### What it does
Checks for manual swapping.

### Why is this bad?
The `std::mem::swap` function exposes the intent better
without deinitializing or copying either variable.

### Example
```
let mut a = 42;
let mut b = 1337;

let t = b;
b = a;
a = t;
```
Use std::mem::swap():
```
let mut a = 1;
let mut b = 2;
std::mem::swap(&mut a, &mut b);
```