From a84e3fab307f562312c54991e3c7386246ba3f75 Mon Sep 17 00:00:00 2001
From: jackh726 <jack.huey@umassmed.edu>
Date: Sun, 26 Sep 2021 15:58:24 -0400
Subject: [PATCH] Don't normalize opaque types with escaping late-bound
 regions.

Turns out, this has some really bad perf implications in large types (issue #88862). While we technically can handle them fine, it doesn't change test output either way. For now, revert with an added benchmark. Future attempts to change this back will have to consider perf.
---
 compiler/rustc_trait_selection/src/traits/project.rs      | 8 ++++----
 .../rustc_trait_selection/src/traits/query/normalize.rs   | 8 ++++----
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 7038f16a2c9c4..7b6775786fcda 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -388,15 +388,15 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
         // to make sure we don't forget to fold the substs regardless.
 
         match *ty.kind() {
-            ty::Opaque(def_id, substs) => {
+            // This is really important. While we *can* handle this, this has
+            // severe performance implications for large opaque types with
+            // late-bound regions. See `issue-88862` benchmark.
+            ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => {
                 // Only normalize `impl Trait` after type-checking, usually in codegen.
                 match self.param_env.reveal() {
                     Reveal::UserFacing => ty.super_fold_with(self),
 
                     Reveal::All => {
-                        // N.b. there is an assumption here all this code can handle
-                        // escaping bound vars.
-
                         let recursion_limit = self.tcx().recursion_limit();
                         if !recursion_limit.value_within_limit(self.depth) {
                             let obligation = Obligation::with_depth(
diff --git a/compiler/rustc_trait_selection/src/traits/query/normalize.rs b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
index ed5fe466c69f7..1364cf1c99535 100644
--- a/compiler/rustc_trait_selection/src/traits/query/normalize.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
@@ -206,15 +206,15 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
 
         // Wrap this in a closure so we don't accidentally return from the outer function
         let res = (|| match *ty.kind() {
-            ty::Opaque(def_id, substs) => {
+            // This is really important. While we *can* handle this, this has
+            // severe performance implications for large opaque types with
+            // late-bound regions. See `issue-88862` benchmark.
+            ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => {
                 // Only normalize `impl Trait` after type-checking, usually in codegen.
                 match self.param_env.reveal() {
                     Reveal::UserFacing => ty.super_fold_with(self),
 
                     Reveal::All => {
-                        // N.b. there is an assumption here all this code can handle
-                        // escaping bound vars.
-
                         let substs = substs.super_fold_with(self);
                         let recursion_limit = self.tcx().recursion_limit();
                         if !recursion_limit.value_within_limit(self.anon_depth) {