src/librustc_mir/transform/elaborate_drops.rs

   1 use crate::dataflow::move_paths::{HasMoveData, MoveData, MovePathIndex, LookupResult};
   2 use crate::dataflow::{MaybeInitializedPlaces, MaybeUninitializedPlaces};
   3 use crate::dataflow::{DataflowResults};
   4 use crate::dataflow::{on_all_children_bits, on_all_drop_children_bits};
   5 use crate::dataflow::{drop_flag_effects_for_location, on_lookup_result_bits};
   6 use crate::dataflow::MoveDataParamEnv;
   7 use crate::dataflow::{self, do_dataflow, DebugFormatted};
   8 use crate::transform::{MirPass, MirSource};
   9 use crate::util::patch::MirPatch;
  10 use crate::util::elaborate_drops::{DropFlagState, Unwind, elaborate_drop};
  11 use crate::util::elaborate_drops::{DropElaborator, DropStyle, DropFlagMode};
  12 use rustc::ty::{self, TyCtxt};
  13 use rustc::ty::layout::VariantIdx;
  14 use rustc::hir;
  15 use rustc::mir::*;
  16 use rustc::util::nodemap::FxHashMap;
  17 use rustc_data_structures::bit_set::BitSet;
  18 use std::fmt;
  19 use syntax_pos::Span;
  20
  21 pub struct ElaborateDrops;
  22
  23 impl<'tcx> MirPass<'tcx> for ElaborateDrops {
  24     fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut Body<'tcx>) {
  25         debug!("elaborate_drops({:?} @ {:?})", src, body.span);
  26
  27         let def_id = src.def_id();
  28         let param_env = tcx.param_env(src.def_id()).with_reveal_all();
  29         let move_data = match MoveData::gather_moves(body, tcx) {
  30             Ok(move_data) => move_data,
  31             Err((move_data, _move_errors)) => {
  32                 // The only way we should be allowing any move_errors
  33                 // in here is if we are in the migration path for the
  34                 // NLL-based MIR-borrowck.
  35                 //
  36                 // If we are in the migration path, we have already
  37                 // reported these errors as warnings to the user. So
  38                 // we will just ignore them here.
  39                 assert!(tcx.migrate_borrowck());
  40                 move_data
  41             }
  42         };
  43         let elaborate_patch = {
  44             let body = &*body;
  45             let env = MoveDataParamEnv {
  46                 move_data,
  47                 param_env,
  48             };
  49             let dead_unwinds = find_dead_unwinds(tcx, body, def_id, &env);
  50             let flow_inits =
  51                 do_dataflow(tcx, body, def_id, &[], &dead_unwinds,
  52                             MaybeInitializedPlaces::new(tcx, body, &env),
  53                             |bd, p| DebugFormatted::new(&bd.move_data().move_paths[p]));
  54             let flow_uninits =
  55                 do_dataflow(tcx, body, def_id, &[], &dead_unwinds,
  56                             MaybeUninitializedPlaces::new(tcx, body, &env),
  57                             |bd, p| DebugFormatted::new(&bd.move_data().move_paths[p]));
  58
  59             ElaborateDropsCtxt {
  60                 tcx,
  61                 body,
  62                 env: &env,
  63                 flow_inits,
  64                 flow_uninits,
  65                 drop_flags: Default::default(),
  66                 patch: MirPatch::new(body),
  67             }.elaborate()
  68         };
  69         elaborate_patch.apply(body);
  70     }
  71 }
  72
  73 /// Returns the set of basic blocks whose unwind edges are known
  74 /// to not be reachable, because they are `drop` terminators
  75 /// that can't drop anything.
  76 fn find_dead_unwinds<'tcx>(
  77     tcx: TyCtxt<'tcx>,
  78     body: &Body<'tcx>,
  79     def_id: hir::def_id::DefId,
  80     env: &MoveDataParamEnv<'tcx>,
  81 ) -> BitSet<BasicBlock> {
  82     debug!("find_dead_unwinds({:?})", body.span);
  83     // We only need to do this pass once, because unwind edges can only
  84     // reach cleanup blocks, which can't have unwind edges themselves.
  85     let mut dead_unwinds = BitSet::new_empty(body.basic_blocks().len());
  86     let flow_inits =
  87         do_dataflow(tcx, body, def_id, &[], &dead_unwinds,
  88                     MaybeInitializedPlaces::new(tcx, body, &env),
  89                     |bd, p| DebugFormatted::new(&bd.move_data().move_paths[p]));
  90     for (bb, bb_data) in body.basic_blocks().iter_enumerated() {
  91         let location = match bb_data.terminator().kind {
  92             TerminatorKind::Drop { ref location, unwind: Some(_), .. } |
  93             TerminatorKind::DropAndReplace { ref location, unwind: Some(_), .. } => location,
  94             _ => continue,
  95         };
  96
  97         let mut init_data = InitializationData {
  98             live: flow_inits.sets().entry_set_for(bb.index()).to_owned(),
  99             dead: BitSet::new_empty(env.move_data.move_paths.len()),
 100         };
 101         debug!("find_dead_unwinds @ {:?}: {:?}; init_data={:?}",
 102                bb, bb_data, init_data.live);
 103         for stmt in 0..bb_data.statements.len() {
 104             let loc = Location { block: bb, statement_index: stmt };
 105             init_data.apply_location(tcx, body, env, loc);
 106         }
 107
 108         let path = match env.move_data.rev_lookup.find(location.as_ref()) {
 109             LookupResult::Exact(e) => e,
 110             LookupResult::Parent(..) => {
 111                 debug!("find_dead_unwinds: has parent; skipping");
 112                 continue
 113             }
 114         };
 115
 116         debug!("find_dead_unwinds @ {:?}: path({:?})={:?}", bb, location, path);
 117
 118         let mut maybe_live = false;
 119         on_all_drop_children_bits(tcx, body, &env, path, |child| {
 120             let (child_maybe_live, _) = init_data.state(child);
 121             maybe_live |= child_maybe_live;
 122         });
 123
 124         debug!("find_dead_unwinds @ {:?}: maybe_live={}", bb, maybe_live);
 125         if !maybe_live {
 126             dead_unwinds.insert(bb);
 127         }
 128     }
 129
 130     dead_unwinds
 131 }
 132
 133 struct InitializationData {
 134     live: BitSet<MovePathIndex>,
 135     dead: BitSet<MovePathIndex>
 136 }
 137
 138 impl InitializationData {
 139     fn apply_location<'tcx>(
 140         &mut self,
 141         tcx: TyCtxt<'tcx>,
 142         body: &Body<'tcx>,
 143         env: &MoveDataParamEnv<'tcx>,
 144         loc: Location,
 145     ) {
 146         drop_flag_effects_for_location(tcx, body, env, loc, |path, df| {
 147             debug!("at location {:?}: setting {:?} to {:?}",
 148                    loc, path, df);
 149             match df {
 150                 DropFlagState::Present => {
 151                     self.live.insert(path);
 152                     self.dead.remove(path);
 153                 }
 154                 DropFlagState::Absent => {
 155                     self.dead.insert(path);
 156                     self.live.remove(path);
 157                 }
 158             }
 159         });
 160     }
 161
 162     fn state(&self, path: MovePathIndex) -> (bool, bool) {
 163         (self.live.contains(path), self.dead.contains(path))
 164     }
 165 }
 166
 167 struct Elaborator<'a, 'b, 'tcx> {
 168     init_data: &'a InitializationData,
 169     ctxt: &'a mut ElaborateDropsCtxt<'b, 'tcx>,
 170 }
 171
 172 impl<'a, 'b, 'tcx> fmt::Debug for Elaborator<'a, 'b, 'tcx> {
 173     fn fmt(&self, _f: &mut fmt::Formatter<'_>) -> fmt::Result {
 174         Ok(())
 175     }
 176 }
 177
 178 impl<'a, 'b, 'tcx> DropElaborator<'a, 'tcx> for Elaborator<'a, 'b, 'tcx> {
 179     type Path = MovePathIndex;
 180
 181     fn patch(&mut self) -> &mut MirPatch<'tcx> {
 182         &mut self.ctxt.patch
 183     }
 184
 185     fn body(&self) -> &'a Body<'tcx> {
 186         self.ctxt.body
 187     }
 188
 189     fn tcx(&self) -> TyCtxt<'tcx> {
 190         self.ctxt.tcx
 191     }
 192
 193     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 194         self.ctxt.param_env()
 195     }
 196
 197     fn drop_style(&self, path: Self::Path, mode: DropFlagMode) -> DropStyle {
 198         let ((maybe_live, maybe_dead), multipart) = match mode {
 199             DropFlagMode::Shallow => (self.init_data.state(path), false),
 200             DropFlagMode::Deep => {
 201                 let mut some_live = false;
 202                 let mut some_dead = false;
 203                 let mut children_count = 0;
 204                 on_all_drop_children_bits(
 205                     self.tcx(), self.body(), self.ctxt.env, path, |child| {
 206                         let (live, dead) = self.init_data.state(child);
 207                         debug!("elaborate_drop: state({:?}) = {:?}",
 208                                child, (live, dead));
 209                         some_live |= live;
 210                         some_dead |= dead;
 211                         children_count += 1;
 212                     });
 213                 ((some_live, some_dead), children_count != 1)
 214             }
 215         };
 216         match (maybe_live, maybe_dead, multipart) {
 217             (false, _, _) => DropStyle::Dead,
 218             (true, false, _) => DropStyle::Static,
 219             (true, true, false) => DropStyle::Conditional,
 220             (true, true, true) => DropStyle::Open,
 221         }
 222     }
 223
 224     fn clear_drop_flag(&mut self, loc: Location, path: Self::Path, mode: DropFlagMode) {
 225         match mode {
 226             DropFlagMode::Shallow => {
 227                 self.ctxt.set_drop_flag(loc, path, DropFlagState::Absent);
 228             }
 229             DropFlagMode::Deep => {
 230                 on_all_children_bits(
 231                     self.tcx(), self.body(), self.ctxt.move_data(), path,
 232                     |child| self.ctxt.set_drop_flag(loc, child, DropFlagState::Absent)
 233                  );
 234             }
 235         }
 236     }
 237
 238     fn field_subpath(&self, path: Self::Path, field: Field) -> Option<Self::Path> {
 239         dataflow::move_path_children_matching(self.ctxt.move_data(), path, |p| match p {
 240             [.., ProjectionElem::Field(idx, _)] => *idx == field,
 241             _ => false,
 242         })
 243     }
 244
 245     fn array_subpath(&self, path: Self::Path, index: u32, size: u32) -> Option<Self::Path> {
 246         dataflow::move_path_children_matching(self.ctxt.move_data(), path, |p| match p {
 247             [.., ProjectionElem::ConstantIndex { offset, min_length: _, from_end: false }] => {
 248                 *offset == index
 249             }
 250             [.., ProjectionElem::ConstantIndex { offset, min_length: _, from_end: true }] => {
 251                 size - offset == index
 252             }
 253             _ => false,
 254         })
 255     }
 256
 257     fn deref_subpath(&self, path: Self::Path) -> Option<Self::Path> {
 258         dataflow::move_path_children_matching(self.ctxt.move_data(), path, |p| {
 259             match p {
 260                 [.., ProjectionElem::Deref] => true,
 261                 _ => false
 262             }
 263         })
 264     }
 265
 266     fn downcast_subpath(&self, path: Self::Path, variant: VariantIdx) -> Option<Self::Path> {
 267         dataflow::move_path_children_matching(self.ctxt.move_data(), path, |p| match p {
 268             [.., ProjectionElem::Downcast(_, idx)] => *idx == variant,
 269             _ => false
 270         })
 271     }
 272
 273     fn get_drop_flag(&mut self, path: Self::Path) -> Option<Operand<'tcx>> {
 274         self.ctxt.drop_flag(path).map(Operand::Copy)
 275     }
 276 }
 277
 278 struct ElaborateDropsCtxt<'a, 'tcx> {
 279     tcx: TyCtxt<'tcx>,
 280     body: &'a Body<'tcx>,
 281     env: &'a MoveDataParamEnv<'tcx>,
 282     flow_inits: DataflowResults<'tcx, MaybeInitializedPlaces<'a, 'tcx>>,
 283     flow_uninits: DataflowResults<'tcx, MaybeUninitializedPlaces<'a, 'tcx>>,
 284     drop_flags: FxHashMap<MovePathIndex, Local>,
 285     patch: MirPatch<'tcx>,
 286 }
 287
 288 impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
 289     fn move_data(&self) -> &'b MoveData<'tcx> { &self.env.move_data }
 290
 291     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 292         self.env.param_env
 293     }
 294
 295     fn initialization_data_at(&self, loc: Location) -> InitializationData {
 296         let mut data = InitializationData {
 297             live: self.flow_inits.sets().entry_set_for(loc.block.index())
 298                 .to_owned(),
 299             dead: self.flow_uninits.sets().entry_set_for(loc.block.index())
 300                 .to_owned(),
 301         };
 302         for stmt in 0..loc.statement_index {
 303             data.apply_location(self.tcx, self.body, self.env,
 304                                 Location { block: loc.block, statement_index: stmt });
 305         }
 306         data
 307     }
 308
 309     fn create_drop_flag(&mut self, index: MovePathIndex, span: Span) {
 310         let tcx = self.tcx;
 311         let patch = &mut self.patch;
 312         debug!("create_drop_flag({:?})", self.body.span);
 313         self.drop_flags.entry(index).or_insert_with(|| {
 314             patch.new_internal(tcx.types.bool, span)
 315         });
 316     }
 317
 318     fn drop_flag(&mut self, index: MovePathIndex) -> Option<Place<'tcx>> {
 319         self.drop_flags.get(&index).map(|t| Place::from(*t))
 320     }
 321
 322     /// create a patch that elaborates all drops in the input
 323     /// MIR.
 324     fn elaborate(mut self) -> MirPatch<'tcx>
 325     {
 326         self.collect_drop_flags();
 327
 328         self.elaborate_drops();
 329
 330         self.drop_flags_on_init();
 331         self.drop_flags_for_fn_rets();
 332         self.drop_flags_for_args();
 333         self.drop_flags_for_locs();
 334
 335         self.patch
 336     }
 337
 338     fn collect_drop_flags(&mut self)
 339     {
 340         for (bb, data) in self.body.basic_blocks().iter_enumerated() {
 341             let terminator = data.terminator();
 342             let location = match terminator.kind {
 343                 TerminatorKind::Drop { ref location, .. } |
 344                 TerminatorKind::DropAndReplace { ref location, .. } => location,
 345                 _ => continue
 346             };
 347
 348             let init_data = self.initialization_data_at(Location {
 349                 block: bb,
 350                 statement_index: data.statements.len()
 351             });
 352
 353             let path = self.move_data().rev_lookup.find(location.as_ref());
 354             debug!("collect_drop_flags: {:?}, place {:?} ({:?})",
 355                    bb, location, path);
 356
 357             let path = match path {
 358                 LookupResult::Exact(e) => e,
 359                 LookupResult::Parent(None) => continue,
 360                 LookupResult::Parent(Some(parent)) => {
 361                     let (_maybe_live, maybe_dead) = init_data.state(parent);
 362                     if maybe_dead {
 363                         span_bug!(terminator.source_info.span,
 364                                   "drop of untracked, uninitialized value {:?}, place {:?} ({:?})",
 365                                   bb, location, path);
 366                     }
 367                     continue
 368                 }
 369             };
 370
 371             on_all_drop_children_bits(self.tcx, self.body, self.env, path, |child| {
 372                 let (maybe_live, maybe_dead) = init_data.state(child);
 373                 debug!("collect_drop_flags: collecting {:?} from {:?}@{:?} - {:?}",
 374                        child, location, path, (maybe_live, maybe_dead));
 375                 if maybe_live && maybe_dead {
 376                     self.create_drop_flag(child, terminator.source_info.span)
 377                 }
 378             });
 379         }
 380     }
 381
 382     fn elaborate_drops(&mut self)
 383     {
 384         for (bb, data) in self.body.basic_blocks().iter_enumerated() {
 385             let loc = Location { block: bb, statement_index: data.statements.len() };
 386             let terminator = data.terminator();
 387
 388             let resume_block = self.patch.resume_block();
 389             match terminator.kind {
 390                 TerminatorKind::Drop { ref location, target, unwind } => {
 391                     let init_data = self.initialization_data_at(loc);
 392                     match self.move_data().rev_lookup.find(location.as_ref()) {
 393                         LookupResult::Exact(path) => {
 394                             elaborate_drop(
 395                                 &mut Elaborator {
 396                                     init_data: &init_data,
 397                                     ctxt: self
 398                                 },
 399                                 terminator.source_info,
 400                                 location,
 401                                 path,
 402                                 target,
 403                                 if data.is_cleanup {
 404                                     Unwind::InCleanup
 405                                 } else {
 406                                     Unwind::To(Option::unwrap_or(unwind, resume_block))
 407                                 },
 408                                 bb)
 409                         }
 410                         LookupResult::Parent(..) => {
 411                             span_bug!(terminator.source_info.span,
 412                                       "drop of untracked value {:?}", bb);
 413                         }
 414                     }
 415                 }
 416                 TerminatorKind::DropAndReplace { ref location, ref value,
 417                                                  target, unwind } =>
 418                 {
 419                     assert!(!data.is_cleanup);
 420
 421                     self.elaborate_replace(
 422                         loc,
 423                         location, value,
 424                         target, unwind
 425                     );
 426                 }
 427                 _ => continue
 428             }
 429         }
 430     }
 431
 432     /// Elaborate a MIR `replace` terminator. This instruction
 433     /// is not directly handled by codegen, and therefore
 434     /// must be desugared.
 435     ///
 436     /// The desugaring drops the location if needed, and then writes
 437     /// the value (including setting the drop flag) over it in *both* arms.
 438     ///
 439     /// The `replace` terminator can also be called on places that
 440     /// are not tracked by elaboration (for example,
 441     /// `replace x[i] <- tmp0`). The borrow checker requires that
 442     /// these locations are initialized before the assignment,
 443     /// so we just generate an unconditional drop.
 444     fn elaborate_replace(
 445         &mut self,
 446         loc: Location,
 447         location: &Place<'tcx>,
 448         value: &Operand<'tcx>,
 449         target: BasicBlock,
 450         unwind: Option<BasicBlock>)
 451     {
 452         let bb = loc.block;
 453         let data = &self.body[bb];
 454         let terminator = data.terminator();
 455         assert!(!data.is_cleanup, "DropAndReplace in unwind path not supported");
 456
 457         let assign = Statement {
 458             kind: StatementKind::Assign(location.clone(), box Rvalue::Use(value.clone())),
 459             source_info: terminator.source_info
 460         };
 461
 462         let unwind = unwind.unwrap_or_else(|| self.patch.resume_block());
 463         let unwind = self.patch.new_block(BasicBlockData {
 464             statements: vec![assign.clone()],
 465             terminator: Some(Terminator {
 466                 kind: TerminatorKind::Goto { target: unwind },
 467                 ..*terminator
 468             }),
 469             is_cleanup: true
 470         });
 471
 472         let target = self.patch.new_block(BasicBlockData {
 473             statements: vec![assign],
 474             terminator: Some(Terminator {
 475                 kind: TerminatorKind::Goto { target },
 476                 ..*terminator
 477             }),
 478             is_cleanup: false,
 479         });
 480
 481         match self.move_data().rev_lookup.find(location.as_ref()) {
 482             LookupResult::Exact(path) => {
 483                 debug!("elaborate_drop_and_replace({:?}) - tracked {:?}", terminator, path);
 484                 let init_data = self.initialization_data_at(loc);
 485
 486                 elaborate_drop(
 487                     &mut Elaborator {
 488                         init_data: &init_data,
 489                         ctxt: self
 490                     },
 491                     terminator.source_info,
 492                     location,
 493                     path,
 494                     target,
 495                     Unwind::To(unwind),
 496                     bb);
 497                 on_all_children_bits(self.tcx, self.body, self.move_data(), path, |child| {
 498                     self.set_drop_flag(Location { block: target, statement_index: 0 },
 499                                        child, DropFlagState::Present);
 500                     self.set_drop_flag(Location { block: unwind, statement_index: 0 },
 501                                        child, DropFlagState::Present);
 502                 });
 503             }
 504             LookupResult::Parent(parent) => {
 505                 // drop and replace behind a pointer/array/whatever. The location
 506                 // must be initialized.
 507                 debug!("elaborate_drop_and_replace({:?}) - untracked {:?}", terminator, parent);
 508                 self.patch.patch_terminator(bb, TerminatorKind::Drop {
 509                     location: location.clone(),
 510                     target,
 511                     unwind: Some(unwind)
 512                 });
 513             }
 514         }
 515     }
 516
 517     fn constant_bool(&self, span: Span, val: bool) -> Rvalue<'tcx> {
 518         Rvalue::Use(Operand::Constant(Box::new(Constant {
 519             span,
 520             user_ty: None,
 521             literal: ty::Const::from_bool(self.tcx, val),
 522         })))
 523     }
 524
 525     fn set_drop_flag(&mut self, loc: Location, path: MovePathIndex, val: DropFlagState) {
 526         if let Some(&flag) = self.drop_flags.get(&path) {
 527             let span = self.patch.source_info_for_location(self.body, loc).span;
 528             let val = self.constant_bool(span, val.value());
 529             self.patch.add_assign(loc, Place::from(flag), val);
 530         }
 531     }
 532
 533     fn drop_flags_on_init(&mut self) {
 534         let loc = Location::START;
 535         let span = self.patch.source_info_for_location(self.body, loc).span;
 536         let false_ = self.constant_bool(span, false);
 537         for flag in self.drop_flags.values() {
 538             self.patch.add_assign(loc, Place::from(*flag), false_.clone());
 539         }
 540     }
 541
 542     fn drop_flags_for_fn_rets(&mut self) {
 543         for (bb, data) in self.body.basic_blocks().iter_enumerated() {
 544             if let TerminatorKind::Call {
 545                 destination: Some((ref place, tgt)), cleanup: Some(_), ..
 546             } = data.terminator().kind {
 547                 assert!(!self.patch.is_patched(bb));
 548
 549                 let loc = Location { block: tgt, statement_index: 0 };
 550                 let path = self.move_data().rev_lookup.find(place.as_ref());
 551                 on_lookup_result_bits(
 552                     self.tcx, self.body, self.move_data(), path,
 553                     |child| self.set_drop_flag(loc, child, DropFlagState::Present)
 554                 );
 555             }
 556         }
 557     }
 558
 559     fn drop_flags_for_args(&mut self) {
 560         let loc = Location::START;
 561         dataflow::drop_flag_effects_for_function_entry(
 562             self.tcx, self.body, self.env, |path, ds| {
 563                 self.set_drop_flag(loc, path, ds);
 564             }
 565         )
 566     }
 567
 568     fn drop_flags_for_locs(&mut self) {
 569         // We intentionally iterate only over the *old* basic blocks.
 570         //
 571         // Basic blocks created by drop elaboration update their
 572         // drop flags by themselves, to avoid the drop flags being
 573         // clobbered before they are read.
 574
 575         for (bb, data) in self.body.basic_blocks().iter_enumerated() {
 576             debug!("drop_flags_for_locs({:?})", data);
 577             for i in 0..(data.statements.len()+1) {
 578                 debug!("drop_flag_for_locs: stmt {}", i);
 579                 let mut allow_initializations = true;
 580                 if i == data.statements.len() {
 581                     match data.terminator().kind {
 582                         TerminatorKind::Drop { .. } => {
 583                             // drop elaboration should handle that by itself
 584                             continue
 585                         }
 586                         TerminatorKind::DropAndReplace { .. } => {
 587                             // this contains the move of the source and
 588                             // the initialization of the destination. We
 589                             // only want the former - the latter is handled
 590                             // by the elaboration code and must be done
 591                             // *after* the destination is dropped.
 592                             assert!(self.patch.is_patched(bb));
 593                             allow_initializations = false;
 594                         }
 595                         TerminatorKind::Resume => {
 596                             // It is possible for `Resume` to be patched
 597                             // (in particular it can be patched to be replaced with
 598                             // a Goto; see `MirPatch::new`).
 599                         }
 600                         _ => {
 601                             assert!(!self.patch.is_patched(bb));
 602                         }
 603                     }
 604                 }
 605                 let loc = Location { block: bb, statement_index: i };
 606                 dataflow::drop_flag_effects_for_location(
 607                     self.tcx, self.body, self.env, loc, |path, ds| {
 608                         if ds == DropFlagState::Absent || allow_initializations {
 609                             self.set_drop_flag(loc, path, ds)
 610                         }
 611                     }
 612                 )
 613             }
 614
 615             // There may be a critical edge after this call,
 616             // so mark the return as initialized *before* the
 617             // call.
 618             if let TerminatorKind::Call {
 619                 destination: Some((ref place, _)), cleanup: None, ..
 620             } = data.terminator().kind {
 621                 assert!(!self.patch.is_patched(bb));
 622
 623                 let loc = Location { block: bb, statement_index: data.statements.len() };
 624                 let path = self.move_data().rev_lookup.find(place.as_ref());
 625                 on_lookup_result_bits(
 626                     self.tcx, self.body, self.move_data(), path,
 627                     |child| self.set_drop_flag(loc, child, DropFlagState::Present)
 628                 );
 629             }
 630         }
 631     }
 632 }