### What it does
Lints for suspicious operations in impls of OpAssign, e.g.
subtracting elements in an AddAssign impl.

### Why is this bad?
This is probably a typo or copy-and-paste error and not intended.

### Example
```
impl AddAssign for Foo {
    fn add_assign(&mut self, other: Foo) {
        *self = *self - other;
    }
}
```