Improve #[rpc] error message if Base<T> field is missing

Author: Bromeon

godot-core/src/obj/traits.rs

@@ -244,7 +244,7 @@ pub trait IndexEnum: EngineEnum {
 #[diagnostic::on_unimplemented(
     message = "Class `{Self}` requires a `Base<T>` field",
     label = "missing field `_base: Base<...>`",
-    note = "A base field is required to access the base from within `self`, or when using script virtual functions",
+    note = "A base field is required to access the base from within `self`, for script-virtual functions or #[rpc] methods",
     note = "see also: https://godot-rust.github.io/book/register/classes.html#the-base-field"
 )]
 pub trait WithBaseField: GodotClass + Bounds<Declarer = bounds::DeclUser> {

godot-macros/src/class/data_models/rpc.rs

@@ -74,7 +74,8 @@ pub fn make_rpc_registrations_fn(class_name: &Ident, funcs: &[FuncDefinition]) -
     }
 
     quote! {
-        #[allow(clippy::needless_update)] // clippy complains about using `..RpcConfig::default()` if all fields are overridden
+        // Clippy complains about using `..RpcConfig::default()` if all fields are overridden.
+        #[allow(clippy::needless_update)]
         fn __register_rpcs(object: &mut dyn ::std::any::Any) {
             use ::std::any::Any;
             use ::godot::register::RpcConfig;
@@ -83,10 +84,12 @@ pub fn make_rpc_registrations_fn(class_name: &Ident, funcs: &[FuncDefinition]) -
             use ::godot::classes::Node;
             use ::godot::obj::{WithBaseField, Gd};
 
-            let mut gd = object
-                .downcast_mut::<#class_name>()
-                .expect("bad type erasure when registering RPCs")
-                .to_gd();
+            let this = object
+                .downcast_ref::<#class_name>()
+                .expect("bad type erasure when registering RPCs");
+
+            // Use fully-qualified syntax, so that error message isn't just "no method named `to_gd` found".
+            let mut gd = ::godot::obj::WithBaseField::to_gd(this);
 
             let node = gd.upcast_mut::<Node>();
             #( #rpc_registrations )*