Unitful breaks the type-stability of Rational:

julia> using Unitful
INFO: Recompiling stale cache file /home/tim/.julia/lib/v0.5/Unitful.ji for module Unitful.

julia> @code_warntype Rational(5463, 20)
Variables:
  #self#::Type{Rational}
  n::Int64
  d::Int64

Body:
  begin 
      return $(Expr(:invoke, LambdaInfo for Rational{Int64}(::Int64, ::Int64), Rational{Int64}, :(n), :(d)))
  end::Any

If you comment out this line and this function, then it's back to normal. Indeed, just the signature of the latter function (i.e., defining a blank body) is sufficient to break the type-stability of Rational.

What I find curious is that the latter function does not obviously have anything to do with Rational. Seems to suggest some kind of corruption.