Add an always-ambiguous predicate to make sure that we don't accidentlally allow trait resolution to prove false things during coherence

compiler/rustc_infer/src/infer/canonical/query_response.rs

@@ -23,7 +23,6 @@ use rustc_index::vec::Idx;
 use rustc_index::vec::IndexVec;
 use rustc_middle::arena::ArenaAllocatable;
 use rustc_middle::mir::ConstraintCategory;
-use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::relate::TypeRelation;
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind};
@@ -741,11 +740,7 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for QueryTypeRelatingDelegate<'_, 'tcx> {
         true
     }
 
-    fn register_opaque_type_obligations(
-        &mut self,
-        obligations: PredicateObligations<'tcx>,
-    ) -> Result<(), TypeError<'tcx>> {
+    fn register_opaque_type_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
         self.obligations.extend(obligations);
-        Ok(())
     }
 }

compiler/rustc_infer/src/infer/combine.rs

@@ -450,6 +450,15 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
             ty::Binder::dummy(predicate),
         ));
     }
+
+    pub fn mark_ambiguous(&mut self) {
+        self.obligations.push(Obligation::new(
+            self.tcx(),
+            self.trace.cause.clone(),
+            self.param_env,
+            ty::Binder::dummy(ty::PredicateKind::Ambiguous),
+        ));
+    }
 }
 
 struct Generalizer<'cx, 'tcx> {
@@ -538,6 +547,11 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
         true
     }
 
+    fn mark_ambiguous(&mut self) {
+        // The generalizer always compares types against themselves,
+        // and thus doesn't really take part in coherence.
+    }
+
     fn binders<T>(
         &mut self,
         a: ty::Binder<'tcx, T>,
@@ -820,6 +834,10 @@ impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
         true
     }
 
+    fn mark_ambiguous(&mut self) {
+        bug!()
+    }
+
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         _variance: ty::Variance,

compiler/rustc_infer/src/infer/equate.rs

@@ -44,6 +44,10 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
         self.a_is_expected
     }
 
+    fn mark_ambiguous(&mut self) {
+        self.fields.mark_ambiguous();
+    }
+
     fn relate_item_substs(
         &mut self,
         _item_def_id: DefId,

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2954,6 +2954,10 @@ impl<'tcx> TypeRelation<'tcx> for SameTypeModuloInfer<'_, 'tcx> {
         true
     }
 
+    fn mark_ambiguous(&mut self) {
+        bug!()
+    }
+
     fn relate_with_variance<T: relate::Relate<'tcx>>(
         &mut self,
         _variance: ty::Variance,

compiler/rustc_infer/src/infer/glb.rs

@@ -46,6 +46,10 @@ impl<'tcx> TypeRelation<'tcx> for Glb<'_, '_, 'tcx> {
         self.a_is_expected
     }
 
+    fn mark_ambiguous(&mut self) {
+        self.fields.mark_ambiguous();
+    }
+
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         variance: ty::Variance,

compiler/rustc_infer/src/infer/lub.rs

@@ -46,6 +46,10 @@ impl<'tcx> TypeRelation<'tcx> for Lub<'_, '_, 'tcx> {
         self.a_is_expected
     }
 
+    fn mark_ambiguous(&mut self) {
+        self.fields.mark_ambiguous();
+    }
+
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         variance: ty::Variance,

compiler/rustc_infer/src/infer/nll_relate/mod.rs

@@ -93,10 +93,7 @@ pub trait TypeRelatingDelegate<'tcx> {
     );
 
     fn const_equate(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>);
-    fn register_opaque_type_obligations(
-        &mut self,
-        obligations: Vec<PredicateObligation<'tcx>>,
-    ) -> Result<(), TypeError<'tcx>>;
+    fn register_opaque_type_obligations(&mut self, obligations: Vec<PredicateObligation<'tcx>>);
 
     /// Creates a new universe index. Used when instantiating placeholders.
     fn create_next_universe(&mut self) -> ty::UniverseIndex;
@@ -419,7 +416,7 @@ where
             .infcx
             .handle_opaque_type(a, b, true, &cause, self.delegate.param_env())?
             .obligations;
-        self.delegate.register_opaque_type_obligations(obligations)?;
+        self.delegate.register_opaque_type_obligations(obligations);
         trace!(a = ?a.kind(), b = ?b.kind(), "opaque type instantiated");
         Ok(a)
     }
@@ -547,6 +544,10 @@ where
         true
     }
 
+    fn mark_ambiguous(&mut self) {
+        bug!()
+    }
+
     #[instrument(skip(self, info), level = "trace", ret)]
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
@@ -918,6 +919,10 @@ where
         true
     }
 
+    fn mark_ambiguous(&mut self) {
+        bug!()
+    }
+
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         variance: ty::Variance,

compiler/rustc_infer/src/infer/outlives/mod.rs

@@ -29,6 +29,7 @@ pub fn explicit_outlives_bounds<'tcx>(
             | ty::PredicateKind::TypeOutlives(..)
             | ty::PredicateKind::ConstEvaluatable(..)
             | ty::PredicateKind::ConstEquate(..)
+            | ty::PredicateKind::Ambiguous
             | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
             ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(r_a, r_b)) => {
                 Some(OutlivesBound::RegionSubRegion(r_b, r_a))

compiler/rustc_infer/src/infer/outlives/test_type_match.rs

@@ -151,6 +151,10 @@ impl<'tcx> TypeRelation<'tcx> for Match<'tcx> {
         true
     } // irrelevant
 
+    fn mark_ambiguous(&mut self) {
+        bug!()
+    }
+
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         _: ty::Variance,

compiler/rustc_infer/src/infer/sub.rs

@@ -52,6 +52,10 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
         self.a_is_expected
     }
 
+    fn mark_ambiguous(&mut self) {
+        self.fields.mark_ambiguous()
+    }
+
     fn with_cause<F, R>(&mut self, cause: Cause, f: F) -> R
     where
         F: FnOnce(&mut Self) -> R,

compiler/rustc_infer/src/traits/util.rs

@@ -285,6 +285,7 @@ impl<'tcx> Elaborator<'tcx> {
             ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                 // Nothing to elaborate
             }
+            ty::PredicateKind::Ambiguous => {}
         }
     }
 }