LibGUI: Optimize GUI::Variant move constructor

Author: https://github.com/ccapitalK
Commit: https://github.com/SerenityOS/serenity/commit/4062add8edd
Pull-request: https://github.com/SerenityOS/serenity/pull/4315

Libraries/LibGUI/Variant.cpp

@@ -26,6 +26,7 @@
 
 #include <AK/FlyString.h>
 #include <AK/JsonValue.h>
+#include <AK/RefPtr.h>
 #include <LibGUI/Variant.h>
 
 namespace GUI {
@@ -255,10 +256,8 @@ Variant& Variant::operator=(Variant&& other)
 {
     if (&other == this)
         return *this;
-    // FIXME: Move, not copy!
     clear();
-    copy_from(other);
-    other.clear();
+    move_from(AK::move(other));
     return *this;
 }
 
@@ -267,6 +266,14 @@ Variant::Variant(const Variant& other)
     copy_from(other);
 }
 
+void Variant::move_from(Variant&& other)
+{
+    m_type = other.m_type;
+    m_value = other.m_value;
+    other.m_type = Type::Invalid;
+    other.m_value.as_string = nullptr;
+}
+
 void Variant::copy_from(const Variant& other)
 {
     ASSERT(!is_valid());

Libraries/LibGUI/Variant.h

@@ -250,6 +250,7 @@ public:
 
 private:
     void copy_from(const Variant&);
+    void move_from(Variant&&);
 
     struct RawPoint {
         int x;