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 two 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::Substs;
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::{self, IdentityMap};
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::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::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 struct SuspensionPoint {
150 drop: Option<BasicBlock>,
151 storage_liveness: liveness::LiveVarSet<Local>,
154 struct TransformVisitor<'a, 'tcx: 'a> {
155 tcx: TyCtxt<'a, 'tcx, 'tcx>,
156 state_adt_ref: &'tcx AdtDef,
157 state_substs: &'tcx Substs<'tcx>,
159 // The index of the generator state in the generator struct
162 // Mapping from Local to (type of local, generator struct index)
163 // FIXME(eddyb) This should use `IndexVec<Local, Option<_>>`.
164 remap: FxHashMap<Local, (Ty<'tcx>, usize)>,
166 // A map from a suspension point in a block to the locals which have live storage at that point
167 // FIXME(eddyb) This should use `IndexVec<BasicBlock, Option<_>>`.
168 storage_liveness: FxHashMap<BasicBlock, liveness::LiveVarSet<Local>>,
170 // A list of suspension points, generated during the transform
171 suspension_points: Vec<SuspensionPoint>,
173 // The original RETURN_PLACE local
174 new_ret_local: Local,
177 impl<'a, 'tcx> TransformVisitor<'a, 'tcx> {
178 // Make a GeneratorState rvalue
179 fn make_state(&self, idx: VariantIdx, val: Operand<'tcx>) -> Rvalue<'tcx> {
180 let adt = AggregateKind::Adt(self.state_adt_ref, idx, self.state_substs, None, None);
181 Rvalue::Aggregate(box adt, vec![val])
184 // Create a Place referencing a generator struct field
185 fn make_field(&self, idx: usize, ty: Ty<'tcx>) -> Place<'tcx> {
186 let base = Place::Local(self_arg());
187 let field = Projection {
189 elem: ProjectionElem::Field(Field::new(idx), ty),
191 Place::Projection(Box::new(field))
194 // Create a statement which changes the generator state
195 fn set_state(&self, state_disc: u32, source_info: SourceInfo) -> Statement<'tcx> {
196 let state = self.make_field(self.state_field, self.tcx.types.u32);
197 let val = Operand::Constant(box Constant {
198 span: source_info.span,
199 ty: self.tcx.types.u32,
201 literal: self.tcx.mk_lazy_const(ty::LazyConst::Evaluated(ty::Const::from_bits(
204 ty::ParamEnv::empty().and(self.tcx.types.u32)
209 kind: StatementKind::Assign(state, box Rvalue::Use(val)),
214 impl<'a, 'tcx> MutVisitor<'tcx> for TransformVisitor<'a, 'tcx> {
215 fn visit_local(&mut self,
217 _: PlaceContext<'tcx>,
219 assert_eq!(self.remap.get(local), None);
222 fn visit_place(&mut self,
223 place: &mut Place<'tcx>,
224 context: PlaceContext<'tcx>,
225 location: Location) {
226 if let Place::Local(l) = *place {
227 // Replace an Local in the remap with a generator struct access
228 if let Some(&(ty, idx)) = self.remap.get(&l) {
229 *place = self.make_field(idx, ty);
232 self.super_place(place, context, location);
236 fn visit_basic_block_data(&mut self,
238 data: &mut BasicBlockData<'tcx>) {
239 // Remove StorageLive and StorageDead statements for remapped locals
240 data.retain_statements(|s| {
242 StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => {
243 !self.remap.contains_key(&l)
249 let ret_val = match data.terminator().kind {
250 TerminatorKind::Return => Some((VariantIdx::new(1),
252 Operand::Move(Place::Local(self.new_ret_local)),
254 TerminatorKind::Yield { ref value, resume, drop } => Some((VariantIdx::new(0),
261 if let Some((state_idx, resume, v, drop)) = ret_val {
262 let source_info = data.terminator().source_info;
263 // We must assign the value first in case it gets declared dead below
264 data.statements.push(Statement {
266 kind: StatementKind::Assign(Place::Local(RETURN_PLACE),
267 box self.make_state(state_idx, v)),
269 let state = if let Some(resume) = resume { // Yield
270 let state = 3 + self.suspension_points.len() as u32;
272 self.suspension_points.push(SuspensionPoint {
276 storage_liveness: self.storage_liveness.get(&block).unwrap().clone(),
281 1 // state for returned
283 data.statements.push(self.set_state(state, source_info));
284 data.terminator.as_mut().unwrap().kind = TerminatorKind::Return;
287 self.super_basic_block_data(block, data);
291 fn make_generator_state_argument_indirect<'a, 'tcx>(
292 tcx: TyCtxt<'a, 'tcx, 'tcx>,
294 mir: &mut Mir<'tcx>) {
295 let gen_ty = mir.local_decls.raw[1].ty;
297 let region = ty::ReFree(ty::FreeRegion {
299 bound_region: ty::BoundRegion::BrEnv,
302 let region = tcx.mk_region(region);
304 let ref_gen_ty = tcx.mk_ref(region, ty::TypeAndMut {
306 mutbl: hir::MutMutable
309 // Replace the by value generator argument
310 mir.local_decls.raw[1].ty = ref_gen_ty;
312 // Add a deref to accesses of the generator state
313 DerefArgVisitor.visit_mir(mir);
316 fn make_generator_state_argument_pinned<'a, 'tcx>(
317 tcx: TyCtxt<'a, 'tcx, 'tcx>,
318 mir: &mut Mir<'tcx>) {
319 let ref_gen_ty = mir.local_decls.raw[1].ty;
321 let pin_did = tcx.lang_items().pin_type().unwrap();
322 let pin_adt_ref = tcx.adt_def(pin_did);
323 let substs = tcx.intern_substs(&[ref_gen_ty.into()]);
324 let pin_ref_gen_ty = tcx.mk_adt(pin_adt_ref, substs);
326 // Replace the by ref generator argument
327 mir.local_decls.raw[1].ty = pin_ref_gen_ty;
329 // Add the Pin field access to accesses of the generator state
330 PinArgVisitor { ref_gen_ty }.visit_mir(mir);
333 fn replace_result_variable<'tcx>(
337 let source_info = source_info(mir);
338 let new_ret = LocalDecl {
339 mutability: Mutability::Mut,
341 user_ty: UserTypeProjections::none(),
344 visibility_scope: source_info.scope,
347 is_user_variable: None,
349 let new_ret_local = Local::new(mir.local_decls.len());
350 mir.local_decls.push(new_ret);
351 mir.local_decls.swap(RETURN_PLACE, new_ret_local);
361 struct StorageIgnored(liveness::LiveVarSet<Local>);
363 impl<'tcx> Visitor<'tcx> for StorageIgnored {
364 fn visit_statement(&mut self,
366 statement: &Statement<'tcx>,
367 _location: Location) {
368 match statement.kind {
369 StatementKind::StorageLive(l) |
370 StatementKind::StorageDead(l) => { self.0.remove(l); }
376 fn locals_live_across_suspend_points(
377 tcx: TyCtxt<'a, 'tcx, 'tcx>,
379 source: MirSource<'tcx>,
382 liveness::LiveVarSet<Local>,
383 FxHashMap<BasicBlock, liveness::LiveVarSet<Local>>,
385 let dead_unwinds = BitSet::new_empty(mir.basic_blocks().len());
386 let node_id = tcx.hir().as_local_node_id(source.def_id()).unwrap();
388 // Calculate when MIR locals have live storage. This gives us an upper bound of their
390 let storage_live_analysis = MaybeStorageLive::new(mir);
392 do_dataflow(tcx, mir, node_id, &[], &dead_unwinds, storage_live_analysis,
393 |bd, p| DebugFormatted::new(&bd.mir().local_decls[p]));
395 // Find the MIR locals which do not use StorageLive/StorageDead statements.
396 // The storage of these locals are always live.
397 let mut ignored = StorageIgnored(BitSet::new_filled(mir.local_decls.len()));
398 ignored.visit_mir(mir);
400 // Calculate the MIR locals which have been previously
401 // borrowed (even if they are still active).
402 // This is only used for immovable generators.
403 let borrowed_locals = if !movable {
404 let analysis = HaveBeenBorrowedLocals::new(mir);
406 do_dataflow(tcx, mir, node_id, &[], &dead_unwinds, analysis,
407 |bd, p| DebugFormatted::new(&bd.mir().local_decls[p]));
408 Some((analysis, result))
413 // Calculate the liveness of MIR locals ignoring borrows.
414 let mut set = liveness::LiveVarSet::new_empty(mir.local_decls.len());
415 let mut liveness = liveness::liveness_of_locals(
417 &IdentityMap::new(mir),
421 "generator_liveness",
424 &IdentityMap::new(mir),
428 let mut storage_liveness_map = FxHashMap::default();
430 for (block, data) in mir.basic_blocks().iter_enumerated() {
431 if let TerminatorKind::Yield { .. } = data.terminator().kind {
434 statement_index: data.statements.len(),
437 if let Some((ref analysis, ref result)) = borrowed_locals {
438 let borrowed_locals = state_for_location(loc,
442 // The `liveness` variable contains the liveness of MIR locals ignoring borrows.
443 // This is correct for movable generators since borrows cannot live across
444 // suspension points. However for immovable generators we need to account for
445 // borrows, so we conseratively assume that all borrowed locals are live until
446 // we find a StorageDead statement referencing the locals.
447 // To do this we just union our `liveness` result with `borrowed_locals`, which
448 // contains all the locals which has been borrowed before this suspension point.
449 // If a borrow is converted to a raw reference, we must also assume that it lives
450 // forever. Note that the final liveness is still bounded by the storage liveness
451 // of the local, which happens using the `intersect` operation below.
452 liveness.outs[block].union(&borrowed_locals);
455 let mut storage_liveness = state_for_location(loc,
456 &storage_live_analysis,
460 // Store the storage liveness for later use so we can restore the state
461 // after a suspension point
462 storage_liveness_map.insert(block, storage_liveness.clone());
464 // Mark locals without storage statements as always having live storage
465 storage_liveness.union(&ignored.0);
467 // Locals live are live at this point only if they are used across
468 // suspension points (the `liveness` variable)
469 // and their storage is live (the `storage_liveness` variable)
470 storage_liveness.intersect(&liveness.outs[block]);
472 let live_locals = storage_liveness;
474 // Add the locals life at this suspension point to the set of locals which live across
475 // any suspension points
476 set.union(&live_locals);
480 // The generator argument is ignored
481 set.remove(self_arg());
483 (set, storage_liveness_map)
486 fn compute_layout<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
487 source: MirSource<'tcx>,
488 upvars: Vec<Ty<'tcx>>,
492 -> (FxHashMap<Local, (Ty<'tcx>, usize)>,
493 GeneratorLayout<'tcx>,
494 FxHashMap<BasicBlock, liveness::LiveVarSet<Local>>)
496 // Use a liveness analysis to compute locals which are live across a suspension point
497 let (live_locals, storage_liveness) = locals_live_across_suspend_points(tcx,
501 // Erase regions from the types passed in from typeck so we can compare them with
503 let allowed_upvars = tcx.erase_regions(&upvars);
504 let allowed = match interior.sty {
505 ty::GeneratorWitness(s) => tcx.erase_late_bound_regions(&s),
509 for (local, decl) in mir.local_decls.iter_enumerated() {
510 // Ignore locals which are internal or not live
511 if !live_locals.contains(local) || decl.internal {
515 // Sanity check that typeck knows about the type of locals which are
516 // live across a suspension point
517 if !allowed.contains(&decl.ty) && !allowed_upvars.contains(&decl.ty) {
519 "Broken MIR: generator contains type {} in MIR, \
520 but typeck only knows about {}",
526 let upvar_len = mir.upvar_decls.len();
527 let dummy_local = LocalDecl::new_internal(tcx.mk_unit(), mir.span);
529 // Gather live locals and their indices replacing values in mir.local_decls with a dummy
530 // to avoid changing local indices
531 let live_decls = live_locals.iter().map(|local| {
532 let var = mem::replace(&mut mir.local_decls[local], dummy_local.clone());
536 // Create a map from local indices to generator struct indices.
537 // These are offset by (upvar_len + 1) because of fields which comes before locals.
538 // We also create a vector of the LocalDecls of these locals.
539 let (remap, vars) = live_decls.enumerate().map(|(idx, (local, var))| {
540 ((local, (var.ty, upvar_len + 1 + idx)), var)
543 let layout = GeneratorLayout {
547 (remap, layout, storage_liveness)
550 fn insert_switch<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
552 cases: Vec<(u32, BasicBlock)>,
553 transform: &TransformVisitor<'a, 'tcx>,
554 default: TerminatorKind<'tcx>) {
555 let default_block = insert_term_block(mir, default);
557 let switch = TerminatorKind::SwitchInt {
558 discr: Operand::Copy(transform.make_field(transform.state_field, tcx.types.u32)),
559 switch_ty: tcx.types.u32,
560 values: Cow::from(cases.iter().map(|&(i, _)| i.into()).collect::<Vec<_>>()),
561 targets: cases.iter().map(|&(_, d)| d).chain(once(default_block)).collect(),
564 let source_info = source_info(mir);
565 mir.basic_blocks_mut().raw.insert(0, BasicBlockData {
566 statements: Vec::new(),
567 terminator: Some(Terminator {
574 let blocks = mir.basic_blocks_mut().iter_mut();
576 for target in blocks.flat_map(|b| b.terminator_mut().successors_mut()) {
577 *target = BasicBlock::new(target.index() + 1);
581 fn elaborate_generator_drops<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
583 mir: &mut Mir<'tcx>) {
584 use crate::util::elaborate_drops::{elaborate_drop, Unwind};
585 use crate::util::patch::MirPatch;
586 use crate::shim::DropShimElaborator;
588 // Note that `elaborate_drops` only drops the upvars of a generator, and
589 // this is ok because `open_drop` can only be reached within that own
590 // generator's resume function.
592 let param_env = tcx.param_env(def_id);
593 let gen = self_arg();
595 for block in mir.basic_blocks().indices() {
596 let (target, unwind, source_info) = match mir.basic_blocks()[block].terminator() {
599 kind: TerminatorKind::Drop {
600 location: Place::Local(local),
604 } if local == gen => (target, unwind, source_info),
607 let unwind = if let Some(unwind) = unwind {
613 let mut elaborator = DropShimElaborator {
615 patch: MirPatch::new(mir),
634 fn create_generator_drop_shim<'a, 'tcx>(
635 tcx: TyCtxt<'a, 'tcx, 'tcx>,
636 transform: &TransformVisitor<'a, 'tcx>,
638 source: MirSource<'tcx>,
641 drop_clean: BasicBlock) -> Mir<'tcx> {
642 let mut mir = mir.clone();
644 let source_info = source_info(&mir);
646 let mut cases = create_cases(&mut mir, transform, |point| point.drop);
648 cases.insert(0, (0, drop_clean));
650 // The returned state (1) and the poisoned state (2) falls through to
651 // the default case which is just to return
653 insert_switch(tcx, &mut mir, cases, &transform, TerminatorKind::Return);
655 for block in mir.basic_blocks_mut() {
656 let kind = &mut block.terminator_mut().kind;
657 if let TerminatorKind::GeneratorDrop = *kind {
658 *kind = TerminatorKind::Return;
662 // Replace the return variable
663 mir.local_decls[RETURN_PLACE] = LocalDecl {
664 mutability: Mutability::Mut,
666 user_ty: UserTypeProjections::none(),
669 visibility_scope: source_info.scope,
672 is_user_variable: None,
675 make_generator_state_argument_indirect(tcx, def_id, &mut mir);
677 // Change the generator argument from &mut to *mut
678 mir.local_decls[self_arg()] = LocalDecl {
679 mutability: Mutability::Mut,
680 ty: tcx.mk_ptr(ty::TypeAndMut {
682 mutbl: hir::Mutability::MutMutable,
684 user_ty: UserTypeProjections::none(),
687 visibility_scope: source_info.scope,
690 is_user_variable: None,
692 if tcx.sess.opts.debugging_opts.mir_emit_retag {
693 // Alias tracking must know we changed the type
694 mir.basic_blocks_mut()[START_BLOCK].statements.insert(0, Statement {
696 kind: StatementKind::Retag(RetagKind::Raw, Place::Local(self_arg())),
700 no_landing_pads(tcx, &mut mir);
702 // Make sure we remove dead blocks to remove
703 // unrelated code from the resume part of the function
704 simplify::remove_dead_blocks(&mut mir);
706 dump_mir(tcx, None, "generator_drop", &0, source, &mut mir, |_, _| Ok(()) );
711 fn insert_term_block<'tcx>(mir: &mut Mir<'tcx>, kind: TerminatorKind<'tcx>) -> BasicBlock {
712 let term_block = BasicBlock::new(mir.basic_blocks().len());
713 let source_info = source_info(mir);
714 mir.basic_blocks_mut().push(BasicBlockData {
715 statements: Vec::new(),
716 terminator: Some(Terminator {
725 fn insert_panic_block<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
727 message: AssertMessage<'tcx>) -> BasicBlock {
728 let assert_block = BasicBlock::new(mir.basic_blocks().len());
729 let term = TerminatorKind::Assert {
730 cond: Operand::Constant(box Constant {
734 literal: tcx.mk_lazy_const(ty::LazyConst::Evaluated(
735 ty::Const::from_bool(tcx, false),
740 target: assert_block,
744 let source_info = source_info(mir);
745 mir.basic_blocks_mut().push(BasicBlockData {
746 statements: Vec::new(),
747 terminator: Some(Terminator {
757 fn create_generator_resume_function<'a, 'tcx>(
758 tcx: TyCtxt<'a, 'tcx, 'tcx>,
759 transform: TransformVisitor<'a, 'tcx>,
761 source: MirSource<'tcx>,
762 mir: &mut Mir<'tcx>) {
763 // Poison the generator when it unwinds
764 for block in mir.basic_blocks_mut() {
765 let source_info = block.terminator().source_info;
766 if let &TerminatorKind::Resume = &block.terminator().kind {
767 block.statements.push(transform.set_state(1, source_info));
771 let mut cases = create_cases(mir, &transform, |point| Some(point.resume));
773 use rustc::mir::interpret::EvalErrorKind::{
774 GeneratorResumedAfterPanic,
775 GeneratorResumedAfterReturn,
778 // Jump to the entry point on the 0 state
779 cases.insert(0, (0, BasicBlock::new(0)));
780 // Panic when resumed on the returned (1) state
781 cases.insert(1, (1, insert_panic_block(tcx, mir, GeneratorResumedAfterReturn)));
782 // Panic when resumed on the poisoned (2) state
783 cases.insert(2, (2, insert_panic_block(tcx, mir, GeneratorResumedAfterPanic)));
785 insert_switch(tcx, mir, cases, &transform, TerminatorKind::Unreachable);
787 make_generator_state_argument_indirect(tcx, def_id, mir);
788 make_generator_state_argument_pinned(tcx, mir);
790 no_landing_pads(tcx, mir);
792 // Make sure we remove dead blocks to remove
793 // unrelated code from the drop part of the function
794 simplify::remove_dead_blocks(mir);
796 dump_mir(tcx, None, "generator_resume", &0, source, mir, |_, _| Ok(()) );
799 fn source_info<'a, 'tcx>(mir: &Mir<'tcx>) -> SourceInfo {
802 scope: OUTERMOST_SOURCE_SCOPE,
806 fn insert_clean_drop<'a, 'tcx>(mir: &mut Mir<'tcx>) -> BasicBlock {
807 let return_block = insert_term_block(mir, TerminatorKind::Return);
809 // Create a block to destroy an unresumed generators. This can only destroy upvars.
810 let drop_clean = BasicBlock::new(mir.basic_blocks().len());
811 let term = TerminatorKind::Drop {
812 location: Place::Local(self_arg()),
813 target: return_block,
816 let source_info = source_info(mir);
817 mir.basic_blocks_mut().push(BasicBlockData {
818 statements: Vec::new(),
819 terminator: Some(Terminator {
829 fn create_cases<'a, 'tcx, F>(mir: &mut Mir<'tcx>,
830 transform: &TransformVisitor<'a, 'tcx>,
831 target: F) -> Vec<(u32, BasicBlock)>
832 where F: Fn(&SuspensionPoint) -> Option<BasicBlock> {
833 let source_info = source_info(mir);
835 transform.suspension_points.iter().filter_map(|point| {
836 // Find the target for this suspension point, if applicable
837 target(point).map(|target| {
838 let block = BasicBlock::new(mir.basic_blocks().len());
839 let mut statements = Vec::new();
841 // Create StorageLive instructions for locals with live storage
842 for i in 0..(mir.local_decls.len()) {
843 let l = Local::new(i);
844 if point.storage_liveness.contains(l) && !transform.remap.contains_key(&l) {
845 statements.push(Statement {
847 kind: StatementKind::StorageLive(l),
852 // Then jump to the real target
853 mir.basic_blocks_mut().push(BasicBlockData {
855 terminator: Some(Terminator {
857 kind: TerminatorKind::Goto {
869 impl MirPass for StateTransform {
870 fn run_pass<'a, 'tcx>(&self,
871 tcx: TyCtxt<'a, 'tcx, 'tcx>,
872 source: MirSource<'tcx>,
873 mir: &mut Mir<'tcx>) {
874 let yield_ty = if let Some(yield_ty) = mir.yield_ty {
877 // This only applies to generators
881 assert!(mir.generator_drop.is_none());
883 let def_id = source.def_id();
885 // The first argument is the generator type passed by value
886 let gen_ty = mir.local_decls.raw[1].ty;
888 // Get the interior types and substs which typeck computed
889 let (upvars, interior, movable) = match gen_ty.sty {
890 ty::Generator(_, substs, movability) => {
891 (substs.upvar_tys(def_id, tcx).collect(),
892 substs.witness(def_id, tcx),
893 movability == hir::GeneratorMovability::Movable)
898 // Compute GeneratorState<yield_ty, return_ty>
899 let state_did = tcx.lang_items().gen_state().unwrap();
900 let state_adt_ref = tcx.adt_def(state_did);
901 let state_substs = tcx.intern_substs(&[
903 mir.return_ty().into(),
905 let ret_ty = tcx.mk_adt(state_adt_ref, state_substs);
907 // We rename RETURN_PLACE which has type mir.return_ty to new_ret_local
908 // RETURN_PLACE then is a fresh unused local with type ret_ty.
909 let new_ret_local = replace_result_variable(ret_ty, mir);
911 // Extract locals which are live across suspension point into `layout`
912 // `remap` gives a mapping from local indices onto generator struct indices
913 // `storage_liveness` tells us which locals have live storage at suspension points
914 let (remap, layout, storage_liveness) = compute_layout(
922 let state_field = mir.upvar_decls.len();
924 // Run the transformation which converts Places from Local to generator struct
925 // accesses for locals in `remap`.
926 // It also rewrites `return x` and `yield y` as writing a new generator state and returning
927 // GeneratorState::Complete(x) and GeneratorState::Yielded(y) respectively.
928 let mut transform = TransformVisitor {
934 suspension_points: Vec::new(),
938 transform.visit_mir(mir);
940 // Update our MIR struct to reflect the changed we've made
943 mir.spread_arg = None;
944 mir.generator_layout = Some(layout);
946 // Insert `drop(generator_struct)` which is used to drop upvars for generators in
947 // the unresumed (0) state.
948 // This is expanded to a drop ladder in `elaborate_generator_drops`.
949 let drop_clean = insert_clean_drop(mir);
951 dump_mir(tcx, None, "generator_pre-elab", &0, source, mir, |_, _| Ok(()) );
953 // Expand `drop(generator_struct)` to a drop ladder which destroys upvars.
954 // If any upvars are moved out of, drop elaboration will handle upvar destruction.
955 // However we need to also elaborate the code generated by `insert_clean_drop`.
956 elaborate_generator_drops(tcx, def_id, mir);
958 dump_mir(tcx, None, "generator_post-transform", &0, source, mir, |_, _| Ok(()) );
960 // Create a copy of our MIR and use it to create the drop shim for the generator
961 let drop_shim = create_generator_drop_shim(tcx,
969 mir.generator_drop = Some(box drop_shim);
971 // Create the Generator::resume function
972 create_generator_resume_function(tcx, transform, def_id, source, mir);