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!