Fix attaching __gc metamethod

Bug introdused in ddebf56

Author: khvzak

src/state/raw.rs

@@ -961,9 +961,9 @@ impl RawLua {
         #[cfg(feature = "luau")]
         let extra_init = None;
         #[cfg(not(feature = "luau"))]
-        let extra_init: Option<fn(*mut ffi::lua_State) -> Result<()>> = Some(|state| {
+        let extra_init: Option<fn(*mut ffi::lua_State, c_int) -> Result<()>> = Some(|state, mt_idx| {
             ffi::lua_pushcfunction(state, crate::util::userdata_destructor::<UserDataStorage<T>>);
-            rawset_field(state, -2, "__gc")
+            rawset_field(state, mt_idx, "__gc")
         });
 
         init_userdata_metatable(

src/util/userdata.rs

@@ -152,7 +152,7 @@ pub(crate) unsafe fn init_userdata_metatable(
     field_getters: Option<c_int>,
     field_setters: Option<c_int>,
     methods: Option<c_int>,
-    extra_init: Option<fn(*mut ffi::lua_State) -> Result<()>>,
+    extra_init: Option<fn(*mut ffi::lua_State, c_int) -> Result<()>>,
 ) -> Result<()> {
     if field_getters.is_some() || methods.is_some() {
         // Push `__index` generator function
@@ -197,7 +197,7 @@ pub(crate) unsafe fn init_userdata_metatable(
 
     // Additional initialization
     if let Some(extra_init) = extra_init {
-        extra_init(state)?;
+        extra_init(state, metatable)?;
     }
 
     ffi::lua_pushboolean(state, 0);