{{ code }} => {
let msg = if custom_ptr.is_null() {
String::new()
} else {
// SAFETY: caller guarantees this is a valid heap CString.
let cstr = unsafe { std::ffi::CString::from_raw(custom_ptr) };
cstr.to_string_lossy().into_owned()
};
VisitorResult::{{ variant_name }}(msg)
},