add FIXME for missing test

compiler/rustc_trait_selection/src/solve/search_graph/mod.rs

@@ -253,6 +253,15 @@ impl<'tcx> SearchGraph<'tcx> {
                     debug_assert!(entry.cycle_root_depth >= root_depth);
                     entry.cycle_root_depth = root_depth;
                     root.cycle_participants.insert(entry.input);
+                    // FIXME(@lcnr): I believe that this line is needed as we could
+                    // otherwise access a cache entry for the root of a cycle while
+                    // computing the result of a cycle participant. This can result
+                    // in unstable results due to incompleteness.
+                    //
+                    // However, a test for this would be an even more complex version of
+                    // tests/ui/traits/new-solver/coinduction/incompleteness-unstable-result.rs.
+                    // I did not bother to write such a test and we have no regression test
+                    // for this. It would be good to have such a test :)
                     #[allow(rustc::potential_query_instability)]
                     root.cycle_participants.extend(entry.cycle_participants.drain());
                 }