Coverage: Don't produce ModBreaks if not HscInterpreted

emptyModBreaks contains a bottom and consequently it's important that we
don't use it unless necessary.

(cherry picked from commit f684a7d5)

compiler/deSugar/Coverage.hs

@@ -111,7 +111,7 @@ addTicksToBinds hsc_env mod mod_loc exports tyCons binds
 
      dumpIfSet_dyn dflags Opt_D_dump_ticked "HPC" (pprLHsBinds binds1)
 
-     return (binds1, HpcInfo tickCount hashNo, Just modBreaks)
+     return (binds1, HpcInfo tickCount hashNo, modBreaks)
 
   | otherwise = return (binds, emptyHpcInfo False, Nothing)
 
@@ -128,23 +128,23 @@ guessSourceFile binds orig_file =
         _ -> orig_file
 
 
-mkModBreaks :: HscEnv -> Module -> Int -> [MixEntry_] -> IO ModBreaks
+mkModBreaks :: HscEnv -> Module -> Int -> [MixEntry_] -> IO (Maybe ModBreaks)
 mkModBreaks hsc_env mod count entries
-  | HscInterpreted <- hscTarget (hsc_dflags hsc_env) = do
+  | breakpointsEnabled (hsc_dflags hsc_env) = do
     breakArray <- GHCi.newBreakArray hsc_env (length entries)
     ccs <- mkCCSArray hsc_env mod count entries
     let
            locsTicks  = listArray (0,count-1) [ span  | (span,_,_,_)  <- entries ]
            varsTicks  = listArray (0,count-1) [ vars  | (_,_,vars,_)  <- entries ]
            declsTicks = listArray (0,count-1) [ decls | (_,decls,_,_) <- entries ]
-    return emptyModBreaks
+    return $ Just $ emptyModBreaks
                        { modBreaks_flags = breakArray
                        , modBreaks_locs  = locsTicks
                        , modBreaks_vars  = varsTicks
                        , modBreaks_decls = declsTicks
                        , modBreaks_ccs   = ccs
                        }
-  | otherwise = return emptyModBreaks
+  | otherwise = return Nothing
 
 mkCCSArray
   :: HscEnv -> Module -> Int -> [MixEntry_]
@@ -1038,7 +1038,7 @@ data TickishType = ProfNotes | HpcTicks | Breakpoints | SourceNotes
 
 coveragePasses :: DynFlags -> [TickishType]
 coveragePasses dflags =
-    ifa (hscTarget dflags == HscInterpreted) Breakpoints $
+    ifa (breakpointsEnabled dflags)          Breakpoints $
     ifa (gopt Opt_Hpc dflags)                HpcTicks $
     ifa (gopt Opt_SccProfilingOn dflags &&
          profAuto dflags /= NoProfAuto)      ProfNotes $
@@ -1046,6 +1046,10 @@ coveragePasses dflags =
   where ifa f x xs | f         = x:xs
                    | otherwise = xs
 
+-- | Should we produce 'Breakpoint' ticks?
+breakpointsEnabled :: DynFlags -> Bool
+breakpointsEnabled dflags = hscTarget dflags == HscInterpreted
+
 -- | Tickishs that only make sense when their source code location
 -- refers to the current file. This might not always be true due to
 -- LINE pragmas in the code - which would confuse at least HPC.