Rollup merge of rust-lang#105250 - Swatinem:async-rm-resumety, r=oli-obk

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.

fixes rust-lang#104828 and rust-lang#104321 (comment)

r? `@oli-obk`

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