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