### What it does
Checks for the use of bindings with a single leading
underscore.

### Why is this bad?
A single leading underscore is usually used to indicate
that a binding will not be used. Using such a binding breaks this
expectation.

### Known problems
The lint does not work properly with desugaring and
macro, it has been allowed in the mean time.

### Example
```
let _x = 0;
let y = _x + 1; // Here we are using `_x`, even though it has a leading
                // underscore. We should rename `_x` to `x`
```