convert the example into a test

examples/hello_world/main.rs

@@ -5,6 +5,16 @@ mod implementation;
 use self::class_table::ClassTableQueryContext;
 use self::implementation::QueryContextImpl;
 
+#[test]
+fn test() {
+    let query = QueryContextImpl::default();
+    let all_def_ids = query.all_fields().of(());
+    assert_eq!(
+        format!("{:?}", all_def_ids),
+        "[DefId(1), DefId(2), DefId(11), DefId(12)]"
+    );
+}
+
 fn main() {
     let query = QueryContextImpl::default();
     for f in query.all_fields().of(()).iter() {

src/lib.rs

@@ -48,7 +48,7 @@ pub trait BaseQueryContext: Sized {
 pub trait Query<QC: BaseQueryContext>: Debug + Default + Sized + 'static {
     type Key: Clone + Debug + Hash + Eq + Send;
     type Value: Clone + Debug + Hash + Eq + Send;
-    type Storage: QueryStorageOps<QC, Self>;
+    type Storage: QueryStorageOps<QC, Self> + Send;
 
     fn execute(query: &QC, key: Self::Key) -> Self::Value;
 }