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!