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_data_structures::indexed_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: 'a> : fmt::Debug {
  74     type Path : Copy + fmt::Debug;
  75
  76     fn patch(&mut self) -> &mut MirPatch<'tcx>;
  77     fn mir(&self) -> &'a Body<'tcx>;
  78     fn tcx(&self) -> TyCtxt<'a, 'tcx, '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: 'l, 'tcx: 'b, D>
  94     where D : DropElaborator<'b, 'tcx> + 'l
  95 {
  96     elaborator: &'l mut D,
  97
  98     source_info: SourceInfo,
  99
 100     place: &'l Place<'tcx>,
 101     path: D::Path,
 102     succ: BasicBlock,
 103     unwind: Unwind,
 104 }
 105
 106 pub fn elaborate_drop<'b, 'tcx, D>(
 107     elaborator: &mut D,
 108     source_info: SourceInfo,
 109     place: &Place<'tcx>,
 110     path: D::Path,
 111     succ: BasicBlock,
 112     unwind: Unwind,
 113     bb: BasicBlock)
 114     where D: DropElaborator<'b, 'tcx>
 115 {
 116     DropCtxt {
 117         elaborator, source_info, place, path, succ, unwind
 118     }.elaborate_drop(bb)
 119 }
 120
 121 impl<'l, 'b, 'tcx, D> DropCtxt<'l, 'b, 'tcx, D>
 122     where D: DropElaborator<'b, 'tcx>
 123 {
 124     fn place_ty(&self, place: &Place<'tcx>) -> Ty<'tcx> {
 125         place.ty(self.elaborator.mir(), self.tcx()).ty
 126     }
 127
 128     fn tcx(&self) -> TyCtxt<'b, 'tcx, 'tcx> {
 129         self.elaborator.tcx()
 130     }
 131
 132     /// This elaborates a single drop instruction, located at `bb`, and
 133     /// patches over it.
 134     ///
 135     /// The elaborated drop checks the drop flags to only drop what
 136     /// is initialized.
 137     ///
 138     /// In addition, the relevant drop flags also need to be cleared
 139     /// to avoid double-drops. However, in the middle of a complex
 140     /// drop, one must avoid clearing some of the flags before they
 141     /// are read, as that would cause a memory leak.
 142     ///
 143     /// In particular, when dropping an ADT, multiple fields may be
 144     /// joined together under the `rest` subpath. They are all controlled
 145     /// by the primary drop flag, but only the last rest-field dropped
 146     /// should clear it (and it must also not clear anything else).
 147     //
 148     // FIXME: I think we should just control the flags externally,
 149     // and then we do not need this machinery.
 150     pub fn elaborate_drop<'a>(&mut self, bb: BasicBlock) {
 151         debug!("elaborate_drop({:?})", self);
 152         let style = self.elaborator.drop_style(self.path, DropFlagMode::Deep);
 153         debug!("elaborate_drop({:?}): live - {:?}", self, style);
 154         match style {
 155             DropStyle::Dead => {
 156                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Goto {
 157                     target: self.succ
 158                 });
 159             }
 160             DropStyle::Static => {
 161                 let loc = self.terminator_loc(bb);
 162                 self.elaborator.clear_drop_flag(loc, self.path, DropFlagMode::Deep);
 163                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Drop {
 164                     location: self.place.clone(),
 165                     target: self.succ,
 166                     unwind: self.unwind.into_option(),
 167                 });
 168             }
 169             DropStyle::Conditional => {
 170                 let unwind = self.unwind; // FIXME(#43234)
 171                 let succ = self.succ;
 172                 let drop_bb = self.complete_drop(Some(DropFlagMode::Deep), succ, unwind);
 173                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Goto {
 174                     target: drop_bb
 175                 });
 176             }
 177             DropStyle::Open => {
 178                 let drop_bb = self.open_drop();
 179                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Goto {
 180                     target: drop_bb
 181                 });
 182             }
 183         }
 184     }
 185
 186     /// Returns the place and move path for each field of `variant`,
 187     /// (the move path is `None` if the field is a rest field).
 188     fn move_paths_for_fields(&self,
 189                              base_place: &Place<'tcx>,
 190                              variant_path: D::Path,
 191                              variant: &'tcx ty::VariantDef,
 192                              substs: SubstsRef<'tcx>)
 193                              -> Vec<(Place<'tcx>, Option<D::Path>)>
 194     {
 195         variant.fields.iter().enumerate().map(|(i, f)| {
 196             let field = Field::new(i);
 197             let subpath = self.elaborator.field_subpath(variant_path, field);
 198
 199             assert_eq!(self.elaborator.param_env().reveal, Reveal::All);
 200             let field_ty = self.tcx().normalize_erasing_regions(
 201                 self.elaborator.param_env(),
 202                 f.ty(self.tcx(), substs),
 203             );
 204             (base_place.clone().field(field, field_ty), subpath)
 205         }).collect()
 206     }
 207
 208     fn drop_subpath(&mut self,
 209                     place: &Place<'tcx>,
 210                     path: Option<D::Path>,
 211                     succ: BasicBlock,
 212                     unwind: Unwind)
 213                     -> BasicBlock
 214     {
 215         if let Some(path) = path {
 216             debug!("drop_subpath: for std field {:?}", place);
 217
 218             DropCtxt {
 219                 elaborator: self.elaborator,
 220                 source_info: self.source_info,
 221                 path, place, succ, unwind,
 222             }.elaborated_drop_block()
 223         } else {
 224             debug!("drop_subpath: for rest field {:?}", place);
 225
 226             DropCtxt {
 227                 elaborator: self.elaborator,
 228                 source_info: self.source_info,
 229                 place, succ, unwind,
 230                 // Using `self.path` here to condition the drop on
 231                 // our own drop flag.
 232                 path: self.path
 233             }.complete_drop(None, succ, unwind)
 234         }
 235     }
 236
 237     /// Creates one-half of the drop ladder for a list of fields, and return
 238     /// the list of steps in it in reverse order, with the first step
 239     /// dropping 0 fields and so on.
 240     ///
 241     /// `unwind_ladder` is such a list of steps in reverse order,
 242     /// which is called if the matching step of the drop glue panics.
 243     fn drop_halfladder(&mut self,
 244                        unwind_ladder: &[Unwind],
 245                        mut succ: BasicBlock,
 246                        fields: &[(Place<'tcx>, Option<D::Path>)])
 247                        -> Vec<BasicBlock>
 248     {
 249         Some(succ).into_iter().chain(
 250             fields.iter().rev().zip(unwind_ladder)
 251                 .map(|(&(ref place, path), &unwind_succ)| {
 252                     succ = self.drop_subpath(place, path, succ, unwind_succ);
 253                     succ
 254                 })
 255         ).collect()
 256     }
 257
 258     fn drop_ladder_bottom(&mut self) -> (BasicBlock, Unwind) {
 259         // Clear the "master" drop flag at the end. This is needed
 260         // because the "master" drop protects the ADT's discriminant,
 261         // which is invalidated after the ADT is dropped.
 262         let (succ, unwind) = (self.succ, self.unwind); // FIXME(#43234)
 263         (
 264             self.drop_flag_reset_block(DropFlagMode::Shallow, succ, unwind),
 265             unwind.map(|unwind| {
 266                 self.drop_flag_reset_block(DropFlagMode::Shallow, unwind, Unwind::InCleanup)
 267             })
 268         )
 269     }
 270
 271     /// Creates a full drop ladder, consisting of 2 connected half-drop-ladders
 272     ///
 273     /// For example, with 3 fields, the drop ladder is
 274     ///
 275     /// .d0:
 276     ///     ELAB(drop location.0 [target=.d1, unwind=.c1])
 277     /// .d1:
 278     ///     ELAB(drop location.1 [target=.d2, unwind=.c2])
 279     /// .d2:
 280     ///     ELAB(drop location.2 [target=`self.succ`, unwind=`self.unwind`])
 281     /// .c1:
 282     ///     ELAB(drop location.1 [target=.c2])
 283     /// .c2:
 284     ///     ELAB(drop location.2 [target=`self.unwind`])
 285     ///
 286     /// NOTE: this does not clear the master drop flag, so you need
 287     /// to point succ/unwind on a `drop_ladder_bottom`.
 288     fn drop_ladder<'a>(&mut self,
 289                        fields: Vec<(Place<'tcx>, Option<D::Path>)>,
 290                        succ: BasicBlock,
 291                        unwind: Unwind)
 292                        -> (BasicBlock, Unwind)
 293     {
 294         debug!("drop_ladder({:?}, {:?})", self, fields);
 295
 296         let mut fields = fields;
 297         fields.retain(|&(ref place, _)| {
 298             self.place_ty(place).needs_drop(self.tcx(), self.elaborator.param_env())
 299         });
 300
 301         debug!("drop_ladder - fields needing drop: {:?}", fields);
 302
 303         let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 304         let unwind_ladder: Vec<_> = if let Unwind::To(target) = unwind {
 305             let halfladder = self.drop_halfladder(&unwind_ladder, target, &fields);
 306             halfladder.into_iter().map(Unwind::To).collect()
 307         } else {
 308             unwind_ladder
 309         };
 310
 311         let normal_ladder =
 312             self.drop_halfladder(&unwind_ladder, succ, &fields);
 313
 314         (*normal_ladder.last().unwrap(), *unwind_ladder.last().unwrap())
 315     }
 316
 317     fn open_drop_for_tuple<'a>(&mut self, tys: &[Ty<'tcx>])
 318                                -> BasicBlock
 319     {
 320         debug!("open_drop_for_tuple({:?}, {:?})", self, tys);
 321
 322         let fields = tys.iter().enumerate().map(|(i, &ty)| {
 323             (self.place.clone().field(Field::new(i), ty),
 324              self.elaborator.field_subpath(self.path, Field::new(i)))
 325         }).collect();
 326
 327         let (succ, unwind) = self.drop_ladder_bottom();
 328         self.drop_ladder(fields, succ, unwind).0
 329     }
 330
 331     fn open_drop_for_box<'a>(&mut self, adt: &'tcx ty::AdtDef, substs: SubstsRef<'tcx>)
 332                              -> BasicBlock
 333     {
 334         debug!("open_drop_for_box({:?}, {:?}, {:?})", self, adt, substs);
 335
 336         let interior = self.place.clone().deref();
 337         let interior_path = self.elaborator.deref_subpath(self.path);
 338
 339         let succ = self.succ; // FIXME(#43234)
 340         let unwind = self.unwind;
 341         let succ = self.box_free_block(adt, substs, succ, unwind);
 342         let unwind_succ = self.unwind.map(|unwind| {
 343             self.box_free_block(adt, substs, unwind, Unwind::InCleanup)
 344         });
 345
 346         self.drop_subpath(&interior, interior_path, succ, unwind_succ)
 347     }
 348
 349     fn open_drop_for_adt<'a>(&mut self, adt: &'tcx ty::AdtDef, substs: SubstsRef<'tcx>)
 350                              -> 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::Base(PlaceBase::Local(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<'a>(&mut self, (succ, unwind): (BasicBlock, Unwind))
 509                                  -> BasicBlock
 510     {
 511         debug!("destructor_call_block({:?}, {:?})", self, succ);
 512         let tcx = self.tcx();
 513         let drop_trait = tcx.lang_items().drop_trait().unwrap();
 514         let drop_fn = tcx.associated_items(drop_trait).next().unwrap();
 515         let ty = self.place_ty(self.place);
 516         let substs = tcx.mk_substs_trait(ty, &[]);
 517
 518         let ref_ty = tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut {
 519             ty,
 520             mutbl: hir::Mutability::MutMutable
 521         });
 522         let ref_place = self.new_temp(ref_ty);
 523         let unit_temp = Place::Base(PlaceBase::Local(self.new_temp(tcx.mk_unit())));
 524
 525         let result = BasicBlockData {
 526             statements: vec![self.assign(
 527                 &Place::Base(PlaceBase::Local(ref_place)),
 528                 Rvalue::Ref(tcx.lifetimes.re_erased,
 529                             BorrowKind::Mut { allow_two_phase_borrow: false },
 530                             self.place.clone())
 531             )],
 532             terminator: Some(Terminator {
 533                 kind: TerminatorKind::Call {
 534                     func: Operand::function_handle(tcx, drop_fn.def_id, substs,
 535                                                    self.source_info.span),
 536                     args: vec![Operand::Move(Place::Base(PlaceBase::Local(ref_place)))],
 537                     destination: Some((unit_temp, succ)),
 538                     cleanup: unwind.into_option(),
 539                     from_hir_call: true,
 540                 },
 541                 source_info: self.source_info,
 542             }),
 543             is_cleanup: unwind.is_cleanup(),
 544         };
 545         self.elaborator.patch().new_block(result)
 546     }
 547
 548     /// Create a loop that drops an array:
 549     ///
 550     /// ```text
 551     /// loop-block:
 552     ///    can_go = cur == length_or_end
 553     ///    if can_go then succ else drop-block
 554     /// drop-block:
 555     ///    if ptr_based {
 556     ///        ptr = &mut *cur
 557     ///        cur = cur.offset(1)
 558     ///    } else {
 559     ///        ptr = &mut P[cur]
 560     ///        cur = cur + 1
 561     ///    }
 562     ///    drop(ptr)
 563     /// ```
 564     fn drop_loop(
 565         &mut self,
 566         succ: BasicBlock,
 567         cur: Local,
 568         length_or_end: &Place<'tcx>,
 569         ety: Ty<'tcx>,
 570         unwind: Unwind,
 571         ptr_based: bool,
 572     ) -> BasicBlock {
 573         let copy = |place: &Place<'tcx>| Operand::Copy(place.clone());
 574         let move_ = |place: &Place<'tcx>| Operand::Move(place.clone());
 575         let tcx = self.tcx();
 576
 577         let ref_ty = tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut {
 578             ty: ety,
 579             mutbl: hir::Mutability::MutMutable
 580         });
 581         let ptr = &Place::Base(PlaceBase::Local(self.new_temp(ref_ty)));
 582         let can_go = &Place::Base(PlaceBase::Local(self.new_temp(tcx.types.bool)));
 583
 584         let one = self.constant_usize(1);
 585         let (ptr_next, cur_next) = if ptr_based {
 586             (Rvalue::Ref(
 587                 tcx.lifetimes.re_erased,
 588                 BorrowKind::Mut { allow_two_phase_borrow: false },
 589                 Place::Projection(Box::new(Projection {
 590                     base: Place::Base(PlaceBase::Local(cur)),
 591                     elem: ProjectionElem::Deref,
 592                 }))
 593              ),
 594              Rvalue::BinaryOp(BinOp::Offset, move_(&Place::Base(PlaceBase::Local(cur))), one))
 595         } else {
 596             (Rvalue::Ref(
 597                  tcx.lifetimes.re_erased,
 598                  BorrowKind::Mut { allow_two_phase_borrow: false },
 599                  self.place.clone().index(cur)),
 600              Rvalue::BinaryOp(BinOp::Add, move_(&Place::Base(PlaceBase::Local(cur))), one))
 601         };
 602
 603         let drop_block = BasicBlockData {
 604             statements: vec![
 605                 self.assign(ptr, ptr_next),
 606                 self.assign(&Place::Base(PlaceBase::Local(cur)), cur_next)
 607             ],
 608             is_cleanup: unwind.is_cleanup(),
 609             terminator: Some(Terminator {
 610                 source_info: self.source_info,
 611                 // this gets overwritten by drop elaboration.
 612                 kind: TerminatorKind::Unreachable,
 613             })
 614         };
 615         let drop_block = self.elaborator.patch().new_block(drop_block);
 616
 617         let loop_block = BasicBlockData {
 618             statements: vec![
 619                 self.assign(can_go, Rvalue::BinaryOp(BinOp::Eq,
 620                                                      copy(&Place::Base(PlaceBase::Local(cur))),
 621                                                      copy(length_or_end)))
 622             ],
 623             is_cleanup: unwind.is_cleanup(),
 624             terminator: Some(Terminator {
 625                 source_info: self.source_info,
 626                 kind: TerminatorKind::if_(tcx, move_(can_go), succ, drop_block)
 627             })
 628         };
 629         let loop_block = self.elaborator.patch().new_block(loop_block);
 630
 631         self.elaborator.patch().patch_terminator(drop_block, TerminatorKind::Drop {
 632             location: ptr.clone().deref(),
 633             target: loop_block,
 634             unwind: unwind.into_option()
 635         });
 636
 637         loop_block
 638     }
 639
 640     fn open_drop_for_array(&mut self, ety: Ty<'tcx>, opt_size: Option<u64>) -> BasicBlock {
 641         debug!("open_drop_for_array({:?}, {:?})", ety, opt_size);
 642
 643         // if size_of::<ety>() == 0 {
 644         //     index_based_loop
 645         // } else {
 646         //     ptr_based_loop
 647         // }
 648
 649         if let Some(size) = opt_size {
 650             let size: u32 = size.try_into().unwrap_or_else(|_| {
 651                 bug!("move out check isn't implemented for array sizes bigger than u32::MAX");
 652             });
 653             let fields: Vec<(Place<'tcx>, Option<D::Path>)> = (0..size).map(|i| {
 654                 (self.place.clone().elem(ProjectionElem::ConstantIndex{
 655                     offset: i,
 656                     min_length: size,
 657                     from_end: false
 658                 }),
 659                  self.elaborator.array_subpath(self.path, i, size))
 660             }).collect();
 661
 662             if fields.iter().any(|(_,path)| path.is_some()) {
 663                 let (succ, unwind) = self.drop_ladder_bottom();
 664                 return self.drop_ladder(fields, succ, unwind).0
 665             }
 666         }
 667
 668         let move_ = |place: &Place<'tcx>| Operand::Move(place.clone());
 669         let tcx = self.tcx();
 670         let elem_size = &Place::Base(PlaceBase::Local(self.new_temp(tcx.types.usize)));
 671         let len = &Place::Base(PlaceBase::Local(self.new_temp(tcx.types.usize)));
 672
 673         static USIZE_SWITCH_ZERO: &[u128] = &[0];
 674
 675         let base_block = BasicBlockData {
 676             statements: vec![
 677                 self.assign(elem_size, Rvalue::NullaryOp(NullOp::SizeOf, ety)),
 678                 self.assign(len, Rvalue::Len(self.place.clone())),
 679             ],
 680             is_cleanup: self.unwind.is_cleanup(),
 681             terminator: Some(Terminator {
 682                 source_info: self.source_info,
 683                 kind: TerminatorKind::SwitchInt {
 684                     discr: move_(elem_size),
 685                     switch_ty: tcx.types.usize,
 686                     values: From::from(USIZE_SWITCH_ZERO),
 687                     targets: vec![
 688                         self.drop_loop_pair(ety, false, len.clone()),
 689                         self.drop_loop_pair(ety, true, len.clone()),
 690                     ],
 691                 },
 692             })
 693         };
 694         self.elaborator.patch().new_block(base_block)
 695     }
 696
 697     /// Ceates a pair of drop-loops of `place`, which drops its contents, even
 698     /// in the case of 1 panic. If `ptr_based`, creates a pointer loop,
 699     /// otherwise create an index loop.
 700     fn drop_loop_pair(
 701         &mut self,
 702         ety: Ty<'tcx>,
 703         ptr_based: bool,
 704         length: Place<'tcx>,
 705     ) -> BasicBlock {
 706         debug!("drop_loop_pair({:?}, {:?})", ety, ptr_based);
 707         let tcx = self.tcx();
 708         let iter_ty = if ptr_based {
 709             tcx.mk_mut_ptr(ety)
 710         } else {
 711             tcx.types.usize
 712         };
 713
 714         let cur = self.new_temp(iter_ty);
 715         let length_or_end = if ptr_based {
 716             // FIXME check if we want to make it return a `Place` directly
 717             // if all use sites want a `Place::Base` anyway.
 718             let temp = self.new_temp(iter_ty);
 719             Place::Base(PlaceBase::Local(temp))
 720         } else {
 721             length.clone()
 722         };
 723
 724         let unwind = self.unwind.map(|unwind| {
 725             self.drop_loop(unwind,
 726                            cur,
 727                            &length_or_end,
 728                            ety,
 729                            Unwind::InCleanup,
 730                            ptr_based)
 731         });
 732
 733         let loop_block = self.drop_loop(
 734             self.succ,
 735             cur,
 736             &length_or_end,
 737             ety,
 738             unwind,
 739             ptr_based);
 740
 741         let cur = Place::Base(PlaceBase::Local(cur));
 742         let drop_block_stmts = if ptr_based {
 743             let tmp_ty = tcx.mk_mut_ptr(self.place_ty(self.place));
 744             let tmp = Place::Base(PlaceBase::Local(self.new_temp(tmp_ty)));
 745             // tmp = &mut P;
 746             // cur = tmp as *mut T;
 747             // end = Offset(cur, len);
 748             vec![
 749                 self.assign(&tmp, Rvalue::Ref(
 750                     tcx.lifetimes.re_erased,
 751                     BorrowKind::Mut { allow_two_phase_borrow: false },
 752                     self.place.clone()
 753                 )),
 754                 self.assign(
 755                     &cur,
 756                     Rvalue::Cast(CastKind::Misc, Operand::Move(tmp), iter_ty),
 757                 ),
 758                 self.assign(
 759                     &length_or_end,
 760                     Rvalue::BinaryOp(BinOp::Offset, Operand::Copy(cur), Operand::Move(length)
 761                 )),
 762             ]
 763         } else {
 764             // cur = 0 (length already pushed)
 765             let zero = self.constant_usize(0);
 766             vec![self.assign(&cur, Rvalue::Use(zero))]
 767         };
 768         let drop_block = self.elaborator.patch().new_block(BasicBlockData {
 769             statements: drop_block_stmts,
 770             is_cleanup: unwind.is_cleanup(),
 771             terminator: Some(Terminator {
 772                 source_info: self.source_info,
 773                 kind: TerminatorKind::Goto { target: loop_block }
 774             })
 775         });
 776
 777         // FIXME(#34708): handle partially-dropped array/slice elements.
 778         let reset_block = self.drop_flag_reset_block(DropFlagMode::Deep, drop_block, unwind);
 779         self.drop_flag_test_block(reset_block, self.succ, unwind)
 780     }
 781
 782     /// The slow-path - create an "open", elaborated drop for a type
 783     /// which is moved-out-of only partially, and patch `bb` to a jump
 784     /// to it. This must not be called on ADTs with a destructor,
 785     /// as these can't be moved-out-of, except for `Box<T>`, which is
 786     /// special-cased.
 787     ///
 788     /// This creates a "drop ladder" that drops the needed fields of the
 789     /// ADT, both in the success case or if one of the destructors fail.
 790     fn open_drop<'a>(&mut self) -> BasicBlock {
 791         let ty = self.place_ty(self.place);
 792         match ty.sty {
 793             ty::Closure(def_id, substs) => {
 794                 let tys : Vec<_> = substs.upvar_tys(def_id, self.tcx()).collect();
 795                 self.open_drop_for_tuple(&tys)
 796             }
 797             // Note that `elaborate_drops` only drops the upvars of a generator,
 798             // and this is ok because `open_drop` here can only be reached
 799             // within that own generator's resume function.
 800             // This should only happen for the self argument on the resume function.
 801             // It effetively only contains upvars until the generator transformation runs.
 802             // See librustc_mir/transform/generator.rs for more details.
 803             ty::Generator(def_id, substs, _) => {
 804                 let tys : Vec<_> = substs.upvar_tys(def_id, self.tcx()).collect();
 805                 self.open_drop_for_tuple(&tys)
 806             }
 807             ty::Tuple(tys) => {
 808                 let tys: Vec<_> = tys.iter().map(|k| k.expect_ty()).collect();
 809                 self.open_drop_for_tuple(&tys)
 810             }
 811             ty::Adt(def, substs) => {
 812                 if def.is_box() {
 813                     self.open_drop_for_box(def, substs)
 814                 } else {
 815                     self.open_drop_for_adt(def, substs)
 816                 }
 817             }
 818             ty::Dynamic(..) => {
 819                 let unwind = self.unwind; // FIXME(#43234)
 820                 let succ = self.succ;
 821                 self.complete_drop(Some(DropFlagMode::Deep), succ, unwind)
 822             }
 823             ty::Array(ety, size) => {
 824                 let size = size.assert_usize(self.tcx());
 825                 self.open_drop_for_array(ety, size)
 826             },
 827             ty::Slice(ety) => self.open_drop_for_array(ety, None),
 828
 829             _ => bug!("open drop from non-ADT `{:?}`", ty)
 830         }
 831     }
 832
 833     /// Returns a basic block that drop a place using the context
 834     /// and path in `c`. If `mode` is something, also clear `c`
 835     /// according to it.
 836     ///
 837     /// if FLAG(self.path)
 838     ///     if let Some(mode) = mode: FLAG(self.path)[mode] = false
 839     ///     drop(self.place)
 840     fn complete_drop<'a>(&mut self,
 841                          drop_mode: Option<DropFlagMode>,
 842                          succ: BasicBlock,
 843                          unwind: Unwind) -> BasicBlock
 844     {
 845         debug!("complete_drop({:?},{:?})", self, drop_mode);
 846
 847         let drop_block = self.drop_block(succ, unwind);
 848         let drop_block = if let Some(mode) = drop_mode {
 849             self.drop_flag_reset_block(mode, drop_block, unwind)
 850         } else {
 851             drop_block
 852         };
 853
 854         self.drop_flag_test_block(drop_block, succ, unwind)
 855     }
 856
 857     fn drop_flag_reset_block(&mut self,
 858                              mode: DropFlagMode,
 859                              succ: BasicBlock,
 860                              unwind: Unwind) -> BasicBlock
 861     {
 862         debug!("drop_flag_reset_block({:?},{:?})", self, mode);
 863
 864         let block = self.new_block(unwind, TerminatorKind::Goto { target: succ });
 865         let block_start = Location { block: block, statement_index: 0 };
 866         self.elaborator.clear_drop_flag(block_start, self.path, mode);
 867         block
 868     }
 869
 870     fn elaborated_drop_block<'a>(&mut self) -> BasicBlock {
 871         debug!("elaborated_drop_block({:?})", self);
 872         let unwind = self.unwind; // FIXME(#43234)
 873         let succ = self.succ;
 874         let blk = self.drop_block(succ, unwind);
 875         self.elaborate_drop(blk);
 876         blk
 877     }
 878
 879     fn box_free_block<'a>(
 880         &mut self,
 881         adt: &'tcx ty::AdtDef,
 882         substs: SubstsRef<'tcx>,
 883         target: BasicBlock,
 884         unwind: Unwind,
 885     ) -> BasicBlock {
 886         let block = self.unelaborated_free_block(adt, substs, target, unwind);
 887         self.drop_flag_test_block(block, target, unwind)
 888     }
 889
 890     fn unelaborated_free_block<'a>(
 891         &mut self,
 892         adt: &'tcx ty::AdtDef,
 893         substs: SubstsRef<'tcx>,
 894         target: BasicBlock,
 895         unwind: Unwind
 896     ) -> BasicBlock {
 897         let tcx = self.tcx();
 898         let unit_temp = Place::Base(PlaceBase::Local(self.new_temp(tcx.mk_unit())));
 899         let free_func = tcx.require_lang_item(lang_items::BoxFreeFnLangItem);
 900         let args = adt.variants[VariantIdx::new(0)].fields.iter().enumerate().map(|(i, f)| {
 901             let field = Field::new(i);
 902             let field_ty = f.ty(self.tcx(), substs);
 903             Operand::Move(self.place.clone().field(field, field_ty))
 904         }).collect();
 905
 906         let call = TerminatorKind::Call {
 907             func: Operand::function_handle(tcx, free_func, substs, self.source_info.span),
 908             args: args,
 909             destination: Some((unit_temp, target)),
 910             cleanup: None,
 911             from_hir_call: false,
 912         }; // FIXME(#43234)
 913         let free_block = self.new_block(unwind, call);
 914
 915         let block_start = Location { block: free_block, statement_index: 0 };
 916         self.elaborator.clear_drop_flag(block_start, self.path, DropFlagMode::Shallow);
 917         free_block
 918     }
 919
 920     fn drop_block<'a>(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
 921         let block = TerminatorKind::Drop {
 922             location: self.place.clone(),
 923             target,
 924             unwind: unwind.into_option()
 925         };
 926         self.new_block(unwind, block)
 927     }
 928
 929     fn drop_flag_test_block(&mut self,
 930                             on_set: BasicBlock,
 931                             on_unset: BasicBlock,
 932                             unwind: Unwind)
 933                             -> BasicBlock
 934     {
 935         let style = self.elaborator.drop_style(self.path, DropFlagMode::Shallow);
 936         debug!("drop_flag_test_block({:?},{:?},{:?},{:?}) - {:?}",
 937                self, on_set, on_unset, unwind, style);
 938
 939         match style {
 940             DropStyle::Dead => on_unset,
 941             DropStyle::Static => on_set,
 942             DropStyle::Conditional | DropStyle::Open => {
 943                 let flag = self.elaborator.get_drop_flag(self.path).unwrap();
 944                 let term = TerminatorKind::if_(self.tcx(), flag, on_set, on_unset);
 945                 self.new_block(unwind, term)
 946             }
 947         }
 948     }
 949
 950     fn new_block<'a>(&mut self,
 951                      unwind: Unwind,
 952                      k: TerminatorKind<'tcx>)
 953                      -> BasicBlock
 954     {
 955         self.elaborator.patch().new_block(BasicBlockData {
 956             statements: vec![],
 957             terminator: Some(Terminator {
 958                 source_info: self.source_info, kind: k
 959             }),
 960             is_cleanup: unwind.is_cleanup()
 961         })
 962     }
 963
 964     fn new_temp(&mut self, ty: Ty<'tcx>) -> Local {
 965         self.elaborator.patch().new_temp(ty, self.source_info.span)
 966     }
 967
 968     fn terminator_loc(&mut self, bb: BasicBlock) -> Location {
 969         let mir = self.elaborator.mir();
 970         self.elaborator.patch().terminator_loc(mir, bb)
 971     }
 972
 973     fn constant_usize(&self, val: u16) -> Operand<'tcx> {
 974         Operand::Constant(box Constant {
 975             span: self.source_info.span,
 976             ty: self.tcx().types.usize,
 977             user_ty: None,
 978             literal: ty::Const::from_usize(self.tcx(), val.into()),
 979         })
 980     }
 981
 982     fn assign(&self, lhs: &Place<'tcx>, rhs: Rvalue<'tcx>) -> Statement<'tcx> {
 983         Statement {
 984             source_info: self.source_info,
 985             kind: StatementKind::Assign(lhs.clone(), box rhs)
 986         }
 987     }
 988 }