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, 16 insertions, 0 deletions

diff --git a/testsuite/tests/th/T10820.hs b/testsuite/tests/th/T10820.hs
new file mode 100644
index 0000000000..0e53bbde72
--- /dev/null
+++ b/testsuite/tests/th/T10820.hs
@@ -0,0 +1,16 @@
+{-# LANGUAGE LiberalTypeSynonyms #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE MagicHash #-}
+
+module Main where
+
+import Language.Haskell.TH.Syntax
+import GHC.LanguageExtensions
+
+main = do
+  print $(isExtEnabled Cpp                  >>= lift)
+  print $(isExtEnabled LiberalTypeSynonyms  >>= lift)
+  print $(isExtEnabled RankNTypes           >>= lift)
+  print $(isExtEnabled TypeSynonymInstances >>= lift)
+  print $(isExtEnabled MagicHash            >>= lift)