### 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 we only need the keys or the values.

### Example

```
let map: HashMap<u32, u32> = HashMap::new();
let values = map.iter().map(|(_, value)| value).collect::<Vec<_>>();
```

Use instead:
```
let map: HashMap<u32, u32> = HashMap::new();
let values = map.values().collect::<Vec<_>>();
```