Solver: Only link packages to other unlinked packages.

If 1.A is already linked to 0.A, then linking 2.A to 1.A gives the same result
as linking 2.A to 0.A. This commit avoids creating options to link to packages
that are already linked, to reduce the branching factor of the search tree.

cabal-install/Distribution/Solver/Modular/Linking.hs

@@ -50,14 +50,17 @@ type Linker       = Reader RelatedGoals
 -- "related" if they are for the same instance of the same package (but have
 -- different prefixes). A link option is available in a package choice node
 -- whenever we can choose an instance that has already been chosen for a related
--- goal at a higher position in the tree.
+-- goal at a higher position in the tree. We only create link options for
+-- related goals that are not themselves linked, because the choice to link to a
+-- linked goal is the same as the choice to link to the target of that goal's
+-- linking.
 --
 -- The code here proceeds by maintaining a finite map recording choices that
 -- have been made at higher positions in the tree. For each pair of package name
 -- and instance, it stores the prefixes at which we have made a choice for this
--- package instance. Whenever we make a choice, we extend the map. Whenever we
--- find a choice, we look into the map in order to find out what link options we
--- have to add.
+-- package instance. Whenever we make an unlinked choice, we extend the map.
+-- Whenever we find a choice, we look into the map in order to find out what
+-- link options we have to add.
 addLinking :: Tree a -> Tree a
 addLinking = (`runReader` M.empty) .  cata go
   where
@@ -66,9 +69,10 @@ addLinking = (`runReader` M.empty) .  cata go
     -- The only nodes of interest are package nodes
     go (PChoiceF qpn gr cs) = do
       env <- ask
-      cs' <- T.sequence $ P.mapWithKey (goP qpn) cs
-      let newCs = concatMap (linkChoices env qpn) (P.toList cs')
-      return $ PChoice qpn gr (cs' `P.union` P.fromList newCs)
+      let linkedCs = P.fromList $ concatMap (linkChoices env qpn) (P.toList cs)
+          unlinkedCs = P.mapWithKey (goP qpn) cs
+      allCs <- T.sequence $ unlinkedCs `P.union` linkedCs
+      return $ PChoice qpn gr allCs
     go _otherwise =
       innM _otherwise