You're missing how rust works. The function is explicitly allowed to fail, which is why it returns a Result<(), Error>. They're using the function calls within for their side effects. The ? at the end of each line signals that the function will short-circuit return with an error if the function call fails, and only if it is successful it returns the actual value: they just don't care about this value, hence the let _ =. Basically, they are doing the equivalent of:

  let _, err = function_call();
  if err {
    return err
  }
  ...