### What it does
Checks for a `#[must_use]` attribute without
further information on functions and methods that return a type already
marked as `#[must_use]`.

### Why is this bad?
The attribute isn't needed. Not using the result
will already be reported. Alternatively, one can add some text to the
attribute to improve the lint message.

### Examples
```
#[must_use]
fn double_must_use() -> Result<(), ()> {
    unimplemented!();
}
```