[POC] create an MVP for using Destruct for custom dtors

compiler/rustc_attr_ir/src/lang_items.rs

@@ -192,6 +192,7 @@ language_item_table! {
 
     Drop,                    sym::drop,                drop_trait,                 Target::Trait,          GenericRequirement::None;
     Destruct,                sym::destruct,            destruct_trait,             Target::Trait,          GenericRequirement::None;
+    DestructDropInPlace,      sym::destruct_drop_in_place, destruct_drop_in_place,  Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
     AsyncDrop,               sym::async_drop,          async_drop_trait,           Target::Trait,          GenericRequirement::None;
     AsyncDropInPlace,        sym::async_drop_in_place, async_drop_in_place_fn,     Target::Fn,             GenericRequirement::Exact(1);
 

compiler/rustc_codegen_cranelift/example/mini_core.rs

@@ -50,7 +50,13 @@ pub trait MetaSized: PointeeSized {}
 pub trait Sized: MetaSized {}
 
 #[lang = "destruct"]
-pub trait Destruct {}
+pub trait Destruct {
+    /// Entrypoint for drop
+    ///
+    /// Generated by default if not implemented manually.
+    #[lang = "destruct_drop_in_place"]
+    unsafe fn drop_in_place(_to_drop: &mut Self);
+}
 
 #[lang = "tuple_trait"]
 pub trait Tuple {}

compiler/rustc_codegen_cranelift/src/abi/mod.rs

@@ -743,7 +743,7 @@ pub(crate) fn codegen_drop<'tcx>(
     unwind: UnwindAction,
 ) {
     let ty = drop_place.layout().ty;
-    let drop_instance = Instance::resolve_drop_glue(fx.tcx, ty);
+    let drop_instance = Instance::resolve_drop_in_place(fx.tcx, ty);
     let ret_block = fx.get_block(target);
 
     // AsyncDropGlueCtorShim can't be here

compiler/rustc_codegen_gcc/example/mini_core.rs

@@ -54,7 +54,13 @@ pub trait MetaSized: PointeeSized {}
 pub trait Sized: MetaSized {}
 
 #[lang = "destruct"]
-pub trait Destruct {}
+pub trait Destruct {
+    /// Entrypoint for drop
+    ///
+    /// Generated by default if not implemented manually.
+    #[lang = "destruct_drop_in_place"]
+    unsafe fn drop_in_place(_to_drop: &mut Self);
+}
 
 #[lang = "tuple_trait"]
 pub trait Tuple {}

compiler/rustc_codegen_ssa/src/back/symbol_export.rs

@@ -503,7 +503,7 @@ fn upstream_monomorphizations_provider(
 
     let mut instances: DefIdMap<UnordMap<_, _>> = Default::default();
 
-    let drop_glue_fn_def_id = tcx.lang_items().drop_glue_fn();
+    let drop_in_place_fn_def_id = tcx.lang_items().destruct_drop_in_place();
     let async_drop_in_place_fn_def_id = tcx.lang_items().async_drop_in_place_fn();
 
     for &cnum in cnums.iter() {
@@ -519,7 +519,7 @@ fn upstream_monomorphizations_provider(
             let (def_id, args) = match *exported_symbol {
                 ExportedSymbol::Generic(def_id, args) => (def_id, args),
                 ExportedSymbol::DropGlue(ty) => {
-                    if let Some(drop_in_place_fn_def_id) = drop_glue_fn_def_id {
+                    if let Some(drop_in_place_fn_def_id) = drop_in_place_fn_def_id {
                         (drop_in_place_fn_def_id, tcx.mk_args(&[ty.into()]))
                     } else {
                         // `drop_glue` does not exist, don't try to use it.
@@ -572,7 +572,7 @@ fn upstream_drop_glue_for_provider<'tcx>(
     tcx: TyCtxt<'tcx>,
     args: GenericArgsRef<'tcx>,
 ) -> Option<CrateNum> {
-    let def_id = tcx.lang_items().drop_glue_fn()?;
+    let def_id = tcx.lang_items().destruct_drop_in_place()?;
     tcx.upstream_monomorphizations_for(def_id)?.get(&args).cloned()
 }
 
@@ -694,7 +694,7 @@ pub(crate) fn symbol_name_for_instance_in_crate<'tcx>(
         }
         ExportedSymbol::DropGlue(ty) => rustc_symbol_mangling::symbol_name_for_instance_in_crate(
             tcx,
-            Instance::resolve_drop_glue(tcx, ty),
+            Instance::resolve_drop_in_place(tcx, ty),
             instantiating_crate,
         ),
         ExportedSymbol::AsyncDropGlueCtorShim(ty) => {

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -641,7 +641,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     ) -> MergingSucc {
         let ty = location.ty(self.mir, bx.tcx()).ty;
         let ty = self.monomorphize(ty);
-        let drop_fn = Instance::resolve_drop_glue(bx.tcx(), ty);
+        let drop_fn = Instance::resolve_drop_in_place(bx.tcx(), ty);
 
         if let ty::InstanceKind::Shim(ty::ShimKind::DropGlue(_, None)) = drop_fn.def {
             // we don't actually need to drop anything.

compiler/rustc_const_eval/src/interpret/call.rs

@@ -1015,15 +1015,15 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             _ => {
                 debug_assert_eq!(
                     instance,
-                    ty::Instance::resolve_drop_glue(*self.tcx, place.layout.ty)
+                    ty::Instance::resolve_drop_in_place(*self.tcx, place.layout.ty)
                 );
                 place
             }
         };
 
         let instance = {
             let _trace = enter_trace_span!(M, resolve::resolve_drop_glue, ty = ?place.layout.ty);
-            ty::Instance::resolve_drop_glue(*self.tcx, place.layout.ty)
+            ty::Instance::resolve_drop_in_place(*self.tcx, place.layout.ty)
         };
         let fn_abi = self.fn_abi_of_instance_no_deduced_attrs(instance, ty::List::empty())?;
 

compiler/rustc_const_eval/src/interpret/step.rs

@@ -631,7 +631,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 let instance = {
                     let _trace =
                         enter_trace_span!(M, resolve::resolve_drop_glue, ty = ?place.layout.ty);
-                    Instance::resolve_drop_glue(*self.tcx, place.layout.ty)
+                    Instance::resolve_drop_in_place(*self.tcx, place.layout.ty)
                 };
                 if let ty::InstanceKind::Shim(ty::ShimKind::DropGlue(_, None)) = instance.def {
                     // This is the branch we enter if and only if the dropped type has no drop glue

compiler/rustc_middle/src/middle/exported_symbols.rs

@@ -66,7 +66,7 @@ impl<'tcx> ExportedSymbol<'tcx> {
                 tcx.symbol_name(ty::Instance::new_raw(def_id, args))
             }
             ExportedSymbol::DropGlue(ty) => {
-                tcx.symbol_name(ty::Instance::resolve_drop_glue(tcx, ty))
+                tcx.symbol_name(ty::Instance::resolve_drop_in_place(tcx, ty))
             }
             ExportedSymbol::AsyncDropGlueCtorShim(ty) => {
                 tcx.symbol_name(ty::Instance::resolve_async_drop_in_place(tcx, ty))

compiler/rustc_middle/src/ty/instance.rs

@@ -160,7 +160,7 @@ pub enum ShimKind<'tcx> {
     /// Proxy shim for async drop of future (def_id, proxy_cor_ty, impl_cor_ty)
     FutureDropPoll(DefId, Ty<'tcx>, Ty<'tcx>),
 
-    /// `core::ptr::drop_glue::<T>`.
+    /// `Destruct::drop_in_place()`
     ///
     /// The `DefId` is for `core::ptr::drop_glue`.
     /// The `Option<Ty<'tcx>>` is either `Some(T)`, or `None` for empty drop glue.
@@ -796,8 +796,8 @@ impl<'tcx> Instance<'tcx> {
         }
     }
 
-    pub fn resolve_drop_glue(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> ty::Instance<'tcx> {
-        let def_id = tcx.require_lang_item(LangItem::DropGlue, DUMMY_SP);
+    pub fn resolve_drop_in_place(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> ty::Instance<'tcx> {
+        let def_id = tcx.require_lang_item(LangItem::DestructDropInPlace, DUMMY_SP);
         let args = tcx.mk_args(&[ty.into()]);
         Instance::expect_resolve(
             tcx,
@@ -808,6 +808,16 @@ impl<'tcx> Instance<'tcx> {
         )
     }
 
+    pub fn try_resolve_drop_in_place(
+        tcx: TyCtxt<'tcx>,
+        typing_env: ty::TypingEnv<'tcx>,
+        ty: Ty<'tcx>,
+    ) -> Result<Option<Instance<'tcx>>, ErrorGuaranteed> {
+        let def_id = tcx.require_lang_item(LangItem::DestructDropInPlace, DUMMY_SP);
+        let args = tcx.mk_args(&[ty.into()]);
+        Instance::try_resolve(tcx, typing_env, def_id, args)
+    }
+
     pub fn resolve_async_drop_in_place(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> ty::Instance<'tcx> {
         let def_id = tcx.require_lang_item(LangItem::AsyncDropInPlace, DUMMY_SP);
         let args = tcx.mk_args(&[ty.into()]);

compiler/rustc_middle/src/ty/vtable.rs

@@ -122,7 +122,7 @@ pub(super) fn vtable_allocation_provider<'tcx>(
         let scalar = match *entry {
             VtblEntry::MetadataDropInPlace => {
                 if ty.needs_drop(tcx, ty::TypingEnv::fully_monomorphized()) {
-                    let instance = ty::Instance::resolve_drop_glue(tcx, ty);
+                    let instance = ty::Instance::resolve_drop_in_place(tcx, ty);
                     let fn_alloc_id = tcx.reserve_and_set_fn_alloc(instance, CTFE_ALLOC_SALT);
                     let fn_ptr = Pointer::from(fn_alloc_id);
                     Scalar::from_pointer(fn_ptr, &tcx)

compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs

@@ -128,7 +128,7 @@ impl RemoveNoopLandingPads {
                         extra.typing_env,
                         ty::EarlyBinder::bind(extra.tcx, ty),
                     );
-                    let drop_fn = Instance::resolve_drop_glue(extra.tcx, ty);
+                    let drop_fn = Instance::resolve_drop_in_place(extra.tcx, ty);
                     if let ty::InstanceKind::Shim(ty::ShimKind::DropGlue(_, None)) = drop_fn.def {
                         // no need to drop anything, if all of our successors are also no-op then we
                         // can be skipped.

compiler/rustc_monomorphize/src/collector.rs

@@ -953,7 +953,7 @@ fn visit_drop_use<'tcx>(
     source: Span,
     output: &mut MonoItems<'tcx>,
 ) {
-    let instance = Instance::resolve_drop_glue(tcx, ty);
+    let instance = Instance::resolve_drop_in_place(tcx, ty);
     visit_instance_use(tcx, instance, is_direct_call, source, output);
 }
 
@@ -1074,6 +1074,9 @@ fn visit_instance_use<'tcx>(
 /// Returns `true` if we should codegen an instance in the local crate, or returns `false` if we
 /// can just link to the upstream crate and therefore don't need a mono item.
 fn should_codegen_locally<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) -> bool {
+    if let ty::InstanceKind::Shim(ty::ShimKind::DropGlue(_, Some(_))) = instance.def {
+        return instance.upstream_monomorphization(tcx).is_none();
+    }
     let Some(def_id) = instance.def.def_id_if_not_guaranteed_local_codegen() else {
         return true;
     };

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

@@ -478,37 +478,50 @@ where
 
         match assemble_from {
             AssembleCandidatesFrom::All => {
-                self.assemble_builtin_impl_candidates(goal, &mut candidates)?;
-                // For performance we only assemble impls if there are no candidates
-                // which would shadow them. This is necessary to avoid hangs in rayon,
-                // see trait-system-refactor-initiative#109 for more details.
-                //
-                // We always assemble builtin impls as trivial builtin impls have a higher
-                // priority than where-clauses.
-                //
-                // We only do this if any such candidate applies without any constraints
-                // as we may want to weaken inference guidance in the future and don't want
-                // to worry about causing major performance regressions when doing so.
-                // See trait-system-refactor-initiative#226 for some ideas here.
-                let assemble_impls = match self.typing_mode() {
-                    TypingMode::Coherence => true,
-                    TypingMode::Typeck { .. }
-                    | TypingMode::PostTypeckUntilBorrowck { .. }
-                    | TypingMode::Reflection
-                    | TypingMode::PostBorrowck { .. }
-                    | TypingMode::PostAnalysis
-                    | TypingMode::Codegen
-                    | TypingMode::ErasedNotCoherence(MayBeErased) => !candidates.iter().any(|c| {
-                        matches!(
-                            c.source,
-                            CandidateSource::ParamEnv(ParamEnvSource::NonGlobal)
-                                | CandidateSource::AliasBound(_)
-                        ) && has_no_inference_or_external_constraints(c.result)
-                    }),
-                };
-                if assemble_impls {
+                let trait_def_id = goal.predicate.trait_def_id(self.cx());
+                // Check if there are any user defined impls for Destruct. If there are,
+                // use those, and fallback to builtin drop glue impl if none are present
+                if self.cx().is_trait_lang_item(trait_def_id, SolverTraitLangItem::Destruct) {
                     self.assemble_impl_candidates(goal, &mut candidates)?;
+                    let has_impl_candidate =
+                        candidates.iter().any(|c| matches!(c.source, CandidateSource::Impl(_)));
+                    if !has_impl_candidate {
+                        self.assemble_builtin_impl_candidates(goal, &mut candidates)?;
+                    }
                     self.assemble_object_bound_candidates(goal, &mut candidates);
+                } else {
+                    self.assemble_builtin_impl_candidates(goal, &mut candidates)?;
+                    // For performance we only assemble impls if there are no candidates
+                    // which would shadow them. This is necessary to avoid hangs in rayon,
+                    // see trait-system-refactor-initiative#109 for more details.
+                    //
+                    // We always assemble builtin impls as trivial builtin impls have a higher
+                    // priority than where-clauses.
+                    //
+                    // We only do this if any such candidate applies without any constraints
+                    // as we may want to weaken inference guidance in the future and don't want
+                    // to worry about causing major performance regressions when doing so.
+                    // See trait-system-refactor-initiative#226 for some ideas here.
+                    let assemble_impls = match self.typing_mode() {
+                        TypingMode::Coherence => true,
+                        TypingMode::Typeck { .. }
+                        | TypingMode::PostTypeckUntilBorrowck { .. }
+                        | TypingMode::PostBorrowck { .. }
+                        | TypingMode::Reflection
+                        | TypingMode::PostAnalysis
+                        | TypingMode::Codegen
+                        | TypingMode::ErasedNotCoherence(_) => !candidates.iter().any(|c| {
+                            matches!(
+                                c.source,
+                                CandidateSource::ParamEnv(ParamEnvSource::NonGlobal)
+                                    | CandidateSource::AliasBound(_)
+                            ) && has_no_inference_or_external_constraints(c.result)
+                        }),
+                    };
+                    if assemble_impls {
+                        self.assemble_impl_candidates(goal, &mut candidates)?;
+                        self.assemble_object_bound_candidates(goal, &mut candidates);
+                    }
                 }
             }
             AssembleCandidatesFrom::EnvAndBounds => {

compiler/rustc_public_bridge/src/context/impls.rs

@@ -701,7 +701,7 @@ impl<'tcx, B: Bridge> CompilerCtxt<'tcx, B> {
 
     /// Resolve an instance for drop_in_place for the given type.
     pub fn resolve_drop_in_place(&self, internal_ty: Ty<'tcx>) -> Instance<'tcx> {
-        let instance = Instance::resolve_drop_glue(self.tcx, internal_ty);
+        let instance = Instance::resolve_drop_in_place(self.tcx, internal_ty);
         instance
     }
 

compiler/rustc_span/src/symbol.rs

@@ -814,6 +814,7 @@ symbols! {
         derive_from,
         derive_smart_pointer,
         destruct,
+        destruct_drop_in_place,
         destructuring_assignment,
         diagnostic,
         diagnostic_namespace,

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

@@ -114,7 +114,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     self.assemble_candidates_for_unsizing(obligation, &mut candidates);
                 }
                 Some(LangItem::Destruct) => {
-                    self.assemble_const_destruct_candidates(obligation, &mut candidates);
+                    let before = candidates.vec.len();
+                    self.assemble_candidates_from_impls(obligation, &mut candidates);
+                    let added_impl =
+                        candidates.vec[before..].iter().any(|c| matches!(c, ImplCandidate(_)));
+                    if !added_impl {
+                        self.assemble_const_destruct_candidates(obligation, &mut candidates);
+                    }
                 }
                 Some(LangItem::TransmuteTrait) => {
                     // User-defined transmutability impls are permitted.