Function __riterator_next_shim 

#[doc(hidden)]
unsafe extern "C" fn __riterator_next_shim<__ImplT: RIterator + IntoR>(
    data: *mut c_void,
    argc: i32,
    argv: *const SEXP,
) -> SEXP
where
    Option<<__ImplT as RIterator>::Item>: IntoR,
    Vec<<__ImplT as RIterator>::Item>: IntoR,

Method shim for RIterator::next.

Converts SEXP arguments, calls the method, and returns SEXP result. Both Rust panics and R longjmps are caught via with_r_unwind_protect.