Trick to cause compile error when missing #[godot_api] attribute

Idea by @chitoyuu

Author: Bromeon

godot-core/src/bind.rs

@@ -25,7 +25,7 @@ use crate::obj::GodotClass;
 /// Do not call any of these methods directly -- they are an interface to Godot. Functionality
 /// described here is available through other means (e.g. `init` via `Gd::new_default`).
 #[allow(unused_variables)]
-pub trait GodotExt
+pub trait GodotExt: crate::private::You_forgot_the_attribute__godot_api
 where
     Self: GodotClass,
 {

godot-core/src/lib.rs

@@ -22,6 +22,10 @@ pub use godot_ffi as sys;
 
 #[doc(hidden)]
 pub mod private {
+    // If someone forgets #[godot_api], this causes a compile error, rather than virtual functions not being called at runtime.
+    #[allow(non_camel_case_types)]
+    pub trait You_forgot_the_attribute__godot_api {}
+
     pub use crate::builtin::func_callbacks;
     pub use crate::engine::gen::classes::class_macros;
     pub use crate::registry::{callbacks, ClassPlugin, ErasedRegisterFn, PluginComponent};

godot-macros/src/godot_api.rs

@@ -282,6 +282,8 @@ fn transform_trait_impl(original_impl: Impl) -> Result<TokenStream, Error> {
         #original_impl
         #godot_init_impl
 
+        impl ::godot::private::You_forgot_the_attribute__godot_api for #class_name {}
+
         impl ::godot::obj::cap::ImplementsGodotExt for #class_name {
             fn __virtual_call(name: &str) -> ::godot::sys::GDNativeExtensionClassCallVirtual {
                 //println!("virtual_call: {}.{}", std::any::type_name::<Self>(), name);