compiler/rustc_mir/src/util/elaborate_drops.rs

   1 use crate::util::patch::MirPatch;
   2 use rustc_hir as hir;
   3 use rustc_hir::lang_items::LangItem;
   4 use rustc_index::vec::Idx;
   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;
  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         Some(succ)
 333             .into_iter()
 334             .chain(fields.iter().rev().zip(unwind_ladder).map(|(&(place, path), &unwind_succ)| {
 335                 succ = self.drop_subpath(place, path, succ, unwind_succ);
 336                 succ
 337             }))
 338             .collect()
 339     }
 340
 341     fn drop_ladder_bottom(&mut self) -> (BasicBlock, Unwind) {
 342         // Clear the "master" drop flag at the end. This is needed
 343         // because the "master" drop protects the ADT's discriminant,
 344         // which is invalidated after the ADT is dropped.
 345         (self.drop_flag_reset_block(DropFlagMode::Shallow, self.succ, self.unwind), self.unwind)
 346     }
 347
 348     /// Creates a full drop ladder, consisting of 2 connected half-drop-ladders
 349     ///
 350     /// For example, with 3 fields, the drop ladder is
 351     ///
 352     /// .d0:
 353     ///     ELAB(drop location.0 [target=.d1, unwind=.c1])
 354     /// .d1:
 355     ///     ELAB(drop location.1 [target=.d2, unwind=.c2])
 356     /// .d2:
 357     ///     ELAB(drop location.2 [target=`self.succ`, unwind=`self.unwind`])
 358     /// .c1:
 359     ///     ELAB(drop location.1 [target=.c2])
 360     /// .c2:
 361     ///     ELAB(drop location.2 [target=`self.unwind`])
 362     ///
 363     /// NOTE: this does not clear the master drop flag, so you need
 364     /// to point succ/unwind on a `drop_ladder_bottom`.
 365     fn drop_ladder(
 366         &mut self,
 367         fields: Vec<(Place<'tcx>, Option<D::Path>)>,
 368         succ: BasicBlock,
 369         unwind: Unwind,
 370     ) -> (BasicBlock, Unwind) {
 371         debug!("drop_ladder({:?}, {:?})", self, fields);
 372
 373         let mut fields = fields;
 374         fields.retain(|&(place, _)| {
 375             self.place_ty(place).needs_drop(self.tcx(), self.elaborator.param_env())
 376         });
 377
 378         debug!("drop_ladder - fields needing drop: {:?}", fields);
 379
 380         let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 381         let unwind_ladder: Vec<_> = if let Unwind::To(target) = unwind {
 382             let halfladder = self.drop_halfladder(&unwind_ladder, target, &fields);
 383             halfladder.into_iter().map(Unwind::To).collect()
 384         } else {
 385             unwind_ladder
 386         };
 387
 388         let normal_ladder = self.drop_halfladder(&unwind_ladder, succ, &fields);
 389
 390         (*normal_ladder.last().unwrap(), *unwind_ladder.last().unwrap())
 391     }
 392
 393     fn open_drop_for_tuple(&mut self, tys: &[Ty<'tcx>]) -> BasicBlock {
 394         debug!("open_drop_for_tuple({:?}, {:?})", self, tys);
 395
 396         let fields = tys
 397             .iter()
 398             .enumerate()
 399             .map(|(i, &ty)| {
 400                 (
 401                     self.tcx().mk_place_field(self.place, Field::new(i), ty),
 402                     self.elaborator.field_subpath(self.path, Field::new(i)),
 403                 )
 404             })
 405             .collect();
 406
 407         let (succ, unwind) = self.drop_ladder_bottom();
 408         self.drop_ladder(fields, succ, unwind).0
 409     }
 410
 411     fn open_drop_for_box(&mut self, adt: &'tcx ty::AdtDef, substs: SubstsRef<'tcx>) -> BasicBlock {
 412         debug!("open_drop_for_box({:?}, {:?}, {:?})", self, adt, substs);
 413
 414         let interior = self.tcx().mk_place_deref(self.place);
 415         let interior_path = self.elaborator.deref_subpath(self.path);
 416
 417         let succ = self.box_free_block(adt, substs, self.succ, self.unwind);
 418         let unwind_succ =
 419             self.unwind.map(|unwind| self.box_free_block(adt, substs, unwind, Unwind::InCleanup));
 420
 421         self.drop_subpath(interior, interior_path, succ, unwind_succ)
 422     }
 423
 424     fn open_drop_for_adt(&mut self, adt: &'tcx ty::AdtDef, substs: SubstsRef<'tcx>) -> BasicBlock {
 425         debug!("open_drop_for_adt({:?}, {:?}, {:?})", self, adt, substs);
 426         if adt.variants.is_empty() {
 427             return self.elaborator.patch().new_block(BasicBlockData {
 428                 statements: vec![],
 429                 terminator: Some(Terminator {
 430                     source_info: self.source_info,
 431                     kind: TerminatorKind::Unreachable,
 432                 }),
 433                 is_cleanup: self.unwind.is_cleanup(),
 434             });
 435         }
 436
 437         let skip_contents =
 438             adt.is_union() || Some(adt.did) == self.tcx().lang_items().manually_drop();
 439         let contents_drop = if skip_contents {
 440             (self.succ, self.unwind)
 441         } else {
 442             self.open_drop_for_adt_contents(adt, substs)
 443         };
 444
 445         if adt.has_dtor(self.tcx()) {
 446             self.destructor_call_block(contents_drop)
 447         } else {
 448             contents_drop.0
 449         }
 450     }
 451
 452     fn open_drop_for_adt_contents(
 453         &mut self,
 454         adt: &'tcx ty::AdtDef,
 455         substs: SubstsRef<'tcx>,
 456     ) -> (BasicBlock, Unwind) {
 457         let (succ, unwind) = self.drop_ladder_bottom();
 458         if !adt.is_enum() {
 459             let fields = self.move_paths_for_fields(
 460                 self.place,
 461                 self.path,
 462                 &adt.variants[VariantIdx::new(0)],
 463                 substs,
 464             );
 465             self.drop_ladder(fields, succ, unwind)
 466         } else {
 467             self.open_drop_for_multivariant(adt, substs, succ, unwind)
 468         }
 469     }
 470
 471     fn open_drop_for_multivariant(
 472         &mut self,
 473         adt: &'tcx ty::AdtDef,
 474         substs: SubstsRef<'tcx>,
 475         succ: BasicBlock,
 476         unwind: Unwind,
 477     ) -> (BasicBlock, Unwind) {
 478         let mut values = Vec::with_capacity(adt.variants.len());
 479         let mut normal_blocks = Vec::with_capacity(adt.variants.len());
 480         let mut unwind_blocks =
 481             if unwind.is_cleanup() { None } else { Some(Vec::with_capacity(adt.variants.len())) };
 482
 483         let mut have_otherwise_with_drop_glue = false;
 484         let mut have_otherwise = false;
 485         let tcx = self.tcx();
 486
 487         for (variant_index, discr) in adt.discriminants(tcx) {
 488             let variant = &adt.variants[variant_index];
 489             let subpath = self.elaborator.downcast_subpath(self.path, variant_index);
 490
 491             if let Some(variant_path) = subpath {
 492                 let base_place = tcx.mk_place_elem(
 493                     self.place,
 494                     ProjectionElem::Downcast(Some(variant.ident.name), variant_index),
 495                 );
 496                 let fields = self.move_paths_for_fields(base_place, variant_path, &variant, substs);
 497                 values.push(discr.val);
 498                 if let Unwind::To(unwind) = unwind {
 499                     // We can't use the half-ladder from the original
 500                     // drop ladder, because this breaks the
 501                     // "funclet can't have 2 successor funclets"
 502                     // requirement from MSVC:
 503                     //
 504                     //           switch       unwind-switch
 505                     //          /      \         /        \
 506                     //         v1.0    v2.0  v2.0-unwind  v1.0-unwind
 507                     //         |        |      /             |
 508                     //    v1.1-unwind  v2.1-unwind           |
 509                     //      ^                                |
 510                     //       \-------------------------------/
 511                     //
 512                     // Create a duplicate half-ladder to avoid that. We
 513                     // could technically only do this on MSVC, but I
 514                     // I want to minimize the divergence between MSVC
 515                     // and non-MSVC.
 516
 517                     let unwind_blocks = unwind_blocks.as_mut().unwrap();
 518                     let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 519                     let halfladder = self.drop_halfladder(&unwind_ladder, unwind, &fields);
 520                     unwind_blocks.push(halfladder.last().cloned().unwrap());
 521                 }
 522                 let (normal, _) = self.drop_ladder(fields, succ, unwind);
 523                 normal_blocks.push(normal);
 524             } else {
 525                 have_otherwise = true;
 526
 527                 let param_env = self.elaborator.param_env();
 528                 let have_field_with_drop_glue = variant
 529                     .fields
 530                     .iter()
 531                     .any(|field| field.ty(tcx, substs).needs_drop(tcx, param_env));
 532                 if have_field_with_drop_glue {
 533                     have_otherwise_with_drop_glue = true;
 534                 }
 535             }
 536         }
 537
 538         if !have_otherwise {
 539             values.pop();
 540         } else if !have_otherwise_with_drop_glue {
 541             normal_blocks.push(self.goto_block(succ, unwind));
 542             if let Unwind::To(unwind) = unwind {
 543                 unwind_blocks.as_mut().unwrap().push(self.goto_block(unwind, Unwind::InCleanup));
 544             }
 545         } else {
 546             normal_blocks.push(self.drop_block(succ, unwind));
 547             if let Unwind::To(unwind) = unwind {
 548                 unwind_blocks.as_mut().unwrap().push(self.drop_block(unwind, Unwind::InCleanup));
 549             }
 550         }
 551
 552         (
 553             self.adt_switch_block(adt, normal_blocks, &values, succ, unwind),
 554             unwind.map(|unwind| {
 555                 self.adt_switch_block(
 556                     adt,
 557                     unwind_blocks.unwrap(),
 558                     &values,
 559                     unwind,
 560                     Unwind::InCleanup,
 561                 )
 562             }),
 563         )
 564     }
 565
 566     fn adt_switch_block(
 567         &mut self,
 568         adt: &'tcx ty::AdtDef,
 569         blocks: Vec<BasicBlock>,
 570         values: &[u128],
 571         succ: BasicBlock,
 572         unwind: Unwind,
 573     ) -> BasicBlock {
 574         // If there are multiple variants, then if something
 575         // is present within the enum the discriminant, tracked
 576         // by the rest path, must be initialized.
 577         //
 578         // Additionally, we do not want to switch on the
 579         // discriminant after it is free-ed, because that
 580         // way lies only trouble.
 581         let discr_ty = adt.repr.discr_type().to_ty(self.tcx());
 582         let discr = Place::from(self.new_temp(discr_ty));
 583         let discr_rv = Rvalue::Discriminant(self.place);
 584         let switch_block = BasicBlockData {
 585             statements: vec![self.assign(discr, discr_rv)],
 586             terminator: Some(Terminator {
 587                 source_info: self.source_info,
 588                 kind: TerminatorKind::SwitchInt {
 589                     discr: Operand::Move(discr),
 590                     switch_ty: discr_ty,
 591                     targets: SwitchTargets::new(
 592                         values.iter().copied().zip(blocks.iter().copied()),
 593                         *blocks.last().unwrap(),
 594                     ),
 595                 },
 596             }),
 597             is_cleanup: unwind.is_cleanup(),
 598         };
 599         let switch_block = self.elaborator.patch().new_block(switch_block);
 600         self.drop_flag_test_block(switch_block, succ, unwind)
 601     }
 602
 603     fn destructor_call_block(&mut self, (succ, unwind): (BasicBlock, Unwind)) -> BasicBlock {
 604         debug!("destructor_call_block({:?}, {:?})", self, succ);
 605         let tcx = self.tcx();
 606         let drop_trait = tcx.require_lang_item(LangItem::Drop, None);
 607         let drop_fn = tcx.associated_items(drop_trait).in_definition_order().next().unwrap();
 608         let ty = self.place_ty(self.place);
 609         let substs = tcx.mk_substs_trait(ty, &[]);
 610
 611         let ref_ty =
 612             tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut { ty, mutbl: hir::Mutability::Mut });
 613         let ref_place = self.new_temp(ref_ty);
 614         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 615
 616         let result = BasicBlockData {
 617             statements: vec![self.assign(
 618                 Place::from(ref_place),
 619                 Rvalue::Ref(
 620                     tcx.lifetimes.re_erased,
 621                     BorrowKind::Mut { allow_two_phase_borrow: false },
 622                     self.place,
 623                 ),
 624             )],
 625             terminator: Some(Terminator {
 626                 kind: TerminatorKind::Call {
 627                     func: Operand::function_handle(
 628                         tcx,
 629                         drop_fn.def_id,
 630                         substs,
 631                         self.source_info.span,
 632                     ),
 633                     args: vec![Operand::Move(Place::from(ref_place))],
 634                     destination: Some((unit_temp, succ)),
 635                     cleanup: unwind.into_option(),
 636                     from_hir_call: true,
 637                     fn_span: self.source_info.span,
 638                 },
 639                 source_info: self.source_info,
 640             }),
 641             is_cleanup: unwind.is_cleanup(),
 642         };
 643         self.elaborator.patch().new_block(result)
 644     }
 645
 646     /// Create a loop that drops an array:
 647     ///
 648     /// ```text
 649     /// loop-block:
 650     ///    can_go = cur == length_or_end
 651     ///    if can_go then succ else drop-block
 652     /// drop-block:
 653     ///    if ptr_based {
 654     ///        ptr = cur
 655     ///        cur = cur.offset(1)
 656     ///    } else {
 657     ///        ptr = &raw mut P[cur]
 658     ///        cur = cur + 1
 659     ///    }
 660     ///    drop(ptr)
 661     /// ```
 662     fn drop_loop(
 663         &mut self,
 664         succ: BasicBlock,
 665         cur: Local,
 666         length_or_end: Place<'tcx>,
 667         ety: Ty<'tcx>,
 668         unwind: Unwind,
 669         ptr_based: bool,
 670     ) -> BasicBlock {
 671         let copy = |place: Place<'tcx>| Operand::Copy(place);
 672         let move_ = |place: Place<'tcx>| Operand::Move(place);
 673         let tcx = self.tcx();
 674
 675         let ptr_ty = tcx.mk_ptr(ty::TypeAndMut { ty: ety, mutbl: hir::Mutability::Mut });
 676         let ptr = Place::from(self.new_temp(ptr_ty));
 677         let can_go = Place::from(self.new_temp(tcx.types.bool));
 678
 679         let one = self.constant_usize(1);
 680         let (ptr_next, cur_next) = if ptr_based {
 681             (Rvalue::Use(copy(cur.into())), Rvalue::BinaryOp(BinOp::Offset, move_(cur.into()), one))
 682         } else {
 683             (
 684                 Rvalue::AddressOf(Mutability::Mut, tcx.mk_place_index(self.place, cur)),
 685                 Rvalue::BinaryOp(BinOp::Add, move_(cur.into()), one),
 686             )
 687         };
 688
 689         let drop_block = BasicBlockData {
 690             statements: vec![self.assign(ptr, ptr_next), self.assign(Place::from(cur), cur_next)],
 691             is_cleanup: unwind.is_cleanup(),
 692             terminator: Some(Terminator {
 693                 source_info: self.source_info,
 694                 // this gets overwritten by drop elaboration.
 695                 kind: TerminatorKind::Unreachable,
 696             }),
 697         };
 698         let drop_block = self.elaborator.patch().new_block(drop_block);
 699
 700         let loop_block = BasicBlockData {
 701             statements: vec![self.assign(
 702                 can_go,
 703                 Rvalue::BinaryOp(BinOp::Eq, copy(Place::from(cur)), copy(length_or_end)),
 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         static USIZE_SWITCH_ZERO: &[u128; 1] = &[0];
 764
 765         let base_block = BasicBlockData {
 766             statements: vec![
 767                 self.assign(elem_size, Rvalue::NullaryOp(NullOp::SizeOf, ety)),
 768                 self.assign(len, Rvalue::Len(self.place)),
 769             ],
 770             is_cleanup: self.unwind.is_cleanup(),
 771             terminator: Some(Terminator {
 772                 source_info: self.source_info,
 773                 kind: TerminatorKind::SwitchInt {
 774                     discr: move_(elem_size),
 775                     switch_ty: tcx.types.usize,
 776                     targets: SwitchTargets::static_if(
 777                         USIZE_SWITCH_ZERO,
 778                         self.drop_loop_pair(ety, false, len),
 779                         self.drop_loop_pair(ety, true, len),
 780                     ),
 781                 },
 782             }),
 783         };
 784         self.elaborator.patch().new_block(base_block)
 785     }
 786
 787     /// Creates a pair of drop-loops of `place`, which drops its contents, even
 788     /// in the case of 1 panic. If `ptr_based`, creates a pointer loop,
 789     /// otherwise create an index loop.
 790     fn drop_loop_pair(
 791         &mut self,
 792         ety: Ty<'tcx>,
 793         ptr_based: bool,
 794         length: Place<'tcx>,
 795     ) -> BasicBlock {
 796         debug!("drop_loop_pair({:?}, {:?})", ety, ptr_based);
 797         let tcx = self.tcx();
 798         let iter_ty = if ptr_based { tcx.mk_mut_ptr(ety) } else { tcx.types.usize };
 799
 800         let cur = self.new_temp(iter_ty);
 801         let length_or_end = if ptr_based { Place::from(self.new_temp(iter_ty)) } else { length };
 802
 803         let unwind = self.unwind.map(|unwind| {
 804             self.drop_loop(unwind, cur, length_or_end, ety, Unwind::InCleanup, ptr_based)
 805         });
 806
 807         let loop_block = self.drop_loop(self.succ, cur, length_or_end, ety, unwind, ptr_based);
 808
 809         let cur = Place::from(cur);
 810         let drop_block_stmts = if ptr_based {
 811             let tmp_ty = tcx.mk_mut_ptr(self.place_ty(self.place));
 812             let tmp = Place::from(self.new_temp(tmp_ty));
 813             // tmp = &raw mut P;
 814             // cur = tmp as *mut T;
 815             // end = Offset(cur, len);
 816             vec![
 817                 self.assign(tmp, Rvalue::AddressOf(Mutability::Mut, self.place)),
 818                 self.assign(cur, Rvalue::Cast(CastKind::Misc, Operand::Move(tmp), iter_ty)),
 819                 self.assign(
 820                     length_or_end,
 821                     Rvalue::BinaryOp(BinOp::Offset, Operand::Copy(cur), Operand::Move(length)),
 822                 ),
 823             ]
 824         } else {
 825             // cur = 0 (length already pushed)
 826             let zero = self.constant_usize(0);
 827             vec![self.assign(cur, Rvalue::Use(zero))]
 828         };
 829         let drop_block = self.elaborator.patch().new_block(BasicBlockData {
 830             statements: drop_block_stmts,
 831             is_cleanup: unwind.is_cleanup(),
 832             terminator: Some(Terminator {
 833                 source_info: self.source_info,
 834                 kind: TerminatorKind::Goto { target: loop_block },
 835             }),
 836         });
 837
 838         // FIXME(#34708): handle partially-dropped array/slice elements.
 839         let reset_block = self.drop_flag_reset_block(DropFlagMode::Deep, drop_block, unwind);
 840         self.drop_flag_test_block(reset_block, self.succ, unwind)
 841     }
 842
 843     /// The slow-path - create an "open", elaborated drop for a type
 844     /// which is moved-out-of only partially, and patch `bb` to a jump
 845     /// to it. This must not be called on ADTs with a destructor,
 846     /// as these can't be moved-out-of, except for `Box<T>`, which is
 847     /// special-cased.
 848     ///
 849     /// This creates a "drop ladder" that drops the needed fields of the
 850     /// ADT, both in the success case or if one of the destructors fail.
 851     fn open_drop(&mut self) -> BasicBlock {
 852         let ty = self.place_ty(self.place);
 853         match ty.kind() {
 854             ty::Closure(_, substs) => {
 855                 let tys: Vec<_> = substs.as_closure().upvar_tys().collect();
 856                 self.open_drop_for_tuple(&tys)
 857             }
 858             // Note that `elaborate_drops` only drops the upvars of a generator,
 859             // and this is ok because `open_drop` here can only be reached
 860             // within that own generator's resume function.
 861             // This should only happen for the self argument on the resume function.
 862             // It effetively only contains upvars until the generator transformation runs.
 863             // See librustc_body/transform/generator.rs for more details.
 864             ty::Generator(_, substs, _) => {
 865                 let tys: Vec<_> = substs.as_generator().upvar_tys().collect();
 866                 self.open_drop_for_tuple(&tys)
 867             }
 868             ty::Tuple(..) => {
 869                 let tys: Vec<_> = ty.tuple_fields().collect();
 870                 self.open_drop_for_tuple(&tys)
 871             }
 872             ty::Adt(def, substs) => {
 873                 if def.is_box() {
 874                     self.open_drop_for_box(def, substs)
 875                 } else {
 876                     self.open_drop_for_adt(def, substs)
 877                 }
 878             }
 879             ty::Dynamic(..) => self.complete_drop(self.succ, self.unwind),
 880             ty::Array(ety, size) => {
 881                 let size = size.try_eval_usize(self.tcx(), self.elaborator.param_env());
 882                 self.open_drop_for_array(ety, size)
 883             }
 884             ty::Slice(ety) => self.open_drop_for_array(ety, None),
 885
 886             _ => bug!("open drop from non-ADT `{:?}`", ty),
 887         }
 888     }
 889
 890     fn complete_drop(&mut self, succ: BasicBlock, unwind: Unwind) -> BasicBlock {
 891         debug!("complete_drop(succ={:?}, unwind={:?})", succ, unwind);
 892
 893         let drop_block = self.drop_block(succ, unwind);
 894
 895         self.drop_flag_test_block(drop_block, succ, unwind)
 896     }
 897
 898     /// Creates a block that resets the drop flag. If `mode` is deep, all children drop flags will
 899     /// also be cleared.
 900     fn drop_flag_reset_block(
 901         &mut self,
 902         mode: DropFlagMode,
 903         succ: BasicBlock,
 904         unwind: Unwind,
 905     ) -> BasicBlock {
 906         debug!("drop_flag_reset_block({:?},{:?})", self, mode);
 907
 908         if unwind.is_cleanup() {
 909             // The drop flag isn't read again on the unwind path, so don't
 910             // bother setting it.
 911             return succ;
 912         }
 913         let block = self.new_block(unwind, TerminatorKind::Goto { target: succ });
 914         let block_start = Location { block, statement_index: 0 };
 915         self.elaborator.clear_drop_flag(block_start, self.path, mode);
 916         block
 917     }
 918
 919     fn elaborated_drop_block(&mut self) -> BasicBlock {
 920         debug!("elaborated_drop_block({:?})", self);
 921         let blk = self.drop_block(self.succ, self.unwind);
 922         self.elaborate_drop(blk);
 923         blk
 924     }
 925
 926     /// Creates a block that frees the backing memory of a `Box` if its drop is required (either
 927     /// statically or by checking its drop flag).
 928     ///
 929     /// The contained value will not be dropped.
 930     fn box_free_block(
 931         &mut self,
 932         adt: &'tcx ty::AdtDef,
 933         substs: SubstsRef<'tcx>,
 934         target: BasicBlock,
 935         unwind: Unwind,
 936     ) -> BasicBlock {
 937         let block = self.unelaborated_free_block(adt, substs, target, unwind);
 938         self.drop_flag_test_block(block, target, unwind)
 939     }
 940
 941     /// Creates a block that frees the backing memory of a `Box` (without dropping the contained
 942     /// value).
 943     fn unelaborated_free_block(
 944         &mut self,
 945         adt: &'tcx ty::AdtDef,
 946         substs: SubstsRef<'tcx>,
 947         target: BasicBlock,
 948         unwind: Unwind,
 949     ) -> BasicBlock {
 950         let tcx = self.tcx();
 951         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 952         let free_func = tcx.require_lang_item(LangItem::BoxFree, Some(self.source_info.span));
 953         let args = adt.variants[VariantIdx::new(0)]
 954             .fields
 955             .iter()
 956             .enumerate()
 957             .map(|(i, f)| {
 958                 let field = Field::new(i);
 959                 let field_ty = f.ty(tcx, substs);
 960                 Operand::Move(tcx.mk_place_field(self.place, field, field_ty))
 961             })
 962             .collect();
 963
 964         let call = TerminatorKind::Call {
 965             func: Operand::function_handle(tcx, free_func, substs, self.source_info.span),
 966             args,
 967             destination: Some((unit_temp, target)),
 968             cleanup: None,
 969             from_hir_call: false,
 970             fn_span: self.source_info.span,
 971         }; // FIXME(#43234)
 972         let free_block = self.new_block(unwind, call);
 973
 974         let block_start = Location { block: free_block, statement_index: 0 };
 975         self.elaborator.clear_drop_flag(block_start, self.path, DropFlagMode::Shallow);
 976         free_block
 977     }
 978
 979     fn drop_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
 980         let block =
 981             TerminatorKind::Drop { place: self.place, target, unwind: unwind.into_option() };
 982         self.new_block(unwind, block)
 983     }
 984
 985     fn goto_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
 986         let block = TerminatorKind::Goto { target };
 987         self.new_block(unwind, block)
 988     }
 989
 990     /// Returns the block to jump to in order to test the drop flag and execute the drop.
 991     ///
 992     /// Depending on the required `DropStyle`, this might be a generated block with an `if`
 993     /// terminator (for dynamic/open drops), or it might be `on_set` or `on_unset` itself, in case
 994     /// the drop can be statically determined.
 995     fn drop_flag_test_block(
 996         &mut self,
 997         on_set: BasicBlock,
 998         on_unset: BasicBlock,
 999         unwind: Unwind,
1000     ) -> BasicBlock {
1001         let style = self.elaborator.drop_style(self.path, DropFlagMode::Shallow);
1002         debug!(
1003             "drop_flag_test_block({:?},{:?},{:?},{:?}) - {:?}",
1004             self, on_set, on_unset, unwind, style
1005         );
1006
1007         match style {
1008             DropStyle::Dead => on_unset,
1009             DropStyle::Static => on_set,
1010             DropStyle::Conditional | DropStyle::Open => {
1011                 let flag = self.elaborator.get_drop_flag(self.path).unwrap();
1012                 let term = TerminatorKind::if_(self.tcx(), flag, on_set, on_unset);
1013                 self.new_block(unwind, term)
1014             }
1015         }
1016     }
1017
1018     fn new_block(&mut self, unwind: Unwind, k: TerminatorKind<'tcx>) -> BasicBlock {
1019         self.elaborator.patch().new_block(BasicBlockData {
1020             statements: vec![],
1021             terminator: Some(Terminator { source_info: self.source_info, kind: k }),
1022             is_cleanup: unwind.is_cleanup(),
1023         })
1024     }
1025
1026     fn new_temp(&mut self, ty: Ty<'tcx>) -> Local {
1027         self.elaborator.patch().new_temp(ty, self.source_info.span)
1028     }
1029
1030     fn constant_usize(&self, val: u16) -> Operand<'tcx> {
1031         Operand::Constant(box Constant {
1032             span: self.source_info.span,
1033             user_ty: None,
1034             literal: ty::Const::from_usize(self.tcx(), val.into()),
1035         })
1036     }
1037
1038     fn assign(&self, lhs: Place<'tcx>, rhs: Rvalue<'tcx>) -> Statement<'tcx> {
1039         Statement { source_info: self.source_info, kind: StatementKind::Assign(box (lhs, rhs)) }
1040     }
1041 }