This change is worrisome to me, both because:
1. I thought the rules in RFC 599 imply that the `Box<Trait>` without `'static`
in the first case would expand to the second case, but their behaviors
here differ. And,
2. The explicit handling of `'static` should mean `dropck` has no application
here and thus we should have seen no change to the expected error messages.
Nonetheless, the error messages changed.
specialized to closures, and invoke them as soon as we know the
closure kind. I thought initially we would need a fixed-point
inference algorithm but it appears I was mistaken, so we can do this.