When I originally implemented bug 1458161, this is how it was done, but
it was suggested to use a configure-time check. This turned out to not
be great, because the rust compiler changes regularly, and we don't run
the configure tests when the version changes. When people upgraded their
rust compiler to 1.27, the code subsequently failed to build because the
features were still set for the previous version they had installed.
c3cc42fd41