Document runtime class in editor requirement.

- Explicitly declare type while `cast`ing, so type inference won't try anything funny
- Make `resolve_instance_ptr` safe. Returning (and casting) null ptrs is totally safe.

Author: Yarwin

godot-core/src/classes/class_runtime.rs

@@ -102,6 +102,30 @@ pub(crate) fn ensure_object_inherits(derived: ClassName, base: ClassName, instan
     )
 }
 
+#[cfg(debug_assertions)]
+pub(crate) fn ensure_binding_not_null<T>(binding: sys::GDExtensionClassInstancePtr)
+where
+    T: GodotClass + Bounds<Declarer = bounds::DeclUser>,
+{
+    if !binding.is_null() {
+        return;
+    }
+
+    // Non-tool classes can't be instantiated in the editor.
+    if crate::classes::Engine::singleton().is_editor_hint() {
+        panic!(
+            "Class {} -- null instance; does the class have a Godot creator function? \
+            Ensure that the given class is a tool class with #[class(tool)], if it is being accessed in the editor.",
+            std::any::type_name::<T>()
+        )
+    } else {
+        panic!(
+            "Class {} -- null instance; does the class have a Godot creator function?",
+            std::any::type_name::<T>()
+        );
+    }
+}
+
 // ----------------------------------------------------------------------------------------------------------------------------------------------
 // Implementation of this file
 

godot-core/src/obj/raw_gd.rs

@@ -425,8 +425,19 @@ where
         }
     }
 
-    // TODO: document unsafety in this function, and double check that it actually needs to be unsafe.
-    unsafe fn resolve_instance_ptr(&self) -> sys::GDExtensionClassInstancePtr {
+    /// Retrieves and caches pointer to this class instance if `self.obj` is non-null.
+    /// Returns a null pointer otherwise.
+    ///
+    /// Note: The returned pointer to the GDExtensionClass instance (even when `self.obj` is non-null)
+    /// might still be null when:
+    /// - The class isn't instantiable in the current context.
+    /// - The instance is a placeholder (e.g., non-`tool` classes in the editor).
+    ///
+    /// However, null pointers might also occur in other, undocumented contexts.
+    ///
+    /// # Panics
+    /// In Debug mode, if binding is null.
+    fn resolve_instance_ptr(&self) -> sys::GDExtensionClassInstancePtr {
         if self.is_null() {
             return ptr::null_mut();
         }
@@ -437,16 +448,21 @@ where
         }
 
         let callbacks = crate::storage::nop_instance_callbacks();
-        let token = sys::get_library() as *mut std::ffi::c_void;
-        let binding = interface_fn!(object_get_instance_binding)(self.obj_sys(), token, &callbacks);
 
-        debug_assert!(
-            !binding.is_null(),
-            "Class {} -- null instance; does the class have a Godot creator function?",
-            std::any::type_name::<T>()
-        );
+        // SAFETY: library is already initialized.
+        let token = unsafe { sys::get_library() };
+        let token = token.cast::<std::ffi::c_void>();
+
+        // SAFETY: ensured that `self.obj` is non-null and valid.
+        let binding = unsafe {
+            interface_fn!(object_get_instance_binding)(self.obj_sys(), token, &callbacks)
+        };
+
+        let ptr: sys::GDExtensionClassInstancePtr = binding.cast();
+
+        #[cfg(debug_assertions)]
+        crate::classes::ensure_binding_not_null::<T>(ptr);
 
-        let ptr = binding as sys::GDExtensionClassInstancePtr;
         self.cached_storage_ptr.set(ptr);
         ptr
     }

godot-macros/src/lib.rs

@@ -325,7 +325,9 @@ use crate::util::{bail, ident, KvParser};
 /// See [`ExtensionLibrary::editor_run_behavior()`](../init/trait.ExtensionLibrary.html#method.editor_run_behavior)
 /// for more information and further customization.
 ///
-/// This is very similar to [GDScript's `@tool` feature](https://docs.godotengine.org/en/stable/tutorials/plugins/running_code_in_the_editor.html).
+/// This behaves similarly to [GDScript's `@tool` feature](https://docs.godotengine.org/en/stable/tutorials/plugins/running_code_in_the_editor.html).
+///
+/// **Note**: As in GDScript, the class must be marked as a `tool` to be accessible in the editor (e.g., for use by editor plugins and inspectors).
 ///
 /// ## Editor plugins
 ///