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](rust-lang/compiler-team#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 (#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

Author: bors

compiler/rustc_codegen_ssa/src/back/symbol_export.rs

@@ -363,6 +363,24 @@ fn exported_symbols_provider_local(
                         },
                     ));
                 }
+                MonoItem::Fn(Instance {
+                    def: InstanceDef::AsyncDropGlueCtorShim(def_id, Some(ty)),
+                    args,
+                }) => {
+                    // A little sanity-check
+                    debug_assert_eq!(
+                        args.non_erasable_generics(tcx, def_id).skip(1).next(),
+                        Some(GenericArgKind::Type(ty))
+                    );
+                    symbols.push((
+                        ExportedSymbol::AsyncDropGlueCtorShim(ty),
+                        SymbolExportInfo {
+                            level: SymbolExportLevel::Rust,
+                            kind: SymbolExportKind::Text,
+                            used: false,
+                        },
+                    ));
+                }
                 _ => {
                     // Any other symbols don't qualify for sharing
                 }
@@ -385,6 +403,7 @@ fn upstream_monomorphizations_provider(
     let mut instances: DefIdMap<UnordMap<_, _>> = Default::default();
 
     let drop_in_place_fn_def_id = tcx.lang_items().drop_in_place_fn();
+    let async_drop_in_place_fn_def_id = tcx.lang_items().async_drop_in_place_fn();
 
     for &cnum in cnums.iter() {
         for (exported_symbol, _) in tcx.exported_symbols(cnum).iter() {
@@ -399,6 +418,18 @@ fn upstream_monomorphizations_provider(
                         continue;
                     }
                 }
+                ExportedSymbol::AsyncDropGlueCtorShim(ty) => {
+                    if let Some(async_drop_in_place_fn_def_id) = async_drop_in_place_fn_def_id {
+                        (
+                            async_drop_in_place_fn_def_id,
+                            tcx.mk_args(&[tcx.lifetimes.re_erased.into(), ty.into()]),
+                        )
+                    } else {
+                        // `drop_in_place` in place does not exist, don't try
+                        // to use it.
+                        continue;
+                    }
+                }
                 ExportedSymbol::NonGeneric(..)
                 | ExportedSymbol::ThreadLocalShim(..)
                 | ExportedSymbol::NoDefId(..) => {
@@ -534,6 +565,13 @@ pub fn symbol_name_for_instance_in_crate<'tcx>(
             Instance::resolve_drop_in_place(tcx, ty),
             instantiating_crate,
         ),
+        ExportedSymbol::AsyncDropGlueCtorShim(ty) => {
+            rustc_symbol_mangling::symbol_name_for_instance_in_crate(
+                tcx,
+                Instance::resolve_async_drop_in_place(tcx, ty),
+                instantiating_crate,
+            )
+        }
         ExportedSymbol::NoDefId(symbol_name) => symbol_name.to_string(),
     }
 }
@@ -582,6 +620,9 @@ pub fn linking_symbol_name_for_instance_in_crate<'tcx>(
         // DropGlue always use the Rust calling convention and thus follow the target's default
         // symbol decoration scheme.
         ExportedSymbol::DropGlue(..) => None,
+        // AsyncDropGlueCtorShim always use the Rust calling convention and thus follow the
+        // target's default symbol decoration scheme.
+        ExportedSymbol::AsyncDropGlueCtorShim(..) => None,
         // NoDefId always follow the target's default symbol decoration scheme.
         ExportedSymbol::NoDefId(..) => None,
         // ThreadLocalShim always follow the target's default symbol decoration scheme.

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -835,7 +835,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
         let def = instance.map(|i| i.def);
 
-        if let Some(ty::InstanceDef::DropGlue(_, None)) = def {
+        if let Some(
+            ty::InstanceDef::DropGlue(_, None) | ty::InstanceDef::AsyncDropGlueCtorShim(_, None),
+        ) = def
+        {
             // Empty drop glue; a no-op.
             let target = target.unwrap();
             return helper.funclet_br(self, bx, target, mergeable_succ);