### What it does
Detects `().hash(_)`.

### Why is this bad?
Hashing a unit value doesn't do anything as the implementation of `Hash` for `()` is a no-op.

### Example
```
match my_enum {
	Empty => ().hash(&mut state),
	WithValue(x) => x.hash(&mut state),
}
```
Use instead:
```
match my_enum {
	Empty => 0_u8.hash(&mut state),
	WithValue(x) => x.hash(&mut state),
}
```