Hot reload: work around #[class(no_init)] limitation before Godot 4.5

Author: Bromeon

godot-core/src/registry/callbacks.rs

@@ -43,6 +43,24 @@ pub unsafe extern "C" fn create<T: cap::GodotDefault>(
     create_custom(T::__godot_user_init).unwrap_or(std::ptr::null_mut())
 }
 
+/// Workaround for <https://github.com/godot-rust/gdext/issues/874> before Godot 4.5.
+///
+/// Godot expects a creator function, but doesn't require an actual object to be instantiated.
+#[cfg(all(since_api = "4.4", before_api = "4.5"))]
+pub unsafe extern "C" fn create_null<T>(
+    _class_userdata: *mut std::ffi::c_void,
+    _notify_postinitialize: sys::GDExtensionBool,
+) -> sys::GDExtensionObjectPtr {
+    std::ptr::null_mut()
+}
+
+#[cfg(before_api = "4.4")]
+pub unsafe extern "C" fn create_null<T>(
+    _class_userdata: *mut std::ffi::c_void,
+) -> sys::GDExtensionObjectPtr {
+    std::ptr::null_mut()
+}
+
 /// Godot FFI function for recreating a GDExtension instance, e.g. after a hot reload.
 ///
 /// If the `init()` constructor panics, null is returned.
@@ -55,6 +73,17 @@ pub unsafe extern "C" fn recreate<T: cap::GodotDefault>(
         .unwrap_or(std::ptr::null_mut())
 }
 
+/// Workaround for <https://github.com/godot-rust/gdext/issues/874> before Godot 4.5.
+///
+/// Godot expects a creator function, but doesn't require an actual object to be instantiated.
+#[cfg(all(since_api = "4.2", before_api = "4.5"))]
+pub unsafe extern "C" fn recreate_null<T>(
+    _class_userdata: *mut std::ffi::c_void,
+    _object: sys::GDExtensionObjectPtr,
+) -> sys::GDExtensionClassInstancePtr {
+    std::ptr::null_mut()
+}
+
 pub(crate) fn create_custom<T, F>(
     make_user_instance: F,
 ) -> Result<sys::GDExtensionObjectPtr, PanicPayload>

godot-core/src/registry/plugin.rs

@@ -147,6 +147,10 @@ pub struct Struct {
 
     /// Godot low-level `create` function, wired up to library-generated `init`.
     ///
+    /// For `#[class(no_init)]`, behavior depends on Godot version:
+    /// - 4.5 and later: `None`
+    /// - until 4.4: a dummy function that fails, to not break hot reloading.
+    ///
     /// This is mutually exclusive with [`ITraitImpl::user_create_fn`].
     pub(crate) generated_create_fn: Option<GodotCreateFn>,
 
@@ -222,6 +226,19 @@ impl Struct {
         self
     }
 
+    // Workaround for https://github.com/godot-rust/gdext/issues/874, before https://github.com/godotengine/godot/pull/99133 is merged in 4.5.
+    #[cfg(before_api = "4.5")]
+    pub fn with_generated_no_default<T: GodotClass>(mut self) -> Self {
+        set(&mut self.generated_create_fn, callbacks::create_null::<T>);
+
+        #[cfg(since_api = "4.2")]
+        set(
+            &mut self.generated_recreate_fn,
+            callbacks::recreate_null::<T>,
+        );
+        self
+    }
+
     pub fn with_default_get_virtual_fn<T: GodotClass + UserClass>(mut self) -> Self {
         set(
             &mut self.default_get_virtual_fn,

godot-macros/src/class/derive_godot_class.rs

@@ -122,6 +122,10 @@ pub fn derive_godot_class(item: venial::Item) -> ParseResult<TokenStream> {
         }
         InitStrategy::Absent => {
             is_instantiable = false;
+
+            // Workaround for https://github.com/godot-rust/gdext/issues/874 before Godot 4.5.
+            #[cfg(before_api = "4.5")]
+            modifiers.push(quote! { with_generated_no_default::<#class_name> });
         }
     };
     if is_instantiable {