Lots of changes, not sure if actually safe yet.

* Make Lua Send
* Add Send bounds to (nearly) all instances where userdata and functions are
  passed to Lua
* Add a "scope" method which takes a callback that accepts a `Scope`, and give
  `Scope` the ability to create functions and userdata that are !Send, *and also
  functions that are not even 'static!*.

8 files changed, 300 insertions, 79 deletions

diff --git a/src/conversion.rs b/src/conversion.rs
index 39baaa2..5d09f5d 100644
--- a/src/conversion.rs
+++ b/src/conversion.rs
@@ -112,7 +112,7 @@ impl<'lua> FromLua<'lua> for AnyUserData<'lua> {
     }
 }
 
-impl<'lua, T: UserData> ToLua<'lua> for T {
+impl<'lua, T: Send + UserData> ToLua<'lua> for T {
     fn to_lua(self, lua: &'lua Lua) -> Result<Value<'lua>> {
         Ok(Value::UserData(lua.create_userdata(self)?))
     }
diff --git a/src/ffi.rs b/src/ffi.rs
index 9130afd..dfb2878 100644
--- a/src/ffi.rs
+++ b/src/ffi.rs
@@ -124,6 +124,9 @@ extern "C" {
     pub fn lua_setuservalue(state: *mut lua_State, index: c_int);
     pub fn lua_getuservalue(state: *mut lua_State, index: c_int) -> c_int;
 
+    pub fn lua_getupvalue(state: *mut lua_State, funcindex: c_int, n: c_int) -> *const c_char;
+    pub fn lua_setupvalue(state: *mut lua_State, funcindex: c_int, n: c_int) -> *const c_char;
+
     pub fn lua_settable(state: *mut lua_State, index: c_int);
     pub fn lua_rawset(state: *mut lua_State, index: c_int);
     pub fn lua_setmetatable(state: *mut lua_State, index: c_int);
diff --git a/src/lua.rs b/src/lua.rs
index d8dd809..948d142 100644
--- a/src/lua.rs
+++ b/src/lua.rs
@@ -1,4 +1,4 @@
-use std::{ptr, str};
+use std::{mem, process, ptr, str};
 use std::sync::{Arc, Mutex};
 use std::ops::DerefMut;
 use std::cell::RefCell;
@@ -7,8 +7,7 @@ use std::any::TypeId;
 use std::marker::PhantomData;
 use std::collections::HashMap;
 use std::os::raw::{c_char, c_int, c_void};
-use std::mem;
-use std::process;
+use std::panic::{catch_unwind, AssertUnwindSafe};
 
 use libc;
 
@@ -30,12 +29,21 @@ pub struct Lua {
     ephemeral: bool,
 }
 
+/// Constructed by the `Lua::scope` method, allows temporarily passing to Lua userdata that is
+/// !Send, and callbacks that are !Send and not 'static.
+pub struct Scope<'lua> {
+    lua: &'lua Lua,
+    destructors: RefCell<Vec<Box<FnMut(*mut ffi::lua_State)>>>,
+}
+
 // Data associated with the main lua_State via lua_getextraspace.
 struct ExtraData {
     registered_userdata: HashMap<TypeId, c_int>,
-    registry_drop_list: Arc<Mutex<Vec<c_int>>>,
+    registry_unref_list: Arc<Mutex<Vec<c_int>>>,
 }
 
+unsafe impl Send for Lua {}
+
 impl Drop for Lua {
     fn drop(&mut self) {
         unsafe {
@@ -259,7 +267,7 @@ impl Lua {
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,
-        F: 'static + FnMut(&'lua Lua, A) -> Result<R>,
+        F: 'static + Send + FnMut(&'lua Lua, A) -> Result<R>,
     {
         self.create_callback_function(Box::new(move |lua, args| {
             func(lua, A::from_lua_multi(args, lua)?)?.to_lua_multi(lua)
@@ -286,25 +294,9 @@ impl Lua {
     /// Create a Lua userdata object from a custom userdata type.
     pub fn create_userdata<T>(&self, data: T) -> Result<AnyUserData>
     where
-        T: UserData,
+        T: Send + UserData,
     {
-        unsafe {
-            stack_err_guard(self.state, 0, move || {
-                check_stack(self.state, 3);
-
-                push_userdata::<RefCell<T>>(self.state, RefCell::new(data))?;
-
-                ffi::lua_rawgeti(
-                    self.state,
-                    ffi::LUA_REGISTRYINDEX,
-                    self.userdata_metatable::<T>()? as ffi::lua_Integer,
-                );
-
-                ffi::lua_setmetatable(self.state, -2);
-
-                Ok(AnyUserData(self.pop_ref(self.state)))
-            })
-        }
+        self.do_create_userdata(data)
     }
 
     /// Returns a handle to the global environment.
@@ -318,6 +310,28 @@ impl Lua {
         }
     }
 
+    /// Calls the given function with a `Scope` parameter, giving the function the ability to create
+    /// userdata from rust types that are !Send, and rust callbacks that are !Send and not 'static.
+    /// The lifetime of any function or userdata created through `Scope` lasts only until the
+    /// completion of this method call, on completion all such created values are automatically
+    /// dropped and Lua references to them are invalidated.  If a script accesses a value created
+    /// through `Scope` outside of this method, a Lua error will result.  Since we can ensure the
+    /// lifetime of values created through `Scope`, and we know that `Lua` cannot be sent to another
+    /// thread while `Scope` is live, it is safe to allow !Send datatypes and functions whose
+    /// lifetimes only outlive the scope lifetime.
+    pub fn scope<'lua, F, R>(&'lua self, f: F) -> R
+    where
+        F: FnOnce(&mut Scope<'lua>) -> R,
+    {
+        let mut scope = Scope {
+            lua: self,
+            destructors: RefCell::new(Vec::new()),
+        };
+        let r = f(&mut scope);
+        drop(scope);
+        r
+    }
+
     /// Coerces a Lua value to a string.
     ///
     /// The value must be a string (in which case this is a no-op) or a number.
@@ -492,7 +506,8 @@ impl Lua {
 
                 Ok(RegistryKey {
                     registry_id,
-                    drop_list: (*self.extra()).registry_drop_list.clone(),
+                    unref_list: (*self.extra()).registry_unref_list.clone(),
+                    drop_unref: true,
                 })
             })
         }
@@ -504,7 +519,7 @@ impl Lua {
     /// value previously placed by `create_registry_value`.
     pub fn registry_value<'lua, T: FromLua<'lua>>(&'lua self, key: &RegistryKey) -> Result<T> {
         unsafe {
-            if !Arc::ptr_eq(&key.drop_list, &(*self.extra()).registry_drop_list) {
+            if !Arc::ptr_eq(&key.unref_list, &(*self.extra()).registry_unref_list) {
                 return Err(Error::MismatchedRegistryKey);
             }
 
@@ -528,13 +543,12 @@ impl Lua {
     /// `RegistryKey`s have been dropped.
     pub fn remove_registry_value(&self, mut key: RegistryKey) -> Result<()> {
         unsafe {
-            if !Arc::ptr_eq(&key.drop_list, &(*self.extra()).registry_drop_list) {
+            if !Arc::ptr_eq(&key.unref_list, &(*self.extra()).registry_unref_list) {
                 return Err(Error::MismatchedRegistryKey);
             }
 
             ffi::luaL_unref(self.state, ffi::LUA_REGISTRYINDEX, key.registry_id);
-            // Don't adding to the registry drop list when dropping the key
-            key.registry_id = ffi::LUA_REFNIL;
+            key.drop_unref = false;
             Ok(())
         }
     }
@@ -546,7 +560,7 @@ impl Lua {
     /// `Error::MismatchedRegistryKey` if passed a `RegistryKey` that was not created with a
     /// matching `Lua` state.
     pub fn owns_registry_value(&self, key: &RegistryKey) -> bool {
-        unsafe { Arc::ptr_eq(&key.drop_list, &(*self.extra()).registry_drop_list) }
+        unsafe { Arc::ptr_eq(&key.unref_list, &(*self.extra()).registry_unref_list) }
     }
 
     /// Remove any registry values whose `RegistryKey`s have all been dropped.  Unlike normal handle
@@ -554,11 +568,11 @@ impl Lua {
     /// can call this method to remove any unreachable registry values.
     pub fn expire_registry_values(&self) {
         unsafe {
-            let drop_list = mem::replace(
-                (*self.extra()).registry_drop_list.lock().unwrap().as_mut(),
+            let unref_list = mem::replace(
+                (*self.extra()).registry_unref_list.lock().unwrap().as_mut(),
                 Vec::new(),
             );
-            for id in drop_list {
+            for id in unref_list {
                 ffi::luaL_unref(self.state, ffi::LUA_REGISTRYINDEX, id);
             }
         }
@@ -694,6 +708,7 @@ impl Lua {
         LuaRef {
             lua: self,
             registry_id: registry_id,
+            drop_unref: true,
         }
     }
 
@@ -920,7 +935,7 @@ impl Lua {
 
             let extra_data = Box::into_raw(Box::new(ExtraData {
                 registered_userdata: HashMap::new(),
-                registry_drop_list: Arc::new(Mutex::new(Vec::new())),
+                registry_unref_list: Arc::new(Mutex::new(Vec::new())),
             }));
             *(ffi::lua_getextraspace(state) as *mut *mut ExtraData) = extra_data;
         });
@@ -934,6 +949,11 @@ impl Lua {
 
     fn create_callback_function<'lua>(&'lua self, func: Callback<'lua>) -> Result<Function<'lua>> {
         unsafe extern "C" fn callback_call_impl(state: *mut ffi::lua_State) -> c_int {
+            if ffi::lua_type(state, ffi::lua_upvalueindex(1)) == ffi::LUA_TNIL {
+                ffi::lua_pushstring(state, cstr!("rust callback has been destructed"));
+                ffi::lua_error(state)
+            }
+
             callback_error(state, || {
                 let lua = Lua {
                     state: state,
@@ -976,7 +996,7 @@ impl Lua {
                     self.state,
                     &FUNCTION_METATABLE_REGISTRY_KEY as *const u8 as *mut c_void,
                 );
-                ffi::lua_gettable(self.state, ffi::LUA_REGISTRYINDEX);
+                ffi::lua_rawget(self.state, ffi::LUA_REGISTRYINDEX);
                 ffi::lua_setmetatable(self.state, -2);
 
                 protect_lua_call(self.state, 1, 1, |state| {
@@ -988,9 +1008,104 @@ impl Lua {
         }
     }
 
+    fn do_create_userdata<T>(&self, data: T) -> Result<AnyUserData>
+    where
+        T: UserData,
+    {
+        unsafe {
+            stack_err_guard(self.state, 0, move || {
+                check_stack(self.state, 3);
+
+                push_userdata::<RefCell<T>>(self.state, RefCell::new(data))?;
+
+                ffi::lua_rawgeti(
+                    self.state,
+                    ffi::LUA_REGISTRYINDEX,
+                    self.userdata_metatable::<T>()? as ffi::lua_Integer,
+                );
+
+                ffi::lua_setmetatable(self.state, -2);
+
+                Ok(AnyUserData(self.pop_ref(self.state)))
+            })
+        }
+    }
+
     unsafe fn extra(&self) -> *mut ExtraData {
         *(ffi::lua_getextraspace(self.main_state) as *mut *mut ExtraData)
     }
 }
 
+impl<'lua> Scope<'lua> {
+    pub fn create_function<'scope, A, R, F>(&'scope self, mut func: F) -> Result<Function<'scope>>
+    where
+        A: FromLuaMulti<'scope>,
+        R: ToLuaMulti<'scope>,
+        F: 'scope + FnMut(&'scope Lua, A) -> Result<R>,
+    {
+        unsafe {
+            let mut f = self.lua
+                .create_callback_function(Box::new(move |lua, args| {
+                    func(lua, A::from_lua_multi(args, lua)?)?.to_lua_multi(lua)
+                }))?;
+            f.0.drop_unref = false;
+            let mut destructors = self.destructors.borrow_mut();
+            let registry_id = f.0.registry_id;
+            destructors.push(Box::new(move |state| {
+                check_stack(state, 2);
+                ffi::lua_rawgeti(
+                    state,
+                    ffi::LUA_REGISTRYINDEX,
+                    registry_id as ffi::lua_Integer,
+                );
+                ffi::lua_getupvalue(state, -1, 1);
+                destruct_userdata::<RefCell<Callback>>(state);
+
+                ffi::lua_pushnil(state);
+                ffi::lua_setupvalue(state, -2, 1);
+
+                ffi::lua_pop(state, 1);
+            }));
+            Ok(f)
+        }
+    }
+
+    pub fn create_userdata<T>(&self, data: T) -> Result<AnyUserData>
+    where
+        T: UserData,
+    {
+        unsafe {
+            let mut u = self.lua.do_create_userdata(data)?;
+            u.0.drop_unref = false;
+            let mut destructors = self.destructors.borrow_mut();
+            let registry_id = u.0.registry_id;
+            destructors.push(Box::new(move |state| {
+                check_stack(state, 1);
+                ffi::lua_rawgeti(
+                    state,
+                    ffi::LUA_REGISTRYINDEX,
+                    registry_id as ffi::lua_Integer,
+                );
+                destruct_userdata::<RefCell<T>>(state);
+            }));
+            Ok(u)
+        }
+    }
+}
+
+impl<'lua> Drop for Scope<'lua> {
+    fn drop(&mut self) {
+        let state = self.lua.state;
+        for mut destructor in self.destructors.get_mut().drain(..) {
+            match catch_unwind(AssertUnwindSafe(move || destructor(state))) {
+                Ok(_) => {}
+                Err(_) => {
+                    eprintln!("Scope userdata Drop impl has panicked, aborting!");
+                    process::abort()
+                }
+            }
+        }
+    }
+}
+
 static FUNCTION_METATABLE_REGISTRY_KEY: u8 = 0;
diff --git a/src/table.rs b/src/table.rs
index 679849a..29af395 100644
--- a/src/table.rs
+++ b/src/table.rs
@@ -266,6 +266,7 @@ impl<'lua> Table<'lua> {
         let next_key = Some(LuaRef {
             lua: self.0.lua,
             registry_id: ffi::LUA_REFNIL,
+            drop_unref: true,
         });
 
         TablePairs {
diff --git a/src/tests.rs b/src/tests.rs
index 1185b1b..bb721d8 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -1,6 +1,8 @@
 use std::fmt;
 use std::error;
 use std::rc::Rc;
+use std::cell::Cell;
+use std::sync::Arc;
 use std::panic::catch_unwind;
 
 use {Error, ExternalError, Function, Lua, Nil, Result, Table, UserData, Value, Variadic};
@@ -539,16 +541,16 @@ fn test_registry_value() {
 
 #[test]
 fn test_drop_registry_value() {
-    struct MyUserdata(Rc<()>);
+    struct MyUserdata(Arc<()>);
 
     impl UserData for MyUserdata {}
 
     let lua = Lua::new();
 
-    let rc = Rc::new(());
+    let rc = Arc::new(());
 
     let r = lua.create_registry_value(MyUserdata(rc.clone())).unwrap();
-    assert_eq!(Rc::strong_count(&rc), 2);
+    assert_eq!(Arc::strong_count(&rc), 2);
 
     drop(r);
     lua.expire_registry_values();
@@ -556,7 +558,7 @@ fn test_drop_registry_value() {
     lua.exec::<()>(r#"collectgarbage("collect")"#, None)
         .unwrap();
 
-    assert_eq!(Rc::strong_count(&rc), 1);
+    assert_eq!(Arc::strong_count(&rc), 1);
 }
 
 #[test]
@@ -597,6 +599,72 @@ fn test_mismatched_registry_key() {
     };
 }
 
+#[test]
+fn scope_func() {
+    let rc = Rc::new(Cell::new(0));
+
+    let lua = Lua::new();
+    lua.scope(|scope| {
+        let r = rc.clone();
+        let f = scope
+            .create_function(move |_, ()| {
+                r.set(42);
+                Ok(())
+            })
+            .unwrap();
+        lua.globals().set("bad", f.clone()).unwrap();
+        f.call::<_, ()>(()).unwrap();
+    });
+    assert_eq!(rc.get(), 42);
+    assert_eq!(Rc::strong_count(&rc), 1);
+
+    assert!(
+        lua.globals()
+            .get::<_, Function>("bad")
+            .unwrap()
+            .call::<_, ()>(())
+            .is_err()
+    );
+}
+
+#[test]
+fn scope_drop() {
+    struct MyUserdata(Rc<()>);
+    impl UserData for MyUserdata {}
+
+    let rc = Rc::new(());
+
+    let lua = Lua::new();
+    lua.scope(|scope| {
+        lua.globals()
+            .set(
+                "test",
+                scope.create_userdata(MyUserdata(rc.clone())).unwrap(),
+            )
+            .unwrap();
+        assert_eq!(Rc::strong_count(&rc), 2);
+    });
+    assert_eq!(Rc::strong_count(&rc), 1);
+}
+
+#[test]
+fn scope_capture() {
+    let mut i = 0;
+
+    let lua = Lua::new();
+    lua.scope(|scope| {
+        scope
+            .create_function(|_, ()| {
+                i = 42;
+                Ok(())
+            })
+            .unwrap()
+            .call::<_, ()>(())
+            .unwrap();
+    });
+    assert_eq!(i, 42);
+}
+
 // TODO: Need to use compiletest-rs or similar to make sure these don't compile.
 /*
 #[test]
@@ -619,5 +687,15 @@ fn should_not_compile() {
     globals.set("boom", lua.create_function(|_, _| {
         lua.eval::<i32>("1 + 1", None)
     })).unwrap();
+
+    // Should not allow Scope references to leak
+    struct MyUserdata(Rc<()>);
+    impl UserData for MyUserdata {}
+
+    let lua = Lua::new();
+    let mut r = None;
+    lua.scope(|scope| {
+    r = Some(scope.create_userdata(MyUserdata(Rc::new(()))).unwrap());
+        });
 }
 */
diff --git a/src/types.rs b/src/types.rs
index 5aa531b..de9f4e9 100644
--- a/src/types.rs
+++ b/src/types.rs
@@ -25,13 +25,14 @@ pub struct LightUserData(pub *mut c_void);
 /// can be used in many situations where it would be impossible to store a regular handle value.
 pub struct RegistryKey {
     pub(crate) registry_id: c_int,
-    pub(crate) drop_list: Arc<Mutex<Vec<c_int>>>,
+    pub(crate) unref_list: Arc<Mutex<Vec<c_int>>>,
+    pub(crate) drop_unref: bool,
 }
 
 impl Drop for RegistryKey {
     fn drop(&mut self) {
-        if self.registry_id != ffi::LUA_REFNIL {
-            self.drop_list.lock().unwrap().push(self.registry_id);
+        if self.drop_unref {
+            self.unref_list.lock().unwrap().push(self.registry_id);
         }
     }
 }
@@ -42,6 +43,7 @@ pub(crate) type Callback<'lua> =
 pub(crate) struct LuaRef<'lua> {
     pub lua: &'lua Lua,
     pub registry_id: c_int,
+    pub drop_unref: bool,
 }
 
 impl<'lua> fmt::Debug for LuaRef<'lua> {
@@ -52,17 +54,27 @@ impl<'lua> fmt::Debug for LuaRef<'lua> {
 
 impl<'lua> Clone for LuaRef<'lua> {
     fn clone(&self) -> Self {
-        unsafe {
-            self.lua.push_ref(self.lua.state, self);
-            self.lua.pop_ref(self.lua.state)
+        if self.drop_unref {
+            unsafe {
+                self.lua.push_ref(self.lua.state, self);
+                self.lua.pop_ref(self.lua.state)
+            }
+        } else {
+            LuaRef {
+                lua: self.lua,
+                registry_id: self.registry_id,
+                drop_unref: self.drop_unref,
+            }
         }
     }
 }
 
 impl<'lua> Drop for LuaRef<'lua> {
     fn drop(&mut self) {
-        unsafe {
-            ffi::luaL_unref(self.lua.state, ffi::LUA_REGISTRYINDEX, self.registry_id);
+        if self.drop_unref {
+            unsafe {
+                ffi::luaL_unref(self.lua.state, ffi::LUA_REGISTRYINDEX, self.registry_id);
+            }
         }
     }
 }
diff --git a/src/userdata.rs b/src/userdata.rs
index 8c154b5..536c853 100644
--- a/src/userdata.rs
+++ b/src/userdata.rs
@@ -89,7 +89,7 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> {
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,
-        M: 'static + for<'a> FnMut(&'lua Lua, &'a T, A) -> Result<R>,
+        M: 'static + Send + for<'a> FnMut(&'lua Lua, &'a T, A) -> Result<R>,
     {
         self.methods
             .insert(name.to_owned(), Self::box_method(method));
@@ -104,7 +104,7 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> {
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,
-        M: 'static + for<'a> FnMut(&'lua Lua, &'a mut T, A) -> Result<R>,
+        M: 'static + Send + for<'a> FnMut(&'lua Lua, &'a mut T, A) -> Result<R>,
     {
         self.methods
             .insert(name.to_owned(), Self::box_method_mut(method));
@@ -121,7 +121,7 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> {
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,
-        F: 'static + FnMut(&'lua Lua, A) -> Result<R>,
+        F: 'static + Send + FnMut(&'lua Lua, A) -> Result<R>,
     {
         self.methods
             .insert(name.to_owned(), Self::box_function(function));
@@ -139,7 +139,7 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> {
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,
-        M: 'static + for<'a> FnMut(&'lua Lua, &'a T, A) -> Result<R>,
+        M: 'static + Send + for<'a> FnMut(&'lua Lua, &'a T, A) -> Result<R>,
     {
         self.meta_methods.insert(meta, Self::box_method(method));
     }
@@ -156,7 +156,7 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> {
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,
-        M: 'static + for<'a> FnMut(&'lua Lua, &'a mut T, A) -> Result<R>,
+        M: 'static + Send + for<'a> FnMut(&'lua Lua, &'a mut T, A) -> Result<R>,
     {
         self.meta_methods.insert(meta, Self::box_method_mut(method));
     }
@@ -170,7 +170,7 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> {
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,
-        F: 'static + FnMut(&'lua Lua, A) -> Result<R>,
+        F: 'static + Send + FnMut(&'lua Lua, A) -> Result<R>,
     {
         self.meta_methods.insert(meta, Self::box_function(function));
     }
@@ -179,7 +179,7 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> {
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,
-        F: 'static + FnMut(&'lua Lua, A) -> Result<R>,
+        F: 'static + Send + FnMut(&'lua Lua, A) -> Result<R>,
     {
         Box::new(move |lua, args| function(lua, A::from_lua_multi(args, lua)?)?.to_lua_multi(lua))
     }
@@ -188,7 +188,7 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> {
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,
-        M: 'static + for<'a> FnMut(&'lua Lua, &'a T, A) -> Result<R>,
+        M: 'static + Send + for<'a> FnMut(&'lua Lua, &'a T, A) -> Result<R>,
     {
         Box::new(move |lua, mut args| {
             if let Some(front) = args.pop_front() {
@@ -209,7 +209,7 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> {
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,
-        M: 'static + for<'a> FnMut(&'lua Lua, &'a mut T, A) -> Result<R>,
+        M: 'static + Send + for<'a> FnMut(&'lua Lua, &'a mut T, A) -> Result<R>,
     {
         Box::new(move |lua, mut args| {
             if let Some(front) = args.pop_front() {
@@ -433,7 +433,7 @@ impl<'lua> AnyUserData<'lua> {
 
 #[cfg(test)]
 mod tests {
-    use std::rc::Rc;
+    use std::sync::Arc;
 
     use super::{MetaMethod, UserData, UserDataMethods};
     use error::ExternalError;
@@ -590,11 +590,11 @@ mod tests {
 
     #[test]
     fn detroys_userdata() {
-        struct MyUserdata(Rc<()>);
+        struct MyUserdata(Arc<()>);
 
         impl UserData for MyUserdata {}
 
-        let rc = Rc::new(());
+        let rc = Arc::new(());
 
         let lua = Lua::new();
         {
@@ -602,9 +602,9 @@ mod tests {
             globals.set("userdata", MyUserdata(rc.clone())).unwrap();
         }
 
-        assert_eq!(Rc::strong_count(&rc), 2);
+        assert_eq!(Arc::strong_count(&rc), 2);
         drop(lua); // should destroy all objects
-        assert_eq!(Rc::strong_count(&rc), 1);
+        assert_eq!(Arc::strong_count(&rc), 1);
     }
 
     #[test]
diff --git a/src/util.rs b/src/util.rs
index fa36028..1de9b9f 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -250,18 +250,24 @@ pub unsafe fn get_userdata<T>(state: *mut ffi::lua_State, index: c_int) -> *mut
 
 pub unsafe extern "C" fn userdata_destructor<T>(state: *mut ffi::lua_State) -> c_int {
     callback_error(state, || {
-        // We set the metatable of userdata on __gc to a special table with no __gc method and with
-        // metamethods that trigger an error on access.  We do this so that it will not be double
-        // dropped, and also so that it cannot be used or identified as any particular userdata type
-        // after the first call to __gc.
-        get_gc_userdata_metatable(state);
-        ffi::lua_setmetatable(state, -2);
-        let ud = &mut *(ffi::lua_touserdata(state, 1) as *mut T);
-        mem::replace(ud, mem::uninitialized());
+        destruct_userdata::<T>(state);
         Ok(0)
     })
 }
 
+// Pops the userdata off of the top of the stack and drops it
+pub unsafe fn destruct_userdata<T>(state: *mut ffi::lua_State) {
+    // We set the metatable of userdata on __gc to a special table with no __gc method and with
+    // metamethods that trigger an error on access.  We do this so that it will not be double
+    // dropped, and also so that it cannot be used or identified as any particular userdata type
+    // after the first call to __gc.
+    get_destructed_userdata_metatable(state);
+    ffi::lua_setmetatable(state, -2);
+    let ud = &mut *(ffi::lua_touserdata(state, -1) as *mut T);
+    ffi::lua_pop(state, 1);
+    mem::replace(ud, mem::uninitialized());
+}
+
 // In the context of a lua callback, this will call the given function and if the given function
 // returns an error, *or if the given function panics*, this will result in a call to lua_error (a
 // longjmp).  The error or panic is wrapped in such a way that when calling pop_error back on
@@ -517,7 +523,7 @@ unsafe fn get_error_metatable(state: *mut ffi::lua_State) -> c_int {
         state,
         &ERROR_METATABLE_REGISTRY_KEY as *const u8 as *mut c_void,
     );
-    let t = ffi::lua_gettable(state, ffi::LUA_REGISTRYINDEX);
+    let t = ffi::lua_rawget(state, ffi::LUA_REGISTRYINDEX);
 
     if t != ffi::LUA_TTABLE {
         ffi::lua_pop(state, 1);
@@ -558,7 +564,7 @@ unsafe fn get_panic_metatable(state: *mut ffi::lua_State) -> c_int {
         state,
         &PANIC_METATABLE_REGISTRY_KEY as *const u8 as *mut c_void,
     );
-    let t = ffi::lua_gettable(state, ffi::LUA_REGISTRYINDEX);
+    let t = ffi::lua_rawget(state, ffi::LUA_REGISTRYINDEX);
 
     if t != ffi::LUA_TTABLE {
         ffi::lua_pop(state, 1);
@@ -588,16 +594,19 @@ unsafe fn get_panic_metatable(state: *mut ffi::lua_State) -> c_int {
     ffi::LUA_TTABLE
 }
 
-unsafe fn get_gc_userdata_metatable(state: *mut ffi::lua_State) -> c_int {
-    static GC_USERDATA_METATABLE: u8 = 0;
+unsafe fn get_destructed_userdata_metatable(state: *mut ffi::lua_State) -> c_int {
+    static DESTRUCTED_USERDATA_METATABLE: u8 = 0;
 
-    unsafe extern "C" fn gc_error(state: *mut ffi::lua_State) -> c_int {
-        ffi::lua_pushstring(state, cstr!("userdata has been garbage collected"));
+    unsafe extern "C" fn destructed_error(state: *mut ffi::lua_State) -> c_int {
+        ffi::lua_pushstring(state, cstr!("userdata has been destructed"));
         ffi::lua_error(state)
     }
 
-    ffi::lua_pushlightuserdata(state, &GC_USERDATA_METATABLE as *const u8 as *mut c_void);
-    let t = ffi::lua_gettable(state, ffi::LUA_REGISTRYINDEX);
+    ffi::lua_pushlightuserdata(
+        state,
+        &DESTRUCTED_USERDATA_METATABLE as *const u8 as *mut c_void,
+    );
+    let t = ffi::lua_rawget(state, ffi::LUA_REGISTRYINDEX);
 
     if t != ffi::LUA_TTABLE {
         ffi::lua_pop(state, 1);
@@ -606,7 +615,10 @@ unsafe fn get_gc_userdata_metatable(state: *mut ffi::lua_State) -> c_int {
 
         gc_guard(state, || {
             ffi::lua_newtable(state);
-            ffi::lua_pushlightuserdata(state, &GC_USERDATA_METATABLE as *const u8 as *mut c_void);
+            ffi::lua_pushlightuserdata(
+                state,
+                &DESTRUCTED_USERDATA_METATABLE as *const u8 as *mut c_void,
+            );
             ffi::lua_pushvalue(state, -2);
 
             for &method in &[
@@ -637,7 +649,7 @@ unsafe fn get_gc_userdata_metatable(state: *mut ffi::lua_State) -> c_int {
                 cstr!("__ipairs"),
             ] {
                 ffi::lua_pushstring(state, method);
-                ffi::lua_pushcfunction(state, gc_error);
+                ffi::lua_pushcfunction(state, destructed_error);
                 ffi::lua_rawset(state, -3);
             }