Function gen_extendr_kwargs_constructor 

pub fn gen_extendr_kwargs_constructor(
    typ: &TypeDef,
    type_mapper: &dyn Fn(&TypeRef) -> String,
    enum_names: &AHashSet<String>,
) -> String

Generate an extendr (R) kwargs constructor for a type with has_default.

Rust does not support function-parameter defaults, and extendr 0.9 only allows defaults via the per-parameter #[extendr(default = "...")] attribute (not via param: T = expr syntax). Rather than encode every default in attribute form, we accept each field as Option<T> and unwrap it via T::default() (or via the type’s own Default::default() for the whole struct as the base) inside the body. The R-side wrapper generated in generate_public_api already supplies named arguments with NULL defaults, so callers see ergonomic kwargs at the R level.

enum_names is the set of type names that are enums in this API surface. For fields whose type resolves to a Named enum, the parameter is widened to Option<String> (extendr has no TryFrom<&Robj> for binding enums) and the body deserialises the string back to the enum via serde_json::from_str.