Default to using a stack-installed, isolated GHC #2221

Address option compatibility issues around the new default setting:

* Add notes to ghc-variant and setup --reinstall help texts
* Add a check to catch uses of ghc-variant in combination with
  no-system-ghc

ChangeLog.md

@@ -6,6 +6,15 @@ Release notes:
 
 Major changes:
 
+* Stack will now always use its own GHC installation, even when a suitable GHC
+  installation is available on the PATH. To get the old behaviour, use
+  the `--system-ghc` flag or set `system-ghc: true` in your ~/.stack/config.yaml.
+
+  NB: Scripts that previously used stack in combination with a system GHC
+  installation should now include a `stack setup` line or use the `--install-ghc`
+  flag.
+  [#2221](https://github.com/commercialhaskell/stack/issues/2221)
+
 Behavior changes:
 
 * Switch the "Run from outside project" messages to debug-level, to

doc/yaml_configuration.md

@@ -306,13 +306,13 @@ other projects by installing into your shared snapshot database.
 
 ### system-ghc
 
-Enables or disables using the GHC available on the PATH. Useful to disable if
-you want to force stack to use its own installed GHC (via `stack setup`), in
-cases where your system GHC my be incomplete for some reason. Default is `true`.
+Enables or disables using the GHC available on the PATH.
+Useful to disable if you want to save the time, bandwidth or storage space needed to setup an isolated GHC.
+Default is `false`.
 
 ```yaml
-# Turn off system GHC
-system-ghc: false
+# Turn on system GHC
+system-ghc: true
 ```
 
 ### install-ghc
@@ -452,6 +452,8 @@ Specify a variant binary distribution of GHC to use.  Known values:
   [setup-info](#setup-info) so `stack setup` knows where to download it, or
   pass the `stack setup --ghc-bindist` argument on the command-line
 
+This option is incompatible with `system-ghc: true`.
+
 ### ghc-build
 
 (Since 1.2.1)

src/Stack/Config.hs

@@ -228,9 +228,12 @@ configFromConfigMonoid configStackRoot configUserConfigPath mresolver mproject C
 
          configGHCVariant0 = getFirst configMonoidGHCVariant
          configGHCBuild = getFirst configMonoidGHCBuild
+         configSystemGHC = fromFirst False configMonoidSystemGHC
 
-         configSystemGHC = fromFirst (isNothing configGHCVariant0) configMonoidSystemGHC
-         configInstallGHC = fromFirst False configMonoidInstallGHC
+     when (isJust configGHCVariant0 && configSystemGHC) $
+         throwM ManualGHCVariantSettingsAreIncompatibleWithSystemGHC
+
+     let configInstallGHC = fromFirst False configMonoidInstallGHC
          configSkipGHCCheck = fromFirst False configMonoidSkipGHCCheck
          configSkipMsys = fromFirst False configMonoidSkipMsys
 

src/Stack/Options/ConfigParser.hs

@@ -57,7 +57,7 @@ configOptsParser hide0 =
     <*> nixOptsParser True
     <*> firstBoolFlags
             "system-ghc"
-            "using the system installed GHC (on the PATH) if available and a matching version"
+            "using the system installed GHC (on the PATH) if available and a matching version. Disabled by default."
             hide
     <*> firstBoolFlags
             "install-ghc"

src/Stack/Options/GhcVariantParser.hs

@@ -13,7 +13,7 @@ ghcVariantParser hide =
         readGHCVariant
         (long "ghc-variant" <> metavar "VARIANT" <>
          help
-             "Specialized GHC variant, e.g. integersimple (implies --no-system-ghc)" <>
+             "Specialized GHC variant, e.g. integersimple (incompatible with --system-ghc)" <>
          hideMods hide
         )
   where

src/Stack/Setup.hs

@@ -432,7 +432,8 @@ ensureCompiler sopts = do
                             (soptsStackYaml sopts)
                             (fromMaybe
                                 ("Try running \"stack setup\" to install the correct GHC into "
-                                <> T.pack (toFilePath (configLocalPrograms config)))
+                                <> T.pack (toFilePath (configLocalPrograms config))
+                                <> " or use \"--system-ghc\" to use a suitable compiler available on your PATH.")
                                 $ soptsResolveMissingGHC sopts)
 
             -- Install msys2 on windows, if necessary

src/Stack/SetupCmd.hs

@@ -46,7 +46,7 @@ setupParser = SetupCmdOpts
                       "The default is to install the version implied by the resolver.")))
     <*> OA.boolFlags False
             "reinstall"
-            "reinstalling GHC, even if available (implies no-system-ghc)"
+            "reinstalling GHC, even if available (incompatible with --system-ghc)"
             OA.idm
     <*> OA.boolFlags False
             "upgrade-cabal"

src/Stack/Types/Config.hs

@@ -1086,6 +1086,7 @@ data ConfigException
   | BadStackRoot (Path Abs Dir)
   | Won'tCreateStackRootInDirectoryOwnedByDifferentUser (Path Abs Dir) (Path Abs Dir) -- ^ @$STACK_ROOT@, parent dir
   | UserDoesn'tOwnDirectory (Path Abs Dir)
+  | ManualGHCVariantSettingsAreIncompatibleWithSystemGHC
   deriving Typeable
 instance Show ConfigException where
     show (ParseConfigFileException configFile exception) = concat
@@ -1181,6 +1182,13 @@ instance Show ConfigException where
         , T.unpack configMonoidAllowDifferentUserName
         , "' to disable this precaution."
         ]
+    show ManualGHCVariantSettingsAreIncompatibleWithSystemGHC = T.unpack $ T.concat
+        [ "stack can only control the "
+        , configMonoidGHCVariantName
+        , " of its own GHC installations. Please use '--no-"
+        , configMonoidSystemGHCName
+        , "'."
+        ]
 instance Exception ConfigException
 
 showOptions :: WhichSolverCmd -> String