simple-rust-tests/__std/misc/src/bin/conditional_test.rs

#[cfg(target_os = "macos")]
fn get_os() -> &'static str {
    "macos"
}

#[cfg(not(target_os = "macos"))]
fn get_os() -> &'static str {
    "unknown"
}

fn main() {
    println!("OS: {}", get_os());
}