compiler/rustc_mir_dataflow/src/elaborate_drops.rs

   1 use rustc_hir as hir;
   2 use rustc_hir::lang_items::LangItem;
   3 use rustc_index::vec::Idx;
   4 use rustc_middle::mir::patch::MirPatch;
   5 use rustc_middle::mir::*;
   6 use rustc_middle::traits::Reveal;
   7 use rustc_middle::ty::subst::SubstsRef;
   8 use rustc_middle::ty::util::IntTypeExt;
   9 use rustc_middle::ty::{self, Ty, TyCtxt};
  10 use rustc_target::abi::VariantIdx;
  11 use std::{fmt, iter};
  12
  13 /// The value of an inserted drop flag.
  14 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
  15 pub enum DropFlagState {
  16     /// The tracked value is initialized and needs to be dropped when leaving its scope.
  17     Present,
  18
  19     /// The tracked value is uninitialized or was moved out of and does not need to be dropped when
  20     /// leaving its scope.
  21     Absent,
  22 }
  23
  24 impl DropFlagState {
  25     pub fn value(self) -> bool {
  26         match self {
  27             DropFlagState::Present => true,
  28             DropFlagState::Absent => false,
  29         }
  30     }
  31 }
  32
  33 /// Describes how/if a value should be dropped.
  34 #[derive(Debug)]
  35 pub enum DropStyle {
  36     /// The value is already dead at the drop location, no drop will be executed.
  37     Dead,
  38
  39     /// The value is known to always be initialized at the drop location, drop will always be
  40     /// executed.
  41     Static,
  42
  43     /// Whether the value needs to be dropped depends on its drop flag.
  44     Conditional,
  45
  46     /// An "open" drop is one where only the fields of a value are dropped.
  47     ///
  48     /// For example, this happens when moving out of a struct field: The rest of the struct will be
  49     /// dropped in such an "open" drop. It is also used to generate drop glue for the individual
  50     /// components of a value, for example for dropping array elements.
  51     Open,
  52 }
  53
  54 /// Which drop flags to affect/check with an operation.
  55 #[derive(Debug)]
  56 pub enum DropFlagMode {
  57     /// Only affect the top-level drop flag, not that of any contained fields.
  58     Shallow,
  59     /// Affect all nested drop flags in addition to the top-level one.
  60     Deep,
  61 }
  62
  63 /// Describes if unwinding is necessary and where to unwind to if a panic occurs.
  64 #[derive(Copy, Clone, Debug)]
  65 pub enum Unwind {
  66     /// Unwind to this block.
  67     To(BasicBlock),
  68     /// Already in an unwind path, any panic will cause an abort.
  69     InCleanup,
  70 }
  71
  72 impl Unwind {
  73     fn is_cleanup(self) -> bool {
  74         match self {
  75             Unwind::To(..) => false,
  76             Unwind::InCleanup => true,
  77         }
  78     }
  79
  80     fn into_option(self) -> Option<BasicBlock> {
  81         match self {
  82             Unwind::To(bb) => Some(bb),
  83             Unwind::InCleanup => None,
  84         }
  85     }
  86
  87     fn map<F>(self, f: F) -> Self
  88     where
  89         F: FnOnce(BasicBlock) -> BasicBlock,
  90     {
  91         match self {
  92             Unwind::To(bb) => Unwind::To(f(bb)),
  93             Unwind::InCleanup => Unwind::InCleanup,
  94         }
  95     }
  96 }
  97
  98 pub trait DropElaborator<'a, 'tcx>: fmt::Debug {
  99     /// The type representing paths that can be moved out of.
 100     ///
 101     /// Users can move out of individual fields of a struct, such as `a.b.c`. This type is used to
 102     /// represent such move paths. Sometimes tracking individual move paths is not necessary, in
 103     /// which case this may be set to (for example) `()`.
 104     type Path: Copy + fmt::Debug;
 105
 106     // Accessors
 107
 108     fn patch(&mut self) -> &mut MirPatch<'tcx>;
 109     fn body(&self) -> &'a Body<'tcx>;
 110     fn tcx(&self) -> TyCtxt<'tcx>;
 111     fn param_env(&self) -> ty::ParamEnv<'tcx>;
 112
 113     // Drop logic
 114
 115     /// Returns how `path` should be dropped, given `mode`.
 116     fn drop_style(&self, path: Self::Path, mode: DropFlagMode) -> DropStyle;
 117
 118     /// Returns the drop flag of `path` as a MIR `Operand` (or `None` if `path` has no drop flag).
 119     fn get_drop_flag(&mut self, path: Self::Path) -> Option<Operand<'tcx>>;
 120
 121     /// Modifies the MIR patch so that the drop flag of `path` (if any) is cleared at `location`.
 122     ///
 123     /// If `mode` is deep, drop flags of all child paths should also be cleared by inserting
 124     /// additional statements.
 125     fn clear_drop_flag(&mut self, location: Location, path: Self::Path, mode: DropFlagMode);
 126
 127     // Subpaths
 128
 129     /// Returns the subpath of a field of `path` (or `None` if there is no dedicated subpath).
 130     ///
 131     /// If this returns `None`, `field` will not get a dedicated drop flag.
 132     fn field_subpath(&self, path: Self::Path, field: Field) -> Option<Self::Path>;
 133
 134     /// Returns the subpath of a dereference of `path` (or `None` if there is no dedicated subpath).
 135     ///
 136     /// If this returns `None`, `*path` will not get a dedicated drop flag.
 137     ///
 138     /// This is only relevant for `Box<T>`, where the contained `T` can be moved out of the box.
 139     fn deref_subpath(&self, path: Self::Path) -> Option<Self::Path>;
 140
 141     /// Returns the subpath of downcasting `path` to one of its variants.
 142     ///
 143     /// If this returns `None`, the downcast of `path` will not get a dedicated drop flag.
 144     fn downcast_subpath(&self, path: Self::Path, variant: VariantIdx) -> Option<Self::Path>;
 145
 146     /// Returns the subpath of indexing a fixed-size array `path`.
 147     ///
 148     /// If this returns `None`, elements of `path` will not get a dedicated drop flag.
 149     ///
 150     /// This is only relevant for array patterns, which can move out of individual array elements.
 151     fn array_subpath(&self, path: Self::Path, index: u64, size: u64) -> Option<Self::Path>;
 152 }
 153
 154 #[derive(Debug)]
 155 struct DropCtxt<'l, 'b, 'tcx, D>
 156 where
 157     D: DropElaborator<'b, 'tcx>,
 158 {
 159     elaborator: &'l mut D,
 160
 161     source_info: SourceInfo,
 162
 163     place: Place<'tcx>,
 164     path: D::Path,
 165     succ: BasicBlock,
 166     unwind: Unwind,
 167 }
 168
 169 /// "Elaborates" a drop of `place`/`path` and patches `bb`'s terminator to execute it.
 170 ///
 171 /// The passed `elaborator` is used to determine what should happen at the drop terminator. It
 172 /// decides whether the drop can be statically determined or whether it needs a dynamic drop flag,
 173 /// and whether the drop is "open", ie. should be expanded to drop all subfields of the dropped
 174 /// value.
 175 ///
 176 /// When this returns, the MIR patch in the `elaborator` contains the necessary changes.
 177 pub fn elaborate_drop<'b, 'tcx, D>(
 178     elaborator: &mut D,
 179     source_info: SourceInfo,
 180     place: Place<'tcx>,
 181     path: D::Path,
 182     succ: BasicBlock,
 183     unwind: Unwind,
 184     bb: BasicBlock,
 185 ) where
 186     D: DropElaborator<'b, 'tcx>,
 187     'tcx: 'b,
 188 {
 189     DropCtxt { elaborator, source_info, place, path, succ, unwind }.elaborate_drop(bb)
 190 }
 191
 192 impl<'l, 'b, 'tcx, D> DropCtxt<'l, 'b, 'tcx, D>
 193 where
 194     D: DropElaborator<'b, 'tcx>,
 195     'tcx: 'b,
 196 {
 197     fn place_ty(&self, place: Place<'tcx>) -> Ty<'tcx> {
 198         place.ty(self.elaborator.body(), self.tcx()).ty
 199     }
 200
 201     fn tcx(&self) -> TyCtxt<'tcx> {
 202         self.elaborator.tcx()
 203     }
 204
 205     /// This elaborates a single drop instruction, located at `bb`, and
 206     /// patches over it.
 207     ///
 208     /// The elaborated drop checks the drop flags to only drop what
 209     /// is initialized.
 210     ///
 211     /// In addition, the relevant drop flags also need to be cleared
 212     /// to avoid double-drops. However, in the middle of a complex
 213     /// drop, one must avoid clearing some of the flags before they
 214     /// are read, as that would cause a memory leak.
 215     ///
 216     /// In particular, when dropping an ADT, multiple fields may be
 217     /// joined together under the `rest` subpath. They are all controlled
 218     /// by the primary drop flag, but only the last rest-field dropped
 219     /// should clear it (and it must also not clear anything else).
 220     //
 221     // FIXME: I think we should just control the flags externally,
 222     // and then we do not need this machinery.
 223     pub fn elaborate_drop(&mut self, bb: BasicBlock) {
 224         debug!("elaborate_drop({:?}, {:?})", bb, self);
 225         let style = self.elaborator.drop_style(self.path, DropFlagMode::Deep);
 226         debug!("elaborate_drop({:?}, {:?}): live - {:?}", bb, self, style);
 227         match style {
 228             DropStyle::Dead => {
 229                 self.elaborator
 230                     .patch()
 231                     .patch_terminator(bb, TerminatorKind::Goto { target: self.succ });
 232             }
 233             DropStyle::Static => {
 234                 self.elaborator.patch().patch_terminator(
 235                     bb,
 236                     TerminatorKind::Drop {
 237                         place: self.place,
 238                         target: self.succ,
 239                         unwind: self.unwind.into_option(),
 240                     },
 241                 );
 242             }
 243             DropStyle::Conditional => {
 244                 let drop_bb = self.complete_drop(self.succ, self.unwind);
 245                 self.elaborator
 246                     .patch()
 247                     .patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
 248             }
 249             DropStyle::Open => {
 250                 let drop_bb = self.open_drop();
 251                 self.elaborator
 252                     .patch()
 253                     .patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
 254             }
 255         }
 256     }
 257
 258     /// Returns the place and move path for each field of `variant`,
 259     /// (the move path is `None` if the field is a rest field).
 260     fn move_paths_for_fields(
 261         &self,
 262         base_place: Place<'tcx>,
 263         variant_path: D::Path,
 264         variant: &'tcx ty::VariantDef,
 265         substs: SubstsRef<'tcx>,
 266     ) -> Vec<(Place<'tcx>, Option<D::Path>)> {
 267         variant
 268             .fields
 269             .iter()
 270             .enumerate()
 271             .map(|(i, f)| {
 272                 let field = Field::new(i);
 273                 let subpath = self.elaborator.field_subpath(variant_path, field);
 274                 let tcx = self.tcx();
 275
 276                 assert_eq!(self.elaborator.param_env().reveal(), Reveal::All);
 277                 let field_ty =
 278                     tcx.normalize_erasing_regions(self.elaborator.param_env(), f.ty(tcx, substs));
 279                 (tcx.mk_place_field(base_place, field, field_ty), subpath)
 280             })
 281             .collect()
 282     }
 283
 284     fn drop_subpath(
 285         &mut self,
 286         place: Place<'tcx>,
 287         path: Option<D::Path>,
 288         succ: BasicBlock,
 289         unwind: Unwind,
 290     ) -> BasicBlock {
 291         if let Some(path) = path {
 292             debug!("drop_subpath: for std field {:?}", place);
 293
 294             DropCtxt {
 295                 elaborator: self.elaborator,
 296                 source_info: self.source_info,
 297                 path,
 298                 place,
 299                 succ,
 300                 unwind,
 301             }
 302             .elaborated_drop_block()
 303         } else {
 304             debug!("drop_subpath: for rest field {:?}", place);
 305
 306             DropCtxt {
 307                 elaborator: self.elaborator,
 308                 source_info: self.source_info,
 309                 place,
 310                 succ,
 311                 unwind,
 312                 // Using `self.path` here to condition the drop on
 313                 // our own drop flag.
 314                 path: self.path,
 315             }
 316             .complete_drop(succ, unwind)
 317         }
 318     }
 319
 320     /// Creates one-half of the drop ladder for a list of fields, and return
 321     /// the list of steps in it in reverse order, with the first step
 322     /// dropping 0 fields and so on.
 323     ///
 324     /// `unwind_ladder` is such a list of steps in reverse order,
 325     /// which is called if the matching step of the drop glue panics.
 326     fn drop_halfladder(
 327         &mut self,
 328         unwind_ladder: &[Unwind],
 329         mut succ: BasicBlock,
 330         fields: &[(Place<'tcx>, Option<D::Path>)],
 331     ) -> Vec<BasicBlock> {
 332         iter::once(succ)
 333             .chain(fields.iter().rev().zip(unwind_ladder).map(|(&(place, path), &unwind_succ)| {
 334                 succ = self.drop_subpath(place, path, succ, unwind_succ);
 335                 succ
 336             }))
 337             .collect()
 338     }
 339
 340     fn drop_ladder_bottom(&mut self) -> (BasicBlock, Unwind) {
 341         // Clear the "master" drop flag at the end. This is needed
 342         // because the "master" drop protects the ADT's discriminant,
 343         // which is invalidated after the ADT is dropped.
 344         (self.drop_flag_reset_block(DropFlagMode::Shallow, self.succ, self.unwind), self.unwind)
 345     }
 346
 347     /// Creates a full drop ladder, consisting of 2 connected half-drop-ladders
 348     ///
 349     /// For example, with 3 fields, the drop ladder is
 350     ///
 351     /// .d0:
 352     ///     ELAB(drop location.0 [target=.d1, unwind=.c1])
 353     /// .d1:
 354     ///     ELAB(drop location.1 [target=.d2, unwind=.c2])
 355     /// .d2:
 356     ///     ELAB(drop location.2 [target=`self.succ`, unwind=`self.unwind`])
 357     /// .c1:
 358     ///     ELAB(drop location.1 [target=.c2])
 359     /// .c2:
 360     ///     ELAB(drop location.2 [target=`self.unwind`])
 361     ///
 362     /// NOTE: this does not clear the master drop flag, so you need
 363     /// to point succ/unwind on a `drop_ladder_bottom`.
 364     fn drop_ladder(
 365         &mut self,
 366         fields: Vec<(Place<'tcx>, Option<D::Path>)>,
 367         succ: BasicBlock,
 368         unwind: Unwind,
 369     ) -> (BasicBlock, Unwind) {
 370         debug!("drop_ladder({:?}, {:?})", self, fields);
 371
 372         let mut fields = fields;
 373         fields.retain(|&(place, _)| {
 374             self.place_ty(place).needs_drop(self.tcx(), self.elaborator.param_env())
 375         });
 376
 377         debug!("drop_ladder - fields needing drop: {:?}", fields);
 378
 379         let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 380         let unwind_ladder: Vec<_> = if let Unwind::To(target) = unwind {
 381             let halfladder = self.drop_halfladder(&unwind_ladder, target, &fields);
 382             halfladder.into_iter().map(Unwind::To).collect()
 383         } else {
 384             unwind_ladder
 385         };
 386
 387         let normal_ladder = self.drop_halfladder(&unwind_ladder, succ, &fields);
 388
 389         (*normal_ladder.last().unwrap(), *unwind_ladder.last().unwrap())
 390     }
 391
 392     fn open_drop_for_tuple(&mut self, tys: &[Ty<'tcx>]) -> BasicBlock {
 393         debug!("open_drop_for_tuple({:?}, {:?})", self, tys);
 394
 395         let fields = tys
 396             .iter()
 397             .enumerate()
 398             .map(|(i, &ty)| {
 399                 (
 400                     self.tcx().mk_place_field(self.place, Field::new(i), ty),
 401                     self.elaborator.field_subpath(self.path, Field::new(i)),
 402                 )
 403             })
 404             .collect();
 405
 406         let (succ, unwind) = self.drop_ladder_bottom();
 407         self.drop_ladder(fields, succ, unwind).0
 408     }
 409
 410     fn open_drop_for_box(&mut self, adt: ty::AdtDef<'tcx>, substs: SubstsRef<'tcx>) -> BasicBlock {
 411         debug!("open_drop_for_box({:?}, {:?}, {:?})", self, adt, substs);
 412
 413         let interior = self.tcx().mk_place_deref(self.place);
 414         let interior_path = self.elaborator.deref_subpath(self.path);
 415
 416         let succ = self.box_free_block(adt, substs, self.succ, self.unwind);
 417         let unwind_succ =
 418             self.unwind.map(|unwind| self.box_free_block(adt, substs, unwind, Unwind::InCleanup));
 419
 420         self.drop_subpath(interior, interior_path, succ, unwind_succ)
 421     }
 422
 423     fn open_drop_for_adt(&mut self, adt: ty::AdtDef<'tcx>, substs: SubstsRef<'tcx>) -> BasicBlock {
 424         debug!("open_drop_for_adt({:?}, {:?}, {:?})", self, adt, substs);
 425         if adt.variants().is_empty() {
 426             return self.elaborator.patch().new_block(BasicBlockData {
 427                 statements: vec![],
 428                 terminator: Some(Terminator {
 429                     source_info: self.source_info,
 430                     kind: TerminatorKind::Unreachable,
 431                 }),
 432                 is_cleanup: self.unwind.is_cleanup(),
 433             });
 434         }
 435
 436         let skip_contents =
 437             adt.is_union() || Some(adt.did()) == self.tcx().lang_items().manually_drop();
 438         let contents_drop = if skip_contents {
 439             (self.succ, self.unwind)
 440         } else {
 441             self.open_drop_for_adt_contents(adt, substs)
 442         };
 443
 444         if adt.has_dtor(self.tcx()) {
 445             self.destructor_call_block(contents_drop)
 446         } else {
 447             contents_drop.0
 448         }
 449     }
 450
 451     fn open_drop_for_adt_contents(
 452         &mut self,
 453         adt: ty::AdtDef<'tcx>,
 454         substs: SubstsRef<'tcx>,
 455     ) -> (BasicBlock, Unwind) {
 456         let (succ, unwind) = self.drop_ladder_bottom();
 457         if !adt.is_enum() {
 458             let fields = self.move_paths_for_fields(
 459                 self.place,
 460                 self.path,
 461                 &adt.variant(VariantIdx::new(0)),
 462                 substs,
 463             );
 464             self.drop_ladder(fields, succ, unwind)
 465         } else {
 466             self.open_drop_for_multivariant(adt, substs, succ, unwind)
 467         }
 468     }
 469
 470     fn open_drop_for_multivariant(
 471         &mut self,
 472         adt: ty::AdtDef<'tcx>,
 473         substs: SubstsRef<'tcx>,
 474         succ: BasicBlock,
 475         unwind: Unwind,
 476     ) -> (BasicBlock, Unwind) {
 477         let mut values = Vec::with_capacity(adt.variants().len());
 478         let mut normal_blocks = Vec::with_capacity(adt.variants().len());
 479         let mut unwind_blocks =
 480             if unwind.is_cleanup() { None } else { Some(Vec::with_capacity(adt.variants().len())) };
 481
 482         let mut have_otherwise_with_drop_glue = false;
 483         let mut have_otherwise = false;
 484         let tcx = self.tcx();
 485
 486         for (variant_index, discr) in adt.discriminants(tcx) {
 487             let variant = &adt.variant(variant_index);
 488             let subpath = self.elaborator.downcast_subpath(self.path, variant_index);
 489
 490             if let Some(variant_path) = subpath {
 491                 let base_place = tcx.mk_place_elem(
 492                     self.place,
 493                     ProjectionElem::Downcast(Some(variant.name), variant_index),
 494                 );
 495                 let fields = self.move_paths_for_fields(base_place, variant_path, &variant, substs);
 496                 values.push(discr.val);
 497                 if let Unwind::To(unwind) = unwind {
 498                     // We can't use the half-ladder from the original
 499                     // drop ladder, because this breaks the
 500                     // "funclet can't have 2 successor funclets"
 501                     // requirement from MSVC:
 502                     //
 503                     //           switch       unwind-switch
 504                     //          /      \         /        \
 505                     //         v1.0    v2.0  v2.0-unwind  v1.0-unwind
 506                     //         |        |      /             |
 507                     //    v1.1-unwind  v2.1-unwind           |
 508                     //      ^                                |
 509                     //       \-------------------------------/
 510                     //
 511                     // Create a duplicate half-ladder to avoid that. We
 512                     // could technically only do this on MSVC, but I
 513                     // I want to minimize the divergence between MSVC
 514                     // and non-MSVC.
 515
 516                     let unwind_blocks = unwind_blocks.as_mut().unwrap();
 517                     let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 518                     let halfladder = self.drop_halfladder(&unwind_ladder, unwind, &fields);
 519                     unwind_blocks.push(halfladder.last().cloned().unwrap());
 520                 }
 521                 let (normal, _) = self.drop_ladder(fields, succ, unwind);
 522                 normal_blocks.push(normal);
 523             } else {
 524                 have_otherwise = true;
 525
 526                 let param_env = self.elaborator.param_env();
 527                 let have_field_with_drop_glue = variant
 528                     .fields
 529                     .iter()
 530                     .any(|field| field.ty(tcx, substs).needs_drop(tcx, param_env));
 531                 if have_field_with_drop_glue {
 532                     have_otherwise_with_drop_glue = true;
 533                 }
 534             }
 535         }
 536
 537         if !have_otherwise {
 538             values.pop();
 539         } else if !have_otherwise_with_drop_glue {
 540             normal_blocks.push(self.goto_block(succ, unwind));
 541             if let Unwind::To(unwind) = unwind {
 542                 unwind_blocks.as_mut().unwrap().push(self.goto_block(unwind, Unwind::InCleanup));
 543             }
 544         } else {
 545             normal_blocks.push(self.drop_block(succ, unwind));
 546             if let Unwind::To(unwind) = unwind {
 547                 unwind_blocks.as_mut().unwrap().push(self.drop_block(unwind, Unwind::InCleanup));
 548             }
 549         }
 550
 551         (
 552             self.adt_switch_block(adt, normal_blocks, &values, succ, unwind),
 553             unwind.map(|unwind| {
 554                 self.adt_switch_block(
 555                     adt,
 556                     unwind_blocks.unwrap(),
 557                     &values,
 558                     unwind,
 559                     Unwind::InCleanup,
 560                 )
 561             }),
 562         )
 563     }
 564
 565     fn adt_switch_block(
 566         &mut self,
 567         adt: ty::AdtDef<'tcx>,
 568         blocks: Vec<BasicBlock>,
 569         values: &[u128],
 570         succ: BasicBlock,
 571         unwind: Unwind,
 572     ) -> BasicBlock {
 573         // If there are multiple variants, then if something
 574         // is present within the enum the discriminant, tracked
 575         // by the rest path, must be initialized.
 576         //
 577         // Additionally, we do not want to switch on the
 578         // discriminant after it is free-ed, because that
 579         // way lies only trouble.
 580         let discr_ty = adt.repr().discr_type().to_ty(self.tcx());
 581         let discr = Place::from(self.new_temp(discr_ty));
 582         let discr_rv = Rvalue::Discriminant(self.place);
 583         let switch_block = BasicBlockData {
 584             statements: vec![self.assign(discr, discr_rv)],
 585             terminator: Some(Terminator {
 586                 source_info: self.source_info,
 587                 kind: TerminatorKind::SwitchInt {
 588                     discr: Operand::Move(discr),
 589                     switch_ty: discr_ty,
 590                     targets: SwitchTargets::new(
 591                         values.iter().copied().zip(blocks.iter().copied()),
 592                         *blocks.last().unwrap(),
 593                     ),
 594                 },
 595             }),
 596             is_cleanup: unwind.is_cleanup(),
 597         };
 598         let switch_block = self.elaborator.patch().new_block(switch_block);
 599         self.drop_flag_test_block(switch_block, succ, unwind)
 600     }
 601
 602     fn destructor_call_block(&mut self, (succ, unwind): (BasicBlock, Unwind)) -> BasicBlock {
 603         debug!("destructor_call_block({:?}, {:?})", self, succ);
 604         let tcx = self.tcx();
 605         let drop_trait = tcx.require_lang_item(LangItem::Drop, None);
 606         let drop_fn = tcx.associated_item_def_ids(drop_trait)[0];
 607         let ty = self.place_ty(self.place);
 608         let substs = tcx.mk_substs_trait(ty, &[]);
 609
 610         let ref_ty =
 611             tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut { ty, mutbl: hir::Mutability::Mut });
 612         let ref_place = self.new_temp(ref_ty);
 613         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 614
 615         let result = BasicBlockData {
 616             statements: vec![self.assign(
 617                 Place::from(ref_place),
 618                 Rvalue::Ref(
 619                     tcx.lifetimes.re_erased,
 620                     BorrowKind::Mut { allow_two_phase_borrow: false },
 621                     self.place,
 622                 ),
 623             )],
 624             terminator: Some(Terminator {
 625                 kind: TerminatorKind::Call {
 626                     func: Operand::function_handle(tcx, drop_fn, substs, self.source_info.span),
 627                     args: vec![Operand::Move(Place::from(ref_place))],
 628                     destination: Some((unit_temp, succ)),
 629                     cleanup: unwind.into_option(),
 630                     from_hir_call: true,
 631                     fn_span: self.source_info.span,
 632                 },
 633                 source_info: self.source_info,
 634             }),
 635             is_cleanup: unwind.is_cleanup(),
 636         };
 637         self.elaborator.patch().new_block(result)
 638     }
 639
 640     /// Create a loop that drops an array:
 641     ///
 642     /// ```text
 643     /// loop-block:
 644     ///    can_go = cur == length_or_end
 645     ///    if can_go then succ else drop-block
 646     /// drop-block:
 647     ///    if ptr_based {
 648     ///        ptr = cur
 649     ///        cur = cur.offset(1)
 650     ///    } else {
 651     ///        ptr = &raw mut P[cur]
 652     ///        cur = cur + 1
 653     ///    }
 654     ///    drop(ptr)
 655     /// ```
 656     fn drop_loop(
 657         &mut self,
 658         succ: BasicBlock,
 659         cur: Local,
 660         length_or_end: Place<'tcx>,
 661         ety: Ty<'tcx>,
 662         unwind: Unwind,
 663         ptr_based: bool,
 664     ) -> BasicBlock {
 665         let copy = |place: Place<'tcx>| Operand::Copy(place);
 666         let move_ = |place: Place<'tcx>| Operand::Move(place);
 667         let tcx = self.tcx();
 668
 669         let ptr_ty = tcx.mk_ptr(ty::TypeAndMut { ty: ety, mutbl: hir::Mutability::Mut });
 670         let ptr = Place::from(self.new_temp(ptr_ty));
 671         let can_go = Place::from(self.new_temp(tcx.types.bool));
 672
 673         let one = self.constant_usize(1);
 674         let (ptr_next, cur_next) = if ptr_based {
 675             (
 676                 Rvalue::Use(copy(cur.into())),
 677                 Rvalue::BinaryOp(BinOp::Offset, Box::new((move_(cur.into()), one))),
 678             )
 679         } else {
 680             (
 681                 Rvalue::AddressOf(Mutability::Mut, tcx.mk_place_index(self.place, cur)),
 682                 Rvalue::BinaryOp(BinOp::Add, Box::new((move_(cur.into()), one))),
 683             )
 684         };
 685
 686         let drop_block = BasicBlockData {
 687             statements: vec![self.assign(ptr, ptr_next), self.assign(Place::from(cur), cur_next)],
 688             is_cleanup: unwind.is_cleanup(),
 689             terminator: Some(Terminator {
 690                 source_info: self.source_info,
 691                 // this gets overwritten by drop elaboration.
 692                 kind: TerminatorKind::Unreachable,
 693             }),
 694         };
 695         let drop_block = self.elaborator.patch().new_block(drop_block);
 696
 697         let loop_block = BasicBlockData {
 698             statements: vec![self.assign(
 699                 can_go,
 700                 Rvalue::BinaryOp(
 701                     BinOp::Eq,
 702                     Box::new((copy(Place::from(cur)), copy(length_or_end))),
 703                 ),
 704             )],
 705             is_cleanup: unwind.is_cleanup(),
 706             terminator: Some(Terminator {
 707                 source_info: self.source_info,
 708                 kind: TerminatorKind::if_(tcx, move_(can_go), succ, drop_block),
 709             }),
 710         };
 711         let loop_block = self.elaborator.patch().new_block(loop_block);
 712
 713         self.elaborator.patch().patch_terminator(
 714             drop_block,
 715             TerminatorKind::Drop {
 716                 place: tcx.mk_place_deref(ptr),
 717                 target: loop_block,
 718                 unwind: unwind.into_option(),
 719             },
 720         );
 721
 722         loop_block
 723     }
 724
 725     fn open_drop_for_array(&mut self, ety: Ty<'tcx>, opt_size: Option<u64>) -> BasicBlock {
 726         debug!("open_drop_for_array({:?}, {:?})", ety, opt_size);
 727
 728         // if size_of::<ety>() == 0 {
 729         //     index_based_loop
 730         // } else {
 731         //     ptr_based_loop
 732         // }
 733
 734         let tcx = self.tcx();
 735
 736         if let Some(size) = opt_size {
 737             let fields: Vec<(Place<'tcx>, Option<D::Path>)> = (0..size)
 738                 .map(|i| {
 739                     (
 740                         tcx.mk_place_elem(
 741                             self.place,
 742                             ProjectionElem::ConstantIndex {
 743                                 offset: i,
 744                                 min_length: size,
 745                                 from_end: false,
 746                             },
 747                         ),
 748                         self.elaborator.array_subpath(self.path, i, size),
 749                     )
 750                 })
 751                 .collect();
 752
 753             if fields.iter().any(|(_, path)| path.is_some()) {
 754                 let (succ, unwind) = self.drop_ladder_bottom();
 755                 return self.drop_ladder(fields, succ, unwind).0;
 756             }
 757         }
 758
 759         let move_ = |place: Place<'tcx>| Operand::Move(place);
 760         let elem_size = Place::from(self.new_temp(tcx.types.usize));
 761         let len = Place::from(self.new_temp(tcx.types.usize));
 762
 763         let base_block = BasicBlockData {
 764             statements: vec![
 765                 self.assign(elem_size, Rvalue::NullaryOp(NullOp::SizeOf, ety)),
 766                 self.assign(len, Rvalue::Len(self.place)),
 767             ],
 768             is_cleanup: self.unwind.is_cleanup(),
 769             terminator: Some(Terminator {
 770                 source_info: self.source_info,
 771                 kind: TerminatorKind::SwitchInt {
 772                     discr: move_(elem_size),
 773                     switch_ty: tcx.types.usize,
 774                     targets: SwitchTargets::static_if(
 775                         0,
 776                         self.drop_loop_pair(ety, false, len),
 777                         self.drop_loop_pair(ety, true, len),
 778                     ),
 779                 },
 780             }),
 781         };
 782         self.elaborator.patch().new_block(base_block)
 783     }
 784
 785     /// Creates a pair of drop-loops of `place`, which drops its contents, even
 786     /// in the case of 1 panic. If `ptr_based`, creates a pointer loop,
 787     /// otherwise create an index loop.
 788     fn drop_loop_pair(
 789         &mut self,
 790         ety: Ty<'tcx>,
 791         ptr_based: bool,
 792         length: Place<'tcx>,
 793     ) -> BasicBlock {
 794         debug!("drop_loop_pair({:?}, {:?})", ety, ptr_based);
 795         let tcx = self.tcx();
 796         let iter_ty = if ptr_based { tcx.mk_mut_ptr(ety) } else { tcx.types.usize };
 797
 798         let cur = self.new_temp(iter_ty);
 799         let length_or_end = if ptr_based { Place::from(self.new_temp(iter_ty)) } else { length };
 800
 801         let unwind = self.unwind.map(|unwind| {
 802             self.drop_loop(unwind, cur, length_or_end, ety, Unwind::InCleanup, ptr_based)
 803         });
 804
 805         let loop_block = self.drop_loop(self.succ, cur, length_or_end, ety, unwind, ptr_based);
 806
 807         let cur = Place::from(cur);
 808         let drop_block_stmts = if ptr_based {
 809             let tmp_ty = tcx.mk_mut_ptr(self.place_ty(self.place));
 810             let tmp = Place::from(self.new_temp(tmp_ty));
 811             // tmp = &raw mut P;
 812             // cur = tmp as *mut T;
 813             // end = Offset(cur, len);
 814             vec![
 815                 self.assign(tmp, Rvalue::AddressOf(Mutability::Mut, self.place)),
 816                 self.assign(cur, Rvalue::Cast(CastKind::Misc, Operand::Move(tmp), iter_ty)),
 817                 self.assign(
 818                     length_or_end,
 819                     Rvalue::BinaryOp(
 820                         BinOp::Offset,
 821                         Box::new((Operand::Copy(cur), Operand::Move(length))),
 822                     ),
 823                 ),
 824             ]
 825         } else {
 826             // cur = 0 (length already pushed)
 827             let zero = self.constant_usize(0);
 828             vec![self.assign(cur, Rvalue::Use(zero))]
 829         };
 830         let drop_block = self.elaborator.patch().new_block(BasicBlockData {
 831             statements: drop_block_stmts,
 832             is_cleanup: unwind.is_cleanup(),
 833             terminator: Some(Terminator {
 834                 source_info: self.source_info,
 835                 kind: TerminatorKind::Goto { target: loop_block },
 836             }),
 837         });
 838
 839         // FIXME(#34708): handle partially-dropped array/slice elements.
 840         let reset_block = self.drop_flag_reset_block(DropFlagMode::Deep, drop_block, unwind);
 841         self.drop_flag_test_block(reset_block, self.succ, unwind)
 842     }
 843
 844     /// The slow-path - create an "open", elaborated drop for a type
 845     /// which is moved-out-of only partially, and patch `bb` to a jump
 846     /// to it. This must not be called on ADTs with a destructor,
 847     /// as these can't be moved-out-of, except for `Box<T>`, which is
 848     /// special-cased.
 849     ///
 850     /// This creates a "drop ladder" that drops the needed fields of the
 851     /// ADT, both in the success case or if one of the destructors fail.
 852     fn open_drop(&mut self) -> BasicBlock {
 853         let ty = self.place_ty(self.place);
 854         match ty.kind() {
 855             ty::Closure(_, substs) => {
 856                 let tys: Vec<_> = substs.as_closure().upvar_tys().collect();
 857                 self.open_drop_for_tuple(&tys)
 858             }
 859             // Note that `elaborate_drops` only drops the upvars of a generator,
 860             // and this is ok because `open_drop` here can only be reached
 861             // within that own generator's resume function.
 862             // This should only happen for the self argument on the resume function.
 863             // It effectively only contains upvars until the generator transformation runs.
 864             // See librustc_body/transform/generator.rs for more details.
 865             ty::Generator(_, substs, _) => {
 866                 let tys: Vec<_> = substs.as_generator().upvar_tys().collect();
 867                 self.open_drop_for_tuple(&tys)
 868             }
 869             ty::Tuple(fields) => self.open_drop_for_tuple(fields),
 870             ty::Adt(def, substs) => {
 871                 if def.is_box() {
 872                     self.open_drop_for_box(*def, substs)
 873                 } else {
 874                     self.open_drop_for_adt(*def, substs)
 875                 }
 876             }
 877             ty::Dynamic(..) => self.complete_drop(self.succ, self.unwind),
 878             ty::Array(ety, size) => {
 879                 let size = size.try_eval_usize(self.tcx(), self.elaborator.param_env());
 880                 self.open_drop_for_array(*ety, size)
 881             }
 882             ty::Slice(ety) => self.open_drop_for_array(*ety, None),
 883
 884             _ => bug!("open drop from non-ADT `{:?}`", ty),
 885         }
 886     }
 887
 888     fn complete_drop(&mut self, succ: BasicBlock, unwind: Unwind) -> BasicBlock {
 889         debug!("complete_drop(succ={:?}, unwind={:?})", succ, unwind);
 890
 891         let drop_block = self.drop_block(succ, unwind);
 892
 893         self.drop_flag_test_block(drop_block, succ, unwind)
 894     }
 895
 896     /// Creates a block that resets the drop flag. If `mode` is deep, all children drop flags will
 897     /// also be cleared.
 898     fn drop_flag_reset_block(
 899         &mut self,
 900         mode: DropFlagMode,
 901         succ: BasicBlock,
 902         unwind: Unwind,
 903     ) -> BasicBlock {
 904         debug!("drop_flag_reset_block({:?},{:?})", self, mode);
 905
 906         if unwind.is_cleanup() {
 907             // The drop flag isn't read again on the unwind path, so don't
 908             // bother setting it.
 909             return succ;
 910         }
 911         let block = self.new_block(unwind, TerminatorKind::Goto { target: succ });
 912         let block_start = Location { block, statement_index: 0 };
 913         self.elaborator.clear_drop_flag(block_start, self.path, mode);
 914         block
 915     }
 916
 917     fn elaborated_drop_block(&mut self) -> BasicBlock {
 918         debug!("elaborated_drop_block({:?})", self);
 919         let blk = self.drop_block(self.succ, self.unwind);
 920         self.elaborate_drop(blk);
 921         blk
 922     }
 923
 924     /// Creates a block that frees the backing memory of a `Box` if its drop is required (either
 925     /// statically or by checking its drop flag).
 926     ///
 927     /// The contained value will not be dropped.
 928     fn box_free_block(
 929         &mut self,
 930         adt: ty::AdtDef<'tcx>,
 931         substs: SubstsRef<'tcx>,
 932         target: BasicBlock,
 933         unwind: Unwind,
 934     ) -> BasicBlock {
 935         let block = self.unelaborated_free_block(adt, substs, target, unwind);
 936         self.drop_flag_test_block(block, target, unwind)
 937     }
 938
 939     /// Creates a block that frees the backing memory of a `Box` (without dropping the contained
 940     /// value).
 941     fn unelaborated_free_block(
 942         &mut self,
 943         adt: ty::AdtDef<'tcx>,
 944         substs: SubstsRef<'tcx>,
 945         target: BasicBlock,
 946         unwind: Unwind,
 947     ) -> BasicBlock {
 948         let tcx = self.tcx();
 949         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 950         let free_func = tcx.require_lang_item(LangItem::BoxFree, Some(self.source_info.span));
 951         let args = adt
 952             .variant(VariantIdx::new(0))
 953             .fields
 954             .iter()
 955             .enumerate()
 956             .map(|(i, f)| {
 957                 let field = Field::new(i);
 958                 let field_ty = f.ty(tcx, substs);
 959                 Operand::Move(tcx.mk_place_field(self.place, field, field_ty))
 960             })
 961             .collect();
 962
 963         let call = TerminatorKind::Call {
 964             func: Operand::function_handle(tcx, free_func, substs, self.source_info.span),
 965             args,
 966             destination: Some((unit_temp, target)),
 967             cleanup: None,
 968             from_hir_call: false,
 969             fn_span: self.source_info.span,
 970         }; // FIXME(#43234)
 971         let free_block = self.new_block(unwind, call);
 972
 973         let block_start = Location { block: free_block, statement_index: 0 };
 974         self.elaborator.clear_drop_flag(block_start, self.path, DropFlagMode::Shallow);
 975         free_block
 976     }
 977
 978     fn drop_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
 979         let block =
 980             TerminatorKind::Drop { place: self.place, target, unwind: unwind.into_option() };
 981         self.new_block(unwind, block)
 982     }
 983
 984     fn goto_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
 985         let block = TerminatorKind::Goto { target };
 986         self.new_block(unwind, block)
 987     }
 988
 989     /// Returns the block to jump to in order to test the drop flag and execute the drop.
 990     ///
 991     /// Depending on the required `DropStyle`, this might be a generated block with an `if`
 992     /// terminator (for dynamic/open drops), or it might be `on_set` or `on_unset` itself, in case
 993     /// the drop can be statically determined.
 994     fn drop_flag_test_block(
 995         &mut self,
 996         on_set: BasicBlock,
 997         on_unset: BasicBlock,
 998         unwind: Unwind,
 999     ) -> BasicBlock {
1000         let style = self.elaborator.drop_style(self.path, DropFlagMode::Shallow);
1001         debug!(
1002             "drop_flag_test_block({:?},{:?},{:?},{:?}) - {:?}",
1003             self, on_set, on_unset, unwind, style
1004         );
1005
1006         match style {
1007             DropStyle::Dead => on_unset,
1008             DropStyle::Static => on_set,
1009             DropStyle::Conditional | DropStyle::Open => {
1010                 let flag = self.elaborator.get_drop_flag(self.path).unwrap();
1011                 let term = TerminatorKind::if_(self.tcx(), flag, on_set, on_unset);
1012                 self.new_block(unwind, term)
1013             }
1014         }
1015     }
1016
1017     fn new_block(&mut self, unwind: Unwind, k: TerminatorKind<'tcx>) -> BasicBlock {
1018         self.elaborator.patch().new_block(BasicBlockData {
1019             statements: vec![],
1020             terminator: Some(Terminator { source_info: self.source_info, kind: k }),
1021             is_cleanup: unwind.is_cleanup(),
1022         })
1023     }
1024
1025     fn new_temp(&mut self, ty: Ty<'tcx>) -> Local {
1026         self.elaborator.patch().new_temp(ty, self.source_info.span)
1027     }
1028
1029     fn constant_usize(&self, val: u16) -> Operand<'tcx> {
1030         Operand::Constant(Box::new(Constant {
1031             span: self.source_info.span,
1032             user_ty: None,
1033             literal: ty::Const::from_usize(self.tcx(), val.into()).into(),
1034         }))
1035     }
1036
1037     fn assign(&self, lhs: Place<'tcx>, rhs: Rvalue<'tcx>) -> Statement<'tcx> {
1038         Statement {
1039             source_info: self.source_info,
1040             kind: StatementKind::Assign(Box::new((lhs, rhs))),
1041         }
1042     }
1043 }