Auto merge of #121801 - zetanumbers:async_drop_glue, r=oli-obk

Add simple async drop glue generation

This is a prototype of the async drop glue generation for some simple types. Async drop glue is intended to behave very similar to the regular drop glue except for being asynchronous. Currently it does not execute synchronous drops but only calls user implementations of `AsyncDrop::async_drop` associative function and awaits the returned future. It is not complete as it only recurses into arrays, slices, tuples, and structs and does not have same sensible restrictions as the old `Drop` trait implementation like having the same bounds as the type definition, while code assumes their existence (requires a future work).

This current design uses a workaround as it does not create any custom async destructor state machine types for ADTs, but instead uses types defined in the std library called future combinators (deferred_async_drop, chain, ready_unit).

Also I recommend reading my [explainer](https://zetanumbers.github.io/book/async-drop-design.html).

This is a part of the [MCP: Low level components for async drop](https://github.com/rust-lang/compiler-team/issues/727) work.

Feature completeness:

 - [x] `AsyncDrop` trait
 - [ ] `async_drop_in_place_raw`/async drop glue generation support for
   - [x] Trivially destructible types (integers, bools, floats, string slices, pointers, references, etc.)
   - [x] Arrays and slices (array pointer is unsized into slice pointer)
   - [x] ADTs (enums, structs, unions)
   - [x] tuple-like types (tuples, closures)
   - [ ] Dynamic types (`dyn Trait`, see explainer's [proposed design](https://github.com/zetanumbers/posts/blob/main/async-drop-design.md#async-drop-glue-for-dyn-trait))
   - [ ] coroutines (https://github.com/rust-lang/rust/pull/123948)
 - [x] Async drop glue includes sync drop glue code
 - [x] Cleanup branch generation for `async_drop_in_place_raw`
 - [ ] Union rejects non-trivially async destructible fields
 - [ ] `AsyncDrop` implementation requires same bounds as type definition
 - [ ] Skip trivially destructible fields (optimization)
 - [ ] New [`TyKind::AdtAsyncDestructor`](https://github.com/zetanumbers/posts/blob/main/async-drop-design.md#adt-async-destructor-types) and get rid of combinators
 - [ ] [Synchronously undroppable types](https://github.com/zetanumbers/posts/blob/main/async-drop-design.md#exclusively-async-drop)
 - [ ] Automatic async drop at the end of the scope in async context

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

@@ -240,6 +240,11 @@ pub(super) trait GoalKind<'tcx>:
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx>;
 
+    fn consider_builtin_async_destruct_candidate(
+        ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+    ) -> QueryResult<'tcx>;
+
     fn consider_builtin_destruct_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
@@ -520,6 +525,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             G::consider_builtin_coroutine_candidate(self, goal)
         } else if lang_items.discriminant_kind_trait() == Some(trait_def_id) {
             G::consider_builtin_discriminant_kind_candidate(self, goal)
+        } else if lang_items.async_destruct_trait() == Some(trait_def_id) {
+            G::consider_builtin_async_destruct_candidate(self, goal)
         } else if lang_items.destruct_trait() == Some(trait_def_id) {
             G::consider_builtin_destruct_candidate(self, goal)
         } else if lang_items.transmute_trait() == Some(trait_def_id) {

compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs

@@ -814,6 +814,59 @@ impl<'tcx> assembly::GoalKind<'tcx> for NormalizesTo<'tcx> {
         })
     }
 
+    fn consider_builtin_async_destruct_candidate(
+        ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+    ) -> QueryResult<'tcx> {
+        let self_ty = goal.predicate.self_ty();
+        let async_destructor_ty = match *self_ty.kind() {
+            ty::Bool
+            | ty::Char
+            | ty::Int(..)
+            | ty::Uint(..)
+            | ty::Float(..)
+            | ty::Array(..)
+            | ty::RawPtr(..)
+            | ty::Ref(..)
+            | ty::FnDef(..)
+            | ty::FnPtr(..)
+            | ty::Closure(..)
+            | ty::CoroutineClosure(..)
+            | ty::Infer(ty::IntVar(..) | ty::FloatVar(..))
+            | ty::Never
+            | ty::Adt(_, _)
+            | ty::Str
+            | ty::Slice(_)
+            | ty::Tuple(_)
+            | ty::Error(_) => self_ty.async_destructor_ty(ecx.tcx(), goal.param_env),
+
+            // We do not call `Ty::async_destructor_ty` on alias, param, or placeholder
+            // types, which return `<self_ty as AsyncDestruct>::AsyncDestructor`
+            // (or ICE in the case of placeholders). Projecting a type to itself
+            // is never really productive.
+            ty::Alias(_, _) | ty::Param(_) | ty::Placeholder(..) => {
+                return Err(NoSolution);
+            }
+
+            ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_))
+            | ty::Foreign(..)
+            | ty::Bound(..) => bug!(
+                "unexpected self ty `{:?}` when normalizing `<T as AsyncDestruct>::AsyncDestructor`",
+                goal.predicate.self_ty()
+            ),
+
+            ty::Pat(..) | ty::Dynamic(..) | ty::Coroutine(..) | ty::CoroutineWitness(..) => bug!(
+                "`consider_builtin_async_destruct_candidate` is not yet implemented for type: {self_ty:?}"
+            ),
+        };
+
+        ecx.probe_misc_candidate("builtin async destruct").enter(|ecx| {
+            ecx.eq(goal.param_env, goal.predicate.term, async_destructor_ty.into())
+                .expect("expected goal term to be fully unconstrained");
+            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+        })
+    }
+
     fn consider_builtin_destruct_candidate(
         _ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -544,6 +544,18 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
     }
 
+    fn consider_builtin_async_destruct_candidate(
+        ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+    ) -> QueryResult<'tcx> {
+        if goal.predicate.polarity != ty::PredicatePolarity::Positive {
+            return Err(NoSolution);
+        }
+
+        // `AsyncDestruct` is automatically implemented for every type.
+        ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+    }
+
     fn consider_builtin_destruct_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1074,6 +1074,42 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                         | ty::Infer(..)
                         | ty::Error(_) => false,
                     }
+                } else if lang_items.async_destruct_trait() == Some(trait_ref.def_id) {
+                    match self_ty.kind() {
+                        ty::Bool
+                        | ty::Char
+                        | ty::Int(_)
+                        | ty::Uint(_)
+                        | ty::Float(_)
+                        | ty::Adt(..)
+                        | ty::Str
+                        | ty::Array(..)
+                        | ty::Slice(_)
+                        | ty::RawPtr(..)
+                        | ty::Ref(..)
+                        | ty::FnDef(..)
+                        | ty::FnPtr(..)
+                        | ty::Dynamic(..)
+                        | ty::Closure(..)
+                        | ty::CoroutineClosure(..)
+                        | ty::Coroutine(..)
+                        | ty::CoroutineWitness(..)
+                        | ty::Pat(..)
+                        | ty::Never
+                        | ty::Tuple(..)
+                        | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
+
+                        // type parameters, opaques, and unnormalized projections don't have
+                        // a known async destructor and may need to be normalized further or rely
+                        // on param env for async destructor projections
+                        ty::Param(_)
+                        | ty::Foreign(_)
+                        | ty::Alias(..)
+                        | ty::Bound(..)
+                        | ty::Placeholder(..)
+                        | ty::Infer(_)
+                        | ty::Error(_) => false,
+                    }
                 } else if lang_items.pointee_trait() == Some(trait_ref.def_id) {
                     let tail = selcx.tcx().struct_tail_with_normalize(
                         self_ty,
@@ -1488,15 +1524,20 @@ fn confirm_builtin_candidate<'cx, 'tcx>(
 ) -> Progress<'tcx> {
     let tcx = selcx.tcx();
     let self_ty = obligation.predicate.self_ty();
-    let args = tcx.mk_args(&[self_ty.into()]);
     let lang_items = tcx.lang_items();
     let item_def_id = obligation.predicate.def_id;
     let trait_def_id = tcx.trait_of_item(item_def_id).unwrap();
+    let args = tcx.mk_args(&[self_ty.into()]);
     let (term, obligations) = if lang_items.discriminant_kind_trait() == Some(trait_def_id) {
         let discriminant_def_id = tcx.require_lang_item(LangItem::Discriminant, None);
         assert_eq!(discriminant_def_id, item_def_id);
 
         (self_ty.discriminant_ty(tcx).into(), Vec::new())
+    } else if lang_items.async_destruct_trait() == Some(trait_def_id) {
+        let destructor_def_id = tcx.associated_item_def_ids(trait_def_id)[0];
+        assert_eq!(destructor_def_id, item_def_id);
+
+        (self_ty.async_destructor_ty(tcx, obligation.param_env).into(), Vec::new())
     } else if lang_items.pointee_trait() == Some(trait_def_id) {
         let metadata_def_id = tcx.require_lang_item(LangItem::Metadata, None);
         assert_eq!(metadata_def_id, item_def_id);

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

@@ -81,6 +81,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             } else if lang_items.discriminant_kind_trait() == Some(def_id) {
                 // `DiscriminantKind` is automatically implemented for every type.
                 candidates.vec.push(BuiltinCandidate { has_nested: false });
+            } else if lang_items.async_destruct_trait() == Some(def_id) {
+                // `AsyncDestruct` is automatically implemented for every type.
+                candidates.vec.push(BuiltinCandidate { has_nested: false });
             } else if lang_items.pointee_trait() == Some(def_id) {
                 // `Pointee` is automatically implemented for every type.
                 candidates.vec.push(BuiltinCandidate { has_nested: false });