    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    The Desugarer: turning HsSyn into Core.
    -}

    {-# LANGUAGE CPP #-}

    module Desugar (
        -- * Desugaring operations
        deSugar, deSugarExpr,
        -- * Dependency/fingerprinting code (used by MkIface)
        mkUsageInfo, mkUsedNames, mkDependencies
        ) where

    #include "HsVersions.h"

    import DynFlags
    import HscTypes
    import HsSyn
    import TcRnTypes
    import TcRnMonad ( finalSafeMode, fixSafeInstances )
    import Id
    import Name
    import Type
    import FamInstEnv
    import InstEnv
    import Class
    import Avail
    import CoreSyn
    import CoreFVs( exprsSomeFreeVarsList )
    import CoreSubst
    import PprCore
    import DsMonad
    import DsExpr
    import DsBinds
    import DsForeign
    import PrelNames   ( coercibleTyConKey )
    import TysPrim     ( eqReprPrimTyCon )
    import Unique      ( hasKey )
    import Coercion    ( mkCoVarCo )
    import TysWiredIn  ( coercibleDataCon )
    import DataCon     ( dataConWrapId )
    import MkCore      ( mkCoreLet )
    import Module
    import NameSet
    import NameEnv
    import Rules
    import BasicTypes       ( Activation(.. ), competesWith, pprRuleName )
    import CoreMonad        ( CoreToDo(..) )
    import CoreLint         ( endPassIO )
    import VarSet
    import FastString
    import ErrUtils
    import Outputable
    import SrcLoc
    import Coverage
    import Util
    import MonadUtils
    import OrdList
    import StaticPtrTable
    import UniqFM
    import ListSetOps
    import Fingerprint
    import Maybes

    import Data.Function
    import Data.List
    import Data.IORef
    import Control.Monad( when )
    import Data.Map (Map)
    import qualified Data.Map as Map

    -- | Extract information from the rename and typecheck phases to produce
    -- a dependencies information for the module being compiled.
    mkDependencies :: TcGblEnv -> IO Dependencies
    mkDependencies
              TcGblEnv{ tcg_mod = mod,
                        tcg_imports = imports,
                        tcg_th_used = th_var
                      }
     = do
          -- Template Haskell used?
          th_used <- readIORef th_var
          let dep_mods = eltsUFM (delFromUFM (imp_dep_mods imports) (moduleName mod))
                    -- M.hi-boot can be in the imp_dep_mods, but we must remove
                    -- it before recording the modules on which this one depends!
                    -- (We want to retain M.hi-boot in imp_dep_mods so that
                    --  loadHiBootInterface can see if M's direct imports depend
                    --  on M.hi-boot, and hence that we should do the hi-boot consistency
                    --  check.)

              pkgs | th_used   = insertList thUnitId (imp_dep_pkgs imports)
                   | otherwise = imp_dep_pkgs imports

              -- Set the packages required to be Safe according to Safe Haskell.
              -- See Note [RnNames . Tracking Trust Transitively]
              sorted_pkgs = sortBy stableUnitIdCmp pkgs
              trust_pkgs  = imp_trust_pkgs imports
              dep_pkgs'   = map (\x -> (x, x `elem` trust_pkgs)) sorted_pkgs

          return Deps { dep_mods   = sortBy (stableModuleNameCmp `on` fst) dep_mods,
                        dep_pkgs   = dep_pkgs',
                        dep_orphs  = sortBy stableModuleCmp (imp_orphs  imports),
                        dep_finsts = sortBy stableModuleCmp (imp_finsts imports) }
                        -- sort to get into canonical order
                        -- NB. remember to use lexicographic ordering

    mkUsedNames :: TcGblEnv -> NameSet
    mkUsedNames TcGblEnv{ tcg_dus = dus } = allUses dus

    mkUsageInfo :: HscEnv -> Module -> ImportedMods -> NameSet -> [FilePath] -> IO [Usage]
    mkUsageInfo hsc_env this_mod dir_imp_mods used_names dependent_files
      = do
        eps <- hscEPS hsc_env
        hashes <- mapM getFileHash dependent_files
        let mod_usages = mk_mod_usage_info (eps_PIT eps) hsc_env this_mod
                                           dir_imp_mods used_names
        let usages = mod_usages ++ [ UsageFile { usg_file_path = f
                                               , usg_file_hash = hash }
                                   | (f, hash) <- zip dependent_files hashes ]
        usages `seqList` return usages
        -- seq the list of Usages returned: occasionally these
        -- don't get evaluated for a while and we can end up hanging on to
        -- the entire collection of Ifaces.

    mk_mod_usage_info :: PackageIfaceTable
                  -> HscEnv
                  -> Module
                  -> ImportedMods
                  -> NameSet
                  -> [Usage]
    mk_mod_usage_info pit hsc_env this_mod direct_imports used_names
      = mapMaybe mkUsage usage_mods
      where
        hpt = hsc_HPT hsc_env
        dflags = hsc_dflags hsc_env
        this_pkg = thisPackage dflags

        used_mods    = moduleEnvKeys ent_map
        dir_imp_mods = moduleEnvKeys direct_imports
        all_mods     = used_mods ++ filter (`notElem` used_mods) dir_imp_mods
        usage_mods   = sortBy stableModuleCmp all_mods
                            -- canonical order is imported, to avoid interface-file
                            -- wobblage.

        -- ent_map groups together all the things imported and used
        -- from a particular module
        ent_map :: ModuleEnv [OccName]
        ent_map  = foldNameSet add_mv emptyModuleEnv used_names
         where
          add_mv name mv_map
            | isWiredInName name = mv_map  -- ignore wired-in names
            | otherwise
            = case nameModule_maybe name of
                 Nothing  -> ASSERT2( isSystemName name, ppr name ) mv_map
                    -- See Note [Internal used_names]

                 Just mod -> -- This lambda function is really just a
                             -- specialised (++); originally came about to
                             -- avoid quadratic behaviour (trac #2680)
                             extendModuleEnvWith (\_ xs -> occ:xs) mv_map mod [occ]
                    where occ = nameOccName name

        -- We want to create a Usage for a home module if
        --  a) we used something from it; has something in used_names
        --  b) we imported it, even if we used nothing from it
        --     (need to recompile if its export list changes: export_fprint)
        mkUsage :: Module -> Maybe Usage
        mkUsage mod
          | isNothing maybe_iface           -- We can't depend on it if we didn't
                                            -- load its interface.
          || mod == this_mod                -- We don't care about usages of
                                            -- things in *this* module
          = Nothing

          | moduleUnitId mod /= this_pkg
          = Just UsagePackageModule{ usg_mod      = mod,
                                     usg_mod_hash = mod_hash,
                                     usg_safe     = imp_safe }
            -- for package modules, we record the module hash only

          | (null used_occs
              && isNothing export_hash
              && not is_direct_import
              && not finsts_mod)
          = Nothing                 -- Record no usage info
            -- for directly-imported modules, we always want to record a usage
            -- on the orphan hash.  This is what triggers a recompilation if
            -- an orphan is added or removed somewhere below us in the future.

          | otherwise
          = Just UsageHomeModule {
                          usg_mod_name = moduleName mod,
                          usg_mod_hash = mod_hash,
                          usg_exports  = export_hash,
                          usg_entities = Map.toList ent_hashs,
                          usg_safe     = imp_safe }
          where
            maybe_iface  = lookupIfaceByModule dflags hpt pit mod
                    -- In one-shot mode, the interfaces for home-package
                    -- modules accumulate in the PIT not HPT.  Sigh.

            Just iface   = maybe_iface
            finsts_mod   = mi_finsts    iface
            hash_env     = mi_hash_fn   iface
            mod_hash     = mi_mod_hash  iface
            export_hash | depend_on_exports = Just (mi_exp_hash iface)
                        | otherwise         = Nothing

            (is_direct_import, imp_safe)
                = case lookupModuleEnv direct_imports mod of
                    Just (imv : _xs) -> (True, imv_is_safe imv)
                    Just _           -> pprPanic "mkUsage: empty direct import" Outputable.empty
                    Nothing          -> (False, safeImplicitImpsReq dflags)
                    -- Nothing case is for implicit imports like 'System.IO' when 'putStrLn'
                    -- is used in the source code. We require them to be safe in Safe Haskell

            used_occs = lookupModuleEnv ent_map mod `orElse` []

            -- Making a Map here ensures that (a) we remove duplicates
            -- when we have usages on several subordinates of a single parent,
            -- and (b) that the usages emerge in a canonical order, which
            -- is why we use Map rather than OccEnv: Map works
            -- using Ord on the OccNames, which is a lexicographic ordering.
            ent_hashs :: Map OccName Fingerprint
            ent_hashs = Map.fromList (map lookup_occ used_occs)

            lookup_occ occ =
                case hash_env occ of
                    Nothing -> pprPanic "mkUsage" (ppr mod <+> ppr occ <+> ppr used_names)
                    Just r  -> r

            depend_on_exports = is_direct_import
            {- True
                  Even if we used 'import M ()', we have to register a
                  usage on the export list because we are sensitive to
                  changes in orphan instances/rules.
               False
                  In GHC 6.8.x we always returned true, and in
                  fact it recorded a dependency on *all* the
                  modules underneath in the dependency tree.  This
                  happens to make orphans work right, but is too
                  expensive: it'll read too many interface files.
                  The 'isNothing maybe_iface' check above saved us
                  from generating many of these usages (at least in
                  one-shot mode), but that's even more bogus!
            -}

    {-
    ************************************************************************
    *                                                                      *
    *              The main function: deSugar
    *                                                                      *
    ************************************************************************
    -}

    -- | Main entry point to the desugarer.
    deSugar :: HscEnv -> ModLocation -> TcGblEnv -> IO (Messages, Maybe ModGuts)
    -- Can modify PCS by faulting in more declarations

    deSugar hsc_env
            mod_loc
            tcg_env@(TcGblEnv { tcg_mod          = mod,
                                tcg_src          = hsc_src,
                                tcg_type_env     = type_env,
                                tcg_imports      = imports,
                                tcg_exports      = exports,
                                tcg_keep         = keep_var,
                                tcg_th_splice_used = tc_splice_used,
                                tcg_rdr_env      = rdr_env,
                                tcg_fix_env      = fix_env,
                                tcg_inst_env     = inst_env,
                                tcg_fam_inst_env = fam_inst_env,
                                tcg_warns        = warns,
                                tcg_anns         = anns,
                                tcg_binds        = binds,
                                tcg_imp_specs    = imp_specs,
                                tcg_dependent_files = dependent_files,
                                tcg_ev_binds     = ev_binds,
                                tcg_fords        = fords,
                                tcg_rules        = rules,
                                tcg_vects        = vects,
                                tcg_patsyns      = patsyns,
                                tcg_tcs          = tcs,
                                tcg_insts        = insts,
                                tcg_fam_insts    = fam_insts,
                                tcg_hpc          = other_hpc_info})

      = do { let dflags = hsc_dflags hsc_env
                 print_unqual = mkPrintUnqualified dflags rdr_env
            ; withTiming (pure dflags)
                         (text "Desugar"<+>brackets (ppr mod))
                         (const ()) $
         do { -- Desugar the program
            ; let export_set = availsToNameSet exports
                  target     = hscTarget dflags
                  hpcInfo    = emptyHpcInfo other_hpc_info

            ; (binds_cvr, ds_hpc_info, modBreaks)
                             <- if not (isHsBootOrSig hsc_src)
                                  then addTicksToBinds hsc_env mod mod_loc
                                           export_set (typeEnvTyCons type_env) binds
                                  else return (binds, hpcInfo, Nothing)

            ; (msgs, mb_res) <- initDs hsc_env mod rdr_env type_env fam_inst_env $
                           do { ds_ev_binds <- dsEvBinds ev_binds
                              ; core_prs <- dsTopLHsBinds binds_cvr
                              ; (spec_prs, spec_rules) <- dsImpSpecs imp_specs
                              ; (ds_fords, foreign_prs) <- dsForeigns fords
                              ; ds_rules <- mapMaybeM dsRule rules
                              ; ds_vects <- mapM dsVect vects
                              ; stBinds <- dsGetStaticBindsVar >>=
                                               liftIO . readIORef
                              ; let hpc_init
                                      | gopt Opt_Hpc dflags = hpcInitCode mod ds_hpc_info
                                      | otherwise = empty
                                    -- Stub to insert the static entries of the
                                    -- module into the static pointer table
                                    spt_init = sptInitCode mod stBinds
                              ; return ( ds_ev_binds
                                       , foreign_prs `appOL` core_prs `appOL` spec_prs
                                                     `appOL` toOL (map snd stBinds)
                                       , spec_rules ++ ds_rules, ds_vects
                                       , ds_fords `appendStubC` hpc_init
                                                  `appendStubC` spt_init) }

            ; case mb_res of {
               Nothing -> return (msgs, Nothing) ;
               Just (ds_ev_binds, all_prs, all_rules, vects0, ds_fords) ->

         do {       -- Add export flags to bindings
              keep_alive <- readIORef keep_var
            ; let (rules_for_locals, rules_for_imps) = partition isLocalRule all_rules
                  final_prs = addExportFlagsAndRules target export_set keep_alive
                                                     rules_for_locals (fromOL all_prs)

                  final_pgm = combineEvBinds ds_ev_binds final_prs
            -- Notice that we put the whole lot in a big Rec, even the foreign binds
            -- When compiling PrelFloat, which defines data Float = F# Float#
            -- we want F# to be in scope in the foreign marshalling code!
            -- You might think it doesn't matter, but the simplifier brings all top-level
            -- things into the in-scope set before simplifying; so we get no unfolding for F#!

    #ifdef DEBUG
              -- Debug only as pre-simple-optimisation program may be really big
            ; endPassIO hsc_env print_unqual CoreDesugar final_pgm rules_for_imps
    #endif
            ; (ds_binds, ds_rules_for_imps, ds_vects)
                <- simpleOptPgm dflags mod final_pgm rules_for_imps vects0
                             -- The simpleOptPgm gets rid of type
                             -- bindings plus any stupid dead code

            ; endPassIO hsc_env print_unqual CoreDesugarOpt ds_binds ds_rules_for_imps

            ; let used_names = mkUsedNames tcg_env
            ; deps <- mkDependencies tcg_env

            ; used_th <- readIORef tc_splice_used
            ; dep_files <- readIORef dependent_files
            ; safe_mode <- finalSafeMode dflags tcg_env
            ; usages <- mkUsageInfo hsc_env mod (imp_mods imports) used_names dep_files

            ; let mod_guts = ModGuts {
                    mg_module       = mod,
                    mg_hsc_src      = hsc_src,
                    mg_loc          = mkFileSrcSpan mod_loc,
                    mg_exports      = exports,
                    mg_usages       = usages,
                    mg_deps         = deps,
                    mg_used_th      = used_th,
                    mg_rdr_env      = rdr_env,
                    mg_fix_env      = fix_env,
                    mg_warns        = warns,
                    mg_anns         = anns,
                    mg_tcs          = tcs,
                    mg_insts        = fixSafeInstances safe_mode insts,
                    mg_fam_insts    = fam_insts,
                    mg_inst_env     = inst_env,
                    mg_fam_inst_env = fam_inst_env,
                    mg_patsyns      = patsyns,
                    mg_rules        = ds_rules_for_imps,
                    mg_binds        = ds_binds,
                    mg_foreign      = ds_fords,
                    mg_hpc_info     = ds_hpc_info,
                    mg_modBreaks    = modBreaks,
                    mg_vect_decls   = ds_vects,
                    mg_vect_info    = noVectInfo,
                    mg_safe_haskell = safe_mode,
                    mg_trust_pkg    = imp_trust_own_pkg imports
                  }
            ; return (msgs, Just mod_guts)
            }}}}

    mkFileSrcSpan :: ModLocation -> SrcSpan
    mkFileSrcSpan mod_loc
      = case ml_hs_file mod_loc of
          Just file_path -> mkGeneralSrcSpan (mkFastString file_path)
          Nothing        -> interactiveSrcSpan   -- Presumably

    dsImpSpecs :: [LTcSpecPrag] -> DsM (OrdList (Id,CoreExpr), [CoreRule])
    dsImpSpecs imp_specs
     = do { spec_prs <- mapMaybeM (dsSpec Nothing) imp_specs
          ; let (spec_binds, spec_rules) = unzip spec_prs
          ; return (concatOL spec_binds, spec_rules) }

    combineEvBinds :: [CoreBind] -> [(Id,CoreExpr)] -> [CoreBind]
    -- Top-level bindings can include coercion bindings, but not via superclasses
    -- See Note [Top-level evidence]
    combineEvBinds [] val_prs
      = [Rec val_prs]
    combineEvBinds (NonRec b r : bs) val_prs
      | isId b    = combineEvBinds bs ((b,r):val_prs)
      | otherwise = NonRec b r : combineEvBinds bs val_prs
    combineEvBinds (Rec prs : bs) val_prs
      = combineEvBinds bs (prs ++ val_prs)

    {-
    Note [Top-level evidence]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Top-level evidence bindings may be mutually recursive with the top-level value
    bindings, so we must put those in a Rec.  But we can't put them *all* in a Rec
    because the occurrence analyser doesn't teke account of type/coercion variables
    when computing dependencies.

    So we pull out the type/coercion variables (which are in dependency order),
    and Rec the rest.
    -}

    deSugarExpr :: HscEnv -> LHsExpr Id -> IO (Messages, Maybe CoreExpr)

    deSugarExpr hsc_env tc_expr
      = do { let dflags       = hsc_dflags hsc_env
                 icntxt       = hsc_IC hsc_env
                 rdr_env      = ic_rn_gbl_env icntxt
                 type_env     = mkTypeEnvWithImplicits (ic_tythings icntxt)
                 fam_insts    = snd (ic_instances icntxt)
                 fam_inst_env = extendFamInstEnvList emptyFamInstEnv fam_insts
                 -- This stuff is a half baked version of TcRnDriver.setInteractiveContext

           ; showPass dflags "Desugar"

             -- Do desugaring
           ; (msgs, mb_core_expr) <- initDs hsc_env (icInteractiveModule icntxt) rdr_env
                                            type_env fam_inst_env $
                                     dsLExpr tc_expr

           ; case mb_core_expr of
                Nothing   -> return ()
                Just expr -> dumpIfSet_dyn dflags Opt_D_dump_ds "Desugared" (pprCoreExpr expr)

           ; return (msgs, mb_core_expr) }

    {-
    ************************************************************************
    *                                                                      *
    *              Add rules and export flags to binders
    *                                                                      *
    ************************************************************************
    -}

    addExportFlagsAndRules
        :: HscTarget -> NameSet -> NameSet -> [CoreRule]
        -> [(Id, t)] -> [(Id, t)]
    addExportFlagsAndRules target exports keep_alive rules prs
      = mapFst add_one prs
      where
        add_one bndr = add_rules name (add_export name bndr)
           where
             name = idName bndr

        ---------- Rules --------
            -- See Note [Attach rules to local ids]
            -- NB: the binder might have some existing rules,
            -- arising from specialisation pragmas
        add_rules name bndr
            | Just rules <- lookupNameEnv rule_base name
            = bndr `addIdSpecialisations` rules
            | otherwise
            = bndr
        rule_base = extendRuleBaseList emptyRuleBase rules

        ---------- Export flag --------
        -- See Note [Adding export flags]
        add_export name bndr
            | dont_discard name = setIdExported bndr
            | otherwise         = bndr

        dont_discard :: Name -> Bool
        dont_discard name = is_exported name
                         || name `elemNameSet` keep_alive

            -- In interactive mode, we don't want to discard any top-level
            -- entities at all (eg. do not inline them away during
            -- simplification), and retain them all in the TypeEnv so they are
            -- available from the command line.
            --
            -- isExternalName separates the user-defined top-level names from those
            -- introduced by the type checker.
        is_exported :: Name -> Bool
        is_exported | targetRetainsAllBindings target = isExternalName
                    | otherwise                       = (`elemNameSet` exports)

    {-
    Note [Adding export flags]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Set the no-discard flag if either
            a) the Id is exported
            b) it's mentioned in the RHS of an orphan rule
            c) it's in the keep-alive set

    It means that the binding won't be discarded EVEN if the binding
    ends up being trivial (v = w) -- the simplifier would usually just
    substitute w for v throughout, but we don't apply the substitution to
    the rules (maybe we should?), so this substitution would make the rule
    bogus.

    You might wonder why exported Ids aren't already marked as such;
    it's just because the type checker is rather busy already and
    I didn't want to pass in yet another mapping.

    Note [Attach rules to local ids]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Find the rules for locally-defined Ids; then we can attach them
    to the binders in the top-level bindings

    Reason
      - It makes the rules easier to look up
      - It means that transformation rules and specialisations for
        locally defined Ids are handled uniformly
      - It keeps alive things that are referred to only from a rule
        (the occurrence analyser knows about rules attached to Ids)
      - It makes sure that, when we apply a rule, the free vars
        of the RHS are more likely to be in scope
      - The imported rules are carried in the in-scope set
        which is extended on each iteration by the new wave of
        local binders; any rules which aren't on the binding will
        thereby get dropped

    ************************************************************************
    *                                                                      *
    *              Desugaring transformation rules
    *                                                                      *
    ************************************************************************
    -}

    dsRule :: LRuleDecl Id -> DsM (Maybe CoreRule)
    dsRule (L loc (HsRule name rule_act vars lhs _tv_lhs rhs _fv_rhs))
      = putSrcSpanDs loc $
        do  { let bndrs' = [var | L _ (RuleBndr (L _ var)) <- vars]

            ; lhs' <- unsetGOptM Opt_EnableRewriteRules $
                      unsetWOptM Opt_WarnIdentities $
                      dsLExpr lhs   -- Note [Desugaring RULE left hand sides]

            ; rhs' <- dsLExpr rhs
            ; this_mod <- getModule

            ; (bndrs'', lhs'', rhs'') <- unfold_coerce bndrs' lhs' rhs'

            -- Substitute the dict bindings eagerly,
            -- and take the body apart into a (f args) form
            ; case decomposeRuleLhs bndrs'' lhs'' of {
                    Left msg -> do { warnDs NoReason msg; return Nothing } ;
                    Right (final_bndrs, fn_id, args) -> do

            { let is_local = isLocalId fn_id
                    -- NB: isLocalId is False of implicit Ids.  This is good because
                    -- we don't want to attach rules to the bindings of implicit Ids,
                    -- because they don't show up in the bindings until just before code gen
                  fn_name   = idName fn_id
                  final_rhs = simpleOptExpr rhs''    -- De-crap it
                  rule_name = snd (unLoc name)
                  final_bndrs_set = mkVarSet final_bndrs
                  arg_ids = filterOut (`elemVarSet` final_bndrs_set) $
                            exprsSomeFreeVarsList isId args

            ; dflags <- getDynFlags
            ; rule <- dsMkUserRule this_mod is_local
                             rule_name rule_act fn_name final_bndrs args
                             final_rhs
            ; when (wopt Opt_WarnInlineRuleShadowing dflags) $
              warnRuleShadowing rule_name rule_act fn_id arg_ids

            ; return (Just rule)
            } } }

    warnRuleShadowing :: RuleName -> Activation -> Id -> [Id] -> DsM ()
    -- See Note [Rules and inlining/other rules]
    warnRuleShadowing rule_name rule_act fn_id arg_ids
      = do { check False fn_id    -- We often have multiple rules for the same Id in a
                                  -- module. Maybe we should check that they don't overlap
                                  -- but currently we don't
           ; mapM_ (check True) arg_ids }
      where
        check check_rules_too lhs_id
          | isLocalId lhs_id || canUnfold (idUnfolding lhs_id)
                           -- If imported with no unfolding, no worries
          , idInlineActivation lhs_id `competesWith` rule_act
          = warnDs (Reason Opt_WarnInlineRuleShadowing)
                   (vcat [ hang (text "Rule" <+> pprRuleName rule_name
                                   <+> text "may never fire")
                                2 (text "because" <+> quotes (ppr lhs_id)
                                   <+> text "might inline first")
                         , text "Probable fix: add an INLINE[n] or NOINLINE[n] pragma for"
                           <+> quotes (ppr lhs_id)
                         , ifPprDebug (ppr (idInlineActivation lhs_id) $$ ppr rule_act) ])

          | check_rules_too
          , bad_rule : _ <- get_bad_rules lhs_id
          = warnDs (Reason Opt_WarnInlineRuleShadowing)
                   (vcat [ hang (text "Rule" <+> pprRuleName rule_name
                                   <+> text "may never fire")
                                2 (text "because rule" <+> pprRuleName (ruleName bad_rule)
                                   <+> text "for"<+> quotes (ppr lhs_id)
                                   <+> text "might fire first")
                          , text "Probable fix: add phase [n] or [~n] to the competing rule"
                          , ifPprDebug (ppr bad_rule) ])

          | otherwise
          = return ()

        get_bad_rules lhs_id
          = [ rule | rule <- idCoreRules lhs_id
                   , ruleActivation rule `competesWith` rule_act ]

    -- See Note [Desugaring coerce as cast]
    unfold_coerce :: [Id] -> CoreExpr -> CoreExpr -> DsM ([Var], CoreExpr, CoreExpr)
    unfold_coerce bndrs lhs rhs = do
        (bndrs', wrap) <- go bndrs
        return (bndrs', wrap lhs, wrap rhs)
      where
        go :: [Id] -> DsM ([Id], CoreExpr -> CoreExpr)
        go []     = return ([], id)
        go (v:vs)
            | Just (tc, [k, t1, t2]) <- splitTyConApp_maybe (idType v)
            , tc `hasKey` coercibleTyConKey = do
                u <- newUnique

                let ty' = mkTyConApp eqReprPrimTyCon [k, k, t1, t2]
                    v'  = mkLocalCoVar
                            (mkDerivedInternalName mkRepEqOcc u (getName v)) ty'
                    box = Var (dataConWrapId coercibleDataCon) `mkTyApps`
                          [k, t1, t2] `App`
                          Coercion (mkCoVarCo v')

                (bndrs, wrap) <- go vs
                return (v':bndrs, mkCoreLet (NonRec v box) . wrap)
            | otherwise = do
                (bndrs,wrap) <- go vs
                return (v:bndrs, wrap)

    {- Note [Desugaring RULE left hand sides]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For the LHS of a RULE we do *not* want to desugar
        [x]   to    build (\cn. x `c` n)
    We want to leave explicit lists simply as chains
    of cons's. We can achieve that slightly indirectly by
    switching off EnableRewriteRules.  See DsExpr.dsExplicitList.

    That keeps the desugaring of list comprehensions simple too.

    Nor do we want to warn of conversion identities on the LHS;
    the rule is precisly to optimise them:
      {-# RULES "fromRational/id" fromRational = id :: Rational -> Rational #-}

    Note [Desugaring coerce as cast]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We want the user to express a rule saying roughly “mapping a coercion over a
    list can be replaced by a coercion”. But the cast operator of Core (▷) cannot
    be written in Haskell. So we use `coerce` for that (#2110). The user writes
        map coerce = coerce
    as a RULE, and this optimizes any kind of mapped' casts aways, including `map
    MkNewtype`.

    For that we replace any forall'ed `c :: Coercible a b` value in a RULE by
    corresponding `co :: a ~#R b` and wrap the LHS and the RHS in
    `let c = MkCoercible co in ...`. This is later simplified to the desired form
    by simpleOptExpr (for the LHS) resp. the simplifiers (for the RHS).
    See also Note [Getting the map/coerce RULE to work] in CoreSubst.

    Note [Rules and inlining/other rules]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If you have
      f x = ...
      g x = ...
      {-# RULES "rule-for-f" forall x. f (g x) = ... #-}
    then there's a good chance that in a potential rule redex
        ...f (g e)...
    then 'f' or 'g' will inline befor the rule can fire.  Solution: add an
    INLINE [n] or NOINLINE [n] pragma to 'f' and 'g'.

    Note that this applies to all the free variables on the LHS, both the
    main function and things in its arguments.

    We also check if there are Ids on the LHS that have competing RULES.
    In the above example, suppose we had
      {-# RULES "rule-for-g" forally. g [y] = ... #-}
    Then "rule-for-f" and "rule-for-g" would compete.  Better to add phase
    control, so "rule-for-f" has a chance to fire before "rule-for-g" becomes
    active; or perhpas after "rule-for-g" has become inactive. This is checked
    by 'competesWith'

    Class methods have a built-in RULE to select the method from the dictionary,
    so you can't change the phase on this.  That makes id very dubious to
    match on class methods in RULE lhs's.   See Trac #10595.   I'm not happy
    about this. For exmaple in Control.Arrow we have

    {-# RULES "compose/arr"   forall f g .
                              (arr f) . (arr g) = arr (f . g) #-}

    and similar, which will elicit exactly these warnings, and risk never
    firing.  But it's not clear what to do instead.  We could make the
    class methocd rules inactive in phase 2, but that would delay when
    subsequent transformations could fire.

    ************************************************************************
    *                                                                      *
    *              Desugaring vectorisation declarations
    *                                                                      *
    ************************************************************************
    -}

    dsVect :: LVectDecl Id -> DsM CoreVect
    dsVect (L loc (HsVect _ (L _ v) rhs))
      = putSrcSpanDs loc $
        do { rhs' <- dsLExpr rhs
           ; return $ Vect v rhs'
           }
    dsVect (L _loc (HsNoVect _ (L _ v)))
      = return $ NoVect v
    dsVect (L _loc (HsVectTypeOut isScalar tycon rhs_tycon))
      = return $ VectType isScalar tycon' rhs_tycon
      where
        tycon' | Just ty <- coreView $ mkTyConTy tycon
               , (tycon', []) <- splitTyConApp ty      = tycon'
               | otherwise                             = tycon
    dsVect vd@(L _ (HsVectTypeIn _ _ _ _))
      = pprPanic "Desugar.dsVect: unexpected 'HsVectTypeIn'" (ppr vd)
    dsVect (L _loc (HsVectClassOut cls))
      = return $ VectClass (classTyCon cls)
    dsVect vc@(L _ (HsVectClassIn _ _))
      = pprPanic "Desugar.dsVect: unexpected 'HsVectClassIn'" (ppr vc)
    dsVect (L _loc (HsVectInstOut inst))
      = return $ VectInst (instanceDFunId inst)
    dsVect vi@(L _ (HsVectInstIn _))
      = pprPanic "Desugar.dsVect: unexpected 'HsVectInstIn'" (ppr vi)