compiler/rustc_mir_dataflow/src/elaborate_drops.rs

   1 use rustc_hir as hir;
   2 use rustc_hir::lang_items::LangItem;
   3 use rustc_index::vec::Idx;
   4 use rustc_middle::mir::patch::MirPatch;
   5 use rustc_middle::mir::*;
   6 use rustc_middle::traits::Reveal;
   7 use rustc_middle::ty::subst::SubstsRef;
   8 use rustc_middle::ty::util::IntTypeExt;
   9 use rustc_middle::ty::{self, Ty, TyCtxt};
  10 use rustc_target::abi::VariantIdx;
  11 use std::{fmt, iter};
  12
  13 /// The value of an inserted drop flag.
  14 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
  15 pub enum DropFlagState {
  16     /// The tracked value is initialized and needs to be dropped when leaving its scope.
  17     Present,
  18
  19     /// The tracked value is uninitialized or was moved out of and does not need to be dropped when
  20     /// leaving its scope.
  21     Absent,
  22 }
  23
  24 impl DropFlagState {
  25     pub fn value(self) -> bool {
  26         match self {
  27             DropFlagState::Present => true,
  28             DropFlagState::Absent => false,
  29         }
  30     }
  31 }
  32
  33 /// Describes how/if a value should be dropped.
  34 #[derive(Debug)]
  35 pub enum DropStyle {
  36     /// The value is already dead at the drop location, no drop will be executed.
  37     Dead,
  38
  39     /// The value is known to always be initialized at the drop location, drop will always be
  40     /// executed.
  41     Static,
  42
  43     /// Whether the value needs to be dropped depends on its drop flag.
  44     Conditional,
  45
  46     /// An "open" drop is one where only the fields of a value are dropped.
  47     ///
  48     /// For example, this happens when moving out of a struct field: The rest of the struct will be
  49     /// dropped in such an "open" drop. It is also used to generate drop glue for the individual
  50     /// components of a value, for example for dropping array elements.
  51     Open,
  52 }
  53
  54 /// Which drop flags to affect/check with an operation.
  55 #[derive(Debug)]
  56 pub enum DropFlagMode {
  57     /// Only affect the top-level drop flag, not that of any contained fields.
  58     Shallow,
  59     /// Affect all nested drop flags in addition to the top-level one.
  60     Deep,
  61 }
  62
  63 /// Describes if unwinding is necessary and where to unwind to if a panic occurs.
  64 #[derive(Copy, Clone, Debug)]
  65 pub enum Unwind {
  66     /// Unwind to this block.
  67     To(BasicBlock),
  68     /// Already in an unwind path, any panic will cause an abort.
  69     InCleanup,
  70 }
  71
  72 impl Unwind {
  73     fn is_cleanup(self) -> bool {
  74         match self {
  75             Unwind::To(..) => false,
  76             Unwind::InCleanup => true,
  77         }
  78     }
  79
  80     fn into_option(self) -> Option<BasicBlock> {
  81         match self {
  82             Unwind::To(bb) => Some(bb),
  83             Unwind::InCleanup => None,
  84         }
  85     }
  86
  87     fn map<F>(self, f: F) -> Self
  88     where
  89         F: FnOnce(BasicBlock) -> BasicBlock,
  90     {
  91         match self {
  92             Unwind::To(bb) => Unwind::To(f(bb)),
  93             Unwind::InCleanup => Unwind::InCleanup,
  94         }
  95     }
  96 }
  97
  98 pub trait DropElaborator<'a, 'tcx>: fmt::Debug {
  99     /// The type representing paths that can be moved out of.
 100     ///
 101     /// Users can move out of individual fields of a struct, such as `a.b.c`. This type is used to
 102     /// represent such move paths. Sometimes tracking individual move paths is not necessary, in
 103     /// which case this may be set to (for example) `()`.
 104     type Path: Copy + fmt::Debug;
 105
 106     // Accessors
 107
 108     fn patch(&mut self) -> &mut MirPatch<'tcx>;
 109     fn body(&self) -> &'a Body<'tcx>;
 110     fn tcx(&self) -> TyCtxt<'tcx>;
 111     fn param_env(&self) -> ty::ParamEnv<'tcx>;
 112
 113     // Drop logic
 114
 115     /// Returns how `path` should be dropped, given `mode`.
 116     fn drop_style(&self, path: Self::Path, mode: DropFlagMode) -> DropStyle;
 117
 118     /// Returns the drop flag of `path` as a MIR `Operand` (or `None` if `path` has no drop flag).
 119     fn get_drop_flag(&mut self, path: Self::Path) -> Option<Operand<'tcx>>;
 120
 121     /// Modifies the MIR patch so that the drop flag of `path` (if any) is cleared at `location`.
 122     ///
 123     /// If `mode` is deep, drop flags of all child paths should also be cleared by inserting
 124     /// additional statements.
 125     fn clear_drop_flag(&mut self, location: Location, path: Self::Path, mode: DropFlagMode);
 126
 127     // Subpaths
 128
 129     /// Returns the subpath of a field of `path` (or `None` if there is no dedicated subpath).
 130     ///
 131     /// If this returns `None`, `field` will not get a dedicated drop flag.
 132     fn field_subpath(&self, path: Self::Path, field: Field) -> Option<Self::Path>;
 133
 134     /// Returns the subpath of a dereference of `path` (or `None` if there is no dedicated subpath).
 135     ///
 136     /// If this returns `None`, `*path` will not get a dedicated drop flag.
 137     ///
 138     /// This is only relevant for `Box<T>`, where the contained `T` can be moved out of the box.
 139     fn deref_subpath(&self, path: Self::Path) -> Option<Self::Path>;
 140
 141     /// Returns the subpath of downcasting `path` to one of its variants.
 142     ///
 143     /// If this returns `None`, the downcast of `path` will not get a dedicated drop flag.
 144     fn downcast_subpath(&self, path: Self::Path, variant: VariantIdx) -> Option<Self::Path>;
 145
 146     /// Returns the subpath of indexing a fixed-size array `path`.
 147     ///
 148     /// If this returns `None`, elements of `path` will not get a dedicated drop flag.
 149     ///
 150     /// This is only relevant for array patterns, which can move out of individual array elements.
 151     fn array_subpath(&self, path: Self::Path, index: u64, size: u64) -> Option<Self::Path>;
 152 }
 153
 154 #[derive(Debug)]
 155 struct DropCtxt<'l, 'b, 'tcx, D>
 156 where
 157     D: DropElaborator<'b, 'tcx>,
 158 {
 159     elaborator: &'l mut D,
 160
 161     source_info: SourceInfo,
 162
 163     place: Place<'tcx>,
 164     path: D::Path,
 165     succ: BasicBlock,
 166     unwind: Unwind,
 167 }
 168
 169 /// "Elaborates" a drop of `place`/`path` and patches `bb`'s terminator to execute it.
 170 ///
 171 /// The passed `elaborator` is used to determine what should happen at the drop terminator. It
 172 /// decides whether the drop can be statically determined or whether it needs a dynamic drop flag,
 173 /// and whether the drop is "open", ie. should be expanded to drop all subfields of the dropped
 174 /// value.
 175 ///
 176 /// When this returns, the MIR patch in the `elaborator` contains the necessary changes.
 177 pub fn elaborate_drop<'b, 'tcx, D>(
 178     elaborator: &mut D,
 179     source_info: SourceInfo,
 180     place: Place<'tcx>,
 181     path: D::Path,
 182     succ: BasicBlock,
 183     unwind: Unwind,
 184     bb: BasicBlock,
 185 ) where
 186     D: DropElaborator<'b, 'tcx>,
 187     'tcx: 'b,
 188 {
 189     DropCtxt { elaborator, source_info, place, path, succ, unwind }.elaborate_drop(bb)
 190 }
 191
 192 impl<'l, 'b, 'tcx, D> DropCtxt<'l, 'b, 'tcx, D>
 193 where
 194     D: DropElaborator<'b, 'tcx>,
 195     'tcx: 'b,
 196 {
 197     fn place_ty(&self, place: Place<'tcx>) -> Ty<'tcx> {
 198         place.ty(self.elaborator.body(), self.tcx()).ty
 199     }
 200
 201     fn tcx(&self) -> TyCtxt<'tcx> {
 202         self.elaborator.tcx()
 203     }
 204
 205     /// This elaborates a single drop instruction, located at `bb`, and
 206     /// patches over it.
 207     ///
 208     /// The elaborated drop checks the drop flags to only drop what
 209     /// is initialized.
 210     ///
 211     /// In addition, the relevant drop flags also need to be cleared
 212     /// to avoid double-drops. However, in the middle of a complex
 213     /// drop, one must avoid clearing some of the flags before they
 214     /// are read, as that would cause a memory leak.
 215     ///
 216     /// In particular, when dropping an ADT, multiple fields may be
 217     /// joined together under the `rest` subpath. They are all controlled
 218     /// by the primary drop flag, but only the last rest-field dropped
 219     /// should clear it (and it must also not clear anything else).
 220     //
 221     // FIXME: I think we should just control the flags externally,
 222     // and then we do not need this machinery.
 223     pub fn elaborate_drop(&mut self, bb: BasicBlock) {
 224         debug!("elaborate_drop({:?}, {:?})", bb, self);
 225         let style = self.elaborator.drop_style(self.path, DropFlagMode::Deep);
 226         debug!("elaborate_drop({:?}, {:?}): live - {:?}", bb, self, style);
 227         match style {
 228             DropStyle::Dead => {
 229                 self.elaborator
 230                     .patch()
 231                     .patch_terminator(bb, TerminatorKind::Goto { target: self.succ });
 232             }
 233             DropStyle::Static => {
 234                 self.elaborator.patch().patch_terminator(
 235                     bb,
 236                     TerminatorKind::Drop {
 237                         place: self.place,
 238                         target: self.succ,
 239                         unwind: self.unwind.into_option(),
 240                     },
 241                 );
 242             }
 243             DropStyle::Conditional => {
 244                 let drop_bb = self.complete_drop(self.succ, self.unwind);
 245                 self.elaborator
 246                     .patch()
 247                     .patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
 248             }
 249             DropStyle::Open => {
 250                 let drop_bb = self.open_drop();
 251                 self.elaborator
 252                     .patch()
 253                     .patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
 254             }
 255         }
 256     }
 257
 258     /// Returns the place and move path for each field of `variant`,
 259     /// (the move path is `None` if the field is a rest field).
 260     fn move_paths_for_fields(
 261         &self,
 262         base_place: Place<'tcx>,
 263         variant_path: D::Path,
 264         variant: &'tcx ty::VariantDef,
 265         substs: SubstsRef<'tcx>,
 266     ) -> Vec<(Place<'tcx>, Option<D::Path>)> {
 267         variant
 268             .fields
 269             .iter()
 270             .enumerate()
 271             .map(|(i, f)| {
 272                 let field = Field::new(i);
 273                 let subpath = self.elaborator.field_subpath(variant_path, field);
 274                 let tcx = self.tcx();
 275
 276                 assert_eq!(self.elaborator.param_env().reveal(), Reveal::All);
 277                 let field_ty =
 278                     tcx.normalize_erasing_regions(self.elaborator.param_env(), f.ty(tcx, substs));
 279                 (tcx.mk_place_field(base_place, field, field_ty), subpath)
 280             })
 281             .collect()
 282     }
 283
 284     fn drop_subpath(
 285         &mut self,
 286         place: Place<'tcx>,
 287         path: Option<D::Path>,
 288         succ: BasicBlock,
 289         unwind: Unwind,
 290     ) -> BasicBlock {
 291         if let Some(path) = path {
 292             debug!("drop_subpath: for std field {:?}", place);
 293
 294             DropCtxt {
 295                 elaborator: self.elaborator,
 296                 source_info: self.source_info,
 297                 path,
 298                 place,
 299                 succ,
 300                 unwind,
 301             }
 302             .elaborated_drop_block()
 303         } else {
 304             debug!("drop_subpath: for rest field {:?}", place);
 305
 306             DropCtxt {
 307                 elaborator: self.elaborator,
 308                 source_info: self.source_info,
 309                 place,
 310                 succ,
 311                 unwind,
 312                 // Using `self.path` here to condition the drop on
 313                 // our own drop flag.
 314                 path: self.path,
 315             }
 316             .complete_drop(succ, unwind)
 317         }
 318     }
 319
 320     /// Creates one-half of the drop ladder for a list of fields, and return
 321     /// the list of steps in it in reverse order, with the first step
 322     /// dropping 0 fields and so on.
 323     ///
 324     /// `unwind_ladder` is such a list of steps in reverse order,
 325     /// which is called if the matching step of the drop glue panics.
 326     fn drop_halfladder(
 327         &mut self,
 328         unwind_ladder: &[Unwind],
 329         mut succ: BasicBlock,
 330         fields: &[(Place<'tcx>, Option<D::Path>)],
 331     ) -> Vec<BasicBlock> {
 332         iter::once(succ)
 333             .chain(fields.iter().rev().zip(unwind_ladder).map(|(&(place, path), &unwind_succ)| {
 334                 succ = self.drop_subpath(place, path, succ, unwind_succ);
 335                 succ
 336             }))
 337             .collect()
 338     }
 339
 340     fn drop_ladder_bottom(&mut self) -> (BasicBlock, Unwind) {
 341         // Clear the "master" drop flag at the end. This is needed
 342         // because the "master" drop protects the ADT's discriminant,
 343         // which is invalidated after the ADT is dropped.
 344         (self.drop_flag_reset_block(DropFlagMode::Shallow, self.succ, self.unwind), self.unwind)
 345     }
 346
 347     /// Creates a full drop ladder, consisting of 2 connected half-drop-ladders
 348     ///
 349     /// For example, with 3 fields, the drop ladder is
 350     ///
 351     /// .d0:
 352     ///     ELAB(drop location.0 [target=.d1, unwind=.c1])
 353     /// .d1:
 354     ///     ELAB(drop location.1 [target=.d2, unwind=.c2])
 355     /// .d2:
 356     ///     ELAB(drop location.2 [target=`self.succ`, unwind=`self.unwind`])
 357     /// .c1:
 358     ///     ELAB(drop location.1 [target=.c2])
 359     /// .c2:
 360     ///     ELAB(drop location.2 [target=`self.unwind`])
 361     ///
 362     /// NOTE: this does not clear the master drop flag, so you need
 363     /// to point succ/unwind on a `drop_ladder_bottom`.
 364     fn drop_ladder(
 365         &mut self,
 366         fields: Vec<(Place<'tcx>, Option<D::Path>)>,
 367         succ: BasicBlock,
 368         unwind: Unwind,
 369     ) -> (BasicBlock, Unwind) {
 370         debug!("drop_ladder({:?}, {:?})", self, fields);
 371
 372         let mut fields = fields;
 373         fields.retain(|&(place, _)| {
 374             self.place_ty(place).needs_drop(self.tcx(), self.elaborator.param_env())
 375         });
 376
 377         debug!("drop_ladder - fields needing drop: {:?}", fields);
 378
 379         let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 380         let unwind_ladder: Vec<_> = if let Unwind::To(target) = unwind {
 381             let halfladder = self.drop_halfladder(&unwind_ladder, target, &fields);
 382             halfladder.into_iter().map(Unwind::To).collect()
 383         } else {
 384             unwind_ladder
 385         };
 386
 387         let normal_ladder = self.drop_halfladder(&unwind_ladder, succ, &fields);
 388
 389         (*normal_ladder.last().unwrap(), *unwind_ladder.last().unwrap())
 390     }
 391
 392     fn open_drop_for_tuple(&mut self, tys: &[Ty<'tcx>]) -> BasicBlock {
 393         debug!("open_drop_for_tuple({:?}, {:?})", self, tys);
 394
 395         let fields = tys
 396             .iter()
 397             .enumerate()
 398             .map(|(i, &ty)| {
 399                 (
 400                     self.tcx().mk_place_field(self.place, Field::new(i), ty),
 401                     self.elaborator.field_subpath(self.path, Field::new(i)),
 402                 )
 403             })
 404             .collect();
 405
 406         let (succ, unwind) = self.drop_ladder_bottom();
 407         self.drop_ladder(fields, succ, unwind).0
 408     }
 409
 410     #[instrument(level = "debug", ret)]
 411     fn open_drop_for_box(&mut self, adt: ty::AdtDef<'tcx>, substs: SubstsRef<'tcx>) -> BasicBlock {
 412         // drop glue is sent straight to codegen
 413         // box cannot be directly dereferenced
 414         let unique_ty = adt.non_enum_variant().fields[0].ty(self.tcx(), substs);
 415         let nonnull_ty =
 416             unique_ty.ty_adt_def().unwrap().non_enum_variant().fields[0].ty(self.tcx(), substs);
 417         let ptr_ty = self.tcx().mk_imm_ptr(substs[0].expect_ty());
 418
 419         let unique_place = self.tcx().mk_place_field(self.place, Field::new(0), unique_ty);
 420         let nonnull_place = self.tcx().mk_place_field(unique_place, Field::new(0), nonnull_ty);
 421         let ptr_place = self.tcx().mk_place_field(nonnull_place, Field::new(0), ptr_ty);
 422         let interior = self.tcx().mk_place_deref(ptr_place);
 423
 424         let interior_path = self.elaborator.deref_subpath(self.path);
 425
 426         let succ = self.box_free_block(adt, substs, self.succ, self.unwind);
 427         let unwind_succ =
 428             self.unwind.map(|unwind| self.box_free_block(adt, substs, unwind, Unwind::InCleanup));
 429
 430         self.drop_subpath(interior, interior_path, succ, unwind_succ)
 431     }
 432
 433     #[instrument(level = "debug", ret)]
 434     fn open_drop_for_adt(&mut self, adt: ty::AdtDef<'tcx>, substs: SubstsRef<'tcx>) -> BasicBlock {
 435         if adt.variants().is_empty() {
 436             return self.elaborator.patch().new_block(BasicBlockData {
 437                 statements: vec![],
 438                 terminator: Some(Terminator {
 439                     source_info: self.source_info,
 440                     kind: TerminatorKind::Unreachable,
 441                 }),
 442                 is_cleanup: self.unwind.is_cleanup(),
 443             });
 444         }
 445
 446         let skip_contents =
 447             adt.is_union() || Some(adt.did()) == self.tcx().lang_items().manually_drop();
 448         let contents_drop = if skip_contents {
 449             (self.succ, self.unwind)
 450         } else {
 451             self.open_drop_for_adt_contents(adt, substs)
 452         };
 453
 454         if adt.has_dtor(self.tcx()) {
 455             self.destructor_call_block(contents_drop)
 456         } else {
 457             contents_drop.0
 458         }
 459     }
 460
 461     fn open_drop_for_adt_contents(
 462         &mut self,
 463         adt: ty::AdtDef<'tcx>,
 464         substs: SubstsRef<'tcx>,
 465     ) -> (BasicBlock, Unwind) {
 466         let (succ, unwind) = self.drop_ladder_bottom();
 467         if !adt.is_enum() {
 468             let fields = self.move_paths_for_fields(
 469                 self.place,
 470                 self.path,
 471                 &adt.variant(VariantIdx::new(0)),
 472                 substs,
 473             );
 474             self.drop_ladder(fields, succ, unwind)
 475         } else {
 476             self.open_drop_for_multivariant(adt, substs, succ, unwind)
 477         }
 478     }
 479
 480     fn open_drop_for_multivariant(
 481         &mut self,
 482         adt: ty::AdtDef<'tcx>,
 483         substs: SubstsRef<'tcx>,
 484         succ: BasicBlock,
 485         unwind: Unwind,
 486     ) -> (BasicBlock, Unwind) {
 487         let mut values = Vec::with_capacity(adt.variants().len());
 488         let mut normal_blocks = Vec::with_capacity(adt.variants().len());
 489         let mut unwind_blocks =
 490             if unwind.is_cleanup() { None } else { Some(Vec::with_capacity(adt.variants().len())) };
 491
 492         let mut have_otherwise_with_drop_glue = false;
 493         let mut have_otherwise = false;
 494         let tcx = self.tcx();
 495
 496         for (variant_index, discr) in adt.discriminants(tcx) {
 497             let variant = &adt.variant(variant_index);
 498             let subpath = self.elaborator.downcast_subpath(self.path, variant_index);
 499
 500             if let Some(variant_path) = subpath {
 501                 let base_place = tcx.mk_place_elem(
 502                     self.place,
 503                     ProjectionElem::Downcast(Some(variant.name), variant_index),
 504                 );
 505                 let fields = self.move_paths_for_fields(base_place, variant_path, &variant, substs);
 506                 values.push(discr.val);
 507                 if let Unwind::To(unwind) = unwind {
 508                     // We can't use the half-ladder from the original
 509                     // drop ladder, because this breaks the
 510                     // "funclet can't have 2 successor funclets"
 511                     // requirement from MSVC:
 512                     //
 513                     //           switch       unwind-switch
 514                     //          /      \         /        \
 515                     //         v1.0    v2.0  v2.0-unwind  v1.0-unwind
 516                     //         |        |      /             |
 517                     //    v1.1-unwind  v2.1-unwind           |
 518                     //      ^                                |
 519                     //       \-------------------------------/
 520                     //
 521                     // Create a duplicate half-ladder to avoid that. We
 522                     // could technically only do this on MSVC, but I
 523                     // I want to minimize the divergence between MSVC
 524                     // and non-MSVC.
 525
 526                     let unwind_blocks = unwind_blocks.as_mut().unwrap();
 527                     let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 528                     let halfladder = self.drop_halfladder(&unwind_ladder, unwind, &fields);
 529                     unwind_blocks.push(halfladder.last().cloned().unwrap());
 530                 }
 531                 let (normal, _) = self.drop_ladder(fields, succ, unwind);
 532                 normal_blocks.push(normal);
 533             } else {
 534                 have_otherwise = true;
 535
 536                 let param_env = self.elaborator.param_env();
 537                 let have_field_with_drop_glue = variant
 538                     .fields
 539                     .iter()
 540                     .any(|field| field.ty(tcx, substs).needs_drop(tcx, param_env));
 541                 if have_field_with_drop_glue {
 542                     have_otherwise_with_drop_glue = true;
 543                 }
 544             }
 545         }
 546
 547         if !have_otherwise {
 548             values.pop();
 549         } else if !have_otherwise_with_drop_glue {
 550             normal_blocks.push(self.goto_block(succ, unwind));
 551             if let Unwind::To(unwind) = unwind {
 552                 unwind_blocks.as_mut().unwrap().push(self.goto_block(unwind, Unwind::InCleanup));
 553             }
 554         } else {
 555             normal_blocks.push(self.drop_block(succ, unwind));
 556             if let Unwind::To(unwind) = unwind {
 557                 unwind_blocks.as_mut().unwrap().push(self.drop_block(unwind, Unwind::InCleanup));
 558             }
 559         }
 560
 561         (
 562             self.adt_switch_block(adt, normal_blocks, &values, succ, unwind),
 563             unwind.map(|unwind| {
 564                 self.adt_switch_block(
 565                     adt,
 566                     unwind_blocks.unwrap(),
 567                     &values,
 568                     unwind,
 569                     Unwind::InCleanup,
 570                 )
 571             }),
 572         )
 573     }
 574
 575     fn adt_switch_block(
 576         &mut self,
 577         adt: ty::AdtDef<'tcx>,
 578         blocks: Vec<BasicBlock>,
 579         values: &[u128],
 580         succ: BasicBlock,
 581         unwind: Unwind,
 582     ) -> BasicBlock {
 583         // If there are multiple variants, then if something
 584         // is present within the enum the discriminant, tracked
 585         // by the rest path, must be initialized.
 586         //
 587         // Additionally, we do not want to switch on the
 588         // discriminant after it is free-ed, because that
 589         // way lies only trouble.
 590         let discr_ty = adt.repr().discr_type().to_ty(self.tcx());
 591         let discr = Place::from(self.new_temp(discr_ty));
 592         let discr_rv = Rvalue::Discriminant(self.place);
 593         let switch_block = BasicBlockData {
 594             statements: vec![self.assign(discr, discr_rv)],
 595             terminator: Some(Terminator {
 596                 source_info: self.source_info,
 597                 kind: TerminatorKind::SwitchInt {
 598                     discr: Operand::Move(discr),
 599                     targets: SwitchTargets::new(
 600                         values.iter().copied().zip(blocks.iter().copied()),
 601                         *blocks.last().unwrap(),
 602                     ),
 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_item_def_ids(drop_trait)[0];
 616         let ty = self.place_ty(self.place);
 617
 618         let ref_ty =
 619             tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut { ty, mutbl: hir::Mutability::Mut });
 620         let ref_place = self.new_temp(ref_ty);
 621         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 622
 623         let result = BasicBlockData {
 624             statements: vec![self.assign(
 625                 Place::from(ref_place),
 626                 Rvalue::Ref(
 627                     tcx.lifetimes.re_erased,
 628                     BorrowKind::Mut { allow_two_phase_borrow: false },
 629                     self.place,
 630                 ),
 631             )],
 632             terminator: Some(Terminator {
 633                 kind: TerminatorKind::Call {
 634                     func: Operand::function_handle(
 635                         tcx,
 636                         drop_fn,
 637                         [ty.into()],
 638                         self.source_info.span,
 639                     ),
 640                     args: vec![Operand::Move(Place::from(ref_place))],
 641                     destination: unit_temp,
 642                     target: Some(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             (
 690                 Rvalue::Use(copy(cur.into())),
 691                 Rvalue::BinaryOp(BinOp::Offset, Box::new((move_(cur.into()), one))),
 692             )
 693         } else {
 694             (
 695                 Rvalue::AddressOf(Mutability::Mut, tcx.mk_place_index(self.place, cur)),
 696                 Rvalue::BinaryOp(BinOp::Add, Box::new((move_(cur.into()), one))),
 697             )
 698         };
 699
 700         let drop_block = BasicBlockData {
 701             statements: vec![self.assign(ptr, ptr_next), self.assign(Place::from(cur), cur_next)],
 702             is_cleanup: unwind.is_cleanup(),
 703             terminator: Some(Terminator {
 704                 source_info: self.source_info,
 705                 // this gets overwritten by drop elaboration.
 706                 kind: TerminatorKind::Unreachable,
 707             }),
 708         };
 709         let drop_block = self.elaborator.patch().new_block(drop_block);
 710
 711         let loop_block = BasicBlockData {
 712             statements: vec![self.assign(
 713                 can_go,
 714                 Rvalue::BinaryOp(
 715                     BinOp::Eq,
 716                     Box::new((copy(Place::from(cur)), copy(length_or_end))),
 717                 ),
 718             )],
 719             is_cleanup: unwind.is_cleanup(),
 720             terminator: Some(Terminator {
 721                 source_info: self.source_info,
 722                 kind: TerminatorKind::if_(move_(can_go), succ, drop_block),
 723             }),
 724         };
 725         let loop_block = self.elaborator.patch().new_block(loop_block);
 726
 727         self.elaborator.patch().patch_terminator(
 728             drop_block,
 729             TerminatorKind::Drop {
 730                 place: tcx.mk_place_deref(ptr),
 731                 target: loop_block,
 732                 unwind: unwind.into_option(),
 733             },
 734         );
 735
 736         loop_block
 737     }
 738
 739     fn open_drop_for_array(&mut self, ety: Ty<'tcx>, opt_size: Option<u64>) -> BasicBlock {
 740         debug!("open_drop_for_array({:?}, {:?})", ety, opt_size);
 741
 742         // if size_of::<ety>() == 0 {
 743         //     index_based_loop
 744         // } else {
 745         //     ptr_based_loop
 746         // }
 747
 748         let tcx = self.tcx();
 749
 750         if let Some(size) = opt_size {
 751             let fields: Vec<(Place<'tcx>, Option<D::Path>)> = (0..size)
 752                 .map(|i| {
 753                     (
 754                         tcx.mk_place_elem(
 755                             self.place,
 756                             ProjectionElem::ConstantIndex {
 757                                 offset: i,
 758                                 min_length: size,
 759                                 from_end: false,
 760                             },
 761                         ),
 762                         self.elaborator.array_subpath(self.path, i, size),
 763                     )
 764                 })
 765                 .collect();
 766
 767             if fields.iter().any(|(_, path)| path.is_some()) {
 768                 let (succ, unwind) = self.drop_ladder_bottom();
 769                 return self.drop_ladder(fields, succ, unwind).0;
 770             }
 771         }
 772
 773         let move_ = |place: Place<'tcx>| Operand::Move(place);
 774         let elem_size = Place::from(self.new_temp(tcx.types.usize));
 775         let len = Place::from(self.new_temp(tcx.types.usize));
 776
 777         let base_block = BasicBlockData {
 778             statements: vec![
 779                 self.assign(elem_size, Rvalue::NullaryOp(NullOp::SizeOf, ety)),
 780                 self.assign(len, Rvalue::Len(self.place)),
 781             ],
 782             is_cleanup: self.unwind.is_cleanup(),
 783             terminator: Some(Terminator {
 784                 source_info: self.source_info,
 785                 kind: TerminatorKind::SwitchInt {
 786                     discr: move_(elem_size),
 787                     targets: SwitchTargets::static_if(
 788                         0,
 789                         self.drop_loop_pair(ety, false, len),
 790                         self.drop_loop_pair(ety, true, len),
 791                     ),
 792                 },
 793             }),
 794         };
 795         self.elaborator.patch().new_block(base_block)
 796     }
 797
 798     /// Creates a pair of drop-loops of `place`, which drops its contents, even
 799     /// in the case of 1 panic. If `ptr_based`, creates a pointer loop,
 800     /// otherwise create an index loop.
 801     fn drop_loop_pair(
 802         &mut self,
 803         ety: Ty<'tcx>,
 804         ptr_based: bool,
 805         length: Place<'tcx>,
 806     ) -> BasicBlock {
 807         debug!("drop_loop_pair({:?}, {:?})", ety, ptr_based);
 808         let tcx = self.tcx();
 809         let iter_ty = if ptr_based { tcx.mk_mut_ptr(ety) } else { tcx.types.usize };
 810
 811         let cur = self.new_temp(iter_ty);
 812         let length_or_end = if ptr_based { Place::from(self.new_temp(iter_ty)) } else { length };
 813
 814         let unwind = self.unwind.map(|unwind| {
 815             self.drop_loop(unwind, cur, length_or_end, ety, Unwind::InCleanup, ptr_based)
 816         });
 817
 818         let loop_block = self.drop_loop(self.succ, cur, length_or_end, ety, unwind, ptr_based);
 819
 820         let cur = Place::from(cur);
 821         let drop_block_stmts = if ptr_based {
 822             let tmp_ty = tcx.mk_mut_ptr(self.place_ty(self.place));
 823             let tmp = Place::from(self.new_temp(tmp_ty));
 824             // tmp = &raw mut P;
 825             // cur = tmp as *mut T;
 826             // end = Offset(cur, len);
 827             let mir_cast_kind = ty::cast::mir_cast_kind(iter_ty, tmp_ty);
 828             vec![
 829                 self.assign(tmp, Rvalue::AddressOf(Mutability::Mut, self.place)),
 830                 self.assign(cur, Rvalue::Cast(mir_cast_kind, Operand::Move(tmp), iter_ty)),
 831                 self.assign(
 832                     length_or_end,
 833                     Rvalue::BinaryOp(
 834                         BinOp::Offset,
 835                         Box::new((Operand::Copy(cur), Operand::Move(length))),
 836                     ),
 837                 ),
 838             ]
 839         } else {
 840             // cur = 0 (length already pushed)
 841             let zero = self.constant_usize(0);
 842             vec![self.assign(cur, Rvalue::Use(zero))]
 843         };
 844         let drop_block = self.elaborator.patch().new_block(BasicBlockData {
 845             statements: drop_block_stmts,
 846             is_cleanup: unwind.is_cleanup(),
 847             terminator: Some(Terminator {
 848                 source_info: self.source_info,
 849                 kind: TerminatorKind::Goto { target: loop_block },
 850             }),
 851         });
 852
 853         // FIXME(#34708): handle partially-dropped array/slice elements.
 854         let reset_block = self.drop_flag_reset_block(DropFlagMode::Deep, drop_block, unwind);
 855         self.drop_flag_test_block(reset_block, self.succ, unwind)
 856     }
 857
 858     /// The slow-path - create an "open", elaborated drop for a type
 859     /// which is moved-out-of only partially, and patch `bb` to a jump
 860     /// to it. This must not be called on ADTs with a destructor,
 861     /// as these can't be moved-out-of, except for `Box<T>`, which is
 862     /// special-cased.
 863     ///
 864     /// This creates a "drop ladder" that drops the needed fields of the
 865     /// ADT, both in the success case or if one of the destructors fail.
 866     fn open_drop(&mut self) -> BasicBlock {
 867         let ty = self.place_ty(self.place);
 868         match ty.kind() {
 869             ty::Closure(_, substs) => {
 870                 let tys: Vec<_> = substs.as_closure().upvar_tys().collect();
 871                 self.open_drop_for_tuple(&tys)
 872             }
 873             // Note that `elaborate_drops` only drops the upvars of a generator,
 874             // and this is ok because `open_drop` here can only be reached
 875             // within that own generator's resume function.
 876             // This should only happen for the self argument on the resume function.
 877             // It effectively only contains upvars until the generator transformation runs.
 878             // See librustc_body/transform/generator.rs for more details.
 879             ty::Generator(_, substs, _) => {
 880                 let tys: Vec<_> = substs.as_generator().upvar_tys().collect();
 881                 self.open_drop_for_tuple(&tys)
 882             }
 883             ty::Tuple(fields) => self.open_drop_for_tuple(fields),
 884             ty::Adt(def, substs) => {
 885                 if def.is_box() {
 886                     self.open_drop_for_box(*def, substs)
 887                 } else {
 888                     self.open_drop_for_adt(*def, substs)
 889                 }
 890             }
 891             ty::Dynamic(..) => self.complete_drop(self.succ, self.unwind),
 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             _ => span_bug!(self.source_info.span, "open drop from non-ADT `{:?}`", ty),
 899         }
 900     }
 901
 902     fn complete_drop(&mut self, succ: BasicBlock, unwind: Unwind) -> BasicBlock {
 903         debug!("complete_drop(succ={:?}, unwind={:?})", succ, unwind);
 904
 905         let drop_block = self.drop_block(succ, unwind);
 906
 907         self.drop_flag_test_block(drop_block, succ, unwind)
 908     }
 909
 910     /// Creates a block that resets the drop flag. If `mode` is deep, all children drop flags will
 911     /// also be cleared.
 912     fn drop_flag_reset_block(
 913         &mut self,
 914         mode: DropFlagMode,
 915         succ: BasicBlock,
 916         unwind: Unwind,
 917     ) -> BasicBlock {
 918         debug!("drop_flag_reset_block({:?},{:?})", self, mode);
 919
 920         if unwind.is_cleanup() {
 921             // The drop flag isn't read again on the unwind path, so don't
 922             // bother setting it.
 923             return succ;
 924         }
 925         let block = self.new_block(unwind, TerminatorKind::Goto { target: succ });
 926         let block_start = Location { block, statement_index: 0 };
 927         self.elaborator.clear_drop_flag(block_start, self.path, mode);
 928         block
 929     }
 930
 931     fn elaborated_drop_block(&mut self) -> BasicBlock {
 932         debug!("elaborated_drop_block({:?})", self);
 933         let blk = self.drop_block(self.succ, self.unwind);
 934         self.elaborate_drop(blk);
 935         blk
 936     }
 937
 938     /// Creates a block that frees the backing memory of a `Box` if its drop is required (either
 939     /// statically or by checking its drop flag).
 940     ///
 941     /// The contained value will not be dropped.
 942     fn box_free_block(
 943         &mut self,
 944         adt: ty::AdtDef<'tcx>,
 945         substs: SubstsRef<'tcx>,
 946         target: BasicBlock,
 947         unwind: Unwind,
 948     ) -> BasicBlock {
 949         let block = self.unelaborated_free_block(adt, substs, target, unwind);
 950         self.drop_flag_test_block(block, target, unwind)
 951     }
 952
 953     /// Creates a block that frees the backing memory of a `Box` (without dropping the contained
 954     /// value).
 955     fn unelaborated_free_block(
 956         &mut self,
 957         adt: ty::AdtDef<'tcx>,
 958         substs: SubstsRef<'tcx>,
 959         target: BasicBlock,
 960         unwind: Unwind,
 961     ) -> BasicBlock {
 962         let tcx = self.tcx();
 963         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 964         let free_func = tcx.require_lang_item(LangItem::BoxFree, Some(self.source_info.span));
 965         let args = adt
 966             .variant(VariantIdx::new(0))
 967             .fields
 968             .iter()
 969             .enumerate()
 970             .map(|(i, f)| {
 971                 let field = Field::new(i);
 972                 let field_ty = f.ty(tcx, substs);
 973                 Operand::Move(tcx.mk_place_field(self.place, field, field_ty))
 974             })
 975             .collect();
 976
 977         let call = TerminatorKind::Call {
 978             func: Operand::function_handle(tcx, free_func, substs, self.source_info.span),
 979             args,
 980             destination: unit_temp,
 981             target: Some(target),
 982             cleanup: None,
 983             from_hir_call: false,
 984             fn_span: self.source_info.span,
 985         }; // FIXME(#43234)
 986         let free_block = self.new_block(unwind, call);
 987
 988         let block_start = Location { block: free_block, statement_index: 0 };
 989         self.elaborator.clear_drop_flag(block_start, self.path, DropFlagMode::Shallow);
 990         free_block
 991     }
 992
 993     fn drop_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
 994         let block =
 995             TerminatorKind::Drop { place: self.place, target, unwind: unwind.into_option() };
 996         self.new_block(unwind, block)
 997     }
 998
 999     fn goto_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
1000         let block = TerminatorKind::Goto { target };
1001         self.new_block(unwind, block)
1002     }
1003
1004     /// Returns the block to jump to in order to test the drop flag and execute the drop.
1005     ///
1006     /// Depending on the required `DropStyle`, this might be a generated block with an `if`
1007     /// terminator (for dynamic/open drops), or it might be `on_set` or `on_unset` itself, in case
1008     /// the drop can be statically determined.
1009     fn drop_flag_test_block(
1010         &mut self,
1011         on_set: BasicBlock,
1012         on_unset: BasicBlock,
1013         unwind: Unwind,
1014     ) -> BasicBlock {
1015         let style = self.elaborator.drop_style(self.path, DropFlagMode::Shallow);
1016         debug!(
1017             "drop_flag_test_block({:?},{:?},{:?},{:?}) - {:?}",
1018             self, on_set, on_unset, unwind, style
1019         );
1020
1021         match style {
1022             DropStyle::Dead => on_unset,
1023             DropStyle::Static => on_set,
1024             DropStyle::Conditional | DropStyle::Open => {
1025                 let flag = self.elaborator.get_drop_flag(self.path).unwrap();
1026                 let term = TerminatorKind::if_(flag, on_set, on_unset);
1027                 self.new_block(unwind, term)
1028             }
1029         }
1030     }
1031
1032     fn new_block(&mut self, unwind: Unwind, k: TerminatorKind<'tcx>) -> BasicBlock {
1033         self.elaborator.patch().new_block(BasicBlockData {
1034             statements: vec![],
1035             terminator: Some(Terminator { source_info: self.source_info, kind: k }),
1036             is_cleanup: unwind.is_cleanup(),
1037         })
1038     }
1039
1040     fn new_temp(&mut self, ty: Ty<'tcx>) -> Local {
1041         self.elaborator.patch().new_temp(ty, self.source_info.span)
1042     }
1043
1044     fn constant_usize(&self, val: u16) -> Operand<'tcx> {
1045         Operand::Constant(Box::new(Constant {
1046             span: self.source_info.span,
1047             user_ty: None,
1048             literal: ConstantKind::from_usize(self.tcx(), val.into()),
1049         }))
1050     }
1051
1052     fn assign(&self, lhs: Place<'tcx>, rhs: Rvalue<'tcx>) -> Statement<'tcx> {
1053         Statement {
1054             source_info: self.source_info,
1055             kind: StatementKind::Assign(Box::new((lhs, rhs))),
1056         }
1057     }
1058 }