Add a flag to emit error messages as JSON

This patch adds the flag `-ddump-json` which dumps all the compiler
output as a JSON array. This allows tooling to more easily parse GHC's
output to display to users.

The flag is currently experimental and will hopefully be refined for the
next release.  In particular I have avoided any changes which involve
significant refactoring and provided what is easy given the current
infrastructure.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: DanielG, gracjan, thomie

Differential Revision: https://phabricator.haskell.org/D3010

GHC Trac Issues: #13190

1 files changed, 8 insertions, 0 deletions

diff --git a/compiler/main/ErrUtils.hs b/compiler/main/ErrUtils.hs
index c410f06099..2aeddc26a7 100644
--- a/compiler/main/ErrUtils.hs
+++ b/compiler/main/ErrUtils.hs
@@ -6,6 +6,7 @@
 
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE RecordWildCards #-}
 
 module ErrUtils (
         -- * Basic types
@@ -63,6 +64,7 @@ import SrcLoc
 import DynFlags
 import FastString (unpackFS)
 import StringBuffer (hGetStringBuffer, len, lexemeToString)
+import Json
 
 import System.Directory
 import System.Exit      ( ExitCode(..), exitWith )
@@ -127,6 +129,7 @@ data ErrMsg = ErrMsg {
         }
         -- The SrcSpan is used for sorting errors into line-number order
 
+
 -- | Categorise error msgs by their importance.  This is so each section can
 -- be rendered visually distinct.  See Note [Error report] for where these come
 -- from.
@@ -164,6 +167,11 @@ data Severity
     --     plus "warning:" or "error:",
     --     added by mkLocMessags
     --   o Output is intended for end users
+  deriving Show
+
+
+instance ToJson Severity where
+  json s = JSString (show s)
 
 
 instance Show ErrMsg where