Rollup merge of #93064 - Aaron1011:provisional-dep-node, r=michaelwoerister

Properly track `DepNode`s in trait evaluation provisional cache

Fixes #92987

During evaluation of an auto trait predicate, we may encounter a cycle.
This causes us to store the evaluation result in a special 'provisional
cache;. If we later end up determining that the type can legitimately
implement the auto trait despite the cycle, we remove the entry from
the provisional cache, and insert it into the evaluation cache.

Additionally, trait evaluation creates a special anonymous `DepNode`.
All queries invoked during the predicate evaluation are added as
outoging dependency edges from the `DepNode`. This `DepNode` is then
store in the evaluation cache - if a different query ends up reading
from the cache entry, it will also perform a read of the stored
`DepNode`. As a result, the cached evaluation will still end up
(transitively) incurring all of the same dependencies that it would
if it actually performed the uncached evaluation (e.g. a call to
`type_of` to determine constituent types).

Previously, we did not correctly handle the interaction between the
provisional cache and the created `DepNode`. Storing an evaluation
result in the provisional cache would cause us to lose the `DepNode`
created during the evaluation. If we later moved the entry from the
provisional cache to the evaluation cache, we would use the `DepNode`
associated with the evaluation that caused us to 'complete' the cycle,
not the evaluatoon where we first discovered the cycle. As a result,
future reads from the evaluation cache would miss some incremental
compilation dependencies that would have otherwise been added if the
evaluation was *not* cached.

Under the right circumstances, this could lead to us trying to force
a query with a no-longer-existing `DefPathHash`, since we were missing
the (red) dependency edge that would have caused us to bail out before
attempting forcing.

This commit makes the provisional cache store the `DepNode` create
during the provisional evaluation. When we move an entry from the
provisional cache to the evaluation cache, we create a *new* `DepNode`
that has dependencies going to *both* of the evaluation `DepNodes` we
have available. This ensures that cached reads will incur all of
the necessary dependency edges.

Author: matthiaskrgr

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -765,14 +765,38 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             debug!(?result, "CACHE MISS");
             self.insert_evaluation_cache(param_env, fresh_trait_pred, dep_node, result);
 
-            stack.cache().on_completion(stack.dfn, |fresh_trait_pred, provisional_result| {
-                self.insert_evaluation_cache(
-                    param_env,
-                    fresh_trait_pred,
-                    dep_node,
-                    provisional_result.max(result),
-                );
-            });
+            stack.cache().on_completion(
+                stack.dfn,
+                |fresh_trait_pred, provisional_result, provisional_dep_node| {
+                    // Create a new `DepNode` that has dependencies on:
+                    // * The `DepNode` for the original evaluation that resulted in a provisional cache
+                    // entry being crated
+                    // * The `DepNode` for the *current* evaluation, which resulted in us completing
+                    // provisional caches entries and inserting them into the evaluation cache
+                    //
+                    // This ensures that when a query reads this entry from the evaluation cache,
+                    // it will end up (transitively) dependening on all of the incr-comp dependencies
+                    // created during the evaluation of this trait. For example, evaluating a trait
+                    // will usually require us to invoke `type_of(field_def_id)` to determine the
+                    // constituent types, and we want any queries reading from this evaluation
+                    // cache entry to end up with a transitive `type_of(field_def_id`)` dependency.
+                    //
+                    // By using `in_task`, we're also creating an edge from the *current* query
+                    // to the newly-created `combined_dep_node`. This is probably redundant,
+                    // but it's better to add too many dep graph edges than to add too few
+                    // dep graph edges.
+                    let ((), combined_dep_node) = self.in_task(|this| {
+                        this.tcx().dep_graph.read_index(provisional_dep_node);
+                        this.tcx().dep_graph.read_index(dep_node);
+                    });
+                    self.insert_evaluation_cache(
+                        param_env,
+                        fresh_trait_pred,
+                        combined_dep_node,
+                        provisional_result.max(result),
+                    );
+                },
+            );
         } else {
             debug!(?result, "PROVISIONAL");
             debug!(
@@ -781,7 +805,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 fresh_trait_pred, stack.depth, reached_depth,
             );
 
-            stack.cache().insert_provisional(stack.dfn, reached_depth, fresh_trait_pred, result);
+            stack.cache().insert_provisional(
+                stack.dfn,
+                reached_depth,
+                fresh_trait_pred,
+                result,
+                dep_node,
+            );
         }
 
         Ok(result)
@@ -2506,6 +2536,11 @@ struct ProvisionalEvaluation {
     from_dfn: usize,
     reached_depth: usize,
     result: EvaluationResult,
+    /// The `DepNodeIndex` created for the `evaluate_stack` call for this provisional
+    /// evaluation. When we create an entry in the evaluation cache using this provisional
+    /// cache entry (see `on_completion`), we use this `dep_node` to ensure that future reads from
+    /// the cache will have all of the necessary incr comp dependencies tracked.
+    dep_node: DepNodeIndex,
 }
 
 impl<'tcx> Default for ProvisionalEvaluationCache<'tcx> {
@@ -2548,6 +2583,7 @@ impl<'tcx> ProvisionalEvaluationCache<'tcx> {
         reached_depth: usize,
         fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
         result: EvaluationResult,
+        dep_node: DepNodeIndex,
     ) {
         debug!(?from_dfn, ?fresh_trait_pred, ?result, "insert_provisional");
 
@@ -2573,7 +2609,10 @@ impl<'tcx> ProvisionalEvaluationCache<'tcx> {
             }
         }
 
-        map.insert(fresh_trait_pred, ProvisionalEvaluation { from_dfn, reached_depth, result });
+        map.insert(
+            fresh_trait_pred,
+            ProvisionalEvaluation { from_dfn, reached_depth, result, dep_node },
+        );
     }
 
     /// Invoked when the node with dfn `dfn` does not get a successful
@@ -2624,7 +2663,7 @@ impl<'tcx> ProvisionalEvaluationCache<'tcx> {
     fn on_completion(
         &self,
         dfn: usize,
-        mut op: impl FnMut(ty::PolyTraitPredicate<'tcx>, EvaluationResult),
+        mut op: impl FnMut(ty::PolyTraitPredicate<'tcx>, EvaluationResult, DepNodeIndex),
     ) {
         debug!(?dfn, "on_completion");
 
@@ -2633,7 +2672,7 @@ impl<'tcx> ProvisionalEvaluationCache<'tcx> {
         {
             debug!(?fresh_trait_pred, ?eval, "on_completion");
 
-            op(fresh_trait_pred, eval.result);
+            op(fresh_trait_pred, eval.result, eval.dep_node);
         }
     }
 }

src/test/incremental/issue-92987-provisional-dep-node.rs

@@ -0,0 +1,24 @@
+// revisions: rpass1 rpass2
+
+// Regression test for issue #92987
+// Tests that we properly manage `DepNode`s during trait evaluation
+// involing an auto-trait cycle.
+
+#[cfg(rpass1)]
+struct CycleOne(Box<CycleTwo>);
+
+#[cfg(rpass2)]
+enum CycleOne {
+    Variant(Box<CycleTwo>)
+}
+
+struct CycleTwo(CycleOne);
+
+fn assert_send<T: Send>() {}
+
+fn bar() {
+    assert_send::<CycleOne>();
+    assert_send::<CycleTwo>();
+}
+
+fn main() {}