Attempt to speed solving when test is enable projectwide

cabal-install-solver/src/Distribution/Solver/Modular/Preference.hs

@@ -127,11 +127,18 @@ preferPackagePreferences pcs =
 
 -- | Traversal that tries to establish package stanza enable\/disable
 -- preferences. Works by reordering the branches of stanza choices.
+-- Note that this works on packages lower in the path as well as at the top level.
+-- This is because stanza preferences apply to local packages only
+-- and for local packages, a single version is fixed, which means
+-- (for now) that all stanza preferences must be uniform at all levels.
+-- Further, even when we can have multiple versions of the same package,
+-- the build plan will be more efficient if we can attempt to keep
+-- stanza preferences aligned at all levels.
 preferPackageStanzaPreferences :: (PN -> PackagePreferences) -> Tree d c -> Tree d c
 preferPackageStanzaPreferences pcs = trav go
   where
-    go (SChoiceF qsn@(SN (Q pp pn) s) rdm gr _tr ts)
-      | primaryPP pp && enableStanzaPref pn s =
+    go (SChoiceF qsn@(SN (Q _pp pn) s) rdm gr _tr ts)
+      | enableStanzaPref pn s =
           -- move True case first to try enabling the stanza
           let ts' = W.mapWeightsWithKey (\k w -> weight k : w) ts
               weight k = if k then 0 else 1

cabal-install/src/Distribution/Client/ProjectPlanning.hs

@@ -1024,13 +1024,14 @@ planPackages verbosity comp platform solver SolverSettings{..}
             | (pc, src) <- solverSettingConstraints ]
 
       . addPreferences
-          -- enable stanza preference where the user did not specify
+          -- enable stanza preference unilaterally, regardless if the user asked
+          -- accordingly or expressed no preference, to help hint the solver
           [ PackageStanzasPreference pkgname stanzas
           | pkg <- localPackages
           , let pkgname = pkgSpecifierTarget pkg
                 stanzaM = Map.findWithDefault Map.empty pkgname pkgStanzasEnable
                 stanzas = [ stanza | stanza <- [minBound..maxBound]
-                          , Map.lookup stanza stanzaM == Nothing ]
+                          , Map.lookup stanza stanzaM /= Just False ]
           , not (null stanzas)
           ]
 
@@ -2592,7 +2593,7 @@ nubComponentTargets =
                             -> [(ComponentTarget, NonEmpty a)]
     wholeComponentOverrides ts =
       case [ ta | ta@(ComponentTarget _ WholeComponent, _) <- ts ] of
-        ((t, x):_) -> 
+        ((t, x):_) ->
                 let
                     -- Delete tuple (t, x) from original list to avoid duplicates.
                     -- Use 'deleteBy', to avoid additional Class constraint on 'nubComponentTargets'.