| Some conditionals like `target_os` are implicitly provided by `rustc`, but | |
| custom conditionals must be passed to `rustc` using the `--cfg` flag. | |
| {custom.rs} | |
| Without the custom `cfg` flag: | |
| {custom.out} | |
| With the custom `cfg` flag: | |
| ``` | |
| $ rustc --cfg some_condition custom.rs && ./custom | |
| condition met! | |
| ``` |