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