From 12b9040cdab9895f7152e09c5401ae446ff12a1e Mon Sep 17 00:00:00 2001
From: lcnr <rust@lcnr.de>
Date: Thu, 22 Feb 2024 12:35:59 +0100
Subject: [PATCH] add comment

---
 compiler/rustc_middle/src/infer/unify_key.rs | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_middle/src/infer/unify_key.rs b/compiler/rustc_middle/src/infer/unify_key.rs
index 237a465534565..aa9b5ede60579 100644
--- a/compiler/rustc_middle/src/infer/unify_key.rs
+++ b/compiler/rustc_middle/src/infer/unify_key.rs
@@ -77,7 +77,14 @@ impl<'tcx> UnifyValue for RegionVariableValue<'tcx> {
             (
                 RegionVariableValue::Unknown { universe: a },
                 RegionVariableValue::Unknown { universe: b },
-            ) => Ok(RegionVariableValue::Unknown { universe: a.min(b) }),
+            ) => {
+                // If we unify two unconstrained regions then whatever
+                // value they wind up taking (which must be the same value) must
+                // be nameable by both universes. Therefore, the resulting
+                // universe is the minimum of the two universes, because that is
+                // the one which contains the fewest names in scope.
+                Ok(RegionVariableValue::Unknown { universe: a.min(b) })
+            }
         }
     }
 }