convert a (pinned) bytearray to a bytestring without copying

ByteArrayToByteString.hs

byteArrayToByteString :: ByteArray -> ByteString
byteArrayToByteString (ByteArray b#) =
  let contents# = byteArrayContents# b#
      fp = ForeignPtr contents# (PlainPtr (byteArrayToMutableByteArray b#))
      len = lenAddr contents#
  in PS fp 0 len

byteArrayToMutableByteArray :: ByteArray# -> MutableByteArray# RealWorld
byteArrayToMutableByteArray = unsafeCoerce#

lenAddr :: Addr# -> Int
lenAddr addr# = fromIntegral . accursedUnutterablePerformIO $ c_strlen (Ptr addr#)