src/librustc_mir/util/elaborate_drops.rs

   1 // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use std::fmt;
  12 use rustc::mir::*;
  13 use rustc::middle::lang_items;
  14 use rustc::ty::{self, Ty};
  15 use rustc::ty::subst::{Kind, Subst, Substs};
  16 use rustc::ty::util::IntTypeExt;
  17 use rustc_data_structures::indexed_vec::Idx;
  18 use util::patch::MirPatch;
  19
  20 use std::iter;
  21
  22 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
  23 pub enum DropFlagState {
  24     Present, // i.e. initialized
  25     Absent, // i.e. deinitialized or "moved"
  26 }
  27
  28 impl DropFlagState {
  29     pub fn value(self) -> bool {
  30         match self {
  31             DropFlagState::Present => true,
  32             DropFlagState::Absent => false
  33         }
  34     }
  35 }
  36
  37 #[derive(Debug)]
  38 pub enum DropStyle {
  39     Dead,
  40     Static,
  41     Conditional,
  42     Open,
  43 }
  44
  45 #[derive(Debug)]
  46 pub enum DropFlagMode {
  47     Shallow,
  48     Deep
  49 }
  50
  51 pub trait DropElaborator<'a, 'tcx: 'a> : fmt::Debug {
  52     type Path : Copy + fmt::Debug;
  53
  54     fn patch(&mut self) -> &mut MirPatch<'tcx>;
  55     fn mir(&self) -> &'a Mir<'tcx>;
  56     fn tcx(&self) -> ty::TyCtxt<'a, 'tcx, 'tcx>;
  57     fn param_env(&self) -> &'a ty::ParameterEnvironment<'tcx>;
  58
  59     fn drop_style(&self, path: Self::Path, mode: DropFlagMode) -> DropStyle;
  60     fn get_drop_flag(&mut self, path: Self::Path) -> Option<Operand<'tcx>>;
  61     fn clear_drop_flag(&mut self, location: Location, path: Self::Path, mode: DropFlagMode);
  62
  63
  64     fn field_subpath(&self, path: Self::Path, field: Field) -> Option<Self::Path>;
  65     fn deref_subpath(&self, path: Self::Path) -> Option<Self::Path>;
  66     fn downcast_subpath(&self, path: Self::Path, variant: usize) -> Option<Self::Path>;
  67 }
  68
  69 #[derive(Debug)]
  70 struct DropCtxt<'l, 'b: 'l, 'tcx: 'b, D>
  71     where D : DropElaborator<'b, 'tcx> + 'l
  72 {
  73     elaborator: &'l mut D,
  74
  75     source_info: SourceInfo,
  76     is_cleanup: bool,
  77
  78     lvalue: &'l Lvalue<'tcx>,
  79     path: D::Path,
  80     succ: BasicBlock,
  81     unwind: Option<BasicBlock>,
  82 }
  83
  84 pub fn elaborate_drop<'b, 'tcx, D>(
  85     elaborator: &mut D,
  86     source_info: SourceInfo,
  87     is_cleanup: bool,
  88     lvalue: &Lvalue<'tcx>,
  89     path: D::Path,
  90     succ: BasicBlock,
  91     unwind: Option<BasicBlock>,
  92     bb: BasicBlock)
  93     where D: DropElaborator<'b, 'tcx>
  94 {
  95     DropCtxt {
  96         elaborator, source_info, is_cleanup, lvalue, path, succ, unwind
  97     }.elaborate_drop(bb)
  98 }
  99
 100 impl<'l, 'b, 'tcx, D> DropCtxt<'l, 'b, 'tcx, D>
 101     where D: DropElaborator<'b, 'tcx>
 102 {
 103     fn lvalue_ty(&self, lvalue: &Lvalue<'tcx>) -> Ty<'tcx> {
 104         lvalue.ty(self.elaborator.mir(), self.tcx()).to_ty(self.tcx())
 105     }
 106
 107     fn tcx(&self) -> ty::TyCtxt<'b, 'tcx, 'tcx> {
 108         self.elaborator.tcx()
 109     }
 110
 111     /// This elaborates a single drop instruction, located at `bb`, and
 112     /// patches over it.
 113     ///
 114     /// The elaborated drop checks the drop flags to only drop what
 115     /// is initialized.
 116     ///
 117     /// In addition, the relevant drop flags also need to be cleared
 118     /// to avoid double-drops. However, in the middle of a complex
 119     /// drop, one must avoid clearing some of the flags before they
 120     /// are read, as that would cause a memory leak.
 121     ///
 122     /// In particular, when dropping an ADT, multiple fields may be
 123     /// joined together under the `rest` subpath. They are all controlled
 124     /// by the primary drop flag, but only the last rest-field dropped
 125     /// should clear it (and it must also not clear anything else).
 126     ///
 127     /// FIXME: I think we should just control the flags externally
 128     /// and then we do not need this machinery.
 129     pub fn elaborate_drop<'a>(&mut self, bb: BasicBlock) {
 130         debug!("elaborate_drop({:?})", self);
 131         let style = self.elaborator.drop_style(self.path, DropFlagMode::Deep);
 132         debug!("elaborate_drop({:?}): live - {:?}", self, style);
 133         match style {
 134             DropStyle::Dead => {
 135                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Goto {
 136                     target: self.succ
 137                 });
 138             }
 139             DropStyle::Static => {
 140                 let loc = self.terminator_loc(bb);
 141                 self.elaborator.clear_drop_flag(loc, self.path, DropFlagMode::Deep);
 142                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Drop {
 143                     location: self.lvalue.clone(),
 144                     target: self.succ,
 145                     unwind: self.unwind
 146                 });
 147             }
 148             DropStyle::Conditional => {
 149                 let drop_bb = self.complete_drop(Some(DropFlagMode::Deep));
 150                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Goto {
 151                     target: drop_bb
 152                 });
 153             }
 154             DropStyle::Open => {
 155                 let drop_bb = self.open_drop();
 156                 self.elaborator.patch().patch_terminator(bb, TerminatorKind::Goto {
 157                     target: drop_bb
 158                 });
 159             }
 160         }
 161     }
 162
 163     /// Return the lvalue and move path for each field of `variant`,
 164     /// (the move path is `None` if the field is a rest field).
 165     fn move_paths_for_fields(&self,
 166                              base_lv: &Lvalue<'tcx>,
 167                              variant_path: D::Path,
 168                              variant: &'tcx ty::VariantDef,
 169                              substs: &'tcx Substs<'tcx>)
 170                              -> Vec<(Lvalue<'tcx>, Option<D::Path>)>
 171     {
 172         variant.fields.iter().enumerate().map(|(i, f)| {
 173             let field = Field::new(i);
 174             let subpath = self.elaborator.field_subpath(variant_path, field);
 175
 176             let field_ty =
 177                 self.tcx().normalize_associated_type_in_env(
 178                     &f.ty(self.tcx(), substs),
 179                     self.elaborator.param_env()
 180                 );
 181             (base_lv.clone().field(field, field_ty), subpath)
 182         }).collect()
 183     }
 184
 185     fn drop_subpath(&mut self,
 186                     is_cleanup: bool,
 187                     lvalue: &Lvalue<'tcx>,
 188                     path: Option<D::Path>,
 189                     succ: BasicBlock,
 190                     unwind: Option<BasicBlock>)
 191                     -> BasicBlock
 192     {
 193         if let Some(path) = path {
 194             debug!("drop_subpath: for std field {:?}", lvalue);
 195
 196             DropCtxt {
 197                 elaborator: self.elaborator,
 198                 source_info: self.source_info,
 199                 path, lvalue, succ, unwind, is_cleanup
 200             }.elaborated_drop_block()
 201         } else {
 202             debug!("drop_subpath: for rest field {:?}", lvalue);
 203
 204             DropCtxt {
 205                 elaborator: self.elaborator,
 206                 source_info: self.source_info,
 207                 lvalue, succ, unwind, is_cleanup,
 208                 // Using `self.path` here to condition the drop on
 209                 // our own drop flag.
 210                 path: self.path
 211             }.complete_drop(None)
 212         }
 213     }
 214
 215     /// Create one-half of the drop ladder for a list of fields, and return
 216     /// the list of steps in it in reverse order.
 217     ///
 218     /// `unwind_ladder` is such a list of steps in reverse order,
 219     /// which is called instead of the next step if the drop unwinds
 220     /// (the first field is never reached). If it is `None`, all
 221     /// unwind targets are left blank.
 222     fn drop_halfladder<'a>(&mut self,
 223                            unwind_ladder: Option<Vec<BasicBlock>>,
 224                            succ: BasicBlock,
 225                            fields: &[(Lvalue<'tcx>, Option<D::Path>)],
 226                            is_cleanup: bool)
 227                            -> Vec<BasicBlock>
 228     {
 229         let mut unwind_succ = if is_cleanup {
 230             None
 231         } else {
 232             self.unwind
 233         };
 234
 235         let goto = TerminatorKind::Goto { target: succ };
 236         let mut succ = self.new_block(is_cleanup, goto);
 237
 238         // Always clear the "master" drop flag at the bottom of the
 239         // ladder. This is needed because the "master" drop flag
 240         // protects the ADT's discriminant, which is invalidated
 241         // after the ADT is dropped.
 242         let succ_loc = Location { block: succ, statement_index: 0 };
 243         self.elaborator.clear_drop_flag(succ_loc, self.path, DropFlagMode::Shallow);
 244
 245         fields.iter().rev().enumerate().map(|(i, &(ref lv, path))| {
 246             succ = self.drop_subpath(is_cleanup, lv, path, succ, unwind_succ);
 247             unwind_succ = unwind_ladder.as_ref().map(|p| p[i]);
 248             succ
 249         }).collect()
 250     }
 251
 252     /// Create a full drop ladder, consisting of 2 connected half-drop-ladders
 253     ///
 254     /// For example, with 3 fields, the drop ladder is
 255     ///
 256     /// .d0:
 257     ///     ELAB(drop location.0 [target=.d1, unwind=.c1])
 258     /// .d1:
 259     ///     ELAB(drop location.1 [target=.d2, unwind=.c2])
 260     /// .d2:
 261     ///     ELAB(drop location.2 [target=`self.succ`, unwind=`self.unwind`])
 262     /// .c1:
 263     ///     ELAB(drop location.1 [target=.c2])
 264     /// .c2:
 265     ///     ELAB(drop location.2 [target=`self.unwind])
 266     fn drop_ladder<'a>(&mut self,
 267                        fields: Vec<(Lvalue<'tcx>, Option<D::Path>)>)
 268                        -> BasicBlock
 269     {
 270         debug!("drop_ladder({:?}, {:?})", self, fields);
 271
 272         let mut fields = fields;
 273         fields.retain(|&(ref lvalue, _)| {
 274             self.tcx().type_needs_drop_given_env(
 275                 self.lvalue_ty(lvalue), self.elaborator.param_env())
 276         });
 277
 278         debug!("drop_ladder - fields needing drop: {:?}", fields);
 279
 280         let unwind_ladder = if self.is_cleanup {
 281             None
 282         } else {
 283             let unwind = self.unwind.unwrap(); // FIXME(#6393)
 284             Some(self.drop_halfladder(None, unwind, &fields, true))
 285         };
 286
 287         let succ = self.succ; // FIXME(#6393)
 288         let is_cleanup = self.is_cleanup;
 289         self.drop_halfladder(unwind_ladder, succ, &fields, is_cleanup)
 290             .last().cloned().unwrap_or(succ)
 291     }
 292
 293     fn open_drop_for_tuple<'a>(&mut self, tys: &[Ty<'tcx>])
 294                                -> BasicBlock
 295     {
 296         debug!("open_drop_for_tuple({:?}, {:?})", self, tys);
 297
 298         let fields = tys.iter().enumerate().map(|(i, &ty)| {
 299             (self.lvalue.clone().field(Field::new(i), ty),
 300              self.elaborator.field_subpath(self.path, Field::new(i)))
 301         }).collect();
 302
 303         self.drop_ladder(fields)
 304     }
 305
 306     fn open_drop_for_box<'a>(&mut self, ty: Ty<'tcx>) -> BasicBlock
 307     {
 308         debug!("open_drop_for_box({:?}, {:?})", self, ty);
 309
 310         let interior = self.lvalue.clone().deref();
 311         let interior_path = self.elaborator.deref_subpath(self.path);
 312
 313         let succ = self.succ; // FIXME(#6393)
 314         let is_cleanup = self.is_cleanup;
 315         let succ = self.box_free_block(ty, succ, is_cleanup);
 316         let unwind_succ = self.unwind.map(|u| {
 317             self.box_free_block(ty, u, true)
 318         });
 319
 320         self.drop_subpath(is_cleanup, &interior, interior_path, succ, unwind_succ)
 321     }
 322
 323     fn open_drop_for_adt<'a>(&mut self, adt: &'tcx ty::AdtDef, substs: &'tcx Substs<'tcx>)
 324                              -> BasicBlock {
 325         debug!("open_drop_for_adt({:?}, {:?}, {:?})", self, adt, substs);
 326
 327         match adt.variants.len() {
 328             1 => {
 329                 let fields = self.move_paths_for_fields(
 330                     self.lvalue,
 331                     self.path,
 332                     &adt.variants[0],
 333                     substs
 334                 );
 335                 self.drop_ladder(fields)
 336             }
 337             _ => {
 338                 let mut values = Vec::with_capacity(adt.variants.len());
 339                 let mut blocks = Vec::with_capacity(adt.variants.len());
 340                 let mut otherwise = None;
 341                 for (variant_index, discr) in adt.discriminants(self.tcx()).enumerate() {
 342                     let subpath = self.elaborator.downcast_subpath(
 343                         self.path, variant_index);
 344                     if let Some(variant_path) = subpath {
 345                         let base_lv = self.lvalue.clone().elem(
 346                             ProjectionElem::Downcast(adt, variant_index)
 347                         );
 348                         let fields = self.move_paths_for_fields(
 349                             &base_lv,
 350                             variant_path,
 351                             &adt.variants[variant_index],
 352                             substs);
 353                         values.push(discr);
 354                         blocks.push(self.drop_ladder(fields));
 355                     } else {
 356                         // variant not found - drop the entire enum
 357                         if let None = otherwise {
 358                             otherwise =
 359                                 Some(self.complete_drop(Some(DropFlagMode::Shallow)));
 360                         }
 361                     }
 362                 }
 363                 if let Some(block) = otherwise {
 364                     blocks.push(block);
 365                 } else {
 366                     values.pop();
 367                 }
 368                 // If there are multiple variants, then if something
 369                 // is present within the enum the discriminant, tracked
 370                 // by the rest path, must be initialized.
 371                 //
 372                 // Additionally, we do not want to switch on the
 373                 // discriminant after it is free-ed, because that
 374                 // way lies only trouble.
 375                 let discr_ty = adt.repr.discr_type().to_ty(self.tcx());
 376                 let discr = Lvalue::Local(self.new_temp(discr_ty));
 377                 let discr_rv = Rvalue::Discriminant(self.lvalue.clone());
 378                 let switch_block = self.elaborator.patch().new_block(BasicBlockData {
 379                     statements: vec![
 380                         Statement {
 381                             source_info: self.source_info,
 382                             kind: StatementKind::Assign(discr.clone(), discr_rv),
 383                         }
 384                     ],
 385                     terminator: Some(Terminator {
 386                         source_info: self.source_info,
 387                         kind: TerminatorKind::SwitchInt {
 388                             discr: Operand::Consume(discr),
 389                             switch_ty: discr_ty,
 390                             values: From::from(values),
 391                             targets: blocks,
 392                         }
 393                     }),
 394                     is_cleanup: self.is_cleanup,
 395                 });
 396                 self.drop_flag_test_block(switch_block)
 397             }
 398         }
 399     }
 400
 401     /// The slow-path - create an "open", elaborated drop for a type
 402     /// which is moved-out-of only partially, and patch `bb` to a jump
 403     /// to it. This must not be called on ADTs with a destructor,
 404     /// as these can't be moved-out-of, except for `Box<T>`, which is
 405     /// special-cased.
 406     ///
 407     /// This creates a "drop ladder" that drops the needed fields of the
 408     /// ADT, both in the success case or if one of the destructors fail.
 409     fn open_drop<'a>(&mut self) -> BasicBlock {
 410         let ty = self.lvalue_ty(self.lvalue);
 411         match ty.sty {
 412             ty::TyClosure(def_id, substs) => {
 413                 let tys : Vec<_> = substs.upvar_tys(def_id, self.tcx()).collect();
 414                 self.open_drop_for_tuple(&tys)
 415             }
 416             ty::TyTuple(tys, _) => {
 417                 self.open_drop_for_tuple(tys)
 418             }
 419             ty::TyAdt(def, _) if def.is_box() => {
 420                 self.open_drop_for_box(ty.boxed_ty())
 421             }
 422             ty::TyAdt(def, substs) => {
 423                 self.open_drop_for_adt(def, substs)
 424             }
 425             _ => bug!("open drop from non-ADT `{:?}`", ty)
 426         }
 427     }
 428
 429     /// Return a basic block that drop an lvalue using the context
 430     /// and path in `c`. If `mode` is something, also clear `c`
 431     /// according to it.
 432     ///
 433     /// if FLAG(self.path)
 434     ///     if let Some(mode) = mode: FLAG(self.path)[mode] = false
 435     ///     drop(self.lv)
 436     fn complete_drop<'a>(&mut self, drop_mode: Option<DropFlagMode>) -> BasicBlock
 437     {
 438         debug!("complete_drop({:?},{:?})", self, drop_mode);
 439
 440         let drop_block = self.drop_block();
 441         if let Some(mode) = drop_mode {
 442             let block_start = Location { block: drop_block, statement_index: 0 };
 443             self.elaborator.clear_drop_flag(block_start, self.path, mode);
 444         }
 445
 446         self.drop_flag_test_block(drop_block)
 447     }
 448
 449     fn elaborated_drop_block<'a>(&mut self) -> BasicBlock {
 450         debug!("elaborated_drop_block({:?})", self);
 451         let blk = self.drop_block();
 452         self.elaborate_drop(blk);
 453         blk
 454     }
 455
 456     fn box_free_block<'a>(
 457         &mut self,
 458         ty: Ty<'tcx>,
 459         target: BasicBlock,
 460         is_cleanup: bool
 461     ) -> BasicBlock {
 462         let block = self.unelaborated_free_block(ty, target, is_cleanup);
 463         self.drop_flag_test_block_with_succ(is_cleanup, block, target)
 464     }
 465
 466     fn unelaborated_free_block<'a>(
 467         &mut self,
 468         ty: Ty<'tcx>,
 469         target: BasicBlock,
 470         is_cleanup: bool
 471     ) -> BasicBlock {
 472         let tcx = self.tcx();
 473         let unit_temp = Lvalue::Local(self.new_temp(tcx.mk_nil()));
 474         let free_func = tcx.require_lang_item(lang_items::BoxFreeFnLangItem);
 475         let substs = tcx.mk_substs(iter::once(Kind::from(ty)));
 476         let fty = tcx.item_type(free_func).subst(tcx, substs);
 477
 478         let free_block = self.elaborator.patch().new_block(BasicBlockData {
 479             statements: vec![],
 480             terminator: Some(Terminator {
 481                 source_info: self.source_info, kind: TerminatorKind::Call {
 482                     func: Operand::Constant(Constant {
 483                         span: self.source_info.span,
 484                         ty: fty,
 485                         literal: Literal::Item {
 486                             def_id: free_func,
 487                             substs: substs
 488                         }
 489                     }),
 490                     args: vec![Operand::Consume(self.lvalue.clone())],
 491                     destination: Some((unit_temp, target)),
 492                     cleanup: None
 493                 }
 494             }),
 495             is_cleanup: is_cleanup
 496         });
 497         let block_start = Location { block: free_block, statement_index: 0 };
 498         self.elaborator.clear_drop_flag(block_start, self.path, DropFlagMode::Shallow);
 499         free_block
 500     }
 501
 502     fn drop_block<'a>(&mut self) -> BasicBlock {
 503         let block = TerminatorKind::Drop {
 504             location: self.lvalue.clone(),
 505             target: self.succ,
 506             unwind: self.unwind
 507         };
 508         let is_cleanup = self.is_cleanup; // FIXME(#6393)
 509         self.new_block(is_cleanup, block)
 510     }
 511
 512     fn drop_flag_test_block<'a>(&mut self, on_set: BasicBlock) -> BasicBlock {
 513         let is_cleanup = self.is_cleanup;
 514         let succ = self.succ; // FIXME(#6393)
 515         self.drop_flag_test_block_with_succ(is_cleanup, on_set, succ)
 516     }
 517
 518     fn drop_flag_test_block_with_succ<'a>(&mut self,
 519                                           is_cleanup: bool,
 520                                           on_set: BasicBlock,
 521                                           on_unset: BasicBlock)
 522                                           -> BasicBlock
 523     {
 524         let style = self.elaborator.drop_style(self.path, DropFlagMode::Shallow);
 525         debug!("drop_flag_test_block({:?},{:?},{:?}) - {:?}",
 526                self, is_cleanup, on_set, style);
 527
 528         match style {
 529             DropStyle::Dead => on_unset,
 530             DropStyle::Static => on_set,
 531             DropStyle::Conditional | DropStyle::Open => {
 532                 let flag = self.elaborator.get_drop_flag(self.path).unwrap();
 533                 let term = TerminatorKind::if_(self.tcx(), flag, on_set, on_unset);
 534                 self.new_block(is_cleanup, term)
 535             }
 536         }
 537     }
 538
 539     fn new_block<'a>(&mut self,
 540                      is_cleanup: bool,
 541                      k: TerminatorKind<'tcx>)
 542                      -> BasicBlock
 543     {
 544         self.elaborator.patch().new_block(BasicBlockData {
 545             statements: vec![],
 546             terminator: Some(Terminator {
 547                 source_info: self.source_info, kind: k
 548             }),
 549             is_cleanup: is_cleanup
 550         })
 551     }
 552
 553     fn new_temp(&mut self, ty: Ty<'tcx>) -> Local {
 554         self.elaborator.patch().new_temp(ty)
 555     }
 556
 557     fn terminator_loc(&mut self, bb: BasicBlock) -> Location {
 558         let mir = self.elaborator.mir();
 559         self.elaborator.patch().terminator_loc(mir, bb)
 560     }
 561 }