panic when reading fields of tracked structs from older revisions

components/salsa-2022/src/function/maybe_changed_after.rs

@@ -170,12 +170,18 @@ where
                 // during this revision or is otherwise stale.
                 return false;
             }
-            QueryOrigin::BaseInput | QueryOrigin::Field => {
-                // BaseInput: This value was `set` by the mutator thread -- ie, it's a base input and it cannot be out of date.
-                // Field: This value is the value of a field of some tracked struct S. It is always updated whenever S is created.
-                // So if a query has access to S, then they will have an up-to-date value.
+            QueryOrigin::BaseInput => {
+                // This value was `set` by the mutator thread -- ie, it's a base input and it cannot be out of date.
                 return true;
             }
+            QueryOrigin::Field => {
+                // This value is the value of a field of some tracked struct S.
+                // The fact that we are here means that we are accessing fields from old revisions, which is not allowed.
+                panic!(
+                    "accessing fields of tracked struct {:?} from older revisions",
+                    std::any::type_name::<<C as Configuration>::SalsaStruct>()
+                )
+            }
             QueryOrigin::DerivedUntracked(_) => {
                 // Untracked inputs? Have to assume that it changed.
                 return false;

salsa-2022-tests/tests/panic-when-reading-fields-of-tracked-structs-from-older-revisions.rs

@@ -0,0 +1,46 @@
+use test_log::test;
+
+#[salsa::jar(db = Db)]
+struct Jar(MyInput, MyTracked, tracked_fn);
+
+trait Db: salsa::DbWithJar<Jar> {}
+
+#[salsa::input(jar = Jar)]
+struct MyInput {
+    field: u32,
+}
+
+#[salsa::tracked(jar = Jar)]
+struct MyTracked {
+    field: u32,
+}
+
+#[salsa::tracked(jar = Jar)]
+fn tracked_fn(db: &dyn Db, input: MyInput) -> MyTracked {
+    MyTracked::new(db, input.field(db) / 2)
+}
+
+#[salsa::db(Jar)]
+#[derive(Default)]
+struct Database {
+    storage: salsa::Storage<Self>,
+}
+
+impl salsa::Database for Database {}
+
+impl Db for Database {}
+
+#[test]
+#[should_panic(expected = "accessing fields of tracked struct")]
+fn execute() {
+    let mut db = Database::default();
+
+    let input = MyInput::new(&mut db, 22);
+    let tracked = tracked_fn(&db, input);
+
+    // modify the input and change the revision
+    input.set_field(&mut db).to(24);
+
+    // panic when reading fields of tracked structs from older revisions
+    tracked.field(&db);
+}