address review comment

compiler/rustc_infer/src/infer/region_constraints/mod.rs

@@ -420,7 +420,7 @@ pub fn take_and_reset_data(&mut self) -> RegionConstraintData<'tcx> {
         // `RegionConstraintData` contains the relationship here.
         if *any_unifications {
             *any_unifications = false;
-            self.unification_table_mut().reset_unifications(|_| UnifiedRegion(None));
+            self.unification_table_mut().reset_unifications(|_| UnifiedRegion::new(None));
         }
 
         data
@@ -447,7 +447,7 @@ pub(super) fn new_region_var(
     ) -> RegionVid {
         let vid = self.var_infos.push(RegionVariableInfo { origin, universe });
 
-        let u_vid = self.unification_table_mut().new_key(UnifiedRegion(None));
+        let u_vid = self.unification_table_mut().new_key(UnifiedRegion::new(None));
         assert_eq!(vid, u_vid.vid);
         self.undo_log.push(AddVar(vid));
         debug!("created new region variable {:?} in {:?} with origin {:?}", vid, universe, origin);
@@ -522,7 +522,7 @@ pub(super) fn make_eqregion(
                 (Region(Interned(ReVar(vid), _)), value)
                 | (value, Region(Interned(ReVar(vid), _))) => {
                     debug!("make_eqregion: unifying {:?} with {:?}", vid, value);
-                    self.unification_table_mut().union_value(*vid, UnifiedRegion(Some(value)));
+                    self.unification_table_mut().union_value(*vid, UnifiedRegion::new(Some(value)));
                     self.any_unifications = true;
                 }
                 (_, _) => {}
@@ -642,7 +642,10 @@ pub fn opportunistic_resolve_var(
     ) -> ty::Region<'tcx> {
         let mut ut = self.unification_table_mut(); // FIXME(rust-lang/ena#42): unnecessary mut
         let root_vid = ut.find(vid).vid;
-        let resolved = ut.probe_value(root_vid).0.unwrap_or_else(|| tcx.mk_re_var(root_vid));
+        let resolved = ut
+            .probe_value(root_vid)
+            .get_value_ignoring_universes()
+            .unwrap_or_else(|| tcx.mk_re_var(root_vid));
 
         // Don't resolve a variable to a region that it cannot name.
         if self.var_universe(vid).can_name(self.universe(resolved)) {

compiler/rustc_middle/src/infer/unify_key.rs

@@ -1,4 +1,4 @@
-use crate::ty::{self, Ty, TyCtxt};
+use crate::ty::{self, Region, Ty, TyCtxt};
 use rustc_data_structures::unify::{NoError, UnifyKey, UnifyValue};
 use rustc_span::def_id::DefId;
 use rustc_span::symbol::Symbol;
@@ -11,7 +11,20 @@ pub trait ToType {
 }
 
 #[derive(PartialEq, Copy, Clone, Debug)]
-pub struct UnifiedRegion<'tcx>(pub Option<ty::Region<'tcx>>);
+pub struct UnifiedRegion<'tcx> {
+    value: Option<ty::Region<'tcx>>,
+}
+
+impl<'tcx> UnifiedRegion<'tcx> {
+    pub fn new(value: Option<Region<'tcx>>) -> Self {
+        Self { value }
+    }
+
+    /// The caller is responsible for checking universe compatibility before using this value.
+    pub fn get_value_ignoring_universes(self) -> Option<Region<'tcx>> {
+        self.value
+    }
+}
 
 #[derive(PartialEq, Copy, Clone, Debug)]
 pub struct RegionVidKey<'tcx> {
@@ -44,7 +57,7 @@ impl<'tcx> UnifyValue for UnifiedRegion<'tcx> {
     type Error = NoError;
 
     fn unify_values(value1: &Self, value2: &Self) -> Result<Self, NoError> {
-        Ok(match (value1.0, value2.0) {
+        Ok(match (value1.value, value2.value) {
             // Here we can just pick one value, because the full constraints graph
             // will be handled later. Ideally, we might want a `MultipleValues`
             // variant or something. For now though, this is fine.