WIP: Remove ResumeTy from async lowering

Instead of using the stdlib supported `ResumeTy`, which is being converting to a `&mut Context<'_>` during the Generator MIR pass,
this will use `&mut Context<'_>` directly in HIR lowering.

It pretty much reverts rust-lang#105977 and re-applies an updated version of rust-lang#105250.

This still fails the testcase added in rust-lang#106264 however, for reasons I don’t understand.

Author: Swatinem

compiler/rustc_ast_lowering/src/expr.rs

@@ -12,7 +12,7 @@ use rustc_middle::span_bug;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::errors::report_lit_error;
 use rustc_span::source_map::{Spanned, respan};
-use rustc_span::{DUMMY_SP, DesugaringKind, Ident, Span, Symbol, sym};
+use rustc_span::{DUMMY_SP, DesugaringKind, Ident, Span, Symbol, kw, sym};
 use thin_vec::{ThinVec, thin_vec};
 use visit::{Visitor, walk_expr};
 
@@ -718,18 +718,30 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // whereas a generator does not.
         let (inputs, params, task_context): (&[_], &[_], _) = match desugaring_kind {
             hir::CoroutineDesugaring::Async | hir::CoroutineDesugaring::AsyncGen => {
-                // Resume argument type: `ResumeTy`
-                let unstable_span = self.mark_span_with_reason(
-                    DesugaringKind::Async,
-                    self.lower_span(span),
-                    Some(Arc::clone(&self.allow_gen_future)),
-                );
-                let resume_ty =
-                    self.make_lang_item_qpath(hir::LangItem::ResumeTy, unstable_span, None);
+                // Resume argument type: `&mut Context<'_>`.
+                let context_lifetime_ident = Ident::with_dummy_span(kw::UnderscoreLifetime);
+                let context_lifetime = self.arena.alloc(hir::Lifetime {
+                    hir_id: self.next_id(),
+                    ident: context_lifetime_ident,
+                    kind: hir::LifetimeKind::Infer,
+                    source: hir::LifetimeSource::Other,
+                    syntax: hir::LifetimeSyntax::Implicit,
+                });
+                let context_path =
+                    hir::QPath::LangItem(hir::LangItem::Context, self.lower_span(span));
+                let context_ty = hir::MutTy {
+                    ty: self.arena.alloc(hir::Ty {
+                        hir_id: self.next_id(),
+                        kind: hir::TyKind::Path(context_path),
+                        span: self.lower_span(span),
+                    }),
+                    mutbl: hir::Mutability::Mut,
+                };
+
                 let input_ty = hir::Ty {
                     hir_id: self.next_id(),
-                    kind: hir::TyKind::Path(resume_ty),
-                    span: unstable_span,
+                    kind: hir::TyKind::Ref(context_lifetime, context_ty),
+                    span: self.lower_span(span),
                 };
                 let inputs = arena_vec![self; input_ty];
 
@@ -830,7 +842,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     ///     mut __awaitee => loop {
     ///         match unsafe { ::std::future::Future::poll(
     ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
-    ///             ::std::future::get_context(task_context),
+    ///             task_context,
     ///         ) } {
     ///             ::std::task::Poll::Ready(result) => break result,
     ///             ::std::task::Poll::Pending => {}
@@ -889,26 +901,21 @@ impl<'hir> LoweringContext<'_, 'hir> {
             FutureKind::AsyncIterator => Some(Arc::clone(&self.allow_for_await)),
         };
         let span = self.mark_span_with_reason(DesugaringKind::Await, await_kw_span, features);
-        let gen_future_span = self.mark_span_with_reason(
-            DesugaringKind::Await,
-            full_span,
-            Some(Arc::clone(&self.allow_gen_future)),
-        );
         let expr_hir_id = expr.hir_id;
 
         // Note that the name of this binding must not be changed to something else because
         // debuggers and debugger extensions expect it to be called `__awaitee`. They use
         // this name to identify what is being awaited by a suspended async functions.
         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
         let (awaitee_pat, awaitee_pat_hid) =
-            self.pat_ident_binding_mode(gen_future_span, awaitee_ident, hir::BindingMode::MUT);
+            self.pat_ident_binding_mode(full_span, awaitee_ident, hir::BindingMode::MUT);
 
         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 
         // unsafe {
         //     ::std::future::Future::poll(
         //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
-        //         ::std::future::get_context(task_context),
+        //         task_context,
         //     )
         // }
         let poll_expr = {
@@ -926,21 +933,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 hir::LangItem::PinNewUnchecked,
                 arena_vec![self; ref_mut_awaitee],
             );
-            let get_context = self.expr_call_lang_item_fn_mut(
-                gen_future_span,
-                hir::LangItem::GetContext,
-                arena_vec![self; task_context],
-            );
             let call = match await_kind {
                 FutureKind::Future => self.expr_call_lang_item_fn(
                     span,
                     hir::LangItem::FuturePoll,
-                    arena_vec![self; new_unchecked, get_context],
+                    arena_vec![self; new_unchecked, task_context],
                 ),
                 FutureKind::AsyncIterator => self.expr_call_lang_item_fn(
                     span,
                     hir::LangItem::AsyncIteratorPollNext,
-                    arena_vec![self; new_unchecked, get_context],
+                    arena_vec![self; new_unchecked, task_context],
                 ),
             };
             self.arena.alloc(self.expr_unsafe(call))
@@ -951,14 +953,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let loop_hir_id = self.lower_node_id(loop_node_id);
         let ready_arm = {
             let x_ident = Ident::with_dummy_span(sym::result);
-            let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
-            let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
-            let ready_field = self.single_pat_field(gen_future_span, x_pat);
+            let (x_pat, x_pat_hid) = self.pat_ident(full_span, x_ident);
+            let x_expr = self.expr_ident(full_span, x_ident, x_pat_hid);
+            let ready_field = self.single_pat_field(full_span, x_pat);
             let ready_pat = self.pat_lang_item_variant(span, hir::LangItem::PollReady, ready_field);
             let break_x = self.with_loop_scope(loop_hir_id, move |this| {
                 let expr_break =
                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
-                this.arena.alloc(this.expr(gen_future_span, expr_break))
+                this.arena.alloc(this.expr(full_span, expr_break))
             });
             self.arm(ready_pat, break_x)
         };

compiler/rustc_hir/src/lang_items.rs

@@ -379,11 +379,6 @@ language_item_table! {
     AsyncGenPending,         sym::AsyncGenPending,     async_gen_pending,          Target::AssocConst,     GenericRequirement::Exact(1);
     AsyncGenFinished,        sym::AsyncGenFinished,    async_gen_finished,         Target::AssocConst,     GenericRequirement::Exact(1);
 
-    // FIXME(swatinem): the following lang items are used for async lowering and
-    // should become obsolete eventually.
-    ResumeTy,                sym::ResumeTy,            resume_ty,                  Target::Struct,         GenericRequirement::None;
-    GetContext,              sym::get_context,         get_context_fn,             Target::Fn,             GenericRequirement::None;
-
     Context,                 sym::Context,             context,                    Target::Struct,         GenericRequirement::None;
     FuturePoll,              sym::poll,                future_poll_fn,             Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
 

compiler/rustc_mir_transform/src/coroutine.rs

@@ -533,108 +533,14 @@ fn replace_local<'tcx>(
     new_local
 }
 
-/// Transforms the `body` of the coroutine applying the following transforms:
-///
-/// - Eliminates all the `get_context` calls that async lowering created.
-/// - Replace all `Local` `ResumeTy` types with `&mut Context<'_>` (`context_mut_ref`).
-///
-/// The `Local`s that have their types replaced are:
-/// - The `resume` argument itself.
-/// - The argument to `get_context`.
-/// - The yielded value of a `yield`.
-///
-/// The `ResumeTy` hides a `&mut Context<'_>` behind an unsafe raw pointer, and the
-/// `get_context` function is being used to convert that back to a `&mut Context<'_>`.
-///
-/// Ideally the async lowering would not use the `ResumeTy`/`get_context` indirection,
-/// but rather directly use `&mut Context<'_>`, however that would currently
-/// lead to higher-kinded lifetime errors.
-/// See <https://github.com/rust-lang/rust/issues/105501>.
-///
-/// The async lowering step and the type / lifetime inference / checking are
-/// still using the `ResumeTy` indirection for the time being, and that indirection
-/// is removed here. After this transform, the coroutine body only knows about `&mut Context<'_>`.
-fn transform_async_context<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) -> Ty<'tcx> {
-    let context_mut_ref = Ty::new_task_context(tcx);
-
-    // replace the type of the `resume` argument
-    replace_resume_ty_local(tcx, body, CTX_ARG, context_mut_ref);
-
-    let get_context_def_id = tcx.require_lang_item(LangItem::GetContext, body.span);
-
-    for bb in body.basic_blocks.indices() {
-        let bb_data = &body[bb];
-        if bb_data.is_cleanup {
-            continue;
-        }
-
-        match &bb_data.terminator().kind {
-            TerminatorKind::Call { func, .. } => {
-                let func_ty = func.ty(body, tcx);
-                if let ty::FnDef(def_id, _) = *func_ty.kind()
-                    && def_id == get_context_def_id
-                {
-                    let local = eliminate_get_context_call(&mut body[bb]);
-                    replace_resume_ty_local(tcx, body, local, context_mut_ref);
-                }
-            }
-            TerminatorKind::Yield { resume_arg, .. } => {
-                replace_resume_ty_local(tcx, body, resume_arg.local, context_mut_ref);
-            }
-            _ => {}
-        }
-    }
-    context_mut_ref
-}
-
-fn eliminate_get_context_call<'tcx>(bb_data: &mut BasicBlockData<'tcx>) -> Local {
-    let terminator = bb_data.terminator.take().unwrap();
-    let TerminatorKind::Call { args, destination, target, .. } = terminator.kind else {
-        bug!();
-    };
-    let [arg] = *Box::try_from(args).unwrap();
-    let local = arg.node.place().unwrap().local;
-
-    let arg = Rvalue::Use(arg.node);
-    let assign =
-        Statement::new(terminator.source_info, StatementKind::Assign(Box::new((destination, arg))));
-    bb_data.statements.push(assign);
-    bb_data.terminator = Some(Terminator {
-        source_info: terminator.source_info,
-        kind: TerminatorKind::Goto { target: target.unwrap() },
-    });
-    local
-}
-
-#[cfg_attr(not(debug_assertions), allow(unused))]
-fn replace_resume_ty_local<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    body: &mut Body<'tcx>,
-    local: Local,
-    context_mut_ref: Ty<'tcx>,
-) {
-    let local_ty = std::mem::replace(&mut body.local_decls[local].ty, context_mut_ref);
-    // We have to replace the `ResumeTy` that is used for type and borrow checking
-    // with `&mut Context<'_>` in MIR.
-    #[cfg(debug_assertions)]
-    {
-        if let ty::Adt(resume_ty_adt, _) = local_ty.kind() {
-            let expected_adt = tcx.adt_def(tcx.require_lang_item(LangItem::ResumeTy, body.span));
-            assert_eq!(*resume_ty_adt, expected_adt);
-        } else {
-            panic!("expected `ResumeTy`, found `{:?}`", local_ty);
-        };
-    }
-}
-
 /// Transforms the `body` of the coroutine applying the following transform:
 ///
 /// - Remove the `resume` argument.
 ///
 /// Ideally the async lowering would not add the `resume` argument.
 ///
 /// The async lowering step and the type / lifetime inference / checking are
-/// still using the `resume` argument for the time being. After this transform,
+/// still using the `resume` argument for the time being. After this transform
 /// the coroutine body doesn't have the `resume` argument.
 fn transform_gen_context<'tcx>(body: &mut Body<'tcx>) {
     // This leaves the local representing the `resume` argument in place,
@@ -1504,7 +1410,7 @@ impl<'tcx> crate::MirPass<'tcx> for StateTransform {
             coroutine_kind,
             CoroutineKind::Desugared(CoroutineDesugaring::Async | CoroutineDesugaring::AsyncGen, _)
         ) {
-            let context_mut_ref = transform_async_context(tcx, body);
+            let context_mut_ref = Ty::new_task_context(tcx);
             expand_async_drops(tcx, body, context_mut_ref, coroutine_kind, coroutine_ty);
             dump_mir(tcx, false, "coroutine_async_drop_expand", &0, body, |_, _| Ok(()));
         } else {

compiler/rustc_mir_transform/src/shim/async_destructor_ctor.rs

@@ -64,11 +64,10 @@ pub(super) fn build_async_drop_shim<'tcx>(
     let needs_async_drop = drop_ty.needs_async_drop(tcx, typing_env);
     let needs_sync_drop = !needs_async_drop && drop_ty.needs_drop(tcx, typing_env);
 
-    let resume_adt = tcx.adt_def(tcx.require_lang_item(LangItem::ResumeTy, DUMMY_SP));
-    let resume_ty = Ty::new_adt(tcx, resume_adt, ty::List::empty());
+    let context_mut_ref = Ty::new_task_context(tcx);
 
     let fn_sig = ty::Binder::dummy(tcx.mk_fn_sig(
-        [ty, resume_ty],
+        [ty, context_mut_ref],
         tcx.types.unit,
         false,
         Safety::Safe,

compiler/rustc_span/src/symbol.rs

@@ -336,7 +336,6 @@ symbols! {
         Relaxed,
         Release,
         Result,
-        ResumeTy,
         Return,
         Reverse,
         Right,
@@ -1107,7 +1106,6 @@ symbols! {
         generic_const_parameter_types,
         generic_param_attrs,
         generic_pattern_types,
-        get_context,
         global_alloc_ty,
         global_allocator,
         global_asm,

compiler/rustc_ty_utils/src/abi.rs

@@ -155,21 +155,7 @@ fn fn_sig_for_fn_abi<'tcx>(
                     let poll_args = tcx.mk_args(&[sig.return_ty.into()]);
                     let ret_ty = Ty::new_adt(tcx, poll_adt_ref, poll_args);
 
-                    // We have to replace the `ResumeTy` that is used for type and borrow checking
-                    // with `&mut Context<'_>` which is used in codegen.
-                    #[cfg(debug_assertions)]
-                    {
-                        if let ty::Adt(resume_ty_adt, _) = sig.resume_ty.kind() {
-                            let expected_adt =
-                                tcx.adt_def(tcx.require_lang_item(LangItem::ResumeTy, DUMMY_SP));
-                            assert_eq!(*resume_ty_adt, expected_adt);
-                        } else {
-                            panic!("expected `ResumeTy`, found `{:?}`", sig.resume_ty);
-                        };
-                    }
-                    let context_mut_ref = Ty::new_task_context(tcx);
-
-                    (Some(context_mut_ref), ret_ty)
+                    (Some(sig.resume_ty), ret_ty)
                 }
                 hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Gen, _) => {
                     // The signature should be `Iterator::next(_) -> Option<Yield>`
@@ -191,21 +177,7 @@ fn fn_sig_for_fn_abi<'tcx>(
                     // Yield type is already `Poll<Option<yield_ty>>`
                     let ret_ty = sig.yield_ty;
 
-                    // We have to replace the `ResumeTy` that is used for type and borrow checking
-                    // with `&mut Context<'_>` which is used in codegen.
-                    #[cfg(debug_assertions)]
-                    {
-                        if let ty::Adt(resume_ty_adt, _) = sig.resume_ty.kind() {
-                            let expected_adt =
-                                tcx.adt_def(tcx.require_lang_item(LangItem::ResumeTy, DUMMY_SP));
-                            assert_eq!(*resume_ty_adt, expected_adt);
-                        } else {
-                            panic!("expected `ResumeTy`, found `{:?}`", sig.resume_ty);
-                        };
-                    }
-                    let context_mut_ref = Ty::new_task_context(tcx);
-
-                    (Some(context_mut_ref), ret_ty)
+                    (Some(sig.resume_ty), ret_ty)
                 }
                 hir::CoroutineKind::Coroutine(_) => {
                     // The signature should be `Coroutine::resume(_, Resume) -> CoroutineState<Yield, Return>`

library/core/src/future/mod.rs

@@ -9,9 +9,6 @@
 //! [`await`]: ../../std/keyword.await.html
 //! [async book]: https://rust-lang.github.io/async-book/
 
-use crate::ptr::NonNull;
-use crate::task::Context;
-
 mod async_drop;
 mod future;
 mod into_future;
@@ -35,34 +32,3 @@ pub use ready::{Ready, ready};
 pub use self::future::Future;
 #[unstable(feature = "future_join", issue = "91642")]
 pub use self::join::join;
-
-/// This type is needed because:
-///
-/// a) Coroutines cannot implement `for<'a, 'b> Coroutine<&'a mut Context<'b>>`, so we need to pass
-///    a raw pointer (see <https://github.com/rust-lang/rust/issues/68923>).
-/// b) Raw pointers and `NonNull` aren't `Send` or `Sync`, so that would make every single future
-///    non-Send/Sync as well, and we don't want that.
-///
-/// It also simplifies the HIR lowering of `.await`.
-#[lang = "ResumeTy"]
-#[doc(hidden)]
-#[unstable(feature = "gen_future", issue = "none")]
-#[derive(Debug, Copy, Clone)]
-pub struct ResumeTy(NonNull<Context<'static>>);
-
-#[unstable(feature = "gen_future", issue = "none")]
-unsafe impl Send for ResumeTy {}
-
-#[unstable(feature = "gen_future", issue = "none")]
-unsafe impl Sync for ResumeTy {}
-
-#[lang = "get_context"]
-#[doc(hidden)]
-#[unstable(feature = "gen_future", issue = "none")]
-#[must_use]
-#[inline]
-pub unsafe fn get_context<'a, 'b>(cx: ResumeTy) -> &'a mut Context<'b> {
-    // SAFETY: the caller must guarantee that `cx.0` is a valid pointer
-    // that fulfills all the requirements for a mutable reference.
-    unsafe { &mut *cx.0.as_ptr().cast() }
-}

tests/ui/async-await/unreachable-lint.rs

@@ -1,13 +1,13 @@
-//@ check-pass
 //@ edition:2018
 #![deny(unreachable_code)]
 
 async fn foo() {
     endless().await;
+    //~^ ERROR unreachable expression
 }
 
 async fn endless() -> ! {
     loop {}
 }
 
-fn main() { }
+fn main() {}