Rollup merge of #62709 - nhynes:test-maplike-fromiter, r=cuviper

Test that maplike FromIter satisfies uniqueness

This PR adds a simple assertion to the `HashMap` and `HashSet` tests to ensure that uniqueness is satisfied when `FromIter`ing. This is useful for people who want to test their custom type against the Map/Set interfaces since they'll copy the tests wholesale but possibly miss this bug (where _they_ = _me_).

src/libstd/collections/hash/map.rs

@@ -3138,13 +3138,15 @@ fn test_reserve_shrink_to_fit() {
 
     #[test]
     fn test_from_iter() {
-        let xs = [(1, 1), (2, 2), (3, 3), (4, 4), (5, 5), (6, 6)];
+        let xs = [(1, 1), (2, 2), (2, 2), (3, 3), (4, 4), (5, 5), (6, 6)];
 
         let map: HashMap<_, _> = xs.iter().cloned().collect();
 
         for &(k, v) in &xs {
             assert_eq!(map.get(&k), Some(&v));
         }
+
+        assert_eq!(map.iter().len(), xs.len() - 1);
     }
 
     #[test]

src/libstd/collections/hash/set.rs

@@ -1782,13 +1782,15 @@ fn test_union() {
 
     #[test]
     fn test_from_iter() {
-        let xs = [1, 2, 3, 4, 5, 6, 7, 8, 9];
+        let xs = [1, 2, 2, 3, 4, 5, 6, 7, 8, 9];
 
         let set: HashSet<_> = xs.iter().cloned().collect();
 
         for x in &xs {
             assert!(set.contains(x));
         }
+
+        assert_eq!(set.iter().len(), xs.len() - 1);
     }
 
     #[test]