Auto merge of #130617 - lcnr:nalgebra-hang-3, r=<try>

bail if there are too many non-region infer vars in the query response

A minimal fix for the hang in nalgebra. If the query response would result in too many distinct non-region inference variables, simply overwrite the result with overflow. This should either happen if the result already has too many distinct type inference variables, or if evaluating the query encountered a lot of ambiguous associated types. In both cases it's straightforward to wait until the aliases are no longer ambiguous and then try again.

r? `@compiler-errors`

Author: bors

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

@@ -157,6 +157,13 @@ where
             },
         );
 
+        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 {
+                suggest_increasing_limit: true,
+            }));
+        }
+
         Ok(canonical)
     }
 

tests/ui/traits/coherence-alias-hang.rs

@@ -1,4 +1,6 @@
 //@ check-pass
+//@ revisions: current next
+//[next]@ compile-flags: -Znext-solver
 
 // Regression test for nalgebra hang <https://github.com/rust-lang/rust/issues/130056>.