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,
  10     TerminatorKind, Terminator,  ClearCrossCrate, SourceInfo, BinOp,
  11     SourceScope, SourceScopeLocalData, LocalDecl,
  12 };
  13 use rustc::mir::visit::{
  14     Visitor, PlaceContext, MutatingUseContext, MutVisitor, NonMutatingUseContext,
  15 };
  16 use rustc::mir::interpret::{Scalar, 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, StackPopCleanup, LocalValue, LocalState,
  28 };
  29 use crate::const_eval::{
  30     CompileTimeInterpreter, error_to_const_error, 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                 use rustc::mir::interpret::InterpError::*;
 241                 match error.kind {
 242                     Exit(_) => bug!("the CTFE program cannot exit"),
 243                     Unsupported(_)
 244                     | UndefinedBehavior(_)
 245                     | InvalidProgram(_)
 246                     | ResourceExhaustion(_) => {
 247                         // Ignore these errors.
 248                     }
 249                     Panic(_) => {
 250                         let diagnostic = error_to_const_error(&self.ecx, error);
 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         self.use_ecx(source_info, |this| {
 286             this.ecx.eval_place_to_op(place, None)
 287         })
 288     }
 289
 290     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 291         match *op {
 292             Operand::Constant(ref c) => self.eval_constant(c),
 293             | Operand::Move(ref place)
 294             | Operand::Copy(ref place) => self.eval_place(place, source_info),
 295         }
 296     }
 297
 298     fn const_prop(
 299         &mut self,
 300         rvalue: &Rvalue<'tcx>,
 301         place_layout: TyLayout<'tcx>,
 302         source_info: SourceInfo,
 303         place: &Place<'tcx>,
 304     ) -> Option<Const<'tcx>> {
 305         let span = source_info.span;
 306         match *rvalue {
 307             Rvalue::Use(_) |
 308             Rvalue::Len(_) |
 309             Rvalue::Cast(..) => {
 310                 self.use_ecx(source_info, |this| {
 311                     this.ecx.eval_rvalue_into_place(rvalue, place)?;
 312                     this.ecx.eval_place_to_op(place, Some(place_layout))
 313                 })
 314             },
 315             Rvalue::Ref(_, _, ref place) => {
 316                 let src = self.eval_place(place, source_info)?;
 317                 let mplace = src.try_as_mplace().ok()?;
 318                 Some(ImmTy::from_scalar(mplace.ptr.into(), place_layout).into())
 319             },
 320             Rvalue::Repeat(..) |
 321             Rvalue::Aggregate(..) |
 322             Rvalue::NullaryOp(NullOp::Box, _) |
 323             Rvalue::Discriminant(..) => None,
 324
 325             Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
 326                 type_size_of(self.tcx, self.param_env, ty).and_then(|n| Some(
 327                     ImmTy::from_uint(
 328                         n,
 329                         self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
 330                     ).into()
 331                 ))
 332             }
 333             Rvalue::UnaryOp(op, ref arg) => {
 334                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 335                     self.tcx.closure_base_def_id(self.source.def_id())
 336                 } else {
 337                     self.source.def_id()
 338                 };
 339                 let generics = self.tcx.generics_of(def_id);
 340                 if generics.requires_monomorphization(self.tcx) {
 341                     // FIXME: can't handle code with generics
 342                     return None;
 343                 }
 344
 345                 let arg = self.eval_operand(arg, source_info)?;
 346                 let oflo_check = self.tcx.sess.overflow_checks();
 347                 let val = self.use_ecx(source_info, |this| {
 348                     let prim = this.ecx.read_immediate(arg)?;
 349                     match op {
 350                         UnOp::Neg => {
 351                             // We check overflow in debug mode already
 352                             // so should only check in release mode.
 353                             if !oflo_check
 354                             && prim.layout.ty.is_signed()
 355                             && prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
 356                                 throw_panic!(OverflowNeg)
 357                             }
 358                         }
 359                         UnOp::Not => {
 360                             // Cannot overflow
 361                         }
 362                     }
 363                     // Now run the actual operation.
 364                     this.ecx.unary_op(op, prim)
 365                 })?;
 366                 Some(val.into())
 367             }
 368             Rvalue::CheckedBinaryOp(op, ref left, ref right) |
 369             Rvalue::BinaryOp(op, ref left, ref right) => {
 370                 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
 371                 let right = self.eval_operand(right, source_info)?;
 372                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 373                     self.tcx.closure_base_def_id(self.source.def_id())
 374                 } else {
 375                     self.source.def_id()
 376                 };
 377                 let generics = self.tcx.generics_of(def_id);
 378                 if generics.requires_monomorphization(self.tcx) {
 379                     // FIXME: can't handle code with generics
 380                     return None;
 381                 }
 382
 383                 let r = self.use_ecx(source_info, |this| {
 384                     this.ecx.read_immediate(right)
 385                 })?;
 386                 if op == BinOp::Shr || op == BinOp::Shl {
 387                     let left_ty = left.ty(&self.local_decls, self.tcx);
 388                     let left_bits = self
 389                         .tcx
 390                         .layout_of(self.param_env.and(left_ty))
 391                         .unwrap()
 392                         .size
 393                         .bits();
 394                     let right_size = right.layout.size;
 395                     let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
 396                     if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
 397                         let source_scope_local_data = match self.source_scope_local_data {
 398                             ClearCrossCrate::Set(ref data) => data,
 399                             ClearCrossCrate::Clear => return None,
 400                         };
 401                         let dir = if op == BinOp::Shr {
 402                             "right"
 403                         } else {
 404                             "left"
 405                         };
 406                         let hir_id = source_scope_local_data[source_info.scope].lint_root;
 407                         self.tcx.lint_hir(
 408                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 409                             hir_id,
 410                             span,
 411                             &format!("attempt to shift {} with overflow", dir));
 412                         return None;
 413                     }
 414                 }
 415                 let left = self.eval_operand(left, source_info)?;
 416                 let l = self.use_ecx(source_info, |this| {
 417                     this.ecx.read_immediate(left)
 418                 })?;
 419                 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
 420                 let (val, overflow, _ty) = self.use_ecx(source_info, |this| {
 421                     this.ecx.overflowing_binary_op(op, l, r)
 422                 })?;
 423                 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
 424                     Immediate::ScalarPair(
 425                         val.into(),
 426                         Scalar::from_bool(overflow).into(),
 427                     )
 428                 } else {
 429                     // We check overflow in debug mode already
 430                     // so should only check in release mode.
 431                     if !self.tcx.sess.overflow_checks() && overflow {
 432                         let err = err_panic!(Overflow(op)).into();
 433                         let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
 434                         return None;
 435                     }
 436                     Immediate::Scalar(val.into())
 437                 };
 438                 let res = ImmTy {
 439                     imm: val,
 440                     layout: place_layout,
 441                 };
 442                 Some(res.into())
 443             },
 444         }
 445     }
 446
 447     fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
 448         Operand::Constant(Box::new(
 449             Constant {
 450                 span,
 451                 user_ty: None,
 452                 literal: self.tcx.mk_const(*ty::Const::from_scalar(
 453                     self.tcx,
 454                     scalar,
 455                     ty,
 456                 ))
 457             }
 458         ))
 459     }
 460
 461     fn replace_with_const(
 462         &mut self,
 463         rval: &mut Rvalue<'tcx>,
 464         value: Const<'tcx>,
 465         source_info: SourceInfo,
 466     ) {
 467         trace!("attepting to replace {:?} with {:?}", rval, value);
 468         if let Err(e) = self.ecx.validate_operand(
 469             value,
 470             vec![],
 471             // FIXME: is ref tracking too expensive?
 472             Some(&mut interpret::RefTracking::empty()),
 473         ) {
 474             trace!("validation error, attempt failed: {:?}", e);
 475             return;
 476         }
 477
 478         // FIXME> figure out what tho do when try_read_immediate fails
 479         let imm = self.use_ecx(source_info, |this| {
 480             this.ecx.try_read_immediate(value)
 481         });
 482
 483         if let Some(Ok(imm)) = imm {
 484             match *imm {
 485                 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
 486                     *rval = Rvalue::Use(
 487                         self.operand_from_scalar(scalar, value.layout.ty, source_info.span));
 488                 },
 489                 Immediate::ScalarPair(
 490                     ScalarMaybeUndef::Scalar(one),
 491                     ScalarMaybeUndef::Scalar(two)
 492                 ) => {
 493                     let ty = &value.layout.ty.kind;
 494                     if let ty::Tuple(substs) = ty {
 495                         *rval = Rvalue::Aggregate(
 496                             Box::new(AggregateKind::Tuple),
 497                             vec![
 498                                 self.operand_from_scalar(
 499                                     one, substs[0].expect_ty(), source_info.span
 500                                 ),
 501                                 self.operand_from_scalar(
 502                                     two, substs[1].expect_ty(), source_info.span
 503                                 ),
 504                             ],
 505                         );
 506                     }
 507                 },
 508                 _ => { }
 509             }
 510         }
 511     }
 512
 513     fn should_const_prop(&self) -> bool {
 514         self.tcx.sess.opts.debugging_opts.mir_opt_level >= 2
 515     }
 516 }
 517
 518 fn type_size_of<'tcx>(
 519     tcx: TyCtxt<'tcx>,
 520     param_env: ty::ParamEnv<'tcx>,
 521     ty: Ty<'tcx>,
 522 ) -> Option<u64> {
 523     tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
 524 }
 525
 526 struct CanConstProp {
 527     can_const_prop: IndexVec<Local, bool>,
 528     // false at the beginning, once set, there are not allowed to be any more assignments
 529     found_assignment: IndexVec<Local, bool>,
 530 }
 531
 532 impl CanConstProp {
 533     /// returns true if `local` can be propagated
 534     fn check(body: &Body<'_>) -> IndexVec<Local, bool> {
 535         let mut cpv = CanConstProp {
 536             can_const_prop: IndexVec::from_elem(true, &body.local_decls),
 537             found_assignment: IndexVec::from_elem(false, &body.local_decls),
 538         };
 539         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 540             // cannot use args at all
 541             // cannot use locals because if x < y { y - x } else { x - y } would
 542             //        lint for x != y
 543             // FIXME(oli-obk): lint variables until they are used in a condition
 544             // FIXME(oli-obk): lint if return value is constant
 545             *val = body.local_kind(local) == LocalKind::Temp;
 546
 547             if !*val {
 548                 trace!("local {:?} can't be propagated because it's not a temporary", local);
 549             }
 550         }
 551         cpv.visit_body(body);
 552         cpv.can_const_prop
 553     }
 554 }
 555
 556 impl<'tcx> Visitor<'tcx> for CanConstProp {
 557     fn visit_local(
 558         &mut self,
 559         &local: &Local,
 560         context: PlaceContext,
 561         _: Location,
 562     ) {
 563         use rustc::mir::visit::PlaceContext::*;
 564         match context {
 565             // Constants must have at most one write
 566             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 567             // only occur in independent execution paths
 568             MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
 569                 trace!("local {:?} can't be propagated because of multiple assignments", local);
 570                 self.can_const_prop[local] = false;
 571             } else {
 572                 self.found_assignment[local] = true
 573             },
 574             // Reading constants is allowed an arbitrary number of times
 575             NonMutatingUse(NonMutatingUseContext::Copy) |
 576             NonMutatingUse(NonMutatingUseContext::Move) |
 577             NonMutatingUse(NonMutatingUseContext::Inspect) |
 578             NonMutatingUse(NonMutatingUseContext::Projection) |
 579             MutatingUse(MutatingUseContext::Projection) |
 580             NonUse(_) => {},
 581             _ => {
 582                 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
 583                 self.can_const_prop[local] = false;
 584             },
 585         }
 586     }
 587 }
 588
 589 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
 590     fn visit_constant(
 591         &mut self,
 592         constant: &mut Constant<'tcx>,
 593         location: Location,
 594     ) {
 595         trace!("visit_constant: {:?}", constant);
 596         self.super_constant(constant, location);
 597         self.eval_constant(constant);
 598     }
 599
 600     fn visit_statement(
 601         &mut self,
 602         statement: &mut Statement<'tcx>,
 603         location: Location,
 604     ) {
 605         trace!("visit_statement: {:?}", statement);
 606         if let StatementKind::Assign(box(ref place, ref mut rval)) = statement.kind {
 607             let place_ty: Ty<'tcx> = place
 608                 .ty(&self.local_decls, self.tcx)
 609                 .ty;
 610             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 611                 if let Place {
 612                     base: PlaceBase::Local(local),
 613                     projection: box [],
 614                 } = *place {
 615                     if let Some(value) = self.const_prop(rval, place_layout, statement.source_info, place) {
 616                         trace!("checking whether {:?} can be stored to {:?}", value, local);
 617                         if self.can_const_prop[local] {
 618                             trace!("storing {:?} to {:?}", value, local);
 619                             assert!(self.get_const(local).is_none() || self.get_const(local) == Some(value));
 620                             self.set_const(local, value);
 621
 622                             if self.should_const_prop() {
 623                                 self.replace_with_const(
 624                                     rval,
 625                                     value,
 626                                     statement.source_info,
 627                                 );
 628                             }
 629                         }
 630                     }
 631                 }
 632             }
 633         } else if let StatementKind::StorageLive(local) = statement.kind {
 634             if self.can_const_prop[local] {
 635                 let frame = self.ecx.frame_mut();
 636
 637                 frame.locals[local].value = LocalValue::Uninitialized;
 638             }
 639         } else if let StatementKind::StorageDead(local) = statement.kind {
 640             if self.can_const_prop[local] {
 641                 let frame = self.ecx.frame_mut();
 642
 643                 frame.locals[local].value = LocalValue::Dead;
 644             }
 645         }
 646         self.super_statement(statement, location);
 647     }
 648
 649     fn visit_terminator(
 650         &mut self,
 651         terminator: &mut Terminator<'tcx>,
 652         location: Location,
 653     ) {
 654         self.super_terminator(terminator, location);
 655         let source_info = terminator.source_info;
 656         match &mut terminator.kind {
 657             TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
 658                 if let Some(value) = self.eval_operand(&cond, source_info) {
 659                     trace!("assertion on {:?} should be {:?}", value, expected);
 660                     let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
 661                     let value_const = self.ecx.read_scalar(value).unwrap();
 662                     if expected != value_const {
 663                         // poison all places this operand references so that further code
 664                         // doesn't use the invalid value
 665                         match cond {
 666                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 667                                 if let PlaceBase::Local(local) = place.base {
 668                                     self.remove_const(local);
 669                                 }
 670                             },
 671                             Operand::Constant(_) => {}
 672                         }
 673                         let span = terminator.source_info.span;
 674                         let hir_id = self
 675                             .tcx
 676                             .hir()
 677                             .as_local_hir_id(self.source.def_id())
 678                             .expect("some part of a failing const eval must be local");
 679                         let msg = match msg {
 680                             PanicInfo::Overflow(_) |
 681                             PanicInfo::OverflowNeg |
 682                             PanicInfo::DivisionByZero |
 683                             PanicInfo::RemainderByZero =>
 684                                 msg.description().to_owned(),
 685                             PanicInfo::BoundsCheck { ref len, ref index } => {
 686                                 let len = self
 687                                     .eval_operand(len, source_info)
 688                                     .expect("len must be const");
 689                                 let len = match self.ecx.read_scalar(len) {
 690                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 691                                         data, ..
 692                                     })) => data,
 693                                     other => bug!("const len not primitive: {:?}", other),
 694                                 };
 695                                 let index = self
 696                                     .eval_operand(index, source_info)
 697                                     .expect("index must be const");
 698                                 let index = match self.ecx.read_scalar(index) {
 699                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 700                                         data, ..
 701                                     })) => data,
 702                                     other => bug!("const index not primitive: {:?}", other),
 703                                 };
 704                                 format!(
 705                                     "index out of bounds: \
 706                                     the len is {} but the index is {}",
 707                                     len,
 708                                     index,
 709                                 )
 710                             },
 711                             // Need proper const propagator for these
 712                             _ => return,
 713                         };
 714                         self.tcx.lint_hir(
 715                             ::rustc::lint::builtin::CONST_ERR,
 716                             hir_id,
 717                             span,
 718                             &msg,
 719                         );
 720                     } else {
 721                         if self.should_const_prop() {
 722                             if let ScalarMaybeUndef::Scalar(scalar) = value_const {
 723                                 *cond = self.operand_from_scalar(
 724                                     scalar,
 725                                     self.tcx.types.bool,
 726                                     source_info.span,
 727                                 );
 728                             }
 729                         }
 730                     }
 731                 }
 732             },
 733             TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
 734                 if self.should_const_prop() {
 735                     if let Some(value) = self.eval_operand(&discr, source_info) {
 736                         if let ScalarMaybeUndef::Scalar(scalar) =
 737                                 self.ecx.read_scalar(value).unwrap() {
 738                             *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
 739                         }
 740                     }
 741                 }
 742             },
 743             //none of these have Operands to const-propagate
 744             TerminatorKind::Goto { .. } |
 745             TerminatorKind::Resume |
 746             TerminatorKind::Abort |
 747             TerminatorKind::Return |
 748             TerminatorKind::Unreachable |
 749             TerminatorKind::Drop { .. } |
 750             TerminatorKind::DropAndReplace { .. } |
 751             TerminatorKind::Yield { .. } |
 752             TerminatorKind::GeneratorDrop |
 753             TerminatorKind::FalseEdges { .. } |
 754             TerminatorKind::FalseUnwind { .. } => { }
 755             //FIXME(wesleywiser) Call does have Operands that could be const-propagated
 756             TerminatorKind::Call { .. } => { }
 757         }
 758     }
 759 }