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::{Scalar, GlobalId, InterpResult, InterpError, 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, Size,
  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 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(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: InterpCx<'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(
 192         self,
 193     ) -> (
 194         ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
 195         IndexVec<Promoted, Body<'tcx>>,
 196     ) {
 197         (self.source_scope_local_data, self.promoted)
 198     }
 199
 200     fn get_const(&self, local: Local) -> Option<Const<'tcx>> {
 201         let l = &self.ecx.frame().locals[local];
 202
 203         // If the local is `Unitialized` or `Dead` then we haven't propagated a value into it.
 204         //
 205         // `InterpCx::access_local()` mostly takes care of this for us however, for ZSTs,
 206         // it will synthesize a value for us. In doing so, that will cause the
 207         // `get_const(l).is_empty()` assert right before we call `set_const()` in `visit_statement`
 208         // to fail.
 209         if let LocalValue::Uninitialized | LocalValue::Dead = l.value {
 210             return None;
 211         }
 212
 213         self.ecx.access_local(self.ecx.frame(), local, None).ok()
 214     }
 215
 216     fn set_const(&mut self, local: Local, c: Const<'tcx>) {
 217         let frame = self.ecx.frame_mut();
 218
 219         if let Some(layout) = frame.locals[local].layout.get() {
 220             debug_assert_eq!(c.layout, layout);
 221         }
 222
 223         frame.locals[local] = LocalState {
 224             value: LocalValue::Live(*c),
 225             layout: Cell::new(Some(c.layout)),
 226         };
 227     }
 228
 229     fn remove_const(&mut self, local: Local) {
 230         self.ecx.frame_mut().locals[local] = LocalState {
 231             value: LocalValue::Uninitialized,
 232             layout: Cell::new(None),
 233         };
 234     }
 235
 236     fn use_ecx<F, T>(
 237         &mut self,
 238         source_info: SourceInfo,
 239         f: F
 240     ) -> Option<T>
 241     where
 242         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 243     {
 244         self.ecx.tcx.span = source_info.span;
 245         let lint_root = match self.source_scope_local_data {
 246             ClearCrossCrate::Set(ref ivs) => {
 247                 //FIXME(#51314): remove this check
 248                 if source_info.scope.index() >= ivs.len() {
 249                     return None;
 250                 }
 251                 ivs[source_info.scope].lint_root
 252             },
 253             ClearCrossCrate::Clear => return None,
 254         };
 255         let r = match f(self) {
 256             Ok(val) => Some(val),
 257             Err(error) => {
 258                 let diagnostic = error_to_const_error(&self.ecx, error);
 259                 use rustc::mir::interpret::InterpError::*;
 260                 match diagnostic.error {
 261                     Exit(_) => bug!("the CTFE program cannot exit"),
 262                     | Unsupported(_) => {},
 263                     | UndefinedBehaviour(_) => {},
 264                     | InvalidProgram(_) => {},
 265                     | ResourceExhaustion(_) => {},
 266                     | Panic(_)
 267                     => {
 268                         diagnostic.report_as_lint(
 269                             self.ecx.tcx,
 270                             "this expression will panic at runtime",
 271                             lint_root,
 272                             None,
 273                         );
 274                     }
 275                 }
 276                 None
 277             },
 278         };
 279         self.ecx.tcx.span = DUMMY_SP;
 280         r
 281     }
 282
 283     fn eval_constant(
 284         &mut self,
 285         c: &Constant<'tcx>,
 286     ) -> Option<Const<'tcx>> {
 287         self.ecx.tcx.span = c.span;
 288         match self.ecx.eval_const_to_op(c.literal, None) {
 289             Ok(op) => {
 290                 Some(op)
 291             },
 292             Err(error) => {
 293                 let err = error_to_const_error(&self.ecx, error);
 294                 err.report_as_error(self.ecx.tcx, "erroneous constant used");
 295                 None
 296             },
 297         }
 298     }
 299
 300     fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 301         trace!("eval_place(place={:?})", place);
 302         place.iterate(|place_base, place_projection| {
 303             let mut eval = match place_base {
 304                 PlaceBase::Local(loc) => self.get_const(*loc).clone()?,
 305                 PlaceBase::Static(box Static {kind: StaticKind::Promoted(promoted), ..}) => {
 306                     let generics = self.tcx.generics_of(self.source.def_id());
 307                     if generics.requires_monomorphization(self.tcx) {
 308                         // FIXME: can't handle code with generics
 309                         return None;
 310                     }
 311                     let substs = InternalSubsts::identity_for_item(self.tcx, self.source.def_id());
 312                     let instance = Instance::new(self.source.def_id(), substs);
 313                     let cid = GlobalId {
 314                         instance,
 315                         promoted: Some(*promoted),
 316                     };
 317                     // cannot use `const_eval` here, because that would require having the MIR
 318                     // for the current function available, but we're producing said MIR right now
 319                     let res = self.use_ecx(source_info, |this| {
 320                         let body = &this.promoted[*promoted];
 321                         eval_promoted(this.tcx, cid, body, this.param_env)
 322                     })?;
 323                     trace!("evaluated promoted {:?} to {:?}", promoted, res);
 324                     res.into()
 325                 }
 326                 _ => return None,
 327             };
 328
 329             for proj in place_projection {
 330                 match proj.elem {
 331                     ProjectionElem::Field(field, _) => {
 332                         trace!("field proj on {:?}", proj.base);
 333                         eval = self.use_ecx(source_info, |this| {
 334                             this.ecx.operand_field(eval, field.index() as u64)
 335                         })?;
 336                     },
 337                     ProjectionElem::Deref => {
 338                         trace!("processing deref");
 339                         eval = self.use_ecx(source_info, |this| {
 340                             this.ecx.deref_operand(eval)
 341                         })?.into();
 342                     }
 343                     // We could get more projections by using e.g., `operand_projection`,
 344                     // but we do not even have the stack frame set up properly so
 345                     // an `Index` projection would throw us off-track.
 346                     _ => return None,
 347                 }
 348             }
 349
 350             Some(eval)
 351         })
 352     }
 353
 354     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 355         match *op {
 356             Operand::Constant(ref c) => self.eval_constant(c),
 357             | Operand::Move(ref place)
 358             | Operand::Copy(ref place) => self.eval_place(place, source_info),
 359         }
 360     }
 361
 362     fn const_prop(
 363         &mut self,
 364         rvalue: &Rvalue<'tcx>,
 365         place_layout: TyLayout<'tcx>,
 366         source_info: SourceInfo,
 367     ) -> Option<Const<'tcx>> {
 368         let span = source_info.span;
 369         match *rvalue {
 370             Rvalue::Use(ref op) => {
 371                 self.eval_operand(op, source_info)
 372             },
 373             Rvalue::Ref(_, _, ref place) => {
 374                 let src = self.eval_place(place, source_info)?;
 375                 let mplace = src.try_as_mplace().ok()?;
 376                 Some(ImmTy::from_scalar(mplace.ptr.into(), place_layout).into())
 377             },
 378             Rvalue::Repeat(..) |
 379             Rvalue::Aggregate(..) |
 380             Rvalue::NullaryOp(NullOp::Box, _) |
 381             Rvalue::Discriminant(..) => None,
 382
 383             Rvalue::Cast(kind, ref operand, _) => {
 384                 let op = self.eval_operand(operand, source_info)?;
 385                 self.use_ecx(source_info, |this| {
 386                     let dest = this.ecx.allocate(place_layout, MemoryKind::Stack);
 387                     this.ecx.cast(op, kind, dest.into())?;
 388                     Ok(dest.into())
 389                 })
 390             },
 391             Rvalue::Len(ref place) => {
 392                 let place = self.eval_place(&place, source_info)?;
 393                 let mplace = place.try_as_mplace().ok()?;
 394
 395                 if let ty::Slice(_) = mplace.layout.ty.sty {
 396                     let len = mplace.meta.unwrap().to_usize(&self.ecx).unwrap();
 397
 398                     Some(ImmTy {
 399                         imm: Immediate::Scalar(
 400                             Scalar::from_uint(
 401                                 len,
 402                                 Size::from_bits(
 403                                     self.tcx.sess.target.usize_ty.bit_width().unwrap() as u64
 404                                 )
 405                             ).into(),
 406                         ),
 407                         layout: self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
 408                     }.into())
 409                 } else {
 410                     trace!("not slice: {:?}", mplace.layout.ty.sty);
 411                     None
 412                 }
 413             },
 414             Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
 415                 type_size_of(self.tcx, self.param_env, ty).and_then(|n| Some(
 416                     ImmTy {
 417                         imm: Immediate::Scalar(
 418                             Scalar::from_uint(n, self.tcx.data_layout.pointer_size).into()
 419                         ),
 420                         layout: self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
 421                     }.into()
 422                 ))
 423             }
 424             Rvalue::UnaryOp(op, ref arg) => {
 425                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 426                     self.tcx.closure_base_def_id(self.source.def_id())
 427                 } else {
 428                     self.source.def_id()
 429                 };
 430                 let generics = self.tcx.generics_of(def_id);
 431                 if generics.requires_monomorphization(self.tcx) {
 432                     // FIXME: can't handle code with generics
 433                     return None;
 434                 }
 435
 436                 let arg = self.eval_operand(arg, source_info)?;
 437                 let val = self.use_ecx(source_info, |this| {
 438                     let prim = this.ecx.read_immediate(arg)?;
 439                     match op {
 440                         UnOp::Neg => {
 441                             // Need to do overflow check here: For actual CTFE, MIR
 442                             // generation emits code that does this before calling the op.
 443                             if prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
 444                                 throw_panic!(OverflowNeg)
 445                             }
 446                         }
 447                         UnOp::Not => {
 448                             // Cannot overflow
 449                         }
 450                     }
 451                     // Now run the actual operation.
 452                     this.ecx.unary_op(op, prim)
 453                 })?;
 454                 let res = ImmTy {
 455                     imm: Immediate::Scalar(val.into()),
 456                     layout: place_layout,
 457                 };
 458                 Some(res.into())
 459             }
 460             Rvalue::CheckedBinaryOp(op, ref left, ref right) |
 461             Rvalue::BinaryOp(op, ref left, ref right) => {
 462                 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
 463                 let right = self.eval_operand(right, source_info)?;
 464                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 465                     self.tcx.closure_base_def_id(self.source.def_id())
 466                 } else {
 467                     self.source.def_id()
 468                 };
 469                 let generics = self.tcx.generics_of(def_id);
 470                 if generics.requires_monomorphization(self.tcx) {
 471                     // FIXME: can't handle code with generics
 472                     return None;
 473                 }
 474
 475                 let r = self.use_ecx(source_info, |this| {
 476                     this.ecx.read_immediate(right)
 477                 })?;
 478                 if op == BinOp::Shr || op == BinOp::Shl {
 479                     let left_ty = left.ty(&self.local_decls, self.tcx);
 480                     let left_bits = self
 481                         .tcx
 482                         .layout_of(self.param_env.and(left_ty))
 483                         .unwrap()
 484                         .size
 485                         .bits();
 486                     let right_size = right.layout.size;
 487                     let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
 488                     if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
 489                         let source_scope_local_data = match self.source_scope_local_data {
 490                             ClearCrossCrate::Set(ref data) => data,
 491                             ClearCrossCrate::Clear => return None,
 492                         };
 493                         let dir = if op == BinOp::Shr {
 494                             "right"
 495                         } else {
 496                             "left"
 497                         };
 498                         let hir_id = source_scope_local_data[source_info.scope].lint_root;
 499                         self.tcx.lint_hir(
 500                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 501                             hir_id,
 502                             span,
 503                             &format!("attempt to shift {} with overflow", dir));
 504                         return None;
 505                     }
 506                 }
 507                 let left = self.eval_operand(left, source_info)?;
 508                 let l = self.use_ecx(source_info, |this| {
 509                     this.ecx.read_immediate(left)
 510                 })?;
 511                 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
 512                 let (val, overflow) = self.use_ecx(source_info, |this| {
 513                     this.ecx.binary_op(op, l, r)
 514                 })?;
 515                 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
 516                     Immediate::ScalarPair(
 517                         val.into(),
 518                         Scalar::from_bool(overflow).into(),
 519                     )
 520                 } else {
 521                     if overflow {
 522                         let err = InterpError::Panic(PanicInfo::Overflow(op)).into();
 523                         let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
 524                         return None;
 525                     }
 526                     Immediate::Scalar(val.into())
 527                 };
 528                 let res = ImmTy {
 529                     imm: val,
 530                     layout: place_layout,
 531                 };
 532                 Some(res.into())
 533             },
 534         }
 535     }
 536
 537     fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
 538         Operand::Constant(Box::new(
 539             Constant {
 540                 span,
 541                 ty,
 542                 user_ty: None,
 543                 literal: self.tcx.mk_const(*ty::Const::from_scalar(
 544                     self.tcx,
 545                     scalar,
 546                     ty,
 547                 ))
 548             }
 549         ))
 550     }
 551
 552     fn replace_with_const(
 553         &mut self,
 554         rval: &mut Rvalue<'tcx>,
 555         value: Const<'tcx>,
 556         source_info: SourceInfo,
 557     ) {
 558         trace!("attepting to replace {:?} with {:?}", rval, value);
 559         if let Err(e) = self.ecx.validate_operand(
 560             value,
 561             vec![],
 562             // FIXME: is ref tracking too expensive?
 563             Some(&mut interpret::RefTracking::empty()),
 564         ) {
 565             trace!("validation error, attempt failed: {:?}", e);
 566             return;
 567         }
 568
 569         // FIXME> figure out what tho do when try_read_immediate fails
 570         let imm = self.use_ecx(source_info, |this| {
 571             this.ecx.try_read_immediate(value)
 572         });
 573
 574         if let Some(Ok(imm)) = imm {
 575             match *imm {
 576                 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
 577                     *rval = Rvalue::Use(
 578                         self.operand_from_scalar(scalar, value.layout.ty, source_info.span));
 579                 },
 580                 Immediate::ScalarPair(
 581                     ScalarMaybeUndef::Scalar(one),
 582                     ScalarMaybeUndef::Scalar(two)
 583                 ) => {
 584                     let ty = &value.layout.ty.sty;
 585                     if let ty::Tuple(substs) = ty {
 586                         *rval = Rvalue::Aggregate(
 587                             Box::new(AggregateKind::Tuple),
 588                             vec![
 589                                 self.operand_from_scalar(
 590                                     one, substs[0].expect_ty(), source_info.span
 591                                 ),
 592                                 self.operand_from_scalar(
 593                                     two, substs[1].expect_ty(), source_info.span
 594                                 ),
 595                             ],
 596                         );
 597                     }
 598                 },
 599                 _ => { }
 600             }
 601         }
 602     }
 603
 604     fn should_const_prop(&self) -> bool {
 605         self.tcx.sess.opts.debugging_opts.mir_opt_level >= 2
 606     }
 607 }
 608
 609 fn type_size_of<'tcx>(
 610     tcx: TyCtxt<'tcx>,
 611     param_env: ty::ParamEnv<'tcx>,
 612     ty: Ty<'tcx>,
 613 ) -> Option<u64> {
 614     tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
 615 }
 616
 617 struct CanConstProp {
 618     can_const_prop: IndexVec<Local, bool>,
 619     // false at the beginning, once set, there are not allowed to be any more assignments
 620     found_assignment: IndexVec<Local, bool>,
 621 }
 622
 623 impl CanConstProp {
 624     /// returns true if `local` can be propagated
 625     fn check(body: &Body<'_>) -> IndexVec<Local, bool> {
 626         let mut cpv = CanConstProp {
 627             can_const_prop: IndexVec::from_elem(true, &body.local_decls),
 628             found_assignment: IndexVec::from_elem(false, &body.local_decls),
 629         };
 630         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 631             // cannot use args at all
 632             // cannot use locals because if x < y { y - x } else { x - y } would
 633             //        lint for x != y
 634             // FIXME(oli-obk): lint variables until they are used in a condition
 635             // FIXME(oli-obk): lint if return value is constant
 636             *val = body.local_kind(local) == LocalKind::Temp;
 637
 638             if !*val {
 639                 trace!("local {:?} can't be propagated because it's not a temporary", local);
 640             }
 641         }
 642         cpv.visit_body(body);
 643         cpv.can_const_prop
 644     }
 645 }
 646
 647 impl<'tcx> Visitor<'tcx> for CanConstProp {
 648     fn visit_local(
 649         &mut self,
 650         &local: &Local,
 651         context: PlaceContext,
 652         _: Location,
 653     ) {
 654         use rustc::mir::visit::PlaceContext::*;
 655         match context {
 656             // Constants must have at most one write
 657             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 658             // only occur in independent execution paths
 659             MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
 660                 trace!("local {:?} can't be propagated because of multiple assignments", local);
 661                 self.can_const_prop[local] = false;
 662             } else {
 663                 self.found_assignment[local] = true
 664             },
 665             // Reading constants is allowed an arbitrary number of times
 666             NonMutatingUse(NonMutatingUseContext::Copy) |
 667             NonMutatingUse(NonMutatingUseContext::Move) |
 668             NonMutatingUse(NonMutatingUseContext::Inspect) |
 669             NonMutatingUse(NonMutatingUseContext::Projection) |
 670             MutatingUse(MutatingUseContext::Projection) |
 671             NonUse(_) => {},
 672             _ => {
 673                 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
 674                 self.can_const_prop[local] = false;
 675             },
 676         }
 677     }
 678 }
 679
 680 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
 681     fn visit_constant(
 682         &mut self,
 683         constant: &mut Constant<'tcx>,
 684         location: Location,
 685     ) {
 686         trace!("visit_constant: {:?}", constant);
 687         self.super_constant(constant, location);
 688         self.eval_constant(constant);
 689     }
 690
 691     fn visit_statement(
 692         &mut self,
 693         statement: &mut Statement<'tcx>,
 694         location: Location,
 695     ) {
 696         trace!("visit_statement: {:?}", statement);
 697         if let StatementKind::Assign(ref place, ref mut rval) = statement.kind {
 698             let place_ty: Ty<'tcx> = place
 699                 .ty(&self.local_decls, self.tcx)
 700                 .ty;
 701             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 702                 if let Some(value) = self.const_prop(rval, place_layout, statement.source_info) {
 703                     if let Place {
 704                         base: PlaceBase::Local(local),
 705                         projection: None,
 706                     } = *place {
 707                         trace!("checking whether {:?} can be stored to {:?}", value, local);
 708                         if self.can_const_prop[local] {
 709                             trace!("storing {:?} to {:?}", value, local);
 710                             assert!(self.get_const(local).is_none());
 711                             self.set_const(local, value);
 712
 713                             if self.should_const_prop() {
 714                                 self.replace_with_const(
 715                                     rval,
 716                                     value,
 717                                     statement.source_info,
 718                                 );
 719                             }
 720                         }
 721                     }
 722                 }
 723             }
 724         }
 725         self.super_statement(statement, location);
 726     }
 727
 728     fn visit_terminator(
 729         &mut self,
 730         terminator: &mut Terminator<'tcx>,
 731         location: Location,
 732     ) {
 733         self.super_terminator(terminator, location);
 734         let source_info = terminator.source_info;
 735         match &mut terminator.kind {
 736             TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
 737                 if let Some(value) = self.eval_operand(&cond, source_info) {
 738                     trace!("assertion on {:?} should be {:?}", value, expected);
 739                     let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
 740                     let value_const = self.ecx.read_scalar(value).unwrap();
 741                     if expected != value_const {
 742                         // poison all places this operand references so that further code
 743                         // doesn't use the invalid value
 744                         match cond {
 745                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 746                                 if let PlaceBase::Local(local) = place.base {
 747                                     self.remove_const(local);
 748                                 }
 749                             },
 750                             Operand::Constant(_) => {}
 751                         }
 752                         let span = terminator.source_info.span;
 753                         let hir_id = self
 754                             .tcx
 755                             .hir()
 756                             .as_local_hir_id(self.source.def_id())
 757                             .expect("some part of a failing const eval must be local");
 758                         let msg = match msg {
 759                             PanicInfo::Overflow(_) |
 760                             PanicInfo::OverflowNeg |
 761                             PanicInfo::DivisionByZero |
 762                             PanicInfo::RemainderByZero =>
 763                                 msg.description().to_owned(),
 764                             PanicInfo::BoundsCheck { ref len, ref index } => {
 765                                 let len = self
 766                                     .eval_operand(len, source_info)
 767                                     .expect("len must be const");
 768                                 let len = match self.ecx.read_scalar(len) {
 769                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 770                                         data, ..
 771                                     })) => data,
 772                                     other => bug!("const len not primitive: {:?}", other),
 773                                 };
 774                                 let index = self
 775                                     .eval_operand(index, source_info)
 776                                     .expect("index must be const");
 777                                 let index = match self.ecx.read_scalar(index) {
 778                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 779                                         data, ..
 780                                     })) => data,
 781                                     other => bug!("const index not primitive: {:?}", other),
 782                                 };
 783                                 format!(
 784                                     "index out of bounds: \
 785                                     the len is {} but the index is {}",
 786                                     len,
 787                                     index,
 788                                 )
 789                             },
 790                             // Need proper const propagator for these
 791                             _ => return,
 792                         };
 793                         self.tcx.lint_hir(
 794                             ::rustc::lint::builtin::CONST_ERR,
 795                             hir_id,
 796                             span,
 797                             &msg,
 798                         );
 799                     } else {
 800                         if self.should_const_prop() {
 801                             if let ScalarMaybeUndef::Scalar(scalar) = value_const {
 802                                 *cond = self.operand_from_scalar(
 803                                     scalar,
 804                                     self.tcx.types.bool,
 805                                     source_info.span,
 806                                 );
 807                             }
 808                         }
 809                     }
 810                 }
 811             },
 812             TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
 813                 if self.should_const_prop() {
 814                     if let Some(value) = self.eval_operand(&discr, source_info) {
 815                         if let ScalarMaybeUndef::Scalar(scalar) =
 816                                 self.ecx.read_scalar(value).unwrap() {
 817                             *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
 818                         }
 819                     }
 820                 }
 821             },
 822             //none of these have Operands to const-propagate
 823             TerminatorKind::Goto { .. } |
 824             TerminatorKind::Resume |
 825             TerminatorKind::Abort |
 826             TerminatorKind::Return |
 827             TerminatorKind::Unreachable |
 828             TerminatorKind::Drop { .. } |
 829             TerminatorKind::DropAndReplace { .. } |
 830             TerminatorKind::Yield { .. } |
 831             TerminatorKind::GeneratorDrop |
 832             TerminatorKind::FalseEdges { .. } |
 833             TerminatorKind::FalseUnwind { .. } => { }
 834             //FIXME(wesleywiser) Call does have Operands that could be const-propagated
 835             TerminatorKind::Call { .. } => { }
 836         }
 837     }
 838 }