{"record":{"id":"68952769b58455f7","repo":"bevyengine/bevy","slug":"settingsgroup-cannot-be-derived-for-unions","errorCode":null,"errorMessage":"SettingsGroup cannot be derived for unions","messagePattern":"SettingsGroup cannot be derived for unions","errorType":"validation","errorClass":"syn::Error","httpStatus":null,"severity":"error","filePath":"crates/bevy_ecs/macros/src/lib.rs","lineNumber":721,"sourceCode":"\n    let key_name = match &input.data {\n        Data::Struct(data) => match data.fields {\n            Fields::Named(_) if override_key_name.is_some() => {\n                return syn::Error::new(\n                    Span::call_site(),\n                    \"The `key` attribute is not supported for structs with named fields\",\n                )\n                .into_compile_error()\n                .into();\n            }\n            Fields::Named(_) => None,\n            Fields::Unnamed(_) | Fields::Unit => {\n                override_key_name.or_else(|| Some(pascal_to_snake_case(&name.to_string())))\n            }\n        },\n        Data::Enum(_) => override_key_name.or(Some(pascal_to_snake_case(&name.to_string()))),\n        Data::Union(_) => {\n            return syn::Error::new(\n                Span::call_site(),\n                \"SettingsGroup cannot be derived for unions\",\n            )\n            .into_compile_error()\n            .into();\n        }\n    };\n\n    let group_name = override_group_name.unwrap_or(pascal_to_snake_case(&name.to_string()));\n    let key_name = key_name\n        .map(|f| quote! { #FQOption::Some(#f) })\n        .unwrap_or(quote! { #FQOption::None });\n    let file_name = override_file\n        .map(|f| quote! { #FQOption::Some(#f) })\n        .unwrap_or(quote! { #FQOption::None });\n\n    let expanded = quote! {\n        impl #path::SettingsGroup for #name {","sourceCodeStart":703,"sourceCodeEnd":739,"githubUrl":"https://github.com/bevyengine/bevy/blob/396ca727080776bd313bb892423b7d94e03b81b4/crates/bevy_ecs/macros/src/lib.rs#L703-L739","documentation":"Compile error from derive_settings_group: the DeriveInput's data is a DataUnion. SettingsGroup codegen only handles structs (named, unnamed, or unit), so the match on input.data has no productive arm for unions and raises this shape guard.","triggerScenarios":"Thrown at crates/bevy_ecs/macros/src/lib.rs:721 when the library encounters an invalid state.","commonSituations":"See trigger scenarios.","solutions":["Change the union into a struct; unions cannot represent a settings hierarchy","Model the variants as separate settings-group structs and select at runtime"],"exampleFix":null,"handlingStrategy":"validation","validationCode":null,"typeGuard":null,"tryCatchPattern":null,"preventionTips":[],"tags":[],"backgroundTag":null,"analyzedSha":"396ca727080776bd313bb892423b7d94e03b81b4","analyzedAt":"2026-08-20T16:12:39.808Z","contentChangedAt":"2026-08-20T16:12:39.808Z","schemaVersion":2},"datasetVersion":"2026-09-14T00:17:10.932Z"}