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,
  12 };
  13 use rustc::mir::visit::{
  14     Visitor, PlaceContext, MutatingUseContext, MutVisitor, NonMutatingUseContext,
  15 };
  16 use rustc::mir::interpret::{Scalar, GlobalId, InterpResult, PanicInfo};
  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,
  23 };
  24
  25 use crate::interpret::{
  26     self, InterpCx, 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<'tcx> MirPass<'tcx> for ConstProp {
  37     fn run_pass(&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(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
  68         let dummy_body =
  69             &Body::new(
  70                 body.basic_blocks().clone(),
  71                 Default::default(),
  72                 ClearCrossCrate::Clear,
  73                 None,
  74                 body.local_decls.clone(),
  75                 Default::default(),
  76                 body.arg_count,
  77                 Default::default(),
  78                 tcx.def_span(source.def_id()),
  79                 Default::default(),
  80             );
  81
  82         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
  83         // constants, instead of just checking for const-folding succeeding.
  84         // That would require an uniform one-def no-mutation analysis
  85         // and RPO (or recursing when needing the value of a local).
  86         let mut optimization_finder = ConstPropagator::new(
  87             body,
  88             dummy_body,
  89             source_scope_local_data,
  90             tcx,
  91             source
  92         );
  93         optimization_finder.visit_body(body);
  94
  95         // put back the data we stole from `mir`
  96         let source_scope_local_data = optimization_finder.release_stolen_data();
  97         std::mem::replace(
  98             &mut body.source_scope_local_data,
  99             source_scope_local_data
 100         );
 101
 102         trace!("ConstProp done for {:?}", source.def_id());
 103     }
 104 }
 105
 106 type Const<'tcx> = OpTy<'tcx>;
 107
 108 /// Finds optimization opportunities on the MIR.
 109 struct ConstPropagator<'mir, 'tcx> {
 110     ecx: InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>>,
 111     tcx: TyCtxt<'tcx>,
 112     source: MirSource<'tcx>,
 113     can_const_prop: IndexVec<Local, bool>,
 114     param_env: ParamEnv<'tcx>,
 115     source_scope_local_data: ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
 116     local_decls: IndexVec<Local, LocalDecl<'tcx>>,
 117 }
 118
 119 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
 120     type Ty = Ty<'tcx>;
 121     type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
 122
 123     fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyLayout {
 124         self.tcx.layout_of(self.param_env.and(ty))
 125     }
 126 }
 127
 128 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
 129     #[inline]
 130     fn data_layout(&self) -> &TargetDataLayout {
 131         &self.tcx.data_layout
 132     }
 133 }
 134
 135 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
 136     #[inline]
 137     fn tcx(&self) -> TyCtxt<'tcx> {
 138         self.tcx
 139     }
 140 }
 141
 142 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 143     fn new(
 144         body: &Body<'tcx>,
 145         dummy_body: &'mir Body<'tcx>,
 146         source_scope_local_data: ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
 147         tcx: TyCtxt<'tcx>,
 148         source: MirSource<'tcx>,
 149     ) -> ConstPropagator<'mir, 'tcx> {
 150         let def_id = source.def_id();
 151         let param_env = tcx.param_env(def_id);
 152         let span = tcx.def_span(def_id);
 153         let mut ecx = mk_eval_cx(tcx, span, param_env);
 154         let can_const_prop = CanConstProp::check(body);
 155
 156         ecx.push_stack_frame(
 157             Instance::new(def_id, &InternalSubsts::identity_for_item(tcx, def_id)),
 158             span,
 159             dummy_body,
 160             None,
 161             StackPopCleanup::None {
 162                 cleanup: false,
 163             },
 164         ).expect("failed to push initial stack frame");
 165
 166         ConstPropagator {
 167             ecx,
 168             tcx,
 169             source,
 170             param_env,
 171             can_const_prop,
 172             source_scope_local_data,
 173             //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
 174             local_decls: body.local_decls.clone(),
 175         }
 176     }
 177
 178     fn release_stolen_data(self) -> ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>> {
 179         self.source_scope_local_data
 180     }
 181
 182     fn get_const(&self, local: Local) -> Option<Const<'tcx>> {
 183         let l = &self.ecx.frame().locals[local];
 184
 185         // If the local is `Unitialized` or `Dead` then we haven't propagated a value into it.
 186         //
 187         // `InterpCx::access_local()` mostly takes care of this for us however, for ZSTs,
 188         // it will synthesize a value for us. In doing so, that will cause the
 189         // `get_const(l).is_empty()` assert right before we call `set_const()` in `visit_statement`
 190         // to fail.
 191         if let LocalValue::Uninitialized | LocalValue::Dead = l.value {
 192             return None;
 193         }
 194
 195         self.ecx.access_local(self.ecx.frame(), local, None).ok()
 196     }
 197
 198     fn set_const(&mut self, local: Local, c: Const<'tcx>) {
 199         let frame = self.ecx.frame_mut();
 200
 201         if let Some(layout) = frame.locals[local].layout.get() {
 202             debug_assert_eq!(c.layout, layout);
 203         }
 204
 205         frame.locals[local] = LocalState {
 206             value: LocalValue::Live(*c),
 207             layout: Cell::new(Some(c.layout)),
 208         };
 209     }
 210
 211     fn remove_const(&mut self, local: Local) {
 212         self.ecx.frame_mut().locals[local] = LocalState {
 213             value: LocalValue::Uninitialized,
 214             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                     Exit(_) => bug!("the CTFE program cannot exit"),
 244                     Unsupported(_)
 245                     | UndefinedBehavior(_)
 246                     | InvalidProgram(_)
 247                     | ResourceExhaustion(_) => {
 248                         // Ignore these errors.
 249                     }
 250                     Panic(_) => {
 251                         diagnostic.report_as_lint(
 252                             self.ecx.tcx,
 253                             "this expression will panic at runtime",
 254                             lint_root,
 255                             None,
 256                         );
 257                     }
 258                 }
 259                 None
 260             },
 261         };
 262         self.ecx.tcx.span = DUMMY_SP;
 263         r
 264     }
 265
 266     fn eval_constant(
 267         &mut self,
 268         c: &Constant<'tcx>,
 269     ) -> Option<Const<'tcx>> {
 270         self.ecx.tcx.span = c.span;
 271         match self.ecx.eval_const_to_op(c.literal, None) {
 272             Ok(op) => {
 273                 Some(op)
 274             },
 275             Err(error) => {
 276                 let err = error_to_const_error(&self.ecx, error);
 277                 err.report_as_error(self.ecx.tcx, "erroneous constant used");
 278                 None
 279             },
 280         }
 281     }
 282
 283     fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 284         trace!("eval_place(place={:?})", place);
 285         place.iterate(|place_base, place_projection| {
 286             let mut eval = match place_base {
 287                 PlaceBase::Local(loc) => self.get_const(*loc).clone()?,
 288                 PlaceBase::Static(box Static {kind: StaticKind::Promoted(promoted), ..}) => {
 289                     let generics = self.tcx.generics_of(self.source.def_id());
 290                     if generics.requires_monomorphization(self.tcx) {
 291                         // FIXME: can't handle code with generics
 292                         return None;
 293                     }
 294                     let substs = InternalSubsts::identity_for_item(self.tcx, self.source.def_id());
 295                     let instance = Instance::new(self.source.def_id(), substs);
 296                     let cid = GlobalId {
 297                         instance,
 298                         promoted: Some(*promoted),
 299                     };
 300                     // cannot use `const_eval` here, because that would require having the MIR
 301                     // for the current function available, but we're producing said MIR right now
 302                     let res = self.use_ecx(source_info, |this| {
 303                         let body = &this.tcx.promoted_mir(this.source.def_id())[*promoted];
 304                         eval_promoted(this.tcx, cid, body, this.param_env)
 305                     })?;
 306                     trace!("evaluated promoted {:?} to {:?}", promoted, res);
 307                     res.into()
 308                 }
 309                 _ => return None,
 310             };
 311
 312             for proj in place_projection {
 313                 match proj.elem {
 314                     ProjectionElem::Field(field, _) => {
 315                         trace!("field proj on {:?}", proj.base);
 316                         eval = self.use_ecx(source_info, |this| {
 317                             this.ecx.operand_field(eval, field.index() as u64)
 318                         })?;
 319                     },
 320                     ProjectionElem::Deref => {
 321                         trace!("processing deref");
 322                         eval = self.use_ecx(source_info, |this| {
 323                             this.ecx.deref_operand(eval)
 324                         })?.into();
 325                     }
 326                     // We could get more projections by using e.g., `operand_projection`,
 327                     // but we do not even have the stack frame set up properly so
 328                     // an `Index` projection would throw us off-track.
 329                     _ => return None,
 330                 }
 331             }
 332
 333             Some(eval)
 334         })
 335     }
 336
 337     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 338         match *op {
 339             Operand::Constant(ref c) => self.eval_constant(c),
 340             | Operand::Move(ref place)
 341             | Operand::Copy(ref place) => self.eval_place(place, source_info),
 342         }
 343     }
 344
 345     fn const_prop(
 346         &mut self,
 347         rvalue: &Rvalue<'tcx>,
 348         place_layout: TyLayout<'tcx>,
 349         source_info: SourceInfo,
 350     ) -> Option<Const<'tcx>> {
 351         let span = source_info.span;
 352         match *rvalue {
 353             Rvalue::Use(ref op) => {
 354                 self.eval_operand(op, source_info)
 355             },
 356             Rvalue::Ref(_, _, ref place) => {
 357                 let src = self.eval_place(place, source_info)?;
 358                 let mplace = src.try_as_mplace().ok()?;
 359                 Some(ImmTy::from_scalar(mplace.ptr.into(), place_layout).into())
 360             },
 361             Rvalue::Repeat(..) |
 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             Rvalue::Len(ref place) => {
 375                 let place = self.eval_place(&place, source_info)?;
 376                 let mplace = place.try_as_mplace().ok()?;
 377
 378                 if let ty::Slice(_) = mplace.layout.ty.sty {
 379                     let len = mplace.meta.unwrap().to_usize(&self.ecx).unwrap();
 380
 381                     Some(ImmTy::from_uint(
 382                         len,
 383                         self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
 384                     ).into())
 385                 } else {
 386                     trace!("not slice: {:?}", mplace.layout.ty.sty);
 387                     None
 388                 }
 389             },
 390             Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
 391                 type_size_of(self.tcx, self.param_env, ty).and_then(|n| Some(
 392                     ImmTy::from_uint(
 393                         n,
 394                         self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
 395                     ).into()
 396                 ))
 397             }
 398             Rvalue::UnaryOp(op, ref arg) => {
 399                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 400                     self.tcx.closure_base_def_id(self.source.def_id())
 401                 } else {
 402                     self.source.def_id()
 403                 };
 404                 let generics = self.tcx.generics_of(def_id);
 405                 if generics.requires_monomorphization(self.tcx) {
 406                     // FIXME: can't handle code with generics
 407                     return None;
 408                 }
 409
 410                 let arg = self.eval_operand(arg, source_info)?;
 411                 let val = self.use_ecx(source_info, |this| {
 412                     let prim = this.ecx.read_immediate(arg)?;
 413                     match op {
 414                         UnOp::Neg => {
 415                             // Need to do overflow check here: For actual CTFE, MIR
 416                             // generation emits code that does this before calling the op.
 417                             if prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
 418                                 throw_panic!(OverflowNeg)
 419                             }
 420                         }
 421                         UnOp::Not => {
 422                             // Cannot overflow
 423                         }
 424                     }
 425                     // Now run the actual operation.
 426                     this.ecx.unary_op(op, prim)
 427                 })?;
 428                 Some(val.into())
 429             }
 430             Rvalue::CheckedBinaryOp(op, ref left, ref right) |
 431             Rvalue::BinaryOp(op, ref left, ref right) => {
 432                 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
 433                 let right = self.eval_operand(right, source_info)?;
 434                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 435                     self.tcx.closure_base_def_id(self.source.def_id())
 436                 } else {
 437                     self.source.def_id()
 438                 };
 439                 let generics = self.tcx.generics_of(def_id);
 440                 if generics.requires_monomorphization(self.tcx) {
 441                     // FIXME: can't handle code with generics
 442                     return None;
 443                 }
 444
 445                 let r = self.use_ecx(source_info, |this| {
 446                     this.ecx.read_immediate(right)
 447                 })?;
 448                 if op == BinOp::Shr || op == BinOp::Shl {
 449                     let left_ty = left.ty(&self.local_decls, self.tcx);
 450                     let left_bits = self
 451                         .tcx
 452                         .layout_of(self.param_env.and(left_ty))
 453                         .unwrap()
 454                         .size
 455                         .bits();
 456                     let right_size = right.layout.size;
 457                     let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
 458                     if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
 459                         let source_scope_local_data = match self.source_scope_local_data {
 460                             ClearCrossCrate::Set(ref data) => data,
 461                             ClearCrossCrate::Clear => return None,
 462                         };
 463                         let dir = if op == BinOp::Shr {
 464                             "right"
 465                         } else {
 466                             "left"
 467                         };
 468                         let hir_id = source_scope_local_data[source_info.scope].lint_root;
 469                         self.tcx.lint_hir(
 470                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 471                             hir_id,
 472                             span,
 473                             &format!("attempt to shift {} with overflow", dir));
 474                         return None;
 475                     }
 476                 }
 477                 let left = self.eval_operand(left, source_info)?;
 478                 let l = self.use_ecx(source_info, |this| {
 479                     this.ecx.read_immediate(left)
 480                 })?;
 481                 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
 482                 let (val, overflow, _ty) = self.use_ecx(source_info, |this| {
 483                     this.ecx.overflowing_binary_op(op, l, r)
 484                 })?;
 485                 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
 486                     Immediate::ScalarPair(
 487                         val.into(),
 488                         Scalar::from_bool(overflow).into(),
 489                     )
 490                 } else {
 491                     if overflow {
 492                         let err = err_panic!(Overflow(op)).into();
 493                         let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
 494                         return None;
 495                     }
 496                     Immediate::Scalar(val.into())
 497                 };
 498                 let res = ImmTy {
 499                     imm: val,
 500                     layout: place_layout,
 501                 };
 502                 Some(res.into())
 503             },
 504         }
 505     }
 506
 507     fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
 508         Operand::Constant(Box::new(
 509             Constant {
 510                 span,
 511                 user_ty: None,
 512                 literal: self.tcx.mk_const(*ty::Const::from_scalar(
 513                     self.tcx,
 514                     scalar,
 515                     ty,
 516                 ))
 517             }
 518         ))
 519     }
 520
 521     fn replace_with_const(
 522         &mut self,
 523         rval: &mut Rvalue<'tcx>,
 524         value: Const<'tcx>,
 525         source_info: SourceInfo,
 526     ) {
 527         trace!("attepting to replace {:?} with {:?}", rval, value);
 528         if let Err(e) = self.ecx.validate_operand(
 529             value,
 530             vec![],
 531             // FIXME: is ref tracking too expensive?
 532             Some(&mut interpret::RefTracking::empty()),
 533         ) {
 534             trace!("validation error, attempt failed: {:?}", e);
 535             return;
 536         }
 537
 538         // FIXME> figure out what tho do when try_read_immediate fails
 539         let imm = self.use_ecx(source_info, |this| {
 540             this.ecx.try_read_immediate(value)
 541         });
 542
 543         if let Some(Ok(imm)) = imm {
 544             match *imm {
 545                 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
 546                     *rval = Rvalue::Use(
 547                         self.operand_from_scalar(scalar, value.layout.ty, source_info.span));
 548                 },
 549                 Immediate::ScalarPair(
 550                     ScalarMaybeUndef::Scalar(one),
 551                     ScalarMaybeUndef::Scalar(two)
 552                 ) => {
 553                     let ty = &value.layout.ty.sty;
 554                     if let ty::Tuple(substs) = ty {
 555                         *rval = Rvalue::Aggregate(
 556                             Box::new(AggregateKind::Tuple),
 557                             vec![
 558                                 self.operand_from_scalar(
 559                                     one, substs[0].expect_ty(), source_info.span
 560                                 ),
 561                                 self.operand_from_scalar(
 562                                     two, substs[1].expect_ty(), source_info.span
 563                                 ),
 564                             ],
 565                         );
 566                     }
 567                 },
 568                 _ => { }
 569             }
 570         }
 571     }
 572
 573     fn should_const_prop(&self) -> bool {
 574         self.tcx.sess.opts.debugging_opts.mir_opt_level >= 2
 575     }
 576 }
 577
 578 fn type_size_of<'tcx>(
 579     tcx: TyCtxt<'tcx>,
 580     param_env: ty::ParamEnv<'tcx>,
 581     ty: Ty<'tcx>,
 582 ) -> Option<u64> {
 583     tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
 584 }
 585
 586 struct CanConstProp {
 587     can_const_prop: IndexVec<Local, bool>,
 588     // false at the beginning, once set, there are not allowed to be any more assignments
 589     found_assignment: IndexVec<Local, bool>,
 590 }
 591
 592 impl CanConstProp {
 593     /// returns true if `local` can be propagated
 594     fn check(body: &Body<'_>) -> IndexVec<Local, bool> {
 595         let mut cpv = CanConstProp {
 596             can_const_prop: IndexVec::from_elem(true, &body.local_decls),
 597             found_assignment: IndexVec::from_elem(false, &body.local_decls),
 598         };
 599         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 600             // cannot use args at all
 601             // cannot use locals because if x < y { y - x } else { x - y } would
 602             //        lint for x != y
 603             // FIXME(oli-obk): lint variables until they are used in a condition
 604             // FIXME(oli-obk): lint if return value is constant
 605             *val = body.local_kind(local) == LocalKind::Temp;
 606
 607             if !*val {
 608                 trace!("local {:?} can't be propagated because it's not a temporary", local);
 609             }
 610         }
 611         cpv.visit_body(body);
 612         cpv.can_const_prop
 613     }
 614 }
 615
 616 impl<'tcx> Visitor<'tcx> for CanConstProp {
 617     fn visit_local(
 618         &mut self,
 619         &local: &Local,
 620         context: PlaceContext,
 621         _: Location,
 622     ) {
 623         use rustc::mir::visit::PlaceContext::*;
 624         match context {
 625             // Constants must have at most one write
 626             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 627             // only occur in independent execution paths
 628             MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
 629                 trace!("local {:?} can't be propagated because of multiple assignments", local);
 630                 self.can_const_prop[local] = false;
 631             } else {
 632                 self.found_assignment[local] = true
 633             },
 634             // Reading constants is allowed an arbitrary number of times
 635             NonMutatingUse(NonMutatingUseContext::Copy) |
 636             NonMutatingUse(NonMutatingUseContext::Move) |
 637             NonMutatingUse(NonMutatingUseContext::Inspect) |
 638             NonMutatingUse(NonMutatingUseContext::Projection) |
 639             MutatingUse(MutatingUseContext::Projection) |
 640             NonUse(_) => {},
 641             _ => {
 642                 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
 643                 self.can_const_prop[local] = false;
 644             },
 645         }
 646     }
 647 }
 648
 649 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
 650     fn visit_constant(
 651         &mut self,
 652         constant: &mut Constant<'tcx>,
 653         location: Location,
 654     ) {
 655         trace!("visit_constant: {:?}", constant);
 656         self.super_constant(constant, location);
 657         self.eval_constant(constant);
 658     }
 659
 660     fn visit_statement(
 661         &mut self,
 662         statement: &mut Statement<'tcx>,
 663         location: Location,
 664     ) {
 665         trace!("visit_statement: {:?}", statement);
 666         if let StatementKind::Assign(ref place, ref mut rval) = statement.kind {
 667             let place_ty: Ty<'tcx> = place
 668                 .ty(&self.local_decls, self.tcx)
 669                 .ty;
 670             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 671                 if let Some(value) = self.const_prop(rval, place_layout, statement.source_info) {
 672                     if let Place {
 673                         base: PlaceBase::Local(local),
 674                         projection: None,
 675                     } = *place {
 676                         trace!("checking whether {:?} can be stored to {:?}", value, local);
 677                         if self.can_const_prop[local] {
 678                             trace!("storing {:?} to {:?}", value, local);
 679                             assert!(self.get_const(local).is_none());
 680                             self.set_const(local, value);
 681
 682                             if self.should_const_prop() {
 683                                 self.replace_with_const(
 684                                     rval,
 685                                     value,
 686                                     statement.source_info,
 687                                 );
 688                             }
 689                         }
 690                     }
 691                 }
 692             }
 693         }
 694         self.super_statement(statement, location);
 695     }
 696
 697     fn visit_terminator(
 698         &mut self,
 699         terminator: &mut Terminator<'tcx>,
 700         location: Location,
 701     ) {
 702         self.super_terminator(terminator, location);
 703         let source_info = terminator.source_info;
 704         match &mut terminator.kind {
 705             TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
 706                 if let Some(value) = self.eval_operand(&cond, source_info) {
 707                     trace!("assertion on {:?} should be {:?}", value, expected);
 708                     let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
 709                     let value_const = self.ecx.read_scalar(value).unwrap();
 710                     if expected != value_const {
 711                         // poison all places this operand references so that further code
 712                         // doesn't use the invalid value
 713                         match cond {
 714                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 715                                 if let PlaceBase::Local(local) = place.base {
 716                                     self.remove_const(local);
 717                                 }
 718                             },
 719                             Operand::Constant(_) => {}
 720                         }
 721                         let span = terminator.source_info.span;
 722                         let hir_id = self
 723                             .tcx
 724                             .hir()
 725                             .as_local_hir_id(self.source.def_id())
 726                             .expect("some part of a failing const eval must be local");
 727                         let msg = match msg {
 728                             PanicInfo::Overflow(_) |
 729                             PanicInfo::OverflowNeg |
 730                             PanicInfo::DivisionByZero |
 731                             PanicInfo::RemainderByZero =>
 732                                 msg.description().to_owned(),
 733                             PanicInfo::BoundsCheck { ref len, ref index } => {
 734                                 let len = self
 735                                     .eval_operand(len, source_info)
 736                                     .expect("len must be const");
 737                                 let len = match self.ecx.read_scalar(len) {
 738                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 739                                         data, ..
 740                                     })) => data,
 741                                     other => bug!("const len not primitive: {:?}", other),
 742                                 };
 743                                 let index = self
 744                                     .eval_operand(index, source_info)
 745                                     .expect("index must be const");
 746                                 let index = match self.ecx.read_scalar(index) {
 747                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 748                                         data, ..
 749                                     })) => data,
 750                                     other => bug!("const index not primitive: {:?}", other),
 751                                 };
 752                                 format!(
 753                                     "index out of bounds: \
 754                                     the len is {} but the index is {}",
 755                                     len,
 756                                     index,
 757                                 )
 758                             },
 759                             // Need proper const propagator for these
 760                             _ => return,
 761                         };
 762                         self.tcx.lint_hir(
 763                             ::rustc::lint::builtin::CONST_ERR,
 764                             hir_id,
 765                             span,
 766                             &msg,
 767                         );
 768                     } else {
 769                         if self.should_const_prop() {
 770                             if let ScalarMaybeUndef::Scalar(scalar) = value_const {
 771                                 *cond = self.operand_from_scalar(
 772                                     scalar,
 773                                     self.tcx.types.bool,
 774                                     source_info.span,
 775                                 );
 776                             }
 777                         }
 778                     }
 779                 }
 780             },
 781             TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
 782                 if self.should_const_prop() {
 783                     if let Some(value) = self.eval_operand(&discr, source_info) {
 784                         if let ScalarMaybeUndef::Scalar(scalar) =
 785                                 self.ecx.read_scalar(value).unwrap() {
 786                             *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
 787                         }
 788                     }
 789                 }
 790             },
 791             //none of these have Operands to const-propagate
 792             TerminatorKind::Goto { .. } |
 793             TerminatorKind::Resume |
 794             TerminatorKind::Abort |
 795             TerminatorKind::Return |
 796             TerminatorKind::Unreachable |
 797             TerminatorKind::Drop { .. } |
 798             TerminatorKind::DropAndReplace { .. } |
 799             TerminatorKind::Yield { .. } |
 800             TerminatorKind::GeneratorDrop |
 801             TerminatorKind::FalseEdges { .. } |
 802             TerminatorKind::FalseUnwind { .. } => { }
 803             //FIXME(wesleywiser) Call does have Operands that could be const-propagated
 804             TerminatorKind::Call { .. } => { }
 805         }
 806     }
 807 }