Add a --write-ghc-environment-files setting.

Fixes haskell#4542.

cabal-install/Distribution/Client/Config.hs

@@ -363,7 +363,9 @@ instance Semigroup SavedConfig where
         configPreferences   = lastNonEmpty configPreferences,
         configSolver        = combine configSolver,
         configAllowNewer    = combineMonoid savedConfigureExFlags configAllowNewer,
-        configAllowOlder    = combineMonoid savedConfigureExFlags configAllowOlder
+        configAllowOlder    = combineMonoid savedConfigureExFlags configAllowOlder,
+        configWriteGhcEnvironmentFilesPolicy
+                            = combine configWriteGhcEnvironmentFilesPolicy
         }
         where
           combine      = combine' savedConfigureExFlags

cabal-install/Distribution/Client/ProjectConfig/Legacy.hs

@@ -334,7 +334,9 @@ convertLegacyAllPackageFlags globalFlags configFlags
       configPreferences         = projectConfigPreferences,
       configSolver              = projectConfigSolver,
       configAllowOlder          = projectConfigAllowOlder,
-      configAllowNewer          = projectConfigAllowNewer
+      configAllowNewer          = projectConfigAllowNewer,
+      configWriteGhcEnvironmentFilesPolicy
+                                = projectConfigWriteGhcEnvironmentFilesPolicy
     } = configExFlags
 
     InstallFlags {
@@ -555,8 +557,9 @@ convertToLegacySharedConfig
       configPreferences   = projectConfigPreferences,
       configSolver        = projectConfigSolver,
       configAllowOlder    = projectConfigAllowOlder,
-      configAllowNewer    = projectConfigAllowNewer
-
+      configAllowNewer    = projectConfigAllowNewer,
+      configWriteGhcEnvironmentFilesPolicy
+                          = projectConfigWriteGhcEnvironmentFilesPolicy
     }
 
     installFlags = InstallFlags {
@@ -925,7 +928,7 @@ legacySharedConfigFieldDescrs =
         (\v conf -> conf { configAllowNewer = fmap AllowNewer v })
       ]
   . filterFields
-      [ "cabal-lib-version", "solver"
+      [ "cabal-lib-version", "solver", "write-ghc-environment-files"
         -- not "constraint" or "preference", we use our own plural ones above
       ]
   . commandOptionsToFields

cabal-install/Distribution/Client/ProjectConfig/Types.hs

@@ -21,7 +21,8 @@ module Distribution.Client.ProjectConfig.Types (
   ) where
 
 import Distribution.Client.Types
-         ( RemoteRepo, AllowNewer(..), AllowOlder(..) )
+         ( RemoteRepo, AllowNewer(..), AllowOlder(..)
+         , WriteGhcEnvironmentFilesPolicy )
 import Distribution.Client.Dependency.Types
          ( PreSolver )
 import Distribution.Client.Targets
@@ -187,6 +188,8 @@ data ProjectConfigShared
        projectConfigSolver            :: Flag PreSolver,
        projectConfigAllowOlder        :: Maybe AllowOlder,
        projectConfigAllowNewer        :: Maybe AllowNewer,
+       projectConfigWriteGhcEnvironmentFilesPolicy
+                                      :: Flag WriteGhcEnvironmentFilesPolicy,
        projectConfigMaxBackjumps      :: Flag Int,
        projectConfigReorderGoals      :: Flag ReorderGoals,
        projectConfigCountConflicts    :: Flag CountConflicts,

cabal-install/Distribution/Client/ProjectOrchestration.hs

@@ -113,7 +113,8 @@ import           Distribution.Client.ProjectPlanOutput
 import           Distribution.Client.Types
                    ( GenericReadyPackage(..), UnresolvedSourcePackage
                    , PackageSpecifier(..)
-                   , SourcePackageDb(..) )
+                   , SourcePackageDb(..)
+                   , WriteGhcEnvironmentFilesPolicy(..) )
 import           Distribution.Solver.Types.PackageIndex
                    ( lookupPackageName )
 import qualified Distribution.Client.InstallPlan as InstallPlan
@@ -124,6 +125,8 @@ import           Distribution.Client.TargetSelector
 import           Distribution.Client.DistDirLayout
 import           Distribution.Client.Config (getCabalDir)
 import           Distribution.Client.Setup hiding (packageName)
+import           Distribution.Compiler
+                   ( CompilerFlavor(GHC) )
 import           Distribution.Types.ComponentName
                    ( componentNameString )
 import           Distribution.Types.UnqualComponentName
@@ -138,16 +141,20 @@ import           Distribution.PackageDescription
                    , diffFlagAssignment )
 import           Distribution.Simple.LocalBuildInfo
                    ( ComponentName(..), pkgComponents )
+import           Distribution.Simple.Flag
+                   ( fromFlagOrDefault )
 import qualified Distribution.Simple.Setup as Setup
 import           Distribution.Simple.Command (commandShowOptions)
 import           Distribution.Simple.Configure (computeEffectiveProfiling)
 
 import           Distribution.Simple.Utils
                    ( die', warn, notice, noticeNoWrap, debugNoWrap )
 import           Distribution.Verbosity
+import           Distribution.Version
+                   ( mkVersion )
 import           Distribution.Text
 import           Distribution.Simple.Compiler
-                   ( showCompilerId
+                   ( compilerCompatVersion, showCompilerId
                    , OptimisationLevel(..))
 
 import qualified Data.Monoid as Mon
@@ -391,10 +398,27 @@ runProjectPostBuildPhase verbosity
                          pkgsBuildStatus
                          buildOutcomes
 
-    void $ writePlanGhcEnvironment (distProjectRootDirectory distDirLayout)
-                                   elaboratedPlanOriginal
-                                   elaboratedShared
-                                   postBuildStatus
+    -- Write the .ghc.environment file (if allowed by the env file write policy).
+    let writeGhcEnvFilesPolicy =
+          projectConfigWriteGhcEnvironmentFilesPolicy . projectConfigShared
+          $ projectConfig
+
+        shouldWriteGhcEnvironment =
+          case fromFlagOrDefault WriteGhcEnvironmentFilesOnlyForGhc844AndNewer
+               writeGhcEnvFilesPolicy
+          of
+            AlwaysWriteGhcEnvironmentFiles                -> True
+            NeverWriteGhcEnvironmentFiles                 -> False
+            WriteGhcEnvironmentFilesOnlyForGhc844AndNewer ->
+              let compiler         = pkgConfigCompiler elaboratedShared
+                  ghcCompatVersion = compilerCompatVersion GHC compiler
+              in maybe False (>= mkVersion [8,4,4]) ghcCompatVersion
+
+    when shouldWriteGhcEnvironment $
+      void $ writePlanGhcEnvironment (distProjectRootDirectory distDirLayout)
+                                     elaboratedPlanOriginal
+                                     elaboratedShared
+                                     postBuildStatus
 
     -- Finally if there were any build failures then report them and throw
     -- an exception to terminate the program

cabal-install/Distribution/Client/Setup.hs

@@ -1,7 +1,8 @@
 {-# LANGUAGE ScopedTypeVariables #-}
-{-# LANGUAGE RecordWildCards #-}
-{-# LANGUAGE RankNTypes #-}
-{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE RecordWildCards     #-}
+{-# LANGUAGE RankNTypes          #-}
+{-# LANGUAGE DeriveGeneric       #-}
+{-# LANGUAGE LambdaCase          #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Distribution.Client.Setup
@@ -70,6 +71,7 @@ import Distribution.Client.Compat.Prelude hiding (get)
 import Distribution.Client.Types
          ( Username(..), Password(..), RemoteRepo(..)
          , AllowNewer(..), AllowOlder(..), RelaxDeps(..)
+         , WriteGhcEnvironmentFilesPolicy(..)
          )
 import Distribution.Client.BuildReports.Types
          ( ReportLevel(..) )
@@ -637,12 +639,14 @@ configCompilerAux' configFlags =
 -- | cabal configure takes some extra flags beyond runghc Setup configure
 --
 data ConfigExFlags = ConfigExFlags {
-    configCabalVersion :: Flag Version,
-    configExConstraints:: [(UserConstraint, ConstraintSource)],
-    configPreferences  :: [PackageVersionConstraint],
-    configSolver       :: Flag PreSolver,
-    configAllowNewer   :: Maybe AllowNewer,
-    configAllowOlder   :: Maybe AllowOlder
+    configCabalVersion  :: Flag Version,
+    configExConstraints :: [(UserConstraint, ConstraintSource)],
+    configPreferences   :: [PackageVersionConstraint],
+    configSolver        :: Flag PreSolver,
+    configAllowNewer    :: Maybe AllowNewer,
+    configAllowOlder    :: Maybe AllowOlder,
+    configWriteGhcEnvironmentFilesPolicy
+      :: Flag WriteGhcEnvironmentFilesPolicy
   }
   deriving (Eq, Generic)
 
@@ -707,9 +711,34 @@ configureExOptions _showOrParseArgs src =
      (readP_to_E ("Cannot parse the list of packages: " ++) relaxDepsParser)
      (Just RelaxDepsAll) relaxDepsPrinter)
 
+  , option [] ["write-ghc-environment-files"]
+    ("Whether to create a .ghc.environment file after a successful build"
+      ++ " (v2-build only)")
+    configWriteGhcEnvironmentFilesPolicy
+    (\v flags -> flags { configWriteGhcEnvironmentFilesPolicy = v})
+    (reqArg "always|never|ghc8.4.4+"
+     writeGhcEnvironmentFilesPolicyParser
+     writeGhcEnvironmentFilesPolicyPrinter)
   ]
 
 
+writeGhcEnvironmentFilesPolicyParser :: ReadE (Flag WriteGhcEnvironmentFilesPolicy)
+writeGhcEnvironmentFilesPolicyParser = ReadE $ \case
+  "always"    -> Right $ Flag AlwaysWriteGhcEnvironmentFiles
+  "never"     -> Right $ Flag NeverWriteGhcEnvironmentFiles
+  "ghc8.4.4+" -> Right $ Flag WriteGhcEnvironmentFilesOnlyForGhc844AndNewer
+  policy      -> Left  $ "Cannot parse the GHC environment file write policy '"
+                 <> policy <> "'"
+
+writeGhcEnvironmentFilesPolicyPrinter
+  :: Flag WriteGhcEnvironmentFilesPolicy -> [String]
+writeGhcEnvironmentFilesPolicyPrinter = \case
+  (Flag AlwaysWriteGhcEnvironmentFiles)                -> ["always"]
+  (Flag NeverWriteGhcEnvironmentFiles)                 -> ["never"]
+  (Flag WriteGhcEnvironmentFilesOnlyForGhc844AndNewer) -> ["ghc8.4.4+"]
+  NoFlag                                               -> []
+
+
 relaxDepsParser :: Parse.ReadP r (Maybe RelaxDeps)
 relaxDepsParser =
   (Just . RelaxDepsSome) `fmap` Parse.sepBy1 parse (Parse.char ',')

cabal-install/Distribution/Client/Types.hs

@@ -591,3 +591,19 @@ instance Monoid AllowNewer where
 instance Monoid AllowOlder where
   mempty  = AllowOlder mempty
   mappend = (<>)
+
+-- ------------------------------------------------------------
+-- * --write-ghc-environment-file
+-- ------------------------------------------------------------
+
+-- | Whether 'v2-build' should write a .ghc.environment file after
+-- success. Possible values: 'always', 'never', 'ghc8.4.4+' (the
+-- default; GHC 8.4.4 is the earliest version that supports
+-- '-pkg-env -').
+data WriteGhcEnvironmentFilesPolicy
+  = AlwaysWriteGhcEnvironmentFiles
+  | NeverWriteGhcEnvironmentFiles
+  | WriteGhcEnvironmentFilesOnlyForGhc844AndNewer
+  deriving (Eq, Generic, Show)
+
+instance Binary WriteGhcEnvironmentFilesPolicy