Allow #[cfg] to be used in #[godot_api] virtual impls

Author: PgBiel

godot-macros/src/class/godot_api.rs

@@ -398,6 +398,7 @@ fn transform_trait_impl(original_impl: Impl) -> Result<TokenStream, Error> {
     let mut on_notification_fn = quote! { None };
 
     let mut virtual_methods = vec![];
+    let mut virtual_method_cfg_attrs = vec![];
     let mut virtual_method_names = vec![];
 
     let prv = quote! { ::godot::private };
@@ -409,52 +410,85 @@ fn transform_trait_impl(original_impl: Impl) -> Result<TokenStream, Error> {
             continue;
         };
 
+        // Transport #[cfg] attributes to the virtual method's FFI glue, to ensure it won't be
+        // registered in Godot if conditionally removed from compilation.
+        let cfg_attrs = util::extract_cfg_attrs(&method.attributes)
+            .into_iter()
+            .collect::<Vec<_>>();
         let method_name = method.name.to_string();
         match method_name.as_str() {
             "register_class" => {
                 register_class_impl = quote! {
+                    #(#cfg_attrs)*
                     impl ::godot::obj::cap::GodotRegisterClass for #class_name {
                         fn __godot_register_class(builder: &mut ::godot::builder::GodotBuilder<Self>) {
                             <Self as #trait_name>::register_class(builder)
                         }
                     }
                 };
 
+                // Use 'match' as a way to only return 'Some(...)' if the given cfg attrs allow.
+                // Needs '#[allow(unreachable_patterns)]' to avoid warnings about the last arm.
                 register_fn = quote! {
-                    Some(#prv::ErasedRegisterFn {
-                        raw: #prv::callbacks::register_class_by_builder::<#class_name>
-                    })
+                    match () {
+                        #(#cfg_attrs)*
+                        () => Some(#prv::ErasedRegisterFn {
+                            raw: #prv::callbacks::register_class_by_builder::<#class_name>
+                        }),
+                        _ => None,
+                    }
                 };
             }
 
             "init" => {
                 godot_init_impl = quote! {
+                    #(#cfg_attrs)*
                     impl ::godot::obj::cap::GodotInit for #class_name {
                         fn __godot_init(base: ::godot::obj::Base<Self::Base>) -> Self {
                             <Self as #trait_name>::init(base)
                         }
                     }
                 };
-                create_fn = quote! { Some(#prv::callbacks::create::<#class_name>) };
+                create_fn = quote! {
+                    match () {
+                        #(#cfg_attrs)*
+                        () => Some(#prv::callbacks::create::<#class_name>),
+                        _ => None,
+                    }
+                };
                 if cfg!(since_api = "4.2") {
-                    recreate_fn = quote! { Some(#prv::callbacks::recreate::<#class_name>) };
+                    recreate_fn = quote! {
+                        match () {
+                            #(#cfg_attrs)*
+                            () => Some(#prv::callbacks::recreate::<#class_name>),
+                            _ => None,
+                        }
+                    };
                 }
             }
 
             "to_string" => {
                 to_string_impl = quote! {
+                    #(#cfg_attrs)*
                     impl ::godot::obj::cap::GodotToString for #class_name {
                         fn __godot_to_string(&self) -> ::godot::builtin::GodotString {
                             <Self as #trait_name>::to_string(self)
                         }
                     }
                 };
 
-                to_string_fn = quote! { Some(#prv::callbacks::to_string::<#class_name>) };
+                to_string_fn = quote! {
+                    match () {
+                      #(#cfg_attrs)*
+                      () => Some(#prv::callbacks::to_string::<#class_name>),
+                      _ => None,
+                    }
+                };
             }
 
             "on_notification" => {
                 on_notification_impl = quote! {
+                    #(#cfg_attrs)*
                     impl ::godot::obj::cap::GodotNotification for #class_name {
                         fn __godot_notification(&mut self, what: i32) {
                             if ::godot::private::is_class_inactive(Self::__config().is_tool) {
@@ -467,7 +501,11 @@ fn transform_trait_impl(original_impl: Impl) -> Result<TokenStream, Error> {
                 };
 
                 on_notification_fn = quote! {
-                    Some(#prv::callbacks::on_notification::<#class_name>)
+                    match () {
+                        #(#cfg_attrs)*
+                        () => Some(#prv::callbacks::on_notification::<#class_name>),
+                        _ => None,
+                    }
                 };
             }
 
@@ -487,6 +525,7 @@ fn transform_trait_impl(original_impl: Impl) -> Result<TokenStream, Error> {
                 } else {
                     format!("_{method_name}")
                 };
+                virtual_method_cfg_attrs.push(cfg_attrs);
                 virtual_method_names.push(virtual_method_name);
                 virtual_methods.push(method);
             }
@@ -517,6 +556,7 @@ fn transform_trait_impl(original_impl: Impl) -> Result<TokenStream, Error> {
 
                 match name {
                     #(
+                       #(#virtual_method_cfg_attrs)*
                        #virtual_method_names => #virtual_method_callbacks,
                     )*
                     _ => None,
@@ -526,6 +566,8 @@ fn transform_trait_impl(original_impl: Impl) -> Result<TokenStream, Error> {
 
         ::godot::sys::plugin_add!(__GODOT_PLUGIN_REGISTRY in #prv; #prv::ClassPlugin {
             class_name: #class_name_obj,
+            #[allow(unreachable_patterns)] // due to the cfg-based match statements
+            #[allow(clippy::match_single_binding)] // avoid warning on single-arm matches
             component: #prv::PluginComponent::UserVirtuals {
                 user_register_fn: #register_fn,
                 user_create_fn: #create_fn,

godot-macros/src/util/mod.rs

@@ -227,6 +227,15 @@ pub(crate) fn path_ends_with(path: &[TokenTree], expected: &str) -> bool {
         .unwrap_or(false)
 }
 
+pub(crate) fn extract_cfg_attrs(
+    attrs: &[venial::Attribute],
+) -> impl IntoIterator<Item = &venial::Attribute> {
+    attrs.iter().filter(|attr| {
+        attr.get_single_path_segment()
+            .map_or(false, |name| name == "cfg")
+    })
+}
+
 pub(crate) struct DeclInfo {
     pub where_: Option<WhereClause>,
     pub generic_params: Option<GenericParamList>,

itest/rust/src/object_tests/virtual_methods_test.rs

@@ -75,6 +75,11 @@ impl Node2DVirtual for ReadyVirtualTest {
     fn ready(&mut self) {
         self.implementation_value += 1;
     }
+
+    #[cfg(any())]
+    fn to_string(&self) -> GodotString {
+        compile_error!("Removed by #[cfg]")
+    }
 }
 
 // ----------------------------------------------------------------------------------------------------------------------------------------------

itest/rust/src/register_tests/func_test.rs

@@ -147,4 +147,34 @@ impl RefCountedVirtual for GdSelfReference {
             base,
         }
     }
+
+    #[cfg(any())]
+    fn init(base: Base<Self::Base>) -> Self {
+        compile_error!("Removed by #[cfg]")
+    }
+
+    #[cfg(all())]
+    fn to_string(&self) -> GodotString {
+        GodotString::new()
+    }
+
+    #[cfg(any())]
+    fn register_class() {
+        compile_error!("Removed by #[cfg]");
+    }
+
+    #[cfg(all())]
+    fn on_notification(&mut self, _: godot::engine::notify::ObjectNotification) {
+        godot_print!("Hello!");
+    }
+
+    #[cfg(any())]
+    fn on_notification(&mut self, _: godot::engine::notify::ObjectNotification) {
+        compile_error!("Removed by #[cfg]");
+    }
+
+    #[cfg(any())]
+    fn cfg_removes_this() {
+        compile_error!("Removed by #[cfg]");
+    }
 }