Dont call unsafeGlobalDynFlags if it is not set

Parsing of static and mode flags happens before any session is started,
i.e., before the first call to 'GHC.withGhc'. Therefore, to report
errors for invalid usage of these two types of flags, we can not call
any function that needs DynFlags, as there are no DynFlags available yet
(unsafeGlobalDynFlags is not set either). So we always print "on the
commandline" as the location, which is true except for Api users, which
is probably ok.

When reporting errors for invalid usage of dynamic flags we /can/ make
use of DynFlags, and we do so explicitly in
DynFlags.parseDynamicFlagsFull.

Before, we called unsafeGlobalDynFlags when an invalid (combination of)
flag(s) was given on the commandline, resulting in panics (#9963). This
regression was introduced in 1d6124de.

Also rename showSDocSimple to showSDocUnsafe, to hopefully prevent this
from happening again.

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D730

GHC Trac Issues: #9963

6 files changed, 43 insertions, 17 deletions

diff --git a/compiler/basicTypes/SrcLoc.hs b/compiler/basicTypes/SrcLoc.hs
index 4f6cc1a17d..362a925992 100644
--- a/compiler/basicTypes/SrcLoc.hs
+++ b/compiler/basicTypes/SrcLoc.hs
@@ -48,7 +48,7 @@ module SrcLoc (
         srcSpanStart, srcSpanEnd,
         realSrcSpanStart, realSrcSpanEnd,
         srcSpanFileName_maybe,
-        showUserSpan, pprUserRealSpan,
+        pprUserRealSpan,
 
         -- ** Unsafely deconstructing SrcSpan
         -- These are dubious exports, because they crash on some inputs
@@ -484,9 +484,6 @@ instance Outputable SrcSpan where
 --           UnhelpfulSpan _ -> panic "Outputable UnhelpfulSpan"
 --           RealSrcSpan s -> ppr s
 
-showUserSpan :: Bool -> SrcSpan -> String
-showUserSpan show_path span = showSDocSimple (pprUserSpan show_path span)
-
 pprUserSpan :: Bool -> SrcSpan -> SDoc
 pprUserSpan _         (UnhelpfulSpan s) = ftext s
 pprUserSpan show_path (RealSrcSpan s)   = pprUserRealSpan show_path s
diff --git a/compiler/main/CmdLineParser.hs b/compiler/main/CmdLineParser.hs
index 422fa13eb5..dad7ea7ae2 100644
--- a/compiler/main/CmdLineParser.hs
+++ b/compiler/main/CmdLineParser.hs
@@ -295,8 +295,26 @@ missingArgErr f = Left ("missing argument for flag: " ++ f)
 -- Utils
 --------------------------------------------------------
 
-errorsToGhcException :: [Located String] -> GhcException
+
+-- See Note [Handling errors when parsing flags]
+errorsToGhcException :: [(String,    -- Location
+                          String)]   -- Error
+                     -> GhcException
 errorsToGhcException errs =
-    UsageError $
-        intercalate "\n" [ showUserSpan True l ++ ": " ++ e | L l e <- errs ]
+    UsageError $ intercalate "\n" $ [ l ++ ": " ++ e | (l, e) <- errs ]
+
+{- Note [Handling errors when parsing commandline flags]
+
+Parsing of static and mode flags happens before any session is started, i.e.,
+before the first call to 'GHC.withGhc'. Therefore, to report errors for
+invalid usage of these two types of flags, we can not call any function that
+needs DynFlags, as there are no DynFlags available yet (unsafeGlobalDynFlags
+is not set either). So we always print "on the commandline" as the location,
+which is true except for Api users, which is probably ok.
+
+When reporting errors for invalid usage of dynamic flags we /can/ make use of
+DynFlags, and we do so explicitly in DynFlags.parseDynamicFlagsFull.
 
+Before, we called unsafeGlobalDynFlags when an invalid (combination of)
+flag(s) was given on the commandline, resulting in panics (#9963).
+-}
diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index aa6b7f9308..2c1a82c787 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -178,6 +178,7 @@ import {-# SOURCE #-} ErrUtils ( Severity(..), MsgDoc, mkLocMessage )
 
 import System.IO.Unsafe ( unsafePerformIO )
 import Data.IORef
+import Control.Arrow ((&&&))
 import Control.Monad
 import Control.Exception (throwIO)
 
@@ -2108,8 +2109,10 @@ parseDynamicFlagsFull :: MonadIO m
 parseDynamicFlagsFull activeFlags cmdline dflags0 args = do
   let ((leftover, errs, warns), dflags1)
           = runCmdLine (processArgs activeFlags args) dflags0
-  when (not (null errs)) $ liftIO $
-      throwGhcExceptionIO $ errorsToGhcException errs
+
+  -- See Note [Handling errors when parsing commandline flags]
+  unless (null errs) $ liftIO $ throwGhcExceptionIO $
+      errorsToGhcException . map (showPpr dflags0 . getLoc &&& unLoc) $ errs
 
   -- check for disabled flags in safe haskell
   let (dflags2, sh_warns) = safeFlagCheck cmdline dflags1
@@ -4200,7 +4203,8 @@ makeDynFlagsConsistent dflags
 -- to show SDocs when tracing, but we don't always have DynFlags
 -- available.
 --
--- Do not use it if you can help it. You may get the wrong value!
+-- Do not use it if you can help it. You may get the wrong value, or this
+-- panic!
 
 GLOBAL_VAR(v_unsafeGlobalDynFlags, panic "v_unsafeGlobalDynFlags: not initialised", DynFlags)
 
diff --git a/compiler/main/StaticFlags.hs b/compiler/main/StaticFlags.hs
index 4b4403a3ea..914a1459df 100644
--- a/compiler/main/StaticFlags.hs
+++ b/compiler/main/StaticFlags.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE CPP #-}
+{-# LANGUAGE CPP, TupleSections #-}
 {-# OPTIONS_GHC -fno-cse #-}
 -- -fno-cse is needed for GLOBAL_VAR's to behave properly
 
@@ -82,7 +82,10 @@ parseStaticFlagsFull flagsAvailable args = do
   when ready $ throwGhcExceptionIO (ProgramError "Too late for parseStaticFlags: call it before runGhc or runGhcT")
 
   (leftover, errs, warns) <- processArgs flagsAvailable args
-  when (not (null errs)) $ throwGhcExceptionIO $ errorsToGhcException errs
+
+  -- See Note [Handling errors when parsing commandline flags]
+  unless (null errs) $ throwGhcExceptionIO $
+      errorsToGhcException . map (("on the commandline", ) . unLoc) $ errs
 
     -- see sanity code in staticOpts
   writeIORef v_opt_C_ready True
diff --git a/compiler/typecheck/TcGenDeriv.hs b/compiler/typecheck/TcGenDeriv.hs
index 7802a22f87..6216ec2079 100644
--- a/compiler/typecheck/TcGenDeriv.hs
+++ b/compiler/typecheck/TcGenDeriv.hs
@@ -2274,8 +2274,10 @@ mkAuxBinderName parent occ_fun
     uniq_parent_occ = mkOccName (occNameSpace parent_occ) uniq_string
 
     uniq_string
-      | opt_PprStyle_Debug = showSDocSimple (ppr parent_occ <> underscore <> ppr parent_uniq)
-      | otherwise          = show parent_uniq
+      | opt_PprStyle_Debug
+      = showSDocUnsafe (ppr parent_occ <> underscore <> ppr parent_uniq)
+      | otherwise
+      = show parent_uniq
       -- The debug thing is just to generate longer, but perhaps more perspicuous, names
 
     parent_uniq = nameUnique parent
diff --git a/compiler/utils/Outputable.hs b/compiler/utils/Outputable.hs
index c557224fc1..e6e8e02dda 100644
--- a/compiler/utils/Outputable.hs
+++ b/compiler/utils/Outputable.hs
@@ -40,7 +40,7 @@ module Outputable (
         -- * Converting 'SDoc' into strings and outputing it
         printForC, printForAsm, printForUser, printForUserPartWay,
         pprCode, mkCodeStyle,
-        showSDoc, showSDocSimple, showSDocOneLine,
+        showSDoc, showSDocUnsafe, showSDocOneLine,
         showSDocForUser, showSDocDebug, showSDocDump, showSDocDumpOneLine,
         showSDocUnqual, showPpr,
         renderWithStyle,
@@ -406,8 +406,10 @@ mkCodeStyle = PprCode
 showSDoc :: DynFlags -> SDoc -> String
 showSDoc dflags sdoc = renderWithStyle dflags sdoc defaultUserStyle
 
-showSDocSimple :: SDoc -> String
-showSDocSimple sdoc = showSDoc unsafeGlobalDynFlags sdoc
+-- showSDocUnsafe is unsafe, because `unsafeGlobalDynFlags` might not be
+-- initialised yet.
+showSDocUnsafe :: SDoc -> String
+showSDocUnsafe sdoc = showSDoc unsafeGlobalDynFlags sdoc
 
 showPpr :: Outputable a => DynFlags -> a -> String
 showPpr dflags thing = showSDoc dflags (ppr thing)