1 //! This is the implementation of the pass which transforms generators into state machines.
3 //! MIR generation for generators creates a function which has a self argument which
4 //! passes by value. This argument is effectively a generator type which only contains upvars and
5 //! is only used for this argument inside the MIR for the generator.
6 //! It is passed by value to enable upvars to be moved out of it. Drop elaboration runs on that
7 //! MIR before this pass and creates drop flags for MIR locals.
8 //! It will also drop the generator argument (which only consists of upvars) if any of the upvars
9 //! are moved out of. This pass elaborates the drops of upvars / generator argument in the case
10 //! that none of the upvars were moved out of. This is because we cannot have any drops of this
11 //! generator in the MIR, since it is used to create the drop glue for the generator. We'd get
12 //! infinite recursion otherwise.
14 //! This pass creates the implementation for either the `Generator::resume` or `Future::poll`
15 //! function and the drop shim for the generator based on the MIR input.
16 //! It converts the generator argument from Self to &mut Self adding derefs in the MIR as needed.
17 //! It computes the final layout of the generator struct which looks like this:
18 //! First upvars are stored
19 //! It is followed by the generator state field.
20 //! Then finally the MIR locals which are live across a suspension point are stored.
21 //! ```ignore (illustrative)
22 //! struct Generator {
28 //! This pass computes the meaning of the state field and the MIR locals which are live
29 //! across a suspension point. There are however three hardcoded generator states:
30 //! 0 - Generator have not been resumed yet
31 //! 1 - Generator has returned / is completed
32 //! 2 - Generator has been poisoned
34 //! It also rewrites `return x` and `yield y` as setting a new generator state and returning
35 //! `GeneratorState::Complete(x)` and `GeneratorState::Yielded(y)`,
36 //! or `Poll::Ready(x)` and `Poll::Pending` respectively.
37 //! MIR locals which are live across a suspension point are moved to the generator struct
38 //! with references to them being updated with references to the generator struct.
40 //! The pass creates two functions which have a switch on the generator state giving
41 //! the action to take.
43 //! One of them is the implementation of `Generator::resume` / `Future::poll`.
44 //! For generators with state 0 (unresumed) it starts the execution of the generator.
45 //! For generators with state 1 (returned) and state 2 (poisoned) it panics.
46 //! Otherwise it continues the execution from the last suspension point.
48 //! The other function is the drop glue for the generator.
49 //! For generators with state 0 (unresumed) it drops the upvars of the generator.
50 //! For generators with state 1 (returned) and state 2 (poisoned) it does nothing.
51 //! Otherwise it drops all the values in scope at the last suspension point.
53 use crate::deref_separator::deref_finder;
55 use crate::util::expand_aggregate;
57 use rustc_data_structures::fx::FxHashMap;
59 use rustc_hir::lang_items::LangItem;
60 use rustc_hir::GeneratorKind;
61 use rustc_index::bit_set::{BitMatrix, BitSet, GrowableBitSet};
62 use rustc_index::vec::{Idx, IndexVec};
63 use rustc_middle::mir::dump_mir;
64 use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
65 use rustc_middle::mir::*;
66 use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt};
67 use rustc_middle::ty::{GeneratorSubsts, SubstsRef};
68 use rustc_mir_dataflow::impls::{
69 MaybeBorrowedLocals, MaybeLiveLocals, MaybeRequiresStorage, MaybeStorageLive,
71 use rustc_mir_dataflow::storage::always_storage_live_locals;
72 use rustc_mir_dataflow::{self, Analysis};
73 use rustc_target::abi::VariantIdx;
74 use rustc_target::spec::PanicStrategy;
77 pub struct StateTransform;
79 struct RenameLocalVisitor<'tcx> {
85 impl<'tcx> MutVisitor<'tcx> for RenameLocalVisitor<'tcx> {
86 fn tcx(&self) -> TyCtxt<'tcx> {
90 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
91 if *local == self.from {
96 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
97 match terminator.kind {
98 TerminatorKind::Return => {
99 // Do not replace the implicit `_0` access here, as that's not possible. The
100 // transform already handles `return` correctly.
102 _ => self.super_terminator(terminator, location),
107 struct DerefArgVisitor<'tcx> {
111 impl<'tcx> MutVisitor<'tcx> for DerefArgVisitor<'tcx> {
112 fn tcx(&self) -> TyCtxt<'tcx> {
116 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
117 assert_ne!(*local, SELF_ARG);
120 fn visit_place(&mut self, place: &mut Place<'tcx>, context: PlaceContext, location: Location) {
121 if place.local == SELF_ARG {
126 projection: self.tcx().intern_place_elems(&[ProjectionElem::Deref]),
131 self.visit_local(&mut place.local, context, location);
133 for elem in place.projection.iter() {
134 if let PlaceElem::Index(local) = elem {
135 assert_ne!(local, SELF_ARG);
142 struct PinArgVisitor<'tcx> {
143 ref_gen_ty: Ty<'tcx>,
147 impl<'tcx> MutVisitor<'tcx> for PinArgVisitor<'tcx> {
148 fn tcx(&self) -> TyCtxt<'tcx> {
152 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
153 assert_ne!(*local, SELF_ARG);
156 fn visit_place(&mut self, place: &mut Place<'tcx>, context: PlaceContext, location: Location) {
157 if place.local == SELF_ARG {
162 projection: self.tcx().intern_place_elems(&[ProjectionElem::Field(
170 self.visit_local(&mut place.local, context, location);
172 for elem in place.projection.iter() {
173 if let PlaceElem::Index(local) = elem {
174 assert_ne!(local, SELF_ARG);
181 fn replace_base<'tcx>(place: &mut Place<'tcx>, new_base: Place<'tcx>, tcx: TyCtxt<'tcx>) {
182 place.local = new_base.local;
184 let mut new_projection = new_base.projection.to_vec();
185 new_projection.append(&mut place.projection.to_vec());
187 place.projection = tcx.intern_place_elems(&new_projection);
190 const SELF_ARG: Local = Local::from_u32(1);
192 /// Generator has not been resumed yet.
193 const UNRESUMED: usize = GeneratorSubsts::UNRESUMED;
194 /// Generator has returned / is completed.
195 const RETURNED: usize = GeneratorSubsts::RETURNED;
196 /// Generator has panicked and is poisoned.
197 const POISONED: usize = GeneratorSubsts::POISONED;
199 /// Number of variants to reserve in generator state. Corresponds to
200 /// `UNRESUMED` (beginning of a generator) and `RETURNED`/`POISONED`
201 /// (end of a generator) states.
202 const RESERVED_VARIANTS: usize = 3;
204 /// A `yield` point in the generator.
205 struct SuspensionPoint<'tcx> {
206 /// State discriminant used when suspending or resuming at this point.
208 /// The block to jump to after resumption.
210 /// Where to move the resume argument after resumption.
211 resume_arg: Place<'tcx>,
212 /// Which block to jump to if the generator is dropped in this state.
213 drop: Option<BasicBlock>,
214 /// Set of locals that have live storage while at this suspension point.
215 storage_liveness: GrowableBitSet<Local>,
218 struct TransformVisitor<'tcx> {
221 state_adt_ref: AdtDef<'tcx>,
222 state_substs: SubstsRef<'tcx>,
224 // The type of the discriminant in the generator struct
227 // Mapping from Local to (type of local, generator struct index)
228 // FIXME(eddyb) This should use `IndexVec<Local, Option<_>>`.
229 remap: FxHashMap<Local, (Ty<'tcx>, VariantIdx, usize)>,
231 // A map from a suspension point in a block to the locals which have live storage at that point
232 storage_liveness: IndexVec<BasicBlock, Option<BitSet<Local>>>,
234 // A list of suspension points, generated during the transform
235 suspension_points: Vec<SuspensionPoint<'tcx>>,
237 // The set of locals that have no `StorageLive`/`StorageDead` annotations.
238 always_live_locals: BitSet<Local>,
240 // The original RETURN_PLACE local
241 new_ret_local: Local,
244 impl<'tcx> TransformVisitor<'tcx> {
245 // Make a `GeneratorState` or `Poll` variant assignment.
247 // `core::ops::GeneratorState` only has single element tuple variants,
248 // so we can just write to the downcasted first field and then set the
249 // discriminant to the appropriate variant.
253 source_info: SourceInfo,
255 statements: &mut Vec<Statement<'tcx>>,
257 let idx = VariantIdx::new(match (is_return, self.is_async_kind) {
258 (true, false) => 1, // GeneratorState::Complete
259 (false, false) => 0, // GeneratorState::Yielded
260 (true, true) => 0, // Poll::Ready
261 (false, true) => 1, // Poll::Pending
264 let kind = AggregateKind::Adt(self.state_adt_ref.did(), idx, self.state_substs, None, None);
267 if self.is_async_kind && idx == VariantIdx::new(1) {
268 assert_eq!(self.state_adt_ref.variant(idx).fields.len(), 0);
270 // FIXME(swatinem): assert that `val` is indeed unit?
271 statements.extend(expand_aggregate(
272 Place::return_place(),
281 // else: `Poll::Ready(x)`, `GeneratorState::Yielded(x)` or `GeneratorState::Complete(x)`
282 assert_eq!(self.state_adt_ref.variant(idx).fields.len(), 1);
286 .bound_type_of(self.state_adt_ref.variant(idx).fields[0].did)
287 .subst(self.tcx, self.state_substs);
289 statements.extend(expand_aggregate(
290 Place::return_place(),
291 std::iter::once((val, ty)),
298 // Create a Place referencing a generator struct field
299 fn make_field(&self, variant_index: VariantIdx, idx: usize, ty: Ty<'tcx>) -> Place<'tcx> {
300 let self_place = Place::from(SELF_ARG);
301 let base = self.tcx.mk_place_downcast_unnamed(self_place, variant_index);
302 let mut projection = base.projection.to_vec();
303 projection.push(ProjectionElem::Field(Field::new(idx), ty));
305 Place { local: base.local, projection: self.tcx.intern_place_elems(&projection) }
308 // Create a statement which changes the discriminant
309 fn set_discr(&self, state_disc: VariantIdx, source_info: SourceInfo) -> Statement<'tcx> {
310 let self_place = Place::from(SELF_ARG);
313 kind: StatementKind::SetDiscriminant {
314 place: Box::new(self_place),
315 variant_index: state_disc,
320 // Create a statement which reads the discriminant into a temporary
321 fn get_discr(&self, body: &mut Body<'tcx>) -> (Statement<'tcx>, Place<'tcx>) {
322 let temp_decl = LocalDecl::new(self.discr_ty, body.span).internal();
323 let local_decls_len = body.local_decls.push(temp_decl);
324 let temp = Place::from(local_decls_len);
326 let self_place = Place::from(SELF_ARG);
327 let assign = Statement {
328 source_info: SourceInfo::outermost(body.span),
329 kind: StatementKind::Assign(Box::new((temp, Rvalue::Discriminant(self_place)))),
335 impl<'tcx> MutVisitor<'tcx> for TransformVisitor<'tcx> {
336 fn tcx(&self) -> TyCtxt<'tcx> {
340 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
341 assert_eq!(self.remap.get(local), None);
346 place: &mut Place<'tcx>,
347 _context: PlaceContext,
350 // Replace an Local in the remap with a generator struct access
351 if let Some(&(ty, variant_index, idx)) = self.remap.get(&place.local) {
352 replace_base(place, self.make_field(variant_index, idx, ty), self.tcx);
356 fn visit_basic_block_data(&mut self, block: BasicBlock, data: &mut BasicBlockData<'tcx>) {
357 // Remove StorageLive and StorageDead statements for remapped locals
358 data.retain_statements(|s| match s.kind {
359 StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => {
360 !self.remap.contains_key(&l)
365 let ret_val = match data.terminator().kind {
366 TerminatorKind::Return => {
367 Some((true, None, Operand::Move(Place::from(self.new_ret_local)), None))
369 TerminatorKind::Yield { ref value, resume, resume_arg, drop } => {
370 Some((false, Some((resume, resume_arg)), value.clone(), drop))
375 if let Some((is_return, resume, v, drop)) = ret_val {
376 let source_info = data.terminator().source_info;
377 // We must assign the value first in case it gets declared dead below
378 self.make_state(v, source_info, is_return, &mut data.statements);
379 let state = if let Some((resume, mut resume_arg)) = resume {
381 let state = RESERVED_VARIANTS + self.suspension_points.len();
383 // The resume arg target location might itself be remapped if its base local is
384 // live across a yield.
386 if let Some(&(ty, variant, idx)) = self.remap.get(&resume_arg.local) {
387 replace_base(&mut resume_arg, self.make_field(variant, idx, ty), self.tcx);
393 self.suspension_points.push(SuspensionPoint {
398 storage_liveness: self.storage_liveness[block].clone().unwrap().into(),
401 VariantIdx::new(state)
404 VariantIdx::new(RETURNED) // state for returned
406 data.statements.push(self.set_discr(state, source_info));
407 data.terminator_mut().kind = TerminatorKind::Return;
410 self.super_basic_block_data(block, data);
414 fn make_generator_state_argument_indirect<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
415 let gen_ty = body.local_decls.raw[1].ty;
418 tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut { ty: gen_ty, mutbl: Mutability::Mut });
420 // Replace the by value generator argument
421 body.local_decls.raw[1].ty = ref_gen_ty;
423 // Add a deref to accesses of the generator state
424 DerefArgVisitor { tcx }.visit_body(body);
427 fn make_generator_state_argument_pinned<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
428 let ref_gen_ty = body.local_decls.raw[1].ty;
430 let pin_did = tcx.require_lang_item(LangItem::Pin, Some(body.span));
431 let pin_adt_ref = tcx.adt_def(pin_did);
432 let substs = tcx.intern_substs(&[ref_gen_ty.into()]);
433 let pin_ref_gen_ty = tcx.mk_adt(pin_adt_ref, substs);
435 // Replace the by ref generator argument
436 body.local_decls.raw[1].ty = pin_ref_gen_ty;
438 // Add the Pin field access to accesses of the generator state
439 PinArgVisitor { ref_gen_ty, tcx }.visit_body(body);
442 /// Allocates a new local and replaces all references of `local` with it. Returns the new local.
444 /// `local` will be changed to a new local decl with type `ty`.
446 /// Note that the new local will be uninitialized. It is the caller's responsibility to assign some
447 /// valid value to it before its first use.
448 fn replace_local<'tcx>(
451 body: &mut Body<'tcx>,
454 let new_decl = LocalDecl::new(ty, body.span);
455 let new_local = body.local_decls.push(new_decl);
456 body.local_decls.swap(local, new_local);
458 RenameLocalVisitor { from: local, to: new_local, tcx }.visit_body(body);
463 /// Transforms the `body` of the generator applying the following transforms:
465 /// - Eliminates all the `get_context` calls that async lowering created.
466 /// - Replace all `Local` `ResumeTy` types with `&mut Context<'_>` (`context_mut_ref`).
468 /// The `Local`s that have their types replaced are:
469 /// - The `resume` argument itself.
470 /// - The argument to `get_context`.
471 /// - The yielded value of a `yield`.
473 /// The `ResumeTy` hides a `&mut Context<'_>` behind an unsafe raw pointer, and the
474 /// `get_context` function is being used to convert that back to a `&mut Context<'_>`.
476 /// Ideally the async lowering would not use the `ResumeTy`/`get_context` indirection,
477 /// but rather directly use `&mut Context<'_>`, however that would currently
478 /// lead to higher-kinded lifetime errors.
479 /// See <https://github.com/rust-lang/rust/issues/105501>.
481 /// The async lowering step and the type / lifetime inference / checking are
482 /// still using the `ResumeTy` indirection for the time being, and that indirection
483 /// is removed here. After this transform, the generator body only knows about `&mut Context<'_>`.
484 fn transform_async_context<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
485 let context_mut_ref = tcx.mk_task_context();
487 // replace the type of the `resume` argument
488 replace_resume_ty_local(tcx, body, Local::new(2), context_mut_ref);
490 let get_context_def_id = tcx.require_lang_item(LangItem::GetContext, None);
492 for bb in BasicBlock::new(0)..body.basic_blocks.next_index() {
493 let bb_data = &body[bb];
494 if bb_data.is_cleanup {
498 match &bb_data.terminator().kind {
499 TerminatorKind::Call { func, .. } => {
500 let func_ty = func.ty(body, tcx);
501 if let ty::FnDef(def_id, _) = *func_ty.kind() {
502 if def_id == get_context_def_id {
503 let local = eliminate_get_context_call(&mut body[bb]);
504 replace_resume_ty_local(tcx, body, local, context_mut_ref);
510 TerminatorKind::Yield { resume_arg, .. } => {
511 replace_resume_ty_local(tcx, body, resume_arg.local, context_mut_ref);
518 fn eliminate_get_context_call<'tcx>(bb_data: &mut BasicBlockData<'tcx>) -> Local {
519 let terminator = bb_data.terminator.take().unwrap();
520 if let TerminatorKind::Call { mut args, destination, target, .. } = terminator.kind {
521 let arg = args.pop().unwrap();
522 let local = arg.place().unwrap().local;
524 let arg = Rvalue::Use(arg);
525 let assign = Statement {
526 source_info: terminator.source_info,
527 kind: StatementKind::Assign(Box::new((destination, arg))),
529 bb_data.statements.push(assign);
530 bb_data.terminator = Some(Terminator {
531 source_info: terminator.source_info,
532 kind: TerminatorKind::Goto { target: target.unwrap() },
540 #[cfg_attr(not(debug_assertions), allow(unused))]
541 fn replace_resume_ty_local<'tcx>(
543 body: &mut Body<'tcx>,
545 context_mut_ref: Ty<'tcx>,
547 let local_ty = std::mem::replace(&mut body.local_decls[local].ty, context_mut_ref);
548 // We have to replace the `ResumeTy` that is used for type and borrow checking
549 // with `&mut Context<'_>` in MIR.
550 #[cfg(debug_assertions)]
552 if let ty::Adt(resume_ty_adt, _) = local_ty.kind() {
553 let expected_adt = tcx.adt_def(tcx.require_lang_item(LangItem::ResumeTy, None));
554 assert_eq!(*resume_ty_adt, expected_adt);
556 panic!("expected `ResumeTy`, found `{:?}`", local_ty);
561 struct LivenessInfo {
562 /// Which locals are live across any suspension point.
563 saved_locals: GeneratorSavedLocals,
565 /// The set of saved locals live at each suspension point.
566 live_locals_at_suspension_points: Vec<BitSet<GeneratorSavedLocal>>,
568 /// Parallel vec to the above with SourceInfo for each yield terminator.
569 source_info_at_suspension_points: Vec<SourceInfo>,
571 /// For every saved local, the set of other saved locals that are
572 /// storage-live at the same time as this local. We cannot overlap locals in
573 /// the layout which have conflicting storage.
574 storage_conflicts: BitMatrix<GeneratorSavedLocal, GeneratorSavedLocal>,
576 /// For every suspending block, the locals which are storage-live across
577 /// that suspension point.
578 storage_liveness: IndexVec<BasicBlock, Option<BitSet<Local>>>,
581 fn locals_live_across_suspend_points<'tcx>(
584 always_live_locals: &BitSet<Local>,
587 let body_ref: &Body<'_> = &body;
589 // Calculate when MIR locals have live storage. This gives us an upper bound of their
591 let mut storage_live = MaybeStorageLive::new(std::borrow::Cow::Borrowed(always_live_locals))
592 .into_engine(tcx, body_ref)
593 .iterate_to_fixpoint()
594 .into_results_cursor(body_ref);
596 // Calculate the MIR locals which have been previously
597 // borrowed (even if they are still active).
598 let borrowed_locals_results =
599 MaybeBorrowedLocals.into_engine(tcx, body_ref).pass_name("generator").iterate_to_fixpoint();
601 let mut borrowed_locals_cursor =
602 rustc_mir_dataflow::ResultsCursor::new(body_ref, &borrowed_locals_results);
604 // Calculate the MIR locals that we actually need to keep storage around
606 let requires_storage_results = MaybeRequiresStorage::new(body, &borrowed_locals_results)
607 .into_engine(tcx, body_ref)
608 .iterate_to_fixpoint();
609 let mut requires_storage_cursor =
610 rustc_mir_dataflow::ResultsCursor::new(body_ref, &requires_storage_results);
612 // Calculate the liveness of MIR locals ignoring borrows.
613 let mut liveness = MaybeLiveLocals
614 .into_engine(tcx, body_ref)
615 .pass_name("generator")
616 .iterate_to_fixpoint()
617 .into_results_cursor(body_ref);
619 let mut storage_liveness_map = IndexVec::from_elem(None, &body.basic_blocks);
620 let mut live_locals_at_suspension_points = Vec::new();
621 let mut source_info_at_suspension_points = Vec::new();
622 let mut live_locals_at_any_suspension_point = BitSet::new_empty(body.local_decls.len());
624 for (block, data) in body.basic_blocks.iter_enumerated() {
625 if let TerminatorKind::Yield { .. } = data.terminator().kind {
626 let loc = Location { block, statement_index: data.statements.len() };
628 liveness.seek_to_block_end(block);
629 let mut live_locals: BitSet<_> = BitSet::new_empty(body.local_decls.len());
630 live_locals.union(liveness.get());
633 // The `liveness` variable contains the liveness of MIR locals ignoring borrows.
634 // This is correct for movable generators since borrows cannot live across
635 // suspension points. However for immovable generators we need to account for
636 // borrows, so we conservatively assume that all borrowed locals are live until
637 // we find a StorageDead statement referencing the locals.
638 // To do this we just union our `liveness` result with `borrowed_locals`, which
639 // contains all the locals which has been borrowed before this suspension point.
640 // If a borrow is converted to a raw reference, we must also assume that it lives
641 // forever. Note that the final liveness is still bounded by the storage liveness
642 // of the local, which happens using the `intersect` operation below.
643 borrowed_locals_cursor.seek_before_primary_effect(loc);
644 live_locals.union(borrowed_locals_cursor.get());
647 // Store the storage liveness for later use so we can restore the state
648 // after a suspension point
649 storage_live.seek_before_primary_effect(loc);
650 storage_liveness_map[block] = Some(storage_live.get().clone());
652 // Locals live are live at this point only if they are used across
653 // suspension points (the `liveness` variable)
654 // and their storage is required (the `storage_required` variable)
655 requires_storage_cursor.seek_before_primary_effect(loc);
656 live_locals.intersect(requires_storage_cursor.get());
658 // The generator argument is ignored.
659 live_locals.remove(SELF_ARG);
661 debug!("loc = {:?}, live_locals = {:?}", loc, live_locals);
663 // Add the locals live at this suspension point to the set of locals which live across
664 // any suspension points
665 live_locals_at_any_suspension_point.union(&live_locals);
667 live_locals_at_suspension_points.push(live_locals);
668 source_info_at_suspension_points.push(data.terminator().source_info);
672 debug!("live_locals_anywhere = {:?}", live_locals_at_any_suspension_point);
673 let saved_locals = GeneratorSavedLocals(live_locals_at_any_suspension_point);
675 // Renumber our liveness_map bitsets to include only the locals we are
677 let live_locals_at_suspension_points = live_locals_at_suspension_points
679 .map(|live_here| saved_locals.renumber_bitset(&live_here))
682 let storage_conflicts = compute_storage_conflicts(
685 always_live_locals.clone(),
686 requires_storage_results,
691 live_locals_at_suspension_points,
692 source_info_at_suspension_points,
694 storage_liveness: storage_liveness_map,
698 /// The set of `Local`s that must be saved across yield points.
700 /// `GeneratorSavedLocal` is indexed in terms of the elements in this set;
701 /// i.e. `GeneratorSavedLocal::new(1)` corresponds to the second local
702 /// included in this set.
703 struct GeneratorSavedLocals(BitSet<Local>);
705 impl GeneratorSavedLocals {
706 /// Returns an iterator over each `GeneratorSavedLocal` along with the `Local` it corresponds
708 fn iter_enumerated(&self) -> impl '_ + Iterator<Item = (GeneratorSavedLocal, Local)> {
709 self.iter().enumerate().map(|(i, l)| (GeneratorSavedLocal::from(i), l))
712 /// Transforms a `BitSet<Local>` that contains only locals saved across yield points to the
713 /// equivalent `BitSet<GeneratorSavedLocal>`.
714 fn renumber_bitset(&self, input: &BitSet<Local>) -> BitSet<GeneratorSavedLocal> {
715 assert!(self.superset(&input), "{:?} not a superset of {:?}", self.0, input);
716 let mut out = BitSet::new_empty(self.count());
717 for (saved_local, local) in self.iter_enumerated() {
718 if input.contains(local) {
719 out.insert(saved_local);
725 fn get(&self, local: Local) -> Option<GeneratorSavedLocal> {
726 if !self.contains(local) {
730 let idx = self.iter().take_while(|&l| l < local).count();
731 Some(GeneratorSavedLocal::new(idx))
735 impl ops::Deref for GeneratorSavedLocals {
736 type Target = BitSet<Local>;
738 fn deref(&self) -> &Self::Target {
743 /// For every saved local, looks for which locals are StorageLive at the same
744 /// time. Generates a bitset for every local of all the other locals that may be
745 /// StorageLive simultaneously with that local. This is used in the layout
746 /// computation; see `GeneratorLayout` for more.
747 fn compute_storage_conflicts<'mir, 'tcx>(
748 body: &'mir Body<'tcx>,
749 saved_locals: &GeneratorSavedLocals,
750 always_live_locals: BitSet<Local>,
751 requires_storage: rustc_mir_dataflow::Results<'tcx, MaybeRequiresStorage<'mir, 'tcx>>,
752 ) -> BitMatrix<GeneratorSavedLocal, GeneratorSavedLocal> {
753 assert_eq!(body.local_decls.len(), saved_locals.domain_size());
755 debug!("compute_storage_conflicts({:?})", body.span);
756 debug!("always_live = {:?}", always_live_locals);
758 // Locals that are always live or ones that need to be stored across
759 // suspension points are not eligible for overlap.
760 let mut ineligible_locals = always_live_locals;
761 ineligible_locals.intersect(&**saved_locals);
763 // Compute the storage conflicts for all eligible locals.
764 let mut visitor = StorageConflictVisitor {
766 saved_locals: &saved_locals,
767 local_conflicts: BitMatrix::from_row_n(&ineligible_locals, body.local_decls.len()),
770 requires_storage.visit_reachable_with(body, &mut visitor);
772 let local_conflicts = visitor.local_conflicts;
774 // Compress the matrix using only stored locals (Local -> GeneratorSavedLocal).
776 // NOTE: Today we store a full conflict bitset for every local. Technically
777 // this is twice as many bits as we need, since the relation is symmetric.
778 // However, in practice these bitsets are not usually large. The layout code
779 // also needs to keep track of how many conflicts each local has, so it's
780 // simpler to keep it this way for now.
781 let mut storage_conflicts = BitMatrix::new(saved_locals.count(), saved_locals.count());
782 for (saved_local_a, local_a) in saved_locals.iter_enumerated() {
783 if ineligible_locals.contains(local_a) {
784 // Conflicts with everything.
785 storage_conflicts.insert_all_into_row(saved_local_a);
787 // Keep overlap information only for stored locals.
788 for (saved_local_b, local_b) in saved_locals.iter_enumerated() {
789 if local_conflicts.contains(local_a, local_b) {
790 storage_conflicts.insert(saved_local_a, saved_local_b);
798 struct StorageConflictVisitor<'mir, 'tcx, 's> {
799 body: &'mir Body<'tcx>,
800 saved_locals: &'s GeneratorSavedLocals,
801 // FIXME(tmandry): Consider using sparse bitsets here once we have good
802 // benchmarks for generators.
803 local_conflicts: BitMatrix<Local, Local>,
806 impl<'mir, 'tcx> rustc_mir_dataflow::ResultsVisitor<'mir, 'tcx>
807 for StorageConflictVisitor<'mir, 'tcx, '_>
809 type FlowState = BitSet<Local>;
811 fn visit_statement_before_primary_effect(
813 state: &Self::FlowState,
814 _statement: &'mir Statement<'tcx>,
817 self.apply_state(state, loc);
820 fn visit_terminator_before_primary_effect(
822 state: &Self::FlowState,
823 _terminator: &'mir Terminator<'tcx>,
826 self.apply_state(state, loc);
830 impl StorageConflictVisitor<'_, '_, '_> {
831 fn apply_state(&mut self, flow_state: &BitSet<Local>, loc: Location) {
832 // Ignore unreachable blocks.
833 if self.body.basic_blocks[loc.block].terminator().kind == TerminatorKind::Unreachable {
837 let mut eligible_storage_live = flow_state.clone();
838 eligible_storage_live.intersect(&**self.saved_locals);
840 for local in eligible_storage_live.iter() {
841 self.local_conflicts.union_row_with(&eligible_storage_live, local);
844 if eligible_storage_live.count() > 1 {
845 trace!("at {:?}, eligible_storage_live={:?}", loc, eligible_storage_live);
850 /// Validates the typeck view of the generator against the actual set of types saved between
852 fn sanitize_witness<'tcx>(
856 upvars: Vec<Ty<'tcx>>,
857 saved_locals: &GeneratorSavedLocals,
859 let did = body.source.def_id();
860 let param_env = tcx.param_env(did);
862 let allowed_upvars = tcx.normalize_erasing_regions(param_env, upvars);
863 let allowed = match witness.kind() {
864 &ty::GeneratorWitness(interior_tys) => {
865 tcx.normalize_erasing_late_bound_regions(param_env, interior_tys)
868 tcx.sess.delay_span_bug(
870 &format!("unexpected generator witness type {:?}", witness.kind()),
876 for (local, decl) in body.local_decls.iter_enumerated() {
877 // Ignore locals which are internal or not saved between yields.
878 if !saved_locals.contains(local) || decl.internal {
881 let decl_ty = tcx.normalize_erasing_regions(param_env, decl.ty);
883 // Sanity check that typeck knows about the type of locals which are
884 // live across a suspension point
885 if !allowed.contains(&decl_ty) && !allowed_upvars.contains(&decl_ty) {
888 "Broken MIR: generator contains type {} in MIR, \
889 but typeck only knows about {} and {:?}",
898 fn compute_layout<'tcx>(
899 liveness: LivenessInfo,
900 body: &mut Body<'tcx>,
902 FxHashMap<Local, (Ty<'tcx>, VariantIdx, usize)>,
903 GeneratorLayout<'tcx>,
904 IndexVec<BasicBlock, Option<BitSet<Local>>>,
908 live_locals_at_suspension_points,
909 source_info_at_suspension_points,
914 // Gather live local types and their indices.
915 let mut locals = IndexVec::<GeneratorSavedLocal, _>::new();
916 let mut tys = IndexVec::<GeneratorSavedLocal, _>::new();
917 for (saved_local, local) in saved_locals.iter_enumerated() {
919 tys.push(body.local_decls[local].ty);
920 debug!("generator saved local {:?} => {:?}", saved_local, local);
923 // Leave empty variants for the UNRESUMED, RETURNED, and POISONED states.
924 // In debuginfo, these will correspond to the beginning (UNRESUMED) or end
925 // (RETURNED, POISONED) of the function.
926 let body_span = body.source_scopes[OUTERMOST_SOURCE_SCOPE].span;
927 let mut variant_source_info: IndexVec<VariantIdx, SourceInfo> = [
928 SourceInfo::outermost(body_span.shrink_to_lo()),
929 SourceInfo::outermost(body_span.shrink_to_hi()),
930 SourceInfo::outermost(body_span.shrink_to_hi()),
936 // Build the generator variant field list.
937 // Create a map from local indices to generator struct indices.
938 let mut variant_fields: IndexVec<VariantIdx, IndexVec<Field, GeneratorSavedLocal>> =
939 iter::repeat(IndexVec::new()).take(RESERVED_VARIANTS).collect();
940 let mut remap = FxHashMap::default();
941 for (suspension_point_idx, live_locals) in live_locals_at_suspension_points.iter().enumerate() {
942 let variant_index = VariantIdx::from(RESERVED_VARIANTS + suspension_point_idx);
943 let mut fields = IndexVec::new();
944 for (idx, saved_local) in live_locals.iter().enumerate() {
945 fields.push(saved_local);
946 // Note that if a field is included in multiple variants, we will
947 // just use the first one here. That's fine; fields do not move
948 // around inside generators, so it doesn't matter which variant
949 // index we access them by.
950 remap.entry(locals[saved_local]).or_insert((tys[saved_local], variant_index, idx));
952 variant_fields.push(fields);
953 variant_source_info.push(source_info_at_suspension_points[suspension_point_idx]);
955 debug!("generator variant_fields = {:?}", variant_fields);
956 debug!("generator storage_conflicts = {:#?}", storage_conflicts);
959 GeneratorLayout { field_tys: tys, variant_fields, variant_source_info, storage_conflicts };
961 (remap, layout, storage_liveness)
964 /// Replaces the entry point of `body` with a block that switches on the generator discriminant and
965 /// dispatches to blocks according to `cases`.
967 /// After this function, the former entry point of the function will be bb1.
968 fn insert_switch<'tcx>(
969 body: &mut Body<'tcx>,
970 cases: Vec<(usize, BasicBlock)>,
971 transform: &TransformVisitor<'tcx>,
972 default: TerminatorKind<'tcx>,
974 let default_block = insert_term_block(body, default);
975 let (assign, discr) = transform.get_discr(body);
977 SwitchTargets::new(cases.iter().map(|(i, bb)| ((*i) as u128, *bb)), default_block);
978 let switch = TerminatorKind::SwitchInt { discr: Operand::Move(discr), targets: switch_targets };
980 let source_info = SourceInfo::outermost(body.span);
981 body.basic_blocks_mut().raw.insert(
984 statements: vec![assign],
985 terminator: Some(Terminator { source_info, kind: switch }),
990 let blocks = body.basic_blocks_mut().iter_mut();
992 for target in blocks.flat_map(|b| b.terminator_mut().successors_mut()) {
993 *target = BasicBlock::new(target.index() + 1);
997 fn elaborate_generator_drops<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
998 use crate::shim::DropShimElaborator;
999 use rustc_middle::mir::patch::MirPatch;
1000 use rustc_mir_dataflow::elaborate_drops::{elaborate_drop, Unwind};
1002 // Note that `elaborate_drops` only drops the upvars of a generator, and
1003 // this is ok because `open_drop` can only be reached within that own
1004 // generator's resume function.
1006 let def_id = body.source.def_id();
1007 let param_env = tcx.param_env(def_id);
1009 let mut elaborator = DropShimElaborator { body, patch: MirPatch::new(body), tcx, param_env };
1011 for (block, block_data) in body.basic_blocks.iter_enumerated() {
1012 let (target, unwind, source_info) = match block_data.terminator() {
1013 Terminator { source_info, kind: TerminatorKind::Drop { place, target, unwind } } => {
1014 if let Some(local) = place.as_local() {
1015 if local == SELF_ARG {
1016 (target, unwind, source_info)
1026 let unwind = if block_data.is_cleanup {
1029 Unwind::To(unwind.unwrap_or_else(|| elaborator.patch.resume_block()))
1034 Place::from(SELF_ARG),
1041 elaborator.patch.apply(body);
1044 fn create_generator_drop_shim<'tcx>(
1046 transform: &TransformVisitor<'tcx>,
1048 body: &mut Body<'tcx>,
1049 drop_clean: BasicBlock,
1051 let mut body = body.clone();
1052 body.arg_count = 1; // make sure the resume argument is not included here
1054 let source_info = SourceInfo::outermost(body.span);
1056 let mut cases = create_cases(&mut body, transform, Operation::Drop);
1058 cases.insert(0, (UNRESUMED, drop_clean));
1060 // The returned state and the poisoned state fall through to the default
1061 // case which is just to return
1063 insert_switch(&mut body, cases, &transform, TerminatorKind::Return);
1065 for block in body.basic_blocks_mut() {
1066 let kind = &mut block.terminator_mut().kind;
1067 if let TerminatorKind::GeneratorDrop = *kind {
1068 *kind = TerminatorKind::Return;
1072 // Replace the return variable
1073 body.local_decls[RETURN_PLACE] = LocalDecl::with_source_info(tcx.mk_unit(), source_info);
1075 make_generator_state_argument_indirect(tcx, &mut body);
1077 // Change the generator argument from &mut to *mut
1078 body.local_decls[SELF_ARG] = LocalDecl::with_source_info(
1079 tcx.mk_ptr(ty::TypeAndMut { ty: gen_ty, mutbl: hir::Mutability::Mut }),
1083 // Make sure we remove dead blocks to remove
1084 // unrelated code from the resume part of the function
1085 simplify::remove_dead_blocks(tcx, &mut body);
1087 dump_mir(tcx, false, "generator_drop", &0, &body, |_, _| Ok(()));
1092 fn insert_term_block<'tcx>(body: &mut Body<'tcx>, kind: TerminatorKind<'tcx>) -> BasicBlock {
1093 let source_info = SourceInfo::outermost(body.span);
1094 body.basic_blocks_mut().push(BasicBlockData {
1095 statements: Vec::new(),
1096 terminator: Some(Terminator { source_info, kind }),
1101 fn insert_panic_block<'tcx>(
1103 body: &mut Body<'tcx>,
1104 message: AssertMessage<'tcx>,
1106 let assert_block = BasicBlock::new(body.basic_blocks.len());
1107 let term = TerminatorKind::Assert {
1108 cond: Operand::Constant(Box::new(Constant {
1111 literal: ConstantKind::from_bool(tcx, false),
1115 target: assert_block,
1119 let source_info = SourceInfo::outermost(body.span);
1120 body.basic_blocks_mut().push(BasicBlockData {
1121 statements: Vec::new(),
1122 terminator: Some(Terminator { source_info, kind: term }),
1129 fn can_return<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, param_env: ty::ParamEnv<'tcx>) -> bool {
1130 // Returning from a function with an uninhabited return type is undefined behavior.
1131 if body.return_ty().is_privately_uninhabited(tcx, param_env) {
1135 // If there's a return terminator the function may return.
1136 for block in body.basic_blocks.iter() {
1137 if let TerminatorKind::Return = block.terminator().kind {
1142 // Otherwise the function can't return.
1146 fn can_unwind<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> bool {
1147 // Nothing can unwind when landing pads are off.
1148 if tcx.sess.panic_strategy() == PanicStrategy::Abort {
1152 // Unwinds can only start at certain terminators.
1153 for block in body.basic_blocks.iter() {
1154 match block.terminator().kind {
1155 // These never unwind.
1156 TerminatorKind::Goto { .. }
1157 | TerminatorKind::SwitchInt { .. }
1158 | TerminatorKind::Abort
1159 | TerminatorKind::Return
1160 | TerminatorKind::Unreachable
1161 | TerminatorKind::GeneratorDrop
1162 | TerminatorKind::FalseEdge { .. }
1163 | TerminatorKind::FalseUnwind { .. } => {}
1165 // Resume will *continue* unwinding, but if there's no other unwinding terminator it
1166 // will never be reached.
1167 TerminatorKind::Resume => {}
1169 TerminatorKind::Yield { .. } => {
1170 unreachable!("`can_unwind` called before generator transform")
1173 // These may unwind.
1174 TerminatorKind::Drop { .. }
1175 | TerminatorKind::DropAndReplace { .. }
1176 | TerminatorKind::Call { .. }
1177 | TerminatorKind::InlineAsm { .. }
1178 | TerminatorKind::Assert { .. } => return true,
1182 // If we didn't find an unwinding terminator, the function cannot unwind.
1186 fn create_generator_resume_function<'tcx>(
1188 transform: TransformVisitor<'tcx>,
1189 body: &mut Body<'tcx>,
1192 let can_unwind = can_unwind(tcx, body);
1194 // Poison the generator when it unwinds
1196 let source_info = SourceInfo::outermost(body.span);
1197 let poison_block = body.basic_blocks_mut().push(BasicBlockData {
1198 statements: vec![transform.set_discr(VariantIdx::new(POISONED), source_info)],
1199 terminator: Some(Terminator { source_info, kind: TerminatorKind::Resume }),
1203 for (idx, block) in body.basic_blocks_mut().iter_enumerated_mut() {
1204 let source_info = block.terminator().source_info;
1206 if let TerminatorKind::Resume = block.terminator().kind {
1207 // An existing `Resume` terminator is redirected to jump to our dedicated
1208 // "poisoning block" above.
1209 if idx != poison_block {
1210 *block.terminator_mut() = Terminator {
1212 kind: TerminatorKind::Goto { target: poison_block },
1215 } else if !block.is_cleanup {
1216 // Any terminators that *can* unwind but don't have an unwind target set are also
1217 // pointed at our poisoning block (unless they're part of the cleanup path).
1218 if let Some(unwind @ None) = block.terminator_mut().unwind_mut() {
1219 *unwind = Some(poison_block);
1225 let mut cases = create_cases(body, &transform, Operation::Resume);
1227 use rustc_middle::mir::AssertKind::{ResumedAfterPanic, ResumedAfterReturn};
1229 // Jump to the entry point on the unresumed
1230 cases.insert(0, (UNRESUMED, BasicBlock::new(0)));
1232 // Panic when resumed on the returned or poisoned state
1233 let generator_kind = body.generator_kind().unwrap();
1238 (POISONED, insert_panic_block(tcx, body, ResumedAfterPanic(generator_kind))),
1245 (RETURNED, insert_panic_block(tcx, body, ResumedAfterReturn(generator_kind))),
1249 insert_switch(body, cases, &transform, TerminatorKind::Unreachable);
1251 make_generator_state_argument_indirect(tcx, body);
1252 make_generator_state_argument_pinned(tcx, body);
1254 // Make sure we remove dead blocks to remove
1255 // unrelated code from the drop part of the function
1256 simplify::remove_dead_blocks(tcx, body);
1258 dump_mir(tcx, false, "generator_resume", &0, body, |_, _| Ok(()));
1261 fn insert_clean_drop(body: &mut Body<'_>) -> BasicBlock {
1262 let return_block = insert_term_block(body, TerminatorKind::Return);
1265 TerminatorKind::Drop { place: Place::from(SELF_ARG), target: return_block, unwind: None };
1266 let source_info = SourceInfo::outermost(body.span);
1268 // Create a block to destroy an unresumed generators. This can only destroy upvars.
1269 body.basic_blocks_mut().push(BasicBlockData {
1270 statements: Vec::new(),
1271 terminator: Some(Terminator { source_info, kind: term }),
1276 /// An operation that can be performed on a generator.
1277 #[derive(PartialEq, Copy, Clone)]
1284 fn target_block(self, point: &SuspensionPoint<'_>) -> Option<BasicBlock> {
1286 Operation::Resume => Some(point.resume),
1287 Operation::Drop => point.drop,
1292 fn create_cases<'tcx>(
1293 body: &mut Body<'tcx>,
1294 transform: &TransformVisitor<'tcx>,
1295 operation: Operation,
1296 ) -> Vec<(usize, BasicBlock)> {
1297 let source_info = SourceInfo::outermost(body.span);
1302 .filter_map(|point| {
1303 // Find the target for this suspension point, if applicable
1304 operation.target_block(point).map(|target| {
1305 let mut statements = Vec::new();
1307 // Create StorageLive instructions for locals with live storage
1308 for i in 0..(body.local_decls.len()) {
1310 // The resume argument is live on function entry. Don't insert a
1311 // `StorageLive`, or the following `Assign` will read from uninitialized
1316 let l = Local::new(i);
1317 let needs_storage_live = point.storage_liveness.contains(l)
1318 && !transform.remap.contains_key(&l)
1319 && !transform.always_live_locals.contains(l);
1320 if needs_storage_live {
1322 .push(Statement { source_info, kind: StatementKind::StorageLive(l) });
1326 if operation == Operation::Resume {
1327 // Move the resume argument to the destination place of the `Yield` terminator
1328 let resume_arg = Local::new(2); // 0 = return, 1 = self
1329 statements.push(Statement {
1331 kind: StatementKind::Assign(Box::new((
1333 Rvalue::Use(Operand::Move(resume_arg.into())),
1338 // Then jump to the real target
1339 let block = body.basic_blocks_mut().push(BasicBlockData {
1341 terminator: Some(Terminator {
1343 kind: TerminatorKind::Goto { target },
1348 (point.state, block)
1354 impl<'tcx> MirPass<'tcx> for StateTransform {
1355 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
1356 let Some(yield_ty) = body.yield_ty() else {
1357 // This only applies to generators
1361 assert!(body.generator_drop().is_none());
1363 // The first argument is the generator type passed by value
1364 let gen_ty = body.local_decls.raw[1].ty;
1366 // Get the interior types and substs which typeck computed
1367 let (upvars, interior, discr_ty, movable) = match *gen_ty.kind() {
1368 ty::Generator(_, substs, movability) => {
1369 let substs = substs.as_generator();
1371 substs.upvar_tys().collect(),
1373 substs.discr_ty(tcx),
1374 movability == hir::Movability::Movable,
1379 .delay_span_bug(body.span, &format!("unexpected generator type {}", gen_ty));
1384 let is_async_kind = matches!(body.generator_kind(), Some(GeneratorKind::Async(_)));
1385 let (state_adt_ref, state_substs) = if is_async_kind {
1386 // Compute Poll<return_ty>
1387 let poll_did = tcx.require_lang_item(LangItem::Poll, None);
1388 let poll_adt_ref = tcx.adt_def(poll_did);
1389 let poll_substs = tcx.intern_substs(&[body.return_ty().into()]);
1390 (poll_adt_ref, poll_substs)
1392 // Compute GeneratorState<yield_ty, return_ty>
1393 let state_did = tcx.require_lang_item(LangItem::GeneratorState, None);
1394 let state_adt_ref = tcx.adt_def(state_did);
1395 let state_substs = tcx.intern_substs(&[yield_ty.into(), body.return_ty().into()]);
1396 (state_adt_ref, state_substs)
1398 let ret_ty = tcx.mk_adt(state_adt_ref, state_substs);
1400 // We rename RETURN_PLACE which has type mir.return_ty to new_ret_local
1401 // RETURN_PLACE then is a fresh unused local with type ret_ty.
1402 let new_ret_local = replace_local(RETURN_PLACE, ret_ty, body, tcx);
1404 // Replace all occurrences of `ResumeTy` with `&mut Context<'_>` within async bodies.
1406 transform_async_context(tcx, body);
1409 // We also replace the resume argument and insert an `Assign`.
1410 // This is needed because the resume argument `_2` might be live across a `yield`, in which
1411 // case there is no `Assign` to it that the transform can turn into a store to the generator
1412 // state. After the yield the slot in the generator state would then be uninitialized.
1413 let resume_local = Local::new(2);
1415 if is_async_kind { tcx.mk_task_context() } else { body.local_decls[resume_local].ty };
1416 let new_resume_local = replace_local(resume_local, resume_ty, body, tcx);
1418 // When first entering the generator, move the resume argument into its new local.
1419 let source_info = SourceInfo::outermost(body.span);
1420 let stmts = &mut body.basic_blocks_mut()[BasicBlock::new(0)].statements;
1425 kind: StatementKind::Assign(Box::new((
1426 new_resume_local.into(),
1427 Rvalue::Use(Operand::Move(resume_local.into())),
1432 let always_live_locals = always_storage_live_locals(&body);
1435 locals_live_across_suspend_points(tcx, body, &always_live_locals, movable);
1437 sanitize_witness(tcx, body, interior, upvars, &liveness_info.saved_locals);
1439 if tcx.sess.opts.unstable_opts.validate_mir {
1440 let mut vis = EnsureGeneratorFieldAssignmentsNeverAlias {
1441 assigned_local: None,
1442 saved_locals: &liveness_info.saved_locals,
1443 storage_conflicts: &liveness_info.storage_conflicts,
1446 vis.visit_body(body);
1449 // Extract locals which are live across suspension point into `layout`
1450 // `remap` gives a mapping from local indices onto generator struct indices
1451 // `storage_liveness` tells us which locals have live storage at suspension points
1452 let (remap, layout, storage_liveness) = compute_layout(liveness_info, body);
1454 let can_return = can_return(tcx, body, tcx.param_env(body.source.def_id()));
1456 // Run the transformation which converts Places from Local to generator struct
1457 // accesses for locals in `remap`.
1458 // It also rewrites `return x` and `yield y` as writing a new generator state and returning
1459 // either GeneratorState::Complete(x) and GeneratorState::Yielded(y),
1460 // or Poll::Ready(x) and Poll::Pending respectively depending on `is_async_kind`.
1461 let mut transform = TransformVisitor {
1469 suspension_points: Vec::new(),
1473 transform.visit_body(body);
1475 // Update our MIR struct to reflect the changes we've made
1476 body.arg_count = 2; // self, resume arg
1477 body.spread_arg = None;
1479 body.generator.as_mut().unwrap().yield_ty = None;
1480 body.generator.as_mut().unwrap().generator_layout = Some(layout);
1482 // Insert `drop(generator_struct)` which is used to drop upvars for generators in
1483 // the unresumed state.
1484 // This is expanded to a drop ladder in `elaborate_generator_drops`.
1485 let drop_clean = insert_clean_drop(body);
1487 dump_mir(tcx, false, "generator_pre-elab", &0, body, |_, _| Ok(()));
1489 // Expand `drop(generator_struct)` to a drop ladder which destroys upvars.
1490 // If any upvars are moved out of, drop elaboration will handle upvar destruction.
1491 // However we need to also elaborate the code generated by `insert_clean_drop`.
1492 elaborate_generator_drops(tcx, body);
1494 dump_mir(tcx, false, "generator_post-transform", &0, body, |_, _| Ok(()));
1496 // Create a copy of our MIR and use it to create the drop shim for the generator
1497 let drop_shim = create_generator_drop_shim(tcx, &transform, gen_ty, body, drop_clean);
1499 body.generator.as_mut().unwrap().generator_drop = Some(drop_shim);
1501 // Create the Generator::resume / Future::poll function
1502 create_generator_resume_function(tcx, transform, body, can_return);
1504 // Run derefer to fix Derefs that are not in the first place
1505 deref_finder(tcx, body);
1509 /// Looks for any assignments between locals (e.g., `_4 = _5`) that will both be converted to fields
1510 /// in the generator state machine but whose storage is not marked as conflicting
1512 /// Validation needs to happen immediately *before* `TransformVisitor` is invoked, not after.
1514 /// This condition would arise when the assignment is the last use of `_5` but the initial
1515 /// definition of `_4` if we weren't extra careful to mark all locals used inside a statement as
1516 /// conflicting. Non-conflicting generator saved locals may be stored at the same location within
1517 /// the generator state machine, which would result in ill-formed MIR: the left-hand and right-hand
1518 /// sides of an assignment may not alias. This caused a miscompilation in [#73137].
1520 /// [#73137]: https://github.com/rust-lang/rust/issues/73137
1521 struct EnsureGeneratorFieldAssignmentsNeverAlias<'a> {
1522 saved_locals: &'a GeneratorSavedLocals,
1523 storage_conflicts: &'a BitMatrix<GeneratorSavedLocal, GeneratorSavedLocal>,
1524 assigned_local: Option<GeneratorSavedLocal>,
1527 impl EnsureGeneratorFieldAssignmentsNeverAlias<'_> {
1528 fn saved_local_for_direct_place(&self, place: Place<'_>) -> Option<GeneratorSavedLocal> {
1529 if place.is_indirect() {
1533 self.saved_locals.get(place.local)
1536 fn check_assigned_place(&mut self, place: Place<'_>, f: impl FnOnce(&mut Self)) {
1537 if let Some(assigned_local) = self.saved_local_for_direct_place(place) {
1538 assert!(self.assigned_local.is_none(), "`check_assigned_place` must not recurse");
1540 self.assigned_local = Some(assigned_local);
1542 self.assigned_local = None;
1547 impl<'tcx> Visitor<'tcx> for EnsureGeneratorFieldAssignmentsNeverAlias<'_> {
1548 fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, location: Location) {
1549 let Some(lhs) = self.assigned_local else {
1550 // This visitor only invokes `visit_place` for the right-hand side of an assignment
1551 // and only after setting `self.assigned_local`. However, the default impl of
1552 // `Visitor::super_body` may call `visit_place` with a `NonUseContext` for places
1553 // with debuginfo. Ignore them here.
1554 assert!(!context.is_use());
1558 let Some(rhs) = self.saved_local_for_direct_place(*place) else { return };
1560 if !self.storage_conflicts.contains(lhs, rhs) {
1562 "Assignment between generator saved locals whose storage is not \
1563 marked as conflicting: {:?}: {:?} = {:?}",
1571 fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
1572 match &statement.kind {
1573 StatementKind::Assign(box (lhs, rhs)) => {
1574 self.check_assigned_place(*lhs, |this| this.visit_rvalue(rhs, location));
1577 StatementKind::FakeRead(..)
1578 | StatementKind::SetDiscriminant { .. }
1579 | StatementKind::Deinit(..)
1580 | StatementKind::StorageLive(_)
1581 | StatementKind::StorageDead(_)
1582 | StatementKind::Retag(..)
1583 | StatementKind::AscribeUserType(..)
1584 | StatementKind::Coverage(..)
1585 | StatementKind::Intrinsic(..)
1586 | StatementKind::Nop => {}
1590 fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
1591 // Checking for aliasing in terminators is probably overkill, but until we have actual
1592 // semantics, we should be conservative here.
1593 match &terminator.kind {
1594 TerminatorKind::Call {
1603 self.check_assigned_place(*destination, |this| {
1604 this.visit_operand(func, location);
1606 this.visit_operand(arg, location);
1611 TerminatorKind::Yield { value, resume: _, resume_arg, drop: _ } => {
1612 self.check_assigned_place(*resume_arg, |this| this.visit_operand(value, location));
1615 // FIXME: Does `asm!` have any aliasing requirements?
1616 TerminatorKind::InlineAsm { .. } => {}
1618 TerminatorKind::Call { .. }
1619 | TerminatorKind::Goto { .. }
1620 | TerminatorKind::SwitchInt { .. }
1621 | TerminatorKind::Resume
1622 | TerminatorKind::Abort
1623 | TerminatorKind::Return
1624 | TerminatorKind::Unreachable
1625 | TerminatorKind::Drop { .. }
1626 | TerminatorKind::DropAndReplace { .. }
1627 | TerminatorKind::Assert { .. }
1628 | TerminatorKind::GeneratorDrop
1629 | TerminatorKind::FalseEdge { .. }
1630 | TerminatorKind::FalseUnwind { .. } => {}