used to indicate when anonymous regions (i.e., &T) are permitted

it. This should eventually be merged with the Subst trait.

… old

region-parameterization/variance inference. We now compute variance for
type parameters but do not make use of it (most of the way towards rust-lang#3598).

This code fits better in check because it is checking that the impl
matches the interface. This allows us to avoid the awkward constructions
that lazilly collect traits and so forth. It also permits us to make
use of the results of variance inference.

all the information it has at its disposal. Unfortunately this also
reveals that we need to improve the reporting heuristics further,
as sometimes the errors it chooses to emit seem somewhat mystifying
and are not related to the actual problem.

annotated.

…inter to emit the same thing twice in a row