prefer universal from lower universe

In case a variable is unified with two universal regions from different
universes, use the one with the lower universe as it has a higher chance
of being compatible with the variable.

Author: aliemjay

compiler/rustc_middle/src/infer/unify_key.rs

@@ -57,11 +57,27 @@ impl<'tcx> UnifyValue for UnifiedRegion<'tcx> {
     type Error = NoError;
 
     fn unify_values(value1: &Self, value2: &Self) -> Result<Self, NoError> {
+        // We pick the value of the least universe because it is compatible with more variables.
+        // This is *not* neccessary for soundness, but it allows more region variables to be
+        // resolved to the said value.
+        #[cold]
+        fn min_universe<'tcx>(r1: Region<'tcx>, r2: Region<'tcx>) -> Region<'tcx> {
+            cmp::min_by_key(r1, r2, |r| match r.kind() {
+                ty::ReStatic
+                | ty::ReErased
+                | ty::ReFree(..)
+                | ty::ReEarlyBound(..)
+                | ty::ReError(_) => ty::UniverseIndex::ROOT,
+                ty::RePlaceholder(placeholder) => placeholder.universe,
+                ty::ReVar(..) | ty::ReLateBound(..) => bug!("not a universal region"),
+            })
+        }
+
         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.
-            (Some(_), Some(_)) => *value1,
+            (Some(val1), Some(val2)) => Self { value: Some(min_universe(val1, val2)) },
 
             (Some(_), _) => *value1,
             (_, Some(_)) => *value2,