src/librustc_mir/util/elaborate_drops.rs

   1 // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use std::fmt;
  12 use rustc::hir;
  13 use rustc::mir::*;
  14 use rustc::middle::lang_items;
  15 use rustc::traits::Reveal;
  16 use rustc::ty::{self, Ty, TyCtxt};
  17 use rustc::ty::subst::Substs;
  18 use rustc::ty::util::IntTypeExt;
  19 use rustc_data_structures::indexed_vec::Idx;
  20 use util::patch::MirPatch;
  21
  22 use std::u32;
  23
  24 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
  25 pub enum DropFlagState {
  26     Present, // i.e. initialized
  27     Absent, // i.e. deinitialized or "moved"
  28 }
  29
  30 impl DropFlagState {
  31     pub fn value(self) -> bool {
  32         match self {
  33             DropFlagState::Present => true,
  34             DropFlagState::Absent => false
  35         }
  36     }
  37 }
  38
  39 #[derive(Debug)]
  40 pub enum DropStyle {
  41     Dead,
  42     Static,
  43     Conditional,
  44     Open,
  45 }
  46
  47 #[derive(Debug)]
  48 pub enum DropFlagMode {
  49     Shallow,
  50     Deep
  51 }
  52
  53 #[derive(Copy, Clone, Debug)]
  54 pub enum Unwind {
  55     To(BasicBlock),
  56     InCleanup
  57 }
  58
  59 impl Unwind {
  60     fn is_cleanup(self) -> bool {
  61         match self {
  62             Unwind::To(..) => false,
  63             Unwind::InCleanup => true
  64         }
  65     }
  66
  67     fn into_option(self) -> Option<BasicBlock> {
  68         match self {
  69             Unwind::To(bb) => Some(bb),
  70             Unwind::InCleanup => None,
  71         }
  72     }
  73
  74     fn map<F>(self, f: F) -> Self where F: FnOnce(BasicBlock) -> BasicBlock {
  75         match self {
  76             Unwind::To(bb) => Unwind::To(f(bb)),
  77             Unwind::InCleanup => Unwind::InCleanup
  78         }
  79     }
  80 }
  81
  82 pub trait DropElaborator<'a, 'tcx: 'a> : fmt::Debug {
  83     type Path : Copy + fmt::Debug;
  84
  85     fn patch(&mut self) -> &mut MirPatch<'tcx>;
  86     fn mir(&self) -> &'a Mir<'tcx>;
  87     fn tcx(&self) -> TyCtxt<'a, 'tcx, 'tcx>;
  88     fn param_env(&self) -> ty::ParamEnv<'tcx>;
  89
  90     fn drop_style(&self, path: Self::Path, mode: DropFlagMode) -> DropStyle;
  91     fn get_drop_flag(&mut self, path: Self::Path) -> Option<Operand<'tcx>>;
  92     fn clear_drop_flag(&mut self, location: Location, path: Self::Path, mode: DropFlagMode);
  93
  94
  95     fn field_subpath(&self, path: Self::Path, field: Field) -> Option<Self::Path>;
  96     fn deref_subpath(&self, path: Self::Path) -> Option<Self::Path>;
  97     fn downcast_subpath(&self, path: Self::Path, variant: usize) -> Option<Self::Path>;
  98     fn array_subpath(&self, path: Self::Path, index: u32, size: u32) -> Option<Self::Path>;
  99 }
 100
 101 #[derive(Debug)]
 102 struct DropCtxt<'l, 'b: 'l, 'tcx: 'b, D>
 103     where D : DropElaborator<'b, 'tcx> + 'l
 104 {
 105     elaborator: &'l mut D,
 106
 107     source_info: SourceInfo,
 108
 109     place: &'l Place<'tcx>,
 110     path: D::Path,
 111     succ: BasicBlock,
 112     unwind: Unwind,
 113 }
 114
 115 pub fn elaborate_drop<'b, 'tcx, D>(
 116     elaborator: &mut D,
 117     source_info: SourceInfo,
 118     place: &Place<'tcx>,
 119     path: D::Path,
 120     succ: BasicBlock,
 121     unwind: Unwind,
 122     bb: BasicBlock)
 123     where D: DropElaborator<'b, 'tcx>
 124 {
 125     DropCtxt {
 126         elaborator, source_info, place, path, succ, unwind
 127     }.elaborate_drop(bb)
 128 }
 129
 130 impl<'l, 'b, 'tcx, D> DropCtxt<'l, 'b, 'tcx, D>
 131     where D: DropElaborator<'b, 'tcx>
 132 {
 133     fn place_ty(&self, place: &Place<'tcx>) -> Ty<'tcx> {
 134         place.ty(self.elaborator.mir(), self.tcx()).to_ty(self.tcx())
 135     }
 136
 137     fn tcx(&self) -> TyCtxt<'b, 'tcx, 'tcx> {
 138         self.elaborator.tcx()
 139     }
 140
 141     /// This elaborates a single drop instruction, located at `bb`, and
 142     /// patches over it.
 143     ///
 144     /// The elaborated drop checks the drop flags to only drop what
 145     /// is initialized.
 146     ///
 147     /// In addition, the relevant drop flags also need to be cleared
 148     /// to avoid double-drops. However, in the middle of a complex
 149     /// drop, one must avoid clearing some of the flags before they
 150     /// are read, as that would cause a memory leak.
 151     ///
 152     /// In particular, when dropping an ADT, multiple fields may be
 153     /// joined together under the `rest` subpath. They are all controlled
 154     /// by the primary drop flag, but only the last rest-field dropped
 155     /// should clear it (and it must also not clear anything else).
 156     ///
 157     /// FIXME: I think we should just control the flags externally
 158     /// and then we do not need this machinery.
 159     pub fn elaborate_drop<'a>(&mut self, bb: BasicBlock) {
 160         debug!("elaborate_drop({:?})", self);
 161         let style = self.elaborator.drop_style(self.path, DropFlagMode::Deep);
 162         debug!("elaborate_drop({:?}): live - {:?}", self, style);
 163         match style {
 164             DropStyle::Dead => {
 165                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Goto {
 166                     target: self.succ
 167                 });
 168             }
 169             DropStyle::Static => {
 170                 let loc = self.terminator_loc(bb);
 171                 self.elaborator.clear_drop_flag(loc, self.path, DropFlagMode::Deep);
 172                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Drop {
 173                     location: self.place.clone(),
 174                     target: self.succ,
 175                     unwind: self.unwind.into_option(),
 176                 });
 177             }
 178             DropStyle::Conditional => {
 179                 let unwind = self.unwind; // FIXME(#43234)
 180                 let succ = self.succ;
 181                 let drop_bb = self.complete_drop(Some(DropFlagMode::Deep), succ, unwind);
 182                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Goto {
 183                     target: drop_bb
 184                 });
 185             }
 186             DropStyle::Open => {
 187                 let drop_bb = self.open_drop();
 188                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Goto {
 189                     target: drop_bb
 190                 });
 191             }
 192         }
 193     }
 194
 195     /// Return the place and move path for each field of `variant`,
 196     /// (the move path is `None` if the field is a rest field).
 197     fn move_paths_for_fields(&self,
 198                              base_place: &Place<'tcx>,
 199                              variant_path: D::Path,
 200                              variant: &'tcx ty::VariantDef,
 201                              substs: &'tcx Substs<'tcx>)
 202                              -> Vec<(Place<'tcx>, Option<D::Path>)>
 203     {
 204         variant.fields.iter().enumerate().map(|(i, f)| {
 205             let field = Field::new(i);
 206             let subpath = self.elaborator.field_subpath(variant_path, field);
 207
 208             assert_eq!(self.elaborator.param_env().reveal, Reveal::All);
 209             let field_ty = self.tcx().normalize_erasing_regions(
 210                 self.elaborator.param_env(),
 211                 f.ty(self.tcx(), substs),
 212             );
 213             (base_place.clone().field(field, field_ty), subpath)
 214         }).collect()
 215     }
 216
 217     fn drop_subpath(&mut self,
 218                     place: &Place<'tcx>,
 219                     path: Option<D::Path>,
 220                     succ: BasicBlock,
 221                     unwind: Unwind)
 222                     -> BasicBlock
 223     {
 224         if let Some(path) = path {
 225             debug!("drop_subpath: for std field {:?}", place);
 226
 227             DropCtxt {
 228                 elaborator: self.elaborator,
 229                 source_info: self.source_info,
 230                 path, place, succ, unwind,
 231             }.elaborated_drop_block()
 232         } else {
 233             debug!("drop_subpath: for rest field {:?}", place);
 234
 235             DropCtxt {
 236                 elaborator: self.elaborator,
 237                 source_info: self.source_info,
 238                 place, succ, unwind,
 239                 // Using `self.path` here to condition the drop on
 240                 // our own drop flag.
 241                 path: self.path
 242             }.complete_drop(None, succ, unwind)
 243         }
 244     }
 245
 246     /// Create one-half of the drop ladder for a list of fields, and return
 247     /// the list of steps in it in reverse order, with the first step
 248     /// dropping 0 fields and so on.
 249     ///
 250     /// `unwind_ladder` is such a list of steps in reverse order,
 251     /// which is called if the matching step of the drop glue panics.
 252     fn drop_halfladder(&mut self,
 253                        unwind_ladder: &[Unwind],
 254                        mut succ: BasicBlock,
 255                        fields: &[(Place<'tcx>, Option<D::Path>)])
 256                        -> Vec<BasicBlock>
 257     {
 258         Some(succ).into_iter().chain(
 259             fields.iter().rev().zip(unwind_ladder)
 260                 .map(|(&(ref place, path), &unwind_succ)| {
 261                     succ = self.drop_subpath(place, path, succ, unwind_succ);
 262                     succ
 263                 })
 264         ).collect()
 265     }
 266
 267     fn drop_ladder_bottom(&mut self) -> (BasicBlock, Unwind) {
 268         // Clear the "master" drop flag at the end. This is needed
 269         // because the "master" drop protects the ADT's discriminant,
 270         // which is invalidated after the ADT is dropped.
 271         let (succ, unwind) = (self.succ, self.unwind); // FIXME(#43234)
 272         (
 273             self.drop_flag_reset_block(DropFlagMode::Shallow, succ, unwind),
 274             unwind.map(|unwind| {
 275                 self.drop_flag_reset_block(DropFlagMode::Shallow, unwind, Unwind::InCleanup)
 276             })
 277         )
 278     }
 279
 280     /// Create a full drop ladder, consisting of 2 connected half-drop-ladders
 281     ///
 282     /// For example, with 3 fields, the drop ladder is
 283     ///
 284     /// .d0:
 285     ///     ELAB(drop location.0 [target=.d1, unwind=.c1])
 286     /// .d1:
 287     ///     ELAB(drop location.1 [target=.d2, unwind=.c2])
 288     /// .d2:
 289     ///     ELAB(drop location.2 [target=`self.succ`, unwind=`self.unwind`])
 290     /// .c1:
 291     ///     ELAB(drop location.1 [target=.c2])
 292     /// .c2:
 293     ///     ELAB(drop location.2 [target=`self.unwind`])
 294     ///
 295     /// NOTE: this does not clear the master drop flag, so you need
 296     /// to point succ/unwind on a `drop_ladder_bottom`.
 297     fn drop_ladder<'a>(&mut self,
 298                        fields: Vec<(Place<'tcx>, Option<D::Path>)>,
 299                        succ: BasicBlock,
 300                        unwind: Unwind)
 301                        -> (BasicBlock, Unwind)
 302     {
 303         debug!("drop_ladder({:?}, {:?})", self, fields);
 304
 305         let mut fields = fields;
 306         fields.retain(|&(ref place, _)| {
 307             self.place_ty(place).needs_drop(self.tcx(), self.elaborator.param_env())
 308         });
 309
 310         debug!("drop_ladder - fields needing drop: {:?}", fields);
 311
 312         let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 313         let unwind_ladder: Vec<_> = if let Unwind::To(target) = unwind {
 314             let halfladder = self.drop_halfladder(&unwind_ladder, target, &fields);
 315             halfladder.into_iter().map(Unwind::To).collect()
 316         } else {
 317             unwind_ladder
 318         };
 319
 320         let normal_ladder =
 321             self.drop_halfladder(&unwind_ladder, succ, &fields);
 322
 323         (*normal_ladder.last().unwrap(), *unwind_ladder.last().unwrap())
 324     }
 325
 326     fn open_drop_for_tuple<'a>(&mut self, tys: &[Ty<'tcx>])
 327                                -> BasicBlock
 328     {
 329         debug!("open_drop_for_tuple({:?}, {:?})", self, tys);
 330
 331         let fields = tys.iter().enumerate().map(|(i, &ty)| {
 332             (self.place.clone().field(Field::new(i), ty),
 333              self.elaborator.field_subpath(self.path, Field::new(i)))
 334         }).collect();
 335
 336         let (succ, unwind) = self.drop_ladder_bottom();
 337         self.drop_ladder(fields, succ, unwind).0
 338     }
 339
 340     fn open_drop_for_box<'a>(&mut self, adt: &'tcx ty::AdtDef, substs: &'tcx Substs<'tcx>)
 341                              -> BasicBlock
 342     {
 343         debug!("open_drop_for_box({:?}, {:?}, {:?})", self, adt, substs);
 344
 345         let interior = self.place.clone().deref();
 346         let interior_path = self.elaborator.deref_subpath(self.path);
 347
 348         let succ = self.succ; // FIXME(#43234)
 349         let unwind = self.unwind;
 350         let succ = self.box_free_block(adt, substs, succ, unwind);
 351         let unwind_succ = self.unwind.map(|unwind| {
 352             self.box_free_block(adt, substs, unwind, Unwind::InCleanup)
 353         });
 354
 355         self.drop_subpath(&interior, interior_path, succ, unwind_succ)
 356     }
 357
 358     fn open_drop_for_adt<'a>(&mut self, adt: &'tcx ty::AdtDef, substs: &'tcx Substs<'tcx>)
 359                              -> BasicBlock {
 360         debug!("open_drop_for_adt({:?}, {:?}, {:?})", self, adt, substs);
 361         if adt.variants.len() == 0 {
 362             return self.elaborator.patch().new_block(BasicBlockData {
 363                 statements: vec![],
 364                 terminator: Some(Terminator {
 365                     source_info: self.source_info,
 366                     kind: TerminatorKind::Unreachable
 367                 }),
 368                 is_cleanup: self.unwind.is_cleanup()
 369             });
 370         }
 371
 372         let contents_drop = if adt.is_union() {
 373             (self.succ, self.unwind)
 374         } else {
 375             self.open_drop_for_adt_contents(adt, substs)
 376         };
 377
 378         if adt.has_dtor(self.tcx()) {
 379             self.destructor_call_block(contents_drop)
 380         } else {
 381             contents_drop.0
 382         }
 383     }
 384
 385     fn open_drop_for_adt_contents(&mut self, adt: &'tcx ty::AdtDef,
 386                                   substs: &'tcx Substs<'tcx>)
 387                                   -> (BasicBlock, Unwind) {
 388         let (succ, unwind) = self.drop_ladder_bottom();
 389         if !adt.is_enum() {
 390             let fields = self.move_paths_for_fields(
 391                 self.place,
 392                 self.path,
 393                 &adt.variants[0],
 394                 substs
 395             );
 396             self.drop_ladder(fields, succ, unwind)
 397         } else {
 398             self.open_drop_for_multivariant(adt, substs, succ, unwind)
 399         }
 400     }
 401
 402     fn open_drop_for_multivariant(&mut self, adt: &'tcx ty::AdtDef,
 403                                   substs: &'tcx Substs<'tcx>,
 404                                   succ: BasicBlock,
 405                                   unwind: Unwind)
 406                                   -> (BasicBlock, Unwind) {
 407         let mut values = Vec::with_capacity(adt.variants.len());
 408         let mut normal_blocks = Vec::with_capacity(adt.variants.len());
 409         let mut unwind_blocks = if unwind.is_cleanup() {
 410             None
 411         } else {
 412             Some(Vec::with_capacity(adt.variants.len()))
 413         };
 414
 415         let mut have_otherwise = false;
 416
 417         for (variant_index, discr) in adt.discriminants(self.tcx()).enumerate() {
 418             let subpath = self.elaborator.downcast_subpath(
 419                 self.path, variant_index);
 420             if let Some(variant_path) = subpath {
 421                 let base_place = self.place.clone().elem(
 422                     ProjectionElem::Downcast(adt, variant_index)
 423                         );
 424                 let fields = self.move_paths_for_fields(
 425                     &base_place,
 426                     variant_path,
 427                     &adt.variants[variant_index],
 428                     substs);
 429                 values.push(discr.val);
 430                 if let Unwind::To(unwind) = unwind {
 431                     // We can't use the half-ladder from the original
 432                     // drop ladder, because this breaks the
 433                     // "funclet can't have 2 successor funclets"
 434                     // requirement from MSVC:
 435                     //
 436                     //           switch       unwind-switch
 437                     //          /      \         /        \
 438                     //         v1.0    v2.0  v2.0-unwind  v1.0-unwind
 439                     //         |        |      /             |
 440                     //    v1.1-unwind  v2.1-unwind           |
 441                     //      ^                                |
 442                     //       \-------------------------------/
 443                     //
 444                     // Create a duplicate half-ladder to avoid that. We
 445                     // could technically only do this on MSVC, but I
 446                     // I want to minimize the divergence between MSVC
 447                     // and non-MSVC.
 448
 449                     let unwind_blocks = unwind_blocks.as_mut().unwrap();
 450                     let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 451                     let halfladder =
 452                         self.drop_halfladder(&unwind_ladder, unwind, &fields);
 453                     unwind_blocks.push(halfladder.last().cloned().unwrap());
 454                 }
 455                 let (normal, _) = self.drop_ladder(fields, succ, unwind);
 456                 normal_blocks.push(normal);
 457             } else {
 458                 have_otherwise = true;
 459             }
 460         }
 461
 462         if have_otherwise {
 463             normal_blocks.push(self.drop_block(succ, unwind));
 464             if let Unwind::To(unwind) = unwind {
 465                 unwind_blocks.as_mut().unwrap().push(
 466                     self.drop_block(unwind, Unwind::InCleanup)
 467                         );
 468             }
 469         } else {
 470             values.pop();
 471         }
 472
 473         (self.adt_switch_block(adt, normal_blocks, &values, succ, unwind),
 474          unwind.map(|unwind| {
 475              self.adt_switch_block(
 476                  adt, unwind_blocks.unwrap(), &values, unwind, Unwind::InCleanup
 477              )
 478          }))
 479     }
 480
 481     fn adt_switch_block(&mut self,
 482                         adt: &'tcx ty::AdtDef,
 483                         blocks: Vec<BasicBlock>,
 484                         values: &[u128],
 485                         succ: BasicBlock,
 486                         unwind: Unwind)
 487                         -> BasicBlock {
 488         // If there are multiple variants, then if something
 489         // is present within the enum the discriminant, tracked
 490         // by the rest path, must be initialized.
 491         //
 492         // Additionally, we do not want to switch on the
 493         // discriminant after it is free-ed, because that
 494         // way lies only trouble.
 495         let discr_ty = adt.repr.discr_type().to_ty(self.tcx());
 496         let discr = Place::Local(self.new_temp(discr_ty));
 497         let discr_rv = Rvalue::Discriminant(self.place.clone());
 498         let switch_block = BasicBlockData {
 499             statements: vec![self.assign(&discr, discr_rv)],
 500             terminator: Some(Terminator {
 501                 source_info: self.source_info,
 502                 kind: TerminatorKind::SwitchInt {
 503                     discr: Operand::Move(discr),
 504                     switch_ty: discr_ty,
 505                     values: From::from(values.to_owned()),
 506                     targets: blocks,
 507                 }
 508             }),
 509             is_cleanup: unwind.is_cleanup(),
 510         };
 511         let switch_block = self.elaborator.patch().new_block(switch_block);
 512         self.drop_flag_test_block(switch_block, succ, unwind)
 513     }
 514
 515     fn destructor_call_block<'a>(&mut self, (succ, unwind): (BasicBlock, Unwind))
 516                                  -> BasicBlock
 517     {
 518         debug!("destructor_call_block({:?}, {:?})", self, succ);
 519         let tcx = self.tcx();
 520         let drop_trait = tcx.lang_items().drop_trait().unwrap();
 521         let drop_fn = tcx.associated_items(drop_trait).next().unwrap();
 522         let ty = self.place_ty(self.place);
 523         let substs = tcx.mk_substs_trait(ty, &[]);
 524
 525         let ref_ty = tcx.mk_ref(tcx.types.re_erased, ty::TypeAndMut {
 526             ty,
 527             mutbl: hir::Mutability::MutMutable
 528         });
 529         let ref_place = self.new_temp(ref_ty);
 530         let unit_temp = Place::Local(self.new_temp(tcx.mk_nil()));
 531
 532         let result = BasicBlockData {
 533             statements: vec![self.assign(
 534                 &Place::Local(ref_place),
 535                 Rvalue::Ref(tcx.types.re_erased,
 536                             BorrowKind::Mut { allow_two_phase_borrow: false },
 537                             self.place.clone())
 538             )],
 539             terminator: Some(Terminator {
 540                 kind: TerminatorKind::Call {
 541                     func: Operand::function_handle(tcx, drop_fn.def_id, substs,
 542                                                    self.source_info.span),
 543                     args: vec![Operand::Move(Place::Local(ref_place))],
 544                     destination: Some((unit_temp, succ)),
 545                     cleanup: unwind.into_option(),
 546                 },
 547                 source_info: self.source_info
 548             }),
 549             is_cleanup: unwind.is_cleanup(),
 550         };
 551         self.elaborator.patch().new_block(result)
 552     }
 553
 554     /// create a loop that drops an array:
 555     ///
 556
 557     ///
 558     /// loop-block:
 559     ///    can_go = cur == length_or_end
 560     ///    if can_go then succ else drop-block
 561     /// drop-block:
 562     ///    if ptr_based {
 563     ///        ptr = cur
 564     ///        cur = cur.offset(1)
 565     ///    } else {
 566     ///        ptr = &mut P[cur]
 567     ///        cur = cur + 1
 568     ///    }
 569     ///    drop(ptr)
 570     fn drop_loop(&mut self,
 571                  succ: BasicBlock,
 572                  cur: Local,
 573                  length_or_end: &Place<'tcx>,
 574                  ety: Ty<'tcx>,
 575                  unwind: Unwind,
 576                  ptr_based: bool)
 577                  -> BasicBlock
 578     {
 579         let copy = |place: &Place<'tcx>| Operand::Copy(place.clone());
 580         let move_ = |place: &Place<'tcx>| Operand::Move(place.clone());
 581         let tcx = self.tcx();
 582
 583         let ref_ty = tcx.mk_ref(tcx.types.re_erased, ty::TypeAndMut {
 584             ty: ety,
 585             mutbl: hir::Mutability::MutMutable
 586         });
 587         let ptr = &Place::Local(self.new_temp(ref_ty));
 588         let can_go = &Place::Local(self.new_temp(tcx.types.bool));
 589
 590         let one = self.constant_usize(1);
 591         let (ptr_next, cur_next) = if ptr_based {
 592             (Rvalue::Use(copy(&Place::Local(cur))),
 593              Rvalue::BinaryOp(BinOp::Offset, copy(&Place::Local(cur)), one))
 594         } else {
 595             (Rvalue::Ref(
 596                  tcx.types.re_erased,
 597                  BorrowKind::Mut { allow_two_phase_borrow: false },
 598                  self.place.clone().index(cur)),
 599              Rvalue::BinaryOp(BinOp::Add, copy(&Place::Local(cur)), one))
 600         };
 601
 602         let drop_block = BasicBlockData {
 603             statements: vec![
 604                 self.assign(ptr, ptr_next),
 605                 self.assign(&Place::Local(cur), cur_next)
 606             ],
 607             is_cleanup: unwind.is_cleanup(),
 608             terminator: Some(Terminator {
 609                 source_info: self.source_info,
 610                 // this gets overwritten by drop elaboration.
 611                 kind: TerminatorKind::Unreachable,
 612             })
 613         };
 614         let drop_block = self.elaborator.patch().new_block(drop_block);
 615
 616         let loop_block = BasicBlockData {
 617             statements: vec![
 618                 self.assign(can_go, Rvalue::BinaryOp(BinOp::Eq,
 619                                                      copy(&Place::Local(cur)),
 620                                                      copy(length_or_end)))
 621             ],
 622             is_cleanup: unwind.is_cleanup(),
 623             terminator: Some(Terminator {
 624                 source_info: self.source_info,
 625                 kind: TerminatorKind::if_(tcx, move_(can_go), succ, drop_block)
 626             })
 627         };
 628         let loop_block = self.elaborator.patch().new_block(loop_block);
 629
 630         self.elaborator.patch().patch_terminator(drop_block, TerminatorKind::Drop {
 631             location: ptr.clone().deref(),
 632             target: loop_block,
 633             unwind: unwind.into_option()
 634         });
 635
 636         loop_block
 637     }
 638
 639     fn open_drop_for_array(&mut self, ety: Ty<'tcx>, opt_size: Option<u64>) -> BasicBlock {
 640         debug!("open_drop_for_array({:?}, {:?})", ety, opt_size);
 641
 642         // if size_of::<ety>() == 0 {
 643         //     index_based_loop
 644         // } else {
 645         //     ptr_based_loop
 646         // }
 647
 648         if let Some(size) = opt_size {
 649             assert!(size <= (u32::MAX as u64),
 650                     "move out check doesn't implemented for array bigger then u32");
 651             let size = size as u32;
 652             let fields: Vec<(Place<'tcx>, Option<D::Path>)> = (0..size).map(|i| {
 653                 (self.place.clone().elem(ProjectionElem::ConstantIndex{
 654                     offset: i,
 655                     min_length: size,
 656                     from_end: false
 657                 }),
 658                  self.elaborator.array_subpath(self.path, i, size))
 659             }).collect();
 660
 661             if fields.iter().any(|(_,path)| path.is_some()) {
 662                 let (succ, unwind) = self.drop_ladder_bottom();
 663                 return self.drop_ladder(fields, succ, unwind).0
 664             }
 665         }
 666
 667         let move_ = |place: &Place<'tcx>| Operand::Move(place.clone());
 668         let tcx = self.tcx();
 669         let size = &Place::Local(self.new_temp(tcx.types.usize));
 670         let size_is_zero = &Place::Local(self.new_temp(tcx.types.bool));
 671         let base_block = BasicBlockData {
 672             statements: vec![
 673                 self.assign(size, Rvalue::NullaryOp(NullOp::SizeOf, ety)),
 674                 self.assign(size_is_zero, Rvalue::BinaryOp(BinOp::Eq,
 675                                                            move_(size),
 676                                                            self.constant_usize(0)))
 677             ],
 678             is_cleanup: self.unwind.is_cleanup(),
 679             terminator: Some(Terminator {
 680                 source_info: self.source_info,
 681                 kind: TerminatorKind::if_(
 682                     tcx,
 683                     move_(size_is_zero),
 684                     self.drop_loop_pair(ety, false),
 685                     self.drop_loop_pair(ety, true)
 686                 )
 687             })
 688         };
 689         self.elaborator.patch().new_block(base_block)
 690     }
 691
 692     // create a pair of drop-loops of `place`, which drops its contents
 693     // even in the case of 1 panic. If `ptr_based`, create a pointer loop,
 694     // otherwise create an index loop.
 695     fn drop_loop_pair(&mut self, ety: Ty<'tcx>, ptr_based: bool) -> BasicBlock {
 696         debug!("drop_loop_pair({:?}, {:?})", ety, ptr_based);
 697         let tcx = self.tcx();
 698         let iter_ty = if ptr_based {
 699             tcx.mk_mut_ptr(ety)
 700         } else {
 701             tcx.types.usize
 702         };
 703
 704         let cur = self.new_temp(iter_ty);
 705         let length = Place::Local(self.new_temp(tcx.types.usize));
 706         let length_or_end = if ptr_based {
 707             Place::Local(self.new_temp(iter_ty))
 708         } else {
 709             length.clone()
 710         };
 711
 712         let unwind = self.unwind.map(|unwind| {
 713             self.drop_loop(unwind,
 714                            cur,
 715                            &length_or_end,
 716                            ety,
 717                            Unwind::InCleanup,
 718                            ptr_based)
 719         });
 720
 721         let succ = self.succ; // FIXME(#43234)
 722         let loop_block = self.drop_loop(
 723             succ,
 724             cur,
 725             &length_or_end,
 726             ety,
 727             unwind,
 728             ptr_based);
 729
 730         let cur = Place::Local(cur);
 731         let zero = self.constant_usize(0);
 732         let mut drop_block_stmts = vec![];
 733         drop_block_stmts.push(self.assign(&length, Rvalue::Len(self.place.clone())));
 734         if ptr_based {
 735             let tmp_ty = tcx.mk_mut_ptr(self.place_ty(self.place));
 736             let tmp = Place::Local(self.new_temp(tmp_ty));
 737             // tmp = &P;
 738             // cur = tmp as *mut T;
 739             // end = Offset(cur, len);
 740             drop_block_stmts.push(self.assign(&tmp, Rvalue::Ref(
 741                 tcx.types.re_erased,
 742                 BorrowKind::Mut { allow_two_phase_borrow: false },
 743                 self.place.clone()
 744             )));
 745             drop_block_stmts.push(self.assign(&cur, Rvalue::Cast(
 746                 CastKind::Misc, Operand::Move(tmp.clone()), iter_ty
 747             )));
 748             drop_block_stmts.push(self.assign(&length_or_end,
 749                 Rvalue::BinaryOp(BinOp::Offset,
 750                      Operand::Copy(cur.clone()), Operand::Move(length.clone())
 751             )));
 752         } else {
 753             // index = 0 (length already pushed)
 754             drop_block_stmts.push(self.assign(&cur, Rvalue::Use(zero)));
 755         }
 756         let drop_block = self.elaborator.patch().new_block(BasicBlockData {
 757             statements: drop_block_stmts,
 758             is_cleanup: unwind.is_cleanup(),
 759             terminator: Some(Terminator {
 760                 source_info: self.source_info,
 761                 kind: TerminatorKind::Goto { target: loop_block }
 762             })
 763         });
 764
 765         // FIXME(#34708): handle partially-dropped array/slice elements.
 766         let reset_block = self.drop_flag_reset_block(DropFlagMode::Deep, drop_block, unwind);
 767         self.drop_flag_test_block(reset_block, succ, unwind)
 768     }
 769
 770     /// The slow-path - create an "open", elaborated drop for a type
 771     /// which is moved-out-of only partially, and patch `bb` to a jump
 772     /// to it. This must not be called on ADTs with a destructor,
 773     /// as these can't be moved-out-of, except for `Box<T>`, which is
 774     /// special-cased.
 775     ///
 776     /// This creates a "drop ladder" that drops the needed fields of the
 777     /// ADT, both in the success case or if one of the destructors fail.
 778     fn open_drop<'a>(&mut self) -> BasicBlock {
 779         let ty = self.place_ty(self.place);
 780         match ty.sty {
 781             ty::TyClosure(def_id, substs) => {
 782                 let tys : Vec<_> = substs.upvar_tys(def_id, self.tcx()).collect();
 783                 self.open_drop_for_tuple(&tys)
 784             }
 785             // Note that `elaborate_drops` only drops the upvars of a generator,
 786             // and this is ok because `open_drop` here can only be reached
 787             // within that own generator's resume function.
 788             // This should only happen for the self argument on the resume function.
 789             // It effetively only contains upvars until the generator transformation runs.
 790             // See librustc_mir/transform/generator.rs for more details.
 791             ty::TyGenerator(def_id, substs, _) => {
 792                 let tys : Vec<_> = substs.upvar_tys(def_id, self.tcx()).collect();
 793                 self.open_drop_for_tuple(&tys)
 794             }
 795             ty::TyTuple(tys) => {
 796                 self.open_drop_for_tuple(tys)
 797             }
 798             ty::TyAdt(def, substs) => {
 799                 if def.is_box() {
 800                     self.open_drop_for_box(def, substs)
 801                 } else {
 802                     self.open_drop_for_adt(def, substs)
 803                 }
 804             }
 805             ty::TyDynamic(..) => {
 806                 let unwind = self.unwind; // FIXME(#43234)
 807                 let succ = self.succ;
 808                 self.complete_drop(Some(DropFlagMode::Deep), succ, unwind)
 809             }
 810             ty::TyArray(ety, size) => {
 811                 let size = size.assert_usize(self.tcx());
 812                 self.open_drop_for_array(ety, size)
 813             },
 814             ty::TySlice(ety) => self.open_drop_for_array(ety, None),
 815
 816             _ => bug!("open drop from non-ADT `{:?}`", ty)
 817         }
 818     }
 819
 820     /// Return a basic block that drop a place using the context
 821     /// and path in `c`. If `mode` is something, also clear `c`
 822     /// according to it.
 823     ///
 824     /// if FLAG(self.path)
 825     ///     if let Some(mode) = mode: FLAG(self.path)[mode] = false
 826     ///     drop(self.place)
 827     fn complete_drop<'a>(&mut self,
 828                          drop_mode: Option<DropFlagMode>,
 829                          succ: BasicBlock,
 830                          unwind: Unwind) -> BasicBlock
 831     {
 832         debug!("complete_drop({:?},{:?})", self, drop_mode);
 833
 834         let drop_block = self.drop_block(succ, unwind);
 835         let drop_block = if let Some(mode) = drop_mode {
 836             self.drop_flag_reset_block(mode, drop_block, unwind)
 837         } else {
 838             drop_block
 839         };
 840
 841         self.drop_flag_test_block(drop_block, succ, unwind)
 842     }
 843
 844     fn drop_flag_reset_block(&mut self,
 845                              mode: DropFlagMode,
 846                              succ: BasicBlock,
 847                              unwind: Unwind) -> BasicBlock
 848     {
 849         debug!("drop_flag_reset_block({:?},{:?})", self, mode);
 850
 851         let block = self.new_block(unwind, TerminatorKind::Goto { target: succ });
 852         let block_start = Location { block: block, statement_index: 0 };
 853         self.elaborator.clear_drop_flag(block_start, self.path, mode);
 854         block
 855     }
 856
 857     fn elaborated_drop_block<'a>(&mut self) -> BasicBlock {
 858         debug!("elaborated_drop_block({:?})", self);
 859         let unwind = self.unwind; // FIXME(#43234)
 860         let succ = self.succ;
 861         let blk = self.drop_block(succ, unwind);
 862         self.elaborate_drop(blk);
 863         blk
 864     }
 865
 866     fn box_free_block<'a>(
 867         &mut self,
 868         adt: &'tcx ty::AdtDef,
 869         substs: &'tcx Substs<'tcx>,
 870         target: BasicBlock,
 871         unwind: Unwind,
 872     ) -> BasicBlock {
 873         let block = self.unelaborated_free_block(adt, substs, target, unwind);
 874         self.drop_flag_test_block(block, target, unwind)
 875     }
 876
 877     fn unelaborated_free_block<'a>(
 878         &mut self,
 879         adt: &'tcx ty::AdtDef,
 880         substs: &'tcx Substs<'tcx>,
 881         target: BasicBlock,
 882         unwind: Unwind
 883     ) -> BasicBlock {
 884         let tcx = self.tcx();
 885         let unit_temp = Place::Local(self.new_temp(tcx.mk_nil()));
 886         let free_func = tcx.require_lang_item(lang_items::BoxFreeFnLangItem);
 887         let args = adt.variants[0].fields.iter().enumerate().map(|(i, f)| {
 888             let field = Field::new(i);
 889             let field_ty = f.ty(self.tcx(), substs);
 890             Operand::Move(self.place.clone().field(field, field_ty))
 891         }).collect();
 892
 893         let call = TerminatorKind::Call {
 894             func: Operand::function_handle(tcx, free_func, substs, self.source_info.span),
 895             args: args,
 896             destination: Some((unit_temp, target)),
 897             cleanup: None
 898         }; // FIXME(#43234)
 899         let free_block = self.new_block(unwind, call);
 900
 901         let block_start = Location { block: free_block, statement_index: 0 };
 902         self.elaborator.clear_drop_flag(block_start, self.path, DropFlagMode::Shallow);
 903         free_block
 904     }
 905
 906     fn drop_block<'a>(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
 907         let block = TerminatorKind::Drop {
 908             location: self.place.clone(),
 909             target,
 910             unwind: unwind.into_option()
 911         };
 912         self.new_block(unwind, block)
 913     }
 914
 915     fn drop_flag_test_block(&mut self,
 916                             on_set: BasicBlock,
 917                             on_unset: BasicBlock,
 918                             unwind: Unwind)
 919                             -> BasicBlock
 920     {
 921         let style = self.elaborator.drop_style(self.path, DropFlagMode::Shallow);
 922         debug!("drop_flag_test_block({:?},{:?},{:?},{:?}) - {:?}",
 923                self, on_set, on_unset, unwind, style);
 924
 925         match style {
 926             DropStyle::Dead => on_unset,
 927             DropStyle::Static => on_set,
 928             DropStyle::Conditional | DropStyle::Open => {
 929                 let flag = self.elaborator.get_drop_flag(self.path).unwrap();
 930                 let term = TerminatorKind::if_(self.tcx(), flag, on_set, on_unset);
 931                 self.new_block(unwind, term)
 932             }
 933         }
 934     }
 935
 936     fn new_block<'a>(&mut self,
 937                      unwind: Unwind,
 938                      k: TerminatorKind<'tcx>)
 939                      -> BasicBlock
 940     {
 941         self.elaborator.patch().new_block(BasicBlockData {
 942             statements: vec![],
 943             terminator: Some(Terminator {
 944                 source_info: self.source_info, kind: k
 945             }),
 946             is_cleanup: unwind.is_cleanup()
 947         })
 948     }
 949
 950     fn new_temp(&mut self, ty: Ty<'tcx>) -> Local {
 951         self.elaborator.patch().new_temp(ty, self.source_info.span)
 952     }
 953
 954     fn terminator_loc(&mut self, bb: BasicBlock) -> Location {
 955         let mir = self.elaborator.mir();
 956         self.elaborator.patch().terminator_loc(mir, bb)
 957     }
 958
 959     fn constant_usize(&self, val: u16) -> Operand<'tcx> {
 960         Operand::Constant(box Constant {
 961             span: self.source_info.span,
 962             ty: self.tcx().types.usize,
 963             literal: Literal::Value {
 964                 value: ty::Const::from_usize(self.tcx(), val.into())
 965             }
 966         })
 967     }
 968
 969     fn assign(&self, lhs: &Place<'tcx>, rhs: Rvalue<'tcx>) -> Statement<'tcx> {
 970         Statement {
 971             source_info: self.source_info,
 972             kind: StatementKind::Assign(lhs.clone(), rhs)
 973         }
 974     }
 975 }