Class without `init` cannot be returned from Rust to GDScript

[ttencate]

#[derive(GodotClass)]
#[class]
pub struct World {
}

#[derive(GodotClass)]
#[class(init)]
pub struct WorldGen {
}

#[godot_api]
impl WorldGen {
    #[func]
    fn generate(&mut self) -> Gd<World> {
        Gd::new(World {})
    }
}

When calling WorldGen.new().generate() from GDScript, Godot prints an error:

ERROR: Class 'World' has no native extension.
   at: set_object_extension_instance (core/object/class_db.cpp:356)

If I put #[class(init)] on World instead of bare #[class], it starts working. Same if I manually implement init on GodotExt.

I did expect World not to be instantiable from GDScript or addable in the editor, but when instantiated from Rust it should still be possible tot pass it back to GDScript.

[ttencate]

Notice that the editor calls init sometimes, even if you're not creating any instances yourself.

#[godot_api]
impl GodotExt for World {
    func init(_base: Base<RefCounted>) {
        unimplemented!("should never be called");
    }
}

It seems to have something to do with generating documentation:

...
#14 godot_core::registry::callbacks::create<my_crate::World> (_class_userdata=<optimized out>)
    at .../godot-core/src/registry.rs:282
#15 0x0000555559d9f6da in ClassDB::class_get_default_property_value (p_class=..., p_property=..., 
    r_valid=0x7fffffffd4e0) at core/object/class_db.cpp:1464
#16 0x0000555557190e66 in get_documentation_default_value (p_class_name=..., p_property_name=..., 
    r_default_value_valid=@0x7fffffffd4e0: false) at editor/doc_tools.cpp:339
#17 0x00005555571dffe4 in DocTools::generate (this=0x55555f96ef80, p_basic_types=true)
    at editor/doc_tools.cpp:453
#18 0x0000555557331bdd in EditorHelp::generate_doc () at editor/editor_help.cpp:2190
#19 EditorNode::EditorNode (this=<optimized out>, this=<optimized out>) at editor/editor_node.cpp:6625
#20 0x0000555556470a01 in Main::start () at main/main.cpp:2795
#21 0x00005555563de303 in main (argc=<optimized out>, argv=0x7fffffffe508)
    at platform/linuxbsd/godot_linuxbsd.cpp:71

[lilizoey]

Checking the godot source-code, we need to set the create_instance_func in GDExtensionClassCreationInfo to be a null-pointer for godot to understand that a class cannot be instantiated:

bool ClassDB::can_instantiate(const StringName &p_class) {
	OBJTYPE_RLOCK;

	ClassInfo *ti = classes.getptr(p_class);
	ERR_FAIL_COND_V_MSG(!ti, false, "Cannot get class '" + String(p_class) + "'.");
#ifdef TOOLS_ENABLED
	if (ti->api == API_EDITOR && !Engine::get_singleton()->is_editor_hint()) {
		return false;
	}
#endif
	return (!ti->disabled && ti->creation_func != nullptr && !(ti->gdextension && !ti->gdextension->create_instance));
}

Alternatively we can set a class to be virtual.

When generating documentation godot will try to instantiate a new object of that class. This is so that godot can call get_property_list on that object to generate documentation. If we make the class either not instantiable or virtual, godot will not try to instantiate a new object of that class and will instead attempt to instantiate a direct inheriter of that class. failing this i believe godot just fails to generate docs for that class's properties.

[lilizoey]

https://github.com/godotengine/godot/pull/58972 see also here for a guide on what abstract vs virtual means and such. but the basic point is that we do need to properly set our class as not being instantiable.

[Bromeon]

I just tested the original snippet. Nowadays, the API expects explicit init or no_init, and the error message is quite detailed if forgotten:

error[E0277]: Class `World` requires either an `init` constructor, or explicit opt-out
  --> itest/rust/src/register_tests/var_test.rs:27:12
   |
27 | pub struct World {
   |            ^^^^^ needs `init`
   |
   = help: the trait `GodotDefault` is not implemented for `World`
   = note: To provide a default constructor, use `#[class(init)]` or implement an `init` method
   = note: To opt out, use `#[class(no_init)]`
   = note: see also: https://godot-rust.github.io/book/register/constructors.html

So, after adding no_init, we have this updated code:

#[derive(GodotClass)]
#[class(no_init)]
pub struct World {}

#[derive(GodotClass)]
#[class(init)]
pub struct WorldGen {}

#[godot_api]
impl WorldGen {
    #[func]
    fn generate(&mut self) -> Gd<World> {
        Gd::from_object(World {})
    }
}

Now, calling from GDScript:

func test_world_gen():
    var w = WorldGen.new().generate()
    print(w)

This works as expected, prints <RefCounted#-9223371997294098691>. The issue should thus be solved :beer: