### What it does
Checks for functions that expect closures of type
Fn(...) -> Ord where the implemented closure returns the unit type.
The lint also suggests to remove the semi-colon at the end of the statement if present.

### Why is this bad?
Likely, returning the unit type is unintentional, and
could simply be caused by an extra semi-colon. Since () implements Ord
it doesn't cause a compilation error.
This is the same reasoning behind the unit_cmp lint.

### Known problems
If returning unit is intentional, then there is no
way of specifying this without triggering needless_return lint

### Example
```
let mut twins = vec!((1, 1), (2, 2));
twins.sort_by_key(|x| { x.1; });
```