//! 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 either the `Generator::resume` or `Future::poll` //! 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. //! ```ignore (illustrative) //! 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 three 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)`, //! or `Poll::Ready(x)` and `Poll::Pending` 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` / `Future::poll`. //! 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 crate::deref_separator::deref_finder; use crate::simplify; use crate::util::expand_aggregate; use crate::MirPass; use rustc_data_structures::fx::FxHashMap; use rustc_hir as hir; use rustc_hir::lang_items::LangItem; use rustc_hir::GeneratorKind; use rustc_index::bit_set::{BitMatrix, BitSet, GrowableBitSet}; use rustc_index::vec::{Idx, IndexVec}; use rustc_middle::mir::dump_mir; use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor}; use rustc_middle::mir::*; use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt}; use rustc_middle::ty::{GeneratorSubsts, SubstsRef}; use rustc_mir_dataflow::impls::{ MaybeBorrowedLocals, MaybeLiveLocals, MaybeRequiresStorage, MaybeStorageLive, }; use rustc_mir_dataflow::storage::always_storage_live_locals; use rustc_mir_dataflow::{self, Analysis}; use rustc_target::abi::VariantIdx; use rustc_target::spec::PanicStrategy; use std::{iter, ops}; pub struct StateTransform; struct RenameLocalVisitor<'tcx> { from: Local, to: Local, tcx: TyCtxt<'tcx>, } impl<'tcx> MutVisitor<'tcx> for RenameLocalVisitor<'tcx> { fn tcx(&self) -> TyCtxt<'tcx> { self.tcx } fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) { if *local == self.from { *local = self.to; } } fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) { match terminator.kind { TerminatorKind::Return => { // Do not replace the implicit `_0` access here, as that's not possible. The // transform already handles `return` correctly. } _ => self.super_terminator(terminator, location), } } } struct DerefArgVisitor<'tcx> { tcx: TyCtxt<'tcx>, } impl<'tcx> MutVisitor<'tcx> for DerefArgVisitor<'tcx> { fn tcx(&self) -> TyCtxt<'tcx> { self.tcx } fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) { assert_ne!(*local, SELF_ARG); } fn visit_place(&mut self, place: &mut Place<'tcx>, context: PlaceContext, location: Location) { if place.local == SELF_ARG { replace_base( place, Place { local: SELF_ARG, projection: self.tcx().intern_place_elems(&[ProjectionElem::Deref]), }, self.tcx, ); } else { self.visit_local(&mut place.local, context, location); for elem in place.projection.iter() { if let PlaceElem::Index(local) = elem { assert_ne!(local, SELF_ARG); } } } } } struct PinArgVisitor<'tcx> { ref_gen_ty: Ty<'tcx>, tcx: TyCtxt<'tcx>, } impl<'tcx> MutVisitor<'tcx> for PinArgVisitor<'tcx> { fn tcx(&self) -> TyCtxt<'tcx> { self.tcx } fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) { assert_ne!(*local, SELF_ARG); } fn visit_place(&mut self, place: &mut Place<'tcx>, context: PlaceContext, location: Location) { if place.local == SELF_ARG { replace_base( place, Place { local: SELF_ARG, projection: self.tcx().intern_place_elems(&[ProjectionElem::Field( Field::new(0), self.ref_gen_ty, )]), }, self.tcx, ); } else { self.visit_local(&mut place.local, context, location); for elem in place.projection.iter() { if let PlaceElem::Index(local) = elem { assert_ne!(local, SELF_ARG); } } } } } fn replace_base<'tcx>(place: &mut Place<'tcx>, new_base: Place<'tcx>, tcx: TyCtxt<'tcx>) { place.local = new_base.local; let mut new_projection = new_base.projection.to_vec(); new_projection.append(&mut place.projection.to_vec()); place.projection = tcx.intern_place_elems(&new_projection); } const SELF_ARG: Local = Local::from_u32(1); /// Generator has not been resumed yet. const UNRESUMED: usize = GeneratorSubsts::UNRESUMED; /// Generator has returned / is completed. const RETURNED: usize = GeneratorSubsts::RETURNED; /// Generator has panicked and is poisoned. const POISONED: usize = GeneratorSubsts::POISONED; /// Number of variants to reserve in generator state. Corresponds to /// `UNRESUMED` (beginning of a generator) and `RETURNED`/`POISONED` /// (end of a generator) states. const RESERVED_VARIANTS: usize = 3; /// A `yield` point in the generator. struct SuspensionPoint<'tcx> { /// State discriminant used when suspending or resuming at this point. state: usize, /// The block to jump to after resumption. resume: BasicBlock, /// Where to move the resume argument after resumption. resume_arg: Place<'tcx>, /// Which block to jump to if the generator is dropped in this state. drop: Option, /// Set of locals that have live storage while at this suspension point. storage_liveness: GrowableBitSet, } struct TransformVisitor<'tcx> { tcx: TyCtxt<'tcx>, is_async_kind: bool, state_adt_ref: AdtDef<'tcx>, state_substs: SubstsRef<'tcx>, // The type of the discriminant in the generator struct discr_ty: Ty<'tcx>, // Mapping from Local to (type of local, generator struct index) // FIXME(eddyb) This should use `IndexVec>`. remap: FxHashMap, VariantIdx, usize)>, // A map from a suspension point in a block to the locals which have live storage at that point storage_liveness: IndexVec>>, // A list of suspension points, generated during the transform suspension_points: Vec>, // The set of locals that have no `StorageLive`/`StorageDead` annotations. always_live_locals: BitSet, // The original RETURN_PLACE local new_ret_local: Local, } impl<'tcx> TransformVisitor<'tcx> { // Make a `GeneratorState` or `Poll` variant assignment. // // `core::ops::GeneratorState` only has single element tuple variants, // so we can just write to the downcasted first field and then set the // discriminant to the appropriate variant. fn make_state( &self, val: Operand<'tcx>, source_info: SourceInfo, is_return: bool, statements: &mut Vec>, ) { let idx = VariantIdx::new(match (is_return, self.is_async_kind) { (true, false) => 1, // GeneratorState::Complete (false, false) => 0, // GeneratorState::Yielded (true, true) => 0, // Poll::Ready (false, true) => 1, // Poll::Pending }); let kind = AggregateKind::Adt(self.state_adt_ref.did(), idx, self.state_substs, None, None); // `Poll::Pending` if self.is_async_kind && idx == VariantIdx::new(1) { assert_eq!(self.state_adt_ref.variant(idx).fields.len(), 0); // FIXME(swatinem): assert that `val` is indeed unit? statements.extend(expand_aggregate( Place::return_place(), std::iter::empty(), kind, source_info, self.tcx, )); return; } // else: `Poll::Ready(x)`, `GeneratorState::Yielded(x)` or `GeneratorState::Complete(x)` assert_eq!(self.state_adt_ref.variant(idx).fields.len(), 1); let ty = self .tcx .bound_type_of(self.state_adt_ref.variant(idx).fields[0].did) .subst(self.tcx, self.state_substs); statements.extend(expand_aggregate( Place::return_place(), std::iter::once((val, ty)), kind, source_info, self.tcx, )); } // Create a Place referencing a generator struct field fn make_field(&self, variant_index: VariantIdx, idx: usize, ty: Ty<'tcx>) -> Place<'tcx> { let self_place = Place::from(SELF_ARG); let base = self.tcx.mk_place_downcast_unnamed(self_place, variant_index); let mut projection = base.projection.to_vec(); projection.push(ProjectionElem::Field(Field::new(idx), ty)); Place { local: base.local, projection: self.tcx.intern_place_elems(&projection) } } // Create a statement which changes the discriminant fn set_discr(&self, state_disc: VariantIdx, source_info: SourceInfo) -> Statement<'tcx> { let self_place = Place::from(SELF_ARG); Statement { source_info, kind: StatementKind::SetDiscriminant { place: Box::new(self_place), variant_index: state_disc, }, } } // Create a statement which reads the discriminant into a temporary fn get_discr(&self, body: &mut Body<'tcx>) -> (Statement<'tcx>, Place<'tcx>) { let temp_decl = LocalDecl::new(self.discr_ty, body.span).internal(); let local_decls_len = body.local_decls.push(temp_decl); let temp = Place::from(local_decls_len); let self_place = Place::from(SELF_ARG); let assign = Statement { source_info: SourceInfo::outermost(body.span), kind: StatementKind::Assign(Box::new((temp, Rvalue::Discriminant(self_place)))), }; (assign, temp) } } impl<'tcx> MutVisitor<'tcx> for TransformVisitor<'tcx> { fn tcx(&self) -> TyCtxt<'tcx> { self.tcx } fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) { assert_eq!(self.remap.get(local), None); } fn visit_place( &mut self, place: &mut Place<'tcx>, _context: PlaceContext, _location: Location, ) { // Replace an Local in the remap with a generator struct access if let Some(&(ty, variant_index, idx)) = self.remap.get(&place.local) { replace_base(place, self.make_field(variant_index, idx, ty), self.tcx); } } 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((true, None, Operand::Move(Place::from(self.new_ret_local)), None)) } TerminatorKind::Yield { ref value, resume, resume_arg, drop } => { Some((false, Some((resume, resume_arg)), value.clone(), drop)) } _ => None, }; if let Some((is_return, 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 self.make_state(v, source_info, is_return, &mut data.statements); let state = if let Some((resume, mut resume_arg)) = resume { // Yield let state = RESERVED_VARIANTS + self.suspension_points.len(); // The resume arg target location might itself be remapped if its base local is // live across a yield. let resume_arg = if let Some(&(ty, variant, idx)) = self.remap.get(&resume_arg.local) { replace_base(&mut resume_arg, self.make_field(variant, idx, ty), self.tcx); resume_arg } else { resume_arg }; self.suspension_points.push(SuspensionPoint { state, resume, resume_arg, drop, storage_liveness: self.storage_liveness[block].clone().unwrap().into(), }); VariantIdx::new(state) } else { // Return VariantIdx::new(RETURNED) // state for returned }; data.statements.push(self.set_discr(state, source_info)); data.terminator_mut().kind = TerminatorKind::Return; } self.super_basic_block_data(block, data); } } fn make_generator_state_argument_indirect<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { let gen_ty = body.local_decls.raw[1].ty; let ref_gen_ty = tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut { ty: gen_ty, mutbl: Mutability::Mut }); // Replace the by value generator argument body.local_decls.raw[1].ty = ref_gen_ty; // Add a deref to accesses of the generator state DerefArgVisitor { tcx }.visit_body(body); } fn make_generator_state_argument_pinned<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { let ref_gen_ty = body.local_decls.raw[1].ty; let pin_did = tcx.require_lang_item(LangItem::Pin, Some(body.span)); let pin_adt_ref = tcx.adt_def(pin_did); let substs = tcx.intern_substs(&[ref_gen_ty.into()]); let pin_ref_gen_ty = tcx.mk_adt(pin_adt_ref, substs); // Replace the by ref generator argument body.local_decls.raw[1].ty = pin_ref_gen_ty; // Add the Pin field access to accesses of the generator state PinArgVisitor { ref_gen_ty, tcx }.visit_body(body); } /// Allocates a new local and replaces all references of `local` with it. Returns the new local. /// /// `local` will be changed to a new local decl with type `ty`. /// /// Note that the new local will be uninitialized. It is the caller's responsibility to assign some /// valid value to it before its first use. fn replace_local<'tcx>( local: Local, ty: Ty<'tcx>, body: &mut Body<'tcx>, tcx: TyCtxt<'tcx>, ) -> Local { let new_decl = LocalDecl::new(ty, body.span); let new_local = body.local_decls.push(new_decl); body.local_decls.swap(local, new_local); RenameLocalVisitor { from: local, to: new_local, tcx }.visit_body(body); new_local } /// Transforms the `body` of the generator applying the following transforms: /// /// - Eliminates all the `get_context` calls that async lowering created. /// - Replace all `Local` `ResumeTy` types with `&mut Context<'_>` (`context_mut_ref`). /// /// The `Local`s that have their types replaced are: /// - The `resume` argument itself. /// - The argument to `get_context`. /// - The yielded value of a `yield`. /// /// The `ResumeTy` hides a `&mut Context<'_>` behind an unsafe raw pointer, and the /// `get_context` function is being used to convert that back to a `&mut Context<'_>`. /// /// Ideally the async lowering would not use the `ResumeTy`/`get_context` indirection, /// but rather directly use `&mut Context<'_>`, however that would currently /// lead to higher-kinded lifetime errors. /// See . /// /// The async lowering step and the type / lifetime inference / checking are /// still using the `ResumeTy` indirection for the time being, and that indirection /// is removed here. After this transform, the generator body only knows about `&mut Context<'_>`. fn transform_async_context<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { let context_mut_ref = tcx.mk_task_context(); // replace the type of the `resume` argument replace_resume_ty_local(tcx, body, Local::new(2), context_mut_ref); let get_context_def_id = tcx.require_lang_item(LangItem::GetContext, None); for bb in BasicBlock::new(0)..body.basic_blocks.next_index() { let bb_data = &body[bb]; if bb_data.is_cleanup { continue; } match &bb_data.terminator().kind { TerminatorKind::Call { func, .. } => { let func_ty = func.ty(body, tcx); if let ty::FnDef(def_id, _) = *func_ty.kind() { if def_id == get_context_def_id { let local = eliminate_get_context_call(&mut body[bb]); replace_resume_ty_local(tcx, body, local, context_mut_ref); } } else { continue; } } TerminatorKind::Yield { resume_arg, .. } => { replace_resume_ty_local(tcx, body, resume_arg.local, context_mut_ref); } _ => {} } } } fn eliminate_get_context_call<'tcx>(bb_data: &mut BasicBlockData<'tcx>) -> Local { let terminator = bb_data.terminator.take().unwrap(); if let TerminatorKind::Call { mut args, destination, target, .. } = terminator.kind { let arg = args.pop().unwrap(); let local = arg.place().unwrap().local; let arg = Rvalue::Use(arg); let assign = Statement { source_info: terminator.source_info, kind: StatementKind::Assign(Box::new((destination, arg))), }; bb_data.statements.push(assign); bb_data.terminator = Some(Terminator { source_info: terminator.source_info, kind: TerminatorKind::Goto { target: target.unwrap() }, }); local } else { bug!(); } } #[cfg_attr(not(debug_assertions), allow(unused))] fn replace_resume_ty_local<'tcx>( tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, local: Local, context_mut_ref: Ty<'tcx>, ) { let local_ty = std::mem::replace(&mut body.local_decls[local].ty, context_mut_ref); // We have to replace the `ResumeTy` that is used for type and borrow checking // with `&mut Context<'_>` in MIR. #[cfg(debug_assertions)] { if let ty::Adt(resume_ty_adt, _) = local_ty.kind() { let expected_adt = tcx.adt_def(tcx.require_lang_item(LangItem::ResumeTy, None)); assert_eq!(*resume_ty_adt, expected_adt); } else { panic!("expected `ResumeTy`, found `{:?}`", local_ty); }; } } struct LivenessInfo { /// Which locals are live across any suspension point. saved_locals: GeneratorSavedLocals, /// The set of saved locals live at each suspension point. live_locals_at_suspension_points: Vec>, /// Parallel vec to the above with SourceInfo for each yield terminator. source_info_at_suspension_points: Vec, /// For every saved local, the set of other saved locals that are /// storage-live at the same time as this local. We cannot overlap locals in /// the layout which have conflicting storage. storage_conflicts: BitMatrix, /// For every suspending block, the locals which are storage-live across /// that suspension point. storage_liveness: IndexVec>>, } fn locals_live_across_suspend_points<'tcx>( tcx: TyCtxt<'tcx>, body: &Body<'tcx>, always_live_locals: &BitSet, movable: bool, ) -> LivenessInfo { let body_ref: &Body<'_> = &body; // Calculate when MIR locals have live storage. This gives us an upper bound of their // lifetimes. let mut storage_live = MaybeStorageLive::new(std::borrow::Cow::Borrowed(always_live_locals)) .into_engine(tcx, body_ref) .iterate_to_fixpoint() .into_results_cursor(body_ref); // Calculate the MIR locals which have been previously // borrowed (even if they are still active). let borrowed_locals_results = MaybeBorrowedLocals.into_engine(tcx, body_ref).pass_name("generator").iterate_to_fixpoint(); let mut borrowed_locals_cursor = rustc_mir_dataflow::ResultsCursor::new(body_ref, &borrowed_locals_results); // Calculate the MIR locals that we actually need to keep storage around // for. let requires_storage_results = MaybeRequiresStorage::new(body, &borrowed_locals_results) .into_engine(tcx, body_ref) .iterate_to_fixpoint(); let mut requires_storage_cursor = rustc_mir_dataflow::ResultsCursor::new(body_ref, &requires_storage_results); // Calculate the liveness of MIR locals ignoring borrows. let mut liveness = MaybeLiveLocals .into_engine(tcx, body_ref) .pass_name("generator") .iterate_to_fixpoint() .into_results_cursor(body_ref); let mut storage_liveness_map = IndexVec::from_elem(None, &body.basic_blocks); let mut live_locals_at_suspension_points = Vec::new(); let mut source_info_at_suspension_points = Vec::new(); let mut live_locals_at_any_suspension_point = BitSet::new_empty(body.local_decls.len()); for (block, data) in body.basic_blocks.iter_enumerated() { if let TerminatorKind::Yield { .. } = data.terminator().kind { let loc = Location { block, statement_index: data.statements.len() }; liveness.seek_to_block_end(block); let mut live_locals: BitSet<_> = BitSet::new_empty(body.local_decls.len()); live_locals.union(liveness.get()); if !movable { // 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 conservatively 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. borrowed_locals_cursor.seek_before_primary_effect(loc); live_locals.union(borrowed_locals_cursor.get()); } // Store the storage liveness for later use so we can restore the state // after a suspension point storage_live.seek_before_primary_effect(loc); storage_liveness_map[block] = Some(storage_live.get().clone()); // Locals live are live at this point only if they are used across // suspension points (the `liveness` variable) // and their storage is required (the `storage_required` variable) requires_storage_cursor.seek_before_primary_effect(loc); live_locals.intersect(requires_storage_cursor.get()); // The generator argument is ignored. live_locals.remove(SELF_ARG); debug!("loc = {:?}, live_locals = {:?}", loc, live_locals); // Add the locals live at this suspension point to the set of locals which live across // any suspension points live_locals_at_any_suspension_point.union(&live_locals); live_locals_at_suspension_points.push(live_locals); source_info_at_suspension_points.push(data.terminator().source_info); } } debug!("live_locals_anywhere = {:?}", live_locals_at_any_suspension_point); let saved_locals = GeneratorSavedLocals(live_locals_at_any_suspension_point); // Renumber our liveness_map bitsets to include only the locals we are // saving. let live_locals_at_suspension_points = live_locals_at_suspension_points .iter() .map(|live_here| saved_locals.renumber_bitset(&live_here)) .collect(); let storage_conflicts = compute_storage_conflicts( body_ref, &saved_locals, always_live_locals.clone(), requires_storage_results, ); LivenessInfo { saved_locals, live_locals_at_suspension_points, source_info_at_suspension_points, storage_conflicts, storage_liveness: storage_liveness_map, } } /// The set of `Local`s that must be saved across yield points. /// /// `GeneratorSavedLocal` is indexed in terms of the elements in this set; /// i.e. `GeneratorSavedLocal::new(1)` corresponds to the second local /// included in this set. struct GeneratorSavedLocals(BitSet); impl GeneratorSavedLocals { /// Returns an iterator over each `GeneratorSavedLocal` along with the `Local` it corresponds /// to. fn iter_enumerated(&self) -> impl '_ + Iterator { self.iter().enumerate().map(|(i, l)| (GeneratorSavedLocal::from(i), l)) } /// Transforms a `BitSet` that contains only locals saved across yield points to the /// equivalent `BitSet`. fn renumber_bitset(&self, input: &BitSet) -> BitSet { assert!(self.superset(&input), "{:?} not a superset of {:?}", self.0, input); let mut out = BitSet::new_empty(self.count()); for (saved_local, local) in self.iter_enumerated() { if input.contains(local) { out.insert(saved_local); } } out } fn get(&self, local: Local) -> Option { if !self.contains(local) { return None; } let idx = self.iter().take_while(|&l| l < local).count(); Some(GeneratorSavedLocal::new(idx)) } } impl ops::Deref for GeneratorSavedLocals { type Target = BitSet; fn deref(&self) -> &Self::Target { &self.0 } } /// For every saved local, looks for which locals are StorageLive at the same /// time. Generates a bitset for every local of all the other locals that may be /// StorageLive simultaneously with that local. This is used in the layout /// computation; see `GeneratorLayout` for more. fn compute_storage_conflicts<'mir, 'tcx>( body: &'mir Body<'tcx>, saved_locals: &GeneratorSavedLocals, always_live_locals: BitSet, requires_storage: rustc_mir_dataflow::Results<'tcx, MaybeRequiresStorage<'mir, 'tcx>>, ) -> BitMatrix { assert_eq!(body.local_decls.len(), saved_locals.domain_size()); debug!("compute_storage_conflicts({:?})", body.span); debug!("always_live = {:?}", always_live_locals); // Locals that are always live or ones that need to be stored across // suspension points are not eligible for overlap. let mut ineligible_locals = always_live_locals; ineligible_locals.intersect(&**saved_locals); // Compute the storage conflicts for all eligible locals. let mut visitor = StorageConflictVisitor { body, saved_locals: &saved_locals, local_conflicts: BitMatrix::from_row_n(&ineligible_locals, body.local_decls.len()), }; requires_storage.visit_reachable_with(body, &mut visitor); let local_conflicts = visitor.local_conflicts; // Compress the matrix using only stored locals (Local -> GeneratorSavedLocal). // // NOTE: Today we store a full conflict bitset for every local. Technically // this is twice as many bits as we need, since the relation is symmetric. // However, in practice these bitsets are not usually large. The layout code // also needs to keep track of how many conflicts each local has, so it's // simpler to keep it this way for now. let mut storage_conflicts = BitMatrix::new(saved_locals.count(), saved_locals.count()); for (saved_local_a, local_a) in saved_locals.iter_enumerated() { if ineligible_locals.contains(local_a) { // Conflicts with everything. storage_conflicts.insert_all_into_row(saved_local_a); } else { // Keep overlap information only for stored locals. for (saved_local_b, local_b) in saved_locals.iter_enumerated() { if local_conflicts.contains(local_a, local_b) { storage_conflicts.insert(saved_local_a, saved_local_b); } } } } storage_conflicts } struct StorageConflictVisitor<'mir, 'tcx, 's> { body: &'mir Body<'tcx>, saved_locals: &'s GeneratorSavedLocals, // FIXME(tmandry): Consider using sparse bitsets here once we have good // benchmarks for generators. local_conflicts: BitMatrix, } impl<'mir, 'tcx> rustc_mir_dataflow::ResultsVisitor<'mir, 'tcx> for StorageConflictVisitor<'mir, 'tcx, '_> { type FlowState = BitSet; fn visit_statement_before_primary_effect( &mut self, state: &Self::FlowState, _statement: &'mir Statement<'tcx>, loc: Location, ) { self.apply_state(state, loc); } fn visit_terminator_before_primary_effect( &mut self, state: &Self::FlowState, _terminator: &'mir Terminator<'tcx>, loc: Location, ) { self.apply_state(state, loc); } } impl StorageConflictVisitor<'_, '_, '_> { fn apply_state(&mut self, flow_state: &BitSet, loc: Location) { // Ignore unreachable blocks. if self.body.basic_blocks[loc.block].terminator().kind == TerminatorKind::Unreachable { return; } let mut eligible_storage_live = flow_state.clone(); eligible_storage_live.intersect(&**self.saved_locals); for local in eligible_storage_live.iter() { self.local_conflicts.union_row_with(&eligible_storage_live, local); } if eligible_storage_live.count() > 1 { trace!("at {:?}, eligible_storage_live={:?}", loc, eligible_storage_live); } } } /// Validates the typeck view of the generator against the actual set of types saved between /// yield points. fn sanitize_witness<'tcx>( tcx: TyCtxt<'tcx>, body: &Body<'tcx>, witness: Ty<'tcx>, upvars: Vec>, saved_locals: &GeneratorSavedLocals, ) { let did = body.source.def_id(); let param_env = tcx.param_env(did); let allowed_upvars = tcx.normalize_erasing_regions(param_env, upvars); let allowed = match witness.kind() { &ty::GeneratorWitness(interior_tys) => { tcx.normalize_erasing_late_bound_regions(param_env, interior_tys) } _ => { tcx.sess.delay_span_bug( body.span, &format!("unexpected generator witness type {:?}", witness.kind()), ); return; } }; for (local, decl) in body.local_decls.iter_enumerated() { // Ignore locals which are internal or not saved between yields. if !saved_locals.contains(local) || decl.internal { continue; } let decl_ty = tcx.normalize_erasing_regions(param_env, decl.ty); // 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!( body.span, "Broken MIR: generator contains type {} in MIR, \ but typeck only knows about {} and {:?}", decl_ty, allowed, allowed_upvars ); } } } fn compute_layout<'tcx>( liveness: LivenessInfo, body: &mut Body<'tcx>, ) -> ( FxHashMap, VariantIdx, usize)>, GeneratorLayout<'tcx>, IndexVec>>, ) { let LivenessInfo { saved_locals, live_locals_at_suspension_points, source_info_at_suspension_points, storage_conflicts, storage_liveness, } = liveness; // Gather live local types and their indices. let mut locals = IndexVec::::new(); let mut tys = IndexVec::::new(); for (saved_local, local) in saved_locals.iter_enumerated() { locals.push(local); tys.push(body.local_decls[local].ty); debug!("generator saved local {:?} => {:?}", saved_local, local); } // Leave empty variants for the UNRESUMED, RETURNED, and POISONED states. // In debuginfo, these will correspond to the beginning (UNRESUMED) or end // (RETURNED, POISONED) of the function. let body_span = body.source_scopes[OUTERMOST_SOURCE_SCOPE].span; let mut variant_source_info: IndexVec = [ SourceInfo::outermost(body_span.shrink_to_lo()), SourceInfo::outermost(body_span.shrink_to_hi()), SourceInfo::outermost(body_span.shrink_to_hi()), ] .iter() .copied() .collect(); // Build the generator variant field list. // Create a map from local indices to generator struct indices. let mut variant_fields: IndexVec> = iter::repeat(IndexVec::new()).take(RESERVED_VARIANTS).collect(); let mut remap = FxHashMap::default(); for (suspension_point_idx, live_locals) in live_locals_at_suspension_points.iter().enumerate() { let variant_index = VariantIdx::from(RESERVED_VARIANTS + suspension_point_idx); let mut fields = IndexVec::new(); for (idx, saved_local) in live_locals.iter().enumerate() { fields.push(saved_local); // Note that if a field is included in multiple variants, we will // just use the first one here. That's fine; fields do not move // around inside generators, so it doesn't matter which variant // index we access them by. remap.entry(locals[saved_local]).or_insert((tys[saved_local], variant_index, idx)); } variant_fields.push(fields); variant_source_info.push(source_info_at_suspension_points[suspension_point_idx]); } debug!("generator variant_fields = {:?}", variant_fields); debug!("generator storage_conflicts = {:#?}", storage_conflicts); let layout = GeneratorLayout { field_tys: tys, variant_fields, variant_source_info, storage_conflicts }; (remap, layout, storage_liveness) } /// Replaces the entry point of `body` with a block that switches on the generator discriminant and /// dispatches to blocks according to `cases`. /// /// After this function, the former entry point of the function will be bb1. fn insert_switch<'tcx>( body: &mut Body<'tcx>, cases: Vec<(usize, BasicBlock)>, transform: &TransformVisitor<'tcx>, default: TerminatorKind<'tcx>, ) { let default_block = insert_term_block(body, default); let (assign, discr) = transform.get_discr(body); let switch_targets = SwitchTargets::new(cases.iter().map(|(i, bb)| ((*i) as u128, *bb)), default_block); let switch = TerminatorKind::SwitchInt { discr: Operand::Move(discr), targets: switch_targets }; let source_info = SourceInfo::outermost(body.span); body.basic_blocks_mut().raw.insert( 0, BasicBlockData { statements: vec![assign], terminator: Some(Terminator { source_info, kind: switch }), is_cleanup: false, }, ); let blocks = body.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<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { use crate::shim::DropShimElaborator; use rustc_middle::mir::patch::MirPatch; use rustc_mir_dataflow::elaborate_drops::{elaborate_drop, Unwind}; // 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 def_id = body.source.def_id(); let param_env = tcx.param_env(def_id); let mut elaborator = DropShimElaborator { body, patch: MirPatch::new(body), tcx, param_env }; for (block, block_data) in body.basic_blocks.iter_enumerated() { let (target, unwind, source_info) = match block_data.terminator() { Terminator { source_info, kind: TerminatorKind::Drop { place, target, unwind } } => { if let Some(local) = place.as_local() { if local == SELF_ARG { (target, unwind, source_info) } else { continue; } } else { continue; } } _ => continue, }; let unwind = if block_data.is_cleanup { Unwind::InCleanup } else { Unwind::To(unwind.unwrap_or_else(|| elaborator.patch.resume_block())) }; elaborate_drop( &mut elaborator, *source_info, Place::from(SELF_ARG), (), *target, unwind, block, ); } elaborator.patch.apply(body); } fn create_generator_drop_shim<'tcx>( tcx: TyCtxt<'tcx>, transform: &TransformVisitor<'tcx>, gen_ty: Ty<'tcx>, body: &mut Body<'tcx>, drop_clean: BasicBlock, ) -> Body<'tcx> { let mut body = body.clone(); body.arg_count = 1; // make sure the resume argument is not included here let source_info = SourceInfo::outermost(body.span); let mut cases = create_cases(&mut body, transform, Operation::Drop); cases.insert(0, (UNRESUMED, drop_clean)); // The returned state and the poisoned state fall through to the default // case which is just to return insert_switch(&mut body, cases, &transform, TerminatorKind::Return); for block in body.basic_blocks_mut() { let kind = &mut block.terminator_mut().kind; if let TerminatorKind::GeneratorDrop = *kind { *kind = TerminatorKind::Return; } } // Replace the return variable body.local_decls[RETURN_PLACE] = LocalDecl::with_source_info(tcx.mk_unit(), source_info); make_generator_state_argument_indirect(tcx, &mut body); // Change the generator argument from &mut to *mut body.local_decls[SELF_ARG] = LocalDecl::with_source_info( tcx.mk_ptr(ty::TypeAndMut { ty: gen_ty, mutbl: hir::Mutability::Mut }), source_info, ); // Make sure we remove dead blocks to remove // unrelated code from the resume part of the function simplify::remove_dead_blocks(tcx, &mut body); dump_mir(tcx, false, "generator_drop", &0, &body, |_, _| Ok(())); body } fn insert_term_block<'tcx>(body: &mut Body<'tcx>, kind: TerminatorKind<'tcx>) -> BasicBlock { let source_info = SourceInfo::outermost(body.span); body.basic_blocks_mut().push(BasicBlockData { statements: Vec::new(), terminator: Some(Terminator { source_info, kind }), is_cleanup: false, }) } fn insert_panic_block<'tcx>( tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, message: AssertMessage<'tcx>, ) -> BasicBlock { let assert_block = BasicBlock::new(body.basic_blocks.len()); let term = TerminatorKind::Assert { cond: Operand::Constant(Box::new(Constant { span: body.span, user_ty: None, literal: ConstantKind::from_bool(tcx, false), })), expected: true, msg: message, target: assert_block, cleanup: None, }; let source_info = SourceInfo::outermost(body.span); body.basic_blocks_mut().push(BasicBlockData { statements: Vec::new(), terminator: Some(Terminator { source_info, kind: term }), is_cleanup: false, }); assert_block } fn can_return<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, param_env: ty::ParamEnv<'tcx>) -> bool { // Returning from a function with an uninhabited return type is undefined behavior. if body.return_ty().is_privately_uninhabited(tcx, param_env) { return false; } // If there's a return terminator the function may return. for block in body.basic_blocks.iter() { if let TerminatorKind::Return = block.terminator().kind { return true; } } // Otherwise the function can't return. false } fn can_unwind<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> bool { // Nothing can unwind when landing pads are off. if tcx.sess.panic_strategy() == PanicStrategy::Abort { return false; } // Unwinds can only start at certain terminators. for block in body.basic_blocks.iter() { match block.terminator().kind { // These never unwind. TerminatorKind::Goto { .. } | TerminatorKind::SwitchInt { .. } | TerminatorKind::Abort | TerminatorKind::Return | TerminatorKind::Unreachable | TerminatorKind::GeneratorDrop | TerminatorKind::FalseEdge { .. } | TerminatorKind::FalseUnwind { .. } => {} // Resume will *continue* unwinding, but if there's no other unwinding terminator it // will never be reached. TerminatorKind::Resume => {} TerminatorKind::Yield { .. } => { unreachable!("`can_unwind` called before generator transform") } // These may unwind. TerminatorKind::Drop { .. } | TerminatorKind::DropAndReplace { .. } | TerminatorKind::Call { .. } | TerminatorKind::InlineAsm { .. } | TerminatorKind::Assert { .. } => return true, } } // If we didn't find an unwinding terminator, the function cannot unwind. false } fn create_generator_resume_function<'tcx>( tcx: TyCtxt<'tcx>, transform: TransformVisitor<'tcx>, body: &mut Body<'tcx>, can_return: bool, ) { let can_unwind = can_unwind(tcx, body); // Poison the generator when it unwinds if can_unwind { let source_info = SourceInfo::outermost(body.span); let poison_block = body.basic_blocks_mut().push(BasicBlockData { statements: vec![transform.set_discr(VariantIdx::new(POISONED), source_info)], terminator: Some(Terminator { source_info, kind: TerminatorKind::Resume }), is_cleanup: true, }); for (idx, block) in body.basic_blocks_mut().iter_enumerated_mut() { let source_info = block.terminator().source_info; if let TerminatorKind::Resume = block.terminator().kind { // An existing `Resume` terminator is redirected to jump to our dedicated // "poisoning block" above. if idx != poison_block { *block.terminator_mut() = Terminator { source_info, kind: TerminatorKind::Goto { target: poison_block }, }; } } else if !block.is_cleanup { // Any terminators that *can* unwind but don't have an unwind target set are also // pointed at our poisoning block (unless they're part of the cleanup path). if let Some(unwind @ None) = block.terminator_mut().unwind_mut() { *unwind = Some(poison_block); } } } } let mut cases = create_cases(body, &transform, Operation::Resume); use rustc_middle::mir::AssertKind::{ResumedAfterPanic, ResumedAfterReturn}; // Jump to the entry point on the unresumed cases.insert(0, (UNRESUMED, BasicBlock::new(0))); // Panic when resumed on the returned or poisoned state let generator_kind = body.generator_kind().unwrap(); if can_unwind { cases.insert( 1, (POISONED, insert_panic_block(tcx, body, ResumedAfterPanic(generator_kind))), ); } if can_return { cases.insert( 1, (RETURNED, insert_panic_block(tcx, body, ResumedAfterReturn(generator_kind))), ); } insert_switch(body, cases, &transform, TerminatorKind::Unreachable); make_generator_state_argument_indirect(tcx, body); make_generator_state_argument_pinned(tcx, body); // Make sure we remove dead blocks to remove // unrelated code from the drop part of the function simplify::remove_dead_blocks(tcx, body); dump_mir(tcx, false, "generator_resume", &0, body, |_, _| Ok(())); } fn insert_clean_drop(body: &mut Body<'_>) -> BasicBlock { let return_block = insert_term_block(body, TerminatorKind::Return); let term = TerminatorKind::Drop { place: Place::from(SELF_ARG), target: return_block, unwind: None }; let source_info = SourceInfo::outermost(body.span); // Create a block to destroy an unresumed generators. This can only destroy upvars. body.basic_blocks_mut().push(BasicBlockData { statements: Vec::new(), terminator: Some(Terminator { source_info, kind: term }), is_cleanup: false, }) } /// An operation that can be performed on a generator. #[derive(PartialEq, Copy, Clone)] enum Operation { Resume, Drop, } impl Operation { fn target_block(self, point: &SuspensionPoint<'_>) -> Option { match self { Operation::Resume => Some(point.resume), Operation::Drop => point.drop, } } } fn create_cases<'tcx>( body: &mut Body<'tcx>, transform: &TransformVisitor<'tcx>, operation: Operation, ) -> Vec<(usize, BasicBlock)> { let source_info = SourceInfo::outermost(body.span); transform .suspension_points .iter() .filter_map(|point| { // Find the target for this suspension point, if applicable operation.target_block(point).map(|target| { let mut statements = Vec::new(); // Create StorageLive instructions for locals with live storage for i in 0..(body.local_decls.len()) { if i == 2 { // The resume argument is live on function entry. Don't insert a // `StorageLive`, or the following `Assign` will read from uninitialized // memory. continue; } let l = Local::new(i); let needs_storage_live = point.storage_liveness.contains(l) && !transform.remap.contains_key(&l) && !transform.always_live_locals.contains(l); if needs_storage_live { statements .push(Statement { source_info, kind: StatementKind::StorageLive(l) }); } } if operation == Operation::Resume { // Move the resume argument to the destination place of the `Yield` terminator let resume_arg = Local::new(2); // 0 = return, 1 = self statements.push(Statement { source_info, kind: StatementKind::Assign(Box::new(( point.resume_arg, Rvalue::Use(Operand::Move(resume_arg.into())), ))), }); } // Then jump to the real target let block = body.basic_blocks_mut().push(BasicBlockData { statements, terminator: Some(Terminator { source_info, kind: TerminatorKind::Goto { target }, }), is_cleanup: false, }); (point.state, block) }) }) .collect() } impl<'tcx> MirPass<'tcx> for StateTransform { fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { let Some(yield_ty) = body.yield_ty() else { // This only applies to generators return; }; assert!(body.generator_drop().is_none()); // The first argument is the generator type passed by value let gen_ty = body.local_decls.raw[1].ty; // Get the interior types and substs which typeck computed let (upvars, interior, discr_ty, movable) = match *gen_ty.kind() { ty::Generator(_, substs, movability) => { let substs = substs.as_generator(); ( substs.upvar_tys().collect(), substs.witness(), substs.discr_ty(tcx), movability == hir::Movability::Movable, ) } _ => { tcx.sess .delay_span_bug(body.span, &format!("unexpected generator type {}", gen_ty)); return; } }; let is_async_kind = matches!(body.generator_kind(), Some(GeneratorKind::Async(_))); let (state_adt_ref, state_substs) = if is_async_kind { // Compute Poll let poll_did = tcx.require_lang_item(LangItem::Poll, None); let poll_adt_ref = tcx.adt_def(poll_did); let poll_substs = tcx.intern_substs(&[body.return_ty().into()]); (poll_adt_ref, poll_substs) } else { // Compute GeneratorState let state_did = tcx.require_lang_item(LangItem::GeneratorState, None); let state_adt_ref = tcx.adt_def(state_did); let state_substs = tcx.intern_substs(&[yield_ty.into(), body.return_ty().into()]); (state_adt_ref, state_substs) }; 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_local(RETURN_PLACE, ret_ty, body, tcx); // Replace all occurrences of `ResumeTy` with `&mut Context<'_>` within async bodies. if is_async_kind { transform_async_context(tcx, body); } // We also replace the resume argument and insert an `Assign`. // This is needed because the resume argument `_2` might be live across a `yield`, in which // case there is no `Assign` to it that the transform can turn into a store to the generator // state. After the yield the slot in the generator state would then be uninitialized. let resume_local = Local::new(2); let resume_ty = if is_async_kind { tcx.mk_task_context() } else { body.local_decls[resume_local].ty }; let new_resume_local = replace_local(resume_local, resume_ty, body, tcx); // When first entering the generator, move the resume argument into its new local. let source_info = SourceInfo::outermost(body.span); let stmts = &mut body.basic_blocks_mut()[BasicBlock::new(0)].statements; stmts.insert( 0, Statement { source_info, kind: StatementKind::Assign(Box::new(( new_resume_local.into(), Rvalue::Use(Operand::Move(resume_local.into())), ))), }, ); let always_live_locals = always_storage_live_locals(&body); let liveness_info = locals_live_across_suspend_points(tcx, body, &always_live_locals, movable); sanitize_witness(tcx, body, interior, upvars, &liveness_info.saved_locals); if tcx.sess.opts.unstable_opts.validate_mir { let mut vis = EnsureGeneratorFieldAssignmentsNeverAlias { assigned_local: None, saved_locals: &liveness_info.saved_locals, storage_conflicts: &liveness_info.storage_conflicts, }; vis.visit_body(body); } // 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(liveness_info, body); let can_return = can_return(tcx, body, tcx.param_env(body.source.def_id())); // 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 // either GeneratorState::Complete(x) and GeneratorState::Yielded(y), // or Poll::Ready(x) and Poll::Pending respectively depending on `is_async_kind`. let mut transform = TransformVisitor { tcx, is_async_kind, state_adt_ref, state_substs, remap, storage_liveness, always_live_locals, suspension_points: Vec::new(), new_ret_local, discr_ty, }; transform.visit_body(body); // Update our MIR struct to reflect the changes we've made body.arg_count = 2; // self, resume arg body.spread_arg = None; body.generator.as_mut().unwrap().yield_ty = None; body.generator.as_mut().unwrap().generator_layout = Some(layout); // Insert `drop(generator_struct)` which is used to drop upvars for generators in // the unresumed state. // This is expanded to a drop ladder in `elaborate_generator_drops`. let drop_clean = insert_clean_drop(body); dump_mir(tcx, false, "generator_pre-elab", &0, body, |_, _| 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, body); dump_mir(tcx, false, "generator_post-transform", &0, body, |_, _| 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, gen_ty, body, drop_clean); body.generator.as_mut().unwrap().generator_drop = Some(drop_shim); // Create the Generator::resume / Future::poll function create_generator_resume_function(tcx, transform, body, can_return); // Run derefer to fix Derefs that are not in the first place deref_finder(tcx, body); } } /// Looks for any assignments between locals (e.g., `_4 = _5`) that will both be converted to fields /// in the generator state machine but whose storage is not marked as conflicting /// /// Validation needs to happen immediately *before* `TransformVisitor` is invoked, not after. /// /// This condition would arise when the assignment is the last use of `_5` but the initial /// definition of `_4` if we weren't extra careful to mark all locals used inside a statement as /// conflicting. Non-conflicting generator saved locals may be stored at the same location within /// the generator state machine, which would result in ill-formed MIR: the left-hand and right-hand /// sides of an assignment may not alias. This caused a miscompilation in [#73137]. /// /// [#73137]: https://github.com/rust-lang/rust/issues/73137 struct EnsureGeneratorFieldAssignmentsNeverAlias<'a> { saved_locals: &'a GeneratorSavedLocals, storage_conflicts: &'a BitMatrix, assigned_local: Option, } impl EnsureGeneratorFieldAssignmentsNeverAlias<'_> { fn saved_local_for_direct_place(&self, place: Place<'_>) -> Option { if place.is_indirect() { return None; } self.saved_locals.get(place.local) } fn check_assigned_place(&mut self, place: Place<'_>, f: impl FnOnce(&mut Self)) { if let Some(assigned_local) = self.saved_local_for_direct_place(place) { assert!(self.assigned_local.is_none(), "`check_assigned_place` must not recurse"); self.assigned_local = Some(assigned_local); f(self); self.assigned_local = None; } } } impl<'tcx> Visitor<'tcx> for EnsureGeneratorFieldAssignmentsNeverAlias<'_> { fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, location: Location) { let Some(lhs) = self.assigned_local else { // This visitor only invokes `visit_place` for the right-hand side of an assignment // and only after setting `self.assigned_local`. However, the default impl of // `Visitor::super_body` may call `visit_place` with a `NonUseContext` for places // with debuginfo. Ignore them here. assert!(!context.is_use()); return; }; let Some(rhs) = self.saved_local_for_direct_place(*place) else { return }; if !self.storage_conflicts.contains(lhs, rhs) { bug!( "Assignment between generator saved locals whose storage is not \ marked as conflicting: {:?}: {:?} = {:?}", location, lhs, rhs, ); } } fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) { match &statement.kind { StatementKind::Assign(box (lhs, rhs)) => { self.check_assigned_place(*lhs, |this| this.visit_rvalue(rhs, location)); } StatementKind::FakeRead(..) | StatementKind::SetDiscriminant { .. } | StatementKind::Deinit(..) | StatementKind::StorageLive(_) | StatementKind::StorageDead(_) | StatementKind::Retag(..) | StatementKind::AscribeUserType(..) | StatementKind::Coverage(..) | StatementKind::Intrinsic(..) | StatementKind::Nop => {} } } fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) { // Checking for aliasing in terminators is probably overkill, but until we have actual // semantics, we should be conservative here. match &terminator.kind { TerminatorKind::Call { func, args, destination, target: Some(_), cleanup: _, from_hir_call: _, fn_span: _, } => { self.check_assigned_place(*destination, |this| { this.visit_operand(func, location); for arg in args { this.visit_operand(arg, location); } }); } TerminatorKind::Yield { value, resume: _, resume_arg, drop: _ } => { self.check_assigned_place(*resume_arg, |this| this.visit_operand(value, location)); } // FIXME: Does `asm!` have any aliasing requirements? TerminatorKind::InlineAsm { .. } => {} TerminatorKind::Call { .. } | TerminatorKind::Goto { .. } | TerminatorKind::SwitchInt { .. } | TerminatorKind::Resume | TerminatorKind::Abort | TerminatorKind::Return | TerminatorKind::Unreachable | TerminatorKind::Drop { .. } | TerminatorKind::DropAndReplace { .. } | TerminatorKind::Assert { .. } | TerminatorKind::GeneratorDrop | TerminatorKind::FalseEdge { .. } | TerminatorKind::FalseUnwind { .. } => {} } } }