remove a performance hack

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

@@ -1308,36 +1308,6 @@ where
             },
         );
 
-        // HACK: We bail with overflow if the response would have too many non-region
-        // inference variables. This tends to only happen if we encounter a lot of
-        // ambiguous alias types which get replaced with fresh inference variables
-        // during generalization. This prevents hangs caused by an exponential blowup,
-        // see tests/ui/traits/next-solver/coherence-alias-hang.rs.
-        match self.current_goal_kind {
-            // We don't do so for `NormalizesTo` goals as we erased the expected term and
-            // bailing with overflow here would prevent us from detecting a type-mismatch,
-            // causing a coherence error in diesel, see #131969. We still bail with overflow
-            // when later returning from the parent AliasRelate goal.
-            CurrentGoalKind::NormalizesTo => {}
-            CurrentGoalKind::Misc | CurrentGoalKind::CoinductiveTrait => {
-                let num_non_region_vars = canonical
-                    .variables
-                    .iter()
-                    .filter(|c| !c.is_region() && c.is_existential())
-                    .count();
-                if num_non_region_vars > self.cx().recursion_limit() {
-                    debug!(?num_non_region_vars, "too many inference variables -> overflow");
-                    return Ok(self.make_ambiguous_response_no_constraints(
-                        MaybeCause::Overflow {
-                            suggest_increasing_limit: true,
-                            keep_constraints: false,
-                        },
-                        OpaqueTypesJank::AllGood,
-                    ));
-                }
-            }
-        }
-
         Ok(canonical)
     }
 

tests/ui/traits/next-solver/cycles/coinduction/fixpoint-exponential-growth.stderr

@@ -4,7 +4,6 @@ error[E0275]: overflow evaluating the requirement `W<_>: Trait`
 LL |     impls::<W<_>>();
    |             ^^^^
    |
-   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`fixpoint_exponential_growth`)
 note: required by a bound in `impls`
   --> $DIR/fixpoint-exponential-growth.rs:30:13
    |