Replace usage of ResumeTy in async lowering with Context

Replaces using `ResumeTy` / `get_context` in favor of using `&'static mut Context<'_>`.

Usage of the `'static` lifetime here is technically "cheating", and replaces
the raw pointer in `ResumeTy` and the `get_context` fn that pulls the
correct lifetimes out of thin air.

Author: Swatinem

compiler/rustc_ast_lowering/src/expr.rs

@@ -16,7 +16,7 @@ use rustc_hir::def::Res;
 use rustc_hir::definitions::DefPathData;
 use rustc_session::errors::report_lit_error;
 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
-use rustc_span::symbol::{sym, Ident};
+use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::DUMMY_SP;
 use thin_vec::thin_vec;
 
@@ -592,14 +592,38 @@ impl<'hir> LoweringContext<'_, 'hir> {
     ) -> hir::ExprKind<'hir> {
         let output = ret_ty.unwrap_or_else(|| hir::FnRetTy::DefaultReturn(self.lower_span(span)));
 
-        // Resume argument type: `ResumeTy`
-        let unstable_span =
-            self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
-        let resume_ty = hir::QPath::LangItem(hir::LangItem::ResumeTy, unstable_span, None);
+        // Resume argument type, which should be `&mut Context<'_>`.
+        // NOTE: Using the `'static` lifetime here is technically cheating.
+        // The `Future::poll` argument really is `&'a mut Context<'b>`, but we cannot
+        // express the fact that we are not storing it across yield-points yet,
+        // and we would thus run into lifetime errors.
+        // See <https://github.com/rust-lang/rust/issues/68923>.
+        // Our lowering makes sure we are not mis-using the `_task_context` input type
+        // in the sense that we are indeed not using it across yield points. We
+        // get a fresh `&mut Context` for each resume / call of `Future::poll`.
+        // This "cheating" was previously done with a `ResumeTy` that contained a raw
+        // pointer, and a `get_context` accessor that pulled the `Context` lifetimes
+        // out of thin air.
+        let context_lifetime_ident = Ident::with_dummy_span(kw::StaticLifetime);
+        let context_lifetime = self.arena.alloc(hir::Lifetime {
+            hir_id: self.next_id(),
+            ident: context_lifetime_ident,
+            res: hir::LifetimeName::Static,
+        });
+        let context_path =
+            hir::QPath::LangItem(hir::LangItem::Context, self.lower_span(span), None);
+        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::Rptr(context_lifetime, context_ty),
+            span: self.lower_span(span),
         };
 
         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
@@ -710,7 +734,7 @@ 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 => {}
@@ -751,7 +775,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // unsafe {
         //     ::std::future::Future::poll(
         //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
-        //         ::std::future::get_context(task_context),
+        //         task_context,
         //     )
         // }
         let poll_expr = {
@@ -769,16 +793,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 arena_vec![self; ref_mut_awaitee],
                 Some(expr_hir_id),
             );
-            let get_context = self.expr_call_lang_item_fn_mut(
-                gen_future_span,
-                hir::LangItem::GetContext,
-                arena_vec![self; task_context],
-                Some(expr_hir_id),
-            );
             let call = self.expr_call_lang_item_fn(
                 span,
                 hir::LangItem::FuturePoll,
-                arena_vec![self; new_unchecked, get_context],
+                arena_vec![self; new_unchecked, task_context],
                 Some(expr_hir_id),
             );
             self.arena.alloc(self.expr_unsafe(call))

compiler/rustc_hir/src/lang_items.rs

@@ -286,10 +286,9 @@ language_item_table! {
 
     // 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;
     IdentityFuture,          sym::identity_future,     identity_future_fn,         Target::Fn,             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;
 
     FromFrom,                sym::from,                from_fn,                    Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;

compiler/rustc_span/src/symbol.rs

@@ -165,6 +165,7 @@ symbols! {
         Capture,
         Center,
         Clone,
+        Context,
         Continue,
         Copy,
         Count,
@@ -264,7 +265,6 @@ symbols! {
         Relaxed,
         Release,
         Result,
-        ResumeTy,
         Return,
         Right,
         Rust,
@@ -754,7 +754,6 @@ symbols! {
         generic_associated_types_extended,
         generic_const_exprs,
         generic_param_attrs,
-        get_context,
         global_allocator,
         global_asm,
         globs,

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`.
-#[cfg_attr(not(bootstrap), lang = "ResumeTy")]
+// FIXME(swatinem): This type can be removed when bumping the bootstrap compiler
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
 #[derive(Debug, Copy, Clone)]
@@ -61,6 +61,7 @@ unsafe impl Sync for ResumeTy {}
 /// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
 /// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
 // This is `const` to avoid extra errors after we recover from `const async fn`
+// FIXME(swatinem): This fn can be removed when bumping the bootstrap compiler
 #[cfg_attr(bootstrap, lang = "from_generator")]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
@@ -102,7 +103,8 @@ where
     GenFuture(gen)
 }
 
-#[lang = "get_context"]
+// FIXME(swatinem): This fn can be removed when bumping the bootstrap compiler
+#[cfg_attr(bootstrap, lang = "get_context")]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
 #[must_use]
@@ -113,6 +115,10 @@ pub unsafe fn get_context<'a, 'b>(cx: ResumeTy) -> &'a mut Context<'b> {
     unsafe { &mut *cx.0.as_ptr().cast() }
 }
 
+// FIXME(swatinem): This fn is currently needed to work around shortcomings
+// in type and lifetime inference.
+// See the comment at the bottom of `LoweringContext::make_async_expr` and
+// <https://github.com/rust-lang/rust/issues/104826>.
 #[cfg_attr(not(bootstrap), lang = "identity_future")]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]

library/core/src/task/wake.rs

@@ -174,6 +174,7 @@ impl RawWakerVTable {
 /// Currently, `Context` only serves to provide access to a [`&Waker`](Waker)
 /// which can be used to wake the current task.
 #[stable(feature = "futures_api", since = "1.36.0")]
+#[cfg_attr(not(bootstrap), lang = "Context")]
 pub struct Context<'a> {
     waker: &'a Waker,
     // Ensure we future-proof against variance changes by forcing

src/test/ui/async-await/async-await-let-else.drop-tracking.stderr

@@ -40,7 +40,7 @@ LL |   async fn bar2<T>(_: T) -> ! {
 LL | |     panic!()
 LL | | }
    | |_^
-   = note: required because it captures the following types: `ResumeTy`, `Option<bool>`, `impl Future<Output = !>`, `()`
+   = note: required because it captures the following types: `&mut Context<'_>`, `Option<bool>`, `impl Future<Output = !>`, `()`
 note: required because it's used within this `async fn` body
   --> $DIR/async-await-let-else.rs:21:32
    |

src/test/ui/async-await/issue-68112.drop_tracking.stderr

@@ -57,7 +57,7 @@ note: required because it appears within the type `impl Future<Output = Arc<RefC
    |
 LL | fn make_non_send_future2() -> impl Future<Output = Arc<RefCell<i32>>> {
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: required because it captures the following types: `ResumeTy`, `impl Future<Output = Arc<RefCell<i32>>>`, `()`, `Ready<i32>`
+   = note: required because it captures the following types: `&mut Context<'_>`, `impl Future<Output = Arc<RefCell<i32>>>`, `()`, `Ready<i32>`
 note: required because it's used within this `async` block
   --> $DIR/issue-68112.rs:60:20
    |

src/test/ui/async-await/issue-68112.no_drop_tracking.stderr

@@ -57,7 +57,7 @@ note: required because it appears within the type `impl Future<Output = Arc<RefC
    |
 LL | fn make_non_send_future2() -> impl Future<Output = Arc<RefCell<i32>>> {
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: required because it captures the following types: `ResumeTy`, `impl Future<Output = Arc<RefCell<i32>>>`, `()`, `i32`, `Ready<i32>`
+   = note: required because it captures the following types: `&mut Context<'_>`, `impl Future<Output = Arc<RefCell<i32>>>`, `()`, `i32`, `Ready<i32>`
 note: required because it's used within this `async` block
   --> $DIR/issue-68112.rs:60:20
    |

src/test/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 previous error
 

src/test/ui/async-await/issue-70935-complex-spans.drop_tracking.stderr

@@ -18,7 +18,7 @@ LL |   async fn baz<T>(_c: impl FnMut() -> T) where T: Future<Output=()> {
    |  ___________________________________________________________________^
 LL | | }
    | |_^
-   = note: required because it captures the following types: `ResumeTy`, `impl Future<Output = ()>`, `()`
+   = note: required because it captures the following types: `&mut Context<'_>`, `impl Future<Output = ()>`, `()`
 note: required because it's used within this `async` block
   --> $DIR/issue-70935-complex-spans.rs:16:5
    |

src/test/ui/async-await/partial-drop-partial-reinit.drop_tracking.stderr

@@ -11,7 +11,7 @@ LL | async fn foo() {
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `NotSend`
    = note: required because it appears within the type `(NotSend,)`
-   = note: required because it captures the following types: `ResumeTy`, `(NotSend,)`, `()`, `impl Future<Output = ()>`
+   = note: required because it captures the following types: `&mut Context<'_>`, `(NotSend,)`, `()`, `impl Future<Output = ()>`
 note: required because it's used within this `async fn` body
   --> $DIR/partial-drop-partial-reinit.rs:31:16
    |

src/test/ui/async-await/partial-drop-partial-reinit.no_drop_tracking.stderr

@@ -11,7 +11,7 @@ LL | async fn foo() {
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `NotSend`
    = note: required because it appears within the type `(NotSend,)`
-   = note: required because it captures the following types: `ResumeTy`, `(NotSend,)`, `impl Future<Output = ()>`, `()`
+   = note: required because it captures the following types: `&mut Context<'_>`, `(NotSend,)`, `impl Future<Output = ()>`, `()`
 note: required because it's used within this `async fn` body
   --> $DIR/partial-drop-partial-reinit.rs:31:16
    |

src/test/ui/regions/closure-in-projection-issue-97405.rs

@@ -22,11 +22,11 @@ fn good_generic_fn<T>() {
 // This should fail because `T` ends up in the upvars of the closure.
 fn bad_generic_fn<T: Copy>(t: T) {
     assert_static(opaque(async move { t; }).next());
-    //~^ ERROR the associated type `<impl Iterator as Iterator>::Item` may not live long enough
+    //~^ ERROR the parameter type `T` may not live long enough
     assert_static(opaque(move || { t; }).next());
     //~^ ERROR the associated type `<impl Iterator as Iterator>::Item` may not live long enough
     assert_static(opaque(opaque(async move { t; }).next()).next());
-    //~^ ERROR the associated type `<impl Iterator as Iterator>::Item` may not live long enough
+    //~^ ERROR the parameter type `T` may not live long enough
 }
 
 fn main() {}

src/test/ui/regions/closure-in-projection-issue-97405.stderr

@@ -1,11 +1,13 @@
-error[E0310]: the associated type `<impl Iterator as Iterator>::Item` may not live long enough
+error[E0310]: the parameter type `T` may not live long enough
   --> $DIR/closure-in-projection-issue-97405.rs:24:5
    |
 LL |     assert_static(opaque(async move { t; }).next());
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds
    |
-   = help: consider adding an explicit lifetime bound `<impl Iterator as Iterator>::Item: 'static`...
-   = note: ...so that the type `<impl Iterator as Iterator>::Item` will meet its required lifetime bounds
+help: consider adding an explicit lifetime bound...
+   |
+LL | fn bad_generic_fn<T: Copy + 'static>(t: T) {
+   |                           +++++++++
 
 error[E0310]: the associated type `<impl Iterator as Iterator>::Item` may not live long enough
   --> $DIR/closure-in-projection-issue-97405.rs:26:5
@@ -16,14 +18,16 @@ LL |     assert_static(opaque(move || { t; }).next());
    = help: consider adding an explicit lifetime bound `<impl Iterator as Iterator>::Item: 'static`...
    = note: ...so that the type `<impl Iterator as Iterator>::Item` will meet its required lifetime bounds
 
-error[E0310]: the associated type `<impl Iterator as Iterator>::Item` may not live long enough
+error[E0310]: the parameter type `T` may not live long enough
   --> $DIR/closure-in-projection-issue-97405.rs:28:5
    |
 LL |     assert_static(opaque(opaque(async move { t; }).next()).next());
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds
    |
-   = help: consider adding an explicit lifetime bound `<impl Iterator as Iterator>::Item: 'static`...
-   = note: ...so that the type `<impl Iterator as Iterator>::Item` will meet its required lifetime bounds
+help: consider adding an explicit lifetime bound...
+   |
+LL | fn bad_generic_fn<T: Copy + 'static>(t: T) {
+   |                           +++++++++
 
 error: aborting due to 3 previous errors