Remove the BoundConstness::NotConst variant

compiler/rustc_hir_analysis/src/bounds.rs

@@ -51,7 +51,7 @@ impl<'tcx> Bounds<'tcx> {
         bound_trait_ref: ty::PolyTraitRef<'tcx>,
         span: Span,
         polarity: ty::PredicatePolarity,
-        constness: ty::BoundConstness,
+        constness: Option<ty::BoundConstness>,
         predicate_filter: PredicateFilter,
     ) {
         let clause = (
@@ -88,19 +88,20 @@ impl<'tcx> Bounds<'tcx> {
         // associated type of `<T as Tr>` and make sure that the effect is compatible.
         let compat_val = match (tcx.def_kind(defining_def_id), constness) {
             // FIXME(effects): revisit the correctness of this
-            (_, ty::BoundConstness::Const) => tcx.consts.false_,
+            (_, Some(ty::BoundConstness::Const)) => tcx.consts.false_,
             // body owners that can have trait bounds
-            (DefKind::Const | DefKind::Fn | DefKind::AssocFn, ty::BoundConstness::ConstIfConst) => {
-                tcx.expected_host_effect_param_for_body(defining_def_id)
-            }
+            (
+                DefKind::Const | DefKind::Fn | DefKind::AssocFn,
+                Some(ty::BoundConstness::ConstIfConst),
+            ) => tcx.expected_host_effect_param_for_body(defining_def_id),
 
-            (_, ty::BoundConstness::NotConst) => {
+            (_, None) => {
                 if !tcx.is_const_trait(bound_trait_ref.def_id()) {
                     return;
                 }
                 tcx.consts.true_
             }
-            (DefKind::Trait, ty::BoundConstness::ConstIfConst) => {
+            (DefKind::Trait, Some(ty::BoundConstness::ConstIfConst)) => {
                 // we are in a trait, where `bound_trait_ref` could be:
                 // (1) a super trait `trait Foo: ~const Bar`.
                 //     - This generates `<Self as Foo>::Effects: TyCompat<<Self as Bar>::Effects>`
@@ -138,7 +139,7 @@ impl<'tcx> Bounds<'tcx> {
                 return;
             }
 
-            (DefKind::Impl { of_trait: true }, ty::BoundConstness::ConstIfConst) => {
+            (DefKind::Impl { of_trait: true }, Some(ty::BoundConstness::ConstIfConst)) => {
                 // this is a where clause on an impl header.
                 // push `<T as Tr>::Effects` into the set for the `Min` bound.
                 let Some(assoc) = tcx.associated_type_for_effects(bound_trait_ref.def_id()) else {
@@ -172,12 +173,12 @@ impl<'tcx> Bounds<'tcx> {
             //
             // FIXME(effects) this is equality for now, which wouldn't be helpful for a non-const implementor
             // that uses a `Bar` that implements `Trait` with `Maybe` effects.
-            (DefKind::AssocTy, ty::BoundConstness::ConstIfConst) => {
+            (DefKind::AssocTy, Some(ty::BoundConstness::ConstIfConst)) => {
                 // FIXME(effects): implement this
                 return;
             }
             // probably illegal in this position.
-            (_, ty::BoundConstness::ConstIfConst) => {
+            (_, Some(ty::BoundConstness::ConstIfConst)) => {
                 tcx.dcx().span_delayed_bug(span, "invalid `~const` encountered");
                 return;
             }

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -171,16 +171,15 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 hir::GenericBound::Trait(poly_trait_ref) => {
                     let (constness, polarity) = match poly_trait_ref.modifiers {
                         hir::TraitBoundModifier::Const => {
-                            (ty::BoundConstness::Const, ty::PredicatePolarity::Positive)
-                        }
-                        hir::TraitBoundModifier::MaybeConst => {
-                            (ty::BoundConstness::ConstIfConst, ty::PredicatePolarity::Positive)
-                        }
-                        hir::TraitBoundModifier::None => {
-                            (ty::BoundConstness::NotConst, ty::PredicatePolarity::Positive)
+                            (Some(ty::BoundConstness::Const), ty::PredicatePolarity::Positive)
                         }
+                        hir::TraitBoundModifier::MaybeConst => (
+                            Some(ty::BoundConstness::ConstIfConst),
+                            ty::PredicatePolarity::Positive,
+                        ),
+                        hir::TraitBoundModifier::None => (None, ty::PredicatePolarity::Positive),
                         hir::TraitBoundModifier::Negative => {
-                            (ty::BoundConstness::NotConst, ty::PredicatePolarity::Negative)
+                            (None, ty::PredicatePolarity::Negative)
                         }
                         hir::TraitBoundModifier::Maybe => continue,
                     };

compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs

@@ -51,7 +51,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             } = self.lower_poly_trait_ref(
                 &trait_bound.trait_ref,
                 trait_bound.span,
-                ty::BoundConstness::NotConst,
+                None,
                 ty::PredicatePolarity::Positive,
                 dummy_self,
                 &mut bounds,

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -652,7 +652,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         &self,
         trait_ref: &hir::TraitRef<'tcx>,
         span: Span,
-        constness: ty::BoundConstness,
+        constness: Option<ty::BoundConstness>,
         polarity: ty::PredicatePolarity,
         self_ty: Ty<'tcx>,
         bounds: &mut Bounds<'tcx>,
@@ -675,7 +675,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             Some(self_ty),
         );
 
-        if let ty::BoundConstness::Const | ty::BoundConstness::ConstIfConst = constness
+        if let Some(constness) = constness
             && !self.tcx().is_const_trait(trait_def_id)
         {
             self.dcx().emit_err(crate::errors::ConstBoundForNonConstTrait {

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

@@ -1956,7 +1956,6 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
         define_scoped_cx!(self);
 
         match constness {
-            ty::BoundConstness::NotConst => {}
             ty::BoundConstness::Const => {
                 p!("const ");
             }
@@ -2948,7 +2947,10 @@ impl<'tcx> ty::TraitPredicate<'tcx> {
 }
 
 #[derive(Copy, Clone, TypeFoldable, TypeVisitable, Lift)]
-pub struct TraitPredPrintWithBoundConstness<'tcx>(ty::TraitPredicate<'tcx>, ty::BoundConstness);
+pub struct TraitPredPrintWithBoundConstness<'tcx>(
+    ty::TraitPredicate<'tcx>,
+    Option<ty::BoundConstness>,
+);
 
 impl<'tcx> fmt::Debug for TraitPredPrintWithBoundConstness<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -2966,7 +2968,7 @@ impl<'tcx> ty::PolyTraitPredicate<'tcx> {
 
     fn print_with_bound_constness(
         self,
-        constness: ty::BoundConstness,
+        constness: Option<ty::BoundConstness>,
     ) -> ty::Binder<'tcx, TraitPredPrintWithBoundConstness<'tcx>> {
         self.map_bound(|trait_pred| TraitPredPrintWithBoundConstness(trait_pred, constness))
     }
@@ -3206,7 +3208,9 @@ define_print_and_forward_display! {
 
     TraitPredPrintWithBoundConstness<'tcx> {
         p!(print(self.0.trait_ref.self_ty()), ": ");
-        p!(pretty_print_bound_constness(self.1));
+        if let Some(constness) = self.1 {
+            p!(pretty_print_bound_constness(constness));
+        }
         if let ty::PredicatePolarity::Negative = self.0.polarity {
             p!("!");
         }

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -290,7 +290,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         }
 
                         if tcx.is_lang_item(leaf_trait_ref.def_id(), LangItem::Drop)
-                            && matches!(predicate_constness, ty::BoundConstness::ConstIfConst | ty::BoundConstness::Const)
+                            && matches!(predicate_constness, Some(ty::BoundConstness::ConstIfConst | ty::BoundConstness::Const))
                         {
                             err.note("`~const Drop` was renamed to `~const Destruct`");
                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
@@ -2192,27 +2192,29 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         &self,
         trait_predicate: ty::PolyTraitPredicate<'tcx>,
         message: Option<String>,
-        predicate_constness: ty::BoundConstness,
+        predicate_constness: Option<ty::BoundConstness>,
         append_const_msg: Option<AppendConstMessage>,
         post_message: String,
     ) -> String {
         message
             .and_then(|cannot_do_this| {
                 match (predicate_constness, append_const_msg) {
                     // do nothing if predicate is not const
-                    (ty::BoundConstness::NotConst, _) => Some(cannot_do_this),
+                    (None, _) => Some(cannot_do_this),
                     // suggested using default post message
                     (
-                        ty::BoundConstness::Const | ty::BoundConstness::ConstIfConst,
+                        Some(ty::BoundConstness::Const | ty::BoundConstness::ConstIfConst),
                         Some(AppendConstMessage::Default),
                     ) => Some(format!("{cannot_do_this} in const contexts")),
                     // overridden post message
                     (
-                        ty::BoundConstness::Const | ty::BoundConstness::ConstIfConst,
+                        Some(ty::BoundConstness::Const | ty::BoundConstness::ConstIfConst),
                         Some(AppendConstMessage::Custom(custom_msg, _)),
                     ) => Some(format!("{cannot_do_this}{custom_msg}")),
                     // fallback to generic message
-                    (ty::BoundConstness::Const | ty::BoundConstness::ConstIfConst, None) => None,
+                    (Some(ty::BoundConstness::Const | ty::BoundConstness::ConstIfConst), None) => {
+                        None
+                    }
                 }
             })
             .unwrap_or_else(|| {
@@ -2358,26 +2360,27 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         p: ty::PolyTraitPredicate<'tcx>,
         leaf: ty::PolyTraitPredicate<'tcx>,
         span: Span,
-    ) -> (ty::PolyTraitPredicate<'tcx>, ty::PolyTraitPredicate<'tcx>, ty::BoundConstness) {
+    ) -> (ty::PolyTraitPredicate<'tcx>, ty::PolyTraitPredicate<'tcx>, Option<ty::BoundConstness>)
+    {
         let trait_ref = p.to_poly_trait_ref();
         if !self.tcx.is_lang_item(trait_ref.def_id(), LangItem::EffectsCompat) {
-            return (p, leaf, ty::BoundConstness::NotConst);
+            return (p, leaf, None);
         }
 
         let Some(ty::Alias(ty::AliasTyKind::Projection, projection)) =
             trait_ref.self_ty().no_bound_vars().map(Ty::kind)
         else {
-            return (p, leaf, ty::BoundConstness::NotConst);
+            return (p, leaf, None);
         };
 
         let constness = trait_ref.skip_binder().args.const_at(1);
 
         let constness = if constness == self.tcx.consts.true_ || constness.is_ct_infer() {
-            ty::BoundConstness::NotConst
+            None
         } else if constness == self.tcx.consts.false_ {
-            ty::BoundConstness::Const
+            Some(ty::BoundConstness::Const)
         } else if matches!(constness.kind(), ty::ConstKind::Param(_)) {
-            ty::BoundConstness::ConstIfConst
+            Some(ty::BoundConstness::ConstIfConst)
         } else {
             self.dcx().span_bug(span, format!("Unknown constness argument: {constness:?}"));
         };

compiler/rustc_type_ir/src/predicate.rs

@@ -726,9 +726,9 @@ pub struct CoercePredicate<I: Interner> {
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 #[cfg_attr(feature = "nightly", derive(HashStable_NoContext, TyEncodable, TyDecodable))]
 pub enum BoundConstness {
-    /// `Type: Trait`
-    NotConst,
     /// `Type: const Trait`
+    ///
+    /// A bound is required to be unconditionally const, even in a runtime function.
     Const,
     /// `Type: ~const Trait`
     ///
@@ -739,7 +739,6 @@ pub enum BoundConstness {
 impl BoundConstness {
     pub fn as_str(self) -> &'static str {
         match self {
-            Self::NotConst => "",
             Self::Const => "const",
             Self::ConstIfConst => "~const",
         }
@@ -749,7 +748,6 @@ impl BoundConstness {
 impl fmt::Display for BoundConstness {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
-            Self::NotConst => f.write_str("normal"),
             Self::Const => f.write_str("const"),
             Self::ConstIfConst => f.write_str("~const"),
         }