Prevent Gd::default_instance bypassing placeholder instances.

----

- Forward object creation through ClassDB to handle placeholder instances logic.
- Remove blanket implementation of `__godot_default` – requiring repeatedly implementing `__godot_default()` for all user classes.

Author: Yarwin

godot-core/src/obj/base.rs

@@ -80,7 +80,7 @@ pub struct Base<T: GodotClass> {
     // 2.
     obj: ManuallyDrop<Gd<T>>,
 
-    /// Tracks the initialization state of this `Base<T>` in Debug mode.
+    /// Tracks the initialization state of this `Base<T>` if safeguards are enabled.
     ///
     /// Rc allows to "copy-construct" the base from an existing one, while still affecting the user-instance through the original `Base<T>`.
     #[cfg(safeguards_balanced)]

godot-core/src/obj/bounds.rs

@@ -420,7 +420,7 @@ impl Declarer for DeclUser {
     where
         T: GodotDefault + Bounds<Declarer = Self>,
     {
-        Gd::default_instance()
+        Gd::default_user_instance()
     }
 }
 

godot-core/src/obj/gd.rs

@@ -19,12 +19,14 @@ use crate::meta::{
     ToGodot,
 };
 use crate::obj::{
-    bounds, cap, Bounds, DynGd, GdDerefTarget, GdMut, GdRef, GodotClass, Inherits, InstanceId,
-    OnEditor, RawGd, WithBaseField, WithSignals,
+    bounds, cap, Base, Bounds, DynGd, GdDerefTarget, GdMut, GdRef, GodotClass, Inherits,
+    InstanceId, OnEditor, RawGd, WithBaseField, WithSignals,
 };
+use crate::private::callbacks::postinit;
 use crate::private::{callbacks, PanicPayload};
 use crate::registry::class::try_dynify_object;
 use crate::registry::property::{object_export_element_type_string, Export, Var};
+use crate::storage::Storage;
 use crate::{classes, meta, out};
 
 /// Smart pointer to objects owned by the Godot engine.
@@ -204,6 +206,39 @@ where
     pub fn bind_mut(&mut self) -> GdMut<'_, T> {
         self.raw.bind_mut()
     }
+
+    /// Creates a new initialized instance.
+    ///
+    /// Creation goes through ClassDb, which handles creation of placeholders and other specific logic.
+    ///
+    /// Since Godot 4.4 the caller (in this case godot-rust) is responsible for emitting the `POSTINITIALIZE`
+    /// notification, which should be handled by a not fully-initialized object. This is done so to maintain the consistency –
+    /// the notification must be handled the same way regardless of how and where the instance was created (in other words,
+    /// `T::new_gd()` and `T::default()` must be equivalent to the GDScript expression `T.new()`).
+    ///
+    /// Before Godot 4.4, Godot itself was responsible for emitting the `POSTINITIALIZE` notification, so we mark the instance as initialized
+    /// during creation.
+    ///
+    /// See also: [`Base`].
+    #[doc(hidden)]
+    pub fn default_user_instance() -> Gd<T>
+    where
+        T: cap::GodotDefault,
+    {
+        // SAFETY: All safety requirements of classdb_construct_object/from_obj_sys are being held.
+        let obj = unsafe {
+            let object_ptr = sys::classdb_construct_object(T::class_id().string_sys());
+            postinit(object_ptr);
+            Gd::<T>::from_obj_sys(object_ptr)
+        };
+
+        // Mark base as initialized after postinit.
+        #[cfg(since_api = "4.4")]
+        if let Some(storage) = obj.raw.storage() {
+            unsafe { Base::from_base(storage.base()).mark_initialized() };
+        }
+        obj
+    }
 }
 
 /// _The methods in this impl block are available for any `T`._ <br><br>
@@ -524,24 +559,6 @@ impl<T: GodotClass> Gd<T> {
             .map_err(Self::from_ffi)
     }
 
-    /// Create default instance for all types that have `GodotDefault`.
-    ///
-    /// Deliberately more loose than `Gd::default()`, does not require ref-counted memory strategy for user types.
-    pub(crate) fn default_instance() -> Self
-    where
-        T: cap::GodotDefault,
-    {
-        unsafe {
-            // Default value (and compat one) for `p_notify_postinitialize` is true in Godot.
-            #[cfg(since_api = "4.4")]
-            let object_ptr = callbacks::create::<T>(std::ptr::null_mut(), sys::conv::SYS_TRUE);
-            #[cfg(before_api = "4.4")]
-            let object_ptr = callbacks::create::<T>(std::ptr::null_mut());
-
-            Gd::from_obj_sys(object_ptr)
-        }
-    }
-
     /// Upgrades to a `DynGd<T, D>` pointer, enabling the `D` abstraction.
     ///
     /// The `D` parameter can typically be inferred when there is a single `AsDyn<...>` implementation for `T`.  \

godot-core/src/obj/traits.rs

@@ -773,7 +773,7 @@ pub mod cap {
     use super::*;
     use crate::builtin::{StringName, Variant};
     use crate::meta::PropertyInfo;
-    use crate::obj::{Base, Bounds, Gd};
+    use crate::obj::{Base, Gd};
     use crate::storage::{IntoVirtualMethodReceiver, VirtualMethodReceiver};
 
     /// Trait for all classes that are default-constructible from the Godot engine.
@@ -804,19 +804,7 @@ pub mod cap {
         /// - For user-defined classes, this calls `T::init()` or the generated init-constructor.
         /// - For engine classes, this calls `T::new()`.
         #[doc(hidden)]
-        fn __godot_default() -> Gd<Self> {
-            // This is a bit hackish, but the alternatives are:
-            // 1. Separate trait `GodotUserDefault` for user classes, which then proliferates through all APIs and makes abstraction harder.
-            // 2. Repeatedly implementing __godot_default() that forwards to something like Gd::default_user_instance(). Possible, but this
-            //    will make the step toward builder APIs more difficult, as users would need to re-implement this as well.
-            sys::strict_assert_eq!(
-                std::any::TypeId::of::<<Self as Bounds>::Declarer>(),
-                std::any::TypeId::of::<bounds::DeclUser>(),
-                "__godot_default() called on engine class; must be overridden for engine classes"
-            );
-
-            Gd::default_instance()
-        }
+        fn __godot_default() -> Gd<Self>;
 
         /// Only provided for user classes.
         #[doc(hidden)]

godot-core/src/registry/callbacks.rs

@@ -13,6 +13,7 @@
 use std::any::Any;
 
 use godot_ffi as sys;
+use godot_ffi::GDExtensionObjectPtr;
 use sys::conv::u32_to_usize;
 use sys::interface_fn;
 
@@ -75,7 +76,7 @@ pub unsafe extern "C" fn recreate<T: cap::GodotDefault>(
     _class_userdata: *mut std::ffi::c_void,
     object: sys::GDExtensionObjectPtr,
 ) -> sys::GDExtensionClassInstancePtr {
-    create_rust_part_for_existing_godot_part(T::__godot_user_init, object, |_| {})
+    create_rust_part_for_existing_godot_part(T::__godot_user_init, object, true)
         .unwrap_or(std::ptr::null_mut())
 }
 
@@ -90,6 +91,18 @@ pub unsafe extern "C" fn recreate_null<T>(
     std::ptr::null_mut()
 }
 
+#[cfg(since_api = "4.4")]
+pub(crate) fn postinit(base_ptr: GDExtensionObjectPtr) {
+    // Should notify it with a weak pointer, during `NOTIFICATION_POSTINITIALIZE`, ref-counted object is not yet fully-initialized.
+    // SAFETY: `Gd::from_obj_sys_weak` will raise panic if base_ptr is null or invalid.
+    let mut obj = unsafe { Gd::<Object>::from_obj_sys_weak(base_ptr) };
+    obj.notify(crate::classes::notify::ObjectNotification::POSTINITIALIZE);
+    obj.drop_weak();
+}
+
+#[cfg(before_api = "4.4")]
+pub(crate) fn postinit(_base_ptr: GDExtensionObjectPtr) {}
+
 pub(crate) fn create_custom<T, F>(
     make_user_instance: F,
     notify_postinitialize: bool,
@@ -101,17 +114,11 @@ where
     let base_class_name = T::Base::class_id();
     let base_ptr = unsafe { sys::classdb_construct_object(base_class_name.string_sys()) };
 
-    let postinit = |base_ptr| {
-        #[cfg(since_api = "4.4")]
-        if notify_postinitialize {
-            // Should notify it with a weak pointer, during `NOTIFICATION_POSTINITIALIZE`, ref-counted object is not yet fully-initialized.
-            let mut obj = unsafe { Gd::<Object>::from_obj_sys_weak(base_ptr) };
-            obj.notify(crate::classes::notify::ObjectNotification::POSTINITIALIZE);
-            obj.drop_weak();
-        }
-    };
-
-    match create_rust_part_for_existing_godot_part(make_user_instance, base_ptr, postinit) {
+    match create_rust_part_for_existing_godot_part(
+        make_user_instance,
+        base_ptr,
+        notify_postinitialize,
+    ) {
         Ok(_extension_ptr) => Ok(base_ptr),
         Err(payload) => {
             // Creation of extension object failed; we must now also destroy the base object to avoid leak.
@@ -130,15 +137,14 @@ where
 /// With godot-rust, custom objects consist of two parts: the Godot object and the Rust object. This method takes the Godot part by pointer,
 /// creates the Rust part with the supplied state, and links them together. This is used for both brand-new object creation and hot reload.
 /// During hot reload, Rust objects are disposed of and then created again with updated code, so it's necessary to re-link them to Godot objects.
-fn create_rust_part_for_existing_godot_part<T, F, P>(
+fn create_rust_part_for_existing_godot_part<T, F>(
     make_user_instance: F,
     base_ptr: sys::GDExtensionObjectPtr,
-    postinit: P,
+    notify_postinitialize: bool,
 ) -> Result<sys::GDExtensionClassInstancePtr, PanicPayload>
 where
     T: GodotClass,
     F: FnOnce(Base<T::Base>) -> T,
-    P: Fn(sys::GDExtensionObjectPtr),
 {
     let class_name = T::class_id();
     //out!("create callback: {}", class_name.backing);
@@ -170,10 +176,11 @@ where
         );
     }
 
-    postinit(base_ptr);
-
-    // Mark initialization as complete, now that user constructor has finished.
-    base_copy.mark_initialized();
+    if notify_postinitialize {
+        postinit(base_ptr);
+        // Mark initialization as complete, now that user constructor has finished.
+        base_copy.mark_initialized();
+    }
 
     // No std::mem::forget(base_copy) here, since Base may stores other fields that need deallocation.
     Ok(instance_ptr)

godot-macros/src/class/data_models/interface_trait_impl.rs

@@ -289,6 +289,10 @@ fn handle_init<'a>(
 
         #(#cfg_attrs)*
         impl ::godot::obj::cap::GodotDefault for #class_name {
+            fn __godot_default() -> ::godot::obj::Gd<Self> {
+                ::godot::obj::Gd::default_user_instance()
+            }
+
             fn __godot_user_init(base: ::godot::obj::Base<Self::Base>) -> Self {
                 <Self as #trait_path>::init(base)
             }

godot-macros/src/class/derive_godot_class.rs

@@ -362,6 +362,10 @@ fn make_godot_init_impl(class_name: &Ident, fields: &Fields) -> TokenStream {
 
     quote! {
         impl ::godot::obj::cap::GodotDefault for #class_name {
+            fn __godot_default() -> ::godot::obj::Gd<Self> {
+                ::godot::obj::Gd::default_user_instance()
+            }
+
             fn __godot_user_init(base: ::godot::obj::Base<<#class_name as ::godot::obj::GodotClass>::Base>) -> Self {
                 Self {
                     #( #rest_init )*