compiler/rustc_borrowck/src/dataflow.rs

   1 #![deny(rustc::untranslatable_diagnostic)]
   2 #![deny(rustc::diagnostic_outside_of_impl)]
   3 use rustc_data_structures::fx::FxHashMap;
   4 use rustc_index::bit_set::BitSet;
   5 use rustc_middle::mir::{self, BasicBlock, Body, Location, Place};
   6 use rustc_middle::ty::RegionVid;
   7 use rustc_middle::ty::TyCtxt;
   8 use rustc_mir_dataflow::impls::{EverInitializedPlaces, MaybeUninitializedPlaces};
   9 use rustc_mir_dataflow::ResultsVisitable;
  10 use rustc_mir_dataflow::{self, fmt::DebugWithContext, CallReturnPlaces, GenKill};
  11 use rustc_mir_dataflow::{Analysis, Direction, Results};
  12 use std::fmt;
  13
  14 use crate::{
  15     places_conflict, BorrowSet, PlaceConflictBias, PlaceExt, RegionInferenceContext, ToRegionVid,
  16 };
  17
  18 /// A tuple with named fields that can hold either the results or the transient state of the
  19 /// dataflow analyses used by the borrow checker.
  20 #[derive(Debug)]
  21 pub struct BorrowckAnalyses<B, U, E> {
  22     pub borrows: B,
  23     pub uninits: U,
  24     pub ever_inits: E,
  25 }
  26
  27 /// The results of the dataflow analyses used by the borrow checker.
  28 pub type BorrowckResults<'mir, 'tcx> = BorrowckAnalyses<
  29     Results<'tcx, Borrows<'mir, 'tcx>>,
  30     Results<'tcx, MaybeUninitializedPlaces<'mir, 'tcx>>,
  31     Results<'tcx, EverInitializedPlaces<'mir, 'tcx>>,
  32 >;
  33
  34 /// The transient state of the dataflow analyses used by the borrow checker.
  35 pub type BorrowckFlowState<'mir, 'tcx> =
  36     <BorrowckResults<'mir, 'tcx> as ResultsVisitable<'tcx>>::FlowState;
  37
  38 macro_rules! impl_visitable {
  39     ( $(
  40         $T:ident { $( $field:ident : $A:ident ),* $(,)? }
  41     )* ) => { $(
  42         impl<'tcx, $($A),*, D: Direction> ResultsVisitable<'tcx> for $T<$( Results<'tcx, $A> ),*>
  43         where
  44             $( $A: Analysis<'tcx, Direction = D>, )*
  45         {
  46             type Direction = D;
  47             type FlowState = $T<$( $A::Domain ),*>;
  48
  49             fn new_flow_state(&self, body: &mir::Body<'tcx>) -> Self::FlowState {
  50                 $T {
  51                     $( $field: self.$field.analysis.bottom_value(body) ),*
  52                 }
  53             }
  54
  55             fn reset_to_block_entry(
  56                 &self,
  57                 state: &mut Self::FlowState,
  58                 block: BasicBlock,
  59             ) {
  60                 $( state.$field.clone_from(&self.$field.entry_set_for_block(block)); )*
  61             }
  62
  63             fn reconstruct_before_statement_effect(
  64                 &self,
  65                 state: &mut Self::FlowState,
  66                 stmt: &mir::Statement<'tcx>,
  67                 loc: Location,
  68             ) {
  69                 $( self.$field.analysis
  70                     .apply_before_statement_effect(&mut state.$field, stmt, loc); )*
  71             }
  72
  73             fn reconstruct_statement_effect(
  74                 &self,
  75                 state: &mut Self::FlowState,
  76                 stmt: &mir::Statement<'tcx>,
  77                 loc: Location,
  78             ) {
  79                 $( self.$field.analysis
  80                     .apply_statement_effect(&mut state.$field, stmt, loc); )*
  81             }
  82
  83             fn reconstruct_before_terminator_effect(
  84                 &self,
  85                 state: &mut Self::FlowState,
  86                 term: &mir::Terminator<'tcx>,
  87                 loc: Location,
  88             ) {
  89                 $( self.$field.analysis
  90                     .apply_before_terminator_effect(&mut state.$field, term, loc); )*
  91             }
  92
  93             fn reconstruct_terminator_effect(
  94                 &self,
  95                 state: &mut Self::FlowState,
  96                 term: &mir::Terminator<'tcx>,
  97                 loc: Location,
  98             ) {
  99                 $( self.$field.analysis
 100                     .apply_terminator_effect(&mut state.$field, term, loc); )*
 101             }
 102         }
 103     )* }
 104 }
 105
 106 impl_visitable! {
 107     BorrowckAnalyses { borrows: B, uninits: U, ever_inits: E }
 108 }
 109
 110 rustc_index::newtype_index! {
 111     pub struct BorrowIndex {
 112         DEBUG_FORMAT = "bw{}"
 113     }
 114 }
 115
 116 /// `Borrows` stores the data used in the analyses that track the flow
 117 /// of borrows.
 118 ///
 119 /// It uniquely identifies every borrow (`Rvalue::Ref`) by a
 120 /// `BorrowIndex`, and maps each such index to a `BorrowData`
 121 /// describing the borrow. These indexes are used for representing the
 122 /// borrows in compact bitvectors.
 123 pub struct Borrows<'a, 'tcx> {
 124     tcx: TyCtxt<'tcx>,
 125     body: &'a Body<'tcx>,
 126
 127     borrow_set: &'a BorrowSet<'tcx>,
 128     borrows_out_of_scope_at_location: FxHashMap<Location, Vec<BorrowIndex>>,
 129 }
 130
 131 struct StackEntry {
 132     bb: mir::BasicBlock,
 133     lo: usize,
 134     hi: usize,
 135 }
 136
 137 struct OutOfScopePrecomputer<'a, 'tcx> {
 138     visited: BitSet<mir::BasicBlock>,
 139     visit_stack: Vec<StackEntry>,
 140     body: &'a Body<'tcx>,
 141     regioncx: &'a RegionInferenceContext<'tcx>,
 142     borrows_out_of_scope_at_location: FxHashMap<Location, Vec<BorrowIndex>>,
 143 }
 144
 145 impl<'a, 'tcx> OutOfScopePrecomputer<'a, 'tcx> {
 146     fn new(body: &'a Body<'tcx>, regioncx: &'a RegionInferenceContext<'tcx>) -> Self {
 147         OutOfScopePrecomputer {
 148             visited: BitSet::new_empty(body.basic_blocks.len()),
 149             visit_stack: vec![],
 150             body,
 151             regioncx,
 152             borrows_out_of_scope_at_location: FxHashMap::default(),
 153         }
 154     }
 155 }
 156
 157 impl<'tcx> OutOfScopePrecomputer<'_, 'tcx> {
 158     fn precompute_borrows_out_of_scope(
 159         &mut self,
 160         borrow_index: BorrowIndex,
 161         borrow_region: RegionVid,
 162         location: Location,
 163     ) {
 164         // We visit one BB at a time. The complication is that we may start in the
 165         // middle of the first BB visited (the one containing `location`), in which
 166         // case we may have to later on process the first part of that BB if there
 167         // is a path back to its start.
 168
 169         // For visited BBs, we record the index of the first statement processed.
 170         // (In fully processed BBs this index is 0.) Note also that we add BBs to
 171         // `visited` once they are added to `stack`, before they are actually
 172         // processed, because this avoids the need to look them up again on
 173         // completion.
 174         self.visited.insert(location.block);
 175
 176         let mut first_lo = location.statement_index;
 177         let first_hi = self.body[location.block].statements.len();
 178
 179         self.visit_stack.push(StackEntry { bb: location.block, lo: first_lo, hi: first_hi });
 180
 181         while let Some(StackEntry { bb, lo, hi }) = self.visit_stack.pop() {
 182             // If we process the first part of the first basic block (i.e. we encounter that block
 183             // for the second time), we no longer have to visit its successors again.
 184             let mut finished_early = bb == location.block && hi != first_hi;
 185             for i in lo..=hi {
 186                 let location = Location { block: bb, statement_index: i };
 187                 // If region does not contain a point at the location, then add to list and skip
 188                 // successor locations.
 189                 if !self.regioncx.region_contains(borrow_region, location) {
 190                     debug!("borrow {:?} gets killed at {:?}", borrow_index, location);
 191                     self.borrows_out_of_scope_at_location
 192                         .entry(location)
 193                         .or_default()
 194                         .push(borrow_index);
 195                     finished_early = true;
 196                     break;
 197                 }
 198             }
 199
 200             if !finished_early {
 201                 // Add successor BBs to the work list, if necessary.
 202                 let bb_data = &self.body[bb];
 203                 debug_assert!(hi == bb_data.statements.len());
 204                 for succ_bb in bb_data.terminator().successors() {
 205                     if !self.visited.insert(succ_bb) {
 206                         if succ_bb == location.block && first_lo > 0 {
 207                             // `succ_bb` has been seen before. If it wasn't
 208                             // fully processed, add its first part to `stack`
 209                             // for processing.
 210                             self.visit_stack.push(StackEntry {
 211                                 bb: succ_bb,
 212                                 lo: 0,
 213                                 hi: first_lo - 1,
 214                             });
 215
 216                             // And update this entry with 0, to represent the
 217                             // whole BB being processed.
 218                             first_lo = 0;
 219                         }
 220                     } else {
 221                         // succ_bb hasn't been seen before. Add it to
 222                         // `stack` for processing.
 223                         self.visit_stack.push(StackEntry {
 224                             bb: succ_bb,
 225                             lo: 0,
 226                             hi: self.body[succ_bb].statements.len(),
 227                         });
 228                     }
 229                 }
 230             }
 231         }
 232
 233         self.visited.clear();
 234     }
 235 }
 236
 237 impl<'a, 'tcx> Borrows<'a, 'tcx> {
 238     pub(crate) fn new(
 239         tcx: TyCtxt<'tcx>,
 240         body: &'a Body<'tcx>,
 241         nonlexical_regioncx: &'a RegionInferenceContext<'tcx>,
 242         borrow_set: &'a BorrowSet<'tcx>,
 243     ) -> Self {
 244         let mut prec = OutOfScopePrecomputer::new(body, nonlexical_regioncx);
 245         for (borrow_index, borrow_data) in borrow_set.iter_enumerated() {
 246             let borrow_region = borrow_data.region.to_region_vid();
 247             let location = borrow_data.reserve_location;
 248
 249             prec.precompute_borrows_out_of_scope(borrow_index, borrow_region, location);
 250         }
 251
 252         Borrows {
 253             tcx,
 254             body,
 255             borrow_set,
 256             borrows_out_of_scope_at_location: prec.borrows_out_of_scope_at_location,
 257         }
 258     }
 259
 260     pub fn location(&self, idx: BorrowIndex) -> &Location {
 261         &self.borrow_set[idx].reserve_location
 262     }
 263
 264     /// Add all borrows to the kill set, if those borrows are out of scope at `location`.
 265     /// That means they went out of a nonlexical scope
 266     fn kill_loans_out_of_scope_at_location(
 267         &self,
 268         trans: &mut impl GenKill<BorrowIndex>,
 269         location: Location,
 270     ) {
 271         // NOTE: The state associated with a given `location`
 272         // reflects the dataflow on entry to the statement.
 273         // Iterate over each of the borrows that we've precomputed
 274         // to have went out of scope at this location and kill them.
 275         //
 276         // We are careful always to call this function *before* we
 277         // set up the gen-bits for the statement or
 278         // terminator. That way, if the effect of the statement or
 279         // terminator *does* introduce a new loan of the same
 280         // region, then setting that gen-bit will override any
 281         // potential kill introduced here.
 282         if let Some(indices) = self.borrows_out_of_scope_at_location.get(&location) {
 283             trans.kill_all(indices.iter().copied());
 284         }
 285     }
 286
 287     /// Kill any borrows that conflict with `place`.
 288     fn kill_borrows_on_place(&self, trans: &mut impl GenKill<BorrowIndex>, place: Place<'tcx>) {
 289         debug!("kill_borrows_on_place: place={:?}", place);
 290
 291         let other_borrows_of_local = self
 292             .borrow_set
 293             .local_map
 294             .get(&place.local)
 295             .into_iter()
 296             .flat_map(|bs| bs.iter())
 297             .copied();
 298
 299         // If the borrowed place is a local with no projections, all other borrows of this
 300         // local must conflict. This is purely an optimization so we don't have to call
 301         // `places_conflict` for every borrow.
 302         if place.projection.is_empty() {
 303             if !self.body.local_decls[place.local].is_ref_to_static() {
 304                 trans.kill_all(other_borrows_of_local);
 305             }
 306             return;
 307         }
 308
 309         // By passing `PlaceConflictBias::NoOverlap`, we conservatively assume that any given
 310         // pair of array indices are unequal, so that when `places_conflict` returns true, we
 311         // will be assured that two places being compared definitely denotes the same sets of
 312         // locations.
 313         let definitely_conflicting_borrows = other_borrows_of_local.filter(|&i| {
 314             places_conflict(
 315                 self.tcx,
 316                 self.body,
 317                 self.borrow_set[i].borrowed_place,
 318                 place,
 319                 PlaceConflictBias::NoOverlap,
 320             )
 321         });
 322
 323         trans.kill_all(definitely_conflicting_borrows);
 324     }
 325 }
 326
 327 impl<'tcx> rustc_mir_dataflow::AnalysisDomain<'tcx> for Borrows<'_, 'tcx> {
 328     type Domain = BitSet<BorrowIndex>;
 329
 330     const NAME: &'static str = "borrows";
 331
 332     fn bottom_value(&self, _: &mir::Body<'tcx>) -> Self::Domain {
 333         // bottom = nothing is reserved or activated yet;
 334         BitSet::new_empty(self.borrow_set.len() * 2)
 335     }
 336
 337     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
 338         // no borrows of code region_scopes have been taken prior to
 339         // function execution, so this method has no effect.
 340     }
 341 }
 342
 343 impl<'tcx> rustc_mir_dataflow::GenKillAnalysis<'tcx> for Borrows<'_, 'tcx> {
 344     type Idx = BorrowIndex;
 345
 346     fn before_statement_effect(
 347         &self,
 348         trans: &mut impl GenKill<Self::Idx>,
 349         _statement: &mir::Statement<'tcx>,
 350         location: Location,
 351     ) {
 352         self.kill_loans_out_of_scope_at_location(trans, location);
 353     }
 354
 355     fn statement_effect(
 356         &self,
 357         trans: &mut impl GenKill<Self::Idx>,
 358         stmt: &mir::Statement<'tcx>,
 359         location: Location,
 360     ) {
 361         match stmt.kind {
 362             mir::StatementKind::Assign(box (lhs, ref rhs)) => {
 363                 if let mir::Rvalue::Ref(_, _, place) = *rhs {
 364                     if place.ignore_borrow(
 365                         self.tcx,
 366                         self.body,
 367                         &self.borrow_set.locals_state_at_exit,
 368                     ) {
 369                         return;
 370                     }
 371                     let index = self.borrow_set.get_index_of(&location).unwrap_or_else(|| {
 372                         panic!("could not find BorrowIndex for location {:?}", location);
 373                     });
 374
 375                     trans.gen(index);
 376                 }
 377
 378                 // Make sure there are no remaining borrows for variables
 379                 // that are assigned over.
 380                 self.kill_borrows_on_place(trans, lhs);
 381             }
 382
 383             mir::StatementKind::StorageDead(local) => {
 384                 // Make sure there are no remaining borrows for locals that
 385                 // are gone out of scope.
 386                 self.kill_borrows_on_place(trans, Place::from(local));
 387             }
 388
 389             mir::StatementKind::FakeRead(..)
 390             | mir::StatementKind::SetDiscriminant { .. }
 391             | mir::StatementKind::Deinit(..)
 392             | mir::StatementKind::StorageLive(..)
 393             | mir::StatementKind::Retag { .. }
 394             | mir::StatementKind::AscribeUserType(..)
 395             | mir::StatementKind::Coverage(..)
 396             | mir::StatementKind::Intrinsic(..)
 397             | mir::StatementKind::Nop => {}
 398         }
 399     }
 400
 401     fn before_terminator_effect(
 402         &self,
 403         trans: &mut impl GenKill<Self::Idx>,
 404         _terminator: &mir::Terminator<'tcx>,
 405         location: Location,
 406     ) {
 407         self.kill_loans_out_of_scope_at_location(trans, location);
 408     }
 409
 410     fn terminator_effect(
 411         &self,
 412         trans: &mut impl GenKill<Self::Idx>,
 413         terminator: &mir::Terminator<'tcx>,
 414         _location: Location,
 415     ) {
 416         if let mir::TerminatorKind::InlineAsm { operands, .. } = &terminator.kind {
 417             for op in operands {
 418                 if let mir::InlineAsmOperand::Out { place: Some(place), .. }
 419                 | mir::InlineAsmOperand::InOut { out_place: Some(place), .. } = *op
 420                 {
 421                     self.kill_borrows_on_place(trans, place);
 422                 }
 423             }
 424         }
 425     }
 426
 427     fn call_return_effect(
 428         &self,
 429         _trans: &mut impl GenKill<Self::Idx>,
 430         _block: mir::BasicBlock,
 431         _return_places: CallReturnPlaces<'_, 'tcx>,
 432     ) {
 433     }
 434 }
 435
 436 impl DebugWithContext<Borrows<'_, '_>> for BorrowIndex {
 437     fn fmt_with(&self, ctxt: &Borrows<'_, '_>, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 438         write!(f, "{:?}", ctxt.location(*self))
 439     }
 440 }