Update compiler/rustc_borrowck/src/handle_placeholders.rs

Co-authored-by: lcnr <[email protected]>

Author: amandasystems

compiler/rustc_borrowck/src/handle_placeholders.rs

@@ -163,6 +163,9 @@ impl RegionTracker {
 
     /// Determine if the tracked universes of the two SCCs are compatible.
     pub(crate) fn universe_compatible_with(&self, other: Self) -> bool {
+        // HACK: We first check whether we can name the highest existential universe
+        // of `other`. This only exists to avoid errors in case that scc already
+        // depends on a placeholder it cannot name itself.
         self.max_nameable_universe().can_name(other.max_nameable_universe())
             || other.reachable_placeholders.can_be_named_by(self.max_nameable_universe())
     }