shave this yak some more, make Callback type alias have two lifetimes

src/lua.rs

@@ -973,7 +973,7 @@ impl Lua {
 
     fn create_callback_function<'lua, 'callback>(
         &'lua self,
-        func: Callback<'callback>,
+        func: Callback<'callback, 'static>,
     ) -> Result<Function<'lua>> {
         unsafe extern "C" fn callback_call_impl(state: *mut ffi::lua_State) -> c_int {
             callback_error(state, || {
@@ -1066,16 +1066,11 @@ impl<'lua, 'scope> Scope<'lua, 'scope> {
         R: ToLuaMulti<'callback>,
         F: 'scope + Fn(&'callback Lua, A) -> Result<R>,
     {
-        let f: Box<
-            Fn(&'callback Lua, MultiValue<'callback>) -> Result<MultiValue<'callback>> + 'scope,
-        > = Box::new(move |lua, args| func(lua, A::from_lua_multi(args, lua)?)?.to_lua_multi(lua));
-
         unsafe {
-            // SCARY, we are transmuting the 'scope lifetime to 'static.
-            let f: Box<
-                Fn(&'callback Lua, MultiValue<'callback>) -> Result<MultiValue<'callback>>,
-            > = mem::transmute(f);
-
+            let f = Box::new(move |lua, args| {
+                func(lua, A::from_lua_multi(args, lua)?)?.to_lua_multi(lua)
+            });
+            let f = mem::transmute::<Callback<'callback, 'scope>, Callback<'callback, 'static>>(f);
             let mut f = self.lua.create_callback_function(f)?;
 
             f.0.drop_unref = false;

src/types.rs

@@ -44,7 +44,8 @@ impl Drop for RegistryKey {
     }
 }
 
-pub(crate) type Callback<'lua> = Box<Fn(&'lua Lua, MultiValue<'lua>) -> Result<MultiValue<'lua>>>;
+pub(crate) type Callback<'lua, 'a> =
+    Box<Fn(&'lua Lua, MultiValue<'lua>) -> Result<MultiValue<'lua>> + 'a>;
 
 pub(crate) struct LuaRef<'lua> {
     pub lua: &'lua Lua,

src/userdata.rs

@@ -72,8 +72,8 @@ pub enum MetaMethod {
 ///
 /// [`UserData`]: trait.UserData.html
 pub struct UserDataMethods<'lua, T> {
-    pub(crate) methods: HashMap<StdString, Callback<'lua>>,
-    pub(crate) meta_methods: HashMap<MetaMethod, Callback<'lua>>,
+    pub(crate) methods: HashMap<StdString, Callback<'lua, 'static>>,
+    pub(crate) meta_methods: HashMap<MetaMethod, Callback<'lua, 'static>>,
     pub(crate) _type: PhantomData<T>,
 }
 
@@ -175,7 +175,7 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> {
         self.meta_methods.insert(meta, Self::box_function(function));
     }
 
-    fn box_function<A, R, F>(function: F) -> Callback<'lua>
+    fn box_function<A, R, F>(function: F) -> Callback<'lua, 'static>
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,
@@ -184,7 +184,7 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> {
         Box::new(move |lua, args| function(lua, A::from_lua_multi(args, lua)?)?.to_lua_multi(lua))
     }
 
-    fn box_method<A, R, M>(method: M) -> Callback<'lua>
+    fn box_method<A, R, M>(method: M) -> Callback<'lua, 'static>
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,
@@ -205,7 +205,7 @@ impl<'lua, T: UserData> UserDataMethods<'lua, T> {
         })
     }
 
-    fn box_method_mut<A, R, M>(method: M) -> Callback<'lua>
+    fn box_method_mut<A, R, M>(method: M) -> Callback<'lua, 'static>
     where
         A: FromLuaMulti<'lua>,
         R: ToLuaMulti<'lua>,