trait A<'g> { }

trait B<'g> { }

trait C<T, U>
where
    T: for<'g> A<'g> + ?Sized,
    U: for<'g> B<'g> + ?Sized,
{
    fn get_a_or_b(&self) -> AorB<T, U>;
    fn get_c(&self) -> &U;
}

enum AorB<'g, T, U>
where
    T: A<'g> + ?Sized,
    U: B<'g> + ?Sized,
{
    A(&'g T),
    B(&'g U),
}

struct MyStruct {
    a: String,
    b: String,
    c: String
}

impl<'g> A<'g> for str {}

impl<'g> B<'g> for str {}

impl C<str, str> for MyStruct {
    fn get_a_or_b(&self) -> AorB<str, str> {
        AorB::A(&self.a)
    }

    fn get_c(&self) -> &str {
        &self.c
    }
}