massert: implement HasKey

mtest/massert/massert.go

@@ -375,3 +375,21 @@ func Has(set, elem interface{}) Assertion {
 		return errors.New("value not in set")
 	}, toStr(set)+" has value "+toStr(elem), 0)
 }
+
+// HasKey asserts that the given set (which must be a map type) has the given
+// element as a key in it.
+func HasKey(set, elem interface{}) Assertion {
+	return newAssertion(func() error {
+		v := reflect.ValueOf(set)
+		if v.Kind() != reflect.Map {
+			return fmt.Errorf("type %s is not a map", v.Type())
+		}
+
+		for _, key := range v.MapKeys() {
+			if reflect.DeepEqual(key.Interface(), elem) {
+				return nil
+			}
+		}
+		return errors.New("value not a key in the map")
+	}, toStr(set)+" has key "+toStr(elem), 0)
+}

mtest/massert/massert_test.go

@@ -157,3 +157,18 @@ func TestHas(t *T) {
 		Has(map[int]int{1: 2, 2: 1}, 3),
 	))
 }
+
+func TestHasKey(t *T) {
+	Fatal(t, All(
+		HasKey(map[int]int{1: 1}, 1),
+		HasKey(map[int]int{1: 1, 2: 2}, 1),
+		HasKey(map[int]int{1: 1, 2: 2}, 2),
+	))
+
+	Fatal(t, None(
+		HasKey([]int{}, 1),
+		HasKey([]int{1}, 1),
+		HasKey(map[int]int{}, 1),
+		HasKey(map[int]int{2: 2}, 1),
+	))
+}