Add stability test for sort_by_cached_key

src/liballoc/tests/slice.rs

@@ -485,24 +485,29 @@ fn test_sort_stability() {
  // the second item represents which occurrence of that
  // number this element is, i.e. the second elements
  // will occur in sorted order.
- let mut v: Vec<_> = (0..len)
+ let mut orig: Vec<_> = (0..len)
  .map(|_| {
  let n = thread_rng().gen::<usize>() % 10;
  counts[n] += 1;
  (n, counts[n])
  })
  .collect();
 
- // only sort on the first element, so an unstable sort
+ let mut v = orig.clone();
+ // Only sort on the first element, so an unstable sort
  // may mix up the counts.
  v.sort_by(|&(a, _), &(b, _)| a.cmp(&b));
 
- // this comparison includes the count (the second item
+ // This comparison includes the count (the second item
  // of the tuple), so elements with equal first items
  // will need to be ordered with increasing
  // counts... i.e. exactly asserting that this sort is
  // stable.
  assert!(v.windows(2).all(|w| w[0] <= w[1]));
+
+ let mut v = orig.clone();
+ v.sort_by_cached_key(|&(x, _)| x);
+ assert!(v.windows(2).all(|w| w[0] <= w[1]));
  }
  }
 }