Auto merge of #88558 - fee1-dead:const-drop, r=oli-obk

Const drop

The changes are pretty primitive at this point. But at least it works. ^-^

Problems with the current change that I can think of now:
 - [x] `~const Drop` shouldn't change anything in the non-const world.
 - [x] types that do not have drop glues shouldn't fail to satisfy `~const Drop` in const contexts. `struct S { a: u8, b: u16 }` This might not fail for `needs_non_const_drop`, but it will fail in `rustc_trait_selection`.
 - [x] The current change accepts types that have `const Drop` impls but have non-const `Drop` glue.

Fixes #88424.

Significant Changes:

- `~const Drop` is no longer treated as a normal trait bound. In non-const contexts, this bound has no effect, but in const contexts, this restricts the input type and all of its transitive fields to either a) have a `const Drop` impl or b) can be trivially dropped (i.e. no drop glue)
- `T: ~const Drop` will not be linted like `T: Drop`.
- Instead of recursing and iterating through the type in `rustc_mir::transform::check_consts`, we use the trait system to special case `~const Drop`. See [`rustc_trait_selection::...::candidate_assembly#assemble_const_drop_candidates`](https://github.com/fee1-dead/rust/blob/const-drop/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs#L817) and others.

Changes not related to `const Drop`ping and/or changes that are insignificant:

 - `Node.constness_for_typeck` no longer returns `hir::Constness::Const` for type aliases in traits. This was previously used to hack how we determine default bound constness for items. But because we now use an explicit opt-in, it is no longer needed.
 - Removed `is_const_impl_raw` query. We have `impl_constness`, and the only existing use of that query uses `HirId`, which means we can just operate it with hir.
 - `ty::Destructor` now has a field `constness`, which represents the constness of the destructor.

r? `@oli-obk`

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1669,7 +1669,9 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 walk_list!(self, visit_ty, ty);
             }
             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, ref body))
-                if self.in_const_trait_impl || ctxt == AssocCtxt::Trait =>
+                if self.in_const_trait_impl
+                    || ctxt == AssocCtxt::Trait
+                    || matches!(sig.header.constness, Const::Yes(_)) =>
             {
                 self.visit_vis(&item.vis);
                 self.visit_ident(item.ident);

compiler/rustc_const_eval/src/const_eval/fn_queries.rs

@@ -1,5 +1,5 @@
 use rustc_hir as hir;
-use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::def_id::DefId;
 use rustc_middle::hir::map::blocks::FnLikeNode;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::TyCtxt;
@@ -34,8 +34,14 @@ pub fn is_unstable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Symbol> {
 }
 
 pub fn is_parent_const_impl_raw(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
-    let parent_id = tcx.hir().get_parent_did(hir_id);
-    if !parent_id.is_top_level_module() { is_const_impl_raw(tcx, parent_id) } else { false }
+    let parent_id = tcx.hir().get_parent_node(hir_id);
+    matches!(
+        tcx.hir().get(parent_id),
+        hir::Node::Item(hir::Item {
+            kind: hir::ItemKind::Impl(hir::Impl { constness: hir::Constness::Const, .. }),
+            ..
+        })
+    )
 }
 
 /// Checks whether the function has a `const` modifier or, in case it is an intrinsic, whether
@@ -70,19 +76,6 @@ fn is_const_fn_raw(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
     }
 }
 
-/// Checks whether the given item is an `impl` that has a `const` modifier.
-fn is_const_impl_raw(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
-    let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
-    let node = tcx.hir().get(hir_id);
-    matches!(
-        node,
-        hir::Node::Item(hir::Item {
-            kind: hir::ItemKind::Impl(hir::Impl { constness: hir::Constness::Const, .. }),
-            ..
-        })
-    )
-}
-
 fn is_promotable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
     is_const_fn(tcx, def_id)
         && match tcx.lookup_const_stability(def_id) {
@@ -103,10 +96,5 @@ fn is_promotable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 }
 
 pub fn provide(providers: &mut Providers) {
-    *providers = Providers {
-        is_const_fn_raw,
-        is_const_impl_raw: |tcx, def_id| is_const_impl_raw(tcx, def_id.expect_local()),
-        is_promotable_const_fn,
-        ..*providers
-    };
+    *providers = Providers { is_const_fn_raw, is_promotable_const_fn, ..*providers };
 }

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

@@ -22,7 +22,7 @@ use std::mem;
 use std::ops::Deref;
 
 use super::ops::{self, NonConstOp, Status};
-use super::qualifs::{self, CustomEq, HasMutInterior, NeedsDrop};
+use super::qualifs::{self, CustomEq, HasMutInterior, NeedsNonConstDrop};
 use super::resolver::FlowSensitiveAnalysis;
 use super::{is_lang_panic_fn, ConstCx, Qualif};
 use crate::const_eval::is_unstable_const_fn;
@@ -39,7 +39,7 @@ type QualifResults<'mir, 'tcx, Q> =
 #[derive(Default)]
 pub struct Qualifs<'mir, 'tcx> {
     has_mut_interior: Option<QualifResults<'mir, 'tcx, HasMutInterior>>,
-    needs_drop: Option<QualifResults<'mir, 'tcx, NeedsDrop>>,
+    needs_drop: Option<QualifResults<'mir, 'tcx, NeedsNonConstDrop>>,
     indirectly_mutable: Option<IndirectlyMutableResults<'mir, 'tcx>>,
 }
 
@@ -80,14 +80,14 @@ impl Qualifs<'mir, 'tcx> {
         location: Location,
     ) -> bool {
         let ty = ccx.body.local_decls[local].ty;
-        if !NeedsDrop::in_any_value_of_ty(ccx, ty) {
+        if !NeedsNonConstDrop::in_any_value_of_ty(ccx, ty) {
             return false;
         }
 
         let needs_drop = self.needs_drop.get_or_insert_with(|| {
             let ConstCx { tcx, body, .. } = *ccx;
 
-            FlowSensitiveAnalysis::new(NeedsDrop, ccx)
+            FlowSensitiveAnalysis::new(NeedsNonConstDrop, ccx)
                 .into_engine(tcx, &body)
                 .iterate_to_fixpoint()
                 .into_results_cursor(&body)
@@ -988,12 +988,12 @@ impl Visitor<'tcx> for Checker<'mir, 'tcx> {
 
                 let mut err_span = self.span;
 
-                // Check to see if the type of this place can ever have a drop impl. If not, this
-                // `Drop` terminator is frivolous.
-                let ty_needs_drop =
-                    dropped_place.ty(self.body, self.tcx).ty.needs_drop(self.tcx, self.param_env);
+                let ty_needs_non_const_drop = qualifs::NeedsNonConstDrop::in_any_value_of_ty(
+                    self.ccx,
+                    dropped_place.ty(self.body, self.tcx).ty,
+                );
 
-                if !ty_needs_drop {
+                if !ty_needs_non_const_drop {
                     return;
                 }
 

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

@@ -5,7 +5,7 @@ use rustc_span::Span;
 
 use super::check::Qualifs;
 use super::ops::{self, NonConstOp};
-use super::qualifs::{NeedsDrop, Qualif};
+use super::qualifs::{NeedsNonConstDrop, Qualif};
 use super::ConstCx;
 
 /// Returns `true` if we should use the more precise live drop checker that runs after drop
@@ -78,10 +78,10 @@ impl Visitor<'tcx> for CheckLiveDrops<'mir, 'tcx> {
         match &terminator.kind {
             mir::TerminatorKind::Drop { place: dropped_place, .. } => {
                 let dropped_ty = dropped_place.ty(self.body, self.tcx).ty;
-                if !NeedsDrop::in_any_value_of_ty(self.ccx, dropped_ty) {
-                    bug!(
-                        "Drop elaboration left behind a Drop for a type that does not need dropping"
-                    );
+                if !NeedsNonConstDrop::in_any_value_of_ty(self.ccx, dropped_ty) {
+                    // Instead of throwing a bug, we just return here. This is because we have to
+                    // run custom `const Drop` impls.
+                    return;
                 }
 
                 if dropped_place.is_indirect() {

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

@@ -3,10 +3,14 @@
 //! 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};
 use rustc_span::DUMMY_SP;
-use rustc_trait_selection::traits;
+use rustc_trait_selection::traits::{
+    self, ImplSource, Obligation, ObligationCause, SelectionContext,
+};
 
 use super::ConstCx;
 
@@ -17,7 +21,7 @@ pub fn in_any_value_of_ty(
 ) -> ConstQualifs {
     ConstQualifs {
         has_mut_interior: HasMutInterior::in_any_value_of_ty(cx, ty),
-        needs_drop: NeedsDrop::in_any_value_of_ty(cx, ty),
+        needs_drop: NeedsNonConstDrop::in_any_value_of_ty(cx, ty),
         custom_eq: CustomEq::in_any_value_of_ty(cx, ty),
         error_occured,
     }
@@ -97,22 +101,48 @@ impl Qualif for HasMutInterior {
 /// This must be ruled out (a) because we cannot run `Drop` during compile-time
 /// as that might not be a `const fn`, and (b) because implicit promotion would
 /// remove side-effects that occur as part of dropping that value.
-pub struct NeedsDrop;
+pub struct NeedsNonConstDrop;
 
-impl Qualif for NeedsDrop {
-    const ANALYSIS_NAME: &'static str = "flow_needs_drop";
+impl Qualif for NeedsNonConstDrop {
+    const ANALYSIS_NAME: &'static str = "flow_needs_nonconst_drop";
     const IS_CLEARED_ON_MOVE: bool = true;
 
     fn in_qualifs(qualifs: &ConstQualifs) -> bool {
         qualifs.needs_drop
     }
 
     fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
-        ty.needs_drop(cx.tcx, cx.param_env)
+        let drop_trait = if let Some(did) = cx.tcx.lang_items().drop_trait() {
+            did
+        } else {
+            // there is no way to define a type that needs non-const drop
+            // 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,
+                constness: ty::BoundConstness::ConstIfConst,
+            }),
+        );
+
+        let implsrc = cx.tcx.infer_ctxt().enter(|infcx| {
+            let mut selcx = SelectionContext::with_constness(&infcx, hir::Constness::Const);
+            selcx.select(&obligation)
+        });
+        match implsrc {
+            Ok(Some(ImplSource::ConstDrop(_)))
+            | Ok(Some(ImplSource::Param(_, ty::BoundConstness::ConstIfConst))) => false,
+            _ => true,
+        }
     }
 
     fn in_adt_inherently(cx: &ConstCx<'_, 'tcx>, adt: &'tcx AdtDef, _: SubstsRef<'tcx>) -> bool {
-        adt.has_dtor(cx.tcx)
+        adt.has_non_const_dtor(cx.tcx)
     }
 }
 

compiler/rustc_const_eval/src/transform/promote_consts.rs

@@ -231,7 +231,7 @@ impl<'tcx> Validator<'_, 'tcx> {
 
                         // We cannot promote things that need dropping, since the promoted value
                         // would not get dropped.
-                        if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
+                        if self.qualif_local::<qualifs::NeedsNonConstDrop>(place.local) {
                             return Err(Unpromotable);
                         }
 

compiler/rustc_hir/src/hir.rs

@@ -3234,12 +3234,7 @@ impl<'hir> Node<'hir> {
         }
     }
 
-    /// Returns `Constness::Const` when this node is a const fn/impl/item,
-    ///
-    /// HACK(fee1-dead): or an associated type in a trait. This works because
-    /// only typeck cares about const trait predicates, so although the predicates
-    /// query would return const predicates when it does not need to be const,
-    /// it wouldn't have any effect.
+    /// Returns `Constness::Const` when this node is a const fn/impl/item.
     pub fn constness_for_typeck(&self) -> Constness {
         match self {
             Node::Item(Item {
@@ -3258,7 +3253,6 @@ impl<'hir> Node<'hir> {
 
             Node::Item(Item { kind: ItemKind::Const(..), .. })
             | Node::TraitItem(TraitItem { kind: TraitItemKind::Const(..), .. })
-            | Node::TraitItem(TraitItem { kind: TraitItemKind::Type(..), .. })
             | Node::ImplItem(ImplItem { kind: ImplItemKind::Const(..), .. }) => Constness::Const,
 
             _ => Constness::NotConst,

compiler/rustc_lint/src/traits.rs

@@ -86,6 +86,7 @@ declare_lint_pass!(
 
 impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
+        use rustc_middle::ty;
         use rustc_middle::ty::PredicateKind::*;
 
         let predicates = cx.tcx.explicit_predicates_of(item.def_id);
@@ -94,6 +95,10 @@ impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
                 Trait(trait_predicate) => trait_predicate,
                 _ => continue,
             };
+            if trait_predicate.constness == ty::BoundConstness::ConstIfConst {
+                // `~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/query/mod.rs

@@ -552,14 +552,6 @@ rustc_queries! {
         desc { |tcx| "checking if item is const fn: `{}`", tcx.def_path_str(key) }
     }
 
-    /// Returns `true` if this is a const `impl`. **Do not call this function manually.**
-    ///
-    /// This query caches the base data for the `is_const_impl` helper function, which also
-    /// takes into account stability attributes (e.g., `#[rustc_const_unstable]`).
-    query is_const_impl_raw(key: DefId) -> bool {
-        desc { |tcx| "checking if item is const impl: `{}`", tcx.def_path_str(key) }
-    }
-
     query asyncness(key: DefId) -> hir::IsAsync {
         desc { |tcx| "checking if the function is async: `{}`", tcx.def_path_str(key) }
     }

compiler/rustc_middle/src/traits/mod.rs

@@ -529,6 +529,9 @@ pub enum ImplSource<'tcx, N> {
 
     /// ImplSource for a trait alias.
     TraitAlias(ImplSourceTraitAliasData<'tcx, N>),
+
+    /// ImplSource for a `const Drop` implementation.
+    ConstDrop(ImplSourceConstDropData),
 }
 
 impl<'tcx, N> ImplSource<'tcx, N> {
@@ -543,7 +546,8 @@ impl<'tcx, N> ImplSource<'tcx, N> {
             ImplSource::Object(d) => d.nested,
             ImplSource::FnPointer(d) => d.nested,
             ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
-            | ImplSource::Pointee(ImplSourcePointeeData) => Vec::new(),
+            | ImplSource::Pointee(ImplSourcePointeeData)
+            | ImplSource::ConstDrop(ImplSourceConstDropData) => Vec::new(),
             ImplSource::TraitAlias(d) => d.nested,
             ImplSource::TraitUpcasting(d) => d.nested,
         }
@@ -560,7 +564,8 @@ 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[..],
         }
@@ -621,6 +626,9 @@ impl<'tcx, N> ImplSource<'tcx, N> {
                     nested: d.nested.into_iter().map(f).collect(),
                 })
             }
+            ImplSource::ConstDrop(ImplSourceConstDropData) => {
+                ImplSource::ConstDrop(ImplSourceConstDropData)
+            }
         }
     }
 }
@@ -712,6 +720,9 @@ pub struct ImplSourceDiscriminantKindData;
 #[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
 pub struct ImplSourcePointeeData;
 
+#[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
+pub struct ImplSourceConstDropData;
+
 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
 pub struct ImplSourceTraitAliasData<'tcx, N> {
     pub alias_def_id: DefId,

compiler/rustc_middle/src/traits/select.rs

@@ -143,6 +143,9 @@ pub enum SelectionCandidate<'tcx> {
     BuiltinObjectCandidate,
 
     BuiltinUnsizeCandidate,
+
+    /// Implementation of `const Drop`.
+    ConstDropCandidate,
 }
 
 /// The result of trait evaluation. The order is important

compiler/rustc_middle/src/traits/structural_impls.rs

@@ -32,6 +32,8 @@ impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSource<'tcx, N> {
             super::ImplSource::TraitAlias(ref d) => write!(f, "{:?}", d),
 
             super::ImplSource::TraitUpcasting(ref d) => write!(f, "{:?}", d),
+
+            super::ImplSource::ConstDrop(ref d) => write!(f, "{:?}", d),
         }
     }
 }
@@ -125,4 +127,5 @@ TrivialTypeFoldableAndLiftImpls! {
     super::IfExpressionCause,
     super::ImplSourceDiscriminantKindData,
     super::ImplSourcePointeeData,
+    super::ImplSourceConstDropData,
 }

compiler/rustc_middle/src/ty/adt.rs

@@ -7,6 +7,7 @@ use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_errors::ErrorReported;
+use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
 use rustc_index::vec::{Idx, IndexVec};
@@ -288,6 +289,10 @@ impl<'tcx> AdtDef {
         self.destructor(tcx).is_some()
     }
 
+    pub fn has_non_const_dtor(&self, tcx: TyCtxt<'tcx>) -> bool {
+        matches!(self.destructor(tcx), Some(Destructor { constness: hir::Constness::NotConst, .. }))
+    }
+
     /// Asserts this is a struct or union and returns its unique variant.
     pub fn non_enum_variant(&self) -> &VariantDef {
         assert!(self.is_struct() || self.is_union());

compiler/rustc_middle/src/ty/mod.rs

@@ -1377,6 +1377,8 @@ where
 pub struct Destructor {
     /// The `DefId` of the destructor method
     pub did: DefId,
+    /// The constness of the destructor method
+    pub constness: hir::Constness,
 }
 
 bitflags! {