Possible optimization of useAsCString?

[Lysxia]

useAsCString uses withForeignPtr, however the pointer is only used for the initial copy. Would lifting the subsequent actions out of that block allow you to use unsafeWithForeignPtr instead?

bytestring/Data/ByteString.hs

Lines 1735 to 1742 in 0f49510

	useAsCString :: ByteString -> (CString -> IO a) -> IO a
	useAsCString (BS fp l) action =
	allocaBytes (l+1) $ \buf ->
	-- Cannot use unsafeWithForeignPtr, because action can diverge
	withForeignPtr fp $ \p -> do
	memcpy buf p l
	pokeByteOff buf l (0::Word8)
	action (castPtr buf)

allocaBytes (l+1) $ \buf -> do
 unsafeWithForeignPtr fp $ \p -> memcpy buf p l
 pokeByteOff buf l (0::Word8) 
 action (castPtr buf) 

[clyring]

It looks that way to me. A pull request would be welcome, I believe.