compiler/rustc_mir_build/src/build/expr/as_place.rs

   1 //! See docs in build/expr/mod.rs
   2
   3 use crate::build::expr::category::Category;
   4 use crate::build::ForGuard::{OutsideGuard, RefWithinGuard};
   5 use crate::build::{BlockAnd, BlockAndExtension, Builder};
   6 use crate::thir::*;
   7 use rustc_middle::middle::region;
   8 use rustc_middle::mir::AssertKind::BoundsCheck;
   9 use rustc_middle::mir::*;
  10 use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty, TyCtxt, Variance};
  11 use rustc_span::Span;
  12
  13 use rustc_index::vec::Idx;
  14
  15 /// `PlaceBuilder` is used to create places during MIR construction. It allows you to "build up" a
  16 /// place by pushing more and more projections onto the end, and then convert the final set into a
  17 /// place using the `into_place` method.
  18 ///
  19 /// This is used internally when building a place for an expression like `a.b.c`. The fields `b`
  20 /// and `c` can be progressively pushed onto the place builder that is created when converting `a`.
  21 #[derive(Clone)]
  22 struct PlaceBuilder<'tcx> {
  23     local: Local,
  24     projection: Vec<PlaceElem<'tcx>>,
  25 }
  26
  27 impl<'tcx> PlaceBuilder<'tcx> {
  28     fn into_place(self, tcx: TyCtxt<'tcx>) -> Place<'tcx> {
  29         Place { local: self.local, projection: tcx.intern_place_elems(&self.projection) }
  30     }
  31
  32     fn field(self, f: Field, ty: Ty<'tcx>) -> Self {
  33         self.project(PlaceElem::Field(f, ty))
  34     }
  35
  36     fn deref(self) -> Self {
  37         self.project(PlaceElem::Deref)
  38     }
  39
  40     fn index(self, index: Local) -> Self {
  41         self.project(PlaceElem::Index(index))
  42     }
  43
  44     fn project(mut self, elem: PlaceElem<'tcx>) -> Self {
  45         self.projection.push(elem);
  46         self
  47     }
  48 }
  49
  50 impl<'tcx> From<Local> for PlaceBuilder<'tcx> {
  51     fn from(local: Local) -> Self {
  52         Self { local, projection: Vec::new() }
  53     }
  54 }
  55
  56 impl<'a, 'tcx> Builder<'a, 'tcx> {
  57     /// Compile `expr`, yielding a place that we can move from etc.
  58     ///
  59     /// WARNING: Any user code might:
  60     /// * Invalidate any slice bounds checks performed.
  61     /// * Change the address that this `Place` refers to.
  62     /// * Modify the memory that this place refers to.
  63     /// * Invalidate the memory that this place refers to, this will be caught
  64     ///   by borrow checking.
  65     ///
  66     /// Extra care is needed if any user code is allowed to run between calling
  67     /// this method and using it, as is the case for `match` and index
  68     /// expressions.
  69     crate fn as_place<M>(&mut self, mut block: BasicBlock, expr: M) -> BlockAnd<Place<'tcx>>
  70     where
  71         M: Mirror<'tcx, Output = Expr<'tcx>>,
  72     {
  73         let place_builder = unpack!(block = self.as_place_builder(block, expr));
  74         block.and(place_builder.into_place(self.hir.tcx()))
  75     }
  76
  77     /// This is used when constructing a compound `Place`, so that we can avoid creating
  78     /// intermediate `Place` values until we know the full set of projections.
  79     fn as_place_builder<M>(&mut self, block: BasicBlock, expr: M) -> BlockAnd<PlaceBuilder<'tcx>>
  80     where
  81         M: Mirror<'tcx, Output = Expr<'tcx>>,
  82     {
  83         let expr = self.hir.mirror(expr);
  84         self.expr_as_place(block, expr, Mutability::Mut, None)
  85     }
  86
  87     /// Compile `expr`, yielding a place that we can move from etc.
  88     /// Mutability note: The caller of this method promises only to read from the resulting
  89     /// place. The place itself may or may not be mutable:
  90     /// * If this expr is a place expr like a.b, then we will return that place.
  91     /// * Otherwise, a temporary is created: in that event, it will be an immutable temporary.
  92     crate fn as_read_only_place<M>(
  93         &mut self,
  94         mut block: BasicBlock,
  95         expr: M,
  96     ) -> BlockAnd<Place<'tcx>>
  97     where
  98         M: Mirror<'tcx, Output = Expr<'tcx>>,
  99     {
 100         let place_builder = unpack!(block = self.as_read_only_place_builder(block, expr));
 101         block.and(place_builder.into_place(self.hir.tcx()))
 102     }
 103
 104     /// This is used when constructing a compound `Place`, so that we can avoid creating
 105     /// intermediate `Place` values until we know the full set of projections.
 106     /// Mutability note: The caller of this method promises only to read from the resulting
 107     /// place. The place itself may or may not be mutable:
 108     /// * If this expr is a place expr like a.b, then we will return that place.
 109     /// * Otherwise, a temporary is created: in that event, it will be an immutable temporary.
 110     fn as_read_only_place_builder<M>(
 111         &mut self,
 112         block: BasicBlock,
 113         expr: M,
 114     ) -> BlockAnd<PlaceBuilder<'tcx>>
 115     where
 116         M: Mirror<'tcx, Output = Expr<'tcx>>,
 117     {
 118         let expr = self.hir.mirror(expr);
 119         self.expr_as_place(block, expr, Mutability::Not, None)
 120     }
 121
 122     fn expr_as_place(
 123         &mut self,
 124         mut block: BasicBlock,
 125         expr: Expr<'tcx>,
 126         mutability: Mutability,
 127         fake_borrow_temps: Option<&mut Vec<Local>>,
 128     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 129         debug!("expr_as_place(block={:?}, expr={:?}, mutability={:?})", block, expr, mutability);
 130
 131         let this = self;
 132         let expr_span = expr.span;
 133         let source_info = this.source_info(expr_span);
 134         match expr.kind {
 135             ExprKind::Scope { region_scope, lint_level, value } => {
 136                 this.in_scope((region_scope, source_info), lint_level, |this| {
 137                     let value = this.hir.mirror(value);
 138                     this.expr_as_place(block, value, mutability, fake_borrow_temps)
 139                 })
 140             }
 141             ExprKind::Field { lhs, name } => {
 142                 let lhs = this.hir.mirror(lhs);
 143                 let place_builder =
 144                     unpack!(block = this.expr_as_place(block, lhs, mutability, fake_borrow_temps,));
 145                 block.and(place_builder.field(name, expr.ty))
 146             }
 147             ExprKind::Deref { arg } => {
 148                 let arg = this.hir.mirror(arg);
 149                 let place_builder =
 150                     unpack!(block = this.expr_as_place(block, arg, mutability, fake_borrow_temps,));
 151                 block.and(place_builder.deref())
 152             }
 153             ExprKind::Index { lhs, index } => this.lower_index_expression(
 154                 block,
 155                 lhs,
 156                 index,
 157                 mutability,
 158                 fake_borrow_temps,
 159                 expr.temp_lifetime,
 160                 expr_span,
 161                 source_info,
 162             ),
 163             ExprKind::UpvarRef { closure_def_id, var_hir_id } => {
 164                 let capture = this
 165                     .hir
 166                     .typeck_results
 167                     .closure_captures
 168                     .get(&closure_def_id)
 169                     .and_then(|captures| captures.get_full(&var_hir_id));
 170
 171                 if capture.is_none() {
 172                     if !this.hir.tcx().features().capture_disjoint_fields {
 173                         bug!(
 174                             "No associated capture found for {:?} even though \
 175                             capture_disjoint_fields isn't enabled",
 176                             expr.kind
 177                         )
 178                     }
 179                     // FIXME(project-rfc-2229#24): Handle this case properly
 180                 }
 181
 182                 // Unwrap until the FIXME has been resolved
 183                 let (capture_index, _, upvar_id) = capture.unwrap();
 184                 this.lower_closure_capture(block, capture_index, *upvar_id)
 185             }
 186
 187             ExprKind::VarRef { id } => {
 188                 let place_builder = if this.is_bound_var_in_guard(id) {
 189                     let index = this.var_local_id(id, RefWithinGuard);
 190                     PlaceBuilder::from(index).deref()
 191                 } else {
 192                     let index = this.var_local_id(id, OutsideGuard);
 193                     PlaceBuilder::from(index)
 194                 };
 195                 block.and(place_builder)
 196             }
 197
 198             ExprKind::PlaceTypeAscription { source, user_ty } => {
 199                 let source = this.hir.mirror(source);
 200                 let place_builder = unpack!(
 201                     block = this.expr_as_place(block, source, mutability, fake_borrow_temps,)
 202                 );
 203                 if let Some(user_ty) = user_ty {
 204                     let annotation_index =
 205                         this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
 206                             span: source_info.span,
 207                             user_ty,
 208                             inferred_ty: expr.ty,
 209                         });
 210
 211                     let place = place_builder.clone().into_place(this.hir.tcx());
 212                     this.cfg.push(
 213                         block,
 214                         Statement {
 215                             source_info,
 216                             kind: StatementKind::AscribeUserType(
 217                                 box (
 218                                     place,
 219                                     UserTypeProjection { base: annotation_index, projs: vec![] },
 220                                 ),
 221                                 Variance::Invariant,
 222                             ),
 223                         },
 224                     );
 225                 }
 226                 block.and(place_builder)
 227             }
 228             ExprKind::ValueTypeAscription { source, user_ty } => {
 229                 let source = this.hir.mirror(source);
 230                 let temp =
 231                     unpack!(block = this.as_temp(block, source.temp_lifetime, source, mutability));
 232                 if let Some(user_ty) = user_ty {
 233                     let annotation_index =
 234                         this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
 235                             span: source_info.span,
 236                             user_ty,
 237                             inferred_ty: expr.ty,
 238                         });
 239                     this.cfg.push(
 240                         block,
 241                         Statement {
 242                             source_info,
 243                             kind: StatementKind::AscribeUserType(
 244                                 box (
 245                                     Place::from(temp),
 246                                     UserTypeProjection { base: annotation_index, projs: vec![] },
 247                                 ),
 248                                 Variance::Invariant,
 249                             ),
 250                         },
 251                     );
 252                 }
 253                 block.and(PlaceBuilder::from(temp))
 254             }
 255
 256             ExprKind::Array { .. }
 257             | ExprKind::Tuple { .. }
 258             | ExprKind::Adt { .. }
 259             | ExprKind::Closure { .. }
 260             | ExprKind::Unary { .. }
 261             | ExprKind::Binary { .. }
 262             | ExprKind::LogicalOp { .. }
 263             | ExprKind::Box { .. }
 264             | ExprKind::Cast { .. }
 265             | ExprKind::Use { .. }
 266             | ExprKind::NeverToAny { .. }
 267             | ExprKind::Pointer { .. }
 268             | ExprKind::Repeat { .. }
 269             | ExprKind::Borrow { .. }
 270             | ExprKind::AddressOf { .. }
 271             | ExprKind::Match { .. }
 272             | ExprKind::Loop { .. }
 273             | ExprKind::Block { .. }
 274             | ExprKind::Assign { .. }
 275             | ExprKind::AssignOp { .. }
 276             | ExprKind::Break { .. }
 277             | ExprKind::Continue { .. }
 278             | ExprKind::Return { .. }
 279             | ExprKind::Literal { .. }
 280             | ExprKind::ConstBlock { .. }
 281             | ExprKind::StaticRef { .. }
 282             | ExprKind::InlineAsm { .. }
 283             | ExprKind::LlvmInlineAsm { .. }
 284             | ExprKind::Yield { .. }
 285             | ExprKind::ThreadLocalRef(_)
 286             | ExprKind::Call { .. } => {
 287                 // these are not places, so we need to make a temporary.
 288                 debug_assert!(!matches!(Category::of(&expr.kind), Some(Category::Place)));
 289                 let temp =
 290                     unpack!(block = this.as_temp(block, expr.temp_lifetime, expr, mutability));
 291                 block.and(PlaceBuilder::from(temp))
 292             }
 293         }
 294     }
 295
 296     /// Lower a closure/generator capture by representing it as a field
 297     /// access within the desugared closure/generator.
 298     ///
 299     /// `capture_index` is the index of the capture within the desugared
 300     /// closure/generator.
 301     fn lower_closure_capture(
 302         &mut self,
 303         block: BasicBlock,
 304         capture_index: usize,
 305         upvar_id: ty::UpvarId,
 306     )  -> BlockAnd<PlaceBuilder<'tcx>> {
 307         let closure_ty = self
 308             .hir
 309             .typeck_results()
 310             .node_type(self.hir.tcx().hir().local_def_id_to_hir_id(upvar_id.closure_expr_id));
 311
 312         // Captures are represented using fields inside a structure.
 313         // This represents accessing self in the closure structure
 314         let mut place_builder = PlaceBuilder::from(Local::new(1));
 315
 316         // In case of Fn/FnMut closures we must deref to access the fields
 317         // Generators are considered FnOnce, so we ignore this step for them.
 318         if let ty::Closure(_, closure_substs) = closure_ty.kind() {
 319             match self.hir.infcx().closure_kind(closure_substs).unwrap() {
 320                 ty::ClosureKind::Fn | ty::ClosureKind::FnMut => {
 321                     place_builder = place_builder.deref();
 322                 }
 323                 ty::ClosureKind::FnOnce => {}
 324             }
 325         }
 326
 327         let substs = match closure_ty.kind() {
 328             ty::Closure(_, substs) => ty::UpvarSubsts::Closure(substs),
 329             ty::Generator(_, substs, _) => ty::UpvarSubsts::Generator(substs),
 330             _ => bug!("Lowering capture for non-closure type {:?}", closure_ty)
 331         };
 332
 333         // Access the capture by accessing the field within the Closure struct.
 334         //
 335         // We must have inferred the capture types since we are building MIR, therefore
 336         // it's safe to call `upvar_tys` and we can unwrap here because
 337         // we know that the capture exists and is the `capture_index`-th capture.
 338         let var_ty = substs.upvar_tys().nth(capture_index).unwrap();
 339         place_builder = place_builder.field(Field::new(capture_index), var_ty);
 340
 341         // If the variable is captured via ByRef(Immutable/Mutable) Borrow,
 342         // we need to deref it
 343         match self.hir.typeck_results.upvar_capture(upvar_id) {
 344             ty::UpvarCapture::ByRef(_) => {
 345                 block.and(place_builder.deref())
 346             }
 347             ty::UpvarCapture::ByValue(_) => block.and(place_builder),
 348         }
 349     }
 350
 351     /// Lower an index expression
 352     ///
 353     /// This has two complications;
 354     ///
 355     /// * We need to do a bounds check.
 356     /// * We need to ensure that the bounds check can't be invalidated using an
 357     ///   expression like `x[1][{x = y; 2}]`. We use fake borrows here to ensure
 358     ///   that this is the case.
 359     fn lower_index_expression(
 360         &mut self,
 361         mut block: BasicBlock,
 362         base: ExprRef<'tcx>,
 363         index: ExprRef<'tcx>,
 364         mutability: Mutability,
 365         fake_borrow_temps: Option<&mut Vec<Local>>,
 366         temp_lifetime: Option<region::Scope>,
 367         expr_span: Span,
 368         source_info: SourceInfo,
 369     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 370         let lhs = self.hir.mirror(base);
 371
 372         let base_fake_borrow_temps = &mut Vec::new();
 373         let is_outermost_index = fake_borrow_temps.is_none();
 374         let fake_borrow_temps = fake_borrow_temps.unwrap_or(base_fake_borrow_temps);
 375
 376         let base_place =
 377             unpack!(block = self.expr_as_place(block, lhs, mutability, Some(fake_borrow_temps),));
 378
 379         // Making this a *fresh* temporary means we do not have to worry about
 380         // the index changing later: Nothing will ever change this temporary.
 381         // The "retagging" transformation (for Stacked Borrows) relies on this.
 382         let idx = unpack!(block = self.as_temp(block, temp_lifetime, index, Mutability::Not,));
 383
 384         block = self.bounds_check(
 385             block,
 386             base_place.clone().into_place(self.hir.tcx()),
 387             idx,
 388             expr_span,
 389             source_info,
 390         );
 391
 392         if is_outermost_index {
 393             self.read_fake_borrows(block, fake_borrow_temps, source_info)
 394         } else {
 395             self.add_fake_borrows_of_base(
 396                 &base_place,
 397                 block,
 398                 fake_borrow_temps,
 399                 expr_span,
 400                 source_info,
 401             );
 402         }
 403
 404         block.and(base_place.index(idx))
 405     }
 406
 407     fn bounds_check(
 408         &mut self,
 409         block: BasicBlock,
 410         slice: Place<'tcx>,
 411         index: Local,
 412         expr_span: Span,
 413         source_info: SourceInfo,
 414     ) -> BasicBlock {
 415         let usize_ty = self.hir.usize_ty();
 416         let bool_ty = self.hir.bool_ty();
 417         // bounds check:
 418         let len = self.temp(usize_ty, expr_span);
 419         let lt = self.temp(bool_ty, expr_span);
 420
 421         // len = len(slice)
 422         self.cfg.push_assign(block, source_info, len, Rvalue::Len(slice));
 423         // lt = idx < len
 424         self.cfg.push_assign(
 425             block,
 426             source_info,
 427             lt,
 428             Rvalue::BinaryOp(BinOp::Lt, Operand::Copy(Place::from(index)), Operand::Copy(len)),
 429         );
 430         let msg = BoundsCheck { len: Operand::Move(len), index: Operand::Copy(Place::from(index)) };
 431         // assert!(lt, "...")
 432         self.assert(block, Operand::Move(lt), true, msg, expr_span)
 433     }
 434
 435     fn add_fake_borrows_of_base(
 436         &mut self,
 437         base_place: &PlaceBuilder<'tcx>,
 438         block: BasicBlock,
 439         fake_borrow_temps: &mut Vec<Local>,
 440         expr_span: Span,
 441         source_info: SourceInfo,
 442     ) {
 443         let tcx = self.hir.tcx();
 444         let place_ty =
 445             Place::ty_from(base_place.local, &base_place.projection, &self.local_decls, tcx);
 446         if let ty::Slice(_) = place_ty.ty.kind() {
 447             // We need to create fake borrows to ensure that the bounds
 448             // check that we just did stays valid. Since we can't assign to
 449             // unsized values, we only need to ensure that none of the
 450             // pointers in the base place are modified.
 451             for (idx, elem) in base_place.projection.iter().enumerate().rev() {
 452                 match elem {
 453                     ProjectionElem::Deref => {
 454                         let fake_borrow_deref_ty = Place::ty_from(
 455                             base_place.local,
 456                             &base_place.projection[..idx],
 457                             &self.local_decls,
 458                             tcx,
 459                         )
 460                         .ty;
 461                         let fake_borrow_ty =
 462                             tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty);
 463                         let fake_borrow_temp =
 464                             self.local_decls.push(LocalDecl::new(fake_borrow_ty, expr_span));
 465                         let projection = tcx.intern_place_elems(&base_place.projection[..idx]);
 466                         self.cfg.push_assign(
 467                             block,
 468                             source_info,
 469                             fake_borrow_temp.into(),
 470                             Rvalue::Ref(
 471                                 tcx.lifetimes.re_erased,
 472                                 BorrowKind::Shallow,
 473                                 Place { local: base_place.local, projection },
 474                             ),
 475                         );
 476                         fake_borrow_temps.push(fake_borrow_temp);
 477                     }
 478                     ProjectionElem::Index(_) => {
 479                         let index_ty = Place::ty_from(
 480                             base_place.local,
 481                             &base_place.projection[..idx],
 482                             &self.local_decls,
 483                             tcx,
 484                         );
 485                         match index_ty.ty.kind() {
 486                             // The previous index expression has already
 487                             // done any index expressions needed here.
 488                             ty::Slice(_) => break,
 489                             ty::Array(..) => (),
 490                             _ => bug!("unexpected index base"),
 491                         }
 492                     }
 493                     ProjectionElem::Field(..)
 494                     | ProjectionElem::Downcast(..)
 495                     | ProjectionElem::ConstantIndex { .. }
 496                     | ProjectionElem::Subslice { .. } => (),
 497                 }
 498             }
 499         }
 500     }
 501
 502     fn read_fake_borrows(
 503         &mut self,
 504         bb: BasicBlock,
 505         fake_borrow_temps: &mut Vec<Local>,
 506         source_info: SourceInfo,
 507     ) {
 508         // All indexes have been evaluated now, read all of the
 509         // fake borrows so that they are live across those index
 510         // expressions.
 511         for temp in fake_borrow_temps {
 512             self.cfg.push_fake_read(bb, source_info, FakeReadCause::ForIndex, Place::from(*temp));
 513         }
 514     }
 515 }