src/librustc_mir/transform/const_prop.rs

   1 //! Propagates constants for early reporting of statically known
   2 //! assertion failures
   3
   4 use rustc::hir::def::DefKind;
   5 use rustc::mir::{
   6     AggregateKind, Constant, Location, Place, PlaceBase, Mir, Operand, Rvalue, Local,
   7     NullOp, UnOp, StatementKind, Statement, LocalKind, Static, StaticKind,
   8     TerminatorKind, Terminator,  ClearCrossCrate, SourceInfo, BinOp, ProjectionElem,
   9     SourceScope, SourceScopeLocalData, LocalDecl, Promoted,
  10 };
  11 use rustc::mir::visit::{
  12     Visitor, PlaceContext, MutatingUseContext, MutVisitor, NonMutatingUseContext,
  13 };
  14 use rustc::mir::interpret::{InterpError, Scalar, GlobalId, EvalResult};
  15 use rustc::ty::{self, Instance, ParamEnv, Ty, TyCtxt};
  16 use syntax_pos::{Span, DUMMY_SP};
  17 use rustc::ty::subst::InternalSubsts;
  18 use rustc_data_structures::indexed_vec::IndexVec;
  19 use rustc::ty::layout::{
  20     LayoutOf, TyLayout, LayoutError,
  21     HasTyCtxt, TargetDataLayout, HasDataLayout,
  22 };
  23
  24 use crate::interpret::{self, InterpretCx, ScalarMaybeUndef, Immediate, OpTy, ImmTy, MemoryKind};
  25 use crate::const_eval::{
  26     CompileTimeInterpreter, error_to_const_error, eval_promoted, mk_eval_cx,
  27 };
  28 use crate::transform::{MirPass, MirSource};
  29
  30 pub struct ConstProp;
  31
  32 impl MirPass for ConstProp {
  33     fn run_pass<'a, 'tcx>(&self,
  34                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
  35                           source: MirSource<'tcx>,
  36                           mir: &mut Mir<'tcx>) {
  37         // will be evaluated by miri and produce its errors there
  38         if source.promoted.is_some() {
  39             return;
  40         }
  41
  42         use rustc::hir::map::blocks::FnLikeNode;
  43         let hir_id = tcx.hir().as_local_hir_id(source.def_id())
  44                               .expect("Non-local call to local provider is_const_fn");
  45
  46         let is_fn_like = FnLikeNode::from_node(tcx.hir().get_by_hir_id(hir_id)).is_some();
  47         let is_assoc_const = match tcx.def_kind(source.def_id()) {
  48             Some(DefKind::AssociatedConst) => true,
  49             _ => false,
  50         };
  51
  52         // Only run const prop on functions, methods, closures and associated constants
  53         if !is_fn_like && !is_assoc_const  {
  54             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  55             trace!("ConstProp skipped for {:?}", source.def_id());
  56             return
  57         }
  58
  59         trace!("ConstProp starting for {:?}", source.def_id());
  60
  61         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
  62         // constants, instead of just checking for const-folding succeeding.
  63         // That would require an uniform one-def no-mutation analysis
  64         // and RPO (or recursing when needing the value of a local).
  65         let mut optimization_finder = ConstPropagator::new(mir, tcx, source);
  66         optimization_finder.visit_mir(mir);
  67
  68         // put back the data we stole from `mir`
  69         std::mem::replace(
  70             &mut mir.source_scope_local_data,
  71             optimization_finder.source_scope_local_data
  72         );
  73         std::mem::replace(
  74             &mut mir.promoted,
  75             optimization_finder.promoted
  76         );
  77
  78         trace!("ConstProp done for {:?}", source.def_id());
  79     }
  80 }
  81
  82 type Const<'tcx> = OpTy<'tcx>;
  83
  84 /// Finds optimization opportunities on the MIR.
  85 struct ConstPropagator<'a, 'mir, 'tcx:'a+'mir> {
  86     ecx: InterpretCx<'a, 'mir, 'tcx, CompileTimeInterpreter<'a, 'mir, 'tcx>>,
  87     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  88     source: MirSource<'tcx>,
  89     places: IndexVec<Local, Option<Const<'tcx>>>,
  90     can_const_prop: IndexVec<Local, bool>,
  91     param_env: ParamEnv<'tcx>,
  92     source_scope_local_data: ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
  93     local_decls: IndexVec<Local, LocalDecl<'tcx>>,
  94     promoted: IndexVec<Promoted, Mir<'tcx>>,
  95 }
  96
  97 impl<'a, 'b, 'tcx> LayoutOf for ConstPropagator<'a, 'b, 'tcx> {
  98     type Ty = Ty<'tcx>;
  99     type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
 100
 101     fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyLayout {
 102         self.tcx.layout_of(self.param_env.and(ty))
 103     }
 104 }
 105
 106 impl<'a, 'b, 'tcx> HasDataLayout for ConstPropagator<'a, 'b, 'tcx> {
 107     #[inline]
 108     fn data_layout(&self) -> &TargetDataLayout {
 109         &self.tcx.data_layout
 110     }
 111 }
 112
 113 impl<'a, 'b, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'a, 'b, 'tcx> {
 114     #[inline]
 115     fn tcx<'c>(&'c self) -> TyCtxt<'c, 'tcx, 'tcx> {
 116         self.tcx
 117     }
 118 }
 119
 120 impl<'a, 'mir, 'tcx> ConstPropagator<'a, 'mir, 'tcx> {
 121     fn new(
 122         mir: &mut Mir<'tcx>,
 123         tcx: TyCtxt<'a, 'tcx, 'tcx>,
 124         source: MirSource<'tcx>,
 125     ) -> ConstPropagator<'a, 'mir, 'tcx> {
 126         let param_env = tcx.param_env(source.def_id());
 127         let ecx = mk_eval_cx(tcx, tcx.def_span(source.def_id()), param_env);
 128         let can_const_prop = CanConstProp::check(mir);
 129         let source_scope_local_data = std::mem::replace(
 130             &mut mir.source_scope_local_data,
 131             ClearCrossCrate::Clear
 132         );
 133         let promoted = std::mem::replace(
 134             &mut mir.promoted,
 135             IndexVec::new()
 136         );
 137
 138         ConstPropagator {
 139             ecx,
 140             tcx,
 141             source,
 142             param_env,
 143             can_const_prop,
 144             places: IndexVec::from_elem(None, &mir.local_decls),
 145             source_scope_local_data,
 146             //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_mir()` needs it
 147             local_decls: mir.local_decls.clone(),
 148             promoted,
 149         }
 150     }
 151
 152     fn use_ecx<F, T>(
 153         &mut self,
 154         source_info: SourceInfo,
 155         f: F
 156     ) -> Option<T>
 157     where
 158         F: FnOnce(&mut Self) -> EvalResult<'tcx, T>,
 159     {
 160         self.ecx.tcx.span = source_info.span;
 161         let lint_root = match self.source_scope_local_data {
 162             ClearCrossCrate::Set(ref ivs) => {
 163                 //FIXME(#51314): remove this check
 164                 if source_info.scope.index() >= ivs.len() {
 165                     return None;
 166                 }
 167                 ivs[source_info.scope].lint_root
 168             },
 169             ClearCrossCrate::Clear => return None,
 170         };
 171         let r = match f(self) {
 172             Ok(val) => Some(val),
 173             Err(error) => {
 174                 let diagnostic = error_to_const_error(&self.ecx, error);
 175                 use rustc::mir::interpret::InterpError::*;
 176                 match diagnostic.error {
 177                     // don't report these, they make no sense in a const prop context
 178                     | MachineError(_)
 179                     | Exit(_)
 180                     // at runtime these transformations might make sense
 181                     // FIXME: figure out the rules and start linting
 182                     | FunctionAbiMismatch(..)
 183                     | FunctionArgMismatch(..)
 184                     | FunctionRetMismatch(..)
 185                     | FunctionArgCountMismatch
 186                     // fine at runtime, might be a register address or sth
 187                     | ReadBytesAsPointer
 188                     // fine at runtime
 189                     | ReadForeignStatic
 190                     | Unimplemented(_)
 191                     // don't report const evaluator limits
 192                     | StackFrameLimitReached
 193                     | NoMirFor(..)
 194                     | InlineAsm
 195                     => {},
 196
 197                     | InvalidMemoryAccess
 198                     | DanglingPointerDeref
 199                     | DoubleFree
 200                     | InvalidFunctionPointer
 201                     | InvalidBool
 202                     | InvalidDiscriminant(..)
 203                     | PointerOutOfBounds { .. }
 204                     | InvalidNullPointerUsage
 205                     | ValidationFailure(..)
 206                     | InvalidPointerMath
 207                     | ReadUndefBytes(_)
 208                     | DeadLocal
 209                     | InvalidBoolOp(_)
 210                     | DerefFunctionPointer
 211                     | ExecuteMemory
 212                     | Intrinsic(..)
 213                     | InvalidChar(..)
 214                     | AbiViolation(_)
 215                     | AlignmentCheckFailed{..}
 216                     | CalledClosureAsFunction
 217                     | VtableForArgumentlessMethod
 218                     | ModifiedConstantMemory
 219                     | ModifiedStatic
 220                     | AssumptionNotHeld
 221                     // FIXME: should probably be removed and turned into a bug! call
 222                     | TypeNotPrimitive(_)
 223                     | ReallocatedWrongMemoryKind(_, _)
 224                     | DeallocatedWrongMemoryKind(_, _)
 225                     | ReallocateNonBasePtr
 226                     | DeallocateNonBasePtr
 227                     | IncorrectAllocationInformation(..)
 228                     | UnterminatedCString(_)
 229                     | HeapAllocZeroBytes
 230                     | HeapAllocNonPowerOfTwoAlignment(_)
 231                     | Unreachable
 232                     | ReadFromReturnPointer
 233                     | GeneratorResumedAfterReturn
 234                     | GeneratorResumedAfterPanic
 235                     | ReferencedConstant
 236                     | InfiniteLoop
 237                     => {
 238                         // FIXME: report UB here
 239                     },
 240
 241                     | OutOfTls
 242                     | TlsOutOfBounds
 243                     | PathNotFound(_)
 244                     => bug!("these should not be in rustc, but in miri's machine errors"),
 245
 246                     | Layout(_)
 247                     | UnimplementedTraitSelection
 248                     | TypeckError
 249                     | TooGeneric
 250                     // these are just noise
 251                     => {},
 252
 253                     // non deterministic
 254                     | ReadPointerAsBytes
 255                     // FIXME: implement
 256                     => {},
 257
 258                     | Panic { .. }
 259                     | BoundsCheck{..}
 260                     | Overflow(_)
 261                     | OverflowNeg
 262                     | DivisionByZero
 263                     | RemainderByZero
 264                     => {
 265                         diagnostic.report_as_lint(
 266                             self.ecx.tcx,
 267                             "this expression will panic at runtime",
 268                             lint_root,
 269                             None,
 270                         );
 271                     }
 272                 }
 273                 None
 274             },
 275         };
 276         self.ecx.tcx.span = DUMMY_SP;
 277         r
 278     }
 279
 280     fn eval_constant(
 281         &mut self,
 282         c: &Constant<'tcx>,
 283     ) -> Option<Const<'tcx>> {
 284         self.ecx.tcx.span = c.span;
 285         match self.ecx.eval_const_to_op(*c.literal, None) {
 286             Ok(op) => {
 287                 Some(op)
 288             },
 289             Err(error) => {
 290                 let err = error_to_const_error(&self.ecx, error);
 291                 err.report_as_error(self.ecx.tcx, "erroneous constant used");
 292                 None
 293             },
 294         }
 295     }
 296
 297     fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 298         match *place {
 299             Place::Base(PlaceBase::Local(loc)) => self.places[loc].clone(),
 300             Place::Projection(ref proj) => match proj.elem {
 301                 ProjectionElem::Field(field, _) => {
 302                     trace!("field proj on {:?}", proj.base);
 303                     let base = self.eval_place(&proj.base, source_info)?;
 304                     let res = self.use_ecx(source_info, |this| {
 305                         this.ecx.operand_field(base, field.index() as u64)
 306                     })?;
 307                     Some(res)
 308                 },
 309                 // We could get more projections by using e.g., `operand_projection`,
 310                 // but we do not even have the stack frame set up properly so
 311                 // an `Index` projection would throw us off-track.
 312                 _ => None,
 313             },
 314             Place::Base(
 315                 PlaceBase::Static(box Static {kind: StaticKind::Promoted(promoted), ..})
 316             ) => {
 317                 let generics = self.tcx.generics_of(self.source.def_id());
 318                 if generics.requires_monomorphization(self.tcx) {
 319                     // FIXME: can't handle code with generics
 320                     return None;
 321                 }
 322                 let substs = InternalSubsts::identity_for_item(self.tcx, self.source.def_id());
 323                 let instance = Instance::new(self.source.def_id(), substs);
 324                 let cid = GlobalId {
 325                     instance,
 326                     promoted: Some(promoted),
 327                 };
 328                 // cannot use `const_eval` here, because that would require having the MIR
 329                 // for the current function available, but we're producing said MIR right now
 330                 let res = self.use_ecx(source_info, |this| {
 331                     let mir = &this.promoted[promoted];
 332                     eval_promoted(this.tcx, cid, mir, this.param_env)
 333                 })?;
 334                 trace!("evaluated promoted {:?} to {:?}", promoted, res);
 335                 Some(res.into())
 336             },
 337             _ => None,
 338         }
 339     }
 340
 341     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 342         match *op {
 343             Operand::Constant(ref c) => self.eval_constant(c),
 344             | Operand::Move(ref place)
 345             | Operand::Copy(ref place) => self.eval_place(place, source_info),
 346         }
 347     }
 348
 349     fn const_prop(
 350         &mut self,
 351         rvalue: &Rvalue<'tcx>,
 352         place_layout: TyLayout<'tcx>,
 353         source_info: SourceInfo,
 354     ) -> Option<Const<'tcx>> {
 355         let span = source_info.span;
 356         match *rvalue {
 357             Rvalue::Use(ref op) => {
 358                 self.eval_operand(op, source_info)
 359             },
 360             Rvalue::Repeat(..) |
 361             Rvalue::Ref(..) |
 362             Rvalue::Aggregate(..) |
 363             Rvalue::NullaryOp(NullOp::Box, _) |
 364             Rvalue::Discriminant(..) => None,
 365
 366             Rvalue::Cast(kind, ref operand, _) => {
 367                 let op = self.eval_operand(operand, source_info)?;
 368                 self.use_ecx(source_info, |this| {
 369                     let dest = this.ecx.allocate(place_layout, MemoryKind::Stack);
 370                     this.ecx.cast(op, kind, dest.into())?;
 371                     Ok(dest.into())
 372                 })
 373             }
 374
 375             // FIXME(oli-obk): evaluate static/constant slice lengths
 376             Rvalue::Len(_) => None,
 377             Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
 378                 type_size_of(self.tcx, self.param_env, ty).and_then(|n| Some(
 379                     ImmTy {
 380                         imm: Immediate::Scalar(
 381                             Scalar::Bits {
 382                                 bits: n as u128,
 383                                 size: self.tcx.data_layout.pointer_size.bytes() as u8,
 384                             }.into()
 385                         ),
 386                         layout: self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
 387                     }.into()
 388                 ))
 389             }
 390             Rvalue::UnaryOp(op, ref arg) => {
 391                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 392                     self.tcx.closure_base_def_id(self.source.def_id())
 393                 } else {
 394                     self.source.def_id()
 395                 };
 396                 let generics = self.tcx.generics_of(def_id);
 397                 if generics.requires_monomorphization(self.tcx) {
 398                     // FIXME: can't handle code with generics
 399                     return None;
 400                 }
 401
 402                 let arg = self.eval_operand(arg, source_info)?;
 403                 let val = self.use_ecx(source_info, |this| {
 404                     let prim = this.ecx.read_immediate(arg)?;
 405                     match op {
 406                         UnOp::Neg => {
 407                             // Need to do overflow check here: For actual CTFE, MIR
 408                             // generation emits code that does this before calling the op.
 409                             if prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
 410                                 return err!(OverflowNeg);
 411                             }
 412                         }
 413                         UnOp::Not => {
 414                             // Cannot overflow
 415                         }
 416                     }
 417                     // Now run the actual operation.
 418                     this.ecx.unary_op(op, prim)
 419                 })?;
 420                 let res = ImmTy {
 421                     imm: Immediate::Scalar(val.into()),
 422                     layout: place_layout,
 423                 };
 424                 Some(res.into())
 425             }
 426             Rvalue::CheckedBinaryOp(op, ref left, ref right) |
 427             Rvalue::BinaryOp(op, ref left, ref right) => {
 428                 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
 429                 let right = self.eval_operand(right, source_info)?;
 430                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 431                     self.tcx.closure_base_def_id(self.source.def_id())
 432                 } else {
 433                     self.source.def_id()
 434                 };
 435                 let generics = self.tcx.generics_of(def_id);
 436                 if generics.requires_monomorphization(self.tcx) {
 437                     // FIXME: can't handle code with generics
 438                     return None;
 439                 }
 440
 441                 let r = self.use_ecx(source_info, |this| {
 442                     this.ecx.read_immediate(right)
 443                 })?;
 444                 if op == BinOp::Shr || op == BinOp::Shl {
 445                     let left_ty = left.ty(&self.local_decls, self.tcx);
 446                     let left_bits = self
 447                         .tcx
 448                         .layout_of(self.param_env.and(left_ty))
 449                         .unwrap()
 450                         .size
 451                         .bits();
 452                     let right_size = right.layout.size;
 453                     let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
 454                     if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
 455                         let source_scope_local_data = match self.source_scope_local_data {
 456                             ClearCrossCrate::Set(ref data) => data,
 457                             ClearCrossCrate::Clear => return None,
 458                         };
 459                         let dir = if op == BinOp::Shr {
 460                             "right"
 461                         } else {
 462                             "left"
 463                         };
 464                         let hir_id = source_scope_local_data[source_info.scope].lint_root;
 465                         self.tcx.lint_hir(
 466                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 467                             hir_id,
 468                             span,
 469                             &format!("attempt to shift {} with overflow", dir));
 470                         return None;
 471                     }
 472                 }
 473                 let left = self.eval_operand(left, source_info)?;
 474                 let l = self.use_ecx(source_info, |this| {
 475                     this.ecx.read_immediate(left)
 476                 })?;
 477                 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
 478                 let (val, overflow) = self.use_ecx(source_info, |this| {
 479                     this.ecx.binary_op(op, l, r)
 480                 })?;
 481                 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
 482                     Immediate::ScalarPair(
 483                         val.into(),
 484                         Scalar::from_bool(overflow).into(),
 485                     )
 486                 } else {
 487                     if overflow {
 488                         let err = InterpError::Overflow(op).into();
 489                         let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
 490                         return None;
 491                     }
 492                     Immediate::Scalar(val.into())
 493                 };
 494                 let res = ImmTy {
 495                     imm: val,
 496                     layout: place_layout,
 497                 };
 498                 Some(res.into())
 499             },
 500         }
 501     }
 502
 503     fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
 504         Operand::Constant(Box::new(
 505             Constant {
 506                 span,
 507                 ty,
 508                 user_ty: None,
 509                 literal: self.tcx.mk_const(ty::Const::from_scalar(
 510                     scalar,
 511                     ty,
 512                 ))
 513             }
 514         ))
 515     }
 516
 517     fn replace_with_const(&self, rval: &mut Rvalue<'tcx>, value: Const<'tcx>, span: Span) {
 518         self.ecx.validate_operand(
 519             value,
 520             vec![],
 521             None,
 522             true,
 523         ).expect("value should already be a valid const");
 524
 525         if let interpret::Operand::Immediate(im) = *value {
 526             match im {
 527                 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
 528                     *rval = Rvalue::Use(self.operand_from_scalar(scalar, value.layout.ty, span));
 529                 },
 530                 Immediate::ScalarPair(
 531                     ScalarMaybeUndef::Scalar(one),
 532                     ScalarMaybeUndef::Scalar(two)
 533                 ) => {
 534                     let ty = &value.layout.ty.sty;
 535                     if let ty::Tuple(substs) = ty {
 536                         *rval = Rvalue::Aggregate(
 537                             Box::new(AggregateKind::Tuple),
 538                             vec![
 539                                 self.operand_from_scalar(one, substs[0].expect_ty(), span),
 540                                 self.operand_from_scalar(two, substs[1].expect_ty(), span),
 541                             ],
 542                         );
 543                     }
 544                 },
 545                 _ => { }
 546             }
 547         }
 548     }
 549
 550     fn should_const_prop(&self) -> bool {
 551         self.tcx.sess.opts.debugging_opts.mir_opt_level >= 2
 552     }
 553 }
 554
 555 fn type_size_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 556                           param_env: ty::ParamEnv<'tcx>,
 557                           ty: Ty<'tcx>) -> Option<u64> {
 558     tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
 559 }
 560
 561 struct CanConstProp {
 562     can_const_prop: IndexVec<Local, bool>,
 563     // false at the beginning, once set, there are not allowed to be any more assignments
 564     found_assignment: IndexVec<Local, bool>,
 565 }
 566
 567 impl CanConstProp {
 568     /// returns true if `local` can be propagated
 569     fn check(mir: &Mir<'_>) -> IndexVec<Local, bool> {
 570         let mut cpv = CanConstProp {
 571             can_const_prop: IndexVec::from_elem(true, &mir.local_decls),
 572             found_assignment: IndexVec::from_elem(false, &mir.local_decls),
 573         };
 574         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 575             // cannot use args at all
 576             // cannot use locals because if x < y { y - x } else { x - y } would
 577             //        lint for x != y
 578             // FIXME(oli-obk): lint variables until they are used in a condition
 579             // FIXME(oli-obk): lint if return value is constant
 580             *val = mir.local_kind(local) == LocalKind::Temp;
 581         }
 582         cpv.visit_mir(mir);
 583         cpv.can_const_prop
 584     }
 585 }
 586
 587 impl<'tcx> Visitor<'tcx> for CanConstProp {
 588     fn visit_local(
 589         &mut self,
 590         &local: &Local,
 591         context: PlaceContext,
 592         _: Location,
 593     ) {
 594         use rustc::mir::visit::PlaceContext::*;
 595         match context {
 596             // Constants must have at most one write
 597             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 598             // only occur in independent execution paths
 599             MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
 600                 self.can_const_prop[local] = false;
 601             } else {
 602                 self.found_assignment[local] = true
 603             },
 604             // Reading constants is allowed an arbitrary number of times
 605             NonMutatingUse(NonMutatingUseContext::Copy) |
 606             NonMutatingUse(NonMutatingUseContext::Move) |
 607             NonMutatingUse(NonMutatingUseContext::Inspect) |
 608             NonMutatingUse(NonMutatingUseContext::Projection) |
 609             MutatingUse(MutatingUseContext::Projection) |
 610             NonUse(_) => {},
 611             _ => self.can_const_prop[local] = false,
 612         }
 613     }
 614 }
 615
 616 impl<'b, 'a, 'tcx> MutVisitor<'tcx> for ConstPropagator<'b, 'a, 'tcx> {
 617     fn visit_constant(
 618         &mut self,
 619         constant: &mut Constant<'tcx>,
 620         location: Location,
 621     ) {
 622         trace!("visit_constant: {:?}", constant);
 623         self.super_constant(constant, location);
 624         self.eval_constant(constant);
 625     }
 626
 627     fn visit_statement(
 628         &mut self,
 629         statement: &mut Statement<'tcx>,
 630         location: Location,
 631     ) {
 632         trace!("visit_statement: {:?}", statement);
 633         if let StatementKind::Assign(ref place, ref mut rval) = statement.kind {
 634             let place_ty: Ty<'tcx> = place
 635                 .ty(&self.local_decls, self.tcx)
 636                 .ty;
 637             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 638                 if let Some(value) = self.const_prop(rval, place_layout, statement.source_info) {
 639                     if let Place::Base(PlaceBase::Local(local)) = *place {
 640                         trace!("checking whether {:?} can be stored to {:?}", value, local);
 641                         if self.can_const_prop[local] {
 642                             trace!("storing {:?} to {:?}", value, local);
 643                             assert!(self.places[local].is_none());
 644                             self.places[local] = Some(value);
 645
 646                             if self.should_const_prop() {
 647                                 self.replace_with_const(rval, value, statement.source_info.span);
 648                             }
 649                         }
 650                     }
 651                 }
 652             }
 653         }
 654         self.super_statement(statement, location);
 655     }
 656
 657     fn visit_terminator(
 658         &mut self,
 659         terminator: &mut Terminator<'tcx>,
 660         location: Location,
 661     ) {
 662         self.super_terminator(terminator, location);
 663         let source_info = terminator.source_info;
 664         match &mut terminator.kind {
 665             TerminatorKind::Assert { expected, msg, ref mut cond, .. } => {
 666                 if let Some(value) = self.eval_operand(&cond, source_info) {
 667                     trace!("assertion on {:?} should be {:?}", value, expected);
 668                     let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
 669                     let value_const = self.ecx.read_scalar(value).unwrap();
 670                     if expected != value_const {
 671                         // poison all places this operand references so that further code
 672                         // doesn't use the invalid value
 673                         match cond {
 674                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 675                                 let mut place = place;
 676                                 while let Place::Projection(ref proj) = *place {
 677                                     place = &proj.base;
 678                                 }
 679                                 if let Place::Base(PlaceBase::Local(local)) = *place {
 680                                     self.places[local] = None;
 681                                 }
 682                             },
 683                             Operand::Constant(_) => {}
 684                         }
 685                         let span = terminator.source_info.span;
 686                         let hir_id = self
 687                             .tcx
 688                             .hir()
 689                             .as_local_hir_id(self.source.def_id())
 690                             .expect("some part of a failing const eval must be local");
 691                         use rustc::mir::interpret::InterpError::*;
 692                         let msg = match msg {
 693                             Overflow(_) |
 694                             OverflowNeg |
 695                             DivisionByZero |
 696                             RemainderByZero => msg.description().to_owned(),
 697                             BoundsCheck { ref len, ref index } => {
 698                                 let len = self
 699                                     .eval_operand(len, source_info)
 700                                     .expect("len must be const");
 701                                 let len = match self.ecx.read_scalar(len) {
 702                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Bits {
 703                                         bits, ..
 704                                     })) => bits,
 705                                     other => bug!("const len not primitive: {:?}", other),
 706                                 };
 707                                 let index = self
 708                                     .eval_operand(index, source_info)
 709                                     .expect("index must be const");
 710                                 let index = match self.ecx.read_scalar(index) {
 711                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Bits {
 712                                         bits, ..
 713                                     })) => bits,
 714                                     other => bug!("const index not primitive: {:?}", other),
 715                                 };
 716                                 format!(
 717                                     "index out of bounds: \
 718                                     the len is {} but the index is {}",
 719                                     len,
 720                                     index,
 721                                 )
 722                             },
 723                             // Need proper const propagator for these
 724                             _ => return,
 725                         };
 726                         self.tcx.lint_hir(
 727                             ::rustc::lint::builtin::CONST_ERR,
 728                             hir_id,
 729                             span,
 730                             &msg,
 731                         );
 732                     } else {
 733                         if self.should_const_prop() {
 734                             if let ScalarMaybeUndef::Scalar(scalar) = value_const {
 735                                 *cond = self.operand_from_scalar(
 736                                     scalar,
 737                                     self.tcx.types.bool,
 738                                     source_info.span,
 739                                 );
 740                             }
 741                         }
 742                     }
 743                 }
 744             },
 745             TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
 746                 if self.should_const_prop() {
 747                     if let Some(value) = self.eval_operand(&discr, source_info) {
 748                         if let ScalarMaybeUndef::Scalar(scalar) =
 749                                 self.ecx.read_scalar(value).unwrap() {
 750                             *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
 751                         }
 752                     }
 753                 }
 754             },
 755             //none of these have Operands to const-propagate
 756             TerminatorKind::Goto { .. } |
 757             TerminatorKind::Resume |
 758             TerminatorKind::Abort |
 759             TerminatorKind::Return |
 760             TerminatorKind::Unreachable |
 761             TerminatorKind::Drop { .. } |
 762             TerminatorKind::DropAndReplace { .. } |
 763             TerminatorKind::Yield { .. } |
 764             TerminatorKind::GeneratorDrop |
 765             TerminatorKind::FalseEdges { .. } |
 766             TerminatorKind::FalseUnwind { .. } => { }
 767             //FIXME(wesleywiser) Call does have Operands that could be const-propagated
 768             TerminatorKind::Call { .. } => { }
 769         }
 770     }
 771 }