Clarify Node::duplicate() semantics on #[var] and #[export] fields

itest/rust/src/object_tests/property_template_test.rs

@@ -107,6 +107,11 @@ fn property_template_test(ctx: &TestContext) {
                 "mismatch in property {name}:\n  GDScript: {gdscript_prop:?}\n  Rust:     {rust_prop:?}"
             ));
         }
+        /*else { // Keep around for debugging.
+            println!(
+                "good property {name}:\n  GDScript: {gdscript_prop:?}\n  Rust:     {rust_prop:?}"
+            );
+        }*/
     }
 
     rust_properties.free();

itest/rust/src/object_tests/property_test.rs

@@ -537,3 +537,61 @@ fn test_var_with_renamed_funcs() {
 
     obj.free();
 }
+
+// ----------------------------------------------------------------------------------------------------------------------------------------------
+
+#[derive(GodotClass)]
+#[class(init, base=Node)]
+struct Duplicator {
+    // #[export] would also make tests pass, but #[export(storage)] additionally hides the properties from the editor.
+    // See https://docs.godotengine.org/en/stable/tutorials/scripting/gdscript/gdscript_exports.html#export-storage.
+    #[export(storage)]
+    int_export: i32,
+
+    // Low-level #[var] should also work.
+    #[var(usage_flags = [STORAGE])]
+    int_var: i32,
+
+    // Not copied because not marked as serialize ("storage").
+    #[var]
+    int_ignored: i32,
+
+    #[export(storage)]
+    optional_node: Option<Gd<Node>>,
+
+    #[export(storage)]
+    oneditor_node: OnEditor<Gd<Node>>,
+}
+
+#[itest]
+fn test_duplicate_retains_properties() {
+    let optional_node = Node::new_alloc();
+    let oneditor_node = Node::new_alloc();
+
+    // Set up original node.
+    let mut original = Duplicator::new_alloc();
+    {
+        let mut original = original.bind_mut();
+        original.int_export = 5;
+        original.int_var = 7;
+        original.int_ignored = 9; // Will not be copied.
+        original.optional_node = Some(optional_node.clone());
+        original.oneditor_node.init(oneditor_node.clone());
+    }
+
+    // Create duplicate and verify all properties are copied correctly.
+    let duplicated: Gd<Duplicator> = original.duplicate().unwrap().cast();
+    {
+        let duplicated = duplicated.bind();
+        assert_eq!(duplicated.int_export, 5);
+        assert_eq!(duplicated.int_var, 7);
+        assert_eq!(duplicated.int_ignored, 0); // Not copied.
+        assert_eq!(duplicated.optional_node.as_ref().unwrap(), &optional_node);
+        assert_eq!(&*duplicated.oneditor_node, &oneditor_node);
+    }
+
+    optional_node.free();
+    oneditor_node.free();
+    duplicated.free();
+    original.free();
+}