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

6 files changed, 11 insertions, 0 deletions

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