Rollup merge of #68140 - ecstatic-morse:const-trait-bound-opt-out, r=oli-obk

Implement `?const` opt-out for trait bounds

For now, such bounds are treated exactly the same as unprefixed ones in all contexts. [RFC 2632](https://github.com/rust-lang/rfcs/pull/2632) does not specify whether such bounds are forbidden outside of `const` contexts, so they are allowed at the moment.

Prior to this PR, the constness of a trait bound/impl was stored in `TraitRef`. Now, the constness of an `impl` is stored in `ast::ItemKind::Impl` and the constness of a bound in `ast::TraitBoundModifer`. Additionally, constness of trait bounds is now stored in an additional field of `ty::Predicate::Trait`, and the combination of the constness of the item along with any `TraitBoundModifier` determines the constness of the bound in accordance with the RFC. Encoding the constness of impls at the `ty` level is left for a later PR.

After a discussion in \#wg-grammar on Discord, it was decided that the grammar should not encode the mutual exclusivity of trait bound modifiers. The grammar for trait bound modifiers remains `[?const] [?]`. To encode this, I add a dummy variant to `ast::TraitBoundModifier` that is used when the syntax `?const ?` appears. This variant causes an error in AST validation and disappears during HIR lowering.

cc #67794

r? @oli-obk

17 files changed, 179 insertions, 76 deletions

diff --git a/src/librustc/traits/auto_trait.rs b/src/librustc/traits/auto_trait.rs
index 89b28aeda1c..c97c5c2077f 100644
--- a/src/librustc/traits/auto_trait.rs
+++ b/src/librustc/traits/auto_trait.rs
@@ -337,7 +337,10 @@ impl AutoTraitFinder<'tcx> {
                 &Err(SelectionError::Unimplemented) => {
                     if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
                         already_visited.remove(&pred);
-                        self.add_user_pred(&mut user_computed_preds, ty::Predicate::Trait(pred));
+                        self.add_user_pred(
+                            &mut user_computed_preds,
+                            ty::Predicate::Trait(pred, ast::Constness::NotConst),
+                        );
                         predicates.push_back(pred);
                     } else {
                         debug!(
@@ -405,7 +408,7 @@ impl AutoTraitFinder<'tcx> {
         let mut should_add_new = true;
         user_computed_preds.retain(|&old_pred| {
             match (&new_pred, old_pred) {
-                (&ty::Predicate::Trait(new_trait), ty::Predicate::Trait(old_trait)) => {
+                (&ty::Predicate::Trait(new_trait, _), ty::Predicate::Trait(old_trait, _)) => {
                     if new_trait.def_id() == old_trait.def_id() {
                         let new_substs = new_trait.skip_binder().trait_ref.substs;
                         let old_substs = old_trait.skip_binder().trait_ref.substs;
@@ -627,7 +630,7 @@ impl AutoTraitFinder<'tcx> {
             // We check this by calling is_of_param on the relevant types
             // from the various possible predicates
             match &predicate {
-                &ty::Predicate::Trait(p) => {
+                &ty::Predicate::Trait(p, _) => {
                     if self.is_param_no_infer(p.skip_binder().trait_ref.substs)
                         && !only_projections
                         && is_new_pred
diff --git a/src/librustc/traits/engine.rs b/src/librustc/traits/engine.rs
index 5b804480119..84bfc86e6a9 100644
--- a/src/librustc/traits/engine.rs
+++ b/src/librustc/traits/engine.rs
@@ -1,6 +1,6 @@
 use crate::infer::InferCtxt;
 use crate::traits::Obligation;
-use crate::ty::{self, ToPredicate, Ty, TyCtxt};
+use crate::ty::{self, ToPredicate, Ty, TyCtxt, WithConstness};
 use rustc_hir::def_id::DefId;
 
 use super::{ChalkFulfillmentContext, FulfillmentContext, FulfillmentError};
@@ -33,7 +33,7 @@ pub trait TraitEngine<'tcx>: 'tcx {
                 cause,
                 recursion_depth: 0,
                 param_env,
-                predicate: trait_ref.to_predicate(),
+                predicate: trait_ref.without_const().to_predicate(),
             },
         );
     }
diff --git a/src/librustc/traits/error_reporting/mod.rs b/src/librustc/traits/error_reporting/mod.rs
index 646cb80bffd..d1c369d0abb 100644
--- a/src/librustc/traits/error_reporting/mod.rs
+++ b/src/librustc/traits/error_reporting/mod.rs
@@ -19,7 +19,9 @@ use crate::ty::error::ExpectedFound;
 use crate::ty::fast_reject;
 use crate::ty::fold::TypeFolder;
 use crate::ty::SubtypePredicate;
-use crate::ty::{self, AdtKind, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable};
+use crate::ty::{
+    self, AdtKind, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
+};
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder};
@@ -128,7 +130,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
 
         let (cond, error) = match (cond, error) {
-            (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error)) => (cond, error),
+            (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error, _)) => (cond, error),
             _ => {
                 // FIXME: make this work in other cases too.
                 return false;
@@ -136,7 +138,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         };
 
         for implication in super::elaborate_predicates(self.tcx, vec![cond.clone()]) {
-            if let ty::Predicate::Trait(implication) = implication {
+            if let ty::Predicate::Trait(implication, _) = implication {
                 let error = error.to_poly_trait_ref();
                 let implication = implication.to_poly_trait_ref();
                 // FIXME: I'm just not taking associated types at all here.
@@ -528,7 +530,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     return;
                 }
                 match obligation.predicate {
-                    ty::Predicate::Trait(ref trait_predicate) => {
+                    ty::Predicate::Trait(ref trait_predicate, _) => {
                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 
                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
@@ -581,7 +583,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             "{}",
                             message.unwrap_or_else(|| format!(
                                 "the trait bound `{}` is not satisfied{}",
-                                trait_ref.to_predicate(),
+                                trait_ref.without_const().to_predicate(),
                                 post_message,
                             ))
                         );
@@ -693,7 +695,10 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                 trait_pred
                             });
                             let unit_obligation = Obligation {
-                                predicate: ty::Predicate::Trait(predicate),
+                                predicate: ty::Predicate::Trait(
+                                    predicate,
+                                    ast::Constness::NotConst,
+                                ),
                                 ..obligation.clone()
                             };
                             if self.predicate_may_hold(&unit_obligation) {
@@ -986,7 +991,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     ) -> PredicateObligation<'tcx> {
         let new_trait_ref =
             ty::TraitRef { def_id, substs: self.tcx.mk_substs_trait(output_ty, &[]) };
-        Obligation::new(cause, param_env, new_trait_ref.to_predicate())
+        Obligation::new(cause, param_env, new_trait_ref.without_const().to_predicate())
     }
 }
 
@@ -1074,7 +1079,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
 
         let mut err = match predicate {
-            ty::Predicate::Trait(ref data) => {
+            ty::Predicate::Trait(ref data, _) => {
                 let trait_ref = data.to_poly_trait_ref();
                 let self_ty = trait_ref.self_ty();
                 debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind, trait_ref);
@@ -1267,8 +1272,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             )
             .value;
 
-            let obligation =
-                Obligation::new(ObligationCause::dummy(), param_env, cleaned_pred.to_predicate());
+            let obligation = Obligation::new(
+                ObligationCause::dummy(),
+                param_env,
+                cleaned_pred.without_const().to_predicate(),
+            );
 
             self.predicate_may_hold(&obligation)
         })
diff --git a/src/librustc/traits/error_reporting/suggestions.rs b/src/librustc/traits/error_reporting/suggestions.rs
index c18e17581ce..4559007ea42 100644
--- a/src/librustc/traits/error_reporting/suggestions.rs
+++ b/src/librustc/traits/error_reporting/suggestions.rs
@@ -6,7 +6,7 @@ use super::{
 use crate::infer::InferCtxt;
 use crate::traits::object_safety::object_safety_violations;
 use crate::ty::TypeckTables;
-use crate::ty::{self, AdtKind, DefIdTree, ToPredicate, Ty, TyCtxt, TypeFoldable};
+use crate::ty::{self, AdtKind, DefIdTree, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
 
 use rustc_errors::{
     error_code, pluralize, struct_span_err, Applicability, DiagnosticBuilder, Style,
@@ -48,7 +48,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             } else {
                                 " where"
                             },
-                            trait_ref.to_predicate(),
+                            trait_ref.without_const().to_predicate(),
                         ),
                         Applicability::MachineApplicable,
                     );
@@ -338,8 +338,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             let new_self_ty = self.tcx.mk_imm_ref(self.tcx.lifetimes.re_static, self_ty);
             let substs = self.tcx.mk_substs_trait(new_self_ty, &[]);
             let new_trait_ref = ty::TraitRef::new(obligation.parent_trait_ref.def_id(), substs);
-            let new_obligation =
-                Obligation::new(ObligationCause::dummy(), param_env, new_trait_ref.to_predicate());
+            let new_obligation = Obligation::new(
+                ObligationCause::dummy(),
+                param_env,
+                new_trait_ref.without_const().to_predicate(),
+            );
             if self.predicate_must_hold_modulo_regions(&new_obligation) {
                 if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
                     // We have a very specific type of error, where just borrowing this argument
@@ -1120,7 +1123,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         // the type. The last generator has information about where the bound was introduced. At
         // least one generator should be present for this diagnostic to be modified.
         let (mut trait_ref, mut target_ty) = match obligation.predicate {
-            ty::Predicate::Trait(p) => {
+            ty::Predicate::Trait(p, _) => {
                 (Some(p.skip_binder().trait_ref), Some(p.skip_binder().self_ty()))
             }
             _ => (None, None),
@@ -1543,7 +1546,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 err.note(&format!("required because it appears within the type `{}`", ty));
                 obligated_types.push(ty);
 
-                let parent_predicate = parent_trait_ref.to_predicate();
+                let parent_predicate = parent_trait_ref.without_const().to_predicate();
                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
                     self.note_obligation_cause_code(
                         err,
@@ -1560,7 +1563,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     parent_trait_ref.print_only_trait_path(),
                     parent_trait_ref.skip_binder().self_ty()
                 ));
-                let parent_predicate = parent_trait_ref.to_predicate();
+                let parent_predicate = parent_trait_ref.without_const().to_predicate();
                 self.note_obligation_cause_code(
                     err,
                     &parent_predicate,
diff --git a/src/librustc/traits/fulfill.rs b/src/librustc/traits/fulfill.rs
index 46ece6fc405..0aac6fb81e4 100644
--- a/src/librustc/traits/fulfill.rs
+++ b/src/librustc/traits/fulfill.rs
@@ -311,7 +311,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
         }
 
         match obligation.predicate {
-            ty::Predicate::Trait(ref data) => {
+            ty::Predicate::Trait(ref data, _) => {
                 let trait_obligation = obligation.with(data.clone());
 
                 if data.is_global() {
diff --git a/src/librustc/traits/mod.rs b/src/librustc/traits/mod.rs
index e93529186ac..daaba95cf6b 100644
--- a/src/librustc/traits/mod.rs
+++ b/src/librustc/traits/mod.rs
@@ -29,7 +29,7 @@ use crate::mir::interpret::ErrorHandled;
 use crate::ty::error::{ExpectedFound, TypeError};
 use crate::ty::fold::{TypeFoldable, TypeFolder, TypeVisitor};
 use crate::ty::subst::{InternalSubsts, SubstsRef};
-use crate::ty::{self, AdtKind, GenericParamDefKind, List, ToPredicate, Ty, TyCtxt};
+use crate::ty::{self, AdtKind, GenericParamDefKind, List, ToPredicate, Ty, TyCtxt, WithConstness};
 use crate::util::common::ErrorReported;
 use chalk_engine;
 use rustc_hir as hir;
@@ -732,7 +732,7 @@ pub fn type_known_to_meet_bound_modulo_regions<'a, 'tcx>(
         param_env,
         cause: ObligationCause::misc(span, hir::DUMMY_HIR_ID),
         recursion_depth: 0,
-        predicate: trait_ref.to_predicate(),
+        predicate: trait_ref.without_const().to_predicate(),
     };
 
     let result = infcx.predicate_must_hold_modulo_regions(&obligation);
diff --git a/src/librustc/traits/object_safety.rs b/src/librustc/traits/object_safety.rs
index ce57fb81104..15f81bb3f47 100644
--- a/src/librustc/traits/object_safety.rs
+++ b/src/librustc/traits/object_safety.rs
@@ -12,7 +12,7 @@ use super::elaborate_predicates;
 
 use crate::traits::{self, Obligation, ObligationCause};
 use crate::ty::subst::{InternalSubsts, Subst};
-use crate::ty::{self, Predicate, ToPredicate, Ty, TyCtxt, TypeFoldable};
+use crate::ty::{self, Predicate, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
@@ -234,7 +234,7 @@ fn predicates_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId, supertraits_o
         .map(|(predicate, _)| predicate.subst_supertrait(tcx, &trait_ref))
         .any(|predicate| {
             match predicate {
-                ty::Predicate::Trait(ref data) => {
+                ty::Predicate::Trait(ref data, _) => {
                     // In the case of a trait predicate, we can skip the "self" type.
                     data.skip_binder().input_types().skip(1).any(has_self_ty)
                 }
@@ -285,7 +285,7 @@ fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
     let predicates = tcx.predicates_of(def_id);
     let predicates = predicates.instantiate_identity(tcx).predicates;
     elaborate_predicates(tcx, predicates).any(|predicate| match predicate {
-        ty::Predicate::Trait(ref trait_pred) => {
+        ty::Predicate::Trait(ref trait_pred, _) => {
             trait_pred.def_id() == sized_def_id && trait_pred.skip_binder().self_ty().is_param(0)
         }
         ty::Predicate::Projection(..)
@@ -585,6 +585,7 @@ fn receiver_is_dispatchable<'tcx>(
             def_id: unsize_did,
             substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
         }
+        .without_const()
         .to_predicate();
 
         // U: Trait<Arg1, ..., ArgN>
@@ -598,7 +599,7 @@ fn receiver_is_dispatchable<'tcx>(
                     }
                 });
 
-            ty::TraitRef { def_id: unsize_did, substs }.to_predicate()
+            ty::TraitRef { def_id: unsize_did, substs }.without_const().to_predicate()
         };
 
         let caller_bounds: Vec<Predicate<'tcx>> = param_env
@@ -620,6 +621,7 @@ fn receiver_is_dispatchable<'tcx>(
             def_id: dispatch_from_dyn_did,
             substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
         }
+        .without_const()
         .to_predicate();
 
         Obligation::new(ObligationCause::dummy(), param_env, predicate)
diff --git a/src/librustc/traits/project.rs b/src/librustc/traits/project.rs
index 5bc211ade40..62672a78104 100644
--- a/src/librustc/traits/project.rs
+++ b/src/librustc/traits/project.rs
@@ -16,7 +16,7 @@ use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::infer::{InferCtxt, InferOk, LateBoundRegionConversionTime};
 use crate::ty::fold::{TypeFoldable, TypeFolder};
 use crate::ty::subst::{InternalSubsts, Subst};
-use crate::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt};
+use crate::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, WithConstness};
 use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
 use rustc_hir::def_id::DefId;
 use rustc_macros::HashStable;
@@ -738,7 +738,12 @@ fn get_paranoid_cache_value_obligation<'a, 'tcx>(
     depth: usize,
 ) -> PredicateObligation<'tcx> {
     let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
-    Obligation { cause, recursion_depth: depth, param_env, predicate: trait_ref.to_predicate() }
+    Obligation {
+        cause,
+        recursion_depth: depth,
+        param_env,
+        predicate: trait_ref.without_const().to_predicate(),
+    }
 }
 
 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
@@ -772,7 +777,7 @@ fn normalize_to_error<'a, 'tcx>(
         cause,
         recursion_depth: depth,
         param_env,
-        predicate: trait_ref.to_predicate(),
+        predicate: trait_ref.without_const().to_predicate(),
     };
     let tcx = selcx.infcx().tcx;
     let def_id = projection_ty.item_def_id;
diff --git a/src/librustc/traits/query/type_op/prove_predicate.rs b/src/librustc/traits/query/type_op/prove_predicate.rs
index c0a0cbe9a38..15870ec95d8 100644
--- a/src/librustc/traits/query/type_op/prove_predicate.rs
+++ b/src/librustc/traits/query/type_op/prove_predicate.rs
@@ -24,7 +24,7 @@ impl<'tcx> super::QueryTypeOp<'tcx> for ProvePredicate<'tcx> {
         // `&T`, accounts for about 60% percentage of the predicates
         // we have to prove. No need to canonicalize and all that for
         // such cases.
-        if let Predicate::Trait(trait_ref) = key.value.predicate {
+        if let Predicate::Trait(trait_ref, _) = key.value.predicate {
             if let Some(sized_def_id) = tcx.lang_items().sized_trait() {
                 if trait_ref.def_id() == sized_def_id {
                     if trait_ref.skip_binder().self_ty().is_trivially_sized(tcx) {
diff --git a/src/librustc/traits/select.rs b/src/librustc/traits/select.rs
index 3bfe542baab..ac1ca4db9d6 100644
--- a/src/librustc/traits/select.rs
+++ b/src/librustc/traits/select.rs
@@ -37,7 +37,7 @@ use crate::middle::lang_items;
 use crate::ty::fast_reject;
 use crate::ty::relate::TypeRelation;
 use crate::ty::subst::{Subst, SubstsRef};
-use crate::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable};
+use crate::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
 use rustc_hir::def_id::DefId;
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
@@ -51,7 +51,7 @@ use std::cmp;
 use std::fmt::{self, Display};
 use std::iter;
 use std::rc::Rc;
-use syntax::attr;
+use syntax::{ast, attr};
 
 pub struct SelectionContext<'cx, 'tcx> {
     infcx: &'cx InferCtxt<'cx, 'tcx>,
@@ -718,7 +718,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
 
         match obligation.predicate {
-            ty::Predicate::Trait(ref t) => {
+            ty::Predicate::Trait(ref t, _) => {
                 debug_assert!(!t.has_escaping_bound_vars());
                 let obligation = obligation.with(t.clone());
                 self.evaluate_trait_predicate_recursively(previous_stack, obligation)
@@ -945,7 +945,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             // trait refs. This is important because it's only a cycle
             // if the regions match exactly.
             let cycle = stack.iter().skip(1).take_while(|s| s.depth >= cycle_depth);
-            let cycle = cycle.map(|stack| ty::Predicate::Trait(stack.obligation.predicate));
+            let cycle = cycle.map(|stack| {
+                ty::Predicate::Trait(stack.obligation.predicate, ast::Constness::NotConst)
+            });
             if self.coinductive_match(cycle) {
                 debug!("evaluate_stack({:?}) --> recursive, coinductive", stack.fresh_trait_ref);
                 Some(EvaluatedToOk)
@@ -1060,7 +1062,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
     fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
         let result = match predicate {
-            ty::Predicate::Trait(ref data) => self.tcx().trait_is_auto(data.def_id()),
+            ty::Predicate::Trait(ref data, _) => self.tcx().trait_is_auto(data.def_id()),
             _ => false,
         };
         debug!("coinductive_predicate({:?}) = {:?}", predicate, result);
@@ -3366,7 +3368,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
                     tcx.mk_substs_trait(source, &[]),
                 );
-                nested.push(predicate_to_obligation(tr.to_predicate()));
+                nested.push(predicate_to_obligation(tr.without_const().to_predicate()));
 
                 // If the type is `Foo + 'a`, ensure that the type
                 // being cast to `Foo + 'a` outlives `'a`:
diff --git a/src/librustc/traits/util.rs b/src/librustc/traits/util.rs
index f058a4d2df2..f3bd98b8551 100644
--- a/src/librustc/traits/util.rs
+++ b/src/librustc/traits/util.rs
@@ -4,7 +4,7 @@ use smallvec::SmallVec;
 
 use crate::ty::outlives::Component;
 use crate::ty::subst::{GenericArg, Subst, SubstsRef};
-use crate::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt};
+use crate::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, WithConstness};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
@@ -13,8 +13,8 @@ use super::{Normalized, Obligation, ObligationCause, PredicateObligation, Select
 
 fn anonymize_predicate<'tcx>(tcx: TyCtxt<'tcx>, pred: &ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
     match *pred {
-        ty::Predicate::Trait(ref data) => {
-            ty::Predicate::Trait(tcx.anonymize_late_bound_regions(data))
+        ty::Predicate::Trait(ref data, constness) => {
+            ty::Predicate::Trait(tcx.anonymize_late_bound_regions(data), constness)
         }
 
         ty::Predicate::RegionOutlives(ref data) => {
@@ -99,14 +99,14 @@ pub fn elaborate_trait_ref<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_ref: ty::PolyTraitRef<'tcx>,
 ) -> Elaborator<'tcx> {
-    elaborate_predicates(tcx, vec![trait_ref.to_predicate()])
+    elaborate_predicates(tcx, vec![trait_ref.without_const().to_predicate()])
 }
 
 pub fn elaborate_trait_refs<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_refs: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
 ) -> Elaborator<'tcx> {
-    let predicates = trait_refs.map(|trait_ref| trait_ref.to_predicate()).collect();
+    let predicates = trait_refs.map(|trait_ref| trait_ref.without_const().to_predicate()).collect();
     elaborate_predicates(tcx, predicates)
 }
 
@@ -127,7 +127,7 @@ impl Elaborator<'tcx> {
     fn elaborate(&mut self, predicate: &ty::Predicate<'tcx>) {
         let tcx = self.visited.tcx;
         match *predicate {
-            ty::Predicate::Trait(ref data) => {
+            ty::Predicate::Trait(ref data, _) => {
                 // Get predicates declared on the trait.
                 let predicates = tcx.super_predicates_of(data.def_id());
 
@@ -358,7 +358,7 @@ impl<'tcx> TraitAliasExpander<'tcx> {
     fn expand(&mut self, item: &TraitAliasExpansionInfo<'tcx>) -> bool {
         let tcx = self.tcx;
         let trait_ref = item.trait_ref();
-        let pred = trait_ref.to_predicate();
+        let pred = trait_ref.without_const().to_predicate();
 
         debug!("expand_trait_aliases: trait_ref={:?}", trait_ref);
 
@@ -370,13 +370,9 @@ impl<'tcx> TraitAliasExpander<'tcx> {
 
         // Don't recurse if this trait alias is already on the stack for the DFS search.
         let anon_pred = anonymize_predicate(tcx, &pred);
-        if item
-            .path
-            .iter()
-            .rev()
-            .skip(1)
-            .any(|(tr, _)| anonymize_predicate(tcx, &tr.to_predicate()) == anon_pred)
-        {
+        if item.path.iter().rev().skip(1).any(|(tr, _)| {
+            anonymize_predicate(tcx, &tr.without_const().to_predicate()) == anon_pred
+        }) {
             return false;
         }
 
@@ -471,7 +467,7 @@ impl<'tcx, I: Iterator<Item = ty::Predicate<'tcx>>> Iterator for FilterToTraits<
 
     fn next(&mut self) -> Option<ty::PolyTraitRef<'tcx>> {
         while let Some(pred) = self.base_iterator.next() {
-            if let ty::Predicate::Trait(data) = pred {
+            if let ty::Predicate::Trait(data, _) = pred {
                 return Some(data.to_poly_trait_ref());
             }
         }
@@ -545,7 +541,12 @@ pub fn predicate_for_trait_ref<'tcx>(
     trait_ref: ty::TraitRef<'tcx>,
     recursion_depth: usize,
 ) -> PredicateObligation<'tcx> {
-    Obligation { cause, param_env, recursion_depth, predicate: trait_ref.to_predicate() }
+    Obligation {
+        cause,
+        param_env,
+        recursion_depth,
+        predicate: trait_ref.without_const().to_predicate(),
+    }
 }
 
 pub fn predicate_for_trait_def(
diff --git a/src/librustc/traits/wf.rs b/src/librustc/traits/wf.rs
index 869ba5315c1..a0cb8446c92 100644
--- a/src/librustc/traits/wf.rs
+++ b/src/librustc/traits/wf.rs
@@ -3,7 +3,7 @@ use crate::infer::InferCtxt;
 use crate::middle::lang_items;
 use crate::traits::{self, AssocTypeBoundData};
 use crate::ty::subst::SubstsRef;
-use crate::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
+use crate::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_span::symbol::{kw, Ident};
@@ -62,7 +62,7 @@ pub fn predicate_obligations<'a, 'tcx>(
 
     // (*) ok to skip binders, because wf code is prepared for it
     match *predicate {
-        ty::Predicate::Trait(ref t) => {
+        ty::Predicate::Trait(ref t, _) => {
             wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*)
         }
         ty::Predicate::RegionOutlives(..) => {}
@@ -245,7 +245,7 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
                             }
                         }
                     }
-                    ty::Predicate::Trait(proj) => {
+                    ty::Predicate::Trait(proj, _) => {
                         // An associated item obligation born out of the `trait` failed to be met.
                         // Point at the `impl` that failed the obligation, the associated item that
                         // needed to meet the obligation, and the definition of that associated item,
@@ -350,7 +350,7 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
         self.compute_trait_ref(&trait_ref, Elaborate::None);
 
         if !data.has_escaping_bound_vars() {
-            let predicate = trait_ref.to_predicate();
+            let predicate = trait_ref.without_const().to_predicate();
             let cause = self.cause(traits::ProjectionWf(data));
             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
         }
@@ -378,7 +378,11 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
             };
-            self.out.push(traits::Obligation::new(cause, self.param_env, trait_ref.to_predicate()));
+            self.out.push(traits::Obligation::new(
+                cause,
+                self.param_env,
+                trait_ref.without_const().to_predicate(),
+            ));
         }
     }
 
diff --git a/src/librustc/ty/fold.rs b/src/librustc/ty/fold.rs
index b16db6ae5b1..0dddca98c62 100644
--- a/src/librustc/ty/fold.rs
+++ b/src/librustc/ty/fold.rs
@@ -150,6 +150,15 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
     }
 }
 
+impl TypeFoldable<'tcx> for syntax::ast::Constness {
+    fn super_fold_with<F: TypeFolder<'tcx>>(&self, _: &mut F) -> Self {
+        *self
+    }
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> bool {
+        false
+    }
+}
+
 /// The `TypeFolder` trait defines the actual *folding*. There is a
 /// method defined for every foldable type. Each of these has a
 /// default implementation that does an "identity" fold. Within each
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 3040ecf90ed..0470ab20dc4 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -52,7 +52,7 @@ use std::ops::Deref;
 use std::ops::Range;
 use std::slice;
 use std::{mem, ptr};
-use syntax::ast::{self, Ident, Name, NodeId};
+use syntax::ast::{self, Constness, Ident, Name, NodeId};
 use syntax::attr;
 
 pub use self::sty::BoundRegion::*;
@@ -1068,7 +1068,11 @@ pub enum Predicate<'tcx> {
     /// Corresponds to `where Foo: Bar<A, B, C>`. `Foo` here would be
     /// the `Self` type of the trait reference and `A`, `B`, and `C`
     /// would be the type parameters.
-    Trait(PolyTraitPredicate<'tcx>),
+    ///
+    /// A trait predicate will have `Constness::Const` if it originates
+    /// from a bound on a `const fn` without the `?const` opt-out (e.g.,
+    /// `const fn foobar<Foo: Bar>() {}`).
+    Trait(PolyTraitPredicate<'tcx>, Constness),
 
     /// `where 'a: 'b`
     RegionOutlives(PolyRegionOutlivesPredicate<'tcx>),
@@ -1191,8 +1195,8 @@ impl<'tcx> Predicate<'tcx> {
 
         let substs = &trait_ref.skip_binder().substs;
         match *self {
-            Predicate::Trait(ref binder) => {
-                Predicate::Trait(binder.map_bound(|data| data.subst(tcx, substs)))
+            Predicate::Trait(ref binder, constness) => {
+                Predicate::Trait(binder.map_bound(|data| data.subst(tcx, substs)), constness)
             }
             Predicate::Subtype(ref binder) => {
                 Predicate::Subtype(binder.map_bound(|data| data.subst(tcx, substs)))
@@ -1336,15 +1340,33 @@ pub trait ToPredicate<'tcx> {
     fn to_predicate(&self) -> Predicate<'tcx>;
 }
 
-impl<'tcx> ToPredicate<'tcx> for TraitRef<'tcx> {
+impl<'tcx> ToPredicate<'tcx> for ConstnessAnd<TraitRef<'tcx>> {
+    fn to_predicate(&self) -> Predicate<'tcx> {
+        ty::Predicate::Trait(
+            ty::Binder::dummy(ty::TraitPredicate { trait_ref: self.value.clone() }),
+            self.constness,
+        )
+    }
+}
+
+impl<'tcx> ToPredicate<'tcx> for ConstnessAnd<&TraitRef<'tcx>> {
+    fn to_predicate(&self) -> Predicate<'tcx> {
+        ty::Predicate::Trait(
+            ty::Binder::dummy(ty::TraitPredicate { trait_ref: self.value.clone() }),
+            self.constness,
+        )
+    }
+}
+
+impl<'tcx> ToPredicate<'tcx> for ConstnessAnd<PolyTraitRef<'tcx>> {
     fn to_predicate(&self) -> Predicate<'tcx> {
-        ty::Predicate::Trait(ty::Binder::dummy(ty::TraitPredicate { trait_ref: self.clone() }))
+        ty::Predicate::Trait(self.value.to_poly_trait_predicate(), self.constness)
     }
 }
 
-impl<'tcx> ToPredicate<'tcx> for PolyTraitRef<'tcx> {
+impl<'tcx> ToPredicate<'tcx> for ConstnessAnd<&PolyTraitRef<'tcx>> {
     fn to_predicate(&self) -> Predicate<'tcx> {
-        ty::Predicate::Trait(self.to_poly_trait_predicate())
+        ty::Predicate::Trait(self.value.to_poly_trait_predicate(), self.constness)
     }
 }
 
@@ -1413,7 +1435,7 @@ impl<'tcx> Predicate<'tcx> {
     /// with depth 0 are bound by the predicate.
     pub fn walk_tys(&'a self) -> impl Iterator<Item = Ty<'tcx>> + 'a {
         match *self {
-            ty::Predicate::Trait(ref data) => {
+            ty::Predicate::Trait(ref data, _) => {
                 WalkTysIter::InputTypes(data.skip_binder().input_types())
             }
             ty::Predicate::Subtype(binder) => {
@@ -1439,7 +1461,7 @@ impl<'tcx> Predicate<'tcx> {
 
     pub fn to_opt_poly_trait_ref(&self) -> Option<PolyTraitRef<'tcx>> {
         match *self {
-            Predicate::Trait(ref t) => Some(t.to_poly_trait_ref()),
+            Predicate::Trait(ref t, _) => Some(t.to_poly_trait_ref()),
             Predicate::Projection(..)
             | Predicate::Subtype(..)
             | Predicate::RegionOutlives(..)
@@ -1700,6 +1722,33 @@ impl<'tcx> ParamEnv<'tcx> {
     }
 }
 
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct ConstnessAnd<T> {
+    pub constness: Constness,
+    pub value: T,
+}
+
+// FIXME(ecstaticmorse): Audit all occurrences of `without_const().to_predicate()` to ensure that
+// the constness of trait bounds is being propagated correctly.
+pub trait WithConstness: Sized {
+    #[inline]
+    fn with_constness(self, constness: Constness) -> ConstnessAnd<Self> {
+        ConstnessAnd { constness, value: self }
+    }
+
+    #[inline]
+    fn with_const(self) -> ConstnessAnd<Self> {
+        self.with_constness(Constness::Const)
+    }
+
+    #[inline]
+    fn without_const(self) -> ConstnessAnd<Self> {
+        self.with_constness(Constness::NotConst)
+    }
+}
+
+impl<T> WithConstness for T {}
+
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TypeFoldable)]
 pub struct ParamEnvAnd<'tcx, T> {
     pub param_env: ParamEnv<'tcx>,
diff --git a/src/librustc/ty/print/pretty.rs b/src/librustc/ty/print/pretty.rs
index 8b1b2bb5865..9091de55b7d 100644
--- a/src/librustc/ty/print/pretty.rs
+++ b/src/librustc/ty/print/pretty.rs
@@ -1791,7 +1791,12 @@ define_print_and_forward_display! {
 
     ty::Predicate<'tcx> {
         match *self {
-            ty::Predicate::Trait(ref data) => p!(print(data)),
+            ty::Predicate::Trait(ref data, constness) => {
+                if let ast::Constness::Const = constness {
+                    p!(write("const "));
+                }
+                p!(print(data))
+            }
             ty::Predicate::Subtype(ref predicate) => p!(print(predicate)),
             ty::Predicate::RegionOutlives(ref predicate) => p!(print(predicate)),
             ty::Predicate::TypeOutlives(ref predicate) => p!(print(predicate)),
diff --git a/src/librustc/ty/structural_impls.rs b/src/librustc/ty/structural_impls.rs
index 62e895af7f3..25f9dc5b0c7 100644
--- a/src/librustc/ty/structural_impls.rs
+++ b/src/librustc/ty/structural_impls.rs
@@ -15,6 +15,7 @@ use smallvec::SmallVec;
 use std::fmt;
 use std::rc::Rc;
 use std::sync::Arc;
+use syntax::ast;
 
 impl fmt::Debug for ty::GenericParamDef {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -234,7 +235,12 @@ impl fmt::Debug for ty::ProjectionPredicate<'tcx> {
 impl fmt::Debug for ty::Predicate<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match *self {
-            ty::Predicate::Trait(ref a) => a.fmt(f),
+            ty::Predicate::Trait(ref a, constness) => {
+                if let ast::Constness::Const = constness {
+                    write!(f, "const ")?;
+                }
+                a.fmt(f)
+            }
             ty::Predicate::Subtype(ref pair) => pair.fmt(f),
             ty::Predicate::RegionOutlives(ref pair) => pair.fmt(f),
             ty::Predicate::TypeOutlives(ref pair) => pair.fmt(f),
@@ -474,7 +480,9 @@ impl<'a, 'tcx> Lift<'tcx> for ty::Predicate<'a> {
     type Lifted = ty::Predicate<'tcx>;
     fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
         match *self {
-            ty::Predicate::Trait(ref binder) => tcx.lift(binder).map(ty::Predicate::Trait),
+            ty::Predicate::Trait(ref binder, constness) => {
+                tcx.lift(binder).map(|binder| ty::Predicate::Trait(binder, constness))
+            }
             ty::Predicate::Subtype(ref binder) => tcx.lift(binder).map(ty::Predicate::Subtype),
             ty::Predicate::RegionOutlives(ref binder) => {
                 tcx.lift(binder).map(ty::Predicate::RegionOutlives)
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index 84236128482..13f623aadb1 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -12,7 +12,9 @@ use crate::mir::interpret::Scalar;
 use crate::mir::Promoted;
 use crate::ty::layout::VariantIdx;
 use crate::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, Subst, SubstsRef};
-use crate::ty::{self, AdtDef, DefIdTree, Discr, Ty, TyCtxt, TypeFlags, TypeFoldable};
+use crate::ty::{
+    self, AdtDef, DefIdTree, Discr, Ty, TyCtxt, TypeFlags, TypeFoldable, WithConstness,
+};
 use crate::ty::{List, ParamEnv, ParamEnvAnd, TyS};
 use polonius_engine::Atom;
 use rustc_data_structures::captures::Captures;
@@ -665,14 +667,16 @@ impl<'tcx> Binder<ExistentialPredicate<'tcx>> {
     pub fn with_self_ty(&self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> ty::Predicate<'tcx> {
         use crate::ty::ToPredicate;
         match *self.skip_binder() {
-            ExistentialPredicate::Trait(tr) => Binder(tr).with_self_ty(tcx, self_ty).to_predicate(),
+            ExistentialPredicate::Trait(tr) => {
+                Binder(tr).with_self_ty(tcx, self_ty).without_const().to_predicate()
+            }
             ExistentialPredicate::Projection(p) => {
                 ty::Predicate::Projection(Binder(p.with_self_ty(tcx, self_ty)))
             }
             ExistentialPredicate::AutoTrait(did) => {
                 let trait_ref =
                     Binder(ty::TraitRef { def_id: did, substs: tcx.mk_substs_trait(self_ty, &[]) });
-                trait_ref.to_predicate()
+                trait_ref.without_const().to_predicate()
             }
         }
     }