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

    Pattern-matching bindings (HsBinds and MonoBinds)

    Handles @HsBinds@; those at the top level require different handling,
    in that the @Rec@/@NonRec@/etc structure is thrown away (whereas at
    lower levels it is preserved with @let@/@letrec@s).
    -}

    {-# LANGUAGE CPP #-}

    module DsBinds ( dsTopLHsBinds, dsLHsBinds, decomposeRuleLhs, dsSpec,
                     dsHsWrapper, dsTcEvBinds, dsEvBinds
      ) where

    #include "HsVersions.h"

    import {-# SOURCE #-}   DsExpr( dsLExpr )
    import {-# SOURCE #-}   Match( matchWrapper )

    import DsMonad
    import DsGRHSs
    import DsUtils

    import HsSyn            -- lots of things
    import CoreSyn          -- lots of things
    import Literal          ( Literal(MachStr) )
    import CoreSubst
    import OccurAnal        ( occurAnalyseExpr )
    import MkCore
    import CoreUtils
    import CoreArity ( etaExpand )
    import CoreUnfold
    import CoreFVs
    import UniqSupply
    import Digraph
    import PrelNames
    import TysPrim ( mkProxyPrimTy )
    import TyCon      ( isTupleTyCon, tyConDataCons_maybe
                      , tyConName, isPromotedTyCon, isPromotedDataCon, tyConKind )
    import TcEvidence
    import TcType
    import Type
    import Kind (returnsConstraintKind)
    import Coercion hiding (substCo)
    import TysWiredIn ( eqBoxDataCon, coercibleDataCon, tupleCon, mkListTy
                      , mkBoxedTupleTy, stringTy )
    import Id
    import MkId(proxyHashId)
    import Class
    import DataCon  ( dataConTyCon, dataConWorkId )
    import Name
    import IdInfo   ( IdDetails(..) )
    import Var
    import VarSet
    import Rules
    import VarEnv
    import Outputable
    import Module
    import SrcLoc
    import Maybes
    import OrdList
    import Bag
    import BasicTypes hiding ( TopLevel )
    import DynFlags
    import FastString
    import ErrUtils( MsgDoc )
    import ListSetOps( getNth )
    import Util
    import Control.Monad( when )
    import MonadUtils
    import Control.Monad(liftM)
    import Fingerprint(Fingerprint(..), fingerprintString)

    {-
    ************************************************************************
    *                                                                      *
    \subsection[dsMonoBinds]{Desugaring a @MonoBinds@}
    *                                                                      *
    ************************************************************************
    -}

    dsTopLHsBinds :: LHsBinds Id -> DsM (OrdList (Id,CoreExpr))
    dsTopLHsBinds binds = ds_lhs_binds binds

    dsLHsBinds :: LHsBinds Id -> DsM [(Id,CoreExpr)]
    dsLHsBinds binds = do { binds' <- ds_lhs_binds binds
                          ; return (fromOL binds') }

    ------------------------
    ds_lhs_binds :: LHsBinds Id -> DsM (OrdList (Id,CoreExpr))

    ds_lhs_binds binds = do { ds_bs <- mapBagM dsLHsBind binds
                            ; return (foldBag appOL id nilOL ds_bs) }

    dsLHsBind :: LHsBind Id -> DsM (OrdList (Id,CoreExpr))
    dsLHsBind (L loc bind) = putSrcSpanDs loc $ dsHsBind bind

    dsHsBind :: HsBind Id -> DsM (OrdList (Id,CoreExpr))

    dsHsBind (VarBind { var_id = var, var_rhs = expr, var_inline = inline_regardless })
      = do  { dflags <- getDynFlags
            ; core_expr <- dsLExpr expr

                    -- Dictionary bindings are always VarBinds,
                    -- so we only need do this here
            ; let var' | inline_regardless = var `setIdUnfolding` mkCompulsoryUnfolding core_expr
                       | otherwise         = var

            ; return (unitOL (makeCorePair dflags var' False 0 core_expr)) }

    dsHsBind (FunBind { fun_id = L _ fun, fun_matches = matches
                      , fun_co_fn = co_fn, fun_tick = tick
                      , fun_infix = inf })
     = do   { dflags <- getDynFlags
            ; (args, body) <- matchWrapper (FunRhs (idName fun) inf) matches
            ; let body' = mkOptTickBox tick body
            ; rhs <- dsHsWrapper co_fn (mkLams args body')
            ; {- pprTrace "dsHsBind" (ppr fun <+> ppr (idInlinePragma fun)) $ -}
               return (unitOL (makeCorePair dflags fun False 0 rhs)) }

    dsHsBind (PatBind { pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty
                      , pat_ticks = (rhs_tick, var_ticks) })
      = do  { body_expr <- dsGuarded grhss ty
            ; let body' = mkOptTickBox rhs_tick body_expr
            ; sel_binds <- mkSelectorBinds var_ticks pat body'
              -- We silently ignore inline pragmas; no makeCorePair
              -- Not so cool, but really doesn't matter
        ; return (toOL sel_binds) }

            -- A common case: one exported variable
            -- Non-recursive bindings come through this way
            -- So do self-recursive bindings, and recursive bindings
            -- that have been chopped up with type signatures
    dsHsBind (AbsBinds { abs_tvs = tyvars, abs_ev_vars = dicts
                       , abs_exports = [export]
                       , abs_ev_binds = ev_binds, abs_binds = binds })
      | ABE { abe_wrap = wrap, abe_poly = global
            , abe_mono = local, abe_prags = prags } <- export
      = do  { dflags <- getDynFlags
            ; bind_prs    <- ds_lhs_binds binds
            ; let   core_bind = Rec (fromOL bind_prs)
            ; ds_binds <- dsTcEvBinds ev_binds
            ; rhs <- dsHsWrapper wrap $  -- Usually the identity
                                mkLams tyvars $ mkLams dicts $
                                mkCoreLets ds_binds $
                                Let core_bind $
                                Var local

            ; (spec_binds, rules) <- dsSpecs rhs prags

            ; let   global'   = addIdSpecialisations global rules
                    main_bind = makeCorePair dflags global' (isDefaultMethod prags)
                                             (dictArity dicts) rhs

            ; return (main_bind `consOL` spec_binds) }

    dsHsBind (AbsBinds { abs_tvs = tyvars, abs_ev_vars = dicts
                       , abs_exports = exports, abs_ev_binds = ev_binds
                       , abs_binds = binds })
             -- See Note [Desugaring AbsBinds]
      = do  { dflags <- getDynFlags
            ; bind_prs    <- ds_lhs_binds binds
            ; let core_bind = Rec [ makeCorePair dflags (add_inline lcl_id) False 0 rhs
                                  | (lcl_id, rhs) <- fromOL bind_prs ]
                    -- Monomorphic recursion possible, hence Rec

                  locals       = map abe_mono exports
                  tup_expr     = mkBigCoreVarTup locals
                  tup_ty       = exprType tup_expr
            ; ds_binds <- dsTcEvBinds ev_binds
            ; let poly_tup_rhs = mkLams tyvars $ mkLams dicts $
                                 mkCoreLets ds_binds $
                                 Let core_bind $
                                 tup_expr

            ; poly_tup_id <- newSysLocalDs (exprType poly_tup_rhs)

            ; let mk_bind (ABE { abe_wrap = wrap, abe_poly = global
                               , abe_mono = local, abe_prags = spec_prags })
                    = do { tup_id  <- newSysLocalDs tup_ty
                         ; rhs <- dsHsWrapper wrap $
                                     mkLams tyvars $ mkLams dicts $
                                     mkTupleSelector locals local tup_id $
                                     mkVarApps (Var poly_tup_id) (tyvars ++ dicts)
                         ; let rhs_for_spec = Let (NonRec poly_tup_id poly_tup_rhs) rhs
                         ; (spec_binds, rules) <- dsSpecs rhs_for_spec spec_prags
                         ; let global' = (global `setInlinePragma` defaultInlinePragma)
                                                 `addIdSpecialisations` rules
                               -- Kill the INLINE pragma because it applies to
                               -- the user written (local) function.  The global
                               -- Id is just the selector.  Hmm.
                         ; return ((global', rhs) `consOL` spec_binds) }

            ; export_binds_s <- mapM mk_bind exports

            ; return ((poly_tup_id, poly_tup_rhs) `consOL`
                        concatOL export_binds_s) }
      where
        inline_env :: IdEnv Id   -- Maps a monomorphic local Id to one with
                                 -- the inline pragma from the source
                                 -- The type checker put the inline pragma
                                 -- on the *global* Id, so we need to transfer it
        inline_env = mkVarEnv [ (lcl_id, setInlinePragma lcl_id prag)
                              | ABE { abe_mono = lcl_id, abe_poly = gbl_id } <- exports
                              , let prag = idInlinePragma gbl_id ]

        add_inline :: Id -> Id    -- tran
        add_inline lcl_id = lookupVarEnv inline_env lcl_id `orElse` lcl_id

    dsHsBind (PatSynBind{}) = panic "dsHsBind: PatSynBind"

    ------------------------
    makeCorePair :: DynFlags -> Id -> Bool -> Arity -> CoreExpr -> (Id, CoreExpr)
    makeCorePair dflags gbl_id is_default_method dict_arity rhs
      | is_default_method                 -- Default methods are *always* inlined
      = (gbl_id `setIdUnfolding` mkCompulsoryUnfolding rhs, rhs)

      | DFunId _ is_newtype <- idDetails gbl_id
      = (mk_dfun_w_stuff is_newtype, rhs)

      | otherwise
      = case inlinePragmaSpec inline_prag of
              EmptyInlineSpec -> (gbl_id, rhs)
              NoInline        -> (gbl_id, rhs)
              Inlinable       -> (gbl_id `setIdUnfolding` inlinable_unf, rhs)
              Inline          -> inline_pair

      where
        inline_prag   = idInlinePragma gbl_id
        inlinable_unf = mkInlinableUnfolding dflags rhs
        inline_pair
           | Just arity <- inlinePragmaSat inline_prag
            -- Add an Unfolding for an INLINE (but not for NOINLINE)
            -- And eta-expand the RHS; see Note [Eta-expanding INLINE things]
           , let real_arity = dict_arity + arity
            -- NB: The arity in the InlineRule takes account of the dictionaries
           = ( gbl_id `setIdUnfolding` mkInlineUnfolding (Just real_arity) rhs
             , etaExpand real_arity rhs)

           | otherwise
           = pprTrace "makeCorePair: arity missing" (ppr gbl_id) $
             (gbl_id `setIdUnfolding` mkInlineUnfolding Nothing rhs, rhs)

                    -- See Note [ClassOp/DFun selection] in TcInstDcls
                    -- See Note [Single-method classes]  in TcInstDcls
        mk_dfun_w_stuff is_newtype
           | is_newtype
           = gbl_id `setIdUnfolding`  mkInlineUnfolding (Just 0) rhs
                    `setInlinePragma` alwaysInlinePragma { inl_sat = Just 0 }
           | otherwise
           = gbl_id `setIdUnfolding`  mkDFunUnfolding dfun_bndrs dfun_constr dfun_args
                    `setInlinePragma` dfunInlinePragma
        (dfun_bndrs, dfun_body) = collectBinders (simpleOptExpr rhs)
        (dfun_con, dfun_args)   = collectArgs dfun_body
        dfun_constr | Var id <- dfun_con
                    , DataConWorkId con <- idDetails id
                    = con
                    | otherwise = pprPanic "makeCorePair: dfun" (ppr rhs)

    dictArity :: [Var] -> Arity
    -- Don't count coercion variables in arity
    dictArity dicts = count isId dicts

    {-
    [Desugaring AbsBinds]
    ~~~~~~~~~~~~~~~~~~~~~
    In the general AbsBinds case we desugar the binding to this:

           tup a (d:Num a) = let fm = ...gm...
                                 gm = ...fm...
                             in (fm,gm)
           f a d = case tup a d of { (fm,gm) -> fm }
           g a d = case tup a d of { (fm,gm) -> fm }

    Note [Rules and inlining]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Common special case: no type or dictionary abstraction
    This is a bit less trivial than you might suppose
    The naive way woudl be to desguar to something like
            f_lcl = ...f_lcl...     -- The "binds" from AbsBinds
            M.f = f_lcl             -- Generated from "exports"
    But we don't want that, because if M.f isn't exported,
    it'll be inlined unconditionally at every call site (its rhs is
    trivial).  That would be ok unless it has RULES, which would
    thereby be completely lost.  Bad, bad, bad.

    Instead we want to generate
            M.f = ...f_lcl...
            f_lcl = M.f
    Now all is cool. The RULES are attached to M.f (by SimplCore),
    and f_lcl is rapidly inlined away.

    This does not happen in the same way to polymorphic binds,
    because they desugar to
            M.f = /\a. let f_lcl = ...f_lcl... in f_lcl
    Although I'm a bit worried about whether full laziness might
    float the f_lcl binding out and then inline M.f at its call site

    Note [Specialising in no-dict case]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Even if there are no tyvars or dicts, we may have specialisation pragmas.
    Class methods can generate
          AbsBinds [] [] [( ... spec-prag]
             { AbsBinds [tvs] [dicts] ...blah }
    So the overloading is in the nested AbsBinds. A good example is in GHC.Float:

      class  (Real a, Fractional a) => RealFrac a  where
        round :: (Integral b) => a -> b

      instance  RealFrac Float  where
        {-# SPECIALIZE round :: Float -> Int #-}

    The top-level AbsBinds for $cround has no tyvars or dicts (because the
    instance does not).  But the method is locally overloaded!

    Note [Abstracting over tyvars only]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When abstracting over type variable only (not dictionaries), we don't really need to
    built a tuple and select from it, as we do in the general case. Instead we can take

            AbsBinds [a,b] [ ([a,b], fg, fl, _),
                             ([b],   gg, gl, _) ]
                    { fl = e1
                      gl = e2
                       h = e3 }

    and desugar it to

            fg = /\ab. let B in e1
            gg = /\b. let a = () in let B in S(e2)
            h  = /\ab. let B in e3

    where B is the *non-recursive* binding
            fl = fg a b
            gl = gg b
            h  = h a b    -- See (b); note shadowing!

    Notice (a) g has a different number of type variables to f, so we must
                 use the mkArbitraryType thing to fill in the gaps.
                 We use a type-let to do that.

             (b) The local variable h isn't in the exports, and rather than
                 clone a fresh copy we simply replace h by (h a b), where
                 the two h's have different types!  Shadowing happens here,
                 which looks confusing but works fine.

             (c) The result is *still* quadratic-sized if there are a lot of
                 small bindings.  So if there are more than some small
                 number (10), we filter the binding set B by the free
                 variables of the particular RHS.  Tiresome.

    Why got to this trouble?  It's a common case, and it removes the
    quadratic-sized tuple desugaring.  Less clutter, hopefully faster
    compilation, especially in a case where there are a *lot* of
    bindings.

    Note [Eta-expanding INLINE things]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       foo :: Eq a => a -> a
       {-# INLINE foo #-}
       foo x = ...

    If (foo d) ever gets floated out as a common sub-expression (which can
    happen as a result of method sharing), there's a danger that we never
    get to do the inlining, which is a Terribly Bad thing given that the
    user said "inline"!

    To avoid this we pre-emptively eta-expand the definition, so that foo
    has the arity with which it is declared in the source code.  In this
    example it has arity 2 (one for the Eq and one for x). Doing this
    should mean that (foo d) is a PAP and we don't share it.

    Note [Nested arities]
    ~~~~~~~~~~~~~~~~~~~~~
    For reasons that are not entirely clear, method bindings come out looking like
    this:

      AbsBinds [] [] [$cfromT <= [] fromT]
        $cfromT [InlPrag=INLINE] :: T Bool -> Bool
        { AbsBinds [] [] [fromT <= [] fromT_1]
            fromT :: T Bool -> Bool
            { fromT_1 ((TBool b)) = not b } } }

    Note the nested AbsBind.  The arity for the InlineRule on $cfromT should be
    gotten from the binding for fromT_1.

    It might be better to have just one level of AbsBinds, but that requires more
    thought!

    Note [Implementing SPECIALISE pragmas]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Example:
            f :: (Eq a, Ix b) => a -> b -> Bool
            {-# SPECIALISE f :: (Ix p, Ix q) => Int -> (p,q) -> Bool #-}
            f = <poly_rhs>

    From this the typechecker generates

        AbsBinds [ab] [d1,d2] [([ab], f, f_mono, prags)] binds

        SpecPrag (wrap_fn :: forall a b. (Eq a, Ix b) => XXX
                          -> forall p q. (Ix p, Ix q) => XXX[ Int/a, (p,q)/b ])

    Note that wrap_fn can transform *any* function with the right type prefix
        forall ab. (Eq a, Ix b) => XXX
    regardless of XXX.  It's sort of polymorphic in XXX.  This is
    useful: we use the same wrapper to transform each of the class ops, as
    well as the dict.

    From these we generate:

        Rule:       forall p, q, (dp:Ix p), (dq:Ix q).
                        f Int (p,q) dInt ($dfInPair dp dq) = f_spec p q dp dq

        Spec bind:  f_spec = wrap_fn <poly_rhs>

    Note that

      * The LHS of the rule may mention dictionary *expressions* (eg
        $dfIxPair dp dq), and that is essential because the dp, dq are
        needed on the RHS.

      * The RHS of f_spec, <poly_rhs> has a *copy* of 'binds', so that it
        can fully specialise it.
    -}

    ------------------------
    dsSpecs :: CoreExpr     -- Its rhs
            -> TcSpecPrags
            -> DsM ( OrdList (Id,CoreExpr)  -- Binding for specialised Ids
                   , [CoreRule] )           -- Rules for the Global Ids
    -- See Note [Implementing SPECIALISE pragmas]
    dsSpecs _ IsDefaultMethod = return (nilOL, [])
    dsSpecs poly_rhs (SpecPrags sps)
      = do { pairs <- mapMaybeM (dsSpec (Just poly_rhs)) sps
           ; let (spec_binds_s, rules) = unzip pairs
           ; return (concatOL spec_binds_s, rules) }

    dsSpec :: Maybe CoreExpr        -- Just rhs => RULE is for a local binding
                                    -- Nothing => RULE is for an imported Id
                                    --            rhs is in the Id's unfolding
           -> Located TcSpecPrag
           -> DsM (Maybe (OrdList (Id,CoreExpr), CoreRule))
    dsSpec mb_poly_rhs (L loc (SpecPrag poly_id spec_co spec_inl))
      | isJust (isClassOpId_maybe poly_id)
      = putSrcSpanDs loc $
        do { warnDs (ptext (sLit "Ignoring useless SPECIALISE pragma for class method selector")
                     <+> quotes (ppr poly_id))
           ; return Nothing  }  -- There is no point in trying to specialise a class op
                                -- Moreover, classops don't (currently) have an inl_sat arity set
                                -- (it would be Just 0) and that in turn makes makeCorePair bleat

      | no_act_spec && isNeverActive rule_act
      = putSrcSpanDs loc $
        do { warnDs (ptext (sLit "Ignoring useless SPECIALISE pragma for NOINLINE function:")
                     <+> quotes (ppr poly_id))
           ; return Nothing  }  -- Function is NOINLINE, and the specialiation inherits that
                                -- See Note [Activation pragmas for SPECIALISE]

      | otherwise
      = putSrcSpanDs loc $
        do { uniq <- newUnique
           ; let poly_name = idName poly_id
                 spec_occ  = mkSpecOcc (getOccName poly_name)
                 spec_name = mkInternalName uniq spec_occ (getSrcSpan poly_name)
           ; (bndrs, ds_lhs) <- liftM collectBinders
                                      (dsHsWrapper spec_co (Var poly_id))
           ; let spec_ty = mkPiTypes bndrs (exprType ds_lhs)
           ; -- pprTrace "dsRule" (vcat [ ptext (sLit "Id:") <+> ppr poly_id
             --                         , ptext (sLit "spec_co:") <+> ppr spec_co
             --                         , ptext (sLit "ds_rhs:") <+> ppr ds_lhs ]) $
             case decomposeRuleLhs bndrs ds_lhs of {
               Left msg -> do { warnDs msg; return Nothing } ;
               Right (rule_bndrs, _fn, args) -> do

           { dflags <- getDynFlags
           ; let fn_unf    = realIdUnfolding poly_id
                 unf_fvs   = stableUnfoldingVars fn_unf `orElse` emptyVarSet
                 in_scope  = mkInScopeSet (unf_fvs `unionVarSet` exprsFreeVars args)
                 spec_unf  = specUnfolding dflags (mkEmptySubst in_scope) bndrs args fn_unf
                 spec_id   = mkLocalId spec_name spec_ty
                                `setInlinePragma` inl_prag
                                `setIdUnfolding`  spec_unf
                 rule =  mkRule False {- Not auto -} is_local_id
                            (mkFastString ("SPEC " ++ showPpr dflags poly_name))
                            rule_act poly_name
                            rule_bndrs args
                            (mkVarApps (Var spec_id) bndrs)

           ; spec_rhs <- dsHsWrapper spec_co poly_rhs

           ; when (isInlinePragma id_inl && wopt Opt_WarnPointlessPragmas dflags)
                  (warnDs (specOnInline poly_name))

           ; return (Just (unitOL (spec_id, spec_rhs), rule))
                -- NB: do *not* use makeCorePair on (spec_id,spec_rhs), because
                --     makeCorePair overwrites the unfolding, which we have
                --     just created using specUnfolding
           } } }
      where
        is_local_id = isJust mb_poly_rhs
        poly_rhs | Just rhs <-  mb_poly_rhs
                 = rhs          -- Local Id; this is its rhs
                 | Just unfolding <- maybeUnfoldingTemplate (realIdUnfolding poly_id)
                 = unfolding    -- Imported Id; this is its unfolding
                                -- Use realIdUnfolding so we get the unfolding
                                -- even when it is a loop breaker.
                                -- We want to specialise recursive functions!
                 | otherwise = pprPanic "dsImpSpecs" (ppr poly_id)
                                -- The type checker has checked that it *has* an unfolding

        id_inl = idInlinePragma poly_id

        -- See Note [Activation pragmas for SPECIALISE]
        inl_prag | not (isDefaultInlinePragma spec_inl)    = spec_inl
                 | not is_local_id  -- See Note [Specialising imported functions]
                                     -- in OccurAnal
                 , isStrongLoopBreaker (idOccInfo poly_id) = neverInlinePragma
                 | otherwise                               = id_inl
         -- Get the INLINE pragma from SPECIALISE declaration, or,
         -- failing that, from the original Id

        spec_prag_act = inlinePragmaActivation spec_inl

        -- See Note [Activation pragmas for SPECIALISE]
        -- no_act_spec is True if the user didn't write an explicit
        -- phase specification in the SPECIALISE pragma
        no_act_spec = case inlinePragmaSpec spec_inl of
                        NoInline -> isNeverActive  spec_prag_act
                        _        -> isAlwaysActive spec_prag_act
        rule_act | no_act_spec = inlinePragmaActivation id_inl   -- Inherit
                 | otherwise   = spec_prag_act                   -- Specified by user

    specOnInline :: Name -> MsgDoc
    specOnInline f = ptext (sLit "SPECIALISE pragma on INLINE function probably won't fire:")
                     <+> quotes (ppr f)

    {-
    Note [Activation pragmas for SPECIALISE]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    From a user SPECIALISE pragma for f, we generate
      a) A top-level binding    spec_fn = rhs
      b) A RULE                 f dOrd = spec_fn

    We need two pragma-like things:

    * spec_fn's inline pragma: inherited from f's inline pragma (ignoring
                               activation on SPEC), unless overriden by SPEC INLINE

    * Activation of RULE: from SPECIALISE pragma (if activation given)
                          otherwise from f's inline pragma

    This is not obvious (see Trac #5237)!

    Examples      Rule activation   Inline prag on spec'd fn
    ---------------------------------------------------------------------
    SPEC [n] f :: ty            [n]   Always, or NOINLINE [n]
                                      copy f's prag

    NOINLINE f
    SPEC [n] f :: ty            [n]   NOINLINE
                                      copy f's prag

    NOINLINE [k] f
    SPEC [n] f :: ty            [n]   NOINLINE [k]
                                      copy f's prag

    INLINE [k] f
    SPEC [n] f :: ty            [n]   INLINE [k]
                                      copy f's prag

    SPEC INLINE [n] f :: ty     [n]   INLINE [n]
                                      (ignore INLINE prag on f,
                                      same activation for rule and spec'd fn)

    NOINLINE [k] f
    SPEC f :: ty                [n]   INLINE [k]

    ************************************************************************
    *                                                                      *
    \subsection{Adding inline pragmas}
    *                                                                      *
    ************************************************************************
    -}

    decomposeRuleLhs :: [Var] -> CoreExpr -> Either SDoc ([Var], Id, [CoreExpr])
    -- (decomposeRuleLhs bndrs lhs) takes apart the LHS of a RULE,
    -- The 'bndrs' are the quantified binders of the rules, but decomposeRuleLhs
    -- may add some extra dictionary binders (see Note [Free dictionaries])
    --
    -- Returns Nothing if the LHS isn't of the expected shape
    -- Note [Decomposing the left-hand side of a RULE]
    decomposeRuleLhs orig_bndrs orig_lhs
      | not (null unbound)    -- Check for things unbound on LHS
                              -- See Note [Unused spec binders]
      = Left (vcat (map dead_msg unbound))

      | Just (fn_id, args) <- decompose fun2 args2
      , let extra_dict_bndrs = mk_extra_dict_bndrs fn_id args
      = -- pprTrace "decmposeRuleLhs" (vcat [ ptext (sLit "orig_bndrs:") <+> ppr orig_bndrs
        --                                  , ptext (sLit "orig_lhs:") <+> ppr orig_lhs
        --                                  , ptext (sLit "lhs1:")     <+> ppr lhs1
        --                                  , ptext (sLit "extra_dict_bndrs:") <+> ppr extra_dict_bndrs
        --                                  , ptext (sLit "fn_id:") <+> ppr fn_id
        --                                  , ptext (sLit "args:")   <+> ppr args]) $
        Right (orig_bndrs ++ extra_dict_bndrs, fn_id, args)

      | otherwise
      = Left bad_shape_msg
     where
       lhs1         = drop_dicts orig_lhs
       lhs2         = simpleOptExpr lhs1  -- See Note [Simplify rule LHS]
       (fun2,args2) = collectArgs lhs2

       lhs_fvs    = exprFreeVars lhs2
       unbound    = filterOut (`elemVarSet` lhs_fvs) orig_bndrs

       orig_bndr_set = mkVarSet orig_bndrs

            -- Add extra dict binders: Note [Free dictionaries]
       mk_extra_dict_bndrs fn_id args
         = [ mkLocalId (localiseName (idName d)) (idType d)
           | d <- varSetElems (exprsFreeVars args `delVarSetList` (fn_id : orig_bndrs))
                  -- fn_id: do not quantify over the function itself, which may
                  -- itself be a dictionary (in pathological cases, Trac #10251)
           , isDictId d ]

       decompose (Var fn_id) args
          | not (fn_id `elemVarSet` orig_bndr_set)
          = Just (fn_id, args)

       decompose _ _ = Nothing

       bad_shape_msg = hang (ptext (sLit "RULE left-hand side too complicated to desugar"))
                          2 (vcat [ text "Optimised lhs:" <+> ppr lhs2
                                  , text "Orig lhs:" <+> ppr orig_lhs])
       dead_msg bndr = hang (sep [ ptext (sLit "Forall'd") <+> pp_bndr bndr
                                 , ptext (sLit "is not bound in RULE lhs")])
                          2 (vcat [ text "Orig bndrs:" <+> ppr orig_bndrs
                                  , text "Orig lhs:" <+> ppr orig_lhs
                                  , text "optimised lhs:" <+> ppr lhs2 ])
       pp_bndr bndr
        | isTyVar bndr                      = ptext (sLit "type variable") <+> quotes (ppr bndr)
        | Just pred <- evVarPred_maybe bndr = ptext (sLit "constraint") <+> quotes (ppr pred)
        | otherwise                         = ptext (sLit "variable") <+> quotes (ppr bndr)

       drop_dicts :: CoreExpr -> CoreExpr
       drop_dicts e
           = wrap_lets needed bnds body
         where
           needed = orig_bndr_set `minusVarSet` exprFreeVars body
           (bnds, body) = split_lets (occurAnalyseExpr e)
               -- The occurAnalyseExpr drops dead bindings which is
               -- crucial to ensure that every binding is used later;
               -- which in turn makes wrap_lets work right

       split_lets :: CoreExpr -> ([(DictId,CoreExpr)], CoreExpr)
       split_lets e
         | Let (NonRec d r) body <- e
         , isDictId d
         , (bs, body') <- split_lets body
         = ((d,r):bs, body')
         | otherwise
         = ([], e)

       wrap_lets :: VarSet -> [(DictId,CoreExpr)] -> CoreExpr -> CoreExpr
       wrap_lets _ [] body = body
       wrap_lets needed ((d, r) : bs) body
         | rhs_fvs `intersectsVarSet` needed = Let (NonRec d r) (wrap_lets needed' bs body)
         | otherwise                         = wrap_lets needed bs body
         where
           rhs_fvs = exprFreeVars r
           needed' = (needed `minusVarSet` rhs_fvs) `extendVarSet` d

    {-
    Note [Decomposing the left-hand side of a RULE]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    There are several things going on here.
    * drop_dicts: see Note [Drop dictionary bindings on rule LHS]
    * simpleOptExpr: see Note [Simplify rule LHS]
    * extra_dict_bndrs: see Note [Free dictionaries]

    Note [Drop dictionary bindings on rule LHS]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    drop_dicts drops dictionary bindings on the LHS where possible.
       E.g.  let d:Eq [Int] = $fEqList $fEqInt in f d
         --> f d
       Reasoning here is that there is only one d:Eq [Int], and so we can
       quantify over it. That makes 'd' free in the LHS, but that is later
       picked up by extra_dict_bndrs (Note [Dead spec binders]).

       NB 1: We can only drop the binding if the RHS doesn't bind
             one of the orig_bndrs, which we assume occur on RHS.
             Example
                f :: (Eq a) => b -> a -> a
                {-# SPECIALISE f :: Eq a => b -> [a] -> [a] #-}
             Here we want to end up with
                RULE forall d:Eq a.  f ($dfEqList d) = f_spec d
             Of course, the ($dfEqlist d) in the pattern makes it less likely
             to match, but ther is no other way to get d:Eq a

       NB 2: We do drop_dicts *before* simplOptEpxr, so that we expect all
             the evidence bindings to be wrapped around the outside of the
             LHS.  (After simplOptExpr they'll usually have been inlined.)
             dsHsWrapper does dependency analysis, so that civilised ones
             will be simple NonRec bindings.  We don't handle recursive
             dictionaries!

        NB3: In the common case of a non-overloaded, but perhaps-polymorphic
             specialisation, we don't need to bind *any* dictionaries for use
             in the RHS. For example (Trac #8331)
                 {-# SPECIALIZE INLINE useAbstractMonad :: ReaderST s Int #-}
                 useAbstractMonad :: MonadAbstractIOST m => m Int
             Here, deriving (MonadAbstractIOST (ReaderST s)) is a lot of code
             but the RHS uses no dictionaries, so we want to end up with
                 RULE forall s (d :: MonadBstractIOST (ReaderT s)).
                    useAbstractMonad (ReaderT s) d = $suseAbstractMonad s

       Trac #8848 is a good example of where there are some intersting
       dictionary bindings to discard.

    The drop_dicts algorithm is based on these observations:

      * Given (let d = rhs in e) where d is a DictId,
        matching 'e' will bind e's free variables.

      * So we want to keep the binding if one of the needed variables (for
        which we need a binding) is in fv(rhs) but not already in fv(e).

      * The "needed variables" are simply the orig_bndrs.  Consider
           f :: (Eq a, Show b) => a -> b -> String
           ... SPECIALISE f :: (Show b) => Int -> b -> String ...
        Then orig_bndrs includes the *quantified* dictionaries of the type
        namely (dsb::Show b), but not the one for Eq Int

    So we work inside out, applying the above criterion at each step.

    Note [Simplify rule LHS]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    simplOptExpr occurrence-analyses and simplifies the LHS:

       (a) Inline any remaining dictionary bindings (which hopefully
           occur just once)

       (b) Substitute trivial lets so that they don't get in the way
           Note that we substitute the function too; we might
           have this as a LHS:  let f71 = M.f Int in f71

       (c) Do eta reduction.  To see why, consider the fold/build rule,
           which without simplification looked like:
              fold k z (build (/\a. g a))  ==>  ...
           This doesn't match unless you do eta reduction on the build argument.
           Similarly for a LHS like
             augment g (build h)
           we do not want to get
             augment (\a. g a) (build h)
           otherwise we don't match when given an argument like
              augment (\a. h a a) (build h)

    Note [Matching seqId]
    ~~~~~~~~~~~~~~~~~~~
    The desugarer turns (seq e r) into (case e of _ -> r), via a special-case hack
    and this code turns it back into an application of seq!
    See Note [Rules for seq] in MkId for the details.

    Note [Unused spec binders]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
            f :: a -> a
            ... SPECIALISE f :: Eq a => a -> a ...
    It's true that this *is* a more specialised type, but the rule
    we get is something like this:
            f_spec d = f
            RULE: f = f_spec d
    Note that the rule is bogus, because it mentions a 'd' that is
    not bound on the LHS!  But it's a silly specialisation anyway, because
    the constraint is unused.  We could bind 'd' to (error "unused")
    but it seems better to reject the program because it's almost certainly
    a mistake.  That's what the isDeadBinder call detects.

    Note [Free dictionaries]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    When the LHS of a specialisation rule, (/\as\ds. f es) has a free dict,
    which is presumably in scope at the function definition site, we can quantify
    over it too.  *Any* dict with that type will do.

    So for example when you have
            f :: Eq a => a -> a
            f = <rhs>
            ... SPECIALISE f :: Int -> Int ...

    Then we get the SpecPrag
            SpecPrag (f Int dInt)

    And from that we want the rule

            RULE forall dInt. f Int dInt = f_spec
            f_spec = let f = <rhs> in f Int dInt

    But be careful!  That dInt might be GHC.Base.$fOrdInt, which is an External
    Name, and you can't bind them in a lambda or forall without getting things
    confused.   Likewise it might have an InlineRule or something, which would be
    utterly bogus. So we really make a fresh Id, with the same unique and type
    as the old one, but with an Internal name and no IdInfo.

    ************************************************************************
    *                                                                      *
                    Desugaring evidence
    *                                                                      *
    ************************************************************************

    -}

    dsHsWrapper :: HsWrapper -> CoreExpr -> DsM CoreExpr
    dsHsWrapper WpHole            e = return e
    dsHsWrapper (WpTyApp ty)      e = return $ App e (Type ty)
    dsHsWrapper (WpLet ev_binds)  e = do bs <- dsTcEvBinds ev_binds
                                         return (mkCoreLets bs e)
    dsHsWrapper (WpCompose c1 c2) e = do { e1 <- dsHsWrapper c2 e
                                         ; dsHsWrapper c1 e1 }
    dsHsWrapper (WpFun c1 c2 t1 _) e = do { x <- newSysLocalDs t1
                                          ; e1 <- dsHsWrapper c1 (Var x)
                                          ; e2 <- dsHsWrapper c2 (e `mkCoreAppDs` e1)
                                          ; return (Lam x e2) }
    dsHsWrapper (WpCast co)       e = ASSERT(tcCoercionRole co == Representational)
                                      dsTcCoercion co (mkCast e)
    dsHsWrapper (WpEvLam ev)      e = return $ Lam ev e
    dsHsWrapper (WpTyLam tv)      e = return $ Lam tv e
    dsHsWrapper (WpEvApp    tm)   e = liftM (App e) (dsEvTerm tm)

    --------------------------------------
    dsTcEvBinds :: TcEvBinds -> DsM [CoreBind]
    dsTcEvBinds (TcEvBinds {}) = panic "dsEvBinds"    -- Zonker has got rid of this
    dsTcEvBinds (EvBinds bs)   = dsEvBinds bs

    dsEvBinds :: Bag EvBind -> DsM [CoreBind]
    dsEvBinds bs = mapM ds_scc (sccEvBinds bs)
      where
        ds_scc (AcyclicSCC (EvBind v r)) = liftM (NonRec v) (dsEvTerm r)
        ds_scc (CyclicSCC bs)            = liftM Rec (mapM ds_pair bs)

        ds_pair (EvBind v r) = liftM ((,) v) (dsEvTerm r)

    sccEvBinds :: Bag EvBind -> [SCC EvBind]
    sccEvBinds bs = stronglyConnCompFromEdgedVertices edges
      where
        edges :: [(EvBind, EvVar, [EvVar])]
        edges = foldrBag ((:) . mk_node) [] bs

        mk_node :: EvBind -> (EvBind, EvVar, [EvVar])
        mk_node b@(EvBind var term) = (b, var, varSetElems (evVarsOfTerm term))

    ---------------------------------------
    dsEvTerm :: EvTerm -> DsM CoreExpr
    dsEvTerm (EvId v) = return (Var v)

    dsEvTerm (EvCast tm co)
      = do { tm' <- dsEvTerm tm
           ; dsTcCoercion co $ mkCast tm' }
                            -- 'v' is always a lifted evidence variable so it is
                            -- unnecessary to call varToCoreExpr v here.

    dsEvTerm (EvDFunApp df tys tms) = do { tms' <- mapM dsEvTerm tms
                                         ; return (Var df `mkTyApps` tys `mkApps` tms') }

    dsEvTerm (EvCoercion (TcCoVarCo v)) = return (Var v)  -- See Note [Simple coercions]
    dsEvTerm (EvCoercion co)            = dsTcCoercion co mkEqBox

    dsEvTerm (EvTupleSel tm n)
       = do { tup <- dsEvTerm tm
            ; let scrut_ty  = exprType tup
                  (tc, tys) = splitTyConApp scrut_ty
                  Just [dc] = tyConDataCons_maybe tc
                  xs = mkTemplateLocals tys
                  the_x = getNth xs n
            ; ASSERT( isTupleTyCon tc )
              return $
              Case tup (mkWildValBinder scrut_ty) (idType the_x) [(DataAlt dc, xs, Var the_x)] }

    dsEvTerm (EvTupleMk tms)
      = do { tms' <- mapM dsEvTerm tms
           ; let tys = map exprType tms'
           ; return $ Var (dataConWorkId dc) `mkTyApps` tys `mkApps` tms' }
      where
        dc = tupleCon ConstraintTuple (length tms)

    dsEvTerm (EvSuperClass d n)
      = do { d' <- dsEvTerm d
           ; let (cls, tys) = getClassPredTys (exprType d')
                 sc_sel_id  = classSCSelId cls n    -- Zero-indexed
           ; return $ Var sc_sel_id `mkTyApps` tys `App` d' }
      where

    dsEvTerm (EvDelayedError ty msg) = return $ Var errorId `mkTyApps` [ty] `mkApps` [litMsg]
      where
        errorId = rUNTIME_ERROR_ID
        litMsg  = Lit (MachStr (fastStringToByteString msg))

    dsEvTerm (EvLit l) =
      case l of
        EvNum n -> mkIntegerExpr n
        EvStr s -> mkStringExprFS s

    dsEvTerm (EvTypeable ev) = dsEvTypeable ev

    dsEvTerm (EvCallStack cs) = dsEvCallStack cs

    dsEvTypeable :: EvTypeable -> DsM CoreExpr
    dsEvTypeable ev =
      do tyCl      <- dsLookupTyCon typeableClassName
         typeRepTc <- dsLookupTyCon typeRepTyConName
         let tyRepType = mkTyConApp typeRepTc []

         (ty, rep) <-
            case ev of

              EvTypeableTyCon tc ks ->
                do ctr       <- dsLookupGlobalId mkPolyTyConAppName
                   mkTyCon   <- dsLookupGlobalId mkTyConName
                   dflags    <- getDynFlags
                   let mkRep cRep kReps tReps =
                         mkApps (Var ctr) [ cRep, mkListExpr tyRepType kReps
                                                , mkListExpr tyRepType tReps ]

                   let kindRep k =
                         case splitTyConApp_maybe k of
                           Nothing -> panic "dsEvTypeable: not a kind constructor"
                           Just (kc,ks) ->
                             do kcRep <- tyConRep dflags mkTyCon kc
                                reps  <- mapM kindRep ks
                                return (mkRep kcRep [] reps)

                   tcRep     <- tyConRep dflags mkTyCon tc

                   kReps     <- mapM kindRep ks

                   return ( mkTyConApp tc ks
                          , mkRep tcRep kReps []
                          )

              EvTypeableTyApp t1 t2 ->
                do e1  <- getRep tyCl t1
                   e2  <- getRep tyCl t2
                   ctr <- dsLookupGlobalId mkAppTyName

                   return ( mkAppTy (snd t1) (snd t2)
                          , mkApps (Var ctr) [ e1, e2 ]
                          )

              EvTypeableTyLit ty ->
                do str <- case (isNumLitTy ty, isStrLitTy ty) of
                            (Just n, _) -> return (show n)
                            (_, Just n) -> return (show n)
                            _ -> panic "dsEvTypeable: malformed TyLit evidence"
                   ctr <- dsLookupGlobalId typeLitTypeRepName
                   tag <- mkStringExpr str
                   return (ty, mkApps (Var ctr) [ tag ])

         -- TyRep -> Typeable t
         -- see also: Note [Memoising typeOf]
         repName <- newSysLocalDs tyRepType
         let proxyT = mkProxyPrimTy (typeKind ty) ty
             method = bindNonRec repName rep
                    $ mkLams [mkWildValBinder proxyT] (Var repName)

         -- package up the method as `Typeable` dictionary
         return $ mkCast method $ mkSymCo $ getTypeableCo tyCl ty

      where
      -- co: method -> Typeable k t
      getTypeableCo tc t =
        case instNewTyCon_maybe tc [typeKind t, t] of
          Just (_,co) -> co
          _           -> panic "Class `Typeable` is not a `newtype`."

      -- Typeable t -> TyRep
      getRep tc (ev,t) =
        do typeableExpr <- dsEvTerm ev
           let co     = getTypeableCo tc t
               method = mkCast typeableExpr co
               proxy  = mkTyApps (Var proxyHashId) [typeKind t, t]
           return (mkApps method [proxy])

      -- This part could be cached
      tyConRep dflags mkTyCon tc =
        do pkgStr  <- mkStringExprFS pkg_fs
           modStr  <- mkStringExprFS modl_fs
           nameStr <- mkStringExprFS name_fs
           return (mkApps (Var mkTyCon) [ int64 high, int64 low
                                        , pkgStr, modStr, nameStr
                                        ])
        where
        tycon_name                = tyConName tc
        modl                      = nameModule tycon_name
        pkg                       = modulePackageKey modl

        modl_fs                   = moduleNameFS (moduleName modl)
        pkg_fs                    = packageKeyFS pkg
        name_fs                   = occNameFS (nameOccName tycon_name)
        hash_name_fs
          | isPromotedTyCon tc    = appendFS (mkFastString "$k") name_fs
          | isPromotedDataCon tc  = appendFS (mkFastString "$c") name_fs
          | isTupleTyCon tc &&
            returnsConstraintKind (tyConKind tc)
                                  = appendFS (mkFastString "$p") name_fs
          | otherwise             = name_fs

        hashThis = unwords $ map unpackFS [pkg_fs, modl_fs, hash_name_fs]
        Fingerprint high low = fingerprintString hashThis

        int64
          | wORD_SIZE dflags == 4 = mkWord64LitWord64
          | otherwise             = mkWordLit dflags . fromIntegral

    {- Note [Memoising typeOf]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    See #3245, #9203

    IMPORTANT: we don't want to recalculate the TypeRep once per call with
    the proxy argument.  This is what went wrong in #3245 and #9203. So we
    help GHC by manually keeping the 'rep' *outside* the lambda.
    -}

    dsEvCallStack :: EvCallStack -> DsM CoreExpr
    -- See Note [Overview of implicit CallStacks] in TcEvidence.hs
    dsEvCallStack cs = do
      df              <- getDynFlags
      m               <- getModule
      srcLocDataCon   <- dsLookupDataCon srcLocDataConName
      let srcLocTyCon  = dataConTyCon srcLocDataCon
      let srcLocTy     = mkTyConTy srcLocTyCon
      let mkSrcLoc l =
            liftM (mkCoreConApps srcLocDataCon)
                  (sequence [ mkStringExprFS (packageKeyFS $ modulePackageKey m)
                            , mkStringExprFS (moduleNameFS $ moduleName m)
                            , mkStringExprFS (srcSpanFile l)
                            , return $ mkIntExprInt df (srcSpanStartLine l)
                            , return $ mkIntExprInt df (srcSpanStartCol l)
                            , return $ mkIntExprInt df (srcSpanEndLine l)
                            , return $ mkIntExprInt df (srcSpanEndCol l)
                            ])

      let callSiteTy = mkBoxedTupleTy [stringTy, srcLocTy]

      matchId         <- newSysLocalDs $ mkListTy callSiteTy

      callStackDataCon <- dsLookupDataCon callStackDataConName
      let callStackTyCon = dataConTyCon callStackDataCon
      let callStackTy    = mkTyConTy callStackTyCon
      let emptyCS        = mkCoreConApps callStackDataCon [mkNilExpr callSiteTy]
      let pushCS name loc rest =
            mkWildCase rest callStackTy callStackTy
                       [( DataAlt callStackDataCon
                        , [matchId]
                        , mkCoreConApps callStackDataCon
                           [mkConsExpr callSiteTy
                                       (mkCoreTup [name, loc])
                                       (Var matchId)]
                        )]
      let mkPush name loc tm = do
            nameExpr <- mkStringExprFS name
            locExpr <- mkSrcLoc loc
            case tm of
              EvCallStack EvCsEmpty -> return (pushCS nameExpr locExpr emptyCS)
              _ -> do tmExpr  <- dsEvTerm tm
                      -- at this point tmExpr :: IP sym CallStack
                      -- but we need the actual CallStack to pass to pushCS,
                      -- so we use unwrapIP to strip the dictionary wrapper
                      -- See Note [Overview of implicit CallStacks]
                      let ip_co = unwrapIP (exprType tmExpr)
                      return (pushCS nameExpr locExpr (mkCast tmExpr ip_co))
      case cs of
        EvCsTop name loc tm -> mkPush name loc tm
        EvCsPushCall name loc tm -> mkPush (occNameFS $ getOccName name) loc tm
        EvCsEmpty -> panic "Cannot have an empty CallStack"

    ---------------------------------------
    dsTcCoercion :: TcCoercion -> (Coercion -> CoreExpr) -> DsM CoreExpr
    -- This is the crucial function that moves
    -- from TcCoercions to Coercions; see Note [TcCoercions] in Coercion
    -- e.g.  dsTcCoercion (trans g1 g2) k
    --       = case g1 of EqBox g1# ->
    --         case g2 of EqBox g2# ->
    --         k (trans g1# g2#)
    -- thing_inside will get a coercion at the role requested
    dsTcCoercion co thing_inside
      = do { us <- newUniqueSupply
           ; let eqvs_covs :: [(EqVar,CoVar)]
                 eqvs_covs = zipWith mk_co_var (varSetElems (coVarsOfTcCo co))
                                               (uniqsFromSupply us)

                 subst = mkCvSubst emptyInScopeSet [(eqv, mkCoVarCo cov) | (eqv, cov) <- eqvs_covs]
                 result_expr = thing_inside (ds_tc_coercion subst co)
                 result_ty   = exprType result_expr

           ; return (foldr (wrap_in_case result_ty) result_expr eqvs_covs) }
      where
        mk_co_var :: Id -> Unique -> (Id, Id)
        mk_co_var eqv uniq = (eqv, mkUserLocal occ uniq ty loc)
           where
             eq_nm = idName eqv
             occ = nameOccName eq_nm
             loc = nameSrcSpan eq_nm
             ty  = mkCoercionType (getEqPredRole (evVarPred eqv)) ty1 ty2
             (ty1, ty2) = getEqPredTys (evVarPred eqv)

        wrap_in_case result_ty (eqv, cov) body
          = case getEqPredRole (evVarPred eqv) of
             Nominal          -> Case (Var eqv) eqv result_ty [(DataAlt eqBoxDataCon, [cov], body)]
             Representational -> Case (Var eqv) eqv result_ty [(DataAlt coercibleDataCon, [cov], body)]
             Phantom          -> panic "wrap_in_case/phantom"

    ds_tc_coercion :: CvSubst -> TcCoercion -> Coercion
    -- If the incoming TcCoercion if of type (a ~ b)   (resp.  Coercible a b)
    --                 the result is of type (a ~# b)  (reps.  a ~# b)
    -- The VarEnv maps EqVars of type (a ~ b) to Coercions of type (a ~# b) (resp. and so on)
    -- No need for InScope set etc because the
    ds_tc_coercion subst tc_co
      = go tc_co
      where
        go (TcRefl r ty)            = Refl r (Coercion.substTy subst ty)
        go (TcTyConAppCo r tc cos)  = mkTyConAppCo r tc (map go cos)
        go (TcAppCo co1 co2)        = mkAppCo (go co1) (go co2)
        go (TcForAllCo tv co)       = mkForAllCo tv' (ds_tc_coercion subst' co)
                                  where
                                    (subst', tv') = Coercion.substTyVarBndr subst tv
        go (TcAxiomInstCo ax ind cos)
                                    = AxiomInstCo ax ind (map go cos)
        go (TcPhantomCo ty1 ty2)    = UnivCo (fsLit "ds_tc_coercion") Phantom ty1 ty2
        go (TcSymCo co)             = mkSymCo (go co)
        go (TcTransCo co1 co2)      = mkTransCo (go co1) (go co2)
        go (TcNthCo n co)           = mkNthCo n (go co)
        go (TcLRCo lr co)           = mkLRCo lr (go co)
        go (TcSubCo co)             = mkSubCo (go co)
        go (TcLetCo bs co)          = ds_tc_coercion (ds_co_binds bs) co
        go (TcCastCo co1 co2)       = mkCoCast (go co1) (go co2)
        go (TcCoVarCo v)            = ds_ev_id subst v
        go (TcAxiomRuleCo co ts cs) = AxiomRuleCo co (map (Coercion.substTy subst) ts) (map go cs)
        go (TcCoercion co)          = co

        ds_co_binds :: TcEvBinds -> CvSubst
        ds_co_binds (EvBinds bs)      = foldl ds_scc subst (sccEvBinds bs)
        ds_co_binds eb@(TcEvBinds {}) = pprPanic "ds_co_binds" (ppr eb)

        ds_scc :: CvSubst -> SCC EvBind -> CvSubst
        ds_scc subst (AcyclicSCC (EvBind v ev_term))
          = extendCvSubstAndInScope subst v (ds_co_term subst ev_term)
        ds_scc _ (CyclicSCC other) = pprPanic "ds_scc:cyclic" (ppr other $$ ppr tc_co)

        ds_co_term :: CvSubst -> EvTerm -> Coercion
        ds_co_term subst (EvCoercion tc_co) = ds_tc_coercion subst tc_co
        ds_co_term subst (EvId v)           = ds_ev_id subst v
        ds_co_term subst (EvCast tm co)     = mkCoCast (ds_co_term subst tm) (ds_tc_coercion subst co)
        ds_co_term _ other = pprPanic "ds_co_term" (ppr other $$ ppr tc_co)

        ds_ev_id :: CvSubst -> EqVar -> Coercion
        ds_ev_id subst v
         | Just co <- Coercion.lookupCoVar subst v = co
         | otherwise  = pprPanic "ds_tc_coercion" (ppr v $$ ppr tc_co)

    {-
    Note [Simple coercions]
    ~~~~~~~~~~~~~~~~~~~~~~~
    We have a special case for coercions that are simple variables.
    Suppose   cv :: a ~ b   is in scope
    Lacking the special case, if we see
            f a b cv
    we'd desguar to
            f a b (case cv of EqBox (cv# :: a ~# b) -> EqBox cv#)
    which is a bit stupid.  The special case does the obvious thing.

    This turns out to be important when desugaring the LHS of a RULE
    (see Trac #7837).  Suppose we have
        normalise        :: (a ~ Scalar a) => a -> a
        normalise_Double :: Double -> Double
        {-# RULES "normalise" normalise = normalise_Double #-}

    Then the RULE we want looks like
         forall a, (cv:a~Scalar a).
           normalise a cv = normalise_Double
    But without the special case we generate the redundant box/unbox,
    which simpleOpt (currently) doesn't remove. So the rule never matches.

    Maybe simpleOpt should be smarter.  But it seems like a good plan
    to simply never generate the redundant box/unbox in the first place.
    -}