src/librustc_mir/transform/const_prop.rs

   1 //! Propagates constants for early reporting of statically known
   2 //! assertion failures
   3
   4 use std::cell::Cell;
   5
   6 use rustc::hir::def::DefKind;
   7 use rustc::mir::{
   8     AggregateKind, Constant, Location, Place, PlaceBase, Body, Operand, Rvalue,
   9     Local, NullOp, UnOp, StatementKind, Statement, LocalKind, Static, StaticKind,
  10     TerminatorKind, Terminator,  ClearCrossCrate, SourceInfo, BinOp, ProjectionElem,
  11     SourceScope, SourceScopeLocalData, LocalDecl, Promoted,
  12 };
  13 use rustc::mir::visit::{
  14     Visitor, PlaceContext, MutatingUseContext, MutVisitor, NonMutatingUseContext,
  15 };
  16 use rustc::mir::interpret::{InterpError, Scalar, GlobalId, InterpResult};
  17 use rustc::ty::{self, Instance, ParamEnv, Ty, TyCtxt};
  18 use syntax_pos::{Span, DUMMY_SP};
  19 use rustc::ty::subst::InternalSubsts;
  20 use rustc_data_structures::indexed_vec::IndexVec;
  21 use rustc::ty::layout::{
  22     LayoutOf, TyLayout, LayoutError, HasTyCtxt, TargetDataLayout, HasDataLayout, Size,
  23 };
  24
  25 use crate::interpret::{
  26     self, InterpretCx, ScalarMaybeUndef, Immediate, OpTy,
  27     ImmTy, MemoryKind, StackPopCleanup, LocalValue, LocalState,
  28 };
  29 use crate::const_eval::{
  30     CompileTimeInterpreter, error_to_const_error, eval_promoted, mk_eval_cx,
  31 };
  32 use crate::transform::{MirPass, MirSource};
  33
  34 pub struct ConstProp;
  35
  36 impl MirPass for ConstProp {
  37     fn run_pass<'tcx>(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) {
  38         // will be evaluated by miri and produce its errors there
  39         if source.promoted.is_some() {
  40             return;
  41         }
  42
  43         use rustc::hir::map::blocks::FnLikeNode;
  44         let hir_id = tcx.hir().as_local_hir_id(source.def_id())
  45                               .expect("Non-local call to local provider is_const_fn");
  46
  47         let is_fn_like = FnLikeNode::from_node(tcx.hir().get_by_hir_id(hir_id)).is_some();
  48         let is_assoc_const = match tcx.def_kind(source.def_id()) {
  49             Some(DefKind::AssocConst) => true,
  50             _ => false,
  51         };
  52
  53         // Only run const prop on functions, methods, closures and associated constants
  54         if !is_fn_like && !is_assoc_const  {
  55             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  56             trace!("ConstProp skipped for {:?}", source.def_id());
  57             return
  58         }
  59
  60         trace!("ConstProp starting for {:?}", source.def_id());
  61
  62         // steal some data we need from `body`
  63         let source_scope_local_data = std::mem::replace(
  64             &mut body.source_scope_local_data,
  65             ClearCrossCrate::Clear
  66         );
  67         let promoted = std::mem::replace(
  68             &mut body.promoted,
  69             IndexVec::new()
  70         );
  71
  72         let dummy_body =
  73             &Body::new(
  74                 body.basic_blocks().clone(),
  75                 Default::default(),
  76                 ClearCrossCrate::Clear,
  77                 Default::default(),
  78                 None,
  79                 body.local_decls.clone(),
  80                 Default::default(),
  81                 body.arg_count,
  82                 Default::default(),
  83                 tcx.def_span(source.def_id()),
  84                 Default::default(),
  85             );
  86
  87         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
  88         // constants, instead of just checking for const-folding succeeding.
  89         // That would require an uniform one-def no-mutation analysis
  90         // and RPO (or recursing when needing the value of a local).
  91         let mut optimization_finder = ConstPropagator::new(
  92             body,
  93             dummy_body,
  94             source_scope_local_data,
  95             promoted,
  96             tcx,
  97             source
  98         );
  99         optimization_finder.visit_body(body);
 100
 101         // put back the data we stole from `mir`
 102         let (source_scope_local_data, promoted) = optimization_finder.release_stolen_data();
 103         std::mem::replace(
 104             &mut body.source_scope_local_data,
 105             source_scope_local_data
 106         );
 107         std::mem::replace(
 108             &mut body.promoted,
 109             promoted
 110         );
 111
 112         trace!("ConstProp done for {:?}", source.def_id());
 113     }
 114 }
 115
 116 type Const<'tcx> = OpTy<'tcx>;
 117
 118 /// Finds optimization opportunities on the MIR.
 119 struct ConstPropagator<'mir, 'tcx> {
 120     ecx: InterpretCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>>,
 121     tcx: TyCtxt<'tcx>,
 122     source: MirSource<'tcx>,
 123     can_const_prop: IndexVec<Local, bool>,
 124     param_env: ParamEnv<'tcx>,
 125     source_scope_local_data: ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
 126     local_decls: IndexVec<Local, LocalDecl<'tcx>>,
 127     promoted: IndexVec<Promoted, Body<'tcx>>,
 128 }
 129
 130 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
 131     type Ty = Ty<'tcx>;
 132     type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
 133
 134     fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyLayout {
 135         self.tcx.layout_of(self.param_env.and(ty))
 136     }
 137 }
 138
 139 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
 140     #[inline]
 141     fn data_layout(&self) -> &TargetDataLayout {
 142         &self.tcx.data_layout
 143     }
 144 }
 145
 146 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
 147     #[inline]
 148     fn tcx(&self) -> TyCtxt<'tcx> {
 149         self.tcx
 150     }
 151 }
 152
 153 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 154     fn new(
 155         body: &Body<'tcx>,
 156         dummy_body: &'mir Body<'tcx>,
 157         source_scope_local_data: ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
 158         promoted: IndexVec<Promoted, Body<'tcx>>,
 159         tcx: TyCtxt<'tcx>,
 160         source: MirSource<'tcx>,
 161     ) -> ConstPropagator<'mir, 'tcx> {
 162         let def_id = source.def_id();
 163         let param_env = tcx.param_env(def_id);
 164         let span = tcx.def_span(def_id);
 165         let mut ecx = mk_eval_cx(tcx, span, param_env);
 166         let can_const_prop = CanConstProp::check(body);
 167
 168         ecx.push_stack_frame(
 169             Instance::new(def_id, &InternalSubsts::identity_for_item(tcx, def_id)),
 170             span,
 171             dummy_body,
 172             None,
 173             StackPopCleanup::None {
 174                 cleanup: false,
 175             },
 176         ).expect("failed to push initial stack frame");
 177
 178         ConstPropagator {
 179             ecx,
 180             tcx,
 181             source,
 182             param_env,
 183             can_const_prop,
 184             source_scope_local_data,
 185             //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
 186             local_decls: body.local_decls.clone(),
 187             promoted,
 188         }
 189     }
 190
 191     fn release_stolen_data(self) ->
 192         (ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
 193          IndexVec<Promoted, Body<'tcx>>)
 194     {
 195         (self.source_scope_local_data, self.promoted)
 196     }
 197
 198     fn get_const(&self, local: Local) -> Option<Const<'tcx>> {
 199         let l = &self.ecx.frame().locals[local];
 200         if l.value == LocalValue::Uninitialized || l.value == LocalValue::Dead {
 201             return None;
 202         }
 203
 204         self.ecx.access_local(self.ecx.frame(), local, None).ok()
 205     }
 206
 207     fn set_const(&mut self, local: Local, c: Option<Const<'tcx>>) {
 208         self.ecx.frame_mut().locals[local] =
 209             match c {
 210                 Some(op_ty) => LocalState {
 211                     value: LocalValue::Live(*op_ty),
 212                     layout: Cell::new(Some(op_ty.layout)),
 213                 },
 214                 None => LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) },
 215             };
 216     }
 217
 218     fn use_ecx<F, T>(
 219         &mut self,
 220         source_info: SourceInfo,
 221         f: F
 222     ) -> Option<T>
 223     where
 224         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 225     {
 226         self.ecx.tcx.span = source_info.span;
 227         let lint_root = match self.source_scope_local_data {
 228             ClearCrossCrate::Set(ref ivs) => {
 229                 //FIXME(#51314): remove this check
 230                 if source_info.scope.index() >= ivs.len() {
 231                     return None;
 232                 }
 233                 ivs[source_info.scope].lint_root
 234             },
 235             ClearCrossCrate::Clear => return None,
 236         };
 237         let r = match f(self) {
 238             Ok(val) => Some(val),
 239             Err(error) => {
 240                 let diagnostic = error_to_const_error(&self.ecx, error);
 241                 use rustc::mir::interpret::InterpError::*;
 242                 match diagnostic.error {
 243                     // don't report these, they make no sense in a const prop context
 244                     | MachineError(_)
 245                     | Exit(_)
 246                     // at runtime these transformations might make sense
 247                     // FIXME: figure out the rules and start linting
 248                     | FunctionAbiMismatch(..)
 249                     | FunctionArgMismatch(..)
 250                     | FunctionRetMismatch(..)
 251                     | FunctionArgCountMismatch
 252                     // fine at runtime, might be a register address or sth
 253                     | ReadBytesAsPointer
 254                     // fine at runtime
 255                     | ReadForeignStatic
 256                     | Unimplemented(_)
 257                     // don't report const evaluator limits
 258                     | StackFrameLimitReached
 259                     | NoMirFor(..)
 260                     | InlineAsm
 261                     => {},
 262
 263                     | InvalidMemoryAccess
 264                     | DanglingPointerDeref
 265                     | DoubleFree
 266                     | InvalidFunctionPointer
 267                     | InvalidBool
 268                     | InvalidDiscriminant(..)
 269                     | PointerOutOfBounds { .. }
 270                     | InvalidNullPointerUsage
 271                     | ValidationFailure(..)
 272                     | InvalidPointerMath
 273                     | ReadUndefBytes(_)
 274                     | DeadLocal
 275                     | InvalidBoolOp(_)
 276                     | DerefFunctionPointer
 277                     | ExecuteMemory
 278                     | Intrinsic(..)
 279                     | InvalidChar(..)
 280                     | AbiViolation(_)
 281                     | AlignmentCheckFailed{..}
 282                     | CalledClosureAsFunction
 283                     | VtableForArgumentlessMethod
 284                     | ModifiedConstantMemory
 285                     | ModifiedStatic
 286                     | AssumptionNotHeld
 287                     // FIXME: should probably be removed and turned into a bug! call
 288                     | TypeNotPrimitive(_)
 289                     | ReallocatedWrongMemoryKind(_, _)
 290                     | DeallocatedWrongMemoryKind(_, _)
 291                     | ReallocateNonBasePtr
 292                     | DeallocateNonBasePtr
 293                     | IncorrectAllocationInformation(..)
 294                     | UnterminatedCString(_)
 295                     | HeapAllocZeroBytes
 296                     | HeapAllocNonPowerOfTwoAlignment(_)
 297                     | Unreachable
 298                     | ReadFromReturnPointer
 299                     | GeneratorResumedAfterReturn
 300                     | GeneratorResumedAfterPanic
 301                     | ReferencedConstant
 302                     | InfiniteLoop
 303                     => {
 304                         // FIXME: report UB here
 305                     },
 306
 307                     | OutOfTls
 308                     | TlsOutOfBounds
 309                     | PathNotFound(_)
 310                     => bug!("these should not be in rustc, but in miri's machine errors"),
 311
 312                     | Layout(_)
 313                     | UnimplementedTraitSelection
 314                     | TypeckError
 315                     | TooGeneric
 316                     // these are just noise
 317                     => {},
 318
 319                     // non deterministic
 320                     | ReadPointerAsBytes
 321                     // FIXME: implement
 322                     => {},
 323
 324                     | Panic { .. }
 325                     | BoundsCheck{..}
 326                     | Overflow(_)
 327                     | OverflowNeg
 328                     | DivisionByZero
 329                     | RemainderByZero
 330                     => {
 331                         diagnostic.report_as_lint(
 332                             self.ecx.tcx,
 333                             "this expression will panic at runtime",
 334                             lint_root,
 335                             None,
 336                         );
 337                     }
 338                 }
 339                 None
 340             },
 341         };
 342         self.ecx.tcx.span = DUMMY_SP;
 343         r
 344     }
 345
 346     fn eval_constant(
 347         &mut self,
 348         c: &Constant<'tcx>,
 349     ) -> Option<Const<'tcx>> {
 350         self.ecx.tcx.span = c.span;
 351         match self.ecx.eval_const_to_op(c.literal, None) {
 352             Ok(op) => {
 353                 Some(op)
 354             },
 355             Err(error) => {
 356                 let err = error_to_const_error(&self.ecx, error);
 357                 err.report_as_error(self.ecx.tcx, "erroneous constant used");
 358                 None
 359             },
 360         }
 361     }
 362
 363     fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 364         trace!("eval_place(place={:?})", place);
 365         place.iterate(|place_base, place_projection| {
 366             let mut eval = match place_base {
 367                 PlaceBase::Local(loc) => self.get_const(*loc).clone()?,
 368                 PlaceBase::Static(box Static {kind: StaticKind::Promoted(promoted), ..}) => {
 369                     let generics = self.tcx.generics_of(self.source.def_id());
 370                     if generics.requires_monomorphization(self.tcx) {
 371                         // FIXME: can't handle code with generics
 372                         return None;
 373                     }
 374                     let substs = InternalSubsts::identity_for_item(self.tcx, self.source.def_id());
 375                     let instance = Instance::new(self.source.def_id(), substs);
 376                     let cid = GlobalId {
 377                         instance,
 378                         promoted: Some(*promoted),
 379                     };
 380                     // cannot use `const_eval` here, because that would require having the MIR
 381                     // for the current function available, but we're producing said MIR right now
 382                     let res = self.use_ecx(source_info, |this| {
 383                         let body = &this.promoted[*promoted];
 384                         eval_promoted(this.tcx, cid, body, this.param_env)
 385                     })?;
 386                     trace!("evaluated promoted {:?} to {:?}", promoted, res);
 387                     res.into()
 388                 }
 389                 _ => return None,
 390             };
 391
 392             for proj in place_projection {
 393                 match proj.elem {
 394                     ProjectionElem::Field(field, _) => {
 395                         trace!("field proj on {:?}", proj.base);
 396                         eval = self.use_ecx(source_info, |this| {
 397                             this.ecx.operand_field(eval, field.index() as u64)
 398                         })?;
 399                     },
 400                     ProjectionElem::Deref => {
 401                         trace!("processing deref");
 402                         eval = self.use_ecx(source_info, |this| {
 403                             this.ecx.deref_operand(eval)
 404                         })?.into();
 405                     }
 406                     // We could get more projections by using e.g., `operand_projection`,
 407                     // but we do not even have the stack frame set up properly so
 408                     // an `Index` projection would throw us off-track.
 409                     _ => return None,
 410                 }
 411             }
 412
 413             Some(eval)
 414         })
 415     }
 416
 417     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 418         match *op {
 419             Operand::Constant(ref c) => self.eval_constant(c),
 420             | Operand::Move(ref place)
 421             | Operand::Copy(ref place) => self.eval_place(place, source_info),
 422         }
 423     }
 424
 425     fn const_prop(
 426         &mut self,
 427         rvalue: &Rvalue<'tcx>,
 428         place_layout: TyLayout<'tcx>,
 429         source_info: SourceInfo,
 430     ) -> Option<Const<'tcx>> {
 431         let span = source_info.span;
 432         match *rvalue {
 433             Rvalue::Use(ref op) => {
 434                 self.eval_operand(op, source_info)
 435             },
 436             Rvalue::Ref(_, _, ref place) => {
 437                 let src = self.eval_place(place, source_info)?;
 438                 let mplace = src.try_as_mplace().ok()?;
 439                 Some(ImmTy::from_scalar(mplace.ptr.into(), place_layout).into())
 440             },
 441             Rvalue::Repeat(..) |
 442             Rvalue::Aggregate(..) |
 443             Rvalue::NullaryOp(NullOp::Box, _) |
 444             Rvalue::Discriminant(..) => None,
 445
 446             Rvalue::Cast(kind, ref operand, _) => {
 447                 let op = self.eval_operand(operand, source_info)?;
 448                 self.use_ecx(source_info, |this| {
 449                     let dest = this.ecx.allocate(place_layout, MemoryKind::Stack);
 450                     this.ecx.cast(op, kind, dest.into())?;
 451                     Ok(dest.into())
 452                 })
 453             },
 454             Rvalue::Len(ref place) => {
 455                 let place = self.eval_place(&place, source_info)?;
 456                 let mplace = place.try_as_mplace().ok()?;
 457
 458                 if let ty::Slice(_) = mplace.layout.ty.sty {
 459                     let len = mplace.meta.unwrap().to_usize(&self.ecx).unwrap();
 460
 461                     Some(ImmTy {
 462                         imm: Immediate::Scalar(
 463                             Scalar::from_uint(
 464                                 len,
 465                                 Size::from_bits(
 466                                     self.tcx.sess.target.usize_ty.bit_width().unwrap() as u64
 467                                 )
 468                             ).into(),
 469                         ),
 470                         layout: self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
 471                     }.into())
 472                 } else {
 473                     trace!("not slice: {:?}", mplace.layout.ty.sty);
 474                     None
 475                 }
 476             },
 477             Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
 478                 type_size_of(self.tcx, self.param_env, ty).and_then(|n| Some(
 479                     ImmTy {
 480                         imm: Immediate::Scalar(
 481                             Scalar::from_uint(n, self.tcx.data_layout.pointer_size).into()
 482                         ),
 483                         layout: self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
 484                     }.into()
 485                 ))
 486             }
 487             Rvalue::UnaryOp(op, ref arg) => {
 488                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 489                     self.tcx.closure_base_def_id(self.source.def_id())
 490                 } else {
 491                     self.source.def_id()
 492                 };
 493                 let generics = self.tcx.generics_of(def_id);
 494                 if generics.requires_monomorphization(self.tcx) {
 495                     // FIXME: can't handle code with generics
 496                     return None;
 497                 }
 498
 499                 let arg = self.eval_operand(arg, source_info)?;
 500                 let val = self.use_ecx(source_info, |this| {
 501                     let prim = this.ecx.read_immediate(arg)?;
 502                     match op {
 503                         UnOp::Neg => {
 504                             // Need to do overflow check here: For actual CTFE, MIR
 505                             // generation emits code that does this before calling the op.
 506                             if prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
 507                                 return err!(OverflowNeg);
 508                             }
 509                         }
 510                         UnOp::Not => {
 511                             // Cannot overflow
 512                         }
 513                     }
 514                     // Now run the actual operation.
 515                     this.ecx.unary_op(op, prim)
 516                 })?;
 517                 let res = ImmTy {
 518                     imm: Immediate::Scalar(val.into()),
 519                     layout: place_layout,
 520                 };
 521                 Some(res.into())
 522             }
 523             Rvalue::CheckedBinaryOp(op, ref left, ref right) |
 524             Rvalue::BinaryOp(op, ref left, ref right) => {
 525                 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
 526                 let right = self.eval_operand(right, source_info)?;
 527                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 528                     self.tcx.closure_base_def_id(self.source.def_id())
 529                 } else {
 530                     self.source.def_id()
 531                 };
 532                 let generics = self.tcx.generics_of(def_id);
 533                 if generics.requires_monomorphization(self.tcx) {
 534                     // FIXME: can't handle code with generics
 535                     return None;
 536                 }
 537
 538                 let r = self.use_ecx(source_info, |this| {
 539                     this.ecx.read_immediate(right)
 540                 })?;
 541                 if op == BinOp::Shr || op == BinOp::Shl {
 542                     let left_ty = left.ty(&self.local_decls, self.tcx);
 543                     let left_bits = self
 544                         .tcx
 545                         .layout_of(self.param_env.and(left_ty))
 546                         .unwrap()
 547                         .size
 548                         .bits();
 549                     let right_size = right.layout.size;
 550                     let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
 551                     if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
 552                         let source_scope_local_data = match self.source_scope_local_data {
 553                             ClearCrossCrate::Set(ref data) => data,
 554                             ClearCrossCrate::Clear => return None,
 555                         };
 556                         let dir = if op == BinOp::Shr {
 557                             "right"
 558                         } else {
 559                             "left"
 560                         };
 561                         let hir_id = source_scope_local_data[source_info.scope].lint_root;
 562                         self.tcx.lint_hir(
 563                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 564                             hir_id,
 565                             span,
 566                             &format!("attempt to shift {} with overflow", dir));
 567                         return None;
 568                     }
 569                 }
 570                 let left = self.eval_operand(left, source_info)?;
 571                 let l = self.use_ecx(source_info, |this| {
 572                     this.ecx.read_immediate(left)
 573                 })?;
 574                 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
 575                 let (val, overflow) = self.use_ecx(source_info, |this| {
 576                     this.ecx.binary_op(op, l, r)
 577                 })?;
 578                 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
 579                     Immediate::ScalarPair(
 580                         val.into(),
 581                         Scalar::from_bool(overflow).into(),
 582                     )
 583                 } else {
 584                     if overflow {
 585                         let err = InterpError::Overflow(op).into();
 586                         let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
 587                         return None;
 588                     }
 589                     Immediate::Scalar(val.into())
 590                 };
 591                 let res = ImmTy {
 592                     imm: val,
 593                     layout: place_layout,
 594                 };
 595                 Some(res.into())
 596             },
 597         }
 598     }
 599
 600     fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
 601         Operand::Constant(Box::new(
 602             Constant {
 603                 span,
 604                 ty,
 605                 user_ty: None,
 606                 literal: self.tcx.mk_const(*ty::Const::from_scalar(
 607                     self.tcx,
 608                     scalar,
 609                     ty,
 610                 ))
 611             }
 612         ))
 613     }
 614
 615     fn replace_with_const(
 616         &mut self,
 617         rval: &mut Rvalue<'tcx>,
 618         value: Const<'tcx>,
 619         source_info: SourceInfo,
 620     ) {
 621         trace!("attepting to replace {:?} with {:?}", rval, value);
 622         if let Err(e) = self.ecx.validate_operand(
 623             value,
 624             vec![],
 625             // FIXME: is ref tracking too expensive?
 626             Some(&mut interpret::RefTracking::empty()),
 627         ) {
 628             trace!("validation error, attempt failed: {:?}", e);
 629             return;
 630         }
 631
 632         // FIXME> figure out what tho do when try_read_immediate fails
 633         let imm = self.use_ecx(source_info, |this| {
 634             this.ecx.try_read_immediate(value)
 635         });
 636
 637         if let Some(Ok(imm)) = imm {
 638             match *imm {
 639                 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
 640                     *rval = Rvalue::Use(
 641                         self.operand_from_scalar(scalar, value.layout.ty, source_info.span));
 642                 },
 643                 Immediate::ScalarPair(
 644                     ScalarMaybeUndef::Scalar(one),
 645                     ScalarMaybeUndef::Scalar(two)
 646                 ) => {
 647                     let ty = &value.layout.ty.sty;
 648                     if let ty::Tuple(substs) = ty {
 649                         *rval = Rvalue::Aggregate(
 650                             Box::new(AggregateKind::Tuple),
 651                             vec![
 652                                 self.operand_from_scalar(
 653                                     one, substs[0].expect_ty(), source_info.span
 654                                 ),
 655                                 self.operand_from_scalar(
 656                                     two, substs[1].expect_ty(), source_info.span
 657                                 ),
 658                             ],
 659                         );
 660                     }
 661                 },
 662                 _ => { }
 663             }
 664         }
 665     }
 666
 667     fn should_const_prop(&self) -> bool {
 668         self.tcx.sess.opts.debugging_opts.mir_opt_level >= 2
 669     }
 670 }
 671
 672 fn type_size_of<'tcx>(
 673     tcx: TyCtxt<'tcx>,
 674     param_env: ty::ParamEnv<'tcx>,
 675     ty: Ty<'tcx>,
 676 ) -> Option<u64> {
 677     tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
 678 }
 679
 680 struct CanConstProp {
 681     can_const_prop: IndexVec<Local, bool>,
 682     // false at the beginning, once set, there are not allowed to be any more assignments
 683     found_assignment: IndexVec<Local, bool>,
 684 }
 685
 686 impl CanConstProp {
 687     /// returns true if `local` can be propagated
 688     fn check(body: &Body<'_>) -> IndexVec<Local, bool> {
 689         let mut cpv = CanConstProp {
 690             can_const_prop: IndexVec::from_elem(true, &body.local_decls),
 691             found_assignment: IndexVec::from_elem(false, &body.local_decls),
 692         };
 693         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 694             // cannot use args at all
 695             // cannot use locals because if x < y { y - x } else { x - y } would
 696             //        lint for x != y
 697             // FIXME(oli-obk): lint variables until they are used in a condition
 698             // FIXME(oli-obk): lint if return value is constant
 699             *val = body.local_kind(local) == LocalKind::Temp;
 700
 701             if !*val {
 702                 trace!("local {:?} can't be propagated because it's not a temporary", local);
 703             }
 704         }
 705         cpv.visit_body(body);
 706         cpv.can_const_prop
 707     }
 708 }
 709
 710 impl<'tcx> Visitor<'tcx> for CanConstProp {
 711     fn visit_local(
 712         &mut self,
 713         &local: &Local,
 714         context: PlaceContext,
 715         _: Location,
 716     ) {
 717         use rustc::mir::visit::PlaceContext::*;
 718         match context {
 719             // Constants must have at most one write
 720             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 721             // only occur in independent execution paths
 722             MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
 723                 trace!("local {:?} can't be propagated because of multiple assignments", local);
 724                 self.can_const_prop[local] = false;
 725             } else {
 726                 self.found_assignment[local] = true
 727             },
 728             // Reading constants is allowed an arbitrary number of times
 729             NonMutatingUse(NonMutatingUseContext::Copy) |
 730             NonMutatingUse(NonMutatingUseContext::Move) |
 731             NonMutatingUse(NonMutatingUseContext::Inspect) |
 732             NonMutatingUse(NonMutatingUseContext::Projection) |
 733             MutatingUse(MutatingUseContext::Projection) |
 734             NonUse(_) => {},
 735             _ => {
 736                 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
 737                 self.can_const_prop[local] = false;
 738             },
 739         }
 740     }
 741 }
 742
 743 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
 744     fn visit_constant(
 745         &mut self,
 746         constant: &mut Constant<'tcx>,
 747         location: Location,
 748     ) {
 749         trace!("visit_constant: {:?}", constant);
 750         self.super_constant(constant, location);
 751         self.eval_constant(constant);
 752     }
 753
 754     fn visit_statement(
 755         &mut self,
 756         statement: &mut Statement<'tcx>,
 757         location: Location,
 758     ) {
 759         trace!("visit_statement: {:?}", statement);
 760         if let StatementKind::Assign(ref place, ref mut rval) = statement.kind {
 761             let place_ty: Ty<'tcx> = place
 762                 .ty(&self.local_decls, self.tcx)
 763                 .ty;
 764             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 765                 if let Some(value) = self.const_prop(rval, place_layout, statement.source_info) {
 766                     if let Place::Base(PlaceBase::Local(local)) = *place {
 767                         trace!("checking whether {:?} can be stored to {:?}", value, local);
 768                         if self.can_const_prop[local] {
 769                             trace!("storing {:?} to {:?}", value, local);
 770                             assert!(self.get_const(local).is_none());
 771                             self.set_const(local, Some(value));
 772
 773                             if self.should_const_prop() {
 774                                 self.replace_with_const(
 775                                     rval,
 776                                     value,
 777                                     statement.source_info,
 778                                 );
 779                             }
 780                         }
 781                     }
 782                 }
 783             }
 784         }
 785         self.super_statement(statement, location);
 786     }
 787
 788     fn visit_terminator(
 789         &mut self,
 790         terminator: &mut Terminator<'tcx>,
 791         location: Location,
 792     ) {
 793         self.super_terminator(terminator, location);
 794         let source_info = terminator.source_info;
 795         match &mut terminator.kind {
 796             TerminatorKind::Assert { expected, msg, ref mut cond, .. } => {
 797                 if let Some(value) = self.eval_operand(&cond, source_info) {
 798                     trace!("assertion on {:?} should be {:?}", value, expected);
 799                     let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
 800                     let value_const = self.ecx.read_scalar(value).unwrap();
 801                     if expected != value_const {
 802                         // poison all places this operand references so that further code
 803                         // doesn't use the invalid value
 804                         match cond {
 805                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 806                                 let mut place = place;
 807                                 while let Place::Projection(ref proj) = *place {
 808                                     place = &proj.base;
 809                                 }
 810                                 if let Place::Base(PlaceBase::Local(local)) = *place {
 811                                     self.set_const(local, None);
 812                                 }
 813                             },
 814                             Operand::Constant(_) => {}
 815                         }
 816                         let span = terminator.source_info.span;
 817                         let hir_id = self
 818                             .tcx
 819                             .hir()
 820                             .as_local_hir_id(self.source.def_id())
 821                             .expect("some part of a failing const eval must be local");
 822                         use rustc::mir::interpret::InterpError::*;
 823                         let msg = match msg {
 824                             Overflow(_) |
 825                             OverflowNeg |
 826                             DivisionByZero |
 827                             RemainderByZero => msg.description().to_owned(),
 828                             BoundsCheck { ref len, ref index } => {
 829                                 let len = self
 830                                     .eval_operand(len, source_info)
 831                                     .expect("len must be const");
 832                                 let len = match self.ecx.read_scalar(len) {
 833                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 834                                         data, ..
 835                                     })) => data,
 836                                     other => bug!("const len not primitive: {:?}", other),
 837                                 };
 838                                 let index = self
 839                                     .eval_operand(index, source_info)
 840                                     .expect("index must be const");
 841                                 let index = match self.ecx.read_scalar(index) {
 842                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 843                                         data, ..
 844                                     })) => data,
 845                                     other => bug!("const index not primitive: {:?}", other),
 846                                 };
 847                                 format!(
 848                                     "index out of bounds: \
 849                                     the len is {} but the index is {}",
 850                                     len,
 851                                     index,
 852                                 )
 853                             },
 854                             // Need proper const propagator for these
 855                             _ => return,
 856                         };
 857                         self.tcx.lint_hir(
 858                             ::rustc::lint::builtin::CONST_ERR,
 859                             hir_id,
 860                             span,
 861                             &msg,
 862                         );
 863                     } else {
 864                         if self.should_const_prop() {
 865                             if let ScalarMaybeUndef::Scalar(scalar) = value_const {
 866                                 *cond = self.operand_from_scalar(
 867                                     scalar,
 868                                     self.tcx.types.bool,
 869                                     source_info.span,
 870                                 );
 871                             }
 872                         }
 873                     }
 874                 }
 875             },
 876             TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
 877                 if self.should_const_prop() {
 878                     if let Some(value) = self.eval_operand(&discr, source_info) {
 879                         if let ScalarMaybeUndef::Scalar(scalar) =
 880                                 self.ecx.read_scalar(value).unwrap() {
 881                             *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
 882                         }
 883                     }
 884                 }
 885             },
 886             //none of these have Operands to const-propagate
 887             TerminatorKind::Goto { .. } |
 888             TerminatorKind::Resume |
 889             TerminatorKind::Abort |
 890             TerminatorKind::Return |
 891             TerminatorKind::Unreachable |
 892             TerminatorKind::Drop { .. } |
 893             TerminatorKind::DropAndReplace { .. } |
 894             TerminatorKind::Yield { .. } |
 895             TerminatorKind::GeneratorDrop |
 896             TerminatorKind::FalseEdges { .. } |
 897             TerminatorKind::FalseUnwind { .. } => { }
 898             //FIXME(wesleywiser) Call does have Operands that could be const-propagated
 899             TerminatorKind::Call { .. } => { }
 900         }
 901     }
 902 }