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, Capture, CaptureMap};
   6 use rustc_hir::def_id::LocalDefId;
   7 use rustc_middle::hir::place::Projection as HirProjection;
   8 use rustc_middle::hir::place::ProjectionKind as HirProjectionKind;
   9 use rustc_middle::middle::region;
  10 use rustc_middle::mir::AssertKind::BoundsCheck;
  11 use rustc_middle::mir::*;
  12 use rustc_middle::thir::*;
  13 use rustc_middle::ty::AdtDef;
  14 use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty, TyCtxt, Variance};
  15 use rustc_span::Span;
  16 use rustc_target::abi::VariantIdx;
  17
  18 use rustc_index::vec::Idx;
  19
  20 use std::assert_matches::assert_matches;
  21 use std::iter;
  22
  23 /// The "outermost" place that holds this value.
  24 #[derive(Copy, Clone, Debug, PartialEq)]
  25 pub(crate) enum PlaceBase {
  26     /// Denotes the start of a `Place`.
  27     Local(Local),
  28
  29     /// When building place for an expression within a closure, the place might start off a
  30     /// captured path. When `capture_disjoint_fields` is enabled, we might not know the capture
  31     /// index (within the desugared closure) of the captured path until most of the projections
  32     /// are applied. We use `PlaceBase::Upvar` to keep track of the root variable off of which the
  33     /// captured path starts, the closure the capture belongs to and the trait the closure
  34     /// implements.
  35     ///
  36     /// Once we have figured out the capture index, we can convert the place builder to start from
  37     /// `PlaceBase::Local`.
  38     ///
  39     /// Consider the following example
  40     /// ```rust
  41     /// let t = (((10, 10), 10), 10);
  42     ///
  43     /// let c = || {
  44     ///     println!("{}", t.0.0.0);
  45     /// };
  46     /// ```
  47     /// Here the THIR expression for `t.0.0.0` will be something like
  48     ///
  49     /// ```ignore (illustrative)
  50     /// * Field(0)
  51     ///     * Field(0)
  52     ///         * Field(0)
  53     ///             * UpvarRef(t)
  54     /// ```
  55     ///
  56     /// When `capture_disjoint_fields` is enabled, `t.0.0.0` is captured and we won't be able to
  57     /// figure out that it is captured until all the `Field` projections are applied.
  58     Upvar {
  59         /// HirId of the upvar
  60         var_hir_id: LocalVarId,
  61         /// DefId of the closure
  62         closure_def_id: LocalDefId,
  63     },
  64 }
  65
  66 /// `PlaceBuilder` is used to create places during MIR construction. It allows you to "build up" a
  67 /// place by pushing more and more projections onto the end, and then convert the final set into a
  68 /// place using the `to_place` method.
  69 ///
  70 /// This is used internally when building a place for an expression like `a.b.c`. The fields `b`
  71 /// and `c` can be progressively pushed onto the place builder that is created when converting `a`.
  72 #[derive(Clone, Debug, PartialEq)]
  73 pub(in crate::build) struct PlaceBuilder<'tcx> {
  74     base: PlaceBase,
  75     projection: Vec<PlaceElem<'tcx>>,
  76 }
  77
  78 /// Given a list of MIR projections, convert them to list of HIR ProjectionKind.
  79 /// The projections are truncated to represent a path that might be captured by a
  80 /// closure/generator. This implies the vector returned from this function doesn't contain
  81 /// ProjectionElems `Downcast`, `ConstantIndex`, `Index`, or `Subslice` because those will never be
  82 /// part of a path that is captured by a closure. We stop applying projections once we see the first
  83 /// projection that isn't captured by a closure.
  84 fn convert_to_hir_projections_and_truncate_for_capture<'tcx>(
  85     mir_projections: &[PlaceElem<'tcx>],
  86 ) -> Vec<HirProjectionKind> {
  87     let mut hir_projections = Vec::new();
  88     let mut variant = None;
  89
  90     for mir_projection in mir_projections {
  91         let hir_projection = match mir_projection {
  92             ProjectionElem::Deref => HirProjectionKind::Deref,
  93             ProjectionElem::Field(field, _) => {
  94                 let variant = variant.unwrap_or(VariantIdx::new(0));
  95                 HirProjectionKind::Field(field.index() as u32, variant)
  96             }
  97             ProjectionElem::Downcast(.., idx) => {
  98                 // We don't expect to see multi-variant enums here, as earlier
  99                 // phases will have truncated them already. However, there can
 100                 // still be downcasts, thanks to single-variant enums.
 101                 // We keep track of VariantIdx so we can use this information
 102                 // if the next ProjectionElem is a Field.
 103                 variant = Some(*idx);
 104                 continue;
 105             }
 106             // These do not affect anything, they just make sure we know the right type.
 107             ProjectionElem::OpaqueCast(_) => continue,
 108             ProjectionElem::Index(..)
 109             | ProjectionElem::ConstantIndex { .. }
 110             | ProjectionElem::Subslice { .. } => {
 111                 // We don't capture array-access projections.
 112                 // We can stop here as arrays are captured completely.
 113                 break;
 114             }
 115         };
 116         variant = None;
 117         hir_projections.push(hir_projection);
 118     }
 119
 120     hir_projections
 121 }
 122
 123 /// Return true if the `proj_possible_ancestor` represents an ancestor path
 124 /// to `proj_capture` or `proj_possible_ancestor` is same as `proj_capture`,
 125 /// assuming they both start off of the same root variable.
 126 ///
 127 /// **Note:** It's the caller's responsibility to ensure that both lists of projections
 128 ///           start off of the same root variable.
 129 ///
 130 /// Eg: 1. `foo.x` which is represented using `projections=[Field(x)]` is an ancestor of
 131 ///        `foo.x.y` which is represented using `projections=[Field(x), Field(y)]`.
 132 ///        Note both `foo.x` and `foo.x.y` start off of the same root variable `foo`.
 133 ///     2. Since we only look at the projections here function will return `bar.x` as an a valid
 134 ///        ancestor of `foo.x.y`. It's the caller's responsibility to ensure that both projections
 135 ///        list are being applied to the same root variable.
 136 fn is_ancestor_or_same_capture(
 137     proj_possible_ancestor: &[HirProjectionKind],
 138     proj_capture: &[HirProjectionKind],
 139 ) -> bool {
 140     // We want to make sure `is_ancestor_or_same_capture("x.0.0", "x.0")` to return false.
 141     // Therefore we can't just check if all projections are same in the zipped iterator below.
 142     if proj_possible_ancestor.len() > proj_capture.len() {
 143         return false;
 144     }
 145
 146     iter::zip(proj_possible_ancestor, proj_capture).all(|(a, b)| a == b)
 147 }
 148
 149 /// Given a closure, returns the index of a capture within the desugared closure struct and the
 150 /// `ty::CapturedPlace` which is the ancestor of the Place represented using the `var_hir_id`
 151 /// and `projection`.
 152 ///
 153 /// Note there will be at most one ancestor for any given Place.
 154 ///
 155 /// Returns None, when the ancestor is not found.
 156 fn find_capture_matching_projections<'a, 'tcx>(
 157     upvars: &'a CaptureMap<'tcx>,
 158     var_hir_id: LocalVarId,
 159     projections: &[PlaceElem<'tcx>],
 160 ) -> Option<(usize, &'a Capture<'tcx>)> {
 161     let hir_projections = convert_to_hir_projections_and_truncate_for_capture(projections);
 162
 163     upvars.get_by_key_enumerated(var_hir_id.0).find(|(_, capture)| {
 164         let possible_ancestor_proj_kinds: Vec<_> =
 165             capture.captured_place.place.projections.iter().map(|proj| proj.kind).collect();
 166         is_ancestor_or_same_capture(&possible_ancestor_proj_kinds, &hir_projections)
 167     })
 168 }
 169
 170 /// Takes an upvar place and tries to resolve it into a `PlaceBuilder`
 171 /// with `PlaceBase::Local`
 172 #[instrument(level = "trace", skip(cx), ret)]
 173 fn to_upvars_resolved_place_builder<'tcx>(
 174     cx: &Builder<'_, 'tcx>,
 175     var_hir_id: LocalVarId,
 176     closure_def_id: LocalDefId,
 177     projection: &[PlaceElem<'tcx>],
 178 ) -> Option<PlaceBuilder<'tcx>> {
 179     let Some((capture_index, capture)) =
 180         find_capture_matching_projections(
 181             &cx.upvars,
 182             var_hir_id,
 183             &projection,
 184         ) else {
 185         let closure_span = cx.tcx.def_span(closure_def_id);
 186         if !enable_precise_capture(cx.tcx, closure_span) {
 187             bug!(
 188                 "No associated capture found for {:?}[{:#?}] even though \
 189                     capture_disjoint_fields isn't enabled",
 190                 var_hir_id,
 191                 projection
 192             )
 193         } else {
 194             debug!(
 195                 "No associated capture found for {:?}[{:#?}]",
 196                 var_hir_id, projection,
 197             );
 198         }
 199         return None;
 200     };
 201
 202     // Access the capture by accessing the field within the Closure struct.
 203     let capture_info = &cx.upvars[capture_index];
 204
 205     let mut upvar_resolved_place_builder = PlaceBuilder::from(capture_info.use_place);
 206
 207     // We used some of the projections to build the capture itself,
 208     // now we apply the remaining to the upvar resolved place.
 209     trace!(?capture.captured_place, ?projection);
 210     let remaining_projections = strip_prefix(
 211         capture.captured_place.place.base_ty,
 212         projection,
 213         &capture.captured_place.place.projections,
 214     );
 215     upvar_resolved_place_builder.projection.extend(remaining_projections);
 216
 217     Some(upvar_resolved_place_builder)
 218 }
 219
 220 /// Returns projections remaining after stripping an initial prefix of HIR
 221 /// projections.
 222 ///
 223 /// Supports only HIR projection kinds that represent a path that might be
 224 /// captured by a closure or a generator, i.e., an `Index` or a `Subslice`
 225 /// projection kinds are unsupported.
 226 fn strip_prefix<'a, 'tcx>(
 227     mut base_ty: Ty<'tcx>,
 228     projections: &'a [PlaceElem<'tcx>],
 229     prefix_projections: &[HirProjection<'tcx>],
 230 ) -> impl Iterator<Item = PlaceElem<'tcx>> + 'a {
 231     let mut iter = projections
 232         .iter()
 233         .copied()
 234         // Filter out opaque casts, they are unnecessary in the prefix.
 235         .filter(|elem| !matches!(elem, ProjectionElem::OpaqueCast(..)));
 236     for projection in prefix_projections {
 237         match projection.kind {
 238             HirProjectionKind::Deref => {
 239                 assert_matches!(iter.next(), Some(ProjectionElem::Deref));
 240             }
 241             HirProjectionKind::Field(..) => {
 242                 if base_ty.is_enum() {
 243                     assert_matches!(iter.next(), Some(ProjectionElem::Downcast(..)));
 244                 }
 245                 assert_matches!(iter.next(), Some(ProjectionElem::Field(..)));
 246             }
 247             HirProjectionKind::Index | HirProjectionKind::Subslice => {
 248                 bug!("unexpected projection kind: {:?}", projection);
 249             }
 250         }
 251         base_ty = projection.ty;
 252     }
 253     iter
 254 }
 255
 256 impl<'tcx> PlaceBuilder<'tcx> {
 257     pub(in crate::build) fn to_place(&self, cx: &Builder<'_, 'tcx>) -> Place<'tcx> {
 258         self.try_to_place(cx).unwrap()
 259     }
 260
 261     /// Creates a `Place` or returns `None` if an upvar cannot be resolved
 262     pub(in crate::build) fn try_to_place(&self, cx: &Builder<'_, 'tcx>) -> Option<Place<'tcx>> {
 263         let resolved = self.resolve_upvar(cx);
 264         let builder = resolved.as_ref().unwrap_or(self);
 265         let PlaceBase::Local(local) = builder.base else { return None };
 266         let projection = cx.tcx.intern_place_elems(&builder.projection);
 267         Some(Place { local, projection })
 268     }
 269
 270     /// Attempts to resolve the `PlaceBuilder`.
 271     /// Returns `None` if this is not an upvar.
 272     ///
 273     /// Upvars resolve may fail for a `PlaceBuilder` when attempting to
 274     /// resolve a disjoint field whose root variable is not captured
 275     /// (destructured assignments) or when attempting to resolve a root
 276     /// variable (discriminant matching with only wildcard arm) that is
 277     /// not captured. This can happen because the final mir that will be
 278     /// generated doesn't require a read for this place. Failures will only
 279     /// happen inside closures.
 280     pub(in crate::build) fn resolve_upvar(
 281         &self,
 282         cx: &Builder<'_, 'tcx>,
 283     ) -> Option<PlaceBuilder<'tcx>> {
 284         let PlaceBase::Upvar { var_hir_id, closure_def_id } = self.base else {
 285             return None;
 286         };
 287         to_upvars_resolved_place_builder(cx, var_hir_id, closure_def_id, &self.projection)
 288     }
 289
 290     pub(crate) fn base(&self) -> PlaceBase {
 291         self.base
 292     }
 293
 294     pub(crate) fn projection(&self) -> &[PlaceElem<'tcx>] {
 295         &self.projection
 296     }
 297
 298     pub(crate) fn field(self, f: Field, ty: Ty<'tcx>) -> Self {
 299         self.project(PlaceElem::Field(f, ty))
 300     }
 301
 302     pub(crate) fn deref(self) -> Self {
 303         self.project(PlaceElem::Deref)
 304     }
 305
 306     pub(crate) fn downcast(self, adt_def: AdtDef<'tcx>, variant_index: VariantIdx) -> Self {
 307         self.project(PlaceElem::Downcast(Some(adt_def.variant(variant_index).name), variant_index))
 308     }
 309
 310     fn index(self, index: Local) -> Self {
 311         self.project(PlaceElem::Index(index))
 312     }
 313
 314     pub(crate) fn project(mut self, elem: PlaceElem<'tcx>) -> Self {
 315         self.projection.push(elem);
 316         self
 317     }
 318
 319     /// Same as `.clone().project(..)` but more efficient
 320     pub(crate) fn clone_project(&self, elem: PlaceElem<'tcx>) -> Self {
 321         Self {
 322             base: self.base,
 323             projection: Vec::from_iter(self.projection.iter().copied().chain([elem])),
 324         }
 325     }
 326 }
 327
 328 impl<'tcx> From<Local> for PlaceBuilder<'tcx> {
 329     fn from(local: Local) -> Self {
 330         Self { base: PlaceBase::Local(local), projection: Vec::new() }
 331     }
 332 }
 333
 334 impl<'tcx> From<PlaceBase> for PlaceBuilder<'tcx> {
 335     fn from(base: PlaceBase) -> Self {
 336         Self { base, projection: Vec::new() }
 337     }
 338 }
 339
 340 impl<'tcx> From<Place<'tcx>> for PlaceBuilder<'tcx> {
 341     fn from(p: Place<'tcx>) -> Self {
 342         Self { base: PlaceBase::Local(p.local), projection: p.projection.to_vec() }
 343     }
 344 }
 345
 346 impl<'a, 'tcx> Builder<'a, 'tcx> {
 347     /// Compile `expr`, yielding a place that we can move from etc.
 348     ///
 349     /// WARNING: Any user code might:
 350     /// * Invalidate any slice bounds checks performed.
 351     /// * Change the address that this `Place` refers to.
 352     /// * Modify the memory that this place refers to.
 353     /// * Invalidate the memory that this place refers to, this will be caught
 354     ///   by borrow checking.
 355     ///
 356     /// Extra care is needed if any user code is allowed to run between calling
 357     /// this method and using it, as is the case for `match` and index
 358     /// expressions.
 359     pub(crate) fn as_place(
 360         &mut self,
 361         mut block: BasicBlock,
 362         expr: &Expr<'tcx>,
 363     ) -> BlockAnd<Place<'tcx>> {
 364         let place_builder = unpack!(block = self.as_place_builder(block, expr));
 365         block.and(place_builder.to_place(self))
 366     }
 367
 368     /// This is used when constructing a compound `Place`, so that we can avoid creating
 369     /// intermediate `Place` values until we know the full set of projections.
 370     pub(crate) fn as_place_builder(
 371         &mut self,
 372         block: BasicBlock,
 373         expr: &Expr<'tcx>,
 374     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 375         self.expr_as_place(block, expr, Mutability::Mut, None)
 376     }
 377
 378     /// Compile `expr`, yielding a place that we can move from etc.
 379     /// Mutability note: The caller of this method promises only to read from the resulting
 380     /// place. The place itself may or may not be mutable:
 381     /// * If this expr is a place expr like a.b, then we will return that place.
 382     /// * Otherwise, a temporary is created: in that event, it will be an immutable temporary.
 383     pub(crate) fn as_read_only_place(
 384         &mut self,
 385         mut block: BasicBlock,
 386         expr: &Expr<'tcx>,
 387     ) -> BlockAnd<Place<'tcx>> {
 388         let place_builder = unpack!(block = self.as_read_only_place_builder(block, expr));
 389         block.and(place_builder.to_place(self))
 390     }
 391
 392     /// This is used when constructing a compound `Place`, so that we can avoid creating
 393     /// intermediate `Place` values until we know the full set of projections.
 394     /// Mutability note: The caller of this method promises only to read from the resulting
 395     /// place. The place itself may or may not be mutable:
 396     /// * If this expr is a place expr like a.b, then we will return that place.
 397     /// * Otherwise, a temporary is created: in that event, it will be an immutable temporary.
 398     fn as_read_only_place_builder(
 399         &mut self,
 400         block: BasicBlock,
 401         expr: &Expr<'tcx>,
 402     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 403         self.expr_as_place(block, expr, Mutability::Not, None)
 404     }
 405
 406     fn expr_as_place(
 407         &mut self,
 408         mut block: BasicBlock,
 409         expr: &Expr<'tcx>,
 410         mutability: Mutability,
 411         fake_borrow_temps: Option<&mut Vec<Local>>,
 412     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 413         debug!("expr_as_place(block={:?}, expr={:?}, mutability={:?})", block, expr, mutability);
 414
 415         let this = self;
 416         let expr_span = expr.span;
 417         let source_info = this.source_info(expr_span);
 418         match expr.kind {
 419             ExprKind::Scope { region_scope, lint_level, value } => {
 420                 this.in_scope((region_scope, source_info), lint_level, |this| {
 421                     this.expr_as_place(block, &this.thir[value], mutability, fake_borrow_temps)
 422                 })
 423             }
 424             ExprKind::Field { lhs, variant_index, name } => {
 425                 let lhs = &this.thir[lhs];
 426                 let mut place_builder =
 427                     unpack!(block = this.expr_as_place(block, lhs, mutability, fake_borrow_temps,));
 428                 if let ty::Adt(adt_def, _) = lhs.ty.kind() {
 429                     if adt_def.is_enum() {
 430                         place_builder = place_builder.downcast(*adt_def, variant_index);
 431                     }
 432                 }
 433                 block.and(place_builder.field(name, expr.ty))
 434             }
 435             ExprKind::Deref { arg } => {
 436                 let place_builder = unpack!(
 437                     block =
 438                         this.expr_as_place(block, &this.thir[arg], mutability, fake_borrow_temps,)
 439                 );
 440                 block.and(place_builder.deref())
 441             }
 442             ExprKind::Index { lhs, index } => this.lower_index_expression(
 443                 block,
 444                 &this.thir[lhs],
 445                 &this.thir[index],
 446                 mutability,
 447                 fake_borrow_temps,
 448                 expr.temp_lifetime,
 449                 expr_span,
 450                 source_info,
 451             ),
 452             ExprKind::UpvarRef { closure_def_id, var_hir_id } => {
 453                 this.lower_captured_upvar(block, closure_def_id.expect_local(), var_hir_id)
 454             }
 455
 456             ExprKind::VarRef { id } => {
 457                 let place_builder = if this.is_bound_var_in_guard(id) {
 458                     let index = this.var_local_id(id, RefWithinGuard);
 459                     PlaceBuilder::from(index).deref()
 460                 } else {
 461                     let index = this.var_local_id(id, OutsideGuard);
 462                     PlaceBuilder::from(index)
 463                 };
 464                 block.and(place_builder)
 465             }
 466
 467             ExprKind::PlaceTypeAscription { source, ref user_ty } => {
 468                 let place_builder = unpack!(
 469                     block = this.expr_as_place(
 470                         block,
 471                         &this.thir[source],
 472                         mutability,
 473                         fake_borrow_temps,
 474                     )
 475                 );
 476                 if let Some(user_ty) = user_ty {
 477                     let annotation_index =
 478                         this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
 479                             span: source_info.span,
 480                             user_ty: user_ty.clone(),
 481                             inferred_ty: expr.ty,
 482                         });
 483
 484                     let place = place_builder.to_place(this);
 485                     this.cfg.push(
 486                         block,
 487                         Statement {
 488                             source_info,
 489                             kind: StatementKind::AscribeUserType(
 490                                 Box::new((
 491                                     place,
 492                                     UserTypeProjection { base: annotation_index, projs: vec![] },
 493                                 )),
 494                                 Variance::Invariant,
 495                             ),
 496                         },
 497                     );
 498                 }
 499                 block.and(place_builder)
 500             }
 501             ExprKind::ValueTypeAscription { source, ref user_ty } => {
 502                 let source = &this.thir[source];
 503                 let temp =
 504                     unpack!(block = this.as_temp(block, source.temp_lifetime, source, mutability));
 505                 if let Some(user_ty) = user_ty {
 506                     let annotation_index =
 507                         this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
 508                             span: source_info.span,
 509                             user_ty: user_ty.clone(),
 510                             inferred_ty: expr.ty,
 511                         });
 512                     this.cfg.push(
 513                         block,
 514                         Statement {
 515                             source_info,
 516                             kind: StatementKind::AscribeUserType(
 517                                 Box::new((
 518                                     Place::from(temp),
 519                                     UserTypeProjection { base: annotation_index, projs: vec![] },
 520                                 )),
 521                                 Variance::Invariant,
 522                             ),
 523                         },
 524                     );
 525                 }
 526                 block.and(PlaceBuilder::from(temp))
 527             }
 528
 529             ExprKind::Array { .. }
 530             | ExprKind::Tuple { .. }
 531             | ExprKind::Adt { .. }
 532             | ExprKind::Closure { .. }
 533             | ExprKind::Unary { .. }
 534             | ExprKind::Binary { .. }
 535             | ExprKind::LogicalOp { .. }
 536             | ExprKind::Box { .. }
 537             | ExprKind::Cast { .. }
 538             | ExprKind::Use { .. }
 539             | ExprKind::NeverToAny { .. }
 540             | ExprKind::Pointer { .. }
 541             | ExprKind::Repeat { .. }
 542             | ExprKind::Borrow { .. }
 543             | ExprKind::AddressOf { .. }
 544             | ExprKind::Match { .. }
 545             | ExprKind::If { .. }
 546             | ExprKind::Loop { .. }
 547             | ExprKind::Block { .. }
 548             | ExprKind::Let { .. }
 549             | ExprKind::Assign { .. }
 550             | ExprKind::AssignOp { .. }
 551             | ExprKind::Break { .. }
 552             | ExprKind::Continue { .. }
 553             | ExprKind::Return { .. }
 554             | ExprKind::Literal { .. }
 555             | ExprKind::NamedConst { .. }
 556             | ExprKind::NonHirLiteral { .. }
 557             | ExprKind::ZstLiteral { .. }
 558             | ExprKind::ConstParam { .. }
 559             | ExprKind::ConstBlock { .. }
 560             | ExprKind::StaticRef { .. }
 561             | ExprKind::InlineAsm { .. }
 562             | ExprKind::Yield { .. }
 563             | ExprKind::ThreadLocalRef(_)
 564             | ExprKind::Call { .. } => {
 565                 // these are not places, so we need to make a temporary.
 566                 debug_assert!(!matches!(Category::of(&expr.kind), Some(Category::Place)));
 567                 let temp =
 568                     unpack!(block = this.as_temp(block, expr.temp_lifetime, expr, mutability));
 569                 block.and(PlaceBuilder::from(temp))
 570             }
 571         }
 572     }
 573
 574     /// Lower a captured upvar. Note we might not know the actual capture index,
 575     /// so we create a place starting from `PlaceBase::Upvar`, which will be resolved
 576     /// once all projections that allow us to identify a capture have been applied.
 577     fn lower_captured_upvar(
 578         &mut self,
 579         block: BasicBlock,
 580         closure_def_id: LocalDefId,
 581         var_hir_id: LocalVarId,
 582     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 583         block.and(PlaceBuilder::from(PlaceBase::Upvar { var_hir_id, closure_def_id }))
 584     }
 585
 586     /// Lower an index expression
 587     ///
 588     /// This has two complications;
 589     ///
 590     /// * We need to do a bounds check.
 591     /// * We need to ensure that the bounds check can't be invalidated using an
 592     ///   expression like `x[1][{x = y; 2}]`. We use fake borrows here to ensure
 593     ///   that this is the case.
 594     fn lower_index_expression(
 595         &mut self,
 596         mut block: BasicBlock,
 597         base: &Expr<'tcx>,
 598         index: &Expr<'tcx>,
 599         mutability: Mutability,
 600         fake_borrow_temps: Option<&mut Vec<Local>>,
 601         temp_lifetime: Option<region::Scope>,
 602         expr_span: Span,
 603         source_info: SourceInfo,
 604     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 605         let base_fake_borrow_temps = &mut Vec::new();
 606         let is_outermost_index = fake_borrow_temps.is_none();
 607         let fake_borrow_temps = fake_borrow_temps.unwrap_or(base_fake_borrow_temps);
 608
 609         let base_place =
 610             unpack!(block = self.expr_as_place(block, base, mutability, Some(fake_borrow_temps),));
 611
 612         // Making this a *fresh* temporary means we do not have to worry about
 613         // the index changing later: Nothing will ever change this temporary.
 614         // The "retagging" transformation (for Stacked Borrows) relies on this.
 615         let idx = unpack!(block = self.as_temp(block, temp_lifetime, index, Mutability::Not,));
 616
 617         block = self.bounds_check(block, &base_place, idx, expr_span, source_info);
 618
 619         if is_outermost_index {
 620             self.read_fake_borrows(block, fake_borrow_temps, source_info)
 621         } else {
 622             self.add_fake_borrows_of_base(
 623                 base_place.to_place(self),
 624                 block,
 625                 fake_borrow_temps,
 626                 expr_span,
 627                 source_info,
 628             );
 629         }
 630
 631         block.and(base_place.index(idx))
 632     }
 633
 634     fn bounds_check(
 635         &mut self,
 636         block: BasicBlock,
 637         slice: &PlaceBuilder<'tcx>,
 638         index: Local,
 639         expr_span: Span,
 640         source_info: SourceInfo,
 641     ) -> BasicBlock {
 642         let usize_ty = self.tcx.types.usize;
 643         let bool_ty = self.tcx.types.bool;
 644         // bounds check:
 645         let len = self.temp(usize_ty, expr_span);
 646         let lt = self.temp(bool_ty, expr_span);
 647
 648         // len = len(slice)
 649         self.cfg.push_assign(block, source_info, len, Rvalue::Len(slice.to_place(self)));
 650         // lt = idx < len
 651         self.cfg.push_assign(
 652             block,
 653             source_info,
 654             lt,
 655             Rvalue::BinaryOp(
 656                 BinOp::Lt,
 657                 Box::new((Operand::Copy(Place::from(index)), Operand::Copy(len))),
 658             ),
 659         );
 660         let msg = BoundsCheck { len: Operand::Move(len), index: Operand::Copy(Place::from(index)) };
 661         // assert!(lt, "...")
 662         self.assert(block, Operand::Move(lt), true, msg, expr_span)
 663     }
 664
 665     fn add_fake_borrows_of_base(
 666         &mut self,
 667         base_place: Place<'tcx>,
 668         block: BasicBlock,
 669         fake_borrow_temps: &mut Vec<Local>,
 670         expr_span: Span,
 671         source_info: SourceInfo,
 672     ) {
 673         let tcx = self.tcx;
 674
 675         let place_ty = base_place.ty(&self.local_decls, tcx);
 676         if let ty::Slice(_) = place_ty.ty.kind() {
 677             // We need to create fake borrows to ensure that the bounds
 678             // check that we just did stays valid. Since we can't assign to
 679             // unsized values, we only need to ensure that none of the
 680             // pointers in the base place are modified.
 681             for (idx, elem) in base_place.projection.iter().enumerate().rev() {
 682                 match elem {
 683                     ProjectionElem::Deref => {
 684                         let fake_borrow_deref_ty = Place::ty_from(
 685                             base_place.local,
 686                             &base_place.projection[..idx],
 687                             &self.local_decls,
 688                             tcx,
 689                         )
 690                         .ty;
 691                         let fake_borrow_ty =
 692                             tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty);
 693                         let fake_borrow_temp =
 694                             self.local_decls.push(LocalDecl::new(fake_borrow_ty, expr_span));
 695                         let projection = tcx.intern_place_elems(&base_place.projection[..idx]);
 696                         self.cfg.push_assign(
 697                             block,
 698                             source_info,
 699                             fake_borrow_temp.into(),
 700                             Rvalue::Ref(
 701                                 tcx.lifetimes.re_erased,
 702                                 BorrowKind::Shallow,
 703                                 Place { local: base_place.local, projection },
 704                             ),
 705                         );
 706                         fake_borrow_temps.push(fake_borrow_temp);
 707                     }
 708                     ProjectionElem::Index(_) => {
 709                         let index_ty = Place::ty_from(
 710                             base_place.local,
 711                             &base_place.projection[..idx],
 712                             &self.local_decls,
 713                             tcx,
 714                         );
 715                         match index_ty.ty.kind() {
 716                             // The previous index expression has already
 717                             // done any index expressions needed here.
 718                             ty::Slice(_) => break,
 719                             ty::Array(..) => (),
 720                             _ => bug!("unexpected index base"),
 721                         }
 722                     }
 723                     ProjectionElem::Field(..)
 724                     | ProjectionElem::Downcast(..)
 725                     | ProjectionElem::OpaqueCast(..)
 726                     | ProjectionElem::ConstantIndex { .. }
 727                     | ProjectionElem::Subslice { .. } => (),
 728                 }
 729             }
 730         }
 731     }
 732
 733     fn read_fake_borrows(
 734         &mut self,
 735         bb: BasicBlock,
 736         fake_borrow_temps: &mut Vec<Local>,
 737         source_info: SourceInfo,
 738     ) {
 739         // All indexes have been evaluated now, read all of the
 740         // fake borrows so that they are live across those index
 741         // expressions.
 742         for temp in fake_borrow_temps {
 743             self.cfg.push_fake_read(bb, source_info, FakeReadCause::ForIndex, Place::from(*temp));
 744         }
 745     }
 746 }
 747
 748 /// Precise capture is enabled if the feature gate `capture_disjoint_fields` is enabled or if
 749 /// user is using Rust Edition 2021 or higher.
 750 fn enable_precise_capture(tcx: TyCtxt<'_>, closure_span: Span) -> bool {
 751     tcx.features().capture_disjoint_fields || closure_span.rust_2021()
 752 }