Function ffi_utils::test_utils::call_vec_with_custom

pub unsafe fn call_vec_with_custom<F, E, T, U>(
    ud: &mut UserData, 
    f: F
) -> Result<Vec<T>, i32> where
    F: FnOnce(*mut c_void, extern "C" fn(user_data: *mut c_void, result: *const FfiResult, _: T::C, _: usize)),
    E: Debug,
    T: ReprC<C = *const U, Error = E>, 

Call a FFI function and block until its callback gets called, then copy the array argument which was passed to Vec<T> and return the result. Use this if the callback accepts *const T and usize (length) arguments in addition to user_data and error_code. This version of the function takes a UserData with custom inner data.