Add "defer setup choices" heuristic.

By chosing setup dependencies after regular dependencies we get more
opportunities for linking setup dependencies against regular dependencies.

cabal-install/Distribution/Client/Dependency/Modular/Preference.hs

@@ -259,6 +259,19 @@ preferBaseGoalChoice = trav go
     preferBase _ (OpenGoal (Simple (Dep (Q _pp pn) _) _) _) | unPN pn == "base" = GT
     preferBase _ _                                                              = EQ
 
+-- | Deal with setup dependencies after regular dependencies, so that we can
+-- will link setup depencencies against package dependencies when possible
+deferSetupChoices :: Tree a -> Tree a
+deferSetupChoices = trav go
+  where
+    go (GoalChoiceF xs) = GoalChoiceF (P.sortByKeys deferSetup xs)
+    go x                = x
+
+    deferSetup :: OpenGoal comp -> OpenGoal comp -> Ordering
+    deferSetup (OpenGoal (Simple (Dep (Q (Setup _ _) _) _) _) _) _ = GT
+    deferSetup _ (OpenGoal (Simple (Dep (Q (Setup _ _) _) _) _) _) = LT
+    deferSetup _ _                                                 = EQ
+
 -- | Transformation that sorts choice nodes so that
 -- child nodes with a small branching degree are preferred. As a
 -- special case, choices with 0 branches will be preferred (as they

cabal-install/Distribution/Client/Dependency/Modular/Solver.hs

@@ -42,6 +42,7 @@ solve sc idx userPrefs userConstraints userGoals =
   where
     explorePhase     = exploreTreeLog . backjump
     heuristicsPhase  = P.firstGoal . -- after doing goal-choice heuristics, commit to the first choice (saves space)
+                       P.deferSetupChoices .
                        P.deferWeakFlagChoices .
                        P.preferBaseGoalChoice .
                        P.preferLinked .