Start implementing needs_async_drop and related

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_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,