test VecDeque::iter_mut aliasing

tests/run-pass/vecdeque.rs

@@ -1,5 +1,17 @@
 use std::collections::VecDeque;
 
+fn test_all_refs<'a, T: 'a>(dummy: &mut T, iter: impl Iterator<Item = &'a mut T>) {
+    // Gather all those references.
+    let mut refs: Vec<&mut T> = iter.collect();
+    // Use them all. Twice, to be sure we got all interleavings.
+    for r in refs.iter_mut() {
+        std::mem::swap(dummy, r);
+    }
+    for r in refs {
+        std::mem::swap(dummy, r);
+    }
+}
+
 fn main() {
     let mut dst = VecDeque::new();
     dst.push_front(Box::new(1));
@@ -18,6 +30,21 @@ fn main() {
     println!("{:?}", VecDeque::<u32>::new().iter());
 
     for a in dst {
-      assert_eq!(*a, 2);
+        assert_eq!(*a, 2);
     }
+
+    // # Aliasing tests.
+    let mut v = std::collections::VecDeque::new();
+    v.push_back(1);
+    v.push_back(2);
+
+    // Test `fold` bad aliasing.
+    let mut it = v.iter_mut();
+    let ref0 = it.next().unwrap();
+    let sum = it.fold(0, |x, y| x + *y);
+    assert_eq!(*ref0 + sum, 3);
+
+    // Test general iterator aliasing.
+    v.push_front(0);
+    test_all_refs(&mut 0, v.iter_mut());
 }