src/librustc_mir/transform/const_prop.rs

   1 //! Propagates constants for early reporting of statically known
   2 //! assertion failures
   3
   4
   5 use rustc::hir::def::Def;
   6 use rustc::mir::{Constant, Location, Place, PlaceBase, Mir, Operand, Rvalue, Local};
   7 use rustc::mir::{NullOp, UnOp, StatementKind, Statement, BasicBlock, LocalKind, Static, StaticKind};
   8 use rustc::mir::{TerminatorKind, ClearCrossCrate, SourceInfo, BinOp, ProjectionElem};
   9 use rustc::mir::visit::{Visitor, PlaceContext, MutatingUseContext, NonMutatingUseContext};
  10 use rustc::mir::interpret::{EvalErrorKind, Scalar, GlobalId, EvalResult};
  11 use rustc::ty::{TyCtxt, self, Instance};
  12 use syntax::source_map::{Span, DUMMY_SP};
  13 use rustc::ty::subst::InternalSubsts;
  14 use rustc_data_structures::indexed_vec::IndexVec;
  15 use rustc::ty::ParamEnv;
  16 use rustc::ty::layout::{
  17     LayoutOf, TyLayout, LayoutError,
  18     HasTyCtxt, TargetDataLayout, HasDataLayout,
  19 };
  20
  21 use crate::interpret::{EvalContext, ScalarMaybeUndef, Immediate, OpTy, ImmTy, MemoryKind};
  22 use crate::const_eval::{
  23     CompileTimeInterpreter, error_to_const_error, eval_promoted, mk_eval_cx,
  24 };
  25 use crate::transform::{MirPass, MirSource};
  26
  27 pub struct ConstProp;
  28
  29 impl MirPass for ConstProp {
  30     fn run_pass<'a, 'tcx>(&self,
  31                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
  32                           source: MirSource<'tcx>,
  33                           mir: &mut Mir<'tcx>) {
  34         // will be evaluated by miri and produce its errors there
  35         if source.promoted.is_some() {
  36             return;
  37         }
  38
  39         use rustc::hir::map::blocks::FnLikeNode;
  40         let hir_id = tcx.hir().as_local_hir_id(source.def_id())
  41                               .expect("Non-local call to local provider is_const_fn");
  42
  43         let is_fn_like = FnLikeNode::from_node(tcx.hir().get_by_hir_id(hir_id)).is_some();
  44         let is_assoc_const = match tcx.describe_def(source.def_id()) {
  45             Some(Def::AssociatedConst(_)) => true,
  46             _ => false,
  47         };
  48
  49         // Only run const prop on functions, methods, closures and associated constants
  50         if !is_fn_like && !is_assoc_const  {
  51             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  52             trace!("ConstProp skipped for {:?}", source.def_id());
  53             return
  54         }
  55
  56         trace!("ConstProp starting for {:?}", source.def_id());
  57
  58         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
  59         // constants, instead of just checking for const-folding succeeding.
  60         // That would require an uniform one-def no-mutation analysis
  61         // and RPO (or recursing when needing the value of a local).
  62         let mut optimization_finder = ConstPropagator::new(mir, tcx, source);
  63         optimization_finder.visit_mir(mir);
  64
  65         trace!("ConstProp done for {:?}", source.def_id());
  66     }
  67 }
  68
  69 type Const<'tcx> = (OpTy<'tcx>, Span);
  70
  71 /// Finds optimization opportunities on the MIR.
  72 struct ConstPropagator<'a, 'mir, 'tcx:'a+'mir> {
  73     ecx: EvalContext<'a, 'mir, 'tcx, CompileTimeInterpreter<'a, 'mir, 'tcx>>,
  74     mir: &'mir Mir<'tcx>,
  75     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  76     source: MirSource<'tcx>,
  77     places: IndexVec<Local, Option<Const<'tcx>>>,
  78     can_const_prop: IndexVec<Local, bool>,
  79     param_env: ParamEnv<'tcx>,
  80 }
  81
  82 impl<'a, 'b, 'tcx> LayoutOf for ConstPropagator<'a, 'b, 'tcx> {
  83     type Ty = ty::Ty<'tcx>;
  84     type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
  85
  86     fn layout_of(&self, ty: ty::Ty<'tcx>) -> Self::TyLayout {
  87         self.tcx.layout_of(self.param_env.and(ty))
  88     }
  89 }
  90
  91 impl<'a, 'b, 'tcx> HasDataLayout for ConstPropagator<'a, 'b, 'tcx> {
  92     #[inline]
  93     fn data_layout(&self) -> &TargetDataLayout {
  94         &self.tcx.data_layout
  95     }
  96 }
  97
  98 impl<'a, 'b, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'a, 'b, 'tcx> {
  99     #[inline]
 100     fn tcx<'c>(&'c self) -> TyCtxt<'c, 'tcx, 'tcx> {
 101         self.tcx
 102     }
 103 }
 104
 105 impl<'a, 'mir, 'tcx> ConstPropagator<'a, 'mir, 'tcx> {
 106     fn new(
 107         mir: &'mir Mir<'tcx>,
 108         tcx: TyCtxt<'a, 'tcx, 'tcx>,
 109         source: MirSource<'tcx>,
 110     ) -> ConstPropagator<'a, 'mir, 'tcx> {
 111         let param_env = tcx.param_env(source.def_id());
 112         let ecx = mk_eval_cx(tcx, tcx.def_span(source.def_id()), param_env);
 113         ConstPropagator {
 114             ecx,
 115             mir,
 116             tcx,
 117             source,
 118             param_env,
 119             can_const_prop: CanConstProp::check(mir),
 120             places: IndexVec::from_elem(None, &mir.local_decls),
 121         }
 122     }
 123
 124     fn use_ecx<F, T>(
 125         &mut self,
 126         source_info: SourceInfo,
 127         f: F
 128     ) -> Option<T>
 129     where
 130         F: FnOnce(&mut Self) -> EvalResult<'tcx, T>,
 131     {
 132         self.ecx.tcx.span = source_info.span;
 133         let lint_root = match self.mir.source_scope_local_data {
 134             ClearCrossCrate::Set(ref ivs) => {
 135                 //FIXME(#51314): remove this check
 136                 if source_info.scope.index() >= ivs.len() {
 137                     return None;
 138                 }
 139                 ivs[source_info.scope].lint_root
 140             },
 141             ClearCrossCrate::Clear => return None,
 142         };
 143         let r = match f(self) {
 144             Ok(val) => Some(val),
 145             Err(error) => {
 146                 let diagnostic = error_to_const_error(&self.ecx, error);
 147                 use rustc::mir::interpret::EvalErrorKind::*;
 148                 match diagnostic.error {
 149                     // don't report these, they make no sense in a const prop context
 150                     | MachineError(_)
 151                     // at runtime these transformations might make sense
 152                     // FIXME: figure out the rules and start linting
 153                     | FunctionAbiMismatch(..)
 154                     | FunctionArgMismatch(..)
 155                     | FunctionRetMismatch(..)
 156                     | FunctionArgCountMismatch
 157                     // fine at runtime, might be a register address or sth
 158                     | ReadBytesAsPointer
 159                     // fine at runtime
 160                     | ReadForeignStatic
 161                     | Unimplemented(_)
 162                     // don't report const evaluator limits
 163                     | StackFrameLimitReached
 164                     | NoMirFor(..)
 165                     | InlineAsm
 166                     => {},
 167
 168                     | InvalidMemoryAccess
 169                     | DanglingPointerDeref
 170                     | DoubleFree
 171                     | InvalidFunctionPointer
 172                     | InvalidBool
 173                     | InvalidDiscriminant(..)
 174                     | PointerOutOfBounds { .. }
 175                     | InvalidNullPointerUsage
 176                     | ValidationFailure(..)
 177                     | InvalidPointerMath
 178                     | ReadUndefBytes(_)
 179                     | DeadLocal
 180                     | InvalidBoolOp(_)
 181                     | DerefFunctionPointer
 182                     | ExecuteMemory
 183                     | Intrinsic(..)
 184                     | InvalidChar(..)
 185                     | AbiViolation(_)
 186                     | AlignmentCheckFailed{..}
 187                     | CalledClosureAsFunction
 188                     | VtableForArgumentlessMethod
 189                     | ModifiedConstantMemory
 190                     | ModifiedStatic
 191                     | AssumptionNotHeld
 192                     // FIXME: should probably be removed and turned into a bug! call
 193                     | TypeNotPrimitive(_)
 194                     | ReallocatedWrongMemoryKind(_, _)
 195                     | DeallocatedWrongMemoryKind(_, _)
 196                     | ReallocateNonBasePtr
 197                     | DeallocateNonBasePtr
 198                     | IncorrectAllocationInformation(..)
 199                     | UnterminatedCString(_)
 200                     | HeapAllocZeroBytes
 201                     | HeapAllocNonPowerOfTwoAlignment(_)
 202                     | Unreachable
 203                     | ReadFromReturnPointer
 204                     | GeneratorResumedAfterReturn
 205                     | GeneratorResumedAfterPanic
 206                     | ReferencedConstant
 207                     | InfiniteLoop
 208                     => {
 209                         // FIXME: report UB here
 210                     },
 211
 212                     | OutOfTls
 213                     | TlsOutOfBounds
 214                     | PathNotFound(_)
 215                     => bug!("these should not be in rustc, but in miri's machine errors"),
 216
 217                     | Layout(_)
 218                     | UnimplementedTraitSelection
 219                     | TypeckError
 220                     | TooGeneric
 221                     // these are just noise
 222                     => {},
 223
 224                     // non deterministic
 225                     | ReadPointerAsBytes
 226                     // FIXME: implement
 227                     => {},
 228
 229                     | Panic { .. }
 230                     | BoundsCheck{..}
 231                     | Overflow(_)
 232                     | OverflowNeg
 233                     | DivisionByZero
 234                     | RemainderByZero
 235                     => {
 236                         diagnostic.report_as_lint(
 237                             self.ecx.tcx,
 238                             "this expression will panic at runtime",
 239                             lint_root,
 240                         );
 241                     }
 242                 }
 243                 None
 244             },
 245         };
 246         self.ecx.tcx.span = DUMMY_SP;
 247         r
 248     }
 249
 250     fn eval_constant(
 251         &mut self,
 252         c: &Constant<'tcx>,
 253         source_info: SourceInfo,
 254     ) -> Option<Const<'tcx>> {
 255         self.ecx.tcx.span = source_info.span;
 256         match self.ecx.eval_const_to_op(*c.literal, None) {
 257             Ok(op) => {
 258                 Some((op, c.span))
 259             },
 260             Err(error) => {
 261                 let err = error_to_const_error(&self.ecx, error);
 262                 err.report_as_error(self.ecx.tcx, "erroneous constant used");
 263                 None
 264             },
 265         }
 266     }
 267
 268     fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 269         match *place {
 270             Place::Base(PlaceBase::Local(loc)) => self.places[loc].clone(),
 271             Place::Projection(ref proj) => match proj.elem {
 272                 ProjectionElem::Field(field, _) => {
 273                     trace!("field proj on {:?}", proj.base);
 274                     let (base, span) = self.eval_place(&proj.base, source_info)?;
 275                     let res = self.use_ecx(source_info, |this| {
 276                         this.ecx.operand_field(base, field.index() as u64)
 277                     })?;
 278                     Some((res, span))
 279                 },
 280                 // We could get more projections by using e.g., `operand_projection`,
 281                 // but we do not even have the stack frame set up properly so
 282                 // an `Index` projection would throw us off-track.
 283                 _ => None,
 284             },
 285             Place::Base(
 286                 PlaceBase::Static(box Static {kind: StaticKind::Promoted(promoted), ..})
 287             ) => {
 288                 let generics = self.tcx.generics_of(self.source.def_id());
 289                 if generics.requires_monomorphization(self.tcx) {
 290                     // FIXME: can't handle code with generics
 291                     return None;
 292                 }
 293                 let substs = InternalSubsts::identity_for_item(self.tcx, self.source.def_id());
 294                 let instance = Instance::new(self.source.def_id(), substs);
 295                 let cid = GlobalId {
 296                     instance,
 297                     promoted: Some(promoted),
 298                 };
 299                 // cannot use `const_eval` here, because that would require having the MIR
 300                 // for the current function available, but we're producing said MIR right now
 301                 let res = self.use_ecx(source_info, |this| {
 302                     eval_promoted(this.tcx, cid, this.mir, this.param_env)
 303                 })?;
 304                 trace!("evaluated promoted {:?} to {:?}", promoted, res);
 305                 Some((res.into(), source_info.span))
 306             },
 307             _ => None,
 308         }
 309     }
 310
 311     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 312         match *op {
 313             Operand::Constant(ref c) => self.eval_constant(c, source_info),
 314             | Operand::Move(ref place)
 315             | Operand::Copy(ref place) => self.eval_place(place, source_info),
 316         }
 317     }
 318
 319     fn const_prop(
 320         &mut self,
 321         rvalue: &Rvalue<'tcx>,
 322         place_layout: TyLayout<'tcx>,
 323         source_info: SourceInfo,
 324     ) -> Option<Const<'tcx>> {
 325         let span = source_info.span;
 326         match *rvalue {
 327             Rvalue::Use(ref op) => {
 328                 self.eval_operand(op, source_info)
 329             },
 330             Rvalue::Repeat(..) |
 331             Rvalue::Ref(..) |
 332             Rvalue::Aggregate(..) |
 333             Rvalue::NullaryOp(NullOp::Box, _) |
 334             Rvalue::Discriminant(..) => None,
 335
 336             Rvalue::Cast(kind, ref operand, _) => {
 337                 let (op, span) = self.eval_operand(operand, source_info)?;
 338                 self.use_ecx(source_info, |this| {
 339                     let dest = this.ecx.allocate(place_layout, MemoryKind::Stack);
 340                     this.ecx.cast(op, kind, dest.into())?;
 341                     Ok((dest.into(), span))
 342                 })
 343             }
 344
 345             // FIXME(oli-obk): evaluate static/constant slice lengths
 346             Rvalue::Len(_) => None,
 347             Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
 348                 type_size_of(self.tcx, self.param_env, ty).and_then(|n| Some((
 349                     ImmTy {
 350                         imm: Immediate::Scalar(
 351                             Scalar::Bits {
 352                                 bits: n as u128,
 353                                 size: self.tcx.data_layout.pointer_size.bytes() as u8,
 354                             }.into()
 355                         ),
 356                         layout: self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
 357                     }.into(),
 358                     span,
 359                 )))
 360             }
 361             Rvalue::UnaryOp(op, ref arg) => {
 362                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 363                     self.tcx.closure_base_def_id(self.source.def_id())
 364                 } else {
 365                     self.source.def_id()
 366                 };
 367                 let generics = self.tcx.generics_of(def_id);
 368                 if generics.requires_monomorphization(self.tcx) {
 369                     // FIXME: can't handle code with generics
 370                     return None;
 371                 }
 372
 373                 let (arg, _) = self.eval_operand(arg, source_info)?;
 374                 let val = self.use_ecx(source_info, |this| {
 375                     let prim = this.ecx.read_immediate(arg)?;
 376                     match op {
 377                         UnOp::Neg => {
 378                             // Need to do overflow check here: For actual CTFE, MIR
 379                             // generation emits code that does this before calling the op.
 380                             if prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
 381                                 return err!(OverflowNeg);
 382                             }
 383                         }
 384                         UnOp::Not => {
 385                             // Cannot overflow
 386                         }
 387                     }
 388                     // Now run the actual operation.
 389                     this.ecx.unary_op(op, prim)
 390                 })?;
 391                 let res = ImmTy {
 392                     imm: Immediate::Scalar(val.into()),
 393                     layout: place_layout,
 394                 };
 395                 Some((res.into(), span))
 396             }
 397             Rvalue::CheckedBinaryOp(op, ref left, ref right) |
 398             Rvalue::BinaryOp(op, ref left, ref right) => {
 399                 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
 400                 let right = self.eval_operand(right, source_info)?;
 401                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 402                     self.tcx.closure_base_def_id(self.source.def_id())
 403                 } else {
 404                     self.source.def_id()
 405                 };
 406                 let generics = self.tcx.generics_of(def_id);
 407                 if generics.requires_monomorphization(self.tcx) {
 408                     // FIXME: can't handle code with generics
 409                     return None;
 410                 }
 411
 412                 let r = self.use_ecx(source_info, |this| {
 413                     this.ecx.read_immediate(right.0)
 414                 })?;
 415                 if op == BinOp::Shr || op == BinOp::Shl {
 416                     let left_ty = left.ty(self.mir, self.tcx);
 417                     let left_bits = self
 418                         .tcx
 419                         .layout_of(self.param_env.and(left_ty))
 420                         .unwrap()
 421                         .size
 422                         .bits();
 423                     let right_size = right.0.layout.size;
 424                     let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
 425                     if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
 426                         let source_scope_local_data = match self.mir.source_scope_local_data {
 427                             ClearCrossCrate::Set(ref data) => data,
 428                             ClearCrossCrate::Clear => return None,
 429                         };
 430                         let dir = if op == BinOp::Shr {
 431                             "right"
 432                         } else {
 433                             "left"
 434                         };
 435                         let hir_id = source_scope_local_data[source_info.scope].lint_root;
 436                         self.tcx.lint_hir(
 437                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 438                             hir_id,
 439                             span,
 440                             &format!("attempt to shift {} with overflow", dir));
 441                         return None;
 442                     }
 443                 }
 444                 let left = self.eval_operand(left, source_info)?;
 445                 let l = self.use_ecx(source_info, |this| {
 446                     this.ecx.read_immediate(left.0)
 447                 })?;
 448                 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
 449                 let (val, overflow) = self.use_ecx(source_info, |this| {
 450                     this.ecx.binary_op(op, l, r)
 451                 })?;
 452                 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
 453                     Immediate::ScalarPair(
 454                         val.into(),
 455                         Scalar::from_bool(overflow).into(),
 456                     )
 457                 } else {
 458                     if overflow {
 459                         let err = EvalErrorKind::Overflow(op).into();
 460                         let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
 461                         return None;
 462                     }
 463                     Immediate::Scalar(val.into())
 464                 };
 465                 let res = ImmTy {
 466                     imm: val,
 467                     layout: place_layout,
 468                 };
 469                 Some((res.into(), span))
 470             },
 471         }
 472     }
 473 }
 474
 475 fn type_size_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 476                           param_env: ty::ParamEnv<'tcx>,
 477                           ty: ty::Ty<'tcx>) -> Option<u64> {
 478     tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
 479 }
 480
 481 struct CanConstProp {
 482     can_const_prop: IndexVec<Local, bool>,
 483     // false at the beginning, once set, there are not allowed to be any more assignments
 484     found_assignment: IndexVec<Local, bool>,
 485 }
 486
 487 impl CanConstProp {
 488     /// returns true if `local` can be propagated
 489     fn check(mir: &Mir<'_>) -> IndexVec<Local, bool> {
 490         let mut cpv = CanConstProp {
 491             can_const_prop: IndexVec::from_elem(true, &mir.local_decls),
 492             found_assignment: IndexVec::from_elem(false, &mir.local_decls),
 493         };
 494         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 495             // cannot use args at all
 496             // cannot use locals because if x < y { y - x } else { x - y } would
 497             //        lint for x != y
 498             // FIXME(oli-obk): lint variables until they are used in a condition
 499             // FIXME(oli-obk): lint if return value is constant
 500             *val = mir.local_kind(local) == LocalKind::Temp;
 501         }
 502         cpv.visit_mir(mir);
 503         cpv.can_const_prop
 504     }
 505 }
 506
 507 impl<'tcx> Visitor<'tcx> for CanConstProp {
 508     fn visit_local(
 509         &mut self,
 510         &local: &Local,
 511         context: PlaceContext<'tcx>,
 512         _: Location,
 513     ) {
 514         use rustc::mir::visit::PlaceContext::*;
 515         match context {
 516             // Constants must have at most one write
 517             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 518             // only occur in independent execution paths
 519             MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
 520                 self.can_const_prop[local] = false;
 521             } else {
 522                 self.found_assignment[local] = true
 523             },
 524             // Reading constants is allowed an arbitrary number of times
 525             NonMutatingUse(NonMutatingUseContext::Copy) |
 526             NonMutatingUse(NonMutatingUseContext::Move) |
 527             NonMutatingUse(NonMutatingUseContext::Inspect) |
 528             NonMutatingUse(NonMutatingUseContext::Projection) |
 529             MutatingUse(MutatingUseContext::Projection) |
 530             NonUse(_) => {},
 531             _ => self.can_const_prop[local] = false,
 532         }
 533     }
 534 }
 535
 536 impl<'b, 'a, 'tcx> Visitor<'tcx> for ConstPropagator<'b, 'a, 'tcx> {
 537     fn visit_constant(
 538         &mut self,
 539         constant: &Constant<'tcx>,
 540         location: Location,
 541     ) {
 542         trace!("visit_constant: {:?}", constant);
 543         self.super_constant(constant, location);
 544         let source_info = *self.mir.source_info(location);
 545         self.eval_constant(constant, source_info);
 546     }
 547
 548     fn visit_statement(
 549         &mut self,
 550         block: BasicBlock,
 551         statement: &Statement<'tcx>,
 552         location: Location,
 553     ) {
 554         trace!("visit_statement: {:?}", statement);
 555         if let StatementKind::Assign(ref place, ref rval) = statement.kind {
 556             let place_ty: ty::Ty<'tcx> = place
 557                 .ty(&self.mir.local_decls, self.tcx)
 558                 .to_ty(self.tcx);
 559             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 560                 if let Some(value) = self.const_prop(rval, place_layout, statement.source_info) {
 561                     if let Place::Base(PlaceBase::Local(local)) = *place {
 562                         trace!("checking whether {:?} can be stored to {:?}", value, local);
 563                         if self.can_const_prop[local] {
 564                             trace!("storing {:?} to {:?}", value, local);
 565                             assert!(self.places[local].is_none());
 566                             self.places[local] = Some(value);
 567                         }
 568                     }
 569                 }
 570             }
 571         }
 572         self.super_statement(block, statement, location);
 573     }
 574
 575     fn visit_terminator_kind(
 576         &mut self,
 577         block: BasicBlock,
 578         kind: &TerminatorKind<'tcx>,
 579         location: Location,
 580     ) {
 581         self.super_terminator_kind(block, kind, location);
 582         let source_info = *self.mir.source_info(location);
 583         if let TerminatorKind::Assert { expected, msg, cond, .. } = kind {
 584             if let Some(value) = self.eval_operand(cond, source_info) {
 585                 trace!("assertion on {:?} should be {:?}", value, expected);
 586                 let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
 587                 if expected != self.ecx.read_scalar(value.0).unwrap() {
 588                     // poison all places this operand references so that further code
 589                     // doesn't use the invalid value
 590                     match cond {
 591                         Operand::Move(ref place) | Operand::Copy(ref place) => {
 592                             let mut place = place;
 593                             while let Place::Projection(ref proj) = *place {
 594                                 place = &proj.base;
 595                             }
 596                             if let Place::Base(PlaceBase::Local(local)) = *place {
 597                                 self.places[local] = None;
 598                             }
 599                         },
 600                         Operand::Constant(_) => {}
 601                     }
 602                     let span = self.mir[block]
 603                         .terminator
 604                         .as_ref()
 605                         .unwrap()
 606                         .source_info
 607                         .span;
 608                     let hir_id = self
 609                         .tcx
 610                         .hir()
 611                         .as_local_hir_id(self.source.def_id())
 612                         .expect("some part of a failing const eval must be local");
 613                     use rustc::mir::interpret::EvalErrorKind::*;
 614                     let msg = match msg {
 615                         Overflow(_) |
 616                         OverflowNeg |
 617                         DivisionByZero |
 618                         RemainderByZero => msg.description().to_owned(),
 619                         BoundsCheck { ref len, ref index } => {
 620                             let len = self
 621                                 .eval_operand(len, source_info)
 622                                 .expect("len must be const");
 623                             let len = match self.ecx.read_scalar(len.0) {
 624                                 Ok(ScalarMaybeUndef::Scalar(Scalar::Bits {
 625                                     bits, ..
 626                                 })) => bits,
 627                                 other => bug!("const len not primitive: {:?}", other),
 628                             };
 629                             let index = self
 630                                 .eval_operand(index, source_info)
 631                                 .expect("index must be const");
 632                             let index = match self.ecx.read_scalar(index.0) {
 633                                 Ok(ScalarMaybeUndef::Scalar(Scalar::Bits {
 634                                     bits, ..
 635                                 })) => bits,
 636                                 other => bug!("const index not primitive: {:?}", other),
 637                             };
 638                             format!(
 639                                 "index out of bounds: \
 640                                 the len is {} but the index is {}",
 641                                 len,
 642                                 index,
 643                             )
 644                         },
 645                         // Need proper const propagator for these
 646                         _ => return,
 647                     };
 648                     self.tcx.lint_hir(
 649                         ::rustc::lint::builtin::CONST_ERR,
 650                         hir_id,
 651                         span,
 652                         &msg,
 653                     );
 654                 }
 655             }
 656         }
 657     }
 658 }