When designing configuration systems, provide both replacement and extension options for list-based settings to give users flexibility in how they modify default configurations. For list-based configurations, consider implementing: 1. A base option for complete replacement (`allow_abc_meta_bases`) 2. An extension option that appends to defaults (`extend_abc_meta_bases`) This approach allows users to either: - Override the entire list when they need complete control - Add to the existing defaults when they only need minor adjustments ```rust #[option( default = r#"["typing.Protocol", "typing_extensions.Protocol"]"#, value_type = "list[str]", example = r#"allow-abc-meta-bases = ["my_package.SpecialBaseClass"]"# )] pub allow_abc_meta_bases: Vec, #[option( default = r#"[]"#, value_type = "list[str]", example = r#"extend-abc-meta-bases = ["my_package.SpecialBaseClass"]"# )] pub extend_abc_meta_bases: Vec, ``` When implementing the options, combine them at the settings level rather than requiring consumers to check both settings: ```rust // In your settings structure pub struct Settings { // Combined at initialization time pub allowed_bases: FxHashSet, } // When constructing settings let mut allowed_bases = options.allow_abc_meta_bases.into_iter().collect::>(); allowed_bases.extend(options.extend_abc_meta_bases.iter().cloned()); ``` This pattern promotes cleaner code that consumes the settings, making it less error-prone and easier to maintain.