Actually, I just looked into it, that's no longer possible. I was talking about contravariant lifetime positions, which happen with functions, but now it seems

Actually, I just looked into it, that's no longer possible. I was talking about contravariant lifetime positions, which happen with functions, but now it seems like function pointers are invariant in those positions, not contravariant. The idea was to have something like struct Foo<'a> {f: fn(&'a u8)} should only be able to accept pointers that live shorter than the struct. But contravariance seems to have been removed.