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, 8 insertions, 0 deletions

diff --git a/docs/users_guide/7.12.1-notes.rst b/docs/users_guide/7.12.1-notes.rst
index 79bed2c16d..0604fef7e3 100644
--- a/docs/users_guide/7.12.1-notes.rst
+++ b/docs/users_guide/7.12.1-notes.rst
@@ -290,6 +290,11 @@ Template Haskell
    ``dataD`` and ``newtypeD`` now take a ``CxtQ`` instead of a ``[Name]``
    for the list of derived classes.
 
+-  ``isExtEnabled`` can now be used to determine whether a language extension
+   is enabled in the ``Q`` monad. Similarly, ``extsEnabled`` can be used to list
+   all enabled language extensions.
+
+
 Runtime system
 ~~~~~~~~~~~~~~
 
@@ -416,6 +421,9 @@ ghc-boot
    Haskell, as defined by GHC. (These functions were moved from ``Lexeme``
    in ``ghc``.)
 
+-  Added ``GHC.LanguageExtensions`` which contains a type listing all
+   supported language extensions.
+
 ghc-prim
 ~~~~~~~~