Rust by Example

11.2.1 Custom

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

fn conditional_function() {
    println!("condition met!")

fn main() {

Without the custom cfg flag:

$ rustc && ./custom 7:25 error: unresolved name `conditional_function`     conditional_function();
error: aborting due to previous error

With the custom cfg flag:

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