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

   1 //! See docs in `build/expr/mod.rs`.
   2
   3 use rustc_index::vec::Idx;
   4 use rustc_middle::ty::util::IntTypeExt;
   5
   6 use crate::build::expr::as_place::PlaceBase;
   7 use crate::build::expr::category::{Category, RvalueFunc};
   8 use crate::build::{BlockAnd, BlockAndExtension, Builder, NeedsTemporary};
   9 use rustc_hir::lang_items::LangItem;
  10 use rustc_middle::middle::region;
  11 use rustc_middle::mir::AssertKind;
  12 use rustc_middle::mir::Place;
  13 use rustc_middle::mir::*;
  14 use rustc_middle::thir::*;
  15 use rustc_middle::ty::cast::CastTy;
  16 use rustc_middle::ty::{self, Ty, UpvarSubsts};
  17 use rustc_span::Span;
  18
  19 impl<'a, 'tcx> Builder<'a, 'tcx> {
  20     /// Returns an rvalue suitable for use until the end of the current
  21     /// scope expression.
  22     ///
  23     /// The operand returned from this function will *not be valid* after
  24     /// an ExprKind::Scope is passed, so please do *not* return it from
  25     /// functions to avoid bad miscompiles.
  26     pub(crate) fn as_local_rvalue(
  27         &mut self,
  28         block: BasicBlock,
  29         expr: &Expr<'tcx>,
  30     ) -> BlockAnd<Rvalue<'tcx>> {
  31         let local_scope = self.local_scope();
  32         self.as_rvalue(block, Some(local_scope), expr)
  33     }
  34
  35     /// Compile `expr`, yielding an rvalue.
  36     pub(crate) fn as_rvalue(
  37         &mut self,
  38         mut block: BasicBlock,
  39         scope: Option<region::Scope>,
  40         expr: &Expr<'tcx>,
  41     ) -> BlockAnd<Rvalue<'tcx>> {
  42         debug!("expr_as_rvalue(block={:?}, scope={:?}, expr={:?})", block, scope, expr);
  43
  44         let this = self;
  45         let expr_span = expr.span;
  46         let source_info = this.source_info(expr_span);
  47
  48         match expr.kind {
  49             ExprKind::ThreadLocalRef(did) => block.and(Rvalue::ThreadLocalRef(did)),
  50             ExprKind::Scope { region_scope, lint_level, value } => {
  51                 let region_scope = (region_scope, source_info);
  52                 this.in_scope(region_scope, lint_level, |this| {
  53                     this.as_rvalue(block, scope, &this.thir[value])
  54                 })
  55             }
  56             ExprKind::Repeat { value, count } => {
  57                 if Some(0) == count.try_eval_usize(this.tcx, this.param_env) {
  58                     this.build_zero_repeat(block, value, scope, source_info)
  59                 } else {
  60                     let value_operand = unpack!(
  61                         block = this.as_operand(
  62                             block,
  63                             scope,
  64                             &this.thir[value],
  65                             None,
  66                             NeedsTemporary::No
  67                         )
  68                     );
  69                     block.and(Rvalue::Repeat(value_operand, count))
  70                 }
  71             }
  72             ExprKind::Binary { op, lhs, rhs } => {
  73                 let lhs = unpack!(
  74                     block =
  75                         this.as_operand(block, scope, &this.thir[lhs], None, NeedsTemporary::Maybe)
  76                 );
  77                 let rhs = unpack!(
  78                     block =
  79                         this.as_operand(block, scope, &this.thir[rhs], None, NeedsTemporary::No)
  80                 );
  81                 this.build_binary_op(block, op, expr_span, expr.ty, lhs, rhs)
  82             }
  83             ExprKind::Unary { op, arg } => {
  84                 let arg = unpack!(
  85                     block =
  86                         this.as_operand(block, scope, &this.thir[arg], None, NeedsTemporary::No)
  87                 );
  88                 // Check for -MIN on signed integers
  89                 if this.check_overflow && op == UnOp::Neg && expr.ty.is_signed() {
  90                     let bool_ty = this.tcx.types.bool;
  91
  92                     let minval = this.minval_literal(expr_span, expr.ty);
  93                     let is_min = this.temp(bool_ty, expr_span);
  94
  95                     this.cfg.push_assign(
  96                         block,
  97                         source_info,
  98                         is_min,
  99                         Rvalue::BinaryOp(BinOp::Eq, Box::new((arg.to_copy(), minval))),
 100                     );
 101
 102                     block = this.assert(
 103                         block,
 104                         Operand::Move(is_min),
 105                         false,
 106                         AssertKind::OverflowNeg(arg.to_copy()),
 107                         expr_span,
 108                     );
 109                 }
 110                 block.and(Rvalue::UnaryOp(op, arg))
 111             }
 112             ExprKind::Box { value } => {
 113                 let value = &this.thir[value];
 114                 let tcx = this.tcx;
 115
 116                 // `exchange_malloc` is unsafe but box is safe, so need a new scope.
 117                 let synth_scope = this.new_source_scope(
 118                     expr_span,
 119                     LintLevel::Inherited,
 120                     Some(Safety::BuiltinUnsafe),
 121                 );
 122                 let synth_info = SourceInfo { span: expr_span, scope: synth_scope };
 123
 124                 let size = this.temp(tcx.types.usize, expr_span);
 125                 this.cfg.push_assign(
 126                     block,
 127                     synth_info,
 128                     size,
 129                     Rvalue::NullaryOp(NullOp::SizeOf, value.ty),
 130                 );
 131
 132                 let align = this.temp(tcx.types.usize, expr_span);
 133                 this.cfg.push_assign(
 134                     block,
 135                     synth_info,
 136                     align,
 137                     Rvalue::NullaryOp(NullOp::AlignOf, value.ty),
 138                 );
 139
 140                 // malloc some memory of suitable size and align:
 141                 let exchange_malloc = Operand::function_handle(
 142                     tcx,
 143                     tcx.require_lang_item(LangItem::ExchangeMalloc, Some(expr_span)),
 144                     ty::List::empty(),
 145                     expr_span,
 146                 );
 147                 let storage = this.temp(tcx.mk_mut_ptr(tcx.types.u8), expr_span);
 148                 let success = this.cfg.start_new_block();
 149                 this.cfg.terminate(
 150                     block,
 151                     synth_info,
 152                     TerminatorKind::Call {
 153                         func: exchange_malloc,
 154                         args: vec![Operand::Move(size), Operand::Move(align)],
 155                         destination: storage,
 156                         target: Some(success),
 157                         cleanup: None,
 158                         from_hir_call: false,
 159                         fn_span: expr_span,
 160                     },
 161                 );
 162                 this.diverge_from(block);
 163                 block = success;
 164
 165                 // The `Box<T>` temporary created here is not a part of the HIR,
 166                 // and therefore is not considered during generator auto-trait
 167                 // determination. See the comment about `box` at `yield_in_scope`.
 168                 let result = this.local_decls.push(LocalDecl::new(expr.ty, expr_span).internal());
 169                 this.cfg.push(
 170                     block,
 171                     Statement { source_info, kind: StatementKind::StorageLive(result) },
 172                 );
 173                 if let Some(scope) = scope {
 174                     // schedule a shallow free of that memory, lest we unwind:
 175                     this.schedule_drop_storage_and_value(expr_span, scope, result);
 176                 }
 177
 178                 // Transmute `*mut u8` to the box (thus far, uninitialized):
 179                 let box_ = Rvalue::ShallowInitBox(Operand::Move(storage), value.ty);
 180                 this.cfg.push_assign(block, source_info, Place::from(result), box_);
 181
 182                 // initialize the box contents:
 183                 unpack!(
 184                     block = this.expr_into_dest(
 185                         this.tcx.mk_place_deref(Place::from(result)),
 186                         block,
 187                         value
 188                     )
 189                 );
 190                 block.and(Rvalue::Use(Operand::Move(Place::from(result))))
 191             }
 192             ExprKind::Cast { source } => {
 193                 let source = &this.thir[source];
 194
 195                 // Casting an enum to an integer is equivalent to computing the discriminant and casting the
 196                 // discriminant. Previously every backend had to repeat the logic for this operation. Now we
 197                 // create all the steps directly in MIR with operations all backends need to support anyway.
 198                 let (source, ty) = if let ty::Adt(adt_def, ..) = source.ty.kind() && adt_def.is_enum() {
 199                     let discr_ty = adt_def.repr().discr_type().to_ty(this.tcx);
 200                     let place = unpack!(block = this.as_place(block, source));
 201                     let discr = this.temp(discr_ty, source.span);
 202                     this.cfg.push_assign(
 203                         block,
 204                         source_info,
 205                         discr,
 206                         Rvalue::Discriminant(place),
 207                     );
 208
 209                     (Operand::Move(discr), discr_ty)
 210                 } else {
 211                     let ty = source.ty;
 212                     let source = unpack!(
 213                         block = this.as_operand(block, scope, source, None, NeedsTemporary::No)
 214                     );
 215                     (source, ty)
 216                 };
 217                 let from_ty = CastTy::from_ty(ty);
 218                 let cast_ty = CastTy::from_ty(expr.ty);
 219                 let cast_kind = match (from_ty, cast_ty) {
 220                     (Some(CastTy::Ptr(_) | CastTy::FnPtr), Some(CastTy::Int(_))) => {
 221                         CastKind::PointerExposeAddress
 222                     }
 223                     (Some(CastTy::Int(_)), Some(CastTy::Ptr(_))) => {
 224                         CastKind::PointerFromExposedAddress
 225                     }
 226                     (_, _) => CastKind::Misc,
 227                 };
 228                 block.and(Rvalue::Cast(cast_kind, source, expr.ty))
 229             }
 230             ExprKind::Pointer { cast, source } => {
 231                 let source = unpack!(
 232                     block =
 233                         this.as_operand(block, scope, &this.thir[source], None, NeedsTemporary::No)
 234                 );
 235                 block.and(Rvalue::Cast(CastKind::Pointer(cast), source, expr.ty))
 236             }
 237             ExprKind::Array { ref fields } => {
 238                 // (*) We would (maybe) be closer to codegen if we
 239                 // handled this and other aggregate cases via
 240                 // `into()`, not `as_rvalue` -- in that case, instead
 241                 // of generating
 242                 //
 243                 //     let tmp1 = ...1;
 244                 //     let tmp2 = ...2;
 245                 //     dest = Rvalue::Aggregate(Foo, [tmp1, tmp2])
 246                 //
 247                 // we could just generate
 248                 //
 249                 //     dest.f = ...1;
 250                 //     dest.g = ...2;
 251                 //
 252                 // The problem is that then we would need to:
 253                 //
 254                 // (a) have a more complex mechanism for handling
 255                 //     partial cleanup;
 256                 // (b) distinguish the case where the type `Foo` has a
 257                 //     destructor, in which case creating an instance
 258                 //     as a whole "arms" the destructor, and you can't
 259                 //     write individual fields; and,
 260                 // (c) handle the case where the type Foo has no
 261                 //     fields. We don't want `let x: ();` to compile
 262                 //     to the same MIR as `let x = ();`.
 263
 264                 // first process the set of fields
 265                 let el_ty = expr.ty.sequence_element_type(this.tcx);
 266                 let fields: Vec<_> = fields
 267                     .into_iter()
 268                     .copied()
 269                     .map(|f| {
 270                         unpack!(
 271                             block = this.as_operand(
 272                                 block,
 273                                 scope,
 274                                 &this.thir[f],
 275                                 None,
 276                                 NeedsTemporary::Maybe
 277                             )
 278                         )
 279                     })
 280                     .collect();
 281
 282                 block.and(Rvalue::Aggregate(Box::new(AggregateKind::Array(el_ty)), fields))
 283             }
 284             ExprKind::Tuple { ref fields } => {
 285                 // see (*) above
 286                 // first process the set of fields
 287                 let fields: Vec<_> = fields
 288                     .into_iter()
 289                     .copied()
 290                     .map(|f| {
 291                         unpack!(
 292                             block = this.as_operand(
 293                                 block,
 294                                 scope,
 295                                 &this.thir[f],
 296                                 None,
 297                                 NeedsTemporary::Maybe
 298                             )
 299                         )
 300                     })
 301                     .collect();
 302
 303                 block.and(Rvalue::Aggregate(Box::new(AggregateKind::Tuple), fields))
 304             }
 305             ExprKind::Closure { closure_id, substs, ref upvars, movability, ref fake_reads } => {
 306                 // Convert the closure fake reads, if any, from `ExprRef` to mir `Place`
 307                 // and push the fake reads.
 308                 // This must come before creating the operands. This is required in case
 309                 // there is a fake read and a borrow of the same path, since otherwise the
 310                 // fake read might interfere with the borrow. Consider an example like this
 311                 // one:
 312                 // ```
 313                 // let mut x = 0;
 314                 // let c = || {
 315                 //     &mut x; // mutable borrow of `x`
 316                 //     match x { _ => () } // fake read of `x`
 317                 // };
 318                 // ```
 319                 //
 320                 for (thir_place, cause, hir_id) in fake_reads.into_iter() {
 321                     let place_builder =
 322                         unpack!(block = this.as_place_builder(block, &this.thir[*thir_place]));
 323
 324                     if let Ok(place_builder_resolved) =
 325                         place_builder.try_upvars_resolved(this.tcx, this.typeck_results)
 326                     {
 327                         let mir_place =
 328                             place_builder_resolved.into_place(this.tcx, this.typeck_results);
 329                         this.cfg.push_fake_read(
 330                             block,
 331                             this.source_info(this.tcx.hir().span(*hir_id)),
 332                             *cause,
 333                             mir_place,
 334                         );
 335                     }
 336                 }
 337
 338                 // see (*) above
 339                 let operands: Vec<_> = upvars
 340                     .into_iter()
 341                     .copied()
 342                     .map(|upvar| {
 343                         let upvar = &this.thir[upvar];
 344                         match Category::of(&upvar.kind) {
 345                             // Use as_place to avoid creating a temporary when
 346                             // moving a variable into a closure, so that
 347                             // borrowck knows which variables to mark as being
 348                             // used as mut. This is OK here because the upvar
 349                             // expressions have no side effects and act on
 350                             // disjoint places.
 351                             // This occurs when capturing by copy/move, while
 352                             // by reference captures use as_operand
 353                             Some(Category::Place) => {
 354                                 let place = unpack!(block = this.as_place(block, upvar));
 355                                 this.consume_by_copy_or_move(place)
 356                             }
 357                             _ => {
 358                                 // Turn mutable borrow captures into unique
 359                                 // borrow captures when capturing an immutable
 360                                 // variable. This is sound because the mutation
 361                                 // that caused the capture will cause an error.
 362                                 match upvar.kind {
 363                                     ExprKind::Borrow {
 364                                         borrow_kind:
 365                                             BorrowKind::Mut { allow_two_phase_borrow: false },
 366                                         arg,
 367                                     } => unpack!(
 368                                         block = this.limit_capture_mutability(
 369                                             upvar.span,
 370                                             upvar.ty,
 371                                             scope,
 372                                             block,
 373                                             &this.thir[arg],
 374                                         )
 375                                     ),
 376                                     _ => {
 377                                         unpack!(
 378                                             block = this.as_operand(
 379                                                 block,
 380                                                 scope,
 381                                                 upvar,
 382                                                 None,
 383                                                 NeedsTemporary::Maybe
 384                                             )
 385                                         )
 386                                     }
 387                                 }
 388                             }
 389                         }
 390                     })
 391                     .collect();
 392
 393                 let result = match substs {
 394                     UpvarSubsts::Generator(substs) => {
 395                         // We implicitly set the discriminant to 0. See
 396                         // librustc_mir/transform/deaggregator.rs for details.
 397                         let movability = movability.unwrap();
 398                         Box::new(AggregateKind::Generator(closure_id, substs, movability))
 399                     }
 400                     UpvarSubsts::Closure(substs) => {
 401                         Box::new(AggregateKind::Closure(closure_id, substs))
 402                     }
 403                 };
 404                 block.and(Rvalue::Aggregate(result, operands))
 405             }
 406             ExprKind::Assign { .. } | ExprKind::AssignOp { .. } => {
 407                 block = unpack!(this.stmt_expr(block, expr, None));
 408                 block.and(Rvalue::Use(Operand::Constant(Box::new(Constant {
 409                     span: expr_span,
 410                     user_ty: None,
 411                     literal: ConstantKind::zero_sized(this.tcx.types.unit),
 412                 }))))
 413             }
 414
 415             ExprKind::Literal { .. }
 416             | ExprKind::NamedConst { .. }
 417             | ExprKind::NonHirLiteral { .. }
 418             | ExprKind::ZstLiteral { .. }
 419             | ExprKind::ConstParam { .. }
 420             | ExprKind::ConstBlock { .. }
 421             | ExprKind::StaticRef { .. } => {
 422                 let constant = this.as_constant(expr);
 423                 block.and(Rvalue::Use(Operand::Constant(Box::new(constant))))
 424             }
 425
 426             ExprKind::Yield { .. }
 427             | ExprKind::Block { .. }
 428             | ExprKind::Match { .. }
 429             | ExprKind::If { .. }
 430             | ExprKind::NeverToAny { .. }
 431             | ExprKind::Use { .. }
 432             | ExprKind::Borrow { .. }
 433             | ExprKind::AddressOf { .. }
 434             | ExprKind::Adt { .. }
 435             | ExprKind::Loop { .. }
 436             | ExprKind::LogicalOp { .. }
 437             | ExprKind::Call { .. }
 438             | ExprKind::Field { .. }
 439             | ExprKind::Let { .. }
 440             | ExprKind::Deref { .. }
 441             | ExprKind::Index { .. }
 442             | ExprKind::VarRef { .. }
 443             | ExprKind::UpvarRef { .. }
 444             | ExprKind::Break { .. }
 445             | ExprKind::Continue { .. }
 446             | ExprKind::Return { .. }
 447             | ExprKind::InlineAsm { .. }
 448             | ExprKind::PlaceTypeAscription { .. }
 449             | ExprKind::ValueTypeAscription { .. } => {
 450                 // these do not have corresponding `Rvalue` variants,
 451                 // so make an operand and then return that
 452                 debug_assert!(!matches!(
 453                     Category::of(&expr.kind),
 454                     Some(Category::Rvalue(RvalueFunc::AsRvalue) | Category::Constant)
 455                 ));
 456                 let operand =
 457                     unpack!(block = this.as_operand(block, scope, expr, None, NeedsTemporary::No));
 458                 block.and(Rvalue::Use(operand))
 459             }
 460         }
 461     }
 462
 463     pub(crate) fn build_binary_op(
 464         &mut self,
 465         mut block: BasicBlock,
 466         op: BinOp,
 467         span: Span,
 468         ty: Ty<'tcx>,
 469         lhs: Operand<'tcx>,
 470         rhs: Operand<'tcx>,
 471     ) -> BlockAnd<Rvalue<'tcx>> {
 472         let source_info = self.source_info(span);
 473         let bool_ty = self.tcx.types.bool;
 474         if self.check_overflow && op.is_checkable() && ty.is_integral() {
 475             let result_tup = self.tcx.intern_tup(&[ty, bool_ty]);
 476             let result_value = self.temp(result_tup, span);
 477
 478             self.cfg.push_assign(
 479                 block,
 480                 source_info,
 481                 result_value,
 482                 Rvalue::CheckedBinaryOp(op, Box::new((lhs.to_copy(), rhs.to_copy()))),
 483             );
 484             let val_fld = Field::new(0);
 485             let of_fld = Field::new(1);
 486
 487             let tcx = self.tcx;
 488             let val = tcx.mk_place_field(result_value, val_fld, ty);
 489             let of = tcx.mk_place_field(result_value, of_fld, bool_ty);
 490
 491             let err = AssertKind::Overflow(op, lhs, rhs);
 492
 493             block = self.assert(block, Operand::Move(of), false, err, span);
 494
 495             block.and(Rvalue::Use(Operand::Move(val)))
 496         } else {
 497             if ty.is_integral() && (op == BinOp::Div || op == BinOp::Rem) {
 498                 // Checking division and remainder is more complex, since we 1. always check
 499                 // and 2. there are two possible failure cases, divide-by-zero and overflow.
 500
 501                 let zero_err = if op == BinOp::Div {
 502                     AssertKind::DivisionByZero(lhs.to_copy())
 503                 } else {
 504                     AssertKind::RemainderByZero(lhs.to_copy())
 505                 };
 506                 let overflow_err = AssertKind::Overflow(op, lhs.to_copy(), rhs.to_copy());
 507
 508                 // Check for / 0
 509                 let is_zero = self.temp(bool_ty, span);
 510                 let zero = self.zero_literal(span, ty);
 511                 self.cfg.push_assign(
 512                     block,
 513                     source_info,
 514                     is_zero,
 515                     Rvalue::BinaryOp(BinOp::Eq, Box::new((rhs.to_copy(), zero))),
 516                 );
 517
 518                 block = self.assert(block, Operand::Move(is_zero), false, zero_err, span);
 519
 520                 // We only need to check for the overflow in one case:
 521                 // MIN / -1, and only for signed values.
 522                 if ty.is_signed() {
 523                     let neg_1 = self.neg_1_literal(span, ty);
 524                     let min = self.minval_literal(span, ty);
 525
 526                     let is_neg_1 = self.temp(bool_ty, span);
 527                     let is_min = self.temp(bool_ty, span);
 528                     let of = self.temp(bool_ty, span);
 529
 530                     // this does (rhs == -1) & (lhs == MIN). It could short-circuit instead
 531
 532                     self.cfg.push_assign(
 533                         block,
 534                         source_info,
 535                         is_neg_1,
 536                         Rvalue::BinaryOp(BinOp::Eq, Box::new((rhs.to_copy(), neg_1))),
 537                     );
 538                     self.cfg.push_assign(
 539                         block,
 540                         source_info,
 541                         is_min,
 542                         Rvalue::BinaryOp(BinOp::Eq, Box::new((lhs.to_copy(), min))),
 543                     );
 544
 545                     let is_neg_1 = Operand::Move(is_neg_1);
 546                     let is_min = Operand::Move(is_min);
 547                     self.cfg.push_assign(
 548                         block,
 549                         source_info,
 550                         of,
 551                         Rvalue::BinaryOp(BinOp::BitAnd, Box::new((is_neg_1, is_min))),
 552                     );
 553
 554                     block = self.assert(block, Operand::Move(of), false, overflow_err, span);
 555                 }
 556             }
 557
 558             block.and(Rvalue::BinaryOp(op, Box::new((lhs, rhs))))
 559         }
 560     }
 561
 562     fn build_zero_repeat(
 563         &mut self,
 564         mut block: BasicBlock,
 565         value: ExprId,
 566         scope: Option<region::Scope>,
 567         outer_source_info: SourceInfo,
 568     ) -> BlockAnd<Rvalue<'tcx>> {
 569         let this = self;
 570         let value = &this.thir[value];
 571         let elem_ty = value.ty;
 572         if let Some(Category::Constant) = Category::of(&value.kind) {
 573             // Repeating a const does nothing
 574         } else {
 575             // For a non-const, we may need to generate an appropriate `Drop`
 576             let value_operand =
 577                 unpack!(block = this.as_operand(block, scope, value, None, NeedsTemporary::No));
 578             if let Operand::Move(to_drop) = value_operand {
 579                 let success = this.cfg.start_new_block();
 580                 this.cfg.terminate(
 581                     block,
 582                     outer_source_info,
 583                     TerminatorKind::Drop { place: to_drop, target: success, unwind: None },
 584                 );
 585                 this.diverge_from(block);
 586                 block = success;
 587             }
 588             this.record_operands_moved(&[value_operand]);
 589         }
 590         block.and(Rvalue::Aggregate(Box::new(AggregateKind::Array(elem_ty)), Vec::new()))
 591     }
 592
 593     fn limit_capture_mutability(
 594         &mut self,
 595         upvar_span: Span,
 596         upvar_ty: Ty<'tcx>,
 597         temp_lifetime: Option<region::Scope>,
 598         mut block: BasicBlock,
 599         arg: &Expr<'tcx>,
 600     ) -> BlockAnd<Operand<'tcx>> {
 601         let this = self;
 602
 603         let source_info = this.source_info(upvar_span);
 604         let temp = this.local_decls.push(LocalDecl::new(upvar_ty, upvar_span));
 605
 606         this.cfg.push(block, Statement { source_info, kind: StatementKind::StorageLive(temp) });
 607
 608         let arg_place_builder = unpack!(block = this.as_place_builder(block, arg));
 609
 610         let mutability = match arg_place_builder.base() {
 611             // We are capturing a path that starts off a local variable in the parent.
 612             // The mutability of the current capture is same as the mutability
 613             // of the local declaration in the parent.
 614             PlaceBase::Local(local) => this.local_decls[local].mutability,
 615             // Parent is a closure and we are capturing a path that is captured
 616             // by the parent itself. The mutability of the current capture
 617             // is same as that of the capture in the parent closure.
 618             PlaceBase::Upvar { .. } => {
 619                 let enclosing_upvars_resolved =
 620                     arg_place_builder.clone().into_place(this.tcx, this.typeck_results);
 621
 622                 match enclosing_upvars_resolved.as_ref() {
 623                     PlaceRef {
 624                         local,
 625                         projection: &[ProjectionElem::Field(upvar_index, _), ..],
 626                     }
 627                     | PlaceRef {
 628                         local,
 629                         projection:
 630                             &[ProjectionElem::Deref, ProjectionElem::Field(upvar_index, _), ..],
 631                     } => {
 632                         // Not in a closure
 633                         debug_assert!(
 634                             local == ty::CAPTURE_STRUCT_LOCAL,
 635                             "Expected local to be Local(1), found {:?}",
 636                             local
 637                         );
 638                         // Not in a closure
 639                         debug_assert!(
 640                             this.upvar_mutbls.len() > upvar_index.index(),
 641                             "Unexpected capture place, upvar_mutbls={:#?}, upvar_index={:?}",
 642                             this.upvar_mutbls,
 643                             upvar_index
 644                         );
 645                         this.upvar_mutbls[upvar_index.index()]
 646                     }
 647                     _ => bug!("Unexpected capture place"),
 648                 }
 649             }
 650         };
 651
 652         let borrow_kind = match mutability {
 653             Mutability::Not => BorrowKind::Unique,
 654             Mutability::Mut => BorrowKind::Mut { allow_two_phase_borrow: false },
 655         };
 656
 657         let arg_place = arg_place_builder.into_place(this.tcx, this.typeck_results);
 658
 659         this.cfg.push_assign(
 660             block,
 661             source_info,
 662             Place::from(temp),
 663             Rvalue::Ref(this.tcx.lifetimes.re_erased, borrow_kind, arg_place),
 664         );
 665
 666         // See the comment in `expr_as_temp` and on the `rvalue_scopes` field for why
 667         // this can be `None`.
 668         if let Some(temp_lifetime) = temp_lifetime {
 669             this.schedule_drop_storage_and_value(upvar_span, temp_lifetime, temp);
 670         }
 671
 672         block.and(Operand::Move(Place::from(temp)))
 673     }
 674
 675     // Helper to get a `-1` value of the appropriate type
 676     fn neg_1_literal(&mut self, span: Span, ty: Ty<'tcx>) -> Operand<'tcx> {
 677         let param_ty = ty::ParamEnv::empty().and(ty);
 678         let size = self.tcx.layout_of(param_ty).unwrap().size;
 679         let literal = ConstantKind::from_bits(self.tcx, size.unsigned_int_max(), param_ty);
 680
 681         self.literal_operand(span, literal)
 682     }
 683
 684     // Helper to get the minimum value of the appropriate type
 685     fn minval_literal(&mut self, span: Span, ty: Ty<'tcx>) -> Operand<'tcx> {
 686         assert!(ty.is_signed());
 687         let param_ty = ty::ParamEnv::empty().and(ty);
 688         let bits = self.tcx.layout_of(param_ty).unwrap().size.bits();
 689         let n = 1 << (bits - 1);
 690         let literal = ConstantKind::from_bits(self.tcx, n, param_ty);
 691
 692         self.literal_operand(span, literal)
 693     }
 694 }