src/librustc_mir/util/elaborate_drops.rs

   1 use std::fmt;
   2 use rustc::hir;
   3 use rustc::mir::*;
   4 use rustc::middle::lang_items;
   5 use rustc::traits::Reveal;
   6 use rustc::ty::{self, Ty, TyCtxt};
   7 use rustc::ty::layout::VariantIdx;
   8 use rustc::ty::subst::SubstsRef;
   9 use rustc::ty::util::IntTypeExt;
  10 use rustc_index::vec::Idx;
  11 use crate::util::patch::MirPatch;
  12
  13 use std::convert::TryInto;
  14
  15 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
  16 pub enum DropFlagState {
  17     Present, // i.e., initialized
  18     Absent, // i.e., deinitialized or "moved"
  19 }
  20
  21 impl DropFlagState {
  22     pub fn value(self) -> bool {
  23         match self {
  24             DropFlagState::Present => true,
  25             DropFlagState::Absent => false
  26         }
  27     }
  28 }
  29
  30 #[derive(Debug)]
  31 pub enum DropStyle {
  32     Dead,
  33     Static,
  34     Conditional,
  35     Open,
  36 }
  37
  38 #[derive(Debug)]
  39 pub enum DropFlagMode {
  40     Shallow,
  41     Deep
  42 }
  43
  44 #[derive(Copy, Clone, Debug)]
  45 pub enum Unwind {
  46     To(BasicBlock),
  47     InCleanup
  48 }
  49
  50 impl Unwind {
  51     fn is_cleanup(self) -> bool {
  52         match self {
  53             Unwind::To(..) => false,
  54             Unwind::InCleanup => true
  55         }
  56     }
  57
  58     fn into_option(self) -> Option<BasicBlock> {
  59         match self {
  60             Unwind::To(bb) => Some(bb),
  61             Unwind::InCleanup => None,
  62         }
  63     }
  64
  65     fn map<F>(self, f: F) -> Self where F: FnOnce(BasicBlock) -> BasicBlock {
  66         match self {
  67             Unwind::To(bb) => Unwind::To(f(bb)),
  68             Unwind::InCleanup => Unwind::InCleanup
  69         }
  70     }
  71 }
  72
  73 pub trait DropElaborator<'a, 'tcx>: fmt::Debug {
  74     type Path : Copy + fmt::Debug;
  75
  76     fn patch(&mut self) -> &mut MirPatch<'tcx>;
  77     fn body(&self) -> &'a Body<'tcx>;
  78     fn tcx(&self) -> TyCtxt<'tcx>;
  79     fn param_env(&self) -> ty::ParamEnv<'tcx>;
  80
  81     fn drop_style(&self, path: Self::Path, mode: DropFlagMode) -> DropStyle;
  82     fn get_drop_flag(&mut self, path: Self::Path) -> Option<Operand<'tcx>>;
  83     fn clear_drop_flag(&mut self, location: Location, path: Self::Path, mode: DropFlagMode);
  84
  85
  86     fn field_subpath(&self, path: Self::Path, field: Field) -> Option<Self::Path>;
  87     fn deref_subpath(&self, path: Self::Path) -> Option<Self::Path>;
  88     fn downcast_subpath(&self, path: Self::Path, variant: VariantIdx) -> Option<Self::Path>;
  89     fn array_subpath(&self, path: Self::Path, index: u32, size: u32) -> Option<Self::Path>;
  90 }
  91
  92 #[derive(Debug)]
  93 struct DropCtxt<'l, 'b, 'tcx, D>
  94 where
  95     D: DropElaborator<'b, 'tcx>,
  96 {
  97     elaborator: &'l mut D,
  98
  99     source_info: SourceInfo,
 100
 101     place: &'l Place<'tcx>,
 102     path: D::Path,
 103     succ: BasicBlock,
 104     unwind: Unwind,
 105 }
 106
 107 pub fn elaborate_drop<'b, 'tcx, D>(
 108     elaborator: &mut D,
 109     source_info: SourceInfo,
 110     place: &Place<'tcx>,
 111     path: D::Path,
 112     succ: BasicBlock,
 113     unwind: Unwind,
 114     bb: BasicBlock,
 115 ) where
 116     D: DropElaborator<'b, 'tcx>,
 117     'tcx: 'b,
 118 {
 119     DropCtxt {
 120         elaborator, source_info, place, path, succ, unwind
 121     }.elaborate_drop(bb)
 122 }
 123
 124 impl<'l, 'b, 'tcx, D> DropCtxt<'l, 'b, 'tcx, D>
 125 where
 126     D: DropElaborator<'b, 'tcx>,
 127     'tcx: 'b,
 128 {
 129     fn place_ty(&self, place: &Place<'tcx>) -> Ty<'tcx> {
 130         place.ty(self.elaborator.body(), self.tcx()).ty
 131     }
 132
 133     fn tcx(&self) -> TyCtxt<'tcx> {
 134         self.elaborator.tcx()
 135     }
 136
 137     /// This elaborates a single drop instruction, located at `bb`, and
 138     /// patches over it.
 139     ///
 140     /// The elaborated drop checks the drop flags to only drop what
 141     /// is initialized.
 142     ///
 143     /// In addition, the relevant drop flags also need to be cleared
 144     /// to avoid double-drops. However, in the middle of a complex
 145     /// drop, one must avoid clearing some of the flags before they
 146     /// are read, as that would cause a memory leak.
 147     ///
 148     /// In particular, when dropping an ADT, multiple fields may be
 149     /// joined together under the `rest` subpath. They are all controlled
 150     /// by the primary drop flag, but only the last rest-field dropped
 151     /// should clear it (and it must also not clear anything else).
 152     //
 153     // FIXME: I think we should just control the flags externally,
 154     // and then we do not need this machinery.
 155     pub fn elaborate_drop(&mut self, bb: BasicBlock) {
 156         debug!("elaborate_drop({:?})", self);
 157         let style = self.elaborator.drop_style(self.path, DropFlagMode::Deep);
 158         debug!("elaborate_drop({:?}): live - {:?}", self, style);
 159         match style {
 160             DropStyle::Dead => {
 161                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Goto {
 162                     target: self.succ
 163                 });
 164             }
 165             DropStyle::Static => {
 166                 let loc = self.terminator_loc(bb);
 167                 self.elaborator.clear_drop_flag(loc, self.path, DropFlagMode::Deep);
 168                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Drop {
 169                     location: self.place.clone(),
 170                     target: self.succ,
 171                     unwind: self.unwind.into_option(),
 172                 });
 173             }
 174             DropStyle::Conditional => {
 175                 let unwind = self.unwind; // FIXME(#43234)
 176                 let succ = self.succ;
 177                 let drop_bb = self.complete_drop(Some(DropFlagMode::Deep), succ, unwind);
 178                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Goto {
 179                     target: drop_bb
 180                 });
 181             }
 182             DropStyle::Open => {
 183                 let drop_bb = self.open_drop();
 184                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Goto {
 185                     target: drop_bb
 186                 });
 187             }
 188         }
 189     }
 190
 191     /// Returns the place and move path for each field of `variant`,
 192     /// (the move path is `None` if the field is a rest field).
 193     fn move_paths_for_fields(&self,
 194                              base_place: &Place<'tcx>,
 195                              variant_path: D::Path,
 196                              variant: &'tcx ty::VariantDef,
 197                              substs: SubstsRef<'tcx>)
 198                              -> Vec<(Place<'tcx>, Option<D::Path>)>
 199     {
 200         variant.fields.iter().enumerate().map(|(i, f)| {
 201             let field = Field::new(i);
 202             let subpath = self.elaborator.field_subpath(variant_path, field);
 203
 204             assert_eq!(self.elaborator.param_env().reveal, Reveal::All);
 205             let field_ty = self.tcx().normalize_erasing_regions(
 206                 self.elaborator.param_env(),
 207                 f.ty(self.tcx(), substs),
 208             );
 209             (base_place.clone().field(field, field_ty), subpath)
 210         }).collect()
 211     }
 212
 213     fn drop_subpath(&mut self,
 214                     place: &Place<'tcx>,
 215                     path: Option<D::Path>,
 216                     succ: BasicBlock,
 217                     unwind: Unwind)
 218                     -> BasicBlock
 219     {
 220         if let Some(path) = path {
 221             debug!("drop_subpath: for std field {:?}", place);
 222
 223             DropCtxt {
 224                 elaborator: self.elaborator,
 225                 source_info: self.source_info,
 226                 path, place, succ, unwind,
 227             }.elaborated_drop_block()
 228         } else {
 229             debug!("drop_subpath: for rest field {:?}", place);
 230
 231             DropCtxt {
 232                 elaborator: self.elaborator,
 233                 source_info: self.source_info,
 234                 place, succ, unwind,
 235                 // Using `self.path` here to condition the drop on
 236                 // our own drop flag.
 237                 path: self.path
 238             }.complete_drop(None, succ, unwind)
 239         }
 240     }
 241
 242     /// Creates one-half of the drop ladder for a list of fields, and return
 243     /// the list of steps in it in reverse order, with the first step
 244     /// dropping 0 fields and so on.
 245     ///
 246     /// `unwind_ladder` is such a list of steps in reverse order,
 247     /// which is called if the matching step of the drop glue panics.
 248     fn drop_halfladder(&mut self,
 249                        unwind_ladder: &[Unwind],
 250                        mut succ: BasicBlock,
 251                        fields: &[(Place<'tcx>, Option<D::Path>)])
 252                        -> Vec<BasicBlock>
 253     {
 254         Some(succ).into_iter().chain(
 255             fields.iter().rev().zip(unwind_ladder)
 256                 .map(|(&(ref place, path), &unwind_succ)| {
 257                     succ = self.drop_subpath(place, path, succ, unwind_succ);
 258                     succ
 259                 })
 260         ).collect()
 261     }
 262
 263     fn drop_ladder_bottom(&mut self) -> (BasicBlock, Unwind) {
 264         // Clear the "master" drop flag at the end. This is needed
 265         // because the "master" drop protects the ADT's discriminant,
 266         // which is invalidated after the ADT is dropped.
 267         let (succ, unwind) = (self.succ, self.unwind); // FIXME(#43234)
 268         (
 269             self.drop_flag_reset_block(DropFlagMode::Shallow, succ, unwind),
 270             unwind.map(|unwind| {
 271                 self.drop_flag_reset_block(DropFlagMode::Shallow, unwind, Unwind::InCleanup)
 272             })
 273         )
 274     }
 275
 276     /// Creates a full drop ladder, consisting of 2 connected half-drop-ladders
 277     ///
 278     /// For example, with 3 fields, the drop ladder is
 279     ///
 280     /// .d0:
 281     ///     ELAB(drop location.0 [target=.d1, unwind=.c1])
 282     /// .d1:
 283     ///     ELAB(drop location.1 [target=.d2, unwind=.c2])
 284     /// .d2:
 285     ///     ELAB(drop location.2 [target=`self.succ`, unwind=`self.unwind`])
 286     /// .c1:
 287     ///     ELAB(drop location.1 [target=.c2])
 288     /// .c2:
 289     ///     ELAB(drop location.2 [target=`self.unwind`])
 290     ///
 291     /// NOTE: this does not clear the master drop flag, so you need
 292     /// to point succ/unwind on a `drop_ladder_bottom`.
 293     fn drop_ladder(
 294         &mut self,
 295         fields: Vec<(Place<'tcx>, Option<D::Path>)>,
 296         succ: BasicBlock,
 297         unwind: Unwind,
 298     ) -> (BasicBlock, Unwind) {
 299         debug!("drop_ladder({:?}, {:?})", self, fields);
 300
 301         let mut fields = fields;
 302         fields.retain(|&(ref place, _)| {
 303             self.place_ty(place).needs_drop(self.tcx(), self.elaborator.param_env())
 304         });
 305
 306         debug!("drop_ladder - fields needing drop: {:?}", fields);
 307
 308         let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 309         let unwind_ladder: Vec<_> = if let Unwind::To(target) = unwind {
 310             let halfladder = self.drop_halfladder(&unwind_ladder, target, &fields);
 311             halfladder.into_iter().map(Unwind::To).collect()
 312         } else {
 313             unwind_ladder
 314         };
 315
 316         let normal_ladder =
 317             self.drop_halfladder(&unwind_ladder, succ, &fields);
 318
 319         (*normal_ladder.last().unwrap(), *unwind_ladder.last().unwrap())
 320     }
 321
 322     fn open_drop_for_tuple(&mut self, tys: &[Ty<'tcx>]) -> BasicBlock {
 323         debug!("open_drop_for_tuple({:?}, {:?})", self, tys);
 324
 325         let fields = tys.iter().enumerate().map(|(i, &ty)| {
 326             (self.place.clone().field(Field::new(i), ty),
 327              self.elaborator.field_subpath(self.path, Field::new(i)))
 328         }).collect();
 329
 330         let (succ, unwind) = self.drop_ladder_bottom();
 331         self.drop_ladder(fields, succ, unwind).0
 332     }
 333
 334     fn open_drop_for_box(&mut self, adt: &'tcx ty::AdtDef, substs: SubstsRef<'tcx>) -> BasicBlock {
 335         debug!("open_drop_for_box({:?}, {:?}, {:?})", self, adt, substs);
 336
 337         let interior = self.place.clone().deref();
 338         let interior_path = self.elaborator.deref_subpath(self.path);
 339
 340         let succ = self.succ; // FIXME(#43234)
 341         let unwind = self.unwind;
 342         let succ = self.box_free_block(adt, substs, succ, unwind);
 343         let unwind_succ = self.unwind.map(|unwind| {
 344             self.box_free_block(adt, substs, unwind, Unwind::InCleanup)
 345         });
 346
 347         self.drop_subpath(&interior, interior_path, succ, unwind_succ)
 348     }
 349
 350     fn open_drop_for_adt(&mut self, adt: &'tcx ty::AdtDef, substs: SubstsRef<'tcx>) -> BasicBlock {
 351         debug!("open_drop_for_adt({:?}, {:?}, {:?})", self, adt, substs);
 352         if adt.variants.len() == 0 {
 353             return self.elaborator.patch().new_block(BasicBlockData {
 354                 statements: vec![],
 355                 terminator: Some(Terminator {
 356                     source_info: self.source_info,
 357                     kind: TerminatorKind::Unreachable
 358                 }),
 359                 is_cleanup: self.unwind.is_cleanup()
 360             });
 361         }
 362
 363         let skip_contents =
 364             adt.is_union() || Some(adt.did) == self.tcx().lang_items().manually_drop();
 365         let contents_drop = if skip_contents {
 366             (self.succ, self.unwind)
 367         } else {
 368             self.open_drop_for_adt_contents(adt, substs)
 369         };
 370
 371         if adt.has_dtor(self.tcx()) {
 372             self.destructor_call_block(contents_drop)
 373         } else {
 374             contents_drop.0
 375         }
 376     }
 377
 378     fn open_drop_for_adt_contents(&mut self, adt: &'tcx ty::AdtDef,
 379                                   substs: SubstsRef<'tcx>)
 380                                   -> (BasicBlock, Unwind) {
 381         let (succ, unwind) = self.drop_ladder_bottom();
 382         if !adt.is_enum() {
 383             let fields = self.move_paths_for_fields(
 384                 self.place,
 385                 self.path,
 386                 &adt.variants[VariantIdx::new(0)],
 387                 substs
 388             );
 389             self.drop_ladder(fields, succ, unwind)
 390         } else {
 391             self.open_drop_for_multivariant(adt, substs, succ, unwind)
 392         }
 393     }
 394
 395     fn open_drop_for_multivariant(&mut self, adt: &'tcx ty::AdtDef,
 396                                   substs: SubstsRef<'tcx>,
 397                                   succ: BasicBlock,
 398                                   unwind: Unwind)
 399                                   -> (BasicBlock, Unwind) {
 400         let mut values = Vec::with_capacity(adt.variants.len());
 401         let mut normal_blocks = Vec::with_capacity(adt.variants.len());
 402         let mut unwind_blocks = if unwind.is_cleanup() {
 403             None
 404         } else {
 405             Some(Vec::with_capacity(adt.variants.len()))
 406         };
 407
 408         let mut have_otherwise = false;
 409
 410         for (variant_index, discr) in adt.discriminants(self.tcx()) {
 411             let subpath = self.elaborator.downcast_subpath(
 412                 self.path, variant_index);
 413             if let Some(variant_path) = subpath {
 414                 let base_place = self.place.clone().elem(
 415                     ProjectionElem::Downcast(Some(adt.variants[variant_index].ident.name),
 416                                              variant_index));
 417                 let fields = self.move_paths_for_fields(
 418                     &base_place,
 419                     variant_path,
 420                     &adt.variants[variant_index],
 421                     substs);
 422                 values.push(discr.val);
 423                 if let Unwind::To(unwind) = unwind {
 424                     // We can't use the half-ladder from the original
 425                     // drop ladder, because this breaks the
 426                     // "funclet can't have 2 successor funclets"
 427                     // requirement from MSVC:
 428                     //
 429                     //           switch       unwind-switch
 430                     //          /      \         /        \
 431                     //         v1.0    v2.0  v2.0-unwind  v1.0-unwind
 432                     //         |        |      /             |
 433                     //    v1.1-unwind  v2.1-unwind           |
 434                     //      ^                                |
 435                     //       \-------------------------------/
 436                     //
 437                     // Create a duplicate half-ladder to avoid that. We
 438                     // could technically only do this on MSVC, but I
 439                     // I want to minimize the divergence between MSVC
 440                     // and non-MSVC.
 441
 442                     let unwind_blocks = unwind_blocks.as_mut().unwrap();
 443                     let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 444                     let halfladder =
 445                         self.drop_halfladder(&unwind_ladder, unwind, &fields);
 446                     unwind_blocks.push(halfladder.last().cloned().unwrap());
 447                 }
 448                 let (normal, _) = self.drop_ladder(fields, succ, unwind);
 449                 normal_blocks.push(normal);
 450             } else {
 451                 have_otherwise = true;
 452             }
 453         }
 454
 455         if have_otherwise {
 456             normal_blocks.push(self.drop_block(succ, unwind));
 457             if let Unwind::To(unwind) = unwind {
 458                 unwind_blocks.as_mut().unwrap().push(
 459                     self.drop_block(unwind, Unwind::InCleanup)
 460                         );
 461             }
 462         } else {
 463             values.pop();
 464         }
 465
 466         (self.adt_switch_block(adt, normal_blocks, &values, succ, unwind),
 467          unwind.map(|unwind| {
 468              self.adt_switch_block(
 469                  adt, unwind_blocks.unwrap(), &values, unwind, Unwind::InCleanup
 470              )
 471          }))
 472     }
 473
 474     fn adt_switch_block(&mut self,
 475                         adt: &'tcx ty::AdtDef,
 476                         blocks: Vec<BasicBlock>,
 477                         values: &[u128],
 478                         succ: BasicBlock,
 479                         unwind: Unwind)
 480                         -> BasicBlock {
 481         // If there are multiple variants, then if something
 482         // is present within the enum the discriminant, tracked
 483         // by the rest path, must be initialized.
 484         //
 485         // Additionally, we do not want to switch on the
 486         // discriminant after it is free-ed, because that
 487         // way lies only trouble.
 488         let discr_ty = adt.repr.discr_type().to_ty(self.tcx());
 489         let discr = Place::from(self.new_temp(discr_ty));
 490         let discr_rv = Rvalue::Discriminant(self.place.clone());
 491         let switch_block = BasicBlockData {
 492             statements: vec![self.assign(&discr, discr_rv)],
 493             terminator: Some(Terminator {
 494                 source_info: self.source_info,
 495                 kind: TerminatorKind::SwitchInt {
 496                     discr: Operand::Move(discr),
 497                     switch_ty: discr_ty,
 498                     values: From::from(values.to_owned()),
 499                     targets: blocks,
 500                 }
 501             }),
 502             is_cleanup: unwind.is_cleanup(),
 503         };
 504         let switch_block = self.elaborator.patch().new_block(switch_block);
 505         self.drop_flag_test_block(switch_block, succ, unwind)
 506     }
 507
 508     fn destructor_call_block(&mut self, (succ, unwind): (BasicBlock, Unwind)) -> BasicBlock {
 509         debug!("destructor_call_block({:?}, {:?})", self, succ);
 510         let tcx = self.tcx();
 511         let drop_trait = tcx.lang_items().drop_trait().unwrap();
 512         let drop_fn = tcx.associated_items(drop_trait).next().unwrap();
 513         let ty = self.place_ty(self.place);
 514         let substs = tcx.mk_substs_trait(ty, &[]);
 515
 516         let ref_ty = tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut {
 517             ty,
 518             mutbl: hir::Mutability::MutMutable
 519         });
 520         let ref_place = self.new_temp(ref_ty);
 521         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 522
 523         let result = BasicBlockData {
 524             statements: vec![self.assign(
 525                 &Place::from(ref_place),
 526                 Rvalue::Ref(tcx.lifetimes.re_erased,
 527                             BorrowKind::Mut { allow_two_phase_borrow: false },
 528                             self.place.clone())
 529             )],
 530             terminator: Some(Terminator {
 531                 kind: TerminatorKind::Call {
 532                     func: Operand::function_handle(tcx, drop_fn.def_id, substs,
 533                                                    self.source_info.span),
 534                     args: vec![Operand::Move(Place::from(ref_place))],
 535                     destination: Some((unit_temp, succ)),
 536                     cleanup: unwind.into_option(),
 537                     from_hir_call: true,
 538                 },
 539                 source_info: self.source_info,
 540             }),
 541             is_cleanup: unwind.is_cleanup(),
 542         };
 543         self.elaborator.patch().new_block(result)
 544     }
 545
 546     /// Create a loop that drops an array:
 547     ///
 548     /// ```text
 549     /// loop-block:
 550     ///    can_go = cur == length_or_end
 551     ///    if can_go then succ else drop-block
 552     /// drop-block:
 553     ///    if ptr_based {
 554     ///        ptr = &mut *cur
 555     ///        cur = cur.offset(1)
 556     ///    } else {
 557     ///        ptr = &mut P[cur]
 558     ///        cur = cur + 1
 559     ///    }
 560     ///    drop(ptr)
 561     /// ```
 562     fn drop_loop(
 563         &mut self,
 564         succ: BasicBlock,
 565         cur: Local,
 566         length_or_end: &Place<'tcx>,
 567         ety: Ty<'tcx>,
 568         unwind: Unwind,
 569         ptr_based: bool,
 570     ) -> BasicBlock {
 571         let copy = |place: &Place<'tcx>| Operand::Copy(place.clone());
 572         let move_ = |place: &Place<'tcx>| Operand::Move(place.clone());
 573         let tcx = self.tcx();
 574
 575         let ref_ty = tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut {
 576             ty: ety,
 577             mutbl: hir::Mutability::MutMutable
 578         });
 579         let ptr = &Place::from(self.new_temp(ref_ty));
 580         let can_go = &Place::from(self.new_temp(tcx.types.bool));
 581
 582         let one = self.constant_usize(1);
 583         let (ptr_next, cur_next) = if ptr_based {
 584             (Rvalue::Ref(
 585                 tcx.lifetimes.re_erased,
 586                 BorrowKind::Mut { allow_two_phase_borrow: false },
 587                 Place {
 588                     base: PlaceBase::Local(cur),
 589                     projection: Box::new([ProjectionElem::Deref]),
 590                 }
 591              ),
 592              Rvalue::BinaryOp(BinOp::Offset, move_(&Place::from(cur)), one))
 593         } else {
 594             (Rvalue::Ref(
 595                  tcx.lifetimes.re_erased,
 596                  BorrowKind::Mut { allow_two_phase_borrow: false },
 597                  self.place.clone().index(cur)),
 598              Rvalue::BinaryOp(BinOp::Add, move_(&Place::from(cur)), one))
 599         };
 600
 601         let drop_block = BasicBlockData {
 602             statements: vec![
 603                 self.assign(ptr, ptr_next),
 604                 self.assign(&Place::from(cur), cur_next)
 605             ],
 606             is_cleanup: unwind.is_cleanup(),
 607             terminator: Some(Terminator {
 608                 source_info: self.source_info,
 609                 // this gets overwritten by drop elaboration.
 610                 kind: TerminatorKind::Unreachable,
 611             })
 612         };
 613         let drop_block = self.elaborator.patch().new_block(drop_block);
 614
 615         let loop_block = BasicBlockData {
 616             statements: vec![
 617                 self.assign(can_go, Rvalue::BinaryOp(BinOp::Eq,
 618                                                      copy(&Place::from(cur)),
 619                                                      copy(length_or_end)))
 620             ],
 621             is_cleanup: unwind.is_cleanup(),
 622             terminator: Some(Terminator {
 623                 source_info: self.source_info,
 624                 kind: TerminatorKind::if_(tcx, move_(can_go), succ, drop_block)
 625             })
 626         };
 627         let loop_block = self.elaborator.patch().new_block(loop_block);
 628
 629         self.elaborator.patch().patch_terminator(drop_block, TerminatorKind::Drop {
 630             location: ptr.clone().deref(),
 631             target: loop_block,
 632             unwind: unwind.into_option()
 633         });
 634
 635         loop_block
 636     }
 637
 638     fn open_drop_for_array(&mut self, ety: Ty<'tcx>, opt_size: Option<u64>) -> BasicBlock {
 639         debug!("open_drop_for_array({:?}, {:?})", ety, opt_size);
 640
 641         // if size_of::<ety>() == 0 {
 642         //     index_based_loop
 643         // } else {
 644         //     ptr_based_loop
 645         // }
 646
 647         if let Some(size) = opt_size {
 648             let size: u32 = size.try_into().unwrap_or_else(|_| {
 649                 bug!("move out check isn't implemented for array sizes bigger than u32::MAX");
 650             });
 651             let fields: Vec<(Place<'tcx>, Option<D::Path>)> = (0..size).map(|i| {
 652                 (self.place.clone().elem(ProjectionElem::ConstantIndex{
 653                     offset: i,
 654                     min_length: size,
 655                     from_end: false
 656                 }),
 657                  self.elaborator.array_subpath(self.path, i, size))
 658             }).collect();
 659
 660             if fields.iter().any(|(_,path)| path.is_some()) {
 661                 let (succ, unwind) = self.drop_ladder_bottom();
 662                 return self.drop_ladder(fields, succ, unwind).0
 663             }
 664         }
 665
 666         let move_ = |place: &Place<'tcx>| Operand::Move(place.clone());
 667         let tcx = self.tcx();
 668         let elem_size = &Place::from(self.new_temp(tcx.types.usize));
 669         let len = &Place::from(self.new_temp(tcx.types.usize));
 670
 671         static USIZE_SWITCH_ZERO: &[u128] = &[0];
 672
 673         let base_block = BasicBlockData {
 674             statements: vec![
 675                 self.assign(elem_size, Rvalue::NullaryOp(NullOp::SizeOf, ety)),
 676                 self.assign(len, Rvalue::Len(self.place.clone())),
 677             ],
 678             is_cleanup: self.unwind.is_cleanup(),
 679             terminator: Some(Terminator {
 680                 source_info: self.source_info,
 681                 kind: TerminatorKind::SwitchInt {
 682                     discr: move_(elem_size),
 683                     switch_ty: tcx.types.usize,
 684                     values: From::from(USIZE_SWITCH_ZERO),
 685                     targets: vec![
 686                         self.drop_loop_pair(ety, false, len.clone()),
 687                         self.drop_loop_pair(ety, true, len.clone()),
 688                     ],
 689                 },
 690             })
 691         };
 692         self.elaborator.patch().new_block(base_block)
 693     }
 694
 695     /// Ceates a pair of drop-loops of `place`, which drops its contents, even
 696     /// in the case of 1 panic. If `ptr_based`, creates a pointer loop,
 697     /// otherwise create an index loop.
 698     fn drop_loop_pair(
 699         &mut self,
 700         ety: Ty<'tcx>,
 701         ptr_based: bool,
 702         length: Place<'tcx>,
 703     ) -> BasicBlock {
 704         debug!("drop_loop_pair({:?}, {:?})", ety, ptr_based);
 705         let tcx = self.tcx();
 706         let iter_ty = if ptr_based {
 707             tcx.mk_mut_ptr(ety)
 708         } else {
 709             tcx.types.usize
 710         };
 711
 712         let cur = self.new_temp(iter_ty);
 713         let length_or_end = if ptr_based {
 714             // FIXME check if we want to make it return a `Place` directly
 715             // if all use sites want a `Place::Base` anyway.
 716             Place::from(self.new_temp(iter_ty))
 717         } else {
 718             length.clone()
 719         };
 720
 721         let unwind = self.unwind.map(|unwind| {
 722             self.drop_loop(unwind,
 723                            cur,
 724                            &length_or_end,
 725                            ety,
 726                            Unwind::InCleanup,
 727                            ptr_based)
 728         });
 729
 730         let loop_block = self.drop_loop(
 731             self.succ,
 732             cur,
 733             &length_or_end,
 734             ety,
 735             unwind,
 736             ptr_based);
 737
 738         let cur = Place::from(cur);
 739         let drop_block_stmts = if ptr_based {
 740             let tmp_ty = tcx.mk_mut_ptr(self.place_ty(self.place));
 741             let tmp = Place::from(self.new_temp(tmp_ty));
 742             // tmp = &mut P;
 743             // cur = tmp as *mut T;
 744             // end = Offset(cur, len);
 745             vec![
 746                 self.assign(&tmp, Rvalue::Ref(
 747                     tcx.lifetimes.re_erased,
 748                     BorrowKind::Mut { allow_two_phase_borrow: false },
 749                     self.place.clone()
 750                 )),
 751                 self.assign(
 752                     &cur,
 753                     Rvalue::Cast(CastKind::Misc, Operand::Move(tmp), iter_ty),
 754                 ),
 755                 self.assign(
 756                     &length_or_end,
 757                     Rvalue::BinaryOp(BinOp::Offset, Operand::Copy(cur), Operand::Move(length)
 758                 )),
 759             ]
 760         } else {
 761             // cur = 0 (length already pushed)
 762             let zero = self.constant_usize(0);
 763             vec![self.assign(&cur, Rvalue::Use(zero))]
 764         };
 765         let drop_block = self.elaborator.patch().new_block(BasicBlockData {
 766             statements: drop_block_stmts,
 767             is_cleanup: unwind.is_cleanup(),
 768             terminator: Some(Terminator {
 769                 source_info: self.source_info,
 770                 kind: TerminatorKind::Goto { target: loop_block }
 771             })
 772         });
 773
 774         // FIXME(#34708): handle partially-dropped array/slice elements.
 775         let reset_block = self.drop_flag_reset_block(DropFlagMode::Deep, drop_block, unwind);
 776         self.drop_flag_test_block(reset_block, self.succ, unwind)
 777     }
 778
 779     /// The slow-path - create an "open", elaborated drop for a type
 780     /// which is moved-out-of only partially, and patch `bb` to a jump
 781     /// to it. This must not be called on ADTs with a destructor,
 782     /// as these can't be moved-out-of, except for `Box<T>`, which is
 783     /// special-cased.
 784     ///
 785     /// This creates a "drop ladder" that drops the needed fields of the
 786     /// ADT, both in the success case or if one of the destructors fail.
 787     fn open_drop(&mut self) -> BasicBlock {
 788         let ty = self.place_ty(self.place);
 789         match ty.kind {
 790             ty::Closure(def_id, substs) => {
 791                 let tys : Vec<_> = substs.as_closure().upvar_tys(def_id, self.tcx()).collect();
 792                 self.open_drop_for_tuple(&tys)
 793             }
 794             // Note that `elaborate_drops` only drops the upvars of a generator,
 795             // and this is ok because `open_drop` here can only be reached
 796             // within that own generator's resume function.
 797             // This should only happen for the self argument on the resume function.
 798             // It effetively only contains upvars until the generator transformation runs.
 799             // See librustc_body/transform/generator.rs for more details.
 800             ty::Generator(def_id, substs, _) => {
 801                 let tys : Vec<_> = substs.as_generator().upvar_tys(def_id, self.tcx()).collect();
 802                 self.open_drop_for_tuple(&tys)
 803             }
 804             ty::Tuple(..) => {
 805                 let tys: Vec<_> = ty.tuple_fields().collect();
 806                 self.open_drop_for_tuple(&tys)
 807             }
 808             ty::Adt(def, substs) => {
 809                 if def.is_box() {
 810                     self.open_drop_for_box(def, substs)
 811                 } else {
 812                     self.open_drop_for_adt(def, substs)
 813                 }
 814             }
 815             ty::Dynamic(..) => {
 816                 let unwind = self.unwind; // FIXME(#43234)
 817                 let succ = self.succ;
 818                 self.complete_drop(Some(DropFlagMode::Deep), succ, unwind)
 819             }
 820             ty::Array(ety, size) => {
 821                 let size = size.try_eval_usize(self.tcx(), self.elaborator.param_env());
 822                 self.open_drop_for_array(ety, size)
 823             },
 824             ty::Slice(ety) => self.open_drop_for_array(ety, None),
 825
 826             _ => bug!("open drop from non-ADT `{:?}`", ty)
 827         }
 828     }
 829
 830     /// Returns a basic block that drop a place using the context
 831     /// and path in `c`. If `mode` is something, also clear `c`
 832     /// according to it.
 833     ///
 834     /// if FLAG(self.path)
 835     ///     if let Some(mode) = mode: FLAG(self.path)[mode] = false
 836     ///     drop(self.place)
 837     fn complete_drop(
 838         &mut self,
 839         drop_mode: Option<DropFlagMode>,
 840         succ: BasicBlock,
 841         unwind: Unwind,
 842     ) -> BasicBlock {
 843         debug!("complete_drop({:?},{:?})", self, drop_mode);
 844
 845         let drop_block = self.drop_block(succ, unwind);
 846         let drop_block = if let Some(mode) = drop_mode {
 847             self.drop_flag_reset_block(mode, drop_block, unwind)
 848         } else {
 849             drop_block
 850         };
 851
 852         self.drop_flag_test_block(drop_block, succ, unwind)
 853     }
 854
 855     fn drop_flag_reset_block(&mut self,
 856                              mode: DropFlagMode,
 857                              succ: BasicBlock,
 858                              unwind: Unwind) -> BasicBlock
 859     {
 860         debug!("drop_flag_reset_block({:?},{:?})", self, mode);
 861
 862         let block = self.new_block(unwind, TerminatorKind::Goto { target: succ });
 863         let block_start = Location { block: block, statement_index: 0 };
 864         self.elaborator.clear_drop_flag(block_start, self.path, mode);
 865         block
 866     }
 867
 868     fn elaborated_drop_block(&mut self) -> BasicBlock {
 869         debug!("elaborated_drop_block({:?})", self);
 870         let unwind = self.unwind; // FIXME(#43234)
 871         let succ = self.succ;
 872         let blk = self.drop_block(succ, unwind);
 873         self.elaborate_drop(blk);
 874         blk
 875     }
 876
 877     fn box_free_block(
 878         &mut self,
 879         adt: &'tcx ty::AdtDef,
 880         substs: SubstsRef<'tcx>,
 881         target: BasicBlock,
 882         unwind: Unwind,
 883     ) -> BasicBlock {
 884         let block = self.unelaborated_free_block(adt, substs, target, unwind);
 885         self.drop_flag_test_block(block, target, unwind)
 886     }
 887
 888     fn unelaborated_free_block(
 889         &mut self,
 890         adt: &'tcx ty::AdtDef,
 891         substs: SubstsRef<'tcx>,
 892         target: BasicBlock,
 893         unwind: Unwind,
 894     ) -> BasicBlock {
 895         let tcx = self.tcx();
 896         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 897         let free_func = tcx.require_lang_item(
 898             lang_items::BoxFreeFnLangItem,
 899             Some(self.source_info.span)
 900         );
 901         let args = adt.variants[VariantIdx::new(0)].fields.iter().enumerate().map(|(i, f)| {
 902             let field = Field::new(i);
 903             let field_ty = f.ty(self.tcx(), substs);
 904             Operand::Move(self.place.clone().field(field, field_ty))
 905         }).collect();
 906
 907         let call = TerminatorKind::Call {
 908             func: Operand::function_handle(tcx, free_func, substs, self.source_info.span),
 909             args: args,
 910             destination: Some((unit_temp, target)),
 911             cleanup: None,
 912             from_hir_call: false,
 913         }; // FIXME(#43234)
 914         let free_block = self.new_block(unwind, call);
 915
 916         let block_start = Location { block: free_block, statement_index: 0 };
 917         self.elaborator.clear_drop_flag(block_start, self.path, DropFlagMode::Shallow);
 918         free_block
 919     }
 920
 921     fn drop_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
 922         let block = TerminatorKind::Drop {
 923             location: self.place.clone(),
 924             target,
 925             unwind: unwind.into_option()
 926         };
 927         self.new_block(unwind, block)
 928     }
 929
 930     fn drop_flag_test_block(&mut self,
 931                             on_set: BasicBlock,
 932                             on_unset: BasicBlock,
 933                             unwind: Unwind)
 934                             -> BasicBlock
 935     {
 936         let style = self.elaborator.drop_style(self.path, DropFlagMode::Shallow);
 937         debug!("drop_flag_test_block({:?},{:?},{:?},{:?}) - {:?}",
 938                self, on_set, on_unset, unwind, style);
 939
 940         match style {
 941             DropStyle::Dead => on_unset,
 942             DropStyle::Static => on_set,
 943             DropStyle::Conditional | DropStyle::Open => {
 944                 let flag = self.elaborator.get_drop_flag(self.path).unwrap();
 945                 let term = TerminatorKind::if_(self.tcx(), flag, on_set, on_unset);
 946                 self.new_block(unwind, term)
 947             }
 948         }
 949     }
 950
 951     fn new_block(&mut self, unwind: Unwind, k: TerminatorKind<'tcx>) -> BasicBlock {
 952         self.elaborator.patch().new_block(BasicBlockData {
 953             statements: vec![],
 954             terminator: Some(Terminator {
 955                 source_info: self.source_info, kind: k
 956             }),
 957             is_cleanup: unwind.is_cleanup()
 958         })
 959     }
 960
 961     fn new_temp(&mut self, ty: Ty<'tcx>) -> Local {
 962         self.elaborator.patch().new_temp(ty, self.source_info.span)
 963     }
 964
 965     fn terminator_loc(&mut self, bb: BasicBlock) -> Location {
 966         let body = self.elaborator.body();
 967         self.elaborator.patch().terminator_loc(body, bb)
 968     }
 969
 970     fn constant_usize(&self, val: u16) -> Operand<'tcx> {
 971         Operand::Constant(box Constant {
 972             span: self.source_info.span,
 973             user_ty: None,
 974             literal: ty::Const::from_usize(self.tcx(), val.into()),
 975         })
 976     }
 977
 978     fn assign(&self, lhs: &Place<'tcx>, rhs: Rvalue<'tcx>) -> Statement<'tcx> {
 979         Statement {
 980             source_info: self.source_info,
 981             kind: StatementKind::Assign(box(lhs.clone(), rhs))
 982         }
 983     }
 984 }