Test that HashMap, HashSet, and their iterators are properly covariant

src/libstd/collections/hash/map.rs

@@ -1670,6 +1670,20 @@ impl<K, S, Q: ?Sized> super::Recover<Q> for HashMap<K, (), S>
     }
 }
 
+#[allow(dead_code)]
+fn assert_covariance() {
+    fn map_key<'new>(v: HashMap<&'static str, u8>) -> HashMap<&'new str, u8> { v }
+    fn map_val<'new>(v: HashMap<u8, &'static str>) -> HashMap<u8, &'new str> { v }
+    fn iter_key<'a, 'new>(v: Iter<'a, &'static str, u8>) -> Iter<'a, &'new str, u8> { v }
+    fn iter_val<'a, 'new>(v: Iter<'a, u8, &'static str>) -> Iter<'a, u8, &'new str> { v }
+    fn into_iter_key<'new>(v: IntoIter<&'static str, u8>) -> IntoIter<&'new str, u8> { v }
+    fn into_iter_val<'new>(v: IntoIter<u8, &'static str>) -> IntoIter<u8, &'new str> { v }
+    fn keys_key<'a, 'new>(v: Keys<'a, &'static str, u8>) -> Keys<'a, &'new str, u8> { v }
+    fn keys_val<'a, 'new>(v: Keys<'a, u8, &'static str>) -> Keys<'a, u8, &'new str> { v }
+    fn values_key<'a, 'new>(v: Values<'a, &'static str, u8>) -> Values<'a, &'new str, u8> { v }
+    fn values_val<'a, 'new>(v: Values<'a, u8, &'static str>) -> Values<'a, u8, &'new str> { v }
+}
+
 #[cfg(test)]
 mod test_map {
     use prelude::v1::*;

src/libstd/collections/hash/set.rs

@@ -1024,6 +1024,21 @@ impl<'a, T, S> Iterator for Union<'a, T, S>
     fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
 }
 
+#[allow(dead_code)]
+fn assert_covariance() {
+    fn set<'new>(v: HashSet<&'static str>) -> HashSet<&'new str> { v }
+    fn iter<'a, 'new>(v: Iter<'a, &'static str>) -> Iter<'a, &'new str> { v }
+    fn into_iter<'new>(v: IntoIter<&'static str>) -> IntoIter<&'new str> { v }
+    fn difference<'a, 'new>(v: Difference<'a, &'static str, RandomState>)
+        -> Difference<'a, &'new str, RandomState> { v }
+    fn symmetric_difference<'a, 'new>(v: SymmetricDifference<'a, &'static str, RandomState>)
+        -> SymmetricDifference<'a, &'new str, RandomState> { v }
+    fn intersection<'a, 'new>(v: Intersection<'a, &'static str, RandomState>)
+        -> Intersection<'a, &'new str, RandomState> { v }
+    fn union<'a, 'new>(v: Union<'a, &'static str, RandomState>)
+        -> Union<'a, &'new str, RandomState> { v }
+}
+
 #[cfg(test)]
 mod test_set {
     use prelude::v1::*;