Implement deriving strategies

Allows users to explicitly request which approach to `deriving` to use
via keywords, e.g.,

```
newtype Foo = Foo Bar
  deriving Eq
  deriving stock    Ord
  deriving newtype Show
```

Fixes #10598. Updates haddock submodule.

Test Plan: ./validate

Reviewers: hvr, kosmikus, goldfire, alanz, bgamari, simonpj, austin,
erikd, simonmar

Reviewed By: alanz, bgamari, simonpj

Subscribers: thomie, mpickering, oerjan

Differential Revision: https://phabricator.haskell.org/D2280

GHC Trac Issues: #10598

1 files changed, 26 insertions, 0 deletions

diff --git a/compiler/basicTypes/BasicTypes.hs b/compiler/basicTypes/BasicTypes.hs
index aab0528d1c..0429a43f5d 100644
--- a/compiler/basicTypes/BasicTypes.hs
+++ b/compiler/basicTypes/BasicTypes.hs
@@ -41,6 +41,8 @@ module BasicTypes(
 
         TopLevelFlag(..), isTopLevel, isNotTopLevel,
 
+        DerivStrategy(..),
+
         OverlapFlag(..), OverlapMode(..), setOverlapModeMaybe,
         hasOverlappingFlag, hasOverlappableFlag, hasIncoherentFlag,
 
@@ -479,6 +481,30 @@ instance Outputable Origin where
 {-
 ************************************************************************
 *                                                                      *
+                Deriving strategies
+*                                                                      *
+************************************************************************
+-}
+
+-- | Which technique the user explicitly requested when deriving an instance.
+data DerivStrategy
+  -- See Note [Deriving strategies] in TcDeriv
+  = DerivStock    -- ^ GHC's \"standard\" strategy, which is to implement a
+                  --   custom instance for the data type. This only works for
+                  --   certain types that GHC knows about (e.g., 'Eq', 'Show',
+                  --   'Functor' when @-XDeriveFunctor@ is enabled, etc.)
+  | DerivAnyclass -- ^ @-XDeriveAnyClass@
+  | DerivNewtype  -- ^ @-XGeneralizedNewtypeDeriving@
+  deriving (Eq, Data)
+
+instance Outputable DerivStrategy where
+    ppr DerivStock    = text "stock"
+    ppr DerivAnyclass = text "anyclass"
+    ppr DerivNewtype  = text "newtype"
+
+{-
+************************************************************************
+*                                                                      *
                 Instance overlap flag
 *                                                                      *
 ************************************************************************