Review feedback: name modified UTF-8 NUL constant

Author: hs-viktor

Data/ByteString/Builder/Internal.hs

@@ -143,7 +143,7 @@ import qualified Data.ByteString.Short.Internal as Sh
 import qualified GHC.IO.Buffer as IO (Buffer(..), newByteBuffer)
 import           GHC.IO.Handle.Internals (wantWritableHandle, flushWriteBuffer)
 import           GHC.IO.Handle.Types (Handle__, haByteBuffer, haBufferMode)
-import           GHC.Exts
+import           GHC.Ptr (Ptr(..))
 import           System.IO (hFlush, BufferMode(..), Handle)
 import           Data.IORef
 
@@ -878,6 +878,11 @@ ascLiteralCopy = \ !ip !len -> builder $ \k br -> do
     let !ipe = ip `plusPtr` len
     wrappedBytesCopyStep (BufferRange ip ipe) k br
 
+-- | GHC represents @NUL@ in string literals via an overlong 2-byte encoding,
+-- which is part of "modified UTF-8" (GHC does not also implement CESU-8).
+modifiedUtf8NUL :: CString
+modifiedUtf8NUL = Ptr "\xc0\x80"#
+
 -- | Builder for raw 'Addr#' pointers to null-terminated primitive UTF-8
 -- encoded strings that may contain embedded overlong-encodings (as the
 -- two-byte sequence @0xC0 0x80@) of null characters.
@@ -886,7 +891,7 @@ ascLiteralCopy = \ !ip !len -> builder $ \k br -> do
 {-# INLINABLE modUtf8LitCopy #-}
 modUtf8LitCopy :: Ptr Word8 -> Int -> Builder
 modUtf8LitCopy = \ !ip !len -> builder $ \k br -> do
-    nullAt <- c_strstr (castPtr ip) (Ptr "\xc0\x80"#)
+    nullAt <- c_strstr (castPtr ip) modifiedUtf8NUL
     modUtf8_step ip len nullAt k br
 
 modUtf8_step :: Ptr Word8 -> Int -> Ptr Word8 -> BuildStep r -> BuildStep r
@@ -907,7 +912,7 @@ modUtf8_step !ip !len !nullAt k (BufferRange op0 ope)
             len' = len - used
             !ip' = ip `plusPtr` used
             !op' = op0 `plusPtr` (nullFree + 1)
-        nullAt' <- c_strstr ip' (Ptr "\xc0\x80"#)
+        nullAt' <- c_strstr ip' modifiedUtf8NUL
         modUtf8_step ip' len' nullAt' k (BufferRange op' ope)
     | avail > 0 = do
         -- avail <= nullFree