update test to check Take limits after copying

Author: the8472

Diff for: library/std/src/sys/unix/kernel_copy/tests.rs

@@ -42,8 +42,15 @@ fn copy_specialization() -> Result<()> {
         assert_eq!(sink.buffer(), b"wxyz");
 
         let copied = crate::io::copy(&mut source, &mut sink)?;
-        assert_eq!(copied, 10);
-        assert_eq!(sink.buffer().len(), 0);
+        assert_eq!(copied, 10, "copy obeyed limit imposed by Take");
+        assert_eq!(sink.buffer().len(), 0, "sink buffer was flushed");
+        assert_eq!(source.limit(), 0, "outer Take was exhausted");
+        assert_eq!(source.get_ref().buffer().len(), 0, "source buffer should be drained");
+        assert_eq!(
+            source.get_ref().get_ref().limit(),
+            1,
+            "inner Take allowed reading beyond end of file, some bytes should be left"
+        );
 
         let mut sink = sink.into_inner()?;
         sink.seek(SeekFrom::Start(0))?;