The `not` cfg-predicate was malformed.

Erroneous code example:

```compile_fail,E0536
#[cfg(not())] // error: expected 1 cfg-pattern
pub fn something() {}

pub fn main() {}
```

The `not` predicate expects one cfg-pattern. Example:

```
#[cfg(not(target_os = "linux"))] // ok!
pub fn something() {}

pub fn main() {}
```

For more information about the `cfg` attribute, read the section on
[Conditional Compilation][conditional-compilation] in the Reference.

[conditional-compilation]: https://doc.rust-lang.org/reference/conditional-compilation.html