Modifications to support loading GHC into GHCi

This change was previously part of
[D4904](https://phabricator.haskell.org/D4904), but is being split off
to aid in getting this reviewed and merged.

* The compiler code is built with `NoImplicitPrelude`, but GHCi's
  modules are incompatible with it. So, this adds the pragma to all GHCi
  modules that didn't have it, and adds imports of Prelude.

* In order to run GHC within itself, a `call of 'initGCStatistics`
  needed to be skipped. This uses CPP to skip it when
  `-DGHC_LOADED_INTO_GHCI` is set.

* There is an environment variable workaround suggested by Ben Gamari
  [1], where `_GHC_TOP_DIR` can be used to specify GHC's top dir if `-B`
  isn't provided.  This can be used to solve a problem where the GHC being
  run within GHCi attempts to look in `inplace/lib/lib/` instead of
  `inplace/lib/`.

[1]: https://phabricator.haskell.org/D4904#135438

Reviewers: goldfire, bgamari, erikd, alpmestan

Reviewed By: alpmestan

Subscribers: alpmestan, lelf, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4986

11 files changed, 50 insertions, 8 deletions

diff --git a/compiler/main/SysTools/BaseDir.hs b/compiler/main/SysTools/BaseDir.hs
index 85635df264..625baec8d9 100644
--- a/compiler/main/SysTools/BaseDir.hs
+++ b/compiler/main/SysTools/BaseDir.hs
@@ -22,6 +22,7 @@ import GhcPrelude
 
 import Panic
 
+import System.Environment (lookupEnv)
 import System.FilePath
 import Data.List
 
@@ -115,12 +116,21 @@ findTopDir :: Maybe String -- Maybe TopDir path (without the '-B' prefix).
            -> IO String    -- TopDir (in Unix format '/' separated)
 findTopDir (Just minusb) = return (normalise minusb)
 findTopDir Nothing
-    = do -- Get directory of executable
-         maybe_exec_dir <- getBaseDir
-         case maybe_exec_dir of
-             -- "Just" on Windows, "Nothing" on unix
-             Nothing  -> throwGhcExceptionIO (InstallationError "missing -B<dir> option")
-             Just dir -> return dir
+    = do -- The _GHC_TOP_DIR environment variable can be used to specify
+         -- the top dir when the -B argument is not specified. It is not
+         -- intended for use by users, it was added specifically for the
+         -- purpose of running GHC within GHCi.
+         maybe_env_top_dir <- lookupEnv "_GHC_TOP_DIR"
+         case maybe_env_top_dir of
+             Just env_top_dir -> return env_top_dir
+             Nothing -> do
+                 -- Get directory of executable
+                 maybe_exec_dir <- getBaseDir
+                 case maybe_exec_dir of
+                     -- "Just" on Windows, "Nothing" on unix
+                     Nothing -> throwGhcExceptionIO $
+                         InstallationError "missing -B<dir> option"
+                     Just dir -> return dir
 
 getBaseDir :: IO (Maybe String)
 #if defined(mingw32_HOST_OS)
diff --git a/ghc/GHCi/Leak.hs b/ghc/GHCi/Leak.hs
index aec1ab544e..47fed9c28f 100644
--- a/ghc/GHCi/Leak.hs
+++ b/ghc/GHCi/Leak.hs
@@ -16,6 +16,7 @@ import GHC.Types (IO (..))
 import HscTypes
 import Outputable
 import Platform (target32Bit)
+import Prelude
 import System.Mem
 import System.Mem.Weak
 import UniqDFM
diff --git a/ghc/GHCi/UI/Tags.hs b/ghc/GHCi/UI/Tags.hs
index d8af7f8718..09a8406d96 100644
--- a/ghc/GHCi/UI/Tags.hs
+++ b/ghc/GHCi/UI/Tags.hs
@@ -25,13 +25,14 @@ import OccName (pprOccName)
 import ConLike
 import MonadUtils
 
+import Control.Monad
 import Data.Function
+import Data.List
 import Data.Maybe
 import Data.Ord
 import DriverPhases
 import Panic
-import Data.List
-import Control.Monad
+import Prelude
 import System.Directory
 import System.IO
 import System.IO.Error
diff --git a/ghc/Main.hs b/ghc/Main.hs
index 276546bc95..ea80910afb 100644
--- a/ghc/Main.hs
+++ b/ghc/Main.hs
@@ -74,6 +74,7 @@ import Control.Monad
 import Data.Char
 import Data.List
 import Data.Maybe
+import Prelude
 
 -----------------------------------------------------------------------------
 -- ToDo:
@@ -929,5 +930,18 @@ people since we're linking GHC dynamically, but most things themselves
 link statically.
 -}
 
+-- If GHC_LOADED_INTO_GHCI is not set when GHC is loaded into GHCi, then
+-- running it causes an error like this:
+--
+-- Loading temp shared object failed:
+-- /tmp/ghc13836_0/libghc_1872.so: undefined symbol: initGCStatistics
+--
+-- Skipping the foreign call fixes this problem, and the outer GHCi
+-- should have already made this call anyway.
+#if defined(GHC_LOADED_INTO_GHCI)
+initGCStatistics :: IO ()
+initGCStatistics = return ()
+#else
 foreign import ccall safe "initGCStatistics"
   initGCStatistics :: IO ()
+#endif
diff --git a/ghc/ghc-bin.cabal.in b/ghc/ghc-bin.cabal.in
index 85a92501d2..5c51058d81 100644
--- a/ghc/ghc-bin.cabal.in
+++ b/ghc/ghc-bin.cabal.in
@@ -84,3 +84,8 @@ Executable ghc
         CPP
         NondecreasingIndentation
         TupleSections
+
+    -- This should match the default-extensions used in 'ghc.cabal'. This way,
+    -- GHCi can be used to load it all at once.
+    Default-Extensions:
+        NoImplicitPrelude
diff --git a/libraries/template-haskell/Language/Haskell/TH/Lib.hs b/libraries/template-haskell/Language/Haskell/TH/Lib.hs
index b7966cefac..b0aa580d04 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Lib.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Lib.hs
@@ -151,6 +151,7 @@ import Language.Haskell.TH.Lib.Internal hiding
 import Language.Haskell.TH.Syntax
 
 import Control.Monad (liftM2)
+import Prelude
 
 -- All definitions below represent the "old" API, since their definitions are
 -- different in Language.Haskell.TH.Lib.Internal. Please think carefully before
diff --git a/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs b/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
index cac8ea8643..0ddfddf23a 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
@@ -18,6 +18,7 @@ import Language.Haskell.TH.Syntax hiding (Role, InjectivityAnn)
 import qualified Language.Haskell.TH.Syntax as TH
 import Control.Monad( liftM, liftM2 )
 import Data.Word( Word8 )
+import Prelude
 
 ----------------------------------------------------------
 -- * Type synonyms
diff --git a/libraries/template-haskell/Language/Haskell/TH/Lib/Map.hs b/libraries/template-haskell/Language/Haskell/TH/Lib/Map.hs
index ac241515b8..b11139c2cb 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Lib/Map.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Lib/Map.hs
@@ -16,6 +16,8 @@ module Language.Haskell.TH.Lib.Map
     , Language.Haskell.TH.Lib.Map.lookup
     ) where
 
+import Prelude
+
 data Map k a  = Bin {-# UNPACK #-} !Size !k a !(Map k a) !(Map k a)
               | Tip
 
diff --git a/libraries/template-haskell/Language/Haskell/TH/Quote.hs b/libraries/template-haskell/Language/Haskell/TH/Quote.hs
index 91e37399e6..4ff5a2041b 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Quote.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Quote.hs
@@ -21,6 +21,7 @@ module Language.Haskell.TH.Quote(
     ) where
 
 import Language.Haskell.TH.Syntax
+import Prelude
 
 -- | The 'QuasiQuoter' type, a value @q@ of this type can be used
 -- in the syntax @[q| ... string to parse ...|]@.  In fact, for
diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
index f5f60c38b4..4e0a1c9330 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
@@ -41,6 +41,7 @@ import GHC.Lexeme       ( startsVarSym, startsVarId )
 import GHC.ForeignSrcLang.Type
 import Language.Haskell.TH.LanguageExtensions
 import Numeric.Natural
+import Prelude
 
 import qualified Control.Monad.Fail as Fail
 
diff --git a/libraries/template-haskell/template-haskell.cabal b/libraries/template-haskell/template-haskell.cabal
index 6cd156ccb6..2b2c5db463 100644
--- a/libraries/template-haskell/template-haskell.cabal
+++ b/libraries/template-haskell/template-haskell.cabal
@@ -60,3 +60,8 @@ Library
     -- We need to set the unit ID to template-haskell (without a
     -- version number) as it's magic.
     ghc-options: -this-unit-id template-haskell
+
+    -- This should match the default-extensions used in 'ghc.cabal'. This way,
+    -- GHCi can be used to load it along with the compiler.
+    Default-Extensions:
+        NoImplicitPrelude