Expose enabled language extensions to TH

This exposes `template-haskell` functions for querying the language
extensions which are enabled when compiling a module,

- an `isExtEnabled` function to check whether an extension is enabled
- an `extsEnabled` function to obtain a full list of enabled extensions

To avoid code duplication this adds a `GHC.LanguageExtensions` module to
`ghc-boot` and moves `DynFlags.ExtensionFlag` into it. A happy
consequence of this is that the ungainly `DynFlags` lost around 500
lines. Moreover, flags corresponding to language extensions are now
clearly distinguished from other flags due to the `LangExt.*` prefix.

Updates haddock submodule.

This fixes #10820.

Test Plan: validate

Reviewers: austin, spinda, hvr, goldfire, alanz

Reviewed By: goldfire

Subscribers: mpickering, RyanGlScott, hvr, simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D1200

GHC Trac Issues: #10820

1 files changed, 2 insertions, 1 deletions

diff --git a/compiler/main/HeaderInfo.hs b/compiler/main/HeaderInfo.hs
index 35107c8bc3..3eef327f08 100644
--- a/compiler/main/HeaderInfo.hs
+++ b/compiler/main/HeaderInfo.hs
@@ -38,6 +38,7 @@ import Bag              ( emptyBag, listToBag, unitBag )
 import MonadUtils
 import Exception
 import BasicTypes
+import qualified GHC.LanguageExtensions as LangExt
 
 import Control.Monad
 import System.IO
@@ -83,7 +84,7 @@ getImports dflags buf filename source_filename = do
                 ordinary_imps = filter ((/= moduleName gHC_PRIM) . unLoc . ideclName . unLoc)
                                        ord_idecls
 
-                implicit_prelude = xopt Opt_ImplicitPrelude dflags
+                implicit_prelude = xopt LangExt.ImplicitPrelude dflags
                 implicit_imports = mkPrelImports (unLoc mod) main_loc
                                                  implicit_prelude imps
                 convImport (L _ i) = (fmap sl_fs (ideclPkgQual i), ideclName i)