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, 123 insertions, 0 deletions

diff --git a/libraries/ghc-boot/GHC/LanguageExtensions.hs b/libraries/ghc-boot/GHC/LanguageExtensions.hs
new file mode 100644
index 0000000000..b108013f4b
--- /dev/null
+++ b/libraries/ghc-boot/GHC/LanguageExtensions.hs
@@ -0,0 +1,122 @@
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  GHC.LanguageExtensions
+-- Copyright   :  (c) The GHC Team
+--
+-- Maintainer  :  ghc-devs@haskell.org
+-- Portability :  portable
+--
+-- A data type defining the language extensions supported by GHC.
+--
+module GHC.LanguageExtensions ( Extension(..) ) where
+
+-- | The language extensions known to GHC.
+data Extension
+-- See Note [Updating flag description in the User's Guide] in DynFlags
+   = Cpp
+   | OverlappingInstances
+   | UndecidableInstances
+   | IncoherentInstances
+   | UndecidableSuperClasses
+   | MonomorphismRestriction
+   | MonoPatBinds
+   | MonoLocalBinds
+   | RelaxedPolyRec           -- Deprecated
+   | ExtendedDefaultRules     -- Use GHC's extended rules for defaulting
+   | ForeignFunctionInterface
+   | UnliftedFFITypes
+   | InterruptibleFFI
+   | CApiFFI
+   | GHCForeignImportPrim
+   | JavaScriptFFI
+   | ParallelArrays           -- Syntactic support for parallel arrays
+   | Arrows                   -- Arrow-notation syntax
+   | TemplateHaskell
+   | TemplateHaskellQuotes    -- subset of TH supported by stage1, no splice
+   | QuasiQuotes
+   | ImplicitParams
+   | ImplicitPrelude
+   | ScopedTypeVariables
+   | AllowAmbiguousTypes
+   | UnboxedTuples
+   | BangPatterns
+   | TypeFamilies
+   | TypeInType
+   | OverloadedStrings
+   | OverloadedLists
+   | NumDecimals
+   | DisambiguateRecordFields
+   | RecordWildCards
+   | RecordPuns
+   | ViewPatterns
+   | GADTs
+   | GADTSyntax
+   | NPlusKPatterns
+   | DoAndIfThenElse
+   | RebindableSyntax
+   | ConstraintKinds
+   | PolyKinds                -- Kind polymorphism
+   | DataKinds                -- Datatype promotion
+   | InstanceSigs
+   | ApplicativeDo
+
+   | StandaloneDeriving
+   | DeriveDataTypeable
+   | AutoDeriveTypeable       -- Automatic derivation of Typeable
+   | DeriveFunctor
+   | DeriveTraversable
+   | DeriveFoldable
+   | DeriveGeneric            -- Allow deriving Generic/1
+   | DefaultSignatures        -- Allow extra signatures for defmeths
+   | DeriveAnyClass           -- Allow deriving any class
+   | DeriveLift               -- Allow deriving Lift
+
+   | TypeSynonymInstances
+   | FlexibleContexts
+   | FlexibleInstances
+   | ConstrainedClassMethods
+   | MultiParamTypeClasses
+   | NullaryTypeClasses
+   | FunctionalDependencies
+   | UnicodeSyntax
+   | ExistentialQuantification
+   | MagicHash
+   | EmptyDataDecls
+   | KindSignatures
+   | RoleAnnotations
+   | ParallelListComp
+   | TransformListComp
+   | MonadComprehensions
+   | GeneralizedNewtypeDeriving
+   | RecursiveDo
+   | PostfixOperators
+   | TupleSections
+   | PatternGuards
+   | LiberalTypeSynonyms
+   | RankNTypes
+   | ImpredicativeTypes
+   | TypeOperators
+   | ExplicitNamespaces
+   | PackageImports
+   | ExplicitForAll
+   | AlternativeLayoutRule
+   | AlternativeLayoutRuleTransitional
+   | DatatypeContexts
+   | NondecreasingIndentation
+   | RelaxedLayout
+   | TraditionalRecordSyntax
+   | LambdaCase
+   | MultiWayIf
+   | BinaryLiterals
+   | NegativeLiterals
+   | DuplicateRecordFields
+   | OverloadedLabels
+   | EmptyCase
+   | PatternSynonyms
+   | PartialTypeSignatures
+   | NamedWildCards
+   | StaticPointers
+   | Strict
+   | StrictData
+   | MonadFailDesugaring
+   deriving (Eq, Enum, Show)
diff --git a/libraries/ghc-boot/ghc-boot.cabal b/libraries/ghc-boot/ghc-boot.cabal
index 98929b7f83..7f0f14fc8c 100644
--- a/libraries/ghc-boot/ghc-boot.cabal
+++ b/libraries/ghc-boot/ghc-boot.cabal
@@ -36,6 +36,7 @@ Library
     exposed-modules:
             GHC.Lexeme
             GHC.PackageDb
+            GHC.LanguageExtensions
 
     build-depends: base       >= 4   && < 5,
                    binary     >= 0.7 && < 0.8,