2 Checks for getting the length of something via `.len()`
3 just to compare to zero, and suggests using `.is_empty()` where applicable.
6 Some structures can answer `.is_empty()` much faster
7 than calculating their length. So it is good to get into the habit of using
8 `.is_empty()`, and having it is cheap.
9 Besides, it makes the intent clearer than a manual comparison in some contexts.