Allow #[cfg] to be used with #[constant]

godot-macros/src/class/godot_api.rs

@@ -136,6 +136,7 @@ fn transform_inherent_impl(mut decl: Impl) -> Result<TokenStream, Error> {
         .map(|func_def| make_method_registration(&class_name, func_def));
 
     let consts = process_godot_constants(&mut decl)?;
+    let mut integer_constant_cfg_attrs = Vec::new();
     let mut integer_constant_names = Vec::new();
     let mut integer_constant_values = Vec::new();
 
@@ -146,6 +147,15 @@ fn transform_inherent_impl(mut decl: Impl) -> Result<TokenStream, Error> {
 
         let name = &constant.name;
 
+        // Unlike with #[func] and #[signal], we don't remove the attributes from Constant
+        // signatures within 'process_godot_constants'.
+        let cfg_attrs = util::extract_cfg_attrs(&constant.attributes)
+            .into_iter()
+            .collect::<Vec<_>>();
+
+        // Transport #[cfg] attrs to the FFI glue to ensure constants which were conditionally
+        // removed from compilation don't cause errors.
+        integer_constant_cfg_attrs.push(cfg_attrs);
         integer_constant_names.push(constant.name.to_string());
         integer_constant_values.push(quote! { #class_name::#name });
     }
@@ -157,6 +167,7 @@ fn transform_inherent_impl(mut decl: Impl) -> Result<TokenStream, Error> {
             use ::godot::builtin::StringName;
 
             #(
+                #(#integer_constant_cfg_attrs)*
                 ExportConstant::new(
                     #class_name_obj,
                     ConstantKind::Integer(

itest/rust/src/register_tests/constant_test.rs

@@ -40,15 +40,45 @@ impl HasConstants {
     #[constant]
     #[cfg(all())]
     const CONSTANT_RECOGNIZED_WITH_SIMPLE_PATH_ATTRIBUTE_BELOW_CONST_ATTR: bool = true;
+
+    // The three identically-named definitions below should be mutually exclusive thanks to #[cfg].
+    #[constant]
+    const CFG_REMOVES_DUPLICATE_CONSTANT_DEF: i64 = 5;
+
+    #[cfg(any())]
+    #[constant]
+    const CFG_REMOVES_DUPLICATE_CONSTANT_DEF: i64 = compile_error!("Removed by #[cfg]");
+
+    #[constant]
+    #[cfg(any())]
+    const CFG_REMOVES_DUPLICATE_CONSTANT_DEF: i64 = compile_error!("Removed by #[cfg]");
+
+    // The constant below should end up not being defined at all.
+    #[cfg(any())]
+    #[constant]
+    const CFG_REMOVES_CONSTANT: bool = compile_error!("Removed by #[cfg]");
+
+    #[constant]
+    #[cfg(any())]
+    const CFG_REMOVES_CONSTANT: bool = compile_error!("Removed by #[cfg]");
+}
+
+/// Checks at runtime if a class has a given integer constant through [ClassDb].
+fn class_has_integer_constant<T: GodotClass>(name: &str) -> bool {
+    ClassDb::singleton().class_has_integer_constant(T::class_name().to_string_name(), name.into())
 }
 
 #[itest]
 fn constants_correct_value() {
-    const CONSTANTS: [(&str, i64); 4] = [
+    const CONSTANTS: [(&str, i64); 5] = [
         ("A", HasConstants::A),
         ("B", HasConstants::B as i64),
         ("C", HasConstants::C as i64),
         ("D", HasConstants::D as i64),
+        (
+            "CFG_REMOVES_DUPLICATE_CONSTANT_DEF",
+            HasConstants::CFG_REMOVES_DUPLICATE_CONSTANT_DEF,
+        ),
     ];
 
     let constants = ClassDb::singleton()
@@ -72,6 +102,16 @@ fn constants_correct_value() {
     static_assert!(HasConstants::CONSTANT_RECOGNIZED_WITH_SIMPLE_PATH_ATTRIBUTE_BELOW_CONST_ATTR);
 }
 
+#[itest]
+fn cfg_removes_or_keeps_constants() {
+    assert!(class_has_integer_constant::<HasConstants>(
+        "CFG_REMOVES_DUPLICATE_CONSTANT_DEF"
+    ));
+    assert!(!class_has_integer_constant::<HasConstants>(
+        "CFG_REMOVES_CONSTANT"
+    ));
+}
+
 #[derive(GodotClass)]
 struct HasOtherConstants {}