Auto merge of #93028 - compiler-errors:const_drop_bounds, r=fee1-dead

Check `const Drop` impls considering `~const` Bounds

 This PR adds logic to trait selection to account for `~const` bounds in custom `impl const Drop` for types, elaborates the `const Drop` check in `rustc_const_eval` to check those bounds, and steals some drop linting fixes from #92922, thanks `@DrMeepster.`

r? `@fee1-dead` `@oli-obk` <sup>(edit: guess I can't request review from two people, lol)</sup>
since each of you wrote and reviewed #88558, respectively.

Since the logic here is more complicated than what existed, it's possible that this is a perf regression. But it works correctly with tests, and that makes me happy.

Fixes #92881

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

@@ -4,11 +4,12 @@
 
 use rustc_errors::ErrorReported;
 use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::traits::TraitEngine;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, subst::SubstsRef, AdtDef, Ty};
 use rustc_span::DUMMY_SP;
 use rustc_trait_selection::traits::{
-    self, ImplSource, Obligation, ObligationCause, SelectionContext,
+    self, FulfillmentContext, ImplSource, Obligation, ObligationCause, SelectionContext,
 };
 
 use super::ConstCx;
@@ -145,15 +146,10 @@ impl Qualif for NeedsNonConstDrop {
         qualifs.needs_non_const_drop
     }
 
-    fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, mut ty: Ty<'tcx>) -> bool {
+    fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
-        // Avoid selecting for simple cases.
+        // Avoid selecting for simple cases, such as builtin types.
-        match ty::util::needs_drop_components(ty, &cx.tcx.data_layout).as_deref() {
+        if ty::util::is_trivially_const_drop(ty) {
-            Ok([]) => return false,
+            return false;
-            Err(ty::util::AlwaysRequiresDrop) => return true,
-            // If we've got a single component, select with that
-            // to increase the chance that we hit the selection cache.
-            Ok([t]) => ty = t,
-            Ok([..]) => {}
         }
 
         let Some(drop_trait) = cx.tcx.lang_items().drop_trait() else {
@@ -161,28 +157,50 @@ impl Qualif for NeedsNonConstDrop {
             // without having the lang item present.
             return false;
         };
-        let trait_ref =
+
-            ty::TraitRef { def_id: drop_trait, substs: cx.tcx.mk_substs_trait(ty, &[]) };
         let obligation = Obligation::new(
             ObligationCause::dummy(),
             cx.param_env,
             ty::Binder::dummy(ty::TraitPredicate {
-                trait_ref,
+                trait_ref: ty::TraitRef {
+                    def_id: drop_trait,
+                    substs: cx.tcx.mk_substs_trait(ty, &[]),
+                },
                 constness: ty::BoundConstness::ConstIfConst,
                 polarity: ty::ImplPolarity::Positive,
             }),
         );
 
-        let implsrc = cx.tcx.infer_ctxt().enter(|infcx| {
+        cx.tcx.infer_ctxt().enter(|infcx| {
             let mut selcx = SelectionContext::new(&infcx);
-            selcx.select(&obligation)
+            let Some(impl_src) = selcx.select(&obligation).ok().flatten() else {
-        });
+                // If we couldn't select a const drop candidate, then it's bad
-        !matches!(
+                return true;
-            implsrc,
+            };
-            Ok(Some(
+
+            if !matches!(
+                impl_src,
                 ImplSource::ConstDrop(_) | ImplSource::Param(_, ty::BoundConstness::ConstIfConst)
-            ))
+            ) {
-        )
+                // If our const drop candidate is not ConstDrop or implied by the param env,
+                // then it's bad
+                return true;
+            }
+
+            if impl_src.borrow_nested_obligations().is_empty() {
+                return false;
+            }
+
+            // If we successfully found one, then select all of the predicates
+            // implied by our const drop impl.
+            let mut fcx = FulfillmentContext::new();
+            for nested in impl_src.nested_obligations() {
+                fcx.register_predicate_obligation(&infcx, nested);
+            }
+
+            // If we had any errors, then it's bad
+            !fcx.select_all_or_error(&infcx).is_empty()
+        })
     }
 
     fn in_adt_inherently<'tcx>(

compiler/rustc_middle/src/traits/mod.rs

@@ -566,7 +566,7 @@ pub enum ImplSource<'tcx, N> {
     TraitAlias(ImplSourceTraitAliasData<'tcx, N>),
 
     /// ImplSource for a `const Drop` implementation.
-    ConstDrop(ImplSourceConstDropData),
+    ConstDrop(ImplSourceConstDropData<N>),
 }
 
 impl<'tcx, N> ImplSource<'tcx, N> {
@@ -581,10 +581,10 @@ impl<'tcx, N> ImplSource<'tcx, N> {
             ImplSource::Object(d) => d.nested,
             ImplSource::FnPointer(d) => d.nested,
             ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
-            | ImplSource::Pointee(ImplSourcePointeeData)
+            | ImplSource::Pointee(ImplSourcePointeeData) => Vec::new(),
-            | ImplSource::ConstDrop(ImplSourceConstDropData) => Vec::new(),
             ImplSource::TraitAlias(d) => d.nested,
             ImplSource::TraitUpcasting(d) => d.nested,
+            ImplSource::ConstDrop(i) => i.nested,
         }
     }
 
@@ -599,10 +599,10 @@ impl<'tcx, N> ImplSource<'tcx, N> {
             ImplSource::Object(d) => &d.nested,
             ImplSource::FnPointer(d) => &d.nested,
             ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
-            | ImplSource::Pointee(ImplSourcePointeeData)
+            | ImplSource::Pointee(ImplSourcePointeeData) => &[],
-            | ImplSource::ConstDrop(ImplSourceConstDropData) => &[],
             ImplSource::TraitAlias(d) => &d.nested,
             ImplSource::TraitUpcasting(d) => &d.nested,
+            ImplSource::ConstDrop(i) => &i.nested,
         }
     }
 
@@ -661,9 +661,9 @@ impl<'tcx, N> ImplSource<'tcx, N> {
                     nested: d.nested.into_iter().map(f).collect(),
                 })
             }
-            ImplSource::ConstDrop(ImplSourceConstDropData) => {
+            ImplSource::ConstDrop(i) => ImplSource::ConstDrop(ImplSourceConstDropData {
-                ImplSource::ConstDrop(ImplSourceConstDropData)
+                nested: i.nested.into_iter().map(f).collect(),
-            }
+            }),
         }
     }
 }
@@ -755,8 +755,10 @@ pub struct ImplSourceDiscriminantKindData;
 #[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
 pub struct ImplSourcePointeeData;
 
-#[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
+#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
-pub struct ImplSourceConstDropData;
+pub struct ImplSourceConstDropData<N> {
+    pub nested: Vec<N>,
+}
 
 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
 pub struct ImplSourceTraitAliasData<'tcx, N> {

compiler/rustc_middle/src/traits/select.rs

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

compiler/rustc_middle/src/traits/structural_impls.rs

@@ -120,6 +120,12 @@ impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSourceTraitAliasData<'tcx,
     }
 }
 
+impl<N: fmt::Debug> fmt::Debug for traits::ImplSourceConstDropData<N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "ImplSourceConstDropData(nested={:?})", self.nested)
+    }
+}
+
 ///////////////////////////////////////////////////////////////////////////
 // Lift implementations
 
@@ -127,5 +133,4 @@ TrivialTypeFoldableAndLiftImpls! {
     super::IfExpressionCause,
     super::ImplSourceDiscriminantKindData,
     super::ImplSourcePointeeData,
-    super::ImplSourceConstDropData,
 }

compiler/rustc_middle/src/ty/util.rs

@@ -1041,6 +1041,42 @@ pub fn needs_drop_components<'tcx>(
     }
 }
 
+pub fn is_trivially_const_drop<'tcx>(ty: Ty<'tcx>) -> bool {
+    match *ty.kind() {
+        ty::Bool
+        | ty::Char
+        | ty::Int(_)
+        | ty::Uint(_)
+        | ty::Float(_)
+        | ty::Infer(ty::IntVar(_))
+        | ty::Infer(ty::FloatVar(_))
+        | ty::Str
+        | ty::RawPtr(_)
+        | ty::Ref(..)
+        | ty::FnDef(..)
+        | ty::FnPtr(_)
+        | ty::Never
+        | ty::Foreign(_) => true,
+
+        ty::Opaque(..)
+        | ty::Dynamic(..)
+        | ty::Error(_)
+        | ty::Bound(..)
+        | ty::Param(_)
+        | ty::Placeholder(_)
+        | ty::Projection(_)
+        | ty::Infer(_) => false,
+
+        // Not trivial because they have components, and instead of looking inside,
+        // we'll just perform trait selection.
+        ty::Closure(..) | ty::Generator(..) | ty::GeneratorWitness(_) | ty::Adt(..) => false,
+
+        ty::Array(ty, _) | ty::Slice(ty) => is_trivially_const_drop(ty),
+
+        ty::Tuple(tys) => tys.iter().all(|ty| is_trivially_const_drop(ty.expect_ty())),
+    }
+}
+
 // Does the equivalent of
 // ```
 // let v = self.iter().map(|p| p.fold_with(folder)).collect::<SmallVec<[_; 8]>>();

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

@@ -307,13 +307,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             } else if lang_items.drop_trait() == Some(def_id)
                 && obligation.predicate.skip_binder().constness == ty::BoundConstness::ConstIfConst
             {
-                if obligation.param_env.constness() == hir::Constness::Const {
+                self.assemble_const_drop_candidates(obligation, &mut candidates);
-                    self.assemble_const_drop_candidates(obligation, stack, &mut candidates)?;
-                } else {
-                    debug!("passing ~const Drop bound; in non-const context");
-                    // `~const Drop` when we are not in a const context has no effect.
-                    candidates.vec.push(ConstDropCandidate)
-                }
             } else {
                 if lang_items.clone_trait() == Some(def_id) {
                     // Same builtin conditions as `Copy`, i.e., every type which has builtin support
@@ -918,139 +912,77 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
-    fn assemble_const_drop_candidates<'a>(
+    fn assemble_const_drop_candidates(
         &mut self,
         obligation: &TraitObligation<'tcx>,
-        obligation_stack: &TraitObligationStack<'a, 'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
-    ) -> Result<(), SelectionError<'tcx>> {
+    ) {
-        let mut stack: Vec<(Ty<'tcx>, usize)> = vec![(obligation.self_ty().skip_binder(), 0)];
+        // If the predicate is `~const Drop` in a non-const environment, we don't actually need
-
+        // to check anything. We'll short-circuit checking any obligations in confirmation, too.
-        while let Some((ty, depth)) = stack.pop() {
+        if obligation.param_env.constness() == hir::Constness::NotConst {
-            let mut noreturn = false;
+            candidates.vec.push(ConstDropCandidate(None));
-
+            return;
-            self.check_recursion_depth(depth, obligation)?;
-            let mut new_candidates = SelectionCandidateSet { vec: Vec::new(), ambiguous: false };
-            let mut copy_obligation =
-                obligation.with(obligation.predicate.rebind(ty::TraitPredicate {
-                    trait_ref: ty::TraitRef {
-                        def_id: self.tcx().require_lang_item(hir::LangItem::Copy, None),
-                        substs: self.tcx().mk_substs_trait(ty, &[]),
-                    },
-                    constness: ty::BoundConstness::NotConst,
-                    polarity: ty::ImplPolarity::Positive,
-                }));
-            copy_obligation.recursion_depth = depth + 1;
-            self.assemble_candidates_from_impls(&copy_obligation, &mut new_candidates);
-            let copy_conditions = self.copy_clone_conditions(&copy_obligation);
-            self.assemble_builtin_bound_candidates(copy_conditions, &mut new_candidates);
-            let copy_stack = self.push_stack(obligation_stack.list(), &copy_obligation);
-            self.assemble_candidates_from_caller_bounds(&copy_stack, &mut new_candidates)?;
-
-            let const_drop_obligation =
-                obligation.with(obligation.predicate.rebind(ty::TraitPredicate {
-                    trait_ref: ty::TraitRef {
-                        def_id: self.tcx().require_lang_item(hir::LangItem::Drop, None),
-                        substs: self.tcx().mk_substs_trait(ty, &[]),
-                    },
-                    constness: ty::BoundConstness::ConstIfConst,
-                    polarity: ty::ImplPolarity::Positive,
-                }));
-
-            let const_drop_stack = self.push_stack(obligation_stack.list(), &const_drop_obligation);
-            self.assemble_candidates_from_caller_bounds(&const_drop_stack, &mut new_candidates)?;
-
-            if !new_candidates.vec.is_empty() {
-                noreturn = true;
-            }
-            debug!(?new_candidates.vec, "assemble_const_drop_candidates");
-
-            match ty.kind() {
-                ty::Int(_)
-                | ty::Uint(_)
-                | ty::Float(_)
-                | ty::Infer(ty::IntVar(_))
-                | ty::Infer(ty::FloatVar(_))
-                | ty::FnPtr(_)
-                | ty::Never
-                | ty::Ref(..)
-                | ty::FnDef(..)
-                | ty::RawPtr(_)
-                | ty::Bool
-                | ty::Char
-                | ty::Str
-                | ty::Foreign(_) => {} // Do nothing. These types satisfy `const Drop`.
-
-                ty::Adt(def, subst) => {
-                    let mut set = SelectionCandidateSet { vec: Vec::new(), ambiguous: false };
-                    self.assemble_candidates_from_impls(
-                        &obligation.with(obligation.predicate.map_bound(|mut pred| {
-                            pred.trait_ref.substs = self.tcx().mk_substs_trait(ty, &[]);
-                            pred
-                        })),
-                        &mut set,
-                    );
-                    stack.extend(def.all_fields().map(|f| (f.ty(self.tcx(), subst), depth + 1)));
-
-                    debug!(?set.vec, "assemble_const_drop_candidates - ty::Adt");
-                    if set.vec.into_iter().any(|candidate| {
-                        if let SelectionCandidate::ImplCandidate(did) = candidate {
-                            matches!(self.tcx().impl_constness(did), hir::Constness::NotConst)
-                        } else {
-                            false
-                        }
-                    }) {
-                        if !noreturn {
-                            // has non-const Drop
-                            return Ok(());
-                        }
-                        debug!("not returning");
-                    }
-                }
-
-                ty::Array(ty, _) => stack.push((ty, depth + 1)),
-
-                ty::Tuple(_) => stack.extend(ty.tuple_fields().map(|t| (t, depth + 1))),
-
-                ty::Closure(_, substs) => {
-                    let substs = substs.as_closure();
-                    let ty = self.infcx.shallow_resolve(substs.tupled_upvars_ty());
-                    stack.push((ty, depth + 1));
-                }
-
-                ty::Generator(_, substs, _) => {
-                    let substs = substs.as_generator();
-                    let ty = self.infcx.shallow_resolve(substs.tupled_upvars_ty());
-
-                    stack.push((ty, depth + 1));
-                    stack.push((substs.witness(), depth + 1));
-                }
-
-                ty::GeneratorWitness(tys) => stack.extend(
-                    self.tcx().erase_late_bound_regions(*tys).iter().map(|t| (t, depth + 1)),
-                ),
-
-                ty::Slice(ty) => stack.push((ty, depth + 1)),
-
-                ty::Opaque(..)
-                | ty::Dynamic(..)
-                | ty::Error(_)
-                | ty::Bound(..)
-                | ty::Infer(_)
-                | ty::Placeholder(_)
-                | ty::Projection(..)
-                | ty::Param(..) => {
-                    if !noreturn {
-                        return Ok(());
-                    }
-                    debug!("not returning");
-                }
-            }
-            debug!(?stack, "assemble_const_drop_candidates - in loop");
         }
-        // all types have passed.
-        candidates.vec.push(ConstDropCandidate);
 
-        Ok(())
+        let self_ty = self.infcx().shallow_resolve(obligation.self_ty());
+        match self_ty.skip_binder().kind() {
+            ty::Opaque(..)
+            | ty::Dynamic(..)
+            | ty::Error(_)
+            | ty::Bound(..)
+            | ty::Param(_)
+            | ty::Placeholder(_)
+            | ty::Projection(_) => {
+                // We don't know if these are `~const Drop`, at least
+                // not structurally... so don't push a candidate.
+            }
+
+            ty::Bool
+            | ty::Char
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Infer(ty::IntVar(_))
+            | ty::Infer(ty::FloatVar(_))
+            | ty::Str
+            | ty::RawPtr(_)
+            | ty::Ref(..)
+            | ty::FnDef(..)
+            | ty::FnPtr(_)
+            | ty::Never
+            | ty::Foreign(_)
+            | ty::Array(..)
+            | ty::Slice(_)
+            | ty::Closure(..)
+            | ty::Generator(..)
+            | ty::Tuple(_)
+            | ty::GeneratorWitness(_) => {
+                // These are built-in, and cannot have a custom `impl const Drop`.
+                candidates.vec.push(ConstDropCandidate(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(),
+                    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)));
+                    }
+                } else {
+                    // Otherwise check the ADT like a built-in type (structurally)
+                    candidates.vec.push(ConstDropCandidate(None));
+                }
+            }
+
+            ty::Infer(_) => {
+                candidates.ambiguous = true;
+            }
+        }
     }
 }

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

@@ -80,7 +80,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 param_env: obligation.param_env.without_const(),
                 ..*obligation
             };
-
             obligation = &new_obligation;
         }
 
@@ -159,7 +158,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 Ok(ImplSource::TraitUpcasting(data))
             }
 
-            ConstDropCandidate => Ok(ImplSource::ConstDrop(ImplSourceConstDropData)),
+            ConstDropCandidate(def_id) => {
+                let data = self.confirm_const_drop_candidate(obligation, def_id)?;
+                Ok(ImplSource::ConstDrop(data))
+            }
         }
     }
 
@@ -1087,4 +1089,128 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         Ok(ImplSourceBuiltinData { nested })
     }
+
+    fn confirm_const_drop_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![] });
+        }
+
+        let tcx = self.tcx();
+        let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
+
+        let mut nested = vec![];
+        let cause = obligation.derived_cause(BuiltinDerivedObligation);
+
+        // If we have a custom `impl const Drop`, then
+        // first check it like a regular impl candidate
+        if let Some(impl_def_id) = impl_def_id {
+            nested.extend(self.confirm_impl_candidate(obligation, impl_def_id).nested);
+        }
+
+        // We want to confirm the ADT's fields if we have an ADT
+        let mut stack = match *self_ty.skip_binder().kind() {
+            ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
+            _ => vec![self_ty.skip_binder()],
+        };
+
+        while let Some(nested_ty) = stack.pop() {
+            match *nested_ty.kind() {
+                // We know these types are trivially drop
+                ty::Bool
+                | ty::Char
+                | ty::Int(_)
+                | ty::Uint(_)
+                | ty::Float(_)
+                | ty::Infer(ty::IntVar(_))
+                | ty::Infer(ty::FloatVar(_))
+                | ty::Str
+                | ty::RawPtr(_)
+                | ty::Ref(..)
+                | ty::FnDef(..)
+                | ty::FnPtr(_)
+                | ty::Never
+                | ty::Foreign(_) => {}
+
+                // These types are built-in, so we can fast-track by registering
+                // nested predicates for their constituient type(s)
+                ty::Array(ty, _) | ty::Slice(ty) => {
+                    stack.push(ty);
+                }
+                ty::Tuple(tys) => {
+                    stack.extend(tys.iter().map(|ty| ty.expect_ty()));
+                }
+                ty::Closure(_, substs) => {
+                    stack.push(substs.as_closure().tupled_upvars_ty());
+                }
+                ty::Generator(_, substs, _) => {
+                    let generator = substs.as_generator();
+                    stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
+                }
+                ty::GeneratorWitness(tys) => {
+                    stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
+                }
+
+                // If we have a projection type, make sure to normalize it so we replace it
+                // with a fresh infer variable
+                ty::Projection(..) => {
+                    self.infcx.commit_unconditionally(|_| {
+                        let predicate = normalize_with_depth_to(
+                            self,
+                            obligation.param_env,
+                            cause.clone(),
+                            obligation.recursion_depth + 1,
+                            self_ty
+                                .rebind(ty::TraitPredicate {
+                                    trait_ref: ty::TraitRef {
+                                        def_id: self.tcx().require_lang_item(LangItem::Drop, None),
+                                        substs: self.tcx().mk_substs_trait(nested_ty, &[]),
+                                    },
+                                    constness: ty::BoundConstness::ConstIfConst,
+                                    polarity: ty::ImplPolarity::Positive,
+                                })
+                                .to_predicate(tcx),
+                            &mut nested,
+                        );
+
+                        nested.push(Obligation::with_depth(
+                            cause.clone(),
+                            obligation.recursion_depth + 1,
+                            obligation.param_env,
+                            predicate,
+                        ));
+                    });
+                }
+
+                // If we have any other type (e.g. an ADT), just register a nested obligation
+                // since it's either not `const Drop` (and we raise an error during selection),
+                // or it's an ADT (and we need to check for a custom impl during selection)
+                _ => {
+                    let predicate = self_ty
+                        .rebind(ty::TraitPredicate {
+                            trait_ref: ty::TraitRef {
+                                def_id: self.tcx().require_lang_item(LangItem::Drop, None),
+                                substs: self.tcx().mk_substs_trait(nested_ty, &[]),
+                            },
+                            constness: ty::BoundConstness::ConstIfConst,
+                            polarity: ty::ImplPolarity::Positive,
+                        })
+                        .to_predicate(tcx);
+
+                    nested.push(Obligation::with_depth(
+                        cause.clone(),
+                        obligation.recursion_depth + 1,
+                        obligation.param_env,
+                        predicate,
+                    ));
+                }
+            }
+        }
+
+        Ok(ImplSourceConstDropData { nested })
+    }
 }

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

@@ -1153,7 +1153,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     GeneratorCandidate => {}
                     // FnDef where the function is const
                     FnPointerCandidate { is_const: true } => {}
-                    ConstDropCandidate => {}
+                    ConstDropCandidate(_) => {}
                     _ => {
                         // reject all other types of candidates
                         continue;
@@ -1537,7 +1537,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         };
 
         // (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
-        // and `DiscriminantKindCandidate` to anything else.
+        // `DiscriminantKindCandidate`, and `ConstDropCandidate` to anything else.
         //
         // This is a fix for #53123 and prevents winnowing from accidentally extending the
         // lifetime of a variable.
@@ -1554,7 +1554,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 BuiltinCandidate { has_nested: false }
                 | DiscriminantKindCandidate
                 | PointeeCandidate
-                | ConstDropCandidate,
+                | ConstDropCandidate(_),
                 _,
             ) => true,
             (
@@ -1562,7 +1562,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 BuiltinCandidate { has_nested: false }
                 | DiscriminantKindCandidate
                 | PointeeCandidate
-                | ConstDropCandidate,
+                | ConstDropCandidate(_),
             ) => false,
 
             (ParamCandidate(other), ParamCandidate(victim)) => {

compiler/rustc_typeck/src/check/dropck.rs

@@ -229,7 +229,13 @@ fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
             let p = p.kind();
             match (predicate.skip_binder(), p.skip_binder()) {
                 (ty::PredicateKind::Trait(a), ty::PredicateKind::Trait(b)) => {
-                    relator.relate(predicate.rebind(a), p.rebind(b)).is_ok()
+                    // Since struct predicates cannot have ~const, project the impl predicate
+                    // onto one that ignores the constness. This is equivalent to saying that
+                    // we match a `Trait` bound on the struct with a `Trait` or `~const Trait`
+                    // in the impl.
+                    let non_const_a =
+                        ty::TraitPredicate { constness: ty::BoundConstness::NotConst, ..a };
+                    relator.relate(predicate.rebind(non_const_a), p.rebind(b)).is_ok()
                 }
                 (ty::PredicateKind::Projection(a), ty::PredicateKind::Projection(b)) => {
                     relator.relate(predicate.rebind(a), p.rebind(b)).is_ok()

src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.precise.stderr

@@ -1,13 +1,5 @@
-error: `~const` is not allowed here
-  --> $DIR/const-drop-fail.rs:27:35
-   |
-LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
-   |                                   ^^^^^^^^
-   |
-   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
-
 error[E0277]: the trait bound `NonTrivialDrop: Drop` is not satisfied
-  --> $DIR/const-drop-fail.rs:45:5
+  --> $DIR/const-drop-fail.rs:44:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
@@ -16,50 +8,51 @@ LL |     NonTrivialDrop,
    |     ^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `NonTrivialDrop`
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:36:19
+  --> $DIR/const-drop-fail.rs:35:19
    |
 LL | const fn check<T: ~const Drop>(_: T) {}
    |                   ^^^^^^^^^^^ required by this bound in `check`
 
-error[E0277]: the trait bound `ConstImplWithDropGlue: Drop` is not satisfied
+error[E0277]: the trait bound `NonTrivialDrop: Drop` is not satisfied in `ConstImplWithDropGlue`
-  --> $DIR/const-drop-fail.rs:47:5
+  --> $DIR/const-drop-fail.rs:46:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
 ...
 LL |     ConstImplWithDropGlue(NonTrivialDrop),
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `ConstImplWithDropGlue`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ within `ConstImplWithDropGlue`, the trait `Drop` is not implemented for `NonTrivialDrop`
    |
+note: required because it appears within the type `ConstImplWithDropGlue`
+  --> $DIR/const-drop-fail.rs:17:8
+   |
+LL | struct ConstImplWithDropGlue(NonTrivialDrop);
+   |        ^^^^^^^^^^^^^^^^^^^^^
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:36:19
+  --> $DIR/const-drop-fail.rs:35:19
    |
 LL | const fn check<T: ~const Drop>(_: T) {}
    |                   ^^^^^^^^^^^ required by this bound in `check`
 
 error[E0277]: the trait bound `NonTrivialDrop: A` is not satisfied
-  --> $DIR/const-drop-fail.rs:49:5
+  --> $DIR/const-drop-fail.rs:48:5
-   |
-LL |     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `A` is not implemented for `NonTrivialDrop`
-   |
-note: required by a bound in `ConstDropImplWithBounds`
-  --> $DIR/const-drop-fail.rs:27:35
-   |
-LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
-   |                                   ^^^^^^^^ required by this bound in `ConstDropImplWithBounds`
-
-error[E0277]: the trait bound `NonTrivialDrop: A` is not satisfied
-  --> $DIR/const-drop-fail.rs:49:5
    |
+LL |         const _: () = check($exp);
+   |                       ----- required by a bound introduced by this call
+...
 LL |     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `A` is not implemented for `NonTrivialDrop`
    |
-note: required by a bound in `ConstDropImplWithBounds`
+note: required because of the requirements on the impl of `Drop` for `ConstDropImplWithBounds<NonTrivialDrop>`
-  --> $DIR/const-drop-fail.rs:27:35
+  --> $DIR/const-drop-fail.rs:29:25
    |
-LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
+LL | impl<T: ~const A> const Drop for ConstDropImplWithBounds<T> {
-   |                                   ^^^^^^^^ required by this bound in `ConstDropImplWithBounds`
+   |                         ^^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required by a bound in `check`
+  --> $DIR/const-drop-fail.rs:35:19
+   |
+LL | const fn check<T: ~const Drop>(_: T) {}
+   |                   ^^^^^^^^^^^ required by this bound in `check`
 
-error: aborting due to 5 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.rs

@@ -24,8 +24,7 @@ trait A { fn a() { println!("A"); } }
 
 impl A for NonTrivialDrop {}
 
-struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
+struct ConstDropImplWithBounds<T: A>(PhantomData<T>);
-//~^ ERROR `~const` is not allowed
 
 impl<T: ~const A> const Drop for ConstDropImplWithBounds<T> {
     fn drop(&mut self) {
@@ -48,7 +47,6 @@ check_all! {
     //~^ ERROR the trait bound
     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
     //~^ ERROR the trait bound
-    //~| ERROR the trait bound
 }
 
 fn main() {}

src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.stock.stderr

@@ -1,13 +1,5 @@
-error: `~const` is not allowed here
-  --> $DIR/const-drop-fail.rs:27:35
-   |
-LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
-   |                                   ^^^^^^^^
-   |
-   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
-
 error[E0277]: the trait bound `NonTrivialDrop: Drop` is not satisfied
-  --> $DIR/const-drop-fail.rs:45:5
+  --> $DIR/const-drop-fail.rs:44:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
@@ -16,50 +8,51 @@ LL |     NonTrivialDrop,
    |     ^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `NonTrivialDrop`
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:36:19
+  --> $DIR/const-drop-fail.rs:35:19
    |
 LL | const fn check<T: ~const Drop>(_: T) {}
    |                   ^^^^^^^^^^^ required by this bound in `check`
 
-error[E0277]: the trait bound `ConstImplWithDropGlue: Drop` is not satisfied
+error[E0277]: the trait bound `NonTrivialDrop: Drop` is not satisfied in `ConstImplWithDropGlue`
-  --> $DIR/const-drop-fail.rs:47:5
+  --> $DIR/const-drop-fail.rs:46:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
 ...
 LL |     ConstImplWithDropGlue(NonTrivialDrop),
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `ConstImplWithDropGlue`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ within `ConstImplWithDropGlue`, the trait `Drop` is not implemented for `NonTrivialDrop`
    |
+note: required because it appears within the type `ConstImplWithDropGlue`
+  --> $DIR/const-drop-fail.rs:17:8
+   |
+LL | struct ConstImplWithDropGlue(NonTrivialDrop);
+   |        ^^^^^^^^^^^^^^^^^^^^^
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:36:19
+  --> $DIR/const-drop-fail.rs:35:19
    |
 LL | const fn check<T: ~const Drop>(_: T) {}
    |                   ^^^^^^^^^^^ required by this bound in `check`
 
 error[E0277]: the trait bound `NonTrivialDrop: A` is not satisfied
-  --> $DIR/const-drop-fail.rs:49:5
+  --> $DIR/const-drop-fail.rs:48:5
-   |
-LL |     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `A` is not implemented for `NonTrivialDrop`
-   |
-note: required by a bound in `ConstDropImplWithBounds`
-  --> $DIR/const-drop-fail.rs:27:35
-   |
-LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
-   |                                   ^^^^^^^^ required by this bound in `ConstDropImplWithBounds`
-
-error[E0277]: the trait bound `NonTrivialDrop: A` is not satisfied
-  --> $DIR/const-drop-fail.rs:49:5
    |
+LL |         const _: () = check($exp);
+   |                       ----- required by a bound introduced by this call
+...
 LL |     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `A` is not implemented for `NonTrivialDrop`
    |
-note: required by a bound in `ConstDropImplWithBounds`
+note: required because of the requirements on the impl of `Drop` for `ConstDropImplWithBounds<NonTrivialDrop>`
-  --> $DIR/const-drop-fail.rs:27:35
+  --> $DIR/const-drop-fail.rs:29:25
    |
-LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
+LL | impl<T: ~const A> const Drop for ConstDropImplWithBounds<T> {
-   |                                   ^^^^^^^^ required by this bound in `ConstDropImplWithBounds`
+   |                         ^^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required by a bound in `check`
+  --> $DIR/const-drop-fail.rs:35:19
+   |
+LL | const fn check<T: ~const Drop>(_: T) {}
+   |                   ^^^^^^^^^^^ required by this bound in `check`
 
-error: aborting due to 5 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

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

@@ -3,6 +3,7 @@
 #![feature(const_trait_impl)]
 #![feature(const_fn_trait_bound)]
 #![feature(const_mut_refs)]
+#![feature(never_type)]
 #![cfg_attr(precise, feature(const_precise_live_drops))]
 
 struct S<'a>(&'a mut u8);
@@ -45,6 +46,33 @@ mod t {
 
     pub struct HasConstDrop(pub ConstDrop);
     pub struct TrivialFields(pub u8, pub i8, pub usize, pub isize);
+
+    pub trait SomeTrait {
+        fn foo();
+    }
+    impl const SomeTrait for () {
+        fn foo() {}
+    }
+    // non-const impl
+    impl SomeTrait for i32 {
+        fn foo() {}
+    }
+
+    pub struct ConstDropWithBound<T: SomeTrait>(pub core::marker::PhantomData<T>);
+
+    impl<T: ~const SomeTrait> const Drop for ConstDropWithBound<T> {
+        fn drop(&mut self) {
+            T::foo();
+        }
+    }
+
+    pub struct ConstDropWithNonconstBound<T: SomeTrait>(pub core::marker::PhantomData<T>);
+
+    impl<T: SomeTrait> const Drop for ConstDropWithNonconstBound<T> {
+        fn drop(&mut self) {
+            // Note: we DON'T use the `T: SomeTrait` bound
+        }
+    }
 }
 
 use t::*;
@@ -61,6 +89,9 @@ implements_const_drop! {
     TrivialFields(1, 2, 3, 4),
     &1,
     &1 as *const i32,
+    ConstDropWithBound::<()>,
+    ConstDropWithNonconstBound::<i32>,
+    Result::<i32, !>::Ok(1),
 }
 
 fn main() {