assert that write_at doesn't change the seek position

Author: the8472

library/std/src/fs/tests.rs

@@ -500,8 +500,10 @@ fn file_test_append_write_at() {
     let msg = b"it's not working!";
     check!(fs::write(&filename, &msg));
     // write_at should work even in in append mode
-    let f = check!(fs::File::options().append(true).open(&filename));
+    let mut f = check!(fs::File::options().append(true).open(&filename));
+    assert_eq!(check!(f.stream_position()), 0);
     assert_eq!(check!(f.write_at(b"   ", 5)), 3);
+    assert_eq!(check!(f.stream_position()), 0);
 
     let content = check!(fs::read(&filename));
     assert_eq!(&content, b"it's     working!");

library/std/src/os/unix/fs/tests.rs

@@ -1,5 +1,7 @@
 use super::*;
 
+use crate::io::Seek;
+
 #[test]
 fn read_vectored_at() {
     let msg = b"preadv is working!";
@@ -40,13 +42,15 @@ fn write_vectored_at() {
     }
     let expected = {
         // Open in append mode to test that positioned writes bypass O_APPEND.
-        let file = fs::File::options().append(true).open(&filename).unwrap();
+        let mut file = fs::File::options().append(true).open(&filename).unwrap();
         let buf0 = b"    ";
         let buf1 = b"great  ";
 
         let iovec = [io::IoSlice::new(buf0), io::IoSlice::new(buf1)];
 
+        assert_eq!(file.stream_position().unwrap(), 0);
         let n = file.write_vectored_at(&iovec, 11).unwrap();
+        assert_eq!(file.stream_position().unwrap(), 0);
 
         assert!(n == 4 || n == 11);