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.
53 use rustc::hir::def_id::DefId;
55 use rustc::mir::visit::{PlaceContext, Visitor, MutVisitor};
56 use rustc::ty::{self, TyCtxt, AdtDef, Ty};
57 use rustc::ty::layout::VariantIdx;
58 use rustc::ty::subst::SubstsRef;
59 use rustc_data_structures::fx::FxHashMap;
60 use rustc_data_structures::indexed_vec::Idx;
61 use rustc_data_structures::bit_set::BitSet;
65 use crate::transform::{MirPass, MirSource};
66 use crate::transform::simplify;
67 use crate::transform::no_landing_pads::no_landing_pads;
68 use crate::dataflow::{do_dataflow, DebugFormatted, state_for_location};
69 use crate::dataflow::{MaybeStorageLive, HaveBeenBorrowedLocals};
70 use crate::util::dump_mir;
71 use crate::util::liveness;
73 pub struct StateTransform;
75 struct RenameLocalVisitor {
80 impl<'tcx> MutVisitor<'tcx> for RenameLocalVisitor {
81 fn visit_local(&mut self,
83 _: PlaceContext<'tcx>,
85 if *local == self.from {
91 struct DerefArgVisitor;
93 impl<'tcx> MutVisitor<'tcx> for DerefArgVisitor {
94 fn visit_local(&mut self,
96 _: PlaceContext<'tcx>,
98 assert_ne!(*local, self_arg());
101 fn visit_place(&mut self,
102 place: &mut Place<'tcx>,
103 context: PlaceContext<'tcx>,
104 location: Location) {
105 if *place == Place::Base(PlaceBase::Local(self_arg())) {
106 *place = Place::Projection(Box::new(Projection {
108 elem: ProjectionElem::Deref,
111 self.super_place(place, context, location);
116 struct PinArgVisitor<'tcx> {
117 ref_gen_ty: Ty<'tcx>,
120 impl<'tcx> MutVisitor<'tcx> for PinArgVisitor<'tcx> {
121 fn visit_local(&mut self,
123 _: PlaceContext<'tcx>,
125 assert_ne!(*local, self_arg());
128 fn visit_place(&mut self,
129 place: &mut Place<'tcx>,
130 context: PlaceContext<'tcx>,
131 location: Location) {
132 if *place == Place::Base(PlaceBase::Local(self_arg())) {
133 *place = Place::Projection(Box::new(Projection {
135 elem: ProjectionElem::Field(Field::new(0), self.ref_gen_ty),
138 self.super_place(place, context, location);
143 fn self_arg() -> Local {
147 /// Generator have not been resumed yet
148 const UNRESUMED: u32 = 0;
149 /// Generator has returned / is completed
150 const RETURNED: u32 = 1;
151 /// Generator has been poisoned
152 const POISONED: u32 = 2;
154 struct SuspensionPoint {
157 drop: Option<BasicBlock>,
158 storage_liveness: liveness::LiveVarSet,
161 struct TransformVisitor<'a, 'tcx: 'a> {
162 tcx: TyCtxt<'a, 'tcx, 'tcx>,
163 state_adt_ref: &'tcx AdtDef,
164 state_substs: SubstsRef<'tcx>,
166 // The index of the generator state in the generator struct
169 // Mapping from Local to (type of local, generator struct index)
170 // FIXME(eddyb) This should use `IndexVec<Local, Option<_>>`.
171 remap: FxHashMap<Local, (Ty<'tcx>, usize)>,
173 // A map from a suspension point in a block to the locals which have live storage at that point
174 // FIXME(eddyb) This should use `IndexVec<BasicBlock, Option<_>>`.
175 storage_liveness: FxHashMap<BasicBlock, liveness::LiveVarSet>,
177 // A list of suspension points, generated during the transform
178 suspension_points: Vec<SuspensionPoint>,
180 // The original RETURN_PLACE local
181 new_ret_local: Local,
184 impl<'a, 'tcx> TransformVisitor<'a, 'tcx> {
185 // Make a GeneratorState rvalue
186 fn make_state(&self, idx: VariantIdx, val: Operand<'tcx>) -> Rvalue<'tcx> {
187 let adt = AggregateKind::Adt(self.state_adt_ref, idx, self.state_substs, None, None);
188 Rvalue::Aggregate(box adt, vec![val])
191 // Create a Place referencing a generator struct field
192 fn make_field(&self, idx: usize, ty: Ty<'tcx>) -> Place<'tcx> {
193 let base = Place::Base(PlaceBase::Local(self_arg()));
194 let field = Projection {
196 elem: ProjectionElem::Field(Field::new(idx), ty),
198 Place::Projection(Box::new(field))
201 // Create a statement which changes the generator state
202 fn set_state(&self, state_disc: u32, source_info: SourceInfo) -> Statement<'tcx> {
203 let state = self.make_field(self.state_field, self.tcx.types.u32);
204 let val = Operand::Constant(box Constant {
205 span: source_info.span,
206 ty: self.tcx.types.u32,
208 literal: self.tcx.mk_const(ty::Const::from_bits(
211 ty::ParamEnv::empty().and(self.tcx.types.u32)
216 kind: StatementKind::Assign(state, box Rvalue::Use(val)),
221 impl<'a, 'tcx> MutVisitor<'tcx> for TransformVisitor<'a, 'tcx> {
222 fn visit_local(&mut self,
224 _: PlaceContext<'tcx>,
226 assert_eq!(self.remap.get(local), None);
229 fn visit_place(&mut self,
230 place: &mut Place<'tcx>,
231 context: PlaceContext<'tcx>,
232 location: Location) {
233 if let Place::Base(PlaceBase::Local(l)) = *place {
234 // Replace an Local in the remap with a generator struct access
235 if let Some(&(ty, idx)) = self.remap.get(&l) {
236 *place = self.make_field(idx, ty);
239 self.super_place(place, context, location);
243 fn visit_basic_block_data(&mut self,
245 data: &mut BasicBlockData<'tcx>) {
246 // Remove StorageLive and StorageDead statements for remapped locals
247 data.retain_statements(|s| {
249 StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => {
250 !self.remap.contains_key(&l)
256 let ret_val = match data.terminator().kind {
257 TerminatorKind::Return => Some((VariantIdx::new(1),
259 Operand::Move(Place::Base(PlaceBase::Local(self.new_ret_local))),
261 TerminatorKind::Yield { ref value, resume, drop } => Some((VariantIdx::new(0),
268 if let Some((state_idx, resume, v, drop)) = ret_val {
269 let source_info = data.terminator().source_info;
270 // We must assign the value first in case it gets declared dead below
271 data.statements.push(Statement {
273 kind: StatementKind::Assign(Place::RETURN_PLACE,
274 box self.make_state(state_idx, v)),
276 let state = if let Some(resume) = resume { // Yield
277 let state = 3 + self.suspension_points.len() as u32;
279 self.suspension_points.push(SuspensionPoint {
283 storage_liveness: self.storage_liveness.get(&block).unwrap().clone(),
288 RETURNED // state for returned
290 data.statements.push(self.set_state(state, source_info));
291 data.terminator.as_mut().unwrap().kind = TerminatorKind::Return;
294 self.super_basic_block_data(block, data);
298 fn make_generator_state_argument_indirect<'a, 'tcx>(
299 tcx: TyCtxt<'a, 'tcx, 'tcx>,
301 mir: &mut Mir<'tcx>) {
302 let gen_ty = mir.local_decls.raw[1].ty;
304 let region = ty::ReFree(ty::FreeRegion {
306 bound_region: ty::BoundRegion::BrEnv,
309 let region = tcx.mk_region(region);
311 let ref_gen_ty = tcx.mk_ref(region, ty::TypeAndMut {
313 mutbl: hir::MutMutable
316 // Replace the by value generator argument
317 mir.local_decls.raw[1].ty = ref_gen_ty;
319 // Add a deref to accesses of the generator state
320 DerefArgVisitor.visit_mir(mir);
323 fn make_generator_state_argument_pinned<'a, 'tcx>(
324 tcx: TyCtxt<'a, 'tcx, 'tcx>,
325 mir: &mut Mir<'tcx>) {
326 let ref_gen_ty = mir.local_decls.raw[1].ty;
328 let pin_did = tcx.lang_items().pin_type().unwrap();
329 let pin_adt_ref = tcx.adt_def(pin_did);
330 let substs = tcx.intern_substs(&[ref_gen_ty.into()]);
331 let pin_ref_gen_ty = tcx.mk_adt(pin_adt_ref, substs);
333 // Replace the by ref generator argument
334 mir.local_decls.raw[1].ty = pin_ref_gen_ty;
336 // Add the Pin field access to accesses of the generator state
337 PinArgVisitor { ref_gen_ty }.visit_mir(mir);
340 fn replace_result_variable<'tcx>(
344 let source_info = source_info(mir);
345 let new_ret = LocalDecl {
346 mutability: Mutability::Mut,
348 user_ty: UserTypeProjections::none(),
351 visibility_scope: source_info.scope,
354 is_user_variable: None,
356 let new_ret_local = Local::new(mir.local_decls.len());
357 mir.local_decls.push(new_ret);
358 mir.local_decls.swap(RETURN_PLACE, new_ret_local);
368 struct StorageIgnored(liveness::LiveVarSet);
370 impl<'tcx> Visitor<'tcx> for StorageIgnored {
371 fn visit_statement(&mut self,
373 statement: &Statement<'tcx>,
374 _location: Location) {
375 match statement.kind {
376 StatementKind::StorageLive(l) |
377 StatementKind::StorageDead(l) => { self.0.remove(l); }
383 fn locals_live_across_suspend_points(
384 tcx: TyCtxt<'a, 'tcx, 'tcx>,
386 source: MirSource<'tcx>,
389 liveness::LiveVarSet,
390 FxHashMap<BasicBlock, liveness::LiveVarSet>,
392 let dead_unwinds = BitSet::new_empty(mir.basic_blocks().len());
393 let def_id = source.def_id();
395 // Calculate when MIR locals have live storage. This gives us an upper bound of their
397 let storage_live_analysis = MaybeStorageLive::new(mir);
399 do_dataflow(tcx, mir, def_id, &[], &dead_unwinds, storage_live_analysis,
400 |bd, p| DebugFormatted::new(&bd.mir().local_decls[p]));
402 // Find the MIR locals which do not use StorageLive/StorageDead statements.
403 // The storage of these locals are always live.
404 let mut ignored = StorageIgnored(BitSet::new_filled(mir.local_decls.len()));
405 ignored.visit_mir(mir);
407 // Calculate the MIR locals which have been previously
408 // borrowed (even if they are still active).
409 // This is only used for immovable generators.
410 let borrowed_locals = if !movable {
411 let analysis = HaveBeenBorrowedLocals::new(mir);
413 do_dataflow(tcx, mir, def_id, &[], &dead_unwinds, analysis,
414 |bd, p| DebugFormatted::new(&bd.mir().local_decls[p]));
415 Some((analysis, result))
420 // Calculate the liveness of MIR locals ignoring borrows.
421 let mut set = liveness::LiveVarSet::new_empty(mir.local_decls.len());
422 let mut liveness = liveness::liveness_of_locals(
427 "generator_liveness",
433 let mut storage_liveness_map = FxHashMap::default();
435 for (block, data) in mir.basic_blocks().iter_enumerated() {
436 if let TerminatorKind::Yield { .. } = data.terminator().kind {
439 statement_index: data.statements.len(),
442 if let Some((ref analysis, ref result)) = borrowed_locals {
443 let borrowed_locals = state_for_location(loc,
447 // The `liveness` variable contains the liveness of MIR locals ignoring borrows.
448 // This is correct for movable generators since borrows cannot live across
449 // suspension points. However for immovable generators we need to account for
450 // borrows, so we conseratively assume that all borrowed locals are live until
451 // we find a StorageDead statement referencing the locals.
452 // To do this we just union our `liveness` result with `borrowed_locals`, which
453 // contains all the locals which has been borrowed before this suspension point.
454 // If a borrow is converted to a raw reference, we must also assume that it lives
455 // forever. Note that the final liveness is still bounded by the storage liveness
456 // of the local, which happens using the `intersect` operation below.
457 liveness.outs[block].union(&borrowed_locals);
460 let mut storage_liveness = state_for_location(loc,
461 &storage_live_analysis,
465 // Store the storage liveness for later use so we can restore the state
466 // after a suspension point
467 storage_liveness_map.insert(block, storage_liveness.clone());
469 // Mark locals without storage statements as always having live storage
470 storage_liveness.union(&ignored.0);
472 // Locals live are live at this point only if they are used across
473 // suspension points (the `liveness` variable)
474 // and their storage is live (the `storage_liveness` variable)
475 storage_liveness.intersect(&liveness.outs[block]);
477 let live_locals = storage_liveness;
479 // Add the locals life at this suspension point to the set of locals which live across
480 // any suspension points
481 set.union(&live_locals);
485 // The generator argument is ignored
486 set.remove(self_arg());
488 (set, storage_liveness_map)
491 fn compute_layout<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
492 source: MirSource<'tcx>,
493 upvars: Vec<Ty<'tcx>>,
497 -> (FxHashMap<Local, (Ty<'tcx>, usize)>,
498 GeneratorLayout<'tcx>,
499 FxHashMap<BasicBlock, liveness::LiveVarSet>)
501 // Use a liveness analysis to compute locals which are live across a suspension point
502 let (live_locals, storage_liveness) = locals_live_across_suspend_points(tcx,
506 // Erase regions from the types passed in from typeck so we can compare them with
508 let allowed_upvars = tcx.erase_regions(&upvars);
509 let allowed = match interior.sty {
510 ty::GeneratorWitness(s) => tcx.erase_late_bound_regions(&s),
514 for (local, decl) in mir.local_decls.iter_enumerated() {
515 // Ignore locals which are internal or not live
516 if !live_locals.contains(local) || decl.internal {
520 // Sanity check that typeck knows about the type of locals which are
521 // live across a suspension point
522 if !allowed.contains(&decl.ty) && !allowed_upvars.contains(&decl.ty) {
524 "Broken MIR: generator contains type {} in MIR, \
525 but typeck only knows about {}",
531 let upvar_len = mir.upvar_decls.len();
532 let dummy_local = LocalDecl::new_internal(tcx.mk_unit(), mir.span);
534 // Gather live locals and their indices replacing values in mir.local_decls with a dummy
535 // to avoid changing local indices
536 let live_decls = live_locals.iter().map(|local| {
537 let var = mem::replace(&mut mir.local_decls[local], dummy_local.clone());
541 // Create a map from local indices to generator struct indices.
542 // These are offset by (upvar_len + 1) because of fields which comes before locals.
543 // We also create a vector of the LocalDecls of these locals.
544 let (remap, vars) = live_decls.enumerate().map(|(idx, (local, var))| {
545 ((local, (var.ty, upvar_len + 1 + idx)), var)
548 let layout = GeneratorLayout {
552 (remap, layout, storage_liveness)
555 fn insert_switch<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
557 cases: Vec<(u32, BasicBlock)>,
558 transform: &TransformVisitor<'a, 'tcx>,
559 default: TerminatorKind<'tcx>) {
560 let default_block = insert_term_block(mir, default);
562 let switch = TerminatorKind::SwitchInt {
563 discr: Operand::Copy(transform.make_field(transform.state_field, tcx.types.u32)),
564 switch_ty: tcx.types.u32,
565 values: Cow::from(cases.iter().map(|&(i, _)| i.into()).collect::<Vec<_>>()),
566 targets: cases.iter().map(|&(_, d)| d).chain(once(default_block)).collect(),
569 let source_info = source_info(mir);
570 mir.basic_blocks_mut().raw.insert(0, BasicBlockData {
571 statements: Vec::new(),
572 terminator: Some(Terminator {
579 let blocks = mir.basic_blocks_mut().iter_mut();
581 for target in blocks.flat_map(|b| b.terminator_mut().successors_mut()) {
582 *target = BasicBlock::new(target.index() + 1);
586 fn elaborate_generator_drops<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
588 mir: &mut Mir<'tcx>) {
589 use crate::util::elaborate_drops::{elaborate_drop, Unwind};
590 use crate::util::patch::MirPatch;
591 use crate::shim::DropShimElaborator;
593 // Note that `elaborate_drops` only drops the upvars of a generator, and
594 // this is ok because `open_drop` can only be reached within that own
595 // generator's resume function.
597 let param_env = tcx.param_env(def_id);
598 let gen = self_arg();
600 let mut elaborator = DropShimElaborator {
602 patch: MirPatch::new(mir),
607 for (block, block_data) in mir.basic_blocks().iter_enumerated() {
608 let (target, unwind, source_info) = match block_data.terminator() {
611 kind: TerminatorKind::Drop {
612 location: Place::Base(PlaceBase::Local(local)),
616 } if local == gen => (target, unwind, source_info),
619 let unwind = if block_data.is_cleanup {
622 Unwind::To(unwind.unwrap_or_else(|| elaborator.patch.resume_block()))
627 &Place::Base(PlaceBase::Local(gen)),
634 elaborator.patch.apply(mir);
637 fn create_generator_drop_shim<'a, 'tcx>(
638 tcx: TyCtxt<'a, 'tcx, 'tcx>,
639 transform: &TransformVisitor<'a, 'tcx>,
641 source: MirSource<'tcx>,
644 drop_clean: BasicBlock) -> Mir<'tcx> {
645 let mut mir = mir.clone();
647 let source_info = source_info(&mir);
649 let mut cases = create_cases(&mut mir, transform, |point| point.drop);
651 cases.insert(0, (UNRESUMED, drop_clean));
653 // The returned state and the poisoned state fall through to the default
654 // case which is just to return
656 insert_switch(tcx, &mut mir, cases, &transform, TerminatorKind::Return);
658 for block in mir.basic_blocks_mut() {
659 let kind = &mut block.terminator_mut().kind;
660 if let TerminatorKind::GeneratorDrop = *kind {
661 *kind = TerminatorKind::Return;
665 // Replace the return variable
666 mir.local_decls[RETURN_PLACE] = LocalDecl {
667 mutability: Mutability::Mut,
669 user_ty: UserTypeProjections::none(),
672 visibility_scope: source_info.scope,
675 is_user_variable: None,
678 make_generator_state_argument_indirect(tcx, def_id, &mut mir);
680 // Change the generator argument from &mut to *mut
681 mir.local_decls[self_arg()] = LocalDecl {
682 mutability: Mutability::Mut,
683 ty: tcx.mk_ptr(ty::TypeAndMut {
685 mutbl: hir::Mutability::MutMutable,
687 user_ty: UserTypeProjections::none(),
690 visibility_scope: source_info.scope,
693 is_user_variable: None,
695 if tcx.sess.opts.debugging_opts.mir_emit_retag {
696 // Alias tracking must know we changed the type
697 mir.basic_blocks_mut()[START_BLOCK].statements.insert(0, Statement {
699 kind: StatementKind::Retag(RetagKind::Raw, Place::Base(PlaceBase::Local(self_arg()))),
703 no_landing_pads(tcx, &mut mir);
705 // Make sure we remove dead blocks to remove
706 // unrelated code from the resume part of the function
707 simplify::remove_dead_blocks(&mut mir);
709 dump_mir(tcx, None, "generator_drop", &0, source, &mut mir, |_, _| Ok(()) );
714 fn insert_term_block<'tcx>(mir: &mut Mir<'tcx>, kind: TerminatorKind<'tcx>) -> BasicBlock {
715 let term_block = BasicBlock::new(mir.basic_blocks().len());
716 let source_info = source_info(mir);
717 mir.basic_blocks_mut().push(BasicBlockData {
718 statements: Vec::new(),
719 terminator: Some(Terminator {
728 fn insert_panic_block<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
730 message: AssertMessage<'tcx>) -> BasicBlock {
731 let assert_block = BasicBlock::new(mir.basic_blocks().len());
732 let term = TerminatorKind::Assert {
733 cond: Operand::Constant(box Constant {
737 literal: tcx.mk_const(
738 ty::Const::from_bool(tcx, false),
743 target: assert_block,
747 let source_info = source_info(mir);
748 mir.basic_blocks_mut().push(BasicBlockData {
749 statements: Vec::new(),
750 terminator: Some(Terminator {
760 fn create_generator_resume_function<'a, 'tcx>(
761 tcx: TyCtxt<'a, 'tcx, 'tcx>,
762 transform: TransformVisitor<'a, 'tcx>,
764 source: MirSource<'tcx>,
765 mir: &mut Mir<'tcx>) {
766 // Poison the generator when it unwinds
767 for block in mir.basic_blocks_mut() {
768 let source_info = block.terminator().source_info;
769 if let &TerminatorKind::Resume = &block.terminator().kind {
770 block.statements.push(transform.set_state(POISONED, source_info));
774 let mut cases = create_cases(mir, &transform, |point| Some(point.resume));
776 use rustc::mir::interpret::EvalErrorKind::{
777 GeneratorResumedAfterPanic,
778 GeneratorResumedAfterReturn,
781 // Jump to the entry point on the unresumed
782 cases.insert(0, (UNRESUMED, BasicBlock::new(0)));
783 // Panic when resumed on the returned state
784 cases.insert(1, (RETURNED, insert_panic_block(tcx, mir, GeneratorResumedAfterReturn)));
785 // Panic when resumed on the poisoned state
786 cases.insert(2, (POISONED, insert_panic_block(tcx, mir, GeneratorResumedAfterPanic)));
788 insert_switch(tcx, mir, cases, &transform, TerminatorKind::Unreachable);
790 make_generator_state_argument_indirect(tcx, def_id, mir);
791 make_generator_state_argument_pinned(tcx, mir);
793 no_landing_pads(tcx, mir);
795 // Make sure we remove dead blocks to remove
796 // unrelated code from the drop part of the function
797 simplify::remove_dead_blocks(mir);
799 dump_mir(tcx, None, "generator_resume", &0, source, mir, |_, _| Ok(()) );
802 fn source_info<'a, 'tcx>(mir: &Mir<'tcx>) -> SourceInfo {
805 scope: OUTERMOST_SOURCE_SCOPE,
809 fn insert_clean_drop<'a, 'tcx>(mir: &mut Mir<'tcx>) -> BasicBlock {
810 let return_block = insert_term_block(mir, TerminatorKind::Return);
812 // Create a block to destroy an unresumed generators. This can only destroy upvars.
813 let drop_clean = BasicBlock::new(mir.basic_blocks().len());
814 let term = TerminatorKind::Drop {
815 location: Place::Base(PlaceBase::Local(self_arg())),
816 target: return_block,
819 let source_info = source_info(mir);
820 mir.basic_blocks_mut().push(BasicBlockData {
821 statements: Vec::new(),
822 terminator: Some(Terminator {
832 fn create_cases<'a, 'tcx, F>(mir: &mut Mir<'tcx>,
833 transform: &TransformVisitor<'a, 'tcx>,
834 target: F) -> Vec<(u32, BasicBlock)>
835 where F: Fn(&SuspensionPoint) -> Option<BasicBlock> {
836 let source_info = source_info(mir);
838 transform.suspension_points.iter().filter_map(|point| {
839 // Find the target for this suspension point, if applicable
840 target(point).map(|target| {
841 let block = BasicBlock::new(mir.basic_blocks().len());
842 let mut statements = Vec::new();
844 // Create StorageLive instructions for locals with live storage
845 for i in 0..(mir.local_decls.len()) {
846 let l = Local::new(i);
847 if point.storage_liveness.contains(l) && !transform.remap.contains_key(&l) {
848 statements.push(Statement {
850 kind: StatementKind::StorageLive(l),
855 // Then jump to the real target
856 mir.basic_blocks_mut().push(BasicBlockData {
858 terminator: Some(Terminator {
860 kind: TerminatorKind::Goto {
872 impl MirPass for StateTransform {
873 fn run_pass<'a, 'tcx>(&self,
874 tcx: TyCtxt<'a, 'tcx, 'tcx>,
875 source: MirSource<'tcx>,
876 mir: &mut Mir<'tcx>) {
877 let yield_ty = if let Some(yield_ty) = mir.yield_ty {
880 // This only applies to generators
884 assert!(mir.generator_drop.is_none());
886 let def_id = source.def_id();
888 // The first argument is the generator type passed by value
889 let gen_ty = mir.local_decls.raw[1].ty;
891 // Get the interior types and substs which typeck computed
892 let (upvars, interior, movable) = match gen_ty.sty {
893 ty::Generator(_, substs, movability) => {
894 (substs.upvar_tys(def_id, tcx).collect(),
895 substs.witness(def_id, tcx),
896 movability == hir::GeneratorMovability::Movable)
901 // Compute GeneratorState<yield_ty, return_ty>
902 let state_did = tcx.lang_items().gen_state().unwrap();
903 let state_adt_ref = tcx.adt_def(state_did);
904 let state_substs = tcx.intern_substs(&[
906 mir.return_ty().into(),
908 let ret_ty = tcx.mk_adt(state_adt_ref, state_substs);
910 // We rename RETURN_PLACE which has type mir.return_ty to new_ret_local
911 // RETURN_PLACE then is a fresh unused local with type ret_ty.
912 let new_ret_local = replace_result_variable(ret_ty, mir);
914 // Extract locals which are live across suspension point into `layout`
915 // `remap` gives a mapping from local indices onto generator struct indices
916 // `storage_liveness` tells us which locals have live storage at suspension points
917 let (remap, layout, storage_liveness) = compute_layout(
925 let state_field = mir.upvar_decls.len();
927 // Run the transformation which converts Places from Local to generator struct
928 // accesses for locals in `remap`.
929 // It also rewrites `return x` and `yield y` as writing a new generator state and returning
930 // GeneratorState::Complete(x) and GeneratorState::Yielded(y) respectively.
931 let mut transform = TransformVisitor {
937 suspension_points: Vec::new(),
941 transform.visit_mir(mir);
943 // Update our MIR struct to reflect the changed we've made
946 mir.spread_arg = None;
947 mir.generator_layout = Some(layout);
949 // Insert `drop(generator_struct)` which is used to drop upvars for generators in
950 // the unresumed state.
951 // This is expanded to a drop ladder in `elaborate_generator_drops`.
952 let drop_clean = insert_clean_drop(mir);
954 dump_mir(tcx, None, "generator_pre-elab", &0, source, mir, |_, _| Ok(()) );
956 // Expand `drop(generator_struct)` to a drop ladder which destroys upvars.
957 // If any upvars are moved out of, drop elaboration will handle upvar destruction.
958 // However we need to also elaborate the code generated by `insert_clean_drop`.
959 elaborate_generator_drops(tcx, def_id, mir);
961 dump_mir(tcx, None, "generator_post-transform", &0, source, mir, |_, _| Ok(()) );
963 // Create a copy of our MIR and use it to create the drop shim for the generator
964 let drop_shim = create_generator_drop_shim(tcx,
972 mir.generator_drop = Some(box drop_shim);
974 // Create the Generator::resume function
975 create_generator_resume_function(tcx, transform, def_id, source, mir);