Clarify + test Packed*Array behavior w.r.t. copy-and-write + #[var]

Author: Bromeon

godot-core/src/builtin/collections/packed_array.rs

@@ -68,11 +68,11 @@ macro_rules! impl_packed_array {
         ///
         /// # Registering properties
         ///
-        /// You can use both `#[var]` and `#[export]` with packed arrays. However, since they use copy-on-write, GDScript (for `#[var]`) and the
-        /// editor (for `#[export]`) will effectively keep an independent copy of the array. Writes to the packed array from Rust are thus not
-        /// reflected on the other side -- you may need to replace the entire array.
+        /// You can use both `#[var]` and `#[export]` with packed arrays. In godot-rust, modifications to packed array properties are
+        /// properly synchronized between Rust and GDScript/reflection access.
         ///
-        /// See also [godot/#76150](https://github.com/godotengine/godot/issues/76150) for details.
+        /// In GDScript, mutating methods like `append_array()` may not work on `Packed*Array` properties of engine classes.
+        /// See [godot/#76150](https://github.com/godotengine/godot/issues/76150) for details.
         ///
         /// # Thread safety
         ///

itest/rust/src/object_tests/property_test.rs

@@ -5,7 +5,9 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
  */
 
-use godot::builtin::{vdict, vslice, Color, Dictionary, GString, Variant, VariantType};
+use godot::builtin::{
+    vdict, vslice, Color, Dictionary, GString, PackedInt32Array, Variant, VariantType,
+};
 use godot::classes::{INode, IRefCounted, Node, Object, RefCounted, Resource, Texture};
 use godot::global::{PropertyHint, PropertyUsageFlags};
 use godot::meta::{GodotConvert, PropertyHintInfo, ToGodot};
@@ -48,6 +50,9 @@ struct HasProperty {
 
     #[var(get = get_texture_val, set = set_texture_val, hint = RESOURCE_TYPE, hint_string = "Texture")]
     texture_val_rw: Option<Gd<Texture>>,
+
+    #[var]
+    packed_int_array: PackedInt32Array,
 }
 
 #[godot_api]
@@ -141,6 +146,7 @@ impl INode for HasProperty {
             string_val: GString::new(),
             texture_val: OnEditor::default(),
             texture_val_rw: None,
+            packed_int_array: PackedInt32Array::new(),
         }
     }
 }
@@ -628,6 +634,33 @@ fn test_var_with_renamed_funcs() {
     obj.free();
 }
 
+// Tests that CoW packed-arrays' changes are reflected from Rust. See:
+// * Rust (sync does work): https://github.com/godot-rust/gdext/pull/576
+// * GDScript (not synced): https://github.com/godotengine/godot/issues/76150
+#[itest]
+fn test_copy_on_write_var() {
+    let mut obj = HasProperty::new_alloc();
+
+    // Mutate property via reflection -> verify change is reflected in Rust.
+    obj.set(
+        "packed_int_array",
+        &PackedInt32Array::from([1, 2, 3]).to_variant(),
+    );
+    assert_eq!(
+        obj.bind().packed_int_array,
+        PackedInt32Array::from(&[1, 2, 3])
+    );
+
+    // Mutate property in Rust -> verify change is reflected in Godot.
+    obj.bind_mut().packed_int_array.push(4);
+    assert_eq!(
+        obj.get("packed_int_array").to::<PackedInt32Array>(),
+        PackedInt32Array::from(&[1, 2, 3, 4])
+    );
+
+    obj.free();
+}
+
 // ----------------------------------------------------------------------------------------------------------------------------------------------
 
 #[derive(GodotClass)]