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 use rustc_target::abi::{Abi, Primitive};
   6
   7 use crate::build::expr::as_place::PlaceBase;
   8 use crate::build::expr::category::{Category, RvalueFunc};
   9 use crate::build::{BlockAnd, BlockAndExtension, Builder, NeedsTemporary};
  10 use rustc_hir::lang_items::LangItem;
  11 use rustc_middle::middle::region;
  12 use rustc_middle::mir::AssertKind;
  13 use rustc_middle::mir::Place;
  14 use rustc_middle::mir::*;
  15 use rustc_middle::thir::*;
  16 use rustc_middle::ty::cast::{mir_cast_kind, CastTy};
  17 use rustc_middle::ty::{self, Ty, UpvarSubsts};
  18 use rustc_span::Span;
  19
  20 impl<'a, 'tcx> Builder<'a, 'tcx> {
  21     /// Returns an rvalue suitable for use until the end of the current
  22     /// scope expression.
  23     ///
  24     /// The operand returned from this function will *not be valid* after
  25     /// an ExprKind::Scope is passed, so please do *not* return it from
  26     /// functions to avoid bad miscompiles.
  27     pub(crate) fn as_local_rvalue(
  28         &mut self,
  29         block: BasicBlock,
  30         expr: &Expr<'tcx>,
  31     ) -> BlockAnd<Rvalue<'tcx>> {
  32         let local_scope = self.local_scope();
  33         self.as_rvalue(block, Some(local_scope), expr)
  34     }
  35
  36     /// Compile `expr`, yielding an rvalue.
  37     pub(crate) fn as_rvalue(
  38         &mut self,
  39         mut block: BasicBlock,
  40         scope: Option<region::Scope>,
  41         expr: &Expr<'tcx>,
  42     ) -> BlockAnd<Rvalue<'tcx>> {
  43         debug!("expr_as_rvalue(block={:?}, scope={:?}, expr={:?})", block, scope, expr);
  44
  45         let this = self;
  46         let expr_span = expr.span;
  47         let source_info = this.source_info(expr_span);
  48
  49         match expr.kind {
  50             ExprKind::ThreadLocalRef(did) => block.and(Rvalue::ThreadLocalRef(did)),
  51             ExprKind::Scope { region_scope, lint_level, value } => {
  52                 let region_scope = (region_scope, source_info);
  53                 this.in_scope(region_scope, lint_level, |this| {
  54                     this.as_rvalue(block, scope, &this.thir[value])
  55                 })
  56             }
  57             ExprKind::Repeat { value, count } => {
  58                 if Some(0) == count.try_eval_usize(this.tcx, this.param_env) {
  59                     this.build_zero_repeat(block, value, scope, source_info)
  60                 } else {
  61                     let value_operand = unpack!(
  62                         block = this.as_operand(
  63                             block,
  64                             scope,
  65                             &this.thir[value],
  66                             None,
  67                             NeedsTemporary::No
  68                         )
  69                     );
  70                     block.and(Rvalue::Repeat(value_operand, count))
  71                 }
  72             }
  73             ExprKind::Binary { op, lhs, rhs } => {
  74                 let lhs = unpack!(
  75                     block =
  76                         this.as_operand(block, scope, &this.thir[lhs], None, NeedsTemporary::Maybe)
  77                 );
  78                 let rhs = unpack!(
  79                     block =
  80                         this.as_operand(block, scope, &this.thir[rhs], None, NeedsTemporary::No)
  81                 );
  82                 this.build_binary_op(block, op, expr_span, expr.ty, lhs, rhs)
  83             }
  84             ExprKind::Unary { op, arg } => {
  85                 let arg = unpack!(
  86                     block =
  87                         this.as_operand(block, scope, &this.thir[arg], None, NeedsTemporary::No)
  88                 );
  89                 // Check for -MIN on signed integers
  90                 if this.check_overflow && op == UnOp::Neg && expr.ty.is_signed() {
  91                     let bool_ty = this.tcx.types.bool;
  92
  93                     let minval = this.minval_literal(expr_span, expr.ty);
  94                     let is_min = this.temp(bool_ty, expr_span);
  95
  96                     this.cfg.push_assign(
  97                         block,
  98                         source_info,
  99                         is_min,
 100                         Rvalue::BinaryOp(BinOp::Eq, Box::new((arg.to_copy(), minval))),
 101                     );
 102
 103                     block = this.assert(
 104                         block,
 105                         Operand::Move(is_min),
 106                         false,
 107                         AssertKind::OverflowNeg(arg.to_copy()),
 108                         expr_span,
 109                     );
 110                 }
 111                 block.and(Rvalue::UnaryOp(op, arg))
 112             }
 113             ExprKind::Box { value } => {
 114                 let value = &this.thir[value];
 115                 let tcx = this.tcx;
 116
 117                 // `exchange_malloc` is unsafe but box is safe, so need a new scope.
 118                 let synth_scope = this.new_source_scope(
 119                     expr_span,
 120                     LintLevel::Inherited,
 121                     Some(Safety::BuiltinUnsafe),
 122                 );
 123                 let synth_info = SourceInfo { span: expr_span, scope: synth_scope };
 124
 125                 let size = this.temp(tcx.types.usize, expr_span);
 126                 this.cfg.push_assign(
 127                     block,
 128                     synth_info,
 129                     size,
 130                     Rvalue::NullaryOp(NullOp::SizeOf, value.ty),
 131                 );
 132
 133                 let align = this.temp(tcx.types.usize, expr_span);
 134                 this.cfg.push_assign(
 135                     block,
 136                     synth_info,
 137                     align,
 138                     Rvalue::NullaryOp(NullOp::AlignOf, value.ty),
 139                 );
 140
 141                 // malloc some memory of suitable size and align:
 142                 let exchange_malloc = Operand::function_handle(
 143                     tcx,
 144                     tcx.require_lang_item(LangItem::ExchangeMalloc, Some(expr_span)),
 145                     ty::List::empty(),
 146                     expr_span,
 147                 );
 148                 let storage = this.temp(tcx.mk_mut_ptr(tcx.types.u8), expr_span);
 149                 let success = this.cfg.start_new_block();
 150                 this.cfg.terminate(
 151                     block,
 152                     synth_info,
 153                     TerminatorKind::Call {
 154                         func: exchange_malloc,
 155                         args: vec![Operand::Move(size), Operand::Move(align)],
 156                         destination: storage,
 157                         target: Some(success),
 158                         cleanup: None,
 159                         from_hir_call: false,
 160                         fn_span: expr_span,
 161                     },
 162                 );
 163                 this.diverge_from(block);
 164                 block = success;
 165
 166                 // The `Box<T>` temporary created here is not a part of the HIR,
 167                 // and therefore is not considered during generator auto-trait
 168                 // determination. See the comment about `box` at `yield_in_scope`.
 169                 let result = this.local_decls.push(LocalDecl::new(expr.ty, expr_span).internal());
 170                 this.cfg.push(
 171                     block,
 172                     Statement { source_info, kind: StatementKind::StorageLive(result) },
 173                 );
 174                 if let Some(scope) = scope {
 175                     // schedule a shallow free of that memory, lest we unwind:
 176                     this.schedule_drop_storage_and_value(expr_span, scope, result);
 177                 }
 178
 179                 // Transmute `*mut u8` to the box (thus far, uninitialized):
 180                 let box_ = Rvalue::ShallowInitBox(Operand::Move(storage), value.ty);
 181                 this.cfg.push_assign(block, source_info, Place::from(result), box_);
 182
 183                 // initialize the box contents:
 184                 unpack!(
 185                     block = this.expr_into_dest(
 186                         this.tcx.mk_place_deref(Place::from(result)),
 187                         block,
 188                         value
 189                     )
 190                 );
 191                 block.and(Rvalue::Use(Operand::Move(Place::from(result))))
 192             }
 193             ExprKind::Cast { source } => {
 194                 let source = &this.thir[source];
 195
 196                 // Casting an enum to an integer is equivalent to computing the discriminant and casting the
 197                 // discriminant. Previously every backend had to repeat the logic for this operation. Now we
 198                 // create all the steps directly in MIR with operations all backends need to support anyway.
 199                 let (source, ty) = if let ty::Adt(adt_def, ..) = source.ty.kind() && adt_def.is_enum() {
 200                     let discr_ty = adt_def.repr().discr_type().to_ty(this.tcx);
 201                     let temp = unpack!(block = this.as_temp(block, scope, source, Mutability::Not));
 202                     let layout = this.tcx.layout_of(this.param_env.and(source.ty));
 203                     let discr = this.temp(discr_ty, source.span);
 204                     this.cfg.push_assign(
 205                         block,
 206                         source_info,
 207                         discr,
 208                         Rvalue::Discriminant(temp.into()),
 209                     );
 210                     let (op,ty) = (Operand::Move(discr), discr_ty);
 211
 212                     if let Abi::Scalar(scalar) = layout.unwrap().abi{
 213                         if let Primitive::Int(_, signed) = scalar.primitive() {
 214                             let range = scalar.valid_range(&this.tcx);
 215                             // FIXME: Handle wraparound cases too.
 216                             if range.end >= range.start {
 217                                 let mut assumer = |range: u128, bin_op: BinOp| {
 218                                     // We will be overwriting this val if our scalar is signed value
 219                                     // because sign extension on unsigned types might cause unintended things
 220                                     let mut range_val =
 221                                         ConstantKind::from_bits(this.tcx, range, ty::ParamEnv::empty().and(discr_ty));
 222                                     let bool_ty = this.tcx.types.bool;
 223                                     if signed {
 224                                         let scalar_size_extend = scalar.size(&this.tcx).sign_extend(range);
 225                                         let discr_layout = this.tcx.layout_of(this.param_env.and(discr_ty));
 226                                         let truncated_val = discr_layout.unwrap().size.truncate(scalar_size_extend);
 227                                         range_val = ConstantKind::from_bits(
 228                                             this.tcx,
 229                                             truncated_val,
 230                                             ty::ParamEnv::empty().and(discr_ty),
 231                                         );
 232                                     }
 233                                     let lit_op = this.literal_operand(expr.span, range_val);
 234                                     let is_bin_op = this.temp(bool_ty, expr_span);
 235                                     this.cfg.push_assign(
 236                                         block,
 237                                         source_info,
 238                                         is_bin_op,
 239                                         Rvalue::BinaryOp(bin_op, Box::new(((lit_op), (Operand::Copy(discr))))),
 240                                     );
 241                                     this.cfg.push(
 242                                         block,
 243                                         Statement {
 244                                             source_info,
 245                                             kind: StatementKind::Intrinsic(Box::new(NonDivergingIntrinsic::Assume(
 246                                                 Operand::Copy(is_bin_op),
 247                                             ))),
 248                                         },
 249                                     )
 250                                 };
 251                                 assumer(range.end, BinOp::Ge);
 252                                 assumer(range.start, BinOp::Le);
 253                             }
 254                         }
 255                     }
 256
 257                     (op,ty)
 258
 259                 } else {
 260                     let ty = source.ty;
 261                     let source = unpack!(
 262                         block = this.as_operand(block, scope, source, None, NeedsTemporary::No)
 263                     );
 264                     (source, ty)
 265                 };
 266                 let from_ty = CastTy::from_ty(ty);
 267                 let cast_ty = CastTy::from_ty(expr.ty);
 268                 debug!("ExprKind::Cast from_ty={from_ty:?}, cast_ty={:?}/{cast_ty:?}", expr.ty,);
 269                 let cast_kind = mir_cast_kind(ty, expr.ty);
 270                 block.and(Rvalue::Cast(cast_kind, source, expr.ty))
 271             }
 272             ExprKind::Pointer { cast, source } => {
 273                 let source = unpack!(
 274                     block =
 275                         this.as_operand(block, scope, &this.thir[source], None, NeedsTemporary::No)
 276                 );
 277                 block.and(Rvalue::Cast(CastKind::Pointer(cast), source, expr.ty))
 278             }
 279             ExprKind::Array { ref fields } => {
 280                 // (*) We would (maybe) be closer to codegen if we
 281                 // handled this and other aggregate cases via
 282                 // `into()`, not `as_rvalue` -- in that case, instead
 283                 // of generating
 284                 //
 285                 //     let tmp1 = ...1;
 286                 //     let tmp2 = ...2;
 287                 //     dest = Rvalue::Aggregate(Foo, [tmp1, tmp2])
 288                 //
 289                 // we could just generate
 290                 //
 291                 //     dest.f = ...1;
 292                 //     dest.g = ...2;
 293                 //
 294                 // The problem is that then we would need to:
 295                 //
 296                 // (a) have a more complex mechanism for handling
 297                 //     partial cleanup;
 298                 // (b) distinguish the case where the type `Foo` has a
 299                 //     destructor, in which case creating an instance
 300                 //     as a whole "arms" the destructor, and you can't
 301                 //     write individual fields; and,
 302                 // (c) handle the case where the type Foo has no
 303                 //     fields. We don't want `let x: ();` to compile
 304                 //     to the same MIR as `let x = ();`.
 305
 306                 // first process the set of fields
 307                 let el_ty = expr.ty.sequence_element_type(this.tcx);
 308                 let fields: Vec<_> = fields
 309                     .into_iter()
 310                     .copied()
 311                     .map(|f| {
 312                         unpack!(
 313                             block = this.as_operand(
 314                                 block,
 315                                 scope,
 316                                 &this.thir[f],
 317                                 None,
 318                                 NeedsTemporary::Maybe
 319                             )
 320                         )
 321                     })
 322                     .collect();
 323
 324                 block.and(Rvalue::Aggregate(Box::new(AggregateKind::Array(el_ty)), fields))
 325             }
 326             ExprKind::Tuple { ref fields } => {
 327                 // see (*) above
 328                 // first process the set of fields
 329                 let fields: Vec<_> = fields
 330                     .into_iter()
 331                     .copied()
 332                     .map(|f| {
 333                         unpack!(
 334                             block = this.as_operand(
 335                                 block,
 336                                 scope,
 337                                 &this.thir[f],
 338                                 None,
 339                                 NeedsTemporary::Maybe
 340                             )
 341                         )
 342                     })
 343                     .collect();
 344
 345                 block.and(Rvalue::Aggregate(Box::new(AggregateKind::Tuple), fields))
 346             }
 347             ExprKind::Closure(box ClosureExpr {
 348                 closure_id,
 349                 substs,
 350                 ref upvars,
 351                 movability,
 352                 ref fake_reads,
 353             }) => {
 354                 // Convert the closure fake reads, if any, from `ExprRef` to mir `Place`
 355                 // and push the fake reads.
 356                 // This must come before creating the operands. This is required in case
 357                 // there is a fake read and a borrow of the same path, since otherwise the
 358                 // fake read might interfere with the borrow. Consider an example like this
 359                 // one:
 360                 // ```
 361                 // let mut x = 0;
 362                 // let c = || {
 363                 //     &mut x; // mutable borrow of `x`
 364                 //     match x { _ => () } // fake read of `x`
 365                 // };
 366                 // ```
 367                 //
 368                 for (thir_place, cause, hir_id) in fake_reads.into_iter() {
 369                     let place_builder =
 370                         unpack!(block = this.as_place_builder(block, &this.thir[*thir_place]));
 371
 372                     if let Some(mir_place) = place_builder.try_to_place(this) {
 373                         this.cfg.push_fake_read(
 374                             block,
 375                             this.source_info(this.tcx.hir().span(*hir_id)),
 376                             *cause,
 377                             mir_place,
 378                         );
 379                     }
 380                 }
 381
 382                 // see (*) above
 383                 let operands: Vec<_> = upvars
 384                     .into_iter()
 385                     .copied()
 386                     .map(|upvar| {
 387                         let upvar = &this.thir[upvar];
 388                         match Category::of(&upvar.kind) {
 389                             // Use as_place to avoid creating a temporary when
 390                             // moving a variable into a closure, so that
 391                             // borrowck knows which variables to mark as being
 392                             // used as mut. This is OK here because the upvar
 393                             // expressions have no side effects and act on
 394                             // disjoint places.
 395                             // This occurs when capturing by copy/move, while
 396                             // by reference captures use as_operand
 397                             Some(Category::Place) => {
 398                                 let place = unpack!(block = this.as_place(block, upvar));
 399                                 this.consume_by_copy_or_move(place)
 400                             }
 401                             _ => {
 402                                 // Turn mutable borrow captures into unique
 403                                 // borrow captures when capturing an immutable
 404                                 // variable. This is sound because the mutation
 405                                 // that caused the capture will cause an error.
 406                                 match upvar.kind {
 407                                     ExprKind::Borrow {
 408                                         borrow_kind:
 409                                             BorrowKind::Mut { allow_two_phase_borrow: false },
 410                                         arg,
 411                                     } => unpack!(
 412                                         block = this.limit_capture_mutability(
 413                                             upvar.span,
 414                                             upvar.ty,
 415                                             scope,
 416                                             block,
 417                                             &this.thir[arg],
 418                                         )
 419                                     ),
 420                                     _ => {
 421                                         unpack!(
 422                                             block = this.as_operand(
 423                                                 block,
 424                                                 scope,
 425                                                 upvar,
 426                                                 None,
 427                                                 NeedsTemporary::Maybe
 428                                             )
 429                                         )
 430                                     }
 431                                 }
 432                             }
 433                         }
 434                     })
 435                     .collect();
 436
 437                 let result = match substs {
 438                     UpvarSubsts::Generator(substs) => {
 439                         // We implicitly set the discriminant to 0. See
 440                         // librustc_mir/transform/deaggregator.rs for details.
 441                         let movability = movability.unwrap();
 442                         Box::new(AggregateKind::Generator(closure_id, substs, movability))
 443                     }
 444                     UpvarSubsts::Closure(substs) => {
 445                         Box::new(AggregateKind::Closure(closure_id, substs))
 446                     }
 447                 };
 448                 block.and(Rvalue::Aggregate(result, operands))
 449             }
 450             ExprKind::Assign { .. } | ExprKind::AssignOp { .. } => {
 451                 block = unpack!(this.stmt_expr(block, expr, None));
 452                 block.and(Rvalue::Use(Operand::Constant(Box::new(Constant {
 453                     span: expr_span,
 454                     user_ty: None,
 455                     literal: ConstantKind::zero_sized(this.tcx.types.unit),
 456                 }))))
 457             }
 458
 459             ExprKind::Literal { .. }
 460             | ExprKind::NamedConst { .. }
 461             | ExprKind::NonHirLiteral { .. }
 462             | ExprKind::ZstLiteral { .. }
 463             | ExprKind::ConstParam { .. }
 464             | ExprKind::ConstBlock { .. }
 465             | ExprKind::StaticRef { .. } => {
 466                 let constant = this.as_constant(expr);
 467                 block.and(Rvalue::Use(Operand::Constant(Box::new(constant))))
 468             }
 469
 470             ExprKind::Yield { .. }
 471             | ExprKind::Block { .. }
 472             | ExprKind::Match { .. }
 473             | ExprKind::If { .. }
 474             | ExprKind::NeverToAny { .. }
 475             | ExprKind::Use { .. }
 476             | ExprKind::Borrow { .. }
 477             | ExprKind::AddressOf { .. }
 478             | ExprKind::Adt { .. }
 479             | ExprKind::Loop { .. }
 480             | ExprKind::LogicalOp { .. }
 481             | ExprKind::Call { .. }
 482             | ExprKind::Field { .. }
 483             | ExprKind::Let { .. }
 484             | ExprKind::Deref { .. }
 485             | ExprKind::Index { .. }
 486             | ExprKind::VarRef { .. }
 487             | ExprKind::UpvarRef { .. }
 488             | ExprKind::Break { .. }
 489             | ExprKind::Continue { .. }
 490             | ExprKind::Return { .. }
 491             | ExprKind::InlineAsm { .. }
 492             | ExprKind::PlaceTypeAscription { .. }
 493             | ExprKind::ValueTypeAscription { .. } => {
 494                 // these do not have corresponding `Rvalue` variants,
 495                 // so make an operand and then return that
 496                 debug_assert!(!matches!(
 497                     Category::of(&expr.kind),
 498                     Some(Category::Rvalue(RvalueFunc::AsRvalue) | Category::Constant)
 499                 ));
 500                 let operand =
 501                     unpack!(block = this.as_operand(block, scope, expr, None, NeedsTemporary::No));
 502                 block.and(Rvalue::Use(operand))
 503             }
 504         }
 505     }
 506
 507     pub(crate) fn build_binary_op(
 508         &mut self,
 509         mut block: BasicBlock,
 510         op: BinOp,
 511         span: Span,
 512         ty: Ty<'tcx>,
 513         lhs: Operand<'tcx>,
 514         rhs: Operand<'tcx>,
 515     ) -> BlockAnd<Rvalue<'tcx>> {
 516         let source_info = self.source_info(span);
 517         let bool_ty = self.tcx.types.bool;
 518         if self.check_overflow && op.is_checkable() && ty.is_integral() {
 519             let result_tup = self.tcx.intern_tup(&[ty, bool_ty]);
 520             let result_value = self.temp(result_tup, span);
 521
 522             self.cfg.push_assign(
 523                 block,
 524                 source_info,
 525                 result_value,
 526                 Rvalue::CheckedBinaryOp(op, Box::new((lhs.to_copy(), rhs.to_copy()))),
 527             );
 528             let val_fld = Field::new(0);
 529             let of_fld = Field::new(1);
 530
 531             let tcx = self.tcx;
 532             let val = tcx.mk_place_field(result_value, val_fld, ty);
 533             let of = tcx.mk_place_field(result_value, of_fld, bool_ty);
 534
 535             let err = AssertKind::Overflow(op, lhs, rhs);
 536
 537             block = self.assert(block, Operand::Move(of), false, err, span);
 538
 539             block.and(Rvalue::Use(Operand::Move(val)))
 540         } else {
 541             if ty.is_integral() && (op == BinOp::Div || op == BinOp::Rem) {
 542                 // Checking division and remainder is more complex, since we 1. always check
 543                 // and 2. there are two possible failure cases, divide-by-zero and overflow.
 544
 545                 let zero_err = if op == BinOp::Div {
 546                     AssertKind::DivisionByZero(lhs.to_copy())
 547                 } else {
 548                     AssertKind::RemainderByZero(lhs.to_copy())
 549                 };
 550                 let overflow_err = AssertKind::Overflow(op, lhs.to_copy(), rhs.to_copy());
 551
 552                 // Check for / 0
 553                 let is_zero = self.temp(bool_ty, span);
 554                 let zero = self.zero_literal(span, ty);
 555                 self.cfg.push_assign(
 556                     block,
 557                     source_info,
 558                     is_zero,
 559                     Rvalue::BinaryOp(BinOp::Eq, Box::new((rhs.to_copy(), zero))),
 560                 );
 561
 562                 block = self.assert(block, Operand::Move(is_zero), false, zero_err, span);
 563
 564                 // We only need to check for the overflow in one case:
 565                 // MIN / -1, and only for signed values.
 566                 if ty.is_signed() {
 567                     let neg_1 = self.neg_1_literal(span, ty);
 568                     let min = self.minval_literal(span, ty);
 569
 570                     let is_neg_1 = self.temp(bool_ty, span);
 571                     let is_min = self.temp(bool_ty, span);
 572                     let of = self.temp(bool_ty, span);
 573
 574                     // this does (rhs == -1) & (lhs == MIN). It could short-circuit instead
 575
 576                     self.cfg.push_assign(
 577                         block,
 578                         source_info,
 579                         is_neg_1,
 580                         Rvalue::BinaryOp(BinOp::Eq, Box::new((rhs.to_copy(), neg_1))),
 581                     );
 582                     self.cfg.push_assign(
 583                         block,
 584                         source_info,
 585                         is_min,
 586                         Rvalue::BinaryOp(BinOp::Eq, Box::new((lhs.to_copy(), min))),
 587                     );
 588
 589                     let is_neg_1 = Operand::Move(is_neg_1);
 590                     let is_min = Operand::Move(is_min);
 591                     self.cfg.push_assign(
 592                         block,
 593                         source_info,
 594                         of,
 595                         Rvalue::BinaryOp(BinOp::BitAnd, Box::new((is_neg_1, is_min))),
 596                     );
 597
 598                     block = self.assert(block, Operand::Move(of), false, overflow_err, span);
 599                 }
 600             }
 601
 602             block.and(Rvalue::BinaryOp(op, Box::new((lhs, rhs))))
 603         }
 604     }
 605
 606     fn build_zero_repeat(
 607         &mut self,
 608         mut block: BasicBlock,
 609         value: ExprId,
 610         scope: Option<region::Scope>,
 611         outer_source_info: SourceInfo,
 612     ) -> BlockAnd<Rvalue<'tcx>> {
 613         let this = self;
 614         let value = &this.thir[value];
 615         let elem_ty = value.ty;
 616         if let Some(Category::Constant) = Category::of(&value.kind) {
 617             // Repeating a const does nothing
 618         } else {
 619             // For a non-const, we may need to generate an appropriate `Drop`
 620             let value_operand =
 621                 unpack!(block = this.as_operand(block, scope, value, None, NeedsTemporary::No));
 622             if let Operand::Move(to_drop) = value_operand {
 623                 let success = this.cfg.start_new_block();
 624                 this.cfg.terminate(
 625                     block,
 626                     outer_source_info,
 627                     TerminatorKind::Drop { place: to_drop, target: success, unwind: None },
 628                 );
 629                 this.diverge_from(block);
 630                 block = success;
 631             }
 632             this.record_operands_moved(&[value_operand]);
 633         }
 634         block.and(Rvalue::Aggregate(Box::new(AggregateKind::Array(elem_ty)), Vec::new()))
 635     }
 636
 637     fn limit_capture_mutability(
 638         &mut self,
 639         upvar_span: Span,
 640         upvar_ty: Ty<'tcx>,
 641         temp_lifetime: Option<region::Scope>,
 642         mut block: BasicBlock,
 643         arg: &Expr<'tcx>,
 644     ) -> BlockAnd<Operand<'tcx>> {
 645         let this = self;
 646
 647         let source_info = this.source_info(upvar_span);
 648         let temp = this.local_decls.push(LocalDecl::new(upvar_ty, upvar_span));
 649
 650         this.cfg.push(block, Statement { source_info, kind: StatementKind::StorageLive(temp) });
 651
 652         let arg_place_builder = unpack!(block = this.as_place_builder(block, arg));
 653
 654         let mutability = match arg_place_builder.base() {
 655             // We are capturing a path that starts off a local variable in the parent.
 656             // The mutability of the current capture is same as the mutability
 657             // of the local declaration in the parent.
 658             PlaceBase::Local(local) => this.local_decls[local].mutability,
 659             // Parent is a closure and we are capturing a path that is captured
 660             // by the parent itself. The mutability of the current capture
 661             // is same as that of the capture in the parent closure.
 662             PlaceBase::Upvar { .. } => {
 663                 let enclosing_upvars_resolved = arg_place_builder.to_place(this);
 664
 665                 match enclosing_upvars_resolved.as_ref() {
 666                     PlaceRef {
 667                         local,
 668                         projection: &[ProjectionElem::Field(upvar_index, _), ..],
 669                     }
 670                     | PlaceRef {
 671                         local,
 672                         projection:
 673                             &[ProjectionElem::Deref, ProjectionElem::Field(upvar_index, _), ..],
 674                     } => {
 675                         // Not in a closure
 676                         debug_assert!(
 677                             local == ty::CAPTURE_STRUCT_LOCAL,
 678                             "Expected local to be Local(1), found {:?}",
 679                             local
 680                         );
 681                         // Not in a closure
 682                         debug_assert!(
 683                             this.upvars.len() > upvar_index.index(),
 684                             "Unexpected capture place, upvars={:#?}, upvar_index={:?}",
 685                             this.upvars,
 686                             upvar_index
 687                         );
 688                         this.upvars[upvar_index.index()].mutability
 689                     }
 690                     _ => bug!("Unexpected capture place"),
 691                 }
 692             }
 693         };
 694
 695         let borrow_kind = match mutability {
 696             Mutability::Not => BorrowKind::Unique,
 697             Mutability::Mut => BorrowKind::Mut { allow_two_phase_borrow: false },
 698         };
 699
 700         let arg_place = arg_place_builder.to_place(this);
 701
 702         this.cfg.push_assign(
 703             block,
 704             source_info,
 705             Place::from(temp),
 706             Rvalue::Ref(this.tcx.lifetimes.re_erased, borrow_kind, arg_place),
 707         );
 708
 709         // See the comment in `expr_as_temp` and on the `rvalue_scopes` field for why
 710         // this can be `None`.
 711         if let Some(temp_lifetime) = temp_lifetime {
 712             this.schedule_drop_storage_and_value(upvar_span, temp_lifetime, temp);
 713         }
 714
 715         block.and(Operand::Move(Place::from(temp)))
 716     }
 717
 718     // Helper to get a `-1` value of the appropriate type
 719     fn neg_1_literal(&mut self, span: Span, ty: Ty<'tcx>) -> Operand<'tcx> {
 720         let param_ty = ty::ParamEnv::empty().and(ty);
 721         let size = self.tcx.layout_of(param_ty).unwrap().size;
 722         let literal = ConstantKind::from_bits(self.tcx, size.unsigned_int_max(), param_ty);
 723
 724         self.literal_operand(span, literal)
 725     }
 726
 727     // Helper to get the minimum value of the appropriate type
 728     fn minval_literal(&mut self, span: Span, ty: Ty<'tcx>) -> Operand<'tcx> {
 729         assert!(ty.is_signed());
 730         let param_ty = ty::ParamEnv::empty().and(ty);
 731         let bits = self.tcx.layout_of(param_ty).unwrap().size.bits();
 732         let n = 1 << (bits - 1);
 733         let literal = ConstantKind::from_bits(self.tcx, n, param_ty);
 734
 735         self.literal_operand(span, literal)
 736     }
 737 }