From eee09ec4263ac13d94e79b92bdd7df780330077a Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Fri, 17 Dec 2021 10:27:26 -0500
Subject: [PATCH] Remove 'speculative evaluation' of predicates

Performing 'speculative evaluation' introduces caching bugs that
cannot be fixed without invasive changes to projection.

Hopefully, we can win back most of the performance lost by
re-adding 'cache completion'

Fixes #90662
---
 .../src/traits/project.rs                     | 19 +----------
 .../traits/issue-90662-projection-caching.rs  | 34 +++++++++++++++++++
 2 files changed, 35 insertions(+), 18 deletions(-)
 create mode 100644 src/test/ui/traits/issue-90662-projection-caching.rs

diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index b32fb616e12be..35145fed8c427 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -10,7 +10,6 @@ use super::PredicateObligation;
 use super::Selection;
 use super::SelectionContext;
 use super::SelectionError;
-use super::TraitQueryMode;
 use super::{
     ImplSourceClosureData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
     ImplSourceGeneratorData, ImplSourcePointeeData, ImplSourceUserDefinedData,
@@ -946,27 +945,11 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
             };
 
             let mut deduped: SsoHashSet<_> = Default::default();
-            let mut canonical =
-                SelectionContext::with_query_mode(selcx.infcx(), TraitQueryMode::Canonical);
-
             result.obligations.drain_filter(|projected_obligation| {
                 if !deduped.insert(projected_obligation.clone()) {
                     return true;
                 }
-                // 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())
+                false
             });
 
             if use_cache {
diff --git a/src/test/ui/traits/issue-90662-projection-caching.rs b/src/test/ui/traits/issue-90662-projection-caching.rs
new file mode 100644
index 0000000000000..879f30071bfdb
--- /dev/null
+++ b/src/test/ui/traits/issue-90662-projection-caching.rs
@@ -0,0 +1,34 @@
+// check-pass
+
+// Regression test for issue #90662
+// Tests that projection caching does not cause a spurious error
+
+trait HasProvider<T: ?Sized> {}
+trait Provider<M> {
+    type Interface: ?Sized;
+}
+
+trait Repository {}
+trait Service {}
+
+struct DbConnection;
+impl<M> Provider<M> for DbConnection {
+    type Interface = DbConnection;
+}
+
+struct RepositoryImpl;
+impl<M: HasProvider<DbConnection>> Provider<M> for RepositoryImpl {
+    type Interface = dyn Repository;
+}
+
+struct ServiceImpl;
+impl<M: HasProvider<dyn Repository>> Provider<M> for ServiceImpl {
+    type Interface = dyn Service;
+}
+
+struct TestModule;
+impl HasProvider<<DbConnection as Provider<Self>>::Interface> for TestModule {}
+impl HasProvider<<RepositoryImpl as Provider<Self>>::Interface> for TestModule {}
+impl HasProvider<<ServiceImpl as Provider<Self>>::Interface> for TestModule {}
+
+fn main() {}