src/librustc_mir/transform/const_prop.rs

   1 // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Propagates constants for early reporting of statically known
  12 //! assertion failures
  13
  14
  15 use rustc::hir::def::Def;
  16 use rustc::mir::{Constant, Location, Place, Mir, Operand, Rvalue, Local};
  17 use rustc::mir::{NullOp, UnOp, StatementKind, Statement, BasicBlock, LocalKind};
  18 use rustc::mir::{TerminatorKind, ClearCrossCrate, SourceInfo, BinOp, ProjectionElem};
  19 use rustc::mir::visit::{Visitor, PlaceContext};
  20 use rustc::mir::interpret::{
  21     ConstEvalErr, EvalErrorKind, Scalar, GlobalId, EvalResult
  22 };
  23 use rustc::ty::{TyCtxt, self, Instance};
  24 use interpret::{self, EvalContext, Value, OpTy, MemoryKind, ScalarMaybeUndef};
  25 use const_eval::{CompileTimeInterpreter, eval_promoted, mk_borrowck_eval_cx};
  26 use transform::{MirPass, MirSource};
  27 use syntax::source_map::{Span, DUMMY_SP};
  28 use rustc::ty::subst::Substs;
  29 use rustc_data_structures::indexed_vec::IndexVec;
  30 use rustc::ty::ParamEnv;
  31 use rustc::ty::layout::{
  32     LayoutOf, TyLayout, LayoutError,
  33     HasTyCtxt, TargetDataLayout, HasDataLayout,
  34 };
  35
  36 pub struct ConstProp;
  37
  38 impl MirPass for ConstProp {
  39     fn run_pass<'a, 'tcx>(&self,
  40                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
  41                           source: MirSource,
  42                           mir: &mut Mir<'tcx>) {
  43         // will be evaluated by miri and produce its errors there
  44         if source.promoted.is_some() {
  45             return;
  46         }
  47         match tcx.describe_def(source.def_id) {
  48             // skip statics/consts because they'll be evaluated by miri anyway
  49             Some(Def::Const(..)) |
  50             Some(Def::Static(..)) => return,
  51             // we still run on associated constants, because they might not get evaluated
  52             // within the current crate
  53             _ => {},
  54         }
  55         trace!("ConstProp starting for {:?}", source.def_id);
  56
  57         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
  58         // constants, instead of just checking for const-folding succeeding.
  59         // That would require an uniform one-def no-mutation analysis
  60         // and RPO (or recursing when needing the value of a local).
  61         let mut optimization_finder = ConstPropagator::new(mir, tcx, source);
  62         optimization_finder.visit_mir(mir);
  63
  64         trace!("ConstProp done for {:?}", source.def_id);
  65     }
  66 }
  67
  68 type Const<'tcx> = (OpTy<'tcx>, Span);
  69
  70 /// Finds optimization opportunities on the MIR.
  71 struct ConstPropagator<'a, 'mir, 'tcx:'a+'mir> {
  72     ecx: EvalContext<'a, 'mir, 'tcx, CompileTimeInterpreter<'a, 'mir, 'tcx>>,
  73     mir: &'mir Mir<'tcx>,
  74     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  75     source: MirSource,
  76     places: IndexVec<Local, Option<Const<'tcx>>>,
  77     can_const_prop: IndexVec<Local, bool>,
  78     param_env: ParamEnv<'tcx>,
  79 }
  80
  81 impl<'a, 'b, 'tcx> LayoutOf for &'a ConstPropagator<'a, 'b, 'tcx> {
  82     type Ty = ty::Ty<'tcx>;
  83     type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
  84
  85     fn layout_of(self, ty: ty::Ty<'tcx>) -> Self::TyLayout {
  86         self.tcx.layout_of(self.param_env.and(ty))
  87     }
  88 }
  89
  90 impl<'a, 'b, 'tcx> HasDataLayout for &'a ConstPropagator<'a, 'b, 'tcx> {
  91     #[inline]
  92     fn data_layout(&self) -> &TargetDataLayout {
  93         &self.tcx.data_layout
  94     }
  95 }
  96
  97 impl<'a, 'b, 'tcx> HasTyCtxt<'tcx> for &'a ConstPropagator<'a, 'b, 'tcx> {
  98     #[inline]
  99     fn tcx<'c>(&'c self) -> TyCtxt<'c, 'tcx, 'tcx> {
 100         self.tcx
 101     }
 102 }
 103
 104 impl<'a, 'mir, 'tcx> ConstPropagator<'a, 'mir, 'tcx> {
 105     fn new(
 106         mir: &'mir Mir<'tcx>,
 107         tcx: TyCtxt<'a, 'tcx, 'tcx>,
 108         source: MirSource,
 109     ) -> ConstPropagator<'a, 'mir, 'tcx> {
 110         let param_env = tcx.param_env(source.def_id);
 111         let substs = Substs::identity_for_item(tcx, source.def_id);
 112         let instance = Instance::new(source.def_id, substs);
 113         let ecx = mk_borrowck_eval_cx(tcx, instance, mir, DUMMY_SP).unwrap();
 114         ConstPropagator {
 115             ecx,
 116             mir,
 117             tcx,
 118             source,
 119             param_env,
 120             can_const_prop: CanConstProp::check(mir),
 121             places: IndexVec::from_elem(None, &mir.local_decls),
 122         }
 123     }
 124
 125     fn use_ecx<F, T>(
 126         &mut self,
 127         source_info: SourceInfo,
 128         f: F
 129     ) -> Option<T>
 130     where
 131         F: FnOnce(&mut Self) -> EvalResult<'tcx, T>,
 132     {
 133         self.ecx.tcx.span = source_info.span;
 134         let lint_root = match self.mir.source_scope_local_data {
 135             ClearCrossCrate::Set(ref ivs) => {
 136                 //FIXME(#51314): remove this check
 137                 if source_info.scope.index() >= ivs.len() {
 138                     return None;
 139                 }
 140                 ivs[source_info.scope].lint_root
 141             },
 142             ClearCrossCrate::Clear => return None,
 143         };
 144         let r = match f(self) {
 145             Ok(val) => Some(val),
 146             Err(error) => {
 147                 let (stacktrace, span) = self.ecx.generate_stacktrace(None);
 148                 let diagnostic = ConstEvalErr { span, error, stacktrace };
 149                 use rustc::mir::interpret::EvalErrorKind::*;
 150                 match diagnostic.error.kind {
 151                     // don't report these, they make no sense in a const prop context
 152                     | MachineError(_)
 153                     // at runtime these transformations might make sense
 154                     // FIXME: figure out the rules and start linting
 155                     | FunctionAbiMismatch(..)
 156                     | FunctionArgMismatch(..)
 157                     | FunctionRetMismatch(..)
 158                     | FunctionArgCountMismatch
 159                     // fine at runtime, might be a register address or sth
 160                     | ReadBytesAsPointer
 161                     // fine at runtime
 162                     | ReadForeignStatic
 163                     | Unimplemented(_)
 164                     // don't report const evaluator limits
 165                     | StackFrameLimitReached
 166                     | NoMirFor(..)
 167                     | InlineAsm
 168                     => {},
 169
 170                     | InvalidMemoryAccess
 171                     | DanglingPointerDeref
 172                     | DoubleFree
 173                     | InvalidFunctionPointer
 174                     | InvalidBool
 175                     | InvalidDiscriminant(..)
 176                     | PointerOutOfBounds { .. }
 177                     | InvalidNullPointerUsage
 178                     | MemoryLockViolation { .. }
 179                     | MemoryAcquireConflict { .. }
 180                     | ValidationFailure(..)
 181                     | InvalidMemoryLockRelease { .. }
 182                     | DeallocatedLockedMemory { .. }
 183                     | InvalidPointerMath
 184                     | ReadUndefBytes(_)
 185                     | DeadLocal
 186                     | InvalidBoolOp(_)
 187                     | DerefFunctionPointer
 188                     | ExecuteMemory
 189                     | Intrinsic(..)
 190                     | InvalidChar(..)
 191                     | AbiViolation(_)
 192                     | AlignmentCheckFailed{..}
 193                     | CalledClosureAsFunction
 194                     | VtableForArgumentlessMethod
 195                     | ModifiedConstantMemory
 196                     | AssumptionNotHeld
 197                     // FIXME: should probably be removed and turned into a bug! call
 198                     | TypeNotPrimitive(_)
 199                     | ReallocatedWrongMemoryKind(_, _)
 200                     | DeallocatedWrongMemoryKind(_, _)
 201                     | ReallocateNonBasePtr
 202                     | DeallocateNonBasePtr
 203                     | IncorrectAllocationInformation(..)
 204                     | UnterminatedCString(_)
 205                     | HeapAllocZeroBytes
 206                     | HeapAllocNonPowerOfTwoAlignment(_)
 207                     | Unreachable
 208                     | ReadFromReturnPointer
 209                     | GeneratorResumedAfterReturn
 210                     | GeneratorResumedAfterPanic
 211                     | ReferencedConstant(_)
 212                     | InfiniteLoop
 213                     => {
 214                         // FIXME: report UB here
 215                     },
 216
 217                     | OutOfTls
 218                     | TlsOutOfBounds
 219                     | PathNotFound(_)
 220                     => bug!("these should not be in rustc, but in miri's machine errors"),
 221
 222                     | Layout(_)
 223                     | UnimplementedTraitSelection
 224                     | TypeckError
 225                     | TooGeneric
 226                     | CheckMatchError
 227                     // these are just noise
 228                     => {},
 229
 230                     // non deterministic
 231                     | ReadPointerAsBytes
 232                     // FIXME: implement
 233                     => {},
 234
 235                     | Panic { .. }
 236                     | BoundsCheck{..}
 237                     | Overflow(_)
 238                     | OverflowNeg
 239                     | DivisionByZero
 240                     | RemainderByZero
 241                     => {
 242                         diagnostic.report_as_lint(
 243                             self.ecx.tcx,
 244                             "this expression will panic at runtime",
 245                             lint_root,
 246                         );
 247                     }
 248                 }
 249                 None
 250             },
 251         };
 252         self.ecx.tcx.span = DUMMY_SP;
 253         r
 254     }
 255
 256     fn eval_constant(
 257         &mut self,
 258         c: &Constant<'tcx>,
 259         source_info: SourceInfo,
 260     ) -> Option<Const<'tcx>> {
 261         self.ecx.tcx.span = source_info.span;
 262         match self.ecx.const_to_op(c.literal) {
 263             Ok(op) => {
 264                 Some((op, c.span))
 265             },
 266             Err(error) => {
 267                 let (stacktrace, span) = self.ecx.generate_stacktrace(None);
 268                 let err = ConstEvalErr {
 269                     span,
 270                     error,
 271                     stacktrace,
 272                 };
 273                 err.report_as_error(
 274                     self.tcx.at(source_info.span),
 275                     "could not evaluate constant",
 276                 );
 277                 None
 278             },
 279         }
 280     }
 281
 282     fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 283         match *place {
 284             Place::Local(loc) => self.places[loc].clone(),
 285             Place::Projection(ref proj) => match proj.elem {
 286                 ProjectionElem::Field(field, _) => {
 287                     trace!("field proj on {:?}", proj.base);
 288                     let (base, span) = self.eval_place(&proj.base, source_info)?;
 289                     let res = self.use_ecx(source_info, |this| {
 290                         this.ecx.operand_field(base, field.index() as u64)
 291                     })?;
 292                     Some((res, span))
 293                 },
 294                 // We could get more projections by using e.g. `operand_projection`,
 295                 // but we do not even have the stack frame set up properly so
 296                 // an `Index` projection would throw us off-track.
 297                 _ => None,
 298             },
 299             Place::Promoted(ref promoted) => {
 300                 let generics = self.tcx.generics_of(self.source.def_id);
 301                 if generics.requires_monomorphization(self.tcx) {
 302                     // FIXME: can't handle code with generics
 303                     return None;
 304                 }
 305                 let substs = Substs::identity_for_item(self.tcx, self.source.def_id);
 306                 let instance = Instance::new(self.source.def_id, substs);
 307                 let cid = GlobalId {
 308                     instance,
 309                     promoted: Some(promoted.0),
 310                 };
 311                 // cannot use `const_eval` here, because that would require having the MIR
 312                 // for the current function available, but we're producing said MIR right now
 313                 let res = self.use_ecx(source_info, |this| {
 314                     eval_promoted(this.tcx, cid, this.mir, this.param_env)
 315                 })?;
 316                 trace!("evaluated promoted {:?} to {:?}", promoted, res);
 317                 Some((res, source_info.span))
 318             },
 319             _ => None,
 320         }
 321     }
 322
 323     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 324         match *op {
 325             Operand::Constant(ref c) => self.eval_constant(c, source_info),
 326             | Operand::Move(ref place)
 327             | Operand::Copy(ref place) => self.eval_place(place, source_info),
 328         }
 329     }
 330
 331     fn const_prop(
 332         &mut self,
 333         rvalue: &Rvalue<'tcx>,
 334         place_layout: TyLayout<'tcx>,
 335         source_info: SourceInfo,
 336     ) -> Option<Const<'tcx>> {
 337         let span = source_info.span;
 338         match *rvalue {
 339             Rvalue::Use(ref op) => {
 340                 self.eval_operand(op, source_info)
 341             },
 342             Rvalue::Repeat(..) |
 343             Rvalue::Ref(..) |
 344             Rvalue::Aggregate(..) |
 345             Rvalue::NullaryOp(NullOp::Box, _) |
 346             Rvalue::Discriminant(..) => None,
 347
 348             Rvalue::Cast(kind, ref operand, _) => {
 349                 let (op, span) = self.eval_operand(operand, source_info)?;
 350                 self.use_ecx(source_info, |this| {
 351                     let dest = this.ecx.allocate(place_layout, MemoryKind::Stack)?;
 352                     this.ecx.cast(op, kind, dest.into())?;
 353                     Ok((dest.into(), span))
 354                 })
 355             }
 356
 357             // FIXME(oli-obk): evaluate static/constant slice lengths
 358             Rvalue::Len(_) => None,
 359             Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
 360                 type_size_of(self.tcx, self.param_env, ty).and_then(|n| Some((
 361                     OpTy {
 362                         op: interpret::Operand::Immediate(Value::Scalar(
 363                             Scalar::Bits {
 364                                 bits: n as u128,
 365                                 size: self.tcx.data_layout.pointer_size.bytes() as u8,
 366                             }.into()
 367                         )),
 368                         layout: self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
 369                     },
 370                     span,
 371                 )))
 372             }
 373             Rvalue::UnaryOp(op, ref arg) => {
 374                 let def_id = if self.tcx.is_closure(self.source.def_id) {
 375                     self.tcx.closure_base_def_id(self.source.def_id)
 376                 } else {
 377                     self.source.def_id
 378                 };
 379                 let generics = self.tcx.generics_of(def_id);
 380                 if generics.requires_monomorphization(self.tcx) {
 381                     // FIXME: can't handle code with generics
 382                     return None;
 383                 }
 384
 385                 let (arg, _) = self.eval_operand(arg, source_info)?;
 386                 let val = self.use_ecx(source_info, |this| {
 387                     let prim = this.ecx.read_scalar(arg)?.not_undef()?;
 388                     match op {
 389                         UnOp::Neg => {
 390                             // Need to do overflow check here: For actual CTFE, MIR
 391                             // generation emits code that does this before calling the op.
 392                             let size = arg.layout.size;
 393                             if prim.to_bits(size)? == (1 << (size.bits() - 1)) {
 394                                 return err!(OverflowNeg);
 395                             }
 396                         }
 397                         UnOp::Not => {
 398                             // Cannot overflow
 399                         }
 400                     }
 401                     // Now run the actual operation.
 402                     this.ecx.unary_op(op, prim, arg.layout)
 403                 })?;
 404                 let res = OpTy {
 405                     op: interpret::Operand::Immediate(Value::Scalar(val.into())),
 406                     layout: place_layout,
 407                 };
 408                 Some((res, span))
 409             }
 410             Rvalue::CheckedBinaryOp(op, ref left, ref right) |
 411             Rvalue::BinaryOp(op, ref left, ref right) => {
 412                 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
 413                 let right = self.eval_operand(right, source_info)?;
 414                 let def_id = if self.tcx.is_closure(self.source.def_id) {
 415                     self.tcx.closure_base_def_id(self.source.def_id)
 416                 } else {
 417                     self.source.def_id
 418                 };
 419                 let generics = self.tcx.generics_of(def_id);
 420                 if generics.requires_monomorphization(self.tcx) {
 421                     // FIXME: can't handle code with generics
 422                     return None;
 423                 }
 424
 425                 let r = self.use_ecx(source_info, |this| {
 426                     this.ecx.read_value(right.0)
 427                 })?;
 428                 if op == BinOp::Shr || op == BinOp::Shl {
 429                     let left_ty = left.ty(self.mir, self.tcx);
 430                     let left_bits = self
 431                         .tcx
 432                         .layout_of(self.param_env.and(left_ty))
 433                         .unwrap()
 434                         .size
 435                         .bits();
 436                     let right_size = right.0.layout.size;
 437                     let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
 438                     if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
 439                         let source_scope_local_data = match self.mir.source_scope_local_data {
 440                             ClearCrossCrate::Set(ref data) => data,
 441                             ClearCrossCrate::Clear => return None,
 442                         };
 443                         let dir = if op == BinOp::Shr {
 444                             "right"
 445                         } else {
 446                             "left"
 447                         };
 448                         let node_id = source_scope_local_data[source_info.scope].lint_root;
 449                         self.tcx.lint_node(
 450                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 451                             node_id,
 452                             span,
 453                             &format!("attempt to shift {} with overflow", dir));
 454                         return None;
 455                     }
 456                 }
 457                 let left = self.eval_operand(left, source_info)?;
 458                 let l = self.use_ecx(source_info, |this| {
 459                     this.ecx.read_value(left.0)
 460                 })?;
 461                 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
 462                 let (val, overflow) = self.use_ecx(source_info, |this| {
 463                     this.ecx.binary_op_val(op, l, r)
 464                 })?;
 465                 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
 466                     Value::ScalarPair(
 467                         val.into(),
 468                         Scalar::from_bool(overflow).into(),
 469                     )
 470                 } else {
 471                     if overflow {
 472                         let err = EvalErrorKind::Overflow(op).into();
 473                         let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
 474                         return None;
 475                     }
 476                     Value::Scalar(val.into())
 477                 };
 478                 let res = OpTy {
 479                     op: interpret::Operand::Immediate(val),
 480                     layout: place_layout,
 481                 };
 482                 Some((res, span))
 483             },
 484         }
 485     }
 486 }
 487
 488 fn type_size_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 489                           param_env: ty::ParamEnv<'tcx>,
 490                           ty: ty::Ty<'tcx>) -> Option<u64> {
 491     tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
 492 }
 493
 494 struct CanConstProp {
 495     can_const_prop: IndexVec<Local, bool>,
 496     // false at the beginning, once set, there are not allowed to be any more assignments
 497     found_assignment: IndexVec<Local, bool>,
 498 }
 499
 500 impl CanConstProp {
 501     /// returns true if `local` can be propagated
 502     fn check(mir: &Mir) -> IndexVec<Local, bool> {
 503         let mut cpv = CanConstProp {
 504             can_const_prop: IndexVec::from_elem(true, &mir.local_decls),
 505             found_assignment: IndexVec::from_elem(false, &mir.local_decls),
 506         };
 507         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 508             // cannot use args at all
 509             // cannot use locals because if x < y { y - x } else { x - y } would
 510             //        lint for x != y
 511             // FIXME(oli-obk): lint variables until they are used in a condition
 512             // FIXME(oli-obk): lint if return value is constant
 513             *val = mir.local_kind(local) == LocalKind::Temp;
 514         }
 515         cpv.visit_mir(mir);
 516         cpv.can_const_prop
 517     }
 518 }
 519
 520 impl<'tcx> Visitor<'tcx> for CanConstProp {
 521     fn visit_local(
 522         &mut self,
 523         &local: &Local,
 524         context: PlaceContext<'tcx>,
 525         _: Location,
 526     ) {
 527         use rustc::mir::visit::PlaceContext::*;
 528         match context {
 529             // Constants must have at most one write
 530             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 531             // only occur in independent execution paths
 532             Store => if self.found_assignment[local] {
 533                 self.can_const_prop[local] = false;
 534             } else {
 535                 self.found_assignment[local] = true
 536             },
 537             // Reading constants is allowed an arbitrary number of times
 538             Copy | Move |
 539             StorageDead | StorageLive |
 540             Validate |
 541             Projection(_) |
 542             Inspect => {},
 543             _ => self.can_const_prop[local] = false,
 544         }
 545     }
 546 }
 547
 548 impl<'b, 'a, 'tcx> Visitor<'tcx> for ConstPropagator<'b, 'a, 'tcx> {
 549     fn visit_constant(
 550         &mut self,
 551         constant: &Constant<'tcx>,
 552         location: Location,
 553     ) {
 554         trace!("visit_constant: {:?}", constant);
 555         self.super_constant(constant, location);
 556         let source_info = *self.mir.source_info(location);
 557         self.eval_constant(constant, source_info);
 558     }
 559
 560     fn visit_statement(
 561         &mut self,
 562         block: BasicBlock,
 563         statement: &Statement<'tcx>,
 564         location: Location,
 565     ) {
 566         trace!("visit_statement: {:?}", statement);
 567         if let StatementKind::Assign(ref place, ref rval) = statement.kind {
 568             let place_ty: ty::Ty<'tcx> = place
 569                 .ty(&self.mir.local_decls, self.tcx)
 570                 .to_ty(self.tcx);
 571             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 572                 if let Some(value) = self.const_prop(rval, place_layout, statement.source_info) {
 573                     if let Place::Local(local) = *place {
 574                         trace!("checking whether {:?} can be stored to {:?}", value, local);
 575                         if self.can_const_prop[local] {
 576                             trace!("storing {:?} to {:?}", value, local);
 577                             assert!(self.places[local].is_none());
 578                             self.places[local] = Some(value);
 579                         }
 580                     }
 581                 }
 582             }
 583         }
 584         self.super_statement(block, statement, location);
 585     }
 586
 587     fn visit_terminator_kind(
 588         &mut self,
 589         block: BasicBlock,
 590         kind: &TerminatorKind<'tcx>,
 591         location: Location,
 592     ) {
 593         self.super_terminator_kind(block, kind, location);
 594         let source_info = *self.mir.source_info(location);
 595         if let TerminatorKind::Assert { expected, msg, cond, .. } = kind {
 596             if let Some(value) = self.eval_operand(cond, source_info) {
 597                 trace!("assertion on {:?} should be {:?}", value, expected);
 598                 let expected = Value::Scalar(Scalar::from_bool(*expected).into());
 599                 if expected != value.0.to_immediate() {
 600                     // poison all places this operand references so that further code
 601                     // doesn't use the invalid value
 602                     match cond {
 603                         Operand::Move(ref place) | Operand::Copy(ref place) => {
 604                             let mut place = place;
 605                             while let Place::Projection(ref proj) = *place {
 606                                 place = &proj.base;
 607                             }
 608                             if let Place::Local(local) = *place {
 609                                 self.places[local] = None;
 610                             }
 611                         },
 612                         Operand::Constant(_) => {}
 613                     }
 614                     let span = self.mir[block]
 615                         .terminator
 616                         .as_ref()
 617                         .unwrap()
 618                         .source_info
 619                         .span;
 620                     let node_id = self
 621                         .tcx
 622                         .hir
 623                         .as_local_node_id(self.source.def_id)
 624                         .expect("some part of a failing const eval must be local");
 625                     use rustc::mir::interpret::EvalErrorKind::*;
 626                     let msg = match msg {
 627                         Overflow(_) |
 628                         OverflowNeg |
 629                         DivisionByZero |
 630                         RemainderByZero => msg.description().to_owned(),
 631                         BoundsCheck { ref len, ref index } => {
 632                             let len = self
 633                                 .eval_operand(len, source_info)
 634                                 .expect("len must be const");
 635                             let len = match len.0.to_immediate() {
 636                                 Value::Scalar(ScalarMaybeUndef::Scalar(Scalar::Bits {
 637                                     bits, ..
 638                                 })) => bits,
 639                                 _ => bug!("const len not primitive: {:?}", len),
 640                             };
 641                             let index = self
 642                                 .eval_operand(index, source_info)
 643                                 .expect("index must be const");
 644                             let index = match index.0.to_immediate() {
 645                                 Value::Scalar(ScalarMaybeUndef::Scalar(Scalar::Bits {
 646                                     bits, ..
 647                                 })) => bits,
 648                                 _ => bug!("const index not primitive: {:?}", index),
 649                             };
 650                             format!(
 651                                 "index out of bounds: \
 652                                 the len is {} but the index is {}",
 653                                 len,
 654                                 index,
 655                             )
 656                         },
 657                         // Need proper const propagator for these
 658                         _ => return,
 659                     };
 660                     self.tcx.lint_node(
 661                         ::rustc::lint::builtin::CONST_ERR,
 662                         node_id,
 663                         span,
 664                         &msg,
 665                     );
 666                 }
 667             }
 668         }
 669     }
 670 }