Preserve most sub-obligations in the projection cache

3 files changed, 26 insertions, 49 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 91b9ad0af35..7038f16a2c9 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -10,6 +10,7 @@ use super::PredicateObligation;
 use super::Selection;
 use super::SelectionContext;
 use super::SelectionError;
+use super::TraitQueryMode;
 use super::{
     ImplSourceClosureData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
     ImplSourceGeneratorData, ImplSourcePointeeData, ImplSourceUserDefinedData,
@@ -18,7 +19,7 @@ use super::{Normalized, NormalizedTy, ProjectionCacheEntry, ProjectionCacheKey};
 
 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::infer::{InferCtxt, InferOk, LateBoundRegionConversionTime};
-use crate::traits::error_reporting::InferCtxtExt;
+use crate::traits::error_reporting::InferCtxtExt as _;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_errors::ErrorReported;
 use rustc_hir::def_id::DefId;
@@ -912,6 +913,7 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
     }
 
     let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
+
     match project_type(selcx, &obligation) {
         Ok(ProjectedTy::Progress(Progress {
             ty: projected_ty,
@@ -925,7 +927,7 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
             let projected_ty = selcx.infcx().resolve_vars_if_possible(projected_ty);
             debug!(?projected_ty, ?depth, ?projected_obligations);
 
-            let result = if projected_ty.has_projections() {
+            let mut result = if projected_ty.has_projections() {
                 let mut normalizer = AssocTypeNormalizer::new(
                     selcx,
                     param_env,
@@ -942,8 +944,26 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
                 Normalized { value: projected_ty, obligations: projected_obligations }
             };
 
-            let cache_value = prune_cache_value_obligations(infcx, &result);
-            infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, cache_value);
+            let mut canonical =
+                SelectionContext::with_query_mode(selcx.infcx(), TraitQueryMode::Canonical);
+            result.obligations.drain_filter(|projected_obligation| {
+                // If any global obligations always apply, considering regions, then we don't
+                // need to include them. The `is_global` check rules out inference variables,
+                // so there's no need for the caller of `opt_normalize_projection_type`
+                // to evaluate them.
+                // Note that we do *not* discard obligations that evaluate to
+                // `EvaluatedtoOkModuloRegions`. Evaluating these obligations
+                // inside of a query (e.g. `evaluate_obligation`) can change
+                // the result to `EvaluatedToOkModuloRegions`, while an
+                // `EvaluatedToOk` obligation will never change the result.
+                // See #85360 for more details
+                projected_obligation.is_global(canonical.tcx())
+                    && canonical
+                        .evaluate_root_obligation(projected_obligation)
+                        .map_or(false, |res| res.must_apply_considering_regions())
+            });
+
+            infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, result.clone());
             obligations.extend(result.obligations);
             Ok(Some(result.value))
         }
@@ -974,49 +994,6 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
     }
 }
 
-/// If there are unresolved type variables, then we need to include
-/// any subobligations that bind them, at least until those type
-/// variables are fully resolved.
-fn prune_cache_value_obligations<'a, 'tcx>(
-    infcx: &'a InferCtxt<'a, 'tcx>,
-    result: &NormalizedTy<'tcx>,
-) -> NormalizedTy<'tcx> {
-    if infcx.unresolved_type_vars(&result.value).is_none() {
-        return NormalizedTy { value: result.value, obligations: vec![] };
-    }
-
-    let mut obligations: Vec<_> = result
-        .obligations
-        .iter()
-        .filter(|obligation| {
-            let bound_predicate = obligation.predicate.kind();
-            match bound_predicate.skip_binder() {
-                // We found a `T: Foo<X = U>` predicate, let's check
-                // if `U` references any unresolved type
-                // variables. In principle, we only care if this
-                // projection can help resolve any of the type
-                // variables found in `result.value` -- but we just
-                // check for any type variables here, for fear of
-                // indirect obligations (e.g., we project to `?0`,
-                // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
-                // ?0>`).
-                ty::PredicateKind::Projection(data) => {
-                    infcx.unresolved_type_vars(&bound_predicate.rebind(data.ty)).is_some()
-                }
-
-                // We are only interested in `T: Foo<X = U>` predicates, whre
-                // `U` references one of `unresolved_type_vars`. =)
-                _ => false,
-            }
-        })
-        .cloned()
-        .collect();
-
-    obligations.shrink_to_fit();
-
-    NormalizedTy { value: result.value, obligations }
-}
-
 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
 /// hold. In various error cases, we cannot generate a valid
 /// normalized projection. Therefore, we create an inference variable
diff --git a/compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs b/compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs
index 2dc48e47efc..032d402fec0 100644
--- a/compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs
@@ -71,7 +71,7 @@ impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
         // Run canonical query. If overflow occurs, rerun from scratch but this time
         // in standard trait query mode so that overflow is handled appropriately
         // within `SelectionContext`.
-        self.tcx.evaluate_obligation(c_pred)
+        self.tcx.at(obligation.cause.span(self.tcx)).evaluate_obligation(c_pred)
     }
 
     // Helper function that canonicalizes and runs the query. If an
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 5214277a37d..22013fb79cf 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -682,7 +682,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
         });
 
-        debug!(?result);
+        debug!("finished: {:?} from {:?}", result, obligation);
 
         result
     }