fix eval_outlives for higher-ranked regions

When checking `'sup: 'sub`, take placeholder elements into account.

This wasn't a problem previously (I hope!) because `eval_outlives` was
only used for type_tests, which don't have higher-ranked outlive
relations.

Author: aliemjay

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -1360,6 +1360,14 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             return self.eval_outlives(sup_region, self.universal_regions.fr_static);
         }
 
+        // Check `sup_region` contains all the placeholder regions in `sub_region`.
+        if !self.scc_values.contains_placeholders(sup_region_scc, sub_region_scc) {
+            debug!(
+                "eval_outlives: returning false because sub region contains a placeholder region not present in super"
+            );
+            return false;
+        }
+
         // Both the `sub_region` and `sup_region` consist of the union
         // of some number of universal regions (along with the union
         // of various points in the CFG; ignore those points for

compiler/rustc_borrowck/src/region_infer/values.rs

@@ -303,6 +303,22 @@ impl<N: Idx> RegionValues<N> {
         }
     }
 
+    /// Returns `true` if `sup_region` contains all the placeholder elements that
+    /// `sub_region` contains.
+    pub(crate) fn contains_placeholders(&self, sup_region: N, sub_region: N) -> bool {
+        if let Some(sub_row) = self.placeholders.row(sub_region) {
+            if let Some(sup_row) = self.placeholders.row(sup_region) {
+                sup_row.superset(sub_row)
+            } else {
+                // sup row is empty, so sub row must be empty
+                sub_row.is_empty()
+            }
+        } else {
+            // sub row is empty, always true
+            true
+        }
+    }
+
     /// Returns the locations contained within a given region `r`.
     pub(crate) fn locations_outlived_by<'a>(&'a self, r: N) -> impl Iterator<Item = Location> + 'a {
         self.points.row(r).into_iter().flat_map(move |set| {