rust/src/librustc_mir/borrow_check/borrow_set.rs

use crate::borrow_check::place_ext::PlaceExt;
use crate::borrow_check::nll::ToRegionVid;
use crate::dataflow::indexes::BorrowIndex;
use crate::dataflow::move_paths::MoveData;
use rustc::mir::traversal;
use rustc::mir::visit::{PlaceContext, Visitor, NonUseContext, MutatingUseContext};
use rustc::mir::{self, Location, Mir, Local};
use rustc::ty::{RegionVid, TyCtxt};
use rustc::util::nodemap::{FxHashMap, FxHashSet};
use rustc_data_structures::indexed_vec::IndexVec;
use rustc_data_structures::bit_set::BitSet;
use std::fmt;
use std::ops::Index;

crate struct BorrowSet<'tcx> {
    /// The fundamental map relating bitvector indexes to the borrows
    /// in the MIR.
    crate borrows: IndexVec<BorrowIndex, BorrowData<'tcx>>,

    /// Each borrow is also uniquely identified in the MIR by the
    /// `Location` of the assignment statement in which it appears on
    /// the right hand side; we map each such location to the
    /// corresponding `BorrowIndex`.
    crate location_map: FxHashMap<Location, BorrowIndex>,

    /// Locations which activate borrows.
    /// NOTE: a given location may activate more than one borrow in the future
    /// when more general two-phase borrow support is introduced, but for now we
    /// only need to store one borrow index.
    crate activation_map: FxHashMap<Location, Vec<BorrowIndex>>,

    /// Map from local to all the borrows on that local.
    crate local_map: FxHashMap<mir::Local, FxHashSet<BorrowIndex>>,

    crate locals_state_at_exit: LocalsStateAtExit,
}

impl<'tcx> Index<BorrowIndex> for BorrowSet<'tcx> {
    type Output = BorrowData<'tcx>;

    fn index(&self, index: BorrowIndex) -> &BorrowData<'tcx> {
        &self.borrows[index]
    }
}

/// Location where a two-phase borrow is activated, if a borrow
/// is in fact a two-phase borrow.
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
crate enum TwoPhaseActivation {
    NotTwoPhase,
    NotActivated,
    ActivatedAt(Location),
}

#[derive(Debug, Clone)]
crate struct BorrowData<'tcx> {
    /// Location where the borrow reservation starts.
    /// In many cases, this will be equal to the activation location but not always.
    crate reserve_location: Location,
    /// Location where the borrow is activated.
    crate activation_location: TwoPhaseActivation,
    /// What kind of borrow this is
    crate kind: mir::BorrowKind,
    /// The region for which this borrow is live
    crate region: RegionVid,
    /// Place from which we are borrowing
    crate borrowed_place: mir::Place<'tcx>,
    /// Place to which the borrow was stored
    crate assigned_place: mir::Place<'tcx>,
}

impl<'tcx> fmt::Display for BorrowData<'tcx> {
    fn fmt(&self, w: &mut fmt::Formatter<'_>) -> fmt::Result {
        let kind = match self.kind {
            mir::BorrowKind::Shared => "",
            mir::BorrowKind::Shallow => "shallow ",
            mir::BorrowKind::Unique => "uniq ",
            mir::BorrowKind::Mut { .. } => "mut ",
        };
        write!(w, "&{:?} {}{:?}", self.region, kind, self.borrowed_place)
    }
}

crate enum LocalsStateAtExit {
    AllAreInvalidated,
    SomeAreInvalidated { has_storage_dead_or_moved: BitSet<Local> }
}

impl LocalsStateAtExit {
    fn build(
        locals_are_invalidated_at_exit: bool,
        mir: &Mir<'tcx>,
        move_data: &MoveData<'tcx>
    ) -> Self {
        struct HasStorageDead(BitSet<Local>);

        impl<'tcx> Visitor<'tcx> for HasStorageDead {
            fn visit_local(&mut self, local: &Local, ctx: PlaceContext<'tcx>, _: Location) {
                if ctx == PlaceContext::NonUse(NonUseContext::StorageDead) {
                    self.0.insert(*local);
                }
            }
        }

        if locals_are_invalidated_at_exit {
            LocalsStateAtExit::AllAreInvalidated
        } else {
            let mut has_storage_dead = HasStorageDead(BitSet::new_empty(mir.local_decls.len()));
            has_storage_dead.visit_mir(mir);
            let mut has_storage_dead_or_moved = has_storage_dead.0;
            for move_out in &move_data.moves {
                if let Some(index) = move_data.base_local(move_out.path) {
                    has_storage_dead_or_moved.insert(index);

                }
            }
            LocalsStateAtExit::SomeAreInvalidated{ has_storage_dead_or_moved }
        }
    }
}

impl<'tcx> BorrowSet<'tcx> {
    pub fn build(
        tcx: TyCtxt<'_, '_, 'tcx>,
        mir: &Mir<'tcx>,
        locals_are_invalidated_at_exit: bool,
        move_data: &MoveData<'tcx>
    ) -> Self {

        let mut visitor = GatherBorrows {
            tcx,
            mir,
            idx_vec: IndexVec::new(),
            location_map: Default::default(),
            activation_map: Default::default(),
            local_map: Default::default(),
            pending_activations: Default::default(),
            locals_state_at_exit:
                LocalsStateAtExit::build(locals_are_invalidated_at_exit, mir, move_data),
        };

        for (block, block_data) in traversal::preorder(mir) {
            visitor.visit_basic_block_data(block, block_data);
        }

        BorrowSet {
            borrows: visitor.idx_vec,
            location_map: visitor.location_map,
            activation_map: visitor.activation_map,
            local_map: visitor.local_map,
            locals_state_at_exit: visitor.locals_state_at_exit,
        }
    }

    crate fn activations_at_location(&self, location: Location) -> &[BorrowIndex] {
        self.activation_map
            .get(&location)
            .map(|activations| &activations[..])
            .unwrap_or(&[])
    }
}

struct GatherBorrows<'a, 'gcx: 'tcx, 'tcx: 'a> {
    tcx: TyCtxt<'a, 'gcx, 'tcx>,
    mir: &'a Mir<'tcx>,
    idx_vec: IndexVec<BorrowIndex, BorrowData<'tcx>>,
    location_map: FxHashMap<Location, BorrowIndex>,
    activation_map: FxHashMap<Location, Vec<BorrowIndex>>,
    local_map: FxHashMap<mir::Local, FxHashSet<BorrowIndex>>,

    /// When we encounter a 2-phase borrow statement, it will always
    /// be assigning into a temporary TEMP:
    ///
    ///    TEMP = &foo
    ///
    /// We add TEMP into this map with `b`, where `b` is the index of
    /// the borrow. When we find a later use of this activation, we
    /// remove from the map (and add to the "tombstone" set below).
    pending_activations: FxHashMap<mir::Local, BorrowIndex>,

    locals_state_at_exit: LocalsStateAtExit,
}

impl<'a, 'gcx, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'gcx, 'tcx> {
    fn visit_assign(
        &mut self,
        block: mir::BasicBlock,
        assigned_place: &mir::Place<'tcx>,
        rvalue: &mir::Rvalue<'tcx>,
        location: mir::Location,
    ) {
        if let mir::Rvalue::Ref(region, kind, ref borrowed_place) = *rvalue {
            if borrowed_place.ignore_borrow(
                self.tcx, self.mir, &self.locals_state_at_exit) {
                return;
            }

            let region = region.to_region_vid();

            let borrow = BorrowData {
                kind,
                region,
                reserve_location: location,
                activation_location: TwoPhaseActivation::NotTwoPhase,
                borrowed_place: borrowed_place.clone(),
                assigned_place: assigned_place.clone(),
            };
            let idx = self.idx_vec.push(borrow);
            self.location_map.insert(location, idx);

            self.insert_as_pending_if_two_phase(location, &assigned_place, kind, idx);

            if let Some(local) = borrowed_place.root_local() {
                self.local_map.entry(local).or_default().insert(idx);
            }
        }

        self.super_assign(block, assigned_place, rvalue, location)
    }

    fn visit_local(
        &mut self,
        temp: &Local,
        context: PlaceContext<'tcx>,
        location: Location,
    ) {
        if !context.is_use() {
            return;
        }

        // We found a use of some temporary TMP
        // check whether we (earlier) saw a 2-phase borrow like
        //
        //     TMP = &mut place
        if let Some(&borrow_index) = self.pending_activations.get(temp) {
            let borrow_data = &mut self.idx_vec[borrow_index];

            // Watch out: the use of TMP in the borrow itself
            // doesn't count as an activation. =)
            if borrow_data.reserve_location == location &&
                context == PlaceContext::MutatingUse(MutatingUseContext::Store)
            {
                return;
            }

            if let TwoPhaseActivation::ActivatedAt(other_location) =
                    borrow_data.activation_location {
                span_bug!(
                    self.mir.source_info(location).span,
                    "found two uses for 2-phase borrow temporary {:?}: \
                     {:?} and {:?}",
                    temp,
                    location,
                    other_location,
                );
            }

            // Otherwise, this is the unique later use that we expect.
            // Double check: This borrow is indeed a two-phase borrow (that is,
            // we are 'transitioning' from `NotActivated` to `ActivatedAt`) and
            // we've not found any other activations (checked above).
            assert_eq!(
                borrow_data.activation_location,
                TwoPhaseActivation::NotActivated,
                "never found an activation for this borrow!",
            );
            self.activation_map
                .entry(location)
                .or_default()
                .push(borrow_index);

            borrow_data.activation_location = TwoPhaseActivation::ActivatedAt(location);
        }
    }

    fn visit_rvalue(&mut self, rvalue: &mir::Rvalue<'tcx>, location: mir::Location) {
        if let mir::Rvalue::Ref(region, kind, ref place) = *rvalue {
            // double-check that we already registered a BorrowData for this

            let borrow_index = self.location_map[&location];
            let borrow_data = &self.idx_vec[borrow_index];
            assert_eq!(borrow_data.reserve_location, location);
            assert_eq!(borrow_data.kind, kind);
            assert_eq!(borrow_data.region, region.to_region_vid());
            assert_eq!(borrow_data.borrowed_place, *place);
        }

        return self.super_rvalue(rvalue, location);
    }

    fn visit_statement(
        &mut self,
        block: mir::BasicBlock,
        statement: &mir::Statement<'tcx>,
        location: Location,
    ) {
        return self.super_statement(block, statement, location);
    }
}

impl<'a, 'gcx, 'tcx> GatherBorrows<'a, 'gcx, 'tcx> {
    /// Returns `true` if the borrow represented by `kind` is
    /// allowed to be split into separate Reservation and
    /// Activation phases.
    fn allow_two_phase_borrow(&self, kind: mir::BorrowKind) -> bool {
        kind.allows_two_phase_borrow()
            || self.tcx.sess.opts.debugging_opts.two_phase_beyond_autoref
    }

    /// If this is a two-phase borrow, then we will record it
    /// as "pending" until we find the activating use.
    fn insert_as_pending_if_two_phase(
        &mut self,
        start_location: Location,
        assigned_place: &mir::Place<'tcx>,
        kind: mir::BorrowKind,
        borrow_index: BorrowIndex,
    ) {
        debug!(
            "Borrows::insert_as_pending_if_two_phase({:?}, {:?}, {:?})",
            start_location, assigned_place, borrow_index,
        );

        if !self.allow_two_phase_borrow(kind) {
            debug!("  -> {:?}", start_location);
            return;
        }

        // When we encounter a 2-phase borrow statement, it will always
        // be assigning into a temporary TEMP:
        //
        //    TEMP = &foo
        //
        // so extract `temp`.
        let temp = if let &mir::Place::Base(mir::PlaceBase::Local(temp)) = assigned_place {
            temp
        } else {
            span_bug!(
                self.mir.source_info(start_location).span,
                "expected 2-phase borrow to assign to a local, not `{:?}`",
                assigned_place,
            );
        };

        // Consider the borrow not activated to start. When we find an activation, we'll update
        // this field.
        {
            let borrow_data = &mut self.idx_vec[borrow_index];
            borrow_data.activation_location = TwoPhaseActivation::NotActivated;
        }

        // Insert `temp` into the list of pending activations. From
        // now on, we'll be on the lookout for a use of it. Note that
        // we are guaranteed that this use will come after the
        // assignment.
        let old_value = self.pending_activations.insert(temp, borrow_index);
        if let Some(old_index) = old_value {
            span_bug!(self.mir.source_info(start_location).span,
                      "found already pending activation for temp: {:?} \
                       at borrow_index: {:?} with associated data {:?}",
                      temp, old_index, self.idx_vec[old_index]);
        }
    }
}