Auto merge of #124662 - zetanumbers:needs_async_drop, r=oli-obk

Implement `needs_async_drop` in rustc and optimize async drop glue

This PR expands on #121801 and implements `Ty::needs_async_drop` which works almost exactly the same as `Ty::needs_drop`, which is needed for #123948.

Also made compiler's async drop code to look more like compiler's regular drop code, which enabled me to write an optimization where types which do not use `AsyncDrop` can simply forward async drop glue to `drop_in_place`. This made size of the async block from the [async_drop test](https://github.com/zetanumbers/rust/blob/67980dd6fb11917d23d01a19c2cf4cfc3978aac8/tests/ui/async-await/async-drop.rs) to decrease by 12%.

3 files changed, 19 insertions, 21 deletions

diff --git a/compiler/rustc_ty_utils/src/common_traits.rs b/compiler/rustc_ty_utils/src/common_traits.rs
index cb95239e991..51b908881eb 100644
--- a/compiler/rustc_ty_utils/src/common_traits.rs
+++ b/compiler/rustc_ty_utils/src/common_traits.rs
@@ -22,17 +22,6 @@ fn is_unpin_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>)
     is_item_raw(tcx, query, LangItem::Unpin)
 }
 
-fn has_surface_async_drop_raw<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
-) -> bool {
-    is_item_raw(tcx, query, LangItem::AsyncDrop)
-}
-
-fn has_surface_drop_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
-    is_item_raw(tcx, query, LangItem::Drop)
-}
-
 fn is_item_raw<'tcx>(
     tcx: TyCtxt<'tcx>,
     query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
@@ -45,13 +34,5 @@ fn is_item_raw<'tcx>(
 }
 
 pub(crate) fn provide(providers: &mut Providers) {
-    *providers = Providers {
-        is_copy_raw,
-        is_sized_raw,
-        is_freeze_raw,
-        is_unpin_raw,
-        has_surface_async_drop_raw,
-        has_surface_drop_raw,
-        ..*providers
-    };
+    *providers = Providers { is_copy_raw, is_sized_raw, is_freeze_raw, is_unpin_raw, ..*providers };
 }
diff --git a/compiler/rustc_ty_utils/src/instance.rs b/compiler/rustc_ty_utils/src/instance.rs
index 0d089205c1e..e4dcea785d4 100644
--- a/compiler/rustc_ty_utils/src/instance.rs
+++ b/compiler/rustc_ty_utils/src/instance.rs
@@ -4,6 +4,7 @@ use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::bug;
 use rustc_middle::query::Providers;
 use rustc_middle::traits::{BuiltinImplSource, CodegenObligationError};
+use rustc_middle::ty::util::AsyncDropGlueMorphology;
 use rustc_middle::ty::GenericArgsRef;
 use rustc_middle::ty::{self, Instance, TyCtxt, TypeVisitableExt};
 use rustc_span::sym;
@@ -59,7 +60,7 @@ fn resolve_instance<'tcx>(
         } else if Some(def_id) == tcx.lang_items().async_drop_in_place_fn() {
             let ty = args.type_at(0);
 
-            if !ty.is_async_destructor_noop(tcx, param_env) {
+            if ty.async_drop_glue_morphology(tcx) != AsyncDropGlueMorphology::Noop {
                 match *ty.kind() {
                     ty::Closure(..)
                     | ty::CoroutineClosure(..)
diff --git a/compiler/rustc_ty_utils/src/needs_drop.rs b/compiler/rustc_ty_utils/src/needs_drop.rs
index 72ee1a48249..205b3f2760f 100644
--- a/compiler/rustc_ty_utils/src/needs_drop.rs
+++ b/compiler/rustc_ty_utils/src/needs_drop.rs
@@ -30,6 +30,21 @@ fn needs_drop_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::ParamEnvAnd<'tcx, Ty<'tcx>
     res
 }
 
+fn needs_async_drop_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
+    // If we don't know a type doesn't need async drop, for example if it's a
+    // type parameter without a `Copy` bound, then we conservatively return that
+    // it needs async drop.
+    let adt_has_async_dtor =
+        |adt_def: ty::AdtDef<'tcx>| adt_def.async_destructor(tcx).map(|_| DtorType::Significant);
+    let res = drop_tys_helper(tcx, query.value, query.param_env, adt_has_async_dtor, false)
+        .filter(filter_array_elements(tcx, query.param_env))
+        .next()
+        .is_some();
+
+    debug!("needs_drop_raw({:?}) = {:?}", query, res);
+    res
+}
+
 /// HACK: in order to not mistakenly assume that `[PhantomData<T>; N]` requires drop glue
 /// we check the element type for drop glue. The correct fix would be looking at the
 /// entirety of the code around `needs_drop_components` and this file and come up with
@@ -389,6 +404,7 @@ fn adt_significant_drop_tys(
 pub(crate) fn provide(providers: &mut Providers) {
     *providers = Providers {
         needs_drop_raw,
+        needs_async_drop_raw,
         has_significant_drop_raw,
         adt_drop_tys,
         adt_significant_drop_tys,