Unsafe/Coerce.hs

    -- We don't to strictness analysis on this file to avoid turning loopy unsafe
    -- equality terms below to actual loops. Details in (U5) of
    -- Note [Implementing unsafeCoerce]
    {-# OPTIONS_GHC -fno-strictness #-}

    {-# LANGUAGE DataKinds #-}
    {-# LANGUAGE GADTs #-}
    {-# LANGUAGE MagicHash #-}
    {-# LANGUAGE NoImplicitPrelude #-}
    {-# LANGUAGE PolyKinds #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE TypeApplications #-}
    {-# LANGUAGE Unsafe #-}

    module Unsafe.Coerce
      ( unsafeCoerce, unsafeCoerceUnlifted, unsafeCoerceAddr
      , unsafeEqualityProof
      , UnsafeEquality (..)
      , unsafeCoerce#
      ) where

    import GHC.Arr (amap) -- For amap/unsafeCoerce rule
    import GHC.Base

    import GHC.Types

    {- Note [Implementing unsafeCoerce]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The implementation of unsafeCoerce is surprisingly subtle.
    This Note describes the moving parts.  You will find more
    background in MR !1869 and ticket #16893.

    The key challenge is this.  Suppose we have
       case sameTypeRep t1 t2 of
          False -> blah2
          True  -> ...(case (x |> UnsafeCo @t1 @t2) of { K -> blah })...

    The programmer thinks that the unsafeCoerce from 't1' to 't2' is safe,
    because it is justified by a runtime test (sameTypeRep t1 t2).
    It used to compile to a cast, with a magical 'UnsafeCo' coercion.

    But alas, nothing then stops GHC floating that call to unsafeCoerce
    outwards so we get
       case (x |> UnsafeCo @t1 @t2) of
         K -> case sameTypeRep t1 t2 of
                 False -> blah2
                 True  -> ...blah...

    and this is utterly wrong, because the unsafeCoerce is being performed
    before the dynamic test. This is exactly the setup in #16893.

    The solution is this:

    * In the library Unsafe.Coerce we define:

         unsafeEqualityProof :: forall k (a :: k) (b :: k).
                                UnsafeEquality a b

    * It uses a GADT, Unsafe.Coerce.UnsafeEquality, that is exactly like :~:

        data UnsafeEquality (a :: k) (b :: k) where
          UnsafeRefl :: UnsafeEquality a a

    * We can now define Unsafe.Coerce.unsafeCoerce very simply:

       unsafeCoerce :: forall (a :: Type) (b :: Type) . a -> b
       unsafeCoerce x = case unsafeEqualityProof @a @b of
                          UnsafeRefl -> x

      There is nothing special about unsafeCoerce; it is an
      ordinary library definition, and can be freely inlined.

    Now our bad case can't happen.  We'll have
         case unsafeEqualityProof @t1 @t2 of
            UnsafeRefl (co :: t1 ~ t2) -> ....(x |> co)....

    and the (x |> co) mentions the evidence 'co', which prevents it
    floating.

    But what stops the whole (case unsafeEqualityProof of ...) from
    floating?  Answer: we never float a case on a redex that can fail
    outside a conditional.  See Primop.hs,
    Note [Transformations affected by can_fail and has_side_effects].
    And unsafeEqualityProof (being opaque) is definitely treated as
    can-fail.

    While unsafeCoerce is a perfectly ordinary function that needs no
    special treatment, Unsafe.Coerce.unsafeEqualityProof is magical, in
    several ways

    (U1) unsafeEqualityProof is /never/ inlined.

    (U2) In CoreToStg.Prep, we transform
           case unsafeEqualityProof of UnsafeRefl g -> blah
          ==>
           blah[unsafe-co/g]

         This eliminates the overhead of evaluating the unsafe
         equality proof.

         Any /other/ occurrence of unsafeEqualityProof is left alone.
         For example you could write
             f :: UnsafeEquality a b -> blah
             f eq_proof = case eq_proof of UnsafeRefl -> ...
        (Nothing special about that.)  In a call, you might write
             f unsafeEqualityProof

        and we'll generate code simply by passing the top-level
        unsafeEqualityProof to f.  As (U5) says, it is implemented as
        UnsafeRefl so all is good.

        NB: Don't discard the case if the case-binder is used
               case unsafeEqualityProof of wild_xx { UnsafeRefl ->
               ...wild_xx...
            That rarely happens, but see #18227.

    (U3) In GHC.CoreToStg.Prep.cpeRhsE, if we see
           let x = case unsafeEqualityProof ... of
                     UnsafeRefl -> K e
           in ...

         there is a danger that we'll go to
            let x = case unsafeEqualityProof ... of
                      UnsafeRefl -> let a = e in K a
            in ...

         and produce a thunk even after discarding the unsafeEqualityProof.
         So instead we float out the case to give
            case unsafeEqualityProof ... of { UnsafeRefl ->
            let a = e
                x = K a
            in ...  }
         Floating the case is OK here, even though it broadens the
         scope, because we are done with simplification.

    (U4) Ditto GHC.Core.Unfold.inlineBoringOk we want to treat
         the RHS of unsafeCoerce as very small; see
         Note [Inline unsafeCoerce] in that module.

    (U5) The definition of unsafeEqualityProof in Unsafe.Coerce
         looks very strange:
            unsafeEqualityProof = case unsafeEqualityProof @a @b of
                                     UnsafeRefl -> UnsafeRefl

         It looks recursive!  But the above-mentioned CoreToStg
         transform will change it to
            unsafeEqualityProof = UnsafeRefl
         And that is exactly the code we want!  For example, if we say
            f unsafeEqualityProof
         we want to pass an UnsafeRefl constructor to f.

         We turn off strictness analysis in this module, otherwise
         the strictness analyser will mark unsafeEqualityProof as
         bottom, which is utterly wrong.

    (U6) The UnsafeEquality data type is also special in one way.
         Consider this piece of Core
            case unsafeEqualityProof @Int @Bool of
               UnsafeRefl (g :: Int ~# Bool) -> ...g...

         The simplifier normally eliminates case alternatives with
         contradicatory GADT data constructors; here we bring into
         scope evidence (g :: Int~Bool).  But we do not want to
         eliminate this particular alternative!  So we put a special
         case into DataCon.dataConCannotMatch to account for this.

    (U7) We add a built-in RULE
           unsafeEqualityProof k t t  ==>  UnsafeRefl (Refl t)
         to simplify the ase when the two tpyes are equal.

    (U8) The is a super-magic RULE in GHC.base
             map coerce = coerce
         (see Note [Getting the map/coerce RULE to work] in CoreOpt)
         But it's all about turning coerce into a cast, and unsafeCoerce
         no longer does that.  So we need a separate map/unsafeCoerce
         RULE, in this module.

    There are yet more wrinkles

    (U9) unsafeCoerce works only over types of kind `Type`.
         But what about other types?  In Unsafe.Coerce we also define

          unsafeCoerceUnlifted :: forall (a :: TYPE UnliftedRep)
                                         (b :: TYPE UnliftedRep).
                                  a -> b
          unsafeCoerceUnlifted x
            = case unsafeEqualityProof @a @b of
                  UnsafeRefl -> x

         and similarly for unsafeCoerceAddr, unsafeCoerceInt, etc.

    (U10) We also want a levity-polymorphic unsafeCoerce#:

           unsafeCoerce# :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep)
                            (a :: TYPE r1) (b :: TYPE r2).
                            a -> b

          This is even more dangerous, because it converts between two types
          *with different runtime representations*!!  Our goal is to deprecate
          it entirely.  But for now we want it.

          But having it is hard!  It is defined by a kind of stub in Unsafe.Coerce,
          and overwritten by the desugarer.  See Note [Wiring in unsafeCoerce#]
          in Desugar.  Here's the code for it
            unsafeCoerce# x = case unsafeEqualityProof @r1 @r2 of UnsafeRefl ->
                              case unsafeEqualityProof @a  @b  of UnsafeRefl ->
                              x
          Notice that we can define this kind-/heterogeneous/ function by calling
          the kind-/homogeneous/ unsafeEqualityProof twice.

          See Note [Wiring in unsafeCoerce#] in Desugar.
    -}

    -- | This type is treated magically within GHC. Any pattern match of the
    -- form @case unsafeEqualityProof of UnsafeRefl -> body@ gets transformed just into @body@.
    -- This is ill-typed, but the transformation takes place after type-checking is
    -- complete. It is used to implement 'unsafeCoerce'. You probably don't want to
    -- use 'UnsafeRefl' in an expression, but you might conceivably want to pattern-match
    -- on it. Use 'unsafeEqualityProof' to create one of these.
    data UnsafeEquality a b where
      UnsafeRefl :: UnsafeEquality a a

    {-# NOINLINE unsafeEqualityProof #-}
    unsafeEqualityProof :: forall a b . UnsafeEquality a b
    -- See (U5) of Note [Implementing unsafeCoerce]
    unsafeEqualityProof = case unsafeEqualityProof @a @b of UnsafeRefl -> UnsafeRefl

    {-# INLINE [1] unsafeCoerce #-}
    -- The INLINE will almost certainly happen automatically, but it's almost
    -- certain to generate (slightly) better code, so let's do it.  For example
    --
    --   case (unsafeCoerce blah) of ...
    --
    -- will turn into
    --
    --   case unsafeEqualityProof of UnsafeRefl -> case blah of ...
    --
    -- which is definitely better.

    -- | Coerce a value from one type to another, bypassing the type-checker.
    --
    -- There are several legitimate ways to use 'unsafeCoerce':
    --
    --   1. To coerce e.g. @Int@ to @HValue@, put it in a list of @HValue@,
    --      and then later coerce it back to @Int@ before using it.
    --
    --   2. To produce e.g. @(a+b) :~: (b+a)@ from @unsafeCoerce Refl@.
    --      Here the two sides really are the same type -- so nothing unsafe is happening
    --      -- but GHC is not clever enough to see it.
    --
    --   3. In @Data.Typeable@ we have
    --
    --      @
    --        eqTypeRep :: forall k1 k2 (a :: k1) (b :: k2).
    --                     TypeRep a -> TypeRep b -> Maybe (a :~~: b)
    --        eqTypeRep a b
    --          | sameTypeRep a b = Just (unsafeCoerce HRefl)
    --          | otherwise       = Nothing
    --      @
    --
    --      Here again, the @unsafeCoerce HRefl@ is safe, because the two types really
    --      are the same  -- but the proof of that relies on the complex, trusted
    --      implementation of @Typeable@.
    --
    --   4. The "reflection trick", which takes advantage of the fact that in
    --      @class C a where { op :: ty }@, we can safely coerce between @C a@ and @ty@
    --      (which have different kinds!) because it's really just a newtype.
    --      Note: there is /no guarantee, at all/ that this behavior will be supported
    --      into perpetuity.
    unsafeCoerce :: forall (a :: Type) (b :: Type) . a -> b
    unsafeCoerce x = case unsafeEqualityProof @a @b of UnsafeRefl -> x

    unsafeCoerceUnlifted :: forall (a :: TYPE ('BoxedRep 'Unlifted)) (b :: TYPE ('BoxedRep 'Unlifted)) . a -> b
    -- Kind-homogeneous, but levity monomorphic (TYPE UnliftedRep)
    unsafeCoerceUnlifted x = case unsafeEqualityProof @a @b of UnsafeRefl -> x

    unsafeCoerceAddr :: forall (a :: TYPE 'AddrRep) (b :: TYPE 'AddrRep) . a -> b
    -- Kind-homogeneous, but levity monomorphic (TYPE AddrRep)
    unsafeCoerceAddr x = case unsafeEqualityProof @a @b of UnsafeRefl -> x

    -- | Highly, terribly dangerous coercion from one representation type
    -- to another. Misuse of this function can invite the garbage collector
    -- to trounce upon your data and then laugh in your face. You don't want
    -- this function. Really.
    unsafeCoerce# :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep)
                            (a :: TYPE r1) (b :: TYPE r2).
                     a -> b
    unsafeCoerce# = error "GHC internal error: unsafeCoerce# not unfolded"
    -- See (U10) of Note [Implementing unsafeCorece]
    -- The RHS is updated by Desugar.patchMagicDefns
    -- See Desugar Note [Wiring in unsafeCoerce#]

    {-# RULES
    -- See (U8) in Note [Implementing unsafeCoerce]

    -- unsafeCoerce version of the map/coerce rule defined in GHC.Base
    "map/unsafeCoerce" map unsafeCoerce = unsafeCoerce

    -- unsafeCoerce version of the amap/coerce rule defined in GHC.Arr
    "amap/unsafeCoerce" amap unsafeCoerce = unsafeCoerce
     #-}