src/librustc_mir/borrow_check/borrow_set.rs

   1 use crate::borrow_check::nll::ToRegionVid;
   2 use crate::borrow_check::path_utils::allow_two_phase_borrow;
   3 use crate::borrow_check::place_ext::PlaceExt;
   4 use crate::dataflow::indexes::BorrowIndex;
   5 use crate::dataflow::move_paths::MoveData;
   6 use rustc::mir::traversal;
   7 use rustc::mir::visit::{MutatingUseContext, NonUseContext, PlaceContext, Visitor};
   8 use rustc::mir::{self, Body, Local, Location, ReadOnlyBodyAndCache};
   9 use rustc::ty::{RegionVid, TyCtxt};
  10 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  11 use rustc_index::bit_set::BitSet;
  12 use rustc_index::vec::IndexVec;
  13 use std::fmt;
  14 use std::ops::Index;
  15
  16 crate struct BorrowSet<'tcx> {
  17     /// The fundamental map relating bitvector indexes to the borrows
  18     /// in the MIR.
  19     crate borrows: IndexVec<BorrowIndex, BorrowData<'tcx>>,
  20
  21     /// Each borrow is also uniquely identified in the MIR by the
  22     /// `Location` of the assignment statement in which it appears on
  23     /// the right hand side; we map each such location to the
  24     /// corresponding `BorrowIndex`.
  25     crate location_map: FxHashMap<Location, BorrowIndex>,
  26
  27     /// Locations which activate borrows.
  28     /// NOTE: a given location may activate more than one borrow in the future
  29     /// when more general two-phase borrow support is introduced, but for now we
  30     /// only need to store one borrow index.
  31     crate activation_map: FxHashMap<Location, Vec<BorrowIndex>>,
  32
  33     /// Map from local to all the borrows on that local.
  34     crate local_map: FxHashMap<mir::Local, FxHashSet<BorrowIndex>>,
  35
  36     crate locals_state_at_exit: LocalsStateAtExit,
  37 }
  38
  39 impl<'tcx> Index<BorrowIndex> for BorrowSet<'tcx> {
  40     type Output = BorrowData<'tcx>;
  41
  42     fn index(&self, index: BorrowIndex) -> &BorrowData<'tcx> {
  43         &self.borrows[index]
  44     }
  45 }
  46
  47 /// Location where a two-phase borrow is activated, if a borrow
  48 /// is in fact a two-phase borrow.
  49 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  50 crate enum TwoPhaseActivation {
  51     NotTwoPhase,
  52     NotActivated,
  53     ActivatedAt(Location),
  54 }
  55
  56 #[derive(Debug, Clone)]
  57 crate struct BorrowData<'tcx> {
  58     /// Location where the borrow reservation starts.
  59     /// In many cases, this will be equal to the activation location but not always.
  60     crate reserve_location: Location,
  61     /// Location where the borrow is activated.
  62     crate activation_location: TwoPhaseActivation,
  63     /// What kind of borrow this is
  64     crate kind: mir::BorrowKind,
  65     /// The region for which this borrow is live
  66     crate region: RegionVid,
  67     /// Place from which we are borrowing
  68     crate borrowed_place: mir::Place<'tcx>,
  69     /// Place to which the borrow was stored
  70     crate assigned_place: mir::Place<'tcx>,
  71 }
  72
  73 impl<'tcx> fmt::Display for BorrowData<'tcx> {
  74     fn fmt(&self, w: &mut fmt::Formatter<'_>) -> fmt::Result {
  75         let kind = match self.kind {
  76             mir::BorrowKind::Shared => "",
  77             mir::BorrowKind::Shallow => "shallow ",
  78             mir::BorrowKind::Unique => "uniq ",
  79             mir::BorrowKind::Mut { .. } => "mut ",
  80         };
  81         write!(w, "&{:?} {}{:?}", self.region, kind, self.borrowed_place)
  82     }
  83 }
  84
  85 crate enum LocalsStateAtExit {
  86     AllAreInvalidated,
  87     SomeAreInvalidated { has_storage_dead_or_moved: BitSet<Local> },
  88 }
  89
  90 impl LocalsStateAtExit {
  91     fn build(
  92         locals_are_invalidated_at_exit: bool,
  93         body: ReadOnlyBodyAndCache<'_, 'tcx>,
  94         move_data: &MoveData<'tcx>,
  95     ) -> Self {
  96         struct HasStorageDead(BitSet<Local>);
  97
  98         impl<'tcx> Visitor<'tcx> for HasStorageDead {
  99             fn visit_local(&mut self, local: &Local, ctx: PlaceContext, _: Location) {
 100                 if ctx == PlaceContext::NonUse(NonUseContext::StorageDead) {
 101                     self.0.insert(*local);
 102                 }
 103             }
 104         }
 105
 106         if locals_are_invalidated_at_exit {
 107             LocalsStateAtExit::AllAreInvalidated
 108         } else {
 109             let mut has_storage_dead = HasStorageDead(BitSet::new_empty(body.local_decls.len()));
 110             has_storage_dead.visit_body(body);
 111             let mut has_storage_dead_or_moved = has_storage_dead.0;
 112             for move_out in &move_data.moves {
 113                 if let Some(index) = move_data.base_local(move_out.path) {
 114                     has_storage_dead_or_moved.insert(index);
 115                 }
 116             }
 117             LocalsStateAtExit::SomeAreInvalidated { has_storage_dead_or_moved }
 118         }
 119     }
 120 }
 121
 122 impl<'tcx> BorrowSet<'tcx> {
 123     pub fn build(
 124         tcx: TyCtxt<'tcx>,
 125         body: ReadOnlyBodyAndCache<'_, 'tcx>,
 126         locals_are_invalidated_at_exit: bool,
 127         move_data: &MoveData<'tcx>,
 128     ) -> Self {
 129         let mut visitor = GatherBorrows {
 130             tcx,
 131             body: &body,
 132             idx_vec: IndexVec::new(),
 133             location_map: Default::default(),
 134             activation_map: Default::default(),
 135             local_map: Default::default(),
 136             pending_activations: Default::default(),
 137             locals_state_at_exit: LocalsStateAtExit::build(
 138                 locals_are_invalidated_at_exit,
 139                 body,
 140                 move_data,
 141             ),
 142         };
 143
 144         for (block, block_data) in traversal::preorder(&body) {
 145             visitor.visit_basic_block_data(block, block_data);
 146         }
 147
 148         BorrowSet {
 149             borrows: visitor.idx_vec,
 150             location_map: visitor.location_map,
 151             activation_map: visitor.activation_map,
 152             local_map: visitor.local_map,
 153             locals_state_at_exit: visitor.locals_state_at_exit,
 154         }
 155     }
 156
 157     crate fn activations_at_location(&self, location: Location) -> &[BorrowIndex] {
 158         self.activation_map.get(&location).map(|activations| &activations[..]).unwrap_or(&[])
 159     }
 160 }
 161
 162 struct GatherBorrows<'a, 'tcx> {
 163     tcx: TyCtxt<'tcx>,
 164     body: &'a Body<'tcx>,
 165     idx_vec: IndexVec<BorrowIndex, BorrowData<'tcx>>,
 166     location_map: FxHashMap<Location, BorrowIndex>,
 167     activation_map: FxHashMap<Location, Vec<BorrowIndex>>,
 168     local_map: FxHashMap<mir::Local, FxHashSet<BorrowIndex>>,
 169
 170     /// When we encounter a 2-phase borrow statement, it will always
 171     /// be assigning into a temporary TEMP:
 172     ///
 173     ///    TEMP = &foo
 174     ///
 175     /// We add TEMP into this map with `b`, where `b` is the index of
 176     /// the borrow. When we find a later use of this activation, we
 177     /// remove from the map (and add to the "tombstone" set below).
 178     pending_activations: FxHashMap<mir::Local, BorrowIndex>,
 179
 180     locals_state_at_exit: LocalsStateAtExit,
 181 }
 182
 183 impl<'a, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'tcx> {
 184     fn visit_assign(
 185         &mut self,
 186         assigned_place: &mir::Place<'tcx>,
 187         rvalue: &mir::Rvalue<'tcx>,
 188         location: mir::Location,
 189     ) {
 190         if let mir::Rvalue::Ref(region, kind, ref borrowed_place) = *rvalue {
 191             if borrowed_place.ignore_borrow(self.tcx, self.body, &self.locals_state_at_exit) {
 192                 debug!("ignoring_borrow of {:?}", borrowed_place);
 193                 return;
 194             }
 195
 196             let region = region.to_region_vid();
 197
 198             let borrow = BorrowData {
 199                 kind,
 200                 region,
 201                 reserve_location: location,
 202                 activation_location: TwoPhaseActivation::NotTwoPhase,
 203                 borrowed_place: *borrowed_place,
 204                 assigned_place: *assigned_place,
 205             };
 206             let idx = self.idx_vec.push(borrow);
 207             self.location_map.insert(location, idx);
 208
 209             self.insert_as_pending_if_two_phase(location, &assigned_place, kind, idx);
 210
 211             self.local_map.entry(borrowed_place.local).or_default().insert(idx);
 212         }
 213
 214         self.super_assign(assigned_place, rvalue, location)
 215     }
 216
 217     fn visit_local(&mut self, temp: &Local, context: PlaceContext, location: Location) {
 218         if !context.is_use() {
 219             return;
 220         }
 221
 222         // We found a use of some temporary TMP
 223         // check whether we (earlier) saw a 2-phase borrow like
 224         //
 225         //     TMP = &mut place
 226         if let Some(&borrow_index) = self.pending_activations.get(temp) {
 227             let borrow_data = &mut self.idx_vec[borrow_index];
 228
 229             // Watch out: the use of TMP in the borrow itself
 230             // doesn't count as an activation. =)
 231             if borrow_data.reserve_location == location
 232                 && context == PlaceContext::MutatingUse(MutatingUseContext::Store)
 233             {
 234                 return;
 235             }
 236
 237             if let TwoPhaseActivation::ActivatedAt(other_location) = borrow_data.activation_location
 238             {
 239                 span_bug!(
 240                     self.body.source_info(location).span,
 241                     "found two uses for 2-phase borrow temporary {:?}: \
 242                      {:?} and {:?}",
 243                     temp,
 244                     location,
 245                     other_location,
 246                 );
 247             }
 248
 249             // Otherwise, this is the unique later use that we expect.
 250             // Double check: This borrow is indeed a two-phase borrow (that is,
 251             // we are 'transitioning' from `NotActivated` to `ActivatedAt`) and
 252             // we've not found any other activations (checked above).
 253             assert_eq!(
 254                 borrow_data.activation_location,
 255                 TwoPhaseActivation::NotActivated,
 256                 "never found an activation for this borrow!",
 257             );
 258             self.activation_map.entry(location).or_default().push(borrow_index);
 259
 260             borrow_data.activation_location = TwoPhaseActivation::ActivatedAt(location);
 261         }
 262     }
 263
 264     fn visit_rvalue(&mut self, rvalue: &mir::Rvalue<'tcx>, location: mir::Location) {
 265         if let mir::Rvalue::Ref(region, kind, ref place) = *rvalue {
 266             // double-check that we already registered a BorrowData for this
 267
 268             let borrow_index = self.location_map[&location];
 269             let borrow_data = &self.idx_vec[borrow_index];
 270             assert_eq!(borrow_data.reserve_location, location);
 271             assert_eq!(borrow_data.kind, kind);
 272             assert_eq!(borrow_data.region, region.to_region_vid());
 273             assert_eq!(borrow_data.borrowed_place, *place);
 274         }
 275
 276         return self.super_rvalue(rvalue, location);
 277     }
 278 }
 279
 280 impl<'a, 'tcx> GatherBorrows<'a, 'tcx> {
 281     /// If this is a two-phase borrow, then we will record it
 282     /// as "pending" until we find the activating use.
 283     fn insert_as_pending_if_two_phase(
 284         &mut self,
 285         start_location: Location,
 286         assigned_place: &mir::Place<'tcx>,
 287         kind: mir::BorrowKind,
 288         borrow_index: BorrowIndex,
 289     ) {
 290         debug!(
 291             "Borrows::insert_as_pending_if_two_phase({:?}, {:?}, {:?})",
 292             start_location, assigned_place, borrow_index,
 293         );
 294
 295         if !allow_two_phase_borrow(kind) {
 296             debug!("  -> {:?}", start_location);
 297             return;
 298         }
 299
 300         // When we encounter a 2-phase borrow statement, it will always
 301         // be assigning into a temporary TEMP:
 302         //
 303         //    TEMP = &foo
 304         //
 305         // so extract `temp`.
 306         let temp = if let Some(temp) = assigned_place.as_local() {
 307             temp
 308         } else {
 309             span_bug!(
 310                 self.body.source_info(start_location).span,
 311                 "expected 2-phase borrow to assign to a local, not `{:?}`",
 312                 assigned_place,
 313             );
 314         };
 315
 316         // Consider the borrow not activated to start. When we find an activation, we'll update
 317         // this field.
 318         {
 319             let borrow_data = &mut self.idx_vec[borrow_index];
 320             borrow_data.activation_location = TwoPhaseActivation::NotActivated;
 321         }
 322
 323         // Insert `temp` into the list of pending activations. From
 324         // now on, we'll be on the lookout for a use of it. Note that
 325         // we are guaranteed that this use will come after the
 326         // assignment.
 327         let old_value = self.pending_activations.insert(temp, borrow_index);
 328         if let Some(old_index) = old_value {
 329             span_bug!(
 330                 self.body.source_info(start_location).span,
 331                 "found already pending activation for temp: {:?} \
 332                        at borrow_index: {:?} with associated data {:?}",
 333                 temp,
 334                 old_index,
 335                 self.idx_vec[old_index]
 336             );
 337         }
 338     }
 339 }