Add a flag for the unsupported calling convention warning

compiler/main/DynFlags.hs

@@ -357,6 +357,7 @@ data WarningFlag =
    | Opt_WarnUnsafe
    | Opt_WarnSafe
    | Opt_WarnPointlessPragmas
+   | Opt_WarnUnsupportedCallingConventions
    deriving (Eq, Show, Enum)
 
 data Language = Haskell98 | Haskell2010
@@ -1842,7 +1843,8 @@ fWarningFlags = [
   ( "warn-alternative-layout-rule-transitional", Opt_WarnAlternativeLayoutRuleTransitional, nop ),
   ( "warn-unsafe",                      Opt_WarnUnsafe, setWarnUnsafe ),
   ( "warn-safe",                        Opt_WarnSafe, setWarnSafe ),
-  ( "warn-pointless-pragmas",           Opt_WarnPointlessPragmas, nop ) ]
+  ( "warn-pointless-pragmas",           Opt_WarnPointlessPragmas, nop ),
+  ( "warn-unsupported-calling-conventions", Opt_WarnUnsupportedCallingConventions, nop ) ]
 
 -- | These @-f\<blah\>@ flags can all be reversed with @-fno-\<blah\>@
 fFlags :: [FlagSpec DynFlag]
@@ -2174,7 +2176,8 @@ standardWarnings
         Opt_WarnDodgyForeignImports,
         Opt_WarnWrongDoBind,
         Opt_WarnAlternativeLayoutRuleTransitional,
-        Opt_WarnPointlessPragmas
+        Opt_WarnPointlessPragmas,
+        Opt_WarnUnsupportedCallingConventions
       ]
 
 minusWOpts :: [WarningFlag]

compiler/typecheck/TcForeign.lhs

@@ -48,6 +48,8 @@ import Platform
 import SrcLoc
 import Bag
 import FastString
+
+import Control.Monad
 \end{code}
 
 \begin{code}
@@ -454,7 +456,8 @@ checkCConv StdCallConv  = do dflags <- getDynFlags
                              if platformArch platform == ArchX86
                                  then return StdCallConv
                                  else do -- This is a warning, not an error. see #3336
-                                         addWarnTc (text "the 'stdcall' calling convention is unsupported on this platform," $$ text "treating as ccall")
+                                         when (wopt Opt_WarnUnsupportedCallingConventions dflags) $
+                                             addWarnTc (text "the 'stdcall' calling convention is unsupported on this platform," $$ text "treating as ccall")
                                          return CCallConv
 checkCConv PrimCallConv = do addErrTc (text "The `prim' calling convention can only be used with `foreign import'")
                              return PrimCallConv