normalizes-to disable infer var check

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

@@ -16,7 +16,7 @@
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::relate::solver_relating::RelateExt;
 use rustc_type_ir::{self as ty, Canonical, CanonicalVarValues, InferCtxtLike, Interner};
-use tracing::{instrument, trace};
+use tracing::{debug, instrument, trace};
 
 use crate::canonicalizer::{CanonicalizeMode, Canonicalizer};
 use crate::delegate::SolverDelegate;
@@ -165,12 +165,21 @@ pub(in crate::solve) fn evaluate_added_goals_and_make_canonical_response(
         // 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,
-            }));
+        // during generalization. This prevents hangs caused by an exponential blowup,
+        // see tests/ui/traits/next-solver/coherence-alias-hang.rs.
+        //
+        // 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 verflow
+        // when later returning from the parent AliasRelate goal.
+        if !self.is_normalizes_to_goal {
+            let num_non_region_vars = canonical.variables.iter().filter(|c| !c.is_region()).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,
+                }));
+            }
         }
 
         Ok(canonical)

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -298,6 +298,7 @@ fn equate_impl_headers<'tcx>(
 }
 
 /// The result of [fn impl_intersection_has_impossible_obligation].
+#[derive(Debug)]
 enum IntersectionHasImpossibleObligations<'tcx> {
     Yes,
     No {
@@ -328,6 +329,7 @@ enum IntersectionHasImpossibleObligations<'tcx> {
 /// of the two impls above to be empty.
 ///
 /// Importantly, this works even if there isn't a `impl !Error for MyLocalType`.
+#[instrument(level = "debug", skip(selcx), ret)]
 fn impl_intersection_has_impossible_obligation<'a, 'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     obligations: &'a [PredicateObligation<'tcx>],

tests/ui/traits/next-solver/normalize/normalize-allow-too-many-vars.rs

@@ -0,0 +1,52 @@
+//@ check-pass
+
+// When canonicalizing a response in the trait solver, we bail with overflow
+// if there are too many non-region inference variables. Doing so in normalizes-to
+// goals ends up hiding inference constraints in cases which we want to support,
+// see #131969. To prevent this issue we do not check for too many inference
+// variables in normalizes-to goals.
+#![recursion_limit = "8"]
+
+trait Bound {}
+trait Trait {
+    type Assoc;
+}
+
+
+impl<T0, T1, T2, T3, T4, T5, T6, T7> Trait for (T0, T1, T2, T3, T4, T5, T6, T7)
+where
+    T0: Trait,
+    T1: Trait,
+    T2: Trait,
+    T3: Trait,
+    T4: Trait,
+    T5: Trait,
+    T6: Trait,
+    T7: Trait,
+    (
+        T0::Assoc,
+        T1::Assoc,
+        T2::Assoc,
+        T3::Assoc,
+        T4::Assoc,
+        T5::Assoc,
+        T6::Assoc,
+        T7::Assoc,
+    ): Clone,
+{
+    type Assoc = (
+        T0::Assoc,
+        T1::Assoc,
+        T2::Assoc,
+        T3::Assoc,
+        T4::Assoc,
+        T5::Assoc,
+        T6::Assoc,
+        T7::Assoc,
+    );
+}
+
+trait Overlap {}
+impl<T: Trait<Assoc = ()>> Overlap for T {}
+impl<T0, T1, T2, T3, T4, T5, T6, T7> Overlap for (T0, T1, T2, T3, T4, T5, T6, T7) {}
+fn main() {}

tests/ui/traits/next-solver/overflow/coherence-alias-hang.rs

@@ -1,8 +1,5 @@
 //@ check-pass
-//@ revisions: ai_current ai_next ia_current ia_next ii_current ii_next
-//@[ai_next] compile-flags: -Znext-solver
-//@[ia_next] compile-flags: -Znext-solver
-//@[ii_next] compile-flags: -Znext-solver
+//@ revisions: ai ia ii
 
 // Regression test for nalgebra hang <https://github.com/rust-lang/rust/issues/130056>.
 
@@ -17,11 +14,11 @@ trait Trait {
     type Assoc: ?Sized;
 }
 impl<T: ?Sized + Trait> Trait for W<T, T> {
-    #[cfg(any(ai_current, ai_next))]
+    #[cfg(ai)]
     type Assoc = W<T::Assoc, Id<T::Assoc>>;
-    #[cfg(any(ia_current, ia_next))]
+    #[cfg(ia)]
     type Assoc = W<Id<T::Assoc>, T::Assoc>;
-    #[cfg(any(ii_current, ii_next))]
+    #[cfg(ii)]
     type Assoc = W<Id<T::Assoc>, Id<T::Assoc>>;
 }