Cow<'a, T> — clone on write — is an enum with two variants:

enum Cow<'a, T> {
    Borrowed(&'a T),
    Owned(T::Owned),
}

It lets a function return borrowed data on the common path and owned data only when it actually had to change something:

fn sanitize(input: &str) -> Cow<'_, str> {
    if input.contains(' ') {
        Cow::Owned(input.replace(' ', "_"))   // allocated: we changed it
    } else {
        Cow::Borrowed(input)                  // free: nothing to do
    }
}