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 the Generator::resume function and the drop shim
15 //! for the generator based on the MIR input. It converts the generator argument from Self to
16 //! &mut Self adding derefs in the MIR as needed. It computes the final layout of the generator
17 //! 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.
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) respectively.
36 //! MIR locals which are live across a suspension point are moved to the generator struct
37 //! with references to them being updated with references to the generator struct.
39 //! The pass creates two functions which have a switch on the generator state giving
40 //! the action to take.
42 //! One of them is the implementation of Generator::resume.
43 //! For generators with state 0 (unresumed) it starts the execution of the generator.
44 //! For generators with state 1 (returned) and state 2 (poisoned) it panics.
45 //! Otherwise it continues the execution from the last suspension point.
47 //! The other function is the drop glue for the generator.
48 //! For generators with state 0 (unresumed) it drops the upvars of the generator.
49 //! For generators with state 1 (returned) and state 2 (poisoned) it does nothing.
50 //! Otherwise it drops all the values in scope at the last suspension point.
52 use crate::dataflow::impls::{
53 MaybeBorrowedLocals, MaybeLiveLocals, MaybeRequiresStorage, MaybeStorageLive,
55 use crate::dataflow::{self, Analysis};
56 use crate::transform::no_landing_pads::no_landing_pads;
57 use crate::transform::simplify;
58 use crate::transform::MirPass;
59 use crate::util::dump_mir;
60 use crate::util::expand_aggregate;
61 use crate::util::storage;
62 use rustc_data_structures::fx::FxHashMap;
64 use rustc_hir::def_id::DefId;
65 use rustc_hir::lang_items::LangItem;
66 use rustc_index::bit_set::{BitMatrix, BitSet};
67 use rustc_index::vec::{Idx, IndexVec};
68 use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
69 use rustc_middle::mir::*;
70 use rustc_middle::ty::subst::{Subst, SubstsRef};
71 use rustc_middle::ty::GeneratorSubsts;
72 use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt};
73 use rustc_target::abi::VariantIdx;
74 use rustc_target::spec::PanicStrategy;
78 pub struct StateTransform;
80 struct RenameLocalVisitor<'tcx> {
86 impl<'tcx> MutVisitor<'tcx> for RenameLocalVisitor<'tcx> {
87 fn tcx(&self) -> TyCtxt<'tcx> {
91 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
92 if *local == self.from {
97 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
98 match terminator.kind {
99 TerminatorKind::Return => {
100 // Do not replace the implicit `_0` access here, as that's not possible. The
101 // transform already handles `return` correctly.
103 _ => self.super_terminator(terminator, location),
108 struct DerefArgVisitor<'tcx> {
112 impl<'tcx> MutVisitor<'tcx> for DerefArgVisitor<'tcx> {
113 fn tcx(&self) -> TyCtxt<'tcx> {
117 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
118 assert_ne!(*local, SELF_ARG);
121 fn visit_place(&mut self, place: &mut Place<'tcx>, context: PlaceContext, location: Location) {
122 if place.local == SELF_ARG {
127 projection: self.tcx().intern_place_elems(&[ProjectionElem::Deref]),
132 self.visit_local(&mut place.local, context, location);
134 for elem in place.projection.iter() {
135 if let PlaceElem::Index(local) = elem {
136 assert_ne!(local, SELF_ARG);
143 struct PinArgVisitor<'tcx> {
144 ref_gen_ty: Ty<'tcx>,
148 impl<'tcx> MutVisitor<'tcx> for PinArgVisitor<'tcx> {
149 fn tcx(&self) -> TyCtxt<'tcx> {
153 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
154 assert_ne!(*local, SELF_ARG);
157 fn visit_place(&mut self, place: &mut Place<'tcx>, context: PlaceContext, location: Location) {
158 if place.local == SELF_ARG {
163 projection: self.tcx().intern_place_elems(&[ProjectionElem::Field(
171 self.visit_local(&mut place.local, context, location);
173 for elem in place.projection.iter() {
174 if let PlaceElem::Index(local) = elem {
175 assert_ne!(local, SELF_ARG);
182 fn replace_base<'tcx>(place: &mut Place<'tcx>, new_base: Place<'tcx>, tcx: TyCtxt<'tcx>) {
183 place.local = new_base.local;
185 let mut new_projection = new_base.projection.to_vec();
186 new_projection.append(&mut place.projection.to_vec());
188 place.projection = tcx.intern_place_elems(&new_projection);
191 const SELF_ARG: Local = Local::from_u32(1);
193 /// Generator has not been resumed yet.
194 const UNRESUMED: usize = GeneratorSubsts::UNRESUMED;
195 /// Generator has returned / is completed.
196 const RETURNED: usize = GeneratorSubsts::RETURNED;
197 /// Generator has panicked and is poisoned.
198 const POISONED: usize = GeneratorSubsts::POISONED;
200 /// A `yield` point in the generator.
201 struct SuspensionPoint<'tcx> {
202 /// State discriminant used when suspending or resuming at this point.
204 /// The block to jump to after resumption.
206 /// Where to move the resume argument after resumption.
207 resume_arg: Place<'tcx>,
208 /// Which block to jump to if the generator is dropped in this state.
209 drop: Option<BasicBlock>,
210 /// Set of locals that have live storage while at this suspension point.
211 storage_liveness: BitSet<Local>,
214 struct TransformVisitor<'tcx> {
216 state_adt_ref: &'tcx AdtDef,
217 state_substs: SubstsRef<'tcx>,
219 // The type of the discriminant in the generator struct
222 // Mapping from Local to (type of local, generator struct index)
223 // FIXME(eddyb) This should use `IndexVec<Local, Option<_>>`.
224 remap: FxHashMap<Local, (Ty<'tcx>, VariantIdx, usize)>,
226 // A map from a suspension point in a block to the locals which have live storage at that point
227 storage_liveness: IndexVec<BasicBlock, Option<BitSet<Local>>>,
229 // A list of suspension points, generated during the transform
230 suspension_points: Vec<SuspensionPoint<'tcx>>,
232 // The set of locals that have no `StorageLive`/`StorageDead` annotations.
233 always_live_locals: storage::AlwaysLiveLocals,
235 // The original RETURN_PLACE local
236 new_ret_local: Local,
239 impl TransformVisitor<'tcx> {
240 // Make a GeneratorState variant assignment. `core::ops::GeneratorState` only has single
241 // element tuple variants, so we can just write to the downcasted first field and then set the
242 // discriminant to the appropriate variant.
247 source_info: SourceInfo,
248 ) -> impl Iterator<Item = Statement<'tcx>> {
249 let kind = AggregateKind::Adt(self.state_adt_ref, idx, self.state_substs, None, None);
250 assert_eq!(self.state_adt_ref.variants[idx].fields.len(), 1);
253 .type_of(self.state_adt_ref.variants[idx].fields[0].did)
254 .subst(self.tcx, self.state_substs);
256 Place::return_place(),
257 std::iter::once((val, ty)),
264 // Create a Place referencing a generator struct field
265 fn make_field(&self, variant_index: VariantIdx, idx: usize, ty: Ty<'tcx>) -> Place<'tcx> {
266 let self_place = Place::from(SELF_ARG);
267 let base = self.tcx.mk_place_downcast_unnamed(self_place, variant_index);
268 let mut projection = base.projection.to_vec();
269 projection.push(ProjectionElem::Field(Field::new(idx), ty));
271 Place { local: base.local, projection: self.tcx.intern_place_elems(&projection) }
274 // Create a statement which changes the discriminant
275 fn set_discr(&self, state_disc: VariantIdx, source_info: SourceInfo) -> Statement<'tcx> {
276 let self_place = Place::from(SELF_ARG);
279 kind: StatementKind::SetDiscriminant {
280 place: box self_place,
281 variant_index: state_disc,
286 // Create a statement which reads the discriminant into a temporary
287 fn get_discr(&self, body: &mut Body<'tcx>) -> (Statement<'tcx>, Place<'tcx>) {
288 let temp_decl = LocalDecl::new(self.discr_ty, body.span).internal();
289 let local_decls_len = body.local_decls.push(temp_decl);
290 let temp = Place::from(local_decls_len);
292 let self_place = Place::from(SELF_ARG);
293 let assign = Statement {
294 source_info: SourceInfo::outermost(body.span),
295 kind: StatementKind::Assign(box (temp, Rvalue::Discriminant(self_place))),
301 impl MutVisitor<'tcx> for TransformVisitor<'tcx> {
302 fn tcx(&self) -> TyCtxt<'tcx> {
306 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
307 assert_eq!(self.remap.get(local), None);
312 place: &mut Place<'tcx>,
313 _context: PlaceContext,
316 // Replace an Local in the remap with a generator struct access
317 if let Some(&(ty, variant_index, idx)) = self.remap.get(&place.local) {
318 replace_base(place, self.make_field(variant_index, idx, ty), self.tcx);
322 fn visit_basic_block_data(&mut self, block: BasicBlock, data: &mut BasicBlockData<'tcx>) {
323 // Remove StorageLive and StorageDead statements for remapped locals
324 data.retain_statements(|s| match s.kind {
325 StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => {
326 !self.remap.contains_key(&l)
331 let ret_val = match data.terminator().kind {
332 TerminatorKind::Return => Some((
335 Operand::Move(Place::from(self.new_ret_local)),
338 TerminatorKind::Yield { ref value, resume, resume_arg, drop } => {
339 Some((VariantIdx::new(0), Some((resume, resume_arg)), value.clone(), drop))
344 if let Some((state_idx, resume, v, drop)) = ret_val {
345 let source_info = data.terminator().source_info;
346 // We must assign the value first in case it gets declared dead below
347 data.statements.extend(self.make_state(state_idx, v, source_info));
348 let state = if let Some((resume, resume_arg)) = resume {
350 let state = 3 + self.suspension_points.len();
352 // The resume arg target location might itself be remapped if its base local is
353 // live across a yield.
355 if let Some(&(ty, variant, idx)) = self.remap.get(&resume_arg.local) {
356 self.make_field(variant, idx, ty)
361 self.suspension_points.push(SuspensionPoint {
366 storage_liveness: self.storage_liveness[block].clone().unwrap(),
369 VariantIdx::new(state)
372 VariantIdx::new(RETURNED) // state for returned
374 data.statements.push(self.set_discr(state, source_info));
375 data.terminator_mut().kind = TerminatorKind::Return;
378 self.super_basic_block_data(block, data);
382 fn make_generator_state_argument_indirect<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
383 let gen_ty = body.local_decls.raw[1].ty;
386 tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut { ty: gen_ty, mutbl: Mutability::Mut });
388 // Replace the by value generator argument
389 body.local_decls.raw[1].ty = ref_gen_ty;
391 // Add a deref to accesses of the generator state
392 DerefArgVisitor { tcx }.visit_body(body);
395 fn make_generator_state_argument_pinned<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
396 let ref_gen_ty = body.local_decls.raw[1].ty;
398 let pin_did = tcx.require_lang_item(LangItem::Pin, Some(body.span));
399 let pin_adt_ref = tcx.adt_def(pin_did);
400 let substs = tcx.intern_substs(&[ref_gen_ty.into()]);
401 let pin_ref_gen_ty = tcx.mk_adt(pin_adt_ref, substs);
403 // Replace the by ref generator argument
404 body.local_decls.raw[1].ty = pin_ref_gen_ty;
406 // Add the Pin field access to accesses of the generator state
407 PinArgVisitor { ref_gen_ty, tcx }.visit_body(body);
410 /// Allocates a new local and replaces all references of `local` with it. Returns the new local.
412 /// `local` will be changed to a new local decl with type `ty`.
414 /// Note that the new local will be uninitialized. It is the caller's responsibility to assign some
415 /// valid value to it before its first use.
416 fn replace_local<'tcx>(
419 body: &mut Body<'tcx>,
422 let new_decl = LocalDecl::new(ty, body.span);
423 let new_local = body.local_decls.push(new_decl);
424 body.local_decls.swap(local, new_local);
426 RenameLocalVisitor { from: local, to: new_local, tcx }.visit_body(body);
431 struct LivenessInfo {
432 /// Which locals are live across any suspension point.
433 saved_locals: GeneratorSavedLocals,
435 /// The set of saved locals live at each suspension point.
436 live_locals_at_suspension_points: Vec<BitSet<GeneratorSavedLocal>>,
438 /// Parallel vec to the above with SourceInfo for each yield terminator.
439 source_info_at_suspension_points: Vec<SourceInfo>,
441 /// For every saved local, the set of other saved locals that are
442 /// storage-live at the same time as this local. We cannot overlap locals in
443 /// the layout which have conflicting storage.
444 storage_conflicts: BitMatrix<GeneratorSavedLocal, GeneratorSavedLocal>,
446 /// For every suspending block, the locals which are storage-live across
447 /// that suspension point.
448 storage_liveness: IndexVec<BasicBlock, Option<BitSet<Local>>>,
451 fn locals_live_across_suspend_points(
454 always_live_locals: &storage::AlwaysLiveLocals,
457 let def_id = body.source.def_id();
458 let body_ref: &Body<'_> = &body;
460 // Calculate when MIR locals have live storage. This gives us an upper bound of their
462 let mut storage_live = MaybeStorageLive::new(always_live_locals.clone())
463 .into_engine(tcx, body_ref, def_id)
464 .iterate_to_fixpoint()
465 .into_results_cursor(body_ref);
467 // Calculate the MIR locals which have been previously
468 // borrowed (even if they are still active).
469 let borrowed_locals_results = MaybeBorrowedLocals::all_borrows()
470 .into_engine(tcx, body_ref, def_id)
471 .pass_name("generator")
472 .iterate_to_fixpoint();
474 let mut borrowed_locals_cursor =
475 dataflow::ResultsCursor::new(body_ref, &borrowed_locals_results);
477 // Calculate the MIR locals that we actually need to keep storage around
479 let requires_storage_results = MaybeRequiresStorage::new(body, &borrowed_locals_results)
480 .into_engine(tcx, body_ref, def_id)
481 .iterate_to_fixpoint();
482 let mut requires_storage_cursor =
483 dataflow::ResultsCursor::new(body_ref, &requires_storage_results);
485 // Calculate the liveness of MIR locals ignoring borrows.
486 let mut liveness = MaybeLiveLocals
487 .into_engine(tcx, body_ref, def_id)
488 .pass_name("generator")
489 .iterate_to_fixpoint()
490 .into_results_cursor(body_ref);
492 let mut storage_liveness_map = IndexVec::from_elem(None, body.basic_blocks());
493 let mut live_locals_at_suspension_points = Vec::new();
494 let mut source_info_at_suspension_points = Vec::new();
495 let mut live_locals_at_any_suspension_point = BitSet::new_empty(body.local_decls.len());
497 for (block, data) in body.basic_blocks().iter_enumerated() {
498 if let TerminatorKind::Yield { .. } = data.terminator().kind {
499 let loc = Location { block, statement_index: data.statements.len() };
501 liveness.seek_to_block_end(block);
502 let mut live_locals = liveness.get().clone();
505 // The `liveness` variable contains the liveness of MIR locals ignoring borrows.
506 // This is correct for movable generators since borrows cannot live across
507 // suspension points. However for immovable generators we need to account for
508 // borrows, so we conseratively assume that all borrowed locals are live until
509 // we find a StorageDead statement referencing the locals.
510 // To do this we just union our `liveness` result with `borrowed_locals`, which
511 // contains all the locals which has been borrowed before this suspension point.
512 // If a borrow is converted to a raw reference, we must also assume that it lives
513 // forever. Note that the final liveness is still bounded by the storage liveness
514 // of the local, which happens using the `intersect` operation below.
515 borrowed_locals_cursor.seek_before_primary_effect(loc);
516 live_locals.union(borrowed_locals_cursor.get());
519 // Store the storage liveness for later use so we can restore the state
520 // after a suspension point
521 storage_live.seek_before_primary_effect(loc);
522 storage_liveness_map[block] = Some(storage_live.get().clone());
524 // Locals live are live at this point only if they are used across
525 // suspension points (the `liveness` variable)
526 // and their storage is required (the `storage_required` variable)
527 requires_storage_cursor.seek_before_primary_effect(loc);
528 live_locals.intersect(requires_storage_cursor.get());
530 // The generator argument is ignored.
531 live_locals.remove(SELF_ARG);
533 debug!("loc = {:?}, live_locals = {:?}", loc, live_locals);
535 // Add the locals live at this suspension point to the set of locals which live across
536 // any suspension points
537 live_locals_at_any_suspension_point.union(&live_locals);
539 live_locals_at_suspension_points.push(live_locals);
540 source_info_at_suspension_points.push(data.terminator().source_info);
544 debug!("live_locals_anywhere = {:?}", live_locals_at_any_suspension_point);
545 let saved_locals = GeneratorSavedLocals(live_locals_at_any_suspension_point);
547 // Renumber our liveness_map bitsets to include only the locals we are
549 let live_locals_at_suspension_points = live_locals_at_suspension_points
551 .map(|live_here| saved_locals.renumber_bitset(&live_here))
554 let storage_conflicts = compute_storage_conflicts(
557 always_live_locals.clone(),
558 requires_storage_results,
563 live_locals_at_suspension_points,
564 source_info_at_suspension_points,
566 storage_liveness: storage_liveness_map,
570 /// The set of `Local`s that must be saved across yield points.
572 /// `GeneratorSavedLocal` is indexed in terms of the elements in this set;
573 /// i.e. `GeneratorSavedLocal::new(1)` corresponds to the second local
574 /// included in this set.
575 struct GeneratorSavedLocals(BitSet<Local>);
577 impl GeneratorSavedLocals {
578 /// Returns an iterator over each `GeneratorSavedLocal` along with the `Local` it corresponds
580 fn iter_enumerated(&self) -> impl '_ + Iterator<Item = (GeneratorSavedLocal, Local)> {
581 self.iter().enumerate().map(|(i, l)| (GeneratorSavedLocal::from(i), l))
584 /// Transforms a `BitSet<Local>` that contains only locals saved across yield points to the
585 /// equivalent `BitSet<GeneratorSavedLocal>`.
586 fn renumber_bitset(&self, input: &BitSet<Local>) -> BitSet<GeneratorSavedLocal> {
587 assert!(self.superset(&input), "{:?} not a superset of {:?}", self.0, input);
588 let mut out = BitSet::new_empty(self.count());
589 for (saved_local, local) in self.iter_enumerated() {
590 if input.contains(local) {
591 out.insert(saved_local);
597 fn get(&self, local: Local) -> Option<GeneratorSavedLocal> {
598 if !self.contains(local) {
602 let idx = self.iter().take_while(|&l| l < local).count();
603 Some(GeneratorSavedLocal::new(idx))
607 impl ops::Deref for GeneratorSavedLocals {
608 type Target = BitSet<Local>;
610 fn deref(&self) -> &Self::Target {
615 /// For every saved local, looks for which locals are StorageLive at the same
616 /// time. Generates a bitset for every local of all the other locals that may be
617 /// StorageLive simultaneously with that local. This is used in the layout
618 /// computation; see `GeneratorLayout` for more.
619 fn compute_storage_conflicts(
620 body: &'mir Body<'tcx>,
621 saved_locals: &GeneratorSavedLocals,
622 always_live_locals: storage::AlwaysLiveLocals,
623 requires_storage: dataflow::Results<'tcx, MaybeRequiresStorage<'mir, 'tcx>>,
624 ) -> BitMatrix<GeneratorSavedLocal, GeneratorSavedLocal> {
625 assert_eq!(body.local_decls.len(), saved_locals.domain_size());
627 debug!("compute_storage_conflicts({:?})", body.span);
628 debug!("always_live = {:?}", always_live_locals);
630 // Locals that are always live or ones that need to be stored across
631 // suspension points are not eligible for overlap.
632 let mut ineligible_locals = always_live_locals.into_inner();
633 ineligible_locals.intersect(saved_locals);
635 // Compute the storage conflicts for all eligible locals.
636 let mut visitor = StorageConflictVisitor {
638 saved_locals: &saved_locals,
639 local_conflicts: BitMatrix::from_row_n(&ineligible_locals, body.local_decls.len()),
642 requires_storage.visit_reachable_with(body, &mut visitor);
644 let local_conflicts = visitor.local_conflicts;
646 // Compress the matrix using only stored locals (Local -> GeneratorSavedLocal).
648 // NOTE: Today we store a full conflict bitset for every local. Technically
649 // this is twice as many bits as we need, since the relation is symmetric.
650 // However, in practice these bitsets are not usually large. The layout code
651 // also needs to keep track of how many conflicts each local has, so it's
652 // simpler to keep it this way for now.
653 let mut storage_conflicts = BitMatrix::new(saved_locals.count(), saved_locals.count());
654 for (saved_local_a, local_a) in saved_locals.iter_enumerated() {
655 if ineligible_locals.contains(local_a) {
656 // Conflicts with everything.
657 storage_conflicts.insert_all_into_row(saved_local_a);
659 // Keep overlap information only for stored locals.
660 for (saved_local_b, local_b) in saved_locals.iter_enumerated() {
661 if local_conflicts.contains(local_a, local_b) {
662 storage_conflicts.insert(saved_local_a, saved_local_b);
670 struct StorageConflictVisitor<'mir, 'tcx, 's> {
671 body: &'mir Body<'tcx>,
672 saved_locals: &'s GeneratorSavedLocals,
673 // FIXME(tmandry): Consider using sparse bitsets here once we have good
674 // benchmarks for generators.
675 local_conflicts: BitMatrix<Local, Local>,
678 impl dataflow::ResultsVisitor<'mir, 'tcx> for StorageConflictVisitor<'mir, 'tcx, '_> {
679 type FlowState = BitSet<Local>;
681 fn visit_statement_before_primary_effect(
683 state: &Self::FlowState,
684 _statement: &'mir Statement<'tcx>,
687 self.apply_state(state, loc);
690 fn visit_terminator_before_primary_effect(
692 state: &Self::FlowState,
693 _terminator: &'mir Terminator<'tcx>,
696 self.apply_state(state, loc);
700 impl<'body, 'tcx, 's> StorageConflictVisitor<'body, 'tcx, 's> {
701 fn apply_state(&mut self, flow_state: &BitSet<Local>, loc: Location) {
702 // Ignore unreachable blocks.
703 if self.body.basic_blocks()[loc.block].terminator().kind == TerminatorKind::Unreachable {
707 let mut eligible_storage_live = flow_state.clone();
708 eligible_storage_live.intersect(&self.saved_locals);
710 for local in eligible_storage_live.iter() {
711 self.local_conflicts.union_row_with(&eligible_storage_live, local);
714 if eligible_storage_live.count() > 1 {
715 trace!("at {:?}, eligible_storage_live={:?}", loc, eligible_storage_live);
720 /// Validates the typeck view of the generator against the actual set of types saved between
722 fn sanitize_witness<'tcx>(
727 upvars: &Vec<Ty<'tcx>>,
728 saved_locals: &GeneratorSavedLocals,
730 let allowed_upvars = tcx.erase_regions(upvars);
731 let allowed = match witness.kind() {
732 ty::GeneratorWitness(s) => tcx.erase_late_bound_regions(&s),
734 tcx.sess.delay_span_bug(
736 &format!("unexpected generator witness type {:?}", witness.kind()),
742 let param_env = tcx.param_env(did);
744 for (local, decl) in body.local_decls.iter_enumerated() {
745 // Ignore locals which are internal or not saved between yields.
746 if !saved_locals.contains(local) || decl.internal {
749 let decl_ty = tcx.normalize_erasing_regions(param_env, decl.ty);
751 // Sanity check that typeck knows about the type of locals which are
752 // live across a suspension point
753 if !allowed.contains(&decl_ty) && !allowed_upvars.contains(&decl_ty) {
756 "Broken MIR: generator contains type {} in MIR, \
757 but typeck only knows about {}",
765 fn compute_layout<'tcx>(
766 liveness: LivenessInfo,
767 body: &mut Body<'tcx>,
769 FxHashMap<Local, (Ty<'tcx>, VariantIdx, usize)>,
770 GeneratorLayout<'tcx>,
771 IndexVec<BasicBlock, Option<BitSet<Local>>>,
775 live_locals_at_suspension_points,
776 source_info_at_suspension_points,
781 // Gather live local types and their indices.
782 let mut locals = IndexVec::<GeneratorSavedLocal, _>::new();
783 let mut tys = IndexVec::<GeneratorSavedLocal, _>::new();
784 for (saved_local, local) in saved_locals.iter_enumerated() {
786 tys.push(body.local_decls[local].ty);
787 debug!("generator saved local {:?} => {:?}", saved_local, local);
790 // Leave empty variants for the UNRESUMED, RETURNED, and POISONED states.
791 // In debuginfo, these will correspond to the beginning (UNRESUMED) or end
792 // (RETURNED, POISONED) of the function.
793 const RESERVED_VARIANTS: usize = 3;
794 let body_span = body.source_scopes[OUTERMOST_SOURCE_SCOPE].span;
795 let mut variant_source_info: IndexVec<VariantIdx, SourceInfo> = [
796 SourceInfo::outermost(body_span.shrink_to_lo()),
797 SourceInfo::outermost(body_span.shrink_to_hi()),
798 SourceInfo::outermost(body_span.shrink_to_hi()),
804 // Build the generator variant field list.
805 // Create a map from local indices to generator struct indices.
806 let mut variant_fields: IndexVec<VariantIdx, IndexVec<Field, GeneratorSavedLocal>> =
807 iter::repeat(IndexVec::new()).take(RESERVED_VARIANTS).collect();
808 let mut remap = FxHashMap::default();
809 for (suspension_point_idx, live_locals) in live_locals_at_suspension_points.iter().enumerate() {
810 let variant_index = VariantIdx::from(RESERVED_VARIANTS + suspension_point_idx);
811 let mut fields = IndexVec::new();
812 for (idx, saved_local) in live_locals.iter().enumerate() {
813 fields.push(saved_local);
814 // Note that if a field is included in multiple variants, we will
815 // just use the first one here. That's fine; fields do not move
816 // around inside generators, so it doesn't matter which variant
817 // index we access them by.
818 remap.entry(locals[saved_local]).or_insert((tys[saved_local], variant_index, idx));
820 variant_fields.push(fields);
821 variant_source_info.push(source_info_at_suspension_points[suspension_point_idx]);
823 debug!("generator variant_fields = {:?}", variant_fields);
824 debug!("generator storage_conflicts = {:#?}", storage_conflicts);
827 GeneratorLayout { field_tys: tys, variant_fields, variant_source_info, storage_conflicts };
829 (remap, layout, storage_liveness)
832 /// Replaces the entry point of `body` with a block that switches on the generator discriminant and
833 /// dispatches to blocks according to `cases`.
835 /// After this function, the former entry point of the function will be bb1.
836 fn insert_switch<'tcx>(
837 body: &mut Body<'tcx>,
838 cases: Vec<(usize, BasicBlock)>,
839 transform: &TransformVisitor<'tcx>,
840 default: TerminatorKind<'tcx>,
842 let default_block = insert_term_block(body, default);
843 let (assign, discr) = transform.get_discr(body);
844 let switch = TerminatorKind::SwitchInt {
845 discr: Operand::Move(discr),
846 switch_ty: transform.discr_ty,
847 values: Cow::from(cases.iter().map(|&(i, _)| i as u128).collect::<Vec<_>>()),
848 targets: cases.iter().map(|&(_, d)| d).chain(iter::once(default_block)).collect(),
851 let source_info = SourceInfo::outermost(body.span);
852 body.basic_blocks_mut().raw.insert(
855 statements: vec![assign],
856 terminator: Some(Terminator { source_info, kind: switch }),
861 let blocks = body.basic_blocks_mut().iter_mut();
863 for target in blocks.flat_map(|b| b.terminator_mut().successors_mut()) {
864 *target = BasicBlock::new(target.index() + 1);
868 fn elaborate_generator_drops<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, body: &mut Body<'tcx>) {
869 use crate::shim::DropShimElaborator;
870 use crate::util::elaborate_drops::{elaborate_drop, Unwind};
871 use crate::util::patch::MirPatch;
873 // Note that `elaborate_drops` only drops the upvars of a generator, and
874 // this is ok because `open_drop` can only be reached within that own
875 // generator's resume function.
877 let param_env = tcx.param_env(def_id);
879 let mut elaborator = DropShimElaborator { body, patch: MirPatch::new(body), tcx, param_env };
881 for (block, block_data) in body.basic_blocks().iter_enumerated() {
882 let (target, unwind, source_info) = match block_data.terminator() {
883 Terminator { source_info, kind: TerminatorKind::Drop { place, target, unwind } } => {
884 if let Some(local) = place.as_local() {
885 if local == SELF_ARG {
886 (target, unwind, source_info)
896 let unwind = if block_data.is_cleanup {
899 Unwind::To(unwind.unwrap_or_else(|| elaborator.patch.resume_block()))
904 Place::from(SELF_ARG),
911 elaborator.patch.apply(body);
914 fn create_generator_drop_shim<'tcx>(
916 transform: &TransformVisitor<'tcx>,
918 body: &mut Body<'tcx>,
919 drop_clean: BasicBlock,
921 let mut body = body.clone();
922 body.arg_count = 1; // make sure the resume argument is not included here
924 let source_info = SourceInfo::outermost(body.span);
926 let mut cases = create_cases(&mut body, transform, Operation::Drop);
928 cases.insert(0, (UNRESUMED, drop_clean));
930 // The returned state and the poisoned state fall through to the default
931 // case which is just to return
933 insert_switch(&mut body, cases, &transform, TerminatorKind::Return);
935 for block in body.basic_blocks_mut() {
936 let kind = &mut block.terminator_mut().kind;
937 if let TerminatorKind::GeneratorDrop = *kind {
938 *kind = TerminatorKind::Return;
942 // Replace the return variable
943 body.local_decls[RETURN_PLACE] = LocalDecl::with_source_info(tcx.mk_unit(), source_info);
945 make_generator_state_argument_indirect(tcx, &mut body);
947 // Change the generator argument from &mut to *mut
948 body.local_decls[SELF_ARG] = LocalDecl::with_source_info(
949 tcx.mk_ptr(ty::TypeAndMut { ty: gen_ty, mutbl: hir::Mutability::Mut }),
952 if tcx.sess.opts.debugging_opts.mir_emit_retag {
953 // Alias tracking must know we changed the type
954 body.basic_blocks_mut()[START_BLOCK].statements.insert(
958 kind: StatementKind::Retag(RetagKind::Raw, box Place::from(SELF_ARG)),
963 no_landing_pads(tcx, &mut body);
965 // Make sure we remove dead blocks to remove
966 // unrelated code from the resume part of the function
967 simplify::remove_dead_blocks(&mut body);
969 dump_mir(tcx, None, "generator_drop", &0, &body, |_, _| Ok(()));
974 fn insert_term_block<'tcx>(body: &mut Body<'tcx>, kind: TerminatorKind<'tcx>) -> BasicBlock {
975 let source_info = SourceInfo::outermost(body.span);
976 body.basic_blocks_mut().push(BasicBlockData {
977 statements: Vec::new(),
978 terminator: Some(Terminator { source_info, kind }),
983 fn insert_panic_block<'tcx>(
985 body: &mut Body<'tcx>,
986 message: AssertMessage<'tcx>,
988 let assert_block = BasicBlock::new(body.basic_blocks().len());
989 let term = TerminatorKind::Assert {
990 cond: Operand::Constant(box Constant {
993 literal: ty::Const::from_bool(tcx, false),
997 target: assert_block,
1001 let source_info = SourceInfo::outermost(body.span);
1002 body.basic_blocks_mut().push(BasicBlockData {
1003 statements: Vec::new(),
1004 terminator: Some(Terminator { source_info, kind: term }),
1011 fn can_return<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> bool {
1012 // Returning from a function with an uninhabited return type is undefined behavior.
1013 if body.return_ty().conservative_is_privately_uninhabited(tcx) {
1017 // If there's a return terminator the function may return.
1018 for block in body.basic_blocks() {
1019 if let TerminatorKind::Return = block.terminator().kind {
1024 // Otherwise the function can't return.
1028 fn can_unwind<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> bool {
1029 // Nothing can unwind when landing pads are off.
1030 if tcx.sess.panic_strategy() == PanicStrategy::Abort {
1034 // Unwinds can only start at certain terminators.
1035 for block in body.basic_blocks() {
1036 match block.terminator().kind {
1037 // These never unwind.
1038 TerminatorKind::Goto { .. }
1039 | TerminatorKind::SwitchInt { .. }
1040 | TerminatorKind::Abort
1041 | TerminatorKind::Return
1042 | TerminatorKind::Unreachable
1043 | TerminatorKind::GeneratorDrop
1044 | TerminatorKind::FalseEdge { .. }
1045 | TerminatorKind::FalseUnwind { .. }
1046 | TerminatorKind::InlineAsm { .. } => {}
1048 // Resume will *continue* unwinding, but if there's no other unwinding terminator it
1049 // will never be reached.
1050 TerminatorKind::Resume => {}
1052 TerminatorKind::Yield { .. } => {
1053 unreachable!("`can_unwind` called before generator transform")
1056 // These may unwind.
1057 TerminatorKind::Drop { .. }
1058 | TerminatorKind::DropAndReplace { .. }
1059 | TerminatorKind::Call { .. }
1060 | TerminatorKind::Assert { .. } => return true,
1064 // If we didn't find an unwinding terminator, the function cannot unwind.
1068 fn create_generator_resume_function<'tcx>(
1070 transform: TransformVisitor<'tcx>,
1071 body: &mut Body<'tcx>,
1074 let can_unwind = can_unwind(tcx, body);
1076 // Poison the generator when it unwinds
1078 let source_info = SourceInfo::outermost(body.span);
1079 let poison_block = body.basic_blocks_mut().push(BasicBlockData {
1080 statements: vec![transform.set_discr(VariantIdx::new(POISONED), source_info)],
1081 terminator: Some(Terminator { source_info, kind: TerminatorKind::Resume }),
1085 for (idx, block) in body.basic_blocks_mut().iter_enumerated_mut() {
1086 let source_info = block.terminator().source_info;
1088 if let TerminatorKind::Resume = block.terminator().kind {
1089 // An existing `Resume` terminator is redirected to jump to our dedicated
1090 // "poisoning block" above.
1091 if idx != poison_block {
1092 *block.terminator_mut() = Terminator {
1094 kind: TerminatorKind::Goto { target: poison_block },
1097 } else if !block.is_cleanup {
1098 // Any terminators that *can* unwind but don't have an unwind target set are also
1099 // pointed at our poisoning block (unless they're part of the cleanup path).
1100 if let Some(unwind @ None) = block.terminator_mut().unwind_mut() {
1101 *unwind = Some(poison_block);
1107 let mut cases = create_cases(body, &transform, Operation::Resume);
1109 use rustc_middle::mir::AssertKind::{ResumedAfterPanic, ResumedAfterReturn};
1111 // Jump to the entry point on the unresumed
1112 cases.insert(0, (UNRESUMED, BasicBlock::new(0)));
1114 // Panic when resumed on the returned or poisoned state
1115 let generator_kind = body.generator_kind.unwrap();
1120 (POISONED, insert_panic_block(tcx, body, ResumedAfterPanic(generator_kind))),
1127 (RETURNED, insert_panic_block(tcx, body, ResumedAfterReturn(generator_kind))),
1131 insert_switch(body, cases, &transform, TerminatorKind::Unreachable);
1133 make_generator_state_argument_indirect(tcx, body);
1134 make_generator_state_argument_pinned(tcx, body);
1136 no_landing_pads(tcx, body);
1138 // Make sure we remove dead blocks to remove
1139 // unrelated code from the drop part of the function
1140 simplify::remove_dead_blocks(body);
1142 dump_mir(tcx, None, "generator_resume", &0, body, |_, _| Ok(()));
1145 fn insert_clean_drop(body: &mut Body<'_>) -> BasicBlock {
1146 let return_block = insert_term_block(body, TerminatorKind::Return);
1149 TerminatorKind::Drop { place: Place::from(SELF_ARG), target: return_block, unwind: None };
1150 let source_info = SourceInfo::outermost(body.span);
1152 // Create a block to destroy an unresumed generators. This can only destroy upvars.
1153 body.basic_blocks_mut().push(BasicBlockData {
1154 statements: Vec::new(),
1155 terminator: Some(Terminator { source_info, kind: term }),
1160 /// An operation that can be performed on a generator.
1161 #[derive(PartialEq, Copy, Clone)]
1168 fn target_block(self, point: &SuspensionPoint<'_>) -> Option<BasicBlock> {
1170 Operation::Resume => Some(point.resume),
1171 Operation::Drop => point.drop,
1176 fn create_cases<'tcx>(
1177 body: &mut Body<'tcx>,
1178 transform: &TransformVisitor<'tcx>,
1179 operation: Operation,
1180 ) -> Vec<(usize, BasicBlock)> {
1181 let source_info = SourceInfo::outermost(body.span);
1186 .filter_map(|point| {
1187 // Find the target for this suspension point, if applicable
1188 operation.target_block(point).map(|target| {
1189 let mut statements = Vec::new();
1191 // Create StorageLive instructions for locals with live storage
1192 for i in 0..(body.local_decls.len()) {
1194 // The resume argument is live on function entry. Don't insert a
1195 // `StorageLive`, or the following `Assign` will read from uninitialized
1200 let l = Local::new(i);
1201 let needs_storage_live = point.storage_liveness.contains(l)
1202 && !transform.remap.contains_key(&l)
1203 && !transform.always_live_locals.contains(l);
1204 if needs_storage_live {
1206 .push(Statement { source_info, kind: StatementKind::StorageLive(l) });
1210 if operation == Operation::Resume {
1211 // Move the resume argument to the destination place of the `Yield` terminator
1212 let resume_arg = Local::new(2); // 0 = return, 1 = self
1213 statements.push(Statement {
1215 kind: StatementKind::Assign(box (
1217 Rvalue::Use(Operand::Move(resume_arg.into())),
1222 // Then jump to the real target
1223 let block = body.basic_blocks_mut().push(BasicBlockData {
1225 terminator: Some(Terminator {
1227 kind: TerminatorKind::Goto { target },
1232 (point.state, block)
1238 impl<'tcx> MirPass<'tcx> for StateTransform {
1239 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
1240 let yield_ty = if let Some(yield_ty) = body.yield_ty {
1243 // This only applies to generators
1247 assert!(body.generator_drop.is_none());
1249 let def_id = body.source.def_id();
1251 // The first argument is the generator type passed by value
1252 let gen_ty = body.local_decls.raw[1].ty;
1254 // Get the interior types and substs which typeck computed
1255 let (upvars, interior, discr_ty, movable) = match *gen_ty.kind() {
1256 ty::Generator(_, substs, movability) => {
1257 let substs = substs.as_generator();
1259 substs.upvar_tys().collect(),
1261 substs.discr_ty(tcx),
1262 movability == hir::Movability::Movable,
1267 .delay_span_bug(body.span, &format!("unexpected generator type {}", gen_ty));
1272 // Compute GeneratorState<yield_ty, return_ty>
1273 let state_did = tcx.require_lang_item(LangItem::GeneratorState, None);
1274 let state_adt_ref = tcx.adt_def(state_did);
1275 let state_substs = tcx.intern_substs(&[yield_ty.into(), body.return_ty().into()]);
1276 let ret_ty = tcx.mk_adt(state_adt_ref, state_substs);
1278 // We rename RETURN_PLACE which has type mir.return_ty to new_ret_local
1279 // RETURN_PLACE then is a fresh unused local with type ret_ty.
1280 let new_ret_local = replace_local(RETURN_PLACE, ret_ty, body, tcx);
1282 // We also replace the resume argument and insert an `Assign`.
1283 // This is needed because the resume argument `_2` might be live across a `yield`, in which
1284 // case there is no `Assign` to it that the transform can turn into a store to the generator
1285 // state. After the yield the slot in the generator state would then be uninitialized.
1286 let resume_local = Local::new(2);
1287 let new_resume_local =
1288 replace_local(resume_local, body.local_decls[resume_local].ty, body, tcx);
1290 // When first entering the generator, move the resume argument into its new local.
1291 let source_info = SourceInfo::outermost(body.span);
1292 let stmts = &mut body.basic_blocks_mut()[BasicBlock::new(0)].statements;
1297 kind: StatementKind::Assign(box (
1298 new_resume_local.into(),
1299 Rvalue::Use(Operand::Move(resume_local.into())),
1304 let always_live_locals = storage::AlwaysLiveLocals::new(&body);
1307 locals_live_across_suspend_points(tcx, body, &always_live_locals, movable);
1309 sanitize_witness(tcx, body, def_id, interior, &upvars, &liveness_info.saved_locals);
1311 if tcx.sess.opts.debugging_opts.validate_mir {
1312 let mut vis = EnsureGeneratorFieldAssignmentsNeverAlias {
1313 assigned_local: None,
1314 saved_locals: &liveness_info.saved_locals,
1315 storage_conflicts: &liveness_info.storage_conflicts,
1318 vis.visit_body(body);
1321 // Extract locals which are live across suspension point into `layout`
1322 // `remap` gives a mapping from local indices onto generator struct indices
1323 // `storage_liveness` tells us which locals have live storage at suspension points
1324 let (remap, layout, storage_liveness) = compute_layout(liveness_info, body);
1326 let can_return = can_return(tcx, body);
1328 // Run the transformation which converts Places from Local to generator struct
1329 // accesses for locals in `remap`.
1330 // It also rewrites `return x` and `yield y` as writing a new generator state and returning
1331 // GeneratorState::Complete(x) and GeneratorState::Yielded(y) respectively.
1332 let mut transform = TransformVisitor {
1339 suspension_points: Vec::new(),
1343 transform.visit_body(body);
1345 // Update our MIR struct to reflect the changes we've made
1346 body.yield_ty = None;
1347 body.arg_count = 2; // self, resume arg
1348 body.spread_arg = None;
1349 body.generator_layout = Some(layout);
1351 // Insert `drop(generator_struct)` which is used to drop upvars for generators in
1352 // the unresumed state.
1353 // This is expanded to a drop ladder in `elaborate_generator_drops`.
1354 let drop_clean = insert_clean_drop(body);
1356 dump_mir(tcx, None, "generator_pre-elab", &0, body, |_, _| Ok(()));
1358 // Expand `drop(generator_struct)` to a drop ladder which destroys upvars.
1359 // If any upvars are moved out of, drop elaboration will handle upvar destruction.
1360 // However we need to also elaborate the code generated by `insert_clean_drop`.
1361 elaborate_generator_drops(tcx, def_id, body);
1363 dump_mir(tcx, None, "generator_post-transform", &0, body, |_, _| Ok(()));
1365 // Create a copy of our MIR and use it to create the drop shim for the generator
1366 let drop_shim = create_generator_drop_shim(tcx, &transform, gen_ty, body, drop_clean);
1368 body.generator_drop = Some(box drop_shim);
1370 // Create the Generator::resume function
1371 create_generator_resume_function(tcx, transform, body, can_return);
1375 /// Looks for any assignments between locals (e.g., `_4 = _5`) that will both be converted to fields
1376 /// in the generator state machine but whose storage is not marked as conflicting
1378 /// Validation needs to happen immediately *before* `TransformVisitor` is invoked, not after.
1380 /// This condition would arise when the assignment is the last use of `_5` but the initial
1381 /// definition of `_4` if we weren't extra careful to mark all locals used inside a statement as
1382 /// conflicting. Non-conflicting generator saved locals may be stored at the same location within
1383 /// the generator state machine, which would result in ill-formed MIR: the left-hand and right-hand
1384 /// sides of an assignment may not alias. This caused a miscompilation in [#73137].
1386 /// [#73137]: https://github.com/rust-lang/rust/issues/73137
1387 struct EnsureGeneratorFieldAssignmentsNeverAlias<'a> {
1388 saved_locals: &'a GeneratorSavedLocals,
1389 storage_conflicts: &'a BitMatrix<GeneratorSavedLocal, GeneratorSavedLocal>,
1390 assigned_local: Option<GeneratorSavedLocal>,
1393 impl EnsureGeneratorFieldAssignmentsNeverAlias<'_> {
1394 fn saved_local_for_direct_place(&self, place: Place<'_>) -> Option<GeneratorSavedLocal> {
1395 if place.is_indirect() {
1399 self.saved_locals.get(place.local)
1402 fn check_assigned_place(&mut self, place: Place<'tcx>, f: impl FnOnce(&mut Self)) {
1403 if let Some(assigned_local) = self.saved_local_for_direct_place(place) {
1404 assert!(self.assigned_local.is_none(), "`check_assigned_place` must not recurse");
1406 self.assigned_local = Some(assigned_local);
1408 self.assigned_local = None;
1413 impl Visitor<'tcx> for EnsureGeneratorFieldAssignmentsNeverAlias<'_> {
1414 fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, location: Location) {
1415 let lhs = match self.assigned_local {
1418 // This visitor only invokes `visit_place` for the right-hand side of an assignment
1419 // and only after setting `self.assigned_local`. However, the default impl of
1420 // `Visitor::super_body` may call `visit_place` with a `NonUseContext` for places
1421 // with debuginfo. Ignore them here.
1422 assert!(!context.is_use());
1427 let rhs = match self.saved_local_for_direct_place(*place) {
1432 if !self.storage_conflicts.contains(lhs, rhs) {
1434 "Assignment between generator saved locals whose storage is not \
1435 marked as conflicting: {:?}: {:?} = {:?}",
1443 fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
1444 match &statement.kind {
1445 StatementKind::Assign(box (lhs, rhs)) => {
1446 self.check_assigned_place(*lhs, |this| this.visit_rvalue(rhs, location));
1449 // FIXME: Does `llvm_asm!` have any aliasing requirements?
1450 StatementKind::LlvmInlineAsm(_) => {}
1452 StatementKind::FakeRead(..)
1453 | StatementKind::SetDiscriminant { .. }
1454 | StatementKind::StorageLive(_)
1455 | StatementKind::StorageDead(_)
1456 | StatementKind::Retag(..)
1457 | StatementKind::AscribeUserType(..)
1458 | StatementKind::Coverage(..)
1459 | StatementKind::Nop => {}
1463 fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
1464 // Checking for aliasing in terminators is probably overkill, but until we have actual
1465 // semantics, we should be conservative here.
1466 match &terminator.kind {
1467 TerminatorKind::Call {
1470 destination: Some((dest, _)),
1475 self.check_assigned_place(*dest, |this| {
1476 this.visit_operand(func, location);
1478 this.visit_operand(arg, location);
1483 TerminatorKind::Yield { value, resume: _, resume_arg, drop: _ } => {
1484 self.check_assigned_place(*resume_arg, |this| this.visit_operand(value, location));
1487 // FIXME: Does `asm!` have any aliasing requirements?
1488 TerminatorKind::InlineAsm { .. } => {}
1490 TerminatorKind::Call { .. }
1491 | TerminatorKind::Goto { .. }
1492 | TerminatorKind::SwitchInt { .. }
1493 | TerminatorKind::Resume
1494 | TerminatorKind::Abort
1495 | TerminatorKind::Return
1496 | TerminatorKind::Unreachable
1497 | TerminatorKind::Drop { .. }
1498 | TerminatorKind::DropAndReplace { .. }
1499 | TerminatorKind::Assert { .. }
1500 | TerminatorKind::GeneratorDrop
1501 | TerminatorKind::FalseEdge { .. }
1502 | TerminatorKind::FalseUnwind { .. } => {}