make dyn Trait query implementations work

tests/dyn_trait.rs

@@ -0,0 +1,32 @@
+//! Test that you can implement a query using a `dyn Trait` setup.
+
+#[salsa::database(DynTraitStorage)]
+#[derive(Default)]
+struct DynTraitDatabase {
+    runtime: salsa::Runtime<DynTraitDatabase>,
+}
+
+impl salsa::Database for DynTraitDatabase {
+    fn salsa_runtime(&self) -> &salsa::Runtime<DynTraitDatabase> {
+        &self.runtime
+    }
+}
+
+#[salsa::query_group(DynTraitStorage)]
+trait DynTrait {
+    #[salsa::input]
+    fn input(&self, x: u32) -> u32;
+
+    fn output(&self, x: u32) -> u32;
+}
+
+fn output(db: &dyn DynTrait, x: u32) -> u32 {
+    db.input(x) * 2
+}
+
+#[test]
+fn dyn_trait() {
+    let mut query = DynTraitDatabase::default();
+    query.set_input(22, 23);
+    assert_eq!(query.output(22), 46);
+}