# Error code E0537

An unknown predicate was used inside the `cfg` attribute.

Erroneous code example:

    #[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 the section on Conditional Compilation in the Reference.