Allow trait A: ~const B

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1272,7 +1272,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 self.visit_vis(&item.vis);
                 self.visit_ident(item.ident);
                 self.visit_generics(generics);
-                self.with_banned_tilde_const(|this| {
+                self.with_tilde_const_allowed(|this| {
                     walk_list!(this, visit_param_bound, bounds, BoundKind::SuperTraits)
                 });
                 walk_list!(self, visit_assoc_item, items, AssocCtxt::Trait);

compiler/rustc_infer/src/traits/util.rs

@@ -145,7 +145,12 @@ impl<'tcx> Elaborator<'tcx> {
                 // Get predicates declared on the trait.
                 let predicates = tcx.super_predicates_of(data.def_id());
 
-                let obligations = predicates.predicates.iter().map(|&(pred, _)| {
+                let obligations = predicates.predicates.iter().map(|&(mut pred, _)| {
+                    // when parent predicate is non-const, elaborate it to non-const predicates.
+                    if data.constness == ty::BoundConstness::NotConst {
+                        pred = pred.without_const(tcx);
+                    }
+
                     predicate_obligation(
                         pred.subst_supertrait(tcx, &bound_predicate.rebind(data.trait_ref)),
                         obligation.param_env,

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -85,7 +85,7 @@ pub fn trait_obligations<'a, 'tcx>(
     infcx: &InferCtxt<'a, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     body_id: hir::HirId,
-    trait_ref: &ty::TraitRef<'tcx>,
+    trait_pred: &ty::TraitPredicate<'tcx>,
     span: Span,
     item: &'tcx hir::Item<'tcx>,
 ) -> Vec<traits::PredicateObligation<'tcx>> {
@@ -98,7 +98,7 @@ pub fn trait_obligations<'a, 'tcx>(
         recursion_depth: 0,
         item: Some(item),
     };
-    wf.compute_trait_ref(trait_ref, Elaborate::All);
+    wf.compute_trait_pred(trait_pred, Elaborate::All);
     debug!(obligations = ?wf.out);
     wf.normalize(infcx)
 }
@@ -123,7 +123,7 @@ pub fn predicate_obligations<'a, 'tcx>(
     // It's ok to skip the binder here because wf code is prepared for it
     match predicate.kind().skip_binder() {
         ty::PredicateKind::Trait(t) => {
-            wf.compute_trait_ref(&t.trait_ref, Elaborate::None);
+            wf.compute_trait_pred(&t, Elaborate::None);
         }
         ty::PredicateKind::RegionOutlives(..) => {}
         ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
@@ -301,11 +301,18 @@ impl<'tcx> WfPredicates<'tcx> {
     }
 
     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
-    fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
+    fn compute_trait_pred(&mut self, trait_pred: &ty::TraitPredicate<'tcx>, elaborate: Elaborate) {
         let tcx = self.tcx;
-        let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
+        let trait_ref = &trait_pred.trait_ref;
 
-        debug!("compute_trait_ref obligations {:?}", obligations);
+        // if the trait predicate is not const, the wf obligations should not be const as well.
+        let obligations = if trait_pred.constness == ty::BoundConstness::NotConst {
+            self.nominal_obligations_without_const(trait_ref.def_id, trait_ref.substs)
+        } else {
+            self.nominal_obligations(trait_ref.def_id, trait_ref.substs)
+        };
+
+        debug!("compute_trait_pred obligations {:?}", obligations);
         let param_env = self.param_env;
         let depth = self.recursion_depth;
 
@@ -685,10 +692,11 @@ impl<'tcx> WfPredicates<'tcx> {
     }
 
     #[instrument(level = "debug", skip(self))]
-    fn nominal_obligations(
+    fn nominal_obligations_inner(
         &mut self,
         def_id: DefId,
         substs: SubstsRef<'tcx>,
+        remap_constness: bool,
     ) -> Vec<traits::PredicateObligation<'tcx>> {
         let predicates = self.tcx.predicates_of(def_id);
         let mut origins = vec![def_id; predicates.predicates.len()];
@@ -703,19 +711,38 @@ impl<'tcx> WfPredicates<'tcx> {
         debug_assert_eq!(predicates.predicates.len(), origins.len());
 
         iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
-            .map(|((pred, span), origin_def_id)| {
+            .map(|((mut pred, span), origin_def_id)| {
                 let code = if span.is_dummy() {
                     traits::MiscObligation
                 } else {
                     traits::BindingObligation(origin_def_id, span)
                 };
                 let cause = self.cause(code);
+                if remap_constness {
+                    pred = pred.without_const(self.tcx);
+                }
                 traits::Obligation::with_depth(cause, self.recursion_depth, self.param_env, pred)
             })
             .filter(|pred| !pred.has_escaping_bound_vars())
             .collect()
     }
 
+    fn nominal_obligations(
+        &mut self,
+        def_id: DefId,
+        substs: SubstsRef<'tcx>,
+    ) -> Vec<traits::PredicateObligation<'tcx>> {
+        self.nominal_obligations_inner(def_id, substs, false)
+    }
+
+    fn nominal_obligations_without_const(
+        &mut self,
+        def_id: DefId,
+        substs: SubstsRef<'tcx>,
+    ) -> Vec<traits::PredicateObligation<'tcx>> {
+        self.nominal_obligations_inner(def_id, substs, true)
+    }
+
     fn from_object_ty(
         &mut self,
         ty: Ty<'tcx>,

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -183,7 +183,7 @@ fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) {
             // We match on both `ty::ImplPolarity` and `ast::ImplPolarity` just to get the `!` span.
             match (tcx.impl_polarity(def_id), impl_.polarity) {
                 (ty::ImplPolarity::Positive, _) => {
-                    check_impl(tcx, item, impl_.self_ty, &impl_.of_trait);
+                    check_impl(tcx, item, impl_.self_ty, &impl_.of_trait, impl_.constness);
                 }
                 (ty::ImplPolarity::Negative, ast::ImplPolarity::Negative(span)) => {
                     // FIXME(#27579): what amount of WF checking do we need for neg impls?
@@ -1242,6 +1242,7 @@ fn check_impl<'tcx>(
     item: &'tcx hir::Item<'tcx>,
     ast_self_ty: &hir::Ty<'_>,
     ast_trait_ref: &Option<hir::TraitRef<'_>>,
+    constness: hir::Constness,
 ) {
     enter_wf_checking_ctxt(tcx, item.span, item.def_id, |wfcx| {
         match *ast_trait_ref {
@@ -1251,11 +1252,19 @@ fn check_impl<'tcx>(
                 // won't hold).
                 let trait_ref = tcx.impl_trait_ref(item.def_id).unwrap();
                 let trait_ref = wfcx.normalize(ast_trait_ref.path.span, None, trait_ref);
+                let trait_pred = ty::TraitPredicate {
+                    trait_ref,
+                    constness: match constness {
+                        hir::Constness::Const => ty::BoundConstness::ConstIfConst,
+                        hir::Constness::NotConst => ty::BoundConstness::NotConst,
+                    },
+                    polarity: ty::ImplPolarity::Positive,
+                };
                 let obligations = traits::wf::trait_obligations(
                     wfcx.infcx,
                     wfcx.param_env,
                     wfcx.body_id,
-                    &trait_ref,
+                    &trait_pred,
                     ast_trait_ref.path.span,
                     item,
                 );

src/test/ui/rfc-2632-const-trait-impl/super-traits-fail-2.rs

@@ -0,0 +1,14 @@
+#![feature(const_trait_impl)]
+
+trait Foo {
+    fn a(&self);
+}
+trait Bar: ~const Foo {}
+
+const fn foo<T: Bar>(x: &T) {
+    x.a();
+    //~^ ERROR the trait bound
+    //~| ERROR cannot call
+}
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/super-traits-fail-2.stderr

@@ -0,0 +1,24 @@
+error[E0277]: the trait bound `T: ~const Foo` is not satisfied
+  --> $DIR/super-traits-fail-2.rs:9:7
+   |
+LL |     x.a();
+   |       ^^^ the trait `~const Foo` is not implemented for `T`
+   |
+note: the trait `Foo` is implemented for `T`, but that implementation is not `const`
+  --> $DIR/super-traits-fail-2.rs:9:7
+   |
+LL |     x.a();
+   |       ^^^
+
+error[E0015]: cannot call non-const fn `<T as Foo>::a` in constant functions
+  --> $DIR/super-traits-fail-2.rs:9:7
+   |
+LL |     x.a();
+   |       ^^^
+   |
+   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0015, E0277.
+For more information about an error, try `rustc --explain E0015`.

src/test/ui/rfc-2632-const-trait-impl/super-traits-fail.rs

@@ -0,0 +1,16 @@
+#![feature(const_trait_impl)]
+
+trait Foo {
+    fn a(&self);
+}
+trait Bar: ~const Foo {}
+
+struct S;
+impl Foo for S {
+    fn a(&self) {}
+}
+
+impl const Bar for S {}
+//~^ ERROR the trait bound
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/super-traits-fail.stderr

@@ -0,0 +1,24 @@
+error[E0277]: the trait bound `S: ~const Foo` is not satisfied
+  --> $DIR/super-traits-fail.rs:13:12
+   |
+LL | impl const Bar for S {}
+   |            ^^^ the trait `~const Foo` is not implemented for `S`
+   |
+note: the trait `Foo` is implemented for `S`, but that implementation is not `const`
+  --> $DIR/super-traits-fail.rs:13:12
+   |
+LL | impl const Bar for S {}
+   |            ^^^
+note: required by a bound in `Bar`
+  --> $DIR/super-traits-fail.rs:6:12
+   |
+LL | trait Bar: ~const Foo {}
+   |            ^^^^^^^^^^ required by this bound in `Bar`
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | impl const Bar for S where S: ~const Foo {}
+   |                      +++++++++++++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/rfc-2632-const-trait-impl/super-traits.rs

@@ -0,0 +1,22 @@
+// check-pass
+#![feature(const_trait_impl)]
+
+trait Foo {
+    fn a(&self);
+}
+trait Bar: ~const Foo {}
+
+struct S;
+impl const Foo for S {
+    fn a(&self) {}
+}
+
+impl const Bar for S {}
+
+const fn foo<T: ~const Bar>(t: &T) {
+    t.a();
+}
+
+const _: () = foo(&S);
+
+fn main() {}