lib.rs

struct Foo {
    value: String,
}

impl Foo {

    #[cfg(not(target_os = "macos"))]
    fn xxx(&mut self) {
        self.value.push_str(".not macOS.")
    }

    #[cfg(not(target_os = "linux"))]
    fn xxx(&mut self) {
        self.value.push_str(".not linux.")
    }

    pub fn call(&mut self) -> String {
        self.xxx();
        self.value.clone()
    }
}

fn main() {
    println!("start");

    if cfg!(not(target_os = "macos")) {
        println!("not macOS");
    }

    if cfg!(not(target_os = "linux")) {
        println!("not Linux");
    }

    let mut foo = Foo { value: "str".to_string() };

    println!("> {}", foo.call());

    println!("end");
}