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WIP: Remove ResumeTy from async lowering
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This changes from the stdlib supported `ResumeTy`, and converting that to a `&mut Context<'_>` to directly use `&mut Context<'_>` in lowering.

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

The PR is currently failing as it depends on `-Zdrop-tracking-mir` becoming the default, so that the compiler does not falsly believe the context is being held across await points,which would make async blocks `!Send` and `!UnwindSafe`.
However it also still fails the testcase added in rust-lang#106264 for reasons I don’t understand.
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@Swatinem
Swatinem committed Feb 1, 2023
1 parent 3b63948 commit 304d235
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Showing 7 changed files with 38 additions and 170 deletions.
53 changes: 28 additions & 25 deletions compiler/rustc_ast_lowering/src/expr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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, Symbol};
use rustc_span::symbol::{kw, sym, Ident, Symbol};
use rustc_span::DUMMY_SP;
use thin_vec::thin_vec;

Expand Down Expand Up @@ -599,14 +599,28 @@ 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: `&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), 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::Ref(context_lifetime, context_ty),
span: self.lower_span(span),
};

// The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
Expand Down Expand Up @@ -710,7 +724,7 @@ 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 => {}
Expand All @@ -731,11 +745,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
}
}
let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
let gen_future_span = self.mark_span_with_reason(
DesugaringKind::Await,
full_span,
self.allow_gen_future.clone(),
);
let expr = self.lower_expr_mut(expr);
let expr_hir_id = expr.hir_id;

Expand All @@ -751,7 +760,7 @@ 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 = {
Expand All @@ -769,16 +778,10 @@ 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))
Expand All @@ -789,9 +792,9 @@ 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,
Expand All @@ -801,7 +804,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
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)
};
Expand Down
2 changes: 0 additions & 2 deletions compiler/rustc_hir/src/lang_items.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -295,9 +295,7 @@ 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;
Expand Down
9 changes: 0 additions & 9 deletions compiler/rustc_middle/src/ty/context.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1813,15 +1813,6 @@ impl<'tcx> TyCtxt<'tcx> {
self.mk_ty(GeneratorWitness(types))
}

/// Creates a `&mut Context<'_>` [`Ty`] with erased lifetimes.
pub fn mk_task_context(self) -> Ty<'tcx> {
let context_did = self.require_lang_item(LangItem::Context, None);
let context_adt_ref = self.adt_def(context_did);
let context_substs = self.intern_substs(&[self.lifetimes.re_erased.into()]);
let context_ty = self.mk_adt(context_adt_ref, context_substs);
self.mk_mut_ref(self.lifetimes.re_erased, context_ty)
}

#[inline]
pub fn mk_generator_witness_mir(self, id: DefId, substs: SubstsRef<'tcx>) -> Ty<'tcx> {
self.mk_ty(GeneratorWitnessMIR(id, substs))
Expand Down
108 changes: 2 additions & 106 deletions compiler/rustc_mir_transform/src/generator.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -464,104 +464,6 @@ fn replace_local<'tcx>(
new_local
}

/// Transforms the `body` of the generator 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 generator body only knows about `&mut Context<'_>`.
fn transform_async_context<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let context_mut_ref = tcx.mk_task_context();

// 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 BasicBlock::new(0)..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);
};
}
}

struct LivenessInfo {
/// Which locals are live across any suspension point.
saved_locals: GeneratorSavedLocals,
Expand Down Expand Up @@ -1471,19 +1373,13 @@ impl<'tcx> MirPass<'tcx> for StateTransform {
// RETURN_PLACE then is a fresh unused local with type ret_ty.
let new_ret_local = replace_local(RETURN_PLACE, ret_ty, body, tcx);

// Replace all occurrences of `ResumeTy` with `&mut Context<'_>` within async bodies.
if is_async_kind {
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 generator
// state. After the yield the slot in the generator state would then be uninitialized.
let resume_local = Local::new(2);
let resume_ty =
if is_async_kind { tcx.mk_task_context() } else { body.local_decls[resume_local].ty };
let new_resume_local = replace_local(resume_local, resume_ty, body, tcx);
let new_resume_local =
replace_local(resume_local, body.local_decls[resume_local].ty, body, tcx);

// When first entering the generator, move the resume argument into its new local.
let source_info = SourceInfo::outermost(body.span);
Expand Down
2 changes: 0 additions & 2 deletions compiler/rustc_span/src/symbol.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -266,7 +266,6 @@ symbols! {
Relaxed,
Release,
Result,
ResumeTy,
Return,
Right,
Rust,
Expand Down Expand Up @@ -765,7 +764,6 @@ symbols! {
generic_associated_types_extended,
generic_const_exprs,
generic_param_attrs,
get_context,
global_allocator,
global_asm,
globs,
Expand Down
30 changes: 6 additions & 24 deletions compiler/rustc_ty_utils/src/abi.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -108,41 +108,23 @@ fn fn_sig_for_fn_abi<'tcx>(
// `Generator::resume(...) -> GeneratorState` function in case we
// have an ordinary generator, or the `Future::poll(...) -> Poll`
// function in case this is a special generator backing an async construct.
let (resume_ty, ret_ty) = if tcx.generator_is_async(did) {
// The signature should be `Future::poll(_, &mut Context<'_>) -> Poll<Output>`
let ret_ty = if tcx.generator_is_async(did) {
// The signature should be `Future::poll(..) -> Poll<Output>`
let poll_did = tcx.require_lang_item(LangItem::Poll, None);
let poll_adt_ref = tcx.adt_def(poll_did);
let poll_substs = tcx.intern_substs(&[sig.return_ty.into()]);
let ret_ty = tcx.mk_adt(poll_adt_ref, poll_substs);

// 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 = tcx.mk_task_context();

(context_mut_ref, ret_ty)
tcx.mk_adt(poll_adt_ref, poll_substs)
} else {
// The signature should be `Generator::resume(_, Resume) -> GeneratorState<Yield, Return>`
// The signature should be `Generator::resume(..) -> GeneratorState<Yield, Return>`
let state_did = tcx.require_lang_item(LangItem::GeneratorState, None);
let state_adt_ref = tcx.adt_def(state_did);
let state_substs = tcx.intern_substs(&[sig.yield_ty.into(), sig.return_ty.into()]);
let ret_ty = tcx.mk_adt(state_adt_ref, state_substs);

(sig.resume_ty, ret_ty)
tcx.mk_adt(state_adt_ref, state_substs)
};

ty::Binder::bind_with_vars(
tcx.mk_fn_sig(
[env_ty, resume_ty].iter(),
[env_ty, sig.resume_ty].iter(),
&ret_ty,
false,
hir::Unsafety::Normal,
Expand Down
4 changes: 2 additions & 2 deletions library/core/src/future/mod.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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)]
Expand All @@ -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]
Expand Down

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