Test for checking the properties of GHC byte array alignment

GHCByteArrayAlignmentTest.hs

import Data.Foldable (forM_)
import Data.Word(Word8)
import Foreign.Storable (Storable(..))
import GHC.Ptr (Ptr (..))
import GHC.Base (addr2Int#, int2Word#)
import Streamly.Internal.Data.Unboxed
    ( MutableByteArray(..)
    , Unbox(..)
    , Unboxed
    , castContents
    )

newtype TestAlignmentType = TestAlignmentType Word8

testAlignmentTypeAlignment :: Int
testAlignmentTypeAlignment = 64

instance Storable TestAlignmentType where
    sizeOf _ = 1
    alignment _ = testAlignmentTypeAlignment
    peek = undefined
    poke = undefined

instance Unbox TestAlignmentType where
    box marr i = TestAlignmentType <$> box (castContents marr) i
    unbox marr i (TestAlignmentType a) = unbox (castContents marr) i a

-- If this test passes, the following can be considered true:
-- 1. Alignment affects the entire chunk of the array and not every element.
-- 2. Alignemnt only matters in allocation. It is transparent, thereafter.
testAlignment :: IO ()
testAlignment = do
    let n = 5
        lst = [0 .. (n - 1)]
        lstTA = TestAlignmentType . fromIntegral <$> lst
    arr <- S.fold (Fold.foldlM' MA.snoc (MA.newArray n)) (S.fromList lstTA)
    MA.asPtrUnsafe arr
        $ \(Ptr a) ->
              case fromIntegral (W# (int2Word# (addr2Int# a)))
                       `rem` testAlignmentTypeAlignment of
                  0 -> return ()
                  _ -> expectationFailure "Incorrect initial alignment."
    let mW8Arr = MA.cast arr
    case mW8Arr of
        Nothing -> expectationFailure "Couldn't cast."
        Just w8Arr -> MA.toList w8Arr `shouldReturn` (intToW8 <$> lst)

    where

    intToW8 :: Int -> Word8
    intToW8 = fromIntegral