### What it does
Checks for iterating a map (`HashMap` or `BTreeMap`) and
ignoring either the keys or values.

### Why is this bad?
Readability. There are `keys` and `values` methods that
can be used to express that don't need the values or keys.

### Example
```
for (k, _) in &map {
    ..
}
```

could be replaced by

```
for k in map.keys() {
    ..
}
```