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use std::borrow::Cow; pub trait Moo { fn moo(&self) -> ! { panic!("Moooooooooo!") } } impl<'a, T> Moo for Cow<'a, T> where T: ToOwned + ?Sized { } #[cfg(test)] mod tests { use super::*; #[test] #[should_panic(expected = "Moooooooooo!")] fn it_moos() { Cow::Borrowed("lol").moo() } }