src/librustc_mir/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 use std::convert::TryInto;
  14
  15 /// The value of an inserted drop flag.
  16 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
  17 pub enum DropFlagState {
  18     /// The tracked value is initialized and needs to be dropped when leaving its scope.
  19     Present,
  20
  21     /// The tracked value is uninitialized or was moved out of and does not need to be dropped when
  22     /// leaving its scope.
  23     Absent,
  24 }
  25
  26 impl DropFlagState {
  27     pub fn value(self) -> bool {
  28         match self {
  29             DropFlagState::Present => true,
  30             DropFlagState::Absent => false,
  31         }
  32     }
  33 }
  34
  35 /// Describes how/if a value should be dropped.
  36 #[derive(Debug)]
  37 pub enum DropStyle {
  38     /// The value is already dead at the drop location, no drop will be executed.
  39     Dead,
  40
  41     /// The value is known to always be initialized at the drop location, drop will always be
  42     /// executed.
  43     Static,
  44
  45     /// Whether the value needs to be dropped depends on its drop flag.
  46     Conditional,
  47
  48     /// An "open" drop is one where only the fields of a value are dropped.
  49     ///
  50     /// For example, this happens when moving out of a struct field: The rest of the struct will be
  51     /// dropped in such an "open" drop. It is also used to generate drop glue for the individual
  52     /// components of a value, for example for dropping array elements.
  53     Open,
  54 }
  55
  56 /// Which drop flags to affect/check with an operation.
  57 #[derive(Debug)]
  58 pub enum DropFlagMode {
  59     /// Only affect the top-level drop flag, not that of any contained fields.
  60     Shallow,
  61     /// Affect all nested drop flags in addition to the top-level one.
  62     Deep,
  63 }
  64
  65 /// Describes if unwinding is necessary and where to unwind to if a panic occurs.
  66 #[derive(Copy, Clone, Debug)]
  67 pub enum Unwind {
  68     /// Unwind to this block.
  69     To(BasicBlock),
  70     /// Already in an unwind path, any panic will cause an abort.
  71     InCleanup,
  72 }
  73
  74 impl Unwind {
  75     fn is_cleanup(self) -> bool {
  76         match self {
  77             Unwind::To(..) => false,
  78             Unwind::InCleanup => true,
  79         }
  80     }
  81
  82     fn into_option(self) -> Option<BasicBlock> {
  83         match self {
  84             Unwind::To(bb) => Some(bb),
  85             Unwind::InCleanup => None,
  86         }
  87     }
  88
  89     fn map<F>(self, f: F) -> Self
  90     where
  91         F: FnOnce(BasicBlock) -> BasicBlock,
  92     {
  93         match self {
  94             Unwind::To(bb) => Unwind::To(f(bb)),
  95             Unwind::InCleanup => Unwind::InCleanup,
  96         }
  97     }
  98 }
  99
 100 pub trait DropElaborator<'a, 'tcx>: fmt::Debug {
 101     /// The type representing paths that can be moved out of.
 102     ///
 103     /// Users can move out of individual fields of a struct, such as `a.b.c`. This type is used to
 104     /// represent such move paths. Sometimes tracking individual move paths is not necessary, in
 105     /// which case this may be set to (for example) `()`.
 106     type Path: Copy + fmt::Debug;
 107
 108     // Accessors
 109
 110     fn patch(&mut self) -> &mut MirPatch<'tcx>;
 111     fn body(&self) -> &'a Body<'tcx>;
 112     fn tcx(&self) -> TyCtxt<'tcx>;
 113     fn param_env(&self) -> ty::ParamEnv<'tcx>;
 114
 115     // Drop logic
 116
 117     /// Returns how `path` should be dropped, given `mode`.
 118     fn drop_style(&self, path: Self::Path, mode: DropFlagMode) -> DropStyle;
 119
 120     /// Returns the drop flag of `path` as a MIR `Operand` (or `None` if `path` has no drop flag).
 121     fn get_drop_flag(&mut self, path: Self::Path) -> Option<Operand<'tcx>>;
 122
 123     /// Modifies the MIR patch so that the drop flag of `path` (if any) is cleared at `location`.
 124     ///
 125     /// If `mode` is deep, drop flags of all child paths should also be cleared by inserting
 126     /// additional statements.
 127     fn clear_drop_flag(&mut self, location: Location, path: Self::Path, mode: DropFlagMode);
 128
 129     // Subpaths
 130
 131     /// Returns the subpath of a field of `path` (or `None` if there is no dedicated subpath).
 132     ///
 133     /// If this returns `None`, `field` will not get a dedicated drop flag.
 134     fn field_subpath(&self, path: Self::Path, field: Field) -> Option<Self::Path>;
 135
 136     /// Returns the subpath of a dereference of `path` (or `None` if there is no dedicated subpath).
 137     ///
 138     /// If this returns `None`, `*path` will not get a dedicated drop flag.
 139     ///
 140     /// This is only relevant for `Box<T>`, where the contained `T` can be moved out of the box.
 141     fn deref_subpath(&self, path: Self::Path) -> Option<Self::Path>;
 142
 143     /// Returns the subpath of downcasting `path` to one of its variants.
 144     ///
 145     /// If this returns `None`, the downcast of `path` will not get a dedicated drop flag.
 146     fn downcast_subpath(&self, path: Self::Path, variant: VariantIdx) -> Option<Self::Path>;
 147
 148     /// Returns the subpath of indexing a fixed-size array `path`.
 149     ///
 150     /// If this returns `None`, elements of `path` will not get a dedicated drop flag.
 151     ///
 152     /// This is only relevant for array patterns, which can move out of individual array elements.
 153     fn array_subpath(&self, path: Self::Path, index: u32, size: u32) -> Option<Self::Path>;
 154 }
 155
 156 #[derive(Debug)]
 157 struct DropCtxt<'l, 'b, 'tcx, D>
 158 where
 159     D: DropElaborator<'b, 'tcx>,
 160 {
 161     elaborator: &'l mut D,
 162
 163     source_info: SourceInfo,
 164
 165     place: Place<'tcx>,
 166     path: D::Path,
 167     succ: BasicBlock,
 168     unwind: Unwind,
 169 }
 170
 171 /// "Elaborates" a drop of `place`/`path` and patches `bb`'s terminator to execute it.
 172 ///
 173 /// The passed `elaborator` is used to determine what should happen at the drop terminator. It
 174 /// decides whether the drop can be statically determined or whether it needs a dynamic drop flag,
 175 /// and whether the drop is "open", ie. should be expanded to drop all subfields of the dropped
 176 /// value.
 177 ///
 178 /// When this returns, the MIR patch in the `elaborator` contains the necessary changes.
 179 pub fn elaborate_drop<'b, 'tcx, D>(
 180     elaborator: &mut D,
 181     source_info: SourceInfo,
 182     place: Place<'tcx>,
 183     path: D::Path,
 184     succ: BasicBlock,
 185     unwind: Unwind,
 186     bb: BasicBlock,
 187 ) where
 188     D: DropElaborator<'b, 'tcx>,
 189     'tcx: 'b,
 190 {
 191     DropCtxt { elaborator, source_info, place, path, succ, unwind }.elaborate_drop(bb)
 192 }
 193
 194 impl<'l, 'b, 'tcx, D> DropCtxt<'l, 'b, 'tcx, D>
 195 where
 196     D: DropElaborator<'b, 'tcx>,
 197     'tcx: 'b,
 198 {
 199     fn place_ty(&self, place: Place<'tcx>) -> Ty<'tcx> {
 200         place.ty(self.elaborator.body(), self.tcx()).ty
 201     }
 202
 203     fn tcx(&self) -> TyCtxt<'tcx> {
 204         self.elaborator.tcx()
 205     }
 206
 207     /// This elaborates a single drop instruction, located at `bb`, and
 208     /// patches over it.
 209     ///
 210     /// The elaborated drop checks the drop flags to only drop what
 211     /// is initialized.
 212     ///
 213     /// In addition, the relevant drop flags also need to be cleared
 214     /// to avoid double-drops. However, in the middle of a complex
 215     /// drop, one must avoid clearing some of the flags before they
 216     /// are read, as that would cause a memory leak.
 217     ///
 218     /// In particular, when dropping an ADT, multiple fields may be
 219     /// joined together under the `rest` subpath. They are all controlled
 220     /// by the primary drop flag, but only the last rest-field dropped
 221     /// should clear it (and it must also not clear anything else).
 222     //
 223     // FIXME: I think we should just control the flags externally,
 224     // and then we do not need this machinery.
 225     pub fn elaborate_drop(&mut self, bb: BasicBlock) {
 226         debug!("elaborate_drop({:?}, {:?})", bb, self);
 227         let style = self.elaborator.drop_style(self.path, DropFlagMode::Deep);
 228         debug!("elaborate_drop({:?}, {:?}): live - {:?}", bb, self, style);
 229         match style {
 230             DropStyle::Dead => {
 231                 self.elaborator
 232                     .patch()
 233                     .patch_terminator(bb, TerminatorKind::Goto { target: self.succ });
 234             }
 235             DropStyle::Static => {
 236                 let loc = self.terminator_loc(bb);
 237                 self.elaborator.clear_drop_flag(loc, self.path, DropFlagMode::Deep);
 238                 self.elaborator.patch().patch_terminator(
 239                     bb,
 240                     TerminatorKind::Drop {
 241                         place: self.place,
 242                         target: self.succ,
 243                         unwind: self.unwind.into_option(),
 244                     },
 245                 );
 246             }
 247             DropStyle::Conditional => {
 248                 let unwind = self.unwind; // FIXME(#43234)
 249                 let succ = self.succ;
 250                 let drop_bb = self.complete_drop(Some(DropFlagMode::Deep), succ, unwind);
 251                 self.elaborator
 252                     .patch()
 253                     .patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
 254             }
 255             DropStyle::Open => {
 256                 let drop_bb = self.open_drop();
 257                 self.elaborator
 258                     .patch()
 259                     .patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
 260             }
 261         }
 262     }
 263
 264     /// Returns the place and move path for each field of `variant`,
 265     /// (the move path is `None` if the field is a rest field).
 266     fn move_paths_for_fields(
 267         &self,
 268         base_place: Place<'tcx>,
 269         variant_path: D::Path,
 270         variant: &'tcx ty::VariantDef,
 271         substs: SubstsRef<'tcx>,
 272     ) -> Vec<(Place<'tcx>, Option<D::Path>)> {
 273         variant
 274             .fields
 275             .iter()
 276             .enumerate()
 277             .map(|(i, f)| {
 278                 let field = Field::new(i);
 279                 let subpath = self.elaborator.field_subpath(variant_path, field);
 280                 let tcx = self.tcx();
 281
 282                 assert_eq!(self.elaborator.param_env().reveal(), Reveal::All);
 283                 let field_ty =
 284                     tcx.normalize_erasing_regions(self.elaborator.param_env(), f.ty(tcx, substs));
 285                 (tcx.mk_place_field(base_place, field, field_ty), subpath)
 286             })
 287             .collect()
 288     }
 289
 290     fn drop_subpath(
 291         &mut self,
 292         place: Place<'tcx>,
 293         path: Option<D::Path>,
 294         succ: BasicBlock,
 295         unwind: Unwind,
 296     ) -> BasicBlock {
 297         if let Some(path) = path {
 298             debug!("drop_subpath: for std field {:?}", place);
 299
 300             DropCtxt {
 301                 elaborator: self.elaborator,
 302                 source_info: self.source_info,
 303                 path,
 304                 place,
 305                 succ,
 306                 unwind,
 307             }
 308             .elaborated_drop_block()
 309         } else {
 310             debug!("drop_subpath: for rest field {:?}", place);
 311
 312             DropCtxt {
 313                 elaborator: self.elaborator,
 314                 source_info: self.source_info,
 315                 place,
 316                 succ,
 317                 unwind,
 318                 // Using `self.path` here to condition the drop on
 319                 // our own drop flag.
 320                 path: self.path,
 321             }
 322             .complete_drop(None, succ, unwind)
 323         }
 324     }
 325
 326     /// Creates one-half of the drop ladder for a list of fields, and return
 327     /// the list of steps in it in reverse order, with the first step
 328     /// dropping 0 fields and so on.
 329     ///
 330     /// `unwind_ladder` is such a list of steps in reverse order,
 331     /// which is called if the matching step of the drop glue panics.
 332     fn drop_halfladder(
 333         &mut self,
 334         unwind_ladder: &[Unwind],
 335         mut succ: BasicBlock,
 336         fields: &[(Place<'tcx>, Option<D::Path>)],
 337     ) -> Vec<BasicBlock> {
 338         Some(succ)
 339             .into_iter()
 340             .chain(fields.iter().rev().zip(unwind_ladder).map(|(&(place, path), &unwind_succ)| {
 341                 succ = self.drop_subpath(place, path, succ, unwind_succ);
 342                 succ
 343             }))
 344             .collect()
 345     }
 346
 347     fn drop_ladder_bottom(&mut self) -> (BasicBlock, Unwind) {
 348         // Clear the "master" drop flag at the end. This is needed
 349         // because the "master" drop protects the ADT's discriminant,
 350         // which is invalidated after the ADT is dropped.
 351         let (succ, unwind) = (self.succ, self.unwind); // FIXME(#43234)
 352         (
 353             self.drop_flag_reset_block(DropFlagMode::Shallow, succ, unwind),
 354             unwind.map(|unwind| {
 355                 self.drop_flag_reset_block(DropFlagMode::Shallow, unwind, Unwind::InCleanup)
 356             }),
 357         )
 358     }
 359
 360     /// Creates a full drop ladder, consisting of 2 connected half-drop-ladders
 361     ///
 362     /// For example, with 3 fields, the drop ladder is
 363     ///
 364     /// .d0:
 365     ///     ELAB(drop location.0 [target=.d1, unwind=.c1])
 366     /// .d1:
 367     ///     ELAB(drop location.1 [target=.d2, unwind=.c2])
 368     /// .d2:
 369     ///     ELAB(drop location.2 [target=`self.succ`, unwind=`self.unwind`])
 370     /// .c1:
 371     ///     ELAB(drop location.1 [target=.c2])
 372     /// .c2:
 373     ///     ELAB(drop location.2 [target=`self.unwind`])
 374     ///
 375     /// NOTE: this does not clear the master drop flag, so you need
 376     /// to point succ/unwind on a `drop_ladder_bottom`.
 377     fn drop_ladder(
 378         &mut self,
 379         fields: Vec<(Place<'tcx>, Option<D::Path>)>,
 380         succ: BasicBlock,
 381         unwind: Unwind,
 382     ) -> (BasicBlock, Unwind) {
 383         debug!("drop_ladder({:?}, {:?})", self, fields);
 384
 385         let mut fields = fields;
 386         fields.retain(|&(place, _)| {
 387             self.place_ty(place).needs_drop(self.tcx(), self.elaborator.param_env())
 388         });
 389
 390         debug!("drop_ladder - fields needing drop: {:?}", fields);
 391
 392         let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 393         let unwind_ladder: Vec<_> = if let Unwind::To(target) = unwind {
 394             let halfladder = self.drop_halfladder(&unwind_ladder, target, &fields);
 395             halfladder.into_iter().map(Unwind::To).collect()
 396         } else {
 397             unwind_ladder
 398         };
 399
 400         let normal_ladder = self.drop_halfladder(&unwind_ladder, succ, &fields);
 401
 402         (*normal_ladder.last().unwrap(), *unwind_ladder.last().unwrap())
 403     }
 404
 405     fn open_drop_for_tuple(&mut self, tys: &[Ty<'tcx>]) -> BasicBlock {
 406         debug!("open_drop_for_tuple({:?}, {:?})", self, tys);
 407
 408         let fields = tys
 409             .iter()
 410             .enumerate()
 411             .map(|(i, &ty)| {
 412                 (
 413                     self.tcx().mk_place_field(self.place, Field::new(i), ty),
 414                     self.elaborator.field_subpath(self.path, Field::new(i)),
 415                 )
 416             })
 417             .collect();
 418
 419         let (succ, unwind) = self.drop_ladder_bottom();
 420         self.drop_ladder(fields, succ, unwind).0
 421     }
 422
 423     fn open_drop_for_box(&mut self, adt: &'tcx ty::AdtDef, substs: SubstsRef<'tcx>) -> BasicBlock {
 424         debug!("open_drop_for_box({:?}, {:?}, {:?})", self, adt, substs);
 425
 426         let interior = self.tcx().mk_place_deref(self.place);
 427         let interior_path = self.elaborator.deref_subpath(self.path);
 428
 429         let succ = self.box_free_block(adt, substs, self.succ, self.unwind);
 430         let unwind_succ =
 431             self.unwind.map(|unwind| self.box_free_block(adt, substs, unwind, Unwind::InCleanup));
 432
 433         self.drop_subpath(interior, interior_path, succ, unwind_succ)
 434     }
 435
 436     fn open_drop_for_adt(&mut self, adt: &'tcx ty::AdtDef, substs: SubstsRef<'tcx>) -> BasicBlock {
 437         debug!("open_drop_for_adt({:?}, {:?}, {:?})", self, adt, substs);
 438         if adt.variants.is_empty() {
 439             return self.elaborator.patch().new_block(BasicBlockData {
 440                 statements: vec![],
 441                 terminator: Some(Terminator {
 442                     source_info: self.source_info,
 443                     kind: TerminatorKind::Unreachable,
 444                 }),
 445                 is_cleanup: self.unwind.is_cleanup(),
 446             });
 447         }
 448
 449         let skip_contents =
 450             adt.is_union() || Some(adt.did) == self.tcx().lang_items().manually_drop();
 451         let contents_drop = if skip_contents {
 452             (self.succ, self.unwind)
 453         } else {
 454             self.open_drop_for_adt_contents(adt, substs)
 455         };
 456
 457         if adt.has_dtor(self.tcx()) {
 458             self.destructor_call_block(contents_drop)
 459         } else {
 460             contents_drop.0
 461         }
 462     }
 463
 464     fn open_drop_for_adt_contents(
 465         &mut self,
 466         adt: &'tcx ty::AdtDef,
 467         substs: SubstsRef<'tcx>,
 468     ) -> (BasicBlock, Unwind) {
 469         let (succ, unwind) = self.drop_ladder_bottom();
 470         if !adt.is_enum() {
 471             let fields = self.move_paths_for_fields(
 472                 self.place,
 473                 self.path,
 474                 &adt.variants[VariantIdx::new(0)],
 475                 substs,
 476             );
 477             self.drop_ladder(fields, succ, unwind)
 478         } else {
 479             self.open_drop_for_multivariant(adt, substs, succ, unwind)
 480         }
 481     }
 482
 483     fn open_drop_for_multivariant(
 484         &mut self,
 485         adt: &'tcx ty::AdtDef,
 486         substs: SubstsRef<'tcx>,
 487         succ: BasicBlock,
 488         unwind: Unwind,
 489     ) -> (BasicBlock, Unwind) {
 490         let mut values = Vec::with_capacity(adt.variants.len());
 491         let mut normal_blocks = Vec::with_capacity(adt.variants.len());
 492         let mut unwind_blocks =
 493             if unwind.is_cleanup() { None } else { Some(Vec::with_capacity(adt.variants.len())) };
 494
 495         let mut have_otherwise_with_drop_glue = false;
 496         let mut have_otherwise = false;
 497         let tcx = self.tcx();
 498
 499         for (variant_index, discr) in adt.discriminants(tcx) {
 500             let variant = &adt.variants[variant_index];
 501             let subpath = self.elaborator.downcast_subpath(self.path, variant_index);
 502
 503             if let Some(variant_path) = subpath {
 504                 let base_place = tcx.mk_place_elem(
 505                     self.place,
 506                     ProjectionElem::Downcast(Some(variant.ident.name), variant_index),
 507                 );
 508                 let fields = self.move_paths_for_fields(base_place, variant_path, &variant, substs);
 509                 values.push(discr.val);
 510                 if let Unwind::To(unwind) = unwind {
 511                     // We can't use the half-ladder from the original
 512                     // drop ladder, because this breaks the
 513                     // "funclet can't have 2 successor funclets"
 514                     // requirement from MSVC:
 515                     //
 516                     //           switch       unwind-switch
 517                     //          /      \         /        \
 518                     //         v1.0    v2.0  v2.0-unwind  v1.0-unwind
 519                     //         |        |      /             |
 520                     //    v1.1-unwind  v2.1-unwind           |
 521                     //      ^                                |
 522                     //       \-------------------------------/
 523                     //
 524                     // Create a duplicate half-ladder to avoid that. We
 525                     // could technically only do this on MSVC, but I
 526                     // I want to minimize the divergence between MSVC
 527                     // and non-MSVC.
 528
 529                     let unwind_blocks = unwind_blocks.as_mut().unwrap();
 530                     let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 531                     let halfladder = self.drop_halfladder(&unwind_ladder, unwind, &fields);
 532                     unwind_blocks.push(halfladder.last().cloned().unwrap());
 533                 }
 534                 let (normal, _) = self.drop_ladder(fields, succ, unwind);
 535                 normal_blocks.push(normal);
 536             } else {
 537                 have_otherwise = true;
 538
 539                 let param_env = self.elaborator.param_env();
 540                 let have_field_with_drop_glue = variant
 541                     .fields
 542                     .iter()
 543                     .any(|field| field.ty(tcx, substs).needs_drop(tcx, param_env));
 544                 if have_field_with_drop_glue {
 545                     have_otherwise_with_drop_glue = true;
 546                 }
 547             }
 548         }
 549
 550         if !have_otherwise {
 551             values.pop();
 552         } else if !have_otherwise_with_drop_glue {
 553             normal_blocks.push(self.goto_block(succ, unwind));
 554             if let Unwind::To(unwind) = unwind {
 555                 unwind_blocks.as_mut().unwrap().push(self.goto_block(unwind, Unwind::InCleanup));
 556             }
 557         } else {
 558             normal_blocks.push(self.drop_block(succ, unwind));
 559             if let Unwind::To(unwind) = unwind {
 560                 unwind_blocks.as_mut().unwrap().push(self.drop_block(unwind, Unwind::InCleanup));
 561             }
 562         }
 563
 564         (
 565             self.adt_switch_block(adt, normal_blocks, &values, succ, unwind),
 566             unwind.map(|unwind| {
 567                 self.adt_switch_block(
 568                     adt,
 569                     unwind_blocks.unwrap(),
 570                     &values,
 571                     unwind,
 572                     Unwind::InCleanup,
 573                 )
 574             }),
 575         )
 576     }
 577
 578     fn adt_switch_block(
 579         &mut self,
 580         adt: &'tcx ty::AdtDef,
 581         blocks: Vec<BasicBlock>,
 582         values: &[u128],
 583         succ: BasicBlock,
 584         unwind: Unwind,
 585     ) -> BasicBlock {
 586         // If there are multiple variants, then if something
 587         // is present within the enum the discriminant, tracked
 588         // by the rest path, must be initialized.
 589         //
 590         // Additionally, we do not want to switch on the
 591         // discriminant after it is free-ed, because that
 592         // way lies only trouble.
 593         let discr_ty = adt.repr.discr_type().to_ty(self.tcx());
 594         let discr = Place::from(self.new_temp(discr_ty));
 595         let discr_rv = Rvalue::Discriminant(self.place);
 596         let switch_block = BasicBlockData {
 597             statements: vec![self.assign(discr, discr_rv)],
 598             terminator: Some(Terminator {
 599                 source_info: self.source_info,
 600                 kind: TerminatorKind::SwitchInt {
 601                     discr: Operand::Move(discr),
 602                     switch_ty: discr_ty,
 603                     values: From::from(values.to_owned()),
 604                     targets: blocks,
 605                 },
 606             }),
 607             is_cleanup: unwind.is_cleanup(),
 608         };
 609         let switch_block = self.elaborator.patch().new_block(switch_block);
 610         self.drop_flag_test_block(switch_block, succ, unwind)
 611     }
 612
 613     fn destructor_call_block(&mut self, (succ, unwind): (BasicBlock, Unwind)) -> BasicBlock {
 614         debug!("destructor_call_block({:?}, {:?})", self, succ);
 615         let tcx = self.tcx();
 616         let drop_trait = tcx.require_lang_item(LangItem::Drop, None);
 617         let drop_fn = tcx.associated_items(drop_trait).in_definition_order().next().unwrap();
 618         let ty = self.place_ty(self.place);
 619         let substs = tcx.mk_substs_trait(ty, &[]);
 620
 621         let ref_ty =
 622             tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut { ty, mutbl: hir::Mutability::Mut });
 623         let ref_place = self.new_temp(ref_ty);
 624         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 625
 626         let result = BasicBlockData {
 627             statements: vec![self.assign(
 628                 Place::from(ref_place),
 629                 Rvalue::Ref(
 630                     tcx.lifetimes.re_erased,
 631                     BorrowKind::Mut { allow_two_phase_borrow: false },
 632                     self.place,
 633                 ),
 634             )],
 635             terminator: Some(Terminator {
 636                 kind: TerminatorKind::Call {
 637                     func: Operand::function_handle(
 638                         tcx,
 639                         drop_fn.def_id,
 640                         substs,
 641                         self.source_info.span,
 642                     ),
 643                     args: vec![Operand::Move(Place::from(ref_place))],
 644                     destination: Some((unit_temp, succ)),
 645                     cleanup: unwind.into_option(),
 646                     from_hir_call: true,
 647                     fn_span: self.source_info.span,
 648                 },
 649                 source_info: self.source_info,
 650             }),
 651             is_cleanup: unwind.is_cleanup(),
 652         };
 653         self.elaborator.patch().new_block(result)
 654     }
 655
 656     /// Create a loop that drops an array:
 657     ///
 658     /// ```text
 659     /// loop-block:
 660     ///    can_go = cur == length_or_end
 661     ///    if can_go then succ else drop-block
 662     /// drop-block:
 663     ///    if ptr_based {
 664     ///        ptr = cur
 665     ///        cur = cur.offset(1)
 666     ///    } else {
 667     ///        ptr = &raw mut P[cur]
 668     ///        cur = cur + 1
 669     ///    }
 670     ///    drop(ptr)
 671     /// ```
 672     fn drop_loop(
 673         &mut self,
 674         succ: BasicBlock,
 675         cur: Local,
 676         length_or_end: Place<'tcx>,
 677         ety: Ty<'tcx>,
 678         unwind: Unwind,
 679         ptr_based: bool,
 680     ) -> BasicBlock {
 681         let copy = |place: Place<'tcx>| Operand::Copy(place);
 682         let move_ = |place: Place<'tcx>| Operand::Move(place);
 683         let tcx = self.tcx();
 684
 685         let ptr_ty = tcx.mk_ptr(ty::TypeAndMut { ty: ety, mutbl: hir::Mutability::Mut });
 686         let ptr = Place::from(self.new_temp(ptr_ty));
 687         let can_go = Place::from(self.new_temp(tcx.types.bool));
 688
 689         let one = self.constant_usize(1);
 690         let (ptr_next, cur_next) = if ptr_based {
 691             (Rvalue::Use(copy(cur.into())), Rvalue::BinaryOp(BinOp::Offset, move_(cur.into()), one))
 692         } else {
 693             (
 694                 Rvalue::AddressOf(Mutability::Mut, tcx.mk_place_index(self.place, cur)),
 695                 Rvalue::BinaryOp(BinOp::Add, move_(cur.into()), one),
 696             )
 697         };
 698
 699         let drop_block = BasicBlockData {
 700             statements: vec![self.assign(ptr, ptr_next), self.assign(Place::from(cur), cur_next)],
 701             is_cleanup: unwind.is_cleanup(),
 702             terminator: Some(Terminator {
 703                 source_info: self.source_info,
 704                 // this gets overwritten by drop elaboration.
 705                 kind: TerminatorKind::Unreachable,
 706             }),
 707         };
 708         let drop_block = self.elaborator.patch().new_block(drop_block);
 709
 710         let loop_block = BasicBlockData {
 711             statements: vec![self.assign(
 712                 can_go,
 713                 Rvalue::BinaryOp(BinOp::Eq, copy(Place::from(cur)), copy(length_or_end)),
 714             )],
 715             is_cleanup: unwind.is_cleanup(),
 716             terminator: Some(Terminator {
 717                 source_info: self.source_info,
 718                 kind: TerminatorKind::if_(tcx, move_(can_go), succ, drop_block),
 719             }),
 720         };
 721         let loop_block = self.elaborator.patch().new_block(loop_block);
 722
 723         self.elaborator.patch().patch_terminator(
 724             drop_block,
 725             TerminatorKind::Drop {
 726                 place: tcx.mk_place_deref(ptr),
 727                 target: loop_block,
 728                 unwind: unwind.into_option(),
 729             },
 730         );
 731
 732         loop_block
 733     }
 734
 735     fn open_drop_for_array(&mut self, ety: Ty<'tcx>, opt_size: Option<u64>) -> BasicBlock {
 736         debug!("open_drop_for_array({:?}, {:?})", ety, opt_size);
 737
 738         // if size_of::<ety>() == 0 {
 739         //     index_based_loop
 740         // } else {
 741         //     ptr_based_loop
 742         // }
 743
 744         let tcx = self.tcx();
 745
 746         if let Some(size) = opt_size {
 747             let size: u32 = size.try_into().unwrap_or_else(|_| {
 748                 bug!("move out check isn't implemented for array sizes bigger than u32::MAX");
 749             });
 750             let fields: Vec<(Place<'tcx>, Option<D::Path>)> = (0..size)
 751                 .map(|i| {
 752                     (
 753                         tcx.mk_place_elem(
 754                             self.place,
 755                             ProjectionElem::ConstantIndex {
 756                                 offset: i,
 757                                 min_length: size,
 758                                 from_end: false,
 759                             },
 760                         ),
 761                         self.elaborator.array_subpath(self.path, i, size),
 762                     )
 763                 })
 764                 .collect();
 765
 766             if fields.iter().any(|(_, path)| path.is_some()) {
 767                 let (succ, unwind) = self.drop_ladder_bottom();
 768                 return self.drop_ladder(fields, succ, unwind).0;
 769             }
 770         }
 771
 772         let move_ = |place: Place<'tcx>| Operand::Move(place);
 773         let elem_size = Place::from(self.new_temp(tcx.types.usize));
 774         let len = Place::from(self.new_temp(tcx.types.usize));
 775
 776         static USIZE_SWITCH_ZERO: &[u128] = &[0];
 777
 778         let base_block = BasicBlockData {
 779             statements: vec![
 780                 self.assign(elem_size, Rvalue::NullaryOp(NullOp::SizeOf, ety)),
 781                 self.assign(len, Rvalue::Len(self.place)),
 782             ],
 783             is_cleanup: self.unwind.is_cleanup(),
 784             terminator: Some(Terminator {
 785                 source_info: self.source_info,
 786                 kind: TerminatorKind::SwitchInt {
 787                     discr: move_(elem_size),
 788                     switch_ty: tcx.types.usize,
 789                     values: From::from(USIZE_SWITCH_ZERO),
 790                     targets: vec![
 791                         self.drop_loop_pair(ety, false, len),
 792                         self.drop_loop_pair(ety, true, len),
 793                     ],
 794                 },
 795             }),
 796         };
 797         self.elaborator.patch().new_block(base_block)
 798     }
 799
 800     /// Creates a pair of drop-loops of `place`, which drops its contents, even
 801     /// in the case of 1 panic. If `ptr_based`, creates a pointer loop,
 802     /// otherwise create an index loop.
 803     fn drop_loop_pair(
 804         &mut self,
 805         ety: Ty<'tcx>,
 806         ptr_based: bool,
 807         length: Place<'tcx>,
 808     ) -> BasicBlock {
 809         debug!("drop_loop_pair({:?}, {:?})", ety, ptr_based);
 810         let tcx = self.tcx();
 811         let iter_ty = if ptr_based { tcx.mk_mut_ptr(ety) } else { tcx.types.usize };
 812
 813         let cur = self.new_temp(iter_ty);
 814         let length_or_end = if ptr_based { Place::from(self.new_temp(iter_ty)) } else { length };
 815
 816         let unwind = self.unwind.map(|unwind| {
 817             self.drop_loop(unwind, cur, length_or_end, ety, Unwind::InCleanup, ptr_based)
 818         });
 819
 820         let loop_block = self.drop_loop(self.succ, cur, length_or_end, ety, unwind, ptr_based);
 821
 822         let cur = Place::from(cur);
 823         let drop_block_stmts = if ptr_based {
 824             let tmp_ty = tcx.mk_mut_ptr(self.place_ty(self.place));
 825             let tmp = Place::from(self.new_temp(tmp_ty));
 826             // tmp = &raw mut P;
 827             // cur = tmp as *mut T;
 828             // end = Offset(cur, len);
 829             vec![
 830                 self.assign(tmp, Rvalue::AddressOf(Mutability::Mut, self.place)),
 831                 self.assign(cur, Rvalue::Cast(CastKind::Misc, Operand::Move(tmp), iter_ty)),
 832                 self.assign(
 833                     length_or_end,
 834                     Rvalue::BinaryOp(BinOp::Offset, Operand::Copy(cur), Operand::Move(length)),
 835                 ),
 836             ]
 837         } else {
 838             // cur = 0 (length already pushed)
 839             let zero = self.constant_usize(0);
 840             vec![self.assign(cur, Rvalue::Use(zero))]
 841         };
 842         let drop_block = self.elaborator.patch().new_block(BasicBlockData {
 843             statements: drop_block_stmts,
 844             is_cleanup: unwind.is_cleanup(),
 845             terminator: Some(Terminator {
 846                 source_info: self.source_info,
 847                 kind: TerminatorKind::Goto { target: loop_block },
 848             }),
 849         });
 850
 851         // FIXME(#34708): handle partially-dropped array/slice elements.
 852         let reset_block = self.drop_flag_reset_block(DropFlagMode::Deep, drop_block, unwind);
 853         self.drop_flag_test_block(reset_block, self.succ, unwind)
 854     }
 855
 856     /// The slow-path - create an "open", elaborated drop for a type
 857     /// which is moved-out-of only partially, and patch `bb` to a jump
 858     /// to it. This must not be called on ADTs with a destructor,
 859     /// as these can't be moved-out-of, except for `Box<T>`, which is
 860     /// special-cased.
 861     ///
 862     /// This creates a "drop ladder" that drops the needed fields of the
 863     /// ADT, both in the success case or if one of the destructors fail.
 864     fn open_drop(&mut self) -> BasicBlock {
 865         let ty = self.place_ty(self.place);
 866         match ty.kind {
 867             ty::Closure(_, substs) => {
 868                 let tys: Vec<_> = substs.as_closure().upvar_tys().collect();
 869                 self.open_drop_for_tuple(&tys)
 870             }
 871             // Note that `elaborate_drops` only drops the upvars of a generator,
 872             // and this is ok because `open_drop` here can only be reached
 873             // within that own generator's resume function.
 874             // This should only happen for the self argument on the resume function.
 875             // It effetively only contains upvars until the generator transformation runs.
 876             // See librustc_body/transform/generator.rs for more details.
 877             ty::Generator(_, substs, _) => {
 878                 let tys: Vec<_> = substs.as_generator().upvar_tys().collect();
 879                 self.open_drop_for_tuple(&tys)
 880             }
 881             ty::Tuple(..) => {
 882                 let tys: Vec<_> = ty.tuple_fields().collect();
 883                 self.open_drop_for_tuple(&tys)
 884             }
 885             ty::Adt(def, substs) => {
 886                 if def.is_box() {
 887                     self.open_drop_for_box(def, substs)
 888                 } else {
 889                     self.open_drop_for_adt(def, substs)
 890                 }
 891             }
 892             ty::Dynamic(..) => {
 893                 let unwind = self.unwind; // FIXME(#43234)
 894                 let succ = self.succ;
 895                 self.complete_drop(Some(DropFlagMode::Deep), succ, unwind)
 896             }
 897             ty::Array(ety, size) => {
 898                 let size = size.try_eval_usize(self.tcx(), self.elaborator.param_env());
 899                 self.open_drop_for_array(ety, size)
 900             }
 901             ty::Slice(ety) => self.open_drop_for_array(ety, None),
 902
 903             _ => bug!("open drop from non-ADT `{:?}`", ty),
 904         }
 905     }
 906
 907     fn complete_drop(
 908         &mut self,
 909         drop_mode: Option<DropFlagMode>,
 910         succ: BasicBlock,
 911         unwind: Unwind,
 912     ) -> BasicBlock {
 913         debug!("complete_drop({:?},{:?})", self, drop_mode);
 914
 915         let drop_block = self.drop_block(succ, unwind);
 916         let drop_block = if let Some(mode) = drop_mode {
 917             self.drop_flag_reset_block(mode, drop_block, unwind)
 918         } else {
 919             drop_block
 920         };
 921
 922         self.drop_flag_test_block(drop_block, succ, unwind)
 923     }
 924
 925     /// Creates a block that resets the drop flag. If `mode` is deep, all children drop flags will
 926     /// also be cleared.
 927     fn drop_flag_reset_block(
 928         &mut self,
 929         mode: DropFlagMode,
 930         succ: BasicBlock,
 931         unwind: Unwind,
 932     ) -> BasicBlock {
 933         debug!("drop_flag_reset_block({:?},{:?})", self, mode);
 934
 935         let block = self.new_block(unwind, TerminatorKind::Goto { target: succ });
 936         let block_start = Location { block, statement_index: 0 };
 937         self.elaborator.clear_drop_flag(block_start, self.path, mode);
 938         block
 939     }
 940
 941     fn elaborated_drop_block(&mut self) -> BasicBlock {
 942         debug!("elaborated_drop_block({:?})", self);
 943         let blk = self.drop_block(self.succ, self.unwind);
 944         self.elaborate_drop(blk);
 945         blk
 946     }
 947
 948     /// Creates a block that frees the backing memory of a `Box` if its drop is required (either
 949     /// statically or by checking its drop flag).
 950     ///
 951     /// The contained value will not be dropped.
 952     fn box_free_block(
 953         &mut self,
 954         adt: &'tcx ty::AdtDef,
 955         substs: SubstsRef<'tcx>,
 956         target: BasicBlock,
 957         unwind: Unwind,
 958     ) -> BasicBlock {
 959         let block = self.unelaborated_free_block(adt, substs, target, unwind);
 960         self.drop_flag_test_block(block, target, unwind)
 961     }
 962
 963     /// Creates a block that frees the backing memory of a `Box` (without dropping the contained
 964     /// value).
 965     fn unelaborated_free_block(
 966         &mut self,
 967         adt: &'tcx ty::AdtDef,
 968         substs: SubstsRef<'tcx>,
 969         target: BasicBlock,
 970         unwind: Unwind,
 971     ) -> BasicBlock {
 972         let tcx = self.tcx();
 973         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 974         let free_func = tcx.require_lang_item(LangItem::BoxFree, Some(self.source_info.span));
 975         let args = adt.variants[VariantIdx::new(0)]
 976             .fields
 977             .iter()
 978             .enumerate()
 979             .map(|(i, f)| {
 980                 let field = Field::new(i);
 981                 let field_ty = f.ty(tcx, substs);
 982                 Operand::Move(tcx.mk_place_field(self.place, field, field_ty))
 983             })
 984             .collect();
 985
 986         let call = TerminatorKind::Call {
 987             func: Operand::function_handle(tcx, free_func, substs, self.source_info.span),
 988             args,
 989             destination: Some((unit_temp, target)),
 990             cleanup: None,
 991             from_hir_call: false,
 992             fn_span: self.source_info.span,
 993         }; // FIXME(#43234)
 994         let free_block = self.new_block(unwind, call);
 995
 996         let block_start = Location { block: free_block, statement_index: 0 };
 997         self.elaborator.clear_drop_flag(block_start, self.path, DropFlagMode::Shallow);
 998         free_block
 999     }
1000
1001     fn drop_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
1002         let block =
1003             TerminatorKind::Drop { place: self.place, target, unwind: unwind.into_option() };
1004         self.new_block(unwind, block)
1005     }
1006
1007     fn goto_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
1008         let block = TerminatorKind::Goto { target };
1009         self.new_block(unwind, block)
1010     }
1011
1012     /// Returns the block to jump to in order to test the drop flag and execute the drop.
1013     ///
1014     /// Depending on the required `DropStyle`, this might be a generated block with an `if`
1015     /// terminator (for dynamic/open drops), or it might be `on_set` or `on_unset` itself, in case
1016     /// the drop can be statically determined.
1017     fn drop_flag_test_block(
1018         &mut self,
1019         on_set: BasicBlock,
1020         on_unset: BasicBlock,
1021         unwind: Unwind,
1022     ) -> BasicBlock {
1023         let style = self.elaborator.drop_style(self.path, DropFlagMode::Shallow);
1024         debug!(
1025             "drop_flag_test_block({:?},{:?},{:?},{:?}) - {:?}",
1026             self, on_set, on_unset, unwind, style
1027         );
1028
1029         match style {
1030             DropStyle::Dead => on_unset,
1031             DropStyle::Static => on_set,
1032             DropStyle::Conditional | DropStyle::Open => {
1033                 let flag = self.elaborator.get_drop_flag(self.path).unwrap();
1034                 let term = TerminatorKind::if_(self.tcx(), flag, on_set, on_unset);
1035                 self.new_block(unwind, term)
1036             }
1037         }
1038     }
1039
1040     fn new_block(&mut self, unwind: Unwind, k: TerminatorKind<'tcx>) -> BasicBlock {
1041         self.elaborator.patch().new_block(BasicBlockData {
1042             statements: vec![],
1043             terminator: Some(Terminator { source_info: self.source_info, kind: k }),
1044             is_cleanup: unwind.is_cleanup(),
1045         })
1046     }
1047
1048     fn new_temp(&mut self, ty: Ty<'tcx>) -> Local {
1049         self.elaborator.patch().new_temp(ty, self.source_info.span)
1050     }
1051
1052     fn terminator_loc(&mut self, bb: BasicBlock) -> Location {
1053         let body = self.elaborator.body();
1054         self.elaborator.patch().terminator_loc(body, bb)
1055     }
1056
1057     fn constant_usize(&self, val: u16) -> Operand<'tcx> {
1058         Operand::Constant(box Constant {
1059             span: self.source_info.span,
1060             user_ty: None,
1061             literal: ty::Const::from_usize(self.tcx(), val.into()),
1062         })
1063     }
1064
1065     fn assign(&self, lhs: Place<'tcx>, rhs: Rvalue<'tcx>) -> Statement<'tcx> {
1066         Statement { source_info: self.source_info, kind: StatementKind::Assign(box (lhs, rhs)) }
1067     }
1068 }