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_hir::def_id::DefId;
   8 use rustc_hir::HirId;
   9 use rustc_middle::hir::place::ProjectionKind as HirProjectionKind;
  10 use rustc_middle::middle::region;
  11 use rustc_middle::mir::AssertKind::BoundsCheck;
  12 use rustc_middle::mir::*;
  13 use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty, TyCtxt, Variance};
  14 use rustc_span::Span;
  15 use rustc_target::abi::VariantIdx;
  16
  17 use rustc_index::vec::Idx;
  18
  19 /// The "outermost" place that holds this value.
  20 #[derive(Copy, Clone)]
  21 crate enum PlaceBase {
  22     /// Denotes the start of a `Place`.
  23     Local(Local),
  24
  25     /// When building place for an expression within a closure, the place might start off a
  26     /// captured path. When `capture_disjoint_fields` is enabled, we might not know the capture
  27     /// index (within the desugared closure) of the captured path until most of the projections
  28     /// are applied. We use `PlaceBase::Upvar` to keep track of the root variable off of which the
  29     /// captured path starts, the closure the capture belongs to and the trait the closure
  30     /// implements.
  31     ///
  32     /// Once we have figured out the capture index, we can convert the place builder to start from
  33     /// `PlaceBase::Local`.
  34     ///
  35     /// Consider the following example
  36     /// ```rust
  37     /// let t = (10, (10, (10, 10)));
  38     ///
  39     /// let c = || {
  40     ///     println!("{}", t.0.0.0);
  41     /// };
  42     /// ```
  43     /// Here the THIR expression for `t.0.0.0` will be something like
  44     ///
  45     /// ```
  46     /// * Field(0)
  47     ///     * Field(0)
  48     ///         * Field(0)
  49     ///             * UpvarRef(t)
  50     /// ```
  51     ///
  52     /// When `capture_disjoint_fields` is enabled, `t.0.0.0` is captured and we won't be able to
  53     /// figure out that it is captured until all the `Field` projections are applied.
  54     Upvar {
  55         /// HirId of the upvar
  56         var_hir_id: HirId,
  57         /// DefId of the closure
  58         closure_def_id: DefId,
  59         /// The trait closure implements, `Fn`, `FnMut`, `FnOnce`
  60         closure_kind: ty::ClosureKind,
  61     },
  62 }
  63
  64 /// `PlaceBuilder` is used to create places during MIR construction. It allows you to "build up" a
  65 /// place by pushing more and more projections onto the end, and then convert the final set into a
  66 /// place using the `into_place` method.
  67 ///
  68 /// This is used internally when building a place for an expression like `a.b.c`. The fields `b`
  69 /// and `c` can be progressively pushed onto the place builder that is created when converting `a`.
  70 #[derive(Clone)]
  71 crate struct PlaceBuilder<'tcx> {
  72     base: PlaceBase,
  73     projection: Vec<PlaceElem<'tcx>>,
  74 }
  75
  76 /// Given a list of MIR projections, convert them to list of HIR ProjectionKind.
  77 /// The projections are truncated to represent a path that might be captured by a
  78 /// closure/generator. This implies the vector returned from this function doesn't contain
  79 /// ProjectionElems `Downcast`, `ConstantIndex`, `Index`, or `Subslice` because those will never be
  80 /// part of a path that is captued by a closure. We stop applying projections once we see the first
  81 /// projection that isn't captured by a closure.
  82 fn convert_to_hir_projections_and_truncate_for_capture<'tcx>(
  83     mir_projections: &[PlaceElem<'tcx>],
  84 ) -> Vec<HirProjectionKind> {
  85     let mut hir_projections = Vec::new();
  86
  87     for mir_projection in mir_projections {
  88         let hir_projection = match mir_projection {
  89             ProjectionElem::Deref => HirProjectionKind::Deref,
  90             ProjectionElem::Field(field, _) => {
  91                 // We will never encouter this for multivariant enums,
  92                 // read the comment for `Downcast`.
  93                 HirProjectionKind::Field(field.index() as u32, VariantIdx::new(0))
  94             }
  95             ProjectionElem::Downcast(..) => {
  96                 // This projections exist only for enums that have
  97                 // multiple variants. Since such enums that are captured
  98                 // completely, we can stop here.
  99                 break;
 100             }
 101             ProjectionElem::Index(..)
 102             | ProjectionElem::ConstantIndex { .. }
 103             | ProjectionElem::Subslice { .. } => {
 104                 // We don't capture array-access projections.
 105                 // We can stop here as arrays are captured completely.
 106                 break;
 107             }
 108         };
 109
 110         hir_projections.push(hir_projection);
 111     }
 112
 113     hir_projections
 114 }
 115
 116 /// Return true if the `proj_possible_ancestor` represents an ancestor path
 117 /// to `proj_capture` or `proj_possible_ancestor` is same as `proj_capture`,
 118 /// assuming they both start off of the same root variable.
 119 ///
 120 /// **Note:** It's the caller's responsibility to ensure that both lists of projections
 121 ///           start off of the same root variable.
 122 ///
 123 /// Eg: 1. `foo.x` which is represented using `projections=[Field(x)]` is an ancestor of
 124 ///        `foo.x.y` which is represented using `projections=[Field(x), Field(y)]`.
 125 ///        Note both `foo.x` and `foo.x.y` start off of the same root variable `foo`.
 126 ///     2. Since we only look at the projections here function will return `bar.x` as an a valid
 127 ///        ancestor of `foo.x.y`. It's the caller's responsibility to ensure that both projections
 128 ///        list are being applied to the same root variable.
 129 fn is_ancestor_or_same_capture(
 130     proj_possible_ancestor: &Vec<HirProjectionKind>,
 131     proj_capture: &[HirProjectionKind],
 132 ) -> bool {
 133     // We want to make sure `is_ancestor_or_same_capture("x.0.0", "x.0")` to return false.
 134     // Therefore we can't just check if all projections are same in the zipped iterator below.
 135     if proj_possible_ancestor.len() > proj_capture.len() {
 136         return false;
 137     }
 138
 139     proj_possible_ancestor.iter().zip(proj_capture).all(|(a, b)| a == b)
 140 }
 141
 142 /// Computes the index of a capture within the desugared closure provided the closure's
 143 /// `closure_min_captures` and the capture's index of the capture in the
 144 /// `ty::MinCaptureList` of the root variable `var_hir_id`.
 145 fn compute_capture_idx<'tcx>(
 146     closure_min_captures: &ty::RootVariableMinCaptureList<'tcx>,
 147     var_hir_id: HirId,
 148     root_var_idx: usize,
 149 ) -> usize {
 150     let mut res = 0;
 151     for (var_id, capture_list) in closure_min_captures {
 152         if *var_id == var_hir_id {
 153             res += root_var_idx;
 154             break;
 155         } else {
 156             res += capture_list.len();
 157         }
 158     }
 159
 160     res
 161 }
 162
 163 /// Given a closure, returns the index of a capture within the desugared closure struct and the
 164 /// `ty::CapturedPlace` which is the ancestor of the Place represented using the `var_hir_id`
 165 /// and `projection`.
 166 ///
 167 /// Note there will be at most one ancestor for any given Place.
 168 ///
 169 /// Returns None, when the ancestor is not found.
 170 fn find_capture_matching_projections<'a, 'tcx>(
 171     typeck_results: &'a ty::TypeckResults<'tcx>,
 172     var_hir_id: HirId,
 173     closure_def_id: DefId,
 174     projections: &[PlaceElem<'tcx>],
 175 ) -> Option<(usize, &'a ty::CapturedPlace<'tcx>)> {
 176     let closure_min_captures = typeck_results.closure_min_captures.get(&closure_def_id)?;
 177     let root_variable_min_captures = closure_min_captures.get(&var_hir_id)?;
 178
 179     let hir_projections = convert_to_hir_projections_and_truncate_for_capture(projections);
 180
 181     // If an ancestor is found, `idx` is the index within the list of captured places
 182     // for root variable `var_hir_id` and `capture` is the `ty::CapturedPlace` itself.
 183     let (idx, capture) = root_variable_min_captures.iter().enumerate().find(|(_, capture)| {
 184         let possible_ancestor_proj_kinds =
 185             capture.place.projections.iter().map(|proj| proj.kind).collect();
 186         is_ancestor_or_same_capture(&possible_ancestor_proj_kinds, &hir_projections)
 187     })?;
 188
 189     // Convert index to be from the presepective of the entire closure_min_captures map
 190     // instead of just the root variable capture list
 191     Some((compute_capture_idx(closure_min_captures, var_hir_id, idx), capture))
 192 }
 193
 194 /// Takes a PlaceBuilder and resolves the upvar (if any) within it, so that the
 195 /// `PlaceBuilder` now starts from `PlaceBase::Local`.
 196 ///
 197 /// Returns a Result with the error being the HirId of the Upvar that was not found.
 198 fn to_upvars_resolved_place_builder<'a, 'tcx>(
 199     from_builder: PlaceBuilder<'tcx>,
 200     tcx: TyCtxt<'tcx>,
 201     typeck_results: &'a ty::TypeckResults<'tcx>,
 202 ) -> Result<PlaceBuilder<'tcx>, HirId> {
 203     match from_builder.base {
 204         PlaceBase::Local(_) => Ok(from_builder),
 205         PlaceBase::Upvar { var_hir_id, closure_def_id, closure_kind } => {
 206             // Captures are represented using fields inside a structure.
 207             // This represents accessing self in the closure structure
 208             let mut upvar_resolved_place_builder = PlaceBuilder::from(Local::new(1));
 209             match closure_kind {
 210                 ty::ClosureKind::Fn | ty::ClosureKind::FnMut => {
 211                     upvar_resolved_place_builder = upvar_resolved_place_builder.deref();
 212                 }
 213                 ty::ClosureKind::FnOnce => {}
 214             }
 215
 216             let (capture_index, capture) = if let Some(capture_details) =
 217                 find_capture_matching_projections(
 218                     typeck_results,
 219                     var_hir_id,
 220                     closure_def_id,
 221                     &from_builder.projection,
 222                 ) {
 223                 capture_details
 224             } else {
 225                 if !tcx.features().capture_disjoint_fields {
 226                     bug!(
 227                         "No associated capture found for {:?}[{:#?}] even though \
 228                             capture_disjoint_fields isn't enabled",
 229                         var_hir_id,
 230                         from_builder.projection
 231                     )
 232                 } else {
 233                     // FIXME(project-rfc-2229#24): Handle this case properly
 234                     debug!(
 235                         "No associated capture found for {:?}[{:#?}]",
 236                         var_hir_id, from_builder.projection,
 237                     );
 238                 }
 239                 return Err(var_hir_id);
 240             };
 241
 242             let closure_ty = typeck_results
 243                 .node_type(tcx.hir().local_def_id_to_hir_id(closure_def_id.expect_local()));
 244
 245             let substs = match closure_ty.kind() {
 246                 ty::Closure(_, substs) => ty::UpvarSubsts::Closure(substs),
 247                 ty::Generator(_, substs, _) => ty::UpvarSubsts::Generator(substs),
 248                 _ => bug!("Lowering capture for non-closure type {:?}", closure_ty),
 249             };
 250
 251             // Access the capture by accessing the field within the Closure struct.
 252             //
 253             // We must have inferred the capture types since we are building MIR, therefore
 254             // it's safe to call `tuple_element_ty` and we can unwrap here because
 255             // we know that the capture exists and is the `capture_index`-th capture.
 256             let var_ty = substs.tupled_upvars_ty().tuple_element_ty(capture_index).unwrap();
 257
 258             upvar_resolved_place_builder =
 259                 upvar_resolved_place_builder.field(Field::new(capture_index), var_ty);
 260
 261             // If the variable is captured via ByRef(Immutable/Mutable) Borrow,
 262             // we need to deref it
 263             upvar_resolved_place_builder = match capture.info.capture_kind {
 264                 ty::UpvarCapture::ByRef(_) => upvar_resolved_place_builder.deref(),
 265                 ty::UpvarCapture::ByValue(_) => upvar_resolved_place_builder,
 266             };
 267
 268             let next_projection = capture.place.projections.len();
 269             let mut curr_projections = from_builder.projection;
 270
 271             // We used some of the projections to build the capture itself,
 272             // now we apply the remaining to the upvar resolved place.
 273             upvar_resolved_place_builder
 274                 .projection
 275                 .extend(curr_projections.drain(next_projection..));
 276
 277             Ok(upvar_resolved_place_builder)
 278         }
 279     }
 280 }
 281
 282 impl<'tcx> PlaceBuilder<'tcx> {
 283     crate fn into_place<'a>(
 284         self,
 285         tcx: TyCtxt<'tcx>,
 286         typeck_results: &'a ty::TypeckResults<'tcx>,
 287     ) -> Place<'tcx> {
 288         if let PlaceBase::Local(local) = self.base {
 289             Place { local, projection: tcx.intern_place_elems(&self.projection) }
 290         } else {
 291             self.expect_upvars_resolved(tcx, typeck_results).into_place(tcx, typeck_results)
 292         }
 293     }
 294
 295     fn expect_upvars_resolved<'a>(
 296         self,
 297         tcx: TyCtxt<'tcx>,
 298         typeck_results: &'a ty::TypeckResults<'tcx>,
 299     ) -> PlaceBuilder<'tcx> {
 300         to_upvars_resolved_place_builder(self, tcx, typeck_results).unwrap()
 301     }
 302
 303     crate fn base(&self) -> PlaceBase {
 304         self.base
 305     }
 306
 307     crate fn field(self, f: Field, ty: Ty<'tcx>) -> Self {
 308         self.project(PlaceElem::Field(f, ty))
 309     }
 310
 311     fn deref(self) -> Self {
 312         self.project(PlaceElem::Deref)
 313     }
 314
 315     fn index(self, index: Local) -> Self {
 316         self.project(PlaceElem::Index(index))
 317     }
 318
 319     fn project(mut self, elem: PlaceElem<'tcx>) -> Self {
 320         self.projection.push(elem);
 321         self
 322     }
 323 }
 324
 325 impl<'tcx> From<Local> for PlaceBuilder<'tcx> {
 326     fn from(local: Local) -> Self {
 327         Self { base: PlaceBase::Local(local), projection: Vec::new() }
 328     }
 329 }
 330
 331 impl<'tcx> From<PlaceBase> for PlaceBuilder<'tcx> {
 332     fn from(base: PlaceBase) -> Self {
 333         Self { base, projection: Vec::new() }
 334     }
 335 }
 336
 337 impl<'a, 'tcx> Builder<'a, 'tcx> {
 338     /// Compile `expr`, yielding a place that we can move from etc.
 339     ///
 340     /// WARNING: Any user code might:
 341     /// * Invalidate any slice bounds checks performed.
 342     /// * Change the address that this `Place` refers to.
 343     /// * Modify the memory that this place refers to.
 344     /// * Invalidate the memory that this place refers to, this will be caught
 345     ///   by borrow checking.
 346     ///
 347     /// Extra care is needed if any user code is allowed to run between calling
 348     /// this method and using it, as is the case for `match` and index
 349     /// expressions.
 350     crate fn as_place(
 351         &mut self,
 352         mut block: BasicBlock,
 353         expr: &Expr<'_, 'tcx>,
 354     ) -> BlockAnd<Place<'tcx>> {
 355         let place_builder = unpack!(block = self.as_place_builder(block, expr));
 356         block.and(place_builder.into_place(self.tcx, self.typeck_results))
 357     }
 358
 359     /// This is used when constructing a compound `Place`, so that we can avoid creating
 360     /// intermediate `Place` values until we know the full set of projections.
 361     crate fn as_place_builder(
 362         &mut self,
 363         block: BasicBlock,
 364         expr: &Expr<'_, 'tcx>,
 365     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 366         self.expr_as_place(block, expr, Mutability::Mut, None)
 367     }
 368
 369     /// Compile `expr`, yielding a place that we can move from etc.
 370     /// Mutability note: The caller of this method promises only to read from the resulting
 371     /// place. The place itself may or may not be mutable:
 372     /// * If this expr is a place expr like a.b, then we will return that place.
 373     /// * Otherwise, a temporary is created: in that event, it will be an immutable temporary.
 374     crate fn as_read_only_place(
 375         &mut self,
 376         mut block: BasicBlock,
 377         expr: &Expr<'_, 'tcx>,
 378     ) -> BlockAnd<Place<'tcx>> {
 379         let place_builder = unpack!(block = self.as_read_only_place_builder(block, expr));
 380         block.and(place_builder.into_place(self.tcx, self.typeck_results))
 381     }
 382
 383     /// This is used when constructing a compound `Place`, so that we can avoid creating
 384     /// intermediate `Place` values until we know the full set of projections.
 385     /// Mutability note: The caller of this method promises only to read from the resulting
 386     /// place. The place itself may or may not be mutable:
 387     /// * If this expr is a place expr like a.b, then we will return that place.
 388     /// * Otherwise, a temporary is created: in that event, it will be an immutable temporary.
 389     fn as_read_only_place_builder(
 390         &mut self,
 391         block: BasicBlock,
 392         expr: &Expr<'_, 'tcx>,
 393     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 394         self.expr_as_place(block, expr, Mutability::Not, None)
 395     }
 396
 397     fn expr_as_place(
 398         &mut self,
 399         mut block: BasicBlock,
 400         expr: &Expr<'_, 'tcx>,
 401         mutability: Mutability,
 402         fake_borrow_temps: Option<&mut Vec<Local>>,
 403     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 404         debug!("expr_as_place(block={:?}, expr={:?}, mutability={:?})", block, expr, mutability);
 405
 406         let this = self;
 407         let expr_span = expr.span;
 408         let source_info = this.source_info(expr_span);
 409         match expr.kind {
 410             ExprKind::Scope { region_scope, lint_level, value } => {
 411                 this.in_scope((region_scope, source_info), lint_level, |this| {
 412                     this.expr_as_place(block, value, mutability, fake_borrow_temps)
 413                 })
 414             }
 415             ExprKind::Field { lhs, name } => {
 416                 let place_builder =
 417                     unpack!(block = this.expr_as_place(block, lhs, mutability, fake_borrow_temps,));
 418                 block.and(place_builder.field(name, expr.ty))
 419             }
 420             ExprKind::Deref { arg } => {
 421                 let place_builder =
 422                     unpack!(block = this.expr_as_place(block, arg, mutability, fake_borrow_temps,));
 423                 block.and(place_builder.deref())
 424             }
 425             ExprKind::Index { lhs, index } => this.lower_index_expression(
 426                 block,
 427                 lhs,
 428                 index,
 429                 mutability,
 430                 fake_borrow_temps,
 431                 expr.temp_lifetime,
 432                 expr_span,
 433                 source_info,
 434             ),
 435             ExprKind::UpvarRef { closure_def_id, var_hir_id } => {
 436                 let upvar_id = ty::UpvarId::new(var_hir_id, closure_def_id.expect_local());
 437                 this.lower_captured_upvar(block, upvar_id)
 438             }
 439
 440             ExprKind::VarRef { id } => {
 441                 let place_builder = if this.is_bound_var_in_guard(id) {
 442                     let index = this.var_local_id(id, RefWithinGuard);
 443                     PlaceBuilder::from(index).deref()
 444                 } else {
 445                     let index = this.var_local_id(id, OutsideGuard);
 446                     PlaceBuilder::from(index)
 447                 };
 448                 block.and(place_builder)
 449             }
 450
 451             ExprKind::PlaceTypeAscription { source, user_ty } => {
 452                 let place_builder = unpack!(
 453                     block = this.expr_as_place(block, source, mutability, fake_borrow_temps,)
 454                 );
 455                 if let Some(user_ty) = user_ty {
 456                     let annotation_index =
 457                         this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
 458                             span: source_info.span,
 459                             user_ty,
 460                             inferred_ty: expr.ty,
 461                         });
 462
 463                     let place = place_builder.clone().into_place(this.tcx, this.typeck_results);
 464                     this.cfg.push(
 465                         block,
 466                         Statement {
 467                             source_info,
 468                             kind: StatementKind::AscribeUserType(
 469                                 box (
 470                                     place,
 471                                     UserTypeProjection { base: annotation_index, projs: vec![] },
 472                                 ),
 473                                 Variance::Invariant,
 474                             ),
 475                         },
 476                     );
 477                 }
 478                 block.and(place_builder)
 479             }
 480             ExprKind::ValueTypeAscription { source, user_ty } => {
 481                 let temp =
 482                     unpack!(block = this.as_temp(block, source.temp_lifetime, source, mutability));
 483                 if let Some(user_ty) = user_ty {
 484                     let annotation_index =
 485                         this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
 486                             span: source_info.span,
 487                             user_ty,
 488                             inferred_ty: expr.ty,
 489                         });
 490                     this.cfg.push(
 491                         block,
 492                         Statement {
 493                             source_info,
 494                             kind: StatementKind::AscribeUserType(
 495                                 box (
 496                                     Place::from(temp),
 497                                     UserTypeProjection { base: annotation_index, projs: vec![] },
 498                                 ),
 499                                 Variance::Invariant,
 500                             ),
 501                         },
 502                     );
 503                 }
 504                 block.and(PlaceBuilder::from(temp))
 505             }
 506
 507             ExprKind::Array { .. }
 508             | ExprKind::Tuple { .. }
 509             | ExprKind::Adt { .. }
 510             | ExprKind::Closure { .. }
 511             | ExprKind::Unary { .. }
 512             | ExprKind::Binary { .. }
 513             | ExprKind::LogicalOp { .. }
 514             | ExprKind::Box { .. }
 515             | ExprKind::Cast { .. }
 516             | ExprKind::Use { .. }
 517             | ExprKind::NeverToAny { .. }
 518             | ExprKind::Pointer { .. }
 519             | ExprKind::Repeat { .. }
 520             | ExprKind::Borrow { .. }
 521             | ExprKind::AddressOf { .. }
 522             | ExprKind::Match { .. }
 523             | ExprKind::If { .. }
 524             | ExprKind::Loop { .. }
 525             | ExprKind::Block { .. }
 526             | ExprKind::Assign { .. }
 527             | ExprKind::AssignOp { .. }
 528             | ExprKind::Break { .. }
 529             | ExprKind::Continue { .. }
 530             | ExprKind::Return { .. }
 531             | ExprKind::Literal { .. }
 532             | ExprKind::ConstBlock { .. }
 533             | ExprKind::StaticRef { .. }
 534             | ExprKind::InlineAsm { .. }
 535             | ExprKind::LlvmInlineAsm { .. }
 536             | ExprKind::Yield { .. }
 537             | ExprKind::ThreadLocalRef(_)
 538             | ExprKind::Call { .. } => {
 539                 // these are not places, so we need to make a temporary.
 540                 debug_assert!(!matches!(Category::of(&expr.kind), Some(Category::Place)));
 541                 let temp =
 542                     unpack!(block = this.as_temp(block, expr.temp_lifetime, expr, mutability));
 543                 block.and(PlaceBuilder::from(temp))
 544             }
 545         }
 546     }
 547
 548     /// Lower a captured upvar. Note we might not know the actual capture index,
 549     /// so we create a place starting from `PlaceBase::Upvar`, which will be resolved
 550     /// once all projections that allow us to indentify a capture have been applied.
 551     fn lower_captured_upvar(
 552         &mut self,
 553         block: BasicBlock,
 554         upvar_id: ty::UpvarId,
 555     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 556         let closure_ty = self
 557             .typeck_results
 558             .node_type(self.tcx.hir().local_def_id_to_hir_id(upvar_id.closure_expr_id));
 559
 560         let closure_kind = if let ty::Closure(_, closure_substs) = closure_ty.kind() {
 561             self.infcx.closure_kind(closure_substs).unwrap()
 562         } else {
 563             // Generators are considered FnOnce.
 564             ty::ClosureKind::FnOnce
 565         };
 566
 567         block.and(PlaceBuilder::from(PlaceBase::Upvar {
 568             var_hir_id: upvar_id.var_path.hir_id,
 569             closure_def_id: upvar_id.closure_expr_id.to_def_id(),
 570             closure_kind,
 571         }))
 572     }
 573
 574     /// Lower an index expression
 575     ///
 576     /// This has two complications;
 577     ///
 578     /// * We need to do a bounds check.
 579     /// * We need to ensure that the bounds check can't be invalidated using an
 580     ///   expression like `x[1][{x = y; 2}]`. We use fake borrows here to ensure
 581     ///   that this is the case.
 582     fn lower_index_expression(
 583         &mut self,
 584         mut block: BasicBlock,
 585         base: &Expr<'_, 'tcx>,
 586         index: &Expr<'_, 'tcx>,
 587         mutability: Mutability,
 588         fake_borrow_temps: Option<&mut Vec<Local>>,
 589         temp_lifetime: Option<region::Scope>,
 590         expr_span: Span,
 591         source_info: SourceInfo,
 592     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 593         let base_fake_borrow_temps = &mut Vec::new();
 594         let is_outermost_index = fake_borrow_temps.is_none();
 595         let fake_borrow_temps = fake_borrow_temps.unwrap_or(base_fake_borrow_temps);
 596
 597         let mut base_place =
 598             unpack!(block = self.expr_as_place(block, base, mutability, Some(fake_borrow_temps),));
 599
 600         // Making this a *fresh* temporary means we do not have to worry about
 601         // the index changing later: Nothing will ever change this temporary.
 602         // The "retagging" transformation (for Stacked Borrows) relies on this.
 603         let idx = unpack!(block = self.as_temp(block, temp_lifetime, index, Mutability::Not,));
 604
 605         block = self.bounds_check(
 606             block,
 607             base_place.clone().into_place(self.tcx, self.typeck_results),
 608             idx,
 609             expr_span,
 610             source_info,
 611         );
 612
 613         if is_outermost_index {
 614             self.read_fake_borrows(block, fake_borrow_temps, source_info)
 615         } else {
 616             base_place = base_place.expect_upvars_resolved(self.tcx, self.typeck_results);
 617             self.add_fake_borrows_of_base(
 618                 &base_place,
 619                 block,
 620                 fake_borrow_temps,
 621                 expr_span,
 622                 source_info,
 623             );
 624         }
 625
 626         block.and(base_place.index(idx))
 627     }
 628
 629     fn bounds_check(
 630         &mut self,
 631         block: BasicBlock,
 632         slice: Place<'tcx>,
 633         index: Local,
 634         expr_span: Span,
 635         source_info: SourceInfo,
 636     ) -> BasicBlock {
 637         let usize_ty = self.tcx.types.usize;
 638         let bool_ty = self.tcx.types.bool;
 639         // bounds check:
 640         let len = self.temp(usize_ty, expr_span);
 641         let lt = self.temp(bool_ty, expr_span);
 642
 643         // len = len(slice)
 644         self.cfg.push_assign(block, source_info, len, Rvalue::Len(slice));
 645         // lt = idx < len
 646         self.cfg.push_assign(
 647             block,
 648             source_info,
 649             lt,
 650             Rvalue::BinaryOp(
 651                 BinOp::Lt,
 652                 box (Operand::Copy(Place::from(index)), Operand::Copy(len)),
 653             ),
 654         );
 655         let msg = BoundsCheck { len: Operand::Move(len), index: Operand::Copy(Place::from(index)) };
 656         // assert!(lt, "...")
 657         self.assert(block, Operand::Move(lt), true, msg, expr_span)
 658     }
 659
 660     fn add_fake_borrows_of_base(
 661         &mut self,
 662         base_place: &PlaceBuilder<'tcx>,
 663         block: BasicBlock,
 664         fake_borrow_temps: &mut Vec<Local>,
 665         expr_span: Span,
 666         source_info: SourceInfo,
 667     ) {
 668         let tcx = self.tcx;
 669         let local = match base_place.base {
 670             PlaceBase::Local(local) => local,
 671             PlaceBase::Upvar { .. } => bug!("Expected PlacseBase::Local found Upvar"),
 672         };
 673
 674         let place_ty = Place::ty_from(local, &base_place.projection, &self.local_decls, tcx);
 675         if let ty::Slice(_) = place_ty.ty.kind() {
 676             // We need to create fake borrows to ensure that the bounds
 677             // check that we just did stays valid. Since we can't assign to
 678             // unsized values, we only need to ensure that none of the
 679             // pointers in the base place are modified.
 680             for (idx, elem) in base_place.projection.iter().enumerate().rev() {
 681                 match elem {
 682                     ProjectionElem::Deref => {
 683                         let fake_borrow_deref_ty = Place::ty_from(
 684                             local,
 685                             &base_place.projection[..idx],
 686                             &self.local_decls,
 687                             tcx,
 688                         )
 689                         .ty;
 690                         let fake_borrow_ty =
 691                             tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty);
 692                         let fake_borrow_temp =
 693                             self.local_decls.push(LocalDecl::new(fake_borrow_ty, expr_span));
 694                         let projection = tcx.intern_place_elems(&base_place.projection[..idx]);
 695                         self.cfg.push_assign(
 696                             block,
 697                             source_info,
 698                             fake_borrow_temp.into(),
 699                             Rvalue::Ref(
 700                                 tcx.lifetimes.re_erased,
 701                                 BorrowKind::Shallow,
 702                                 Place { local, projection },
 703                             ),
 704                         );
 705                         fake_borrow_temps.push(fake_borrow_temp);
 706                     }
 707                     ProjectionElem::Index(_) => {
 708                         let index_ty = Place::ty_from(
 709                             local,
 710                             &base_place.projection[..idx],
 711                             &self.local_decls,
 712                             tcx,
 713                         );
 714                         match index_ty.ty.kind() {
 715                             // The previous index expression has already
 716                             // done any index expressions needed here.
 717                             ty::Slice(_) => break,
 718                             ty::Array(..) => (),
 719                             _ => bug!("unexpected index base"),
 720                         }
 721                     }
 722                     ProjectionElem::Field(..)
 723                     | ProjectionElem::Downcast(..)
 724                     | ProjectionElem::ConstantIndex { .. }
 725                     | ProjectionElem::Subslice { .. } => (),
 726                 }
 727             }
 728         }
 729     }
 730
 731     fn read_fake_borrows(
 732         &mut self,
 733         bb: BasicBlock,
 734         fake_borrow_temps: &mut Vec<Local>,
 735         source_info: SourceInfo,
 736     ) {
 737         // All indexes have been evaluated now, read all of the
 738         // fake borrows so that they are live across those index
 739         // expressions.
 740         for temp in fake_borrow_temps {
 741             self.cfg.push_fake_read(bb, source_info, FakeReadCause::ForIndex, Place::from(*temp));
 742         }
 743     }
 744 }