src/librustc_mir/transform/const_prop.rs

   1 //! Propagates constants for early reporting of statically known
   2 //! assertion failures
   3
   4 use std::cell::Cell;
   5
   6 use rustc::hir::def::DefKind;
   7 use rustc::mir::{
   8     AggregateKind, Constant, Location, Place, PlaceBase, Body, Operand, Rvalue,
   9     Local, NullOp, UnOp, StatementKind, Statement, LocalKind, Static, StaticKind,
  10     TerminatorKind, Terminator,  ClearCrossCrate, SourceInfo, BinOp, ProjectionElem,
  11     SourceScope, SourceScopeLocalData, LocalDecl, Promoted,
  12 };
  13 use rustc::mir::visit::{
  14     Visitor, PlaceContext, MutatingUseContext, MutVisitor, NonMutatingUseContext,
  15 };
  16 use rustc::mir::interpret::{InterpError::Panic, Scalar, GlobalId, InterpResult, PanicMessage};
  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                     // don't report these, they make no sense in a const prop context
 262                     | MachineError(_)
 263                     | Exit(_)
 264                     // at runtime these transformations might make sense
 265                     // FIXME: figure out the rules and start linting
 266                     | FunctionAbiMismatch(..)
 267                     | FunctionArgMismatch(..)
 268                     | FunctionRetMismatch(..)
 269                     | FunctionArgCountMismatch
 270                     // fine at runtime, might be a register address or sth
 271                     | ReadBytesAsPointer
 272                     // fine at runtime
 273                     | ReadForeignStatic
 274                     | Unimplemented(_)
 275                     // don't report const evaluator limits
 276                     | StackFrameLimitReached
 277                     | NoMirFor(..)
 278                     | InlineAsm
 279                     => {},
 280
 281                     | InvalidMemoryAccess
 282                     | DanglingPointerDeref
 283                     | DoubleFree
 284                     | InvalidFunctionPointer
 285                     | InvalidBool
 286                     | InvalidDiscriminant(..)
 287                     | PointerOutOfBounds { .. }
 288                     | InvalidNullPointerUsage
 289                     | ValidationFailure(..)
 290                     | InvalidPointerMath
 291                     | ReadUndefBytes(_)
 292                     | DeadLocal
 293                     | InvalidBoolOp(_)
 294                     | DerefFunctionPointer
 295                     | ExecuteMemory
 296                     | Intrinsic(..)
 297                     | InvalidChar(..)
 298                     | AbiViolation(_)
 299                     | AlignmentCheckFailed{..}
 300                     | CalledClosureAsFunction
 301                     | VtableForArgumentlessMethod
 302                     | ModifiedConstantMemory
 303                     | ModifiedStatic
 304                     | AssumptionNotHeld
 305                     // FIXME: should probably be removed and turned into a bug! call
 306                     | TypeNotPrimitive(_)
 307                     | ReallocatedWrongMemoryKind(_, _)
 308                     | DeallocatedWrongMemoryKind(_, _)
 309                     | ReallocateNonBasePtr
 310                     | DeallocateNonBasePtr
 311                     | IncorrectAllocationInformation(..)
 312                     | UnterminatedCString(_)
 313                     | HeapAllocZeroBytes
 314                     | HeapAllocNonPowerOfTwoAlignment(_)
 315                     | Unreachable
 316                     | ReadFromReturnPointer
 317                     | GeneratorResumedAfterReturn
 318                     | GeneratorResumedAfterPanic
 319                     | ReferencedConstant
 320                     | InfiniteLoop
 321                     => {
 322                         // FIXME: report UB here
 323                     },
 324
 325                     | OutOfTls
 326                     | TlsOutOfBounds
 327                     | PathNotFound(_)
 328                     => bug!("these should not be in rustc, but in miri's machine errors"),
 329
 330                     | Layout(_)
 331                     | UnimplementedTraitSelection
 332                     | TypeckError
 333                     | TooGeneric
 334                     // these are just noise
 335                     => {},
 336
 337                     // non deterministic
 338                     | ReadPointerAsBytes
 339                     // FIXME: implement
 340                     => {},
 341
 342                     | Panic(_)
 343                     => {
 344                         diagnostic.report_as_lint(
 345                             self.ecx.tcx,
 346                             "this expression will panic at runtime",
 347                             lint_root,
 348                             None,
 349                         );
 350                     }
 351                 }
 352                 None
 353             },
 354         };
 355         self.ecx.tcx.span = DUMMY_SP;
 356         r
 357     }
 358
 359     fn eval_constant(
 360         &mut self,
 361         c: &Constant<'tcx>,
 362     ) -> Option<Const<'tcx>> {
 363         self.ecx.tcx.span = c.span;
 364         match self.ecx.eval_const_to_op(c.literal, None) {
 365             Ok(op) => {
 366                 Some(op)
 367             },
 368             Err(error) => {
 369                 let err = error_to_const_error(&self.ecx, error);
 370                 err.report_as_error(self.ecx.tcx, "erroneous constant used");
 371                 None
 372             },
 373         }
 374     }
 375
 376     fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 377         trace!("eval_place(place={:?})", place);
 378         place.iterate(|place_base, place_projection| {
 379             let mut eval = match place_base {
 380                 PlaceBase::Local(loc) => self.get_const(*loc).clone()?,
 381                 PlaceBase::Static(box Static {kind: StaticKind::Promoted(promoted), ..}) => {
 382                     let generics = self.tcx.generics_of(self.source.def_id());
 383                     if generics.requires_monomorphization(self.tcx) {
 384                         // FIXME: can't handle code with generics
 385                         return None;
 386                     }
 387                     let substs = InternalSubsts::identity_for_item(self.tcx, self.source.def_id());
 388                     let instance = Instance::new(self.source.def_id(), substs);
 389                     let cid = GlobalId {
 390                         instance,
 391                         promoted: Some(*promoted),
 392                     };
 393                     // cannot use `const_eval` here, because that would require having the MIR
 394                     // for the current function available, but we're producing said MIR right now
 395                     let res = self.use_ecx(source_info, |this| {
 396                         let body = &this.promoted[*promoted];
 397                         eval_promoted(this.tcx, cid, body, this.param_env)
 398                     })?;
 399                     trace!("evaluated promoted {:?} to {:?}", promoted, res);
 400                     res.into()
 401                 }
 402                 _ => return None,
 403             };
 404
 405             for proj in place_projection {
 406                 match proj.elem {
 407                     ProjectionElem::Field(field, _) => {
 408                         trace!("field proj on {:?}", proj.base);
 409                         eval = self.use_ecx(source_info, |this| {
 410                             this.ecx.operand_field(eval, field.index() as u64)
 411                         })?;
 412                     },
 413                     ProjectionElem::Deref => {
 414                         trace!("processing deref");
 415                         eval = self.use_ecx(source_info, |this| {
 416                             this.ecx.deref_operand(eval)
 417                         })?.into();
 418                     }
 419                     // We could get more projections by using e.g., `operand_projection`,
 420                     // but we do not even have the stack frame set up properly so
 421                     // an `Index` projection would throw us off-track.
 422                     _ => return None,
 423                 }
 424             }
 425
 426             Some(eval)
 427         })
 428     }
 429
 430     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 431         match *op {
 432             Operand::Constant(ref c) => self.eval_constant(c),
 433             | Operand::Move(ref place)
 434             | Operand::Copy(ref place) => self.eval_place(place, source_info),
 435         }
 436     }
 437
 438     fn const_prop(
 439         &mut self,
 440         rvalue: &Rvalue<'tcx>,
 441         place_layout: TyLayout<'tcx>,
 442         source_info: SourceInfo,
 443     ) -> Option<Const<'tcx>> {
 444         let span = source_info.span;
 445         match *rvalue {
 446             Rvalue::Use(ref op) => {
 447                 self.eval_operand(op, source_info)
 448             },
 449             Rvalue::Ref(_, _, ref place) => {
 450                 let src = self.eval_place(place, source_info)?;
 451                 let mplace = src.try_as_mplace().ok()?;
 452                 Some(ImmTy::from_scalar(mplace.ptr.into(), place_layout).into())
 453             },
 454             Rvalue::Repeat(..) |
 455             Rvalue::Aggregate(..) |
 456             Rvalue::NullaryOp(NullOp::Box, _) |
 457             Rvalue::Discriminant(..) => None,
 458
 459             Rvalue::Cast(kind, ref operand, _) => {
 460                 let op = self.eval_operand(operand, source_info)?;
 461                 self.use_ecx(source_info, |this| {
 462                     let dest = this.ecx.allocate(place_layout, MemoryKind::Stack);
 463                     this.ecx.cast(op, kind, dest.into())?;
 464                     Ok(dest.into())
 465                 })
 466             },
 467             Rvalue::Len(ref place) => {
 468                 let place = self.eval_place(&place, source_info)?;
 469                 let mplace = place.try_as_mplace().ok()?;
 470
 471                 if let ty::Slice(_) = mplace.layout.ty.sty {
 472                     let len = mplace.meta.unwrap().to_usize(&self.ecx).unwrap();
 473
 474                     Some(ImmTy {
 475                         imm: Immediate::Scalar(
 476                             Scalar::from_uint(
 477                                 len,
 478                                 Size::from_bits(
 479                                     self.tcx.sess.target.usize_ty.bit_width().unwrap() as u64
 480                                 )
 481                             ).into(),
 482                         ),
 483                         layout: self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
 484                     }.into())
 485                 } else {
 486                     trace!("not slice: {:?}", mplace.layout.ty.sty);
 487                     None
 488                 }
 489             },
 490             Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
 491                 type_size_of(self.tcx, self.param_env, ty).and_then(|n| Some(
 492                     ImmTy {
 493                         imm: Immediate::Scalar(
 494                             Scalar::from_uint(n, self.tcx.data_layout.pointer_size).into()
 495                         ),
 496                         layout: self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
 497                     }.into()
 498                 ))
 499             }
 500             Rvalue::UnaryOp(op, ref arg) => {
 501                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 502                     self.tcx.closure_base_def_id(self.source.def_id())
 503                 } else {
 504                     self.source.def_id()
 505                 };
 506                 let generics = self.tcx.generics_of(def_id);
 507                 if generics.requires_monomorphization(self.tcx) {
 508                     // FIXME: can't handle code with generics
 509                     return None;
 510                 }
 511
 512                 let arg = self.eval_operand(arg, source_info)?;
 513                 let val = self.use_ecx(source_info, |this| {
 514                     let prim = this.ecx.read_immediate(arg)?;
 515                     match op {
 516                         UnOp::Neg => {
 517                             // Need to do overflow check here: For actual CTFE, MIR
 518                             // generation emits code that does this before calling the op.
 519                             if prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
 520                                 return err!(Panic(PanicMessage::OverflowNeg));
 521                             }
 522                         }
 523                         UnOp::Not => {
 524                             // Cannot overflow
 525                         }
 526                     }
 527                     // Now run the actual operation.
 528                     this.ecx.unary_op(op, prim)
 529                 })?;
 530                 let res = ImmTy {
 531                     imm: Immediate::Scalar(val.into()),
 532                     layout: place_layout,
 533                 };
 534                 Some(res.into())
 535             }
 536             Rvalue::CheckedBinaryOp(op, ref left, ref right) |
 537             Rvalue::BinaryOp(op, ref left, ref right) => {
 538                 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
 539                 let right = self.eval_operand(right, source_info)?;
 540                 let def_id = if self.tcx.is_closure(self.source.def_id()) {
 541                     self.tcx.closure_base_def_id(self.source.def_id())
 542                 } else {
 543                     self.source.def_id()
 544                 };
 545                 let generics = self.tcx.generics_of(def_id);
 546                 if generics.requires_monomorphization(self.tcx) {
 547                     // FIXME: can't handle code with generics
 548                     return None;
 549                 }
 550
 551                 let r = self.use_ecx(source_info, |this| {
 552                     this.ecx.read_immediate(right)
 553                 })?;
 554                 if op == BinOp::Shr || op == BinOp::Shl {
 555                     let left_ty = left.ty(&self.local_decls, self.tcx);
 556                     let left_bits = self
 557                         .tcx
 558                         .layout_of(self.param_env.and(left_ty))
 559                         .unwrap()
 560                         .size
 561                         .bits();
 562                     let right_size = right.layout.size;
 563                     let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
 564                     if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
 565                         let source_scope_local_data = match self.source_scope_local_data {
 566                             ClearCrossCrate::Set(ref data) => data,
 567                             ClearCrossCrate::Clear => return None,
 568                         };
 569                         let dir = if op == BinOp::Shr {
 570                             "right"
 571                         } else {
 572                             "left"
 573                         };
 574                         let hir_id = source_scope_local_data[source_info.scope].lint_root;
 575                         self.tcx.lint_hir(
 576                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 577                             hir_id,
 578                             span,
 579                             &format!("attempt to shift {} with overflow", dir));
 580                         return None;
 581                     }
 582                 }
 583                 let left = self.eval_operand(left, source_info)?;
 584                 let l = self.use_ecx(source_info, |this| {
 585                     this.ecx.read_immediate(left)
 586                 })?;
 587                 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
 588                 let (val, overflow) = self.use_ecx(source_info, |this| {
 589                     this.ecx.binary_op(op, l, r)
 590                 })?;
 591                 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
 592                     Immediate::ScalarPair(
 593                         val.into(),
 594                         Scalar::from_bool(overflow).into(),
 595                     )
 596                 } else {
 597                     if overflow {
 598                         let err = Panic(PanicMessage::Overflow(op)).into();
 599                         let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
 600                         return None;
 601                     }
 602                     Immediate::Scalar(val.into())
 603                 };
 604                 let res = ImmTy {
 605                     imm: val,
 606                     layout: place_layout,
 607                 };
 608                 Some(res.into())
 609             },
 610         }
 611     }
 612
 613     fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
 614         Operand::Constant(Box::new(
 615             Constant {
 616                 span,
 617                 ty,
 618                 user_ty: None,
 619                 literal: self.tcx.mk_const(*ty::Const::from_scalar(
 620                     self.tcx,
 621                     scalar,
 622                     ty,
 623                 ))
 624             }
 625         ))
 626     }
 627
 628     fn replace_with_const(
 629         &mut self,
 630         rval: &mut Rvalue<'tcx>,
 631         value: Const<'tcx>,
 632         source_info: SourceInfo,
 633     ) {
 634         trace!("attepting to replace {:?} with {:?}", rval, value);
 635         if let Err(e) = self.ecx.validate_operand(
 636             value,
 637             vec![],
 638             // FIXME: is ref tracking too expensive?
 639             Some(&mut interpret::RefTracking::empty()),
 640         ) {
 641             trace!("validation error, attempt failed: {:?}", e);
 642             return;
 643         }
 644
 645         // FIXME> figure out what tho do when try_read_immediate fails
 646         let imm = self.use_ecx(source_info, |this| {
 647             this.ecx.try_read_immediate(value)
 648         });
 649
 650         if let Some(Ok(imm)) = imm {
 651             match *imm {
 652                 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
 653                     *rval = Rvalue::Use(
 654                         self.operand_from_scalar(scalar, value.layout.ty, source_info.span));
 655                 },
 656                 Immediate::ScalarPair(
 657                     ScalarMaybeUndef::Scalar(one),
 658                     ScalarMaybeUndef::Scalar(two)
 659                 ) => {
 660                     let ty = &value.layout.ty.sty;
 661                     if let ty::Tuple(substs) = ty {
 662                         *rval = Rvalue::Aggregate(
 663                             Box::new(AggregateKind::Tuple),
 664                             vec![
 665                                 self.operand_from_scalar(
 666                                     one, substs[0].expect_ty(), source_info.span
 667                                 ),
 668                                 self.operand_from_scalar(
 669                                     two, substs[1].expect_ty(), source_info.span
 670                                 ),
 671                             ],
 672                         );
 673                     }
 674                 },
 675                 _ => { }
 676             }
 677         }
 678     }
 679
 680     fn should_const_prop(&self) -> bool {
 681         self.tcx.sess.opts.debugging_opts.mir_opt_level >= 2
 682     }
 683 }
 684
 685 fn type_size_of<'tcx>(
 686     tcx: TyCtxt<'tcx>,
 687     param_env: ty::ParamEnv<'tcx>,
 688     ty: Ty<'tcx>,
 689 ) -> Option<u64> {
 690     tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
 691 }
 692
 693 struct CanConstProp {
 694     can_const_prop: IndexVec<Local, bool>,
 695     // false at the beginning, once set, there are not allowed to be any more assignments
 696     found_assignment: IndexVec<Local, bool>,
 697 }
 698
 699 impl CanConstProp {
 700     /// returns true if `local` can be propagated
 701     fn check(body: &Body<'_>) -> IndexVec<Local, bool> {
 702         let mut cpv = CanConstProp {
 703             can_const_prop: IndexVec::from_elem(true, &body.local_decls),
 704             found_assignment: IndexVec::from_elem(false, &body.local_decls),
 705         };
 706         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 707             // cannot use args at all
 708             // cannot use locals because if x < y { y - x } else { x - y } would
 709             //        lint for x != y
 710             // FIXME(oli-obk): lint variables until they are used in a condition
 711             // FIXME(oli-obk): lint if return value is constant
 712             *val = body.local_kind(local) == LocalKind::Temp;
 713
 714             if !*val {
 715                 trace!("local {:?} can't be propagated because it's not a temporary", local);
 716             }
 717         }
 718         cpv.visit_body(body);
 719         cpv.can_const_prop
 720     }
 721 }
 722
 723 impl<'tcx> Visitor<'tcx> for CanConstProp {
 724     fn visit_local(
 725         &mut self,
 726         &local: &Local,
 727         context: PlaceContext,
 728         _: Location,
 729     ) {
 730         use rustc::mir::visit::PlaceContext::*;
 731         match context {
 732             // Constants must have at most one write
 733             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 734             // only occur in independent execution paths
 735             MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
 736                 trace!("local {:?} can't be propagated because of multiple assignments", local);
 737                 self.can_const_prop[local] = false;
 738             } else {
 739                 self.found_assignment[local] = true
 740             },
 741             // Reading constants is allowed an arbitrary number of times
 742             NonMutatingUse(NonMutatingUseContext::Copy) |
 743             NonMutatingUse(NonMutatingUseContext::Move) |
 744             NonMutatingUse(NonMutatingUseContext::Inspect) |
 745             NonMutatingUse(NonMutatingUseContext::Projection) |
 746             MutatingUse(MutatingUseContext::Projection) |
 747             NonUse(_) => {},
 748             _ => {
 749                 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
 750                 self.can_const_prop[local] = false;
 751             },
 752         }
 753     }
 754 }
 755
 756 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
 757     fn visit_constant(
 758         &mut self,
 759         constant: &mut Constant<'tcx>,
 760         location: Location,
 761     ) {
 762         trace!("visit_constant: {:?}", constant);
 763         self.super_constant(constant, location);
 764         self.eval_constant(constant);
 765     }
 766
 767     fn visit_statement(
 768         &mut self,
 769         statement: &mut Statement<'tcx>,
 770         location: Location,
 771     ) {
 772         trace!("visit_statement: {:?}", statement);
 773         if let StatementKind::Assign(ref place, ref mut rval) = statement.kind {
 774             let place_ty: Ty<'tcx> = place
 775                 .ty(&self.local_decls, self.tcx)
 776                 .ty;
 777             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 778                 if let Some(value) = self.const_prop(rval, place_layout, statement.source_info) {
 779                     if let Place {
 780                         base: PlaceBase::Local(local),
 781                         projection: None,
 782                     } = *place {
 783                         trace!("checking whether {:?} can be stored to {:?}", value, local);
 784                         if self.can_const_prop[local] {
 785                             trace!("storing {:?} to {:?}", value, local);
 786                             assert!(self.get_const(local).is_none());
 787                             self.set_const(local, value);
 788
 789                             if self.should_const_prop() {
 790                                 self.replace_with_const(
 791                                     rval,
 792                                     value,
 793                                     statement.source_info,
 794                                 );
 795                             }
 796                         }
 797                     }
 798                 }
 799             }
 800         }
 801         self.super_statement(statement, location);
 802     }
 803
 804     fn visit_terminator(
 805         &mut self,
 806         terminator: &mut Terminator<'tcx>,
 807         location: Location,
 808     ) {
 809         self.super_terminator(terminator, location);
 810         let source_info = terminator.source_info;
 811         match &mut terminator.kind {
 812             TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
 813                 if let Some(value) = self.eval_operand(&cond, source_info) {
 814                     trace!("assertion on {:?} should be {:?}", value, expected);
 815                     let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
 816                     let value_const = self.ecx.read_scalar(value).unwrap();
 817                     if expected != value_const {
 818                         // poison all places this operand references so that further code
 819                         // doesn't use the invalid value
 820                         match cond {
 821                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 822                                 if let PlaceBase::Local(local) = place.base {
 823                                     self.remove_const(local);
 824                                 }
 825                             },
 826                             Operand::Constant(_) => {}
 827                         }
 828                         let span = terminator.source_info.span;
 829                         let hir_id = self
 830                             .tcx
 831                             .hir()
 832                             .as_local_hir_id(self.source.def_id())
 833                             .expect("some part of a failing const eval must be local");
 834                         use rustc::mir::interpret::InterpError::*;
 835                         let msg = match msg {
 836                             Panic(PanicMessage::Overflow(_)) |
 837                             Panic(PanicMessage::OverflowNeg) |
 838                             Panic(PanicMessage::DivisionByZero) |
 839                             Panic(PanicMessage::RemainderByZero) =>
 840                                 format!("{:?}", msg),
 841                             Panic(PanicMessage::BoundsCheck { ref len, ref index }) => {
 842                                 let len = self
 843                                     .eval_operand(len, source_info)
 844                                     .expect("len must be const");
 845                                 let len = match self.ecx.read_scalar(len) {
 846                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 847                                         data, ..
 848                                     })) => data,
 849                                     other => bug!("const len not primitive: {:?}", other),
 850                                 };
 851                                 let index = self
 852                                     .eval_operand(index, source_info)
 853                                     .expect("index must be const");
 854                                 let index = match self.ecx.read_scalar(index) {
 855                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 856                                         data, ..
 857                                     })) => data,
 858                                     other => bug!("const index not primitive: {:?}", other),
 859                                 };
 860                                 format!(
 861                                     "index out of bounds: \
 862                                     the len is {} but the index is {}",
 863                                     len,
 864                                     index,
 865                                 )
 866                             },
 867                             // Need proper const propagator for these
 868                             _ => return,
 869                         };
 870                         self.tcx.lint_hir(
 871                             ::rustc::lint::builtin::CONST_ERR,
 872                             hir_id,
 873                             span,
 874                             &msg,
 875                         );
 876                     } else {
 877                         if self.should_const_prop() {
 878                             if let ScalarMaybeUndef::Scalar(scalar) = value_const {
 879                                 *cond = self.operand_from_scalar(
 880                                     scalar,
 881                                     self.tcx.types.bool,
 882                                     source_info.span,
 883                                 );
 884                             }
 885                         }
 886                     }
 887                 }
 888             },
 889             TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
 890                 if self.should_const_prop() {
 891                     if let Some(value) = self.eval_operand(&discr, source_info) {
 892                         if let ScalarMaybeUndef::Scalar(scalar) =
 893                                 self.ecx.read_scalar(value).unwrap() {
 894                             *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
 895                         }
 896                     }
 897                 }
 898             },
 899             //none of these have Operands to const-propagate
 900             TerminatorKind::Goto { .. } |
 901             TerminatorKind::Resume |
 902             TerminatorKind::Abort |
 903             TerminatorKind::Return |
 904             TerminatorKind::Unreachable |
 905             TerminatorKind::Drop { .. } |
 906             TerminatorKind::DropAndReplace { .. } |
 907             TerminatorKind::Yield { .. } |
 908             TerminatorKind::GeneratorDrop |
 909             TerminatorKind::FalseEdges { .. } |
 910             TerminatorKind::FalseUnwind { .. } => { }
 911             //FIXME(wesleywiser) Call does have Operands that could be const-propagated
 912             TerminatorKind::Call { .. } => { }
 913         }
 914     }
 915 }