Fix perf issue for auto trait selection

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!(

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();