An unary operator was used on a type which doesn't implement it.
Example of erroneous code:
```compile_fail,E0600
enum Question {
Yes,
No,
}
!Question::Yes; // error: cannot apply unary operator `!` to type `Question`
```
In this case, `Question` would need to implement the `std::ops::Not` trait in
order to be able to use `!` on it. Let's implement it:
```
use std::ops::Not;
enum Question {
Yes,
No,
}
// We implement the `Not` trait on the enum.
impl Not for Question {
type Output = bool;
fn not(self) -> bool {
match self {
Question::Yes => false, // If the `Answer` is `Yes`, then it
// returns false.
Question::No => true, // And here we do the opposite.
}
}
}
assert_eq!(!Question::Yes, false);
assert_eq!(!Question::No, true);
```