-- 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 {- 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 case when the two types 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 GHC.Core.SimpleOpt) 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. Adding these RULES means we must delay inlinine unsafeCoerce until the RULES have had a chance to fire; hence the INLINE[1] pragma on unsafeCoerce. (Side note: this has the coincidental benefit of making the unsafeCoerce-based version of the `reflection` library work -- see #21575.) 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 representation-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 :: forall {k} (a :: k) (b :: k). UnsafeEquality a b unsafeEqualityProof = case forall (a :: k) (b :: k). UnsafeEquality a b forall {k} (a :: k) (b :: k). UnsafeEquality a b unsafeEqualityProof @a @b of UnsafeEquality a b UnsafeRefl -> UnsafeEquality a a UnsafeEquality a b forall {k} (a :: k). UnsafeEquality a a 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. -- -- Why delay inlining to Phase 1? Because of the RULES for map/unsafeCoerce; -- see (U8) in Note [Implementing unsafeCoerce] -- | `unsafeCoerce` coerces a value from one type to another, bypassing the type-checker. -- -- There are several legitimate ways to use 'unsafeCoerce': -- -- 1. To coerce a lifted type such as @Int@ to @Any@, put it in a list of @Any@, -- 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. (superseded) 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. -- It is now preferred to use `withDict` in @GHC.Magic.Dict@, which -- is type-safe. See Note [withDict] in GHC.Tc.Instance.Class for details. -- -- 5. (superseded) Casting between two types which have exactly the same structure: -- between a newtype of T and T, or between types which differ only -- in "phantom" type parameters. -- It is now preferred to use `coerce` from @Data.Coerce@, which -- is type-safe. -- -- Other uses of 'unsafeCoerce' are undefined. In particular, you should not use -- 'unsafeCoerce' to cast a T to an algebraic data type D, unless T is also -- an algebraic data type. For example, do not cast @'Int'->'Int'@ to 'Bool', even if -- you later cast that 'Bool' back to @'Int'->'Int'@ before applying it. The reasons -- have to do with GHC's internal representation details (for the cognoscenti, data values -- can be entered but function closures cannot). If you want a safe type to cast things -- to, use 'Any', which is not an algebraic data type. -- NB. It is tempting to think that casting a value to a type that it doesn't have is safe -- as long as you don't "do anything" with the value in its cast form, such as seq on it. This -- isn't the case: the compiler can insert seqs itself, and if these happen at the wrong type, -- Bad Things Might Happen. See bug #1616: in this case we cast a function of type (a,b) -> (a,b) -- to () -> () and back again. The strictness analyser saw that the function was strict, but -- the wrapper had type () -> (), and hence the wrapper de-constructed the (), the worker re-constructed -- a new (), with the result that the code ended up with "case () of (a,b) -> ...". unsafeCoerce :: forall (a :: Type) (b :: Type) . a -> b unsafeCoerce :: forall a b. a -> b unsafeCoerce a x = case forall {k} (a :: k) (b :: k). UnsafeEquality a b forall a b. UnsafeEquality a b unsafeEqualityProof @a @b of UnsafeEquality a b UnsafeRefl -> a b x unsafeCoerceUnlifted :: forall (a :: TYPE ('BoxedRep 'Unlifted)) (b :: TYPE ('BoxedRep 'Unlifted)) . a -> b -- Kind-homogeneous, but representation-monomorphic (TYPE UnliftedRep) unsafeCoerceUnlifted :: forall (a :: UnliftedType) (b :: UnliftedType). a -> b unsafeCoerceUnlifted a x = case forall {k} (a :: k) (b :: k). UnsafeEquality a b forall (a :: UnliftedType) (b :: UnliftedType). UnsafeEquality a b unsafeEqualityProof @a @b of UnsafeEquality a b UnsafeRefl -> a b x unsafeCoerceAddr :: forall (a :: TYPE 'AddrRep) (b :: TYPE 'AddrRep) . a -> b -- Kind-homogeneous, but representation-monomorphic (TYPE AddrRep) unsafeCoerceAddr :: forall (a :: TYPE 'AddrRep) (b :: TYPE 'AddrRep). a -> b unsafeCoerceAddr a x = case forall {k} (a :: k) (b :: k). UnsafeEquality a b forall (a :: TYPE 'AddrRep) (b :: TYPE 'AddrRep). UnsafeEquality a b unsafeEqualityProof @a @b of UnsafeEquality a b UnsafeRefl -> a b 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# :: forall a b. a -> b unsafeCoerce# = [Char] -> a -> b forall a. HasCallStack => [Char] -> a error [Char] "GHC internal error: unsafeCoerce# not unfolded" -- See (U10) of Note [Implementing unsafeCoerce] -- 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 #-}