Auto merge of #90361 - Mark-Simulacrum:always-verify, r=michaelwoerister

Enable verification for 1/32th of queries loaded from disk

This is a limited enabling of incremental verification for query results loaded from disk, which previously did not run without -Zincremental-verify-ich. If enabled for all queries, we see a probably unacceptable hit of ~50% in the worst case, so this pairs back the verification to a more limited set based on the hash key.

Per collected [perf results](https://github.com/rust-lang/rust/pull/84227#issuecomment-953350582), this is a regression of at most 7% on coercions opt incr-unchanged, and typically less than 0.5% on other benchmarks (largely limited to incr-unchanged). I believe this is acceptable performance to land, and we can either ratchet it up or down fairly easily.

We have no real sense of whether this will lead to a large amount of assertions in the wild, but since those assertions may lead to miscompilations today, it seems potentially warranted. We have a good bit of lead time until the next stable release, though the holiday season will also start soon; we may wish to discuss the timing of enabling this and weigh the desire to prevent (possible) miscompilations against assertions.

cc `@rust-lang/wg-incr-comp`

compiler/rustc_query_system/src/query/plumbing.rs

@@ -518,9 +518,22 @@ where
         prof_timer.finish_with_query_invocation_id(dep_node_index.into());
 
         if let Some(result) = result {
+            let prev_fingerprint = tcx
+                .dep_context()
+                .dep_graph()
+                .prev_fingerprint_of(dep_node)
+                .unwrap_or(Fingerprint::ZERO);
             // If `-Zincremental-verify-ich` is specified, re-hash results from
             // the cache and make sure that they have the expected fingerprint.
-            if unlikely!(tcx.dep_context().sess().opts.debugging_opts.incremental_verify_ich) {
+            //
+            // If not, we still seek to verify a subset of fingerprints loaded
+            // from disk. Re-hashing results is fairly expensive, so we can't
+            // currently afford to verify every hash. This subset should still
+            // give us some coverage of potential bugs though.
+            let try_verify = prev_fingerprint.as_value().1 % 32 == 0;
+            if unlikely!(
+                try_verify || tcx.dep_context().sess().opts.debugging_opts.incremental_verify_ich
+            ) {
                 incremental_verify_ich(*tcx.dep_context(), &result, dep_node, query);
             }