Merge pull request #593 from godot-rust/feature/no-init

`#[class(no_init)]` to explicitly disable constructor

Author: Bromeon

godot-core/src/lib.rs

@@ -186,21 +186,20 @@ pub mod private {
     // https://github.com/rust-lang/rust/pull/58994. Fortunately, an extra layer of indirection solves most problems: we generate a declarative
     // macro that itself isn't deprecated, but _its_ expansion is. Since the expansion happens in a later step, the warning is emitted.
 
-    #[doc(hidden)]
     #[inline(always)]
     #[deprecated = "#[base] is no longer needed; Base<T> is recognized directly. \n\
         More information on https://github.com/godot-rust/gdext/pull/577."]
-    pub const fn base_attribute_warning() {}
+    pub const fn base_attribute() {}
 
     #[doc(hidden)]
     #[macro_export]
-    macro_rules! __base_attribute_warning_expand {
-        () => {
-            const _: () = $crate::private::base_attribute_warning();
+    macro_rules! __emit_deprecated_warning {
+        ($warning_fn:ident) => {
+            const _: () = $crate::private::$warning_fn();
         };
     }
 
-    pub use crate::__base_attribute_warning_expand;
+    pub use crate::__emit_deprecated_warning;
 }
 
 macro_rules! generate_gdextension_api_version {

godot-core/src/registry.rs

@@ -102,6 +102,9 @@ pub enum PluginItem {
 
         /// Whether `#[class(hidden)]` was used.
         is_hidden: bool,
+
+        /// Whether the class has a default constructor.
+        is_instantiable: bool,
     },
 
     /// Collected from `#[godot_api] impl MyClass`.
@@ -377,9 +380,20 @@ fn fill_class_info(item: PluginItem, c: &mut ClassRegistrationInfo) {
             default_get_virtual_fn,
             is_editor_plugin,
             is_hidden,
+            is_instantiable,
         } => {
             c.parent_class_name = Some(base_class_name);
 
+            // Classes marked #[class(no_init)] are translated to "abstract" in Godot. This disables their default constructor.
+            // "Abstract" is a misnomer -- it's not an abstract base class, but rather a "utility/static class" (although it can have instance
+            // methods). Examples are Input, IP, FileAccess, DisplayServer.
+            //
+            // Abstract base classes on the other hand are called "virtual" in Godot. Examples are Mesh, Material, Texture.
+            // For some reason, certain ABCs like PhysicsBody2D are not marked "virtual" but "abstract".
+            //
+            // See also: https://github.com/godotengine/godot/pull/58972
+            c.godot_params.is_abstract = (!is_instantiable) as sys::GDExtensionBool;
+
             fill_into(
                 &mut c.godot_params.create_instance_func,
                 generated_create_fn,