Handle higher-ranked lifetime conflict errors where the subtype is the sup region

These are happening since the switch to universes, and will now go through the "placeholder error" path, instead of the current fallback of E308 "mismatched types" errors.

Author: lqd

src/librustc/infer/error_reporting/nice_region_error/placeholder_error.rs

@@ -96,6 +96,30 @@ impl NiceRegionError<'me, 'gcx, 'tcx> {
                 ))
             }
 
+            Some(RegionResolutionError::SubSupConflict(
+                vid,
+                _,
+                _,
+                _,
+                SubregionOrigin::Subtype(TypeTrace {
+                    cause,
+                    values: ValuePairs::TraitRefs(ExpectedFound { expected, found }),
+                }),
+                sup_placeholder @ ty::RePlaceholder(_),
+            ))
+                if expected.def_id == found.def_id =>
+            {
+                Some(self.try_report_placeholders_trait(
+                    Some(self.tcx().mk_region(ty::ReVar(*vid))),
+                    cause,
+                    None,
+                    Some(*sup_placeholder),
+                    expected.def_id,
+                    expected.substs,
+                    found.substs,
+                ))
+            }
+
             Some(RegionResolutionError::ConcreteFailure(
                 SubregionOrigin::Subtype(TypeTrace {
                     cause,