fix couple of clippy findings:

filter_map_identity
iter_kv_map
needless_question_mark
redundant_at_rest_pattern
filter_next
derivable_impls

compiler/rustc_query_impl/src/plumbing.rs

@@ -183,7 +183,7 @@ pub(super) fn encode_all_query_results<'tcx>(
     encoder: &mut CacheEncoder<'_, 'tcx>,
     query_result_index: &mut EncodedDepNodeIndex,
 ) {
-    for encode in super::ENCODE_QUERY_RESULTS.iter().copied().filter_map(|e| e) {
+    for encode in super::ENCODE_QUERY_RESULTS.iter().copied().flatten() {
         encode(tcx, encoder, query_result_index);
     }
 }