compiler/rustc_mir_dataflow/src/elaborate_drops.rs

   1 use rustc_hir as hir;
   2 use rustc_hir::lang_items::LangItem;
   3 use rustc_index::vec::Idx;
   4 use rustc_middle::mir::patch::MirPatch;
   5 use rustc_middle::mir::*;
   6 use rustc_middle::traits::Reveal;
   7 use rustc_middle::ty::subst::SubstsRef;
   8 use rustc_middle::ty::util::IntTypeExt;
   9 use rustc_middle::ty::{self, Ty, TyCtxt};
  10 use rustc_target::abi::VariantIdx;
  11 use std::{fmt, iter};
  12
  13 /// The value of an inserted drop flag.
  14 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
  15 pub enum DropFlagState {
  16     /// The tracked value is initialized and needs to be dropped when leaving its scope.
  17     Present,
  18
  19     /// The tracked value is uninitialized or was moved out of and does not need to be dropped when
  20     /// leaving its scope.
  21     Absent,
  22 }
  23
  24 impl DropFlagState {
  25     pub fn value(self) -> bool {
  26         match self {
  27             DropFlagState::Present => true,
  28             DropFlagState::Absent => false,
  29         }
  30     }
  31 }
  32
  33 /// Describes how/if a value should be dropped.
  34 #[derive(Debug)]
  35 pub enum DropStyle {
  36     /// The value is already dead at the drop location, no drop will be executed.
  37     Dead,
  38
  39     /// The value is known to always be initialized at the drop location, drop will always be
  40     /// executed.
  41     Static,
  42
  43     /// Whether the value needs to be dropped depends on its drop flag.
  44     Conditional,
  45
  46     /// An "open" drop is one where only the fields of a value are dropped.
  47     ///
  48     /// For example, this happens when moving out of a struct field: The rest of the struct will be
  49     /// dropped in such an "open" drop. It is also used to generate drop glue for the individual
  50     /// components of a value, for example for dropping array elements.
  51     Open,
  52 }
  53
  54 /// Which drop flags to affect/check with an operation.
  55 #[derive(Debug)]
  56 pub enum DropFlagMode {
  57     /// Only affect the top-level drop flag, not that of any contained fields.
  58     Shallow,
  59     /// Affect all nested drop flags in addition to the top-level one.
  60     Deep,
  61 }
  62
  63 /// Describes if unwinding is necessary and where to unwind to if a panic occurs.
  64 #[derive(Copy, Clone, Debug)]
  65 pub enum Unwind {
  66     /// Unwind to this block.
  67     To(BasicBlock),
  68     /// Already in an unwind path, any panic will cause an abort.
  69     InCleanup,
  70 }
  71
  72 impl Unwind {
  73     fn is_cleanup(self) -> bool {
  74         match self {
  75             Unwind::To(..) => false,
  76             Unwind::InCleanup => true,
  77         }
  78     }
  79
  80     fn into_option(self) -> Option<BasicBlock> {
  81         match self {
  82             Unwind::To(bb) => Some(bb),
  83             Unwind::InCleanup => None,
  84         }
  85     }
  86
  87     fn map<F>(self, f: F) -> Self
  88     where
  89         F: FnOnce(BasicBlock) -> BasicBlock,
  90     {
  91         match self {
  92             Unwind::To(bb) => Unwind::To(f(bb)),
  93             Unwind::InCleanup => Unwind::InCleanup,
  94         }
  95     }
  96 }
  97
  98 pub trait DropElaborator<'a, 'tcx>: fmt::Debug {
  99     /// The type representing paths that can be moved out of.
 100     ///
 101     /// Users can move out of individual fields of a struct, such as `a.b.c`. This type is used to
 102     /// represent such move paths. Sometimes tracking individual move paths is not necessary, in
 103     /// which case this may be set to (for example) `()`.
 104     type Path: Copy + fmt::Debug;
 105
 106     // Accessors
 107
 108     fn patch(&mut self) -> &mut MirPatch<'tcx>;
 109     fn body(&self) -> &'a Body<'tcx>;
 110     fn tcx(&self) -> TyCtxt<'tcx>;
 111     fn param_env(&self) -> ty::ParamEnv<'tcx>;
 112
 113     // Drop logic
 114
 115     /// Returns how `path` should be dropped, given `mode`.
 116     fn drop_style(&self, path: Self::Path, mode: DropFlagMode) -> DropStyle;
 117
 118     /// Returns the drop flag of `path` as a MIR `Operand` (or `None` if `path` has no drop flag).
 119     fn get_drop_flag(&mut self, path: Self::Path) -> Option<Operand<'tcx>>;
 120
 121     /// Modifies the MIR patch so that the drop flag of `path` (if any) is cleared at `location`.
 122     ///
 123     /// If `mode` is deep, drop flags of all child paths should also be cleared by inserting
 124     /// additional statements.
 125     fn clear_drop_flag(&mut self, location: Location, path: Self::Path, mode: DropFlagMode);
 126
 127     // Subpaths
 128
 129     /// Returns the subpath of a field of `path` (or `None` if there is no dedicated subpath).
 130     ///
 131     /// If this returns `None`, `field` will not get a dedicated drop flag.
 132     fn field_subpath(&self, path: Self::Path, field: Field) -> Option<Self::Path>;
 133
 134     /// Returns the subpath of a dereference of `path` (or `None` if there is no dedicated subpath).
 135     ///
 136     /// If this returns `None`, `*path` will not get a dedicated drop flag.
 137     ///
 138     /// This is only relevant for `Box<T>`, where the contained `T` can be moved out of the box.
 139     fn deref_subpath(&self, path: Self::Path) -> Option<Self::Path>;
 140
 141     /// Returns the subpath of downcasting `path` to one of its variants.
 142     ///
 143     /// If this returns `None`, the downcast of `path` will not get a dedicated drop flag.
 144     fn downcast_subpath(&self, path: Self::Path, variant: VariantIdx) -> Option<Self::Path>;
 145
 146     /// Returns the subpath of indexing a fixed-size array `path`.
 147     ///
 148     /// If this returns `None`, elements of `path` will not get a dedicated drop flag.
 149     ///
 150     /// This is only relevant for array patterns, which can move out of individual array elements.
 151     fn array_subpath(&self, path: Self::Path, index: u64, size: u64) -> Option<Self::Path>;
 152 }
 153
 154 #[derive(Debug)]
 155 struct DropCtxt<'l, 'b, 'tcx, D>
 156 where
 157     D: DropElaborator<'b, 'tcx>,
 158 {
 159     elaborator: &'l mut D,
 160
 161     source_info: SourceInfo,
 162
 163     place: Place<'tcx>,
 164     path: D::Path,
 165     succ: BasicBlock,
 166     unwind: Unwind,
 167 }
 168
 169 /// "Elaborates" a drop of `place`/`path` and patches `bb`'s terminator to execute it.
 170 ///
 171 /// The passed `elaborator` is used to determine what should happen at the drop terminator. It
 172 /// decides whether the drop can be statically determined or whether it needs a dynamic drop flag,
 173 /// and whether the drop is "open", ie. should be expanded to drop all subfields of the dropped
 174 /// value.
 175 ///
 176 /// When this returns, the MIR patch in the `elaborator` contains the necessary changes.
 177 pub fn elaborate_drop<'b, 'tcx, D>(
 178     elaborator: &mut D,
 179     source_info: SourceInfo,
 180     place: Place<'tcx>,
 181     path: D::Path,
 182     succ: BasicBlock,
 183     unwind: Unwind,
 184     bb: BasicBlock,
 185 ) where
 186     D: DropElaborator<'b, 'tcx>,
 187     'tcx: 'b,
 188 {
 189     DropCtxt { elaborator, source_info, place, path, succ, unwind }.elaborate_drop(bb)
 190 }
 191
 192 impl<'l, 'b, 'tcx, D> DropCtxt<'l, 'b, 'tcx, D>
 193 where
 194     D: DropElaborator<'b, 'tcx>,
 195     'tcx: 'b,
 196 {
 197     fn place_ty(&self, place: Place<'tcx>) -> Ty<'tcx> {
 198         place.ty(self.elaborator.body(), self.tcx()).ty
 199     }
 200
 201     fn tcx(&self) -> TyCtxt<'tcx> {
 202         self.elaborator.tcx()
 203     }
 204
 205     /// This elaborates a single drop instruction, located at `bb`, and
 206     /// patches over it.
 207     ///
 208     /// The elaborated drop checks the drop flags to only drop what
 209     /// is initialized.
 210     ///
 211     /// In addition, the relevant drop flags also need to be cleared
 212     /// to avoid double-drops. However, in the middle of a complex
 213     /// drop, one must avoid clearing some of the flags before they
 214     /// are read, as that would cause a memory leak.
 215     ///
 216     /// In particular, when dropping an ADT, multiple fields may be
 217     /// joined together under the `rest` subpath. They are all controlled
 218     /// by the primary drop flag, but only the last rest-field dropped
 219     /// should clear it (and it must also not clear anything else).
 220     //
 221     // FIXME: I think we should just control the flags externally,
 222     // and then we do not need this machinery.
 223     pub fn elaborate_drop(&mut self, bb: BasicBlock) {
 224         debug!("elaborate_drop({:?}, {:?})", bb, self);
 225         let style = self.elaborator.drop_style(self.path, DropFlagMode::Deep);
 226         debug!("elaborate_drop({:?}, {:?}): live - {:?}", bb, self, style);
 227         match style {
 228             DropStyle::Dead => {
 229                 self.elaborator
 230                     .patch()
 231                     .patch_terminator(bb, TerminatorKind::Goto { target: self.succ });
 232             }
 233             DropStyle::Static => {
 234                 self.elaborator.patch().patch_terminator(
 235                     bb,
 236                     TerminatorKind::Drop {
 237                         place: self.place,
 238                         target: self.succ,
 239                         unwind: self.unwind.into_option(),
 240                     },
 241                 );
 242             }
 243             DropStyle::Conditional => {
 244                 let drop_bb = self.complete_drop(self.succ, self.unwind);
 245                 self.elaborator
 246                     .patch()
 247                     .patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
 248             }
 249             DropStyle::Open => {
 250                 let drop_bb = self.open_drop();
 251                 self.elaborator
 252                     .patch()
 253                     .patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
 254             }
 255         }
 256     }
 257
 258     /// Returns the place and move path for each field of `variant`,
 259     /// (the move path is `None` if the field is a rest field).
 260     fn move_paths_for_fields(
 261         &self,
 262         base_place: Place<'tcx>,
 263         variant_path: D::Path,
 264         variant: &'tcx ty::VariantDef,
 265         substs: SubstsRef<'tcx>,
 266     ) -> Vec<(Place<'tcx>, Option<D::Path>)> {
 267         variant
 268             .fields
 269             .iter()
 270             .enumerate()
 271             .map(|(i, f)| {
 272                 let field = Field::new(i);
 273                 let subpath = self.elaborator.field_subpath(variant_path, field);
 274                 let tcx = self.tcx();
 275
 276                 assert_eq!(self.elaborator.param_env().reveal(), Reveal::All);
 277                 let field_ty =
 278                     tcx.normalize_erasing_regions(self.elaborator.param_env(), f.ty(tcx, substs));
 279                 (tcx.mk_place_field(base_place, field, field_ty), subpath)
 280             })
 281             .collect()
 282     }
 283
 284     fn drop_subpath(
 285         &mut self,
 286         place: Place<'tcx>,
 287         path: Option<D::Path>,
 288         succ: BasicBlock,
 289         unwind: Unwind,
 290     ) -> BasicBlock {
 291         if let Some(path) = path {
 292             debug!("drop_subpath: for std field {:?}", place);
 293
 294             DropCtxt {
 295                 elaborator: self.elaborator,
 296                 source_info: self.source_info,
 297                 path,
 298                 place,
 299                 succ,
 300                 unwind,
 301             }
 302             .elaborated_drop_block()
 303         } else {
 304             debug!("drop_subpath: for rest field {:?}", place);
 305
 306             DropCtxt {
 307                 elaborator: self.elaborator,
 308                 source_info: self.source_info,
 309                 place,
 310                 succ,
 311                 unwind,
 312                 // Using `self.path` here to condition the drop on
 313                 // our own drop flag.
 314                 path: self.path,
 315             }
 316             .complete_drop(succ, unwind)
 317         }
 318     }
 319
 320     /// Creates one-half of the drop ladder for a list of fields, and return
 321     /// the list of steps in it in reverse order, with the first step
 322     /// dropping 0 fields and so on.
 323     ///
 324     /// `unwind_ladder` is such a list of steps in reverse order,
 325     /// which is called if the matching step of the drop glue panics.
 326     fn drop_halfladder(
 327         &mut self,
 328         unwind_ladder: &[Unwind],
 329         mut succ: BasicBlock,
 330         fields: &[(Place<'tcx>, Option<D::Path>)],
 331     ) -> Vec<BasicBlock> {
 332         iter::once(succ)
 333             .chain(fields.iter().rev().zip(unwind_ladder).map(|(&(place, path), &unwind_succ)| {
 334                 succ = self.drop_subpath(place, path, succ, unwind_succ);
 335                 succ
 336             }))
 337             .collect()
 338     }
 339
 340     fn drop_ladder_bottom(&mut self) -> (BasicBlock, Unwind) {
 341         // Clear the "master" drop flag at the end. This is needed
 342         // because the "master" drop protects the ADT's discriminant,
 343         // which is invalidated after the ADT is dropped.
 344         (self.drop_flag_reset_block(DropFlagMode::Shallow, self.succ, self.unwind), self.unwind)
 345     }
 346
 347     /// Creates a full drop ladder, consisting of 2 connected half-drop-ladders
 348     ///
 349     /// For example, with 3 fields, the drop ladder is
 350     ///
 351     /// .d0:
 352     ///     ELAB(drop location.0 [target=.d1, unwind=.c1])
 353     /// .d1:
 354     ///     ELAB(drop location.1 [target=.d2, unwind=.c2])
 355     /// .d2:
 356     ///     ELAB(drop location.2 [target=`self.succ`, unwind=`self.unwind`])
 357     /// .c1:
 358     ///     ELAB(drop location.1 [target=.c2])
 359     /// .c2:
 360     ///     ELAB(drop location.2 [target=`self.unwind`])
 361     ///
 362     /// NOTE: this does not clear the master drop flag, so you need
 363     /// to point succ/unwind on a `drop_ladder_bottom`.
 364     fn drop_ladder(
 365         &mut self,
 366         fields: Vec<(Place<'tcx>, Option<D::Path>)>,
 367         succ: BasicBlock,
 368         unwind: Unwind,
 369     ) -> (BasicBlock, Unwind) {
 370         debug!("drop_ladder({:?}, {:?})", self, fields);
 371
 372         let mut fields = fields;
 373         fields.retain(|&(place, _)| {
 374             self.place_ty(place).needs_drop(self.tcx(), self.elaborator.param_env())
 375         });
 376
 377         debug!("drop_ladder - fields needing drop: {:?}", fields);
 378
 379         let unwind_ladder = vec![Unwind::InCleanup; fields.len() + 1];
 380         let unwind_ladder: Vec<_> = if let Unwind::To(target) = unwind {
 381             let halfladder = self.drop_halfladder(&unwind_ladder, target, &fields);
 382             halfladder.into_iter().map(Unwind::To).collect()
 383         } else {
 384             unwind_ladder
 385         };
 386
 387         let normal_ladder = self.drop_halfladder(&unwind_ladder, succ, &fields);
 388
 389         (*normal_ladder.last().unwrap(), *unwind_ladder.last().unwrap())
 390     }
 391
 392     fn open_drop_for_tuple(&mut self, tys: &[Ty<'tcx>]) -> BasicBlock {
 393         debug!("open_drop_for_tuple({:?}, {:?})", self, tys);
 394
 395         let fields = tys
 396             .iter()
 397             .enumerate()
 398             .map(|(i, &ty)| {
 399                 (
 400                     self.tcx().mk_place_field(self.place, Field::new(i), ty),
 401                     self.elaborator.field_subpath(self.path, Field::new(i)),
 402                 )
 403             })
 404             .collect();
 405
 406         let (succ, unwind) = self.drop_ladder_bottom();
 407         self.drop_ladder(fields, succ, unwind).0
 408     }
 409
 410     fn open_drop_for_box(&mut self, adt: ty::AdtDef<'tcx>, substs: SubstsRef<'tcx>) -> BasicBlock {
 411         debug!("open_drop_for_box({:?}, {:?}, {:?})", self, adt, substs);
 412
 413         // drop glue is sent straight to codegen
 414         // box cannot be directly dereferenced
 415         let unique_ty = adt.non_enum_variant().fields[0].ty(self.tcx(), substs);
 416         let nonnull_ty =
 417             unique_ty.ty_adt_def().unwrap().non_enum_variant().fields[0].ty(self.tcx(), substs);
 418         let ptr_ty = self.tcx().mk_imm_ptr(substs[0].expect_ty());
 419
 420         let unique_place = self.tcx().mk_place_field(self.place, Field::new(0), unique_ty);
 421         let nonnull_place = self.tcx().mk_place_field(unique_place, Field::new(0), nonnull_ty);
 422         let ptr_place = self.tcx().mk_place_field(nonnull_place, Field::new(0), ptr_ty);
 423         let interior = self.tcx().mk_place_deref(ptr_place);
 424
 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: ty::AdtDef<'tcx>, 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: ty::AdtDef<'tcx>,
 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.variant(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: ty::AdtDef<'tcx>,
 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.variant(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.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: ty::AdtDef<'tcx>,
 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                     targets: SwitchTargets::new(
 602                         values.iter().copied().zip(blocks.iter().copied()),
 603                         *blocks.last().unwrap(),
 604                     ),
 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_item_def_ids(drop_trait)[0];
 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(tcx, drop_fn, substs, self.source_info.span),
 638                     args: vec![Operand::Move(Place::from(ref_place))],
 639                     destination: unit_temp,
 640                     target: Some(succ),
 641                     cleanup: unwind.into_option(),
 642                     from_hir_call: true,
 643                     fn_span: self.source_info.span,
 644                 },
 645                 source_info: self.source_info,
 646             }),
 647             is_cleanup: unwind.is_cleanup(),
 648         };
 649         self.elaborator.patch().new_block(result)
 650     }
 651
 652     /// Create a loop that drops an array:
 653     ///
 654     /// ```text
 655     /// loop-block:
 656     ///    can_go = cur == length_or_end
 657     ///    if can_go then succ else drop-block
 658     /// drop-block:
 659     ///    if ptr_based {
 660     ///        ptr = cur
 661     ///        cur = cur.offset(1)
 662     ///    } else {
 663     ///        ptr = &raw mut P[cur]
 664     ///        cur = cur + 1
 665     ///    }
 666     ///    drop(ptr)
 667     /// ```
 668     fn drop_loop(
 669         &mut self,
 670         succ: BasicBlock,
 671         cur: Local,
 672         length_or_end: Place<'tcx>,
 673         ety: Ty<'tcx>,
 674         unwind: Unwind,
 675         ptr_based: bool,
 676     ) -> BasicBlock {
 677         let copy = |place: Place<'tcx>| Operand::Copy(place);
 678         let move_ = |place: Place<'tcx>| Operand::Move(place);
 679         let tcx = self.tcx();
 680
 681         let ptr_ty = tcx.mk_ptr(ty::TypeAndMut { ty: ety, mutbl: hir::Mutability::Mut });
 682         let ptr = Place::from(self.new_temp(ptr_ty));
 683         let can_go = Place::from(self.new_temp(tcx.types.bool));
 684
 685         let one = self.constant_usize(1);
 686         let (ptr_next, cur_next) = if ptr_based {
 687             (
 688                 Rvalue::Use(copy(cur.into())),
 689                 Rvalue::BinaryOp(BinOp::Offset, Box::new((move_(cur.into()), one))),
 690             )
 691         } else {
 692             (
 693                 Rvalue::AddressOf(Mutability::Mut, tcx.mk_place_index(self.place, cur)),
 694                 Rvalue::BinaryOp(BinOp::Add, Box::new((move_(cur.into()), one))),
 695             )
 696         };
 697
 698         let drop_block = BasicBlockData {
 699             statements: vec![self.assign(ptr, ptr_next), self.assign(Place::from(cur), cur_next)],
 700             is_cleanup: unwind.is_cleanup(),
 701             terminator: Some(Terminator {
 702                 source_info: self.source_info,
 703                 // this gets overwritten by drop elaboration.
 704                 kind: TerminatorKind::Unreachable,
 705             }),
 706         };
 707         let drop_block = self.elaborator.patch().new_block(drop_block);
 708
 709         let loop_block = BasicBlockData {
 710             statements: vec![self.assign(
 711                 can_go,
 712                 Rvalue::BinaryOp(
 713                     BinOp::Eq,
 714                     Box::new((copy(Place::from(cur)), copy(length_or_end))),
 715                 ),
 716             )],
 717             is_cleanup: unwind.is_cleanup(),
 718             terminator: Some(Terminator {
 719                 source_info: self.source_info,
 720                 kind: TerminatorKind::if_(tcx, move_(can_go), succ, drop_block),
 721             }),
 722         };
 723         let loop_block = self.elaborator.patch().new_block(loop_block);
 724
 725         self.elaborator.patch().patch_terminator(
 726             drop_block,
 727             TerminatorKind::Drop {
 728                 place: tcx.mk_place_deref(ptr),
 729                 target: loop_block,
 730                 unwind: unwind.into_option(),
 731             },
 732         );
 733
 734         loop_block
 735     }
 736
 737     fn open_drop_for_array(&mut self, ety: Ty<'tcx>, opt_size: Option<u64>) -> BasicBlock {
 738         debug!("open_drop_for_array({:?}, {:?})", ety, opt_size);
 739
 740         // if size_of::<ety>() == 0 {
 741         //     index_based_loop
 742         // } else {
 743         //     ptr_based_loop
 744         // }
 745
 746         let tcx = self.tcx();
 747
 748         if let Some(size) = opt_size {
 749             let fields: Vec<(Place<'tcx>, Option<D::Path>)> = (0..size)
 750                 .map(|i| {
 751                     (
 752                         tcx.mk_place_elem(
 753                             self.place,
 754                             ProjectionElem::ConstantIndex {
 755                                 offset: i,
 756                                 min_length: size,
 757                                 from_end: false,
 758                             },
 759                         ),
 760                         self.elaborator.array_subpath(self.path, i, size),
 761                     )
 762                 })
 763                 .collect();
 764
 765             if fields.iter().any(|(_, path)| path.is_some()) {
 766                 let (succ, unwind) = self.drop_ladder_bottom();
 767                 return self.drop_ladder(fields, succ, unwind).0;
 768             }
 769         }
 770
 771         let move_ = |place: Place<'tcx>| Operand::Move(place);
 772         let elem_size = Place::from(self.new_temp(tcx.types.usize));
 773         let len = Place::from(self.new_temp(tcx.types.usize));
 774
 775         let base_block = BasicBlockData {
 776             statements: vec![
 777                 self.assign(elem_size, Rvalue::NullaryOp(NullOp::SizeOf, ety)),
 778                 self.assign(len, Rvalue::Len(self.place)),
 779             ],
 780             is_cleanup: self.unwind.is_cleanup(),
 781             terminator: Some(Terminator {
 782                 source_info: self.source_info,
 783                 kind: TerminatorKind::SwitchInt {
 784                     discr: move_(elem_size),
 785                     switch_ty: tcx.types.usize,
 786                     targets: SwitchTargets::static_if(
 787                         0,
 788                         self.drop_loop_pair(ety, false, len),
 789                         self.drop_loop_pair(ety, true, len),
 790                     ),
 791                 },
 792             }),
 793         };
 794         self.elaborator.patch().new_block(base_block)
 795     }
 796
 797     /// Creates a pair of drop-loops of `place`, which drops its contents, even
 798     /// in the case of 1 panic. If `ptr_based`, creates a pointer loop,
 799     /// otherwise create an index loop.
 800     fn drop_loop_pair(
 801         &mut self,
 802         ety: Ty<'tcx>,
 803         ptr_based: bool,
 804         length: Place<'tcx>,
 805     ) -> BasicBlock {
 806         debug!("drop_loop_pair({:?}, {:?})", ety, ptr_based);
 807         let tcx = self.tcx();
 808         let iter_ty = if ptr_based { tcx.mk_mut_ptr(ety) } else { tcx.types.usize };
 809
 810         let cur = self.new_temp(iter_ty);
 811         let length_or_end = if ptr_based { Place::from(self.new_temp(iter_ty)) } else { length };
 812
 813         let unwind = self.unwind.map(|unwind| {
 814             self.drop_loop(unwind, cur, length_or_end, ety, Unwind::InCleanup, ptr_based)
 815         });
 816
 817         let loop_block = self.drop_loop(self.succ, cur, length_or_end, ety, unwind, ptr_based);
 818
 819         let cur = Place::from(cur);
 820         let drop_block_stmts = if ptr_based {
 821             let tmp_ty = tcx.mk_mut_ptr(self.place_ty(self.place));
 822             let tmp = Place::from(self.new_temp(tmp_ty));
 823             // tmp = &raw mut P;
 824             // cur = tmp as *mut T;
 825             // end = Offset(cur, len);
 826             let mir_cast_kind = ty::cast::mir_cast_kind(iter_ty, tmp_ty);
 827             vec![
 828                 self.assign(tmp, Rvalue::AddressOf(Mutability::Mut, self.place)),
 829                 self.assign(cur, Rvalue::Cast(mir_cast_kind, Operand::Move(tmp), iter_ty)),
 830                 self.assign(
 831                     length_or_end,
 832                     Rvalue::BinaryOp(
 833                         BinOp::Offset,
 834                         Box::new((Operand::Copy(cur), Operand::Move(length))),
 835                     ),
 836                 ),
 837             ]
 838         } else {
 839             // cur = 0 (length already pushed)
 840             let zero = self.constant_usize(0);
 841             vec![self.assign(cur, Rvalue::Use(zero))]
 842         };
 843         let drop_block = self.elaborator.patch().new_block(BasicBlockData {
 844             statements: drop_block_stmts,
 845             is_cleanup: unwind.is_cleanup(),
 846             terminator: Some(Terminator {
 847                 source_info: self.source_info,
 848                 kind: TerminatorKind::Goto { target: loop_block },
 849             }),
 850         });
 851
 852         // FIXME(#34708): handle partially-dropped array/slice elements.
 853         let reset_block = self.drop_flag_reset_block(DropFlagMode::Deep, drop_block, unwind);
 854         self.drop_flag_test_block(reset_block, self.succ, unwind)
 855     }
 856
 857     /// The slow-path - create an "open", elaborated drop for a type
 858     /// which is moved-out-of only partially, and patch `bb` to a jump
 859     /// to it. This must not be called on ADTs with a destructor,
 860     /// as these can't be moved-out-of, except for `Box<T>`, which is
 861     /// special-cased.
 862     ///
 863     /// This creates a "drop ladder" that drops the needed fields of the
 864     /// ADT, both in the success case or if one of the destructors fail.
 865     fn open_drop(&mut self) -> BasicBlock {
 866         let ty = self.place_ty(self.place);
 867         match ty.kind() {
 868             ty::Closure(_, substs) => {
 869                 let tys: Vec<_> = substs.as_closure().upvar_tys().collect();
 870                 self.open_drop_for_tuple(&tys)
 871             }
 872             // Note that `elaborate_drops` only drops the upvars of a generator,
 873             // and this is ok because `open_drop` here can only be reached
 874             // within that own generator's resume function.
 875             // This should only happen for the self argument on the resume function.
 876             // It effectively only contains upvars until the generator transformation runs.
 877             // See librustc_body/transform/generator.rs for more details.
 878             ty::Generator(_, substs, _) => {
 879                 let tys: Vec<_> = substs.as_generator().upvar_tys().collect();
 880                 self.open_drop_for_tuple(&tys)
 881             }
 882             ty::Tuple(fields) => self.open_drop_for_tuple(fields),
 883             ty::Adt(def, substs) => {
 884                 if def.is_box() {
 885                     self.open_drop_for_box(*def, substs)
 886                 } else {
 887                     self.open_drop_for_adt(*def, substs)
 888                 }
 889             }
 890             ty::Dynamic(..) => self.complete_drop(self.succ, self.unwind),
 891             ty::Array(ety, size) => {
 892                 let size = size.try_eval_usize(self.tcx(), self.elaborator.param_env());
 893                 self.open_drop_for_array(*ety, size)
 894             }
 895             ty::Slice(ety) => self.open_drop_for_array(*ety, None),
 896
 897             _ => span_bug!(self.source_info.span, "open drop from non-ADT `{:?}`", ty),
 898         }
 899     }
 900
 901     fn complete_drop(&mut self, succ: BasicBlock, unwind: Unwind) -> BasicBlock {
 902         debug!("complete_drop(succ={:?}, unwind={:?})", succ, unwind);
 903
 904         let drop_block = self.drop_block(succ, unwind);
 905
 906         self.drop_flag_test_block(drop_block, succ, unwind)
 907     }
 908
 909     /// Creates a block that resets the drop flag. If `mode` is deep, all children drop flags will
 910     /// also be cleared.
 911     fn drop_flag_reset_block(
 912         &mut self,
 913         mode: DropFlagMode,
 914         succ: BasicBlock,
 915         unwind: Unwind,
 916     ) -> BasicBlock {
 917         debug!("drop_flag_reset_block({:?},{:?})", self, mode);
 918
 919         if unwind.is_cleanup() {
 920             // The drop flag isn't read again on the unwind path, so don't
 921             // bother setting it.
 922             return succ;
 923         }
 924         let block = self.new_block(unwind, TerminatorKind::Goto { target: succ });
 925         let block_start = Location { block, statement_index: 0 };
 926         self.elaborator.clear_drop_flag(block_start, self.path, mode);
 927         block
 928     }
 929
 930     fn elaborated_drop_block(&mut self) -> BasicBlock {
 931         debug!("elaborated_drop_block({:?})", self);
 932         let blk = self.drop_block(self.succ, self.unwind);
 933         self.elaborate_drop(blk);
 934         blk
 935     }
 936
 937     /// Creates a block that frees the backing memory of a `Box` if its drop is required (either
 938     /// statically or by checking its drop flag).
 939     ///
 940     /// The contained value will not be dropped.
 941     fn box_free_block(
 942         &mut self,
 943         adt: ty::AdtDef<'tcx>,
 944         substs: SubstsRef<'tcx>,
 945         target: BasicBlock,
 946         unwind: Unwind,
 947     ) -> BasicBlock {
 948         let block = self.unelaborated_free_block(adt, substs, target, unwind);
 949         self.drop_flag_test_block(block, target, unwind)
 950     }
 951
 952     /// Creates a block that frees the backing memory of a `Box` (without dropping the contained
 953     /// value).
 954     fn unelaborated_free_block(
 955         &mut self,
 956         adt: ty::AdtDef<'tcx>,
 957         substs: SubstsRef<'tcx>,
 958         target: BasicBlock,
 959         unwind: Unwind,
 960     ) -> BasicBlock {
 961         let tcx = self.tcx();
 962         let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
 963         let free_func = tcx.require_lang_item(LangItem::BoxFree, Some(self.source_info.span));
 964         let args = adt
 965             .variant(VariantIdx::new(0))
 966             .fields
 967             .iter()
 968             .enumerate()
 969             .map(|(i, f)| {
 970                 let field = Field::new(i);
 971                 let field_ty = f.ty(tcx, substs);
 972                 Operand::Move(tcx.mk_place_field(self.place, field, field_ty))
 973             })
 974             .collect();
 975
 976         let call = TerminatorKind::Call {
 977             func: Operand::function_handle(tcx, free_func, substs, self.source_info.span),
 978             args,
 979             destination: unit_temp,
 980             target: Some(target),
 981             cleanup: None,
 982             from_hir_call: false,
 983             fn_span: self.source_info.span,
 984         }; // FIXME(#43234)
 985         let free_block = self.new_block(unwind, call);
 986
 987         let block_start = Location { block: free_block, statement_index: 0 };
 988         self.elaborator.clear_drop_flag(block_start, self.path, DropFlagMode::Shallow);
 989         free_block
 990     }
 991
 992     fn drop_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
 993         let block =
 994             TerminatorKind::Drop { place: self.place, target, unwind: unwind.into_option() };
 995         self.new_block(unwind, block)
 996     }
 997
 998     fn goto_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
 999         let block = TerminatorKind::Goto { target };
1000         self.new_block(unwind, block)
1001     }
1002
1003     /// Returns the block to jump to in order to test the drop flag and execute the drop.
1004     ///
1005     /// Depending on the required `DropStyle`, this might be a generated block with an `if`
1006     /// terminator (for dynamic/open drops), or it might be `on_set` or `on_unset` itself, in case
1007     /// the drop can be statically determined.
1008     fn drop_flag_test_block(
1009         &mut self,
1010         on_set: BasicBlock,
1011         on_unset: BasicBlock,
1012         unwind: Unwind,
1013     ) -> BasicBlock {
1014         let style = self.elaborator.drop_style(self.path, DropFlagMode::Shallow);
1015         debug!(
1016             "drop_flag_test_block({:?},{:?},{:?},{:?}) - {:?}",
1017             self, on_set, on_unset, unwind, style
1018         );
1019
1020         match style {
1021             DropStyle::Dead => on_unset,
1022             DropStyle::Static => on_set,
1023             DropStyle::Conditional | DropStyle::Open => {
1024                 let flag = self.elaborator.get_drop_flag(self.path).unwrap();
1025                 let term = TerminatorKind::if_(self.tcx(), flag, on_set, on_unset);
1026                 self.new_block(unwind, term)
1027             }
1028         }
1029     }
1030
1031     fn new_block(&mut self, unwind: Unwind, k: TerminatorKind<'tcx>) -> BasicBlock {
1032         self.elaborator.patch().new_block(BasicBlockData {
1033             statements: vec![],
1034             terminator: Some(Terminator { source_info: self.source_info, kind: k }),
1035             is_cleanup: unwind.is_cleanup(),
1036         })
1037     }
1038
1039     fn new_temp(&mut self, ty: Ty<'tcx>) -> Local {
1040         self.elaborator.patch().new_temp(ty, self.source_info.span)
1041     }
1042
1043     fn constant_usize(&self, val: u16) -> Operand<'tcx> {
1044         Operand::Constant(Box::new(Constant {
1045             span: self.source_info.span,
1046             user_ty: None,
1047             literal: ConstantKind::from_usize(self.tcx(), val.into()),
1048         }))
1049     }
1050
1051     fn assign(&self, lhs: Place<'tcx>, rhs: Rvalue<'tcx>) -> Statement<'tcx> {
1052         Statement {
1053             source_info: self.source_info,
1054             kind: StatementKind::Assign(Box::new((lhs, rhs))),
1055         }
1056     }
1057 }