From ad887322541b1c255b210cf87fae675072d1027f Mon Sep 17 00:00:00 2001
From: Guillaume Gomez <guillaume.gomez@huawei.com>
Date: Sat, 26 Mar 2022 14:58:19 +0100
Subject: [PATCH] Fix perf issue for auto trait selection

---
 compiler/rustc_middle/src/ty/sty.rs            |  9 +++++++++
 .../src/traits/auto_trait.rs                   | 18 ++++++++++++++++++
 .../src/traits/select/mod.rs                   |  4 ++--
 3 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 93ece7537283e..dc16460ca436e 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -999,6 +999,15 @@ impl<'tcx> PolyTraitRef<'tcx> {
             polarity: ty::ImplPolarity::Positive,
         })
     }
+
+    /// Same as [`PolyTraitRef::to_poly_trait_predicate`] but sets a negative polarity instead.
+    pub fn to_poly_trait_predicate_negative_polarity(&self) -> ty::PolyTraitPredicate<'tcx> {
+        self.map_bound(|trait_ref| ty::TraitPredicate {
+            trait_ref,
+            constness: ty::BoundConstness::NotConst,
+            polarity: ty::ImplPolarity::Negative,
+        })
+    }
 }
 
 /// An existential reference to a trait, where `Self` is erased.
diff --git a/compiler/rustc_trait_selection/src/traits/auto_trait.rs b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
index c9398d746d721..6f6f21aa78842 100644
--- a/compiler/rustc_trait_selection/src/traits/auto_trait.rs
+++ b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
@@ -94,6 +94,24 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                 trait_pred.to_poly_trait_predicate(),
             ));
 
+            match result {
+                Ok(Some(ImplSource::UserDefined(_))) => {
+                    debug!(
+                        "find_auto_trait_generics({:?}): \
+                         manual impl found, bailing out",
+                        trait_ref
+                    );
+                    return true;
+                }
+                _ => {}
+            }
+
+            let result = selcx.select(&Obligation::new(
+                ObligationCause::dummy(),
+                orig_env,
+                trait_pred.to_poly_trait_predicate_negative_polarity(),
+            ));
+
             match result {
                 Ok(Some(ImplSource::UserDefined(_))) => {
                     debug!(
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 6d232d86d8a4f..4704d8410129c 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -1272,7 +1272,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // the master cache. Since coherence executes pretty quickly,
         // it's not worth going to more trouble to increase the
         // hit-rate, I don't think.
-        if self.intercrate || self.allow_negative_impls {
+        if self.intercrate {
             return false;
         }
 
@@ -1289,7 +1289,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // mode, so don't do any caching. In particular, we might
         // re-use the same `InferCtxt` with both an intercrate
         // and non-intercrate `SelectionContext`
-        if self.intercrate || self.allow_negative_impls {
+        if self.intercrate {
             return None;
         }
         let tcx = self.tcx();