Deeply normalize associated type bounds before proving them

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -2107,18 +2107,21 @@ pub(super) fn check_type_bounds<'tcx>(
         ObligationCause::new(impl_ty_span, impl_ty_def_id, code)
     };
 
-    let mut obligations: Vec<_> = tcx
-        .explicit_item_bounds(trait_ty.def_id)
-        .iter_instantiated_copied(tcx, rebased_args)
-        .map(|(concrete_ty_bound, span)| {
-            debug!(?concrete_ty_bound);
-            traits::Obligation::new(tcx, mk_cause(span), param_env, concrete_ty_bound)
-        })
-        .collect();
+    let mut obligations: Vec<_> = util::elaborate(
+        tcx,
+        tcx.explicit_item_bounds(trait_ty.def_id).iter_instantiated_copied(tcx, rebased_args).map(
+            |(concrete_ty_bound, span)| {
+                debug!(?concrete_ty_bound);
+                traits::Obligation::new(tcx, mk_cause(span), param_env, concrete_ty_bound)
+            },
+        ),
+    )
+    .collect();
 
     // Only in a const implementation do we need to check that the `~const` item bounds hold.
     if tcx.is_conditionally_const(impl_ty_def_id) {
-        obligations.extend(
+        obligations.extend(util::elaborate(
+            tcx,
             tcx.explicit_implied_const_bounds(trait_ty.def_id)
                 .iter_instantiated_copied(tcx, rebased_args)
                 .map(|(c, span)| {
@@ -2129,34 +2132,27 @@ pub(super) fn check_type_bounds<'tcx>(
                         c.to_host_effect_clause(tcx, ty::BoundConstness::Maybe),
                     )
                 }),
-        );
+        ));
     }
     debug!(item_bounds=?obligations);
 
     // Normalize predicates with the assumption that the GAT may always normalize
     // to its definition type. This should be the param-env we use to *prove* the
     // predicate too, but we don't do that because of performance issues.
     // See <https://github.com/rust-lang/rust/pull/117542#issue-1976337685>.
-    let trait_projection_ty = Ty::new_projection_from_args(tcx, trait_ty.def_id, rebased_args);
-    let impl_identity_ty = tcx.type_of(impl_ty.def_id).instantiate_identity();
     let normalize_param_env = param_env_with_gat_bounds(tcx, impl_ty, impl_trait_ref);
-    for mut obligation in util::elaborate(tcx, obligations) {
-        let normalized_predicate = if infcx.next_trait_solver() {
-            obligation.predicate.fold_with(&mut ReplaceTy {
-                tcx,
-                from: trait_projection_ty,
-                to: impl_identity_ty,
-            })
-        } else {
-            ocx.normalize(&normalize_cause, normalize_param_env, obligation.predicate)
-        };
-        debug!(?normalized_predicate);
-        obligation.predicate = normalized_predicate;
-
-        ocx.register_obligation(obligation);
+    for obligation in &mut obligations {
+        match ocx.deeply_normalize(&normalize_cause, normalize_param_env, obligation.predicate) {
+            Ok(pred) => obligation.predicate = pred,
+            Err(e) => {
+                return Err(infcx.err_ctxt().report_fulfillment_errors(e));
+            }
+        }
     }
+
     // Check that all obligations are satisfied by the implementation's
     // version.
+    ocx.register_obligations(obligations);
     let errors = ocx.select_all_or_error();
     if !errors.is_empty() {
         let reported = infcx.err_ctxt().report_fulfillment_errors(errors);
@@ -2168,22 +2164,6 @@ pub(super) fn check_type_bounds<'tcx>(
     ocx.resolve_regions_and_report_errors(impl_ty_def_id, param_env, assumed_wf_types)
 }
 
-struct ReplaceTy<'tcx> {
-    tcx: TyCtxt<'tcx>,
-    from: Ty<'tcx>,
-    to: Ty<'tcx>,
-}
-
-impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ReplaceTy<'tcx> {
-    fn cx(&self) -> TyCtxt<'tcx> {
-        self.tcx
-    }
-
-    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
-        if self.from == ty { self.to } else { ty.super_fold_with(self) }
-    }
-}
-
 /// Install projection predicates that allow GATs to project to their own
 /// definition types. This is not allowed in general in cases of default
 /// associated types in trait definitions, or when specialization is involved,

tests/ui/impl-trait/in-trait/default-body-with-rpit.rs

@@ -1,5 +1,8 @@
 //@ edition:2021
 //@ check-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
 
 use std::fmt::Debug;
 

tests/ui/impl-trait/in-trait/nested-rpitit-bounds.rs

@@ -1,4 +1,7 @@
 //@ check-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
 
 use std::ops::Deref;
 

tests/ui/traits/const-traits/predicate-entailment-passes.rs

@@ -6,32 +6,21 @@
 #[const_trait] trait Bar {}
 impl const Bar for () {}
 
-
 #[const_trait] trait TildeConst {
-    type Bar<T> where T: ~const Bar;
-
     fn foo<T>() where T: ~const Bar;
 }
 impl TildeConst for () {
-    type Bar<T> = () where T: Bar;
-
     fn foo<T>() where T: Bar {}
 }
 
 
 #[const_trait] trait AlwaysConst {
-    type Bar<T> where T: const Bar;
-
     fn foo<T>() where T: const Bar;
 }
 impl AlwaysConst for i32 {
-    type Bar<T> = () where T: Bar;
-
     fn foo<T>() where T: Bar {}
 }
 impl const AlwaysConst for u32 {
-    type Bar<T> = () where T: ~const Bar;
-
     fn foo<T>() where T: ~const Bar {}
 }
 

tests/ui/traits/next-solver/coherence/trait_ref_is_knowable-norm-overflow.stderr

@@ -1,18 +1,8 @@
-error[E0275]: overflow evaluating the requirement `<T as Overflow>::Assoc: Sized`
+error[E0275]: overflow evaluating the requirement `<T as Overflow>::Assoc == _`
   --> $DIR/trait_ref_is_knowable-norm-overflow.rs:10:18
    |
 LL |     type Assoc = <T as Overflow>::Assoc;
    |                  ^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: required by a bound in `Overflow::Assoc`
-  --> $DIR/trait_ref_is_knowable-norm-overflow.rs:7:5
-   |
-LL |     type Assoc;
-   |     ^^^^^^^^^^^ required by this bound in `Overflow::Assoc`
-help: consider relaxing the implicit `Sized` restriction
-   |
-LL |     type Assoc: ?Sized;
-   |               ++++++++
 
 error[E0119]: conflicting implementations of trait `Trait`
   --> $DIR/trait_ref_is_knowable-norm-overflow.rs:18:1

tests/ui/traits/next-solver/gat-wf.rs

@@ -0,0 +1,14 @@
+// Make sure that, like the old trait solver, we end up requiring that the WC of
+// impl GAT matches that of the trait. This is not a restriction that we *need*,
+// but is a side-effect of registering the where clauses when normalizing the GAT
+// when proving it satisfies its item bounds.
+
+trait Foo {
+    type T<'a>: Sized where Self: 'a;
+}
+
+impl Foo for &() {
+    type T<'a> = ();
+}
+
+fn main() {}