| #### Note: this error code is no longer emitted by the compiler |
| An unknown predicate was used inside the `cfg` attribute. |
| |
| Erroneous code example: |
| |
| ```compile_fail,E0539 |
| #[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][conditional-compilation] in the Reference. |
| |
| [conditional-compilation]: https://doc.rust-lang.org/reference/conditional-compilation.html |
