    }
-
-    fn add_field_function_get<F, R>(&mut self, name: impl AsRef<str>, function: F)
-    where
-        F: Fn(&'lua Lua, AnyUserData<'lua>) -> Result<R> + MaybeSend + 'static,
-        R: IntoLua<'lua>,
-    {
-        let func = NonStaticMethod::Function(Box::new(move |lua, args| {
-            function(lua, AnyUserData::from_lua_multi(args, lua)?)?.into_lua_multi(lua)
-        }));
-        self.field_getters.push((name.as_ref().into(), func));
-    }
-
-    fn add_field_function_set<F, A>(&mut self, name: impl AsRef<str>, mut function: F)
-    where
-        F: FnMut(&'lua Lua, AnyUserData<'lua>, A) -> Result<()> + MaybeSend + 'static,
-        A: FromLua<'lua>,
-    {
-        let func = NonStaticMethod::FunctionMut(Box::new(move |lua, args| {
-            let (ud, val) = <_>::from_lua_multi(args, lua)?;
-            function(lua, ud, val)?.into_lua_multi(lua)
-        }));
-        self.field_setters.push((name.as_ref().into(), func));
-    }
-
-    fn add_meta_field<V>(&mut self, name: impl AsRef<str>, value: V)
-    where
-        V: IntoLua<'lua> + Clone + 'static,
-    {
-        let name = name.as_ref().to_string();
-        let name2 = name.clone();
-        self.meta_fields.push((
-            name,
-            Box::new(move |lua, _| {
-                UserDataRegistry::<()>::check_meta_field(lua, &name2, value.clone())
-            }),
-        ));
-    }
-
-    fn add_meta_field_with<F, R>(&mut self, name: impl AsRef<str>, f: F)
-    where
-        F: Fn(&'lua Lua) -> Result<R> + MaybeSend + 'static,
-        R: IntoLua<'lua>,
-    {
-        let name = name.as_ref().to_string();
-        let name2 = name.clone();
-        self.meta_fields.push((
-            name,
-            Box::new(move |lua, _| UserDataRegistry::<()>::check_meta_field(lua, &name2, f(lua)?)),
-        ));
-    }
-}

src/serde/de.rs

@@ -2,6 +2,7 @@ use std::cell::RefCell;
 use std::convert::TryInto;
 use std::os::raw::c_void;
 use std::rc::Rc;
+use std::result::Result as StdResult;
 use std::string::String as StdString;
 
 use rustc_hash::FxHashSet;
@@ -24,14 +25,14 @@ pub struct Deserializer<'lua> {
 #[derive(Debug, Clone, Copy)]
 #[non_exhaustive]
 pub struct Options {
-    /// If true, an attempt to serialize types such as [`Thread`], [`UserData`], [`LightUserData`]
+    /// If true, an attempt to serialize types such as [`Function`], [`Thread`], [`LightUserData`]
     /// and [`Error`] will cause an error.
     /// Otherwise these types skipped when iterating or serialized as unit type.
     ///
     /// Default: **true**
     ///
+    /// [`Function`]: crate::Function
     /// [`Thread`]: crate::Thread
-    /// [`UserData`]: crate::UserData
     /// [`LightUserData`]: crate::LightUserData
     /// [`Error`]: crate::Error
     pub deny_unsupported_types: bool,
@@ -42,6 +43,11 @@ pub struct Options {
     ///
     /// Default: **true**
     pub deny_recursive_tables: bool,
+
+    /// If true, keys in tables will be iterated in sorted order.
+    ///
+    /// Default: **false**
+    pub sort_keys: bool,
 }
 
 impl Default for Options {
@@ -56,6 +62,7 @@ impl Options {
         Options {
             deny_unsupported_types: true,
             deny_recursive_tables: true,
+            sort_keys: false,
         }
     }
 
@@ -76,6 +83,15 @@ impl Options {
         self.deny_recursive_tables = enabled;
         self
     }
+
+    /// Sets [`sort_keys`] option.
+    ///
+    /// [`sort_keys`]: #structfield.sort_keys
+    #[must_use]
+    pub const fn sort_keys(mut self, enabled: bool) -> Self {
+        self.sort_keys = enabled;
+        self
+    }
 }
 
 impl<'lua> Deserializer<'lua> {
@@ -141,10 +157,8 @@ impl<'lua, 'de> serde::Deserializer<'de> for Deserializer<'lua> {
             | Value::LightUserData(_)
             | Value::Error(_) => {
                 if self.options.deny_unsupported_types {
-                    Err(de::Error::custom(format!(
-                        "unsupported value type `{}`",
-                        self.value.type_name()
-                    )))
+                    let msg = format!("unsupported value type `{}`", self.value.type_name());
+                    Err(de::Error::custom(msg))
                 } else {
                     visitor.visit_unit()
                 }
@@ -195,7 +209,9 @@ impl<'lua, 'de> serde::Deserializer<'de> for Deserializer<'lua> {
                         &"map with a single key",
                     ));
                 }
-                if check_value_if_skip(&value, self.options, &self.visited)? {
+                let skip = check_value_for_skip(&value, self.options, &self.visited)
+                    .map_err(|err| Error::DeserializeError(err.to_string()))?;
+                if skip {
                     return Err(de::Error::custom("bad enum value"));
                 }
 
@@ -235,7 +251,7 @@ impl<'lua, 'de> serde::Deserializer<'de> for Deserializer<'lua> {
             Value::Table(t) => {
                 let _guard = RecursionGuard::new(&t, &self.visited);
 
-                let len = t.raw_len() as usize;
+                let len = t.raw_len();
                 let mut deserializer = SeqDeserializer {
                     seq: t.sequence_values(),
                     options: self.options,
@@ -292,7 +308,7 @@ impl<'lua, 'de> serde::Deserializer<'de> for Deserializer<'lua> {
                 let _guard = RecursionGuard::new(&t, &self.visited);
 
                 let mut deserializer = MapDeserializer {
-                    pairs: t.pairs(),
+                    pairs: MapPairs::new(t, self.options.sort_keys)?,
                     value: None,
                     options: self.options,
                     visited: self.visited,
@@ -390,7 +406,9 @@ impl<'lua, 'de> de::SeqAccess<'de> for SeqDeserializer<'lua> {
             match self.seq.next() {
                 Some(value) => {
                     let value = value?;
-                    if check_value_if_skip(&value, self.options, &self.visited)? {
+                    let skip = check_value_for_skip(&value, self.options, &self.visited)
+                        .map_err(|err| Error::DeserializeError(err.to_string()))?;
+                    if skip {
                         continue;
                     }
                     let visited = Rc::clone(&self.visited);
@@ -443,8 +461,50 @@ impl<'de> de::SeqAccess<'de> for VecDeserializer {
     }
 }
 
+pub(crate) enum MapPairs<'lua> {
+    Iter(TablePairs<'lua, Value<'lua>, Value<'lua>>),
+    Vec(Vec<(Value<'lua>, Value<'lua>)>),
+}
+
+impl<'lua> MapPairs<'lua> {
+    pub(crate) fn new(t: Table<'lua>, sort_keys: bool) -> Result<Self> {
+        if sort_keys {
+            let mut pairs = t.pairs::<Value, Value>().collect::<Result<Vec<_>>>()?;
+            pairs.sort_by(|(a, _), (b, _)| b.cmp(a)); // reverse order as we pop values from the end
+            Ok(MapPairs::Vec(pairs))
+        } else {
+            Ok(MapPairs::Iter(t.pairs::<Value, Value>()))
+        }
+    }
+
+    pub(crate) fn count(self) -> usize {
+        match self {
+            MapPairs::Iter(iter) => iter.count(),
+            MapPairs::Vec(vec) => vec.len(),
+        }
+    }
+
+    pub(crate) fn size_hint(&self) -> (usize, Option<usize>) {
+        match self {
+            MapPairs::Iter(iter) => iter.size_hint(),
+            MapPairs::Vec(vec) => (vec.len(), Some(vec.len())),
+        }
+    }
+}
+
+impl<'lua> Iterator for MapPairs<'lua> {
+    type Item = Result<(Value<'lua>, Value<'lua>)>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self {
+            MapPairs::Iter(iter) => iter.next(),
+            MapPairs::Vec(vec) => vec.pop().map(Ok),
+        }
+    }
+}
+
 struct MapDeserializer<'lua> {
-    pairs: TablePairs<'lua, Value<'lua>, Value<'lua>>,
+    pairs: MapPairs<'lua>,
     value: Option<Value<'lua>>,
     options: Options,
     visited: Rc<RefCell<FxHashSet<*const c_void>>>,
@@ -462,9 +522,11 @@ impl<'lua, 'de> de::MapAccess<'de> for MapDeserializer<'lua> {
             match self.pairs.next() {
                 Some(item) => {
                     let (key, value) = item?;
-                    if check_value_if_skip(&key, self.options, &self.visited)?
-                        || check_value_if_skip(&value, self.options, &self.visited)?
-                    {
+                    let skip_key = check_value_for_skip(&key, self.options, &self.visited)
+                        .map_err(|err| Error::DeserializeError(err.to_string()))?;
+                    let skip_value = check_value_for_skip(&value, self.options, &self.visited)
+                        .map_err(|err| Error::DeserializeError(err.to_string()))?;
+                    if skip_key || skip_value {
                         continue;
                     }
                     self.processed += 1;
@@ -615,17 +677,17 @@ impl Drop for RecursionGuard {
 }
 
 // Checks `options` and decides should we emit an error or skip next element
-fn check_value_if_skip(
+pub(crate) fn check_value_for_skip(
     value: &Value,
     options: Options,
     visited: &RefCell<FxHashSet<*const c_void>>,
-) -> Result<bool> {
+) -> StdResult<bool, &'static str> {
     match value {
         Value::Table(table) => {
             let ptr = table.to_pointer();
             if visited.borrow().contains(&ptr) {
                 if options.deny_recursive_tables {
-                    return Err(de::Error::custom("recursive table detected"));
+                    return Err("recursive table detected");
                 }
                 return Ok(true); // skip
             }

src/table.rs

@@ -6,8 +6,8 @@ use std::os::raw::c_void;
 #[cfg(feature = "serialize")]
 use {
     rustc_hash::FxHashSet,
-    serde::ser::{self, Serialize, SerializeMap, SerializeSeq, Serializer},
-    std::{cell::RefCell, result::Result as StdResult},
+    serde::ser::{Serialize, SerializeMap, SerializeSeq, Serializer},
+    std::{cell::RefCell, rc::Rc, result::Result as StdResult},
 };
 
 use crate::error::{Error, Result};
@@ -86,17 +86,14 @@ impl<'lua> Table<'lua> {
         }
 
         let lua = self.0.lua;
-        let key = key.into_lua(lua)?;
-        let value = value.into_lua(lua)?;
-
         let state = lua.state();
         unsafe {
             let _sg = StackGuard::new(state);
             check_stack(state, 5)?;
 
             lua.push_ref(&self.0);
-            lua.push_value(key)?;
-            lua.push_value(value)?;
+            key.push_into_stack(lua)?;
+            value.push_into_stack(lua)?;
             protect_lua!(state, 3, 0, fn(state) ffi::lua_settable(state, -3))
         }
     }
@@ -133,19 +130,16 @@ impl<'lua> Table<'lua> {
 
         let lua = self.0.lua;
         let state = lua.state();
-        let key = key.into_lua(lua)?;
-
-        let value = unsafe {
+        unsafe {
             let _sg = StackGuard::new(state);
             check_stack(state, 4)?;
 
             lua.push_ref(&self.0);
-            lua.push_value(key)?;
+            key.push_into_stack(lua)?;
             protect_lua!(state, 2, 1, fn(state) ffi::lua_gettable(state, -2))?;
 
-            lua.pop_value()
-        };
-        V::from_lua(value, lua)
+            V::from_stack(-1, lua)
+        }
     }
 
     /// Checks whether the table contains a non-nil value for `key`.
@@ -166,13 +160,12 @@ impl<'lua> Table<'lua> {
 
         let lua = self.0.lua;
         let state = lua.state();
-        let value = value.into_lua(lua)?;
         unsafe {
             let _sg = StackGuard::new(state);
             check_stack(state, 4)?;
 
             lua.push_ref(&self.0);
-            lua.push_value(value)?;
+            value.push_into_stack(lua)?;
             protect_lua!(state, 2, 0, fn(state) {
                 let len = ffi::luaL_len(state, -2) as Integer;
                 ffi::lua_seti(state, -2, len + 1);
@@ -192,7 +185,7 @@ impl<'lua> Table<'lua> {
 
         let lua = self.0.lua;
         let state = lua.state();
-        let value = unsafe {
+        unsafe {
             let _sg = StackGuard::new(state);
             check_stack(state, 4)?;
 
@@ -203,9 +196,8 @@ impl<'lua> Table<'lua> {
                 ffi::lua_pushnil(state);
                 ffi::lua_seti(state, -3, len);
             })?;
-            lua.pop_value()
-        };
-        V::from_lua(value, lua)
+            V::from_stack(-1, lua)
+        }
     }
 
     /// Compares two tables for equality.
@@ -271,16 +263,13 @@ impl<'lua> Table<'lua> {
 
         let lua = self.0.lua;
         let state = lua.state();
-        let key = key.into_lua(lua)?;
-        let value = value.into_lua(lua)?;
-
         unsafe {
             let _sg = StackGuard::new(state);
             check_stack(state, 5)?;
 
             lua.push_ref(&self.0);
-            lua.push_value(key)?;
-            lua.push_value(value)?;
+            key.push_into_stack(lua)?;
+            value.push_into_stack(lua)?;
 
             if lua.unlikely_memory_error() {
                 ffi::lua_rawset(state, -3);
@@ -296,39 +285,34 @@ impl<'lua> Table<'lua> {
     pub fn raw_get<K: IntoLua<'lua>, V: FromLua<'lua>>(&self, key: K) -> Result<V> {
         let lua = self.0.lua;
         let state = lua.state();
-        let key = key.into_lua(lua)?;
-
-        let value = unsafe {
+        unsafe {
             let _sg = StackGuard::new(state);
             check_stack(state, 3)?;
 
             lua.push_ref(&self.0);
-            lua.push_value(key)?;
+            key.push_into_stack(lua)?;
             ffi::lua_rawget(state, -2);
 
-            lua.pop_value()
-        };
-        V::from_lua(value, lua)
+            V::from_stack(-1, lua)
+        }
     }
 
     /// Inserts element value at position `idx` to the table, shifting up the elements from `table[idx]`.
     /// The worst case complexity is O(n), where n is the table length.
     pub fn raw_insert<V: IntoLua<'lua>>(&self, idx: Integer, value: V) -> Result<()> {
-        let lua = self.0.lua;
-        let state = lua.state();
-
-        let size = self.raw_len();
+        let size = self.raw_len() as Integer;
         if idx < 1 || idx > size + 1 {
             return Err(Error::runtime("index out of bounds"));
         }
 
-        let value = value.into_lua(lua)?;
+        let lua = self.0.lua;
+        let state = lua.state();
         unsafe {
             let _sg = StackGuard::new(state);
             check_stack(state, 5)?;
 
             lua.push_ref(&self.0);
-            lua.push_value(value)?;
+            value.push_into_stack(lua)?;
             protect_lua!(state, 2, 0, |state| {
                 for i in (idx..=size).rev() {
                     // table[i+1] = table[i]
@@ -347,14 +331,12 @@ impl<'lua> Table<'lua> {
 
         let lua = self.0.lua;
         let state = lua.state();
-        let value = value.into_lua(lua)?;
-
         unsafe {
             let _sg = StackGuard::new(state);
             check_stack(state, 4)?;
 
             lua.push_ref(&self.0);
-            lua.push_value(value)?;
+            value.push_into_stack(lua)?;
 
             unsafe fn callback(state: *mut ffi::lua_State) {
                 let len = ffi::lua_rawlen(state, -2) as Integer;
@@ -377,7 +359,7 @@ impl<'lua> Table<'lua> {
 
         let lua = self.0.lua;
         let state = lua.state();
-        let value = unsafe {
+        unsafe {
             let _sg = StackGuard::new(state);
             check_stack(state, 3)?;
 
@@ -387,9 +369,9 @@ impl<'lua> Table<'lua> {
             // Set slot to nil (it must be safe to do)
             ffi::lua_pushnil(state);
             ffi::lua_rawseti(state, -3, len);
-            lua.pop_value()
-        };
-        V::from_lua(value, lua)
+
+            V::from_stack(-1, lua)
+        }
     }
 
     /// Removes a key from the table.
@@ -405,7 +387,7 @@ impl<'lua> Table<'lua> {
         let key = key.into_lua(lua)?;
         match key {
             Value::Integer(idx) => {
-                let size = self.raw_len();
+                let size = self.raw_len() as Integer;
                 if idx < 1 || idx > size {
                     return Err(Error::runtime("index out of bounds"));
                 }
@@ -477,7 +459,7 @@ impl<'lua> Table<'lua> {
     pub fn len(&self) -> Result<Integer> {
         // Fast track
         if !self.has_metatable() {
-            return Ok(self.raw_len());
+            return Ok(self.raw_len() as Integer);
         }
 
         let lua = self.0.lua;
@@ -492,9 +474,9 @@ impl<'lua> Table<'lua> {
     }
 
     /// Returns the result of the Lua `#` operator, without invoking the `__len` metamethod.
-    pub fn raw_len(&self) -> Integer {
+    pub fn raw_len(&self) -> usize {
         let ref_thread = self.0.lua.ref_thread();
-        unsafe { ffi::lua_rawlen(ref_thread, self.0.index) as Integer }
+        unsafe { ffi::lua_rawlen(ref_thread, self.0.index) }
     }
 
     /// Returns `true` if the table is empty, without invoking metamethods.
@@ -725,9 +707,9 @@ impl<'lua> Table<'lua> {
     #[cfg(feature = "serialize")]
     pub(crate) fn sequence_values_by_len<V: FromLua<'lua>>(
         self,
-        len: Option<Integer>,
+        len: Option<usize>,
     ) -> TableSequence<'lua, V> {
-        let len = len.unwrap_or_else(|| self.raw_len());
+        let len = len.unwrap_or_else(|| self.raw_len()) as Integer;
         TableSequence {
             table: self.0,
             index: Some(1),
@@ -744,14 +726,12 @@ impl<'lua> Table<'lua> {
 
         let lua = self.0.lua;
         let state = lua.state();
-        let value = value.into_lua(lua)?;
-
         unsafe {
             let _sg = StackGuard::new(state);
             check_stack(state, 5)?;
 
             lua.push_ref(&self.0);
-            lua.push_value(value)?;
+            value.push_into_stack(lua)?;
 
             let idx = idx.try_into().unwrap();
             if lua.unlikely_memory_error() {
@@ -759,8 +739,8 @@ impl<'lua> Table<'lua> {
             } else {
                 protect_lua!(state, 2, 0, |state| ffi::lua_rawseti(state, -2, idx))?;
             }
-            Ok(())
         }
+        Ok(())
     }
 
     #[cfg(feature = "serialize")]
@@ -1036,47 +1016,86 @@ impl<'lua> TableExt<'lua> for Table<'lua> {
     }
 }
 
+/// A wrapped [`Table`] with customized serialization behavior.
+#[cfg(feature = "serialize")]
+pub(crate) struct SerializableTable<'a, 'lua> {
+    table: &'a Table<'lua>,
+    options: crate::serde::de::Options,
+    visited: Rc<RefCell<FxHashSet<*const c_void>>>,
+}
+
 #[cfg(feature = "serialize")]
 impl<'lua> Serialize for Table<'lua> {
+    #[inline]
+    fn serialize<S: Serializer>(&self, serializer: S) -> StdResult<S::Ok, S::Error> {
+        SerializableTable::new(self, Default::default(), Default::default()).serialize(serializer)
+    }
+}
+
+#[cfg(feature = "serialize")]
+impl<'a, 'lua> SerializableTable<'a, 'lua> {
+    #[inline]
+    pub(crate) fn new(
+        table: &'a Table<'lua>,
+        options: crate::serde::de::Options,
+        visited: Rc<RefCell<FxHashSet<*const c_void>>>,
+    ) -> Self {
+        Self {
+            table,
+            options,
+            visited,
+        }
+    }
+}
+
+#[cfg(feature = "serialize")]
+impl<'a, 'lua> Serialize for SerializableTable<'a, 'lua> {
     fn serialize<S>(&self, serializer: S) -> StdResult<S::Ok, S::Error>
     where
         S: Serializer,
     {
-        thread_local! {
-            static VISITED: RefCell<FxHashSet<*const c_void>> = RefCell::new(FxHashSet::default());
+        use crate::serde::de::{check_value_for_skip, MapPairs};
+        use crate::value::SerializableValue;
+
+        let options = self.options;
+        let visited = &self.visited;
+        visited.borrow_mut().insert(self.table.to_pointer());
+
+        // Array
+        let len = self.table.raw_len();
+        if len > 0 || self.table.is_array() {
+            let mut seq = serializer.serialize_seq(Some(len))?;
+            for value in self.table.clone().sequence_values_by_len::<Value>(None) {
+                let value = &value.map_err(serde::ser::Error::custom)?;
+                let skip = check_value_for_skip(value, self.options, &self.visited)
+                    .map_err(serde::ser::Error::custom)?;
+                if skip {
+                    continue;
+                }
+                seq.serialize_element(&SerializableValue::new(value, options, Some(visited)))?;
+            }
+            return seq.end();
         }
 
-        let ptr = self.to_pointer();
-        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);
+        // HashMap
+        let mut map = serializer.serialize_map(None)?;
+        let pairs = MapPairs::new(self.table.clone(), self.options.sort_keys)
+            .map_err(serde::ser::Error::custom)?;
+        for kv in pairs {
+            let (key, value) = kv.map_err(serde::ser::Error::custom)?;
+            let skip_key = check_value_for_skip(&key, self.options, &self.visited)
+                .map_err(serde::ser::Error::custom)?;
+            let skip_value = check_value_for_skip(&value, self.options, &self.visited)
+                .map_err(serde::ser::Error::custom)?;
+            if skip_key || skip_value {
+                continue;
             }
-
-            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().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
+            map.serialize_entry(
+                &SerializableValue::new(&key, options, Some(visited)),
+                &SerializableValue::new(&value, options, Some(visited)),
+            )?;
+        }
+        map.end()
     }
 }
 

src/thread.rs

@@ -1,4 +1,3 @@
-use std::cmp;
 use std::os::raw::c_int;
 
 use crate::error::{Error, Result};
@@ -16,10 +15,7 @@ use crate::{
 
 #[cfg(feature = "async")]
 use {
-    crate::{
-        lua::ASYNC_POLL_PENDING,
-        value::{MultiValue, Value},
-    },
+    crate::{lua::ASYNC_POLL_PENDING, value::MultiValue},
     futures_util::stream::Stream,
     std::{
         future::Future,
@@ -30,7 +26,7 @@ use {
     },
 };
 
-/// Status of a Lua thread (or coroutine).
+/// Status of a Lua thread (coroutine).
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub enum ThreadStatus {
     /// The thread was just created, or is suspended because it has called `coroutine.yield`.
@@ -45,9 +41,33 @@ pub enum ThreadStatus {
     Error,
 }
 
-/// Handle to an internal Lua thread (or coroutine).
+/// Handle to an internal Lua thread (coroutine).
 #[derive(Clone, Debug)]
-pub struct Thread<'lua>(pub(crate) LuaRef<'lua>);
+pub struct Thread<'lua>(pub(crate) LuaRef<'lua>, pub(crate) *mut ffi::lua_State);
+
+/// Owned handle to an internal Lua thread (coroutine).
+///
+/// The owned handle holds a *strong* reference to the current Lua instance.
+/// Be warned, if you place it into a Lua type (eg. [`UserData`] or a Rust callback), it is *very easy*
+/// to accidentally cause reference cycles that would prevent destroying Lua instance.
+///
+/// [`UserData`]: crate::UserData
+#[cfg(feature = "unstable")]
+#[cfg_attr(docsrs, doc(cfg(feature = "unstable")))]
+#[derive(Clone, Debug)]
+pub struct OwnedThread(
+    pub(crate) crate::types::LuaOwnedRef,
+    pub(crate) *mut ffi::lua_State,
+);
+
+#[cfg(feature = "unstable")]
+impl OwnedThread {
+    /// Get borrowed handle to the underlying Lua table.
+    #[cfg_attr(feature = "send", allow(unused))]
+    pub const fn to_ref(&self) -> Thread {
+        Thread(self.0.to_ref(), self.1)
+    }
+}
 
 /// Thread (coroutine) representation as an async [`Future`] or [`Stream`].
 ///
@@ -66,6 +86,16 @@ pub struct AsyncThread<'lua, R> {
 }
 
 impl<'lua> Thread<'lua> {
+    #[inline(always)]
+    pub(crate) fn new(r#ref: LuaRef<'lua>) -> Self {
+        let state = unsafe { ffi::lua_tothread(r#ref.lua.ref_thread(), r#ref.index) };
+        Thread(r#ref, state)
+    }
+
+    const fn state(&self) -> *mut ffi::lua_State {
+        self.1
+    }
+
     /// Resumes execution of this thread.
     ///
     /// Equivalent to `coroutine.resume`.
@@ -114,60 +144,59 @@ impl<'lua> Thread<'lua> {
     {
         let lua = self.0.lua;
         let state = lua.state();
-
-        let mut args = args.into_lua_multi(lua)?;
-        let nargs = args.len() as c_int;
-        let results = unsafe {
+        let thread_state = self.state();
+        unsafe {
             let _sg = StackGuard::new(state);
-            check_stack(state, cmp::max(nargs + 1, 3))?;
+            let _thread_sg = StackGuard::with_top(thread_state, 0);
 
-            let thread_state = ffi::lua_tothread(lua.ref_thread(), self.0.index);
-
-            let status = ffi::lua_status(thread_state);
-            if status != ffi::LUA_YIELD && ffi::lua_gettop(thread_state) == 0 {
-                return Err(Error::CoroutineInactive);
-            }
-
-            check_stack(thread_state, nargs)?;
-            for arg in args.drain_all() {
-                lua.push_value(arg)?;
-            }
-            ffi::lua_xmove(state, thread_state, nargs);
-
-            let mut nresults = 0;
-
-            let ret = ffi::lua_resume(thread_state, state, nargs, &mut nresults as *mut c_int);
-            if ret != ffi::LUA_OK && ret != ffi::LUA_YIELD {
-                if ret == ffi::LUA_ERRMEM {
-                    // Don't call error handler for memory errors
-                    return Err(pop_error(thread_state, ret));
-                }
-                check_stack(state, 3)?;
-                protect_lua!(state, 0, 1, |state| error_traceback_thread(
-                    state,
-                    thread_state
-                ))?;
-                return Err(pop_error(state, ret));
-            }
-
-            let mut results = args; // Reuse MultiValue container
-            check_stack(state, nresults + 2)?; // 2 is extra for `lua.pop_value()` below
+            let nresults = self.resume_inner(args)?;
+            check_stack(state, nresults + 1)?;
             ffi::lua_xmove(thread_state, state, nresults);
 
-            for _ in 0..nresults {
-                results.push_front(lua.pop_value());
+            R::from_stack_multi(nresults, lua)
+        }
+    }
+
+    /// Resumes execution of this thread.
+    ///
+    /// It's similar to `resume()` but leaves `nresults` values on the thread stack.
+    unsafe fn resume_inner<A: IntoLuaMulti<'lua>>(&self, args: A) -> Result<c_int> {
+        let lua = self.0.lua;
+        let state = lua.state();
+        let thread_state = self.state();
+
+        if self.status() != ThreadStatus::Resumable {
+            return Err(Error::CoroutineInactive);
+        }
+
+        let nargs = args.push_into_stack_multi(lua)?;
+        if nargs > 0 {
+            check_stack(thread_state, nargs)?;
+            ffi::lua_xmove(state, thread_state, nargs);
+        }
+
+        let mut nresults = 0;
+        let ret = ffi::lua_resume(thread_state, state, nargs, &mut nresults as *mut c_int);
+        if ret != ffi::LUA_OK && ret != ffi::LUA_YIELD {
+            if ret == ffi::LUA_ERRMEM {
+                // Don't call error handler for memory errors
+                return Err(pop_error(thread_state, ret));
             }
-            results
-        };
-        R::from_lua_multi(results, lua)
+            check_stack(state, 3)?;
+            protect_lua!(state, 0, 1, |state| error_traceback_thread(
+                state,
+                thread_state
+            ))?;
+            return Err(pop_error(state, ret));
+        }
+
+        Ok(nresults)
     }
 
     /// Gets the status of the thread.
     pub fn status(&self) -> ThreadStatus {
-        let lua = self.0.lua;
+        let thread_state = self.state();
         unsafe {
-            let thread_state = ffi::lua_tothread(lua.ref_thread(), self.0.index);
-
             let status = ffi::lua_status(thread_state);
             if status != ffi::LUA_OK && status != ffi::LUA_YIELD {
                 ThreadStatus::Error
@@ -191,8 +220,7 @@ impl<'lua> Thread<'lua> {
     {
         let lua = self.0.lua;
         unsafe {
-            let thread_state = ffi::lua_tothread(lua.ref_thread(), self.0.index);
-            lua.set_thread_hook(thread_state, triggers, callback);
+            lua.set_thread_hook(self.state(), triggers, callback);
         }
     }
 
@@ -214,18 +242,12 @@ impl<'lua> Thread<'lua> {
     #[cfg_attr(docsrs, doc(cfg(not(feature = "luau"))))]
     pub fn reset(&self, func: crate::function::Function<'lua>) -> Result<()> {
         let lua = self.0.lua;
-        let state = lua.state();
+        let thread_state = self.state();
         unsafe {
-            let _sg = StackGuard::new(state);
-            check_stack(state, 2)?;
-
-            lua.push_ref(&self.0);
-            let thread_state = ffi::lua_tothread(state, -1);
-
             #[cfg(all(feature = "lua54", not(feature = "vendored")))]
             let status = ffi::lua_resetthread(thread_state);
             #[cfg(all(feature = "lua54", feature = "vendored"))]
-            let status = ffi::lua_closethread(thread_state, state);
+            let status = ffi::lua_closethread(thread_state, lua.state());
             #[cfg(feature = "lua54")]
             if status != ffi::LUA_OK {
                 return Err(pop_error(thread_state, status));
@@ -233,8 +255,8 @@ impl<'lua> Thread<'lua> {
             #[cfg(feature = "luau")]
             ffi::lua_resetthread(thread_state);
 
-            lua.push_ref(&func.0);
-            ffi::lua_xmove(state, thread_state, 1);
+            // Push function to the top of the thread stack
+            ffi::lua_xpush(lua.ref_thread(), thread_state, func.0.index);
 
             #[cfg(feature = "luau")]
             {
@@ -345,13 +367,21 @@ impl<'lua> Thread<'lua> {
     pub fn sandbox(&self) -> Result<()> {
         let lua = self.0.lua;
         let state = lua.state();
+        let thread_state = self.state();
         unsafe {
-            let thread = ffi::lua_tothread(lua.ref_thread(), self.0.index);
-            check_stack(thread, 3)?;
+            check_stack(thread_state, 3)?;
             check_stack(state, 3)?;
-            protect_lua!(state, 0, 0, |_| ffi::luaL_sandboxthread(thread))
+            protect_lua!(state, 0, 0, |_| ffi::luaL_sandboxthread(thread_state))
         }
     }
+
+    /// Convert this handle to owned version.
+    #[cfg(all(feature = "unstable", any(not(feature = "send"), doc)))]
+    #[cfg_attr(docsrs, doc(cfg(all(feature = "unstable", not(feature = "send")))))]
+    #[inline]
+    pub fn into_owned(self) -> OwnedThread {
+        OwnedThread(self.0.into_owned(), self.1)
+    }
 }
 
 impl<'lua> PartialEq for Thread<'lua> {
@@ -360,6 +390,26 @@ impl<'lua> PartialEq for Thread<'lua> {
     }
 }
 
+// Additional shortcuts
+#[cfg(feature = "unstable")]
+impl OwnedThread {
+    /// Resumes execution of this thread.
+    ///
+    /// See [`Thread::resume()`] for more details.
+    pub fn resume<'lua, A, R>(&'lua self, args: A) -> Result<R>
+    where
+        A: IntoLuaMulti<'lua>,
+        R: FromLuaMulti<'lua>,
+    {
+        self.to_ref().resume(args)
+    }
+
+    /// Gets the status of the thread.
+    pub fn status(&self) -> ThreadStatus {
+        self.to_ref().status()
+    }
+}
+
 #[cfg(feature = "async")]
 impl<'lua, R> AsyncThread<'lua, R> {
     #[inline]
@@ -379,11 +429,10 @@ impl<'lua, R> Drop for AsyncThread<'lua, R> {
                 if !lua.recycle_thread(&mut self.thread) {
                     #[cfg(feature = "lua54")]
                     if self.thread.status() == ThreadStatus::Error {
-                        let thread_state = ffi::lua_tothread(lua.ref_thread(), self.thread.0.index);
                         #[cfg(not(feature = "vendored"))]
-                        ffi::lua_resetthread(thread_state);
+                        ffi::lua_resetthread(self.thread.state());
                         #[cfg(feature = "vendored")]
-                        ffi::lua_closethread(thread_state, lua.state());
+                        ffi::lua_closethread(self.thread.state(), lua.state());
                     }
                 }
             }
@@ -399,29 +448,36 @@ where
     type Item = Result<R>;
 
     fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
-        let lua = self.thread.0.lua;
-
-        match self.thread.status() {
-            ThreadStatus::Resumable => {}
-            _ => return Poll::Ready(None),
-        };
-
-        let _wg = WakerGuard::new(lua, cx.waker());
-
-        // This is safe as we are not moving the whole struct
-        let this = unsafe { self.get_unchecked_mut() };
-        let ret: MultiValue = if let Some(args) = this.init_args.take() {
-            this.thread.resume(args?)?
-        } else {
-            this.thread.resume(())?
-        };
-
-        if is_poll_pending(&ret) {
-            return Poll::Pending;
+        if self.thread.status() != ThreadStatus::Resumable {
+            return Poll::Ready(None);
         }
 
-        cx.waker().wake_by_ref();
-        Poll::Ready(Some(R::from_lua_multi(ret, lua)))
+        let lua = self.thread.0.lua;
+        let state = lua.state();
+        let thread_state = self.thread.state();
+        unsafe {
+            let _sg = StackGuard::new(state);
+            let _thread_sg = StackGuard::with_top(thread_state, 0);
+            let _wg = WakerGuard::new(lua, cx.waker());
+
+            // This is safe as we are not moving the whole struct
+            let this = self.get_unchecked_mut();
+            let nresults = if let Some(args) = this.init_args.take() {
+                this.thread.resume_inner(args?)?
+            } else {
+                this.thread.resume_inner(())?
+            };
+
+            if nresults == 1 && is_poll_pending(thread_state) {
+                return Poll::Pending;
+            }
+
+            check_stack(state, nresults + 1)?;
+            ffi::lua_xmove(thread_state, state, nresults);
+
+            cx.waker().wake_by_ref();
+            Poll::Ready(Some(R::from_stack_multi(nresults, lua)))
+        }
     }
 }
 
@@ -433,46 +489,53 @@ where
     type Output = Result<R>;
 
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        if self.thread.status() != ThreadStatus::Resumable {
+            return Poll::Ready(Err(Error::CoroutineInactive));
+        }
+
         let lua = self.thread.0.lua;
+        let state = lua.state();
+        let thread_state = self.thread.state();
+        unsafe {
+            let _sg = StackGuard::new(state);
+            let _thread_sg = StackGuard::with_top(thread_state, 0);
+            let _wg = WakerGuard::new(lua, cx.waker());
 
-        match self.thread.status() {
-            ThreadStatus::Resumable => {}
-            _ => return Poll::Ready(Err(Error::CoroutineInactive)),
-        };
+            // This is safe as we are not moving the whole struct
+            let this = self.get_unchecked_mut();
+            let nresults = if let Some(args) = this.init_args.take() {
+                this.thread.resume_inner(args?)?
+            } else {
+                this.thread.resume_inner(())?
+            };
 
-        let _wg = WakerGuard::new(lua, cx.waker());
+            if nresults == 1 && is_poll_pending(thread_state) {
+                return Poll::Pending;
+            }
 
-        // This is safe as we are not moving the whole struct
-        let this = unsafe { self.get_unchecked_mut() };
-        let ret: MultiValue = if let Some(args) = this.init_args.take() {
-            this.thread.resume(args?)?
-        } else {
-            this.thread.resume(())?
-        };
+            if ffi::lua_status(thread_state) == ffi::LUA_YIELD {
+                // Ignore value returned via yield()
+                cx.waker().wake_by_ref();
+                return Poll::Pending;
+            }
 
-        if is_poll_pending(&ret) {
-            return Poll::Pending;
+            check_stack(state, nresults + 1)?;
+            ffi::lua_xmove(thread_state, state, nresults);
+
+            Poll::Ready(R::from_stack_multi(nresults, lua))
         }
-
-        if let ThreadStatus::Resumable = this.thread.status() {
-            // Ignore value returned via yield()
-            cx.waker().wake_by_ref();
-            return Poll::Pending;
-        }
-
-        Poll::Ready(R::from_lua_multi(ret, lua))
     }
 }
 
 #[cfg(feature = "async")]
 #[inline(always)]
-fn is_poll_pending(val: &MultiValue) -> bool {
-    match val.iter().enumerate().last() {
-        Some((0, Value::LightUserData(ud))) => {
-            std::ptr::eq(ud.0 as *const u8, &ASYNC_POLL_PENDING as *const u8)
-        }
-        _ => false,
+unsafe fn is_poll_pending(state: *mut ffi::lua_State) -> bool {
+    if ffi::lua_islightuserdata(state, -1) != 0 {
+        let stack_ptr = ffi::lua_touserdata(state, -1) as *const u8;
+        let pending_ptr = &ASYNC_POLL_PENDING as *const u8;
+        return std::ptr::eq(stack_ptr, pending_ptr);
     }
+    false
 }
 
 #[cfg(feature = "async")]
@@ -486,23 +549,19 @@ struct WakerGuard<'lua, 'a> {
 impl<'lua, 'a> WakerGuard<'lua, 'a> {
     #[inline]
     pub fn new(lua: &'lua Lua, waker: &'a Waker) -> Result<WakerGuard<'lua, 'a>> {
-        unsafe {
-            let prev = lua.set_waker(NonNull::from(waker));
-            Ok(WakerGuard {
-                lua,
-                prev,
-                _phantom: PhantomData,
-            })
-        }
+        let prev = unsafe { lua.set_waker(NonNull::from(waker)) };
+        Ok(WakerGuard {
+            lua,
+            prev,
+            _phantom: PhantomData,
+        })
     }
 }
 
 #[cfg(feature = "async")]
 impl<'lua, 'a> Drop for WakerGuard<'lua, 'a> {
     fn drop(&mut self) {
-        unsafe {
-            self.lua.set_waker(self.prev);
-        }
+        unsafe { self.lua.set_waker(self.prev) };
     }
 }
 

src/types.rs

@@ -10,14 +10,13 @@ use std::{fmt, mem, ptr};
 
 use rustc_hash::FxHashMap;
 
-#[cfg(feature = "async")]
-use futures_util::future::LocalBoxFuture;
-
 use crate::error::Result;
 #[cfg(not(feature = "luau"))]
 use crate::hook::Debug;
 use crate::lua::{ExtraData, Lua};
-use crate::value::MultiValue;
+
+#[cfg(feature = "async")]
+use {crate::value::MultiValue, futures_util::future::LocalBoxFuture};
 
 #[cfg(feature = "unstable")]
 use {crate::lua::LuaInner, std::marker::PhantomData};
@@ -34,8 +33,7 @@ pub type Number = ffi::lua_Number;
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub struct LightUserData(pub *mut c_void);
 
-pub(crate) type Callback<'lua, 'a> =
-    Box<dyn Fn(&'lua Lua, MultiValue<'lua>) -> Result<MultiValue<'lua>> + 'a>;
+pub(crate) type Callback<'lua, 'a> = Box<dyn Fn(&'lua Lua, c_int) -> Result<c_int> + 'a>;
 
 pub(crate) struct Upvalue<T> {
     pub(crate) data: T,
@@ -46,13 +44,13 @@ pub(crate) type CallbackUpvalue = Upvalue<Callback<'static, 'static>>;
 
 #[cfg(feature = "async")]
 pub(crate) type AsyncCallback<'lua, 'a> =
-    Box<dyn Fn(&'lua Lua, MultiValue<'lua>) -> LocalBoxFuture<'lua, Result<MultiValue<'lua>>> + 'a>;
+    Box<dyn Fn(&'lua Lua, MultiValue<'lua>) -> LocalBoxFuture<'lua, Result<c_int>> + 'a>;
 
 #[cfg(feature = "async")]
 pub(crate) type AsyncCallbackUpvalue = Upvalue<AsyncCallback<'static, 'static>>;
 
 #[cfg(feature = "async")]
-pub(crate) type AsyncPollUpvalue = Upvalue<LocalBoxFuture<'static, Result<MultiValue<'static>>>>;
+pub(crate) type AsyncPollUpvalue = Upvalue<LocalBoxFuture<'static, Result<c_int>>>;
 
 /// Type to set next Luau VM action after executing interrupt function.
 #[cfg(any(feature = "luau", doc))]

src/userdata.rs

@@ -1104,9 +1104,10 @@ impl<'lua> AnyUserData<'lua> {
         OwnedAnyUserData(self.0.into_owned())
     }
 
-    #[inline(always)]
+    #[cfg(feature = "async")]
+    #[inline]
     pub(crate) fn type_id(&self) -> Result<Option<TypeId>> {
-        unsafe { self.0.lua.get_userdata_type_id(&self.0) }
+        unsafe { self.0.lua.get_userdata_ref_type_id(&self.0) }
     }
 
     /// Returns a type name of this `UserData` (from a metatable field).
@@ -1154,13 +1155,13 @@ impl<'lua> AnyUserData<'lua> {
         Ok(false)
     }
 
-    /// Returns true if this `AnyUserData` is serializable (eg. was created using `create_ser_userdata`).
+    /// Returns `true` if this `AnyUserData` is serializable (eg. was created using `create_ser_userdata`).
     #[cfg(feature = "serialize")]
     pub(crate) fn is_serializable(&self) -> bool {
         let lua = self.0.lua;
         let is_serializable = || unsafe {
             // Userdata can be unregistered or destructed
-            let _ = lua.get_userdata_type_id(&self.0)?;
+            let _ = lua.get_userdata_ref_type_id(&self.0)?;
 
             let ud = &*get_userdata::<UserDataCell<()>>(lua.ref_thread(), self.0.index);
             match &*ud.0.try_borrow().map_err(|_| Error::UserDataBorrowError)? {
@@ -1178,7 +1179,7 @@ impl<'lua> AnyUserData<'lua> {
     {
         let lua = self.0.lua;
         unsafe {
-            let type_id = lua.get_userdata_type_id(&self.0)?;
+            let type_id = lua.get_userdata_ref_type_id(&self.0)?;
             match type_id {
                 Some(type_id) if type_id == TypeId::of::<T>() => {
                     let ref_thread = lua.ref_thread();
@@ -1327,7 +1328,7 @@ impl<'lua> Serialize for AnyUserData<'lua> {
         let lua = self.0.lua;
         let data = unsafe {
             let _ = lua
-                .get_userdata_type_id(&self.0)
+                .get_userdata_ref_type_id(&self.0)
                 .map_err(ser::Error::custom)?;
             let ud = &*get_userdata::<UserDataCell<()>>(lua.ref_thread(), self.0.index);
             ud.0.try_borrow()

src/userdata_impl.rs

@@ -14,17 +14,13 @@ use crate::userdata::{
     AnyUserData, MetaMethod, UserData, UserDataCell, UserDataFields, UserDataMethods,
 };
 use crate::util::{get_userdata, short_type_name};
-use crate::value::{FromLua, FromLuaMulti, IntoLua, IntoLuaMulti, MultiValue, Value};
+use crate::value::{FromLua, FromLuaMulti, IntoLua, IntoLuaMulti, Value};
 
 #[cfg(not(feature = "send"))]
 use std::rc::Rc;
 
 #[cfg(feature = "async")]
-use {
-    crate::types::AsyncCallback,
-    futures_util::future::{self, TryFutureExt},
-    std::future::Future,
-};
+use {crate::types::AsyncCallback, futures_util::future, std::future::Future};
 
 /// Handle to registry for userdata methods and metamethods.
 pub struct UserDataRegistry<'lua, T: 'static> {
@@ -78,63 +74,61 @@ impl<'lua, T: 'static> UserDataRegistry<'lua, T> {
             };
         }
 
-        Box::new(move |lua, mut args| {
-            let front = args
-                .pop_front()
-                .ok_or_else(|| Error::from_lua_conversion("missing argument", "userdata", None));
-            let front = try_self_arg!(front);
-            let call = |ud| {
-                // Self was at index 1, so we pass 2 here
-                let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                method(lua, ud, args)?.into_lua_multi(lua)
-            };
+        Box::new(move |lua, nargs| unsafe {
+            if nargs == 0 {
+                let err = Error::from_lua_conversion("missing argument", "userdata", None);
+                try_self_arg!(Err(err));
+            }
+            let state = lua.state();
+            // Find absolute "self" index before processing args
+            let index = ffi::lua_absindex(state, -nargs);
+            // Self was at position 1, so we pass 2 here
+            let args = A::from_stack_args(nargs - 1, 2, Some(&name), lua);
 
-            let userdata = try_self_arg!(AnyUserData::from_lua(front, lua));
-            let (ref_thread, index) = (lua.ref_thread(), userdata.0.index);
-            match try_self_arg!(userdata.type_id()) {
-                Some(id) if id == TypeId::of::<T>() => unsafe {
-                    let ud = try_self_arg!(get_userdata_ref::<T>(ref_thread, index));
-                    call(&ud)
-                },
+            match try_self_arg!(lua.get_userdata_type_id(index)) {
+                Some(id) if id == TypeId::of::<T>() => {
+                    let ud = try_self_arg!(get_userdata_ref::<T>(state, index));
+                    method(lua, &ud, args?)?.push_into_stack_multi(lua)
+                }
                 #[cfg(not(feature = "send"))]
-                Some(id) if id == TypeId::of::<Rc<T>>() => unsafe {
-                    let ud = try_self_arg!(get_userdata_ref::<Rc<T>>(ref_thread, index));
-                    call(&ud)
-                },
+                Some(id) if id == TypeId::of::<Rc<T>>() => {
+                    let ud = try_self_arg!(get_userdata_ref::<Rc<T>>(state, index));
+                    method(lua, &ud, args?)?.push_into_stack_multi(lua)
+                }
                 #[cfg(not(feature = "send"))]
-                Some(id) if id == TypeId::of::<Rc<RefCell<T>>>() => unsafe {
-                    let ud = try_self_arg!(get_userdata_ref::<Rc<RefCell<T>>>(ref_thread, index));
+                Some(id) if id == TypeId::of::<Rc<RefCell<T>>>() => {
+                    let ud = try_self_arg!(get_userdata_ref::<Rc<RefCell<T>>>(state, index));
                     let ud = try_self_arg!(ud.try_borrow(), Error::UserDataBorrowError);
-                    call(&ud)
-                },
-                Some(id) if id == TypeId::of::<Arc<T>>() => unsafe {
-                    let ud = try_self_arg!(get_userdata_ref::<Arc<T>>(ref_thread, index));
-                    call(&ud)
-                },
-                Some(id) if id == TypeId::of::<Arc<Mutex<T>>>() => unsafe {
-                    let ud = try_self_arg!(get_userdata_ref::<Arc<Mutex<T>>>(ref_thread, index));
+                    method(lua, &ud, args?)?.push_into_stack_multi(lua)
+                }
+                Some(id) if id == TypeId::of::<Arc<T>>() => {
+                    let ud = try_self_arg!(get_userdata_ref::<Arc<T>>(state, index));
+                    method(lua, &ud, args?)?.push_into_stack_multi(lua)
+                }
+                Some(id) if id == TypeId::of::<Arc<Mutex<T>>>() => {
+                    let ud = try_self_arg!(get_userdata_ref::<Arc<Mutex<T>>>(state, index));
                     let ud = try_self_arg!(ud.try_lock(), Error::UserDataBorrowError);
-                    call(&ud)
-                },
+                    method(lua, &ud, args?)?.push_into_stack_multi(lua)
+                }
                 #[cfg(feature = "parking_lot")]
-                Some(id) if id == TypeId::of::<Arc<parking_lot::Mutex<T>>>() => unsafe {
-                    let ud = get_userdata_ref::<Arc<parking_lot::Mutex<T>>>(ref_thread, index);
+                Some(id) if id == TypeId::of::<Arc<parking_lot::Mutex<T>>>() => {
+                    let ud = get_userdata_ref::<Arc<parking_lot::Mutex<T>>>(state, index);
                     let ud = try_self_arg!(ud);
                     let ud = try_self_arg!(ud.try_lock().ok_or(Error::UserDataBorrowError));
-                    call(&ud)
-                },
-                Some(id) if id == TypeId::of::<Arc<RwLock<T>>>() => unsafe {
-                    let ud = try_self_arg!(get_userdata_ref::<Arc<RwLock<T>>>(ref_thread, index));
+                    method(lua, &ud, args?)?.push_into_stack_multi(lua)
+                }
+                Some(id) if id == TypeId::of::<Arc<RwLock<T>>>() => {
+                    let ud = try_self_arg!(get_userdata_ref::<Arc<RwLock<T>>>(state, index));
                     let ud = try_self_arg!(ud.try_read(), Error::UserDataBorrowError);
-                    call(&ud)
-                },
+                    method(lua, &ud, args?)?.push_into_stack_multi(lua)
+                }
                 #[cfg(feature = "parking_lot")]
-                Some(id) if id == TypeId::of::<Arc<parking_lot::RwLock<T>>>() => unsafe {
-                    let ud = get_userdata_ref::<Arc<parking_lot::RwLock<T>>>(ref_thread, index);
+                Some(id) if id == TypeId::of::<Arc<parking_lot::RwLock<T>>>() => {
+                    let ud = get_userdata_ref::<Arc<parking_lot::RwLock<T>>>(state, index);
                     let ud = try_self_arg!(ud);
                     let ud = try_self_arg!(ud.try_read().ok_or(Error::UserDataBorrowError));
-                    call(&ud)
-                },
+                    method(lua, &ud, args?)?.push_into_stack_multi(lua)
+                }
                 _ => Err(Error::bad_self_argument(&name, Error::UserDataTypeMismatch)),
             }
         })
@@ -157,60 +151,58 @@ impl<'lua, T: 'static> UserDataRegistry<'lua, T> {
         }
 
         let method = RefCell::new(method);
-        Box::new(move |lua, mut args| {
+        Box::new(move |lua, nargs| unsafe {
             let mut method = method
                 .try_borrow_mut()
                 .map_err(|_| Error::RecursiveMutCallback)?;
-            let front = args
-                .pop_front()
-                .ok_or_else(|| Error::from_lua_conversion("missing argument", "userdata", None));
-            let front = try_self_arg!(front);
-            let call = |ud| {
-                // Self was at index 1, so we pass 2 here
-                let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                method(lua, ud, args)?.into_lua_multi(lua)
-            };
+            if nargs == 0 {
+                let err = Error::from_lua_conversion("missing argument", "userdata", None);
+                try_self_arg!(Err(err));
+            }
+            let state = lua.state();
+            // Find absolute "self" index before processing args
+            let index = ffi::lua_absindex(state, -nargs);
+            // Self was at position 1, so we pass 2 here
+            let args = A::from_stack_args(nargs - 1, 2, Some(&name), lua);
 
-            let userdata = try_self_arg!(AnyUserData::from_lua(front, lua));
-            let (ref_thread, index) = (lua.ref_thread(), userdata.0.index);
-            match try_self_arg!(userdata.type_id()) {
-                Some(id) if id == TypeId::of::<T>() => unsafe {
-                    let mut ud = try_self_arg!(get_userdata_mut::<T>(ref_thread, index));
-                    call(&mut ud)
-                },
+            match try_self_arg!(lua.get_userdata_type_id(index)) {
+                Some(id) if id == TypeId::of::<T>() => {
+                    let mut ud = try_self_arg!(get_userdata_mut::<T>(state, index));
+                    method(lua, &mut ud, args?)?.push_into_stack_multi(lua)
+                }
                 #[cfg(not(feature = "send"))]
                 Some(id) if id == TypeId::of::<Rc<T>>() => Err(Error::UserDataBorrowMutError),
                 #[cfg(not(feature = "send"))]
-                Some(id) if id == TypeId::of::<Rc<RefCell<T>>>() => unsafe {
-                    let ud = try_self_arg!(get_userdata_mut::<Rc<RefCell<T>>>(ref_thread, index));
+                Some(id) if id == TypeId::of::<Rc<RefCell<T>>>() => {
+                    let ud = try_self_arg!(get_userdata_mut::<Rc<RefCell<T>>>(state, index));
                     let mut ud = try_self_arg!(ud.try_borrow_mut(), Error::UserDataBorrowMutError);
-                    call(&mut ud)
-                },
+                    method(lua, &mut ud, args?)?.push_into_stack_multi(lua)
+                }
                 Some(id) if id == TypeId::of::<Arc<T>>() => Err(Error::UserDataBorrowMutError),
-                Some(id) if id == TypeId::of::<Arc<Mutex<T>>>() => unsafe {
-                    let ud = try_self_arg!(get_userdata_mut::<Arc<Mutex<T>>>(ref_thread, index));
+                Some(id) if id == TypeId::of::<Arc<Mutex<T>>>() => {
+                    let ud = try_self_arg!(get_userdata_mut::<Arc<Mutex<T>>>(state, index));
                     let mut ud = try_self_arg!(ud.try_lock(), Error::UserDataBorrowMutError);
-                    call(&mut ud)
-                },
+                    method(lua, &mut ud, args?)?.push_into_stack_multi(lua)
+                }
                 #[cfg(feature = "parking_lot")]
-                Some(id) if id == TypeId::of::<Arc<parking_lot::Mutex<T>>>() => unsafe {
-                    let ud = get_userdata_mut::<Arc<parking_lot::Mutex<T>>>(ref_thread, index);
+                Some(id) if id == TypeId::of::<Arc<parking_lot::Mutex<T>>>() => {
+                    let ud = get_userdata_mut::<Arc<parking_lot::Mutex<T>>>(state, index);
                     let ud = try_self_arg!(ud);
                     let mut ud = try_self_arg!(ud.try_lock().ok_or(Error::UserDataBorrowMutError));
-                    call(&mut ud)
-                },
-                Some(id) if id == TypeId::of::<Arc<RwLock<T>>>() => unsafe {
-                    let ud = try_self_arg!(get_userdata_mut::<Arc<RwLock<T>>>(ref_thread, index));
+                    method(lua, &mut ud, args?)?.push_into_stack_multi(lua)
+                }
+                Some(id) if id == TypeId::of::<Arc<RwLock<T>>>() => {
+                    let ud = try_self_arg!(get_userdata_mut::<Arc<RwLock<T>>>(state, index));
                     let mut ud = try_self_arg!(ud.try_write(), Error::UserDataBorrowMutError);
-                    call(&mut ud)
-                },
+                    method(lua, &mut ud, args?)?.push_into_stack_multi(lua)
+                }
                 #[cfg(feature = "parking_lot")]
-                Some(id) if id == TypeId::of::<Arc<parking_lot::RwLock<T>>>() => unsafe {
-                    let ud = get_userdata_mut::<Arc<parking_lot::RwLock<T>>>(ref_thread, index);
+                Some(id) if id == TypeId::of::<Arc<parking_lot::RwLock<T>>>() => {
+                    let ud = get_userdata_mut::<Arc<parking_lot::RwLock<T>>>(state, index);
                     let ud = try_self_arg!(ud);
                     let mut ud = try_self_arg!(ud.try_write().ok_or(Error::UserDataBorrowMutError));
-                    call(&mut ud)
-                },
+                    method(lua, &mut ud, args?)?.push_into_stack_multi(lua)
+                }
                 _ => Err(Error::bad_self_argument(&name, Error::UserDataTypeMismatch)),
             }
         })
@@ -229,7 +221,7 @@ impl<'lua, T: 'static> UserDataRegistry<'lua, T> {
         let name = get_function_name::<T>(name);
         let method = Arc::new(method);
 
-        Box::new(move |lua, mut args| {
+        Box::new(move |lua, mut args| unsafe {
             let name = name.clone();
             let method = method.clone();
             macro_rules! try_self_arg {
@@ -242,76 +234,68 @@ impl<'lua, T: 'static> UserDataRegistry<'lua, T> {
             }
 
             Box::pin(async move {
-                let front = args.pop_front().ok_or_else(|| {
+                let this = args.pop_front().ok_or_else(|| {
                     Error::from_lua_conversion("missing argument", "userdata", None)
                 });
-                let front = try_self_arg!(front);
-                let userdata: AnyUserData = try_self_arg!(AnyUserData::from_lua(front, lua));
-                let (ref_thread, index) = (lua.ref_thread(), userdata.0.index);
-                match try_self_arg!(userdata.type_id()) {
-                    Some(id) if id == TypeId::of::<T>() => unsafe {
+                let this = try_self_arg!(AnyUserData::from_lua(try_self_arg!(this), lua));
+                let args = A::from_lua_args(args, 2, Some(&name), lua);
+
+                let (ref_thread, index) = (lua.ref_thread(), this.0.index);
+                match try_self_arg!(this.type_id()) {
+                    Some(id) if id == TypeId::of::<T>() => {
                         let ud = try_self_arg!(get_userdata_ref::<T>(ref_thread, index));
                         let ud = std::mem::transmute::<&T, &T>(&ud);
-                        // Self was at index 1, so we pass 2 here
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
                     #[cfg(not(feature = "send"))]
-                    Some(id) if id == TypeId::of::<Rc<T>>() => unsafe {
+                    Some(id) if id == TypeId::of::<Rc<T>>() => {
                         let ud = try_self_arg!(get_userdata_ref::<Rc<T>>(ref_thread, index));
                         let ud = std::mem::transmute::<&T, &T>(&ud);
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
                     #[cfg(not(feature = "send"))]
-                    Some(id) if id == TypeId::of::<Rc<RefCell<T>>>() => unsafe {
+                    Some(id) if id == TypeId::of::<Rc<RefCell<T>>>() => {
                         let ud =
                             try_self_arg!(get_userdata_ref::<Rc<RefCell<T>>>(ref_thread, index));
                         let ud = try_self_arg!(ud.try_borrow(), Error::UserDataBorrowError);
                         let ud = std::mem::transmute::<&T, &T>(&ud);
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
-                    Some(id) if id == TypeId::of::<Arc<T>>() => unsafe {
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
+                    Some(id) if id == TypeId::of::<Arc<T>>() => {
                         let ud = try_self_arg!(get_userdata_ref::<Arc<T>>(ref_thread, index));
                         let ud = std::mem::transmute::<&T, &T>(&ud);
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
-                    Some(id) if id == TypeId::of::<Arc<Mutex<T>>>() => unsafe {
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
+                    Some(id) if id == TypeId::of::<Arc<Mutex<T>>>() => {
                         let ud =
                             try_self_arg!(get_userdata_ref::<Arc<Mutex<T>>>(ref_thread, index));
                         let ud = try_self_arg!(ud.try_lock(), Error::UserDataBorrowError);
                         let ud = std::mem::transmute::<&T, &T>(&ud);
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
                     #[cfg(feature = "parking_lot")]
-                    Some(id) if id == TypeId::of::<Arc<parking_lot::Mutex<T>>>() => unsafe {
+                    Some(id) if id == TypeId::of::<Arc<parking_lot::Mutex<T>>>() => {
                         let ud = get_userdata_ref::<Arc<parking_lot::Mutex<T>>>(ref_thread, index);
                         let ud = try_self_arg!(ud);
                         let ud = try_self_arg!(ud.try_lock().ok_or(Error::UserDataBorrowError));
                         let ud = std::mem::transmute::<&T, &T>(&ud);
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
-                    Some(id) if id == TypeId::of::<Arc<RwLock<T>>>() => unsafe {
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
+                    Some(id) if id == TypeId::of::<Arc<RwLock<T>>>() => {
                         let ud =
                             try_self_arg!(get_userdata_ref::<Arc<RwLock<T>>>(ref_thread, index));
                         let ud = try_self_arg!(ud.try_read(), Error::UserDataBorrowError);
                         let ud = std::mem::transmute::<&T, &T>(&ud);
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
                     #[cfg(feature = "parking_lot")]
-                    Some(id) if id == TypeId::of::<Arc<parking_lot::RwLock<T>>>() => unsafe {
+                    Some(id) if id == TypeId::of::<Arc<parking_lot::RwLock<T>>>() => {
                         let ud = get_userdata_ref::<Arc<parking_lot::RwLock<T>>>(ref_thread, index);
                         let ud = try_self_arg!(ud);
                         let ud = try_self_arg!(ud.try_read().ok_or(Error::UserDataBorrowError));
                         let ud = std::mem::transmute::<&T, &T>(&ud);
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
                     _ => Err(Error::bad_self_argument(&name, Error::UserDataTypeMismatch)),
                 }
             })
@@ -331,7 +315,7 @@ impl<'lua, T: 'static> UserDataRegistry<'lua, T> {
         let name = get_function_name::<T>(name);
         let method = Arc::new(method);
 
-        Box::new(move |lua, mut args| {
+        Box::new(move |lua, mut args| unsafe {
             let name = name.clone();
             let method = method.clone();
             macro_rules! try_self_arg {
@@ -344,72 +328,66 @@ impl<'lua, T: 'static> UserDataRegistry<'lua, T> {
             }
 
             Box::pin(async move {
-                let front = args.pop_front().ok_or_else(|| {
+                let this = args.pop_front().ok_or_else(|| {
                     Error::from_lua_conversion("missing argument", "userdata", None)
                 });
-                let front = try_self_arg!(front);
-                let userdata: AnyUserData = try_self_arg!(AnyUserData::from_lua(front, lua));
-                let (ref_thread, index) = (lua.ref_thread(), userdata.0.index);
-                match try_self_arg!(userdata.type_id()) {
-                    Some(id) if id == TypeId::of::<T>() => unsafe {
+                let this = try_self_arg!(AnyUserData::from_lua(try_self_arg!(this), lua));
+                let args = A::from_lua_args(args, 2, Some(&name), lua);
+
+                let (ref_thread, index) = (lua.ref_thread(), this.0.index);
+                match try_self_arg!(this.type_id()) {
+                    Some(id) if id == TypeId::of::<T>() => {
                         let mut ud = try_self_arg!(get_userdata_mut::<T>(ref_thread, index));
                         let ud = std::mem::transmute::<&mut T, &mut T>(&mut ud);
-                        // Self was at index 1, so we pass 2 here
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
                     #[cfg(not(feature = "send"))]
                     Some(id) if id == TypeId::of::<Rc<RefCell<T>>>() => {
                         Err(Error::UserDataBorrowMutError)
                     }
                     #[cfg(not(feature = "send"))]
-                    Some(id) if id == TypeId::of::<Rc<RefCell<T>>>() => unsafe {
+                    Some(id) if id == TypeId::of::<Rc<RefCell<T>>>() => {
                         let ud =
                             try_self_arg!(get_userdata_mut::<Rc<RefCell<T>>>(ref_thread, index));
                         let mut ud =
                             try_self_arg!(ud.try_borrow_mut(), Error::UserDataBorrowMutError);
                         let ud = std::mem::transmute::<&mut T, &mut T>(&mut ud);
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
                     #[cfg(not(feature = "send"))]
                     Some(id) if id == TypeId::of::<Arc<T>>() => Err(Error::UserDataBorrowMutError),
-                    Some(id) if id == TypeId::of::<Arc<Mutex<T>>>() => unsafe {
+                    Some(id) if id == TypeId::of::<Arc<Mutex<T>>>() => {
                         let ud =
                             try_self_arg!(get_userdata_mut::<Arc<Mutex<T>>>(ref_thread, index));
                         let mut ud = try_self_arg!(ud.try_lock(), Error::UserDataBorrowMutError);
                         let ud = std::mem::transmute::<&mut T, &mut T>(&mut ud);
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
                     #[cfg(feature = "parking_lot")]
-                    Some(id) if id == TypeId::of::<Arc<parking_lot::Mutex<T>>>() => unsafe {
+                    Some(id) if id == TypeId::of::<Arc<parking_lot::Mutex<T>>>() => {
                         let ud = get_userdata_mut::<Arc<parking_lot::Mutex<T>>>(ref_thread, index);
                         let ud = try_self_arg!(ud);
                         let mut ud =
                             try_self_arg!(ud.try_lock().ok_or(Error::UserDataBorrowMutError));
                         let ud = std::mem::transmute::<&mut T, &mut T>(&mut ud);
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
-                    Some(id) if id == TypeId::of::<Arc<RwLock<T>>>() => unsafe {
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
+                    Some(id) if id == TypeId::of::<Arc<RwLock<T>>>() => {
                         let ud =
                             try_self_arg!(get_userdata_mut::<Arc<RwLock<T>>>(ref_thread, index));
                         let mut ud = try_self_arg!(ud.try_write(), Error::UserDataBorrowMutError);
                         let ud = std::mem::transmute::<&mut T, &mut T>(&mut ud);
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
                     #[cfg(feature = "parking_lot")]
-                    Some(id) if id == TypeId::of::<Arc<parking_lot::RwLock<T>>>() => unsafe {
+                    Some(id) if id == TypeId::of::<Arc<parking_lot::RwLock<T>>>() => {
                         let ud = get_userdata_mut::<Arc<parking_lot::RwLock<T>>>(ref_thread, index);
                         let ud = try_self_arg!(ud);
                         let mut ud =
                             try_self_arg!(ud.try_write().ok_or(Error::UserDataBorrowMutError));
                         let ud = std::mem::transmute::<&mut T, &mut T>(&mut ud);
-                        let args = A::from_lua_multi_args(args, 2, Some(&name), lua)?;
-                        method(lua, ud, args).await?.into_lua_multi(lua)
-                    },
+                        method(lua, ud, args?).await?.push_into_stack_multi(lua)
+                    }
                     _ => Err(Error::bad_self_argument(&name, Error::UserDataTypeMismatch)),
                 }
             })
@@ -423,8 +401,9 @@ impl<'lua, T: 'static> UserDataRegistry<'lua, T> {
         R: IntoLuaMulti<'lua>,
     {
         let name = get_function_name::<T>(name);
-        Box::new(move |lua, args| {
-            function(lua, A::from_lua_multi_args(args, 1, Some(&name), lua)?)?.into_lua_multi(lua)
+        Box::new(move |lua, nargs| unsafe {
+            let args = A::from_stack_args(nargs, 1, Some(&name), lua)?;
+            function(lua, args)?.push_into_stack_multi(lua)
         })
     }
 
@@ -436,11 +415,12 @@ impl<'lua, T: 'static> UserDataRegistry<'lua, T> {
     {
         let name = get_function_name::<T>(name);
         let function = RefCell::new(function);
-        Box::new(move |lua, args| {
+        Box::new(move |lua, nargs| unsafe {
             let function = &mut *function
                 .try_borrow_mut()
                 .map_err(|_| Error::RecursiveMutCallback)?;
-            function(lua, A::from_lua_multi_args(args, 1, Some(&name), lua)?)?.into_lua_multi(lua)
+            let args = A::from_stack_args(nargs, 1, Some(&name), lua)?;
+            function(lua, args)?.push_into_stack_multi(lua)
         })
     }
 
@@ -453,22 +433,17 @@ impl<'lua, T: 'static> UserDataRegistry<'lua, T> {
         R: IntoLuaMulti<'lua>,
     {
         let name = get_function_name::<T>(name);
-        Box::new(move |lua, args| {
-            let args = match A::from_lua_multi_args(args, 1, Some(&name), lua) {
+        Box::new(move |lua, args| unsafe {
+            let args = match A::from_lua_args(args, 1, Some(&name), lua) {
                 Ok(args) => args,
                 Err(e) => return Box::pin(future::err(e)),
             };
-            Box::pin(
-                function(lua, args).and_then(move |ret| future::ready(ret.into_lua_multi(lua))),
-            )
+            let fut = function(lua, args);
+            Box::pin(async move { fut.await?.push_into_stack_multi(lua) })
         })
     }
 
-    pub(crate) fn check_meta_field<V>(
-        lua: &'lua Lua,
-        name: &str,
-        value: V,
-    ) -> Result<MultiValue<'lua>>
+    pub(crate) fn check_meta_field<V>(lua: &'lua Lua, name: &str, value: V) -> Result<Value<'lua>>
     where
         V: IntoLua<'lua>,
     {
@@ -485,7 +460,7 @@ impl<'lua, T: 'static> UserDataRegistry<'lua, T> {
                 }
             }
         }
-        value.into_lua_multi(lua)
+        value.into_lua(lua)
     }
 }
 
@@ -502,7 +477,7 @@ impl<'lua, T: 'static> UserDataFields<'lua, T> for UserDataRegistry<'lua, T> {
         let name = name.as_ref().to_string();
         self.fields.push((
             name,
-            Box::new(move |lua, _| value.clone().into_lua_multi(lua)),
+            Box::new(move |lua, _| unsafe { value.clone().push_into_stack_multi(lua) }),
         ));
     }
 
@@ -554,7 +529,9 @@ impl<'lua, T: 'static> UserDataFields<'lua, T> for UserDataRegistry<'lua, T> {
         let name2 = name.clone();
         self.meta_fields.push((
             name,
-            Box::new(move |lua, _| Self::check_meta_field(lua, &name2, value.clone())),
+            Box::new(move |lua, _| unsafe {
+                Self::check_meta_field(lua, &name2, value.clone())?.push_into_stack_multi(lua)
+            }),
         ));
     }
 
@@ -567,7 +544,9 @@ impl<'lua, T: 'static> UserDataFields<'lua, T> for UserDataRegistry<'lua, T> {
         let name2 = name.clone();
         self.meta_fields.push((
             name,
-            Box::new(move |lua, _| Self::check_meta_field(lua, &name2, f(lua)?)),
+            Box::new(move |lua, _| unsafe {
+                Self::check_meta_field(lua, &name2, f(lua)?)?.push_into_stack_multi(lua)
+            }),
         ));
     }
 

src/util/mod.rs

@@ -63,6 +63,11 @@ impl StackGuard {
             top: ffi::lua_gettop(state),
         }
     }
+
+    // Same as `new()`, but allows specifying the expected stack size at the end of the scope.
+    pub const fn with_top(state: *mut ffi::lua_State, top: c_int) -> StackGuard {
+        StackGuard { state, top }
+    }
 }
 
 impl Drop for StackGuard {

src/value.rs

@@ -1,17 +1,21 @@
+use std::borrow::Cow;
 use std::cmp::Ordering;
 use std::collections::HashSet;
 use std::iter::{self, FromIterator};
 use std::ops::Index;
-use std::os::raw::c_void;
+use std::os::raw::{c_int, c_void};
 use std::string::String as StdString;
 use std::sync::Arc;
-use std::{fmt, ptr, slice, str, vec};
+use std::{fmt, mem, ptr, slice, str, vec};
+
+use num_traits::FromPrimitive;
 
 #[cfg(feature = "serialize")]
 use {
+    crate::table::SerializableTable,
+    rustc_hash::FxHashSet,
     serde::ser::{self, Serialize, Serializer},
-    std::convert::TryInto,
-    std::result::Result as StdResult,
+    std::{cell::RefCell, convert::TryInto, rc::Rc, result::Result as StdResult},
 };
 
 use crate::error::{Error, Result};
@@ -121,7 +125,7 @@ impl<'lua> Value<'lua> {
             Value::String(String(r))
             | Value::Table(Table(r))
             | Value::Function(Function(r))
-            | Value::Thread(Thread(r))
+            | Value::Thread(Thread(r, ..))
             | Value::UserData(AnyUserData(r)) => r.to_pointer(),
             _ => ptr::null(),
         }
@@ -143,7 +147,7 @@ impl<'lua> Value<'lua> {
             Value::String(s) => Ok(s.to_str()?.to_string()),
             Value::Table(Table(r))
             | Value::Function(Function(r))
-            | Value::Thread(Thread(r))
+            | Value::Thread(Thread(r, ..))
             | Value::UserData(AnyUserData(r)) => unsafe {
                 let state = r.lua.state();
                 let _guard = StackGuard::new(state);
@@ -159,6 +163,263 @@ impl<'lua> Value<'lua> {
         }
     }
 
+    /// Returns `true` if the value is a [`Nil`].
+    #[inline]
+    pub fn is_nil(&self) -> bool {
+        self == &Nil
+    }
+
+    /// Returns `true` if the value is a [`NULL`].
+    #[inline]
+    pub fn is_null(&self) -> bool {
+        self == &Self::NULL
+    }
+
+    /// Returns `true` if the value is a boolean.
+    #[inline]
+    pub fn is_boolean(&self) -> bool {
+        self.as_boolean().is_some()
+    }
+
+    /// Cast the value to boolean.
+    ///
+    /// If the value is a Boolean, returns it or `None` otherwise.
+    #[inline]
+    pub fn as_boolean(&self) -> Option<bool> {
+        match *self {
+            Value::Boolean(b) => Some(b),
+            _ => None,
+        }
+    }
+
+    /// Returns `true` if the value is a [`LightUserData`].
+    #[inline]
+    pub fn is_light_userdata(&self) -> bool {
+        self.as_light_userdata().is_some()
+    }
+
+    /// Cast the value to [`LightUserData`].
+    ///
+    /// If the value is a [`LightUserData`], returns it or `None` otherwise.
+    #[inline]
+    pub fn as_light_userdata(&self) -> Option<LightUserData> {
+        match *self {
+            Value::LightUserData(l) => Some(l),
+            _ => None,
+        }
+    }
+
+    /// Returns `true` if the value is an [`Integer`].
+    #[inline]
+    pub fn is_integer(&self) -> bool {
+        self.as_integer().is_some()
+    }
+
+    /// Cast the value to [`Integer`].
+    ///
+    /// If the value is a Lua [`Integer`], returns it or `None` otherwise.
+    #[inline]
+    pub fn as_integer(&self) -> Option<Integer> {
+        match *self {
+            Value::Integer(i) => Some(i),
+            _ => None,
+        }
+    }
+
+    /// Cast the value to `i32`.
+    ///
+    /// If the value is a Lua [`Integer`], try to convert it to `i32` or return `None` otherwise.
+    #[inline]
+    pub fn as_i32(&self) -> Option<i32> {
+        #[allow(clippy::useless_conversion)]
+        self.as_integer().and_then(|i| i32::try_from(i).ok())
+    }
+
+    /// Cast the value to `u32`.
+    ///
+    /// If the value is a Lua [`Integer`], try to convert it to `u32` or return `None` otherwise.
+    #[inline]
+    pub fn as_u32(&self) -> Option<u32> {
+        self.as_integer().and_then(|i| u32::try_from(i).ok())
+    }
+
+    /// Cast the value to `i64`.
+    ///
+    /// If the value is a Lua [`Integer`], try to convert it to `i64` or return `None` otherwise.
+    #[inline]
+    pub fn as_i64(&self) -> Option<i64> {
+        #[allow(clippy::useless_conversion)]
+        self.as_integer().and_then(|i| i64::try_from(i).ok())
+    }
+
+    /// Cast the value to `u64`.
+    ///
+    /// If the value is a Lua [`Integer`], try to convert it to `u64` or return `None` otherwise.
+    #[inline]
+    pub fn as_u64(&self) -> Option<u64> {
+        self.as_integer().and_then(|i| u64::try_from(i).ok())
+    }
+
+    /// Cast the value to `isize`.
+    ///
+    /// If the value is a Lua [`Integer`], try to convert it to `isize` or return `None` otherwise.
+    #[inline]
+    pub fn as_isize(&self) -> Option<isize> {
+        self.as_integer().and_then(|i| isize::try_from(i).ok())
+    }
+
+    /// Cast the value to `usize`.
+    ///
+    /// If the value is a Lua [`Integer`], try to convert it to `usize` or return `None` otherwise.
+    #[inline]
+    pub fn as_usize(&self) -> Option<usize> {
+        self.as_integer().and_then(|i| usize::try_from(i).ok())
+    }
+
+    /// Returns `true` if the value is a Lua [`Number`].
+    #[inline]
+    pub fn is_number(&self) -> bool {
+        self.as_number().is_some()
+    }
+
+    /// Cast the value to [`Number`].
+    ///
+    /// If the value is a Lua [`Number`], returns it or `None` otherwise.
+    #[inline]
+    pub fn as_number(&self) -> Option<Number> {
+        match *self {
+            Value::Number(n) => Some(n),
+            _ => None,
+        }
+    }
+
+    /// Cast the value to `f32`.
+    ///
+    /// If the value is a Lua [`Number`], try to convert it to `f32` or return `None` otherwise.
+    #[inline]
+    pub fn as_f32(&self) -> Option<f32> {
+        self.as_number().and_then(f32::from_f64)
+    }
+
+    /// Cast the value to `f64`.
+    ///
+    /// If the value is a Lua [`Number`], try to convert it to `f64` or return `None` otherwise.
+    #[inline]
+    pub fn as_f64(&self) -> Option<f64> {
+        self.as_number()
+    }
+
+    /// Returns `true` if the value is a Lua [`String`].
+    #[inline]
+    pub fn is_string(&self) -> bool {
+        self.as_string().is_some()
+    }
+
+    /// Cast the value to Lua [`String`].
+    ///
+    /// If the value is a Lua [`String`], returns it or `None` otherwise.
+    #[inline]
+    pub fn as_string(&self) -> Option<&String> {
+        match self {
+            Value::String(s) => Some(s),
+            _ => None,
+        }
+    }
+
+    /// Cast the value to [`str`].
+    ///
+    /// If the value is a Lua [`String`], try to convert it to [`str`] or return `None` otherwise.
+    #[inline]
+    pub fn as_str(&self) -> Option<&str> {
+        self.as_string().and_then(|s| s.to_str().ok())
+    }
+
+    /// Cast the value to [`Cow<str>`].
+    ///
+    /// If the value is a Lua [`String`], converts it to [`Cow<str>`] or returns `None` otherwise.
+    #[inline]
+    pub fn as_string_lossy(&self) -> Option<Cow<str>> {
+        self.as_string().map(|s| s.to_string_lossy())
+    }
+
+    /// Returns `true` if the value is a Lua [`Table`].
+    #[inline]
+    pub fn is_table(&self) -> bool {
+        self.as_table().is_some()
+    }
+
+    /// Cast the value to [`Table`].
+    ///
+    /// If the value is a Lua [`Table`], returns it or `None` otherwise.
+    #[inline]
+    pub fn as_table(&self) -> Option<&Table> {
+        match self {
+            Value::Table(t) => Some(t),
+            _ => None,
+        }
+    }
+
+    /// Returns `true` if the value is a Lua [`Thread`].
+    #[inline]
+    pub fn is_thread(&self) -> bool {
+        self.as_thread().is_some()
+    }
+
+    /// Cast the value to [`Thread`].
+    ///
+    /// If the value is a Lua [`Thread`], returns it or `None` otherwise.
+    #[inline]
+    pub fn as_thread(&self) -> Option<&Thread> {
+        match self {
+            Value::Thread(t) => Some(t),
+            _ => None,
+        }
+    }
+
+    /// Returns `true` if the value is a Lua [`Function`].
+    #[inline]
+    pub fn is_function(&self) -> bool {
+        self.as_function().is_some()
+    }
+
+    /// Cast the value to [`Function`].
+    ///
+    /// If the value is a Lua [`Function`], returns it or `None` otherwise.
+    #[inline]
+    pub fn as_function(&self) -> Option<&Function> {
+        match self {
+            Value::Function(f) => Some(f),
+            _ => None,
+        }
+    }
+
+    /// Returns `true` if the value is an [`AnyUserData`].
+    #[inline]
+    pub fn is_userdata(&self) -> bool {
+        self.as_userdata().is_some()
+    }
+
+    /// Cast the value to [`AnyUserData`].
+    ///
+    /// If the value is an [`AnyUserData`], returns it or `None` otherwise.
+    #[inline]
+    pub fn as_userdata(&self) -> Option<&AnyUserData> {
+        match self {
+            Value::UserData(ud) => Some(ud),
+            _ => None,
+        }
+    }
+
+    /// Wrap reference to this Value into [`SerializableValue`].
+    ///
+    /// This allows customizing serialization behavior using serde.
+    #[cfg(feature = "serialize")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "serialize")))]
+    #[doc(hidden)]
+    pub fn to_serializable(&self) -> SerializableValue<'_, 'lua> {
+        SerializableValue::new(self, Default::default(), None)
+    }
+
     // Compares two values.
     // Used to sort values for Debug printing.
     pub(crate) fn cmp(&self, other: &Self) -> Ordering {
@@ -289,13 +550,86 @@ impl<'lua> AsRef<Value<'lua>> for Value<'lua> {
     }
 }
 
+/// A wrapped [`Value`] with customized serialization behavior.
+#[cfg(feature = "serialize")]
+#[cfg_attr(docsrs, doc(cfg(feature = "serialize")))]
+pub struct SerializableValue<'a, 'lua> {
+    value: &'a Value<'lua>,
+    options: crate::serde::de::Options,
+    // In many cases we don't need `visited` map, so don't allocate memory by default
+    visited: Option<Rc<RefCell<FxHashSet<*const c_void>>>>,
+}
+
 #[cfg(feature = "serialize")]
 impl<'lua> Serialize for Value<'lua> {
+    #[inline]
+    fn serialize<S: Serializer>(&self, serializer: S) -> StdResult<S::Ok, S::Error> {
+        SerializableValue::new(self, Default::default(), None).serialize(serializer)
+    }
+}
+
+#[cfg(feature = "serialize")]
+impl<'a, 'lua> SerializableValue<'a, 'lua> {
+    #[inline]
+    pub(crate) fn new(
+        value: &'a Value<'lua>,
+        options: crate::serde::de::Options,
+        visited: Option<&Rc<RefCell<FxHashSet<*const c_void>>>>,
+    ) -> Self {
+        if let Value::Table(_) = value {
+            return Self {
+                value,
+                options,
+                // We need to always initialize the `visited` map for Tables
+                visited: visited.cloned().or_else(|| Some(Default::default())),
+            };
+        }
+        Self {
+            value,
+            options,
+            visited: None,
+        }
+    }
+
+    /// If true, an attempt to serialize types such as [`Function`], [`Thread`], [`LightUserData`]
+    /// and [`Error`] will cause an error.
+    /// Otherwise these types skipped when iterating or serialized as unit type.
+    ///
+    /// Default: **true**
+    #[must_use]
+    pub const fn deny_unsupported_types(mut self, enabled: bool) -> Self {
+        self.options.deny_unsupported_types = enabled;
+        self
+    }
+
+    /// If true, an attempt to serialize a recursive table (table that refers to itself)
+    /// will cause an error.
+    /// Otherwise subsequent attempts to serialize the same table will be ignored.
+    ///
+    /// Default: **true**
+    #[must_use]
+    pub const fn deny_recursive_tables(mut self, enabled: bool) -> Self {
+        self.options.deny_recursive_tables = enabled;
+        self
+    }
+
+    /// If true, keys in tables will be iterated (and serialized) in sorted order.
+    ///
+    /// Default: **false**
+    #[must_use]
+    pub const fn sort_keys(mut self, enabled: bool) -> Self {
+        self.options.sort_keys = enabled;
+        self
+    }
+}
+
+#[cfg(feature = "serialize")]
+impl<'a, 'lua> Serialize for SerializableValue<'a, 'lua> {
     fn serialize<S>(&self, serializer: S) -> StdResult<S::Ok, S::Error>
     where
         S: Serializer,
     {
-        match self {
+        match self.value {
             Value::Nil => serializer.serialize_unit(),
             Value::Boolean(b) => serializer.serialize_bool(*b),
             #[allow(clippy::useless_conversion)]
@@ -305,21 +639,44 @@ impl<'lua> Serialize for Value<'lua> {
             #[cfg(feature = "luau")]
             Value::Vector(v) => v.serialize(serializer),
             Value::String(s) => s.serialize(serializer),
-            Value::Table(t) => t.serialize(serializer),
-            Value::UserData(ud) => ud.serialize(serializer),
+            Value::Table(t) => {
+                let visited = self.visited.as_ref().unwrap().clone();
+                SerializableTable::new(t, self.options, visited).serialize(serializer)
+            }
             Value::LightUserData(ud) if ud.0.is_null() => serializer.serialize_none(),
-            Value::Error(_) | Value::LightUserData(_) | Value::Function(_) | Value::Thread(_) => {
-                let msg = format!("cannot serialize <{}>", self.type_name());
-                Err(ser::Error::custom(msg))
+            Value::UserData(ud) if ud.is_serializable() || self.options.deny_unsupported_types => {
+                ud.serialize(serializer)
+            }
+            Value::Function(_)
+            | Value::Thread(_)
+            | Value::UserData(_)
+            | Value::LightUserData(_)
+            | Value::Error(_) => {
+                if self.options.deny_unsupported_types {
+                    let msg = format!("cannot serialize <{}>", self.value.type_name());
+                    Err(ser::Error::custom(msg))
+                } else {
+                    serializer.serialize_unit()
+                }
             }
         }
     }
 }
 
 /// Trait for types convertible to `Value`.
-pub trait IntoLua<'lua> {
+pub trait IntoLua<'lua>: Sized {
     /// Performs the conversion.
     fn into_lua(self, lua: &'lua Lua) -> Result<Value<'lua>>;
+
+    /// Pushes the value into the Lua stack.
+    ///
+    /// # Safety
+    /// This method does not check Lua stack space.
+    #[doc(hidden)]
+    #[inline]
+    unsafe fn push_into_stack(self, lua: &'lua Lua) -> Result<()> {
+        lua.push_value(self.into_lua(lua)?)
+    }
 }
 
 /// Trait for types convertible from `Value`.
@@ -332,13 +689,33 @@ pub trait FromLua<'lua>: Sized {
     /// `i` is the argument index (position),
     /// `to` is a function name that received the argument.
     #[doc(hidden)]
-    fn from_lua_arg(
-        value: Value<'lua>,
+    #[inline]
+    fn from_lua_arg(arg: Value<'lua>, i: usize, to: Option<&str>, lua: &'lua Lua) -> Result<Self> {
+        Self::from_lua(arg, lua).map_err(|err| Error::BadArgument {
+            to: to.map(|s| s.to_string()),
+            pos: i,
+            name: None,
+            cause: Arc::new(err),
+        })
+    }
+
+    /// Performs the conversion for a value in the Lua stack at index `idx`.
+    #[doc(hidden)]
+    #[inline]
+    unsafe fn from_stack(idx: c_int, lua: &'lua Lua) -> Result<Self> {
+        Self::from_lua(lua.stack_value(idx), lua)
+    }
+
+    /// Same as `from_lua_arg` but for a value in the Lua stack at index `idx`.
+    #[doc(hidden)]
+    #[inline]
+    unsafe fn from_stack_arg(
+        idx: c_int,
         i: usize,
         to: Option<&str>,
         lua: &'lua Lua,
     ) -> Result<Self> {
-        Self::from_lua(value, lua).map_err(|err| Error::BadArgument {
+        Self::from_stack(idx, lua).map_err(|err| Error::BadArgument {
             to: to.map(|s| s.to_string()),
             pos: i,
             name: None,
@@ -349,24 +726,43 @@ pub trait FromLua<'lua>: Sized {
 
 /// Multiple Lua values used for both argument passing and also for multiple return values.
 #[derive(Debug, Clone)]
-pub struct MultiValue<'lua>(Vec<Value<'lua>>);
+pub struct MultiValue<'lua> {
+    vec: Vec<Value<'lua>>,
+    lua: Option<&'lua Lua>,
+}
+
+impl Drop for MultiValue<'_> {
+    fn drop(&mut self) {
+        if let Some(lua) = self.lua {
+            let vec = mem::take(&mut self.vec);
+            lua.push_multivalue_to_pool(vec);
+        }
+    }
+}
 
 impl<'lua> MultiValue<'lua> {
     /// Creates an empty `MultiValue` containing no values.
     pub const fn new() -> MultiValue<'lua> {
-        MultiValue(Vec::new())
+        MultiValue {
+            vec: Vec::new(),
+            lua: None,
+        }
     }
 
-    /// Similar to `new` but can return previously used container with allocated capacity.
+    /// Similar to `new` but can reuse previously used container with allocated capacity.
     #[inline]
-    pub(crate) fn new_or_pooled(lua: &'lua Lua) -> MultiValue<'lua> {
-        lua.new_multivalue_from_pool()
-    }
-
-    /// Clears and returns previously allocated multivalue container to the pool.
-    #[inline]
-    pub(crate) fn return_to_pool(multivalue: Self, lua: &Lua) {
-        lua.return_multivalue_to_pool(multivalue);
+    pub(crate) fn with_lua_and_capacity(lua: &'lua Lua, capacity: usize) -> MultiValue<'lua> {
+        let vec = lua
+            .pop_multivalue_from_pool()
+            .map(|mut vec| {
+                vec.reserve(capacity);
+                vec
+            })
+            .unwrap_or_else(|| Vec::with_capacity(capacity));
+        MultiValue {
+            vec,
+            lua: Some(lua),
+        }
     }
 }
 
@@ -389,8 +785,10 @@ impl<'lua> IntoIterator for MultiValue<'lua> {
     type IntoIter = iter::Rev<vec::IntoIter<Value<'lua>>>;
 
     #[inline]
-    fn into_iter(self) -> Self::IntoIter {
-        self.0.into_iter().rev()
+    fn into_iter(mut self) -> Self::IntoIter {
+        let vec = mem::take(&mut self.vec);
+        mem::forget(self);
+        vec.into_iter().rev()
     }
 }
 
@@ -400,7 +798,7 @@ impl<'a, 'lua> IntoIterator for &'a MultiValue<'lua> {
 
     #[inline]
     fn into_iter(self) -> Self::IntoIter {
-        self.0.iter().rev()
+        self.vec.iter().rev()
     }
 }
 
@@ -423,64 +821,60 @@ impl<'lua> Index<usize> for MultiValue<'lua> {
 
 impl<'lua> MultiValue<'lua> {
     #[inline]
-    pub fn from_vec(mut v: Vec<Value<'lua>>) -> MultiValue<'lua> {
-        v.reverse();
-        MultiValue(v)
+    pub fn from_vec(mut vec: Vec<Value<'lua>>) -> MultiValue<'lua> {
+        vec.reverse();
+        MultiValue { vec, lua: None }
     }
 
     #[inline]
-    pub fn into_vec(self) -> Vec<Value<'lua>> {
-        let mut v = self.0;
-        v.reverse();
-        v
+    pub fn into_vec(mut self) -> Vec<Value<'lua>> {
+        let mut vec = mem::take(&mut self.vec);
+        mem::forget(self);
+        vec.reverse();
+        vec
     }
 
     #[inline]
     pub fn get(&self, index: usize) -> Option<&Value<'lua>> {
-        if index < self.0.len() {
-            return self.0.get(self.0.len() - index - 1);
+        if index < self.vec.len() {
+            return self.vec.get(self.vec.len() - index - 1);
         }
         None
     }
 
-    #[inline]
-    pub(crate) fn reserve(&mut self, size: usize) {
-        self.0.reserve(size);
-    }
-
     #[inline]
     pub fn pop_front(&mut self) -> Option<Value<'lua>> {
-        self.0.pop()
+        self.vec.pop()
     }
 
     #[inline]
     pub fn push_front(&mut self, value: Value<'lua>) {
-        self.0.push(value);
+        self.vec.push(value);
     }
 
     #[inline]
     pub fn clear(&mut self) {
-        self.0.clear();
+        self.vec.clear();
     }
 
     #[inline]
     pub fn len(&self) -> usize {
-        self.0.len()
+        self.vec.len()
     }
 
     #[inline]
     pub fn is_empty(&self) -> bool {
-        self.0.is_empty()
+        self.vec.is_empty()
     }
 
     #[inline]
     pub fn iter(&self) -> iter::Rev<slice::Iter<Value<'lua>>> {
-        self.0.iter().rev()
+        self.vec.iter().rev()
     }
 
     #[inline]
     pub(crate) fn drain_all(&mut self) -> iter::Rev<vec::Drain<Value<'lua>>> {
-        self.0.drain(..).rev()
+        self.vec.drain(..).rev()
     }
 
     #[inline]
@@ -488,11 +882,11 @@ impl<'lua> MultiValue<'lua> {
         &mut self,
         iter: impl IntoIterator<Item = Result<Value<'lua>>>,
     ) -> Result<()> {
-        self.0.clear();
+        self.vec.clear();
         for value in iter {
-            self.0.push(value?);
+            self.vec.push(value?);
         }
-        self.0.reverse();
+        self.vec.reverse();
         Ok(())
     }
 }
@@ -501,9 +895,26 @@ impl<'lua> MultiValue<'lua> {
 ///
 /// This is a generalization of `IntoLua`, allowing any number of resulting Lua values instead of just
 /// one. Any type that implements `IntoLua` will automatically implement this trait.
-pub trait IntoLuaMulti<'lua> {
+pub trait IntoLuaMulti<'lua>: Sized {
     /// Performs the conversion.
     fn into_lua_multi(self, lua: &'lua Lua) -> Result<MultiValue<'lua>>;
+
+    /// Pushes the values into the Lua stack.
+    ///
+    /// Returns number of pushed values.
+    #[doc(hidden)]
+    #[inline]
+    unsafe fn push_into_stack_multi(self, lua: &'lua Lua) -> Result<c_int> {
+        let mut values = self.into_lua_multi(lua)?;
+        let len: c_int = values.len().try_into().unwrap();
+        unsafe {
+            check_stack(lua.state(), len + 1)?;
+            for v in values.drain_all() {
+                lua.push_value(v)?;
+            }
+        }
+        Ok(len)
+    }
 }
 
 /// Trait for types that can be created from an arbitrary number of Lua values.
@@ -525,15 +936,43 @@ pub trait FromLuaMulti<'lua>: Sized {
     /// `to` is a function name that received the arguments.
     #[doc(hidden)]
     #[inline]
-    fn from_lua_multi_args(
-        values: MultiValue<'lua>,
+    fn from_lua_args(
+        args: MultiValue<'lua>,
         i: usize,
         to: Option<&str>,
         lua: &'lua Lua,
     ) -> Result<Self> {
         let _ = (i, to);
+        Self::from_lua_multi(args, lua)
+    }
+
+    /// Performs the conversion for a number of values in the Lua stack.
+    #[doc(hidden)]
+    #[inline]
+    unsafe fn from_stack_multi(nvals: c_int, lua: &'lua Lua) -> Result<Self> {
+        let mut values = MultiValue::with_lua_and_capacity(lua, nvals as usize);
+        for idx in 1..=nvals {
+            values.push_front(lua.stack_value(-idx));
+        }
+        if nvals > 0 {
+            // It's safe to clear the stack as all references moved to ref thread
+            ffi::lua_pop(lua.state(), nvals);
+        }
         Self::from_lua_multi(values, lua)
     }
+
+    /// Same as `from_lua_args` but for a number of values in the Lua stack.
+    #[doc(hidden)]
+    #[inline]
+    unsafe fn from_stack_args(
+        nargs: c_int,
+        i: usize,
+        to: Option<&str>,
+        lua: &'lua Lua,
+    ) -> Result<Self> {
+        let _ = (i, to);
+        Self::from_stack_multi(nargs, lua)
+    }
 }
 
 #[cfg(test)]

tests/async.rs

@@ -6,8 +6,8 @@ use std::time::Duration;
 use futures_util::stream::TryStreamExt;
 
 use mlua::{
-    AnyUserDataExt, Error, Function, Lua, LuaOptions, Result, StdLib, Table, TableExt, UserData,
-    UserDataMethods, Value,
+    AnyUserDataExt, Error, Function, Lua, LuaOptions, MultiValue, Result, StdLib, Table, TableExt,
+    UserData, UserDataMethods, Value,
 };
 
 async fn sleep_ms(ms: u64) {
@@ -82,6 +82,25 @@ async fn test_async_call() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn test_async_call_many_returns() -> Result<()> {
+    let lua = Lua::new();
+
+    let hello = lua.create_async_function(|_lua, ()| async move {
+        sleep_ms(10).await;
+        Ok(("a", "b", "c", 1))
+    })?;
+
+    let vals = hello.call_async::<_, MultiValue>(()).await?;
+    assert_eq!(vals.len(), 4);
+    assert_eq!(vals[0].to_string()?, "a");
+    assert_eq!(vals[1].to_string()?, "b");
+    assert_eq!(vals[2].to_string()?, "c");
+    assert_eq!(vals[3], Value::Integer(1));
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn test_async_bind_call() -> Result<()> {
     let lua = Lua::new();

tests/chunk.rs

@@ -1,7 +1,7 @@
 use std::fs;
 use std::io;
 
-use mlua::{AnyUserData, Function, Lua, Result};
+use mlua::{Lua, Result};
 
 #[test]
 fn test_chunk_path() -> Result<()> {
@@ -36,8 +36,8 @@ fn test_chunk_macro() -> Result<()> {
     let data = lua.create_table()?;
     data.raw_set("num", 1)?;
 
-    let ud = AnyUserData::wrap("hello");
-    let f = Function::wrap(|_lua, ()| Ok(()));
+    let ud = mlua::AnyUserData::wrap("hello");
+    let f = mlua::Function::wrap(|_lua, ()| Ok(()));
 
     lua.globals().set("g", 123)?;
 

tests/luau.rs

@@ -84,7 +84,7 @@ fn test_vectors() -> Result<()> {
         assert(v.z == 3)
     "#,
     )
-    .set_compiler(Compiler::new().set_vector_ctor(Some("vector".to_string())))
+    .set_compiler(Compiler::new().set_vector_ctor("vector"))
     .exec()?;
 
     Ok(())
@@ -124,7 +124,49 @@ fn test_vectors() -> Result<()> {
         assert(v.w == 4)
     "#,
     )
-    .set_compiler(Compiler::new().set_vector_ctor(Some("vector".to_string())))
+    .set_compiler(Compiler::new().set_vector_ctor("vector"))
+    .exec()?;
+
+    Ok(())
+}
+
+#[cfg(all(not(feature = "luau-vector4"), feature = "unstable"))]
+#[test]
+fn test_vector_metatable() -> Result<()> {
+    let lua = Lua::new();
+
+    let vector_mt = lua
+        .load(
+            r#"
+            {
+                __index = {
+                    new = vector,
+
+                    product = function(a, b)
+                        return vector(a.x * b.x, a.y * b.y, a.z * b.z)
+                    end
+                }
+            }
+    "#,
+        )
+        .eval::<Table>()?;
+    vector_mt.set_metatable(Some(vector_mt.clone()));
+    lua.set_vector_metatable(Some(vector_mt.clone()));
+    lua.globals().set("Vector3", vector_mt)?;
+
+    let compiler = Compiler::new()
+        .set_vector_lib("Vector3")
+        .set_vector_ctor("new");
+
+    // Test vector methods (fastcall)
+    lua.load(
+        r#"
+        local v = Vector3.new(1, 2, 3)
+        local v2 = v:product(Vector3.new(2, 3, 4))
+        assert(v2.x == 2 and v2.y == 6 and v2.z == 12)
+    "#,
+    )
+    .set_compiler(compiler)
     .exec()?;
 
     Ok(())

tests/scope.rs

@@ -1,5 +1,6 @@
 use std::cell::Cell;
 use std::rc::Rc;
+use std::string::String as StdString;
 use std::sync::Arc;
 
 use mlua::{
@@ -76,6 +77,7 @@ fn test_scope_userdata_fields() -> Result<()> {
 
     impl<'a> UserData for MyUserData<'a> {
         fn add_fields<'lua, F: UserDataFields<'lua, Self>>(fields: &mut F) {
+            fields.add_field("field", "hello");
             fields.add_field_method_get("val", |_, data| Ok(data.0.get()));
             fields.add_field_method_set("val", |_, data, val| {
                 data.0.set(val);
@@ -91,6 +93,7 @@ fn test_scope_userdata_fields() -> Result<()> {
         .load(
             r#"
             function(u)
+                assert(u.field == "hello")
                 assert(u.val == 42)
                 u.val = 44
             end
@@ -227,9 +230,19 @@ fn test_scope_userdata_mismatch() -> Result<()> {
         let bu = scope.create_nonstatic_userdata(MyUserData(&b))?;
         assert!(okay.call::<_, ()>((au.clone(), bu.clone())).is_ok());
         match bad.call::<_, ()>((au, bu)) {
-            Err(Error::CallbackError { ref cause, .. }) => match *cause.as_ref() {
-                Error::UserDataTypeMismatch => {}
-                ref other => panic!("wrong error type {:?}", other),
+            Err(Error::CallbackError { ref cause, .. }) => match cause.as_ref() {
+                Error::BadArgument {
+                    to,
+                    pos,
+                    name,
+                    cause,
+                } => {
+                    assert_eq!(to.as_deref(), Some("MyUserData.inc"));
+                    assert_eq!(*pos, 1);
+                    assert_eq!(name.as_deref(), Some("self"));
+                    assert!(matches!(*cause.as_ref(), Error::UserDataTypeMismatch));
+                }
+                other => panic!("wrong error type {:?}", other),
             },
             Err(other) => panic!("wrong error type {:?}", other),
             Ok(_) => panic!("incorrectly returned Ok"),
@@ -417,6 +430,32 @@ fn test_scope_userdata_ref_mut() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_scope_any_userdata() -> Result<()> {
+    let lua = Lua::new();
+
+    lua.register_userdata_type::<StdString>(|reg| {
+        reg.add_meta_method("__tostring", |_, data, ()| Ok(data.clone()));
+    })?;
+
+    lua.scope(|scope| {
+        let ud = scope.create_any_userdata(StdString::from("foo"))?;
+        lua.globals().set("ud", ud)?;
+        lua.load("assert(tostring(ud) == 'foo')").exec()
+    })?;
+
+    // Check that userdata is destructed
+    match lua.load("tostring(ud)").exec() {
+        Err(Error::CallbackError { ref cause, .. }) => match cause.as_ref() {
+            Error::CallbackDestructed => {}
+            err => panic!("expected CallbackDestructed, got {:?}", err),
+        },
+        r => panic!("improper return for destructed userdata: {:?}", r),
+    };
+
+    Ok(())
+}
+
 #[test]
 fn test_scope_any_userdata_ref() -> Result<()> {
     let lua = Lua::new();

tests/serde.rs

@@ -4,8 +4,8 @@ use std::collections::HashMap;
 use std::error::Error as StdError;
 
 use mlua::{
-    DeserializeOptions, Error, Lua, LuaSerdeExt, Result as LuaResult, SerializeOptions, UserData,
-    Value,
+    DeserializeOptions, Error, ExternalResult, Lua, LuaSerdeExt, Result as LuaResult,
+    SerializeOptions, UserData, Value,
 };
 use serde::{Deserialize, Serialize};
 
@@ -191,6 +191,69 @@ fn test_serialize_vector() -> Result<(), Box<dyn StdError>> {
     Ok(())
 }
 
+#[test]
+fn test_serialize_sorted() -> LuaResult<()> {
+    let lua = Lua::new();
+
+    let globals = lua.globals();
+    globals.set("null", lua.null())?;
+
+    let empty_array = lua.create_table()?;
+    empty_array.set_metatable(Some(lua.array_metatable()));
+    globals.set("empty_array", empty_array)?;
+
+    let value = lua
+        .load(
+            r#"
+        {
+            _bool = true,
+            _integer = 123,
+            _number = 321.99,
+            _string = "test string serialization",
+            _table_arr = {nil, "value 1", nil, "value 2", {}},
+            _table_map = {["table"] = "map", ["null"] = null},
+            _bytes = "\240\040\140\040",
+            _null = null,
+            _empty_map = {},
+            _empty_array = empty_array,
+        }
+    "#,
+        )
+        .eval::<Value>()?;
+
+    let json = serde_json::to_string(&value.to_serializable().sort_keys(true)).unwrap();
+    assert_eq!(
+        json,
+        r#"{"_bool":true,"_bytes":[240,40,140,40],"_empty_array":[],"_empty_map":{},"_integer":123,"_null":null,"_number":321.99,"_string":"test string serialization","_table_arr":[null,"value 1",null,"value 2",{}],"_table_map":{"null":null,"table":"map"}}"#
+    );
+
+    Ok(())
+}
+
+#[test]
+fn test_serialize_globals() -> LuaResult<()> {
+    let lua = Lua::new();
+
+    let globals = Value::Table(lua.globals());
+
+    // By default it should not work
+    if let Ok(v) = serde_json::to_value(&globals) {
+        panic!("expected serialization error, got {v:?}");
+    }
+
+    // It should work with `deny_recursive_tables` and `deny_unsupported_types` disabled
+    if let Err(err) = serde_json::to_value(
+        globals
+            .to_serializable()
+            .deny_recursive_tables(false)
+            .deny_unsupported_types(false),
+    ) {
+        panic!("expected no errors, got {err:?}");
+    }
+
+    Ok(())
+}
+
 #[test]
 fn test_to_value_struct() -> LuaResult<()> {
     let lua = Lua::new();
@@ -611,3 +674,26 @@ fn test_from_value_userdata() -> Result<(), Box<dyn StdError>> {
 
     Ok(())
 }
+
+#[test]
+fn test_from_value_sorted() -> Result<(), Box<dyn StdError>> {
+    let lua = Lua::new();
+
+    let to_json = lua.create_function(|lua, value| {
+        let json_value: serde_json::Value =
+            lua.from_value_with(value, DeserializeOptions::new().sort_keys(true))?;
+        serde_json::to_string(&json_value).into_lua_err()
+    })?;
+    lua.globals().set("to_json", to_json)?;
+
+    lua.load(
+        r#"
+        local json = to_json({c = 3, b = 2, hello = "world", x = {1}, ["0a"] = {z = "z", d = "d"}})
+        assert(json == '{"0a":{"d":"d","z":"z"},"b":2,"c":3,"hello":"world","x":[1]}', "invalid json")
+    "#,
+    )
+    .exec()
+    .unwrap();
+
+    Ok(())
+}

tests/tests.rs

@@ -504,25 +504,32 @@ fn test_result_conversions() -> Result<()> {
     let lua = Lua::new();
     let globals = lua.globals();
 
-    let err = lua.create_function(|_, ()| {
-        Ok(Err::<String, _>(
-            "only through failure can we succeed".into_lua_err(),
-        ))
-    })?;
-    let ok = lua.create_function(|_, ()| Ok(Ok::<_, Error>("!".to_owned())))?;
+    let ok = lua.create_function(|_, ()| Ok(Ok::<(), Error>(())))?;
+    let err = lua.create_function(|_, ()| Ok(Err::<(), _>("failure1".into_lua_err())))?;
+    let ok2 = lua.create_function(|_, ()| Ok(Ok::<_, Error>("!".to_owned())))?;
+    let err2 = lua.create_function(|_, ()| Ok(Err::<String, _>("failure2".into_lua_err())))?;
 
-    globals.set("err", err)?;
     globals.set("ok", ok)?;
+    globals.set("ok2", ok2)?;
+    globals.set("err", err)?;
+    globals.set("err2", err2)?;
 
     lua.load(
         r#"
+        local r, e = ok()
+        assert(r == nil and e == nil)
+
         local r, e = err()
         assert(r == nil)
-        assert(tostring(e):find("only through failure can we succeed") ~= nil)
+        assert(tostring(e):find("failure1") ~= nil)
 
-        local r, e = ok()
+        local r, e = ok2()
         assert(r == "!")
         assert(e == nil)
+
+        local r, e = err2()
+        assert(r == nil)
+        assert(tostring(e):find("failure2") ~= nil)
     "#,
     )
     .exec()?;

tests/thread.rs

@@ -190,3 +190,34 @@ fn test_coroutine_panic() {
         Err(p) => assert!(*p.downcast::<&str>().unwrap() == "test_panic"),
     }
 }
+
+#[cfg(all(feature = "unstable", not(feature = "send")))]
+#[test]
+fn test_owned_thread() -> Result<()> {
+    let lua = Lua::new();
+
+    let accumulate = lua
+        .create_thread(
+            lua.load(
+                r#"
+            function (sum)
+                while true do
+                    sum = sum + coroutine.yield(sum)
+                end
+            end
+            "#,
+            )
+            .eval::<Function>()?,
+        )?
+        .into_owned();
+
+    for i in 0..4 {
+        accumulate.resume::<_, ()>(i)?;
+    }
+    assert_eq!(accumulate.resume::<_, i64>(4)?, 10);
+    assert_eq!(accumulate.status(), ThreadStatus::Resumable);
+    assert!(accumulate.resume::<_, ()>("error").is_err());
+    assert_eq!(accumulate.status(), ThreadStatus::Error);
+
+    Ok(())
+}

tests/userdata.rs

@@ -15,7 +15,7 @@ use std::sync::atomic::{AtomicI64, Ordering};
 
 use mlua::{
     AnyUserData, AnyUserDataExt, Error, ExternalError, Function, Lua, MetaMethod, Nil, Result,
-    String, UserData, UserDataFields, UserDataMethods, UserDataRef, Value,
+    String, UserData, UserDataFields, UserDataMethods, UserDataRef, Value, Variadic,
 };
 
 #[test]
@@ -92,6 +92,30 @@ fn test_methods() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_method_variadic() -> Result<()> {
+    struct MyUserData(i64);
+
+    impl UserData for MyUserData {
+        fn add_methods<'lua, M: UserDataMethods<'lua, Self>>(methods: &mut M) {
+            methods.add_method("get", |_, data, ()| Ok(data.0));
+            methods.add_method_mut("add", |_, data, vals: Variadic<i64>| {
+                data.0 += vals.into_iter().sum::<i64>();
+                Ok(())
+            });
+        }
+    }
+
+    let lua = Lua::new();
+    let globals = lua.globals();
+    globals.set("userdata", MyUserData(0))?;
+    lua.load("userdata:add(1, 5, -10)").exec()?;
+    let ud: UserDataRef<MyUserData> = globals.get("userdata")?;
+    assert_eq!(ud.0, -4);
+
+    Ok(())
+}
+
 #[test]
 fn test_metamethods() -> Result<()> {
     #[derive(Copy, Clone)]

tests/value.rs

@@ -158,3 +158,74 @@ fn test_debug_format() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn test_value_conversions() -> Result<()> {
+    let lua = Lua::new();
+
+    assert!(Value::Nil.is_nil());
+    assert!(!Value::NULL.is_nil());
+    assert!(Value::NULL.is_null());
+    assert!(Value::NULL.is_light_userdata());
+    assert!(Value::NULL.as_light_userdata() == Some(LightUserData(ptr::null_mut())));
+    assert!(Value::Boolean(true).is_boolean());
+    assert_eq!(Value::Boolean(false).as_boolean(), Some(false));
+    assert!(Value::Integer(1).is_integer());
+    assert_eq!(Value::Integer(1).as_integer(), Some(1));
+    assert_eq!(Value::Integer(1).as_i32(), Some(1i32));
+    assert_eq!(Value::Integer(1).as_u32(), Some(1u32));
+    assert_eq!(Value::Integer(1).as_i64(), Some(1i64));
+    assert_eq!(Value::Integer(1).as_u64(), Some(1u64));
+    #[cfg(any(feature = "lua54", feature = "lua53"))]
+    {
+        assert_eq!(Value::Integer(mlua::Integer::MAX).as_i32(), None);
+        assert_eq!(Value::Integer(mlua::Integer::MAX).as_u32(), None);
+    }
+    assert_eq!(Value::Integer(1).as_isize(), Some(1isize));
+    assert_eq!(Value::Integer(1).as_usize(), Some(1usize));
+    assert!(Value::Number(1.23).is_number());
+    assert_eq!(Value::Number(1.23).as_number(), Some(1.23));
+    assert_eq!(Value::Number(1.23).as_f32(), Some(1.23f32));
+    assert_eq!(Value::Number(1.23).as_f64(), Some(1.23f64));
+    assert!(Value::String(lua.create_string("hello")?).is_string());
+    assert_eq!(
+        Value::String(lua.create_string("hello")?)
+            .as_string()
+            .unwrap(),
+        "hello"
+    );
+    assert_eq!(
+        Value::String(lua.create_string("hello")?).as_str().unwrap(),
+        "hello"
+    );
+    assert_eq!(
+        Value::String(lua.create_string("hello")?)
+            .as_string_lossy()
+            .unwrap(),
+        "hello"
+    );
+    assert!(Value::Table(lua.create_table()?).is_table());
+    assert!(Value::Table(lua.create_table()?).as_table().is_some());
+    assert!(Value::Function(lua.create_function(|_, ()| Ok(())).unwrap()).is_function());
+    assert!(
+        Value::Function(lua.create_function(|_, ()| Ok(())).unwrap())
+            .as_function()
+            .is_some()
+    );
+    assert!(Value::Thread(lua.create_thread(lua.load("function() end").eval()?)?).is_thread());
+    assert!(
+        Value::Thread(lua.create_thread(lua.load("function() end").eval()?)?)
+            .as_thread()
+            .is_some()
+    );
+    assert!(Value::UserData(lua.create_any_userdata("hello")?).is_userdata());
+    assert_eq!(
+        Value::UserData(lua.create_any_userdata("hello")?)
+            .as_userdata()
+            .and_then(|ud| ud.borrow::<&str>().ok())
+            .as_deref(),
+        Some(&"hello")
+    );
+
+    Ok(())
+}