Some conditionals like target_os are implicitly provided by rustc, but
custom conditionals must be passed to rustc using the --cfg flag.
// custom.rs
#[cfg(some_condition)]
fn conditional_function() {
println!("condition met!")
}
fn main() {
conditional_function();
}
Without the custom cfg flag:
{custom.out}
With the custom cfg flag:
$ rustc --cfg some_condition custom.rs && ./custom
condition met!