On variance: Rust traits had variance before 1.0. RFC 738 and the rustc dev guide document how it used to work. But trait variance required extra annotation for little benefit, and so was removed. I am thinking about whether and how it could be brought back.
That makes things easier, I think. Thanks!
No, a value of type i32 in 1.. is strictly more capable than a value of type i32; knowing more about its contents allows you to do more stuff with it. (A function that needs an i32 in 1.. is less useful than one taking any i32, hence why functions are contravariant with respect to their arguments.)