This warning was introduced on Nov 28, 2015 and got into 1.6 stable, it was later requalified from a hardwired warning to a warn-by-default lint.
If this patch is landed soon enough, then `match_of_unit_variant_via_paren_dotdot` will get into 1.8 stable as a deny-by-default lint.
My intention is to turn it into a hard error after March 3, 2016, then it will hit stable at 1.9.
r? @nikomatsakis
cc @pnkfelix
c894ff6b12