Auto merge of #235 - smmalis37:patch-1, r=Amanieu

Implement `From<HashMap<T, ()>>` for `HashSet<T>`.

Closes #219.

src/set.rs

@@ -1107,6 +1107,15 @@ where
     }
 }
 
+impl<T, S, A> From<HashMap<T, (), S, A>> for HashSet<T, S, A>
+where
+    A: Allocator + Clone,
+{
+    fn from(map: HashMap<T, (), S, A>) -> Self {
+        Self { map }
+    }
+}
+
 impl<T, S, A> FromIterator<T> for HashSet<T, S, A>
 where
     T: Eq + Hash,
@@ -2038,6 +2047,23 @@ mod test_set {
         assert_eq!(i, expected.len());
     }
 
+    #[test]
+    fn test_from_map() {
+        let mut a = crate::HashMap::new();
+        a.insert(1, ());
+        a.insert(2, ());
+        a.insert(3, ());
+        a.insert(4, ());
+
+        let a: HashSet<_> = a.into();
+
+        assert_eq!(a.len(), 4);
+        assert!(a.contains(&1));
+        assert!(a.contains(&2));
+        assert!(a.contains(&3));
+        assert!(a.contains(&4));
+    }
+
     #[test]
     fn test_from_iter() {
         let xs = [1, 2, 2, 3, 4, 5, 6, 7, 8, 9];