# Custom

Some conditionals like `target_os` are implicitly provided by `rustc`, but custom conditionals must be passed to `rustc` using the `--cfg` flag.

    #[cfg(some_condition)]
    fn conditional_function() {
        println!("condition met!");
    }

    fn main() {
        conditional_function();
    }

Try to run this to see what happens without the custom `cfg` flag.

With the custom `cfg` flag:

    $ rustc --cfg some_condition custom.rs && ./custom
    condition met!