Try filtering out non-const impls when we expect const impls

compiler/rustc_hir/src/hir.rs

@@ -2751,6 +2751,15 @@ pub enum Constness {
     NotConst,
 }
 
+impl fmt::Display for Constness {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match *self {
+            Self::Const => "const",
+            Self::NotConst => "non-const",
+        })
+    }
+}
+
 #[derive(Copy, Clone, Encodable, Debug, HashStable_Generic)]
 pub struct FnHeader {
     pub unsafety: Unsafety,
@@ -3252,8 +3261,13 @@ impl<'hir> Node<'hir> {
         }
     }
 
-    /// Returns `Constness::Const` when this node is a const fn/impl.
-    pub fn constness(&self) -> Constness {
+    /// Returns `Constness::Const` when this node is a const fn/impl/item,
+    ///
+    /// HACK(fee1-dead): or an associated type in a trait. This works because
+    /// only typeck cares about const trait predicates, so although the predicates
+    /// query would return const predicates when it does not need to be const,
+    /// it wouldn't have any effect.
+    pub fn constness_for_typeck(&self) -> Constness {
         match self {
             Node::Item(Item {
                 kind: ItemKind::Fn(FnSig { header: FnHeader { constness, .. }, .. }, ..),
@@ -3269,6 +3283,11 @@ impl<'hir> Node<'hir> {
             })
             | Node::Item(Item { kind: ItemKind::Impl(Impl { constness, .. }), .. }) => *constness,
 
+            Node::Item(Item { kind: ItemKind::Const(..), .. })
+            | Node::TraitItem(TraitItem { kind: TraitItemKind::Const(..), .. })
+            | Node::TraitItem(TraitItem { kind: TraitItemKind::Type(..), .. })
+            | Node::ImplItem(ImplItem { kind: ImplItemKind::Const(..), .. }) => Constness::Const,
+
             _ => Constness::NotConst,
         }
     }

compiler/rustc_infer/src/traits/engine.rs

@@ -1,5 +1,6 @@
 use crate::infer::InferCtxt;
 use crate::traits::Obligation;
+use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::{self, ToPredicate, Ty, WithConstness};
 
@@ -49,11 +50,28 @@ pub trait TraitEngine<'tcx>: 'tcx {
         infcx: &InferCtxt<'_, 'tcx>,
     ) -> Result<(), Vec<FulfillmentError<'tcx>>>;
 
+    fn select_all_with_constness_or_error(
+        &mut self,
+        infcx: &InferCtxt<'_, 'tcx>,
+        _constness: hir::Constness,
+    ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
+        self.select_all_or_error(infcx)
+    }
+
     fn select_where_possible(
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
     ) -> Result<(), Vec<FulfillmentError<'tcx>>>;
 
+    // FIXME this should not provide a default body for chalk as chalk should be updated
+    fn select_with_constness_where_possible(
+        &mut self,
+        infcx: &InferCtxt<'_, 'tcx>,
+        _constness: hir::Constness,
+    ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
+        self.select_where_possible(infcx)
+    }
+
     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>>;
 }
 

compiler/rustc_infer/src/traits/util.rs

@@ -124,7 +124,7 @@ impl Elaborator<'tcx> {
 
         let bound_predicate = obligation.predicate.kind();
         match bound_predicate.skip_binder() {
-            ty::PredicateKind::Trait(data, _) => {
+            ty::PredicateKind::Trait(data) => {
                 // Get predicates declared on the trait.
                 let predicates = tcx.super_predicates_of(data.def_id());
 

compiler/rustc_lint/src/traits.rs

@@ -87,7 +87,7 @@ impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
         let predicates = cx.tcx.explicit_predicates_of(item.def_id);
         for &(predicate, span) in predicates.predicates {
             let trait_predicate = match predicate.kind().skip_binder() {
-                Trait(trait_predicate, _constness) => trait_predicate,
+                Trait(trait_predicate) => trait_predicate,
                 _ => continue,
             };
             let def_id = trait_predicate.trait_ref.def_id;

compiler/rustc_lint/src/unused.rs

@@ -211,7 +211,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                     let mut has_emitted = false;
                     for &(predicate, _) in cx.tcx.explicit_item_bounds(def) {
                         // We only look at the `DefId`, so it is safe to skip the binder here.
-                        if let ty::PredicateKind::Trait(ref poly_trait_predicate, _) =
+                        if let ty::PredicateKind::Trait(ref poly_trait_predicate) =
                             predicate.kind().skip_binder()
                         {
                             let def_id = poly_trait_predicate.trait_ref.def_id;

compiler/rustc_middle/src/traits/select.rs

@@ -12,12 +12,12 @@ use rustc_hir::def_id::DefId;
 use rustc_query_system::cache::Cache;
 
 pub type SelectionCache<'tcx> = Cache<
-    ty::ParamEnvAnd<'tcx, ty::TraitRef<'tcx>>,
+    ty::ConstnessAnd<ty::ParamEnvAnd<'tcx, ty::TraitRef<'tcx>>>,
     SelectionResult<'tcx, SelectionCandidate<'tcx>>,
 >;
 
 pub type EvaluationCache<'tcx> =
-    Cache<ty::ParamEnvAnd<'tcx, ty::PolyTraitRef<'tcx>>, EvaluationResult>;
+    Cache<ty::ParamEnvAnd<'tcx, ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>>, EvaluationResult>;
 
 /// The selection process begins by considering all impls, where
 /// clauses, and so forth that might resolve an obligation. Sometimes

compiler/rustc_middle/src/ty/assoc.rs

@@ -16,6 +16,13 @@ pub enum AssocItemContainer {
 }
 
 impl AssocItemContainer {
+    pub fn impl_def_id(&self) -> Option<DefId> {
+        match *self {
+            ImplContainer(id) => Some(id),
+            _ => None,
+        }
+    }
+
     /// Asserts that this is the `DefId` of an associated item declared
     /// in a trait, and returns the trait `DefId`.
     pub fn assert_trait(&self) -> DefId {

compiler/rustc_middle/src/ty/context.rs

@@ -2169,7 +2169,7 @@ impl<'tcx> TyCtxt<'tcx> {
             let generic_predicates = self.super_predicates_of(trait_did);
 
             for (predicate, _) in generic_predicates.predicates {
-                if let ty::PredicateKind::Trait(data, _) = predicate.kind().skip_binder() {
+                if let ty::PredicateKind::Trait(data) = predicate.kind().skip_binder() {
                     if set.insert(data.def_id()) {
                         stack.push(data.def_id());
                     }

compiler/rustc_middle/src/ty/error.rs

@@ -33,6 +33,7 @@ impl<T> ExpectedFound<T> {
 #[derive(Clone, Debug, TypeFoldable)]
 pub enum TypeError<'tcx> {
     Mismatch,
+    ConstnessMismatch(ExpectedFound<hir::Constness>),
     UnsafetyMismatch(ExpectedFound<hir::Unsafety>),
     AbiMismatch(ExpectedFound<abi::Abi>),
     Mutability,
@@ -106,6 +107,9 @@ impl<'tcx> fmt::Display for TypeError<'tcx> {
             CyclicTy(_) => write!(f, "cyclic type of infinite size"),
             CyclicConst(_) => write!(f, "encountered a self-referencing constant"),
             Mismatch => write!(f, "types differ"),
+            ConstnessMismatch(values) => {
+                write!(f, "expected {} fn, found {} fn", values.expected, values.found)
+            }
             UnsafetyMismatch(values) => {
                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
             }
@@ -213,9 +217,11 @@ impl<'tcx> TypeError<'tcx> {
     pub fn must_include_note(&self) -> bool {
         use self::TypeError::*;
         match self {
-            CyclicTy(_) | CyclicConst(_) | UnsafetyMismatch(_) | Mismatch | AbiMismatch(_)
-            | FixedArraySize(_) | ArgumentSorts(..) | Sorts(_) | IntMismatch(_)
-            | FloatMismatch(_) | VariadicMismatch(_) | TargetFeatureCast(_) => false,
+            CyclicTy(_) | CyclicConst(_) | UnsafetyMismatch(_) | ConstnessMismatch(_)
+            | Mismatch | AbiMismatch(_) | FixedArraySize(_) | ArgumentSorts(..) | Sorts(_)
+            | IntMismatch(_) | FloatMismatch(_) | VariadicMismatch(_) | TargetFeatureCast(_) => {
+                false
+            }
 
             Mutability
             | ArgumentMutability(_)

compiler/rustc_middle/src/ty/flags.rs

@@ -216,7 +216,7 @@ impl FlagComputation {
 
     fn add_predicate_atom(&mut self, atom: ty::PredicateKind<'_>) {
         match atom {
-            ty::PredicateKind::Trait(trait_pred, _constness) => {
+            ty::PredicateKind::Trait(trait_pred) => {
                 self.add_substs(trait_pred.trait_ref.substs);
             }
             ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(a, b)) => {

compiler/rustc_middle/src/ty/mod.rs

@@ -456,7 +456,7 @@ pub enum PredicateKind<'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(TraitPredicate<'tcx>, Constness),
+    Trait(TraitPredicate<'tcx>),
 
     /// `where 'a: 'b`
     RegionOutlives(RegionOutlivesPredicate<'tcx>),
@@ -612,6 +612,11 @@ impl<'tcx> Predicate<'tcx> {
 #[derive(HashStable, TypeFoldable)]
 pub struct TraitPredicate<'tcx> {
     pub trait_ref: TraitRef<'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>() {}`).
+    pub constness: hir::Constness,
 }
 
 pub type PolyTraitPredicate<'tcx> = ty::Binder<'tcx, TraitPredicate<'tcx>>;
@@ -745,24 +750,27 @@ impl ToPredicate<'tcx> for PredicateKind<'tcx> {
 
 impl<'tcx> ToPredicate<'tcx> for ConstnessAnd<TraitRef<'tcx>> {
     fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {
-        PredicateKind::Trait(ty::TraitPredicate { trait_ref: self.value }, self.constness)
-            .to_predicate(tcx)
+        PredicateKind::Trait(ty::TraitPredicate {
+            trait_ref: self.value,
+            constness: self.constness,
+        })
+        .to_predicate(tcx)
     }
 }
 
 impl<'tcx> ToPredicate<'tcx> for ConstnessAnd<PolyTraitRef<'tcx>> {
     fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {
         self.value
             .map_bound(|trait_ref| {
-                PredicateKind::Trait(ty::TraitPredicate { trait_ref }, self.constness)
+                PredicateKind::Trait(ty::TraitPredicate { trait_ref, constness: self.constness })
             })
             .to_predicate(tcx)
     }
 }
 
-impl<'tcx> ToPredicate<'tcx> for ConstnessAnd<PolyTraitPredicate<'tcx>> {
+impl<'tcx> ToPredicate<'tcx> for PolyTraitPredicate<'tcx> {
     fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {
-        self.value.map_bound(|value| PredicateKind::Trait(value, self.constness)).to_predicate(tcx)
+        self.map_bound(PredicateKind::Trait).to_predicate(tcx)
     }
 }
 
@@ -788,8 +796,8 @@ impl<'tcx> Predicate<'tcx> {
     pub fn to_opt_poly_trait_ref(self) -> Option<ConstnessAnd<PolyTraitRef<'tcx>>> {
         let predicate = self.kind();
         match predicate.skip_binder() {
-            PredicateKind::Trait(t, constness) => {
-                Some(ConstnessAnd { constness, value: predicate.rebind(t.trait_ref) })
+            PredicateKind::Trait(t) => {
+                Some(ConstnessAnd { constness: t.constness, value: predicate.rebind(t.trait_ref) })
             }
             PredicateKind::Projection(..)
             | PredicateKind::Subtype(..)

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

@@ -630,7 +630,7 @@ pub trait PrettyPrinter<'tcx>:
                     for (predicate, _) in bounds {
                         let predicate = predicate.subst(self.tcx(), substs);
                         let bound_predicate = predicate.kind();
-                        if let ty::PredicateKind::Trait(pred, _) = bound_predicate.skip_binder() {
+                        if let ty::PredicateKind::Trait(pred) = bound_predicate.skip_binder() {
                             let trait_ref = bound_predicate.rebind(pred.trait_ref);
                             // Don't print +Sized, but rather +?Sized if absent.
                             if Some(trait_ref.def_id()) == self.tcx().lang_items().sized_trait() {
@@ -2264,10 +2264,7 @@ define_print_and_forward_display! {
 
     ty::PredicateKind<'tcx> {
         match *self {
-            ty::PredicateKind::Trait(ref data, constness) => {
-                if let hir::Constness::Const = constness {
-                    p!("const ");
-                }
+            ty::PredicateKind::Trait(ref data) => {
                 p!(print(data))
             }
             ty::PredicateKind::Subtype(predicate) => p!(print(predicate)),

compiler/rustc_middle/src/ty/relate.rs

@@ -200,6 +200,33 @@ impl<'tcx> Relate<'tcx> for ty::FnSig<'tcx> {
     }
 }
 
+impl<'tcx> Relate<'tcx> for ast::Constness {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ast::Constness,
+        b: ast::Constness,
+    ) -> RelateResult<'tcx, ast::Constness> {
+        if a != b {
+            Err(TypeError::ConstnessMismatch(expected_found(relation, a, b)))
+        } else {
+            Ok(a)
+        }
+    }
+}
+
+impl<'tcx, T: Relate<'tcx>> Relate<'tcx> for ty::ConstnessAnd<T> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::ConstnessAnd<T>,
+        b: ty::ConstnessAnd<T>,
+    ) -> RelateResult<'tcx, ty::ConstnessAnd<T>> {
+        Ok(ty::ConstnessAnd {
+            constness: relation.relate(a.constness, b.constness)?,
+            value: relation.relate(a.value, b.value)?,
+        })
+    }
+}
+
 impl<'tcx> Relate<'tcx> for ast::Unsafety {
     fn relate<R: TypeRelation<'tcx>>(
         relation: &mut R,
@@ -767,7 +794,10 @@ impl<'tcx> Relate<'tcx> for ty::TraitPredicate<'tcx> {
         a: ty::TraitPredicate<'tcx>,
         b: ty::TraitPredicate<'tcx>,
     ) -> RelateResult<'tcx, ty::TraitPredicate<'tcx>> {
-        Ok(ty::TraitPredicate { trait_ref: relation.relate(a.trait_ref, b.trait_ref)? })
+        Ok(ty::TraitPredicate {
+            trait_ref: relation.relate(a.trait_ref, b.trait_ref)?,
+            constness: relation.relate(a.constness, b.constness)?,
+        })
     }
 }
 

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -155,6 +155,9 @@ impl fmt::Debug for ty::ParamConst {
 
 impl fmt::Debug for ty::TraitPredicate<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let hir::Constness::Const = self.constness {
+            write!(f, "const ")?;
+        }
         write!(f, "TraitPredicate({:?})", self.trait_ref)
     }
 }
@@ -174,12 +177,7 @@ impl fmt::Debug for ty::Predicate<'tcx> {
 impl fmt::Debug for ty::PredicateKind<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match *self {
-            ty::PredicateKind::Trait(ref a, constness) => {
-                if let hir::Constness::Const = constness {
-                    write!(f, "const ")?;
-                }
-                a.fmt(f)
-            }
+            ty::PredicateKind::Trait(ref a) => a.fmt(f),
             ty::PredicateKind::Subtype(ref pair) => pair.fmt(f),
             ty::PredicateKind::RegionOutlives(ref pair) => pair.fmt(f),
             ty::PredicateKind::TypeOutlives(ref pair) => pair.fmt(f),
@@ -366,7 +364,8 @@ impl<'a, 'tcx> Lift<'tcx> for ty::ExistentialPredicate<'a> {
 impl<'a, 'tcx> Lift<'tcx> for ty::TraitPredicate<'a> {
     type Lifted = ty::TraitPredicate<'tcx>;
     fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<ty::TraitPredicate<'tcx>> {
-        tcx.lift(self.trait_ref).map(|trait_ref| ty::TraitPredicate { trait_ref })
+        tcx.lift(self.trait_ref)
+            .map(|trait_ref| ty::TraitPredicate { trait_ref, constness: self.constness })
     }
 }
 
@@ -419,9 +418,7 @@ impl<'a, 'tcx> Lift<'tcx> for ty::PredicateKind<'a> {
     type Lifted = ty::PredicateKind<'tcx>;
     fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
         match self {
-            ty::PredicateKind::Trait(data, constness) => {
-                tcx.lift(data).map(|data| ty::PredicateKind::Trait(data, constness))
-            }
+            ty::PredicateKind::Trait(data) => tcx.lift(data).map(ty::PredicateKind::Trait),
             ty::PredicateKind::Subtype(data) => tcx.lift(data).map(ty::PredicateKind::Subtype),
             ty::PredicateKind::RegionOutlives(data) => {
                 tcx.lift(data).map(ty::PredicateKind::RegionOutlives)
@@ -584,6 +581,7 @@ impl<'a, 'tcx> Lift<'tcx> for ty::error::TypeError<'a> {
 
         Some(match self {
             Mismatch => Mismatch,
+            ConstnessMismatch(x) => ConstnessMismatch(x),
             UnsafetyMismatch(x) => UnsafetyMismatch(x),
             AbiMismatch(x) => AbiMismatch(x),
             Mutability => Mutability,

compiler/rustc_middle/src/ty/sty.rs

@@ -876,7 +876,10 @@ impl<'tcx> PolyTraitRef<'tcx> {
     }
 
     pub fn to_poly_trait_predicate(&self) -> ty::PolyTraitPredicate<'tcx> {
-        self.map_bound(|trait_ref| ty::TraitPredicate { trait_ref })
+        self.map_bound(|trait_ref| ty::TraitPredicate {
+            trait_ref,
+            constness: hir::Constness::NotConst,
+        })
     }
 }