### What it does
Checks for empty `Drop` implementations.

### Why is this bad?
Empty `Drop` implementations have no effect when dropping an instance of the type. They are
most likely useless. However, an empty `Drop` implementation prevents a type from being
destructured, which might be the intention behind adding the implementation as a marker.

### Example
```
struct S;

impl Drop for S {
    fn drop(&mut self) {}
}
```
Use instead:
```
struct S;
```