### What it does
Checks for the use of `.extend(s.chars())` where s is a
`&str` or `String`.

### Why is this bad?
`.push_str(s)` is clearer

### Example
```
let abc = "abc";
let def = String::from("def");
let mut s = String::new();
s.extend(abc.chars());
s.extend(def.chars());
```
The correct use would be:
```
let abc = "abc";
let def = String::from("def");
let mut s = String::new();
s.push_str(abc);
s.push_str(&def);
```