Add a test for tracked function with multiple salsa struct args

tests/tracked_fn_multiple_args.rs

@@ -0,0 +1,25 @@
+//! Test that a `tracked` fn on multiple salsa struct args
+//! compiles and executes successfully.
+
+#[salsa::input]
+struct MyInput {
+    field: u32,
+}
+
+#[salsa::interned]
+struct MyInterned<'db> {
+    field: u32,
+}
+
+#[salsa::tracked]
+fn tracked_fn<'db>(db: &'db dyn salsa::Database, input: MyInput, interned: MyInterned<'db>) -> u32 {
+    input.field(db) + interned.field(db)
+}
+
+#[test]
+fn execute() {
+    let db = salsa::DatabaseImpl::new();
+    let input = MyInput::new(&db, 22);
+    let interned = MyInterned::new(&db, 33);
+    assert_eq!(tracked_fn(&db, input, interned), 55);
+}