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