content: {-
  (c) The University of Glasgow 2006
  (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
  -}
  
  {-# LANGUAGE LambdaCase #-}
  {-# LANGUAGE MagicHash #-}
  {-# LANGUAGE OverloadedStrings #-}
  
  -- |
  -- #name_types#
  -- GHC uses several kinds of name internally:
  --
  -- * 'GHC.Types.Name.Occurrence.OccName' represents names as strings with just a little more information:
  --   the \"namespace\" that the name came from, e.g. the namespace of value, type constructors or
  --   data constructors
  --
  -- * 'GHC.Types.Name.Reader.RdrName': see "GHC.Types.Name.Reader#name_types"
  --
  -- * 'GHC.Types.Name.Name': see "GHC.Types.Name#name_types"
  --
  -- * 'GHC.Types.Id.Id': see "GHC.Types.Id#name_types"
  --
  -- * 'GHC.Types.Var.Var': see "GHC.Types.Var#name_types"
  
  module GHC.Types.Name.Occurrence (
          -- * The 'NameSpace' type
          NameSpace, -- Abstract
  
          -- ** Construction
          -- $real_vs_source_data_constructors
          tcName, clsName, tcClsName, dataName, varName, fieldName,
          tvName, srcDataName,
  
          -- ** Pretty Printing
          pprNameSpace, pprNonVarNameSpace, pprNameSpaceBrief,
  
          -- * The 'OccName' type
          OccName,        -- Abstract, instance of Outputable
          pprOccName, occNameMangledFS,
  
          -- ** Construction
          mkOccName, mkOccNameFS,
          mkVarOcc, mkVarOccFS,
          mkRecFieldOcc, mkRecFieldOccFS,
          mkDataOcc, mkDataOccFS,
          mkTyVarOcc, mkTyVarOccFS,
          mkTcOcc, mkTcOccFS,
          mkClsOcc, mkClsOccFS,
          mkDFunOcc,
          setOccNameSpace,
          demoteOccName, demoteOccTcClsName, demoteOccTvName,
          promoteOccName,
          varToRecFieldOcc,
          recFieldToVarOcc,
          HasOccName(..),
  
          -- ** Derived 'OccName's
          isDerivedOccName,
          mkDataConWrapperOcc, mkWorkerOcc,
          mkMatcherOcc, mkBuilderOcc,
          mkDefaultMethodOcc, isDefaultMethodOcc, isTypeableBindOcc,
          mkNewTyCoOcc, mkClassOpAuxOcc,
          mkCon2TagOcc, mkTag2ConOcc, mkMaxTagOcc,
          mkClassDataConOcc, mkDictOcc, mkIPOcc,
          mkSpecOcc, mkForeignExportOcc, mkRepEqOcc,
          mkGenR, mkGen1R,
          mkDataTOcc, mkDataCOcc, mkDataConWorkerOcc,
          mkSuperDictSelOcc, mkSuperDictAuxOcc,
          mkLocalOcc, mkMethodOcc, mkInstTyTcOcc,
          mkInstTyCoOcc, mkEqPredCoOcc,
          mkTyConRepOcc,
  
          -- ** Deconstruction
          occNameFS, occNameString, occNameSpace,
  
          isVarOcc, isTvOcc, isTcOcc, isDataOcc, isDataSymOcc, isSymOcc, isValOcc,
          isFieldOcc, fieldOcc_maybe,
          parenSymOcc, startsWithUnderscore, isUnderscore,
  
          isTcClsNameSpace, isTvNameSpace, isDataConNameSpace, isVarNameSpace, isValNameSpace,
          isFieldNameSpace, isTermVarOrFieldNameSpace,
  
          -- * The 'OccEnv' type
          OccEnv, emptyOccEnv, unitOccEnv, extendOccEnv,
          mapOccEnv, strictMapOccEnv,
          mapMaybeOccEnv,
          lookupOccEnv, lookupOccEnv_AllNameSpaces,
          lookupOccEnv_WithFields, lookupFieldsOccEnv,
          mkOccEnv, mkOccEnv_C, extendOccEnvList, elemOccEnv,
          nonDetOccEnvElts, nonDetFoldOccEnv,
          plusOccEnv, plusOccEnv_C,
          extendOccEnv_Acc, filterOccEnv, delListFromOccEnv, delFromOccEnv,
          alterOccEnv, minusOccEnv, minusOccEnv_C, minusOccEnv_C_Ns,
          sizeOccEnv,
          pprOccEnv, forceOccEnv,
          intersectOccEnv_C,
  
          -- * The 'OccSet' type
          OccSet, emptyOccSet, unitOccSet, mkOccSet, extendOccSet,
          extendOccSetList,
          unionOccSets, unionManyOccSets, elemOccSet,
          isEmptyOccSet,
  
          -- * Dealing with main
          mainOcc, ppMainFn,
  
          -- * Tidying up
          TidyOccEnv, emptyTidyOccEnv, initTidyOccEnv, trimTidyOccEnv,
          tidyOccName, avoidClashesOccEnv, delTidyOccEnvList,
  
          -- FsEnv
          FastStringEnv, emptyFsEnv, lookupFsEnv, extendFsEnv, mkFsEnv
      ) where
  
  import GHC.Prelude
  
  import GHC.Builtin.Uniques
  import GHC.Utils.Misc
  import GHC.Types.Unique
  import GHC.Types.Unique.FM
  import GHC.Types.Unique.Set
  import GHC.Data.FastString
  import GHC.Data.FastString.Env
  import GHC.Utils.Outputable
  import GHC.Utils.Lexeme
  import GHC.Utils.Binary
  import GHC.Utils.Panic.Plain
  
  import Control.DeepSeq
  import Data.Char
  import Data.Data
  import qualified Data.Semigroup as S
  import GHC.Exts( Int(I#), dataToTag# )
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection{Name space}
  *                                                                      *
  ************************************************************************
  -}
  
  data NameSpace
    -- | Variable name space (including "real" data constructors).
    = VarName
    -- | Record field namespace for the given record.
    | FldName
      { fldParent :: !FastString
        -- ^ The textual name of the parent of the field.
        --
        --   - For a field of a datatype, this is the name of the first constructor
        --     of the datatype (regardless of whether this constructor has this field).
        --   - For a field of a pattern synonym, this is the name of the pattern synonym.
      }
    -- | "Source" data constructor namespace.
    | DataName
    -- | Type variable namespace.
    | TvName
    -- | Type constructor and class namespace.
    | TcClsName
      -- Haskell has type constructors and classes in the same namespace, for now.
     deriving Eq
  
  instance Ord NameSpace where
    compare ns1 ns2 =
      case compare (I# (dataToTag# ns1)) (I# (dataToTag# ns2)) of
        LT -> LT
        GT -> GT
        EQ
          | FldName { fldParent = p1 } <- ns1
          , FldName { fldParent = p2 } <- ns2
          -> lexicalCompareFS p1 p2
          | otherwise
          -> EQ
  
  instance Uniquable NameSpace where
    getUnique (FldName fs) = mkFldNSUnique  fs
    getUnique VarName      = varNSUnique
    getUnique DataName     = dataNSUnique
    getUnique TvName       = tvNSUnique
    getUnique TcClsName    = tcNSUnique
  
  instance NFData NameSpace where
    rnf VarName = ()
    rnf (FldName par) = rnf par
    rnf DataName = ()
    rnf TvName = ()
    rnf TcClsName = ()
  
  {-
  Note [Data Constructors]
  ~~~~~~~~~~~~~~~~~~~~~~~~
  see also: Note [Data Constructor Naming] in GHC.Core.DataCon
  
  $real_vs_source_data_constructors
  There are two forms of data constructor:
  
       [Source data constructors] The data constructors mentioned in Haskell source code
  
       [Real data constructors] The data constructors of the representation type, which may not be the same as the source type
  
  For example:
  
  > data T = T !(Int, Int)
  
  The source datacon has type @(Int, Int) -> T@
  The real   datacon has type @Int -> Int -> T@
  
  GHC chooses a representation based on the strictness etc.
  
  Note [Record field namespacing]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Record fields have a separate namespace from variables, to support
  DuplicateRecordFields, e.g. in
  
    data X = MkX { fld :: Int }
    data Y = MkY { fld :: Bool }
  
    f x = x { fld = 3 }
    g y = y { fld = False }
  
  we want the two occurrences of "fld" to refer to the field names associated with
  the corresponding data type.
  
  The namespace for a record field is as follows:
  
    - for a data type, it is the textual name of the first constructor of the
      datatype, whether this constructor has this field or not;
    - for a pattern synonym, it is the textual name of the pattern synonym itself.
  
  Record fields are initially parsed as variables, but the renamer resolves their
  namespace in GHC.Rename.Names.newRecordFieldLabel, which is called when renaming
  record data declarations and record pattern synonym declarations.
  
  To illustrate the namespacing, consider the record field "fld" in the following datatype
  
    data instance A Int Bool Char
      = MkA1 | MkA2 { fld :: Int } | MkA3 { bar :: Bool, fld :: Int }
  
  Its namespace is `FldName "MkA1"`. This is a convention used throughout GHC
  to circumvent the fact that we don't have a way to refer to the type constructor
  "A Int Bool Char" in the renamer, as data family instances only get given
  'Name's in the typechecker.
  -}
  
  tcName, clsName, tcClsName :: NameSpace
  dataName, srcDataName      :: NameSpace
  tvName, varName            :: NameSpace
  
  -- Though type constructors and classes are in the same name space now,
  -- the NameSpace type is abstract, so we can easily separate them later
  tcName    = TcClsName           -- Type constructors
  clsName   = TcClsName           -- Classes
  tcClsName = TcClsName           -- Not sure which!
  
  dataName    = DataName
  srcDataName = DataName  -- Haskell-source data constructors should be
                          -- in the Data name space
  
  tvName      = TvName
  varName     = VarName
  
  fieldName :: FastString -> NameSpace
  fieldName = FldName
  
  isDataConNameSpace :: NameSpace -> Bool
  isDataConNameSpace DataName = True
  isDataConNameSpace _        = False
  
  isTcClsNameSpace :: NameSpace -> Bool
  isTcClsNameSpace TcClsName = True
  isTcClsNameSpace _         = False
  
  isTvNameSpace :: NameSpace -> Bool
  isTvNameSpace TvName = True
  isTvNameSpace _      = False
  
  isVarNameSpace :: NameSpace -> Bool     -- Variables or type variables, but not constructors
  isVarNameSpace TvName       = True
  isVarNameSpace VarName      = True
  isVarNameSpace (FldName {}) = True
  isVarNameSpace _            = False
  
  -- | Is this a term variable or field name namespace?
  isTermVarOrFieldNameSpace :: NameSpace -> Bool
  isTermVarOrFieldNameSpace VarName      = True
  isTermVarOrFieldNameSpace (FldName {}) = True
  isTermVarOrFieldNameSpace _            = False
  
  isValNameSpace :: NameSpace -> Bool
  isValNameSpace DataName     = True
  isValNameSpace VarName      = True
  isValNameSpace (FldName {}) = True
  isValNameSpace _            = False
  
  isFieldNameSpace :: NameSpace -> Bool
  isFieldNameSpace (FldName {}) = True
  isFieldNameSpace _            = False
  
  pprNameSpace :: NameSpace -> SDoc
  pprNameSpace DataName    = text "data constructor"
  pprNameSpace VarName     = text "variable"
  pprNameSpace TvName      = text "type variable"
  pprNameSpace TcClsName   = text "type constructor or class"
  pprNameSpace (FldName p) = text "record field of" <+> ftext p
  
  pprNonVarNameSpace :: NameSpace -> SDoc
  pprNonVarNameSpace VarName = empty
  pprNonVarNameSpace ns = pprNameSpace ns
  
  pprNameSpaceBrief :: NameSpace -> SDoc
  pprNameSpaceBrief DataName     = char 'd'
  pprNameSpaceBrief VarName      = char 'v'
  pprNameSpaceBrief TvName       = text "tv"
  pprNameSpaceBrief TcClsName    = text "tc"
  pprNameSpaceBrief (FldName {}) = text "fld"
  
  -- | 'demoteNameSpace' lowers the 'NameSpace' to the term-level, if possible.
  --
  -- See Note [Demotion] in GHC.Rename.Env.
  demoteNameSpace :: NameSpace -> Maybe NameSpace
  demoteNameSpace VarName = Nothing
  demoteNameSpace DataName = Nothing
  demoteNameSpace TvName = Just VarName
  demoteNameSpace TcClsName = Just DataName
  demoteNameSpace (FldName {}) = Nothing
  
  demoteTcClsNameSpace :: NameSpace -> Maybe NameSpace
  demoteTcClsNameSpace VarName = Nothing
  demoteTcClsNameSpace DataName = Nothing
  demoteTcClsNameSpace TvName = Nothing
  demoteTcClsNameSpace TcClsName = Just DataName
  demoteTcClsNameSpace (FldName {}) = Nothing
  
  -- demoteTvNameSpace lowers the NameSpace of a type variable.
  -- See Note [Demotion] in GHC.Rename.Env.
  demoteTvNameSpace :: NameSpace -> Maybe NameSpace
  demoteTvNameSpace TvName = Just VarName
  demoteTvNameSpace VarName = Nothing
  demoteTvNameSpace DataName = Nothing
  demoteTvNameSpace TcClsName = Nothing
  demoteTvNameSpace (FldName {}) = Nothing
  
  -- promoteNameSpace promotes the NameSpace as follows.
  -- See Note [Promotion] in GHC.Rename.Env.
  promoteNameSpace :: NameSpace -> Maybe NameSpace
  promoteNameSpace DataName = Just TcClsName
  promoteNameSpace VarName = Just TvName
  promoteNameSpace TcClsName = Nothing
  promoteNameSpace TvName = Nothing
  promoteNameSpace (FldName {}) = Nothing
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection[Name-pieces-datatypes]{The @OccName@ datatypes}
  *                                                                      *
  ************************************************************************
  -}
  
  -- | Occurrence Name
  --
  -- In this context that means:
  -- "classified (i.e. as a type name, value name, etc) but not qualified
  -- and not yet resolved"
  data OccName = OccName
      { occNameSpace  :: !NameSpace
      , occNameFS     :: !FastString
      }
  
  instance Eq OccName where
      (OccName sp1 s1) == (OccName sp2 s2) = s1 == s2 && sp1 == sp2
  
  instance Ord OccName where
          -- Compares lexicographically, *not* by Unique of the string
      compare (OccName sp1 s1) (OccName sp2 s2) =
        lexicalCompareFS s1 s2 S.<> compare sp1 sp2
  
  instance Data OccName where
    -- don't traverse?
    toConstr _   = abstractConstr "OccName"
    gunfold _ _  = error "gunfold"
    dataTypeOf _ = mkNoRepType "OccName"
  
  instance HasOccName OccName where
    occName = id
  
  instance NFData OccName where
    rnf x = x `seq` ()
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection{Printing}
  *                                                                      *
  ************************************************************************
  -}
  
  instance Outputable OccName where
      ppr = pprOccName
  
  instance OutputableBndr OccName where
      pprBndr _ = ppr
      pprInfixOcc n = pprInfixVar (isSymOcc n) (ppr n)
      pprPrefixOcc n = pprPrefixVar (isSymOcc n) (ppr n)
  
  pprOccName :: IsLine doc => OccName -> doc
  pprOccName (OccName sp occ)
    = docWithStyle (ztext (zEncodeFS occ))
      (\_ -> ftext occ <> whenPprDebug (braces (pprNameSpaceBrief sp)))
  {-# SPECIALIZE pprOccName :: OccName -> SDoc #-}
  {-# SPECIALIZE pprOccName :: OccName -> HLine #-} -- see Note [SPECIALIZE to HDoc] in GHC.Utils.Outputable
  
  -- | Mangle field names to avoid duplicate symbols.
  --
  -- See Note [Mangling OccNames].
  occNameMangledFS :: OccName -> FastString
  occNameMangledFS (OccName ns fs) =
    case ns of
      -- Fields need to include the constructor, to ensure that we don't define
      -- duplicate symbols when using DuplicateRecordFields.
      FldName con -> concatFS [fsLit "$fld:", con, ":", fs]
      -- Otherwise, we can ignore the namespace, as there is no risk of name
      -- clashes.
      _           -> fs
  
  {- Note [Mangling OccNames]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~
  When generating a symbol for a Name, we usually discard the NameSpace entirely
  (see GHC.Types.Name.pprName). This is because clashes are usually not possible,
  e.g. a variable and a data constructor can't clash because data constructors
  start with a capital letter or a colon, while variables never do.
  
  However, record field names, in the presence of DuplicateRecordFields, need this
  disambiguation. So, for a record field like
  
    data A = MkA { foo :: Int }
  
  we generate the symbol $fld:MkA:foo. We use the constructor 'MkA' to disambiguate,
  and not the TyCon A as one might naively expect: this is explained in
  Note [Record field namespacing].
  -}
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection{Construction}
  *                                                                      *
  ************************************************************************
  -}
  
  mkOccName :: NameSpace -> String -> OccName
  mkOccName occ_sp str = OccName occ_sp (mkFastString str)
  
  mkOccNameFS :: NameSpace -> FastString -> OccName
  mkOccNameFS occ_sp fs = OccName occ_sp fs
  
  mkVarOcc :: String -> OccName
  mkVarOcc s = mkOccName varName s
  
  mkVarOccFS :: FastString -> OccName
  mkVarOccFS fs = mkOccNameFS varName fs
  
  mkRecFieldOcc :: FastString -> String -> OccName
  mkRecFieldOcc dc = mkOccName (fieldName dc)
  
  mkRecFieldOccFS :: FastString -> FastString -> OccName
  mkRecFieldOccFS dc = mkOccNameFS (fieldName dc)
  
  varToRecFieldOcc :: HasDebugCallStack => FastString -> OccName -> OccName
  varToRecFieldOcc dc (OccName ns s) =
    assert makes_sense $ mkRecFieldOccFS dc s
      where
        makes_sense = case ns of
          VarName    -> True
          FldName {} -> True
            -- NB: it's OK to change the parent data constructor,
            -- see e.g. test T23220 in which we construct with TH
            -- a datatype using the fields of a different datatype.
          _          -> False
  
  recFieldToVarOcc :: HasDebugCallStack => OccName -> OccName
  recFieldToVarOcc (OccName _ns s) = mkVarOccFS s
  
  mkDataOcc :: String -> OccName
  mkDataOcc = mkOccName dataName
  
  mkDataOccFS :: FastString -> OccName
  mkDataOccFS = mkOccNameFS dataName
  
  mkTyVarOcc :: String -> OccName
  mkTyVarOcc = mkOccName tvName
  
  mkTyVarOccFS :: FastString -> OccName
  mkTyVarOccFS fs = mkOccNameFS tvName fs
  
  mkTcOcc :: String -> OccName
  mkTcOcc = mkOccName tcName
  
  mkTcOccFS :: FastString -> OccName
  mkTcOccFS = mkOccNameFS tcName
  
  mkClsOcc :: String -> OccName
  mkClsOcc = mkOccName clsName
  
  mkClsOccFS :: FastString -> OccName
  mkClsOccFS = mkOccNameFS clsName
  
  -- demoteOccName lowers the Namespace of OccName.
  -- See Note [Demotion] in GHC.Rename.Env.
  demoteOccName :: OccName -> Maybe OccName
  demoteOccName (OccName space name) = do
    space' <- demoteNameSpace space
    return $ OccName space' name
  
  demoteOccTcClsName :: OccName -> Maybe OccName
  demoteOccTcClsName (OccName space name) = do
    space' <- demoteTcClsNameSpace space
    return $ OccName space' name
  
  demoteOccTvName :: OccName -> Maybe OccName
  demoteOccTvName (OccName space name) = do
    space' <- demoteTvNameSpace space
    return $ OccName space' name
  
  -- promoteOccName promotes the NameSpace of OccName.
  -- See Note [Promotion] in GHC.Rename.Env.
  promoteOccName :: OccName -> Maybe OccName
  promoteOccName (OccName space name) = do
    promoted_space <- promoteNameSpace space
    let tyop   = isTvNameSpace promoted_space && isLexVarSym name
        space' = if tyop then tcClsName else promoted_space   -- special case for type operators (#24570)
    return $ OccName space' name
  
  {- | Other names in the compiler add additional information to an OccName.
  This class provides a consistent way to access the underlying OccName. -}
  class HasOccName name where
    occName :: name -> OccName
  
  {-
  ************************************************************************
  *                                                                      *
                  Environments
  *                                                                      *
  ************************************************************************
  
  OccEnvs are used for the GlobalRdrEnv and for the envts in ModIface.
  
  Note [OccEnv]
  ~~~~~~~~~~~~~
  An OccEnv is a map keyed on OccName. Recall that an OccEnv consists of two
  components:
  
    - a namespace,
    - a textual name (in the form of a FastString).
  
  In general, for a given textual name, there is only one appropriate namespace.
  However, sometimes we do get an occurrence that belongs to several namespaces:
  
    - Symbolic identifiers such as (:+) can belong to both the data constructor and
      type constructor/class namespaces.
    - With duplicate record fields, a field name can belong to several different
      namespaces, one for each parent datatype (or pattern synonym).
  
  So we represent an OccEnv as a nested data structure
  
    FastStringEnv (UniqFM NameSpace a)
  
  in which we can first look up the textual name, and then choose which of the
  namespaces are relevant. This supports the two main uses of OccEnvs:
  
    1. One wants to look up a specific OccName in the environment, at a specific
       namespace. One looks up the textual name, and then the namespace.
    2. One wants to look up something, but isn't sure in advance of the namespace.
       So one looks up the textual name, and then can decide what to do based on
       the returned map of namespaces.
  
  This data structure isn't performance critical in most situations, but some
  improvements to its performance that might be worth it are as follows:
  
    A. Use a tailor-made data structure for a map keyed on NameSpaces.
  
       Recall that we have:
  
          data IntMap a = Bin !Int !Int !(IntMap a) !(IntMap a)
                        | Tip !Key a
                        | Nil
  
       This is already pretty efficient for singletons, but we don't need the
       empty case (as we would simply omit the parent key in the OccEnv instead
       of storing an empty inner map).
  
    B. Always ensure the inner map (keyed on namespaces) is evaluated, i.e.
       is never a thunk. For this, we would need to use strict operations on
       the outer FastStringEnv (but we'd keep using lazy operations on the inner
       UniqFM).
  -}
  
  -- | A map keyed on 'OccName'. See Note [OccEnv].
  newtype OccEnv a = MkOccEnv (FastStringEnv (UniqFM NameSpace a))
    deriving Functor
  
  -- | The empty 'OccEnv'.
  emptyOccEnv :: OccEnv a
  emptyOccEnv = MkOccEnv emptyFsEnv
  
  -- | A singleton 'OccEnv'.
  unitOccEnv :: OccName -> a -> OccEnv a
  unitOccEnv (OccName ns s) a = MkOccEnv $ unitFsEnv s (unitUFM ns a)
  
  -- | Add a single element to an 'OccEnv'.
  extendOccEnv :: OccEnv a -> OccName -> a -> OccEnv a
  extendOccEnv (MkOccEnv as) (OccName ns s) a =
    MkOccEnv $ extendFsEnv_C plusUFM as s (unitUFM ns a)
  
  -- | Extend an 'OccEnv' by a list.
  --
  -- 'OccName's later on in the list override earlier 'OccName's.
  extendOccEnvList :: OccEnv a -> [(OccName, a)] -> OccEnv a
  extendOccEnvList = foldl' $ \ env (occ, a) -> extendOccEnv env occ a
  
  -- | Look an element up in an 'OccEnv'.
  lookupOccEnv :: OccEnv a -> OccName -> Maybe a
  lookupOccEnv (MkOccEnv as) (OccName ns s)
    = do { m <- lookupFsEnv as s
         ; lookupUFM m ns }
  
  -- | Lookup an element in an 'OccEnv', ignoring 'NameSpace's entirely.
  lookupOccEnv_AllNameSpaces :: OccEnv a -> OccName -> [a]
  lookupOccEnv_AllNameSpaces (MkOccEnv as) (OccName _ s)
    = case lookupFsEnv as s of
        Nothing -> []
        Just r  -> nonDetEltsUFM r
  
  -- | Lookup an element in an 'OccEnv', looking in the record field
  -- namespace for a variable.
  lookupOccEnv_WithFields :: OccEnv a -> OccName -> [a]
  lookupOccEnv_WithFields env occ =
    case lookupOccEnv env occ of
        Nothing  -> fieldGREs
        Just gre -> gre : fieldGREs
    where
      fieldGREs
        -- If the 'OccName' is a variable, also look up
        -- in the record field namespaces.
        | isVarOcc occ
        = lookupFieldsOccEnv env (occNameFS occ)
        | otherwise
        = []
  
  -- | Look up all the record fields that match with the given 'FastString'
  -- in an 'OccEnv'.
  lookupFieldsOccEnv :: OccEnv a -> FastString -> [a]
  lookupFieldsOccEnv (MkOccEnv as) fld =
    case lookupFsEnv as fld of
      Nothing   -> []
      Just flds -> nonDetEltsUFM $ filter_flds flds
    -- NB: non-determinism is OK: in practice we will either end up resolving
    -- to a single field or throwing an error.
    where
      filter_flds = filterUFM_Directly (\ uniq _ -> isFldNSUnique uniq)
  
  -- | Create an 'OccEnv' from a list.
  --
  -- 'OccName's later on in the list override earlier 'OccName's.
  mkOccEnv :: [(OccName,a)] -> OccEnv a
  mkOccEnv = extendOccEnvList emptyOccEnv
  
  -- | Create an 'OccEnv' from a list, combining different values
  -- with the same 'OccName' using the combining function.
  mkOccEnv_C :: (a -> a -> a) -- ^ old -> new -> result
             -> [(OccName,a)]
             -> OccEnv a
  mkOccEnv_C f elts
    = MkOccEnv $ foldl' g emptyFsEnv elts
      where
        g env (OccName ns s, a) =
          extendFsEnv_C (plusUFM_C $ flip f) env s (unitUFM ns a)
  
  -- | Compute whether there is a value keyed by the given 'OccName'.
  elemOccEnv :: OccName -> OccEnv a -> Bool
  elemOccEnv (OccName ns s) (MkOccEnv as)
    = case lookupFsEnv as s of
        Nothing -> False
        Just m  -> ns `elemUFM` m
  
  -- | Fold over an 'OccEnv'. Non-deterministic, unless the folding function
  -- is commutative (i.e. @a1 `f` ( a2 `f` b ) == a2 `f` ( a1 `f` b )@ for all @a1@, @a2@, @b@).
  nonDetFoldOccEnv :: (a -> b -> b) -> b -> OccEnv a -> b
  nonDetFoldOccEnv f b0 (MkOccEnv as) =
    nonDetFoldFsEnv (flip $ nonDetFoldUFM f) b0 as
  
  -- | Obtain the elements of an 'OccEnv'.
  --
  -- The resulting order is non-deterministic.
  nonDetOccEnvElts :: OccEnv a -> [a]
  nonDetOccEnvElts = nonDetFoldOccEnv (:) []
  
  -- | Union of two 'OccEnv's, right-biased.
  plusOccEnv :: OccEnv a -> OccEnv a -> OccEnv a
  plusOccEnv (MkOccEnv env1) (MkOccEnv env2)
    = MkOccEnv $ plusFsEnv_C plusUFM env1 env2
  
  -- | Union of two 'OccEnv's with a combining function.
  plusOccEnv_C :: (a->a->a) -> OccEnv a -> OccEnv a -> OccEnv a
  plusOccEnv_C f (MkOccEnv env1) (MkOccEnv env2)
    = MkOccEnv $ plusFsEnv_C (plusUFM_C f) env1 env2
  
  -- | Map over an 'OccEnv' ('Functor' instance).
  mapOccEnv :: (a->b) -> OccEnv a -> OccEnv b
  mapOccEnv = fmap
  
  -- | 'mapMaybe' for b 'OccEnv'.
  mapMaybeOccEnv :: (a -> Maybe b) -> OccEnv a -> OccEnv b
  mapMaybeOccEnv f (MkOccEnv env)
    = MkOccEnv $ mapMaybeUFM g env
      where
        g as =
          case mapMaybeUFM f as of
            m' | isNullUFM m' -> Nothing
               | otherwise    -> Just m'
  
  -- | Add a single element to an 'OccEnv', using a different function whether
  -- the 'OccName' already exists or not.
  extendOccEnv_Acc :: forall a b
                   .  (a->b->b)    -- ^ add to existing
                   -> (a->b)       -- ^ new element
                   -> OccEnv b     -- ^ old
                   -> OccName -> a -- ^ new
                   -> OccEnv b
  extendOccEnv_Acc f g (MkOccEnv env) (OccName ns s) =
    MkOccEnv . extendFsEnv_Acc f' g' env s
      where
       f' :: a -> UniqFM NameSpace b -> UniqFM NameSpace b
       f' a bs = alterUFM (Just . \ case { Nothing -> g a ; Just b -> f a b }) bs ns
       g' a = unitUFM ns (g a)
  
  -- | Delete one element from an 'OccEnv'.
  delFromOccEnv :: forall a. OccEnv a -> OccName -> OccEnv a
  delFromOccEnv (MkOccEnv env1) (OccName ns s) =
    MkOccEnv $ alterFsEnv f env1 s
      where
        f :: Maybe (UniqFM NameSpace a) -> Maybe (UniqFM NameSpace a)
        f Nothing = Nothing
        f (Just m) =
          case delFromUFM m ns of
            m' | isNullUFM m' -> Nothing
               | otherwise    -> Just m'
  
  -- | Delete multiple elements from an 'OccEnv'.
  delListFromOccEnv :: OccEnv a -> [OccName] -> OccEnv a
  delListFromOccEnv = foldl' delFromOccEnv
  
  -- | Filter out all elements in an 'OccEnv' using a predicate.
  filterOccEnv :: forall a. (a -> Bool) -> OccEnv a -> OccEnv a
  filterOccEnv f (MkOccEnv env) =
    MkOccEnv $ mapMaybeFsEnv g env
      where
        g :: UniqFM NameSpace a -> Maybe (UniqFM NameSpace a)
        g ms =
          case filterUFM f ms of
            m' | isNullUFM m' -> Nothing
               | otherwise    -> Just m'
  
  -- | Alter an 'OccEnv', adding or removing an element at the given key.
  alterOccEnv :: forall a. (Maybe a -> Maybe a) -> OccEnv a -> OccName -> OccEnv a
  alterOccEnv f (MkOccEnv env) (OccName ns s) =
    MkOccEnv $ alterFsEnv g env s
      where
        g :: Maybe (UniqFM NameSpace a) -> Maybe (UniqFM NameSpace a)
        g Nothing  = fmap (unitUFM ns) (f Nothing)
        g (Just m) =
          case alterUFM f m ns of
            m' | isNullUFM m' -> Nothing
               | otherwise    -> Just m'
  
  intersectOccEnv_C :: (a -> b -> c) -> OccEnv a -> OccEnv b -> OccEnv c
  intersectOccEnv_C f (MkOccEnv as) (MkOccEnv bs)
    = MkOccEnv $ intersectUFM_C (intersectUFM_C f) as bs
  
  -- | Remove elements of the first 'OccEnv' that appear in the second 'OccEnv'.
  minusOccEnv :: OccEnv a -> OccEnv b -> OccEnv a
  minusOccEnv = minusOccEnv_C_Ns minusUFM
  
  -- | Alters (replaces or removes) those elements of the first 'OccEnv' that are
  -- mentioned in the second 'OccEnv'.
  --
  -- Same idea as 'Data.Map.differenceWith'.
  minusOccEnv_C :: (a -> b -> Maybe a)
                -> OccEnv a -> OccEnv b -> OccEnv a
  minusOccEnv_C f = minusOccEnv_C_Ns (minusUFM_C f)
  
  minusOccEnv_C_Ns :: forall a b
                   .  (UniqFM NameSpace a -> UniqFM NameSpace b -> UniqFM NameSpace a)
                   -> OccEnv a -> OccEnv b -> OccEnv a
  minusOccEnv_C_Ns f (MkOccEnv as) (MkOccEnv bs) =
    MkOccEnv $ minusUFM_C g as bs
      where
        g :: UniqFM NameSpace a -> UniqFM NameSpace b -> Maybe (UniqFM NameSpace a)
        g as bs =
          let m = f as bs
          in if isNullUFM m
             then Nothing
             else Just m
  
  sizeOccEnv :: OccEnv a -> Int
  sizeOccEnv (MkOccEnv as) =
    nonDetStrictFoldUFM (\ m !acc -> acc + sizeUFM m) 0 as
  
  instance Outputable a => Outputable (OccEnv a) where
      ppr x = pprOccEnv ppr x
  
  pprOccEnv :: (a -> SDoc) -> OccEnv a -> SDoc
  pprOccEnv ppr_elt (MkOccEnv env)
      = brackets $ fsep $ punctuate comma $
      [ ppr uq <+> text ":->" <+> ppr_elt elt
      | (uq, elts) <- nonDetUFMToList env
      , elt <- nonDetEltsUFM elts ]
  
  instance NFData a => NFData (OccEnv a) where
    rnf = forceOccEnv rnf
  
  -- | Map over an 'OccEnv' strictly.
  strictMapOccEnv :: (a -> b) -> OccEnv a -> OccEnv b
  strictMapOccEnv f (MkOccEnv as) =
    MkOccEnv $ strictMapFsEnv (strictMapUFM f) as
  
  -- | Force an 'OccEnv' with the provided function.
  forceOccEnv :: (a -> ()) -> OccEnv a -> ()
  forceOccEnv nf (MkOccEnv fs) = seqEltsUFM (seqEltsUFM nf) fs
  
  --------------------------------------------------------------------------------
  
  newtype OccSet = OccSet (FastStringEnv (UniqSet NameSpace))
  
  emptyOccSet       :: OccSet
  unitOccSet        :: OccName -> OccSet
  mkOccSet          :: [OccName] -> OccSet
  extendOccSet      :: OccSet -> OccName -> OccSet
  extendOccSetList  :: OccSet -> [OccName] -> OccSet
  unionOccSets      :: OccSet -> OccSet -> OccSet
  unionManyOccSets  :: [OccSet] -> OccSet
  elemOccSet        :: OccName -> OccSet -> Bool
  isEmptyOccSet     :: OccSet -> Bool
  
  emptyOccSet       = OccSet emptyFsEnv
  unitOccSet (OccName ns s) = OccSet $ unitFsEnv s (unitUniqSet ns)
  mkOccSet          = extendOccSetList emptyOccSet
  extendOccSet      (OccSet occs) (OccName ns s) = OccSet $ extendFsEnv occs s (unitUniqSet ns)
  extendOccSetList  = foldl' extendOccSet
  unionOccSets      (OccSet xs) (OccSet ys) = OccSet $ plusFsEnv_C unionUniqSets xs ys
  unionManyOccSets  = foldl' unionOccSets emptyOccSet
  elemOccSet (OccName ns s) (OccSet occs) = maybe False (elementOfUniqSet ns) $ lookupFsEnv occs s
  isEmptyOccSet     (OccSet occs) = isNullUFM occs
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection{Predicates and taking them apart}
  *                                                                      *
  ************************************************************************
  -}
  
  occNameString :: OccName -> String
  occNameString (OccName _ s) = unpackFS s
  
  setOccNameSpace :: NameSpace -> OccName -> OccName
  setOccNameSpace sp (OccName _ occ) = OccName sp occ
  
  isVarOcc, isTvOcc, isTcOcc, isDataOcc, isFieldOcc :: OccName -> Bool
  
  isVarOcc (OccName VarName _) = True
  isVarOcc _                   = False
  
  isTvOcc (OccName TvName _) = True
  isTvOcc _                  = False
  
  isTcOcc (OccName TcClsName _) = True
  isTcOcc _                     = False
  
  isFieldOcc (OccName (FldName {}) _) = True
  isFieldOcc _                        = False
  
  fieldOcc_maybe :: OccName -> Maybe FastString
  fieldOcc_maybe (OccName (FldName con) _) = Just con
  fieldOcc_maybe _                         = Nothing
  
  -- | /Value/ 'OccNames's are those that are either in
  -- the variable, field name or data constructor namespaces
  isValOcc :: OccName -> Bool
  isValOcc (OccName VarName      _) = True
  isValOcc (OccName DataName     _) = True
  isValOcc (OccName (FldName {}) _) = True
  isValOcc _                        = False
  
  isDataOcc (OccName DataName _) = True
  isDataOcc _                    = False
  
  -- | Test if the 'OccName' is a data constructor that starts with
  -- a symbol (e.g. @:@, or @[]@)
  isDataSymOcc :: OccName -> Bool
  isDataSymOcc (OccName DataName s) = isLexConSym s
  isDataSymOcc _                    = False
  -- Pretty inefficient!
  
  -- | Test if the 'OccName' is that for any operator (whether
  -- it is a data constructor or variable or whatever)
  isSymOcc :: OccName -> Bool
  isSymOcc (OccName ns s) = case ns of
    DataName   -> isLexConSym s
    TcClsName  -> isLexSym s
    VarName    -> isLexSym s
    TvName     -> isLexSym s
    FldName {} -> isLexSym s
  -- Pretty inefficient!
  
  parenSymOcc :: OccName -> SDoc -> SDoc
  -- ^ Wrap parens around an operator
  parenSymOcc occ doc | isSymOcc occ = parens doc
                      | otherwise    = doc
  
  startsWithUnderscore :: OccName -> Bool
  -- ^ Haskell 98 encourages compilers to suppress warnings about unused
  -- names in a pattern if they start with @_@: this implements that test
  startsWithUnderscore occ = case unpackFS (occNameFS occ) of
    '_':_ -> True
    _     -> False
  
  isUnderscore :: OccName -> Bool
  isUnderscore occ = occNameFS occ == fsLit "_"
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection{Making system names}
  *                                                                      *
  ************************************************************************
  
  Here's our convention for splitting up the interface file name space:
  
     d...         dictionary identifiers
                  (local variables, so no name-clash worries)
  
  All of these other OccNames contain a mixture of alphabetic
  and symbolic characters, and hence cannot possibly clash with
  a user-written type or function name
  
     $f...        Dict-fun identifiers (from inst decls)
     $dmop        Default method for 'op'
     $pnC         n'th superclass selector for class C
     $wf          Worker for function 'f'
     $sf..        Specialised version of f
     D:C          Data constructor for dictionary for class C
     NTCo:T       Coercion connecting newtype T with its representation type
     TFCo:R       Coercion connecting a data family to its representation type R
  
  In encoded form these appear as Zdfxxx etc
  
          :...            keywords (export:, letrec: etc.)
  --- I THINK THIS IS WRONG!
  
  This knowledge is encoded in the following functions.
  
  @mk_deriv@ generates an @OccName@ from the prefix and a string.
  NB: The string must already be encoded!
  -}
  
  -- | Build an 'OccName' derived from another 'OccName'.
  --
  -- Note that the pieces of the name are passed in as a @[FastString]@ so that
  -- the whole name can be constructed with a single 'concatFS', minimizing
  -- unnecessary intermediate allocations.
  mk_deriv :: NameSpace
           -> FastString      -- ^ A prefix which distinguishes one sort of
                              -- derived name from another
           -> [FastString]    -- ^ The name we are deriving from in pieces which
                              -- will be concatenated.
           -> OccName
  mk_deriv occ_sp sys_prefix str =
      mkOccNameFS occ_sp (concatFS $ sys_prefix : str)
  
  isDerivedOccName :: OccName -> Bool
  -- ^ Test for definitions internally generated by GHC.  This predicate
  -- is used to suppress printing of internal definitions in some debug prints
  isDerivedOccName occ =
     case occNameString occ of
       '$':c:_ | isAlphaNum c -> True   -- E.g.  $wfoo
       c:':':_ | isAlphaNum c -> True   -- E.g.  N:blah   newtype coercions
       _other                 -> False
  
  isDefaultMethodOcc :: OccName -> Bool
  isDefaultMethodOcc occ =
     case occNameString occ of
       '$':'d':'m':_ -> True
       _ -> False
  
  -- | Is an 'OccName' one of a Typeable @TyCon@ or @Module@ binding?
  -- This is needed as these bindings are renamed differently.
  -- See Note [Grand plan for Typeable] in "GHC.Tc.Instance.Typeable".
  isTypeableBindOcc :: OccName -> Bool
  isTypeableBindOcc occ =
     case occNameString occ of
       '$':'t':'c':_ -> True  -- mkTyConRepOcc
       '$':'t':'r':_ -> True  -- Module binding
       _ -> False
  
  mkDataConWrapperOcc, mkWorkerOcc,
          mkMatcherOcc, mkBuilderOcc,
          mkDefaultMethodOcc,
          mkClassDataConOcc, mkDictOcc,
          mkIPOcc, mkSpecOcc, mkForeignExportOcc, mkRepEqOcc,
          mkGenR, mkGen1R,
          mkDataConWorkerOcc, mkNewTyCoOcc,
          mkInstTyCoOcc, mkEqPredCoOcc, mkClassOpAuxOcc,
          mkCon2TagOcc, mkTag2ConOcc, mkMaxTagOcc, mkDataTOcc, mkDataCOcc,
          mkTyConRepOcc
     :: OccName -> OccName
  
  -- These derived variables have a prefix that no Haskell value could have
  mkDataConWrapperOcc = mk_simple_deriv varName  "$W"
  mkWorkerOcc         = mk_simple_deriv varName  "$w"
  mkMatcherOcc        = mk_simple_deriv varName  "$m"
  mkBuilderOcc        = mk_simple_deriv varName  "$b"
  mkDefaultMethodOcc  = mk_simple_deriv varName  "$dm"
  mkClassOpAuxOcc     = mk_simple_deriv varName  "$c"
  mkDictOcc           = mk_simple_deriv varName  "$d"
  mkIPOcc             = mk_simple_deriv varName  "$i"
  mkSpecOcc           = mk_simple_deriv varName  "$s"
  mkForeignExportOcc  = mk_simple_deriv varName  "$f"
  mkRepEqOcc          = mk_simple_deriv tvName   "$r"   -- In RULES involving Coercible
  mkClassDataConOcc   = mk_simple_deriv dataName "C:"   -- Data con for a class
  mkNewTyCoOcc        = mk_simple_deriv tcName   "N:"   -- Coercion for newtypes
  mkInstTyCoOcc       = mk_simple_deriv tcName   "D:"   -- Coercion for type functions
  mkEqPredCoOcc       = mk_simple_deriv tcName   "$co"
  
  -- Used in derived instances for the names of auxiliary bindings.
  -- See Note [Auxiliary binders] in GHC.Tc.Deriv.Generate.
  mkCon2TagOcc        = mk_simple_deriv varName  "$con2tag_"
  mkTag2ConOcc        = mk_simple_deriv varName  "$tag2con_"
  mkMaxTagOcc         = mk_simple_deriv varName  "$maxtag_"
  mkDataTOcc          = mk_simple_deriv varName  "$t"
  mkDataCOcc          = mk_simple_deriv varName  "$c"
  
  -- TyConRepName stuff; see Note [Grand plan for Typeable] in GHC.Tc.Instance.Typeable
  mkTyConRepOcc occ = mk_simple_deriv varName prefix occ
    where
      prefix | isDataOcc occ = "$tc'"
             | otherwise     = "$tc"
  
  -- Generic deriving mechanism
  mkGenR   = mk_simple_deriv tcName "Rep_"
  mkGen1R  = mk_simple_deriv tcName "Rep1_"
  
  mk_simple_deriv :: NameSpace -> FastString -> OccName -> OccName
  mk_simple_deriv sp px occ = mk_deriv sp px [occNameFS occ]
  
  -- Data constructor workers are made by setting the name space
  -- of the data constructor OccName (which should be a DataName)
  -- to VarName
  mkDataConWorkerOcc datacon_occ = setOccNameSpace varName datacon_occ
  
  mkSuperDictAuxOcc :: Int -> OccName -> OccName
  mkSuperDictAuxOcc index cls_tc_occ
    = mk_deriv varName "$cp" [fsLit $ show index, occNameFS cls_tc_occ]
  
  mkSuperDictSelOcc :: Int        -- ^ Index of superclass, e.g. 3
                    -> OccName    -- ^ Class, e.g. @Ord@
                    -> OccName    -- ^ Derived 'Occname', e.g. @$p3Ord@
  mkSuperDictSelOcc index cls_tc_occ
    = mk_deriv varName "$p" [fsLit $ show index, occNameFS cls_tc_occ]
  
  mkLocalOcc :: Unique            -- ^ Unique to combine with the 'OccName'
             -> OccName           -- ^ Local name, e.g. @sat@
             -> OccName           -- ^ Nice unique version, e.g. @$L23sat@
  mkLocalOcc uniq occ
     = mk_deriv varName "$L" [fsLit $ show uniq, occNameFS occ]
          -- The Unique might print with characters
          -- that need encoding (e.g. 'z'!)
  
  -- | Derive a name for the representation type constructor of a
  -- @data@\/@newtype@ instance.
  mkInstTyTcOcc :: String                 -- ^ Family name, e.g. @Map@
                -> OccSet                 -- ^ avoid these Occs
                -> OccName                -- ^ @R:Map@
  mkInstTyTcOcc str = chooseUniqueOcc tcName ('R' : ':' : str)
  
  mkDFunOcc :: String             -- ^ Typically the class and type glommed together e.g. @OrdMaybe@.
                                  -- Only used in debug mode, for extra clarity
            -> Bool               -- ^ Is this a hs-boot instance DFun?
            -> OccSet             -- ^ avoid these Occs
            -> OccName            -- ^ E.g. @$f3OrdMaybe@
  
  -- In hs-boot files we make dict funs like $fx7ClsTy, which get bound to the real
  -- thing when we compile the mother module. Reason: we don't know exactly
  -- what the  mother module will call it.
  
  mkDFunOcc info_str is_boot set
    = chooseUniqueOcc VarName (prefix ++ info_str) set
    where
      prefix | is_boot   = "$fx"
             | otherwise = "$f"
  
  {-
  Sometimes we need to pick an OccName that has not already been used,
  given a set of in-use OccNames.
  -}
  
  chooseUniqueOcc :: NameSpace -> String -> OccSet -> OccName
  chooseUniqueOcc ns str set = loop (mkOccName ns str) (0::Int)
    where
    loop occ n
     | occ `elemOccSet` set = loop (mkOccName ns (str ++ show n)) (n+1)
     | otherwise            = occ
  
  {-
  We used to add a '$m' to indicate a method, but that gives rise to bad
  error messages from the type checker when we print the function name or pattern
  of an instance-decl binding.  Why? Because the binding is zapped
  to use the method name in place of the selector name.
  (See GHC.Tc.TyCl.Class.tcMethodBind)
  
  The way it is now, -ddump-xx output may look confusing, but
  you can always say -dppr-debug to get the uniques.
  
  However, we *do* have to zap the first character to be lower case,
  because overloaded constructors (blarg) generate methods too.
  And convert to VarName space
  
  e.g. a call to constructor MkFoo where
          data (Ord a) => Foo a = MkFoo a
  
  If this is necessary, we do it by prefixing '$m'.  These
  guys never show up in error messages.  What a hack.
  -}
  
  mkMethodOcc :: OccName -> OccName
  mkMethodOcc occ@(OccName VarName _) = occ
  mkMethodOcc occ                     = mk_simple_deriv varName "$m" occ
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection{Tidying them up}
  *                                                                      *
  ************************************************************************
  
  Before we print chunks of code we like to rename it so that
  we don't have to print lots of silly uniques in it.  But we mustn't
  accidentally introduce name clashes!  So the idea is that we leave the
  OccName alone unless it accidentally clashes with one that is already
  in scope; if so, we tack on '1' at the end and try again, then '2', and
  so on till we find a unique one.
  
  There's a wrinkle for operators.  Consider '>>='.  We can't use '>>=1'
  because that isn't a single lexeme.  So we encode it to 'lle' and *then*
  tack on the '1', if necessary.
  
  Note [TidyOccEnv]
  ~~~~~~~~~~~~~~~~~
  type TidyOccEnv = UniqFM FastString Int
  
  * Domain = The OccName's FastString. These FastStrings are "taken";
             make sure that we don't re-use
  
  * Int, n = A plausible starting point for new guesses
             There is no guarantee that "FSn" is available;
             you must look that up in the TidyOccEnv.  But
             it's a good place to start looking.
  
  * When looking for a renaming for "foo2" we strip off the "2" and start
    with "foo".  Otherwise if we tidy twice we get silly names like foo23.
  
    However, if it started with digits at the end, we always make a name
    with digits at the end, rather than shortening "foo2" to just "foo",
    even if "foo" is unused.  Reasons:
       - Plain "foo" might be used later
       - We use trailing digits to subtly indicate a unification variable
         in typechecker error message; see TypeRep.tidyTyVarBndr
  
  We have to take care though! Consider a machine-generated module (#10370)
    module Foo where
       a1 = e1
       a2 = e2
       ...
       a2000 = e2000
  Then "a1", "a2" etc are all marked taken.  But now if we come across "a7" again,
  we have to do a linear search to find a free one, "a2001".  That might just be
  acceptable once.  But if we now come across "a8" again, we don't want to repeat
  that search.
  
  So we use the TidyOccEnv mapping for "a" (not "a7" or "a8") as our base for
  starting the search; and we make sure to update the starting point for "a"
  after we allocate a new one.
  
  
  Note [Tidying multiple names at once]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Consider
  
      > :t (id,id,id)
  
  Every id contributes a type variable to the type signature, and all of them are
  "a". If we tidy them one by one, we get
  
      (id,id,id) :: (a2 -> a2, a1 -> a1, a -> a)
  
  which is a bit unfortunate, as it unfairly renames only two of them. What we
  would like to see is
  
      (id,id,id) :: (a3 -> a3, a2 -> a2, a1 -> a1)
  
  To achieve this, the function avoidClashesOccEnv can be used to prepare the
  TidyEnv, by “blocking” every name that occurs twice in the map. This way, none
  of the "a"s will get the privilege of keeping this name, and all of them will
  get a suitable number by tidyOccName.  Thus
  
     avoidNameClashesOccEnv ["a" :-> 7] ["b", "a", "c", "b", "a"]
       = ["a" :-> 7, "b" :-> 1]
  
  Here
  * "a" is already the TidyOccEnv, and so is unaffected
  * "b" occurs twice, so is blocked by adding "b" :-> 1
  * "c" occurs only once, and so is not affected.
  
  This prepared TidyEnv can then be used with tidyOccName. See tidyTyCoVarBndrs
  for an example where this is used.
  
  This is #12382.
  
  -}
  
  type TidyOccEnv = UniqFM FastString Int    -- The in-scope OccNames
    -- See Note [TidyOccEnv]
  
  emptyTidyOccEnv :: TidyOccEnv
  emptyTidyOccEnv = emptyUFM
  
  initTidyOccEnv :: [OccName] -> TidyOccEnv       -- Initialise with names to avoid!
  initTidyOccEnv = foldl' add emptyUFM
    where
      add env (OccName _ fs) = addToUFM env fs 1
  
  delTidyOccEnvList :: TidyOccEnv -> [OccName] -> TidyOccEnv
  delTidyOccEnvList env occs = env `delListFromUFM` map occNameFS occs
  
  -- see Note [Tidying multiple names at once]
  avoidClashesOccEnv :: TidyOccEnv -> [OccName] -> TidyOccEnv
  avoidClashesOccEnv env occs = go env emptyUFM occs
    where
      go env _        [] = env
      go env seenOnce ((OccName _ fs):occs)
        | fs `elemUFM` env      = go env seenOnce                  occs
        | fs `elemUFM` seenOnce = go (addToUFM env fs 1) seenOnce  occs
        | otherwise             = go env (addToUFM seenOnce fs ()) occs
  
  tidyOccName :: TidyOccEnv -> OccName -> (TidyOccEnv, OccName)
  tidyOccName env occ@(OccName occ_sp fs)
    | not (fs `elemUFM` env)
    = -- Desired OccName is free, so use it,
      -- and record in 'env' that it's no longer available
      (addToUFM env fs 1, occ)
  
    | otherwise
    = case lookupUFM env base1 of
         Nothing -> (addToUFM env base1 2, OccName occ_sp base1)
         Just n  -> find 1 n
    where
      base :: String  -- Drop trailing digits (see Note [TidyOccEnv])
      base  = dropWhileEndLE isDigit (unpackFS fs)
      base1 = mkFastString (base ++ "1")
  
      find !k !n
        = case elemUFM new_fs env of
            True -> find (k+1 :: Int) (n+k)
                         -- By using n+k, the n argument to find goes
                         --    1, add 1, add 2, add 3, etc which
                         -- moves at quadratic speed through a dense patch
  
            False -> (new_env, OccName occ_sp new_fs)
         where
           new_fs = mkFastString (base ++ show n)
           new_env = addToUFM (addToUFM env new_fs 1) base1 (n+1)
                       -- Update:  base1,  so that next time we'll start where we left off
                       --          new_fs, so that we know it is taken
                       -- If they are the same (n==1), the former wins
                       -- See Note [TidyOccEnv]
  
  trimTidyOccEnv :: TidyOccEnv -> [OccName] -> TidyOccEnv
  -- Restrict the env to just the [OccName]
  trimTidyOccEnv env vs
    = foldl' add emptyUFM vs
    where
      add :: TidyOccEnv -> OccName -> TidyOccEnv
      add so_far (OccName _ fs)
        = case lookupUFM env fs of
            Just n  -> addToUFM so_far fs n
            Nothing -> so_far
  
  {-
  ************************************************************************
  *                                                                      *
                              Utilies for "main"
  *                                                                      *
  ************************************************************************
  -}
  
  mainOcc :: OccName
  mainOcc = mkVarOccFS (fsLit "main")
  
  ppMainFn :: OccName -> SDoc
  ppMainFn main_occ
    | main_occ == mainOcc
    = text "IO action" <+> quotes (ppr main_occ)
    | otherwise
    = text "main IO action" <+> quotes (ppr main_occ)
  
  {-
  ************************************************************************
  *                                                                      *
                  Binary instance
      Here rather than in GHC.Iface.Binary because OccName is abstract
  *                                                                      *
  ************************************************************************
  -}
  
  instance Binary NameSpace where
      put_ bh VarName =
              putByte bh 0
      put_ bh DataName =
              putByte bh 1
      put_ bh TvName =
              putByte bh 2
      put_ bh TcClsName =
              putByte bh 3
      put_ bh (FldName parent) = do
              putByte bh 4
              put_ bh parent
      get bh = do
              h <- getByte bh
              case h of
                0 -> return VarName
                1 -> return DataName
                2 -> return TvName
                3 -> return TcClsName
                _ -> do
                  parent <- get bh
                  return $ FldName { fldParent = parent }
  
  instance Binary OccName where
      put_ bh (OccName aa ab) = do
              put_ bh aa
              put_ bh ab
      get bh = do
            aa <- get bh
            ab <- get bh
            return (OccName aa ab)
  