Auto merge of #91549 - fee1-dead:const_env, r=spastorino

Eliminate ConstnessAnd again

Closes #91489.
Closes #89432.

Reverts #91491.
Reverts #89450.

r? `@spastorino`

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -31,7 +31,7 @@ use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::subst::{GenericArgKind, SubstsRef, UserSubsts};
 use rustc_middle::ty::{
     self, CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations, OpaqueTypeKey, RegionVid,
-    ToPredicate, Ty, TyCtxt, UserType, UserTypeAnnotationIndex, WithConstness,
+    ToPredicate, Ty, TyCtxt, UserType, UserTypeAnnotationIndex,
 };
 use rustc_span::def_id::CRATE_DEF_ID;
 use rustc_span::{Span, DUMMY_SP};

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -7,6 +7,7 @@ use crate::interpret::{
 };
 
 use rustc_errors::ErrorReported;
+use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::ErrorHandled;
@@ -215,6 +216,7 @@ pub fn eval_to_const_value_raw_provider<'tcx>(
     tcx: TyCtxt<'tcx>,
     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
 ) -> ::rustc_middle::mir::interpret::EvalToConstValueResult<'tcx> {
+    assert!(key.param_env.constness() == hir::Constness::Const);
     // see comment in eval_to_allocation_raw_provider for what we're doing here
     if key.param_env.reveal() == Reveal::All {
         let mut key = key;
@@ -249,6 +251,7 @@ pub fn eval_to_allocation_raw_provider<'tcx>(
     tcx: TyCtxt<'tcx>,
     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
 ) -> ::rustc_middle::mir::interpret::EvalToAllocationRawResult<'tcx> {
+    assert!(key.param_env.constness() == hir::Constness::Const);
     // Because the constant is computed twice (once per value of `Reveal`), we are at risk of
     // reporting the same error twice here. To resolve this, we check whether we can evaluate the
     // constant in the more restrictive `Reveal::UserFacing`, which most likely already was

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -929,6 +929,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         } else {
             self.param_env
         };
+        let param_env = param_env.with_const();
         let val = self.tcx.eval_to_allocation_raw(param_env.and(gid))?;
         self.raw_const_to_mplace(val)
     }

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -817,8 +817,7 @@ impl Visitor<'tcx> for Checker<'mir, 'tcx> {
                     );
 
                     let implsrc = tcx.infer_ctxt().enter(|infcx| {
-                        let mut selcx =
+                        let mut selcx = SelectionContext::new(&infcx);
-                            SelectionContext::with_constness(&infcx, hir::Constness::Const);
                         selcx.select(&obligation)
                     });
 

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -3,7 +3,6 @@
 //! See the `Qualif` trait for more info.
 
 use rustc_errors::ErrorReported;
-use rustc_hir as hir;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, subst::SubstsRef, AdtDef, Ty};
@@ -167,7 +166,7 @@ impl Qualif for NeedsNonConstDrop {
         );
 
         let implsrc = cx.tcx.infer_ctxt().enter(|infcx| {
-            let mut selcx = SelectionContext::with_constness(&infcx, hir::Constness::Const);
+            let mut selcx = SelectionContext::new(&infcx);
             selcx.select(&obligation)
         });
         !matches!(

compiler/rustc_data_structures/src/tagged_ptr/copy.rs

@@ -94,9 +94,11 @@ where
         // SAFETY: pointer_raw returns the original pointer
         unsafe { std::mem::transmute_copy(&self.pointer_raw()) }
     }
+    #[inline]
     pub fn tag(&self) -> T {
         unsafe { T::from_usize(self.packed.get() >> Self::TAG_BIT_SHIFT) }
     }
+    #[inline]
     pub fn set_tag(&mut self, tag: T) {
         let mut packed = self.packed.get();
         let new_tag = T::into_usize(tag) << Self::TAG_BIT_SHIFT;

compiler/rustc_hir/src/hir.rs

@@ -3208,31 +3208,6 @@ impl<'hir> Node<'hir> {
         }
     }
 
-    /// Returns `Constness::Const` when this node is a const fn/impl/item.
-    pub fn constness_for_typeck(&self) -> Constness {
-        match self {
-            Node::Item(Item {
-                kind: ItemKind::Fn(FnSig { header: FnHeader { constness, .. }, .. }, ..),
-                ..
-            })
-            | Node::TraitItem(TraitItem {
-                kind: TraitItemKind::Fn(FnSig { header: FnHeader { constness, .. }, .. }, ..),
-                ..
-            })
-            | Node::ImplItem(ImplItem {
-                kind: ImplItemKind::Fn(FnSig { header: FnHeader { constness, .. }, .. }, ..),
-                ..
-            })
-            | Node::Item(Item { kind: ItemKind::Impl(Impl { constness, .. }), .. }) => *constness,
-
-            Node::Item(Item { kind: ItemKind::Const(..), .. })
-            | Node::TraitItem(TraitItem { kind: TraitItemKind::Const(..), .. })
-            | Node::ImplItem(ImplItem { kind: ImplItemKind::Const(..), .. }) => Constness::Const,
-
-            _ => Constness::NotConst,
-        }
-    }
-
     pub fn as_owner(self) -> Option<OwnerNode<'hir>> {
         match self {
             Node::Item(i) => Some(OwnerNode::Item(i)),

compiler/rustc_infer/src/traits/engine.rs

@@ -1,9 +1,8 @@
 use crate::infer::InferCtxt;
 use crate::traits::Obligation;
 use rustc_data_structures::fx::FxHashMap;
-use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
-use rustc_middle::ty::{self, ToPredicate, Ty, WithConstness};
+use rustc_middle::ty::{self, ToPredicate, Ty};
 
 use super::FulfillmentError;
 use super::{ObligationCause, PredicateObligation};
@@ -48,26 +47,9 @@ pub trait TraitEngine<'tcx>: 'tcx {
 
     fn select_all_or_error(&mut self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<FulfillmentError<'tcx>>;
 
-    fn select_all_with_constness_or_error(
-        &mut self,
-        infcx: &InferCtxt<'_, 'tcx>,
-        _constness: hir::Constness,
-    ) -> Vec<FulfillmentError<'tcx>> {
-        self.select_all_or_error(infcx)
-    }
-
     fn select_where_possible(&mut self, infcx: &InferCtxt<'_, 'tcx>)
     -> Vec<FulfillmentError<'tcx>>;
 
-    // FIXME(fee1-dead) 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,
-    ) -> Vec<FulfillmentError<'tcx>> {
-        self.select_where_possible(infcx)
-    }
-
     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>>;
 
     fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships>;

compiler/rustc_infer/src/traits/mod.rs

@@ -69,6 +69,16 @@ impl PredicateObligation<'tcx> {
     }
 }
 
+impl TraitObligation<'tcx> {
+    /// Returns `true` if the trait predicate is considered `const` in its ParamEnv.
+    pub fn is_const(&self) -> bool {
+        match (self.predicate.skip_binder().constness, self.param_env.constness()) {
+            (ty::BoundConstness::ConstIfConst, hir::Constness::Const) => true,
+            _ => false,
+        }
+    }
+}
+
 // `PredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 static_assert_size!(PredicateObligation<'_>, 32);

compiler/rustc_infer/src/traits/util.rs

@@ -3,7 +3,7 @@ use smallvec::smallvec;
 use crate::infer::outlives::components::{push_outlives_components, Component};
 use crate::traits::{Obligation, ObligationCause, PredicateObligation};
 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
-use rustc_middle::ty::{self, ToPredicate, TyCtxt, WithConstness};
+use rustc_middle::ty::{self, ToPredicate, TyCtxt};
 use rustc_span::symbol::Ident;
 use rustc_span::Span;
 
@@ -328,8 +328,8 @@ pub fn transitive_bounds_that_define_assoc_type<'tcx>(
                 ));
                 for (super_predicate, _) in super_predicates.predicates {
                     let subst_predicate = super_predicate.subst_supertrait(tcx, &trait_ref);
-                    if let Some(binder) = subst_predicate.to_opt_poly_trait_ref() {
+                    if let Some(binder) = subst_predicate.to_opt_poly_trait_pred() {
-                        stack.push(binder.value);
+                        stack.push(binder.map_bound(|t| t.trait_ref));
                     }
                 }
 
@@ -362,8 +362,8 @@ impl<'tcx, I: Iterator<Item = PredicateObligation<'tcx>>> Iterator for FilterToT
 
     fn next(&mut self) -> Option<ty::PolyTraitRef<'tcx>> {
         while let Some(obligation) = self.base_iterator.next() {
-            if let Some(data) = obligation.predicate.to_opt_poly_trait_ref() {
+            if let Some(data) = obligation.predicate.to_opt_poly_trait_pred() {
-                return Some(data.value);
+                return Some(data.map_bound(|t| t.trait_ref));
             }
         }
         None

compiler/rustc_macros/src/query.rs

@@ -58,6 +58,9 @@ enum QueryModifier {
 
     /// Use a separate query provider for local and extern crates
     SeparateProvideExtern(Ident),
+
+    /// Always remap the ParamEnv's constness before hashing and passing to the query provider
+    RemapEnvConstness(Ident),
 }
 
 impl Parse for QueryModifier {
@@ -123,6 +126,8 @@ impl Parse for QueryModifier {
             Ok(QueryModifier::EvalAlways(modifier))
         } else if modifier == "separate_provide_extern" {
             Ok(QueryModifier::SeparateProvideExtern(modifier))
+        } else if modifier == "remap_env_constness" {
+            Ok(QueryModifier::RemapEnvConstness(modifier))
         } else {
             Err(Error::new(modifier.span(), "unknown query modifier"))
         }
@@ -222,6 +227,9 @@ struct QueryModifiers {
 
     /// Use a separate query provider for local and extern crates
     separate_provide_extern: Option<Ident>,
+
+    /// Always remap the ParamEnv's constness before hashing.
+    remap_env_constness: Option<Ident>,
 }
 
 /// Process query modifiers into a struct, erroring on duplicates
@@ -236,6 +244,7 @@ fn process_modifiers(query: &mut Query) -> QueryModifiers {
     let mut anon = None;
     let mut eval_always = None;
     let mut separate_provide_extern = None;
+    let mut remap_env_constness = None;
     for modifier in query.modifiers.0.drain(..) {
         match modifier {
             QueryModifier::LoadCached(tcx, id, block) => {
@@ -335,6 +344,12 @@ fn process_modifiers(query: &mut Query) -> QueryModifiers {
                 }
                 separate_provide_extern = Some(ident);
             }
+            QueryModifier::RemapEnvConstness(ident) => {
+                if remap_env_constness.is_some() {
+                    panic!("duplicate modifier `remap_env_constness` for query `{}`", query.name);
+                }
+                remap_env_constness = Some(ident)
+            }
         }
     }
     let desc = desc.unwrap_or_else(|| {
@@ -351,6 +366,7 @@ fn process_modifiers(query: &mut Query) -> QueryModifiers {
         anon,
         eval_always,
         separate_provide_extern,
+        remap_env_constness,
     }
 }
 
@@ -485,6 +501,10 @@ pub fn rustc_queries(input: TokenStream) -> TokenStream {
         if let Some(separate_provide_extern) = &modifiers.separate_provide_extern {
             attributes.push(quote! { (#separate_provide_extern) });
         }
+        // Pass on the remap_env_constness modifier
+        if let Some(remap_env_constness) = &modifiers.remap_env_constness {
+            attributes.push(quote! { (#remap_env_constness) });
+        }
 
         // This uses the span of the query definition for the commas,
         // which can be important if we later encounter any ambiguity

compiler/rustc_middle/src/hir/map/mod.rs

@@ -474,7 +474,8 @@ impl<'hir> Map<'hir> {
     /// Panics if `LocalDefId` does not have an associated body.
     ///
     /// This should only be used for determining the context of a body, a return
-    /// value of `Some` does not always suggest that the owner of the body is `const`.
+    /// value of `Some` does not always suggest that the owner of the body is `const`,
+    /// just that it has to be checked as if it were.
     pub fn body_const_context(&self, did: LocalDefId) -> Option<ConstContext> {
         let hir_id = self.local_def_id_to_hir_id(did);
         let ccx = match self.body_owner_kind(hir_id) {

compiler/rustc_middle/src/infer/canonical.rs

@@ -246,6 +246,14 @@ impl<'tcx, R> Canonical<'tcx, QueryResponse<'tcx, R>> {
     }
 }
 
+impl<'tcx, R> Canonical<'tcx, ty::ParamEnvAnd<'tcx, R>> {
+    #[inline]
+    pub fn without_const(mut self) -> Self {
+        self.value = self.value.without_const();
+        self
+    }
+}
+
 impl<'tcx, V> Canonical<'tcx, V> {
     /// Allows you to map the `value` of a canonical while keeping the
     /// same set of bound variables.

compiler/rustc_middle/src/mir/interpret/queries.rs

@@ -64,6 +64,7 @@ impl<'tcx> TyCtxt<'tcx> {
         cid: GlobalId<'tcx>,
         span: Option<Span>,
     ) -> EvalToConstValueResult<'tcx> {
+        let param_env = param_env.with_const();
         // Const-eval shouldn't depend on lifetimes at all, so we can erase them, which should
         // improve caching of queries.
         let inputs = self.erase_regions(param_env.and(cid));
@@ -92,6 +93,7 @@ impl<'tcx> TyCtxt<'tcx> {
         gid: GlobalId<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
     ) -> Result<&'tcx mir::Allocation, ErrorHandled> {
+        let param_env = param_env.with_const();
         trace!("eval_to_allocation: Need to compute {:?}", gid);
         let raw_const = self.eval_to_allocation_raw(param_env.and(gid))?;
         Ok(self.global_alloc(raw_const.alloc_id).unwrap_memory())

compiler/rustc_middle/src/query/mod.rs

@@ -880,6 +880,7 @@ rustc_queries! {
         key: ty::ParamEnvAnd<'tcx, ConstAlloc<'tcx>>
     ) -> Option<ty::ValTree<'tcx>> {
         desc { "destructure constant" }
+        remap_env_constness
     }
 
     /// Destructure a constant ADT or array into its variant index and its
@@ -888,6 +889,7 @@ rustc_queries! {
         key: ty::ParamEnvAnd<'tcx, &'tcx ty::Const<'tcx>>
     ) -> mir::DestructuredConst<'tcx> {
         desc { "destructure constant" }
+        remap_env_constness
     }
 
     /// Dereference a constant reference or raw pointer and turn the result into a constant
@@ -896,6 +898,7 @@ rustc_queries! {
         key: ty::ParamEnvAnd<'tcx, &'tcx ty::Const<'tcx>>
     ) -> &'tcx ty::Const<'tcx> {
         desc { "deref constant" }
+        remap_env_constness
     }
 
     query const_caller_location(key: (rustc_span::Symbol, u32, u32)) -> ConstValue<'tcx> {
@@ -1100,26 +1103,32 @@ rustc_queries! {
     /// `ty.is_copy()`, etc, since that will prune the environment where possible.
     query is_copy_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` is `Copy`", env.value }
+        remap_env_constness
     }
     /// Query backing `TyS::is_sized`.
     query is_sized_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` is `Sized`", env.value }
+        remap_env_constness
     }
     /// Query backing `TyS::is_freeze`.
     query is_freeze_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` is freeze", env.value }
+        remap_env_constness
     }
     /// Query backing `TyS::is_unpin`.
     query is_unpin_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` is `Unpin`", env.value }
+        remap_env_constness
     }
     /// Query backing `TyS::needs_drop`.
     query needs_drop_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` needs drop", env.value }
+        remap_env_constness
     }
     /// Query backing `TyS::has_significant_drop_raw`.
     query has_significant_drop_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` has a significant drop", env.value }
+        remap_env_constness
     }
 
     /// Query backing `TyS::is_structural_eq_shallow`.
@@ -1158,6 +1167,7 @@ rustc_queries! {
         key: ty::ParamEnvAnd<'tcx, Ty<'tcx>>
     ) -> Result<ty::layout::TyAndLayout<'tcx>, ty::layout::LayoutError<'tcx>> {
         desc { "computing layout of `{}`", key.value }
+        remap_env_constness
     }
 
     /// Compute a `FnAbi` suitable for indirect calls, i.e. to `fn` pointers.
@@ -1168,6 +1178,7 @@ rustc_queries! {
         key: ty::ParamEnvAnd<'tcx, (ty::PolyFnSig<'tcx>, &'tcx ty::List<Ty<'tcx>>)>
     ) -> Result<&'tcx abi::call::FnAbi<'tcx, Ty<'tcx>>, ty::layout::FnAbiError<'tcx>> {
         desc { "computing call ABI of `{}` function pointers", key.value.0 }
+        remap_env_constness
     }
 
     /// Compute a `FnAbi` suitable for declaring/defining an `fn` instance, and for
@@ -1179,6 +1190,7 @@ rustc_queries! {
         key: ty::ParamEnvAnd<'tcx, (ty::Instance<'tcx>, &'tcx ty::List<Ty<'tcx>>)>
     ) -> Result<&'tcx abi::call::FnAbi<'tcx, Ty<'tcx>>, ty::layout::FnAbiError<'tcx>> {
         desc { "computing call ABI of `{}`", key.value.0 }
+        remap_env_constness
     }
 
     query dylib_dependency_formats(_: CrateNum)
@@ -1463,6 +1475,7 @@ rustc_queries! {
         key: ty::ParamEnvAnd<'tcx, Ty<'tcx>>
     ) -> ty::inhabitedness::DefIdForest {
         desc { "computing the inhabitedness of `{:?}`", key }
+        remap_env_constness
     }
 
     query dep_kind(_: CrateNum) -> CrateDepKind {
@@ -1642,6 +1655,7 @@ rustc_queries! {
         NoSolution,
     > {
         desc { "normalizing `{:?}`", goal }
+        remap_env_constness
     }
 
     // FIXME: Implement `normalize_generic_arg_after_erasing_regions` and
@@ -1654,6 +1668,7 @@ rustc_queries! {
         goal: ParamEnvAnd<'tcx, GenericArg<'tcx>>
     ) -> GenericArg<'tcx> {
         desc { "normalizing `{}`", goal.value }
+        remap_env_constness
     }
 
     /// Do not call this query directly: invoke `normalize_erasing_regions` instead.
@@ -1661,6 +1676,7 @@ rustc_queries! {
         goal: ParamEnvAnd<'tcx, mir::ConstantKind<'tcx>>
     ) -> mir::ConstantKind<'tcx> {
         desc { "normalizing `{}`", goal.value }
+        remap_env_constness
     }
 
     /// Do not call this query directly: invoke `try_normalize_erasing_regions` instead.
@@ -1668,6 +1684,7 @@ rustc_queries! {
         goal: ParamEnvAnd<'tcx, GenericArg<'tcx>>
     ) -> Result<GenericArg<'tcx>, NoSolution> {
         desc { "normalizing `{}`", goal.value }
+        remap_env_constness
     }
 
     /// Do not call this query directly: invoke `try_normalize_erasing_regions` instead.
@@ -1675,6 +1692,7 @@ rustc_queries! {
         goal: ParamEnvAnd<'tcx, mir::ConstantKind<'tcx>>
     ) -> Result<mir::ConstantKind<'tcx>, NoSolution> {
         desc { "normalizing `{}`", goal.value }
+        remap_env_constness
     }
 
     query implied_outlives_bounds(
@@ -1684,6 +1702,7 @@ rustc_queries! {
         NoSolution,
     > {
         desc { "computing implied outlives bounds for `{:?}`", goal }
+        remap_env_constness
     }
 
     /// Do not call this query directly: invoke `infcx.at().dropck_outlives()` instead.
@@ -1694,6 +1713,7 @@ rustc_queries! {
         NoSolution,
     > {
         desc { "computing dropck types for `{:?}`", goal }
+        remap_env_constness
     }
 
     /// Do not call this query directly: invoke `infcx.predicate_may_hold()` or
@@ -1721,6 +1741,7 @@ rustc_queries! {
         NoSolution,
     > {
         desc { "evaluating `type_op_ascribe_user_type` `{:?}`", goal }
+        remap_env_constness
     }
 
     /// Do not call this query directly: part of the `Eq` type-op
@@ -1731,6 +1752,7 @@ rustc_queries! {
         NoSolution,
     > {
         desc { "evaluating `type_op_eq` `{:?}`", goal }
+        remap_env_constness
     }
 
     /// Do not call this query directly: part of the `Subtype` type-op
@@ -1741,6 +1763,7 @@ rustc_queries! {
         NoSolution,
     > {
         desc { "evaluating `type_op_subtype` `{:?}`", goal }
+        remap_env_constness
     }
 
     /// Do not call this query directly: part of the `ProvePredicate` type-op
@@ -1761,6 +1784,7 @@ rustc_queries! {
         NoSolution,
     > {
         desc { "normalizing `{:?}`", goal }
+        remap_env_constness
     }
 
     /// Do not call this query directly: part of the `Normalize` type-op
@@ -1771,6 +1795,7 @@ rustc_queries! {
         NoSolution,
     > {
         desc { "normalizing `{:?}`", goal }
+        remap_env_constness
     }
 
     /// Do not call this query directly: part of the `Normalize` type-op
@@ -1781,6 +1806,7 @@ rustc_queries! {
         NoSolution,
     > {
         desc { "normalizing `{:?}`", goal }
+        remap_env_constness
     }
 
     /// Do not call this query directly: part of the `Normalize` type-op
@@ -1791,6 +1817,7 @@ rustc_queries! {
         NoSolution,
     > {
         desc { "normalizing `{:?}`", goal }
+        remap_env_constness
     }
 
     query subst_and_check_impossible_predicates(key: (DefId, SubstsRef<'tcx>)) -> bool {
@@ -1804,6 +1831,7 @@ rustc_queries! {
         goal: CanonicalTyGoal<'tcx>
     ) -> MethodAutoderefStepsResult<'tcx> {
         desc { "computing autoderef types for `{:?}`", goal }
+        remap_env_constness
     }
 
     query supported_target_features(_: CrateNum) -> FxHashMap<String, Option<Symbol>> {
@@ -1836,6 +1864,7 @@ rustc_queries! {
         key: ty::ParamEnvAnd<'tcx, (DefId, SubstsRef<'tcx>)>
     ) -> Result<Option<ty::Instance<'tcx>>, ErrorReported> {
         desc { "resolving instance `{}`", ty::Instance::new(key.value.0, key.value.1) }
+        remap_env_constness
     }
 
     query resolve_instance_of_const_arg(
@@ -1845,6 +1874,7 @@ rustc_queries! {
             "resolving instance of the const argument `{}`",
             ty::Instance::new(key.value.0.to_def_id(), key.value.2),
         }
+        remap_env_constness
     }
 
     query normalize_opaque_types(key: &'tcx ty::List<ty::Predicate<'tcx>>) -> &'tcx ty::List<ty::Predicate<'tcx>> {
@@ -1859,6 +1889,7 @@ rustc_queries! {
     /// size, to account for partial initialisation. See #49298 for details.)
     query conservative_is_privately_uninhabited(key: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "conservatively checking if {:?} is privately uninhabited", key }
+        remap_env_constness
     }
 
     query limits(key: ()) -> Limits {

compiler/rustc_middle/src/traits/select.rs

@@ -12,14 +12,12 @@ use rustc_hir::def_id::DefId;
 use rustc_query_system::cache::Cache;
 
 pub type SelectionCache<'tcx> = Cache<
-    (ty::ConstnessAnd<ty::ParamEnvAnd<'tcx, ty::TraitRef<'tcx>>>, ty::ImplPolarity),
+    ty::ParamEnvAnd<'tcx, ty::TraitPredicate<'tcx>>,
     SelectionResult<'tcx, SelectionCandidate<'tcx>>,
 >;
 
-pub type EvaluationCache<'tcx> = Cache<
+pub type EvaluationCache<'tcx> =
-    (ty::ParamEnvAnd<'tcx, ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>>, ty::ImplPolarity),
+    Cache<ty::ParamEnvAnd<'tcx, ty::PolyTraitPredicate<'tcx>>, EvaluationResult>;
-    EvaluationResult,
->;
 
 /// The selection process begins by considering all impls, where
 /// clauses, and so forth that might resolve an obligation. Sometimes
@@ -103,7 +101,7 @@ pub enum SelectionCandidate<'tcx> {
         /// `false` if there are no *further* obligations.
         has_nested: bool,
     },
-    ParamCandidate((ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>, ty::ImplPolarity)),
+    ParamCandidate(ty::PolyTraitPredicate<'tcx>),
     ImplCandidate(DefId),
     AutoImplCandidate(DefId),
 

compiler/rustc_middle/src/traits/util.rs

@@ -26,9 +26,9 @@ impl<'tcx> Elaborator<'tcx> {
             .predicates
             .into_iter()
             .flat_map(|(pred, _)| {
-                pred.subst_supertrait(self.tcx, &trait_ref).to_opt_poly_trait_ref()
+                pred.subst_supertrait(self.tcx, &trait_ref).to_opt_poly_trait_pred()
             })
-            .map(|t| t.value)
+            .map(|t| t.map_bound(|pred| pred.trait_ref))
             .filter(|supertrait_ref| self.visited.insert(*supertrait_ref));
 
         self.stack.extend(supertrait_refs);

compiler/rustc_middle/src/ty/mod.rs

@@ -230,6 +230,19 @@ pub enum BoundConstness {
     ConstIfConst,
 }
 
+impl BoundConstness {
+    /// Reduce `self` and `constness` to two possible combined states instead of four.
+    pub fn and(&mut self, constness: hir::Constness) -> hir::Constness {
+        match (constness, self) {
+            (hir::Constness::Const, BoundConstness::ConstIfConst) => hir::Constness::Const,
+            (_, this) => {
+                *this = BoundConstness::NotConst;
+                hir::Constness::NotConst
+            }
+        }
+    }
+}
+
 impl fmt::Display for BoundConstness {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
@@ -846,20 +859,6 @@ impl ToPredicate<'tcx> for Binder<'tcx, PredicateKind<'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,
-                    constness: self.constness,
-                    polarity: ty::ImplPolarity::Positive,
-                })
-            })
-            .to_predicate(tcx)
-    }
-}
-
 impl<'tcx> ToPredicate<'tcx> for PolyTraitPredicate<'tcx> {
     fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {
         self.map_bound(PredicateKind::Trait).to_predicate(tcx)
@@ -885,12 +884,10 @@ impl<'tcx> ToPredicate<'tcx> for PolyProjectionPredicate<'tcx> {
 }
 
 impl<'tcx> Predicate<'tcx> {
-    pub fn to_opt_poly_trait_ref(self) -> Option<ConstnessAnd<PolyTraitRef<'tcx>>> {
+    pub fn to_opt_poly_trait_pred(self) -> Option<PolyTraitPredicate<'tcx>> {
         let predicate = self.kind();
         match predicate.skip_binder() {
-            PredicateKind::Trait(t) => {
+            PredicateKind::Trait(t) => Some(predicate.rebind(t)),
-                Some(ConstnessAnd { constness: t.constness, value: predicate.rebind(t.trait_ref) })
-            }
             PredicateKind::Projection(..)
             | PredicateKind::Subtype(..)
             | PredicateKind::Coerce(..)
@@ -1221,23 +1218,33 @@ pub struct ParamEnv<'tcx> {
     /// want `Reveal::All`.
     ///
     /// Note: This is packed, use the reveal() method to access it.
-    packed: CopyTaggedPtr<&'tcx List<Predicate<'tcx>>, traits::Reveal, true>,
+    packed: CopyTaggedPtr<&'tcx List<Predicate<'tcx>>, ParamTag, true>,
 }
 
-unsafe impl rustc_data_structures::tagged_ptr::Tag for traits::Reveal {
+#[derive(Copy, Clone)]
-    const BITS: usize = 1;
+struct ParamTag {
+    reveal: traits::Reveal,
+    constness: hir::Constness,
+}
+
+unsafe impl rustc_data_structures::tagged_ptr::Tag for ParamTag {
+    const BITS: usize = 2;
     #[inline]
     fn into_usize(self) -> usize {
         match self {
-            traits::Reveal::UserFacing => 0,
+            Self { reveal: traits::Reveal::UserFacing, constness: hir::Constness::NotConst } => 0,
-            traits::Reveal::All => 1,
+            Self { reveal: traits::Reveal::All, constness: hir::Constness::NotConst } => 1,
+            Self { reveal: traits::Reveal::UserFacing, constness: hir::Constness::Const } => 2,
+            Self { reveal: traits::Reveal::All, constness: hir::Constness::Const } => 3,
         }
     }
     #[inline]
     unsafe fn from_usize(ptr: usize) -> Self {
         match ptr {
-            0 => traits::Reveal::UserFacing,
+            0 => Self { reveal: traits::Reveal::UserFacing, constness: hir::Constness::NotConst },
-            1 => traits::Reveal::All,
+            1 => Self { reveal: traits::Reveal::All, constness: hir::Constness::NotConst },
+            2 => Self { reveal: traits::Reveal::UserFacing, constness: hir::Constness::Const },
+            3 => Self { reveal: traits::Reveal::All, constness: hir::Constness::Const },
             _ => std::hint::unreachable_unchecked(),
         }
     }
@@ -1248,6 +1255,7 @@ impl<'tcx> fmt::Debug for ParamEnv<'tcx> {
         f.debug_struct("ParamEnv")
             .field("caller_bounds", &self.caller_bounds())
             .field("reveal", &self.reveal())
+            .field("constness", &self.constness())
             .finish()
     }
 }
@@ -1256,6 +1264,7 @@ impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for ParamEnv<'tcx> {
     fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
         self.caller_bounds().hash_stable(hcx, hasher);
         self.reveal().hash_stable(hcx, hasher);
+        self.constness().hash_stable(hcx, hasher);
     }
 }
 
@@ -1267,12 +1276,14 @@ impl<'tcx> TypeFoldable<'tcx> for ParamEnv<'tcx> {
         Ok(ParamEnv::new(
             self.caller_bounds().try_fold_with(folder)?,
             self.reveal().try_fold_with(folder)?,
+            self.constness().try_fold_with(folder)?,
         ))
     }
 
     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.caller_bounds().visit_with(visitor)?;
-        self.reveal().visit_with(visitor)
+        self.reveal().visit_with(visitor)?;
+        self.constness().visit_with(visitor)
     }
 }
 
@@ -1283,7 +1294,7 @@ impl<'tcx> ParamEnv<'tcx> {
     /// type-checking.
     #[inline]
     pub fn empty() -> Self {
-        Self::new(List::empty(), Reveal::UserFacing)
+        Self::new(List::empty(), Reveal::UserFacing, hir::Constness::NotConst)
     }
 
     #[inline]
@@ -1293,7 +1304,12 @@ impl<'tcx> ParamEnv<'tcx> {
 
     #[inline]
     pub fn reveal(self) -> traits::Reveal {
-        self.packed.tag()
+        self.packed.tag().reveal
+    }
+
+    #[inline]
+    pub fn constness(self) -> hir::Constness {
+        self.packed.tag().constness
     }
 
     /// Construct a trait environment with no where-clauses in scope
@@ -1305,20 +1321,47 @@ impl<'tcx> ParamEnv<'tcx> {
     /// or invoke `param_env.with_reveal_all()`.
     #[inline]
     pub fn reveal_all() -> Self {
-        Self::new(List::empty(), Reveal::All)
+        Self::new(List::empty(), Reveal::All, hir::Constness::NotConst)
     }
 
     /// Construct a trait environment with the given set of predicates.
     #[inline]
-    pub fn new(caller_bounds: &'tcx List<Predicate<'tcx>>, reveal: Reveal) -> Self {
+    pub fn new(
-        ty::ParamEnv { packed: CopyTaggedPtr::new(caller_bounds, reveal) }
+        caller_bounds: &'tcx List<Predicate<'tcx>>,
+        reveal: Reveal,
+        constness: hir::Constness,
+    ) -> Self {
+        ty::ParamEnv { packed: CopyTaggedPtr::new(caller_bounds, ParamTag { reveal, constness }) }
     }
 
     pub fn with_user_facing(mut self) -> Self {
-        self.packed.set_tag(Reveal::UserFacing);
+        self.packed.set_tag(ParamTag { reveal: Reveal::UserFacing, ..self.packed.tag() });
         self
     }
 
+    #[inline]
+    pub fn with_constness(mut self, constness: hir::Constness) -> Self {
+        self.packed.set_tag(ParamTag { constness, ..self.packed.tag() });
+        self
+    }
+
+    #[inline]
+    pub fn with_const(mut self) -> Self {
+        self.packed.set_tag(ParamTag { constness: hir::Constness::Const, ..self.packed.tag() });
+        self
+    }
+
+    #[inline]
+    pub fn without_const(mut self) -> Self {
+        self.packed.set_tag(ParamTag { constness: hir::Constness::NotConst, ..self.packed.tag() });
+        self
+    }
+
+    #[inline]
+    pub fn remap_constness_with(&mut self, mut constness: ty::BoundConstness) {
+        *self = self.with_constness(constness.and(self.constness()))
+    }
+
     /// Returns a new parameter environment with the same clauses, but
     /// which "reveals" the true results of projections in all cases
     /// (even for associated types that are specializable). This is
@@ -1329,17 +1372,21 @@ impl<'tcx> ParamEnv<'tcx> {
     /// will be normalized to their underlying types.
     /// See PR #65989 and issue #65918 for more details
     pub fn with_reveal_all_normalized(self, tcx: TyCtxt<'tcx>) -> Self {
-        if self.packed.tag() == traits::Reveal::All {
+        if self.packed.tag().reveal == traits::Reveal::All {
             return self;
         }
 
-        ParamEnv::new(tcx.normalize_opaque_types(self.caller_bounds()), Reveal::All)
+        ParamEnv::new(
+            tcx.normalize_opaque_types(self.caller_bounds()),
+            Reveal::All,
+            self.constness(),
+        )
     }
 
     /// Returns this same environment but with no caller bounds.
     #[inline]
     pub fn without_caller_bounds(self) -> Self {
-        Self::new(List::empty(), self.reveal())
+        Self::new(List::empty(), self.reveal(), self.constness())
     }
 
     /// Creates a suitable environment in which to perform trait
@@ -1369,33 +1416,23 @@ impl<'tcx> ParamEnv<'tcx> {
     }
 }
 
-#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TypeFoldable)]
-pub struct ConstnessAnd<T> {
-    pub constness: BoundConstness,
-    pub value: T,
-}
-
 // FIXME(ecstaticmorse): Audit all occurrences of `without_const().to_predicate(tcx)` to ensure that
 // the constness of trait bounds is being propagated correctly.
-pub trait WithConstness: Sized {
+impl PolyTraitRef<'tcx> {
     #[inline]
-    fn with_constness(self, constness: BoundConstness) -> ConstnessAnd<Self> {
+    pub fn with_constness(self, constness: BoundConstness) -> PolyTraitPredicate<'tcx> {
-        ConstnessAnd { constness, value: self }
+        self.map_bound(|trait_ref| ty::TraitPredicate {
+            trait_ref,
+            constness,
+            polarity: ty::ImplPolarity::Positive,
+        })
     }
-
     #[inline]
-    fn with_const_if_const(self) -> ConstnessAnd<Self> {
+    pub fn without_const(self) -> PolyTraitPredicate<'tcx> {
-        self.with_constness(BoundConstness::ConstIfConst)
-    }
-
-    #[inline]
-    fn without_const(self) -> ConstnessAnd<Self> {
         self.with_constness(BoundConstness::NotConst)
     }
 }
 
-impl<T> WithConstness for T {}
-
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TypeFoldable)]
 pub struct ParamEnvAnd<'tcx, T> {
     pub param_env: ParamEnv<'tcx>,
@@ -1406,6 +1443,12 @@ impl<'tcx, T> ParamEnvAnd<'tcx, T> {
     pub fn into_parts(self) -> (ParamEnv<'tcx>, T) {
         (self.param_env, self.value)
     }
+
+    #[inline]
+    pub fn without_const(mut self) -> Self {
+        self.param_env = self.param_env.without_const();
+        self
+    }
 }
 
 impl<'a, 'tcx, T> HashStable<StableHashingContext<'a>> for ParamEnvAnd<'tcx, T>

compiler/rustc_middle/src/ty/query.rs

@@ -156,6 +156,16 @@ macro_rules! separate_provide_extern_default {
     };
 }
 
+macro_rules! opt_remap_env_constness {
+    ([][$name:ident]) => {};
+    ([(remap_env_constness) $($rest:tt)*][$name:ident]) => {
+        let $name = $name.without_const();
+    };
+    ([$other:tt $($modifiers:tt)*][$name:ident]) => {
+        opt_remap_env_constness!([$($modifiers)*][$name])
+    };
+}
+
 macro_rules! define_callbacks {
     (<$tcx:tt>
      $($(#[$attr:meta])*
@@ -202,6 +212,8 @@ macro_rules! define_callbacks {
             #[inline(always)]
             pub fn $name(self, key: query_helper_param_ty!($($K)*)) {
                 let key = key.into_query_param();
+                opt_remap_env_constness!([$($modifiers)*][key]);
+
                 let cached = try_get_cached(self.tcx, &self.tcx.query_caches.$name, &key, noop);
 
                 let lookup = match cached {
@@ -229,6 +241,8 @@ macro_rules! define_callbacks {
             pub fn $name(self, key: query_helper_param_ty!($($K)*)) -> query_stored::$name<$tcx>
             {
                 let key = key.into_query_param();
+                opt_remap_env_constness!([$($modifiers)*][key]);
+
                 let cached = try_get_cached(self.tcx, &self.tcx.query_caches.$name, &key, Clone::clone);
 
                 let lookup = match cached {

compiler/rustc_middle/src/ty/relate.rs

@@ -218,19 +218,6 @@ impl<'tcx> Relate<'tcx> for ty::BoundConstness {
     }
 }
 
-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,

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -480,7 +480,7 @@ impl<'a, 'tcx> Lift<'tcx> for ty::ParamEnv<'a> {
     type Lifted = ty::ParamEnv<'tcx>;
     fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
         tcx.lift(self.caller_bounds())
-            .map(|caller_bounds| ty::ParamEnv::new(caller_bounds, self.reveal()))
+            .map(|caller_bounds| ty::ParamEnv::new(caller_bounds, self.reveal(), self.constness()))
     }
 }
 

compiler/rustc_middle/src/ty/sty.rs

@@ -8,9 +8,7 @@ use crate::infer::canonical::Canonical;
 use crate::ty::fold::ValidateBoundVars;
 use crate::ty::subst::{GenericArg, InternalSubsts, Subst, SubstsRef};
 use crate::ty::InferTy::{self, *};
-use crate::ty::{
+use crate::ty::{self, AdtDef, DefIdTree, Discr, Ty, TyCtxt, TypeFlags, TypeFoldable};
-    self, AdtDef, DefIdTree, Discr, Ty, TyCtxt, TypeFlags, TypeFoldable, WithConstness,
-};
 use crate::ty::{DelaySpanBugEmitted, List, ParamEnv, TyS};
 use polonius_engine::Atom;
 use rustc_data_structures::captures::Captures;

compiler/rustc_query_impl/src/plumbing.rs

@@ -231,6 +231,16 @@ macro_rules! get_provider {
     };
 }
 
+macro_rules! opt_remap_env_constness {
+    ([][$name:ident]) => {};
+    ([(remap_env_constness) $($rest:tt)*][$name:ident]) => {
+        let $name = $name.without_const();
+    };
+    ([$other:tt $($modifiers:tt)*][$name:ident]) => {
+        opt_remap_env_constness!([$($modifiers)*][$name])
+    };
+}
+
 macro_rules! define_queries {
     (<$tcx:tt>
      $($(#[$attr:meta])*
@@ -247,6 +257,7 @@ macro_rules! define_queries {
             // Create an eponymous constructor for each query.
             $(#[allow(nonstandard_style)] $(#[$attr])*
             pub fn $name<$tcx>(tcx: QueryCtxt<$tcx>, key: query_keys::$name<$tcx>) -> QueryStackFrame {
+                opt_remap_env_constness!([$($modifiers)*][key]);
                 let kind = dep_graph::DepKind::$name;
                 let name = stringify!($name);
                 // Disable visible paths printing for performance reasons.
@@ -521,6 +532,7 @@ macro_rules! define_queries_struct {
                 lookup: QueryLookup,
                 mode: QueryMode,
             ) -> Option<query_stored::$name<$tcx>> {
+                opt_remap_env_constness!([$($modifiers)*][key]);
                 let qcx = QueryCtxt { tcx, queries: self };
                 get_query::<queries::$name<$tcx>, _>(qcx, span, key, lookup, mode)
             })*

compiler/rustc_trait_selection/src/autoderef.rs

@@ -3,7 +3,7 @@ use crate::traits::{self, TraitEngine};
 use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_infer::infer::InferCtxt;
-use rustc_middle::ty::{self, TraitRef, Ty, TyCtxt, WithConstness};
+use rustc_middle::ty::{self, TraitRef, Ty, TyCtxt};
 use rustc_middle::ty::{ToPredicate, TypeFoldable};
 use rustc_session::{DiagnosticMessageId, Limit};
 use rustc_span::def_id::LOCAL_CRATE;

compiler/rustc_trait_selection/src/infer.rs

@@ -9,7 +9,6 @@ use rustc_middle::infer::canonical::{Canonical, CanonicalizedQueryResponse, Quer
 use rustc_middle::traits::query::Fallible;
 use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::ToPredicate;
-use rustc_middle::ty::WithConstness;
 use rustc_middle::ty::{self, Ty, TypeFoldable};
 use rustc_span::{Span, DUMMY_SP};
 

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -370,12 +370,17 @@ impl AutoTraitFinder<'tcx> {
                 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
             )
             .map(|o| o.predicate);
-            new_env = ty::ParamEnv::new(tcx.mk_predicates(normalized_preds), param_env.reveal());
+            new_env = ty::ParamEnv::new(
+                tcx.mk_predicates(normalized_preds),
+                param_env.reveal(),
+                param_env.constness(),
+            );
         }
 
         let final_user_env = ty::ParamEnv::new(
             tcx.mk_predicates(user_computed_preds.into_iter()),
             user_env.reveal(),
+            user_env.constness(),
         );
         debug!(
             "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \

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

@@ -24,7 +24,7 @@ use rustc_middle::ty::error::ExpectedFound;
 use rustc_middle::ty::fold::TypeFolder;
 use rustc_middle::ty::{
     self, fast_reject, AdtKind, SubtypePredicate, ToPolyTraitRef, ToPredicate, Ty, TyCtxt,
-    TypeFoldable, WithConstness,
+    TypeFoldable,
 };
 use rustc_session::DiagnosticMessageId;
 use rustc_span::symbol::{kw, sym};

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

@@ -21,7 +21,7 @@ use rustc_hir::lang_items::LangItem;
 use rustc_hir::{AsyncGeneratorKind, GeneratorKind, Node};
 use rustc_middle::ty::{
     self, suggest_arbitrary_trait_bound, suggest_constraining_type_param, AdtKind, DefIdTree,
-    Infer, InferTy, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
+    Infer, InferTy, ToPredicate, Ty, TyCtxt, TypeFoldable,
 };
 use rustc_middle::ty::{TypeAndMut, TypeckResults};
 use rustc_session::Limit;

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -4,7 +4,6 @@ use rustc_data_structures::obligation_forest::ProcessResult;
 use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome};
 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
 use rustc_errors::ErrorReported;
-use rustc_hir as hir;
 use rustc_infer::traits::{SelectionError, TraitEngine, TraitEngineExt as _, TraitObligation};
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::thir::abstract_const::NotConstEvaluatable;
@@ -231,21 +230,6 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
     }
 
-    fn select_all_with_constness_or_error(
-        &mut self,
-        infcx: &InferCtxt<'_, 'tcx>,
-        constness: rustc_hir::Constness,
-    ) -> Vec<FulfillmentError<'tcx>> {
-        {
-            let errors = self.select_with_constness_where_possible(infcx, constness);
-            if !errors.is_empty() {
-                return errors;
-            }
-        }
-
-        self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
-    }
-
     fn select_where_possible(
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
@@ -254,15 +238,6 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         self.select(&mut selcx)
     }
 
-    fn select_with_constness_where_possible(
-        &mut self,
-        infcx: &InferCtxt<'_, 'tcx>,
-        constness: hir::Constness,
-    ) -> Vec<FulfillmentError<'tcx>> {
-        let mut selcx = SelectionContext::with_constness(infcx, constness);
-        self.select(&mut selcx)
-    }
-
     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
         self.predicates.map_pending_obligations(|o| o.obligation.clone())
     }
@@ -679,12 +654,7 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
         if obligation.predicate.is_known_global() {
             // no type variables present, can use evaluation for better caching.
             // FIXME: consider caching errors too.
-            //
+            if infcx.predicate_must_hold_considering_regions(obligation) {
-            // If the predicate is considered const, then we cannot use this because
-            // it will cause false negatives in the ui tests.
-            if !self.selcx.is_predicate_const(obligation.predicate)
-                && infcx.predicate_must_hold_considering_regions(obligation)
-            {
                 debug!(
                     "selecting trait at depth {} evaluated to holds",
                     obligation.recursion_depth
@@ -738,12 +708,7 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
         if obligation.predicate.is_global(tcx) {
             // no type variables present, can use evaluation for better caching.
             // FIXME: consider caching errors too.
-            //
+            if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
-            // If the predicate is considered const, then we cannot use this because
-            // it will cause false negatives in the ui tests.
-            if !self.selcx.is_predicate_const(obligation.predicate)
-                && self.selcx.infcx().predicate_must_hold_considering_regions(obligation)
-            {
                 return ProcessResult::Changed(vec![]);
             } else {
                 tracing::debug!("Does NOT hold: {:?}", obligation);

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -33,8 +33,7 @@ use rustc_hir::lang_items::LangItem;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
 use rustc_middle::ty::{
-    self, GenericParamDefKind, ToPredicate, Ty, TyCtxt, VtblEntry, WithConstness,
+    self, GenericParamDefKind, ToPredicate, Ty, TyCtxt, VtblEntry, COMMON_VTABLE_ENTRIES,
-    COMMON_VTABLE_ENTRIES,
 };
 use rustc_span::{sym, Span};
 use smallvec::SmallVec;
@@ -307,8 +306,11 @@ pub fn normalize_param_env_or_error<'tcx>(
 
     debug!("normalize_param_env_or_error: elaborated-predicates={:?}", predicates);
 
-    let elaborated_env =
+    let elaborated_env = ty::ParamEnv::new(
-        ty::ParamEnv::new(tcx.intern_predicates(&predicates), unnormalized_env.reveal());
+        tcx.intern_predicates(&predicates),
+        unnormalized_env.reveal(),
+        unnormalized_env.constness(),
+    );
 
     // HACK: we are trying to normalize the param-env inside *itself*. The problem is that
     // normalization expects its param-env to be already normalized, which means we have
@@ -360,8 +362,11 @@ pub fn normalize_param_env_or_error<'tcx>(
     // predicates here anyway. Keeping them here anyway because it seems safer.
     let outlives_env: Vec<_> =
         non_outlives_predicates.iter().chain(&outlives_predicates).cloned().collect();
-    let outlives_env =
+    let outlives_env = ty::ParamEnv::new(
-        ty::ParamEnv::new(tcx.intern_predicates(&outlives_env), unnormalized_env.reveal());
+        tcx.intern_predicates(&outlives_env),
+        unnormalized_env.reveal(),
+        unnormalized_env.constness(),
+    );
     let outlives_predicates = match do_normalize_predicates(
         tcx,
         region_context,
@@ -381,7 +386,11 @@ pub fn normalize_param_env_or_error<'tcx>(
     let mut predicates = non_outlives_predicates;
     predicates.extend(outlives_predicates);
     debug!("normalize_param_env_or_error: final predicates={:?}", predicates);
-    ty::ParamEnv::new(tcx.intern_predicates(&predicates), unnormalized_env.reveal())
+    ty::ParamEnv::new(
+        tcx.intern_predicates(&predicates),
+        unnormalized_env.reveal(),
+        unnormalized_env.constness(),
+    )
 }
 
 pub fn fully_normalize<'a, 'tcx, T>(
@@ -564,14 +573,17 @@ fn prepare_vtable_segments<'tcx, T>(
                     .predicates
                     .into_iter()
                     .filter_map(move |(pred, _)| {
-                        pred.subst_supertrait(tcx, &inner_most_trait_ref).to_opt_poly_trait_ref()
+                        pred.subst_supertrait(tcx, &inner_most_trait_ref).to_opt_poly_trait_pred()
                     });
 
                 'diving_in_skip_visited_traits: loop {
                     if let Some(next_super_trait) = direct_super_traits_iter.next() {
                         if visited.insert(next_super_trait.to_predicate(tcx)) {
+                            // We're throwing away potential constness of super traits here.
+                            // FIXME: handle ~const super traits
+                            let next_super_trait = next_super_trait.map_bound(|t| t.trait_ref);
                             stack.push((
-                                next_super_trait.value,
+                                next_super_trait,
                                 emit_vptr_on_new_entry,
                                 Some(direct_super_traits_iter),
                             ));
@@ -603,7 +615,11 @@ fn prepare_vtable_segments<'tcx, T>(
                     if let Some(siblings) = siblings_opt {
                         if let Some(next_inner_most_trait_ref) = siblings.next() {
                             if visited.insert(next_inner_most_trait_ref.to_predicate(tcx)) {
-                                *inner_most_trait_ref = next_inner_most_trait_ref.value;
+                                // We're throwing away potential constness of super traits here.
+                                // FIXME: handle ~const super traits
+                                let next_inner_most_trait_ref =
+                                    next_inner_most_trait_ref.map_bound(|t| t.trait_ref);
+                                *inner_most_trait_ref = next_inner_most_trait_ref;
                                 *emit_vptr = emit_vptr_on_new_entry;
                                 break 'exiting_out;
                             } else {

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -18,7 +18,7 @@ use rustc_errors::FatalError;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::subst::{GenericArg, InternalSubsts, Subst};
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeVisitor, WithConstness};
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeVisitor};
 use rustc_middle::ty::{Predicate, ToPredicate};
 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
 use rustc_span::symbol::Symbol;
@@ -694,7 +694,11 @@ fn receiver_is_dispatchable<'tcx>(
         let caller_bounds: Vec<Predicate<'tcx>> =
             param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]).collect();
 
-        ty::ParamEnv::new(tcx.intern_predicates(&caller_bounds), param_env.reveal())
+        ty::ParamEnv::new(
+            tcx.intern_predicates(&caller_bounds),
+            param_env.reveal(),
+            param_env.constness(),
+        )
     };
 
     // Receiver: DispatchFromDyn<Receiver[Self => U]>

compiler/rustc_trait_selection/src/traits/project.rs

@@ -28,7 +28,7 @@ use rustc_hir::lang_items::LangItem;
 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
 use rustc_middle::ty::subst::Subst;
-use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, WithConstness};
+use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt};
 use rustc_span::symbol::sym;
 
 use std::collections::BTreeMap;

compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs

@@ -1,3 +1,5 @@
+use rustc_middle::ty;
+
 use crate::infer::canonical::OriginalQueryValues;
 use crate::infer::InferCtxt;
 use crate::traits::{
@@ -64,10 +66,21 @@ impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
         obligation: &PredicateObligation<'tcx>,
     ) -> Result<EvaluationResult, OverflowError> {
         let mut _orig_values = OriginalQueryValues::default();
-        let c_pred = self.canonicalize_query_keep_static(
+
-            obligation.param_env.and(obligation.predicate),
+        let param_env = match obligation.predicate.kind().skip_binder() {
-            &mut _orig_values,
+            ty::PredicateKind::Trait(pred) => {
-        );
+                // we ignore the value set to it.
+                let mut _constness = pred.constness;
+                obligation
+                    .param_env
+                    .with_constness(_constness.and(obligation.param_env.constness()))
+            }
+            // constness has no effect on the given predicate.
+            _ => obligation.param_env.without_const(),
+        };
+
+        let c_pred = self
+            .canonicalize_query_keep_static(param_env.and(obligation.predicate), &mut _orig_values);
         // Run canonical query. If overflow occurs, rerun from scratch but this time
         // in standard trait query mode so that overflow is handled appropriately
         // within `SelectionContext`.

compiler/rustc_trait_selection/src/traits/query/type_op/prove_predicate.rs

@@ -30,8 +30,14 @@ impl<'tcx> super::QueryTypeOp<'tcx> for ProvePredicate<'tcx> {
 
     fn perform_query(
         tcx: TyCtxt<'tcx>,
-        canonicalized: Canonicalized<'tcx, ParamEnvAnd<'tcx, Self>>,
+        mut canonicalized: Canonicalized<'tcx, ParamEnvAnd<'tcx, Self>>,
     ) -> Fallible<CanonicalizedQueryResponse<'tcx, ()>> {
+        match canonicalized.value.value.predicate.kind().skip_binder() {
+            ty::PredicateKind::Trait(pred) => {
+                canonicalized.value.param_env.remap_constness_with(pred.constness);
+            }
+            _ => canonicalized.value.param_env = canonicalized.value.param_env.without_const(),
+        }
         tcx.type_op_prove_predicate(canonicalized)
     }
 }

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -11,7 +11,7 @@ use rustc_infer::traits::TraitEngine;
 use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
 use rustc_lint_defs::builtin::DEREF_INTO_DYN_SUPERTRAIT;
 use rustc_middle::ty::print::with_no_trimmed_paths;
-use rustc_middle::ty::{self, ToPredicate, Ty, TypeFoldable, WithConstness};
+use rustc_middle::ty::{self, ToPredicate, Ty, TypeFoldable};
 use rustc_target::spec::abi::Abi;
 
 use crate::traits;
@@ -303,7 +303,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             } else if lang_items.drop_trait() == Some(def_id)
                 && obligation.predicate.skip_binder().constness == ty::BoundConstness::ConstIfConst
             {
-                if self.is_in_const_context {
+                if obligation.param_env.constness() == hir::Constness::Const {
                     self.assemble_const_drop_candidates(obligation, stack, &mut candidates)?;
                 } else {
                     debug!("passing ~const Drop bound; in non-const context");
@@ -381,17 +381,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             .param_env
             .caller_bounds()
             .iter()
-            .filter_map(|o| o.to_opt_poly_trait_ref());
+            .filter_map(|o| o.to_opt_poly_trait_pred());
 
         // Micro-optimization: filter out predicates relating to different traits.
         let matching_bounds =
-            all_bounds.filter(|p| p.value.def_id() == stack.obligation.predicate.def_id());
+            all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
 
         // Keep only those bounds which may apply, and propagate overflow if it occurs.
         for bound in matching_bounds {
-            let wc = self.evaluate_where_clause(stack, bound.value)?;
+            // FIXME(oli-obk): it is suspicious that we are dropping the constness and
+            // polarity here.
+            let wc = self.evaluate_where_clause(stack, bound.map_bound(|t| t.trait_ref))?;
             if wc.may_apply() {
-                candidates.vec.push(ParamCandidate((bound, stack.obligation.polarity())));
+                candidates.vec.push(ParamCandidate(bound));
             }
         }
 

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -13,7 +13,7 @@ use rustc_infer::infer::InferOk;
 use rustc_infer::infer::LateBoundRegionConversionTime::HigherRankedType;
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, Subst, SubstsRef};
 use rustc_middle::ty::{self, Ty};
-use rustc_middle::ty::{ToPolyTraitRef, ToPredicate, WithConstness};
+use rustc_middle::ty::{ToPolyTraitRef, ToPredicate};
 use rustc_span::def_id::DefId;
 
 use crate::traits::project::{normalize_with_depth, normalize_with_depth_to};
@@ -58,8 +58,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
 
             ParamCandidate(param) => {
-                let obligations = self.confirm_param_candidate(obligation, param.0.value);
+                let obligations =
-                Ok(ImplSource::Param(obligations, param.0.constness))
+                    self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
+                Ok(ImplSource::Param(obligations, param.skip_binder().constness))
             }
 
             ImplCandidate(impl_def_id) => {
@@ -139,7 +140,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
             let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
             let placeholder_trait_predicate =
-                self.infcx().replace_bound_vars_with_placeholders(trait_predicate);
+                self.infcx().replace_bound_vars_with_placeholders(trait_predicate).trait_ref;
             let placeholder_self_ty = placeholder_trait_predicate.self_ty();
             let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
             let (def_id, substs) = match *placeholder_self_ty.kind() {
@@ -150,8 +151,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
             let candidate_predicate = tcx.item_bounds(def_id)[idx].subst(tcx, substs);
             let candidate = candidate_predicate
-                .to_opt_poly_trait_ref()
+                .to_opt_poly_trait_pred()
-                .expect("projection candidate is not a trait predicate");
+                .expect("projection candidate is not a trait predicate")
+                .map_bound(|t| t.trait_ref);
             let mut obligations = Vec::new();
             let candidate = normalize_with_depth_to(
                 self,
@@ -165,7 +167,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             obligations.extend(self.infcx.commit_if_ok(|_| {
                 self.infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .sup(placeholder_trait_predicate.to_poly_trait_ref(), candidate.value)
+                    .sup(placeholder_trait_predicate, candidate)
                     .map(|InferOk { obligations, .. }| obligations)
                     .map_err(|_| Unimplemented)
             })?);

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -39,7 +39,6 @@ use rustc_middle::ty::fast_reject;
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::relate::TypeRelation;
 use rustc_middle::ty::subst::{GenericArgKind, Subst, SubstsRef};
-use rustc_middle::ty::WithConstness;
 use rustc_middle::ty::{self, PolyProjectionPredicate, ToPolyTraitRef, ToPredicate};
 use rustc_middle::ty::{Ty, TyCtxt, TypeFoldable};
 use rustc_span::symbol::sym;
@@ -128,9 +127,6 @@ pub struct SelectionContext<'cx, 'tcx> {
     /// and a negative impl
     allow_negative_impls: bool,
 
-    /// Are we in a const context that needs `~const` bounds to be const?
-    is_in_const_context: bool,
-
     /// The mode that trait queries run in, which informs our error handling
     /// policy. In essence, canonicalized queries need their errors propagated
     /// rather than immediately reported because we do not have accurate spans.
@@ -141,9 +137,9 @@ pub struct SelectionContext<'cx, 'tcx> {
 struct TraitObligationStack<'prev, 'tcx> {
     obligation: &'prev TraitObligation<'tcx>,
 
-    /// The trait ref from `obligation` but "freshened" with the
+    /// The trait predicate from `obligation` but "freshened" with the
     /// selection-context's freshener. Used to check for recursion.
-    fresh_trait_ref: ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>,
+    fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
 
     /// Starts out equal to `depth` -- if, during evaluation, we
     /// encounter a cycle, then we will set this flag to the minimum
@@ -222,7 +218,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate: false,
             intercrate_ambiguity_causes: None,
             allow_negative_impls: false,
-            is_in_const_context: false,
             query_mode: TraitQueryMode::Standard,
         }
     }
@@ -234,7 +229,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate: true,
             intercrate_ambiguity_causes: None,
             allow_negative_impls: false,
-            is_in_const_context: false,
             query_mode: TraitQueryMode::Standard,
         }
     }
@@ -250,7 +244,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate: false,
             intercrate_ambiguity_causes: None,
             allow_negative_impls,
-            is_in_const_context: false,
             query_mode: TraitQueryMode::Standard,
         }
     }
@@ -266,26 +259,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             intercrate: false,
             intercrate_ambiguity_causes: None,
             allow_negative_impls: false,
-            is_in_const_context: false,
             query_mode,
         }
     }
 
-    pub fn with_constness(
-        infcx: &'cx InferCtxt<'cx, 'tcx>,
-        constness: hir::Constness,
-    ) -> SelectionContext<'cx, 'tcx> {
-        SelectionContext {
-            infcx,
-            freshener: infcx.freshener_keep_static(),
-            intercrate: false,
-            intercrate_ambiguity_causes: None,
-            allow_negative_impls: false,
-            is_in_const_context: matches!(constness, hir::Constness::Const),
-            query_mode: TraitQueryMode::Standard,
-        }
-    }
-
     /// Enables tracking of intercrate ambiguity causes. These are
     /// used in coherence to give improved diagnostics. We don't do
     /// this until we detect a coherence error because it can lead to
@@ -318,20 +295,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         self.intercrate
     }
 
-    /// Returns `true` if the trait predicate is considerd `const` to this selection context.
-    pub fn is_trait_predicate_const(&self, pred: ty::TraitPredicate<'_>) -> bool {
-        matches!(pred.constness, ty::BoundConstness::ConstIfConst) && self.is_in_const_context
-    }
-
-    /// Returns `true` if the predicate is considered `const` to
-    /// this selection context.
-    pub fn is_predicate_const(&self, pred: ty::Predicate<'_>) -> bool {
-        match pred.kind().skip_binder() {
-            ty::PredicateKind::Trait(pred) => self.is_trait_predicate_const(pred),
-            _ => false,
-        }
-    }
-
     ///////////////////////////////////////////////////////////////////////////
     // Selection
     //
@@ -716,20 +679,22 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
 
         let stack = self.push_stack(previous_stack, &obligation);
-        let fresh_trait_ref = stack.fresh_trait_ref;
+        let mut fresh_trait_pred = stack.fresh_trait_pred;
+        let mut param_env = obligation.param_env;
 
-        debug!(?fresh_trait_ref);
+        fresh_trait_pred = fresh_trait_pred.map_bound(|mut pred| {
+            param_env = param_env.with_constness(pred.constness.and(param_env.constness()));
+            pred
+        });
 
-        if let Some(result) = self.check_evaluation_cache(
+        debug!(?fresh_trait_pred);
-            obligation.param_env,
+
-            fresh_trait_ref,
+        if let Some(result) = self.check_evaluation_cache(param_env, fresh_trait_pred) {
-            obligation.polarity(),
-        ) {
             debug!(?result, "CACHE HIT");
             return Ok(result);
         }
 
-        if let Some(result) = stack.cache().get_provisional(fresh_trait_ref) {
+        if let Some(result) = stack.cache().get_provisional(fresh_trait_pred) {
             debug!(?result, "PROVISIONAL CACHE HIT");
             stack.update_reached_depth(result.reached_depth);
             return Ok(result.result);
@@ -754,19 +719,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let reached_depth = stack.reached_depth.get();
         if reached_depth >= stack.depth {
             debug!(?result, "CACHE MISS");
-            self.insert_evaluation_cache(
+            self.insert_evaluation_cache(param_env, fresh_trait_pred, dep_node, result);
-                obligation.param_env,
-                fresh_trait_ref,
-                obligation.polarity(),
-                dep_node,
-                result,
-            );
 
-            stack.cache().on_completion(stack.dfn, |fresh_trait_ref, provisional_result| {
+            stack.cache().on_completion(stack.dfn, |fresh_trait_pred, provisional_result| {
                 self.insert_evaluation_cache(
-                    obligation.param_env,
+                    param_env,
-                    fresh_trait_ref,
+                    fresh_trait_pred,
-                    obligation.polarity(),
                     dep_node,
                     provisional_result.max(result),
                 );
@@ -776,10 +734,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             debug!(
                 "caching provisionally because {:?} \
                  is a cycle participant (at depth {}, reached depth {})",
-                fresh_trait_ref, stack.depth, reached_depth,
+                fresh_trait_pred, stack.depth, reached_depth,
             );
 
-            stack.cache().insert_provisional(stack.dfn, reached_depth, fresh_trait_ref, result);
+            stack.cache().insert_provisional(stack.dfn, reached_depth, fresh_trait_pred, result);
         }
 
         Ok(result)
@@ -813,7 +771,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             .skip(1) // Skip top-most frame.
             .find(|prev| {
                 stack.obligation.param_env == prev.obligation.param_env
-                    && stack.fresh_trait_ref == prev.fresh_trait_ref
+                    && stack.fresh_trait_pred == prev.fresh_trait_pred
             })
             .map(|stack| stack.depth)
         {
@@ -876,7 +834,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // terms of `Fn` etc, but we could probably make this more
         // precise still.
         let unbound_input_types =
-            stack.fresh_trait_ref.value.skip_binder().substs.types().any(|ty| ty.is_fresh());
+            stack.fresh_trait_pred.skip_binder().trait_ref.substs.types().any(|ty| ty.is_fresh());
 
         if stack.obligation.polarity() != ty::ImplPolarity::Negative {
             // This check was an imperfect workaround for a bug in the old
@@ -914,8 +872,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             && stack.iter().skip(1).any(|prev| {
                 stack.obligation.param_env == prev.obligation.param_env
                     && self.match_fresh_trait_refs(
-                        stack.fresh_trait_ref,
+                        stack.fresh_trait_pred,
-                        prev.fresh_trait_ref,
+                        prev.fresh_trait_pred,
                         prev.obligation.param_env,
                     )
             })
@@ -993,7 +951,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // not just the lifetime choice for this particular (non-erased)
         // predicate.
         // See issue #80691
-        if stack.fresh_trait_ref.has_erased_regions() {
+        if stack.fresh_trait_pred.has_erased_regions() {
             result = result.max(EvaluatedToOkModuloRegions);
         }
 
@@ -1004,8 +962,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn check_evaluation_cache(
         &self,
         param_env: ty::ParamEnv<'tcx>,
-        trait_ref: ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
-        polarity: ty::ImplPolarity,
     ) -> Option<EvaluationResult> {
         // Neither the global nor local cache is aware of intercrate
         // mode, so don't do any caching. In particular, we might
@@ -1017,19 +974,17 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         let tcx = self.tcx();
         if self.can_use_global_caches(param_env) {
-            if let Some(res) = tcx.evaluation_cache.get(&(param_env.and(trait_ref), polarity), tcx)
+            if let Some(res) = tcx.evaluation_cache.get(&param_env.and(trait_pred), tcx) {
-            {
                 return Some(res);
             }
         }
-        self.infcx.evaluation_cache.get(&(param_env.and(trait_ref), polarity), tcx)
+        self.infcx.evaluation_cache.get(&param_env.and(trait_pred), tcx)
     }
 
     fn insert_evaluation_cache(
         &mut self,
         param_env: ty::ParamEnv<'tcx>,
-        trait_ref: ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
-        polarity: ty::ImplPolarity,
         dep_node: DepNodeIndex,
         result: EvaluationResult,
     ) {
@@ -1048,23 +1003,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
 
         if self.can_use_global_caches(param_env) {
-            if !trait_ref.needs_infer() {
+            if !trait_pred.needs_infer() {
-                debug!(?trait_ref, ?result, "insert_evaluation_cache global");
+                debug!(?trait_pred, ?result, "insert_evaluation_cache global");
                 // This may overwrite the cache with the same value
                 // FIXME: Due to #50507 this overwrites the different values
                 // This should be changed to use HashMapExt::insert_same
                 // when that is fixed
-                self.tcx().evaluation_cache.insert(
+                self.tcx().evaluation_cache.insert(param_env.and(trait_pred), dep_node, result);
-                    (param_env.and(trait_ref), polarity),
-                    dep_node,
-                    result,
-                );
                 return;
             }
         }
 
-        debug!(?trait_ref, ?result, "insert_evaluation_cache");
+        debug!(?trait_pred, ?result, "insert_evaluation_cache");
-        self.infcx.evaluation_cache.insert((param_env.and(trait_ref), polarity), dep_node, result);
+        self.infcx.evaluation_cache.insert(param_env.and(trait_pred), dep_node, result);
     }
 
     /// For various reasons, it's possible for a subobligation
@@ -1142,16 +1093,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         for candidate in candidates {
             // Respect const trait obligations
-            if self.is_trait_predicate_const(obligation.predicate.skip_binder()) {
+            if obligation.is_const() {
                 match candidate {
                     // const impl
                     ImplCandidate(def_id)
                         if tcx.impl_constness(def_id) == hir::Constness::Const => {}
                     // const param
-                    ParamCandidate((
+                    ParamCandidate(trait_pred)
-                        ty::ConstnessAnd { constness: ty::BoundConstness::ConstIfConst, .. },
+                        if trait_pred.skip_binder().constness
-                        _,
+                            == ty::BoundConstness::ConstIfConst => {}
-                    )) => {}
                     // auto trait impl
                     AutoImplCandidate(..) => {}
                     // generator, this will raise error in other places
@@ -1260,7 +1210,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
     fn check_candidate_cache(
         &mut self,
-        param_env: ty::ParamEnv<'tcx>,
+        mut param_env: ty::ParamEnv<'tcx>,
         cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> {
         // Neither the global nor local cache is aware of intercrate
@@ -1271,19 +1221,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return None;
         }
         let tcx = self.tcx();
-        let pred = &cache_fresh_trait_pred.skip_binder();
+        let mut pred = cache_fresh_trait_pred.skip_binder();
-        let trait_ref = pred.trait_ref;
+        param_env = param_env.with_constness(pred.constness.and(param_env.constness()));
+
         if self.can_use_global_caches(param_env) {
-            if let Some(res) = tcx
+            if let Some(res) = tcx.selection_cache.get(&param_env.and(pred), tcx) {
-                .selection_cache
-                .get(&(param_env.and(trait_ref).with_constness(pred.constness), pred.polarity), tcx)
-            {
                 return Some(res);
             }
         }
-        self.infcx
+        self.infcx.selection_cache.get(&param_env.and(pred), tcx)
-            .selection_cache
-            .get(&(param_env.and(trait_ref).with_constness(pred.constness), pred.polarity), tcx)
     }
 
     /// Determines whether can we safely cache the result
@@ -1321,43 +1267,36 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
     fn insert_candidate_cache(
         &mut self,
-        param_env: ty::ParamEnv<'tcx>,
+        mut param_env: ty::ParamEnv<'tcx>,
         cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
         dep_node: DepNodeIndex,
         candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>,
     ) {
         let tcx = self.tcx();
-        let pred = cache_fresh_trait_pred.skip_binder();
+        let mut pred = cache_fresh_trait_pred.skip_binder();
-        let trait_ref = pred.trait_ref;
+
+        param_env = param_env.with_constness(pred.constness.and(param_env.constness()));
 
         if !self.can_cache_candidate(&candidate) {
-            debug!(?trait_ref, ?candidate, "insert_candidate_cache - candidate is not cacheable");
+            debug!(?pred, ?candidate, "insert_candidate_cache - candidate is not cacheable");
             return;
         }
 
         if self.can_use_global_caches(param_env) {
             if let Err(Overflow) = candidate {
                 // Don't cache overflow globally; we only produce this in certain modes.
-            } else if !trait_ref.needs_infer() {
+            } else if !pred.needs_infer() {
                 if !candidate.needs_infer() {
-                    debug!(?trait_ref, ?candidate, "insert_candidate_cache global");
+                    debug!(?pred, ?candidate, "insert_candidate_cache global");
                     // This may overwrite the cache with the same value.
-                    tcx.selection_cache.insert(
+                    tcx.selection_cache.insert(param_env.and(pred), dep_node, candidate);
-                        (param_env.and(trait_ref).with_constness(pred.constness), pred.polarity),
-                        dep_node,
-                        candidate,
-                    );
                     return;
                 }
             }
         }
 
-        debug!(?trait_ref, ?candidate, "insert_candidate_cache local");
+        debug!(?pred, ?candidate, "insert_candidate_cache local");
-        self.infcx.selection_cache.insert(
+        self.infcx.selection_cache.insert(param_env.and(pred), dep_node, candidate);
-            (param_env.and(trait_ref).with_constness(pred.constness), pred.polarity),
-            dep_node,
-            candidate,
-        );
     }
 
     /// Matches a predicate against the bounds of its self type.
@@ -1548,7 +1487,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // Check if a bound would previously have been removed when normalizing
         // the param_env so that it can be given the lowest priority. See
         // #50825 for the motivation for this.
-        let is_global = |cand: &ty::PolyTraitRef<'tcx>| {
+        let is_global = |cand: &ty::PolyTraitPredicate<'tcx>| {
             cand.is_global(self.infcx.tcx) && !cand.has_late_bound_regions()
         };
 
@@ -1581,25 +1520,22 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | ConstDropCandidate,
             ) => false,
 
-            (
+            (ParamCandidate(other), ParamCandidate(victim)) => {
-                ParamCandidate((other, other_polarity)),
+                let same_except_bound_vars = other.skip_binder().trait_ref
-                ParamCandidate((victim, victim_polarity)),
+                    == victim.skip_binder().trait_ref
-            ) => {
+                    && other.skip_binder().constness == victim.skip_binder().constness
-                let same_except_bound_vars = other.value.skip_binder()
+                    && other.skip_binder().polarity == victim.skip_binder().polarity
-                    == victim.value.skip_binder()
+                    && !other.skip_binder().trait_ref.has_escaping_bound_vars();
-                    && other.constness == victim.constness
-                    && other_polarity == victim_polarity
-                    && !other.value.skip_binder().has_escaping_bound_vars();
                 if same_except_bound_vars {
                     // See issue #84398. In short, we can generate multiple ParamCandidates which are
                     // the same except for unused bound vars. Just pick the one with the fewest bound vars
                     // or the current one if tied (they should both evaluate to the same answer). This is
                     // probably best characterized as a "hack", since we might prefer to just do our
                     // best to *not* create essentially duplicate candidates in the first place.
-                    other.value.bound_vars().len() <= victim.value.bound_vars().len()
+                    other.bound_vars().len() <= victim.bound_vars().len()
-                } else if other.value == victim.value
+                } else if other.skip_binder().trait_ref == victim.skip_binder().trait_ref
-                    && victim.constness == ty::BoundConstness::NotConst
+                    && victim.skip_binder().constness == ty::BoundConstness::NotConst
-                    && other_polarity == victim_polarity
+                    && other.skip_binder().polarity == victim.skip_binder().polarity
                 {
                     // Drop otherwise equivalent non-const candidates in favor of const candidates.
                     true
@@ -1629,11 +1565,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | TraitAliasCandidate(..)
                 | ObjectCandidate(_)
                 | ProjectionCandidate(_),
-            ) => !is_global(&cand.0.value),
+            ) => !is_global(cand),
             (ObjectCandidate(_) | ProjectionCandidate(_), ParamCandidate(ref cand)) => {
                 // Prefer these to a global where-clause bound
                 // (see issue #50825).
-                is_global(&cand.0.value)
+                is_global(cand)
             }
             (
                 ImplCandidate(_)
@@ -1649,7 +1585,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             ) => {
                 // Prefer these to a global where-clause bound
                 // (see issue #50825).
-                is_global(&cand.0.value) && other.evaluation.must_apply_modulo_regions()
+                is_global(cand) && other.evaluation.must_apply_modulo_regions()
             }
 
             (ProjectionCandidate(i), ProjectionCandidate(j))
@@ -2209,8 +2145,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
     fn match_fresh_trait_refs(
         &self,
-        previous: ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>,
+        previous: ty::PolyTraitPredicate<'tcx>,
-        current: ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>,
+        current: ty::PolyTraitPredicate<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
     ) -> bool {
         let mut matcher = ty::_match::Match::new(self.tcx(), param_env);
@@ -2222,17 +2158,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         previous_stack: TraitObligationStackList<'o, 'tcx>,
         obligation: &'o TraitObligation<'tcx>,
     ) -> TraitObligationStack<'o, 'tcx> {
-        let fresh_trait_ref = obligation
+        let fresh_trait_pred = obligation.predicate.fold_with(&mut self.freshener);
-            .predicate
-            .to_poly_trait_ref()
-            .fold_with(&mut self.freshener)
-            .with_constness(obligation.predicate.skip_binder().constness);
 
         let dfn = previous_stack.cache.next_dfn();
         let depth = previous_stack.depth() + 1;
         TraitObligationStack {
             obligation,
-            fresh_trait_ref,
+            fresh_trait_pred,
             reached_depth: Cell::new(depth),
             previous: previous_stack,
             dfn,
@@ -2426,7 +2358,7 @@ impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
         debug!(reached_depth, "update_reached_depth");
         let mut p = self;
         while reached_depth < p.depth {
-            debug!(?p.fresh_trait_ref, "update_reached_depth: marking as cycle participant");
+            debug!(?p.fresh_trait_pred, "update_reached_depth: marking as cycle participant");
             p.reached_depth.set(p.reached_depth.get().min(reached_depth));
             p = p.previous.head.unwrap();
         }
@@ -2505,7 +2437,7 @@ struct ProvisionalEvaluationCache<'tcx> {
     /// - then we determine that `E` is in error -- we will then clear
     ///   all cache values whose DFN is >= 4 -- in this case, that
     ///   means the cached value for `F`.
-    map: RefCell<FxHashMap<ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>, ProvisionalEvaluation>>,
+    map: RefCell<FxHashMap<ty::PolyTraitPredicate<'tcx>, ProvisionalEvaluation>>,
 }
 
 /// A cache value for the provisional cache: contains the depth-first
@@ -2537,28 +2469,28 @@ impl<'tcx> ProvisionalEvaluationCache<'tcx> {
     /// `reached_depth` (from the returned value).
     fn get_provisional(
         &self,
-        fresh_trait_ref: ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>,
+        fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) -> Option<ProvisionalEvaluation> {
         debug!(
-            ?fresh_trait_ref,
+            ?fresh_trait_pred,
             "get_provisional = {:#?}",
-            self.map.borrow().get(&fresh_trait_ref),
+            self.map.borrow().get(&fresh_trait_pred),
         );
-        Some(*self.map.borrow().get(&fresh_trait_ref)?)
+        Some(*self.map.borrow().get(&fresh_trait_pred)?)
     }
 
     /// Insert a provisional result into the cache. The result came
     /// from the node with the given DFN. It accessed a minimum depth
-    /// of `reached_depth` to compute. It evaluated `fresh_trait_ref`
+    /// of `reached_depth` to compute. It evaluated `fresh_trait_pred`
     /// and resulted in `result`.
     fn insert_provisional(
         &self,
         from_dfn: usize,
         reached_depth: usize,
-        fresh_trait_ref: ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>,
+        fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
         result: EvaluationResult,
     ) {
-        debug!(?from_dfn, ?fresh_trait_ref, ?result, "insert_provisional");
+        debug!(?from_dfn, ?fresh_trait_pred, ?result, "insert_provisional");
 
         let mut map = self.map.borrow_mut();
 
@@ -2582,7 +2514,7 @@ impl<'tcx> ProvisionalEvaluationCache<'tcx> {
             }
         }
 
-        map.insert(fresh_trait_ref, ProvisionalEvaluation { from_dfn, reached_depth, result });
+        map.insert(fresh_trait_pred, ProvisionalEvaluation { from_dfn, reached_depth, result });
     }
 
     /// Invoked when the node with dfn `dfn` does not get a successful
@@ -2633,16 +2565,16 @@ impl<'tcx> ProvisionalEvaluationCache<'tcx> {
     fn on_completion(
         &self,
         dfn: usize,
-        mut op: impl FnMut(ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>, EvaluationResult),
+        mut op: impl FnMut(ty::PolyTraitPredicate<'tcx>, EvaluationResult),
     ) {
         debug!(?dfn, "on_completion");
 
-        for (fresh_trait_ref, eval) in
+        for (fresh_trait_pred, eval) in
             self.map.borrow_mut().drain_filter(|_k, eval| eval.from_dfn >= dfn)
         {
-            debug!(?fresh_trait_ref, ?eval, "on_completion");
+            debug!(?fresh_trait_pred, ?eval, "on_completion");
 
-            op(fresh_trait_ref, eval.result);
+            op(fresh_trait_pred, eval.result);
         }
     }
 }

compiler/rustc_trait_selection/src/traits/specialize/mod.rs

@@ -508,9 +508,9 @@ crate fn to_pretty_impl_header(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Option<St
         Vec::with_capacity(predicates.len() + types_without_default_bounds.len());
 
     for (p, _) in predicates {
-        if let Some(poly_trait_ref) = p.to_opt_poly_trait_ref() {
+        if let Some(poly_trait_ref) = p.to_opt_poly_trait_pred() {
-            if Some(poly_trait_ref.value.def_id()) == sized_trait {
+            if Some(poly_trait_ref.def_id()) == sized_trait {
-                types_without_default_bounds.remove(poly_trait_ref.value.self_ty().skip_binder());
+                types_without_default_bounds.remove(poly_trait_ref.self_ty().skip_binder());
                 continue;
             }
         }

compiler/rustc_trait_selection/src/traits/util.rs

@@ -6,7 +6,7 @@ use smallvec::SmallVec;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::subst::{GenericArg, Subst, SubstsRef};
-use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
+use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
 
 use super::{Normalized, Obligation, ObligationCause, PredicateObligation, SelectionContext};
 pub use rustc_infer::traits::{self, util::*};
@@ -126,8 +126,8 @@ impl<'tcx> TraitAliasExpander<'tcx> {
 
         let items = predicates.predicates.iter().rev().filter_map(|(pred, span)| {
             pred.subst_supertrait(tcx, &trait_ref)
-                .to_opt_poly_trait_ref()
+                .to_opt_poly_trait_pred()
-                .map(|trait_ref| item.clone_and_push(trait_ref.value, *span))
+                .map(|trait_ref| item.clone_and_push(trait_ref.map_bound(|t| t.trait_ref), *span))
         });
         debug!("expand_trait_aliases: items={:?}", items.clone());
 
@@ -183,8 +183,8 @@ impl Iterator for SupertraitDefIds<'tcx> {
             predicates
                 .predicates
                 .iter()
-                .filter_map(|(pred, _)| pred.to_opt_poly_trait_ref())
+                .filter_map(|(pred, _)| pred.to_opt_poly_trait_pred())
-                .map(|trait_ref| trait_ref.value.def_id())
+                .map(|trait_ref| trait_ref.def_id())
                 .filter(|&super_def_id| visited.insert(super_def_id)),
         );
         Some(def_id)

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -6,7 +6,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef};
-use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
+use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
 use rustc_span::Span;
 
 use std::iter;
@@ -298,9 +298,10 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 
         let extend = |obligation: traits::PredicateObligation<'tcx>| {
             let mut cause = cause.clone();
-            if let Some(parent_trait_ref) = obligation.predicate.to_opt_poly_trait_ref() {
+            if let Some(parent_trait_ref) = obligation.predicate.to_opt_poly_trait_pred() {
                 let derived_cause = traits::DerivedObligationCause {
-                    parent_trait_ref: parent_trait_ref.value,
+                    // FIXME(fee1-dead): when improving error messages, change this to PolyTraitPredicate
+                    parent_trait_ref: parent_trait_ref.map_bound(|t| t.trait_ref),
                     parent_code: Lrc::new(obligation.cause.code.clone()),
                 };
                 cause.make_mut().code =

compiler/rustc_ty_utils/src/ty.rs

@@ -2,9 +2,7 @@ use rustc_data_structures::fx::FxIndexSet;
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::ty::subst::Subst;
-use rustc_middle::ty::{
+use rustc_middle::ty::{self, Binder, Predicate, PredicateKind, ToPredicate, Ty, TyCtxt};
-    self, Binder, Predicate, PredicateKind, ToPredicate, Ty, TyCtxt, WithConstness,
-};
 use rustc_span::Span;
 use rustc_trait_selection::traits;
 
@@ -282,16 +280,79 @@ fn param_env(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamEnv<'_> {
     // issue #89334
     predicates = tcx.expose_default_const_substs(predicates);
 
-    let unnormalized_env =
+    let local_did = def_id.as_local();
-        ty::ParamEnv::new(tcx.intern_predicates(&predicates), traits::Reveal::UserFacing);
+    let hir_id = local_did.map(|def_id| tcx.hir().local_def_id_to_hir_id(def_id));
 
-    debug!("unnormalized_env caller bounds: {:?}", unnormalized_env.caller_bounds());
+    let constness = match hir_id {
-    let body_id = def_id
+        Some(hir_id) => match tcx.hir().get(hir_id) {
-        .as_local()
+            hir::Node::Item(hir::Item { kind: hir::ItemKind::Const(..), .. })
-        .map(|def_id| tcx.hir().local_def_id_to_hir_id(def_id))
+            | hir::Node::Item(hir::Item { kind: hir::ItemKind::Static(..), .. })
-        .map_or(hir::CRATE_HIR_ID, |id| {
+            | hir::Node::TraitItem(hir::TraitItem {
-            tcx.hir().maybe_body_owned_by(id).map_or(id, |body| body.hir_id)
+                kind: hir::TraitItemKind::Const(..), ..
-        });
+            })
+            | hir::Node::AnonConst(_)
+            | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Const(..), .. })
+            | hir::Node::ImplItem(hir::ImplItem {
+                kind:
+                    hir::ImplItemKind::Fn(
+                        hir::FnSig {
+                            header: hir::FnHeader { constness: hir::Constness::Const, .. },
+                            ..
+                        },
+                        ..,
+                    ),
+                ..
+            }) => hir::Constness::Const,
+
+            hir::Node::ImplItem(hir::ImplItem {
+                kind: hir::ImplItemKind::TyAlias(..) | hir::ImplItemKind::Fn(..),
+                ..
+            }) => {
+                let parent_hir_id = tcx.hir().get_parent_node(hir_id);
+                match tcx.hir().get(parent_hir_id) {
+                    hir::Node::Item(hir::Item {
+                        kind: hir::ItemKind::Impl(hir::Impl { constness, .. }),
+                        ..
+                    }) => *constness,
+                    _ => span_bug!(
+                        tcx.def_span(parent_hir_id.owner),
+                        "impl item's parent node is not an impl",
+                    ),
+                }
+            }
+
+            hir::Node::Item(hir::Item {
+                kind:
+                    hir::ItemKind::Fn(hir::FnSig { header: hir::FnHeader { constness, .. }, .. }, ..),
+                ..
+            })
+            | hir::Node::TraitItem(hir::TraitItem {
+                kind:
+                    hir::TraitItemKind::Fn(
+                        hir::FnSig { header: hir::FnHeader { constness, .. }, .. },
+                        ..,
+                    ),
+                ..
+            })
+            | hir::Node::Item(hir::Item {
+                kind: hir::ItemKind::Impl(hir::Impl { constness, .. }),
+                ..
+            }) => *constness,
+
+            _ => hir::Constness::NotConst,
+        },
+        None => hir::Constness::NotConst,
+    };
+
+    let unnormalized_env = ty::ParamEnv::new(
+        tcx.intern_predicates(&predicates),
+        traits::Reveal::UserFacing,
+        constness,
+    );
+
+    let body_id = hir_id.map_or(hir::CRATE_HIR_ID, |id| {
+        tcx.hir().maybe_body_owned_by(id).map_or(id, |body| body.hir_id)
+    });
     let cause = traits::ObligationCause::misc(tcx.def_span(def_id), body_id);
     traits::normalize_param_env_or_error(tcx, def_id, unnormalized_env, cause)
 }

compiler/rustc_typeck/src/astconv/mod.rs

@@ -1587,7 +1587,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 traits::transitive_bounds_that_define_assoc_type(
                     tcx,
                     predicates.iter().filter_map(|(p, _)| {
-                        p.to_opt_poly_trait_ref().map(|trait_ref| trait_ref.value)
+                        Some(p.to_opt_poly_trait_pred()?.map_bound(|t| t.trait_ref))
                     }),
                     assoc_name,
                 )

compiler/rustc_typeck/src/bounds.rs

@@ -1,7 +1,7 @@
 //! Bounds are restrictions applied to some types after they've been converted into the
 //! `ty` form from the HIR.
 
-use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, WithConstness};
+use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt};
 use rustc_span::Span;
 
 /// Collects together a list of type bounds. These lists of bounds occur in many places

compiler/rustc_typeck/src/check/check.rs

@@ -1506,17 +1506,11 @@ pub(super) fn check_mod_item_types(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
     tcx.hir().visit_item_likes_in_module(module_def_id, &mut CheckItemTypesVisitor { tcx });
 }
 
-pub(super) fn check_item_well_formed(tcx: TyCtxt<'_>, def_id: LocalDefId) {
+pub(super) use wfcheck::check_item_well_formed;
-    wfcheck::check_item_well_formed(tcx, def_id);
-}
 
-pub(super) fn check_trait_item_well_formed(tcx: TyCtxt<'_>, def_id: LocalDefId) {
+pub(super) use wfcheck::check_trait_item as check_trait_item_well_formed;
-    wfcheck::check_trait_item(tcx, def_id);
-}
 
-pub(super) fn check_impl_item_well_formed(tcx: TyCtxt<'_>, def_id: LocalDefId) {
+pub(super) use wfcheck::check_impl_item as check_impl_item_well_formed;
-    wfcheck::check_impl_item(tcx, def_id);
-}
 
 fn async_opaque_type_cycle_error(tcx: TyCtxt<'tcx>, span: Span) {
     struct_span_err!(tcx.sess, span, E0733, "recursion in an `async fn` requires boxing")

compiler/rustc_typeck/src/check/closure.rs

@@ -80,7 +80,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         let generator_types = check_fn(
             self,
-            self.param_env,
+            self.param_env.without_const(),
             liberated_sig,
             decl,
             expr.hir_id,

compiler/rustc_typeck/src/check/compare_method.rs

@@ -208,8 +208,11 @@ fn compare_predicate_entailment<'tcx>(
     // 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_hir_id);
-    let param_env =
+    let param_env = ty::ParamEnv::new(
-        ty::ParamEnv::new(tcx.intern_predicates(&hybrid_preds.predicates), Reveal::UserFacing);
+        tcx.intern_predicates(&hybrid_preds.predicates),
+        Reveal::UserFacing,
+        hir::Constness::NotConst,
+    );
     let param_env =
         traits::normalize_param_env_or_error(tcx, impl_m.def_id, param_env, normalize_cause);
 
@@ -1167,8 +1170,11 @@ fn compare_type_predicate_entailment<'tcx>(
     debug!("compare_type_predicate_entailment: bounds={:?}", hybrid_preds);
 
     let normalize_cause = traits::ObligationCause::misc(impl_ty_span, impl_ty_hir_id);
-    let param_env =
+    let param_env = ty::ParamEnv::new(
-        ty::ParamEnv::new(tcx.intern_predicates(&hybrid_preds.predicates), Reveal::UserFacing);
+        tcx.intern_predicates(&hybrid_preds.predicates),
+        Reveal::UserFacing,
+        hir::Constness::NotConst,
+    );
     let param_env = traits::normalize_param_env_or_error(
         tcx,
         impl_ty.def_id,
@@ -1353,7 +1359,11 @@ pub fn check_type_bounds<'tcx>(
                 .to_predicate(tcx),
             ),
         };
-        ty::ParamEnv::new(tcx.intern_predicates(&predicates), Reveal::UserFacing)
+        ty::ParamEnv::new(
+            tcx.intern_predicates(&predicates),
+            Reveal::UserFacing,
+            param_env.constness(),
+        )
     };
     debug!(?normalize_param_env);
 
@@ -1362,13 +1372,7 @@ pub fn check_type_bounds<'tcx>(
         impl_ty_substs.rebase_onto(tcx, impl_ty.container.id(), impl_trait_ref.substs);
 
     tcx.infer_ctxt().enter(move |infcx| {
-        let constness = impl_ty
+        let inh = Inherited::new(infcx, impl_ty.def_id.expect_local());
-            .container
-            .impl_def_id()
-            .map(|did| tcx.impl_constness(did))
-            .unwrap_or(hir::Constness::NotConst);
-
-        let inh = Inherited::with_constness(infcx, impl_ty.def_id.expect_local(), constness);
         let infcx = &inh.infcx;
         let mut selcx = traits::SelectionContext::new(&infcx);
 
@@ -1412,8 +1416,7 @@ pub fn check_type_bounds<'tcx>(
 
         // Check that all obligations are satisfied by the implementation's
         // version.
-        let errors =
+        let errors = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx);
-            inh.fulfillment_cx.borrow_mut().select_all_with_constness_or_error(&infcx, constness);
         if !errors.is_empty() {
             infcx.report_fulfillment_errors(&errors, None, false);
             return Err(ErrorReported);

compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -616,10 +616,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     #[instrument(skip(self), level = "debug")]
     pub(in super::super) fn select_all_obligations_or_error(&self) {
-        let errors = self
+        let errors = self.fulfillment_cx.borrow_mut().select_all_or_error(&self);
-            .fulfillment_cx
-            .borrow_mut()
-            .select_all_with_constness_or_error(&self, self.inh.constness);
 
         if !errors.is_empty() {
             self.report_fulfillment_errors(&errors, self.inh.body_id, false);
@@ -632,10 +629,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         fallback_has_occurred: bool,
         mutate_fulfillment_errors: impl Fn(&mut Vec<traits::FulfillmentError<'tcx>>),
     ) {
-        let mut result = self
+        let mut result = self.fulfillment_cx.borrow_mut().select_where_possible(self);
-            .fulfillment_cx
-            .borrow_mut()
-            .select_with_constness_where_possible(self, self.inh.constness);
         if !result.is_empty() {
             mutate_fulfillment_errors(&mut result);
             self.report_fulfillment_errors(&result, self.inh.body_id, fallback_has_occurred);

compiler/rustc_typeck/src/check/inherited.rs

@@ -53,9 +53,6 @@ pub struct Inherited<'a, 'tcx> {
     pub(super) deferred_generator_interiors:
         RefCell<Vec<(hir::BodyId, Ty<'tcx>, hir::GeneratorKind)>>,
 
-    /// Reports whether this is in a const context.
-    pub(super) constness: hir::Constness,
-
     pub(super) body_id: Option<hir::BodyId>,
 
     /// Whenever we introduce an adjustment from `!` into a type variable,
@@ -102,16 +99,6 @@ impl<'tcx> InheritedBuilder<'tcx> {
 
 impl Inherited<'a, 'tcx> {
     pub(super) fn new(infcx: InferCtxt<'a, 'tcx>, def_id: LocalDefId) -> Self {
-        let tcx = infcx.tcx;
-        let item_id = tcx.hir().local_def_id_to_hir_id(def_id);
-        Self::with_constness(infcx, def_id, tcx.hir().get(item_id).constness_for_typeck())
-    }
-
-    pub(super) fn with_constness(
-        infcx: InferCtxt<'a, 'tcx>,
-        def_id: LocalDefId,
-        constness: hir::Constness,
-    ) -> Self {
         let tcx = infcx.tcx;
         let item_id = tcx.hir().local_def_id_to_hir_id(def_id);
         let body_id = tcx.hir().maybe_body_owned_by(item_id);
@@ -128,7 +115,6 @@ impl Inherited<'a, 'tcx> {
             deferred_cast_checks: RefCell::new(Vec::new()),
             deferred_generator_interiors: RefCell::new(Vec::new()),
             diverging_type_vars: RefCell::new(Default::default()),
-            constness,
             body_id,
         }
     }

compiler/rustc_typeck/src/check/method/mod.rs

@@ -22,7 +22,7 @@ use rustc_infer::infer::{self, InferOk};
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
 use rustc_middle::ty::GenericParamDefKind;
-use rustc_middle::ty::{self, ToPredicate, Ty, TypeFoldable, WithConstness};
+use rustc_middle::ty::{self, ToPredicate, Ty, TypeFoldable};
 use rustc_span::symbol::Ident;
 use rustc_span::Span;
 use rustc_trait_selection::traits;

compiler/rustc_typeck/src/check/method/probe.rs

@@ -21,7 +21,7 @@ use rustc_infer::infer::{self, InferOk, TyCtxtInferExt};
 use rustc_middle::middle::stability;
 use rustc_middle::ty::subst::{InternalSubsts, Subst, SubstsRef};
 use rustc_middle::ty::GenericParamDefKind;
-use rustc_middle::ty::{self, ParamEnvAnd, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
+use rustc_middle::ty::{self, ParamEnvAnd, ToPredicate, Ty, TyCtxt, TypeFoldable};
 use rustc_session::lint;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::lev_distance::{find_best_match_for_name, lev_distance};

compiler/rustc_typeck/src/check/method/suggest.rs

@@ -12,7 +12,7 @@ use rustc_hir::{ExprKind, Node, QPath};
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_middle::ty::fast_reject::simplify_type;
 use rustc_middle::ty::print::with_crate_prefix;
-use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
+use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
 use rustc_span::lev_distance;
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{source_map, FileName, MultiSpan, Span, Symbol};

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -21,7 +21,6 @@ use rustc_middle::ty::subst::{GenericArgKind, InternalSubsts, Subst};
 use rustc_middle::ty::trait_def::TraitSpecializationKind;
 use rustc_middle::ty::{
     self, AdtKind, GenericParamDefKind, ToPredicate, Ty, TyCtxt, TypeFoldable, TypeVisitor,
-    WithConstness,
 };
 use rustc_session::parse::feature_err;
 use rustc_span::symbol::{sym, Ident, Symbol};

compiler/rustc_typeck/src/collect.rs

@@ -41,7 +41,7 @@ use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::util::Discr;
 use rustc_middle::ty::util::IntTypeExt;
 use rustc_middle::ty::{self, AdtKind, Const, DefIdTree, Ty, TyCtxt};
-use rustc_middle::ty::{ReprOptions, ToPredicate, TypeFoldable, WithConstness};
+use rustc_middle::ty::{ReprOptions, ToPredicate, TypeFoldable};
 use rustc_session::lint;
 use rustc_session::parse::feature_err;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};

library/core/src/convert/mod.rs

@@ -534,9 +534,10 @@ where
 
 // From implies Into
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T, U> Into<U> for T
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<T, U> const Into<U> for T
 where
-    U: From<T>,
+    U: ~const From<T>,
 {
     fn into(self) -> U {
         U::from(self)

library/core/src/lib.rs

@@ -112,6 +112,7 @@
 #![feature(const_float_classify)]
 #![feature(const_fmt_arguments_new)]
 #![feature(const_heap)]
+#![feature(const_convert)]
 #![feature(const_inherent_unchecked_arith)]
 #![feature(const_int_unchecked_arith)]
 #![feature(const_intrinsic_copy)]

library/core/src/option.rs

@@ -2077,7 +2077,8 @@ impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
 }
 
 #[unstable(feature = "try_trait_v2", issue = "84277")]
-impl<T> ops::Try for Option<T> {
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<T> const ops::Try for Option<T> {
     type Output = T;
     type Residual = Option<convert::Infallible>;
 
@@ -2096,6 +2097,7 @@ impl<T> ops::Try for Option<T> {
 }
 
 #[unstable(feature = "try_trait_v2", issue = "84277")]
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 impl<T> const ops::FromResidual for Option<T> {
     #[inline]
     fn from_residual(residual: Option<convert::Infallible>) -> Self {

library/core/src/result.rs

@@ -1945,7 +1945,8 @@ impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
 }
 
 #[unstable(feature = "try_trait_v2", issue = "84277")]
-impl<T, E> ops::Try for Result<T, E> {
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<T, E> const ops::Try for Result<T, E> {
     type Output = T;
     type Residual = Result<convert::Infallible, E>;
 
@@ -1964,7 +1965,10 @@ impl<T, E> ops::Try for Result<T, E> {
 }
 
 #[unstable(feature = "try_trait_v2", issue = "84277")]
-impl<T, E, F: From<E>> ops::FromResidual<Result<convert::Infallible, E>> for Result<T, F> {
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<T, E, F: ~const From<E>> const ops::FromResidual<Result<convert::Infallible, E>>
+    for Result<T, F>
+{
     #[inline]
     fn from_residual(residual: Result<convert::Infallible, E>) -> Self {
         match residual {

library/core/tests/convert.rs

@@ -0,0 +1,16 @@
+#[test]
+fn convert() {
+    const fn from(x: i32) -> i32 {
+        i32::from(x)
+    }
+
+    const FOO: i32 = from(42);
+    assert_eq!(FOO, 42);
+
+    const fn into(x: Vec<String>) -> Vec<String> {
+        x.into()
+    }
+
+    const BAR: Vec<String> = into(Vec::new());
+    assert_eq!(BAR, Vec::<String>::new());
+}

library/core/tests/lib.rs

@@ -12,11 +12,11 @@
 #![feature(const_convert)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_assume_init)]
+#![feature(const_num_from_num)]
 #![feature(const_ptr_read)]
 #![feature(const_ptr_write)]
 #![feature(const_ptr_offset)]
 #![feature(const_trait_impl)]
-#![feature(const_num_from_num)]
 #![feature(core_intrinsics)]
 #![feature(core_private_bignum)]
 #![feature(core_private_diy_float)]
@@ -96,6 +96,7 @@ mod char;
 mod clone;
 mod cmp;
 mod const_ptr;
+mod convert;
 mod fmt;
 mod future;
 mod hash;

src/librustdoc/clean/blanket_impl.rs

@@ -3,7 +3,7 @@ use rustc_hir as hir;
 use rustc_infer::infer::{InferOk, TyCtxtInferExt};
 use rustc_infer::traits;
 use rustc_middle::ty::subst::Subst;
-use rustc_middle::ty::{ToPredicate, WithConstness};
+use rustc_middle::ty::ToPredicate;
 use rustc_span::DUMMY_SP;
 
 use super::*;
@@ -66,7 +66,8 @@ impl<'a, 'tcx> BlanketImplFinder<'a, 'tcx> {
                             .into_iter()
                             .chain(Some(
                                 ty::Binder::dummy(trait_ref)
-                                    .without_const()
+                                    .to_poly_trait_predicate()
+                                    .map_bound(ty::PredicateKind::Trait)
                                     .to_predicate(infcx.tcx),
                             ));
                         for predicate in predicates {

src/test/ui/consts/not_const_clusure_in_const.rs

@@ -1,9 +0,0 @@
-// run-pass
-
-const _FOO: fn() -> String = || "foo".into();
-
-pub fn bar() -> fn() -> String {
-    || "bar".into()
-}
-
-fn main(){}

src/test/ui/consts/try-operator.rs

@@ -0,0 +1,23 @@
+// run-pass
+
+#![feature(try_trait_v2)]
+#![feature(const_trait_impl)]
+#![feature(const_try)]
+#![feature(const_convert)]
+
+fn main() {
+    const fn result() -> Result<bool, ()> {
+        Err(())?;
+        Ok(true)
+    }
+
+    const FOO: Result<bool, ()> = result();
+    assert_eq!(Err(()), FOO);
+
+    const fn option() -> Option<()> {
+        None?;
+        Some(())
+    }
+    const BAR: Option<()> = option();
+    assert_eq!(None, BAR);
+}

src/test/ui/infinite/infinite-struct.stderr

@@ -19,7 +19,7 @@ LL | struct Take(Take);
    | ^^^^^^^^^^^^^^^^^^
    |
    = note: ...which immediately requires computing drop-check constraints for `Take` again
-   = note: cycle used when computing dropck types for `Canonical { max_universe: U0, variables: [], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: UserFacing }, value: Take } }`
+   = note: cycle used when computing dropck types for `Canonical { max_universe: U0, variables: [], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst }, value: Take } }`
 
 error: aborting due to 2 previous errors
 

src/test/ui/infinite/infinite-tag-type-recursion.stderr

@@ -18,7 +18,7 @@ LL | enum MList { Cons(isize, MList), Nil }
    | ^^^^^^^^^^
    |
    = note: ...which immediately requires computing drop-check constraints for `MList` again
-   = note: cycle used when computing dropck types for `Canonical { max_universe: U0, variables: [], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: UserFacing }, value: MList } }`
+   = note: cycle used when computing dropck types for `Canonical { max_universe: U0, variables: [], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst }, value: MList } }`
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-91489.rs

@@ -0,0 +1,40 @@
+// check-pass
+
+// regression test for #91489
+
+use std::borrow::Borrow;
+use std::borrow::Cow;
+
+pub struct VariantType {}
+pub struct VariantTy {}
+
+impl Borrow<VariantTy> for VariantType {
+    fn borrow(&self) -> &VariantTy {
+        unimplemented!()
+    }
+}
+
+impl ToOwned for VariantTy {
+    type Owned = VariantType;
+    fn to_owned(&self) -> VariantType {
+        unimplemented!()
+    }
+}
+
+impl VariantTy {
+    pub fn as_str(&self) -> () {}
+}
+
+// the presence of this was causing all attempts to call `as_str` on
+// `Cow<'_, VariantTy>, including in itself, to not find the method
+static _TYP: () = {
+    let _ = || {
+        // should be found
+        Cow::Borrowed(&VariantTy {}).as_str();
+    };
+};
+
+fn main() {
+    // should be found
+    Cow::Borrowed(&VariantTy {}).as_str()
+}

src/test/ui/rfc-2632-const-trait-impl/assoc-type.rs

@@ -1,6 +1,5 @@
 // FIXME(fee1-dead): this should have a better error message
 #![feature(const_trait_impl)]
-
 struct NonConstAdd(i32);
 
 impl std::ops::Add for NonConstAdd {

src/test/ui/rfc-2632-const-trait-impl/assoc-type.stderr

@@ -1,12 +1,12 @@
 error[E0277]: cannot add `NonConstAdd` to `NonConstAdd`
-  --> $DIR/assoc-type.rs:19:16
+  --> $DIR/assoc-type.rs:18:16
    |
 LL |     type Bar = NonConstAdd;
    |                ^^^^^^^^^^^ no implementation for `NonConstAdd + NonConstAdd`
    |
    = help: the trait `Add` is not implemented for `NonConstAdd`
 note: required by a bound in `Foo::Bar`
-  --> $DIR/assoc-type.rs:15:15
+  --> $DIR/assoc-type.rs:14:15
    |
 LL |     type Bar: ~const std::ops::Add;
    |               ^^^^^^^^^^^^^^^^^^^^ required by this bound in `Foo::Bar`

src/test/ui/rfc-2632-const-trait-impl/non-const-op-in-closure-in-const.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+#![feature(const_trait_impl)]
+#![feature(const_fn_trait_bound)]
+
+trait Convert<T> {
+    fn to(self) -> T;
+}
+
+impl<A, B> const Convert<B> for A where B: ~const From<A> {
+    fn to(self) -> B {
+        B::from(self)
+    }
+}
+
+const FOO: fn() -> String = || "foo".to();
+
+fn main() {}

src/tools/clippy/clippy_lints/src/future_not_send.rs

@@ -68,8 +68,8 @@ impl<'tcx> LateLintPass<'tcx> for FutureNotSend {
             let mut is_future = false;
             for &(p, _span) in preds {
                 let p = p.subst(cx.tcx, subst);
-                if let Some(trait_ref) = p.to_opt_poly_trait_ref() {
+                if let Some(trait_pred) = p.to_opt_poly_trait_pred() {
-                    if Some(trait_ref.value.def_id()) == cx.tcx.lang_items().future_trait() {
+                    if Some(trait_pred.skip_binder().trait_ref.def_id) == cx.tcx.lang_items().future_trait() {
                         is_future = true;
                         break;
                     }