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 /// 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                 let loc = self.terminator_loc(bb);
 235                 self.elaborator.clear_drop_flag(loc, self.path, DropFlagMode::Deep);
 236                 self.elaborator.patch().patch_terminator(
 237                     bb,
 238                     TerminatorKind::Drop {
 239                         place: self.place,
 240                         target: self.succ,
 241                         unwind: self.unwind.into_option(),
 242                     },
 243                 );
 244             }
 245             DropStyle::Conditional => {
 246                 let unwind = self.unwind; // FIXME(#43234)
 247                 let succ = self.succ;
 248                 let drop_bb = self.complete_drop(Some(DropFlagMode::Deep), succ, unwind);
 249                 self.elaborator
 250                     .patch()
 251                     .patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
 252             }
 253             DropStyle::Open => {
 254                 let drop_bb = self.open_drop();
 255                 self.elaborator
 256                     .patch()
 257                     .patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
 258             }
 259         }
 260     }
 261
 262     /// Returns the place and move path for each field of `variant`,
 263     /// (the move path is `None` if the field is a rest field).
 264     fn move_paths_for_fields(
 265         &self,
 266         base_place: Place<'tcx>,
 267         variant_path: D::Path,
 268         variant: &'tcx ty::VariantDef,
 269         substs: SubstsRef<'tcx>,
 270     ) -> Vec<(Place<'tcx>, Option<D::Path>)> {
 271         variant
 272             .fields
 273             .iter()
 274             .enumerate()
 275             .map(|(i, f)| {
 276                 let field = Field::new(i);
 277                 let subpath = self.elaborator.field_subpath(variant_path, field);
 278                 let tcx = self.tcx();
 279
 280                 assert_eq!(self.elaborator.param_env().reveal(), Reveal::All);
 281                 let field_ty =
 282                     tcx.normalize_erasing_regions(self.elaborator.param_env(), f.ty(tcx, substs));
 283                 (tcx.mk_place_field(base_place, field, field_ty), subpath)
 284             })
 285             .collect()
 286     }
 287
 288     fn drop_subpath(
 289         &mut self,
 290         place: Place<'tcx>,
 291         path: Option<D::Path>,
 292         succ: BasicBlock,
 293         unwind: Unwind,
 294     ) -> BasicBlock {
 295         if let Some(path) = path {
 296             debug!("drop_subpath: for std field {:?}", place);
 297
 298             DropCtxt {
 299                 elaborator: self.elaborator,
 300                 source_info: self.source_info,
 301                 path,
 302                 place,
 303                 succ,
 304                 unwind,
 305             }
 306             .elaborated_drop_block()
 307         } else {
 308             debug!("drop_subpath: for rest field {:?}", place);
 309
 310             DropCtxt {
 311                 elaborator: self.elaborator,
 312                 source_info: self.source_info,
 313                 place,
 314                 succ,
 315                 unwind,
 316                 // Using `self.path` here to condition the drop on
 317                 // our own drop flag.
 318                 path: self.path,
 319             }
 320             .complete_drop(None, succ, unwind)
 321         }
 322     }
 323
 324     /// Creates one-half of the drop ladder for a list of fields, and return
 325     /// the list of steps in it in reverse order, with the first step
 326     /// dropping 0 fields and so on.
 327     ///
 328     /// `unwind_ladder` is such a list of steps in reverse order,
 329     /// which is called if the matching step of the drop glue panics.
 330     fn drop_halfladder(
 331         &mut self,
 332         unwind_ladder: &[Unwind],
 333         mut succ: BasicBlock,
 334         fields: &[(Place<'tcx>, Option<D::Path>)],
 335     ) -> Vec<BasicBlock> {
 336         Some(succ)
 337             .into_iter()
 338             .chain(fields.iter().rev().zip(unwind_ladder).map(|(&(place, path), &unwind_succ)| {
 339                 succ = self.drop_subpath(place, path, succ, unwind_succ);
 340                 succ
 341             }))
 342             .collect()
 343     }
 344
 345     fn drop_ladder_bottom(&mut self) -> (BasicBlock, Unwind) {
 346         // Clear the "master" drop flag at the end. This is needed
 347         // because the "master" drop protects the ADT's discriminant,
 348         // which is invalidated after the ADT is dropped.
 349         let (succ, unwind) = (self.succ, self.unwind); // FIXME(#43234)
 350         (
 351             self.drop_flag_reset_block(DropFlagMode::Shallow, succ, unwind),
 352             unwind.map(|unwind| {
 353                 self.drop_flag_reset_block(DropFlagMode::Shallow, unwind, Unwind::InCleanup)
 354             }),
 355         )
 356     }
 357
 358     /// Creates a full drop ladder, consisting of 2 connected half-drop-ladders
 359     ///
 360     /// For example, with 3 fields, the drop ladder is
 361     ///
 362     /// .d0:
 363     ///     ELAB(drop location.0 [target=.d1, unwind=.c1])
 364     /// .d1:
 365     ///     ELAB(drop location.1 [target=.d2, unwind=.c2])
 366     /// .d2:
 367     ///     ELAB(drop location.2 [target=`self.succ`, unwind=`self.unwind`])
 368     /// .c1:
 369     ///     ELAB(drop location.1 [target=.c2])
 370     /// .c2:
 371     ///     ELAB(drop location.2 [target=`self.unwind`])
 372     ///
 373     /// NOTE: this does not clear the master drop flag, so you need
 374     /// to point succ/unwind on a `drop_ladder_bottom`.
 375     fn drop_ladder(
 376         &mut self,
 377         fields: Vec<(Place<'tcx>, Option<D::Path>)>,
 378         succ: BasicBlock,
 379         unwind: Unwind,
 380     ) -> (BasicBlock, Unwind) {
 381         debug!("drop_ladder({:?}, {:?})", self, fields);
 382
 383         let mut fields = fields;
 384         fields.retain(|&(place, _)| {
 385             self.place_ty(place).needs_drop(self.tcx(), self.elaborator.param_env())
 386         });
 387
 388         debug!("drop_ladder - fields needing drop: {:?}", fields);
 389
 390         let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 391         let unwind_ladder: Vec<_> = if let Unwind::To(target) = unwind {
 392             let halfladder = self.drop_halfladder(&unwind_ladder, target, &fields);
 393             halfladder.into_iter().map(Unwind::To).collect()
 394         } else {
 395             unwind_ladder
 396         };
 397
 398         let normal_ladder = self.drop_halfladder(&unwind_ladder, succ, &fields);
 399
 400         (*normal_ladder.last().unwrap(), *unwind_ladder.last().unwrap())
 401     }
 402
 403     fn open_drop_for_tuple(&mut self, tys: &[Ty<'tcx>]) -> BasicBlock {
 404         debug!("open_drop_for_tuple({:?}, {:?})", self, tys);
 405
 406         let fields = tys
 407             .iter()
 408             .enumerate()
 409             .map(|(i, &ty)| {
 410                 (
 411                     self.tcx().mk_place_field(self.place, Field::new(i), ty),
 412                     self.elaborator.field_subpath(self.path, Field::new(i)),
 413                 )
 414             })
 415             .collect();
 416
 417         let (succ, unwind) = self.drop_ladder_bottom();
 418         self.drop_ladder(fields, succ, unwind).0
 419     }
 420
 421     fn open_drop_for_box(&mut self, adt: &'tcx ty::AdtDef, substs: SubstsRef<'tcx>) -> BasicBlock {
 422         debug!("open_drop_for_box({:?}, {:?}, {:?})", self, adt, substs);
 423
 424         let interior = self.tcx().mk_place_deref(self.place);
 425         let interior_path = self.elaborator.deref_subpath(self.path);
 426
 427         let succ = self.box_free_block(adt, substs, self.succ, self.unwind);
 428         let unwind_succ =
 429             self.unwind.map(|unwind| self.box_free_block(adt, substs, unwind, Unwind::InCleanup));
 430
 431         self.drop_subpath(interior, interior_path, succ, unwind_succ)
 432     }
 433
 434     fn open_drop_for_adt(&mut self, adt: &'tcx ty::AdtDef, substs: SubstsRef<'tcx>) -> BasicBlock {
 435         debug!("open_drop_for_adt({:?}, {:?}, {:?})", self, adt, substs);
 436         if adt.variants.is_empty() {
 437             return self.elaborator.patch().new_block(BasicBlockData {
 438                 statements: vec![],
 439                 terminator: Some(Terminator {
 440                     source_info: self.source_info,
 441                     kind: TerminatorKind::Unreachable,
 442                 }),
 443                 is_cleanup: self.unwind.is_cleanup(),
 444             });
 445         }
 446
 447         let skip_contents =
 448             adt.is_union() || Some(adt.did) == self.tcx().lang_items().manually_drop();
 449         let contents_drop = if skip_contents {
 450             (self.succ, self.unwind)
 451         } else {
 452             self.open_drop_for_adt_contents(adt, substs)
 453         };
 454
 455         if adt.has_dtor(self.tcx()) {
 456             self.destructor_call_block(contents_drop)
 457         } else {
 458             contents_drop.0
 459         }
 460     }
 461
 462     fn open_drop_for_adt_contents(
 463         &mut self,
 464         adt: &'tcx ty::AdtDef,
 465         substs: SubstsRef<'tcx>,
 466     ) -> (BasicBlock, Unwind) {
 467         let (succ, unwind) = self.drop_ladder_bottom();
 468         if !adt.is_enum() {
 469             let fields = self.move_paths_for_fields(
 470                 self.place,
 471                 self.path,
 472                 &adt.variants[VariantIdx::new(0)],
 473                 substs,
 474             );
 475             self.drop_ladder(fields, succ, unwind)
 476         } else {
 477             self.open_drop_for_multivariant(adt, substs, succ, unwind)
 478         }
 479     }
 480
 481     fn open_drop_for_multivariant(
 482         &mut self,
 483         adt: &'tcx ty::AdtDef,
 484         substs: SubstsRef<'tcx>,
 485         succ: BasicBlock,
 486         unwind: Unwind,
 487     ) -> (BasicBlock, Unwind) {
 488         let mut values = Vec::with_capacity(adt.variants.len());
 489         let mut normal_blocks = Vec::with_capacity(adt.variants.len());
 490         let mut unwind_blocks =
 491             if unwind.is_cleanup() { None } else { Some(Vec::with_capacity(adt.variants.len())) };
 492
 493         let mut have_otherwise_with_drop_glue = false;
 494         let mut have_otherwise = false;
 495         let tcx = self.tcx();
 496
 497         for (variant_index, discr) in adt.discriminants(tcx) {
 498             let variant = &adt.variants[variant_index];
 499             let subpath = self.elaborator.downcast_subpath(self.path, variant_index);
 500
 501             if let Some(variant_path) = subpath {
 502                 let base_place = tcx.mk_place_elem(
 503                     self.place,
 504                     ProjectionElem::Downcast(Some(variant.ident.name), variant_index),
 505                 );
 506                 let fields = self.move_paths_for_fields(base_place, variant_path, &variant, substs);
 507                 values.push(discr.val);
 508                 if let Unwind::To(unwind) = unwind {
 509                     // We can't use the half-ladder from the original
 510                     // drop ladder, because this breaks the
 511                     // "funclet can't have 2 successor funclets"
 512                     // requirement from MSVC:
 513                     //
 514                     //           switch       unwind-switch
 515                     //          /      \         /        \
 516                     //         v1.0    v2.0  v2.0-unwind  v1.0-unwind
 517                     //         |        |      /             |
 518                     //    v1.1-unwind  v2.1-unwind           |
 519                     //      ^                                |
 520                     //       \-------------------------------/
 521                     //
 522                     // Create a duplicate half-ladder to avoid that. We
 523                     // could technically only do this on MSVC, but I
 524                     // I want to minimize the divergence between MSVC
 525                     // and non-MSVC.
 526
 527                     let unwind_blocks = unwind_blocks.as_mut().unwrap();
 528                     let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 529                     let halfladder = self.drop_halfladder(&unwind_ladder, unwind, &fields);
 530                     unwind_blocks.push(halfladder.last().cloned().unwrap());
 531                 }
 532                 let (normal, _) = self.drop_ladder(fields, succ, unwind);
 533                 normal_blocks.push(normal);
 534             } else {
 535                 have_otherwise = true;
 536
 537                 let param_env = self.elaborator.param_env();
 538                 let have_field_with_drop_glue = variant
 539                     .fields
 540                     .iter()
 541                     .any(|field| field.ty(tcx, substs).needs_drop(tcx, param_env));
 542                 if have_field_with_drop_glue {
 543                     have_otherwise_with_drop_glue = true;
 544                 }
 545             }
 546         }
 547
 548         if !have_otherwise {
 549             values.pop();
 550         } else if !have_otherwise_with_drop_glue {
 551             normal_blocks.push(self.goto_block(succ, unwind));
 552             if let Unwind::To(unwind) = unwind {
 553                 unwind_blocks.as_mut().unwrap().push(self.goto_block(unwind, Unwind::InCleanup));
 554             }
 555         } else {
 556             normal_blocks.push(self.drop_block(succ, unwind));
 557             if let Unwind::To(unwind) = unwind {
 558                 unwind_blocks.as_mut().unwrap().push(self.drop_block(unwind, Unwind::InCleanup));
 559             }
 560         }
 561
 562         (
 563             self.adt_switch_block(adt, normal_blocks, &values, succ, unwind),
 564             unwind.map(|unwind| {
 565                 self.adt_switch_block(
 566                     adt,
 567                     unwind_blocks.unwrap(),
 568                     &values,
 569                     unwind,
 570                     Unwind::InCleanup,
 571                 )
 572             }),
 573         )
 574     }
 575
 576     fn adt_switch_block(
 577         &mut self,
 578         adt: &'tcx ty::AdtDef,
 579         blocks: Vec<BasicBlock>,
 580         values: &[u128],
 581         succ: BasicBlock,
 582         unwind: Unwind,
 583     ) -> BasicBlock {
 584         // If there are multiple variants, then if something
 585         // is present within the enum the discriminant, tracked
 586         // by the rest path, must be initialized.
 587         //
 588         // Additionally, we do not want to switch on the
 589         // discriminant after it is free-ed, because that
 590         // way lies only trouble.
 591         let discr_ty = adt.repr.discr_type().to_ty(self.tcx());
 592         let discr = Place::from(self.new_temp(discr_ty));
 593         let discr_rv = Rvalue::Discriminant(self.place);
 594         let switch_block = BasicBlockData {
 595             statements: vec![self.assign(discr, discr_rv)],
 596             terminator: Some(Terminator {
 597                 source_info: self.source_info,
 598                 kind: TerminatorKind::SwitchInt {
 599                     discr: Operand::Move(discr),
 600                     switch_ty: discr_ty,
 601                     values: From::from(values.to_owned()),
 602                     targets: blocks,
 603                 },
 604             }),
 605             is_cleanup: unwind.is_cleanup(),
 606         };
 607         let switch_block = self.elaborator.patch().new_block(switch_block);
 608         self.drop_flag_test_block(switch_block, succ, unwind)
 609     }
 610
 611     fn destructor_call_block(&mut self, (succ, unwind): (BasicBlock, Unwind)) -> BasicBlock {
 612         debug!("destructor_call_block({:?}, {:?})", self, succ);
 613         let tcx = self.tcx();
 614         let drop_trait = tcx.require_lang_item(LangItem::Drop, None);
 615         let drop_fn = tcx.associated_items(drop_trait).in_definition_order().next().unwrap();
 616         let ty = self.place_ty(self.place);
 617         let substs = tcx.mk_substs_trait(ty, &[]);
 618
 619         let ref_ty =
 620             tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut { ty, mutbl: hir::Mutability::Mut });
 621         let ref_place = self.new_temp(ref_ty);
 622         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 623
 624         let result = BasicBlockData {
 625             statements: vec![self.assign(
 626                 Place::from(ref_place),
 627                 Rvalue::Ref(
 628                     tcx.lifetimes.re_erased,
 629                     BorrowKind::Mut { allow_two_phase_borrow: false },
 630                     self.place,
 631                 ),
 632             )],
 633             terminator: Some(Terminator {
 634                 kind: TerminatorKind::Call {
 635                     func: Operand::function_handle(
 636                         tcx,
 637                         drop_fn.def_id,
 638                         substs,
 639                         self.source_info.span,
 640                     ),
 641                     args: vec![Operand::Move(Place::from(ref_place))],
 642                     destination: Some((unit_temp, succ)),
 643                     cleanup: unwind.into_option(),
 644                     from_hir_call: true,
 645                     fn_span: self.source_info.span,
 646                 },
 647                 source_info: self.source_info,
 648             }),
 649             is_cleanup: unwind.is_cleanup(),
 650         };
 651         self.elaborator.patch().new_block(result)
 652     }
 653
 654     /// Create a loop that drops an array:
 655     ///
 656     /// ```text
 657     /// loop-block:
 658     ///    can_go = cur == length_or_end
 659     ///    if can_go then succ else drop-block
 660     /// drop-block:
 661     ///    if ptr_based {
 662     ///        ptr = cur
 663     ///        cur = cur.offset(1)
 664     ///    } else {
 665     ///        ptr = &raw mut P[cur]
 666     ///        cur = cur + 1
 667     ///    }
 668     ///    drop(ptr)
 669     /// ```
 670     fn drop_loop(
 671         &mut self,
 672         succ: BasicBlock,
 673         cur: Local,
 674         length_or_end: Place<'tcx>,
 675         ety: Ty<'tcx>,
 676         unwind: Unwind,
 677         ptr_based: bool,
 678     ) -> BasicBlock {
 679         let copy = |place: Place<'tcx>| Operand::Copy(place);
 680         let move_ = |place: Place<'tcx>| Operand::Move(place);
 681         let tcx = self.tcx();
 682
 683         let ptr_ty = tcx.mk_ptr(ty::TypeAndMut { ty: ety, mutbl: hir::Mutability::Mut });
 684         let ptr = Place::from(self.new_temp(ptr_ty));
 685         let can_go = Place::from(self.new_temp(tcx.types.bool));
 686
 687         let one = self.constant_usize(1);
 688         let (ptr_next, cur_next) = if ptr_based {
 689             (Rvalue::Use(copy(cur.into())), Rvalue::BinaryOp(BinOp::Offset, move_(cur.into()), one))
 690         } else {
 691             (
 692                 Rvalue::AddressOf(Mutability::Mut, tcx.mk_place_index(self.place, cur)),
 693                 Rvalue::BinaryOp(BinOp::Add, move_(cur.into()), one),
 694             )
 695         };
 696
 697         let drop_block = BasicBlockData {
 698             statements: vec![self.assign(ptr, ptr_next), self.assign(Place::from(cur), cur_next)],
 699             is_cleanup: unwind.is_cleanup(),
 700             terminator: Some(Terminator {
 701                 source_info: self.source_info,
 702                 // this gets overwritten by drop elaboration.
 703                 kind: TerminatorKind::Unreachable,
 704             }),
 705         };
 706         let drop_block = self.elaborator.patch().new_block(drop_block);
 707
 708         let loop_block = BasicBlockData {
 709             statements: vec![self.assign(
 710                 can_go,
 711                 Rvalue::BinaryOp(BinOp::Eq, copy(Place::from(cur)), copy(length_or_end)),
 712             )],
 713             is_cleanup: unwind.is_cleanup(),
 714             terminator: Some(Terminator {
 715                 source_info: self.source_info,
 716                 kind: TerminatorKind::if_(tcx, move_(can_go), succ, drop_block),
 717             }),
 718         };
 719         let loop_block = self.elaborator.patch().new_block(loop_block);
 720
 721         self.elaborator.patch().patch_terminator(
 722             drop_block,
 723             TerminatorKind::Drop {
 724                 place: tcx.mk_place_deref(ptr),
 725                 target: loop_block,
 726                 unwind: unwind.into_option(),
 727             },
 728         );
 729
 730         loop_block
 731     }
 732
 733     fn open_drop_for_array(&mut self, ety: Ty<'tcx>, opt_size: Option<u64>) -> BasicBlock {
 734         debug!("open_drop_for_array({:?}, {:?})", ety, opt_size);
 735
 736         // if size_of::<ety>() == 0 {
 737         //     index_based_loop
 738         // } else {
 739         //     ptr_based_loop
 740         // }
 741
 742         let tcx = self.tcx();
 743
 744         if let Some(size) = opt_size {
 745             let fields: Vec<(Place<'tcx>, Option<D::Path>)> = (0..size)
 746                 .map(|i| {
 747                     (
 748                         tcx.mk_place_elem(
 749                             self.place,
 750                             ProjectionElem::ConstantIndex {
 751                                 offset: i,
 752                                 min_length: size,
 753                                 from_end: false,
 754                             },
 755                         ),
 756                         self.elaborator.array_subpath(self.path, i, size),
 757                     )
 758                 })
 759                 .collect();
 760
 761             if fields.iter().any(|(_, path)| path.is_some()) {
 762                 let (succ, unwind) = self.drop_ladder_bottom();
 763                 return self.drop_ladder(fields, succ, unwind).0;
 764             }
 765         }
 766
 767         let move_ = |place: Place<'tcx>| Operand::Move(place);
 768         let elem_size = Place::from(self.new_temp(tcx.types.usize));
 769         let len = Place::from(self.new_temp(tcx.types.usize));
 770
 771         static USIZE_SWITCH_ZERO: &[u128] = &[0];
 772
 773         let base_block = BasicBlockData {
 774             statements: vec![
 775                 self.assign(elem_size, Rvalue::NullaryOp(NullOp::SizeOf, ety)),
 776                 self.assign(len, Rvalue::Len(self.place)),
 777             ],
 778             is_cleanup: self.unwind.is_cleanup(),
 779             terminator: Some(Terminator {
 780                 source_info: self.source_info,
 781                 kind: TerminatorKind::SwitchInt {
 782                     discr: move_(elem_size),
 783                     switch_ty: tcx.types.usize,
 784                     values: From::from(USIZE_SWITCH_ZERO),
 785                     targets: vec![
 786                         self.drop_loop_pair(ety, false, len),
 787                         self.drop_loop_pair(ety, true, len),
 788                     ],
 789                 },
 790             }),
 791         };
 792         self.elaborator.patch().new_block(base_block)
 793     }
 794
 795     /// Creates a pair of drop-loops of `place`, which drops its contents, even
 796     /// in the case of 1 panic. If `ptr_based`, creates a pointer loop,
 797     /// otherwise create an index loop.
 798     fn drop_loop_pair(
 799         &mut self,
 800         ety: Ty<'tcx>,
 801         ptr_based: bool,
 802         length: Place<'tcx>,
 803     ) -> BasicBlock {
 804         debug!("drop_loop_pair({:?}, {:?})", ety, ptr_based);
 805         let tcx = self.tcx();
 806         let iter_ty = if ptr_based { tcx.mk_mut_ptr(ety) } else { tcx.types.usize };
 807
 808         let cur = self.new_temp(iter_ty);
 809         let length_or_end = if ptr_based { Place::from(self.new_temp(iter_ty)) } else { length };
 810
 811         let unwind = self.unwind.map(|unwind| {
 812             self.drop_loop(unwind, cur, length_or_end, ety, Unwind::InCleanup, ptr_based)
 813         });
 814
 815         let loop_block = self.drop_loop(self.succ, cur, length_or_end, ety, unwind, ptr_based);
 816
 817         let cur = Place::from(cur);
 818         let drop_block_stmts = if ptr_based {
 819             let tmp_ty = tcx.mk_mut_ptr(self.place_ty(self.place));
 820             let tmp = Place::from(self.new_temp(tmp_ty));
 821             // tmp = &raw mut P;
 822             // cur = tmp as *mut T;
 823             // end = Offset(cur, len);
 824             vec![
 825                 self.assign(tmp, Rvalue::AddressOf(Mutability::Mut, self.place)),
 826                 self.assign(cur, Rvalue::Cast(CastKind::Misc, Operand::Move(tmp), iter_ty)),
 827                 self.assign(
 828                     length_or_end,
 829                     Rvalue::BinaryOp(BinOp::Offset, Operand::Copy(cur), Operand::Move(length)),
 830                 ),
 831             ]
 832         } else {
 833             // cur = 0 (length already pushed)
 834             let zero = self.constant_usize(0);
 835             vec![self.assign(cur, Rvalue::Use(zero))]
 836         };
 837         let drop_block = self.elaborator.patch().new_block(BasicBlockData {
 838             statements: drop_block_stmts,
 839             is_cleanup: unwind.is_cleanup(),
 840             terminator: Some(Terminator {
 841                 source_info: self.source_info,
 842                 kind: TerminatorKind::Goto { target: loop_block },
 843             }),
 844         });
 845
 846         // FIXME(#34708): handle partially-dropped array/slice elements.
 847         let reset_block = self.drop_flag_reset_block(DropFlagMode::Deep, drop_block, unwind);
 848         self.drop_flag_test_block(reset_block, self.succ, unwind)
 849     }
 850
 851     /// The slow-path - create an "open", elaborated drop for a type
 852     /// which is moved-out-of only partially, and patch `bb` to a jump
 853     /// to it. This must not be called on ADTs with a destructor,
 854     /// as these can't be moved-out-of, except for `Box<T>`, which is
 855     /// special-cased.
 856     ///
 857     /// This creates a "drop ladder" that drops the needed fields of the
 858     /// ADT, both in the success case or if one of the destructors fail.
 859     fn open_drop(&mut self) -> BasicBlock {
 860         let ty = self.place_ty(self.place);
 861         match ty.kind {
 862             ty::Closure(_, substs) => {
 863                 let tys: Vec<_> = substs.as_closure().upvar_tys().collect();
 864                 self.open_drop_for_tuple(&tys)
 865             }
 866             // Note that `elaborate_drops` only drops the upvars of a generator,
 867             // and this is ok because `open_drop` here can only be reached
 868             // within that own generator's resume function.
 869             // This should only happen for the self argument on the resume function.
 870             // It effetively only contains upvars until the generator transformation runs.
 871             // See librustc_body/transform/generator.rs for more details.
 872             ty::Generator(_, substs, _) => {
 873                 let tys: Vec<_> = substs.as_generator().upvar_tys().collect();
 874                 self.open_drop_for_tuple(&tys)
 875             }
 876             ty::Tuple(..) => {
 877                 let tys: Vec<_> = ty.tuple_fields().collect();
 878                 self.open_drop_for_tuple(&tys)
 879             }
 880             ty::Adt(def, substs) => {
 881                 if def.is_box() {
 882                     self.open_drop_for_box(def, substs)
 883                 } else {
 884                     self.open_drop_for_adt(def, substs)
 885                 }
 886             }
 887             ty::Dynamic(..) => {
 888                 let unwind = self.unwind; // FIXME(#43234)
 889                 let succ = self.succ;
 890                 self.complete_drop(Some(DropFlagMode::Deep), succ, unwind)
 891             }
 892             ty::Array(ety, size) => {
 893                 let size = size.try_eval_usize(self.tcx(), self.elaborator.param_env());
 894                 self.open_drop_for_array(ety, size)
 895             }
 896             ty::Slice(ety) => self.open_drop_for_array(ety, None),
 897
 898             _ => bug!("open drop from non-ADT `{:?}`", ty),
 899         }
 900     }
 901
 902     fn complete_drop(
 903         &mut self,
 904         drop_mode: Option<DropFlagMode>,
 905         succ: BasicBlock,
 906         unwind: Unwind,
 907     ) -> BasicBlock {
 908         debug!("complete_drop({:?},{:?})", self, drop_mode);
 909
 910         let drop_block = self.drop_block(succ, unwind);
 911         let drop_block = if let Some(mode) = drop_mode {
 912             self.drop_flag_reset_block(mode, drop_block, unwind)
 913         } else {
 914             drop_block
 915         };
 916
 917         self.drop_flag_test_block(drop_block, succ, unwind)
 918     }
 919
 920     /// Creates a block that resets the drop flag. If `mode` is deep, all children drop flags will
 921     /// also be cleared.
 922     fn drop_flag_reset_block(
 923         &mut self,
 924         mode: DropFlagMode,
 925         succ: BasicBlock,
 926         unwind: Unwind,
 927     ) -> BasicBlock {
 928         debug!("drop_flag_reset_block({:?},{:?})", self, mode);
 929
 930         let block = self.new_block(unwind, TerminatorKind::Goto { target: succ });
 931         let block_start = Location { block, statement_index: 0 };
 932         self.elaborator.clear_drop_flag(block_start, self.path, mode);
 933         block
 934     }
 935
 936     fn elaborated_drop_block(&mut self) -> BasicBlock {
 937         debug!("elaborated_drop_block({:?})", self);
 938         let blk = self.drop_block(self.succ, self.unwind);
 939         self.elaborate_drop(blk);
 940         blk
 941     }
 942
 943     /// Creates a block that frees the backing memory of a `Box` if its drop is required (either
 944     /// statically or by checking its drop flag).
 945     ///
 946     /// The contained value will not be dropped.
 947     fn box_free_block(
 948         &mut self,
 949         adt: &'tcx ty::AdtDef,
 950         substs: SubstsRef<'tcx>,
 951         target: BasicBlock,
 952         unwind: Unwind,
 953     ) -> BasicBlock {
 954         let block = self.unelaborated_free_block(adt, substs, target, unwind);
 955         self.drop_flag_test_block(block, target, unwind)
 956     }
 957
 958     /// Creates a block that frees the backing memory of a `Box` (without dropping the contained
 959     /// value).
 960     fn unelaborated_free_block(
 961         &mut self,
 962         adt: &'tcx ty::AdtDef,
 963         substs: SubstsRef<'tcx>,
 964         target: BasicBlock,
 965         unwind: Unwind,
 966     ) -> BasicBlock {
 967         let tcx = self.tcx();
 968         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 969         let free_func = tcx.require_lang_item(LangItem::BoxFree, Some(self.source_info.span));
 970         let args = adt.variants[VariantIdx::new(0)]
 971             .fields
 972             .iter()
 973             .enumerate()
 974             .map(|(i, f)| {
 975                 let field = Field::new(i);
 976                 let field_ty = f.ty(tcx, substs);
 977                 Operand::Move(tcx.mk_place_field(self.place, field, field_ty))
 978             })
 979             .collect();
 980
 981         let call = TerminatorKind::Call {
 982             func: Operand::function_handle(tcx, free_func, substs, self.source_info.span),
 983             args,
 984             destination: Some((unit_temp, target)),
 985             cleanup: None,
 986             from_hir_call: false,
 987             fn_span: self.source_info.span,
 988         }; // FIXME(#43234)
 989         let free_block = self.new_block(unwind, call);
 990
 991         let block_start = Location { block: free_block, statement_index: 0 };
 992         self.elaborator.clear_drop_flag(block_start, self.path, DropFlagMode::Shallow);
 993         free_block
 994     }
 995
 996     fn drop_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
 997         let block =
 998             TerminatorKind::Drop { place: self.place, target, unwind: unwind.into_option() };
 999         self.new_block(unwind, block)
1000     }
1001
1002     fn goto_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
1003         let block = TerminatorKind::Goto { target };
1004         self.new_block(unwind, block)
1005     }
1006
1007     /// Returns the block to jump to in order to test the drop flag and execute the drop.
1008     ///
1009     /// Depending on the required `DropStyle`, this might be a generated block with an `if`
1010     /// terminator (for dynamic/open drops), or it might be `on_set` or `on_unset` itself, in case
1011     /// the drop can be statically determined.
1012     fn drop_flag_test_block(
1013         &mut self,
1014         on_set: BasicBlock,
1015         on_unset: BasicBlock,
1016         unwind: Unwind,
1017     ) -> BasicBlock {
1018         let style = self.elaborator.drop_style(self.path, DropFlagMode::Shallow);
1019         debug!(
1020             "drop_flag_test_block({:?},{:?},{:?},{:?}) - {:?}",
1021             self, on_set, on_unset, unwind, style
1022         );
1023
1024         match style {
1025             DropStyle::Dead => on_unset,
1026             DropStyle::Static => on_set,
1027             DropStyle::Conditional | DropStyle::Open => {
1028                 let flag = self.elaborator.get_drop_flag(self.path).unwrap();
1029                 let term = TerminatorKind::if_(self.tcx(), flag, on_set, on_unset);
1030                 self.new_block(unwind, term)
1031             }
1032         }
1033     }
1034
1035     fn new_block(&mut self, unwind: Unwind, k: TerminatorKind<'tcx>) -> BasicBlock {
1036         self.elaborator.patch().new_block(BasicBlockData {
1037             statements: vec![],
1038             terminator: Some(Terminator { source_info: self.source_info, kind: k }),
1039             is_cleanup: unwind.is_cleanup(),
1040         })
1041     }
1042
1043     fn new_temp(&mut self, ty: Ty<'tcx>) -> Local {
1044         self.elaborator.patch().new_temp(ty, self.source_info.span)
1045     }
1046
1047     fn terminator_loc(&mut self, bb: BasicBlock) -> Location {
1048         let body = self.elaborator.body();
1049         self.elaborator.patch().terminator_loc(body, bb)
1050     }
1051
1052     fn constant_usize(&self, val: u16) -> Operand<'tcx> {
1053         Operand::Constant(box Constant {
1054             span: self.source_info.span,
1055             user_ty: None,
1056             literal: ty::Const::from_usize(self.tcx(), val.into()),
1057         })
1058     }
1059
1060     fn assign(&self, lhs: Place<'tcx>, rhs: Rvalue<'tcx>) -> Statement<'tcx> {
1061         Statement { source_info: self.source_info, kind: StatementKind::Assign(box (lhs, rhs)) }
1062     }
1063 }