Add a sanity check in case of any duplicate nodes

compiler/rustc_query_system/src/dep_graph/graph.rs

@@ -1363,7 +1363,10 @@ impl DepNodeColorMap {
             Ordering::Relaxed,
         ) {
             Ok(_) => Ok(()),
-            Err(v) => Err(DepNodeIndex::from_u32(v)),
+            Err(v) => Err({
+                assert_ne!(v, COMPRESSED_RED, "tried to mark a red node as green");
+                DepNodeIndex::from_u32(v)
+            }),
         }
     }
 
@@ -1384,7 +1387,9 @@ impl DepNodeColorMap {
 
     #[inline]
     pub(super) fn insert_red(&self, index: SerializedDepNodeIndex) {
-        self.values[index].store(COMPRESSED_RED, Ordering::Release)
+        let value = self.values[index].swap(COMPRESSED_RED, Ordering::Release);
+        // Sanity check for duplicate nodes
+        assert_eq!(value, COMPRESSED_UNKNOWN, "trying to encode a dep node twice");
     }
 }