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::SelfRef => block.and(PlaceBuilder::from(Local::new(1))),
 164             ExprKind::VarRef { id } => {
 165                 let place_builder = if this.is_bound_var_in_guard(id) {
 166                     let index = this.var_local_id(id, RefWithinGuard);
 167                     PlaceBuilder::from(index).deref()
 168                 } else {
 169                     let index = this.var_local_id(id, OutsideGuard);
 170                     PlaceBuilder::from(index)
 171                 };
 172                 block.and(place_builder)
 173             }
 174
 175             ExprKind::PlaceTypeAscription { source, user_ty } => {
 176                 let source = this.hir.mirror(source);
 177                 let place_builder = unpack!(
 178                     block = this.expr_as_place(block, source, mutability, fake_borrow_temps,)
 179                 );
 180                 if let Some(user_ty) = user_ty {
 181                     let annotation_index =
 182                         this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
 183                             span: source_info.span,
 184                             user_ty,
 185                             inferred_ty: expr.ty,
 186                         });
 187
 188                     let place = place_builder.clone().into_place(this.hir.tcx());
 189                     this.cfg.push(
 190                         block,
 191                         Statement {
 192                             source_info,
 193                             kind: StatementKind::AscribeUserType(
 194                                 box (
 195                                     place,
 196                                     UserTypeProjection { base: annotation_index, projs: vec![] },
 197                                 ),
 198                                 Variance::Invariant,
 199                             ),
 200                         },
 201                     );
 202                 }
 203                 block.and(place_builder)
 204             }
 205             ExprKind::ValueTypeAscription { source, user_ty } => {
 206                 let source = this.hir.mirror(source);
 207                 let temp =
 208                     unpack!(block = this.as_temp(block, source.temp_lifetime, source, mutability));
 209                 if let Some(user_ty) = user_ty {
 210                     let annotation_index =
 211                         this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
 212                             span: source_info.span,
 213                             user_ty,
 214                             inferred_ty: expr.ty,
 215                         });
 216                     this.cfg.push(
 217                         block,
 218                         Statement {
 219                             source_info,
 220                             kind: StatementKind::AscribeUserType(
 221                                 box (
 222                                     Place::from(temp),
 223                                     UserTypeProjection { base: annotation_index, projs: vec![] },
 224                                 ),
 225                                 Variance::Invariant,
 226                             ),
 227                         },
 228                     );
 229                 }
 230                 block.and(PlaceBuilder::from(temp))
 231             }
 232
 233             ExprKind::Array { .. }
 234             | ExprKind::Tuple { .. }
 235             | ExprKind::Adt { .. }
 236             | ExprKind::Closure { .. }
 237             | ExprKind::Unary { .. }
 238             | ExprKind::Binary { .. }
 239             | ExprKind::LogicalOp { .. }
 240             | ExprKind::Box { .. }
 241             | ExprKind::Cast { .. }
 242             | ExprKind::Use { .. }
 243             | ExprKind::NeverToAny { .. }
 244             | ExprKind::Pointer { .. }
 245             | ExprKind::Repeat { .. }
 246             | ExprKind::Borrow { .. }
 247             | ExprKind::AddressOf { .. }
 248             | ExprKind::Match { .. }
 249             | ExprKind::Loop { .. }
 250             | ExprKind::Block { .. }
 251             | ExprKind::Assign { .. }
 252             | ExprKind::AssignOp { .. }
 253             | ExprKind::Break { .. }
 254             | ExprKind::Continue { .. }
 255             | ExprKind::Return { .. }
 256             | ExprKind::Literal { .. }
 257             | ExprKind::ConstBlock { .. }
 258             | ExprKind::StaticRef { .. }
 259             | ExprKind::InlineAsm { .. }
 260             | ExprKind::LlvmInlineAsm { .. }
 261             | ExprKind::Yield { .. }
 262             | ExprKind::ThreadLocalRef(_)
 263             | ExprKind::Call { .. } => {
 264                 // these are not places, so we need to make a temporary.
 265                 debug_assert!(!matches!(Category::of(&expr.kind), Some(Category::Place)));
 266                 let temp =
 267                     unpack!(block = this.as_temp(block, expr.temp_lifetime, expr, mutability));
 268                 block.and(PlaceBuilder::from(temp))
 269             }
 270         }
 271     }
 272
 273     /// Lower an index expression
 274     ///
 275     /// This has two complications;
 276     ///
 277     /// * We need to do a bounds check.
 278     /// * We need to ensure that the bounds check can't be invalidated using an
 279     ///   expression like `x[1][{x = y; 2}]`. We use fake borrows here to ensure
 280     ///   that this is the case.
 281     fn lower_index_expression(
 282         &mut self,
 283         mut block: BasicBlock,
 284         base: ExprRef<'tcx>,
 285         index: ExprRef<'tcx>,
 286         mutability: Mutability,
 287         fake_borrow_temps: Option<&mut Vec<Local>>,
 288         temp_lifetime: Option<region::Scope>,
 289         expr_span: Span,
 290         source_info: SourceInfo,
 291     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 292         let lhs = self.hir.mirror(base);
 293
 294         let base_fake_borrow_temps = &mut Vec::new();
 295         let is_outermost_index = fake_borrow_temps.is_none();
 296         let fake_borrow_temps = fake_borrow_temps.unwrap_or(base_fake_borrow_temps);
 297
 298         let base_place =
 299             unpack!(block = self.expr_as_place(block, lhs, mutability, Some(fake_borrow_temps),));
 300
 301         // Making this a *fresh* temporary means we do not have to worry about
 302         // the index changing later: Nothing will ever change this temporary.
 303         // The "retagging" transformation (for Stacked Borrows) relies on this.
 304         let idx = unpack!(block = self.as_temp(block, temp_lifetime, index, Mutability::Not,));
 305
 306         block = self.bounds_check(
 307             block,
 308             base_place.clone().into_place(self.hir.tcx()),
 309             idx,
 310             expr_span,
 311             source_info,
 312         );
 313
 314         if is_outermost_index {
 315             self.read_fake_borrows(block, fake_borrow_temps, source_info)
 316         } else {
 317             self.add_fake_borrows_of_base(
 318                 &base_place,
 319                 block,
 320                 fake_borrow_temps,
 321                 expr_span,
 322                 source_info,
 323             );
 324         }
 325
 326         block.and(base_place.index(idx))
 327     }
 328
 329     fn bounds_check(
 330         &mut self,
 331         block: BasicBlock,
 332         slice: Place<'tcx>,
 333         index: Local,
 334         expr_span: Span,
 335         source_info: SourceInfo,
 336     ) -> BasicBlock {
 337         let usize_ty = self.hir.usize_ty();
 338         let bool_ty = self.hir.bool_ty();
 339         // bounds check:
 340         let len = self.temp(usize_ty, expr_span);
 341         let lt = self.temp(bool_ty, expr_span);
 342
 343         // len = len(slice)
 344         self.cfg.push_assign(block, source_info, len, Rvalue::Len(slice));
 345         // lt = idx < len
 346         self.cfg.push_assign(
 347             block,
 348             source_info,
 349             lt,
 350             Rvalue::BinaryOp(BinOp::Lt, Operand::Copy(Place::from(index)), Operand::Copy(len)),
 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(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, 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 }