Implement const effect predicate in new solver

compiler/rustc_hir_analysis/src/bounds.rs

@@ -81,6 +81,17 @@ impl<'tcx> Bounds<'tcx> {
         self.clauses.insert(0, (trait_ref.upcast(tcx), span));
     }
 
+    /// Push a `const` or `~const` bound as a `HostEffect` predicate.
+    pub(crate) fn push_const_bound(
+        &mut self,
+        tcx: TyCtxt<'tcx>,
+        bound_trait_ref: ty::PolyTraitRef<'tcx>,
+        host: ty::HostPolarity,
+        span: Span,
+    ) {
+        self.clauses.push((bound_trait_ref.to_host_effect(tcx, host), span));
+    }
+
     pub(crate) fn clauses(
         &self,
         // FIXME(effects): remove tcx

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -187,48 +187,61 @@ fn compare_method_predicate_entailment<'tcx>(
     let impl_to_placeholder_args = GenericArgs::identity_for_item(tcx, impl_m.def_id);
 
     // Create mapping from trait to placeholder.
+    let impl_def_id = impl_m.container_id(tcx);
     let trait_to_placeholder_args =
-        impl_to_placeholder_args.rebase_onto(tcx, impl_m.container_id(tcx), trait_to_impl_args);
+        impl_to_placeholder_args.rebase_onto(tcx, impl_def_id, trait_to_impl_args);
     debug!("compare_impl_method: trait_to_placeholder_args={:?}", trait_to_placeholder_args);
 
     let impl_m_predicates = tcx.predicates_of(impl_m.def_id);
     let trait_m_predicates = tcx.predicates_of(trait_m.def_id);
 
-    // Create obligations for each predicate declared by the impl
-    // definition in the context of the trait's parameter
-    // environment. We can't just use `impl_env.caller_bounds`,
-    // however, because we want to replace all late-bound regions with
-    // region variables.
-    let impl_predicates = tcx.predicates_of(impl_m_predicates.parent.unwrap());
-    let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
-
-    debug!("compare_impl_method: impl_bounds={:?}", hybrid_preds);
-
     // This is the only tricky bit of the new way we check implementation methods
     // We need to build a set of predicates where only the method-level bounds
     // are from the trait and we assume all other bounds from the implementation
     // to be previously satisfied.
     //
     // We then register the obligations from the impl_m and check to see
     // if all constraints hold.
-    hybrid_preds.predicates.extend(
+    let impl_predicates = tcx.predicates_of(impl_m_predicates.parent.unwrap());
+    let mut hybrid_preds = impl_predicates.instantiate_identity(tcx).predicates;
+    hybrid_preds.extend(
         trait_m_predicates
             .instantiate_own(tcx, trait_to_placeholder_args)
             .map(|(predicate, _)| predicate),
     );
 
+    // FIXME(effects): This should be replaced with a more dedicated method.
+    let check_const_if_const = tcx.constness(impl_def_id) == hir::Constness::Const;
+    if check_const_if_const {
+        // Augment the hybrid param-env with the const conditions.
+        hybrid_preds.extend(
+            tcx.const_conditions(impl_def_id)
+                .instantiate_identity(tcx)
+                .into_iter()
+                .chain(
+                    tcx.const_conditions(trait_m.def_id)
+                        .instantiate_own(tcx, trait_to_placeholder_args),
+                )
+                .map(|(trait_ref, _)| trait_ref.to_host_effect(tcx, ty::HostPolarity::Maybe)),
+        );
+    }
+
     // Construct trait parameter environment and then shift it into the placeholder viewpoint.
     // The key step here is to update the caller_bounds's predicates to be
     // the new hybrid bounds we computed.
     let normalize_cause = traits::ObligationCause::misc(impl_m_span, impl_m_def_id);
-    let param_env = ty::ParamEnv::new(tcx.mk_clauses(&hybrid_preds.predicates), Reveal::UserFacing);
+    let param_env = ty::ParamEnv::new(tcx.mk_clauses(&hybrid_preds), Reveal::UserFacing);
     let param_env = traits::normalize_param_env_or_error(tcx, param_env, normalize_cause);
 
     let infcx = &tcx.infer_ctxt().build();
     let ocx = ObligationCtxt::new_with_diagnostics(infcx);
 
     debug!("compare_impl_method: caller_bounds={:?}", param_env.caller_bounds());
 
+    // Create obligations for each predicate declared by the impl
+    // definition in the context of the hybrid param-env. This makes
+    // sure that the impl's method's where clauses are not more
+    // restrictive than the trait's method (and the impl itself).
     let impl_m_own_bounds = impl_m_predicates.instantiate_own(tcx, impl_to_placeholder_args);
     for (predicate, span) in impl_m_own_bounds {
         let normalize_cause = traits::ObligationCause::misc(span, impl_m_def_id);
@@ -243,6 +256,34 @@ fn compare_method_predicate_entailment<'tcx>(
         ocx.register_obligation(traits::Obligation::new(tcx, cause, param_env, predicate));
     }
 
+    // If we're within a const implementation, we need to make sure that the method
+    // does not assume stronger `~const` bounds than the trait definition.
+    //
+    // This registers the `~const` bounds of the impl method, which we will prove
+    // using the hybrid param-env that we earlier augmented with the const conditions
+    // from the impl header and trait method declaration.
+    if check_const_if_const {
+        for (const_condition, span) in
+            tcx.const_conditions(impl_m.def_id).instantiate_own(tcx, impl_to_placeholder_args)
+        {
+            let normalize_cause = traits::ObligationCause::misc(span, impl_m_def_id);
+            let const_condition = ocx.normalize(&normalize_cause, param_env, const_condition);
+
+            let cause =
+                ObligationCause::new(span, impl_m_def_id, ObligationCauseCode::CompareImplItem {
+                    impl_item_def_id: impl_m_def_id,
+                    trait_item_def_id: trait_m.def_id,
+                    kind: impl_m.kind,
+                });
+            ocx.register_obligation(traits::Obligation::new(
+                tcx,
+                cause,
+                param_env,
+                const_condition.to_host_effect(tcx, ty::HostPolarity::Maybe),
+            ));
+        }
+    }
+
     // We now need to check that the signature of the impl method is
     // compatible with that of the trait method. We do this by
     // checking that `impl_fty <: trait_fty`.
@@ -1759,14 +1800,14 @@ fn compare_const_predicate_entailment<'tcx>(
     // The predicates declared by the impl definition, the trait and the
     // associated const in the trait are assumed.
     let impl_predicates = tcx.predicates_of(impl_ct_predicates.parent.unwrap());
-    let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
-    hybrid_preds.predicates.extend(
+    let mut hybrid_preds = impl_predicates.instantiate_identity(tcx).predicates;
+    hybrid_preds.extend(
         trait_ct_predicates
             .instantiate_own(tcx, trait_to_impl_args)
             .map(|(predicate, _)| predicate),
     );
 
-    let param_env = ty::ParamEnv::new(tcx.mk_clauses(&hybrid_preds.predicates), Reveal::UserFacing);
+    let param_env = ty::ParamEnv::new(tcx.mk_clauses(&hybrid_preds), Reveal::UserFacing);
     let param_env = traits::normalize_param_env_or_error(
         tcx,
         param_env,
@@ -1871,14 +1912,16 @@ fn compare_type_predicate_entailment<'tcx>(
     impl_trait_ref: ty::TraitRef<'tcx>,
 ) -> Result<(), ErrorGuaranteed> {
     let impl_args = GenericArgs::identity_for_item(tcx, impl_ty.def_id);
-    let trait_to_impl_args =
-        impl_args.rebase_onto(tcx, impl_ty.container_id(tcx), impl_trait_ref.args);
+    let impl_def_id = impl_ty.container_id(tcx);
+    let trait_to_impl_args = impl_args.rebase_onto(tcx, impl_def_id, impl_trait_ref.args);
 
     let impl_ty_predicates = tcx.predicates_of(impl_ty.def_id);
     let trait_ty_predicates = tcx.predicates_of(trait_ty.def_id);
 
     let impl_ty_own_bounds = impl_ty_predicates.instantiate_own(tcx, impl_args);
-    if impl_ty_own_bounds.len() == 0 {
+    let impl_ty_own_const_conditions =
+        tcx.const_conditions(impl_ty.def_id).instantiate_own(tcx, impl_args);
+    if impl_ty_own_bounds.len() == 0 && impl_ty_own_const_conditions.len() == 0 {
         // Nothing to check.
         return Ok(());
     }
@@ -1892,18 +1935,31 @@ fn compare_type_predicate_entailment<'tcx>(
     // The predicates declared by the impl definition, the trait and the
     // associated type in the trait are assumed.
     let impl_predicates = tcx.predicates_of(impl_ty_predicates.parent.unwrap());
-    let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
-    hybrid_preds.predicates.extend(
+    let mut hybrid_preds = impl_predicates.instantiate_identity(tcx).predicates;
+    hybrid_preds.extend(
         trait_ty_predicates
             .instantiate_own(tcx, trait_to_impl_args)
             .map(|(predicate, _)| predicate),
     );
 
+    let check_const_if_const = tcx.constness(impl_def_id) == hir::Constness::Const;
+    if check_const_if_const {
+        hybrid_preds.extend(
+            tcx.const_conditions(impl_ty_predicates.parent.unwrap())
+                .instantiate_identity(tcx)
+                .into_iter()
+                .chain(
+                    tcx.const_conditions(trait_ty.def_id).instantiate_own(tcx, trait_to_impl_args),
+                )
+                .map(|(trait_ref, _)| trait_ref.to_host_effect(tcx, ty::HostPolarity::Maybe)),
+        );
+    }
+
     debug!("compare_type_predicate_entailment: bounds={:?}", hybrid_preds);
 
     let impl_ty_span = tcx.def_span(impl_ty_def_id);
     let normalize_cause = ObligationCause::misc(impl_ty_span, impl_ty_def_id);
-    let param_env = ty::ParamEnv::new(tcx.mk_clauses(&hybrid_preds.predicates), Reveal::UserFacing);
+    let param_env = ty::ParamEnv::new(tcx.mk_clauses(&hybrid_preds), Reveal::UserFacing);
     let param_env = traits::normalize_param_env_or_error(tcx, param_env, normalize_cause);
     let infcx = tcx.infer_ctxt().build();
     let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
@@ -1923,6 +1979,27 @@ fn compare_type_predicate_entailment<'tcx>(
         ocx.register_obligation(traits::Obligation::new(tcx, cause, param_env, predicate));
     }
 
+    if check_const_if_const {
+        // Validate the const conditions of the impl method.
+        for (const_condition, span) in impl_ty_own_const_conditions {
+            let normalize_cause = traits::ObligationCause::misc(span, impl_ty_def_id);
+            let const_condition = ocx.normalize(&normalize_cause, param_env, const_condition);
+
+            let cause =
+                ObligationCause::new(span, impl_ty_def_id, ObligationCauseCode::CompareImplItem {
+                    impl_item_def_id: impl_ty_def_id,
+                    trait_item_def_id: trait_ty.def_id,
+                    kind: impl_ty.kind,
+                });
+            ocx.register_obligation(traits::Obligation::new(
+                tcx,
+                cause,
+                param_env,
+                const_condition.to_host_effect(tcx, ty::HostPolarity::Maybe),
+            ));
+        }
+    }
+
     // Check that all obligations are satisfied by the implementation's
     // version.
     let errors = ocx.select_all_or_error();
@@ -2005,14 +2082,30 @@ pub(super) fn check_type_bounds<'tcx>(
         ObligationCause::new(impl_ty_span, impl_ty_def_id, code)
     };
 
-    let obligations: Vec<_> = tcx
+    let mut obligations: Vec<_> = tcx
         .explicit_item_bounds(trait_ty.def_id)
         .iter_instantiated_copied(tcx, rebased_args)
         .map(|(concrete_ty_bound, span)| {
             debug!("check_type_bounds: concrete_ty_bound = {:?}", 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.constness(container_id) == hir::Constness::Const {
+        obligations.extend(
+            tcx.implied_const_bounds(trait_ty.def_id)
+                .iter_instantiated_copied(tcx, rebased_args)
+                .map(|(c, span)| {
+                    traits::Obligation::new(
+                        tcx,
+                        mk_cause(span),
+                        param_env,
+                        c.to_host_effect(tcx, ty::HostPolarity::Maybe),
+                    )
+                }),
+        );
+    }
     debug!("check_type_bounds: item_bounds={:?}", obligations);
 
     // Normalize predicates with the assumption that the GAT may always normalize

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -32,7 +32,8 @@ use rustc_trait_selection::traits::misc::{
 use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt as _;
 use rustc_trait_selection::traits::{
-    self, FulfillmentError, ObligationCause, ObligationCauseCode, ObligationCtxt, WellFormedLoc,
+    self, FulfillmentError, Obligation, ObligationCause, ObligationCauseCode, ObligationCtxt,
+    WellFormedLoc,
 };
 use rustc_type_ir::TypeFlags;
 use rustc_type_ir::solve::NoSolution;
@@ -86,7 +87,7 @@ impl<'tcx> WfCheckingCtxt<'_, 'tcx> {
             self.body_def_id,
             ObligationCauseCode::WellFormed(loc),
         );
-        self.ocx.register_obligation(traits::Obligation::new(
+        self.ocx.register_obligation(Obligation::new(
             self.tcx(),
             cause,
             self.param_env,
@@ -1173,7 +1174,7 @@ fn check_type_defn<'tcx>(
                     wfcx.body_def_id,
                     ObligationCauseCode::Misc,
                 );
-                wfcx.register_obligation(traits::Obligation::new(
+                wfcx.register_obligation(Obligation::new(
                     tcx,
                     cause,
                     wfcx.param_env,
@@ -1369,6 +1370,30 @@ fn check_impl<'tcx>(
                         obligation.cause.span = hir_self_ty.span;
                     }
                 }
+
+                // Ensure that the `~const` where clauses of the trait hold for the impl.
+                if tcx.constness(item.owner_id.def_id) == hir::Constness::Const {
+                    for (bound, _) in
+                        tcx.const_conditions(trait_ref.def_id).instantiate(tcx, trait_ref.args)
+                    {
+                        let bound = wfcx.normalize(
+                            item.span,
+                            Some(WellFormedLoc::Ty(item.hir_id().expect_owner().def_id)),
+                            bound,
+                        );
+                        wfcx.register_obligation(Obligation::new(
+                            tcx,
+                            ObligationCause::new(
+                                hir_self_ty.span,
+                                wfcx.body_def_id,
+                                ObligationCauseCode::WellFormed(None),
+                            ),
+                            wfcx.param_env,
+                            bound.to_host_effect(tcx, ty::HostPolarity::Maybe),
+                        ))
+                    }
+                }
+
                 debug!(?obligations);
                 wfcx.register_obligations(obligations);
             }
@@ -1561,7 +1586,7 @@ fn check_where_clauses<'tcx>(wfcx: &WfCheckingCtxt<'_, 'tcx>, span: Span, def_id
                 wfcx.body_def_id,
                 ObligationCauseCode::WhereClause(def_id.to_def_id(), DUMMY_SP),
             );
-            traits::Obligation::new(tcx, cause, wfcx.param_env, pred)
+            Obligation::new(tcx, cause, wfcx.param_env, pred)
         });
 
     let predicates = predicates.instantiate_identity(tcx);
@@ -1852,7 +1877,7 @@ fn receiver_is_implemented<'tcx>(
     let tcx = wfcx.tcx();
     let trait_ref = ty::TraitRef::new(tcx, receiver_trait_def_id, [receiver_ty]);
 
-    let obligation = traits::Obligation::new(tcx, cause, wfcx.param_env, trait_ref);
+    let obligation = Obligation::new(tcx, cause, wfcx.param_env, trait_ref);
 
     if wfcx.infcx.predicate_must_hold_modulo_regions(&obligation) {
         true
@@ -2188,7 +2213,7 @@ impl<'tcx> WfCheckingCtxt<'_, 'tcx> {
                         .unwrap_or(obligation_span);
                 }
 
-                let obligation = traits::Obligation::new(
+                let obligation = Obligation::new(
                     tcx,
                     traits::ObligationCause::new(
                         span,

compiler/rustc_hir_analysis/src/collect.rs

@@ -77,6 +77,8 @@ pub fn provide(providers: &mut Providers) {
         explicit_supertraits_containing_assoc_item:
             predicates_of::explicit_supertraits_containing_assoc_item,
         trait_explicit_predicates_and_bounds: predicates_of::trait_explicit_predicates_and_bounds,
+        const_conditions: predicates_of::const_conditions,
+        implied_const_bounds: predicates_of::implied_const_bounds,
         type_param_predicates: predicates_of::type_param_predicates,
         trait_def,
         adt_def,