### What it does
Checks for items that implement `.len()` but not
`.is_empty()`.

### Why is this bad?
It is good custom to have both methods, because for
some data structures, asking about the length will be a costly operation,
whereas `.is_empty()` can usually answer in constant time. Also it used to
lead to false positives on the [`len_zero`](#len_zero) lint – currently that
lint will ignore such entities.

### Example
```
impl X {
    pub fn len(&self) -> usize {
        ..
    }
}
```