| #### 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 |