Optimize `Lua::create_registry_value` by reusing previously expired registry keys.

src/lua.rs

@@ -1721,14 +1721,29 @@ impl Lua {
             let _sg = StackGuard::new(self.state);
             check_stack(self.state, 4)?;
 
+            let unref_list = (*self.extra.get()).registry_unref_list.clone();
             self.push_value(t)?;
+
+            // Try to reuse previously allocated RegistryKey
+            let unref_list2 = unref_list.clone();
+            let mut unref_list2 = mlua_expect!(unref_list2.lock(), "unref list poisoned");
+            if let Some(registry_id) = unref_list2.as_mut().and_then(|x| x.pop()) {
+                // It must be safe to replace the value without triggering memory error
+                ffi::lua_rawseti(self.state, ffi::LUA_REGISTRYINDEX, registry_id as Integer);
+                return Ok(RegistryKey {
+                    registry_id,
+                    unref_list,
+                });
+            }
+
+            // Allocate a new RegistryKey
             let registry_id = protect_lua!(self.state, 1, 0, |state| {
                 ffi::luaL_ref(state, ffi::LUA_REGISTRYINDEX)
             })?;
 
             Ok(RegistryKey {
                 registry_id,
-                unref_list: (*self.extra.get()).registry_unref_list.clone(),
+                unref_list,
             })
         }
     }
@@ -1787,6 +1802,10 @@ impl Lua {
         key: &RegistryKey,
         t: T,
     ) -> Result<()> {
+        if !self.owns_registry_value(key) {
+            return Err(Error::MismatchedRegistryKey);
+        }
+
         let t = t.to_lua(self)?;
         unsafe {
             let _sg = StackGuard::new(self.state);