fn bar1<'a>(x: &'a mut [i32]) -> impl Fn() -> &'a mut i32 {
    || &mut x[0]
}

struct Foo<'a>(&'a mut i32);

fn bar2<'a>(x: &'a mut [i32]) -> Foo<'a> {
    Foo(&mut x[0])
}

fn bar3<'a>(x: &'a mut [i32]) -> &'a mut i32 {
    &mut x[0]
}