#[cfg(foo)]

     #[cfg(bar = "baz")]

     #[cfg(any(unix, windows))]

     #[cfg(all(unix, target_pointer_width = "32"))]

     #[cfg(not(foo))]

     #[cfg(any(not(unix), all(target_os="macos", target_arch = "powerpc")))]

     [features]
     # no features by default
     default = []

     # The “secure-password” feature depends on the bcrypt package.
     secure-password = ["bcrypt"]

     --cfg feature="${feature_name}"

     #[cfg(feature = "foo")]
     mod foo {
     }

     #[cfg_attr(a, b)]

     if cfg!(target_os = "macos") || cfg!(target_os = "ios") {
         println!("Think Different!");
     }