Derive Macro PreferList 

#[derive(PreferList)]

Derive PreferList: emits an IntoR impl selecting list as the type’s default Rust→R conversion (via IntoList::into_list).

A type carries exactly one representation default: stacking two Prefer* derives is a compile error. Each Prefer* derive emits a fixed-name marker const, so a second one triggers a guided duplicate definitions with name __miniextendr_conflicting_Prefer_derives__keep_ONE_or_use_call_site_As_wrappers error (alongside the raw conflicting-IntoR-impl error) — keep one Prefer*, or drop them all and choose a representation per return value at the call site with an As* wrapper (AsList, AsExternalPtr, AsDataFrame, …).

Example

#[derive(IntoList, PreferList)]
struct Config { verbose: bool, threads: i32 }
// IntoR produces list(verbose = TRUE, threads = 4L)