From 3aee23cb2969a9626b6401664e2acc82086c960b Mon Sep 17 00:00:00 2001
From: Krzysztof Gogolewski <krzysztof.gogolewski@tweag.io>
Date: Tue, 16 May 2023 21:53:43 +0200
Subject: Output Lint errors to stderr instead of stdout

This is a continuation of 7b095b99, which fixed warnings but not errors.

Refs #13342
---
 compiler/GHC/Core/Lint.hs         | 10 ++++++----
 compiler/GHC/Driver/CodeOutput.hs |  2 +-
 compiler/GHC/Stg/Lint.hs          |  2 +-
 3 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/compiler/GHC/Core/Lint.hs b/compiler/GHC/Core/Lint.hs
index 7bb1eb43aa..566b0985e2 100644
--- a/compiler/GHC/Core/Lint.hs
+++ b/compiler/GHC/Core/Lint.hs
@@ -302,6 +302,10 @@ path does not result in allocation in the hot path. This can be surprisingly
 impactful. Changing `lint_app` reduced allocations for one test program I was
 looking at by ~4%.
 
+Note [MCInfo for Lint]
+~~~~~~~~~~~~~~~~~~~~~~
+When printing a Lint message, use the MCInfo severity so that the
+message is printed on stderr rather than stdout (#13342).
 
 ************************************************************************
 *                                                                      *
@@ -425,7 +429,7 @@ displayLintResults :: Logger
                    -> IO ()
 displayLintResults logger display_warnings pp_what pp_pgm (warns, errs)
   | not (isEmptyBag errs)
-  = do { logMsg logger Err.MCDump noSrcSpan
+  = do { logMsg logger Err.MCInfo noSrcSpan  -- See Note [MCInfo for Lint]
            $ withPprStyle defaultDumpStyle
            (vcat [ lint_banner "errors" pp_what, Err.pprMessageBag errs
                  , text "*** Offending Program ***"
@@ -436,9 +440,7 @@ displayLintResults logger display_warnings pp_what pp_pgm (warns, errs)
   | not (isEmptyBag warns)
   , log_enable_debug (logFlags logger)
   , display_warnings
-  -- If the Core linter encounters an error, output to stderr instead of
-  -- stdout (#13342)
-  = logMsg logger Err.MCInfo noSrcSpan
+  = logMsg logger Err.MCInfo noSrcSpan  -- See Note [MCInfo for Lint]
       $ withPprStyle defaultDumpStyle
         (lint_banner "warnings" pp_what $$ Err.pprMessageBag (mapBag ($$ blankLine) warns))
 
diff --git a/compiler/GHC/Driver/CodeOutput.hs b/compiler/GHC/Driver/CodeOutput.hs
index c5c0534d20..458409a204 100644
--- a/compiler/GHC/Driver/CodeOutput.hs
+++ b/compiler/GHC/Driver/CodeOutput.hs
@@ -105,7 +105,7 @@ codeOutput logger tmpfs llvm_config dflags unit_state this_mod filenm location g
                   (const ()) $ do
                 { case cmmLint (targetPlatform dflags) cmm of
                         Just err -> do { logMsg logger
-                                                   MCDump
+                                                   MCInfo -- See Note [MCInfo for Lint] in "GHC.Core.Lint"
                                                    noSrcSpan
                                                    $ withPprStyle defaultDumpStyle err
                                        ; ghcExit logger 1
diff --git a/compiler/GHC/Stg/Lint.hs b/compiler/GHC/Stg/Lint.hs
index 8315e185e0..04d18ac5a5 100644
--- a/compiler/GHC/Stg/Lint.hs
+++ b/compiler/GHC/Stg/Lint.hs
@@ -149,7 +149,7 @@ lintStgTopBindings platform logger diag_opts opts extra_vars this_mod unarised w
       Nothing  ->
         return ()
       Just msg -> do
-        logMsg logger Err.MCDump noSrcSpan
+        logMsg logger Err.MCInfo noSrcSpan   -- See Note [MCInfo for Lint] in "GHC.Core.Lint"
           $ withPprStyle defaultDumpStyle
           (vcat [ text "*** Stg Lint ErrMsgs: in" <+>
                         text whodunit <+> text "***",
-- 
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