This featurelet allows Trac #5237 to be fixed.
The idea is to allow SPECIALISE pragmas to specify
the phases in which the RULE is active, just as you can
do with RULES themselves.
  {-# SPECIALISE [1] foo :: Int -> Int #-}

This feature is so obvious that not having it is really a bug.
There are, needless to say, a few wrinkles.  See
   Note [Activation pragmas for SPECIALISE]
in DsBinds