Pick candidate with fewer bound vars

Author: jackh726

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1366,11 +1366,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     && !other.value.skip_binder().has_escaping_bound_vars();
                 if value_same_except_bound_vars {
                     // See issue #84398. In short, we can generate multiple ParamCandidates which are
-                    // the same except for unused bound vars. Just pick the current one (the should
-                    // both evaluate to the same answer). This is probably best characterized as a
-                    // "hack", since we might prefer to just do our best to *not* create essentially
-                    // duplicate candidates in the first place.
-                    true
+                    // the same except for unused bound vars. Just pick the one with the fewest bound vars
+                    // or the current one if tied (they should both evaluate to the same answer). This is
+                    // probably best characterized as a "hack", since we might prefer to just do our
+                    // best to *not* create essentially duplicate candidates in the first place.
+                    other.value.bound_vars().len() <= victim.value.bound_vars().len()
                 } else if other.value == victim.value && victim.constness == Constness::NotConst {
                     // Drop otherwise equivalent non-const candidates in favor of const candidates.
                     true