Make a proper Opt_WarnLazyUnliftedBindings warning, with a flag etc

compiler/main/DynFlags.hs

@@ -191,6 +191,7 @@ data DynFlag
    | Opt_WarnTabs
    | Opt_WarnUnrecognisedPragmas
    | Opt_WarnDodgyForeignImports
+   | Opt_WarnLazyUnliftedBindings
 
    -- language opts
    | Opt_OverlappingInstances
@@ -906,6 +907,7 @@ standardWarnings
         Opt_WarnMissingFields,
         Opt_WarnMissingMethods,
         Opt_WarnDuplicateExports,
+        Opt_WarnLazyUnliftedBindings,
         Opt_WarnDodgyForeignImports
       ]
 
@@ -1659,6 +1661,8 @@ fFlags = [
   ( "warn-orphans",                     Opt_WarnOrphans, const Supported ),
   ( "warn-tabs",                        Opt_WarnTabs, const Supported ),
   ( "warn-unrecognised-pragmas",        Opt_WarnUnrecognisedPragmas, const Supported ),
+  ( "warn-lazy-unlifted-bindings",      Opt_WarnLazyUnliftedBindings,
+    const $ Deprecated "lazy unlifted bindings will be an error in GHC 6.14, and this flag will no longer exist"),
   ( "print-explicit-foralls",           Opt_PrintExplicitForalls, const Supported ),
   ( "strictness",                       Opt_Strictness, const Supported ),
   ( "static-argument-transformation",   Opt_StaticArgumentTransformation, const Supported ),

compiler/typecheck/TcBinds.lhs

@@ -478,7 +478,8 @@ checkStrictBinds top_lvl rec_group mbind mono_tys infos
         -- This should be a checkTc, not a warnTc, but as of GHC 6.11
         -- the versions of alex and happy available have non-conforming
         -- templates, so the GHC build fails if it's an error:
-        ; warnTc (not bang_pat)
+        ; warnUnlifted <- doptM Opt_WarnLazyUnliftedBindings
+        ; warnTc (warnUnlifted && not bang_pat)
                  (unliftedMustBeBang mbind)
         ; mapM_ check_sig infos
         ; return True }
@@ -495,6 +496,7 @@ unliftedMustBeBang :: LHsBindsLR Var Var -> SDoc
 unliftedMustBeBang mbind
   = hang (text "Bindings containing unlifted types must use an outermost bang pattern:")
          4 (pprLHsBinds mbind)
+ $$ text "*** This will be an error in GHC 6.14! Fix your code now!"
 
 strictBindErr :: String -> Bool -> LHsBindsLR Var Var -> SDoc
 strictBindErr flavour unlifted mbind