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

4 files changed, 23 insertions, 2 deletions

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