```rust
unsafe extern "C" fn json_object_copy_serializer_data(
    mut src: *mut json_object,
    mut dst: *mut json_object,
) -> ::core::ffi::c_int {
    if (*src)._userdata.is_null() && (*src)._user_delete.is_none() {
        return 0 as ::core::ffi::c_int;
    }
    if (*dst)._to_json_string
        == Some(
            json_object_userdata_to_json_string
                as unsafe extern "C" fn(
                    *mut json_object,
                    *mut printbuf,
                    ::core::ffi::c_int,
                    ::core::ffi::c_int,
                ) -> ::core::ffi::c_int,
        )
    {
        (*dst)._userdata =
            strdup((*src)._userdata as *const ::core::ffi::c_char) as *mut ::core::ffi::c_void;
    }
    // else if ... other supported serializers ...
    else {
        _json_c_set_last_err(
            b"json_object_deep_copy: unable to copy unknown serializer data: %p\n\0" as *const u8
                as *const ::core::ffi::c_char,
            (*dst)._to_json_string,
        );
        return -(1 as ::core::ffi::c_int);
    }
    (*dst)._user_delete = (*src)._user_delete;
    return 0 as ::core::ffi::c_int;
}
```