diff --git a/crates/red_knot/src/db.rs b/crates/red_knot/src/db.rs
index f5c366c5d5..4f6659c190 100644
--- a/crates/red_knot/src/db.rs
+++ b/crates/red_knot/src/db.rs
@@ -217,18 +217,27 @@ pub(crate) mod tests {
         fn upcast(&self) -> &(dyn SemanticDb + 'static) {
             self
         }
+        fn upcast_mut(&mut self) -> &mut (dyn SemanticDb + 'static) {
+            self
+        }
     }
 
     impl Upcast<dyn SourceDb> for TestDb {
         fn upcast(&self) -> &(dyn SourceDb + 'static) {
             self
         }
+        fn upcast_mut(&mut self) -> &mut (dyn SourceDb + 'static) {
+            self
+        }
     }
 
     impl Upcast<dyn ResolverDb> for TestDb {
         fn upcast(&self) -> &(dyn ResolverDb + 'static) {
             self
         }
+        fn upcast_mut(&mut self) -> &mut (dyn ResolverDb + 'static) {
+            self
+        }
     }
 
     impl red_knot_module_resolver::Db for TestDb {}