Auto merge of #118725 - lcnr:normalizes-to-projection-split-3, r=BoxyUwU

split `NormalizesTo` out of `Projection` 3

third attempt at #112658. Rebasing #116262 is very annoying, so I am doing it again from scratch. We should now be able to merge it without regressing anything as we handle occurs check failures involving aliases correctly since #117088.

see https://hackmd.io/ktEL8knTSYmtdfrMMnA-Hg

fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/1

r? `@compiler-errors`

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -657,19 +657,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 | ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(..))
                 | ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(..))
                 | ty::PredicateKind::ObjectSafe(..)
+                | ty::PredicateKind::NormalizesTo(..)
                 | ty::PredicateKind::AliasRelate(..)
                 | ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(..))
                 | ty::PredicateKind::ConstEquate(..)
-                // N.B., this predicate is created by breaking down a
+                | ty::PredicateKind::Ambiguous => None,
-                // `ClosureType: FnFoo()` predicate, where
-                // `ClosureType` represents some `Closure`. It can't
-                // possibly be referring to the current closure,
-                // because we haven't produced the `Closure` for
-                // this closure yet; this is exactly why the other
-                // code is looking for a self type of an unresolved
-                // inference variable.
-                | ty::PredicateKind::Ambiguous
-                 => None,
             },
         )
     }

compiler/rustc_infer/src/infer/combine.rs

@@ -35,6 +35,7 @@ use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKin
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::relate::{RelateResult, TypeRelation};
 use rustc_middle::ty::{self, InferConst, ToPredicate, Ty, TyCtxt, TypeVisitableExt};
+use rustc_middle::ty::{AliasRelationDirection, TyVar};
 use rustc_middle::ty::{IntType, UintType};
 use rustc_span::DUMMY_SP;
 
@@ -459,7 +460,12 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
             ambient_variance,
         )?;
 
-        self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
+        // Constrain `b_vid` to the generalized type `b_ty`.
+        if let &ty::Infer(TyVar(b_ty_vid)) = b_ty.kind() {
+            self.infcx.inner.borrow_mut().type_variables().equate(b_vid, b_ty_vid);
+        } else {
+            self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
+        }
 
         if needs_wf {
             self.obligations.push(Obligation::new(
@@ -484,31 +490,46 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
             // cyclic type. We instead delay the unification in case
             // the alias can be normalized to something which does not
             // mention `?0`.
-
+            if self.infcx.next_trait_solver() {
-            // FIXME(-Ztrait-solver=next): replace this with `AliasRelate`
+                let (lhs, rhs, direction) = match ambient_variance {
-            let &ty::Alias(kind, data) = a_ty.kind() else {
+                    ty::Variance::Invariant => {
-                bug!("generalization should only result in infer vars for aliases");
+                        (a_ty.into(), b_ty.into(), AliasRelationDirection::Equate)
-            };
-            if !self.infcx.next_trait_solver() {
-                // The old solver only accepts projection predicates for associated types.
-                match kind {
-                    ty::AliasKind::Projection => {}
-                    ty::AliasKind::Inherent | ty::AliasKind::Weak | ty::AliasKind::Opaque => {
-                        return Err(TypeError::CyclicTy(a_ty));
                     }
+                    ty::Variance::Covariant => {
+                        (a_ty.into(), b_ty.into(), AliasRelationDirection::Subtype)
+                    }
+                    ty::Variance::Contravariant => {
+                        (b_ty.into(), a_ty.into(), AliasRelationDirection::Subtype)
+                    }
+                    ty::Variance::Bivariant => unreachable!("bivariant generalization"),
+                };
+                self.obligations.push(Obligation::new(
+                    self.tcx(),
+                    self.trace.cause.clone(),
+                    self.param_env,
+                    ty::PredicateKind::AliasRelate(lhs, rhs, direction),
+                ));
+            } else {
+                match a_ty.kind() {
+                    &ty::Alias(ty::AliasKind::Projection, data) => {
+                        // FIXME: This does not handle subtyping correctly, we could
+                        // instead create a new inference variable for `a_ty`, emitting
+                        // `Projection(a_ty, a_infer)` and `a_infer <: b_ty`.
+                        self.obligations.push(Obligation::new(
+                            self.tcx(),
+                            self.trace.cause.clone(),
+                            self.param_env,
+                            ty::ProjectionPredicate { projection_ty: data, term: b_ty.into() },
+                        ))
+                    }
+                    // The old solver only accepts projection predicates for associated types.
+                    ty::Alias(
+                        ty::AliasKind::Inherent | ty::AliasKind::Weak | ty::AliasKind::Opaque,
+                        _,
+                    ) => return Err(TypeError::CyclicTy(a_ty)),
+                    _ => bug!("generalizated `{a_ty:?} to infer, not an alias"),
                 }
             }
-
-            // FIXME: This does not handle subtyping correctly, we should switch to
-            // alias-relate in the new solver and could instead create a new inference
-            // variable for `a_ty`, emitting `Projection(a_ty, a_infer)` and
-            // `a_infer <: b_ty`.
-            self.obligations.push(Obligation::new(
-                self.tcx(),
-                self.trace.cause.clone(),
-                self.param_env,
-                ty::ProjectionPredicate { projection_ty: data, term: b_ty.into() },
-            ))
         } else {
             match ambient_variance {
                 ty::Variance::Invariant => self.equate(a_is_expected).relate(a_ty, b_ty),
@@ -519,9 +540,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
                     a_ty,
                     b_ty,
                 ),
-                ty::Variance::Bivariant => {
+                ty::Variance::Bivariant => unreachable!("bivariant generalization"),
-                    unreachable!("no code should be generalizing bivariantly (currently)")
-                }
             }?;
         }
 

compiler/rustc_infer/src/infer/type_variable.rs

@@ -229,12 +229,11 @@ impl<'tcx> TypeVariableTable<'_, 'tcx> {
     /// Precondition: `vid` must not have been previously instantiated.
     pub fn instantiate(&mut self, vid: ty::TyVid, ty: Ty<'tcx>) {
         let vid = self.root_var(vid);
+        debug_assert!(!ty.is_ty_var(), "instantiating ty var with var: {vid:?} {ty:?}");
         debug_assert!(self.probe(vid).is_unknown());
         debug_assert!(
             self.eq_relations().probe_value(vid).is_unknown(),
-            "instantiating type variable `{:?}` twice: new-value = {:?}, old-value={:?}",
+            "instantiating type variable `{vid:?}` twice: new-value = {ty:?}, old-value={:?}",
-            vid,
-            ty,
             self.eq_relations().probe_value(vid)
         );
         self.eq_relations().union_value(vid, TypeVariableValue::Known { value: ty });

compiler/rustc_infer/src/traits/util.rs

@@ -261,9 +261,14 @@ impl<'tcx, O: Elaboratable<'tcx>> Elaborator<'tcx, O> {
     fn elaborate(&mut self, elaboratable: &O) {
         let tcx = self.visited.tcx;
 
-        let bound_predicate = elaboratable.predicate().kind();
+        // We only elaborate clauses.
-        match bound_predicate.skip_binder() {
+        let Some(clause) = elaboratable.predicate().as_clause() else {
-            ty::PredicateKind::Clause(ty::ClauseKind::Trait(data)) => {
+            return;
+        };
+
+        let bound_clause = clause.kind();
+        match bound_clause.skip_binder() {
+            ty::ClauseKind::Trait(data) => {
                 // Negative trait bounds do not imply any supertrait bounds
                 if data.polarity == ty::ImplPolarity::Negative {
                     return;
@@ -280,49 +285,16 @@ impl<'tcx, O: Elaboratable<'tcx>> Elaborator<'tcx, O> {
                 let obligations =
                     predicates.predicates.iter().enumerate().map(|(index, &(clause, span))| {
                         elaboratable.child_with_derived_cause(
-                            clause.subst_supertrait(tcx, &bound_predicate.rebind(data.trait_ref)),
+                            clause.subst_supertrait(tcx, &bound_clause.rebind(data.trait_ref)),
                             span,
-                            bound_predicate.rebind(data),
+                            bound_clause.rebind(data),
                             index,
                         )
                     });
                 debug!(?data, ?obligations, "super_predicates");
                 self.extend_deduped(obligations);
             }
-            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(..)) => {
+            ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(ty_max, r_min)) => {
-                // Currently, we do not elaborate WF predicates,
-                // although we easily could.
-            }
-            ty::PredicateKind::ObjectSafe(..) => {
-                // Currently, we do not elaborate object-safe
-                // predicates.
-            }
-            ty::PredicateKind::Subtype(..) => {
-                // Currently, we do not "elaborate" predicates like `X <: Y`,
-                // though conceivably we might.
-            }
-            ty::PredicateKind::Coerce(..) => {
-                // Currently, we do not "elaborate" predicates like `X -> Y`,
-                // though conceivably we might.
-            }
-            ty::PredicateKind::Clause(ty::ClauseKind::Projection(..)) => {
-                // Nothing to elaborate in a projection predicate.
-            }
-            ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(..)) => {
-                // Currently, we do not elaborate const-evaluatable
-                // predicates.
-            }
-            ty::PredicateKind::ConstEquate(..) => {
-                // Currently, we do not elaborate const-equate
-                // predicates.
-            }
-            ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(..)) => {
-                // Nothing to elaborate from `'a: 'b`.
-            }
-            ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(
-                ty_max,
-                r_min,
-            ))) => {
                 // We know that `T: 'a` for some type `T`. We can
                 // often elaborate this. For example, if we know that
                 // `[U]: 'a`, that implies that `U: 'a`. Similarly, if
@@ -385,15 +357,25 @@ impl<'tcx, O: Elaboratable<'tcx>> Elaborator<'tcx, O> {
                             }
                         })
                         .map(|clause| {
-                            elaboratable.child(bound_predicate.rebind(clause).to_predicate(tcx))
+                            elaboratable.child(bound_clause.rebind(clause).to_predicate(tcx))
                         }),
                 );
             }
-            ty::PredicateKind::Ambiguous => {}
+            ty::ClauseKind::RegionOutlives(..) => {
-            ty::PredicateKind::AliasRelate(..) => {
+                // Nothing to elaborate from `'a: 'b`.
-                // No
             }
-            ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(..)) => {
+            ty::ClauseKind::WellFormed(..) => {
+                // Currently, we do not elaborate WF predicates,
+                // although we easily could.
+            }
+            ty::ClauseKind::Projection(..) => {
+                // Nothing to elaborate in a projection predicate.
+            }
+            ty::ClauseKind::ConstEvaluatable(..) => {
+                // Currently, we do not elaborate const-evaluatable
+                // predicates.
+            }
+            ty::ClauseKind::ConstArgHasType(..) => {
                 // Nothing to elaborate
             }
         }

compiler/rustc_middle/src/ty/context.rs

@@ -121,6 +121,7 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
     type RegionOutlivesPredicate = ty::RegionOutlivesPredicate<'tcx>;
     type TypeOutlivesPredicate = ty::TypeOutlivesPredicate<'tcx>;
     type ProjectionPredicate = ty::ProjectionPredicate<'tcx>;
+    type NormalizesTo = ty::NormalizesTo<'tcx>;
     type SubtypePredicate = ty::SubtypePredicate<'tcx>;
     type CoercePredicate = ty::CoercePredicate<'tcx>;
     type ClosureKind = ty::ClosureKind;

compiler/rustc_middle/src/ty/flags.rs

@@ -272,6 +272,10 @@ impl FlagComputation {
                 self.add_const(found);
             }
             ty::PredicateKind::Ambiguous => {}
+            ty::PredicateKind::NormalizesTo(ty::NormalizesTo { alias, term }) => {
+                self.add_alias_ty(alias);
+                self.add_term(term);
+            }
             ty::PredicateKind::AliasRelate(t1, t2, _) => {
                 self.add_term(t1);
                 self.add_term(t2);

compiler/rustc_middle/src/ty/mod.rs

@@ -553,6 +553,10 @@ impl<'tcx> Predicate<'tcx> {
     pub fn allow_normalization(self) -> bool {
         match self.kind().skip_binder() {
             PredicateKind::Clause(ClauseKind::WellFormed(_)) => false,
+            // `NormalizesTo` is only used in the new solver, so this shouldn't
+            // matter. Normalizing `term` would be 'wrong' however, as it changes whether
+            // `normalizes-to(<T as Trait>::Assoc, <T as Trait>::Assoc)` holds.
+            PredicateKind::NormalizesTo(..) => false,
             PredicateKind::Clause(ClauseKind::Trait(_))
             | PredicateKind::Clause(ClauseKind::RegionOutlives(_))
             | PredicateKind::Clause(ClauseKind::TypeOutlives(_))
@@ -1093,6 +1097,33 @@ impl<'tcx> PolyProjectionPredicate<'tcx> {
     }
 }
 
+/// Used by the new solver. Unlike a `ProjectionPredicate` this can only be
+/// proven by actually normalizing `alias`.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub struct NormalizesTo<'tcx> {
+    pub alias: AliasTy<'tcx>,
+    pub term: Term<'tcx>,
+}
+
+impl<'tcx> NormalizesTo<'tcx> {
+    pub fn self_ty(self) -> Ty<'tcx> {
+        self.alias.self_ty()
+    }
+
+    pub fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> NormalizesTo<'tcx> {
+        Self { alias: self.alias.with_self_ty(tcx, self_ty), ..self }
+    }
+
+    pub fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId {
+        self.alias.trait_def_id(tcx)
+    }
+
+    pub fn def_id(self) -> DefId {
+        self.alias.def_id
+    }
+}
+
 pub trait ToPolyTraitRef<'tcx> {
     fn to_poly_trait_ref(&self) -> PolyTraitRef<'tcx>;
 }
@@ -1274,6 +1305,12 @@ impl<'tcx> ToPredicate<'tcx, Clause<'tcx>> for PolyProjectionPredicate<'tcx> {
     }
 }
 
+impl<'tcx> ToPredicate<'tcx> for NormalizesTo<'tcx> {
+    fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {
+        PredicateKind::NormalizesTo(self).to_predicate(tcx)
+    }
+}
+
 impl<'tcx> Predicate<'tcx> {
     pub fn to_opt_poly_trait_pred(self) -> Option<PolyTraitPredicate<'tcx>> {
         let predicate = self.kind();
@@ -1281,6 +1318,7 @@ impl<'tcx> Predicate<'tcx> {
             PredicateKind::Clause(ClauseKind::Trait(t)) => Some(predicate.rebind(t)),
             PredicateKind::Clause(ClauseKind::Projection(..))
             | PredicateKind::Clause(ClauseKind::ConstArgHasType(..))
+            | PredicateKind::NormalizesTo(..)
             | PredicateKind::AliasRelate(..)
             | PredicateKind::Subtype(..)
             | PredicateKind::Coerce(..)
@@ -1300,6 +1338,7 @@ impl<'tcx> Predicate<'tcx> {
             PredicateKind::Clause(ClauseKind::Projection(t)) => Some(predicate.rebind(t)),
             PredicateKind::Clause(ClauseKind::Trait(..))
             | PredicateKind::Clause(ClauseKind::ConstArgHasType(..))
+            | PredicateKind::NormalizesTo(..)
             | PredicateKind::AliasRelate(..)
             | PredicateKind::Subtype(..)
             | PredicateKind::Coerce(..)

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -2814,6 +2814,7 @@ define_print! {
                 p!("the constant `", print(c1), "` equals `", print(c2), "`")
             }
             ty::PredicateKind::Ambiguous => p!("ambiguous"),
+            ty::PredicateKind::NormalizesTo(data) => p!(print(data)),
             ty::PredicateKind::AliasRelate(t1, t2, dir) => p!(print(t1), write(" {} ", dir), print(t2)),
         }
     }
@@ -2945,6 +2946,12 @@ define_print_and_forward_display! {
         p!(print(self.term))
     }
 
+    ty::NormalizesTo<'tcx> {
+        p!(print(self.alias), " normalizes-to ");
+        cx.reset_type_limit();
+        p!(print(self.term))
+    }
+
     ty::Term<'tcx> {
       match self.unpack() {
         ty::TermKind::Ty(ty) => p!(print(ty)),

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -173,6 +173,12 @@ impl<'tcx> fmt::Debug for ty::ProjectionPredicate<'tcx> {
     }
 }
 
+impl<'tcx> fmt::Debug for ty::NormalizesTo<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "NormalizesTo({:?}, {:?})", self.alias, self.term)
+    }
+}
+
 impl<'tcx> fmt::Debug for ty::Predicate<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "{:?}", self.kind())

compiler/rustc_smir/src/rustc_smir/convert/ty.rs

@@ -601,6 +601,7 @@ impl<'tcx> Stable<'tcx> for ty::PredicateKind<'tcx> {
                 stable_mir::ty::PredicateKind::ConstEquate(a.stable(tables), b.stable(tables))
             }
             PredicateKind::Ambiguous => stable_mir::ty::PredicateKind::Ambiguous,
+            PredicateKind::NormalizesTo(_pred) => unimplemented!(),
             PredicateKind::AliasRelate(a, b, alias_relation_direction) => {
                 stable_mir::ty::PredicateKind::AliasRelate(
                     a.unpack().stable(tables),

compiler/rustc_trait_selection/src/solve/alias_relate.rs

@@ -92,7 +92,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
                     self.add_goal(Goal::new(
                         self.tcx(),
                         param_env,
-                        ty::ProjectionPredicate { projection_ty: alias, term },
+                        ty::NormalizesTo { alias, term },
                     ));
                     self.try_evaluate_added_goals()?;
                     Ok(Some(self.resolve_vars_if_possible(term)))
@@ -109,11 +109,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         opaque: ty::AliasTy<'tcx>,
         term: ty::Term<'tcx>,
     ) -> QueryResult<'tcx> {
-        self.add_goal(Goal::new(
+        self.add_goal(Goal::new(self.tcx(), param_env, ty::NormalizesTo { alias: opaque, term }));
-            self.tcx(),
-            param_env,
-            ty::ProjectionPredicate { projection_ty: opaque, term },
-        ));
         self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
     }
 

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -352,15 +352,13 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         num_steps: usize,
     ) {
         let tcx = self.tcx();
-        let &ty::Alias(_, projection_ty) = goal.predicate.self_ty().kind() else { return };
+        let &ty::Alias(_, alias) = goal.predicate.self_ty().kind() else { return };
 
         candidates.extend(self.probe(|_| ProbeKind::NormalizedSelfTyAssembly).enter(|ecx| {
             if tcx.recursion_limit().value_within_limit(num_steps) {
                 let normalized_ty = ecx.next_ty_infer();
-                let normalizes_to_goal = goal.with(
+                let normalizes_to_goal =
-                    tcx,
+                    goal.with(tcx, ty::NormalizesTo { alias, term: normalized_ty.into() });
-                    ty::ProjectionPredicate { projection_ty, term: normalized_ty.into() },
-                );
                 ecx.add_goal(normalizes_to_goal);
                 if let Err(NoSolution) = ecx.try_evaluate_added_goals() {
                     debug!("self type normalization failed");

compiler/rustc_trait_selection/src/solve/canonicalize.rs

@@ -69,8 +69,8 @@ impl<'a, 'tcx> Canonicalizer<'a, 'tcx> {
         };
 
         let value = value.fold_with(&mut canonicalizer);
-        assert!(!value.has_infer());
+        assert!(!value.has_infer(), "unexpected infer in {value:?}");
-        assert!(!value.has_placeholders());
+        assert!(!value.has_placeholders(), "unexpected placeholders in {value:?}");
 
         let (max_universe, variables) = canonicalizer.finalize();
 

compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs

@@ -103,7 +103,7 @@ pub(super) struct NestedGoals<'tcx> {
     /// with a fresh inference variable when we evaluate this goal. That can result
     /// in a trait solver cycle. This would currently result in overflow but can be
     /// can be unsound with more powerful coinduction in the future.
-    pub(super) normalizes_to_hack_goal: Option<Goal<'tcx, ty::ProjectionPredicate<'tcx>>>,
+    pub(super) normalizes_to_hack_goal: Option<Goal<'tcx, ty::NormalizesTo<'tcx>>>,
     /// The rest of the goals which have not yet processed or remain ambiguous.
     pub(super) goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
 }
@@ -423,6 +423,9 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 ty::PredicateKind::ConstEquate(_, _) => {
                     bug!("ConstEquate should not be emitted when `-Ztrait-solver=next` is active")
                 }
+                ty::PredicateKind::NormalizesTo(predicate) => {
+                    self.compute_normalizes_to_goal(Goal { param_env, predicate })
+                }
                 ty::PredicateKind::AliasRelate(lhs, rhs, direction) => self
                     .compute_alias_relate_goal(Goal {
                         param_env,
@@ -492,10 +495,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             let unconstrained_rhs = self.next_term_infer_of_kind(goal.predicate.term);
             let unconstrained_goal = goal.with(
                 tcx,
-                ty::ProjectionPredicate {
+                ty::NormalizesTo { alias: goal.predicate.alias, term: unconstrained_rhs },
-                    projection_ty: goal.predicate.projection_ty,
-                    term: unconstrained_rhs,
-                },
             );
 
             let (_, certainty, instantiate_goals) = self.evaluate_goal(
@@ -517,9 +517,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             // looking at the "has changed" return from evaluate_goal,
             // because we expect the `unconstrained_rhs` part of the predicate
             // to have changed -- that means we actually normalized successfully!
-            if goal.predicate.projection_ty
+            if goal.predicate.alias != self.resolve_vars_if_possible(goal.predicate.alias) {
-                != self.resolve_vars_if_possible(goal.predicate.projection_ty)
-            {
                 unchanged_certainty = None;
             }
 
@@ -589,9 +587,10 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
     ///
     /// This is the case if the `term` is an inference variable in the innermost universe
     /// and does not occur in any other part of the predicate.
+    #[instrument(level = "debug", skip(self), ret)]
     pub(super) fn term_is_fully_unconstrained(
         &self,
-        goal: Goal<'tcx, ty::ProjectionPredicate<'tcx>>,
+        goal: Goal<'tcx, ty::NormalizesTo<'tcx>>,
     ) -> bool {
         let term_is_infer = match goal.predicate.term.unpack() {
             ty::TermKind::Ty(ty) => {
@@ -655,7 +654,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         let mut visitor = ContainsTerm { infcx: self.infcx, term: goal.predicate.term };
 
         term_is_infer
-            && goal.predicate.projection_ty.visit_with(&mut visitor).is_continue()
+            && goal.predicate.alias.visit_with(&mut visitor).is_continue()
             && goal.param_env.visit_with(&mut visitor).is_continue()
     }
 

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -108,6 +108,11 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
                                             MismatchedProjectionTypes { err: TypeError::Mismatch },
                                         )
                                     }
+                                    ty::PredicateKind::NormalizesTo(..) => {
+                                        FulfillmentErrorCode::CodeProjectionError(
+                                            MismatchedProjectionTypes { err: TypeError::Mismatch },
+                                        )
+                                    }
                                     ty::PredicateKind::AliasRelate(_, _, _) => {
                                         FulfillmentErrorCode::CodeProjectionError(
                                             MismatchedProjectionTypes { err: TypeError::Mismatch },

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -35,6 +35,7 @@ mod eval_ctxt;
 mod fulfill;
 pub mod inspect;
 mod normalize;
+mod normalizes_to;
 mod project_goals;
 mod search_graph;
 mod trait_goals;
@@ -216,7 +217,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {
     #[instrument(level = "debug", skip(self))]
-    fn set_normalizes_to_hack_goal(&mut self, goal: Goal<'tcx, ty::ProjectionPredicate<'tcx>>) {
+    fn set_normalizes_to_hack_goal(&mut self, goal: Goal<'tcx, ty::NormalizesTo<'tcx>>) {
         assert!(
             self.nested_goals.normalizes_to_hack_goal.is_none(),
             "attempted to set the projection eq hack goal when one already exists"
@@ -310,17 +311,17 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             return None;
         }
 
-        let ty::Alias(kind, projection_ty) = *ty.kind() else {
+        let ty::Alias(kind, alias) = *ty.kind() else {
             return Some(ty);
         };
 
         // We do no always define opaque types eagerly to allow non-defining uses in the defining scope.
         if let (DefineOpaqueTypes::No, ty::AliasKind::Opaque) = (define_opaque_types, kind) {
-            if let Some(def_id) = projection_ty.def_id.as_local() {
+            if let Some(def_id) = alias.def_id.as_local() {
                 if self
                     .unify_existing_opaque_tys(
                         param_env,
-                        OpaqueTypeKey { def_id, args: projection_ty.args },
+                        OpaqueTypeKey { def_id, args: alias.args },
                         self.next_ty_infer(),
                     )
                     .is_empty()
@@ -335,7 +336,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             let normalizes_to_goal = Goal::new(
                 this.tcx(),
                 param_env,
-                ty::ProjectionPredicate { projection_ty, term: normalized_ty.into() },
+                ty::NormalizesTo { alias, term: normalized_ty.into() },
             );
             this.add_goal(normalizes_to_goal);
             this.try_evaluate_added_goals()?;

compiler/rustc_trait_selection/src/solve/normalize.rs

@@ -76,7 +76,7 @@ impl<'tcx> NormalizationFolder<'_, 'tcx> {
             tcx,
             self.at.cause.clone(),
             self.at.param_env,
-            ty::ProjectionPredicate { projection_ty: alias, term: new_infer_ty.into() },
+            ty::NormalizesTo { alias, term: new_infer_ty.into() },
         );
 
         // Do not emit an error if normalization is known to fail but instead
@@ -129,8 +129,8 @@ impl<'tcx> NormalizationFolder<'_, 'tcx> {
             tcx,
             self.at.cause.clone(),
             self.at.param_env,
-            ty::ProjectionPredicate {
+            ty::NormalizesTo {
-                projection_ty: AliasTy::new(tcx, uv.def, uv.args),
+                alias: AliasTy::new(tcx, uv.def, uv.args),
                 term: new_infer_ct.into(),
             },
         );

compiler/rustc_trait_selection/src/solve/normalizes_to/inherent.rs

@@ -12,10 +12,10 @@ use super::EvalCtxt;
 impl<'tcx> EvalCtxt<'_, 'tcx> {
     pub(super) fn normalize_inherent_associated_type(
         &mut self,
-        goal: Goal<'tcx, ty::ProjectionPredicate<'tcx>>,
+        goal: Goal<'tcx, ty::NormalizesTo<'tcx>>,
     ) -> QueryResult<'tcx> {
         let tcx = self.tcx();
-        let inherent = goal.predicate.projection_ty;
+        let inherent = goal.predicate.alias;
         let expected = goal.predicate.term.ty().expect("inherent consts are treated separately");
 
         let impl_def_id = tcx.parent(inherent.def_id);

compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs

@@ -13,20 +13,20 @@ use rustc_middle::traits::solve::{
 };
 use rustc_middle::traits::BuiltinImplSource;
 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
-use rustc_middle::ty::ProjectionPredicate;
+use rustc_middle::ty::NormalizesTo;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::ty::{ToPredicate, TypeVisitableExt};
 use rustc_span::{sym, ErrorGuaranteed, DUMMY_SP};
 
-mod inherent_projection;
+mod inherent;
 mod opaques;
 mod weak_types;
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {
     #[instrument(level = "debug", skip(self), ret)]
-    pub(super) fn compute_projection_goal(
+    pub(super) fn compute_normalizes_to_goal(
         &mut self,
-        goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
+        goal: Goal<'tcx, NormalizesTo<'tcx>>,
     ) -> QueryResult<'tcx> {
         let def_id = goal.predicate.def_id();
         match self.tcx().def_kind(def_id) {
@@ -71,16 +71,13 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
     #[instrument(level = "debug", skip(self), ret)]
     fn normalize_anon_const(
         &mut self,
-        goal: Goal<'tcx, ty::ProjectionPredicate<'tcx>>,
+        goal: Goal<'tcx, ty::NormalizesTo<'tcx>>,
     ) -> QueryResult<'tcx> {
         if let Some(normalized_const) = self.try_const_eval_resolve(
             goal.param_env,
-            ty::UnevaluatedConst::new(
+            ty::UnevaluatedConst::new(goal.predicate.alias.def_id, goal.predicate.alias.args),
-                goal.predicate.projection_ty.def_id,
-                goal.predicate.projection_ty.args,
-            ),
             self.tcx()
-                .type_of(goal.predicate.projection_ty.def_id)
+                .type_of(goal.predicate.alias.def_id)
                 .no_bound_vars()
                 .expect("const ty should not rely on other generics"),
         ) {
@@ -92,13 +89,13 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
     }
 }
 
-impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
+impl<'tcx> assembly::GoalKind<'tcx> for NormalizesTo<'tcx> {
     fn self_ty(self) -> Ty<'tcx> {
         self.self_ty()
     }
 
     fn trait_ref(self, tcx: TyCtxt<'tcx>) -> ty::TraitRef<'tcx> {
-        self.projection_ty.trait_ref(tcx)
+        self.alias.trait_ref(tcx)
     }
 
     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
@@ -123,7 +120,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
                         ecx.instantiate_binder_with_infer(projection_pred);
                     ecx.eq(
                         goal.param_env,
-                        goal.predicate.projection_ty,
+                        goal.predicate.alias,
                         assumption_projection_pred.projection_ty,
                     )?;
                     ecx.eq(goal.param_env, goal.predicate.term, assumption_projection_pred.term)
@@ -132,7 +129,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
                     // Add GAT where clauses from the trait's definition
                     ecx.add_goals(
                         tcx.predicates_of(goal.predicate.def_id())
-                            .instantiate_own(tcx, goal.predicate.projection_ty.args)
+                            .instantiate_own(tcx, goal.predicate.alias.args)
                             .map(|(pred, _)| goal.with(tcx, pred)),
                     );
 
@@ -148,12 +145,12 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
 
     fn consider_impl_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
-        goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
+        goal: Goal<'tcx, NormalizesTo<'tcx>>,
         impl_def_id: DefId,
     ) -> Result<Candidate<'tcx>, NoSolution> {
         let tcx = ecx.tcx();
 
-        let goal_trait_ref = goal.predicate.projection_ty.trait_ref(tcx);
+        let goal_trait_ref = goal.predicate.alias.trait_ref(tcx);
         let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
         let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::ForLookup };
         if !drcx.args_may_unify(goal_trait_ref.args, impl_trait_ref.skip_binder().args) {
@@ -177,7 +174,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
             // Add GAT where clauses from the trait's definition
             ecx.add_goals(
                 tcx.predicates_of(goal.predicate.def_id())
-                    .instantiate_own(tcx, goal.predicate.projection_ty.args)
+                    .instantiate_own(tcx, goal.predicate.alias.args)
                     .map(|(pred, _)| goal.with(tcx, pred)),
             );
 
@@ -202,7 +199,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
                         tcx,
                         guar,
                         tcx.type_of(goal.predicate.def_id())
-                            .instantiate(tcx, goal.predicate.projection_ty.args),
+                            .instantiate(tcx, goal.predicate.alias.args),
                     )
                     .into(),
                     ty::AssocKind::Type => Ty::new_error(tcx, guar).into(),
@@ -227,11 +224,8 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
             //
             // And then map these args to the args of the defining impl of `Assoc`, going
             // from `[u32, u64]` to `[u32, i32, u64]`.
-            let impl_args_with_gat = goal.predicate.projection_ty.args.rebase_onto(
+            let impl_args_with_gat =
-                tcx,
+                goal.predicate.alias.args.rebase_onto(tcx, goal_trait_ref.def_id, impl_args);
-                goal_trait_ref.def_id,
-                impl_args,
-            );
             let args = ecx.translate_args(
                 goal.param_env,
                 impl_def_id,

compiler/rustc_trait_selection/src/solve/normalizes_to/opaques.rs

@@ -12,10 +12,10 @@ use crate::solve::{EvalCtxt, SolverMode};
 impl<'tcx> EvalCtxt<'_, 'tcx> {
     pub(super) fn normalize_opaque_type(
         &mut self,
-        goal: Goal<'tcx, ty::ProjectionPredicate<'tcx>>,
+        goal: Goal<'tcx, ty::NormalizesTo<'tcx>>,
     ) -> QueryResult<'tcx> {
         let tcx = self.tcx();
-        let opaque_ty = goal.predicate.projection_ty;
+        let opaque_ty = goal.predicate.alias;
         let expected = goal.predicate.term.ty().expect("no such thing as an opaque const");
 
         match (goal.param_env.reveal(), self.solver_mode()) {

compiler/rustc_trait_selection/src/solve/normalizes_to/weak_types.rs

@@ -11,10 +11,10 @@ use super::EvalCtxt;
 impl<'tcx> EvalCtxt<'_, 'tcx> {
     pub(super) fn normalize_weak_type(
         &mut self,
-        goal: Goal<'tcx, ty::ProjectionPredicate<'tcx>>,
+        goal: Goal<'tcx, ty::NormalizesTo<'tcx>>,
     ) -> QueryResult<'tcx> {
         let tcx = self.tcx();
-        let weak_ty = goal.predicate.projection_ty;
+        let weak_ty = goal.predicate.alias;
         let expected = goal.predicate.term.ty().expect("no such thing as a const alias");
 
         let actual = tcx.type_of(weak_ty.def_id).instantiate(tcx, weak_ty.args);

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -0,0 +1,23 @@
+use super::EvalCtxt;
+use rustc_middle::traits::solve::{Certainty, Goal, QueryResult};
+use rustc_middle::ty::{self, ProjectionPredicate};
+
+impl<'tcx> EvalCtxt<'_, 'tcx> {
+    #[instrument(level = "debug", skip(self), ret)]
+    pub(super) fn compute_projection_goal(
+        &mut self,
+        goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
+    ) -> QueryResult<'tcx> {
+        match goal.predicate.term.unpack() {
+            ty::TermKind::Ty(term) => {
+                let alias = goal.predicate.projection_ty.to_ty(self.tcx());
+                self.eq(goal.param_env, alias, term)?;
+                self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+            }
+            // FIXME(associated_const_equality): actually do something here.
+            ty::TermKind::Const(_) => {
+                self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+            }
+        }
+    }
+}

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -820,6 +820,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                 // the `ParamEnv`.
                 ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(..))
                 | ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(..))
+                | ty::PredicateKind::NormalizesTo(..)
                 | ty::PredicateKind::AliasRelate(..)
                 | ty::PredicateKind::ObjectSafe(..)
                 | ty::PredicateKind::Subtype(..)

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -854,6 +854,11 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 
                     ty::PredicateKind::Ambiguous => span_bug!(span, "ambiguous"),
 
+                    ty::PredicateKind::NormalizesTo(..) => span_bug!(
+                        span,
+                        "NormalizesTo predicate should never be the predicate cause of a SelectionError"
+                    ),
+
                     ty::PredicateKind::AliasRelate(..) => span_bug!(
                         span,
                         "AliasRelate predicate should never be the predicate cause of a SelectionError"

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -360,8 +360,11 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                     ProcessResult::Changed(mk_pending(vec![obligation.with(infcx.tcx, pred)]))
                 }
                 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
+                ty::PredicateKind::NormalizesTo(..) => {
+                    bug!("NormalizesTo is only used by the new solver")
+                }
                 ty::PredicateKind::AliasRelate(..) => {
-                    bug!("AliasRelate is only used for new solver")
+                    bug!("AliasRelate is only used by the new solver")
                 }
             },
             Some(pred) => match pred {
@@ -412,8 +415,11 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 }
 
                 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
+                ty::PredicateKind::NormalizesTo(..) => {
+                    bug!("NormalizesTo is only used by the new solver")
+                }
                 ty::PredicateKind::AliasRelate(..) => {
-                    bug!("AliasRelate is only used for new solver")
+                    bug!("AliasRelate is only used by the new solver")
                 }
 
                 // General case overflow check. Allow `process_trait_obligation`

compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs

@@ -123,9 +123,9 @@ pub fn compute_implied_outlives_bounds_inner<'tcx>(
                 Some(pred) => pred,
             };
             match pred {
-                ty::PredicateKind::Clause(ty::ClauseKind::Trait(..))
                 // FIXME(const_generics): Make sure that `<'a, 'b, const N: &'a &'b u32>` is sound
                 // if we ever support that
+                ty::PredicateKind::Clause(ty::ClauseKind::Trait(..))
                 | ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(..))
                 | ty::PredicateKind::Subtype(..)
                 | ty::PredicateKind::Coerce(..)
@@ -134,8 +134,8 @@ pub fn compute_implied_outlives_bounds_inner<'tcx>(
                 | ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(..))
                 | ty::PredicateKind::ConstEquate(..)
                 | ty::PredicateKind::Ambiguous
-                | ty::PredicateKind::AliasRelate(..)
+                | ty::PredicateKind::NormalizesTo(..)
-                 => {}
+                | ty::PredicateKind::AliasRelate(..) => {}
 
                 // We need to search through *all* WellFormed predicates
                 ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(arg)) => {
@@ -143,10 +143,9 @@ pub fn compute_implied_outlives_bounds_inner<'tcx>(
                 }
 
                 // We need to register region relationships
-                ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(ty::OutlivesPredicate(
+                ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(
-                    r_a,
+                    ty::OutlivesPredicate(r_a, r_b),
-                    r_b,
+                )) => outlives_bounds.push(ty::OutlivesPredicate(r_a.into(), r_b)),
-                ))) => outlives_bounds.push(ty::OutlivesPredicate(r_a.into(), r_b)),
 
                 ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(
                     ty_a,

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -990,8 +990,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         }
                     }
                 }
+                ty::PredicateKind::NormalizesTo(..) => {
+                    bug!("NormalizesTo is only used by the new solver")
+                }
                 ty::PredicateKind::AliasRelate(..) => {
-                    bug!("AliasRelate is only used for new solver")
+                    bug!("AliasRelate is only used by the new solver")
                 }
                 ty::PredicateKind::Ambiguous => Ok(EvaluatedToAmbig),
                 ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, ty)) => {

compiler/rustc_trait_selection/src/traits/structural_normalize.rs

@@ -25,8 +25,7 @@ impl<'tcx> StructurallyNormalizeExt<'tcx> for At<'_, 'tcx> {
             // FIXME(-Ztrait-solver=next): correctly handle
             // overflow here.
             for _ in 0..256 {
-                let ty::Alias(ty::Projection | ty::Inherent | ty::Weak, projection_ty) = *ty.kind()
+                let ty::Alias(ty::Projection | ty::Inherent | ty::Weak, alias) = *ty.kind() else {
-                else {
                     break;
                 };
 
@@ -38,10 +37,7 @@ impl<'tcx> StructurallyNormalizeExt<'tcx> for At<'_, 'tcx> {
                     self.infcx.tcx,
                     self.cause.clone(),
                     self.param_env,
-                    ty::Binder::dummy(ty::ProjectionPredicate {
+                    ty::NormalizesTo { alias, term: new_infer_ty.into() },
-                        projection_ty,
-                        term: new_infer_ty.into(),
-                    }),
                 );
                 if self.infcx.predicate_may_hold(&obligation) {
                     fulfill_cx.register_predicate_obligation(self.infcx, obligation);

compiler/rustc_traits/src/normalize_erasing_regions.rs

@@ -55,6 +55,7 @@ fn not_outlives_predicate(p: ty::Predicate<'_>) -> bool {
         ty::PredicateKind::Clause(ty::ClauseKind::Trait(..))
         | ty::PredicateKind::Clause(ty::ClauseKind::Projection(..))
         | ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(..))
+        | ty::PredicateKind::NormalizesTo(..)
         | ty::PredicateKind::AliasRelate(..)
         | ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(..))
         | ty::PredicateKind::ObjectSafe(..)

compiler/rustc_type_ir/src/interner.rs

@@ -57,6 +57,7 @@ pub trait Interner: Sized {
     type RegionOutlivesPredicate: Clone + Debug + Hash + Eq;
     type TypeOutlivesPredicate: Clone + Debug + Hash + Eq;
     type ProjectionPredicate: Clone + Debug + Hash + Eq;
+    type NormalizesTo: Clone + Debug + Hash + Eq;
     type SubtypePredicate: Clone + Debug + Hash + Eq;
     type CoercePredicate: Clone + Debug + Hash + Eq;
     type ClosureKind: Clone + Debug + Hash + Eq;

compiler/rustc_type_ir/src/predicate_kind.rs

@@ -120,7 +120,7 @@ where
 }
 
 #[derive(derivative::Derivative)]
-#[derivative(Clone(bound = ""), Hash(bound = ""))]
+#[derivative(Clone(bound = ""), Hash(bound = ""), PartialEq(bound = ""), Eq(bound = ""))]
 #[cfg_attr(feature = "nightly", derive(TyEncodable, TyDecodable, HashStable_NoContext))]
 pub enum PredicateKind<I: Interner> {
     /// Prove a clause
@@ -153,6 +153,15 @@ pub enum PredicateKind<I: Interner> {
     /// Used for coherence to mark opaque types as possibly equal to each other but ambiguous.
     Ambiguous,
 
+    /// The alias normalizes to `term`. Unlike `Projection`, this always fails if the alias
+    /// cannot be normalized in the current context.
+    ///
+    /// `Projection(<T as Trait>::Assoc, ?x)` results in `?x == <T as Trait>::Assoc` while
+    /// `NormalizesTo(<T as Trait>::Assoc, ?x)` results in `NoSolution`.
+    ///
+    /// Only used in the new solver.
+    NormalizesTo(I::NormalizesTo),
+
     /// Separate from `ClauseKind::Projection` which is used for normalization in new solver.
     /// This predicate requires two terms to be equal to eachother.
     ///
@@ -168,28 +177,11 @@ where
     I::Term: Copy,
     I::CoercePredicate: Copy,
     I::SubtypePredicate: Copy,
+    I::NormalizesTo: Copy,
     ClauseKind<I>: Copy,
 {
 }
 
-impl<I: Interner> PartialEq for PredicateKind<I> {
-    fn eq(&self, other: &Self) -> bool {
-        match (self, other) {
-            (Self::Clause(l0), Self::Clause(r0)) => l0 == r0,
-            (Self::ObjectSafe(l0), Self::ObjectSafe(r0)) => l0 == r0,
-            (Self::Subtype(l0), Self::Subtype(r0)) => l0 == r0,
-            (Self::Coerce(l0), Self::Coerce(r0)) => l0 == r0,
-            (Self::ConstEquate(l0, l1), Self::ConstEquate(r0, r1)) => l0 == r0 && l1 == r1,
-            (Self::AliasRelate(l0, l1, l2), Self::AliasRelate(r0, r1, r2)) => {
-                l0 == r0 && l1 == r1 && l2 == r2
-            }
-            _ => core::mem::discriminant(self) == core::mem::discriminant(other),
-        }
-    }
-}
-
-impl<I: Interner> Eq for PredicateKind<I> {}
-
 impl<I: Interner> TypeFoldable<I> for PredicateKind<I>
 where
     I::DefId: TypeFoldable<I>,
@@ -198,6 +190,7 @@ where
     I::Term: TypeFoldable<I>,
     I::CoercePredicate: TypeFoldable<I>,
     I::SubtypePredicate: TypeFoldable<I>,
+    I::NormalizesTo: TypeFoldable<I>,
     ClauseKind<I>: TypeFoldable<I>,
 {
     fn try_fold_with<F: FallibleTypeFolder<I>>(self, folder: &mut F) -> Result<Self, F::Error> {
@@ -210,6 +203,7 @@ where
                 PredicateKind::ConstEquate(a.try_fold_with(folder)?, b.try_fold_with(folder)?)
             }
             PredicateKind::Ambiguous => PredicateKind::Ambiguous,
+            PredicateKind::NormalizesTo(p) => PredicateKind::NormalizesTo(p.try_fold_with(folder)?),
             PredicateKind::AliasRelate(a, b, d) => PredicateKind::AliasRelate(
                 a.try_fold_with(folder)?,
                 b.try_fold_with(folder)?,
@@ -227,6 +221,7 @@ where
     I::Term: TypeVisitable<I>,
     I::CoercePredicate: TypeVisitable<I>,
     I::SubtypePredicate: TypeVisitable<I>,
+    I::NormalizesTo: TypeVisitable<I>,
     ClauseKind<I>: TypeVisitable<I>,
 {
     fn visit_with<V: TypeVisitor<I>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
@@ -240,6 +235,7 @@ where
                 b.visit_with(visitor)
             }
             PredicateKind::Ambiguous => ControlFlow::Continue(()),
+            PredicateKind::NormalizesTo(p) => p.visit_with(visitor),
             PredicateKind::AliasRelate(a, b, d) => {
                 a.visit_with(visitor)?;
                 b.visit_with(visitor)?;
@@ -294,6 +290,7 @@ impl<I: Interner> fmt::Debug for PredicateKind<I> {
             }
             PredicateKind::ConstEquate(c1, c2) => write!(f, "ConstEquate({c1:?}, {c2:?})"),
             PredicateKind::Ambiguous => write!(f, "Ambiguous"),
+            PredicateKind::NormalizesTo(p) => p.fmt(f),
             PredicateKind::AliasRelate(t1, t2, dir) => {
                 write!(f, "AliasRelate({t1:?}, {dir:?}, {t2:?})")
             }

tests/ui/traits/new-solver/alias-bound-unsound.rs

@@ -23,7 +23,7 @@ fn main() {
     let x = String::from("hello, world");
     drop(<() as Foo>::copy_me(&x));
     //~^ ERROR overflow evaluating the requirement `<() as Foo>::Item: Sized`
-    //~| ERROR overflow evaluating the requirement `<() as Foo>::Item == _`
+    //~| ERROR overflow evaluating the requirement `<() as Foo>::Item normalizes-to _`
     //~| ERROR overflow evaluating the requirement `<() as Foo>::Item well-formed`
     //~| ERROR overflow evaluating the requirement `String <: <() as Foo>::Item`
     //~| ERROR overflow evaluating the requirement `&<() as Foo>::Item well-formed`

tests/ui/traits/new-solver/alias-bound-unsound.stderr

@@ -19,7 +19,7 @@ LL |     drop(<() as Foo>::copy_me(&x));
    |
    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`alias_bound_unsound`)
 
-error[E0275]: overflow evaluating the requirement `<() as Foo>::Item == _`
+error[E0275]: overflow evaluating the requirement `<() as Foo>::Item normalizes-to _`
   --> $DIR/alias-bound-unsound.rs:24:10
    |
 LL |     drop(<() as Foo>::copy_me(&x));

tests/ui/traits/new-solver/generalize/generalize-proj-new-universe-index-2.rs

@@ -1,5 +1,7 @@
 // compile-flags: -Ztrait-solver=next
 // known-bug: trait-system-refactor-initiative#60
+// dont-check-failure-status
+// dont-check-compiler-stderr
 
 // Generalizing a projection containing an inference variable
 // which cannot be named by the `root_vid` can result in ambiguity.

tests/ui/traits/new-solver/generalize/occurs-check-nested-alias.next.stderr

@@ -1,9 +1,11 @@
-error[E0271]: type mismatch resolving `<<T as Id<_>>::Id as Unnormalizable>::Assoc == _`
+error[E0275]: overflow evaluating the requirement `<<T as Id<_>>::Id as Unnormalizable>::Assoc == _`
-  --> $DIR/occurs-check-nested-alias.rs:35:9
+  --> $DIR/occurs-check-nested-alias.rs:36:9
    |
 LL |     x = y;
-   |         ^ types differ
+   |         ^
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`occurs_check_nested_alias`)
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0271`.
+For more information about this error, try `rustc --explain E0275`.

tests/ui/traits/new-solver/generalize/occurs-check-nested-alias.rs

@@ -1,7 +1,8 @@
 // revisions: old next
 //[old] check-pass
 
-// Need to emit an alias-relate instead of a `Projection` goal here.
+// Currently always fails to generalize the outer alias, even if it
+// is treated as rigid by `alias-relate`.
 //[next] compile-flags: -Ztrait-solver=next
 //[next] known-bug: trait-system-refactor-initiative#8
 #![crate_type = "lib"]

tests/ui/traits/new-solver/projection/param-env-trait-candidate-1.rs

@@ -0,0 +1,14 @@
+// check-pass
+// compile-flags: -Ztrait-solver=next
+
+// See https://github.com/rust-lang/trait-system-refactor-initiative/issues/1
+// a minimization of a pattern in core.
+fn next<T: Iterator<Item = U>, U>(t: &mut T) -> Option<U> {
+    t.next()
+}
+
+fn foo<T: Iterator>(t: &mut T) {
+    let _: Option<T::Item> = next(t);
+}
+
+fn main() {}

tests/ui/traits/new-solver/projection/param-env-trait-candidate-2.rs

@@ -0,0 +1,29 @@
+// check-pass
+// compile-flags: -Ztrait-solver=next
+
+// See https://github.com/rust-lang/trait-system-refactor-initiative/issues/1,
+// a minimization of a pattern in core.
+
+trait Iterator {
+    type Item;
+}
+
+struct Flatten<I>(I);
+
+impl<I, U> Iterator for Flatten<I>
+where
+    I: Iterator<Item = U>,
+{
+    type Item = U;
+}
+
+fn needs_iterator<I: Iterator>() {}
+
+fn environment<J>()
+where
+    J: Iterator,
+{
+    needs_iterator::<Flatten<J>>();
+}
+
+fn main() {}

tests/ui/traits/new-solver/specialization-transmute.stderr

@@ -8,13 +8,11 @@ LL | #![feature(specialization)]
    = help: consider using `min_specialization` instead, which is more stable and complete
    = note: `#[warn(incomplete_features)]` on by default
 
-error[E0284]: type annotations needed
+error[E0284]: type annotations needed: cannot satisfy `<T as Default>::Id normalizes-to _`
   --> $DIR/specialization-transmute.rs:15:23
    |
 LL |     fn intu(&self) -> &Self::Id {
-   |                       ^^^^^^^^^ cannot infer type
+   |                       ^^^^^^^^^ cannot satisfy `<T as Default>::Id normalizes-to _`
-   |
-   = note: cannot satisfy `<T as Default>::Id == _`
 
 error[E0282]: type annotations needed
   --> $DIR/specialization-transmute.rs:13:23