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

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

@@ -157,6 +157,17 @@ 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 {
+                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>.
 

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

@@ -4,6 +4,7 @@ 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
    |

tests/ui/traits/next-solver/overflow/recursion-limit-zero-issue-115351.rs

@@ -1,6 +1,7 @@
+//~ ERROR overflow evaluating the requirement `Self: Trait`
+//~^ ERROR overflow evaluating the requirement `Self well-formed`
 // This is a non-regression test for issue #115351, where a recursion limit of 0 caused an ICE.
 //@ compile-flags: -Znext-solver --crate-type=lib
-//@ check-pass
 
 #![recursion_limit = "0"]
 trait Trait {}

tests/ui/traits/next-solver/overflow/recursion-limit-zero-issue-115351.stderr

@@ -0,0 +1,11 @@
+error[E0275]: overflow evaluating the requirement `Self: Trait`
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "2"]` attribute to your crate (`recursion_limit_zero_issue_115351`)
+
+error[E0275]: overflow evaluating the requirement `Self well-formed`
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "2"]` attribute to your crate (`recursion_limit_zero_issue_115351`)
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0275`.