src/librustc_mir/transform/const_prop.rs

   1 //! Propagates constants for early reporting of statically known
   2 //! assertion failures
   3
   4 use std::borrow::Cow;
   5 use std::cell::Cell;
   6
   7 use rustc::lint;
   8 use rustc::mir::interpret::{InterpResult, Scalar};
   9 use rustc::mir::visit::{
  10     MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
  11 };
  12 use rustc::mir::{
  13     read_only, AggregateKind, AssertKind, BasicBlock, BinOp, Body, BodyAndCache, ClearCrossCrate,
  14     Constant, Local, LocalDecl, LocalKind, Location, Operand, Place, ReadOnlyBodyAndCache, Rvalue,
  15     SourceInfo, SourceScope, SourceScopeData, Statement, StatementKind, Terminator, TerminatorKind,
  16     UnOp, RETURN_PLACE,
  17 };
  18 use rustc::ty::layout::{
  19     HasDataLayout, HasTyCtxt, LayoutError, LayoutOf, Size, TargetDataLayout, TyLayout,
  20 };
  21 use rustc::ty::subst::{InternalSubsts, Subst};
  22 use rustc::ty::{self, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeFoldable};
  23 use rustc_data_structures::fx::FxHashMap;
  24 use rustc_hir::def::DefKind;
  25 use rustc_hir::def_id::DefId;
  26 use rustc_hir::HirId;
  27 use rustc_index::vec::IndexVec;
  28 use rustc_infer::traits;
  29 use rustc_span::Span;
  30 use syntax::ast::Mutability;
  31
  32 use crate::const_eval::error_to_const_error;
  33 use crate::interpret::{
  34     self, intern_const_alloc_recursive, AllocId, Allocation, Frame, ImmTy, Immediate, InternKind,
  35     InterpCx, LocalState, LocalValue, Memory, MemoryKind, OpTy, Operand as InterpOperand, PlaceTy,
  36     Pointer, ScalarMaybeUndef, StackPopCleanup,
  37 };
  38 use crate::transform::{MirPass, MirSource};
  39
  40 /// The maximum number of bytes that we'll allocate space for a return value.
  41 const MAX_ALLOC_LIMIT: u64 = 1024;
  42
  43 pub struct ConstProp;
  44
  45 impl<'tcx> MirPass<'tcx> for ConstProp {
  46     fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
  47         // will be evaluated by miri and produce its errors there
  48         if source.promoted.is_some() {
  49             return;
  50         }
  51
  52         use rustc::hir::map::blocks::FnLikeNode;
  53         let hir_id = tcx
  54             .hir()
  55             .as_local_hir_id(source.def_id())
  56             .expect("Non-local call to local provider is_const_fn");
  57
  58         let is_fn_like = FnLikeNode::from_node(tcx.hir().get(hir_id)).is_some();
  59         let is_assoc_const = match tcx.def_kind(source.def_id()) {
  60             Some(DefKind::AssocConst) => true,
  61             _ => false,
  62         };
  63
  64         // Only run const prop on functions, methods, closures and associated constants
  65         if !is_fn_like && !is_assoc_const {
  66             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  67             trace!("ConstProp skipped for {:?}", source.def_id());
  68             return;
  69         }
  70
  71         let is_generator = tcx.type_of(source.def_id()).is_generator();
  72         // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
  73         // computing their layout.
  74         if is_generator {
  75             trace!("ConstProp skipped for generator {:?}", source.def_id());
  76             return;
  77         }
  78
  79         // Check if it's even possible to satisfy the 'where' clauses
  80         // for this item.
  81         // This branch will never be taken for any normal function.
  82         // However, it's possible to `#!feature(trivial_bounds)]` to write
  83         // a function with impossible to satisfy clauses, e.g.:
  84         // `fn foo() where String: Copy {}`
  85         //
  86         // We don't usually need to worry about this kind of case,
  87         // since we would get a compilation error if the user tried
  88         // to call it. However, since we can do const propagation
  89         // even without any calls to the function, we need to make
  90         // sure that it even makes sense to try to evaluate the body.
  91         // If there are unsatisfiable where clauses, then all bets are
  92         // off, and we just give up.
  93         //
  94         // We manually filter the predicates, skipping anything that's not
  95         // "global". We are in a potentially generic context
  96         // (e.g. we are evaluating a function without substituting generic
  97         // parameters, so this filtering serves two purposes:
  98         //
  99         // 1. We skip evaluating any predicates that we would
 100         // never be able prove are unsatisfiable (e.g. `<T as Foo>`
 101         // 2. We avoid trying to normalize predicates involving generic
 102         // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
 103         // the normalization code (leading to cycle errors), since
 104         // it's usually never invoked in this way.
 105         let predicates = tcx
 106             .predicates_of(source.def_id())
 107             .predicates
 108             .iter()
 109             .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None })
 110             .collect();
 111         if !traits::normalize_and_test_predicates(
 112             tcx,
 113             traits::elaborate_predicates(tcx, predicates).collect(),
 114         ) {
 115             trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", source.def_id());
 116             return;
 117         }
 118
 119         trace!("ConstProp starting for {:?}", source.def_id());
 120
 121         let dummy_body = &Body::new(
 122             body.basic_blocks().clone(),
 123             body.source_scopes.clone(),
 124             body.local_decls.clone(),
 125             Default::default(),
 126             body.arg_count,
 127             Default::default(),
 128             tcx.def_span(source.def_id()),
 129             Default::default(),
 130             body.generator_kind,
 131         );
 132
 133         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
 134         // constants, instead of just checking for const-folding succeeding.
 135         // That would require an uniform one-def no-mutation analysis
 136         // and RPO (or recursing when needing the value of a local).
 137         let mut optimization_finder =
 138             ConstPropagator::new(read_only!(body), dummy_body, tcx, source);
 139         optimization_finder.visit_body(body);
 140
 141         trace!("ConstProp done for {:?}", source.def_id());
 142     }
 143 }
 144
 145 struct ConstPropMachine;
 146
 147 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine {
 148     type MemoryKinds = !;
 149     type PointerTag = ();
 150     type ExtraFnVal = !;
 151
 152     type FrameExtra = ();
 153     type MemoryExtra = ();
 154     type AllocExtra = ();
 155
 156     type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
 157
 158     const STATIC_KIND: Option<!> = None;
 159
 160     const CHECK_ALIGN: bool = false;
 161
 162     #[inline(always)]
 163     fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
 164         false
 165     }
 166
 167     fn find_mir_or_eval_fn(
 168         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 169         _span: Span,
 170         _instance: ty::Instance<'tcx>,
 171         _args: &[OpTy<'tcx>],
 172         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 173         _unwind: Option<BasicBlock>,
 174     ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> {
 175         Ok(None)
 176     }
 177
 178     fn call_extra_fn(
 179         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 180         fn_val: !,
 181         _args: &[OpTy<'tcx>],
 182         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 183         _unwind: Option<BasicBlock>,
 184     ) -> InterpResult<'tcx> {
 185         match fn_val {}
 186     }
 187
 188     fn call_intrinsic(
 189         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 190         _span: Span,
 191         _instance: ty::Instance<'tcx>,
 192         _args: &[OpTy<'tcx>],
 193         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 194         _unwind: Option<BasicBlock>,
 195     ) -> InterpResult<'tcx> {
 196         throw_unsup!(ConstPropUnsupported("calling intrinsics isn't supported in ConstProp"));
 197     }
 198
 199     fn assert_panic(
 200         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 201         _span: Span,
 202         _msg: &rustc::mir::AssertMessage<'tcx>,
 203         _unwind: Option<rustc::mir::BasicBlock>,
 204     ) -> InterpResult<'tcx> {
 205         bug!("panics terminators are not evaluated in ConstProp");
 206     }
 207
 208     fn ptr_to_int(_mem: &Memory<'mir, 'tcx, Self>, _ptr: Pointer) -> InterpResult<'tcx, u64> {
 209         throw_unsup!(ConstPropUnsupported("ptr-to-int casts aren't supported in ConstProp"));
 210     }
 211
 212     fn binary_ptr_op(
 213         _ecx: &InterpCx<'mir, 'tcx, Self>,
 214         _bin_op: BinOp,
 215         _left: ImmTy<'tcx>,
 216         _right: ImmTy<'tcx>,
 217     ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
 218         // We can't do this because aliasing of memory can differ between const eval and llvm
 219         throw_unsup!(ConstPropUnsupported(
 220             "pointer arithmetic or comparisons aren't supported \
 221             in ConstProp"
 222         ));
 223     }
 224
 225     fn find_foreign_static(
 226         _tcx: TyCtxt<'tcx>,
 227         _def_id: DefId,
 228     ) -> InterpResult<'tcx, Cow<'tcx, Allocation<Self::PointerTag>>> {
 229         throw_unsup!(ReadForeignStatic)
 230     }
 231
 232     #[inline(always)]
 233     fn init_allocation_extra<'b>(
 234         _memory_extra: &(),
 235         _id: AllocId,
 236         alloc: Cow<'b, Allocation>,
 237         _kind: Option<MemoryKind<!>>,
 238     ) -> (Cow<'b, Allocation<Self::PointerTag>>, Self::PointerTag) {
 239         // We do not use a tag so we can just cheaply forward the allocation
 240         (alloc, ())
 241     }
 242
 243     #[inline(always)]
 244     fn tag_static_base_pointer(_memory_extra: &(), _id: AllocId) -> Self::PointerTag {
 245         ()
 246     }
 247
 248     fn box_alloc(
 249         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 250         _dest: PlaceTy<'tcx>,
 251     ) -> InterpResult<'tcx> {
 252         throw_unsup!(ConstPropUnsupported("can't const prop `box` keyword"));
 253     }
 254
 255     fn access_local(
 256         _ecx: &InterpCx<'mir, 'tcx, Self>,
 257         frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
 258         local: Local,
 259     ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
 260         let l = &frame.locals[local];
 261
 262         if l.value == LocalValue::Uninitialized {
 263             throw_unsup!(ConstPropUnsupported("tried to access an uninitialized local"));
 264         }
 265
 266         l.access()
 267     }
 268
 269     fn before_access_static(
 270         _memory_extra: &(),
 271         allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
 272     ) -> InterpResult<'tcx> {
 273         // if the static allocation is mutable or if it has relocations (it may be legal to mutate
 274         // the memory behind that in the future), then we can't const prop it
 275         if allocation.mutability == Mutability::Mut || allocation.relocations().len() > 0 {
 276             throw_unsup!(ConstPropUnsupported("can't eval mutable statics in ConstProp"));
 277         }
 278
 279         Ok(())
 280     }
 281
 282     fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
 283         Ok(())
 284     }
 285
 286     #[inline(always)]
 287     fn stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
 288         Ok(())
 289     }
 290 }
 291
 292 /// Finds optimization opportunities on the MIR.
 293 struct ConstPropagator<'mir, 'tcx> {
 294     ecx: InterpCx<'mir, 'tcx, ConstPropMachine>,
 295     tcx: TyCtxt<'tcx>,
 296     can_const_prop: IndexVec<Local, ConstPropMode>,
 297     param_env: ParamEnv<'tcx>,
 298     // FIXME(eddyb) avoid cloning these two fields more than once,
 299     // by accessing them through `ecx` instead.
 300     source_scopes: IndexVec<SourceScope, SourceScopeData>,
 301     local_decls: IndexVec<Local, LocalDecl<'tcx>>,
 302     ret: Option<OpTy<'tcx, ()>>,
 303     // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
 304     // the last known `SourceInfo` here and just keep revisiting it.
 305     source_info: Option<SourceInfo>,
 306 }
 307
 308 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
 309     type Ty = Ty<'tcx>;
 310     type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
 311
 312     fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyLayout {
 313         self.tcx.layout_of(self.param_env.and(ty))
 314     }
 315 }
 316
 317 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
 318     #[inline]
 319     fn data_layout(&self) -> &TargetDataLayout {
 320         &self.tcx.data_layout
 321     }
 322 }
 323
 324 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
 325     #[inline]
 326     fn tcx(&self) -> TyCtxt<'tcx> {
 327         self.tcx
 328     }
 329 }
 330
 331 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 332     fn new(
 333         body: ReadOnlyBodyAndCache<'_, 'tcx>,
 334         dummy_body: &'mir Body<'tcx>,
 335         tcx: TyCtxt<'tcx>,
 336         source: MirSource<'tcx>,
 337     ) -> ConstPropagator<'mir, 'tcx> {
 338         let def_id = source.def_id();
 339         let substs = &InternalSubsts::identity_for_item(tcx, def_id);
 340         let mut param_env = tcx.param_env(def_id);
 341
 342         // If we're evaluating inside a monomorphic function, then use `Reveal::All` because
 343         // we want to see the same instances that codegen will see. This allows us to `resolve()`
 344         // specializations.
 345         if !substs.needs_subst() {
 346             param_env = param_env.with_reveal_all();
 347         }
 348
 349         let span = tcx.def_span(def_id);
 350         let mut ecx = InterpCx::new(tcx.at(span), param_env, ConstPropMachine, ());
 351         let can_const_prop = CanConstProp::check(body);
 352
 353         let ret = ecx
 354             .layout_of(body.return_ty().subst(tcx, substs))
 355             .ok()
 356             // Don't bother allocating memory for ZST types which have no values
 357             // or for large values.
 358             .filter(|ret_layout| {
 359                 !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
 360             })
 361             .map(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack));
 362
 363         ecx.push_stack_frame(
 364             Instance::new(def_id, substs),
 365             span,
 366             dummy_body,
 367             ret.map(Into::into),
 368             StackPopCleanup::None { cleanup: false },
 369         )
 370         .expect("failed to push initial stack frame");
 371
 372         ConstPropagator {
 373             ecx,
 374             tcx,
 375             param_env,
 376             can_const_prop,
 377             // FIXME(eddyb) avoid cloning these two fields more than once,
 378             // by accessing them through `ecx` instead.
 379             source_scopes: body.source_scopes.clone(),
 380             //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
 381             local_decls: body.local_decls.clone(),
 382             ret: ret.map(Into::into),
 383             source_info: None,
 384         }
 385     }
 386
 387     fn get_const(&self, local: Local) -> Option<OpTy<'tcx>> {
 388         if local == RETURN_PLACE {
 389             // Try to read the return place as an immediate so that if it is representable as a
 390             // scalar, we can handle it as such, but otherwise, just return the value as is.
 391             return match self.ret.map(|ret| self.ecx.try_read_immediate(ret)) {
 392                 Some(Ok(Ok(imm))) => Some(imm.into()),
 393                 _ => self.ret,
 394             };
 395         }
 396
 397         self.ecx.access_local(self.ecx.frame(), local, None).ok()
 398     }
 399
 400     fn remove_const(&mut self, local: Local) {
 401         self.ecx.frame_mut().locals[local] =
 402             LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) };
 403     }
 404
 405     fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
 406         match &self.source_scopes[source_info.scope].local_data {
 407             ClearCrossCrate::Set(data) => Some(data.lint_root),
 408             ClearCrossCrate::Clear => None,
 409         }
 410     }
 411
 412     fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
 413     where
 414         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 415     {
 416         let r = match f(self) {
 417             Ok(val) => Some(val),
 418             Err(error) => {
 419                 use rustc::mir::interpret::{
 420                     InterpError::*, UndefinedBehaviorInfo, UnsupportedOpInfo,
 421                 };
 422                 match error.kind {
 423                     MachineStop(_) => bug!("ConstProp does not stop"),
 424
 425                     // Some error shouldn't come up because creating them causes
 426                     // an allocation, which we should avoid. When that happens,
 427                     // dedicated error variants should be introduced instead.
 428                     // Only test this in debug builds though to avoid disruptions.
 429                     Unsupported(UnsupportedOpInfo::Unsupported(_))
 430                     | Unsupported(UnsupportedOpInfo::ValidationFailure(_))
 431                     | UndefinedBehavior(UndefinedBehaviorInfo::Ub(_))
 432                     | UndefinedBehavior(UndefinedBehaviorInfo::UbExperimental(_))
 433                         if cfg!(debug_assertions) =>
 434                     {
 435                         bug!("const-prop encountered allocating error: {:?}", error.kind);
 436                     }
 437
 438                     Unsupported(_)
 439                     | UndefinedBehavior(_)
 440                     | InvalidProgram(_)
 441                     | ResourceExhaustion(_) => {
 442                         // Ignore these errors.
 443                     }
 444                 }
 445                 None
 446             }
 447         };
 448         r
 449     }
 450
 451     fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 452         self.ecx.tcx.span = c.span;
 453
 454         // FIXME we need to revisit this for #67176
 455         if c.needs_subst() {
 456             return None;
 457         }
 458
 459         match self.ecx.eval_const_to_op(c.literal, None) {
 460             Ok(op) => Some(op),
 461             Err(error) => {
 462                 let err = error_to_const_error(&self.ecx, error);
 463                 if let Some(lint_root) = self.lint_root(source_info) {
 464                     let lint_only = match c.literal.val {
 465                         // Promoteds must lint and not error as the user didn't ask for them
 466                         ConstKind::Unevaluated(_, _, Some(_)) => true,
 467                         // Out of backwards compatibility we cannot report hard errors in unused
 468                         // generic functions using associated constants of the generic parameters.
 469                         _ => c.literal.needs_subst(),
 470                     };
 471                     if lint_only {
 472                         // Out of backwards compatibility we cannot report hard errors in unused
 473                         // generic functions using associated constants of the generic parameters.
 474                         err.report_as_lint(
 475                             self.ecx.tcx,
 476                             "erroneous constant used",
 477                             lint_root,
 478                             Some(c.span),
 479                         );
 480                     } else {
 481                         err.report_as_error(self.ecx.tcx, "erroneous constant used");
 482                     }
 483                 } else {
 484                     err.report_as_error(self.ecx.tcx, "erroneous constant used");
 485                 }
 486                 None
 487             }
 488         }
 489     }
 490
 491     fn eval_place(&mut self, place: &Place<'tcx>) -> Option<OpTy<'tcx>> {
 492         trace!("eval_place(place={:?})", place);
 493         self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
 494     }
 495
 496     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 497         match *op {
 498             Operand::Constant(ref c) => self.eval_constant(c, source_info),
 499             Operand::Move(ref place) | Operand::Copy(ref place) => self.eval_place(place),
 500         }
 501     }
 502
 503     fn report_assert_as_lint(
 504         &self,
 505         lint: &'static lint::Lint,
 506         source_info: SourceInfo,
 507         message: &'static str,
 508         panic: AssertKind<u64>,
 509     ) -> Option<()> {
 510         let lint_root = self.lint_root(source_info)?;
 511         self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
 512             let mut err = lint.build(message);
 513             err.span_label(source_info.span, format!("{:?}", panic));
 514             err.emit()
 515         });
 516         return None;
 517     }
 518
 519     fn check_unary_op(
 520         &mut self,
 521         op: UnOp,
 522         arg: &Operand<'tcx>,
 523         source_info: SourceInfo,
 524     ) -> Option<()> {
 525         if self.use_ecx(|this| {
 526             let val = this.ecx.read_immediate(this.ecx.eval_operand(arg, None)?)?;
 527             let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, val)?;
 528             Ok(overflow)
 529         })? {
 530             // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
 531             // appropriate to use.
 532             assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
 533             self.report_assert_as_lint(
 534                 lint::builtin::ARITHMETIC_OVERFLOW,
 535                 source_info,
 536                 "this arithmetic operation will overflow",
 537                 AssertKind::OverflowNeg,
 538             )?;
 539         }
 540
 541         Some(())
 542     }
 543
 544     fn check_binary_op(
 545         &mut self,
 546         op: BinOp,
 547         left: &Operand<'tcx>,
 548         right: &Operand<'tcx>,
 549         source_info: SourceInfo,
 550     ) -> Option<()> {
 551         let r =
 552             self.use_ecx(|this| this.ecx.read_immediate(this.ecx.eval_operand(right, None)?))?;
 553         // Check for exceeding shifts *even if* we cannot evaluate the LHS.
 554         if op == BinOp::Shr || op == BinOp::Shl {
 555             // We need the type of the LHS. We cannot use `place_layout` as that is the type
 556             // of the result, which for checked binops is not the same!
 557             let left_ty = left.ty(&self.local_decls, self.tcx);
 558             let left_size_bits = self.ecx.layout_of(left_ty).ok()?.size.bits();
 559             let right_size = r.layout.size;
 560             let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
 561             if r_bits.map_or(false, |b| b >= left_size_bits as u128) {
 562                 self.report_assert_as_lint(
 563                     lint::builtin::ARITHMETIC_OVERFLOW,
 564                     source_info,
 565                     "this arithmetic operation will overflow",
 566                     AssertKind::Overflow(op),
 567                 )?;
 568             }
 569         }
 570
 571         // The remaining operators are handled through `overflowing_binary_op`.
 572         if self.use_ecx(|this| {
 573             let l = this.ecx.read_immediate(this.ecx.eval_operand(left, None)?)?;
 574             let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
 575             Ok(overflow)
 576         })? {
 577             self.report_assert_as_lint(
 578                 lint::builtin::ARITHMETIC_OVERFLOW,
 579                 source_info,
 580                 "this arithmetic operation will overflow",
 581                 AssertKind::Overflow(op),
 582             )?;
 583         }
 584
 585         Some(())
 586     }
 587
 588     fn const_prop(
 589         &mut self,
 590         rvalue: &Rvalue<'tcx>,
 591         place_layout: TyLayout<'tcx>,
 592         source_info: SourceInfo,
 593         place: &Place<'tcx>,
 594     ) -> Option<()> {
 595         // #66397: Don't try to eval into large places as that can cause an OOM
 596         if place_layout.size >= Size::from_bytes(MAX_ALLOC_LIMIT) {
 597             return None;
 598         }
 599
 600         // FIXME we need to revisit this for #67176
 601         if rvalue.needs_subst() {
 602             return None;
 603         }
 604
 605         // Perform any special handling for specific Rvalue types.
 606         // Generally, checks here fall into one of two categories:
 607         //   1. Additional checking to provide useful lints to the user
 608         //        - In this case, we will do some validation and then fall through to the
 609         //          end of the function which evals the assignment.
 610         //   2. Working around bugs in other parts of the compiler
 611         //        - In this case, we'll return `None` from this function to stop evaluation.
 612         match rvalue {
 613             // Additional checking: give lints to the user if an overflow would occur.
 614             // We do this here and not in the `Assert` terminator as that terminator is
 615             // only sometimes emitted (overflow checks can be disabled), but we want to always
 616             // lint.
 617             Rvalue::UnaryOp(op, arg) => {
 618                 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
 619                 self.check_unary_op(*op, arg, source_info)?;
 620             }
 621             Rvalue::BinaryOp(op, left, right) => {
 622                 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
 623                 self.check_binary_op(*op, left, right, source_info)?;
 624             }
 625             Rvalue::CheckedBinaryOp(op, left, right) => {
 626                 trace!(
 627                     "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
 628                     op,
 629                     left,
 630                     right
 631                 );
 632                 self.check_binary_op(*op, left, right, source_info)?;
 633             }
 634
 635             // Do not try creating references (#67862)
 636             Rvalue::Ref(_, _, place_ref) => {
 637                 trace!("skipping Ref({:?})", place_ref);
 638
 639                 return None;
 640             }
 641
 642             _ => {}
 643         }
 644
 645         self.use_ecx(|this| {
 646             trace!("calling eval_rvalue_into_place(rvalue = {:?}, place = {:?})", rvalue, place);
 647             this.ecx.eval_rvalue_into_place(rvalue, place)?;
 648             Ok(())
 649         })
 650     }
 651
 652     fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
 653         Operand::Constant(Box::new(Constant {
 654             span,
 655             user_ty: None,
 656             literal: self.tcx.mk_const(*ty::Const::from_scalar(self.tcx, scalar, ty)),
 657         }))
 658     }
 659
 660     fn replace_with_const(
 661         &mut self,
 662         rval: &mut Rvalue<'tcx>,
 663         value: OpTy<'tcx>,
 664         source_info: SourceInfo,
 665     ) {
 666         trace!("attepting to replace {:?} with {:?}", rval, value);
 667         if let Err(e) = self.ecx.validate_operand(
 668             value,
 669             vec![],
 670             // FIXME: is ref tracking too expensive?
 671             Some(&mut interpret::RefTracking::empty()),
 672         ) {
 673             trace!("validation error, attempt failed: {:?}", e);
 674             return;
 675         }
 676
 677         // FIXME> figure out what tho do when try_read_immediate fails
 678         let imm = self.use_ecx(|this| this.ecx.try_read_immediate(value));
 679
 680         if let Some(Ok(imm)) = imm {
 681             match *imm {
 682                 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
 683                     *rval = Rvalue::Use(self.operand_from_scalar(
 684                         scalar,
 685                         value.layout.ty,
 686                         source_info.span,
 687                     ));
 688                 }
 689                 Immediate::ScalarPair(
 690                     ScalarMaybeUndef::Scalar(one),
 691                     ScalarMaybeUndef::Scalar(two),
 692                 ) => {
 693                     // Found a value represented as a pair. For now only do cont-prop if type of
 694                     // Rvalue is also a pair with two scalars. The more general case is more
 695                     // complicated to implement so we'll do it later.
 696                     let ty = &value.layout.ty.kind;
 697                     // Only do it for tuples
 698                     if let ty::Tuple(substs) = ty {
 699                         // Only do it if tuple is also a pair with two scalars
 700                         if substs.len() == 2 {
 701                             let opt_ty1_ty2 = self.use_ecx(|this| {
 702                                 let ty1 = substs[0].expect_ty();
 703                                 let ty2 = substs[1].expect_ty();
 704                                 let ty_is_scalar = |ty| {
 705                                     this.ecx.layout_of(ty).ok().map(|ty| ty.details.abi.is_scalar())
 706                                         == Some(true)
 707                                 };
 708                                 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
 709                                     Ok(Some((ty1, ty2)))
 710                                 } else {
 711                                     Ok(None)
 712                                 }
 713                             });
 714
 715                             if let Some(Some((ty1, ty2))) = opt_ty1_ty2 {
 716                                 *rval = Rvalue::Aggregate(
 717                                     Box::new(AggregateKind::Tuple),
 718                                     vec![
 719                                         self.operand_from_scalar(one, ty1, source_info.span),
 720                                         self.operand_from_scalar(two, ty2, source_info.span),
 721                                     ],
 722                                 );
 723                             }
 724                         }
 725                     }
 726                 }
 727                 _ => {}
 728             }
 729         }
 730     }
 731
 732     fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool {
 733         let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level;
 734
 735         if mir_opt_level == 0 {
 736             return false;
 737         }
 738
 739         match *op {
 740             interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUndef::Scalar(s))) => {
 741                 s.is_bits()
 742             }
 743             interpret::Operand::Immediate(Immediate::ScalarPair(
 744                 ScalarMaybeUndef::Scalar(l),
 745                 ScalarMaybeUndef::Scalar(r),
 746             )) => l.is_bits() && r.is_bits(),
 747             interpret::Operand::Indirect(_) if mir_opt_level >= 2 => {
 748                 let mplace = op.assert_mem_place(&self.ecx);
 749                 intern_const_alloc_recursive(&mut self.ecx, InternKind::ConstProp, mplace, false)
 750                     .expect("failed to intern alloc");
 751                 true
 752             }
 753             _ => false,
 754         }
 755     }
 756 }
 757
 758 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
 759 #[derive(Clone, Copy, Debug, PartialEq)]
 760 enum ConstPropMode {
 761     /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
 762     FullConstProp,
 763     /// The `Local` can be propagated into but reads cannot be propagated.
 764     OnlyPropagateInto,
 765     /// No propagation is allowed at all.
 766     NoPropagation,
 767 }
 768
 769 struct CanConstProp {
 770     can_const_prop: IndexVec<Local, ConstPropMode>,
 771     // false at the beginning, once set, there are not allowed to be any more assignments
 772     found_assignment: IndexVec<Local, bool>,
 773 }
 774
 775 impl CanConstProp {
 776     /// returns true if `local` can be propagated
 777     fn check(body: ReadOnlyBodyAndCache<'_, '_>) -> IndexVec<Local, ConstPropMode> {
 778         let mut cpv = CanConstProp {
 779             can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
 780             found_assignment: IndexVec::from_elem(false, &body.local_decls),
 781         };
 782         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 783             // cannot use args at all
 784             // cannot use locals because if x < y { y - x } else { x - y } would
 785             //        lint for x != y
 786             // FIXME(oli-obk): lint variables until they are used in a condition
 787             // FIXME(oli-obk): lint if return value is constant
 788             let local_kind = body.local_kind(local);
 789
 790             if local_kind == LocalKind::Arg || local_kind == LocalKind::Var {
 791                 *val = ConstPropMode::OnlyPropagateInto;
 792                 trace!("local {:?} can't be const propagated because it's not a temporary", local);
 793             }
 794         }
 795         cpv.visit_body(body);
 796         cpv.can_const_prop
 797     }
 798 }
 799
 800 impl<'tcx> Visitor<'tcx> for CanConstProp {
 801     fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
 802         use rustc::mir::visit::PlaceContext::*;
 803         match context {
 804             // Constants must have at most one write
 805             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 806             // only occur in independent execution paths
 807             MutatingUse(MutatingUseContext::Store) => {
 808                 if self.found_assignment[local] {
 809                     trace!("local {:?} can't be propagated because of multiple assignments", local);
 810                     self.can_const_prop[local] = ConstPropMode::NoPropagation;
 811                 } else {
 812                     self.found_assignment[local] = true
 813                 }
 814             }
 815             // Reading constants is allowed an arbitrary number of times
 816             NonMutatingUse(NonMutatingUseContext::Copy)
 817             | NonMutatingUse(NonMutatingUseContext::Move)
 818             | NonMutatingUse(NonMutatingUseContext::Inspect)
 819             | NonMutatingUse(NonMutatingUseContext::Projection)
 820             | MutatingUse(MutatingUseContext::Projection)
 821             | NonUse(_) => {}
 822             _ => {
 823                 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
 824                 self.can_const_prop[local] = ConstPropMode::NoPropagation;
 825             }
 826         }
 827     }
 828 }
 829
 830 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
 831     fn tcx(&self) -> TyCtxt<'tcx> {
 832         self.tcx
 833     }
 834
 835     fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
 836         trace!("visit_constant: {:?}", constant);
 837         self.super_constant(constant, location);
 838         self.eval_constant(constant, self.source_info.unwrap());
 839     }
 840
 841     fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
 842         trace!("visit_statement: {:?}", statement);
 843         let source_info = statement.source_info;
 844         self.source_info = Some(source_info);
 845         if let StatementKind::Assign(box (ref place, ref mut rval)) = statement.kind {
 846             let place_ty: Ty<'tcx> = place.ty(&self.local_decls, self.tcx).ty;
 847             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 848                 if let Some(local) = place.as_local() {
 849                     let can_const_prop = self.can_const_prop[local];
 850                     if let Some(()) = self.const_prop(rval, place_layout, source_info, place) {
 851                         if can_const_prop == ConstPropMode::FullConstProp
 852                             || can_const_prop == ConstPropMode::OnlyPropagateInto
 853                         {
 854                             if let Some(value) = self.get_const(local) {
 855                                 if self.should_const_prop(value) {
 856                                     trace!("replacing {:?} with {:?}", rval, value);
 857                                     self.replace_with_const(rval, value, statement.source_info);
 858
 859                                     if can_const_prop == ConstPropMode::FullConstProp {
 860                                         trace!("propagated into {:?}", local);
 861                                     }
 862                                 }
 863                             }
 864                         }
 865                     }
 866                     if self.can_const_prop[local] != ConstPropMode::FullConstProp {
 867                         trace!("can't propagate into {:?}", local);
 868                         if local != RETURN_PLACE {
 869                             self.remove_const(local);
 870                         }
 871                     }
 872                 }
 873             }
 874         } else {
 875             match statement.kind {
 876                 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
 877                     let frame = self.ecx.frame_mut();
 878                     frame.locals[local].value =
 879                         if let StatementKind::StorageLive(_) = statement.kind {
 880                             LocalValue::Uninitialized
 881                         } else {
 882                             LocalValue::Dead
 883                         };
 884                 }
 885                 _ => {}
 886             }
 887         }
 888
 889         self.super_statement(statement, location);
 890     }
 891
 892     fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
 893         let source_info = terminator.source_info;
 894         self.source_info = Some(source_info);
 895         self.super_terminator(terminator, location);
 896         match &mut terminator.kind {
 897             TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
 898                 if let Some(value) = self.eval_operand(&cond, source_info) {
 899                     trace!("assertion on {:?} should be {:?}", value, expected);
 900                     let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
 901                     let value_const = self.ecx.read_scalar(value).unwrap();
 902                     if expected != value_const {
 903                         // poison all places this operand references so that further code
 904                         // doesn't use the invalid value
 905                         match cond {
 906                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 907                                 self.remove_const(place.local);
 908                             }
 909                             Operand::Constant(_) => {}
 910                         }
 911                         let msg = match msg {
 912                             AssertKind::DivisionByZero => AssertKind::DivisionByZero,
 913                             AssertKind::RemainderByZero => AssertKind::RemainderByZero,
 914                             AssertKind::BoundsCheck { ref len, ref index } => {
 915                                 let len =
 916                                     self.eval_operand(len, source_info).expect("len must be const");
 917                                 let len = self
 918                                     .ecx
 919                                     .read_scalar(len)
 920                                     .unwrap()
 921                                     .to_machine_usize(&self.tcx)
 922                                     .unwrap();
 923                                 let index = self
 924                                     .eval_operand(index, source_info)
 925                                     .expect("index must be const");
 926                                 let index = self
 927                                     .ecx
 928                                     .read_scalar(index)
 929                                     .unwrap()
 930                                     .to_machine_usize(&self.tcx)
 931                                     .unwrap();
 932                                 AssertKind::BoundsCheck { len, index }
 933                             }
 934                             // Overflow is are already covered by checks on the binary operators.
 935                             AssertKind::Overflow(_) | AssertKind::OverflowNeg => return,
 936                             // Need proper const propagator for these.
 937                             _ => return,
 938                         };
 939                         self.report_assert_as_lint(
 940                             lint::builtin::UNCONDITIONAL_PANIC,
 941                             source_info,
 942                             "this operation will panic at runtime",
 943                             msg,
 944                         );
 945                     } else {
 946                         if self.should_const_prop(value) {
 947                             if let ScalarMaybeUndef::Scalar(scalar) = value_const {
 948                                 *cond = self.operand_from_scalar(
 949                                     scalar,
 950                                     self.tcx.types.bool,
 951                                     source_info.span,
 952                                 );
 953                             }
 954                         }
 955                     }
 956                 }
 957             }
 958             TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
 959                 if let Some(value) = self.eval_operand(&discr, source_info) {
 960                     if self.should_const_prop(value) {
 961                         if let ScalarMaybeUndef::Scalar(scalar) =
 962                             self.ecx.read_scalar(value).unwrap()
 963                         {
 964                             *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
 965                         }
 966                     }
 967                 }
 968             }
 969             //none of these have Operands to const-propagate
 970             TerminatorKind::Goto { .. }
 971             | TerminatorKind::Resume
 972             | TerminatorKind::Abort
 973             | TerminatorKind::Return
 974             | TerminatorKind::Unreachable
 975             | TerminatorKind::Drop { .. }
 976             | TerminatorKind::DropAndReplace { .. }
 977             | TerminatorKind::Yield { .. }
 978             | TerminatorKind::GeneratorDrop
 979             | TerminatorKind::FalseEdges { .. }
 980             | TerminatorKind::FalseUnwind { .. } => {}
 981             //FIXME(wesleywiser) Call does have Operands that could be const-propagated
 982             TerminatorKind::Call { .. } => {}
 983         }
 984     }
 985 }