Add extra assertion.

Author: cjgillot

compiler/rustc_query_system/src/dep_graph/graph.rs

@@ -864,6 +864,10 @@ impl<K: DepKind> DepGraphData<K> {
     ) -> Option<DepNodeIndex> {
         let frame = MarkFrame { index: prev_dep_node_index, parent: frame };
 
+        #[cfg(not(parallel_compiler))]
+        self.assert_nonexistent_node(dep_node, || {
+            format!("trying to mark existing {dep_node:?} as green")
+        });
         #[cfg(not(parallel_compiler))]
         debug_assert!(self.colors.get(prev_dep_node_index).is_none());