Pass '-x c++' and '-std=c++11' to `cc` for cpp files, in Hadrian

'-x c++' was found to be required on Darwin Clang 11 and 12.
'-std=c++' was found to be needed on Clang 12 but not 11.

hadrian/src/Builder.hs

 {-# LANGUAGE InstanceSigs, TypeOperators #-}
 module Builder (
     -- * Data types
-    ArMode (..), CcMode (..), ConfigurationInfo (..), GhcMode (..),
-    GhcPkgMode (..), HaddockMode (..), SphinxMode (..), TarMode (..),
-    Builder (..),
+    ArMode (..), CcMode (..), ConfigurationInfo (..), DependencyType (..),
+    GhcMode (..), GhcPkgMode (..), HaddockMode (..), SphinxMode (..),
+    TarMode (..), Builder (..),
 
     -- * Builder properties
     builderProvenance, systemBuilderPath, builderPath, isSpecified, needBuilder,
@@ -41,12 +41,17 @@ import Packages
 -- | C compiler can be used in two different modes:
 -- * Compile or preprocess a source file.
 -- * Extract source dependencies by passing @-MM@ command line argument.
-data CcMode = CompileC | FindCDependencies deriving (Eq, Generic, Show)
+data CcMode = CompileC | FindCDependencies DependencyType deriving (Eq, Generic, Show)
+data DependencyType = CDep | CxxDep deriving (Eq, Generic, Show)
 
 instance Binary   CcMode
 instance Hashable CcMode
 instance NFData   CcMode
 
+instance Binary   DependencyType
+instance Hashable DependencyType
+instance NFData   DependencyType
+
 -- | GHC can be used in four different modes:
 -- * Compile a Haskell source file.
 -- * Compile a C source file.

hadrian/src/Flavour.hs

@@ -391,7 +391,8 @@ builderSetting =
 
         ccBuilder =
           [ ("c", CompileC)
-          , ("deps", FindCDependencies)
+          -- Not sure how to handle the FindCDependencies CxxDep case
+          , ("deps", FindCDependencies CDep)
           ]
 
         stages = map (\stg -> (stageString stg, stg)) [minBound..maxBound]

hadrian/src/Rules/Compile.hs

@@ -239,7 +239,7 @@ compileNonHsObject rs lang path = do
       Cmm -> obj2src "cmm" isGeneratedCmmFile ctx path
       Cxx -> obj2src "cpp" (const False) ctx path
   need [src]
-  needDependencies ctx src (path <.> "d")
+  needDependencies lang ctx src (path <.> "d")
   buildWithResources rs $ target ctx (builder stage) [src] [path]
 
 -- * Helpers
@@ -247,11 +247,11 @@ compileNonHsObject rs lang path = do
 -- | Discover dependencies of a given source file by iteratively calling @gcc@
 -- in the @-MM -MG@ mode and building generated dependencies if they are missing
 -- until reaching a fixed point.
-needDependencies :: Context -> FilePath -> FilePath -> Action ()
-needDependencies context@Context {..} src depFile = discover
+needDependencies :: SourceLang -> Context -> FilePath -> FilePath -> Action ()
+needDependencies lang context@Context {..} src depFile = discover
   where
     discover = do
-        build $ target context (Cc FindCDependencies stage) [src] [depFile]
+        build $ target context (Cc (FindCDependencies depType) stage) [src] [depFile]
         deps <- parseFile depFile
         -- Generated dependencies, if not yet built, will not be found and hence
         -- will be referred to simply by their file names.
@@ -266,6 +266,10 @@ needDependencies context@Context {..} src depFile = discover
             need todo  -- Build newly discovered generated dependencies
             discover   -- Continue the discovery process
 
+    -- We need to pass different flags to cc depending on whether the
+    -- file to compile is a .c or a .cpp file
+    depType = if lang == Cxx then CxxDep else CDep
+
     parseFile :: FilePath -> Action [String]
     parseFile file = do
         input <- liftIO $ readFile file

hadrian/src/Settings/Builders/Cc.hs

@@ -17,12 +17,18 @@ ccBuilderArgs = do
             , arg "-c", arg =<< getInput
             , arg "-o", arg =<< getOutput ]
 
-        , builder (Cc FindCDependencies) ? do
+        , builder (Cc (FindCDependencies CDep)) ? findCDepExpr CDep
+        , builder (Cc (FindCDependencies CxxDep)) ? findCDepExpr CxxDep
+        ]
+    where
+        findCDepExpr depType = do
             output <- getOutput
             mconcat [ arg "-E"
                     , arg "-MM", arg "-MG"
                     , arg "-MF", arg output
                     , arg "-MT", arg $ dropExtension output -<.> "o"
+                    , case depType of CDep -> mempty; CxxDep -> arg "-std=c++11"
                     , cIncludeArgs
-                    , arg "-x", arg "c"
-                    , arg =<< getInput ] ]
+                    , arg "-x", arg (case depType of CDep -> "c"; CxxDep -> "c++")
+                    , arg =<< getInput
+                    ]

hadrian/src/Settings/Packages.hs

@@ -374,7 +374,8 @@ rtsPackageArgs = package rts ? do
           , if not (null libnumaLibraryDir) then arg ("--extra-lib-dirs="++libnumaLibraryDir) else mempty
           , if not (null libnumaIncludeDir) then arg ("--extra-include-dirs="++libnumaIncludeDir) else mempty
           ]
-        , builder (Cc FindCDependencies) ? cArgs
+        , builder (Cc (FindCDependencies CDep)) ? cArgs
+        , builder (Cc (FindCDependencies  CxxDep)) ? cArgs
         , builder (Ghc CompileCWithGhc) ? map ("-optc" ++) <$> cArgs
         , builder (Ghc CompileCppWithGhc) ? map ("-optcxx" ++) <$> cArgs
         , builder Ghc ? ghcArgs