### What it does
Checks for explicit self-assignments.

### Why is this bad?
Self-assignments are redundant and unlikely to be
intentional.

### Known problems
If expression contains any deref coercions or
indexing operations they are assumed not to have any side effects.

### Example
```
struct Event {
    x: i32,
}

fn copy_position(a: &mut Event, b: &Event) {
    a.x = a.x;
}
```

Should be:
```
struct Event {
    x: i32,
}

fn copy_position(a: &mut Event, b: &Event) {
    a.x = b.x;
}
```