Fix t_peek_cstring test

src/Data/Text/Foreign.hs

@@ -158,6 +158,9 @@ asForeignPtr t@(Text _arr _off len) = do
 -- | Marshal a 'Text' into a C string with a trailing NUL byte,
 -- encoded as UTF-8 in temporary storage.
 --
+-- The 'Text' itself must not contain any NUL bytes, this precondition
+-- is not checked. Cf. 'withCStringLen'.
+--
 -- The temporary storage is freed when the subcomputation terminates
 -- (either normally or via an exception), so the pointer to the
 -- temporary storage must /not/ be used after this function returns.

tests/Tests/Properties/LowLevel.hs

@@ -73,7 +73,7 @@ t_use_from0 t = ioProperty $ do
   let t' = t `T.snoc` '\0'
   (== T.takeWhile (/= '\0') t') <$> useAsPtr t' (const . fromPtr0)
 
-t_peek_cstring t = ioProperty $ do
+t_peek_cstring t = T.all (/= '\0') t ==> ioProperty $ do
   roundTrip <- T.withCString t T.peekCString
   assertEqual "cstring" t roundTrip