Auto merge of rust-lang#95031 - compiler-errors:param-env-cache, r=Aaron1011

Do not use `ParamEnv::and` when building a cache key from a param-env and trait eval candidate

Do not use `ParamEnv::and` to cache a param-env with a selection/evaluation candidate.

This is because if the param-env is `RevealAll` mode, and the candidate looks global (i.e. it has erased regions, which can show up when we normalize a projection type under a binder<sup>1</sup>), then when we use `ParamEnv::and` to pair the candidate and the param-env for use as a cache key, we will throw away the param-env's caller bounds, and we'll end up caching a candidate that we inferred from the param-env with a empty param-env, which may cause cache-hit later when we have an empty param-env, and possibly mess with normalization like we see in the referenced issue during codegen.

Not sure how to trigger this with a more structured test, but changing `check-pass` to `build-pass` triggers the case that rust-lang#94903 detected.

<sup>1.</sup> That is, we will replace the late-bound region with a placeholder, which gets canonicalized and turned into an infererence variable, which gets erased during region freshening right before we cache the result. Sorry, it's quite a few steps.

Fixes rust-lang#94903
r? `@Aaron1011` (or reassign as you see fit)

Author: bors

compiler/rustc_middle/src/traits/select.rs

@@ -13,12 +13,19 @@ use rustc_hir::def_id::DefId;
 use rustc_query_system::cache::Cache;
 
 pub type SelectionCache<'tcx> = Cache<
-    ty::ParamEnvAnd<'tcx, ty::TraitPredicate<'tcx>>,
+    // This cache does not use `ParamEnvAnd` in its keys because `ParamEnv::and` can replace
+    // caller bounds with an empty list if the `TraitPredicate` looks global, which may happen
+    // after erasing lifetimes from the predicate.
+    (ty::ParamEnv<'tcx>, ty::TraitPredicate<'tcx>),
     SelectionResult<'tcx, SelectionCandidate<'tcx>>,
 >;
 
-pub type EvaluationCache<'tcx> =
-    Cache<ty::ParamEnvAnd<'tcx, ty::PolyTraitPredicate<'tcx>>, EvaluationResult>;
+pub type EvaluationCache<'tcx> = Cache<
+    // See above: this cache does not use `ParamEnvAnd` in its keys due to sometimes incorrectly
+    // caching with the wrong `ParamEnv`.
+    (ty::ParamEnv<'tcx>, ty::PolyTraitPredicate<'tcx>),
+    EvaluationResult,
+>;
 
 /// The selection process begins by considering all impls, where
 /// clauses, and so forth that might resolve an obligation. Sometimes

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

@@ -1025,11 +1025,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         let tcx = self.tcx();
         if self.can_use_global_caches(param_env) {
-            if let Some(res) = tcx.evaluation_cache.get(&param_env.and(trait_pred), tcx) {
+            if let Some(res) = tcx.evaluation_cache.get(&(param_env, trait_pred), tcx) {
                 return Some(res);
             }
         }
-        self.infcx.evaluation_cache.get(&param_env.and(trait_pred), tcx)
+        self.infcx.evaluation_cache.get(&(param_env, trait_pred), tcx)
     }
 
     fn insert_evaluation_cache(
@@ -1060,13 +1060,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // FIXME: Due to #50507 this overwrites the different values
                 // This should be changed to use HashMapExt::insert_same
                 // when that is fixed
-                self.tcx().evaluation_cache.insert(param_env.and(trait_pred), dep_node, result);
+                self.tcx().evaluation_cache.insert((param_env, trait_pred), dep_node, result);
                 return;
             }
         }
 
         debug!(?trait_pred, ?result, "insert_evaluation_cache");
-        self.infcx.evaluation_cache.insert(param_env.and(trait_pred), dep_node, result);
+        self.infcx.evaluation_cache.insert((param_env, trait_pred), dep_node, result);
     }
 
     /// For various reasons, it's possible for a subobligation
@@ -1275,11 +1275,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         pred.remap_constness(tcx, &mut param_env);
 
         if self.can_use_global_caches(param_env) {
-            if let Some(res) = tcx.selection_cache.get(&param_env.and(pred), tcx) {
+            if let Some(res) = tcx.selection_cache.get(&(param_env, pred), tcx) {
                 return Some(res);
             }
         }
-        self.infcx.selection_cache.get(&param_env.and(pred), tcx)
+        self.infcx.selection_cache.get(&(param_env, pred), tcx)
     }
 
     /// Determines whether can we safely cache the result
@@ -1340,14 +1340,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 if !candidate.needs_infer() {
                     debug!(?pred, ?candidate, "insert_candidate_cache global");
                     // This may overwrite the cache with the same value.
-                    tcx.selection_cache.insert(param_env.and(pred), dep_node, candidate);
+                    tcx.selection_cache.insert((param_env, pred), dep_node, candidate);
                     return;
                 }
             }
         }
 
         debug!(?pred, ?candidate, "insert_candidate_cache local");
-        self.infcx.selection_cache.insert(param_env.and(pred), dep_node, candidate);
+        self.infcx.selection_cache.insert((param_env, pred), dep_node, candidate);
     }
 
     /// Matches a predicate against the bounds of its self type.

src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-80706.rs

@@ -1,4 +1,4 @@
-// check-pass
+// build-pass
 // edition:2018
 
 type BoxFuture<T> = std::pin::Pin<Box<dyn std::future::Future<Output=T>>>;
@@ -65,6 +65,7 @@ async fn run<S>(dep: &str)
 where
     S: Storage,
     for<'a> SaveUser<'a>: StorageRequest<S>,
+    for<'a> SaveUser<'a>: StorageRequestReturnType,
 {
     User { dep }.save().await;
 }