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