Implement ~const Destruct in new solver

Author: compiler-errors

compiler/rustc_const_eval/src/check_consts/qualifs.rs

@@ -2,11 +2,14 @@
 //!
 //! See the `Qualif` trait for more info.
 
+// FIXME(const_trait_impl): This API should be really reworked. It's dangerously general for
+// having basically only two use-cases that act in different ways.
+
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::LangItem;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::*;
-use rustc_middle::ty::{self, AdtDef, GenericArgsRef, Ty};
+use rustc_middle::ty::{self, AdtDef, Ty};
 use rustc_middle::{bug, mir};
 use rustc_trait_selection::traits::{Obligation, ObligationCause, ObligationCtxt};
 use tracing::instrument;
@@ -59,19 +62,9 @@ pub trait Qualif {
     /// It also determines the `Qualif`s for primitive types.
     fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool;
 
-    /// Returns `true` if this `Qualif` is inherent to the given struct or enum.
-    ///
-    /// By default, `Qualif`s propagate into ADTs in a structural way: An ADT only becomes
-    /// qualified if part of it is assigned a value with that `Qualif`. However, some ADTs *always*
-    /// have a certain `Qualif`, regardless of whether their fields have it. For example, a type
-    /// with a custom `Drop` impl is inherently `NeedsDrop`.
-    ///
-    /// Returning `true` for `in_adt_inherently` but `false` for `in_any_value_of_ty` is unsound.
-    fn in_adt_inherently<'tcx>(
-        cx: &ConstCx<'_, 'tcx>,
-        adt: AdtDef<'tcx>,
-        args: GenericArgsRef<'tcx>,
-    ) -> bool;
+    /// Returns `true` if the `Qualif` is not structural, i.e. that we should not recurse
+    /// into the operand.
+    fn is_non_structural<'tcx>(cx: &ConstCx<'_, 'tcx>, adt: AdtDef<'tcx>) -> bool;
 
     /// Returns `true` if this `Qualif` behaves sructurally for pointers and references:
     /// the pointer/reference qualifies if and only if the pointee qualifies.
@@ -101,6 +94,11 @@ impl Qualif for HasMutInterior {
             return false;
         }
 
+        // Avoid selecting for `UnsafeCell` either.
+        if ty.ty_adt_def().is_some_and(|adt| adt.is_unsafe_cell()) {
+            return true;
+        }
+
         // We do not use `ty.is_freeze` here, because that requires revealing opaque types, which
         // requires borrowck, which in turn will invoke mir_const_qualifs again, causing a cycle error.
         // Instead we invoke an obligation context manually, and provide the opaque type inference settings
@@ -129,11 +127,7 @@ impl Qualif for HasMutInterior {
         !errors.is_empty()
     }
 
-    fn in_adt_inherently<'tcx>(
-        _cx: &ConstCx<'_, 'tcx>,
-        adt: AdtDef<'tcx>,
-        _: GenericArgsRef<'tcx>,
-    ) -> bool {
+    fn is_non_structural<'tcx>(_cx: &ConstCx<'_, 'tcx>, adt: AdtDef<'tcx>) -> bool {
         // Exactly one type, `UnsafeCell`, has the `HasMutInterior` qualif inherently.
         // It arises structurally for all other types.
         adt.is_unsafe_cell()
@@ -144,6 +138,7 @@ impl Qualif for HasMutInterior {
     }
 }
 
+// FIXME(const_trait_impl): Get rid of this!
 /// Constant containing an ADT that implements `Drop`.
 /// This must be ruled out because implicit promotion would remove side-effects
 /// that occur as part of dropping that value. N.B., the implicit promotion has
@@ -163,11 +158,7 @@ impl Qualif for NeedsDrop {
         ty.needs_drop(cx.tcx, cx.typing_env)
     }
 
-    fn in_adt_inherently<'tcx>(
-        cx: &ConstCx<'_, 'tcx>,
-        adt: AdtDef<'tcx>,
-        _: GenericArgsRef<'tcx>,
-    ) -> bool {
+    fn is_non_structural<'tcx>(cx: &ConstCx<'_, 'tcx>, adt: AdtDef<'tcx>) -> bool {
         adt.has_dtor(cx.tcx)
     }
 
@@ -196,16 +187,32 @@ impl Qualif for NeedsNonConstDrop {
             return false;
         }
 
-        // FIXME(const_trait_impl): Reimplement const drop checking.
-        NeedsDrop::in_any_value_of_ty(cx, ty)
+        if cx.tcx.features().const_trait_impl() {
+            let destruct_def_id = cx.tcx.require_lang_item(LangItem::Destruct, Some(cx.body.span));
+            let infcx = cx.tcx.infer_ctxt().build(TypingMode::from_param_env(cx.param_env));
+            let ocx = ObligationCtxt::new(&infcx);
+            ocx.register_obligation(Obligation::new(
+                cx.tcx,
+                ObligationCause::misc(cx.body.span, cx.def_id()),
+                cx.param_env,
+                ty::Binder::dummy(ty::TraitRef::new(cx.tcx, destruct_def_id, [ty]))
+                    .to_host_effect_clause(cx.tcx, match cx.const_kind() {
+                        rustc_hir::ConstContext::ConstFn => ty::BoundConstness::Maybe,
+                        rustc_hir::ConstContext::Static(_)
+                        | rustc_hir::ConstContext::Const { .. } => ty::BoundConstness::Const,
+                    }),
+            ));
+            !ocx.select_all_or_error().is_empty()
+        } else {
+            NeedsDrop::in_any_value_of_ty(cx, ty)
+        }
     }
 
-    fn in_adt_inherently<'tcx>(
-        cx: &ConstCx<'_, 'tcx>,
-        adt: AdtDef<'tcx>,
-        _: GenericArgsRef<'tcx>,
-    ) -> bool {
-        adt.has_non_const_dtor(cx.tcx)
+    fn is_non_structural<'tcx>(cx: &ConstCx<'_, 'tcx>, adt: AdtDef<'tcx>) -> bool {
+        // Even a `const` dtor may have `~const` bounds that may need to
+        // be satisfied, so this becomes non-structural as soon as the
+        // ADT gets a destructor at all.
+        adt.has_dtor(cx.tcx)
     }
 
     fn deref_structural<'tcx>(_cx: &ConstCx<'_, 'tcx>) -> bool {
@@ -261,14 +268,10 @@ where
         Rvalue::Aggregate(kind, operands) => {
             // Return early if we know that the struct or enum being constructed is always
             // qualified.
-            if let AggregateKind::Adt(adt_did, _, args, ..) = **kind {
+            if let AggregateKind::Adt(adt_did, ..) = **kind {
                 let def = cx.tcx.adt_def(adt_did);
-                if Q::in_adt_inherently(cx, def, args) {
-                    return true;
-                }
-                // Don't do any value-based reasoning for unions.
-                if def.is_union() && Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) {
-                    return true;
+                if def.is_union() || Q::is_non_structural(cx, def) {
+                    return Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx));
                 }
             }
 

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -269,13 +269,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     );
                 }
                 Adjust::Deref(None) => {
-                    // FIXME(effects): We *could* enforce `&T: ~const Deref` here.
+                    // FIXME(const_trait_impl): We *could* enforce `&T: ~const Deref` here.
                 }
                 Adjust::Pointer(_pointer_coercion) => {
-                    // FIXME(effects): We should probably enforce these.
+                    // FIXME(const_trait_impl): We should probably enforce these.
                 }
                 Adjust::ReborrowPin(_mutability) => {
-                    // FIXME(effects): We could enforce these; they correspond to
+                    // FIXME(const_trait_impl): We could enforce these; they correspond to
                     // `&mut T: DerefMut` tho, so it's kinda moot.
                 }
                 Adjust::Borrow(_) => {

compiler/rustc_middle/src/ty/adt.rs

@@ -18,6 +18,7 @@ use rustc_macros::{HashStable, TyDecodable, TyEncodable};
 use rustc_query_system::ich::StableHashingContext;
 use rustc_session::DataTypeKind;
 use rustc_span::symbol::sym;
+use rustc_type_ir::solve::AdtDestructorKind;
 use tracing::{debug, info, trace};
 
 use super::{
@@ -232,6 +233,13 @@ impl<'tcx> rustc_type_ir::inherent::AdtDef<TyCtxt<'tcx>> for AdtDef<'tcx> {
     fn is_fundamental(self) -> bool {
         self.is_fundamental()
     }
+
+    fn destructor(self, tcx: TyCtxt<'tcx>) -> Option<AdtDestructorKind> {
+        Some(match self.destructor(tcx)?.constness {
+            hir::Constness::Const => AdtDestructorKind::Const,
+            hir::Constness::NotConst => AdtDestructorKind::NotConst,
+        })
+    }
 }
 
 #[derive(Copy, Clone, Debug, Eq, PartialEq, HashStable, TyEncodable, TyDecodable)]
@@ -402,10 +410,6 @@ impl<'tcx> AdtDef<'tcx> {
         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) -> &'tcx VariantDef {
         assert!(self.is_struct() || self.is_union());

compiler/rustc_middle/src/ty/context.rs

@@ -384,6 +384,10 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         self.is_conditionally_const(def_id)
     }
 
+    fn alias_has_const_conditions(self, def_id: DefId) -> bool {
+        self.is_conditionally_const(def_id)
+    }
+
     fn const_conditions(
         self,
         def_id: DefId,
@@ -663,6 +667,7 @@ bidirectional_lang_item_map! {
     CoroutineYield,
     Destruct,
     DiscriminantKind,
+    Drop,
     DynMetadata,
     Fn,
     FnMut,

compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs

@@ -12,7 +12,7 @@ use rustc_type_ir_macros::{TypeFoldable_Generic, TypeVisitable_Generic};
 use tracing::instrument;
 
 use crate::delegate::SolverDelegate;
-use crate::solve::{EvalCtxt, Goal, NoSolution};
+use crate::solve::{AdtDestructorKind, EvalCtxt, Goal, NoSolution};
 
 // Calculates the constituent types of a type for `auto trait` purposes.
 #[instrument(level = "trace", skip(ecx), ret)]
@@ -703,6 +703,78 @@ pub(in crate::solve) fn extract_fn_def_from_const_callable<I: Interner>(
     }
 }
 
+pub(in crate::solve) fn const_conditions_for_destruct<I: Interner>(
+    cx: I,
+    self_ty: I::Ty,
+) -> Result<Vec<ty::TraitRef<I>>, NoSolution> {
+    let destruct_def_id = cx.require_lang_item(TraitSolverLangItem::Destruct);
+
+    match self_ty.kind() {
+        // An ADT is `~const Destruct` only if all of the fields are,
+        // *and* if there is a `Drop` impl, that `Drop` impl is also `~const`.
+        ty::Adt(adt_def, args) => {
+            let mut const_conditions: Vec<_> = adt_def
+                .all_field_tys(cx)
+                .iter_instantiated(cx, args)
+                .map(|field_ty| ty::TraitRef::new(cx, destruct_def_id, [field_ty]))
+                .collect();
+            match adt_def.destructor(cx) {
+                // `Drop` impl exists, but it's not const. Type cannot be `~const Destruct`.
+                Some(AdtDestructorKind::NotConst) => return Err(NoSolution),
+                // `Drop` impl exists, and it's const. Require `Ty: ~const Drop` to hold.
+                Some(AdtDestructorKind::Const) => {
+                    let drop_def_id = cx.require_lang_item(TraitSolverLangItem::Drop);
+                    let drop_trait_ref = ty::TraitRef::new(cx, drop_def_id, [self_ty]);
+                    const_conditions.push(drop_trait_ref);
+                }
+                // No `Drop` impl, no need to require anything else.
+                None => {}
+            }
+            Ok(const_conditions)
+        }
+
+        ty::Array(ty, _) | ty::Pat(ty, _) | ty::Slice(ty) => {
+            Ok(vec![ty::TraitRef::new(cx, destruct_def_id, [ty])])
+        }
+
+        ty::Tuple(tys) => Ok(tys
+            .iter()
+            .map(|field_ty| ty::TraitRef::new(cx, destruct_def_id, [field_ty]))
+            .collect()),
+
+        // Trivially implement `~const Destruct`
+        ty::Bool
+        | ty::Char
+        | ty::Int(..)
+        | ty::Uint(..)
+        | ty::Float(..)
+        | ty::Str
+        | ty::RawPtr(..)
+        | ty::Ref(..)
+        | ty::FnDef(..)
+        | ty::FnPtr(..)
+        | ty::Never
+        | ty::Infer(ty::InferTy::FloatVar(_) | ty::InferTy::IntVar(_))
+        | ty::Error(_) => Ok(vec![]),
+
+        // Coroutines and closures could implement `~const Drop`,
+        // but they don't really need to right now.
+        ty::Closure(_, _)
+        | ty::CoroutineClosure(_, _)
+        | ty::Coroutine(_, _)
+        | ty::CoroutineWitness(_, _) => Err(NoSolution),
+
+        ty::Dynamic(..) | ty::Param(_) | ty::Alias(..) | ty::Placeholder(_) | ty::Foreign(_) => {
+            Err(NoSolution)
+        }
+
+        ty::Bound(..)
+        | ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
+            panic!("unexpected type `{self_ty:?}`")
+        }
+    }
+}
+
 /// Assemble a list of predicates that would be present on a theoretical
 /// user impl for an object type. These predicates must be checked any time
 /// we assemble a built-in object candidate for an object type, since they

compiler/rustc_next_trait_solver/src/solve/effect_goals.rs

@@ -84,6 +84,10 @@ where
         let cx = ecx.cx();
         let mut candidates = vec![];
 
+        if !ecx.cx().alias_has_const_conditions(alias_ty.def_id) {
+            return vec![];
+        }
+
         for clause in elaborate::elaborate(
             cx,
             cx.explicit_implied_const_bounds(alias_ty.def_id)
@@ -338,10 +342,27 @@ where
     }
 
     fn consider_builtin_destruct_candidate(
-        _ecx: &mut EvalCtxt<'_, D>,
-        _goal: Goal<I, Self>,
+        ecx: &mut EvalCtxt<'_, D>,
+        goal: Goal<I, Self>,
     ) -> Result<Candidate<I>, NoSolution> {
-        Err(NoSolution)
+        let cx = ecx.cx();
+
+        let self_ty = goal.predicate.self_ty();
+        let const_conditions = structural_traits::const_conditions_for_destruct(cx, self_ty)?;
+
+        ecx.probe_builtin_trait_candidate(BuiltinImplSource::Misc).enter(|ecx| {
+            ecx.add_goals(
+                GoalSource::Misc,
+                const_conditions.into_iter().map(|trait_ref| {
+                    goal.with(
+                        cx,
+                        ty::Binder::dummy(trait_ref)
+                            .to_host_effect_clause(cx, goal.predicate.constness),
+                    )
+                }),
+            );
+            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+        })
     }
 
     fn consider_builtin_transmute_candidate(

compiler/rustc_type_ir/src/inherent.rs

@@ -11,7 +11,7 @@ use rustc_ast_ir::Mutability;
 use crate::elaborate::Elaboratable;
 use crate::fold::{TypeFoldable, TypeSuperFoldable};
 use crate::relate::Relate;
-use crate::solve::Reveal;
+use crate::solve::{AdtDestructorKind, Reveal};
 use crate::visit::{Flags, TypeSuperVisitable, TypeVisitable};
 use crate::{self as ty, CollectAndApply, Interner, UpcastFrom};
 
@@ -537,6 +537,8 @@ pub trait AdtDef<I: Interner>: Copy + Debug + Hash + Eq {
     fn sized_constraint(self, interner: I) -> Option<ty::EarlyBinder<I, I::Ty>>;
 
     fn is_fundamental(self) -> bool;
+
+    fn destructor(self, interner: I) -> Option<AdtDestructorKind>;
 }
 
 pub trait ParamEnv<I: Interner>: Copy + Debug + Hash + Eq + TypeFoldable<I> {

compiler/rustc_type_ir/src/interner.rs

@@ -225,6 +225,7 @@ pub trait Interner:
 
     fn impl_is_const(self, def_id: Self::DefId) -> bool;
     fn fn_is_const(self, def_id: Self::DefId) -> bool;
+    fn alias_has_const_conditions(self, def_id: Self::DefId) -> bool;
     fn const_conditions(
         self,
         def_id: Self::DefId,

compiler/rustc_type_ir/src/lang_items.rs

@@ -19,6 +19,7 @@ pub enum TraitSolverLangItem {
     CoroutineYield,
     Destruct,
     DiscriminantKind,
+    Drop,
     DynMetadata,
     Fn,
     FnMut,

compiler/rustc_type_ir/src/solve/mod.rs

@@ -326,3 +326,10 @@ impl MaybeCause {
         }
     }
 }
+
+/// Indicates that a `impl Drop for Adt` is `const` or not.
+#[derive(Debug)]
+pub enum AdtDestructorKind {
+    NotConst,
+    Const,
+}