content: {-
  (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
  
  \section{Tidying up Core}
  -}
  
  module Eta.Main.TidyPgm (
         mkBootModDetailsTc, tidyProgram, globaliseAndTidyId
     ) where
  
  import Eta.TypeCheck.TcRnTypes
  import Eta.Main.DynFlags
  import Eta.Core.CoreSyn
  import Eta.Core.CoreUnfold
  import Eta.Core.CoreFVs
  import Eta.Core.CoreTidy
  import Eta.SimplCore.CoreMonad
  import Eta.Core.CorePrep
  import Eta.Core.CoreUtils
  import Eta.Core.CoreLint
  import Eta.BasicTypes.Literal
  import Eta.Specialise.Rules
  import Eta.BasicTypes.PatSyn
  import Eta.BasicTypes.ConLike
  import Eta.Core.CoreArity        ( exprArity, exprBotStrictness_maybe )
  import Eta.BasicTypes.VarEnv
  import Eta.BasicTypes.VarSet
  import Eta.BasicTypes.Var
  import Eta.BasicTypes.Id
  import qualified Eta.BasicTypes.Id as Id
  import Eta.BasicTypes.MkId             ( mkDictSelRhs )
  import Eta.BasicTypes.IdInfo
  import Eta.Types.InstEnv
  import Eta.Types.FamInstEnv
  import Eta.Types.Type             ( tidyTopType )
  import Eta.BasicTypes.Demand           ( appIsBottom, isTopSig, isBottomingSig )
  import Eta.BasicTypes.BasicTypes
  import Eta.BasicTypes.Name hiding (varName)
  import Eta.BasicTypes.NameSet
  import Eta.BasicTypes.NameEnv
  import Eta.BasicTypes.NameCache
  import Eta.BasicTypes.Avail
  import Eta.Iface.IfaceEnv
  import Eta.TypeCheck.TcEnv
  import Eta.TypeCheck.TcRnMonad
  import Eta.BasicTypes.DataCon
  import Eta.Types.TyCon
  import Eta.Types.Class
  import Eta.BasicTypes.Module
  import Eta.Main.Packages( isDllName )
  import Eta.Main.HscTypes
  import Eta.Utils.Maybes
  import Eta.BasicTypes.UniqSupply
  import Eta.Main.ErrUtils (Severity(..))
  import Eta.Utils.Outputable
  import Eta.Utils.FastBool hiding ( fastOr )
  import Eta.BasicTypes.SrcLoc
  import Eta.Utils.FastString
  import qualified Eta.Main.ErrUtils as Err
  
  import Control.Monad
  import Data.Function
  import Data.List        ( sortBy )
  import Data.IORef       ( atomicModifyIORef )
  
  {-
  Constructing the TypeEnv, Instances, Rules, VectInfo from which the
  ModIface is constructed, and which goes on to subsequent modules in
  --make mode.
  
  Most of the interface file is obtained simply by serialising the
  TypeEnv.  One important consequence is that if the *interface file*
  has pragma info if and only if the final TypeEnv does. This is not so
  important for *this* module, but it's essential for ghc --make:
  subsequent compilations must not see (e.g.) the arity if the interface
  file does not contain arity If they do, they'll exploit the arity;
  then the arity might change, but the iface file doesn't change =>
  recompilation does not happen => disaster.
  
  For data types, the final TypeEnv will have a TyThing for the TyCon,
  plus one for each DataCon; the interface file will contain just one
  data type declaration, but it is de-serialised back into a collection
  of TyThings.
  
  ************************************************************************
  *                                                                      *
                  Plan A: simpleTidyPgm
  *                                                                      *
  ************************************************************************
  
  
  Plan A: mkBootModDetails: omit pragmas, make interfaces small
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  * Ignore the bindings
  
  * Drop all WiredIn things from the TypeEnv
          (we never want them in interface files)
  
  * Retain all TyCons and Classes in the TypeEnv, to avoid
          having to find which ones are mentioned in the
          types of exported Ids
  
  * Trim off the constructors of non-exported TyCons, both
          from the TyCon and from the TypeEnv
  
  * Drop non-exported Ids from the TypeEnv
  
  * Tidy the types of the DFunIds of Instances,
    make them into GlobalIds, (they already have External Names)
    and add them to the TypeEnv
  
  * Tidy the types of the (exported) Ids in the TypeEnv,
    make them into GlobalIds (they already have External Names)
  
  * Drop rules altogether
  
  * Tidy the bindings, to ensure that the Caf and Arity
    information is correct for each top-level binder; the
    code generator needs it. And to ensure that local names have
    distinct OccNames in case of object-file splitting
  
  * If this an hsig file, drop the instances altogether too (they'll
    get pulled in by the implicit module import.
  -}
  
  -- This is Plan A: make a small type env when typechecking only,
  -- or when compiling a hs-boot file, or simply when not using -O
  --
  -- We don't look at the bindings at all -- there aren't any
  -- for hs-boot files
  
  mkBootModDetailsTc :: HscEnv -> TcGblEnv -> IO ModDetails
  mkBootModDetailsTc hsc_env
          TcGblEnv{ tcg_exports   = exports,
                    tcg_type_env  = type_env, -- just for the Ids
                    tcg_tcs       = tcs,
                    tcg_patsyns   = pat_syns,
                    tcg_insts     = insts,
                    tcg_fam_insts = fam_insts
                  }
    = do  { let dflags = hsc_dflags hsc_env
          ; showPassIO dflags CoreTidy
  
          ; let { insts'     = map (tidyClsInstDFun globaliseAndTidyId) insts
                ; type_env1  = mkBootTypeEnv (availsToNameSet exports)
                                             (typeEnvIds type_env) tcs fam_insts
                ; pat_syns'  = map (tidyPatSynIds   globaliseAndTidyId) pat_syns
                ; type_env2  = extendTypeEnvWithPatSyns pat_syns' type_env1
                ; dfun_ids   = map instanceDFunId insts'
                ; type_env'  = extendTypeEnvWithIds type_env2 dfun_ids
                }
          ; return (ModDetails { md_types     = type_env'
                               , md_insts     = insts'
                               , md_fam_insts = fam_insts
                               , md_rules     = []
                               , md_anns      = []
                               , md_exports   = exports
                               , md_vect_info = noVectInfo
                               })
          }
    where
  
  mkBootTypeEnv :: NameSet -> [Id] -> [TyCon] -> [FamInst] -> TypeEnv
  mkBootTypeEnv exports ids tcs fam_insts
    = tidyTypeEnv True $
         typeEnvFromEntities final_ids tcs fam_insts
    where
          -- Find the LocalIds in the type env that are exported
          -- Make them into GlobalIds, and tidy their types
          --
          -- It's very important to remove the non-exported ones
          -- because we don't tidy the OccNames, and if we don't remove
          -- the non-exported ones we'll get many things with the
          -- same name in the interface file, giving chaos.
          --
          -- Do make sure that we keep Ids that are already Global.
          -- When typechecking an .hs-boot file, the Ids come through as
          -- GlobalIds.
      final_ids = [ if isLocalId id then globaliseAndTidyId id
                                    else id
                  | id <- ids
                  , keep_it id ]
  
          -- default methods have their export flag set, but everything
          -- else doesn't (yet), because this is pre-desugaring, so we
          -- must test both.
      keep_it id = isExportedId id || idName id `elemNameSet` exports
  
  
  
  globaliseAndTidyId :: Id -> Id
  -- Takes an LocalId with an External Name,
  -- makes it into a GlobalId
  --     * unchanged Name (might be Internal or External)
  --     * unchanged details
  --     * VanillaIdInfo (makes a conservative assumption about Caf-hood)
  globaliseAndTidyId id
    = Id.setIdType (globaliseId id) tidy_type
    where
      tidy_type = tidyTopType (idType id)
  
  {-
  ************************************************************************
  *                                                                      *
          Plan B: tidy bindings, make TypeEnv full of IdInfo
  *                                                                      *
  ************************************************************************
  
  Plan B: include pragmas, make interfaces
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  * Figure out which Ids are externally visible
  
  * Tidy the bindings, externalising appropriate Ids
  
  * Drop all Ids from the TypeEnv, and add all the External Ids from
    the bindings.  (This adds their IdInfo to the TypeEnv; and adds
    floated-out Ids that weren't even in the TypeEnv before.)
  
  Step 1: Figure out external Ids
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Note [choosing external names]
  
  See also the section "Interface stability" in the
  RecompilationAvoidance commentary:
    http://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/RecompilationAvoidance
  
  First we figure out which Ids are "external" Ids.  An
  "external" Id is one that is visible from outside the compilation
  unit.  These are
    a) the user exported ones
    b) ones mentioned in the unfoldings, workers,
       rules of externally-visible ones ,
       or vectorised versions of externally-visible ones
  
  While figuring out which Ids are external, we pick a "tidy" OccName
  for each one.  That is, we make its OccName distinct from the other
  external OccNames in this module, so that in interface files and
  object code we can refer to it unambiguously by its OccName.  The
  OccName for each binder is prefixed by the name of the exported Id
  that references it; e.g. if "f" references "x" in its unfolding, then
  "x" is renamed to "f_x".  This helps distinguish the different "x"s
  from each other, and means that if "f" is later removed, things that
  depend on the other "x"s will not need to be recompiled.  Of course,
  if there are multiple "f_x"s, then we have to disambiguate somehow; we
  use "f_x0", "f_x1" etc.
  
  As far as possible we should assign names in a deterministic fashion.
  Each time this module is compiled with the same options, we should end
  up with the same set of external names with the same types.  That is,
  the ABI hash in the interface should not change.  This turns out to be
  quite tricky, since the order of the bindings going into the tidy
  phase is already non-deterministic, as it is based on the ordering of
  Uniques, which are assigned unpredictably.
  
  To name things in a stable way, we do a depth-first-search of the
  bindings, starting from the exports sorted by name.  This way, as long
  as the bindings themselves are deterministic (they sometimes aren't!),
  the order in which they are presented to the tidying phase does not
  affect the names we assign.
  
  Step 2: Tidy the program
  ~~~~~~~~~~~~~~~~~~~~~~~~
  Next we traverse the bindings top to bottom.  For each *top-level*
  binder
  
   1. Make it into a GlobalId; its IdDetails becomes VanillaGlobal,
      reflecting the fact that from now on we regard it as a global,
      not local, Id
  
   2. Give it a system-wide Unique.
      [Even non-exported things need system-wide Uniques because the
      byte-code generator builds a single Name->BCO symbol table.]
  
      We use the NameCache kept in the HscEnv as the
      source of such system-wide uniques.
  
      For external Ids, use the original-name cache in the NameCache
      to ensure that the unique assigned is the same as the Id had
      in any previous compilation run.
  
   3. Rename top-level Ids according to the names we chose in step 1.
      If it's an external Id, make it have a External Name, otherwise
      make it have an Internal Name.  This is used by the code generator
      to decide whether to make the label externally visible
  
   4. Give it its UTTERLY FINAL IdInfo; in ptic,
          * its unfolding, if it should have one
  
          * its arity, computed from the number of visible lambdas
  
          * its CAF info, computed from what is free in its RHS
  
  
  Finally, substitute these new top-level binders consistently
  throughout, including in unfoldings.  We also tidy binders in
  RHSs, so that they print nicely in interfaces.
  -}
  
  tidyProgram :: HscEnv -> ModGuts -> IO (CgGuts, ModDetails)
  tidyProgram hsc_env  (ModGuts { mg_module    = mod
                                , mg_exports   = exports
                                , mg_rdr_env   = rdr_env
                                , mg_tcs       = tcs
                                , mg_insts     = cls_insts
                                , mg_fam_insts = fam_insts
                                , mg_binds     = binds
                                , mg_patsyns   = patsyns
                                , mg_rules     = imp_rules
                                , mg_vect_info = vect_info
                                , mg_anns      = anns
                                , mg_deps      = deps
                                , mg_foreign   = foreign_stubs
                                , mg_hpc_info  = hpc_info
                                , mg_modBreaks = modBreaks
                                })
  
    = do  { let { dflags     = hsc_dflags hsc_env
                ; omit_prags = gopt Opt_OmitInterfacePragmas dflags
                ; expose_all = gopt Opt_ExposeAllUnfoldings  dflags
                ; print_unqual = mkPrintUnqualified dflags rdr_env
                }
          ; showPassIO dflags CoreTidy
  
          ; let { type_env = typeEnvFromEntities [] tcs fam_insts
  
                ; implicit_binds
                    = concatMap getClassImplicitBinds (typeEnvClasses type_env) ++
                      concatMap getTyConImplicitBinds (typeEnvTyCons type_env)
                }
  
          ; (unfold_env, tidy_occ_env)
                <- chooseExternalIds hsc_env mod omit_prags expose_all
                                     binds implicit_binds imp_rules (vectInfoVar vect_info)
          ; let { (trimmed_binds, trimmed_rules)
                      = findExternalRules omit_prags binds imp_rules unfold_env }
  
          ; (tidy_env, tidy_binds)
                   <- tidyTopBinds hsc_env mod unfold_env tidy_occ_env trimmed_binds
  
          ; let { final_ids  = [ id | id <- bindersOfBinds tidy_binds,
                                      isExternalName (idName id)]
                ; type_env1  = extendTypeEnvWithIds type_env final_ids
  
                ; tidy_cls_insts = map (tidyClsInstDFun (lookup_aux_id tidy_type_env)) cls_insts
                  -- A DFunId will have a binding in tidy_binds, and so will now be in
                  -- tidy_type_env, replete with IdInfo.  Its name will be unchanged since
                  -- it was born, but we want Global, IdInfo-rich (or not) DFunId in the
                  -- tidy_cls_insts.  Similarly the Ids inside a PatSyn.
  
                ; tidy_rules = tidyRules tidy_env trimmed_rules
                  -- You might worry that the tidy_env contains IdInfo-rich stuff
                  -- and indeed it does, but if omit_prags is on, ext_rules is
                  -- empty
  
                ; tidy_vect_info = tidyVectInfo tidy_env vect_info
  
                  -- Tidy the Ids inside each PatSyn, very similarly to DFunIds
                  -- and then override the PatSyns in the type_env with the new tidy ones
                  -- This is really the only reason we keep mg_patsyns at all; otherwise
                  -- they could just stay in type_env
                ; tidy_patsyns = map (tidyPatSynIds (lookup_aux_id tidy_type_env)) patsyns
                ; type_env2    = extendTypeEnvWithPatSyns tidy_patsyns type_env1
  
                ; tidy_type_env = tidyTypeEnv omit_prags type_env2
  
                -- See Note [Injecting implicit bindings]
                ; all_tidy_binds = implicit_binds ++ tidy_binds
  
                -- get the TyCons to generate code for.  Careful!  We must use
                -- the untidied TypeEnv here, because we need
                --  (a) implicit TyCons arising from types and classes defined
                --      in this module
                --  (b) wired-in TyCons, which are normally removed from the
                --      TypeEnv we put in the ModDetails
                --  (c) Constructors even if they are not exported (the
                --      tidied TypeEnv has trimmed these away)
                ; alg_tycons = filter isAlgTyCon (typeEnvTyCons type_env)
                }
  
          ; endPassIO hsc_env print_unqual CoreTidy all_tidy_binds tidy_rules
  
            -- If the endPass didn't print the rules, but ddump-rules is
            -- on, print now
          ; unless (dopt Opt_D_dump_simpl dflags) $
              Err.dumpIfSet_dyn dflags Opt_D_dump_rules
                (showSDoc dflags (ppr CoreTidy <+> ptext (sLit "rules")))
                (pprRulesForUser dflags tidy_rules)
  
            -- Print one-line size info
          ; let cs = coreBindsStats tidy_binds
          ; when (dopt Opt_D_dump_core_stats dflags)
                 (putLogMsg dflags NoReason SevDump noSrcSpan (defaultDumpStyle dflags)
                            (ptext (sLit "Tidy size (terms,types,coercions)")
                             <+> ppr (moduleName mod) <> colon
                             <+> int (cs_tm cs)
                             <+> int (cs_ty cs)
                             <+> int (cs_co cs) ))
  
          ; return (CgGuts { cg_module   = mod,
                             cg_tycons   = alg_tycons,
                             cg_binds    = all_tidy_binds,
                             cg_foreign  = foreign_stubs,
                             cg_dep_pkgs = map fst $ dep_pkgs deps,
                             cg_hpc_info = hpc_info,
                             cg_modBreaks = modBreaks },
  
                     ModDetails { md_types     = tidy_type_env,
                                  md_rules     = tidy_rules,
                                  md_insts     = tidy_cls_insts,
                                  md_vect_info = tidy_vect_info,
                                  md_fam_insts = fam_insts,
                                  md_exports   = exports,
                                  md_anns      = anns      -- are already tidy
                                })
          }
  
  lookup_aux_id :: TypeEnv -> Var -> Id
  lookup_aux_id type_env id
    = case lookupTypeEnv type_env (idName id) of
          Just (AnId id') -> id'
          _other          -> pprPanic "lookup_aux_id" (ppr id)
  
  tidyTypeEnv :: Bool       -- Compiling without -O, so omit prags
              -> TypeEnv -> TypeEnv
  
  -- The competed type environment is gotten from
  --      a) the types and classes defined here (plus implicit things)
  --      b) adding Ids with correct IdInfo, including unfoldings,
  --              gotten from the bindings
  -- From (b) we keep only those Ids with External names;
  --          the CoreTidy pass makes sure these are all and only
  --          the externally-accessible ones
  -- This truncates the type environment to include only the
  -- exported Ids and things needed from them, which saves space
  --
  -- See Note [Don't attempt to trim data types]
  
  tidyTypeEnv omit_prags type_env
   = let
          type_env1 = filterNameEnv (not . isWiredInName . getName) type_env
            -- (1) remove wired-in things
          type_env2 | omit_prags = mapNameEnv trimThing type_env1
                    | otherwise  = type_env1
            -- (2) trimmed if necessary
      in
      type_env2
  
  --------------------------
  trimThing :: TyThing -> TyThing
  -- Trim off inessentials, for boot files and no -O
  trimThing (AnId id)
     | not (isImplicitId id)
     = AnId (id `setIdInfo` vanillaIdInfo)
  
  trimThing other_thing
    = other_thing
  
  extendTypeEnvWithPatSyns :: [PatSyn] -> TypeEnv -> TypeEnv
  extendTypeEnvWithPatSyns tidy_patsyns type_env
    = extendTypeEnvList type_env [AConLike (PatSynCon ps) | ps <- tidy_patsyns ]
  
  tidyVectInfo :: TidyEnv -> VectInfo -> VectInfo
  tidyVectInfo (_, var_env) info@(VectInfo { vectInfoVar          = vars
                                           , vectInfoParallelVars = parallelVars
                                           })
    = info { vectInfoVar          = tidy_vars
           , vectInfoParallelVars = tidy_parallelVars
           }
    where
        -- we only export mappings whose domain and co-domain is exported (otherwise, the iface is
        -- inconsistent)
      tidy_vars = mkVarEnv [ (tidy_var, (tidy_var, tidy_var_v))
                           | (var, var_v) <- varEnvElts vars
                           , let tidy_var   = lookup_var var
                                 tidy_var_v = lookup_var var_v
                           , isExternalId tidy_var   && isExportedId tidy_var
                           , isExternalId tidy_var_v && isExportedId tidy_var_v
                           , isDataConWorkId var || not (isImplicitId var)
                           ]
  
      tidy_parallelVars = mkVarSet [ tidy_var
                                   | var <- varSetElems parallelVars
                                   , let tidy_var = lookup_var var
                                   , isExternalId tidy_var && isExportedId tidy_var
                                   ]
  
      lookup_var var = lookupWithDefaultVarEnv var_env var var
  
      -- We need to make sure that all names getting into the iface version of 'VectInfo' are
      -- external; otherwise, 'MkIface' will bomb out.
      isExternalId = isExternalName . idName
  
  {-
  Note [Don't attempt to trim data types]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  For some time GHC tried to avoid exporting the data constructors
  of a data type if it wasn't strictly necessary to do so; see Trac #835.
  But "strictly necessary" accumulated a longer and longer list
  of exceptions, and finally I gave up the battle:
  
      commit 9a20e540754fc2af74c2e7392f2786a81d8d5f11
      Author: Simon Peyton Jones <simonpj@microsoft.com>
      Date:   Thu Dec 6 16:03:16 2012 +0000
  
      Stop attempting to "trim" data types in interface files
  
      Without -O, we previously tried to make interface files smaller
      by not including the data constructors of data types.  But
      there are a lot of exceptions, notably when Template Haskell is
      involved or, more recently, DataKinds.
  
      However Trac #7445 shows that even without TemplateHaskell, using
      the Data class and invoking Language.Eta.Meta.Quote.dataToExpQ
      is enough to require us to expose the data constructors.
  
      So I've given up on this "optimisation" -- it's probably not
      important anyway.  Now I'm simply not attempting to trim off
      the data constructors.  The gain in simplicity is worth the
      modest cost in interface file growth, which is limited to the
      bits reqd to describe those data constructors.
  
  ************************************************************************
  *                                                                      *
          Implicit bindings
  *                                                                      *
  ************************************************************************
  
  Note [Injecting implicit bindings]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  We inject the implicit bindings right at the end, in CoreTidy.
  Some of these bindings, notably record selectors, are not
  constructed in an optimised form.  E.g. record selector for
          data T = MkT { x :: {-# UNPACK #-} !Int }
  Then the unfolding looks like
          x = \t. case t of MkT x1 -> let x = I# x1 in x
  This generates bad code unless it's first simplified a bit.  That is
  why CoreUnfold.mkImplicitUnfolding uses simleExprOpt to do a bit of
  optimisation first.  (Only matters when the selector is used curried;
  eg map x ys.)  See Trac #2070.
  
  [Oct 09: in fact, record selectors are no longer implicit Ids at all,
  because we really do want to optimise them properly. They are treated
  much like any other Id.  But doing "light" optimisation on an implicit
  Id still makes sense.]
  
  At one time I tried injecting the implicit bindings *early*, at the
  beginning of SimplCore.  But that gave rise to real difficulty,
  because GlobalIds are supposed to have *fixed* IdInfo, but the
  simplifier and other core-to-core passes mess with IdInfo all the
  time.  The straw that broke the camels back was when a class selector
  got the wrong arity -- ie the simplifier gave it arity 2, whereas
  importing modules were expecting it to have arity 1 (Trac #2844).
  It's much safer just to inject them right at the end, after tidying.
  
  Oh: two other reasons for injecting them late:
  
    - If implicit Ids are already in the bindings when we start TidyPgm,
      we'd have to be careful not to treat them as external Ids (in
      the sense of chooseExternalIds); else the Ids mentioned in *their*
      RHSs will be treated as external and you get an interface file
      saying      a18 = <blah>
      but nothing refererring to a18 (because the implicit Id is the
      one that does, and implicit Ids don't appear in interface files).
  
    - More seriously, the tidied type-envt will include the implicit
      Id replete with a18 in its unfolding; but we won't take account
      of a18 when computing a fingerprint for the class; result chaos.
  
  There is one sort of implicit binding that is injected still later,
  namely those for data constructor workers. Reason (I think): it's
  really just a code generation trick.... binding itself makes no sense.
  See Note [Data constructor workers] in CorePrep.
  -}
  
  getTyConImplicitBinds :: TyCon -> [CoreBind]
  getTyConImplicitBinds tc = map get_defn (mapMaybe dataConWrapId_maybe (tyConDataCons tc))
  
  getClassImplicitBinds :: Class -> [CoreBind]
  getClassImplicitBinds cls
    = [ NonRec op (mkDictSelRhs cls val_index)
      | (op, val_index) <- classAllSelIds cls `zip` [0..] ]
  
  get_defn :: Id -> CoreBind
  get_defn id = NonRec id (unfoldingTemplate (realIdUnfolding id))
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection{Step 1: finding externals}
  *                                                                      *
  ************************************************************************
  
  See Note [Choosing external names].
  -}
  
  type UnfoldEnv  = IdEnv (Name{-new name-}, Bool {-show unfolding-})
    -- Maps each top-level Id to its new Name (the Id is tidied in step 2)
    -- The Unique is unchanged.  If the new Name is external, it will be
    -- visible in the interface file.
    --
    -- Bool => expose unfolding or not.
  
  chooseExternalIds :: HscEnv
                    -> Module
                    -> Bool -> Bool
                    -> [CoreBind]
                    -> [CoreBind]
                    -> [CoreRule]
                    -> VarEnv (Var, Var)
                    -> IO (UnfoldEnv, TidyOccEnv)
                    -- Step 1 from the notes above
  
  chooseExternalIds hsc_env mod omit_prags expose_all binds implicit_binds imp_id_rules vect_vars
    = do { (unfold_env1,occ_env1) <- search init_work_list emptyVarEnv init_occ_env
         ; let internal_ids = filter (not . (`elemVarEnv` unfold_env1)) binders
         ; tidy_internal internal_ids unfold_env1 occ_env1 }
   where
    nc_var = hsc_NC hsc_env
  
    -- init_ext_ids is the intial list of Ids that should be
    -- externalised.  It serves as the starting point for finding a
    -- deterministic, tidy, renaming for all external Ids in this
    -- module.
    --
    -- It is sorted, so that it has adeterministic order (i.e. it's the
    -- same list every time this module is compiled), in contrast to the
    -- bindings, which are ordered non-deterministically.
    init_work_list = zip init_ext_ids init_ext_ids
    init_ext_ids   = sortBy (compare `on` getOccName) $
                     filter is_external binders
  
    -- An Id should be external if either (a) it is exported,
    -- (b) it appears in the RHS of a local rule for an imported Id, or
    -- (c) it is the vectorised version of an imported Id
    -- See Note [Which rules to expose]
    is_external id = isExportedId id || id `elemVarSet` rule_rhs_vars || id `elemVarSet` vect_var_vs
    rule_rhs_vars  = mapUnionVarSet ruleRhsFreeVars imp_id_rules
    vect_var_vs    = mkVarSet [var_v | (var, var_v) <- nameEnvElts vect_vars, isGlobalId var]
  
    binders          = bindersOfBinds binds
    implicit_binders = bindersOfBinds implicit_binds
    binder_set       = mkVarSet binders
  
    avoids   = [getOccName name | bndr <- binders ++ implicit_binders,
                                  let name = idName bndr,
                                  isExternalName name ]
                  -- In computing our "avoids" list, we must include
                  --      all implicit Ids
                  --      all things with global names (assigned once and for
                  --                                      all by the renamer)
                  -- since their names are "taken".
                  -- The type environment is a convenient source of such things.
                  -- In particular, the set of binders doesn't include
                  -- implicit Ids at this stage.
  
          -- We also make sure to avoid any exported binders.  Consider
          --      f{-u1-} = 1     -- Local decl
          --      ...
          --      f{-u2-} = 2     -- Exported decl
          --
          -- The second exported decl must 'get' the name 'f', so we
          -- have to put 'f' in the avoids list before we get to the first
          -- decl.  tidyTopId then does a no-op on exported binders.
    init_occ_env = initTidyOccEnv avoids
  
  
    search :: [(Id,Id)]    -- The work-list: (external id, referrring id)
                           -- Make a tidy, external Name for the external id,
                           --   add it to the UnfoldEnv, and do the same for the
                           --   transitive closure of Ids it refers to
                           -- The referring id is used to generate a tidy
                           ---  name for the external id
           -> UnfoldEnv    -- id -> (new Name, show_unfold)
           -> TidyOccEnv   -- occ env for choosing new Names
           -> IO (UnfoldEnv, TidyOccEnv)
  
    search [] unfold_env occ_env = return (unfold_env, occ_env)
  
    search ((idocc,referrer) : rest) unfold_env occ_env
      | idocc `elemVarEnv` unfold_env = search rest unfold_env occ_env
      | otherwise = do
        (occ_env', name') <- tidyTopName mod nc_var (Just referrer) occ_env idocc
        let
            (new_ids, show_unfold)
                  | omit_prags = ([], False)
                  | otherwise  = addExternal expose_all refined_id
  
                  -- add vectorised version if any exists
            new_ids' = new_ids ++ maybeToList (fmap snd $ lookupVarEnv vect_vars idocc)
  
                  -- 'idocc' is an *occurrence*, but we need to see the
                  -- unfolding in the *definition*; so look up in binder_set
            refined_id = case lookupVarSet binder_set idocc of
                           Just id -> id
                           Nothing -> {-WARN( True, ppr idocc )-} idocc
  
            unfold_env' = extendVarEnv unfold_env idocc (name',show_unfold)
            referrer' | isExportedId refined_id = refined_id
                      | otherwise               = referrer
        --
        search (zip new_ids' (repeat referrer') ++ rest) unfold_env' occ_env'
  
    tidy_internal :: [Id] -> UnfoldEnv -> TidyOccEnv
                  -> IO (UnfoldEnv, TidyOccEnv)
    tidy_internal []       unfold_env occ_env = return (unfold_env,occ_env)
    tidy_internal (id:ids) unfold_env occ_env = do
        (occ_env', name') <- tidyTopName mod nc_var Nothing occ_env id
        let unfold_env' = extendVarEnv unfold_env id (name',False)
        tidy_internal ids unfold_env' occ_env'
  
  addExternal :: Bool -> Id -> ([Id], Bool)
  addExternal expose_all id = (new_needed_ids, show_unfold)
    where
      new_needed_ids = bndrFvsInOrder show_unfold id
      idinfo         = idInfo id
      show_unfold    = show_unfolding (unfoldingInfo idinfo)
      never_active   = isNeverActive (inlinePragmaActivation (inlinePragInfo idinfo))
      loop_breaker   = isStrongLoopBreaker (occInfo idinfo)
      bottoming_fn   = isBottomingSig (strictnessInfo idinfo)
  
          -- Stuff to do with the Id's unfolding
          -- We leave the unfolding there even if there is a worker
          -- In GHCi the unfolding is used by importers
  
      show_unfolding (CoreUnfolding { uf_src = src, uf_guidance = guidance })
         =  expose_all         -- 'expose_all' says to expose all
                               -- unfoldings willy-nilly
  
         || isStableSource src     -- Always expose things whose
                                   -- source is an inline rule
  
         || not (bottoming_fn      -- No need to inline bottom functions
             || never_active       -- Or ones that say not to
             || loop_breaker       -- Or that are loop breakers
             || neverUnfoldGuidance guidance)
      show_unfolding (DFunUnfolding {}) = True
      show_unfolding _                  = False
  
  {-
  ************************************************************************
  *                                                                      *
                 Deterministic free variables
  *                                                                      *
  ************************************************************************
  
  We want a deterministic free-variable list.  exprFreeVars gives us
  a VarSet, which is in a non-deterministic order when converted to a
  list.  Hence, here we define a free-variable finder that returns
  the free variables in the order that they are encountered.
  
  See Note [Choosing external names]
  -}
  
  bndrFvsInOrder :: Bool -> Id -> [Id]
  bndrFvsInOrder show_unfold id
    = run (dffvLetBndr show_unfold id)
  
  run :: DFFV () -> [Id]
  run (DFFV m) = case m emptyVarSet (emptyVarSet, []) of
                   ((_,ids),_) -> ids
  
  newtype DFFV a
    = DFFV (VarSet              -- Envt: non-top-level things that are in scope
                                -- we don't want to record these as free vars
        -> (VarSet, [Var])      -- Input State: (set, list) of free vars so far
        -> ((VarSet,[Var]),a))  -- Output state
  
  instance Functor DFFV where
      fmap = liftM
  
  instance Applicative DFFV where
      pure = return
      (<*>) = ap
  
  instance Monad DFFV where
    return a = DFFV $ \_ st -> (st, a)
    (DFFV m) >>= k = DFFV $ \env st ->
      case m env st of
         (st',a) -> case k a of
                       DFFV f -> f env st'
  
  extendScope :: Var -> DFFV a -> DFFV a
  extendScope v (DFFV f) = DFFV (\env st -> f (extendVarSet env v) st)
  
  extendScopeList :: [Var] -> DFFV a -> DFFV a
  extendScopeList vs (DFFV f) = DFFV (\env st -> f (extendVarSetList env vs) st)
  
  insert :: Var -> DFFV ()
  insert v = DFFV $ \ env (set, ids) ->
             let keep_me = isLocalId v &&
                           not (v `elemVarSet` env) &&
                             not (v `elemVarSet` set)
             in if keep_me
                then ((extendVarSet set v, v:ids), ())
                else ((set,                ids),   ())
  
  
  dffvExpr :: CoreExpr -> DFFV ()
  dffvExpr (Var v)              = insert v
  dffvExpr (App e1 e2)          = dffvExpr e1 >> dffvExpr e2
  dffvExpr (Lam v e)            = extendScope v (dffvExpr e)
  dffvExpr (Tick (Breakpoint _ ids) e) = mapM_ insert ids >> dffvExpr e
  dffvExpr (Tick _other e)    = dffvExpr e
  dffvExpr (Cast e _)           = dffvExpr e
  dffvExpr (Let (NonRec x r) e) = dffvBind (x,r) >> extendScope x (dffvExpr e)
  dffvExpr (Let (Rec prs) e)    = extendScopeList (map fst prs) $
                                  (mapM_ dffvBind prs >> dffvExpr e)
  dffvExpr (Case e b _ as)      = dffvExpr e >> extendScope b (mapM_ dffvAlt as)
  dffvExpr _other               = return ()
  
  dffvAlt :: (t, [Var], CoreExpr) -> DFFV ()
  dffvAlt (_,xs,r) = extendScopeList xs (dffvExpr r)
  
  dffvBind :: (Id, CoreExpr) -> DFFV ()
  dffvBind(x,r)
    | not (isId x) = dffvExpr r
    | otherwise    = dffvLetBndr False x >> dffvExpr r
                  -- Pass False because we are doing the RHS right here
                  -- If you say True you'll get *exponential* behaviour!
  
  dffvLetBndr :: Bool -> Id -> DFFV ()
  -- Gather the free vars of the RULES and unfolding of a binder
  -- We always get the free vars of a *stable* unfolding, but
  -- for a *vanilla* one (InlineRhs), the flag controls what happens:
  --   True <=> get fvs of even a *vanilla* unfolding
  --   False <=> ignore an InlineRhs
  -- For nested bindings (call from dffvBind) we always say "False" because
  --       we are taking the fvs of the RHS anyway
  -- For top-level bindings (call from addExternal, via bndrFvsInOrder)
  --       we say "True" if we are exposing that unfolding
  dffvLetBndr vanilla_unfold id
    = do { go_unf (unfoldingInfo idinfo)
         ; mapM_ go_rule (ruleInfoRules (ruleInfo idinfo)) }
    where
      idinfo = idInfo id
  
      go_unf (CoreUnfolding { uf_tmpl = rhs, uf_src = src })
         = case src of
             InlineRhs | vanilla_unfold -> dffvExpr rhs
                       | otherwise      -> return ()
             _                          -> dffvExpr rhs
  
      go_unf (DFunUnfolding { df_bndrs = bndrs, df_args = args })
               = extendScopeList bndrs $ mapM_ dffvExpr args
      go_unf _ = return ()
  
      go_rule (BuiltinRule {}) = return ()
      go_rule (Rule { ru_bndrs = bndrs, ru_rhs = rhs })
        = extendScopeList bndrs (dffvExpr rhs)
  
  {-
  ************************************************************************
  *                                                                      *
                 findExternalRules
  *                                                                      *
  ************************************************************************
  
  Note [Finding external rules]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  The complete rules are gotten by combining
     a) local rules for imported Ids
     b) rules embedded in the top-level Ids
  
  There are two complications:
    * Note [Which rules to expose]
    * Note [Trimming auto-rules]
  
  Note [Which rules to expose]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  The function 'expose_rule' filters out rules that mention, on the LHS,
  Ids that aren't externally visible; these rules can't fire in a client
  module.
  
  The externally-visible binders are computed (by chooseExternalIds)
  assuming that all orphan rules are externalised (see init_ext_ids in
  function 'search'). So in fact it's a bit conservative and we may
  export more than we need.  (It's a sort of mutual recursion.)
  
  Note [Trimming auto-rules]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Second, with auto-specialisation we may specialise local or imported
  dfuns or INLINE functions, and then later inline them.  That may leave
  behind something like
     RULE "foo" forall d. f @ Int d = f_spec
  where f is either local or imported, and there is no remaining
  reference to f_spec except from the RULE.
  
  Now that RULE *might* be useful to an importing module, but that is
  purely speculative, and meanwhile the code is taking up space and
  codegen time.  So is seeems better to drop the binding for f_spec if
  the auto-generated rule is the *only* reason that it is being kept
  alive.
  
  (The RULE still might have been useful in the past; that is, it was
  the right thing to have generated it in the first place.  See Note
  [Inline specialisations] in Specialise.  But now it has served its
  purpose, and can be discarded.)
  
  So findExternalRules does this:
    * Remove all bindings that are kept alive *only* by isAutoRule rules
        (this is done in trim_binds)
    * Remove all auto rules that mention bindings that have been removed
        (this is done by filtering by keep_rule)
  
  So if a binding is kept alive for some *other* reason (e.g. f_spec is
  called in the final code), we keep the rule too.
  
  I found that binary sizes jumped by 6-10% when I started to specialise
  INLINE functions (again, Note [Inline specialisations] in Specialise).
  Adding trimAutoRules removed all this bloat.
  -}
  
  findExternalRules :: Bool       -- Omit pragmas
                    -> [CoreBind]
                    -> [CoreRule] -- Local rules for imported fns
                    -> UnfoldEnv  -- Ids that are exported, so we need their rules
                    -> ([CoreBind], [CoreRule])
  -- See Note [Finding external rules]
  findExternalRules omit_prags binds imp_id_rules unfold_env
    = (trimmed_binds, filter keep_rule all_rules)
    where
      imp_rules         = filter expose_rule imp_id_rules
      imp_user_rule_fvs = mapUnionVarSet user_rule_rhs_fvs imp_rules
  
      user_rule_rhs_fvs rule | isAutoRule rule = emptyVarSet
                             | otherwise       = ruleRhsFreeVars rule
  
      (trimmed_binds, local_bndrs, _, all_rules) = trim_binds binds
  
      keep_rule rule = ruleFreeVars rule `subVarSet` local_bndrs
          -- Remove rules that make no sense, because they mention a
          -- local binder (on LHS or RHS) that we have now discarded.
          -- (NB: ruleFreeVars only includes LocalIds)
          --
          -- LHS: we have alrady filtered out rules that mention internal Ids
          --     on LHS but that isn't enough because we might have by now
          --     discarded a binding with an external Id. (How?
          --     chooseExternalIds is a bit conservative.)
          --
          -- RHS: the auto rules that might mention a binder that has
          --      been discarded; see Note [Trimming auto-rules]
  
      expose_rule rule
          | omit_prags = False
          | otherwise  = all is_external_id (varSetElems (ruleLhsFreeIds rule))
                  -- Don't expose a rule whose LHS mentions a locally-defined
                  -- Id that is completely internal (i.e. not visible to an
                  -- importing module).  NB: ruleLhsFreeIds only returns LocalIds.
                  -- See Note [Which rules to expose]
  
      is_external_id id = case lookupVarEnv unfold_env id of
                            Just (name, _) -> isExternalName name
                            Nothing        -> False
  
      trim_binds :: [CoreBind]
                 -> ( [CoreBind]   -- Trimmed bindings
                    , VarSet       -- Binders of those bindings
                    , VarSet       -- Free vars of those bindings + rhs of user rules
                                   -- (we don't bother to delete the binders)
                    , [CoreRule])  -- All rules, imported + from the bindings
      -- This function removes unnecessary bindings, and gathers up rules from
      -- the bindings we keep.  See Note [Trimming auto-rules]
      trim_binds []  -- Base case, start with imp_user_rule_fvs
         = ([], emptyVarSet, imp_user_rule_fvs, imp_rules)
  
      trim_binds (bind:binds)
         | any needed bndrs    -- Keep binding
         = ( bind : binds', bndr_set', needed_fvs', local_rules ++ rules )
         | otherwise           -- Discard binding altogether
         = stuff
         where
           stuff@(binds', bndr_set, needed_fvs, rules)
                         = trim_binds binds
           needed bndr   = isExportedId bndr || bndr `elemVarSet` needed_fvs
  
           bndrs         = bindersOf  bind
           rhss          = rhssOfBind bind
           bndr_set'     = bndr_set `extendVarSetList` bndrs
  
           needed_fvs'   = needed_fvs                                   `unionVarSet`
                           mapUnionVarSet idUnfoldingVars   bndrs       `unionVarSet`
                                -- Ignore type variables in the type of bndrs
                           mapUnionVarSet exprFreeVars      rhss        `unionVarSet`
                           mapUnionVarSet user_rule_rhs_fvs local_rules
              -- In needed_fvs', we don't bother to delete binders from the fv set
  
           local_rules  = [ rule
                          | id <- bndrs
                          , is_external_id id   -- Only collect rules for external Ids
                          , rule <- idCoreRules id
                          , expose_rule rule ]  -- and ones that can fire in a client
  
  {-
  ************************************************************************
  *                                                                      *
                 tidyTopName
  *                                                                      *
  ************************************************************************
  
  This is where we set names to local/global based on whether they really are
  externally visible (see comment at the top of this module).  If the name
  was previously local, we have to give it a unique occurrence name if
  we intend to externalise it.
  -}
  
  tidyTopName :: Module -> IORef NameCache -> Maybe Id -> TidyOccEnv
              -> Id -> IO (TidyOccEnv, Name)
  tidyTopName mod nc_var maybe_ref occ_env id
    | global && internal = return (occ_env, localiseName name)
  
    | global && external = return (occ_env, name)
          -- Global names are assumed to have been allocated by the renamer,
          -- so they already have the "right" unique
          -- And it's a system-wide unique too
  
    -- Now we get to the real reason that all this is in the IO Monad:
    -- we have to update the name cache in a nice atomic fashion
  
    | local  && internal = do { new_local_name <- atomicModifyIORef nc_var mk_new_local
                              ; return (occ_env', new_local_name) }
          -- Even local, internal names must get a unique occurrence, because
          -- if we do -split-objs we externalise the name later, in the code generator
          --
          -- Similarly, we must make sure it has a system-wide Unique, because
          -- the byte-code generator builds a system-wide Name->BCO symbol table
  
    | local  && external = do { new_external_name <- atomicModifyIORef nc_var mk_new_external
                              ; return (occ_env', new_external_name) }
  
    | otherwise = panic "tidyTopName"
    where
      name        = idName id
      external    = isJust maybe_ref
      global      = isExternalName name
      local       = not global
      internal    = not external
      loc         = nameSrcSpan name
  
      old_occ     = nameOccName name
      new_occ
        | Just ref <- maybe_ref, ref /= id =
            mkOccName (occNameSpace old_occ) $
               let
                   ref_str = occNameString (getOccName ref)
                   occ_str = occNameString old_occ
               in
               case occ_str of
                 '$':'w':_ -> occ_str
                    -- workers: the worker for a function already
                    -- includes the occname for its parent, so there's
                    -- no need to prepend the referrer.
                 _other | isSystemName name -> ref_str
                        | otherwise         -> ref_str ++ '_' : occ_str
                    -- If this name was system-generated, then don't bother
                    -- to retain its OccName, just use the referrer.  These
                    -- system-generated names will become "f1", "f2", etc. for
                    -- a referrer "f".
        | otherwise = old_occ
  
      (occ_env', occ') = tidyOccName occ_env new_occ
  
      mk_new_local nc = (nc { nsUniqs = us }, mkInternalName uniq occ' loc)
                      where
                        (uniq, us) = takeUniqFromSupply (nsUniqs nc)
  
      mk_new_external nc = allocateGlobalBinder nc mod occ' loc
          -- If we want to externalise a currently-local name, check
          -- whether we have already assigned a unique for it.
          -- If so, use it; if not, extend the table.
          -- All this is done by allcoateGlobalBinder.
          -- This is needed when *re*-compiling a module in GHCi; we must
          -- use the same name for externally-visible things as we did before.
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection{Step 2: top-level tidying}
  *                                                                      *
  ************************************************************************
  -}
  
  -- TopTidyEnv: when tidying we need to know
  --   * nc_var: The NameCache, containing a unique supply and any pre-ordained Names.
  --        These may have arisen because the
  --        renamer read in an interface file mentioning M.$wf, say,
  --        and assigned it unique r77.  If, on this compilation, we've
  --        invented an Id whose name is $wf (but with a different unique)
  --        we want to rename it to have unique r77, so that we can do easy
  --        comparisons with stuff from the interface file
  --
  --   * occ_env: The TidyOccEnv, which tells us which local occurrences
  --     are 'used'
  --
  --   * subst_env: A Var->Var mapping that substitutes the new Var for the old
  
  tidyTopBinds :: HscEnv
               -> Module
               -> UnfoldEnv
               -> TidyOccEnv
               -> CoreProgram
               -> IO (TidyEnv, CoreProgram)
  
  tidyTopBinds hsc_env this_mod unfold_env init_occ_env binds
    = do mkIntegerId <- lookupMkIntegerName dflags hsc_env
         integerSDataCon <- lookupIntegerSDataConName dflags hsc_env
         let cvt_integer = cvtLitInteger dflags mkIntegerId integerSDataCon
         return $ tidy cvt_integer init_env binds
    where
      dflags = hsc_dflags hsc_env
  
      init_env = (init_occ_env, emptyVarEnv)
  
      tidy _           env []     = (env, [])
      tidy cvt_integer env (b:bs)
          = let (env1, b')  = tidyTopBind dflags this_mod
                                          cvt_integer unfold_env env b
                (env2, bs') = tidy cvt_integer env1 bs
            in  (env2, b':bs')
  
  ------------------------
  tidyTopBind  :: DynFlags
               -> Module
               -> (Integer -> CoreExpr)
               -> UnfoldEnv
               -> TidyEnv
               -> CoreBind
               -> (TidyEnv, CoreBind)
  
  tidyTopBind dflags this_mod cvt_integer unfold_env
              (occ_env,subst1) (NonRec bndr rhs)
    = (tidy_env2,  NonRec bndr' rhs')
    where
      Just (name',show_unfold) = lookupVarEnv unfold_env bndr
      caf_info      = hasCafRefs dflags this_mod (subst1, cvt_integer) (idArity bndr) rhs
      (bndr', rhs') = tidyTopPair dflags show_unfold tidy_env2 caf_info name' (bndr, rhs)
      subst2        = extendVarEnv subst1 bndr bndr'
      tidy_env2     = (occ_env, subst2)
  
  tidyTopBind dflags this_mod cvt_integer unfold_env
              (occ_env, subst1) (Rec prs)
    = (tidy_env2, Rec prs')
    where
      prs' = [ tidyTopPair dflags show_unfold tidy_env2 caf_info name' (id,rhs)
             | (id,rhs) <- prs,
               let (name',show_unfold) =
                      expectJust "tidyTopBind" $ lookupVarEnv unfold_env id
             ]
  
      subst2    = extendVarEnvList subst1 (bndrs `zip` map fst prs')
      tidy_env2 = (occ_env, subst2)
  
      bndrs = map fst prs
  
          -- the CafInfo for a recursive group says whether *any* rhs in
          -- the group may refer indirectly to a CAF (because then, they all do).
      caf_info
          | or [ mayHaveCafRefs (hasCafRefs dflags this_mod
                                            (subst1, cvt_integer)
                                            (idArity bndr) rhs)
               | (bndr,rhs) <- prs ] = MayHaveCafRefs
          | otherwise                = NoCafRefs
  
  -----------------------------------------------------------
  tidyTopPair :: DynFlags
              -> Bool  -- show unfolding
              -> TidyEnv  -- The TidyEnv is used to tidy the IdInfo
                          -- It is knot-tied: don't look at it!
              -> CafInfo
              -> Name             -- New name
              -> (Id, CoreExpr)   -- Binder and RHS before tidying
              -> (Id, CoreExpr)
          -- This function is the heart of Step 2
          -- The rec_tidy_env is the one to use for the IdInfo
          -- It's necessary because when we are dealing with a recursive
          -- group, a variable late in the group might be mentioned
          -- in the IdInfo of one early in the group
  
  tidyTopPair dflags show_unfold rhs_tidy_env caf_info name' (bndr, rhs)
    = (bndr1, rhs1)
    where
      bndr1    = mkGlobalId details name' ty' idinfo'
      details  = idDetails bndr   -- Preserve the IdDetails
      ty'      = tidyTopType (idType bndr)
      rhs1     = tidyExpr rhs_tidy_env rhs
      idinfo'  = tidyTopIdInfo dflags rhs_tidy_env name' rhs rhs1 (idInfo bndr)
                               show_unfold caf_info
  
  -- tidyTopIdInfo creates the final IdInfo for top-level
  -- binders.  There are two delicate pieces:
  --
  --  * Arity.  After CoreTidy, this arity must not change any more.
  --      Indeed, CorePrep must eta expand where necessary to make
  --      the manifest arity equal to the claimed arity.
  --
  --  * CAF info.  This must also remain valid through to code generation.
  --      We add the info here so that it propagates to all
  --      occurrences of the binders in RHSs, and hence to occurrences in
  --      unfoldings, which are inside Ids imported by GHCi. Ditto RULES.
  --      CoreToStg makes use of this when constructing SRTs.
  tidyTopIdInfo :: DynFlags -> TidyEnv -> Name -> CoreExpr -> CoreExpr
                -> IdInfo -> Bool -> CafInfo -> IdInfo
  tidyTopIdInfo dflags rhs_tidy_env name orig_rhs tidy_rhs idinfo show_unfold caf_info
    | not is_external     -- For internal Ids (not externally visible)
    = vanillaIdInfo       -- we only need enough info for code generation
                          -- Arity and strictness info are enough;
                          --      c.f. CoreTidy.tidyLetBndr
          `setCafInfo`        caf_info
          `setArityInfo`      arity
          `setStrictnessInfo` final_sig
  
    | otherwise           -- Externally-visible Ids get the whole lot
    = vanillaIdInfo
          `setCafInfo`           caf_info
          `setArityInfo`         arity
          `setStrictnessInfo`    final_sig
          `setOccInfo`           robust_occ_info
          `setInlinePragInfo`    (inlinePragInfo idinfo)
          `setUnfoldingInfo`     unfold_info
                  -- NB: we throw away the Rules
                  -- They have already been extracted by findExternalRules
    where
      is_external = isExternalName name
  
      --------- OccInfo ------------
      robust_occ_info = zapFragileOcc (occInfo idinfo)
      -- It's important to keep loop-breaker information
      -- when we are doing -fexpose-all-unfoldings
  
      --------- Strictness ------------
      mb_bot_str = exprBotStrictness_maybe orig_rhs
  
      sig = strictnessInfo idinfo
      final_sig | not $ isTopSig sig
                   = {-WARN( _bottom_hidden sig , ppr name )-} sig
                   -- try a cheap-and-cheerful bottom analyser
                   | Just (_, nsig) <- mb_bot_str = nsig
                   | otherwise                    = sig
  
      _bottom_hidden id_sig = case mb_bot_str of
                                    Nothing         -> False
                                    Just (arity, _) -> not (appIsBottom id_sig arity)
  
      --------- Unfolding ------------
      unf_info = unfoldingInfo idinfo
      unfold_info | show_unfold = tidyUnfolding rhs_tidy_env unf_info unf_from_rhs
                  | otherwise   = noUnfolding
      unf_from_rhs = mkTopUnfolding dflags is_bot tidy_rhs
      is_bot = isBottomingSig final_sig
      -- NB: do *not* expose the worker if show_unfold is off,
      --     because that means this thing is a loop breaker or
      --     marked NOINLINE or something like that
      -- This is important: if you expose the worker for a loop-breaker
      -- then you can make the simplifier go into an infinite loop, because
      -- in effect the unfolding is exposed.  See Trac #1709
      --
      -- You might think that if show_unfold is False, then the thing should
      -- not be w/w'd in the first place.  But a legitimate reason is this:
      --    the function returns bottom
      -- In this case, show_unfold will be false (we don't expose unfoldings
      -- for bottoming functions), but we might still have a worker/wrapper
      -- split (see Note [Worker-wrapper for bottoming functions] in WorkWrap.lhs
  
      --------- Arity ------------
      -- Usually the Id will have an accurate arity on it, because
      -- the simplifier has just run, but not always.
      -- One case I found was when the last thing the simplifier
      -- did was to let-bind a non-atomic argument and then float
      -- it to the top level. So it seems more robust just to
      -- fix it here.
      arity = exprArity orig_rhs
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection{Figuring out CafInfo for an expression}
  *                                                                      *
  ************************************************************************
  
  hasCafRefs decides whether a top-level closure can point into the dynamic heap.
  We mark such things as `MayHaveCafRefs' because this information is
  used to decide whether a particular closure needs to be referenced
  in an SRT or not.
  
  There are two reasons for setting MayHaveCafRefs:
          a) The RHS is a CAF: a top-level updatable thunk.
          b) The RHS refers to something that MayHaveCafRefs
  
  Possible improvement: In an effort to keep the number of CAFs (and
  hence the size of the SRTs) down, we could also look at the expression and
  decide whether it requires a small bounded amount of heap, so we can ignore
  it as a CAF.  In these cases however, we would need to use an additional
  CAF list to keep track of non-collectable CAFs.
  
  Note [Disgusting computation of CafRefs]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  We compute hasCafRefs here, because IdInfo is supposed to be finalised
  after TidyPgm.  But CorePrep does some transformations that affect CAF-hood.
  So we have to *predict* the result here, which is revolting.
  
  In particular CorePrep expands Integer literals.  So in the prediction code
  here we resort to applying the same expansion (cvt_integer). Ugh!
  -}
  
  type CafRefEnv = (VarEnv Id, Integer -> CoreExpr)
    -- The env finds the Caf-ness of the Id
    -- The Integer -> CoreExpr is the desugaring function for Integer literals
    -- See Note [Disgusting computation of CafRefs]
  
  hasCafRefs :: DynFlags -> Module
             -> CafRefEnv -> Arity -> CoreExpr
             -> CafInfo
  hasCafRefs dflags this_mod p@(_,cvt_integer) arity expr
    | is_caf || mentions_cafs = MayHaveCafRefs
    | otherwise               = NoCafRefs
   where
    mentions_cafs   = isFastTrue (cafRefsE p expr)
    is_dynamic_name = isDllName dflags this_mod
    is_caf = not (arity > 0 || rhsIsStatic (targetPlatform dflags) is_dynamic_name cvt_integer expr)
  
    -- NB. we pass in the arity of the expression, which is expected
    -- to be calculated by exprArity.  This is because exprArity
    -- knows how much eta expansion is going to be done by
    -- CorePrep later on, and we don't want to duplicate that
    -- knowledge in rhsIsStatic below.
  
  cafRefsE :: CafRefEnv -> Expr a -> FastBool
  cafRefsE p (Var id)            = cafRefsV p id
  cafRefsE p (Lit lit)           = cafRefsL p lit
  cafRefsE p (App f a)           = fastOr (cafRefsE p f) (cafRefsE p) a
  cafRefsE p (Lam _ e)           = cafRefsE p e
  cafRefsE p (Let b e)           = fastOr (cafRefsEs p (rhssOfBind b)) (cafRefsE p) e
  cafRefsE p (Case e _bndr _ alts) = fastOr (cafRefsE p e) (cafRefsEs p) (rhssOfAlts alts)
  cafRefsE p (Tick _n e)         = cafRefsE p e
  cafRefsE p (Cast e _co)        = cafRefsE p e
  cafRefsE _ (Type _)            = fastBool False
  cafRefsE _ (Coercion _)        = fastBool False
  
  cafRefsEs :: CafRefEnv -> [Expr a] -> FastBool
  cafRefsEs _ []     = fastBool False
  cafRefsEs p (e:es) = fastOr (cafRefsE p e) (cafRefsEs p) es
  
  cafRefsL :: CafRefEnv -> Literal -> FastBool
  -- Don't forget that mk_integer id might have Caf refs!
  -- We first need to convert the Integer into its final form, to
  -- see whether mkInteger is used.
  cafRefsL p@(_, cvt_integer) (LitInteger i _) = cafRefsE p (cvt_integer i)
  cafRefsL _                  _                = fastBool False
  
  cafRefsV :: CafRefEnv -> Id -> FastBool
  cafRefsV (subst, _) id
    | not (isLocalId id)                = fastBool (mayHaveCafRefs (idCafInfo id))
    | Just id' <- lookupVarEnv subst id = fastBool (mayHaveCafRefs (idCafInfo id'))
    | otherwise                         = fastBool False
  
  fastOr :: FastBool -> (a -> FastBool) -> a -> FastBool
  -- hack for lazy-or over FastBool.
  fastOr a f x = fastBool (isFastTrue a || isFastTrue (f x))
  
  {-
  ------------------------------------------------------------------------------
  --               Old, dead, type-trimming code
  -------------------------------------------------------------------------------
  
  We used to try to "trim off" the constructors of data types that are
  not exported, to reduce the size of interface files, at least without
  -O.  But that is not always possible: see the old Note [When we can't
  trim types] below for exceptions.
  
  Then (Trac #7445) I realised that the TH problem arises for any data type
  that we have deriving( Data ), because we can invoke
     Language.Eta.Meta.Quote.dataToExpQ
  to get a TH Exp representation of a value built from that data type.
  You don't even need {-# LANGUAGE TemplateHaskell #-}.
  
  At this point I give up. The pain of trimming constructors just
  doesn't seem worth the gain.  So I've dumped all the code, and am just
  leaving it here at the end of the module in case something like this
  is ever resurrected.
  
  
  Note [When we can't trim types]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  The basic idea of type trimming is to export algebraic data types
  abstractly (without their data constructors) when compiling without
  -O, unless of course they are explicitly exported by the user.
  
  We always export synonyms, because they can be mentioned in the type
  of an exported Id.  We could do a full dependency analysis starting
  from the explicit exports, but that's quite painful, and not done for
  now.
  
  But there are some times we can't do that, indicated by the 'no_trim_types' flag.
  
  First, Template Haskell.  Consider (Trac #2386) this
          module M(T, makeOne) where
            data T = Yay String
            makeOne = [| Yay "Yep" |]
  Notice that T is exported abstractly, but makeOne effectively exports it too!
  A module that splices in $(makeOne) will then look for a declartion of Yay,
  so it'd better be there.  Hence, brutally but simply, we switch off type
  constructor trimming if TH is enabled in this module.
  
  Second, data kinds.  Consider (Trac #5912)
       {-# LANGUAGE DataKinds #-}
       module M() where
       data UnaryTypeC a = UnaryDataC a
       type Bug = 'UnaryDataC
  We always export synonyms, so Bug is exposed, and that means that
  UnaryTypeC must be too, even though it's not explicitly exported.  In
  effect, DataKinds means that we'd need to do a full dependency analysis
  to see what data constructors are mentioned.  But we don't do that yet.
  
  In these two cases we just switch off type trimming altogether.
  
  mustExposeTyCon :: Bool         -- Type-trimming flag
                  -> NameSet      -- Exports
                  -> TyCon        -- The tycon
                  -> Bool         -- Can its rep be hidden?
  -- We are compiling without -O, and thus trying to write as little as
  -- possible into the interface file.  But we must expose the details of
  -- any data types whose constructors or fields are exported
  mustExposeTyCon no_trim_types exports tc
    | no_trim_types               -- See Note [When we can't trim types]
    = True
  
    | not (isAlgTyCon tc)         -- Always expose synonyms (otherwise we'd have to
                                  -- figure out whether it was mentioned in the type
                                  -- of any other exported thing)
    = True
  
    | isEnumerationTyCon tc       -- For an enumeration, exposing the constructors
    = True                        -- won't lead to the need for further exposure
  
    | isFamilyTyCon tc            -- Open type family
    = True
  
    -- Below here we just have data/newtype decls or family instances
  
    | null data_cons              -- Ditto if there are no data constructors
    = True                        -- (NB: empty data types do not count as enumerations
                                  -- see Note [Enumeration types] in TyCon
  
    | any exported_con data_cons  -- Expose rep if any datacon or field is exported
    = True
  
    | isNewTyCon tc && isFFITy (snd (newTyConRhs tc))
    = True   -- Expose the rep for newtypes if the rep is an FFI type.
             -- For a very annoying reason.  'Foreign import' is meant to
             -- be able to look through newtypes transparently, but it
             -- can only do that if it can "see" the newtype representation
  
    | otherwise
    = False
    where
      data_cons = tyConDataCons tc
      exported_con con = any (`elemNameSet` exports)
                             (dataConName con : dataConFieldLabels con)
  -}
  