Rollup merge of #130617 - lcnr:nalgebra-hang-3, r=compiler-errors

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`

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`.