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

2 files changed, 1 insertions, 6 deletions

diff --git a/utils/haddock b/utils/haddock
-Subproject 0fc8cfd532f5dfd12b5504f44a2b3c9fb659cd8
+Subproject d4657f07912416a1b14ddb517696f8ef3ffb85a
diff --git a/utils/mkUserGuidePart/Main.hs b/utils/mkUserGuidePart/Main.hs
index 11283c0b95..b30cc8f01d 100644
--- a/utils/mkUserGuidePart/Main.hs
+++ b/utils/mkUserGuidePart/Main.hs
@@ -1,7 +1,6 @@
 module Main (main) where
 
 import DynFlags
-import Data.List (stripPrefix)
 import Control.Monad (forM_)
 import Types hiding (flag)
 import Table
@@ -43,11 +42,7 @@ whatGlasgowExtsDoes = unlines
     $ [ ".. hlist::", ""]
     ++ map ((" * "++) . parseExt) glasgowExtsFlags
   where
-    parseExt f
-      | Just ext <- stripPrefix "Opt_" (show f)
-      = inlineCode $ "-X" ++ ext
-      | otherwise
-      = error ("Can't parse extension: " ++ show f)
+    parseExt ext = inlineCode $ "-X" ++ show ext
 
 -- | Generate a reference table of the given set of flags. This is used in
 -- the users guide.