Function conversion_error_class_binding 

fn conversion_error_class_binding() -> String

The .miniextendr_conversion_error_class binding that closes the wrappers preamble: the classes .miniextendr_arg_error puts on every R-side argument error, from the [MX_CONVERSION_ERROR_CLASS] entry that miniextendr_init! registers for the package crate (#1591).

The Rust conversion arms carry the same classes, emitted by the macro from the same [package.metadata.miniextendr] key, so the two paths agree for every item of that crate. A package has one miniextendr_init!, hence one entry; with none (or a crate that sets no class) the binding is NULL.