| # Custom | |
| Some conditionals like `target_os` are implicitly provided by `rustc`, but | |
| custom conditionals must be passed to `rustc` using the `--cfg` flag. | |
| ```rust,editable,ignore,mdbook-runnable | |
| #[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: | |
| ```shell | |
| $ rustc --cfg some_condition custom.rs && ./custom | |
| condition met! | |
| ``` |