Fix perf issue for auto trait selection

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.

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -87,13 +87,31 @@ impl<'tcx> AutoTraitFinder<'tcx> {
         let trait_pred = ty::Binder::dummy(trait_ref);
 
         let bail_out = tcx.infer_ctxt().enter(|infcx| {
-            let mut selcx = SelectionContext::with_negative(&infcx, true);
+            let mut selcx = SelectionContext::new(&infcx);
             let result = selcx.select(&Obligation::new(
                 ObligationCause::dummy(),
                 orig_env,
                 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!(
@@ -277,7 +295,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
             fresh_preds.insert(self.clean_pred(infcx, predicate));
         }
 
-        let mut select = SelectionContext::with_negative(&infcx, true);
+        let mut select = SelectionContext::new(&infcx);
 
         let mut already_visited = FxHashSet::default();
         let mut predicates = VecDeque::new();

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

@@ -119,11 +119,6 @@ pub struct SelectionContext<'cx, 'tcx> {
 
     intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
 
-    /// Controls whether or not to filter out negative impls when selecting.
-    /// This is used in librustdoc to distinguish between the lack of an impl
-    /// and a negative impl
-    allow_negative_impls: bool,
-
     /// The mode that trait queries run in, which informs our error handling
     /// policy. In essence, canonicalized queries need their errors propagated
     /// rather than immediately reported because we do not have accurate spans.
@@ -215,7 +210,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             freshener: infcx.freshener_keep_static(),
             intercrate: false,
             intercrate_ambiguity_causes: None,
-            allow_negative_impls: false,
             query_mode: TraitQueryMode::Standard,
         }
     }
@@ -226,22 +220,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             freshener: infcx.freshener_keep_static(),
             intercrate: true,
             intercrate_ambiguity_causes: None,
-            allow_negative_impls: false,
-            query_mode: TraitQueryMode::Standard,
-        }
-    }
-
-    pub fn with_negative(
-        infcx: &'cx InferCtxt<'cx, 'tcx>,
-        allow_negative_impls: bool,
-    ) -> SelectionContext<'cx, 'tcx> {
-        debug!(?allow_negative_impls, "with_negative");
-        SelectionContext {
-            infcx,
-            freshener: infcx.freshener_keep_static(),
-            intercrate: false,
-            intercrate_ambiguity_causes: None,
-            allow_negative_impls,
             query_mode: TraitQueryMode::Standard,
         }
     }
@@ -256,7 +234,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             freshener: infcx.freshener_keep_static(),
             intercrate: false,
             intercrate_ambiguity_causes: None,
-            allow_negative_impls: false,
             query_mode,
         }
     }
@@ -1192,7 +1169,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             if let ImplCandidate(def_id) = candidate {
                 if ty::ImplPolarity::Reservation == tcx.impl_polarity(def_id)
                     || obligation.polarity() == tcx.impl_polarity(def_id)
-                    || self.allow_negative_impls
                 {
                     result.push(candidate);
                 }
@@ -1272,7 +1248,7 @@ 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 +1265,7 @@ 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();