rustc_mir_transform/

coroutine.rs

//! This is the implementation of the pass which transforms coroutines into state machines.
//!
//! MIR generation for coroutines creates a function which has a self argument which
//! passes by value. This argument is effectively a coroutine type which only contains upvars and
//! is only used for this argument inside the MIR for the coroutine.
//! 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 coroutine argument (which only consists of upvars) if any of the upvars
//! are moved out of. This pass elaborates the drops of upvars / coroutine argument in the case
//! that none of the upvars were moved out of. This is because we cannot have any drops of this
//! coroutine in the MIR, since it is used to create the drop glue for the coroutine. We'd get
//! infinite recursion otherwise.
//!
//! This pass creates the implementation for either the `Coroutine::resume` or `Future::poll`
//! function and the drop shim for the coroutine based on the MIR input.
//! It converts the coroutine argument from Self to &mut Self adding derefs in the MIR as needed.
//! It computes the final layout of the coroutine struct which looks like this:
//!     First upvars are stored
//!     It is followed by the coroutine state field.
//!     Then finally the MIR locals which are live across a suspension point are stored.
//!     ```ignore (illustrative)
//!     struct Coroutine {
//!         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 coroutine states:
//!     0 - Coroutine have not been resumed yet
//!     1 - Coroutine has returned / is completed
//!     2 - Coroutine has been poisoned
//!
//! It also rewrites `return x` and `yield y` as setting a new coroutine state and returning
//! `CoroutineState::Complete(x)` and `CoroutineState::Yielded(y)`,
//! or `Poll::Ready(x)` and `Poll::Pending` respectively.
//! MIR locals which are live across a suspension point are moved to the coroutine struct
//! with references to them being updated with references to the coroutine struct.
//!
//! The pass creates two functions which have a switch on the coroutine state giving
//! the action to take.
//!
//! One of them is the implementation of `Coroutine::resume` / `Future::poll`.
//! For coroutines with state 0 (unresumed) it starts the execution of the coroutine.
//! For coroutines 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 coroutine.
//! For coroutines with state 0 (unresumed) it drops the upvars of the coroutine.
//! For coroutines with state 1 (returned) and state 2 (poisoned) it does nothing.
//! Otherwise it drops all the values in scope at the last suspension point.

mod by_move_body;
mod drop;
use std::{iter, ops};

pub(super) use by_move_body::coroutine_by_move_body_def_id;
use drop::{
    cleanup_async_drops, create_coroutine_drop_shim, create_coroutine_drop_shim_async,
    create_coroutine_drop_shim_proxy_async, elaborate_coroutine_drops, expand_async_drops,
    has_expandable_async_drops, insert_clean_drop,
};
use rustc_abi::{FieldIdx, VariantIdx};
use rustc_data_structures::fx::FxHashSet;
use rustc_errors::pluralize;
use rustc_hir as hir;
use rustc_hir::lang_items::LangItem;
use rustc_hir::{CoroutineDesugaring, CoroutineKind};
use rustc_index::bit_set::{BitMatrix, DenseBitSet, GrowableBitSet};
use rustc_index::{Idx, IndexVec};
use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
use rustc_middle::mir::*;
use rustc_middle::ty::util::Discr;
use rustc_middle::ty::{
    self, CoroutineArgs, CoroutineArgsExt, GenericArgsRef, InstanceKind, Ty, TyCtxt, TypingMode,
};
use rustc_middle::{bug, span_bug};
use rustc_mir_dataflow::impls::{
    MaybeBorrowedLocals, MaybeLiveLocals, MaybeRequiresStorage, MaybeStorageLive,
    always_storage_live_locals,
};
use rustc_mir_dataflow::{
    Analysis, Results, ResultsCursor, ResultsVisitor, visit_reachable_results,
};
use rustc_span::def_id::{DefId, LocalDefId};
use rustc_span::source_map::dummy_spanned;
use rustc_span::symbol::sym;
use rustc_span::{DUMMY_SP, Span};
use rustc_target::spec::PanicStrategy;
use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
use rustc_trait_selection::infer::TyCtxtInferExt as _;
use rustc_trait_selection::traits::{ObligationCause, ObligationCauseCode, ObligationCtxt};
use tracing::{debug, instrument, trace};

use crate::deref_separator::deref_finder;
use crate::{abort_unwinding_calls, errors, pass_manager as pm, simplify};

pub(super) 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 SelfArgVisitor<'tcx> {
    tcx: TyCtxt<'tcx>,
    new_base: Place<'tcx>,
}

impl<'tcx> SelfArgVisitor<'tcx> {
    fn new(tcx: TyCtxt<'tcx>, elem: ProjectionElem<Local, Ty<'tcx>>) -> Self {
        Self { tcx, new_base: Place { local: SELF_ARG, projection: tcx.mk_place_elems(&[elem]) } }
    }
}

impl<'tcx> MutVisitor<'tcx> for SelfArgVisitor<'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, self.new_base, 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.mk_place_elems(&new_projection);
}

const SELF_ARG: Local = Local::from_u32(1);
const CTX_ARG: Local = Local::from_u32(2);

/// A `yield` point in the coroutine.
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 coroutine is dropped in this state.
    drop: Option<BasicBlock>,
    /// Set of locals that have live storage while at this suspension point.
    storage_liveness: GrowableBitSet<Local>,
}

struct TransformVisitor<'tcx> {
    tcx: TyCtxt<'tcx>,
    coroutine_kind: hir::CoroutineKind,

    // The type of the discriminant in the coroutine struct
    discr_ty: Ty<'tcx>,

    // Mapping from Local to (type of local, coroutine struct index)
    remap: IndexVec<Local, Option<(Ty<'tcx>, VariantIdx, FieldIdx)>>,

    // A map from a suspension point in a block to the locals which have live storage at that point
    storage_liveness: IndexVec<BasicBlock, Option<DenseBitSet<Local>>>,

    // A list of suspension points, generated during the transform
    suspension_points: Vec<SuspensionPoint<'tcx>>,

    // The set of locals that have no `StorageLive`/`StorageDead` annotations.
    always_live_locals: DenseBitSet<Local>,

    // The original RETURN_PLACE local
    old_ret_local: Local,

    old_yield_ty: Ty<'tcx>,

    old_ret_ty: Ty<'tcx>,
}

impl<'tcx> TransformVisitor<'tcx> {
    fn insert_none_ret_block(&self, body: &mut Body<'tcx>) -> BasicBlock {
        let block = body.basic_blocks.next_index();
        let source_info = SourceInfo::outermost(body.span);

        let none_value = match self.coroutine_kind {
            CoroutineKind::Desugared(CoroutineDesugaring::Async, _) => {
                span_bug!(body.span, "`Future`s are not fused inherently")
            }
            CoroutineKind::Coroutine(_) => span_bug!(body.span, "`Coroutine`s cannot be fused"),
            // `gen` continues return `None`
            CoroutineKind::Desugared(CoroutineDesugaring::Gen, _) => {
                let option_def_id = self.tcx.require_lang_item(LangItem::Option, None);
                make_aggregate_adt(
                    option_def_id,
                    VariantIdx::ZERO,
                    self.tcx.mk_args(&[self.old_yield_ty.into()]),
                    IndexVec::new(),
                )
            }
            // `async gen` continues to return `Poll::Ready(None)`
            CoroutineKind::Desugared(CoroutineDesugaring::AsyncGen, _) => {
                let ty::Adt(_poll_adt, args) = *self.old_yield_ty.kind() else { bug!() };
                let ty::Adt(_option_adt, args) = *args.type_at(0).kind() else { bug!() };
                let yield_ty = args.type_at(0);
                Rvalue::Use(Operand::Constant(Box::new(ConstOperand {
                    span: source_info.span,
                    const_: Const::Unevaluated(
                        UnevaluatedConst::new(
                            self.tcx.require_lang_item(LangItem::AsyncGenFinished, None),
                            self.tcx.mk_args(&[yield_ty.into()]),
                        ),
                        self.old_yield_ty,
                    ),
                    user_ty: None,
                })))
            }
        };

        let statements = vec![Statement {
            kind: StatementKind::Assign(Box::new((Place::return_place(), none_value))),
            source_info,
        }];

        body.basic_blocks_mut().push(BasicBlockData {
            statements,
            terminator: Some(Terminator { source_info, kind: TerminatorKind::Return }),
            is_cleanup: false,
        });

        block
    }

    // Make a `CoroutineState` or `Poll` variant assignment.
    //
    // `core::ops::CoroutineState` 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<Statement<'tcx>>,
    ) {
        const ZERO: VariantIdx = VariantIdx::ZERO;
        const ONE: VariantIdx = VariantIdx::from_usize(1);
        let rvalue = match self.coroutine_kind {
            CoroutineKind::Desugared(CoroutineDesugaring::Async, _) => {
                let poll_def_id = self.tcx.require_lang_item(LangItem::Poll, None);
                let args = self.tcx.mk_args(&[self.old_ret_ty.into()]);
                let (variant_idx, operands) = if is_return {
                    (ZERO, IndexVec::from_raw(vec![val])) // Poll::Ready(val)
                } else {
                    (ONE, IndexVec::new()) // Poll::Pending
                };
                make_aggregate_adt(poll_def_id, variant_idx, args, operands)
            }
            CoroutineKind::Desugared(CoroutineDesugaring::Gen, _) => {
                let option_def_id = self.tcx.require_lang_item(LangItem::Option, None);
                let args = self.tcx.mk_args(&[self.old_yield_ty.into()]);
                let (variant_idx, operands) = if is_return {
                    (ZERO, IndexVec::new()) // None
                } else {
                    (ONE, IndexVec::from_raw(vec![val])) // Some(val)
                };
                make_aggregate_adt(option_def_id, variant_idx, args, operands)
            }
            CoroutineKind::Desugared(CoroutineDesugaring::AsyncGen, _) => {
                if is_return {
                    let ty::Adt(_poll_adt, args) = *self.old_yield_ty.kind() else { bug!() };
                    let ty::Adt(_option_adt, args) = *args.type_at(0).kind() else { bug!() };
                    let yield_ty = args.type_at(0);
                    Rvalue::Use(Operand::Constant(Box::new(ConstOperand {
                        span: source_info.span,
                        const_: Const::Unevaluated(
                            UnevaluatedConst::new(
                                self.tcx.require_lang_item(LangItem::AsyncGenFinished, None),
                                self.tcx.mk_args(&[yield_ty.into()]),
                            ),
                            self.old_yield_ty,
                        ),
                        user_ty: None,
                    })))
                } else {
                    Rvalue::Use(val)
                }
            }
            CoroutineKind::Coroutine(_) => {
                let coroutine_state_def_id =
                    self.tcx.require_lang_item(LangItem::CoroutineState, None);
                let args = self.tcx.mk_args(&[self.old_yield_ty.into(), self.old_ret_ty.into()]);
                let variant_idx = if is_return {
                    ONE // CoroutineState::Complete(val)
                } else {
                    ZERO // CoroutineState::Yielded(val)
                };
                make_aggregate_adt(
                    coroutine_state_def_id,
                    variant_idx,
                    args,
                    IndexVec::from_raw(vec![val]),
                )
            }
        };

        statements.push(Statement {
            kind: StatementKind::Assign(Box::new((Place::return_place(), rvalue))),
            source_info,
        });
    }

    // Create a Place referencing a coroutine struct field
    fn make_field(&self, variant_index: VariantIdx, idx: FieldIdx, 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(idx, ty));

        Place { local: base.local, projection: self.tcx.mk_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);
        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!(!self.remap.contains(*local));
    }

    fn visit_place(
        &mut self,
        place: &mut Place<'tcx>,
        _context: PlaceContext,
        _location: Location,
    ) {
        // Replace an Local in the remap with a coroutine struct access
        if let Some(&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
        for s in &mut data.statements {
            if let StatementKind::StorageLive(l) | StatementKind::StorageDead(l) = s.kind
                && self.remap.contains(l)
            {
                s.make_nop();
            }
        }

        let ret_val = match data.terminator().kind {
            TerminatorKind::Return => {
                Some((true, None, Operand::Move(Place::from(self.old_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 = CoroutineArgs::RESERVED_VARIANTS + self.suspension_points.len();

                // The resume arg target location might itself be remapped if its base local is
                // live across a yield.
                if let Some(&Some((ty, variant, idx))) = self.remap.get(resume_arg.local) {
                    replace_base(&mut resume_arg, self.make_field(variant, idx, ty), self.tcx);
                }

                let storage_liveness: GrowableBitSet<Local> =
                    self.storage_liveness[block].clone().unwrap().into();

                for i in 0..self.always_live_locals.domain_size() {
                    let l = Local::new(i);
                    let needs_storage_dead = storage_liveness.contains(l)
                        && !self.remap.contains(l)
                        && !self.always_live_locals.contains(l);
                    if needs_storage_dead {
                        data.statements
                            .push(Statement { source_info, kind: StatementKind::StorageDead(l) });
                    }
                }

                self.suspension_points.push(SuspensionPoint {
                    state,
                    resume,
                    resume_arg,
                    drop,
                    storage_liveness,
                });

                VariantIdx::new(state)
            } else {
                // Return
                VariantIdx::new(CoroutineArgs::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_aggregate_adt<'tcx>(
    def_id: DefId,
    variant_idx: VariantIdx,
    args: GenericArgsRef<'tcx>,
    operands: IndexVec<FieldIdx, Operand<'tcx>>,
) -> Rvalue<'tcx> {
    Rvalue::Aggregate(Box::new(AggregateKind::Adt(def_id, variant_idx, args, None, None)), operands)
}

fn make_coroutine_state_argument_indirect<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
    let coroutine_ty = body.local_decls.raw[1].ty;

    let ref_coroutine_ty = Ty::new_mut_ref(tcx, tcx.lifetimes.re_erased, coroutine_ty);

    // Replace the by value coroutine argument
    body.local_decls.raw[1].ty = ref_coroutine_ty;

    // Add a deref to accesses of the coroutine state
    SelfArgVisitor::new(tcx, ProjectionElem::Deref).visit_body(body);
}

fn make_coroutine_state_argument_pinned<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
    let ref_coroutine_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 args = tcx.mk_args(&[ref_coroutine_ty.into()]);
    let pin_ref_coroutine_ty = Ty::new_adt(tcx, pin_adt_ref, args);

    // Replace the by ref coroutine argument
    body.local_decls.raw[1].ty = pin_ref_coroutine_ty;

    // Add the Pin field access to accesses of the coroutine state
    SelfArgVisitor::new(tcx, ProjectionElem::Field(FieldIdx::ZERO, ref_coroutine_ty))
        .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 coroutine 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 <https://github.com/rust-lang/rust/issues/105501>.
///
/// 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 coroutine body only knows about `&mut Context<'_>`.
fn transform_async_context<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) -> Ty<'tcx> {
    let context_mut_ref = Ty::new_task_context(tcx);

    // replace the type of the `resume` argument
    replace_resume_ty_local(tcx, body, CTX_ARG, context_mut_ref);

    let get_context_def_id = tcx.require_lang_item(LangItem::GetContext, None);

    for bb in body.basic_blocks.indices() {
        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()
                    && 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);
                }
            }
            TerminatorKind::Yield { resume_arg, .. } => {
                replace_resume_ty_local(tcx, body, resume_arg.local, context_mut_ref);
            }
            _ => {}
        }
    }
    context_mut_ref
}

fn eliminate_get_context_call<'tcx>(bb_data: &mut BasicBlockData<'tcx>) -> Local {
    let terminator = bb_data.terminator.take().unwrap();
    let TerminatorKind::Call { args, destination, target, .. } = terminator.kind else {
        bug!();
    };
    let [arg] = *Box::try_from(args).unwrap();
    let local = arg.node.place().unwrap().local;

    let arg = Rvalue::Use(arg.node);
    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
}

#[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);
        };
    }
}

/// Transforms the `body` of the coroutine applying the following transform:
///
/// - Remove the `resume` argument.
///
/// Ideally the async lowering would not add the `resume` argument.
///
/// The async lowering step and the type / lifetime inference / checking are
/// still using the `resume` argument for the time being. After this transform,
/// the coroutine body doesn't have the `resume` argument.
fn transform_gen_context<'tcx>(body: &mut Body<'tcx>) {
    // This leaves the local representing the `resume` argument in place,
    // but turns it into a regular local variable. This is cheaper than
    // adjusting all local references in the body after removing it.
    body.arg_count = 1;
}

struct LivenessInfo {
    /// Which locals are live across any suspension point.
    saved_locals: CoroutineSavedLocals,

    /// The set of saved locals live at each suspension point.
    live_locals_at_suspension_points: Vec<DenseBitSet<CoroutineSavedLocal>>,

    /// Parallel vec to the above with SourceInfo for each yield terminator.
    source_info_at_suspension_points: Vec<SourceInfo>,

    /// 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<CoroutineSavedLocal, CoroutineSavedLocal>,

    /// For every suspending block, the locals which are storage-live across
    /// that suspension point.
    storage_liveness: IndexVec<BasicBlock, Option<DenseBitSet<Local>>>,
}

/// Computes which locals have to be stored in the state-machine for the
/// given coroutine.
///
/// The basic idea is as follows:
/// - a local is live until we encounter a `StorageDead` statement. In
///   case none exist, the local is considered to be always live.
/// - a local has to be stored if it is either directly used after the
///   the suspend point, or if it is live and has been previously borrowed.
fn locals_live_across_suspend_points<'tcx>(
    tcx: TyCtxt<'tcx>,
    body: &Body<'tcx>,
    always_live_locals: &DenseBitSet<Local>,
    movable: bool,
) -> LivenessInfo {
    // 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))
        .iterate_to_fixpoint(tcx, body, None)
        .into_results_cursor(body);

    // Calculate the MIR locals that have been previously borrowed (even if they are still active).
    let borrowed_locals = MaybeBorrowedLocals.iterate_to_fixpoint(tcx, body, Some("coroutine"));
    let mut borrowed_locals_analysis1 = borrowed_locals.analysis;
    let mut borrowed_locals_analysis2 = borrowed_locals_analysis1.clone(); // trivial
    let borrowed_locals_cursor1 = ResultsCursor::new_borrowing(
        body,
        &mut borrowed_locals_analysis1,
        &borrowed_locals.results,
    );
    let mut borrowed_locals_cursor2 = ResultsCursor::new_borrowing(
        body,
        &mut borrowed_locals_analysis2,
        &borrowed_locals.results,
    );

    // Calculate the MIR locals that we need to keep storage around for.
    let mut requires_storage =
        MaybeRequiresStorage::new(borrowed_locals_cursor1).iterate_to_fixpoint(tcx, body, None);
    let mut requires_storage_cursor = ResultsCursor::new_borrowing(
        body,
        &mut requires_storage.analysis,
        &requires_storage.results,
    );

    // Calculate the liveness of MIR locals ignoring borrows.
    let mut liveness =
        MaybeLiveLocals.iterate_to_fixpoint(tcx, body, Some("coroutine")).into_results_cursor(body);

    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 = DenseBitSet::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 = liveness.get().clone();

            if !movable {
                // The `liveness` variable contains the liveness of MIR locals ignoring borrows.
                // This is correct for movable coroutines since borrows cannot live across
                // suspension points. However for immovable coroutines 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_cursor2.seek_before_primary_effect(loc);
                live_locals.union(borrowed_locals_cursor2.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 coroutine 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 = CoroutineSavedLocals(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,
        &saved_locals,
        always_live_locals.clone(),
        &mut requires_storage.analysis,
        &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.
///
/// `CoroutineSavedLocal` is indexed in terms of the elements in this set;
/// i.e. `CoroutineSavedLocal::new(1)` corresponds to the second local
/// included in this set.
struct CoroutineSavedLocals(DenseBitSet<Local>);

impl CoroutineSavedLocals {
    /// Returns an iterator over each `CoroutineSavedLocal` along with the `Local` it corresponds
    /// to.
    fn iter_enumerated(&self) -> impl '_ + Iterator<Item = (CoroutineSavedLocal, Local)> {
        self.iter().enumerate().map(|(i, l)| (CoroutineSavedLocal::from(i), l))
    }

    /// Transforms a `DenseBitSet<Local>` that contains only locals saved across yield points to the
    /// equivalent `DenseBitSet<CoroutineSavedLocal>`.
    fn renumber_bitset(&self, input: &DenseBitSet<Local>) -> DenseBitSet<CoroutineSavedLocal> {
        assert!(self.superset(input), "{:?} not a superset of {:?}", self.0, input);
        let mut out = DenseBitSet::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<CoroutineSavedLocal> {
        if !self.contains(local) {
            return None;
        }

        let idx = self.iter().take_while(|&l| l < local).count();
        Some(CoroutineSavedLocal::new(idx))
    }
}

impl ops::Deref for CoroutineSavedLocals {
    type Target = DenseBitSet<Local>;

    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 `CoroutineLayout` for more.
fn compute_storage_conflicts<'mir, 'tcx>(
    body: &'mir Body<'tcx>,
    saved_locals: &'mir CoroutineSavedLocals,
    always_live_locals: DenseBitSet<Local>,
    analysis: &mut MaybeRequiresStorage<'mir, 'tcx>,
    results: &Results<DenseBitSet<Local>>,
) -> BitMatrix<CoroutineSavedLocal, CoroutineSavedLocal> {
    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,
        local_conflicts: BitMatrix::from_row_n(&ineligible_locals, body.local_decls.len()),
        eligible_storage_live: DenseBitSet::new_empty(body.local_decls.len()),
    };

    visit_reachable_results(body, analysis, results, &mut visitor);

    let local_conflicts = visitor.local_conflicts;

    // Compress the matrix using only stored locals (Local -> CoroutineSavedLocal).
    //
    // 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<'a, 'tcx> {
    body: &'a Body<'tcx>,
    saved_locals: &'a CoroutineSavedLocals,
    // FIXME(tmandry): Consider using sparse bitsets here once we have good
    // benchmarks for coroutines.
    local_conflicts: BitMatrix<Local, Local>,
    // We keep this bitset as a buffer to avoid reallocating memory.
    eligible_storage_live: DenseBitSet<Local>,
}

impl<'a, 'tcx> ResultsVisitor<'tcx, MaybeRequiresStorage<'a, 'tcx>>
    for StorageConflictVisitor<'a, 'tcx>
{
    fn visit_after_early_statement_effect(
        &mut self,
        _analysis: &mut MaybeRequiresStorage<'a, 'tcx>,
        state: &DenseBitSet<Local>,
        _statement: &Statement<'tcx>,
        loc: Location,
    ) {
        self.apply_state(state, loc);
    }

    fn visit_after_early_terminator_effect(
        &mut self,
        _analysis: &mut MaybeRequiresStorage<'a, 'tcx>,
        state: &DenseBitSet<Local>,
        _terminator: &Terminator<'tcx>,
        loc: Location,
    ) {
        self.apply_state(state, loc);
    }
}

impl StorageConflictVisitor<'_, '_> {
    fn apply_state(&mut self, state: &DenseBitSet<Local>, loc: Location) {
        // Ignore unreachable blocks.
        if let TerminatorKind::Unreachable = self.body.basic_blocks[loc.block].terminator().kind {
            return;
        }

        self.eligible_storage_live.clone_from(state);
        self.eligible_storage_live.intersect(&**self.saved_locals);

        for local in self.eligible_storage_live.iter() {
            self.local_conflicts.union_row_with(&self.eligible_storage_live, local);
        }

        if self.eligible_storage_live.count() > 1 {
            trace!("at {:?}, eligible_storage_live={:?}", loc, self.eligible_storage_live);
        }
    }
}

fn compute_layout<'tcx>(
    liveness: LivenessInfo,
    body: &Body<'tcx>,
) -> (
    IndexVec<Local, Option<(Ty<'tcx>, VariantIdx, FieldIdx)>>,
    CoroutineLayout<'tcx>,
    IndexVec<BasicBlock, Option<DenseBitSet<Local>>>,
) {
    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::<CoroutineSavedLocal, _>::new();
    let mut tys = IndexVec::<CoroutineSavedLocal, _>::new();
    for (saved_local, local) in saved_locals.iter_enumerated() {
        debug!("coroutine saved local {:?} => {:?}", saved_local, local);

        locals.push(local);
        let decl = &body.local_decls[local];
        debug!(?decl);

        // Do not `unwrap_crate_local` here, as post-borrowck cleanup may have already cleared
        // the information. This is alright, since `ignore_for_traits` is only relevant when
        // this code runs on pre-cleanup MIR, and `ignore_for_traits = false` is the safer
        // default.
        let ignore_for_traits = match decl.local_info {
            // Do not include raw pointers created from accessing `static` items, as those could
            // well be re-created by another access to the same static.
            ClearCrossCrate::Set(box LocalInfo::StaticRef { is_thread_local, .. }) => {
                !is_thread_local
            }
            // Fake borrows are only read by fake reads, so do not have any reality in
            // post-analysis MIR.
            ClearCrossCrate::Set(box LocalInfo::FakeBorrow) => true,
            _ => false,
        };
        let decl =
            CoroutineSavedTy { ty: decl.ty, source_info: decl.source_info, ignore_for_traits };
        debug!(?decl);

        tys.push(decl);
    }

    // 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<VariantIdx, SourceInfo> = [
        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 coroutine variant field list.
    // Create a map from local indices to coroutine struct indices.
    let mut variant_fields: IndexVec<VariantIdx, IndexVec<FieldIdx, CoroutineSavedLocal>> =
        iter::repeat(IndexVec::new()).take(CoroutineArgs::RESERVED_VARIANTS).collect();
    let mut remap = IndexVec::from_elem_n(None, saved_locals.domain_size());
    for (suspension_point_idx, live_locals) in live_locals_at_suspension_points.iter().enumerate() {
        let variant_index =
            VariantIdx::from(CoroutineArgs::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 coroutines, so it doesn't matter which variant
            // index we access them by.
            let idx = FieldIdx::from_usize(idx);
            remap[locals[saved_local]] = Some((tys[saved_local].ty, variant_index, idx));
        }
        variant_fields.push(fields);
        variant_source_info.push(source_info_at_suspension_points[suspension_point_idx]);
    }
    debug!("coroutine variant_fields = {:?}", variant_fields);
    debug!("coroutine storage_conflicts = {:#?}", storage_conflicts);

    let mut field_names = IndexVec::from_elem(None, &tys);
    for var in &body.var_debug_info {
        let VarDebugInfoContents::Place(place) = &var.value else { continue };
        let Some(local) = place.as_local() else { continue };
        let Some(&Some((_, variant, field))) = remap.get(local) else {
            continue;
        };

        let saved_local = variant_fields[variant][field];
        field_names.get_or_insert_with(saved_local, || var.name);
    }

    let layout = CoroutineLayout {
        field_tys: tys,
        field_names,
        variant_fields,
        variant_source_info,
        storage_conflicts,
    };
    debug!(?layout);

    (remap, layout, storage_liveness)
}

/// Replaces the entry point of `body` with a block that switches on the coroutine 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_block: BasicBlock,
) {
    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,
        },
    );

    for b in body.basic_blocks_mut().iter_mut() {
        b.terminator_mut().successors_mut(|target| *target += 1);
    }
}

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 return_poll_ready_assign<'tcx>(tcx: TyCtxt<'tcx>, source_info: SourceInfo) -> Statement<'tcx> {
    // Poll::Ready(())
    let poll_def_id = tcx.require_lang_item(LangItem::Poll, None);
    let args = tcx.mk_args(&[tcx.types.unit.into()]);
    let val = Operand::Constant(Box::new(ConstOperand {
        span: source_info.span,
        user_ty: None,
        const_: Const::zero_sized(tcx.types.unit),
    }));
    let ready_val = Rvalue::Aggregate(
        Box::new(AggregateKind::Adt(poll_def_id, VariantIdx::from_usize(0), args, None, None)),
        IndexVec::from_raw(vec![val]),
    );
    Statement {
        kind: StatementKind::Assign(Box::new((Place::return_place(), ready_val))),
        source_info,
    }
}

fn insert_poll_ready_block<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) -> BasicBlock {
    let source_info = SourceInfo::outermost(body.span);
    body.basic_blocks_mut().push(BasicBlockData {
        statements: [return_poll_ready_assign(tcx, source_info)].to_vec(),
        terminator: Some(Terminator { source_info, kind: TerminatorKind::Return }),
        is_cleanup: false,
    })
}

fn insert_panic_block<'tcx>(
    tcx: TyCtxt<'tcx>,
    body: &mut Body<'tcx>,
    message: AssertMessage<'tcx>,
) -> BasicBlock {
    let assert_block = body.basic_blocks.next_index();
    let kind = TerminatorKind::Assert {
        cond: Operand::Constant(Box::new(ConstOperand {
            span: body.span,
            user_ty: None,
            const_: Const::from_bool(tcx, false),
        })),
        expected: true,
        msg: Box::new(message),
        target: assert_block,
        unwind: UnwindAction::Continue,
    };

    insert_term_block(body, kind)
}

fn can_return<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, typing_env: ty::TypingEnv<'tcx>) -> bool {
    // Returning from a function with an uninhabited return type is undefined behavior.
    if body.return_ty().is_privately_uninhabited(tcx, typing_env) {
        return false;
    }

    // If there's a return terminator the function may return.
    body.basic_blocks.iter().any(|block| matches!(block.terminator().kind, TerminatorKind::Return))
    // Otherwise the function can't return.
}

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::UnwindTerminate(_)
            | TerminatorKind::Return
            | TerminatorKind::Unreachable
            | TerminatorKind::CoroutineDrop
            | TerminatorKind::FalseEdge { .. }
            | TerminatorKind::FalseUnwind { .. } => {}

            // Resume will *continue* unwinding, but if there's no other unwinding terminator it
            // will never be reached.
            TerminatorKind::UnwindResume => {}

            TerminatorKind::Yield { .. } => {
                unreachable!("`can_unwind` called before coroutine transform")
            }

            // These may unwind.
            TerminatorKind::Drop { .. }
            | TerminatorKind::Call { .. }
            | TerminatorKind::InlineAsm { .. }
            | TerminatorKind::Assert { .. } => return true,

            TerminatorKind::TailCall { .. } => {
                unreachable!("tail calls can't be present in generators")
            }
        }
    }

    // If we didn't find an unwinding terminator, the function cannot unwind.
    false
}

// Poison the coroutine when it unwinds
fn generate_poison_block_and_redirect_unwinds_there<'tcx>(
    transform: &TransformVisitor<'tcx>,
    body: &mut Body<'tcx>,
) {
    let source_info = SourceInfo::outermost(body.span);
    let poison_block = body.basic_blocks_mut().push(BasicBlockData {
        statements: vec![
            transform.set_discr(VariantIdx::new(CoroutineArgs::POISONED), source_info),
        ],
        terminator: Some(Terminator { source_info, kind: TerminatorKind::UnwindResume }),
        is_cleanup: true,
    });

    for (idx, block) in body.basic_blocks_mut().iter_enumerated_mut() {
        let source_info = block.terminator().source_info;

        if let TerminatorKind::UnwindResume = 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).
            && let Some(unwind @ UnwindAction::Continue) = block.terminator_mut().unwind_mut()
        {
            *unwind = UnwindAction::Cleanup(poison_block);
        }
    }
}

fn create_coroutine_resume_function<'tcx>(
    tcx: TyCtxt<'tcx>,
    transform: TransformVisitor<'tcx>,
    body: &mut Body<'tcx>,
    can_return: bool,
    can_unwind: bool,
) {
    // Poison the coroutine when it unwinds
    if can_unwind {
        generate_poison_block_and_redirect_unwinds_there(&transform, body);
    }

    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, (CoroutineArgs::UNRESUMED, START_BLOCK));

    // Panic when resumed on the returned or poisoned state
    if can_unwind {
        cases.insert(
            1,
            (
                CoroutineArgs::POISONED,
                insert_panic_block(tcx, body, ResumedAfterPanic(transform.coroutine_kind)),
            ),
        );
    }

    if can_return {
        let block = match transform.coroutine_kind {
            CoroutineKind::Desugared(CoroutineDesugaring::Async, _)
            | CoroutineKind::Coroutine(_) => {
                // For `async_drop_in_place<T>::{closure}` we just keep return Poll::Ready,
                // because async drop of such coroutine keeps polling original coroutine
                if tcx.is_async_drop_in_place_coroutine(body.source.def_id()) {
                    insert_poll_ready_block(tcx, body)
                } else {
                    insert_panic_block(tcx, body, ResumedAfterReturn(transform.coroutine_kind))
                }
            }
            CoroutineKind::Desugared(CoroutineDesugaring::AsyncGen, _)
            | CoroutineKind::Desugared(CoroutineDesugaring::Gen, _) => {
                transform.insert_none_ret_block(body)
            }
        };
        cases.insert(1, (CoroutineArgs::RETURNED, block));
    }

    let default_block = insert_term_block(body, TerminatorKind::Unreachable);
    insert_switch(body, cases, &transform, default_block);

    make_coroutine_state_argument_indirect(tcx, body);

    match transform.coroutine_kind {
        CoroutineKind::Coroutine(_)
        | CoroutineKind::Desugared(CoroutineDesugaring::Async | CoroutineDesugaring::AsyncGen, _) =>
        {
            make_coroutine_state_argument_pinned(tcx, body);
        }
        // Iterator::next doesn't accept a pinned argument,
        // unlike for all other coroutine kinds.
        CoroutineKind::Desugared(CoroutineDesugaring::Gen, _) => {}
    }

    // Make sure we remove dead blocks to remove
    // unrelated code from the drop part of the function
    simplify::remove_dead_blocks(body);

    pm::run_passes_no_validate(tcx, body, &[&abort_unwinding_calls::AbortUnwindingCalls], None);

    dump_mir(tcx, false, "coroutine_resume", &0, body, |_, _| Ok(()));
}

/// An operation that can be performed on a coroutine.
#[derive(PartialEq, Copy, Clone)]
enum Operation {
    Resume,
    Drop,
}

impl Operation {
    fn target_block(self, point: &SuspensionPoint<'_>) -> Option<BasicBlock> {
        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 l in body.local_decls.indices() {
                    let needs_storage_live = point.storage_liveness.contains(l)
                        && !transform.remap.contains(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 = CTX_ARG;
                    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()
}

#[instrument(level = "debug", skip(tcx), ret)]
pub(crate) fn mir_coroutine_witnesses<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: LocalDefId,
) -> Option<CoroutineLayout<'tcx>> {
    let (body, _) = tcx.mir_promoted(def_id);
    let body = body.borrow();
    let body = &*body;

    // The first argument is the coroutine type passed by value
    let coroutine_ty = body.local_decls[ty::CAPTURE_STRUCT_LOCAL].ty;

    let movable = match *coroutine_ty.kind() {
        ty::Coroutine(def_id, _) => tcx.coroutine_movability(def_id) == hir::Movability::Movable,
        ty::Error(_) => return None,
        _ => span_bug!(body.span, "unexpected coroutine type {}", coroutine_ty),
    };

    // The witness simply contains all locals live across suspend points.

    let always_live_locals = always_storage_live_locals(body);
    let liveness_info = locals_live_across_suspend_points(tcx, body, &always_live_locals, movable);

    // Extract locals which are live across suspension point into `layout`
    // `remap` gives a mapping from local indices onto coroutine struct indices
    // `storage_liveness` tells us which locals have live storage at suspension points
    let (_, coroutine_layout, _) = compute_layout(liveness_info, body);

    check_suspend_tys(tcx, &coroutine_layout, body);
    check_field_tys_sized(tcx, &coroutine_layout, def_id);

    Some(coroutine_layout)
}

fn check_field_tys_sized<'tcx>(
    tcx: TyCtxt<'tcx>,
    coroutine_layout: &CoroutineLayout<'tcx>,
    def_id: LocalDefId,
) {
    // No need to check if unsized_locals/unsized_fn_params is disabled,
    // since we will error during typeck.
    if !tcx.features().unsized_locals() && !tcx.features().unsized_fn_params() {
        return;
    }

    // FIXME(#132279): @lcnr believes that we may want to support coroutines
    // whose `Sized`-ness relies on the hidden types of opaques defined by the
    // parent function. In this case we'd have to be able to reveal only these
    // opaques here.
    let infcx = tcx.infer_ctxt().ignoring_regions().build(TypingMode::non_body_analysis());
    let param_env = tcx.param_env(def_id);

    let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
    for field_ty in &coroutine_layout.field_tys {
        ocx.register_bound(
            ObligationCause::new(
                field_ty.source_info.span,
                def_id,
                ObligationCauseCode::SizedCoroutineInterior(def_id),
            ),
            param_env,
            field_ty.ty,
            tcx.require_lang_item(hir::LangItem::Sized, Some(field_ty.source_info.span)),
        );
    }

    let errors = ocx.select_all_or_error();
    debug!(?errors);
    if !errors.is_empty() {
        infcx.err_ctxt().report_fulfillment_errors(errors);
    }
}

impl<'tcx> crate::MirPass<'tcx> for StateTransform {
    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
        let Some(old_yield_ty) = body.yield_ty() else {
            // This only applies to coroutines
            return;
        };
        let old_ret_ty = body.return_ty();

        assert!(body.coroutine_drop().is_none() && body.coroutine_drop_async().is_none());

        dump_mir(tcx, false, "coroutine_before", &0, body, |_, _| Ok(()));

        // The first argument is the coroutine type passed by value
        let coroutine_ty = body.local_decls.raw[1].ty;
        let coroutine_kind = body.coroutine_kind().unwrap();

        // Get the discriminant type and args which typeck computed
        let ty::Coroutine(_, args) = coroutine_ty.kind() else {
            tcx.dcx().span_bug(body.span, format!("unexpected coroutine type {coroutine_ty}"));
        };
        let discr_ty = args.as_coroutine().discr_ty(tcx);

        let new_ret_ty = match coroutine_kind {
            CoroutineKind::Desugared(CoroutineDesugaring::Async, _) => {
                // Compute Poll<return_ty>
                let poll_did = tcx.require_lang_item(LangItem::Poll, None);
                let poll_adt_ref = tcx.adt_def(poll_did);
                let poll_args = tcx.mk_args(&[old_ret_ty.into()]);
                Ty::new_adt(tcx, poll_adt_ref, poll_args)
            }
            CoroutineKind::Desugared(CoroutineDesugaring::Gen, _) => {
                // Compute Option<yield_ty>
                let option_did = tcx.require_lang_item(LangItem::Option, None);
                let option_adt_ref = tcx.adt_def(option_did);
                let option_args = tcx.mk_args(&[old_yield_ty.into()]);
                Ty::new_adt(tcx, option_adt_ref, option_args)
            }
            CoroutineKind::Desugared(CoroutineDesugaring::AsyncGen, _) => {
                // The yield ty is already `Poll<Option<yield_ty>>`
                old_yield_ty
            }
            CoroutineKind::Coroutine(_) => {
                // Compute CoroutineState<yield_ty, return_ty>
                let state_did = tcx.require_lang_item(LangItem::CoroutineState, None);
                let state_adt_ref = tcx.adt_def(state_did);
                let state_args = tcx.mk_args(&[old_yield_ty.into(), old_ret_ty.into()]);
                Ty::new_adt(tcx, state_adt_ref, state_args)
            }
        };

        // We rename RETURN_PLACE which has type mir.return_ty to old_ret_local
        // RETURN_PLACE then is a fresh unused local with type ret_ty.
        let old_ret_local = replace_local(RETURN_PLACE, new_ret_ty, body, tcx);

        // We need to insert clean drop for unresumed state and perform drop elaboration
        // (finally in open_drop_for_tuple) before async drop expansion.
        // Async drops, produced by this drop elaboration, will be expanded,
        // and corresponding futures kept in layout.
        let has_async_drops = matches!(
            coroutine_kind,
            CoroutineKind::Desugared(CoroutineDesugaring::Async | CoroutineDesugaring::AsyncGen, _)
        ) && has_expandable_async_drops(tcx, body, coroutine_ty);

        // Replace all occurrences of `ResumeTy` with `&mut Context<'_>` within async bodies.
        if matches!(
            coroutine_kind,
            CoroutineKind::Desugared(CoroutineDesugaring::Async | CoroutineDesugaring::AsyncGen, _)
        ) {
            let context_mut_ref = transform_async_context(tcx, body);
            expand_async_drops(tcx, body, context_mut_ref, coroutine_kind, coroutine_ty);
            dump_mir(tcx, false, "coroutine_async_drop_expand", &0, body, |_, _| Ok(()));
        } else {
            cleanup_async_drops(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 coroutine
        // state. After the yield the slot in the coroutine state would then be uninitialized.
        let resume_local = CTX_ARG;
        let resume_ty = body.local_decls[resume_local].ty;
        let old_resume_local = replace_local(resume_local, resume_ty, body, tcx);

        // When first entering the coroutine, move the resume argument into its old local
        // (which is now a generator interior).
        let source_info = SourceInfo::outermost(body.span);
        let stmts = &mut body.basic_blocks_mut()[START_BLOCK].statements;
        stmts.insert(
            0,
            Statement {
                source_info,
                kind: StatementKind::Assign(Box::new((
                    old_resume_local.into(),
                    Rvalue::Use(Operand::Move(resume_local.into())),
                ))),
            },
        );

        let always_live_locals = always_storage_live_locals(body);

        let movable = coroutine_kind.movability() == hir::Movability::Movable;
        let liveness_info =
            locals_live_across_suspend_points(tcx, body, &always_live_locals, movable);

        if tcx.sess.opts.unstable_opts.validate_mir {
            let mut vis = EnsureCoroutineFieldAssignmentsNeverAlias {
                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 coroutine 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, body.typing_env(tcx));

        // Run the transformation which converts Places from Local to coroutine struct
        // accesses for locals in `remap`.
        // It also rewrites `return x` and `yield y` as writing a new coroutine state and returning
        // either `CoroutineState::Complete(x)` and `CoroutineState::Yielded(y)`,
        // or `Poll::Ready(x)` and `Poll::Pending` respectively depending on the coroutine kind.
        let mut transform = TransformVisitor {
            tcx,
            coroutine_kind,
            remap,
            storage_liveness,
            always_live_locals,
            suspension_points: Vec::new(),
            old_ret_local,
            discr_ty,
            old_ret_ty,
            old_yield_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;

        // Remove the context argument within generator bodies.
        if matches!(coroutine_kind, CoroutineKind::Desugared(CoroutineDesugaring::Gen, _)) {
            transform_gen_context(body);
        }

        // The original arguments to the function are no longer arguments, mark them as such.
        // Otherwise they'll conflict with our new arguments, which although they don't have
        // argument_index set, will get emitted as unnamed arguments.
        for var in &mut body.var_debug_info {
            var.argument_index = None;
        }

        body.coroutine.as_mut().unwrap().yield_ty = None;
        body.coroutine.as_mut().unwrap().resume_ty = None;
        body.coroutine.as_mut().unwrap().coroutine_layout = Some(layout);

        // FIXME: Drops, produced by insert_clean_drop + elaborate_coroutine_drops,
        // are currently sync only. To allow async for them, we need to move those calls
        // before expand_async_drops, and fix the related problems.
        //
        // Insert `drop(coroutine_struct)` which is used to drop upvars for coroutines in
        // the unresumed state.
        // This is expanded to a drop ladder in `elaborate_coroutine_drops`.
        let drop_clean = insert_clean_drop(tcx, body, has_async_drops);

        dump_mir(tcx, false, "coroutine_pre-elab", &0, body, |_, _| Ok(()));

        // Expand `drop(coroutine_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_coroutine_drops(tcx, body);

        dump_mir(tcx, false, "coroutine_post-transform", &0, body, |_, _| Ok(()));

        let can_unwind = can_unwind(tcx, body);

        // Create a copy of our MIR and use it to create the drop shim for the coroutine
        if has_async_drops {
            // If coroutine has async drops, generating async drop shim
            let mut drop_shim =
                create_coroutine_drop_shim_async(tcx, &transform, body, drop_clean, can_unwind);
            // Run derefer to fix Derefs that are not in the first place
            deref_finder(tcx, &mut drop_shim);
            body.coroutine.as_mut().unwrap().coroutine_drop_async = Some(drop_shim);
        } else {
            // If coroutine has no async drops, generating sync drop shim
            let mut drop_shim =
                create_coroutine_drop_shim(tcx, &transform, coroutine_ty, body, drop_clean);
            // Run derefer to fix Derefs that are not in the first place
            deref_finder(tcx, &mut drop_shim);
            body.coroutine.as_mut().unwrap().coroutine_drop = Some(drop_shim);

            // For coroutine with sync drop, generating async proxy for `future_drop_poll` call
            let mut proxy_shim = create_coroutine_drop_shim_proxy_async(tcx, body);
            deref_finder(tcx, &mut proxy_shim);
            body.coroutine.as_mut().unwrap().coroutine_drop_proxy_async = Some(proxy_shim);
        }

        // Create the Coroutine::resume / Future::poll function
        create_coroutine_resume_function(tcx, transform, body, can_return, can_unwind);

        // Run derefer to fix Derefs that are not in the first place
        deref_finder(tcx, body);
    }

    fn is_required(&self) -> bool {
        true
    }
}

/// Looks for any assignments between locals (e.g., `_4 = _5`) that will both be converted to fields
/// in the coroutine 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 coroutine saved locals may be stored at the same location within
/// the coroutine 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 EnsureCoroutineFieldAssignmentsNeverAlias<'a> {
    saved_locals: &'a CoroutineSavedLocals,
    storage_conflicts: &'a BitMatrix<CoroutineSavedLocal, CoroutineSavedLocal>,
    assigned_local: Option<CoroutineSavedLocal>,
}

impl EnsureCoroutineFieldAssignmentsNeverAlias<'_> {
    fn saved_local_for_direct_place(&self, place: Place<'_>) -> Option<CoroutineSavedLocal> {
        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 EnsureCoroutineFieldAssignmentsNeverAlias<'_> {
    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 coroutine 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::PlaceMention(..)
            | StatementKind::Coverage(..)
            | StatementKind::Intrinsic(..)
            | StatementKind::ConstEvalCounter
            | StatementKind::BackwardIncompatibleDropHint { .. }
            | 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(_),
                unwind: _,
                call_source: _,
                fn_span: _,
            } => {
                self.check_assigned_place(*destination, |this| {
                    this.visit_operand(func, location);
                    for arg in args {
                        this.visit_operand(&arg.node, 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::UnwindResume
            | TerminatorKind::UnwindTerminate(_)
            | TerminatorKind::Return
            | TerminatorKind::TailCall { .. }
            | TerminatorKind::Unreachable
            | TerminatorKind::Drop { .. }
            | TerminatorKind::Assert { .. }
            | TerminatorKind::CoroutineDrop
            | TerminatorKind::FalseEdge { .. }
            | TerminatorKind::FalseUnwind { .. } => {}
        }
    }
}

fn check_suspend_tys<'tcx>(tcx: TyCtxt<'tcx>, layout: &CoroutineLayout<'tcx>, body: &Body<'tcx>) {
    let mut linted_tys = FxHashSet::default();

    for (variant, yield_source_info) in
        layout.variant_fields.iter().zip(&layout.variant_source_info)
    {
        debug!(?variant);
        for &local in variant {
            let decl = &layout.field_tys[local];
            debug!(?decl);

            if !decl.ignore_for_traits && linted_tys.insert(decl.ty) {
                let Some(hir_id) = decl.source_info.scope.lint_root(&body.source_scopes) else {
                    continue;
                };

                check_must_not_suspend_ty(
                    tcx,
                    decl.ty,
                    hir_id,
                    SuspendCheckData {
                        source_span: decl.source_info.span,
                        yield_span: yield_source_info.span,
                        plural_len: 1,
                        ..Default::default()
                    },
                );
            }
        }
    }
}

#[derive(Default)]
struct SuspendCheckData<'a> {
    source_span: Span,
    yield_span: Span,
    descr_pre: &'a str,
    descr_post: &'a str,
    plural_len: usize,
}

// Returns whether it emitted a diagnostic or not
// Note that this fn and the proceeding one are based on the code
// for creating must_use diagnostics
//
// Note that this technique was chosen over things like a `Suspend` marker trait
// as it is simpler and has precedent in the compiler
fn check_must_not_suspend_ty<'tcx>(
    tcx: TyCtxt<'tcx>,
    ty: Ty<'tcx>,
    hir_id: hir::HirId,
    data: SuspendCheckData<'_>,
) -> bool {
    if ty.is_unit() {
        return false;
    }

    let plural_suffix = pluralize!(data.plural_len);

    debug!("Checking must_not_suspend for {}", ty);

    match *ty.kind() {
        ty::Adt(_, args) if ty.is_box() => {
            let boxed_ty = args.type_at(0);
            let allocator_ty = args.type_at(1);
            check_must_not_suspend_ty(
                tcx,
                boxed_ty,
                hir_id,
                SuspendCheckData { descr_pre: &format!("{}boxed ", data.descr_pre), ..data },
            ) || check_must_not_suspend_ty(
                tcx,
                allocator_ty,
                hir_id,
                SuspendCheckData { descr_pre: &format!("{}allocator ", data.descr_pre), ..data },
            )
        }
        ty::Adt(def, _) => check_must_not_suspend_def(tcx, def.did(), hir_id, data),
        // FIXME: support adding the attribute to TAITs
        ty::Alias(ty::Opaque, ty::AliasTy { def_id: def, .. }) => {
            let mut has_emitted = false;
            for &(predicate, _) in tcx.explicit_item_bounds(def).skip_binder() {
                // We only look at the `DefId`, so it is safe to skip the binder here.
                if let ty::ClauseKind::Trait(ref poly_trait_predicate) =
                    predicate.kind().skip_binder()
                {
                    let def_id = poly_trait_predicate.trait_ref.def_id;
                    let descr_pre = &format!("{}implementer{} of ", data.descr_pre, plural_suffix);
                    if check_must_not_suspend_def(
                        tcx,
                        def_id,
                        hir_id,
                        SuspendCheckData { descr_pre, ..data },
                    ) {
                        has_emitted = true;
                        break;
                    }
                }
            }
            has_emitted
        }
        ty::Dynamic(binder, _, _) => {
            let mut has_emitted = false;
            for predicate in binder.iter() {
                if let ty::ExistentialPredicate::Trait(ref trait_ref) = predicate.skip_binder() {
                    let def_id = trait_ref.def_id;
                    let descr_post = &format!(" trait object{}{}", plural_suffix, data.descr_post);
                    if check_must_not_suspend_def(
                        tcx,
                        def_id,
                        hir_id,
                        SuspendCheckData { descr_post, ..data },
                    ) {
                        has_emitted = true;
                        break;
                    }
                }
            }
            has_emitted
        }
        ty::Tuple(fields) => {
            let mut has_emitted = false;
            for (i, ty) in fields.iter().enumerate() {
                let descr_post = &format!(" in tuple element {i}");
                if check_must_not_suspend_ty(
                    tcx,
                    ty,
                    hir_id,
                    SuspendCheckData { descr_post, ..data },
                ) {
                    has_emitted = true;
                }
            }
            has_emitted
        }
        ty::Array(ty, len) => {
            let descr_pre = &format!("{}array{} of ", data.descr_pre, plural_suffix);
            check_must_not_suspend_ty(
                tcx,
                ty,
                hir_id,
                SuspendCheckData {
                    descr_pre,
                    // FIXME(must_not_suspend): This is wrong. We should handle printing unevaluated consts.
                    plural_len: len.try_to_target_usize(tcx).unwrap_or(0) as usize + 1,
                    ..data
                },
            )
        }
        // If drop tracking is enabled, we want to look through references, since the referent
        // may not be considered live across the await point.
        ty::Ref(_region, ty, _mutability) => {
            let descr_pre = &format!("{}reference{} to ", data.descr_pre, plural_suffix);
            check_must_not_suspend_ty(tcx, ty, hir_id, SuspendCheckData { descr_pre, ..data })
        }
        _ => false,
    }
}

fn check_must_not_suspend_def(
    tcx: TyCtxt<'_>,
    def_id: DefId,
    hir_id: hir::HirId,
    data: SuspendCheckData<'_>,
) -> bool {
    if let Some(attr) = tcx.get_attr(def_id, sym::must_not_suspend) {
        let reason = attr.value_str().map(|s| errors::MustNotSuspendReason {
            span: data.source_span,
            reason: s.as_str().to_string(),
        });
        tcx.emit_node_span_lint(
            rustc_session::lint::builtin::MUST_NOT_SUSPEND,
            hir_id,
            data.source_span,
            errors::MustNotSupend {
                tcx,
                yield_sp: data.yield_span,
                reason,
                src_sp: data.source_span,
                pre: data.descr_pre,
                def_id,
                post: data.descr_post,
            },
        );

        true
    } else {
        false
    }
}