### What it does
Checks for getting the length of something via `.len()`
just to compare to zero, and suggests using `.is_empty()` where applicable.

### Why is this bad?
Some structures can answer `.is_empty()` much faster
than calculating their length. So it is good to get into the habit of using
`.is_empty()`, and having it is cheap.
Besides, it makes the intent clearer than a manual comparison in some contexts.

### Example
```
if x.len() == 0 {
    ..
}
if y.len() != 0 {
    ..
}
```
instead use
```
if x.is_empty() {
    ..
}
if !y.is_empty() {
    ..
}
```