src/librustc_mir/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::hair::*;
   7 use rustc::middle::region;
   8 use rustc::mir::interpret::PanicInfo::BoundsCheck;
   9 use rustc::mir::*;
  10 use rustc::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 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 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     pub 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     pub fn as_read_only_place<M>(&mut self, mut block: BasicBlock, expr: M) -> BlockAnd<Place<'tcx>>
  93     where
  94         M: Mirror<'tcx, Output = Expr<'tcx>>,
  95     {
  96         let place_builder = unpack!(block = self.as_read_only_place_builder(block, expr));
  97         block.and(place_builder.into_place(self.hir.tcx()))
  98     }
  99
 100     /// This is used when constructing a compound `Place`, so that we can avoid creating
 101     /// intermediate `Place` values until we know the full set of projections.
 102     /// Mutability note: The caller of this method promises only to read from the resulting
 103     /// place. The place itself may or may not be mutable:
 104     /// * If this expr is a place expr like a.b, then we will return that place.
 105     /// * Otherwise, a temporary is created: in that event, it will be an immutable temporary.
 106     fn as_read_only_place_builder<M>(
 107         &mut self,
 108         block: BasicBlock,
 109         expr: M,
 110     ) -> BlockAnd<PlaceBuilder<'tcx>>
 111     where
 112         M: Mirror<'tcx, Output = Expr<'tcx>>,
 113     {
 114         let expr = self.hir.mirror(expr);
 115         self.expr_as_place(block, expr, Mutability::Not, None)
 116     }
 117
 118     fn expr_as_place(
 119         &mut self,
 120         mut block: BasicBlock,
 121         expr: Expr<'tcx>,
 122         mutability: Mutability,
 123         fake_borrow_temps: Option<&mut Vec<Local>>,
 124     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 125         debug!("expr_as_place(block={:?}, expr={:?}, mutability={:?})", block, expr, mutability);
 126
 127         let this = self;
 128         let expr_span = expr.span;
 129         let source_info = this.source_info(expr_span);
 130         match expr.kind {
 131             ExprKind::Scope { region_scope, lint_level, value } => {
 132                 this.in_scope((region_scope, source_info), lint_level, |this| {
 133                     let value = this.hir.mirror(value);
 134                     this.expr_as_place(block, value, mutability, fake_borrow_temps)
 135                 })
 136             }
 137             ExprKind::Field { lhs, name } => {
 138                 let lhs = this.hir.mirror(lhs);
 139                 let place_builder =
 140                     unpack!(block = this.expr_as_place(block, lhs, mutability, fake_borrow_temps,));
 141                 block.and(place_builder.field(name, expr.ty))
 142             }
 143             ExprKind::Deref { arg } => {
 144                 let arg = this.hir.mirror(arg);
 145                 let place_builder =
 146                     unpack!(block = this.expr_as_place(block, arg, mutability, fake_borrow_temps,));
 147                 block.and(place_builder.deref())
 148             }
 149             ExprKind::Index { lhs, index } => this.lower_index_expression(
 150                 block,
 151                 lhs,
 152                 index,
 153                 mutability,
 154                 fake_borrow_temps,
 155                 expr.temp_lifetime,
 156                 expr_span,
 157                 source_info,
 158             ),
 159             ExprKind::SelfRef => block.and(PlaceBuilder::from(Local::new(1))),
 160             ExprKind::VarRef { id } => {
 161                 let place_builder = if this.is_bound_var_in_guard(id) {
 162                     let index = this.var_local_id(id, RefWithinGuard);
 163                     PlaceBuilder::from(index).deref()
 164                 } else {
 165                     let index = this.var_local_id(id, OutsideGuard);
 166                     PlaceBuilder::from(index)
 167                 };
 168                 block.and(place_builder)
 169             }
 170
 171             ExprKind::PlaceTypeAscription { source, user_ty } => {
 172                 let source = this.hir.mirror(source);
 173                 let place_builder = unpack!(
 174                     block = this.expr_as_place(block, source, mutability, fake_borrow_temps,)
 175                 );
 176                 if let Some(user_ty) = user_ty {
 177                     let annotation_index =
 178                         this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
 179                             span: source_info.span,
 180                             user_ty,
 181                             inferred_ty: expr.ty,
 182                         });
 183
 184                     let place = place_builder.clone().into_place(this.hir.tcx());
 185                     this.cfg.push(
 186                         block,
 187                         Statement {
 188                             source_info,
 189                             kind: StatementKind::AscribeUserType(
 190                                 box (
 191                                     place,
 192                                     UserTypeProjection { base: annotation_index, projs: vec![] },
 193                                 ),
 194                                 Variance::Invariant,
 195                             ),
 196                         },
 197                     );
 198                 }
 199                 block.and(place_builder)
 200             }
 201             ExprKind::ValueTypeAscription { source, user_ty } => {
 202                 let source = this.hir.mirror(source);
 203                 let temp =
 204                     unpack!(block = this.as_temp(block, source.temp_lifetime, source, mutability));
 205                 if let Some(user_ty) = user_ty {
 206                     let annotation_index =
 207                         this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
 208                             span: source_info.span,
 209                             user_ty,
 210                             inferred_ty: expr.ty,
 211                         });
 212                     this.cfg.push(
 213                         block,
 214                         Statement {
 215                             source_info,
 216                             kind: StatementKind::AscribeUserType(
 217                                 box (
 218                                     Place::from(temp.clone()),
 219                                     UserTypeProjection { base: annotation_index, projs: vec![] },
 220                                 ),
 221                                 Variance::Invariant,
 222                             ),
 223                         },
 224                     );
 225                 }
 226                 block.and(PlaceBuilder::from(temp))
 227             }
 228
 229             ExprKind::Array { .. }
 230             | ExprKind::Tuple { .. }
 231             | ExprKind::Adt { .. }
 232             | ExprKind::Closure { .. }
 233             | ExprKind::Unary { .. }
 234             | ExprKind::Binary { .. }
 235             | ExprKind::LogicalOp { .. }
 236             | ExprKind::Box { .. }
 237             | ExprKind::Cast { .. }
 238             | ExprKind::Use { .. }
 239             | ExprKind::NeverToAny { .. }
 240             | ExprKind::Pointer { .. }
 241             | ExprKind::Repeat { .. }
 242             | ExprKind::Borrow { .. }
 243             | ExprKind::AddressOf { .. }
 244             | ExprKind::Match { .. }
 245             | ExprKind::Loop { .. }
 246             | ExprKind::Block { .. }
 247             | ExprKind::Assign { .. }
 248             | ExprKind::AssignOp { .. }
 249             | ExprKind::Break { .. }
 250             | ExprKind::Continue { .. }
 251             | ExprKind::Return { .. }
 252             | ExprKind::Literal { .. }
 253             | ExprKind::StaticRef { .. }
 254             | ExprKind::InlineAsm { .. }
 255             | ExprKind::Yield { .. }
 256             | ExprKind::Call { .. } => {
 257                 // these are not places, so we need to make a temporary.
 258                 debug_assert!(match Category::of(&expr.kind) {
 259                     Some(Category::Place) => false,
 260                     _ => true,
 261                 });
 262                 let temp =
 263                     unpack!(block = this.as_temp(block, expr.temp_lifetime, expr, mutability));
 264                 block.and(PlaceBuilder::from(temp))
 265             }
 266         }
 267     }
 268
 269     /// Lower an index expression
 270     ///
 271     /// This has two complications;
 272     ///
 273     /// * We need to do a bounds check.
 274     /// * We need to ensure that the bounds check can't be invalidated using an
 275     ///   expression like `x[1][{x = y; 2}]`. We use fake borrows here to ensure
 276     ///   that this is the case.
 277     fn lower_index_expression(
 278         &mut self,
 279         mut block: BasicBlock,
 280         base: ExprRef<'tcx>,
 281         index: ExprRef<'tcx>,
 282         mutability: Mutability,
 283         fake_borrow_temps: Option<&mut Vec<Local>>,
 284         temp_lifetime: Option<region::Scope>,
 285         expr_span: Span,
 286         source_info: SourceInfo,
 287     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 288         let lhs = self.hir.mirror(base);
 289
 290         let base_fake_borrow_temps = &mut Vec::new();
 291         let is_outermost_index = fake_borrow_temps.is_none();
 292         let fake_borrow_temps = fake_borrow_temps.unwrap_or(base_fake_borrow_temps);
 293
 294         let base_place =
 295             unpack!(block = self.expr_as_place(block, lhs, mutability, Some(fake_borrow_temps),));
 296
 297         // Making this a *fresh* temporary means we do not have to worry about
 298         // the index changing later: Nothing will ever change this temporary.
 299         // The "retagging" transformation (for Stacked Borrows) relies on this.
 300         let idx = unpack!(block = self.as_temp(block, temp_lifetime, index, Mutability::Not,));
 301
 302         block = self.bounds_check(
 303             block,
 304             base_place.clone().into_place(self.hir.tcx()),
 305             idx,
 306             expr_span,
 307             source_info,
 308         );
 309
 310         if is_outermost_index {
 311             self.read_fake_borrows(block, fake_borrow_temps, source_info)
 312         } else {
 313             self.add_fake_borrows_of_base(
 314                 &base_place,
 315                 block,
 316                 fake_borrow_temps,
 317                 expr_span,
 318                 source_info,
 319             );
 320         }
 321
 322         block.and(base_place.index(idx))
 323     }
 324
 325     fn bounds_check(
 326         &mut self,
 327         block: BasicBlock,
 328         slice: Place<'tcx>,
 329         index: Local,
 330         expr_span: Span,
 331         source_info: SourceInfo,
 332     ) -> BasicBlock {
 333         let usize_ty = self.hir.usize_ty();
 334         let bool_ty = self.hir.bool_ty();
 335         // bounds check:
 336         let len = self.temp(usize_ty, expr_span);
 337         let lt = self.temp(bool_ty, expr_span);
 338
 339         // len = len(slice)
 340         self.cfg.push_assign(block, source_info, &len, Rvalue::Len(slice));
 341         // lt = idx < len
 342         self.cfg.push_assign(
 343             block,
 344             source_info,
 345             &lt,
 346             Rvalue::BinaryOp(
 347                 BinOp::Lt,
 348                 Operand::Copy(Place::from(index)),
 349                 Operand::Copy(len.clone()),
 350             ),
 351         );
 352         let msg = BoundsCheck { len: Operand::Move(len), index: Operand::Copy(Place::from(index)) };
 353         // assert!(lt, "...")
 354         self.assert(block, Operand::Move(lt), true, msg, expr_span)
 355     }
 356
 357     fn add_fake_borrows_of_base(
 358         &mut self,
 359         base_place: &PlaceBuilder<'tcx>,
 360         block: BasicBlock,
 361         fake_borrow_temps: &mut Vec<Local>,
 362         expr_span: Span,
 363         source_info: SourceInfo,
 364     ) {
 365         let tcx = self.hir.tcx();
 366         let place_ty =
 367             Place::ty_from(&base_place.local, &base_place.projection, &self.local_decls, tcx);
 368         if let ty::Slice(_) = place_ty.ty.kind {
 369             // We need to create fake borrows to ensure that the bounds
 370             // check that we just did stays valid. Since we can't assign to
 371             // unsized values, we only need to ensure that none of the
 372             // pointers in the base place are modified.
 373             for (idx, elem) in base_place.projection.iter().enumerate().rev() {
 374                 match elem {
 375                     ProjectionElem::Deref => {
 376                         let fake_borrow_deref_ty = Place::ty_from(
 377                             &base_place.local,
 378                             &base_place.projection[..idx],
 379                             &self.local_decls,
 380                             tcx,
 381                         )
 382                         .ty;
 383                         let fake_borrow_ty =
 384                             tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty);
 385                         let fake_borrow_temp =
 386                             self.local_decls.push(LocalDecl::new_temp(fake_borrow_ty, expr_span));
 387                         let projection = tcx.intern_place_elems(&base_place.projection[..idx]);
 388                         self.cfg.push_assign(
 389                             block,
 390                             source_info,
 391                             &fake_borrow_temp.into(),
 392                             Rvalue::Ref(
 393                                 tcx.lifetimes.re_erased,
 394                                 BorrowKind::Shallow,
 395                                 Place { local: base_place.local.clone(), projection },
 396                             ),
 397                         );
 398                         fake_borrow_temps.push(fake_borrow_temp);
 399                     }
 400                     ProjectionElem::Index(_) => {
 401                         let index_ty = Place::ty_from(
 402                             &base_place.local,
 403                             &base_place.projection[..idx],
 404                             &self.local_decls,
 405                             tcx,
 406                         );
 407                         match index_ty.ty.kind {
 408                             // The previous index expression has already
 409                             // done any index expressions needed here.
 410                             ty::Slice(_) => break,
 411                             ty::Array(..) => (),
 412                             _ => bug!("unexpected index base"),
 413                         }
 414                     }
 415                     ProjectionElem::Field(..)
 416                     | ProjectionElem::Downcast(..)
 417                     | ProjectionElem::ConstantIndex { .. }
 418                     | ProjectionElem::Subslice { .. } => (),
 419                 }
 420             }
 421         }
 422     }
 423
 424     fn read_fake_borrows(
 425         &mut self,
 426         bb: BasicBlock,
 427         fake_borrow_temps: &mut Vec<Local>,
 428         source_info: SourceInfo,
 429     ) {
 430         // All indexes have been evaluated now, read all of the
 431         // fake borrows so that they are live across those index
 432         // expressions.
 433         for temp in fake_borrow_temps {
 434             self.cfg.push_fake_read(bb, source_info, FakeReadCause::ForIndex, Place::from(*temp));
 435         }
 436     }
 437 }