Rename ~const Drop to ~const Destruct

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

@@ -3,6 +3,7 @@
 //! See the `Qualif` trait for more info.
 
 use rustc_errors::ErrorGuaranteed;
+use rustc_hir::LangItem;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_infer::traits::TraitEngine;
 use rustc_middle::mir::*;
@@ -152,18 +153,14 @@ impl Qualif for NeedsNonConstDrop {
             return false;
         }
 
-        let Some(drop_trait) = cx.tcx.lang_items().drop_trait() else {
-            // there is no way to define a type that needs non-const drop
-            // without having the lang item present.
-            return false;
-        };
+        let destruct = cx.tcx.require_lang_item(LangItem::Destruct, None);
 
         let obligation = Obligation::new(
             ObligationCause::dummy(),
             cx.param_env,
             ty::Binder::dummy(ty::TraitPredicate {
                 trait_ref: ty::TraitRef {
-                    def_id: drop_trait,
+                    def_id: destruct,
                     substs: cx.tcx.mk_substs_trait(ty, &[]),
                 },
                 constness: ty::BoundConstness::ConstIfConst,
@@ -174,15 +171,16 @@ impl Qualif for NeedsNonConstDrop {
         cx.tcx.infer_ctxt().enter(|infcx| {
             let mut selcx = SelectionContext::new(&infcx);
             let Some(impl_src) = selcx.select(&obligation).ok().flatten() else {
-                // If we couldn't select a const drop candidate, then it's bad
+                // If we couldn't select a const destruct candidate, then it's bad
                 return true;
             };
 
             if !matches!(
                 impl_src,
-                ImplSource::ConstDrop(_) | ImplSource::Param(_, ty::BoundConstness::ConstIfConst)
+                ImplSource::ConstDestruct(_)
+                    | ImplSource::Param(_, ty::BoundConstness::ConstIfConst)
             ) {
-                // If our const drop candidate is not ConstDrop or implied by the param env,
+                // If our const destruct candidate is not ConstDestruct or implied by the param env,
                 // then it's bad
                 return true;
             }

compiler/rustc_hir/src/lang_items.rs

@@ -216,6 +216,7 @@ language_item_table! {
     Freeze,                  sym::freeze,              freeze_trait,               Target::Trait,          GenericRequirement::Exact(0);
 
     Drop,                    sym::drop,                drop_trait,                 Target::Trait,          GenericRequirement::None;
+    Destruct,                sym::destruct,            destruct_trait,             Target::Trait,          GenericRequirement::None;
 
     CoerceUnsized,           sym::coerce_unsized,      coerce_unsized_trait,       Target::Trait,          GenericRequirement::Minimum(1);
     DispatchFromDyn,         sym::dispatch_from_dyn,   dispatch_from_dyn_trait,    Target::Trait,          GenericRequirement::Minimum(1);

compiler/rustc_lint/src/traits.rs

@@ -93,10 +93,6 @@ impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
             let Trait(trait_predicate) = predicate.kind().skip_binder() else {
                 continue
             };
-            if trait_predicate.is_const_if_const() {
-                // `~const Drop` definitely have meanings so avoid linting here.
-                continue;
-            }
             let def_id = trait_predicate.trait_ref.def_id;
             if cx.tcx.lang_items().drop_trait() == Some(def_id) {
                 // Explicitly allow `impl Drop`, a drop-guards-as-Voldemort-type pattern.

compiler/rustc_middle/src/traits/mod.rs

@@ -577,7 +577,7 @@ pub enum ImplSource<'tcx, N> {
     TraitAlias(ImplSourceTraitAliasData<'tcx, N>),
 
     /// ImplSource for a `const Drop` implementation.
-    ConstDrop(ImplSourceConstDropData<N>),
+    ConstDestruct(ImplSourceConstDestructData<N>),
 }
 
 impl<'tcx, N> ImplSource<'tcx, N> {
@@ -595,7 +595,7 @@ impl<'tcx, N> ImplSource<'tcx, N> {
             | ImplSource::Pointee(ImplSourcePointeeData) => Vec::new(),
             ImplSource::TraitAlias(d) => d.nested,
             ImplSource::TraitUpcasting(d) => d.nested,
-            ImplSource::ConstDrop(i) => i.nested,
+            ImplSource::ConstDestruct(i) => i.nested,
         }
     }
 
@@ -613,7 +613,7 @@ impl<'tcx, N> ImplSource<'tcx, N> {
             | ImplSource::Pointee(ImplSourcePointeeData) => &[],
             ImplSource::TraitAlias(d) => &d.nested,
             ImplSource::TraitUpcasting(d) => &d.nested,
-            ImplSource::ConstDrop(i) => &i.nested,
+            ImplSource::ConstDestruct(i) => &i.nested,
         }
     }
 
@@ -672,9 +672,11 @@ impl<'tcx, N> ImplSource<'tcx, N> {
                     nested: d.nested.into_iter().map(f).collect(),
                 })
             }
-            ImplSource::ConstDrop(i) => ImplSource::ConstDrop(ImplSourceConstDropData {
-                nested: i.nested.into_iter().map(f).collect(),
-            }),
+            ImplSource::ConstDestruct(i) => {
+                ImplSource::ConstDestruct(ImplSourceConstDestructData {
+                    nested: i.nested.into_iter().map(f).collect(),
+                })
+            }
         }
     }
 }
@@ -767,7 +769,7 @@ pub struct ImplSourceDiscriminantKindData;
 pub struct ImplSourcePointeeData;
 
 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
-pub struct ImplSourceConstDropData<N> {
+pub struct ImplSourceConstDestructData<N> {
     pub nested: Vec<N>,
 }
 

compiler/rustc_middle/src/traits/select.rs

@@ -146,8 +146,8 @@ pub enum SelectionCandidate<'tcx> {
 
     BuiltinUnsizeCandidate,
 
-    /// Implementation of `const Drop`, optionally from a custom `impl const Drop`.
-    ConstDropCandidate(Option<DefId>),
+    /// Implementation of `const Destruct`, optionally from a custom `impl const Drop`.
+    ConstDestructCandidate(Option<DefId>),
 }
 
 /// The result of trait evaluation. The order is important

compiler/rustc_middle/src/traits/structural_impls.rs

@@ -33,7 +33,7 @@ impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSource<'tcx, N> {
 
             super::ImplSource::TraitUpcasting(ref d) => write!(f, "{:?}", d),
 
-            super::ImplSource::ConstDrop(ref d) => write!(f, "{:?}", d),
+            super::ImplSource::ConstDestruct(ref d) => write!(f, "{:?}", d),
         }
     }
 }
@@ -120,9 +120,9 @@ impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSourceTraitAliasData<'tcx,
     }
 }
 
-impl<N: fmt::Debug> fmt::Debug for traits::ImplSourceConstDropData<N> {
+impl<N: fmt::Debug> fmt::Debug for traits::ImplSourceConstDestructData<N> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "ImplSourceConstDropData(nested={:?})", self.nested)
+        write!(f, "ImplSourceConstDestructData(nested={:?})", self.nested)
     }
 }
 

compiler/rustc_middle/src/ty/mod.rs

@@ -765,6 +765,7 @@ impl<'tcx> TraitPredicate<'tcx> {
         if unlikely!(Some(self.trait_ref.def_id) == tcx.lang_items().drop_trait()) {
             // remap without changing constness of this predicate.
             // this is because `T: ~const Drop` has a different meaning to `T: Drop`
+            // FIXME(fee1-dead): remove this logic after beta bump
             param_env.remap_constness_with(self.constness)
         } else {
             *param_env = param_env.with_constness(self.constness.and(param_env.constness()))

compiler/rustc_span/src/symbol.rs

@@ -571,6 +571,7 @@ symbols! {
         deref_target,
         derive,
         derive_default_enum,
+        destruct,
         destructuring_assignment,
         diagnostic,
         direct,

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1569,7 +1569,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
             super::ImplSource::AutoImpl(..)
             | super::ImplSource::Builtin(..)
             | super::ImplSource::TraitUpcasting(_)
-            | super::ImplSource::ConstDrop(_) => {
+            | super::ImplSource::ConstDestruct(_) => {
                 // These traits have no associated types.
                 selcx.tcx().sess.delay_span_bug(
                     obligation.cause.span,
@@ -1644,7 +1644,7 @@ fn confirm_select_candidate<'cx, 'tcx>(
         | super::ImplSource::Builtin(..)
         | super::ImplSource::TraitUpcasting(_)
         | super::ImplSource::TraitAlias(..)
-        | super::ImplSource::ConstDrop(_) => {
+        | super::ImplSource::ConstDestruct(_) => {
             // we don't create Select candidates with this kind of resolution
             span_bug!(
                 obligation.cause.span,

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

@@ -5,6 +5,7 @@
 //! candidates. See the [rustc dev guide] for more details.
 //!
 //! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
+use hir::LangItem;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_infer::traits::TraitEngine;
@@ -307,7 +308,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             } else if lang_items.drop_trait() == Some(def_id)
                 && obligation.predicate.is_const_if_const()
             {
-                self.assemble_const_drop_candidates(obligation, &mut candidates);
+                // holds to make it easier to transition
+                // FIXME(fee1-dead): add a note for selection error of `~const Drop`
+                // when beta is bumped
+                // FIXME: remove this when beta is bumped
+                #[cfg(bootstrap)]
+                {}
+
+                candidates.vec.push(SelectionCandidate::ConstDestructCandidate(None))
+            } else if lang_items.destruct_trait() == Some(def_id) {
+                self.assemble_const_destruct_candidates(obligation, &mut candidates);
             } else {
                 if lang_items.clone_trait() == Some(def_id) {
                     // Same builtin conditions as `Copy`, i.e., every type which has builtin support
@@ -906,15 +916,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
-    fn assemble_const_drop_candidates(
+    fn assemble_const_destruct_candidates(
         &mut self,
         obligation: &TraitObligation<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
-        // If the predicate is `~const Drop` in a non-const environment, we don't actually need
+        // If the predicate is `~const Destruct` in a non-const environment, we don't actually need
         // to check anything. We'll short-circuit checking any obligations in confirmation, too.
-        if obligation.param_env.constness() == hir::Constness::NotConst {
-            candidates.vec.push(ConstDropCandidate(None));
+        if !obligation.is_const() {
+            candidates.vec.push(ConstDestructCandidate(None));
             return;
         }
 
@@ -927,7 +937,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             | ty::Param(_)
             | ty::Placeholder(_)
             | ty::Projection(_) => {
-                // We don't know if these are `~const Drop`, at least
+                // We don't know if these are `~const Destruct`, at least
                 // not structurally... so don't push a candidate.
             }
 
@@ -951,26 +961,26 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             | ty::Generator(..)
             | ty::Tuple(_)
             | ty::GeneratorWitness(_) => {
-                // These are built-in, and cannot have a custom `impl const Drop`.
-                candidates.vec.push(ConstDropCandidate(None));
+                // These are built-in, and cannot have a custom `impl const Destruct`.
+                candidates.vec.push(ConstDestructCandidate(None));
             }
 
             ty::Adt(..) => {
                 // Find a custom `impl Drop` impl, if it exists
                 let relevant_impl = self.tcx().find_map_relevant_impl(
-                    obligation.predicate.def_id(),
+                    self.tcx().require_lang_item(LangItem::Drop, None),
                     obligation.predicate.skip_binder().trait_ref.self_ty(),
                     Some,
                 );
 
                 if let Some(impl_def_id) = relevant_impl {
                     // Check that `impl Drop` is actually const, if there is a custom impl
                     if self.tcx().impl_constness(impl_def_id) == hir::Constness::Const {
-                        candidates.vec.push(ConstDropCandidate(Some(impl_def_id)));
+                        candidates.vec.push(ConstDestructCandidate(Some(impl_def_id)));
                     }
                 } else {
                     // Otherwise check the ADT like a built-in type (structurally)
-                    candidates.vec.push(ConstDropCandidate(None));
+                    candidates.vec.push(ConstDestructCandidate(None));
                 }
             }
 

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

@@ -8,7 +8,6 @@
 //! https://rustc-dev-guide.rust-lang.org/traits/resolution.html#confirmation
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir::lang_items::LangItem;
-use rustc_hir::Constness;
 use rustc_index::bit_set::GrowableBitSet;
 use rustc_infer::infer::InferOk;
 use rustc_infer::infer::LateBoundRegionConversionTime::HigherRankedType;
@@ -29,9 +28,9 @@ use crate::traits::TraitNotObjectSafe;
 use crate::traits::VtblSegment;
 use crate::traits::{BuiltinDerivedObligation, ImplDerivedObligation};
 use crate::traits::{
-    ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData, ImplSourceConstDropData,
-    ImplSourceDiscriminantKindData, ImplSourceFnPointerData, ImplSourceGeneratorData,
-    ImplSourceObjectData, ImplSourcePointeeData, ImplSourceTraitAliasData,
+    ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData,
+    ImplSourceConstDestructData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
+    ImplSourceGeneratorData, ImplSourceObjectData, ImplSourcePointeeData, ImplSourceTraitAliasData,
     ImplSourceTraitUpcastingData, ImplSourceUserDefinedData,
 };
 use crate::traits::{ObjectCastObligation, PredicateObligation, TraitObligation};
@@ -156,9 +155,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 Ok(ImplSource::TraitUpcasting(data))
             }
 
-            ConstDropCandidate(def_id) => {
-                let data = self.confirm_const_drop_candidate(obligation, def_id)?;
-                Ok(ImplSource::ConstDrop(data))
+            ConstDestructCandidate(def_id) => {
+                let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
+                Ok(ImplSource::ConstDestruct(data))
             }
         }
     }
@@ -1037,14 +1036,23 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         Ok(ImplSourceBuiltinData { nested })
     }
 
-    fn confirm_const_drop_candidate(
+    fn confirm_const_destruct_candidate(
         &mut self,
         obligation: &TraitObligation<'tcx>,
         impl_def_id: Option<DefId>,
-    ) -> Result<ImplSourceConstDropData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
-        // `~const Drop` in a non-const environment is always trivially true, since our type is `Drop`
-        if obligation.param_env.constness() == Constness::NotConst {
-            return Ok(ImplSourceConstDropData { nested: vec![] });
+    ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
+        // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
+        if !obligation.is_const() {
+            return Ok(ImplSourceConstDestructData { nested: vec![] });
+        }
+
+        let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
+        // FIXME: remove if statement below when beta is bumped
+        #[cfg(bootstrap)]
+        {}
+
+        if obligation.predicate.skip_binder().def_id() == drop_trait {
+            return Ok(ImplSourceConstDestructData { nested: vec![] });
         }
 
         let tcx = self.tcx();
@@ -1054,9 +1062,29 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let cause = obligation.derived_cause(BuiltinDerivedObligation);
 
         // If we have a custom `impl const Drop`, then
-        // first check it like a regular impl candidate
+        // first check it like a regular impl candidate.
+        // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
         if let Some(impl_def_id) = impl_def_id {
-            nested.extend(self.confirm_impl_candidate(obligation, impl_def_id).nested);
+            let obligations = self.infcx.commit_unconditionally(|_| {
+                let mut new_obligation = obligation.clone();
+                new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
+                    trait_pred.trait_ref.def_id = drop_trait;
+                    trait_pred
+                });
+                let substs = self.rematch_impl(impl_def_id, &new_obligation);
+                debug!(?substs, "impl substs");
+                let cause = obligation.derived_cause(ImplDerivedObligation);
+                ensure_sufficient_stack(|| {
+                    self.vtable_impl(
+                        impl_def_id,
+                        substs,
+                        cause,
+                        new_obligation.recursion_depth + 1,
+                        new_obligation.param_env,
+                    )
+                })
+            });
+            nested.extend(obligations.nested);
         }
 
         // We want to confirm the ADT's fields if we have an ADT
@@ -1114,7 +1142,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             self_ty
                                 .rebind(ty::TraitPredicate {
                                     trait_ref: ty::TraitRef {
-                                        def_id: self.tcx().require_lang_item(LangItem::Drop, None),
+                                        def_id: self
+                                            .tcx()
+                                            .require_lang_item(LangItem::Destruct, None),
                                         substs: self.tcx().mk_substs_trait(nested_ty, &[]),
                                     },
                                     constness: ty::BoundConstness::ConstIfConst,
@@ -1140,7 +1170,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     let predicate = self_ty
                         .rebind(ty::TraitPredicate {
                             trait_ref: ty::TraitRef {
-                                def_id: self.tcx().require_lang_item(LangItem::Drop, None),
+                                def_id: self.tcx().require_lang_item(LangItem::Destruct, None),
                                 substs: self.tcx().mk_substs_trait(nested_ty, &[]),
                             },
                             constness: ty::BoundConstness::ConstIfConst,
@@ -1158,6 +1188,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
         }
 
-        Ok(ImplSourceConstDropData { nested })
+        Ok(ImplSourceConstDestructData { nested })
     }
 }