//! This is the implementation of the pass which transforms generators into state machines. //! //! MIR generation for generators creates a function which has a self argument which //! passes by value. This argument is effectively a generator type which only contains upvars and //! is only used for this argument inside the MIR for the generator. //! It is passed by value to enable upvars to be moved out of it. Drop elaboration runs on that //! MIR before this pass and creates drop flags for MIR locals. //! It will also drop the generator argument (which only consists of upvars) if any of the upvars //! are moved out of. This pass elaborates the drops of upvars / generator argument in the case //! that none of the upvars were moved out of. This is because we cannot have any drops of this //! generator in the MIR, since it is used to create the drop glue for the generator. We'd get //! infinite recursion otherwise. //! //! This pass creates the implementation for the Generator::resume function and the drop shim //! for the generator based on the MIR input. It converts the generator argument from Self to //! &mut Self adding derefs in the MIR as needed. It computes the final layout of the generator //! struct which looks like this: //! First upvars are stored //! It is followed by the generator state field. //! Then finally the MIR locals which are live across a suspension point are stored. //! //! struct Generator { //! upvars..., //! state: u32, //! mir_locals..., //! } //! //! This pass computes the meaning of the state field and the MIR locals which are live //! across a suspension point. There are however two hardcoded generator states: //! 0 - Generator have not been resumed yet //! 1 - Generator has returned / is completed //! 2 - Generator has been poisoned //! //! It also rewrites `return x` and `yield y` as setting a new generator state and returning //! GeneratorState::Complete(x) and GeneratorState::Yielded(y) respectively. //! MIR locals which are live across a suspension point are moved to the generator struct //! with references to them being updated with references to the generator struct. //! //! The pass creates two functions which have a switch on the generator state giving //! the action to take. //! //! One of them is the implementation of Generator::resume. //! For generators with state 0 (unresumed) it starts the execution of the generator. //! For generators with state 1 (returned) and state 2 (poisoned) it panics. //! Otherwise it continues the execution from the last suspension point. //! //! The other function is the drop glue for the generator. //! For generators with state 0 (unresumed) it drops the upvars of the generator. //! For generators with state 1 (returned) and state 2 (poisoned) it does nothing. //! Otherwise it drops all the values in scope at the last suspension point. use rustc::hir; use rustc::hir::def_id::DefId; use rustc::mir::*; use rustc::mir::visit::{PlaceContext, Visitor, MutVisitor}; use rustc::ty::{self, TyCtxt, AdtDef, Ty}; use rustc::ty::layout::VariantIdx; use rustc::ty::subst::Substs; use util::dump_mir; use util::liveness::{self, IdentityMap}; use rustc_data_structures::fx::FxHashMap; use rustc_data_structures::indexed_vec::Idx; use rustc_data_structures::bit_set::BitSet; use std::borrow::Cow; use std::iter::once; use std::mem; use transform::{MirPass, MirSource}; use transform::simplify; use transform::no_landing_pads::no_landing_pads; use dataflow::{do_dataflow, DebugFormatted, state_for_location}; use dataflow::{MaybeStorageLive, HaveBeenBorrowedLocals}; pub struct StateTransform; struct RenameLocalVisitor { from: Local, to: Local, } impl<'tcx> MutVisitor<'tcx> for RenameLocalVisitor { fn visit_local(&mut self, local: &mut Local, _: PlaceContext<'tcx>, _: Location) { if *local == self.from { *local = self.to; } } } struct DerefArgVisitor; impl<'tcx> MutVisitor<'tcx> for DerefArgVisitor { fn visit_local(&mut self, local: &mut Local, _: PlaceContext<'tcx>, _: Location) { assert_ne!(*local, self_arg()); } fn visit_place(&mut self, place: &mut Place<'tcx>, context: PlaceContext<'tcx>, location: Location) { if *place == Place::Local(self_arg()) { *place = Place::Projection(Box::new(Projection { base: place.clone(), elem: ProjectionElem::Deref, })); } else { self.super_place(place, context, location); } } } fn self_arg() -> Local { Local::new(1) } struct SuspensionPoint { state: u32, resume: BasicBlock, drop: Option, storage_liveness: liveness::LiveVarSet, } struct TransformVisitor<'a, 'tcx: 'a> { tcx: TyCtxt<'a, 'tcx, 'tcx>, state_adt_ref: &'tcx AdtDef, state_substs: &'tcx Substs<'tcx>, // The index of the generator state in the generator struct state_field: usize, // Mapping from Local to (type of local, generator struct index) // FIXME(eddyb) This should use `IndexVec>`. remap: FxHashMap, usize)>, // A map from a suspension point in a block to the locals which have live storage at that point // FIXME(eddyb) This should use `IndexVec>`. storage_liveness: FxHashMap>, // A list of suspension points, generated during the transform suspension_points: Vec, // The original RETURN_PLACE local new_ret_local: Local, } impl<'a, 'tcx> TransformVisitor<'a, 'tcx> { // Make a GeneratorState rvalue fn make_state(&self, idx: VariantIdx, val: Operand<'tcx>) -> Rvalue<'tcx> { let adt = AggregateKind::Adt(self.state_adt_ref, idx, self.state_substs, None, None); Rvalue::Aggregate(box adt, vec![val]) } // Create a Place referencing a generator struct field fn make_field(&self, idx: usize, ty: Ty<'tcx>) -> Place<'tcx> { let base = Place::Local(self_arg()); let field = Projection { base: base, elem: ProjectionElem::Field(Field::new(idx), ty), }; Place::Projection(Box::new(field)) } // Create a statement which changes the generator state fn set_state(&self, state_disc: u32, source_info: SourceInfo) -> Statement<'tcx> { let state = self.make_field(self.state_field, self.tcx.types.u32); let val = Operand::Constant(box Constant { span: source_info.span, ty: self.tcx.types.u32, user_ty: None, literal: self.tcx.intern_lazy_const(ty::LazyConst::Evaluated(ty::Const::from_bits( self.tcx, state_disc.into(), ty::ParamEnv::empty().and(self.tcx.types.u32) ))), }); Statement { source_info, kind: StatementKind::Assign(state, box Rvalue::Use(val)), } } } impl<'a, 'tcx> MutVisitor<'tcx> for TransformVisitor<'a, 'tcx> { fn visit_local(&mut self, local: &mut Local, _: PlaceContext<'tcx>, _: Location) { assert_eq!(self.remap.get(local), None); } fn visit_place(&mut self, place: &mut Place<'tcx>, context: PlaceContext<'tcx>, location: Location) { if let Place::Local(l) = *place { // Replace an Local in the remap with a generator struct access if let Some(&(ty, idx)) = self.remap.get(&l) { *place = self.make_field(idx, ty); } } else { self.super_place(place, context, location); } } fn visit_basic_block_data(&mut self, block: BasicBlock, data: &mut BasicBlockData<'tcx>) { // Remove StorageLive and StorageDead statements for remapped locals data.retain_statements(|s| { match s.kind { StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => { !self.remap.contains_key(&l) } _ => true } }); let ret_val = match data.terminator().kind { TerminatorKind::Return => Some((VariantIdx::new(1), None, Operand::Move(Place::Local(self.new_ret_local)), None)), TerminatorKind::Yield { ref value, resume, drop } => Some((VariantIdx::new(0), Some(resume), value.clone(), drop)), _ => None }; if let Some((state_idx, resume, v, drop)) = ret_val { let source_info = data.terminator().source_info; // We must assign the value first in case it gets declared dead below data.statements.push(Statement { source_info, kind: StatementKind::Assign(Place::Local(RETURN_PLACE), box self.make_state(state_idx, v)), }); let state = if let Some(resume) = resume { // Yield let state = 3 + self.suspension_points.len() as u32; self.suspension_points.push(SuspensionPoint { state, resume, drop, storage_liveness: self.storage_liveness.get(&block).unwrap().clone(), }); state } else { // Return 1 // state for returned }; data.statements.push(self.set_state(state, source_info)); data.terminator.as_mut().unwrap().kind = TerminatorKind::Return; } self.super_basic_block_data(block, data); } } fn make_generator_state_argument_indirect<'a, 'tcx>( tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId, mir: &mut Mir<'tcx>) { let gen_ty = mir.local_decls.raw[1].ty; let region = ty::ReFree(ty::FreeRegion { scope: def_id, bound_region: ty::BoundRegion::BrEnv, }); let region = tcx.mk_region(region); let ref_gen_ty = tcx.mk_ref(region, ty::TypeAndMut { ty: gen_ty, mutbl: hir::MutMutable }); // Replace the by value generator argument mir.local_decls.raw[1].ty = ref_gen_ty; // Add a deref to accesses of the generator state DerefArgVisitor.visit_mir(mir); } fn replace_result_variable<'tcx>( ret_ty: Ty<'tcx>, mir: &mut Mir<'tcx>, ) -> Local { let source_info = source_info(mir); let new_ret = LocalDecl { mutability: Mutability::Mut, ty: ret_ty, user_ty: UserTypeProjections::none(), name: None, source_info, visibility_scope: source_info.scope, internal: false, is_block_tail: None, is_user_variable: None, }; let new_ret_local = Local::new(mir.local_decls.len()); mir.local_decls.push(new_ret); mir.local_decls.swap(RETURN_PLACE, new_ret_local); RenameLocalVisitor { from: RETURN_PLACE, to: new_ret_local, }.visit_mir(mir); new_ret_local } struct StorageIgnored(liveness::LiveVarSet); impl<'tcx> Visitor<'tcx> for StorageIgnored { fn visit_statement(&mut self, _block: BasicBlock, statement: &Statement<'tcx>, _location: Location) { match statement.kind { StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => { self.0.remove(l); } _ => (), } } } fn locals_live_across_suspend_points( tcx: TyCtxt<'a, 'tcx, 'tcx>, mir: &Mir<'tcx>, source: MirSource, movable: bool, ) -> ( liveness::LiveVarSet, FxHashMap>, ) { let dead_unwinds = BitSet::new_empty(mir.basic_blocks().len()); let node_id = tcx.hir().as_local_node_id(source.def_id).unwrap(); // Calculate when MIR locals have live storage. This gives us an upper bound of their // lifetimes. let storage_live_analysis = MaybeStorageLive::new(mir); let storage_live = do_dataflow(tcx, mir, node_id, &[], &dead_unwinds, storage_live_analysis, |bd, p| DebugFormatted::new(&bd.mir().local_decls[p])); // Find the MIR locals which do not use StorageLive/StorageDead statements. // The storage of these locals are always live. let mut ignored = StorageIgnored(BitSet::new_filled(mir.local_decls.len())); ignored.visit_mir(mir); // Calculate the MIR locals which have been previously // borrowed (even if they are still active). // This is only used for immovable generators. let borrowed_locals = if !movable { let analysis = HaveBeenBorrowedLocals::new(mir); let result = do_dataflow(tcx, mir, node_id, &[], &dead_unwinds, analysis, |bd, p| DebugFormatted::new(&bd.mir().local_decls[p])); Some((analysis, result)) } else { None }; // Calculate the liveness of MIR locals ignoring borrows. let mut set = liveness::LiveVarSet::new_empty(mir.local_decls.len()); let mut liveness = liveness::liveness_of_locals( mir, &IdentityMap::new(mir), ); liveness::dump_mir( tcx, "generator_liveness", source, mir, &IdentityMap::new(mir), &liveness, ); let mut storage_liveness_map = FxHashMap::default(); for (block, data) in mir.basic_blocks().iter_enumerated() { if let TerminatorKind::Yield { .. } = data.terminator().kind { let loc = Location { block: block, statement_index: data.statements.len(), }; if let Some((ref analysis, ref result)) = borrowed_locals { let borrowed_locals = state_for_location(loc, analysis, result, mir); // The `liveness` variable contains the liveness of MIR locals ignoring borrows. // This is correct for movable generators since borrows cannot live across // suspension points. However for immovable generators we need to account for // borrows, so we conseratively assume that all borrowed locals are live until // we find a StorageDead statement referencing the locals. // To do this we just union our `liveness` result with `borrowed_locals`, which // contains all the locals which has been borrowed before this suspension point. // If a borrow is converted to a raw reference, we must also assume that it lives // forever. Note that the final liveness is still bounded by the storage liveness // of the local, which happens using the `intersect` operation below. liveness.outs[block].union(&borrowed_locals); } let mut storage_liveness = state_for_location(loc, &storage_live_analysis, &storage_live, mir); // Store the storage liveness for later use so we can restore the state // after a suspension point storage_liveness_map.insert(block, storage_liveness.clone()); // Mark locals without storage statements as always having live storage storage_liveness.union(&ignored.0); // Locals live are live at this point only if they are used across // suspension points (the `liveness` variable) // and their storage is live (the `storage_liveness` variable) storage_liveness.intersect(&liveness.outs[block]); let live_locals = storage_liveness; // Add the locals life at this suspension point to the set of locals which live across // any suspension points set.union(&live_locals); } } // The generator argument is ignored set.remove(self_arg()); (set, storage_liveness_map) } fn compute_layout<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, source: MirSource, upvars: Vec>, interior: Ty<'tcx>, movable: bool, mir: &mut Mir<'tcx>) -> (FxHashMap, usize)>, GeneratorLayout<'tcx>, FxHashMap>) { // Use a liveness analysis to compute locals which are live across a suspension point let (live_locals, storage_liveness) = locals_live_across_suspend_points(tcx, mir, source, movable); // Erase regions from the types passed in from typeck so we can compare them with // MIR types let allowed_upvars = tcx.erase_regions(&upvars); let allowed = match interior.sty { ty::GeneratorWitness(s) => tcx.erase_late_bound_regions(&s), _ => bug!(), }; for (local, decl) in mir.local_decls.iter_enumerated() { // Ignore locals which are internal or not live if !live_locals.contains(local) || decl.internal { continue; } // Sanity check that typeck knows about the type of locals which are // live across a suspension point if !allowed.contains(&decl.ty) && !allowed_upvars.contains(&decl.ty) { span_bug!(mir.span, "Broken MIR: generator contains type {} in MIR, \ but typeck only knows about {}", decl.ty, interior); } } let upvar_len = mir.upvar_decls.len(); let dummy_local = LocalDecl::new_internal(tcx.mk_unit(), mir.span); // Gather live locals and their indices replacing values in mir.local_decls with a dummy // to avoid changing local indices let live_decls = live_locals.iter().map(|local| { let var = mem::replace(&mut mir.local_decls[local], dummy_local.clone()); (local, var) }); // Create a map from local indices to generator struct indices. // These are offset by (upvar_len + 1) because of fields which comes before locals. // We also create a vector of the LocalDecls of these locals. let (remap, vars) = live_decls.enumerate().map(|(idx, (local, var))| { ((local, (var.ty, upvar_len + 1 + idx)), var) }).unzip(); let layout = GeneratorLayout { fields: vars }; (remap, layout, storage_liveness) } fn insert_switch<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, mir: &mut Mir<'tcx>, cases: Vec<(u32, BasicBlock)>, transform: &TransformVisitor<'a, 'tcx>, default: TerminatorKind<'tcx>) { let default_block = insert_term_block(mir, default); let switch = TerminatorKind::SwitchInt { discr: Operand::Copy(transform.make_field(transform.state_field, tcx.types.u32)), switch_ty: tcx.types.u32, values: Cow::from(cases.iter().map(|&(i, _)| i.into()).collect::>()), targets: cases.iter().map(|&(_, d)| d).chain(once(default_block)).collect(), }; let source_info = source_info(mir); mir.basic_blocks_mut().raw.insert(0, BasicBlockData { statements: Vec::new(), terminator: Some(Terminator { source_info, kind: switch, }), is_cleanup: false, }); let blocks = mir.basic_blocks_mut().iter_mut(); for target in blocks.flat_map(|b| b.terminator_mut().successors_mut()) { *target = BasicBlock::new(target.index() + 1); } } fn elaborate_generator_drops<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId, mir: &mut Mir<'tcx>) { use util::elaborate_drops::{elaborate_drop, Unwind}; use util::patch::MirPatch; use shim::DropShimElaborator; // Note that `elaborate_drops` only drops the upvars of a generator, and // this is ok because `open_drop` can only be reached within that own // generator's resume function. let param_env = tcx.param_env(def_id); let gen = self_arg(); for block in mir.basic_blocks().indices() { let (target, unwind, source_info) = match mir.basic_blocks()[block].terminator() { &Terminator { source_info, kind: TerminatorKind::Drop { location: Place::Local(local), target, unwind } } if local == gen => (target, unwind, source_info), _ => continue, }; let unwind = if let Some(unwind) = unwind { Unwind::To(unwind) } else { Unwind::InCleanup }; let patch = { let mut elaborator = DropShimElaborator { mir: &mir, patch: MirPatch::new(mir), tcx, param_env }; elaborate_drop( &mut elaborator, source_info, &Place::Local(gen), (), target, unwind, block ); elaborator.patch }; patch.apply(mir); } } fn create_generator_drop_shim<'a, 'tcx>( tcx: TyCtxt<'a, 'tcx, 'tcx>, transform: &TransformVisitor<'a, 'tcx>, def_id: DefId, source: MirSource, gen_ty: Ty<'tcx>, mir: &Mir<'tcx>, drop_clean: BasicBlock) -> Mir<'tcx> { let mut mir = mir.clone(); let source_info = source_info(&mir); let mut cases = create_cases(&mut mir, transform, |point| point.drop); cases.insert(0, (0, drop_clean)); // The returned state (1) and the poisoned state (2) falls through to // the default case which is just to return insert_switch(tcx, &mut mir, cases, &transform, TerminatorKind::Return); for block in mir.basic_blocks_mut() { let kind = &mut block.terminator_mut().kind; if let TerminatorKind::GeneratorDrop = *kind { *kind = TerminatorKind::Return; } } // Replace the return variable mir.local_decls[RETURN_PLACE] = LocalDecl { mutability: Mutability::Mut, ty: tcx.mk_unit(), user_ty: UserTypeProjections::none(), name: None, source_info, visibility_scope: source_info.scope, internal: false, is_block_tail: None, is_user_variable: None, }; make_generator_state_argument_indirect(tcx, def_id, &mut mir); // Change the generator argument from &mut to *mut mir.local_decls[self_arg()] = LocalDecl { mutability: Mutability::Mut, ty: tcx.mk_ptr(ty::TypeAndMut { ty: gen_ty, mutbl: hir::Mutability::MutMutable, }), user_ty: UserTypeProjections::none(), name: None, source_info, visibility_scope: source_info.scope, internal: false, is_block_tail: None, is_user_variable: None, }; if tcx.sess.opts.debugging_opts.mir_emit_retag { // Alias tracking must know we changed the type mir.basic_blocks_mut()[START_BLOCK].statements.insert(0, Statement { source_info, kind: StatementKind::Retag(RetagKind::Raw, Place::Local(self_arg())), }) } no_landing_pads(tcx, &mut mir); // Make sure we remove dead blocks to remove // unrelated code from the resume part of the function simplify::remove_dead_blocks(&mut mir); dump_mir(tcx, None, "generator_drop", &0, source, &mut mir, |_, _| Ok(()) ); mir } fn insert_term_block<'tcx>(mir: &mut Mir<'tcx>, kind: TerminatorKind<'tcx>) -> BasicBlock { let term_block = BasicBlock::new(mir.basic_blocks().len()); let source_info = source_info(mir); mir.basic_blocks_mut().push(BasicBlockData { statements: Vec::new(), terminator: Some(Terminator { source_info, kind, }), is_cleanup: false, }); term_block } fn insert_panic_block<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, mir: &mut Mir<'tcx>, message: AssertMessage<'tcx>) -> BasicBlock { let assert_block = BasicBlock::new(mir.basic_blocks().len()); let term = TerminatorKind::Assert { cond: Operand::Constant(box Constant { span: mir.span, ty: tcx.types.bool, user_ty: None, literal: tcx.intern_lazy_const(ty::LazyConst::Evaluated( ty::Const::from_bool(tcx, false), )), }), expected: true, msg: message, target: assert_block, cleanup: None, }; let source_info = source_info(mir); mir.basic_blocks_mut().push(BasicBlockData { statements: Vec::new(), terminator: Some(Terminator { source_info, kind: term, }), is_cleanup: false, }); assert_block } fn create_generator_resume_function<'a, 'tcx>( tcx: TyCtxt<'a, 'tcx, 'tcx>, transform: TransformVisitor<'a, 'tcx>, def_id: DefId, source: MirSource, mir: &mut Mir<'tcx>) { // Poison the generator when it unwinds for block in mir.basic_blocks_mut() { let source_info = block.terminator().source_info; if let &TerminatorKind::Resume = &block.terminator().kind { block.statements.push(transform.set_state(1, source_info)); } } let mut cases = create_cases(mir, &transform, |point| Some(point.resume)); use rustc::mir::interpret::EvalErrorKind::{ GeneratorResumedAfterPanic, GeneratorResumedAfterReturn, }; // Jump to the entry point on the 0 state cases.insert(0, (0, BasicBlock::new(0))); // Panic when resumed on the returned (1) state cases.insert(1, (1, insert_panic_block(tcx, mir, GeneratorResumedAfterReturn))); // Panic when resumed on the poisoned (2) state cases.insert(2, (2, insert_panic_block(tcx, mir, GeneratorResumedAfterPanic))); insert_switch(tcx, mir, cases, &transform, TerminatorKind::Unreachable); make_generator_state_argument_indirect(tcx, def_id, mir); no_landing_pads(tcx, mir); // Make sure we remove dead blocks to remove // unrelated code from the drop part of the function simplify::remove_dead_blocks(mir); dump_mir(tcx, None, "generator_resume", &0, source, mir, |_, _| Ok(()) ); } fn source_info<'a, 'tcx>(mir: &Mir<'tcx>) -> SourceInfo { SourceInfo { span: mir.span, scope: OUTERMOST_SOURCE_SCOPE, } } fn insert_clean_drop<'a, 'tcx>(mir: &mut Mir<'tcx>) -> BasicBlock { let return_block = insert_term_block(mir, TerminatorKind::Return); // Create a block to destroy an unresumed generators. This can only destroy upvars. let drop_clean = BasicBlock::new(mir.basic_blocks().len()); let term = TerminatorKind::Drop { location: Place::Local(self_arg()), target: return_block, unwind: None, }; let source_info = source_info(mir); mir.basic_blocks_mut().push(BasicBlockData { statements: Vec::new(), terminator: Some(Terminator { source_info, kind: term, }), is_cleanup: false, }); drop_clean } fn create_cases<'a, 'tcx, F>(mir: &mut Mir<'tcx>, transform: &TransformVisitor<'a, 'tcx>, target: F) -> Vec<(u32, BasicBlock)> where F: Fn(&SuspensionPoint) -> Option { let source_info = source_info(mir); transform.suspension_points.iter().filter_map(|point| { // Find the target for this suspension point, if applicable target(point).map(|target| { let block = BasicBlock::new(mir.basic_blocks().len()); let mut statements = Vec::new(); // Create StorageLive instructions for locals with live storage for i in 0..(mir.local_decls.len()) { let l = Local::new(i); if point.storage_liveness.contains(l) && !transform.remap.contains_key(&l) { statements.push(Statement { source_info, kind: StatementKind::StorageLive(l), }); } } // Then jump to the real target mir.basic_blocks_mut().push(BasicBlockData { statements, terminator: Some(Terminator { source_info, kind: TerminatorKind::Goto { target, }, }), is_cleanup: false, }); (point.state, block) }) }).collect() } impl MirPass for StateTransform { fn run_pass<'a, 'tcx>(&self, tcx: TyCtxt<'a, 'tcx, 'tcx>, source: MirSource, mir: &mut Mir<'tcx>) { let yield_ty = if let Some(yield_ty) = mir.yield_ty { yield_ty } else { // This only applies to generators return }; assert!(mir.generator_drop.is_none()); let def_id = source.def_id; // The first argument is the generator type passed by value let gen_ty = mir.local_decls.raw[1].ty; // Get the interior types and substs which typeck computed let (upvars, interior, movable) = match gen_ty.sty { ty::Generator(_, substs, movability) => { (substs.upvar_tys(def_id, tcx).collect(), substs.witness(def_id, tcx), movability == hir::GeneratorMovability::Movable) } _ => bug!(), }; // Compute GeneratorState let state_did = tcx.lang_items().gen_state().unwrap(); let state_adt_ref = tcx.adt_def(state_did); let state_substs = tcx.intern_substs(&[ yield_ty.into(), mir.return_ty().into(), ]); let ret_ty = tcx.mk_adt(state_adt_ref, state_substs); // We rename RETURN_PLACE which has type mir.return_ty to new_ret_local // RETURN_PLACE then is a fresh unused local with type ret_ty. let new_ret_local = replace_result_variable(ret_ty, mir); // Extract locals which are live across suspension point into `layout` // `remap` gives a mapping from local indices onto generator struct indices // `storage_liveness` tells us which locals have live storage at suspension points let (remap, layout, storage_liveness) = compute_layout( tcx, source, upvars, interior, movable, mir); let state_field = mir.upvar_decls.len(); // Run the transformation which converts Places from Local to generator struct // accesses for locals in `remap`. // It also rewrites `return x` and `yield y` as writing a new generator state and returning // GeneratorState::Complete(x) and GeneratorState::Yielded(y) respectively. let mut transform = TransformVisitor { tcx, state_adt_ref, state_substs, remap, storage_liveness, suspension_points: Vec::new(), new_ret_local, state_field, }; transform.visit_mir(mir); // Update our MIR struct to reflect the changed we've made mir.yield_ty = None; mir.arg_count = 1; mir.spread_arg = None; mir.generator_layout = Some(layout); // Insert `drop(generator_struct)` which is used to drop upvars for generators in // the unresumed (0) state. // This is expanded to a drop ladder in `elaborate_generator_drops`. let drop_clean = insert_clean_drop(mir); dump_mir(tcx, None, "generator_pre-elab", &0, source, mir, |_, _| Ok(()) ); // Expand `drop(generator_struct)` to a drop ladder which destroys upvars. // If any upvars are moved out of, drop elaboration will handle upvar destruction. // However we need to also elaborate the code generated by `insert_clean_drop`. elaborate_generator_drops(tcx, def_id, mir); dump_mir(tcx, None, "generator_post-transform", &0, source, mir, |_, _| Ok(()) ); // Create a copy of our MIR and use it to create the drop shim for the generator let drop_shim = create_generator_drop_shim(tcx, &transform, def_id, source, gen_ty, &mir, drop_clean); mir.generator_drop = Some(box drop_shim); // Create the Generator::resume function create_generator_resume_function(tcx, transform, def_id, source, mir); } }