rustc_borrowck/polonius/

loan_invalidations.rs

use rustc_data_structures::graph::dominators::Dominators;
use rustc_middle::bug;
use rustc_middle::mir::visit::Visitor;
use rustc_middle::mir::{
    self, BasicBlock, Body, BorrowKind, FakeBorrowKind, InlineAsmOperand, Location, Mutability,
    NonDivergingIntrinsic, Operand, Place, Rvalue, Statement, StatementKind, Terminator,
    TerminatorKind,
};
use rustc_middle::ty::TyCtxt;
use tracing::debug;

use crate::borrow_set::BorrowSet;
use crate::facts::AllFacts;
use crate::location::LocationTable;
use crate::path_utils::*;
use crate::{
    AccessDepth, Activation, ArtificialField, BorrowIndex, Deep, LocalMutationIsAllowed, Read,
    ReadKind, ReadOrWrite, Reservation, Shallow, Write, WriteKind,
};

/// Emit `loan_invalidated_at` facts.
pub(super) fn emit_loan_invalidations<'tcx>(
    tcx: TyCtxt<'tcx>,
    all_facts: &mut AllFacts,
    location_table: &LocationTable,
    body: &Body<'tcx>,
    borrow_set: &BorrowSet<'tcx>,
) {
    let dominators = body.basic_blocks.dominators();
    let mut visitor =
        LoanInvalidationsGenerator { all_facts, borrow_set, tcx, location_table, body, dominators };
    visitor.visit_body(body);
}

struct LoanInvalidationsGenerator<'a, 'tcx> {
    tcx: TyCtxt<'tcx>,
    all_facts: &'a mut AllFacts,
    location_table: &'a LocationTable,
    body: &'a Body<'tcx>,
    dominators: &'a Dominators<BasicBlock>,
    borrow_set: &'a BorrowSet<'tcx>,
}

/// Visits the whole MIR and generates `invalidates()` facts.
/// Most of the code implementing this was stolen from `borrow_check/mod.rs`.
impl<'a, 'tcx> Visitor<'tcx> for LoanInvalidationsGenerator<'a, 'tcx> {
    fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
        self.check_activations(location);

        match &statement.kind {
            StatementKind::Assign(box (lhs, rhs)) => {
                self.consume_rvalue(location, rhs);

                self.mutate_place(location, *lhs, Shallow(None));
            }
            StatementKind::FakeRead(box (_, _)) => {
                // Only relevant for initialized/liveness/safety checks.
            }
            StatementKind::Intrinsic(box NonDivergingIntrinsic::Assume(op)) => {
                self.consume_operand(location, op);
            }
            StatementKind::Intrinsic(box NonDivergingIntrinsic::CopyNonOverlapping(mir::CopyNonOverlapping {
                src,
                dst,
                count,
            })) => {
                self.consume_operand(location, src);
                self.consume_operand(location, dst);
                self.consume_operand(location, count);
            }
            // Only relevant for mir typeck
            StatementKind::AscribeUserType(..)
            // Only relevant for liveness and unsafeck
            | StatementKind::PlaceMention(..)
            // Doesn't have any language semantics
            | StatementKind::Coverage(..)
            // Does not actually affect borrowck
            | StatementKind::StorageLive(..) => {}
            StatementKind::StorageDead(local) => {
                self.access_place(
                    location,
                    Place::from(*local),
                    (Shallow(None), Write(WriteKind::StorageDeadOrDrop)),
                    LocalMutationIsAllowed::Yes,
                );
            }
            StatementKind::ConstEvalCounter
            | StatementKind::Nop
            | StatementKind::Retag { .. }
            | StatementKind::Deinit(..)
            | StatementKind::SetDiscriminant { .. } => {
                bug!("Statement not allowed in this MIR phase")
            }
        }

        self.super_statement(statement, location);
    }

    fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
        self.check_activations(location);

        match &terminator.kind {
            TerminatorKind::SwitchInt { discr, targets: _ } => {
                self.consume_operand(location, discr);
            }
            TerminatorKind::Drop { place: drop_place, target: _, unwind: _, replace } => {
                let write_kind =
                    if *replace { WriteKind::Replace } else { WriteKind::StorageDeadOrDrop };
                self.access_place(
                    location,
                    *drop_place,
                    (AccessDepth::Drop, Write(write_kind)),
                    LocalMutationIsAllowed::Yes,
                );
            }
            TerminatorKind::Call {
                func,
                args,
                destination,
                target: _,
                unwind: _,
                call_source: _,
                fn_span: _,
            } => {
                self.consume_operand(location, func);
                for arg in args {
                    self.consume_operand(location, &arg.node);
                }
                self.mutate_place(location, *destination, Deep);
            }
            TerminatorKind::TailCall { func, args, .. } => {
                self.consume_operand(location, func);
                for arg in args {
                    self.consume_operand(location, &arg.node);
                }
            }
            TerminatorKind::Assert { cond, expected: _, msg, target: _, unwind: _ } => {
                self.consume_operand(location, cond);
                use rustc_middle::mir::AssertKind;
                if let AssertKind::BoundsCheck { len, index } = &**msg {
                    self.consume_operand(location, len);
                    self.consume_operand(location, index);
                }
            }
            TerminatorKind::Yield { value, resume, resume_arg, drop: _ } => {
                self.consume_operand(location, value);

                // Invalidate all borrows of local places
                let borrow_set = self.borrow_set;
                let resume = self.location_table.start_index(resume.start_location());
                for (i, data) in borrow_set.iter_enumerated() {
                    if borrow_of_local_data(data.borrowed_place) {
                        self.all_facts.loan_invalidated_at.push((resume, i));
                    }
                }

                self.mutate_place(location, *resume_arg, Deep);
            }
            TerminatorKind::UnwindResume
            | TerminatorKind::Return
            | TerminatorKind::CoroutineDrop => {
                // Invalidate all borrows of local places
                let borrow_set = self.borrow_set;
                let start = self.location_table.start_index(location);
                for (i, data) in borrow_set.iter_enumerated() {
                    if borrow_of_local_data(data.borrowed_place) {
                        self.all_facts.loan_invalidated_at.push((start, i));
                    }
                }
            }
            TerminatorKind::InlineAsm {
                asm_macro: _,
                template: _,
                operands,
                options: _,
                line_spans: _,
                targets: _,
                unwind: _,
            } => {
                for op in operands {
                    match op {
                        InlineAsmOperand::In { reg: _, value } => {
                            self.consume_operand(location, value);
                        }
                        InlineAsmOperand::Out { reg: _, late: _, place, .. } => {
                            if let &Some(place) = place {
                                self.mutate_place(location, place, Shallow(None));
                            }
                        }
                        InlineAsmOperand::InOut { reg: _, late: _, in_value, out_place } => {
                            self.consume_operand(location, in_value);
                            if let &Some(out_place) = out_place {
                                self.mutate_place(location, out_place, Shallow(None));
                            }
                        }
                        InlineAsmOperand::Const { value: _ }
                        | InlineAsmOperand::SymFn { value: _ }
                        | InlineAsmOperand::SymStatic { def_id: _ }
                        | InlineAsmOperand::Label { target_index: _ } => {}
                    }
                }
            }
            TerminatorKind::Goto { target: _ }
            | TerminatorKind::UnwindTerminate(_)
            | TerminatorKind::Unreachable
            | TerminatorKind::FalseEdge { real_target: _, imaginary_target: _ }
            | TerminatorKind::FalseUnwind { real_target: _, unwind: _ } => {
                // no data used, thus irrelevant to borrowck
            }
        }

        self.super_terminator(terminator, location);
    }
}

impl<'a, 'tcx> LoanInvalidationsGenerator<'a, 'tcx> {
    /// Simulates mutation of a place.
    fn mutate_place(&mut self, location: Location, place: Place<'tcx>, kind: AccessDepth) {
        self.access_place(
            location,
            place,
            (kind, Write(WriteKind::Mutate)),
            LocalMutationIsAllowed::ExceptUpvars,
        );
    }

    /// Simulates consumption of an operand.
    fn consume_operand(&mut self, location: Location, operand: &Operand<'tcx>) {
        match *operand {
            Operand::Copy(place) => {
                self.access_place(
                    location,
                    place,
                    (Deep, Read(ReadKind::Copy)),
                    LocalMutationIsAllowed::No,
                );
            }
            Operand::Move(place) => {
                self.access_place(
                    location,
                    place,
                    (Deep, Write(WriteKind::Move)),
                    LocalMutationIsAllowed::Yes,
                );
            }
            Operand::Constant(_) => {}
        }
    }

    // Simulates consumption of an rvalue
    fn consume_rvalue(&mut self, location: Location, rvalue: &Rvalue<'tcx>) {
        match rvalue {
            &Rvalue::Ref(_ /*rgn*/, bk, place) => {
                let access_kind = match bk {
                    BorrowKind::Fake(FakeBorrowKind::Shallow) => {
                        (Shallow(Some(ArtificialField::FakeBorrow)), Read(ReadKind::Borrow(bk)))
                    }
                    BorrowKind::Shared | BorrowKind::Fake(FakeBorrowKind::Deep) => {
                        (Deep, Read(ReadKind::Borrow(bk)))
                    }
                    BorrowKind::Mut { .. } => {
                        let wk = WriteKind::MutableBorrow(bk);
                        if allow_two_phase_borrow(bk) {
                            (Deep, Reservation(wk))
                        } else {
                            (Deep, Write(wk))
                        }
                    }
                };

                self.access_place(location, place, access_kind, LocalMutationIsAllowed::No);
            }

            &Rvalue::RawPtr(mutability, place) => {
                let access_kind = match mutability {
                    Mutability::Mut => (
                        Deep,
                        Write(WriteKind::MutableBorrow(BorrowKind::Mut {
                            kind: mir::MutBorrowKind::Default,
                        })),
                    ),
                    Mutability::Not => (Deep, Read(ReadKind::Borrow(BorrowKind::Shared))),
                };

                self.access_place(location, place, access_kind, LocalMutationIsAllowed::No);
            }

            Rvalue::ThreadLocalRef(_) => {}

            Rvalue::Use(operand)
            | Rvalue::Repeat(operand, _)
            | Rvalue::UnaryOp(_ /*un_op*/, operand)
            | Rvalue::Cast(_ /*cast_kind*/, operand, _ /*ty*/)
            | Rvalue::ShallowInitBox(operand, _ /*ty*/) => self.consume_operand(location, operand),

            &Rvalue::CopyForDeref(place) => {
                let op = &Operand::Copy(place);
                self.consume_operand(location, op);
            }

            &(Rvalue::Len(place) | Rvalue::Discriminant(place)) => {
                let af = match rvalue {
                    Rvalue::Len(..) => Some(ArtificialField::ArrayLength),
                    Rvalue::Discriminant(..) => None,
                    _ => unreachable!(),
                };
                self.access_place(
                    location,
                    place,
                    (Shallow(af), Read(ReadKind::Copy)),
                    LocalMutationIsAllowed::No,
                );
            }

            Rvalue::BinaryOp(_bin_op, box (operand1, operand2)) => {
                self.consume_operand(location, operand1);
                self.consume_operand(location, operand2);
            }

            Rvalue::NullaryOp(_op, _ty) => {}

            Rvalue::Aggregate(_, operands) => {
                for operand in operands {
                    self.consume_operand(location, operand);
                }
            }
        }
    }

    /// Simulates an access to a place.
    fn access_place(
        &mut self,
        location: Location,
        place: Place<'tcx>,
        kind: (AccessDepth, ReadOrWrite),
        _is_local_mutation_allowed: LocalMutationIsAllowed,
    ) {
        let (sd, rw) = kind;
        // note: not doing check_access_permissions checks because they don't generate invalidates
        self.check_access_for_conflict(location, place, sd, rw);
    }

    fn check_access_for_conflict(
        &mut self,
        location: Location,
        place: Place<'tcx>,
        sd: AccessDepth,
        rw: ReadOrWrite,
    ) {
        debug!(
            "check_access_for_conflict(location={:?}, place={:?}, sd={:?}, rw={:?})",
            location, place, sd, rw,
        );
        each_borrow_involving_path(
            self,
            self.tcx,
            self.body,
            (sd, place),
            self.borrow_set,
            |_| true,
            |this, borrow_index, borrow| {
                match (rw, borrow.kind) {
                    // Obviously an activation is compatible with its own
                    // reservation (or even prior activating uses of same
                    // borrow); so don't check if they interfere.
                    //
                    // NOTE: *reservations* do conflict with themselves;
                    // thus aren't injecting unsoundness w/ this check.)
                    (Activation(_, activating), _) if activating == borrow_index => {
                        // Activating a borrow doesn't generate any invalidations, since we
                        // have already taken the reservation
                    }

                    (Read(_), BorrowKind::Fake(_) | BorrowKind::Shared)
                    | (
                        Read(ReadKind::Borrow(BorrowKind::Fake(FakeBorrowKind::Shallow))),
                        BorrowKind::Mut { .. },
                    ) => {
                        // Reads don't invalidate shared or shallow borrows
                    }

                    (Read(_), BorrowKind::Mut { .. }) => {
                        // Reading from mere reservations of mutable-borrows is OK.
                        if !is_active(this.dominators, borrow, location) {
                            // If the borrow isn't active yet, reads don't invalidate it
                            assert!(allow_two_phase_borrow(borrow.kind));
                            return Control::Continue;
                        }

                        // Unique and mutable borrows are invalidated by reads from any
                        // involved path
                        this.emit_loan_invalidated_at(borrow_index, location);
                    }

                    (Reservation(_) | Activation(_, _) | Write(_), _) => {
                        // unique or mutable borrows are invalidated by writes.
                        // Reservations count as writes since we need to check
                        // that activating the borrow will be OK
                        // FIXME(bob_twinkles) is this actually the right thing to do?
                        this.emit_loan_invalidated_at(borrow_index, location);
                    }
                }
                Control::Continue
            },
        );
    }

    /// Generates a new `loan_invalidated_at(L, B)` fact.
    fn emit_loan_invalidated_at(&mut self, b: BorrowIndex, l: Location) {
        let lidx = self.location_table.start_index(l);
        self.all_facts.loan_invalidated_at.push((lidx, b));
    }

    fn check_activations(&mut self, location: Location) {
        // Two-phase borrow support: For each activation that is newly
        // generated at this statement, check if it interferes with
        // another borrow.
        for &borrow_index in self.borrow_set.activations_at_location(location) {
            let borrow = &self.borrow_set[borrow_index];

            // only mutable borrows should be 2-phase
            assert!(match borrow.kind {
                BorrowKind::Shared | BorrowKind::Fake(_) => false,
                BorrowKind::Mut { .. } => true,
            });

            self.access_place(
                location,
                borrow.borrowed_place,
                (Deep, Activation(WriteKind::MutableBorrow(borrow.kind), borrow_index)),
                LocalMutationIsAllowed::No,
            );

            // We do not need to call `check_if_path_or_subpath_is_moved`
            // again, as we already called it when we made the
            // initial reservation.
        }
    }
}