Test sort algorithms using a random cmp function

src/libcollectionstest/slice.rs

@@ -383,9 +383,11 @@ fn test_reverse() {
 
 #[test]
 fn test_sort() {
+    let mut rng = thread_rng();
+
     for len in (2..25).chain(500..510) {
         for _ in 0..100 {
-            let mut v: Vec<_> = thread_rng().gen_iter::<i32>().take(len).collect();
+            let mut v: Vec<_> = rng.gen_iter::<i32>().take(len).collect();
             let mut v1 = v.clone();
 
             v.sort();
@@ -399,6 +401,18 @@ fn test_sort() {
         }
     }
 
+    // Sort using a completely random comparison function.
+    // This will reorder the elements *somehow*, but won't panic.
+    let mut v = [0; 500];
+    for i in 0..v.len() {
+        v[i] = i as i32;
+    }
+    v.sort_by(|_, _| *rng.choose(&[Less, Equal, Greater]).unwrap());
+    v.sort();
+    for i in 0..v.len() {
+        assert_eq!(v[i], i as i32);
+    }
+
     // Should not panic.
     [0i32; 0].sort();
     [(); 10].sort();

src/libcoretest/slice.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use core::cmp::Ordering::{Equal, Greater, Less};
 use core::slice::heapsort;
 use core::result::Result::{Ok, Err};
 use rand::{Rng, XorShiftRng};
@@ -268,6 +269,17 @@ fn sort_unstable() {
         }
     }
 
+    // Sort using a completely random comparison function.
+    // This will reorder the elements *somehow*, but won't panic.
+    for i in 0..v.len() {
+        v[i] = i as i32;
+    }
+    v.sort_unstable_by(|_, _| *rng.choose(&[Less, Equal, Greater]).unwrap());
+    v.sort_unstable();
+    for i in 0..v.len() {
+        assert_eq!(v[i], i as i32);
+    }
+
     // Should not panic.
     [0i32; 0].sort_unstable();
     [(); 10].sort_unstable();