Dispatch to slow cast when buitlin unavailable

- GHC.Float exports them starting from base-4.10.0.0 which starts with
  with ghc 8.2.1

Author: la-wu

Data/ByteString/Builder/RealFloat/D2S.hs

@@ -14,7 +14,6 @@ import Data.ByteString.Builder.RealFloat.Internal
 import Data.ByteString.Builder.RealFloat.TableGenerator
 import Data.Maybe (fromMaybe)
 import GHC.Exts
-import GHC.Float (castDoubleToWord64)
 import GHC.Int (Int32(..), Int64(..))
 import GHC.ST (ST(..), runST)
 import GHC.Word (Word32(..), Word64(..))

Data/ByteString/Builder/RealFloat/F2S.hs

@@ -13,7 +13,6 @@ import Data.ByteString.Builder.Prim (primBounded)
 import Data.ByteString.Builder.RealFloat.Internal
 import Data.ByteString.Builder.RealFloat.TableGenerator
 import GHC.Exts
-import GHC.Float (castFloatToWord32)
 import GHC.Int (Int32(..))
 import GHC.ST (ST(..), runST)
 import GHC.Word (Word32(..), Word64(..))

Data/ByteString/Builder/RealFloat/Internal.hs

@@ -1,5 +1,6 @@
 {-# LANGUAGE ScopedTypeVariables, ExplicitForAll #-}
 {-# LANGUAGE BangPatterns, MagicHash, UnboxedTuples #-}
+{-# LANGUAGE CPP #-}
 
 module Data.ByteString.Builder.RealFloat.Internal
     ( (.>>)
@@ -42,6 +43,8 @@ module Data.ByteString.Builder.RealFloat.Internal
     , box
     , unbox
     , ByteArray(..)
+    , castDoubleToWord64
+    , castFloatToWord32
     ) where
 
 import Control.Monad (foldM)
@@ -56,6 +59,23 @@ import GHC.Word (Word8, Word32(..), Word64(..))
 import Foreign.Ptr (plusPtr)
 import qualified Foreign.Storable as S (poke)
 
+#if __GLASGOW_HASKELL__ >= 802
+import GHC.Float (castFloatToWord32, castDoubleToWord64)
+#else
+import System.IO.Unsafe (unsafePerformIO)
+import Foreign.Marshal.Utils (with)
+import Foreign.Ptr (castPtr)
+import Foreign.Storable (peek)
+{-# NOINLINE castFloatToWord32 #-}
+castFloatToWord32 :: Float -> Word32
+castFloatToWord32 x = unsafePerformIO (with x (peek . castPtr))
+
+-- | Convert a 'Double' to a 'Word64'.
+{-# NOINLINE castDoubleToWord64 #-}
+castDoubleToWord64 :: Double -> Word64
+castDoubleToWord64 x = unsafePerformIO (with x (peek . castPtr))
+#endif
+
 {-# INLINABLE (.>>) #-}
 (.>>) :: (Bits a, Integral b) => a -> b -> a
 a .>> s = unsafeShiftR a (fromIntegral s)