#![allow(unused_variables)]
fn main() {
    use std::borrow::Cow;
    struct Items<'a, X>
    where
        [X]: ToOwned<Owned = Vec<X>>,
    {
        values: Cow<'a, [X]>,
    }
    impl<'a, X: Clone + 'a> Items<'a, X>
    where
        [X]: ToOwned<Owned = Vec<X>>,
    {
        fn new(v: Cow<'a, [X]>) -> Self {
            Items { values: v }
        }
    }
    let readonly = [1, 2];
    let borrowed = Items::new((&readonly[..]).into());
    match borrowed {
        Items { values: Cow::Borrowed(b) } => println!("borrowed {b:?}"),
        _ => panic!("expect borrowed value"),
    }
    let mut clone_on_write = borrowed;
    clone_on_write.values.to_mut().push(3);
    println!("clone_on_write = {:?}", clone_on_write.values);
    match clone_on_write {
        Items { values: Cow::Owned(_) } => println!("clone_on_write contains owned data"),
        _ => panic!("expect owned data"),
    }
}