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

@@ -615,17 +615,28 @@ 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(self.allow_gen_future.clone()),
-                );
-                let resume_ty = self.make_lang_item_qpath(hir::LangItem::ResumeTy, unstable_span);
+                // 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,
+                    res: hir::LifetimeName::Infer,
+                });
+                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];
 
@@ -725,7 +736,7 @@ 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 => {}
@@ -766,29 +777,21 @@ impl<'hir> LoweringContext<'_, 'hir> {
             FutureKind::AsyncIterator => Some(self.allow_for_await.clone()),
         };
         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(self.allow_gen_future.clone()),
-        );
         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::BindingAnnotation::MUT,
-        );
+        let (awaitee_pat, awaitee_pat_hid) =
+            self.pat_ident_binding_mode(full_span, awaitee_ident, hir::BindingAnnotation::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 = {
@@ -806,21 +809,16 @@ 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))
@@ -831,14 +829,14 @@ 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_node_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

@@ -299,11 +299,6 @@ 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_middle/src/ty/sty.rs

@@ -2234,15 +2234,6 @@ impl<'tcx> Ty<'tcx> {
         let def_id = tcx.require_lang_item(LangItem::MaybeUninit, None);
         Ty::new_generic_adt(tcx, def_id, ty)
     }
-
-    /// Creates a `&mut Context<'_>` [`Ty`] with erased lifetimes.
-    pub fn new_task_context(tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
-        let context_did = tcx.require_lang_item(LangItem::Context, None);
-        let context_adt_ref = tcx.adt_def(context_did);
-        let context_args = tcx.mk_args(&[tcx.lifetimes.re_erased.into()]);
-        let context_ty = Ty::new_adt(tcx, context_adt_ref, context_args);
-        Ty::new_mut_ref(tcx, tcx.lifetimes.re_erased, context_ty)
-    }
 }
 
 /// Type utilities

compiler/rustc_mir_transform/src/coroutine.rs

@@ -616,112 +616,14 @@ 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>) {
-    let context_mut_ref = Ty::new_task_context(tcx);
-
-    // replace the type of the `resume` argument
-    replace_resume_ty_local(tcx, body, Local::new(2), context_mut_ref);
-
-    let get_context_def_id = tcx.require_lang_item(LangItem::GetContext, None);
-
-    for bb in START_BLOCK..body.basic_blocks.next_index() {
-        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() {
-                    if 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);
-                    }
-                } else {
-                    continue;
-                }
-            }
-            TerminatorKind::Yield { resume_arg, .. } => {
-                replace_resume_ty_local(tcx, body, resume_arg.local, context_mut_ref);
-            }
-            _ => {}
-        }
-    }
-}
-
-fn eliminate_get_context_call<'tcx>(bb_data: &mut BasicBlockData<'tcx>) -> Local {
-    let terminator = bb_data.terminator.take().unwrap();
-    if let TerminatorKind::Call { mut args, destination, target, .. } = terminator.kind {
-        let arg = args.pop().unwrap();
-        let local = arg.place().unwrap().local;
-
-        let arg = Rvalue::Use(arg);
-        let assign = Statement {
-            source_info: terminator.source_info,
-            kind: 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
-    } else {
-        bug!();
-    }
-}
-
-#[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, None));
-            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>(_tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     // This leaves the local representing the `resume` argument in place,
@@ -1644,14 +1546,6 @@ impl<'tcx> MirPass<'tcx> for StateTransform {
         // RETURN_PLACE then is a fresh unused local with type ret_ty.
         let old_ret_local = replace_local(RETURN_PLACE, new_ret_ty, body, tcx);
 
-        // Replace all occurrences of `ResumeTy` with `&mut Context<'_>` within async bodies.
-        if matches!(
-            coroutine_kind,
-            CoroutineKind::Desugared(CoroutineDesugaring::Async | CoroutineDesugaring::AsyncGen, _)
-        ) {
-            transform_async_context(tcx, body);
-        }
-
         // We also replace the resume argument and insert an `Assign`.
         // This is needed because the resume argument `_2` might be live across a `yield`, in which
         // case there is no `Assign` to it that the transform can turn into a store to the coroutine

compiler/rustc_span/src/symbol.rs

@@ -293,7 +293,6 @@ symbols! {
         Relaxed,
         Release,
         Result,
-        ResumeTy,
         Return,
         Right,
         Rust,
@@ -835,7 +834,6 @@ symbols! {
         generic_const_exprs,
         generic_const_items,
         generic_param_attrs,
-        get_context,
         global_allocator,
         global_asm,
         globs,

compiler/rustc_ty_utils/src/abi.rs

@@ -140,21 +140,7 @@ 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, None));
-                            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>`
@@ -176,21 +162,7 @@ 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, None));
-                            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

@@ -44,7 +44,7 @@ pub use poll_fn::{poll_fn, PollFn};
 ///    non-Send/Sync as well, and we don't want that.
 ///
 /// It also simplifies the HIR lowering of `.await`.
-#[lang = "ResumeTy"]
+#[cfg_attr(bootstrap, lang = "ResumeTy")]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
 #[derive(Debug, Copy, Clone)]
@@ -56,7 +56,7 @@ unsafe impl Send for ResumeTy {}
 #[unstable(feature = "gen_future", issue = "50547")]
 unsafe impl Sync for ResumeTy {}
 
-#[lang = "get_context"]
+#[cfg_attr(bootstrap, lang = "get_context")]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
 #[must_use]

tests/ui/async-await/issue-69446-fnmut-capture.stderr

@@ -14,6 +14,9 @@ LL | |     });
    |
    = note: `FnMut` closures only have access to their captured variables while they are executing...
    = note: ...therefore, they cannot allow references to captured variables to escape
+   = note: requirement occurs because of a mutable reference to `Context<'_>`
+   = note: mutable references are invariant over their type parameter
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to 1 previous error
 

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

@@ -1,9 +1,9 @@
-// check-pass
 // edition:2018
 #![deny(unreachable_code)]
 
 async fn foo() {
     endless().await;
+    //~^ ERROR unreachable expression
 }
 
 async fn endless() -> ! {

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

@@ -0,0 +1,17 @@
+error: unreachable expression
+  --> $DIR/unreachable-lint.rs:5:5
+   |
+LL |     endless().await;
+   |     ^^^^^^^^^^^^^^^
+   |     |
+   |     unreachable expression
+   |     any code following this expression is unreachable
+   |
+note: the lint level is defined here
+  --> $DIR/unreachable-lint.rs:2:9
+   |
+LL | #![deny(unreachable_code)]
+   |         ^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+