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/GHC.hs b/compiler/main/GHC.hs
index 965f7c1439..74860a1c03 100644
--- a/compiler/main/GHC.hs
+++ b/compiler/main/GHC.hs
@@ -341,6 +341,7 @@ import FastString
 import qualified Parser
 import Lexer
 import ApiAnnotation
+import qualified GHC.LanguageExtensions as LangExt
 
 import System.Directory ( doesFileExist )
 import Data.Maybe
@@ -1047,7 +1048,7 @@ getModuleGraph = liftM hsc_mod_graph getSession
 -- have Template Haskell enabled whether it is actually needed or not.
 needsTemplateHaskell :: ModuleGraph -> Bool
 needsTemplateHaskell ms =
-    any (xopt Opt_TemplateHaskell . ms_hspp_opts) ms
+    any (xopt LangExt.TemplateHaskell . ms_hspp_opts) ms
 
 -- | Return @True@ <==> module is loaded.
 isLoaded :: GhcMonad m => ModuleName -> m Bool