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

Author: PgBiel

godot-macros/src/class/godot_api.rs

@@ -381,6 +381,24 @@ where
 
 // ----------------------------------------------------------------------------------------------------------------------------------------------
 
+/// Expects either Some(quote! { () => A, () => B, ... }) or None as the 'tokens' parameter.
+/// The idea is that the () => ... arms can be annotated by cfg attrs, so, if any of them compiles (and assuming the cfg
+/// attrs only allow one arm to 'survive' compilation), their return value (Some(...)) will be prioritized over the
+/// 'None' from the catch-all arm at the end. If, however, none of them compile, then None is returned from the last
+/// match arm.
+fn convert_to_match_expression_or_none(tokens: Option<TokenStream>) -> TokenStream {
+    if let Some(tokens) = tokens {
+        quote! {
+            match () {
+                #tokens
+                _ => None,
+            }
+        }
+    } else {
+        quote! { None }
+    }
+}
+
 /// Codegen for `#[godot_api] impl GodotExt for MyType`
 fn transform_trait_impl(original_impl: Impl) -> Result<TokenStream, Error> {
     let (class_name, trait_name) = util::validate_trait_impl_virtual(&original_impl, "godot_api")?;
@@ -391,13 +409,14 @@ fn transform_trait_impl(original_impl: Impl) -> Result<TokenStream, Error> {
     let mut register_class_impl = TokenStream::new();
     let mut on_notification_impl = TokenStream::new();
 
-    let mut register_fn = quote! { None };
-    let mut create_fn = quote! { None };
-    let mut recreate_fn = quote! { None };
-    let mut to_string_fn = quote! { None };
-    let mut on_notification_fn = quote! { None };
+    let mut register_fn = None;
+    let mut create_fn = None;
+    let mut recreate_fn = None;
+    let mut to_string_fn = None;
+    let mut on_notification_fn = 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 +428,99 @@ 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" => {
+                // Implements the trait once for each implementation of this method, forwarding the cfg attrs of each
+                // implementation to the generated trait impl. If the cfg attrs allow for multiple implementations of
+                // this method to exist, then Rust will generate an error, so we don't have to worry about the multiple
+                // trait implementations actually generating an error, since that can only happen if multiple
+                // implementations of the same method are kept by #[cfg] (due to user error).
+                // Thus, by implementing the trait once for each possible implementation of this method (depending on
+                // what #[cfg] allows), forwarding the cfg attrs, we ensure this trait impl will remain in the code if
+                // at least one of the method impls are kept.
                 register_class_impl = quote! {
+                    #register_class_impl
+
+                    #(#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)
                         }
                     }
                 };
 
-                register_fn = quote! {
-                    Some(#prv::ErasedRegisterFn {
+                // Adds a match arm for each implementation of this method, transferring its respective cfg attrs to
+                // the corresponding match arm (see explanation for the match after this loop).
+                // In principle, the cfg attrs will allow only either 0 or 1 of a function with this name to exist,
+                // unless there are duplicate implementations for the same method, which should error anyway.
+                // Thus, in any correct program, the match arms (which are, in principle, identical) will be reduced to
+                // a single one at most, since we forward the cfg attrs. The idea here is precisely to keep this
+                // specific match arm 'alive' if at least one implementation of the method is also kept (hence why all
+                // the match arms are identical).
+                register_fn = Some(quote! {
+                    #register_fn
+                    #(#cfg_attrs)*
+                    () => Some(#prv::ErasedRegisterFn {
                         raw: #prv::callbacks::register_class_by_builder::<#class_name>
-                    })
-                };
+                    }),
+                });
             }
 
             "init" => {
                 godot_init_impl = quote! {
+                    #godot_init_impl
+
+                    #(#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 = Some(quote! {
+                    #create_fn
+                    #(#cfg_attrs)*
+                    () => Some(#prv::callbacks::create::<#class_name>),
+                });
                 if cfg!(since_api = "4.2") {
-                    recreate_fn = quote! { Some(#prv::callbacks::recreate::<#class_name>) };
+                    recreate_fn = Some(quote! {
+                        #recreate_fn
+                        #(#cfg_attrs)*
+                        () => Some(#prv::callbacks::recreate::<#class_name>),
+                    });
                 }
             }
 
             "to_string" => {
                 to_string_impl = quote! {
+                    #to_string_impl
+
+                    #(#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 = Some(quote! {
+                    #to_string_fn
+                    #(#cfg_attrs)*
+                    () => Some(#prv::callbacks::to_string::<#class_name>),
+                });
             }
 
             "on_notification" => {
                 on_notification_impl = quote! {
+                    #on_notification_impl
+
+                    #(#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) {
@@ -466,9 +532,11 @@ fn transform_trait_impl(original_impl: Impl) -> Result<TokenStream, Error> {
                     }
                 };
 
-                on_notification_fn = quote! {
-                    Some(#prv::callbacks::on_notification::<#class_name>)
-                };
+                on_notification_fn = Some(quote! {
+                    #on_notification_fn
+                    #(#cfg_attrs)*
+                    () => Some(#prv::callbacks::on_notification::<#class_name>),
+                });
             }
 
             // Other virtual methods, like ready, process etc.
@@ -487,6 +555,11 @@ fn transform_trait_impl(original_impl: Impl) -> Result<TokenStream, Error> {
                 } else {
                     format!("_{method_name}")
                 };
+                // Note that, if the same method is implemented multiple times (with different cfg attr combinations),
+                // then there will be multiple match arms annotated with the same cfg attr combinations, thus they will
+                // be reduced to just one arm (at most, if the implementations aren't all removed from compilation) for
+                // each distinct method.
+                virtual_method_cfg_attrs.push(cfg_attrs);
                 virtual_method_names.push(virtual_method_name);
                 virtual_methods.push(method);
             }
@@ -498,6 +571,17 @@ fn transform_trait_impl(original_impl: Impl) -> Result<TokenStream, Error> {
         .map(|method| make_virtual_method_callback(&class_name, method))
         .collect();
 
+    // Use 'match' as a way to only emit 'Some(...)' if the given cfg attrs allow.
+    // This permits users to conditionally remove virtual method impls from compilation while also removing their FFI
+    // glue which would otherwise make them visible to Godot even if not really implemented.
+    // Needs '#[allow(unreachable_patterns)]' to avoid warnings about the last match arm.
+    // Also requires '#[allow(clippy::match_single_binding)]' for similar reasons.
+    let register_fn = convert_to_match_expression_or_none(register_fn);
+    let create_fn = convert_to_match_expression_or_none(create_fn);
+    let recreate_fn = convert_to_match_expression_or_none(recreate_fn);
+    let to_string_fn = convert_to_match_expression_or_none(to_string_fn);
+    let on_notification_fn = convert_to_match_expression_or_none(on_notification_fn);
+
     let result = quote! {
         #original_impl
         #godot_init_impl
@@ -517,6 +601,7 @@ 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 +611,8 @@ 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]");
+    }
 }