An unknown predicate was used inside the `cfg` attribute.
Erroneous code example:
```compile_fail,E0537
#[cfg(unknown())] // error: invalid predicate `unknown`
pub fn something() {}
pub fn main() {}
```
The `cfg` attribute supports only three kinds of predicates:
* any
* all
* not
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