Add test for cycle detection

tests/cycles.rs

@@ -0,0 +1,73 @@
+#![feature(crate_visibility_modifier)]
+
+#[derive(Default)]
+pub struct QueryContextImpl {
+    runtime: salsa::runtime::Runtime<QueryContextImpl>,
+}
+
+impl salsa::QueryContext for QueryContextImpl {
+    fn salsa_runtime(&self) -> &salsa::runtime::Runtime<QueryContextImpl> {
+        &self.runtime
+    }
+}
+
+salsa::query_context_storage! {
+    pub struct QueryContextImplStorage for QueryContextImpl {
+        impl QueryContext {
+            fn memoized_a() for MemoizedA;
+            fn memoized_b() for MemoizedB;
+            fn volatile_a() for VolatileA;
+            fn volatile_b() for VolatileB;
+        }
+    }
+}
+
+trait QueryContext: salsa::QueryContext {
+    salsa::query_prototype! {
+        // `a` and `b` depend on each other and form a cycle
+        fn memoized_a() for MemoizedA;
+        fn memoized_b() for MemoizedB;
+        fn volatile_a() for VolatileA;
+        fn volatile_b() for VolatileB;
+    }
+}
+
+salsa::query_definition! {
+    crate MemoizedA(query: &impl QueryContext, (): ()) -> () {
+        query.memoized_b().get(())
+    }
+}
+
+salsa::query_definition! {
+    crate MemoizedB(query: &impl QueryContext, (): ()) -> () {
+        query.memoized_a().get(())
+    }
+}
+
+salsa::query_definition! {
+    #[storage(volatile)]
+    crate VolatileA(query: &impl QueryContext, (): ()) -> () {
+        query.volatile_b().get(())
+    }
+}
+
+salsa::query_definition! {
+    #[storage(volatile)]
+    crate VolatileB(query: &impl QueryContext, (): ()) -> () {
+        query.volatile_a().get(())
+    }
+}
+
+#[test]
+#[should_panic(expected = "cycle detected")]
+fn cycle_memoized() {
+    let query = QueryContextImpl::default();
+    query.memoized_a().get(());
+}
+
+#[test]
+#[should_panic(expected = "cycle detected")]
+fn cycle_volatile() {
+    let query = QueryContextImpl::default();
+    query.volatile_a().get(());
+}