### What it does
Checks for occurrences where one vector gets extended instead of append

### Why is this bad?
Using `append` instead of `extend` is more concise and faster

### Example
```
let mut a = vec![1, 2, 3];
let mut b = vec![4, 5, 6];

a.extend(b.drain(..));
```

Use instead:
```
let mut a = vec![1, 2, 3];
let mut b = vec![4, 5, 6];

a.append(&mut b);
```