Which trait candidates should be preferred over other candidates? By default, prefer where
bounds over alias bounds. For marker traits, prefer alias bounds over where bounds.
Supporting not-yet-defined opaque types in HIR typeck is somewhat
challenging. Ideally we’d normalize them to a new inference variable
and just defer type inference which relies on the opaque until we’ve
constrained the hidden type.
Whether the given region constraint should be considered/ignored for
leak check. In most part of the compiler, this should be Yes, except
for applying constraints from the nested goals in next-solver.
Unreachable is used in places in which leak check isn’t done, e.g.
borrowck.