Catch preprocessor errors in downsweep

This changes the way preprocessor failures are presented to the
user. Previously the user would simply get an unlocated message on stderr
such as:

    `gcc' failed in phase `C pre-processor'. (Exit code: 1)

Now at the problematic source file is mentioned:

    A.hs:1:1: error:
        `gcc' failed in phase `C pre-processor'. (Exit code: 1)

This also makes live easier for GHC API clients as the preprocessor error
is now thrown as a SourceError exception.

4 files changed, 79 insertions, 11 deletions

diff --git a/testsuite/tests/ghc-api/downsweep/PartialDownsweep.darwin.stderr b/testsuite/tests/ghc-api/downsweep/PartialDownsweep.darwin.stderr
new file mode 100644
index 0000000000..c9cd0f216d
--- /dev/null
+++ b/testsuite/tests/ghc-api/downsweep/PartialDownsweep.darwin.stderr
@@ -0,0 +1,16 @@
+== Parse error in export list
+== Parse error in export list with bypass module
+== Parse error in import list
+== CPP preprocessor error
+
+B.hs:2:2:  #elif without #if
+     #elif <- cpp error here
+ ^
+1 error generated.
+== CPP preprocessor error with bypass
+
+B.hs:2:2:  #elif without #if
+     #elif <- cpp error here
+ ^
+1 error generated.
+== Import error
diff --git a/testsuite/tests/ghc-api/downsweep/PartialDownsweep.hs b/testsuite/tests/ghc-api/downsweep/PartialDownsweep.hs
index f3c379a3fb..fb91fb6c1f 100644
--- a/testsuite/tests/ghc-api/downsweep/PartialDownsweep.hs
+++ b/testsuite/tests/ghc-api/downsweep/PartialDownsweep.hs
@@ -45,7 +45,8 @@ main = do
   runGhc (Just libdir) $ do
     dflags0 <- getSessionDynFlags
     (dflags1, _, _) <- parseDynamicFlags dflags0 $ map noLoc $
-        [ -- "-v3"
+        [ "-fno-diagnostics-show-caret"
+        -- , "-v3"
         ] ++ args
     _ <- setSessionDynFlags dflags1
 
@@ -65,6 +66,23 @@ main = do
          sort (map (moduleNameString . moduleName . ms_mod) mss) == ["A"]
        )
 
+    go "Parse error in export list with bypass module"
+        [ [ "module A where"
+          , "import B"
+          , "import C"
+          ]
+        , [ "module B !parse_error where"
+          , "import D"
+          ]
+        , [ "module C where"
+          , "import D"
+          ]
+        , [ "module D where"
+          ]
+        ]
+       (\mss -> return $
+           sort (map (moduleNameString . moduleName . ms_mod) mss) == ["A", "C", "D"]
+       )
     go "Parse error in import list"
         [ [ "module A where"
           , "import B"
@@ -83,24 +101,40 @@ main = do
          sort (map (moduleNameString . moduleName . ms_mod) mss) == ["A", "B"]
        )
 
-    go "Parse error in export list with bypass module"
+    go "CPP preprocessor error"
         [ [ "module A where"
           , "import B"
-          , "import C"
           ]
-        , [ "module B !parse_error where"
-          , "import D"
+        , [ "{-# LANGUAGE CPP #-}"
+          , "#elif <- cpp error here"
+          , "module B where"
+          , "import C"
           ]
         , [ "module C where"
-          , "import D"
           ]
-        , [ "module D where"
+        ]
+       (\mss -> return $
+         sort (map (moduleNameString . moduleName . ms_mod) mss) == ["A"]
+       )
+
+    go "CPP preprocessor error with bypass"
+        [ [ "module A where"
+          , "import B"
+          , "import C"
+          ]
+        , [ "{-# LANGUAGE CPP #-}"
+          , "#elif <- cpp error here"
+          , "module B where"
+          , "import C"
+          ]
+        , [ "module C where"
           ]
         ]
        (\mss -> return $
-           sort (map (moduleNameString . moduleName . ms_mod) mss) == ["A", "C", "D"]
+         sort (map (moduleNameString . moduleName . ms_mod) mss) == ["A", "C"]
        )
 
+
   errored <- readIORef any_failed
   when errored $ exitFailure
   return ()
@@ -125,5 +159,8 @@ go label mods cnd =
 
 
 writeMod :: [String] -> IO ()
-writeMod src@(head -> stripPrefix "module " -> Just (takeWhile (/=' ') -> mod))
-  = writeFile (mod++".hs") $ unlines src
+writeMod src =
+    writeFile (mod++".hs") $ unlines src
+  where
+    Just modline = find ("module" `isPrefixOf`) src
+    Just (takeWhile (/=' ') -> mod) = stripPrefix "module " modline
diff --git a/testsuite/tests/ghc-api/downsweep/PartialDownsweep.stderr b/testsuite/tests/ghc-api/downsweep/PartialDownsweep.stderr
index 11fd4b73c8..9e5f6d83dc 100644
--- a/testsuite/tests/ghc-api/downsweep/PartialDownsweep.stderr
+++ b/testsuite/tests/ghc-api/downsweep/PartialDownsweep.stderr
@@ -1,3 +1,15 @@
 == Parse error in export list
-== Parse error in import list
 == Parse error in export list with bypass module
+== Parse error in import list
+== CPP preprocessor error
+
+B.hs:2:0: error:
+     error: #elif without #if
+     #elif <- cpp error here
+     
+== CPP preprocessor error with bypass
+
+B.hs:2:0: error:
+     error: #elif without #if
+     #elif <- cpp error here
+     
diff --git a/testsuite/tests/ghc-api/downsweep/all.T b/testsuite/tests/ghc-api/downsweep/all.T
index d7ed778f8e..18ed26ac88 100644
--- a/testsuite/tests/ghc-api/downsweep/all.T
+++ b/testsuite/tests/ghc-api/downsweep/all.T
@@ -1,5 +1,8 @@
 test('PartialDownsweep',
      [ extra_run_opts('"' + config.libdir + '"')
+     , when(opsys('darwin'),
+             use_specs({'stderr' : 'PartialDownsweep.darwin.stderr'})
+           )
      ],
      compile_and_run,
      ['-package ghc'])