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, 3 insertions, 2 deletions

diff --git a/compiler/typecheck/TcPat.hs b/compiler/typecheck/TcPat.hs
index 074532276e..6b22aa6bf6 100644
--- a/compiler/typecheck/TcPat.hs
+++ b/compiler/typecheck/TcPat.hs
@@ -48,6 +48,7 @@ import Util
 import Outputable
 import FastString
 import Maybes( orElse )
+import qualified GHC.LanguageExtensions as LangExt
 import Control.Monad
 
 {-
@@ -674,8 +675,8 @@ tcDataConPat penv (L con_span con_name) data_con pat_ty arg_pats thing_inside
                             LamPat mc -> PatSkol (RealDataCon data_con) mc
                             LetPat {} -> UnkSkol -- Doesn't matter
 
-        ; gadts_on    <- xoptM Opt_GADTs
-        ; families_on <- xoptM Opt_TypeFamilies
+        ; gadts_on    <- xoptM LangExt.GADTs
+        ; families_on <- xoptM LangExt.TypeFamilies
         ; checkTc (no_equalities || gadts_on || families_on)
                   (text "A pattern match on a GADT requires the" <+>
                    text "GADTs or TypeFamilies language extension")