library/std/src/sys/os_str/mod.rs

cfg_if::cfg_if! {
    if #[cfg(any(
        target_os = "windows",
        target_os = "uefi",
    ))] {
        mod wtf8;
        pub use wtf8::{Buf, Slice};
    } else {
        mod bytes;
        pub use bytes::{Buf, Slice};
    }
}