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

@@ -157,6 +157,10 @@ 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 a hang in nalgebra.
         let num_non_region_vars = canonical.variables.iter().filter(|c| !c.is_region()).count();
         if num_non_region_vars > self.cx().recursion_limit() {
             return Ok(self.make_ambiguous_response_no_constraints(MaybeCause::Overflow {