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_ast::ast::Mutability;
   8 use rustc_data_structures::fx::FxHashMap;
   9 use rustc_hir::def::DefKind;
  10 use rustc_hir::HirId;
  11 use rustc_index::vec::IndexVec;
  12 use rustc_middle::mir::interpret::{InterpResult, Scalar};
  13 use rustc_middle::mir::visit::{
  14     MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
  15 };
  16 use rustc_middle::mir::{
  17     AggregateKind, AssertKind, BasicBlock, BinOp, Body, ClearCrossCrate, Constant, Local,
  18     LocalDecl, LocalKind, Location, Operand, Place, Rvalue, SourceInfo, SourceScope,
  19     SourceScopeData, Statement, StatementKind, Terminator, TerminatorKind, UnOp, RETURN_PLACE,
  20 };
  21 use rustc_middle::ty::layout::{HasTyCtxt, LayoutError, TyAndLayout};
  22 use rustc_middle::ty::subst::{InternalSubsts, Subst};
  23 use rustc_middle::ty::{self, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeFoldable};
  24 use rustc_session::lint;
  25 use rustc_span::{def_id::DefId, Span};
  26 use rustc_target::abi::{HasDataLayout, LayoutOf, Size, TargetDataLayout};
  27 use rustc_trait_selection::traits;
  28
  29 use crate::const_eval::error_to_const_error;
  30 use crate::interpret::{
  31     self, intern_const_alloc_recursive, AllocId, Allocation, Frame, ImmTy, Immediate, InternKind,
  32     InterpCx, LocalState, LocalValue, Memory, MemoryKind, OpTy, Operand as InterpOperand, PlaceTy,
  33     Pointer, ScalarMaybeUndef, StackPopCleanup,
  34 };
  35 use crate::transform::{MirPass, MirSource};
  36
  37 /// The maximum number of bytes that we'll allocate space for a return value.
  38 const MAX_ALLOC_LIMIT: u64 = 1024;
  39
  40 /// Macro for machine-specific `InterpError` without allocation.
  41 /// (These will never be shown to the user, but they help diagnose ICEs.)
  42 macro_rules! throw_machine_stop_str {
  43     ($($tt:tt)*) => {{
  44         // We make a new local type for it. The type itself does not carry any information,
  45         // but its vtable (for the `MachineStopType` trait) does.
  46         struct Zst;
  47         // Debug-printing this type shows the desired string.
  48         impl std::fmt::Debug for Zst {
  49             fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
  50                 write!(f, $($tt)*)
  51             }
  52         }
  53         impl rustc_middle::mir::interpret::MachineStopType for Zst {}
  54         throw_machine_stop!(Zst)
  55     }};
  56 }
  57
  58 pub struct ConstProp;
  59
  60 impl<'tcx> MirPass<'tcx> for ConstProp {
  61     fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) {
  62         // will be evaluated by miri and produce its errors there
  63         if source.promoted.is_some() {
  64             return;
  65         }
  66
  67         use rustc_middle::hir::map::blocks::FnLikeNode;
  68         let hir_id = tcx
  69             .hir()
  70             .as_local_hir_id(source.def_id())
  71             .expect("Non-local call to local provider is_const_fn");
  72
  73         let is_fn_like = FnLikeNode::from_node(tcx.hir().get(hir_id)).is_some();
  74         let is_assoc_const = match tcx.def_kind(source.def_id()) {
  75             Some(DefKind::AssocConst) => true,
  76             _ => false,
  77         };
  78
  79         // Only run const prop on functions, methods, closures and associated constants
  80         if !is_fn_like && !is_assoc_const {
  81             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  82             trace!("ConstProp skipped for {:?}", source.def_id());
  83             return;
  84         }
  85
  86         let is_generator = tcx.type_of(source.def_id()).is_generator();
  87         // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
  88         // computing their layout.
  89         if is_generator {
  90             trace!("ConstProp skipped for generator {:?}", source.def_id());
  91             return;
  92         }
  93
  94         // Check if it's even possible to satisfy the 'where' clauses
  95         // for this item.
  96         // This branch will never be taken for any normal function.
  97         // However, it's possible to `#!feature(trivial_bounds)]` to write
  98         // a function with impossible to satisfy clauses, e.g.:
  99         // `fn foo() where String: Copy {}`
 100         //
 101         // We don't usually need to worry about this kind of case,
 102         // since we would get a compilation error if the user tried
 103         // to call it. However, since we can do const propagation
 104         // even without any calls to the function, we need to make
 105         // sure that it even makes sense to try to evaluate the body.
 106         // If there are unsatisfiable where clauses, then all bets are
 107         // off, and we just give up.
 108         //
 109         // We manually filter the predicates, skipping anything that's not
 110         // "global". We are in a potentially generic context
 111         // (e.g. we are evaluating a function without substituting generic
 112         // parameters, so this filtering serves two purposes:
 113         //
 114         // 1. We skip evaluating any predicates that we would
 115         // never be able prove are unsatisfiable (e.g. `<T as Foo>`
 116         // 2. We avoid trying to normalize predicates involving generic
 117         // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
 118         // the normalization code (leading to cycle errors), since
 119         // it's usually never invoked in this way.
 120         let predicates = tcx
 121             .predicates_of(source.def_id())
 122             .predicates
 123             .iter()
 124             .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None })
 125             .collect();
 126         if !traits::normalize_and_test_predicates(
 127             tcx,
 128             traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
 129         ) {
 130             trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", source.def_id());
 131             return;
 132         }
 133
 134         trace!("ConstProp starting for {:?}", source.def_id());
 135
 136         let dummy_body = &Body::new(
 137             body.basic_blocks().clone(),
 138             body.source_scopes.clone(),
 139             body.local_decls.clone(),
 140             Default::default(),
 141             body.arg_count,
 142             Default::default(),
 143             tcx.def_span(source.def_id()),
 144             Default::default(),
 145             body.generator_kind,
 146         );
 147
 148         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
 149         // constants, instead of just checking for const-folding succeeding.
 150         // That would require an uniform one-def no-mutation analysis
 151         // and RPO (or recursing when needing the value of a local).
 152         let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx, source);
 153         optimization_finder.visit_body(body);
 154
 155         trace!("ConstProp done for {:?}", source.def_id());
 156     }
 157 }
 158
 159 struct ConstPropMachine<'mir, 'tcx> {
 160     /// The virtual call stack.
 161     stack: Vec<Frame<'mir, 'tcx, (), ()>>,
 162 }
 163
 164 impl<'mir, 'tcx> ConstPropMachine<'mir, 'tcx> {
 165     fn new() -> Self {
 166         Self { stack: Vec::new() }
 167     }
 168 }
 169
 170 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
 171     type MemoryKind = !;
 172     type PointerTag = ();
 173     type ExtraFnVal = !;
 174
 175     type FrameExtra = ();
 176     type MemoryExtra = ();
 177     type AllocExtra = ();
 178
 179     type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
 180
 181     const GLOBAL_KIND: Option<!> = None; // no copying of globals from `tcx` to machine memory
 182
 183     #[inline(always)]
 184     fn enforce_alignment(_memory_extra: &Self::MemoryExtra) -> bool {
 185         false
 186     }
 187
 188     #[inline(always)]
 189     fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
 190         false
 191     }
 192
 193     fn find_mir_or_eval_fn(
 194         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 195         _instance: ty::Instance<'tcx>,
 196         _args: &[OpTy<'tcx>],
 197         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 198         _unwind: Option<BasicBlock>,
 199     ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> {
 200         Ok(None)
 201     }
 202
 203     fn call_extra_fn(
 204         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 205         fn_val: !,
 206         _args: &[OpTy<'tcx>],
 207         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 208         _unwind: Option<BasicBlock>,
 209     ) -> InterpResult<'tcx> {
 210         match fn_val {}
 211     }
 212
 213     fn call_intrinsic(
 214         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 215         _instance: ty::Instance<'tcx>,
 216         _args: &[OpTy<'tcx>],
 217         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 218         _unwind: Option<BasicBlock>,
 219     ) -> InterpResult<'tcx> {
 220         throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
 221     }
 222
 223     fn assert_panic(
 224         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 225         _msg: &rustc_middle::mir::AssertMessage<'tcx>,
 226         _unwind: Option<rustc_middle::mir::BasicBlock>,
 227     ) -> InterpResult<'tcx> {
 228         bug!("panics terminators are not evaluated in ConstProp")
 229     }
 230
 231     fn ptr_to_int(_mem: &Memory<'mir, 'tcx, Self>, _ptr: Pointer) -> InterpResult<'tcx, u64> {
 232         throw_unsup!(ReadPointerAsBytes)
 233     }
 234
 235     fn binary_ptr_op(
 236         _ecx: &InterpCx<'mir, 'tcx, Self>,
 237         _bin_op: BinOp,
 238         _left: ImmTy<'tcx>,
 239         _right: ImmTy<'tcx>,
 240     ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
 241         // We can't do this because aliasing of memory can differ between const eval and llvm
 242         throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
 243     }
 244
 245     #[inline(always)]
 246     fn init_allocation_extra<'b>(
 247         _memory_extra: &(),
 248         _id: AllocId,
 249         alloc: Cow<'b, Allocation>,
 250         _kind: Option<MemoryKind<!>>,
 251     ) -> (Cow<'b, Allocation<Self::PointerTag>>, Self::PointerTag) {
 252         // We do not use a tag so we can just cheaply forward the allocation
 253         (alloc, ())
 254     }
 255
 256     #[inline(always)]
 257     fn tag_global_base_pointer(_memory_extra: &(), _id: AllocId) -> Self::PointerTag {}
 258
 259     fn box_alloc(
 260         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 261         _dest: PlaceTy<'tcx>,
 262     ) -> InterpResult<'tcx> {
 263         throw_machine_stop_str!("can't const prop heap allocations")
 264     }
 265
 266     fn access_local(
 267         _ecx: &InterpCx<'mir, 'tcx, Self>,
 268         frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
 269         local: Local,
 270     ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
 271         let l = &frame.locals[local];
 272
 273         if l.value == LocalValue::Uninitialized {
 274             throw_machine_stop_str!("tried to access an uninitialized local")
 275         }
 276
 277         l.access()
 278     }
 279
 280     fn before_access_global(
 281         _memory_extra: &(),
 282         _alloc_id: AllocId,
 283         allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
 284         _static_def_id: Option<DefId>,
 285         is_write: bool,
 286     ) -> InterpResult<'tcx> {
 287         if is_write {
 288             throw_machine_stop_str!("can't write to global");
 289         }
 290         // If the static allocation is mutable, then we can't const prop it as its content
 291         // might be different at runtime.
 292         if allocation.mutability == Mutability::Mut {
 293             throw_machine_stop_str!("can't access mutable globals in ConstProp");
 294         }
 295
 296         Ok(())
 297     }
 298
 299     #[inline(always)]
 300     fn init_frame_extra(
 301         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 302         frame: Frame<'mir, 'tcx>,
 303     ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
 304         Ok(frame)
 305     }
 306
 307     #[inline(always)]
 308     fn stack(
 309         ecx: &'a InterpCx<'mir, 'tcx, Self>,
 310     ) -> &'a [Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>] {
 311         &ecx.machine.stack
 312     }
 313
 314     #[inline(always)]
 315     fn stack_mut(
 316         ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
 317     ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>> {
 318         &mut ecx.machine.stack
 319     }
 320 }
 321
 322 /// Finds optimization opportunities on the MIR.
 323 struct ConstPropagator<'mir, 'tcx> {
 324     ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
 325     tcx: TyCtxt<'tcx>,
 326     can_const_prop: IndexVec<Local, ConstPropMode>,
 327     param_env: ParamEnv<'tcx>,
 328     // FIXME(eddyb) avoid cloning these two fields more than once,
 329     // by accessing them through `ecx` instead.
 330     source_scopes: IndexVec<SourceScope, SourceScopeData>,
 331     local_decls: IndexVec<Local, LocalDecl<'tcx>>,
 332     ret: Option<OpTy<'tcx, ()>>,
 333     // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
 334     // the last known `SourceInfo` here and just keep revisiting it.
 335     source_info: Option<SourceInfo>,
 336 }
 337
 338 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
 339     type Ty = Ty<'tcx>;
 340     type TyAndLayout = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
 341
 342     fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyAndLayout {
 343         self.tcx.layout_of(self.param_env.and(ty))
 344     }
 345 }
 346
 347 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
 348     #[inline]
 349     fn data_layout(&self) -> &TargetDataLayout {
 350         &self.tcx.data_layout
 351     }
 352 }
 353
 354 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
 355     #[inline]
 356     fn tcx(&self) -> TyCtxt<'tcx> {
 357         self.tcx
 358     }
 359 }
 360
 361 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 362     fn new(
 363         body: &Body<'tcx>,
 364         dummy_body: &'mir Body<'tcx>,
 365         tcx: TyCtxt<'tcx>,
 366         source: MirSource<'tcx>,
 367     ) -> ConstPropagator<'mir, 'tcx> {
 368         let def_id = source.def_id();
 369         let substs = &InternalSubsts::identity_for_item(tcx, def_id);
 370         let param_env = tcx.param_env(def_id).with_reveal_all();
 371
 372         let span = tcx.def_span(def_id);
 373         let mut ecx = InterpCx::new(tcx.at(span), param_env, ConstPropMachine::new(), ());
 374         let can_const_prop = CanConstProp::check(body);
 375
 376         let ret = ecx
 377             .layout_of(body.return_ty().subst(tcx, substs))
 378             .ok()
 379             // Don't bother allocating memory for ZST types which have no values
 380             // or for large values.
 381             .filter(|ret_layout| {
 382                 !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
 383             })
 384             .map(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack));
 385
 386         ecx.push_stack_frame(
 387             Instance::new(def_id, substs),
 388             dummy_body,
 389             ret.map(Into::into),
 390             StackPopCleanup::None { cleanup: false },
 391         )
 392         .expect("failed to push initial stack frame");
 393
 394         ConstPropagator {
 395             ecx,
 396             tcx,
 397             param_env,
 398             can_const_prop,
 399             // FIXME(eddyb) avoid cloning these two fields more than once,
 400             // by accessing them through `ecx` instead.
 401             source_scopes: body.source_scopes.clone(),
 402             //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
 403             local_decls: body.local_decls.clone(),
 404             ret: ret.map(Into::into),
 405             source_info: None,
 406         }
 407     }
 408
 409     fn get_const(&self, local: Local) -> Option<OpTy<'tcx>> {
 410         if local == RETURN_PLACE {
 411             // Try to read the return place as an immediate so that if it is representable as a
 412             // scalar, we can handle it as such, but otherwise, just return the value as is.
 413             return match self.ret.map(|ret| self.ecx.try_read_immediate(ret)) {
 414                 Some(Ok(Ok(imm))) => Some(imm.into()),
 415                 _ => self.ret,
 416             };
 417         }
 418
 419         self.ecx.access_local(self.ecx.frame(), local, None).ok()
 420     }
 421
 422     fn remove_const(&mut self, local: Local) {
 423         self.ecx.frame_mut().locals[local] =
 424             LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) };
 425     }
 426
 427     fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
 428         match &self.source_scopes[source_info.scope].local_data {
 429             ClearCrossCrate::Set(data) => Some(data.lint_root),
 430             ClearCrossCrate::Clear => None,
 431         }
 432     }
 433
 434     fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
 435     where
 436         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 437     {
 438         match f(self) {
 439             Ok(val) => Some(val),
 440             Err(error) => {
 441                 // Some errors shouldn't come up because creating them causes
 442                 // an allocation, which we should avoid. When that happens,
 443                 // dedicated error variants should be introduced instead.
 444                 assert!(
 445                     !error.kind.allocates(),
 446                     "const-prop encountered allocating error: {}",
 447                     error
 448                 );
 449                 None
 450             }
 451         }
 452     }
 453
 454     fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 455         // FIXME we need to revisit this for #67176
 456         if c.needs_subst() {
 457             return None;
 458         }
 459
 460         match self.ecx.eval_const_to_op(c.literal, None) {
 461             Ok(op) => Some(op),
 462             Err(error) => {
 463                 // Make sure errors point at the constant.
 464                 self.ecx.set_span(c.span);
 465                 let err = error_to_const_error(&self.ecx, error);
 466                 if let Some(lint_root) = self.lint_root(source_info) {
 467                     let lint_only = match c.literal.val {
 468                         // Promoteds must lint and not error as the user didn't ask for them
 469                         ConstKind::Unevaluated(_, _, Some(_)) => true,
 470                         // Out of backwards compatibility we cannot report hard errors in unused
 471                         // generic functions using associated constants of the generic parameters.
 472                         _ => c.literal.needs_subst(),
 473                     };
 474                     if lint_only {
 475                         // Out of backwards compatibility we cannot report hard errors in unused
 476                         // generic functions using associated constants of the generic parameters.
 477                         err.report_as_lint(
 478                             self.ecx.tcx,
 479                             "erroneous constant used",
 480                             lint_root,
 481                             Some(c.span),
 482                         );
 483                     } else {
 484                         err.report_as_error(self.ecx.tcx, "erroneous constant used");
 485                     }
 486                 } else {
 487                     err.report_as_error(self.ecx.tcx, "erroneous constant used");
 488                 }
 489                 None
 490             }
 491         }
 492     }
 493
 494     fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
 495         trace!("eval_place(place={:?})", place);
 496         self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
 497     }
 498
 499     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 500         match *op {
 501             Operand::Constant(ref c) => self.eval_constant(c, source_info),
 502             Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
 503         }
 504     }
 505
 506     fn report_assert_as_lint(
 507         &self,
 508         lint: &'static lint::Lint,
 509         source_info: SourceInfo,
 510         message: &'static str,
 511         panic: AssertKind<u64>,
 512     ) -> Option<()> {
 513         let lint_root = self.lint_root(source_info)?;
 514         self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
 515             let mut err = lint.build(message);
 516             err.span_label(source_info.span, format!("{:?}", panic));
 517             err.emit()
 518         });
 519         None
 520     }
 521
 522     fn check_unary_op(
 523         &mut self,
 524         op: UnOp,
 525         arg: &Operand<'tcx>,
 526         source_info: SourceInfo,
 527     ) -> Option<()> {
 528         if self.use_ecx(|this| {
 529             let val = this.ecx.read_immediate(this.ecx.eval_operand(arg, None)?)?;
 530             let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, val)?;
 531             Ok(overflow)
 532         })? {
 533             // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
 534             // appropriate to use.
 535             assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
 536             self.report_assert_as_lint(
 537                 lint::builtin::ARITHMETIC_OVERFLOW,
 538                 source_info,
 539                 "this arithmetic operation will overflow",
 540                 AssertKind::OverflowNeg,
 541             )?;
 542         }
 543
 544         Some(())
 545     }
 546
 547     fn check_binary_op(
 548         &mut self,
 549         op: BinOp,
 550         left: &Operand<'tcx>,
 551         right: &Operand<'tcx>,
 552         source_info: SourceInfo,
 553     ) -> Option<()> {
 554         let r =
 555             self.use_ecx(|this| this.ecx.read_immediate(this.ecx.eval_operand(right, None)?))?;
 556         // Check for exceeding shifts *even if* we cannot evaluate the LHS.
 557         if op == BinOp::Shr || op == BinOp::Shl {
 558             // We need the type of the LHS. We cannot use `place_layout` as that is the type
 559             // of the result, which for checked binops is not the same!
 560             let left_ty = left.ty(&self.local_decls, self.tcx);
 561             let left_size_bits = self.ecx.layout_of(left_ty).ok()?.size.bits();
 562             let right_size = r.layout.size;
 563             let r_bits = r.to_scalar().ok();
 564             // This is basically `force_bits`.
 565             let r_bits = r_bits.and_then(|r| r.to_bits_or_ptr(right_size, &self.tcx).ok());
 566             if r_bits.map_or(false, |b| b >= left_size_bits as u128) {
 567                 self.report_assert_as_lint(
 568                     lint::builtin::ARITHMETIC_OVERFLOW,
 569                     source_info,
 570                     "this arithmetic operation will overflow",
 571                     AssertKind::Overflow(op),
 572                 )?;
 573             }
 574         }
 575
 576         // The remaining operators are handled through `overflowing_binary_op`.
 577         if self.use_ecx(|this| {
 578             let l = this.ecx.read_immediate(this.ecx.eval_operand(left, None)?)?;
 579             let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
 580             Ok(overflow)
 581         })? {
 582             self.report_assert_as_lint(
 583                 lint::builtin::ARITHMETIC_OVERFLOW,
 584                 source_info,
 585                 "this arithmetic operation will overflow",
 586                 AssertKind::Overflow(op),
 587             )?;
 588         }
 589
 590         Some(())
 591     }
 592
 593     fn const_prop(
 594         &mut self,
 595         rvalue: &Rvalue<'tcx>,
 596         place_layout: TyAndLayout<'tcx>,
 597         source_info: SourceInfo,
 598         place: Place<'tcx>,
 599     ) -> Option<()> {
 600         // #66397: Don't try to eval into large places as that can cause an OOM
 601         if place_layout.size >= Size::from_bytes(MAX_ALLOC_LIMIT) {
 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         // FIXME we need to revisit this for #67176
 646         if rvalue.needs_subst() {
 647             return None;
 648         }
 649
 650         self.use_ecx(|this| {
 651             trace!("calling eval_rvalue_into_place(rvalue = {:?}, place = {:?})", rvalue, place);
 652             this.ecx.eval_rvalue_into_place(rvalue, place)?;
 653             Ok(())
 654         })
 655     }
 656
 657     fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
 658         Operand::Constant(Box::new(Constant {
 659             span,
 660             user_ty: None,
 661             literal: self.tcx.mk_const(*ty::Const::from_scalar(self.tcx, scalar, ty)),
 662         }))
 663     }
 664
 665     fn replace_with_const(
 666         &mut self,
 667         rval: &mut Rvalue<'tcx>,
 668         value: OpTy<'tcx>,
 669         source_info: SourceInfo,
 670     ) {
 671         trace!("attepting to replace {:?} with {:?}", rval, value);
 672         if let Err(e) = self.ecx.const_validate_operand(
 673             value,
 674             vec![],
 675             // FIXME: is ref tracking too expensive?
 676             &mut interpret::RefTracking::empty(),
 677             /*may_ref_to_static*/ true,
 678         ) {
 679             trace!("validation error, attempt failed: {:?}", e);
 680             return;
 681         }
 682
 683         // FIXME> figure out what to do when try_read_immediate fails
 684         let imm = self.use_ecx(|this| this.ecx.try_read_immediate(value));
 685
 686         if let Some(Ok(imm)) = imm {
 687             match *imm {
 688                 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
 689                     *rval = Rvalue::Use(self.operand_from_scalar(
 690                         scalar,
 691                         value.layout.ty,
 692                         source_info.span,
 693                     ));
 694                 }
 695                 Immediate::ScalarPair(
 696                     ScalarMaybeUndef::Scalar(one),
 697                     ScalarMaybeUndef::Scalar(two),
 698                 ) => {
 699                     // Found a value represented as a pair. For now only do cont-prop if type of
 700                     // Rvalue is also a pair with two scalars. The more general case is more
 701                     // complicated to implement so we'll do it later.
 702                     let ty = &value.layout.ty.kind;
 703                     // Only do it for tuples
 704                     if let ty::Tuple(substs) = ty {
 705                         // Only do it if tuple is also a pair with two scalars
 706                         if substs.len() == 2 {
 707                             let opt_ty1_ty2 = self.use_ecx(|this| {
 708                                 let ty1 = substs[0].expect_ty();
 709                                 let ty2 = substs[1].expect_ty();
 710                                 let ty_is_scalar = |ty| {
 711                                     this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
 712                                         == Some(true)
 713                                 };
 714                                 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
 715                                     Ok(Some((ty1, ty2)))
 716                                 } else {
 717                                     Ok(None)
 718                                 }
 719                             });
 720
 721                             if let Some(Some((ty1, ty2))) = opt_ty1_ty2 {
 722                                 *rval = Rvalue::Aggregate(
 723                                     Box::new(AggregateKind::Tuple),
 724                                     vec![
 725                                         self.operand_from_scalar(one, ty1, source_info.span),
 726                                         self.operand_from_scalar(two, ty2, source_info.span),
 727                                     ],
 728                                 );
 729                             }
 730                         }
 731                     }
 732                 }
 733                 _ => {}
 734             }
 735         }
 736     }
 737
 738     fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool {
 739         let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level;
 740
 741         if mir_opt_level == 0 {
 742             return false;
 743         }
 744
 745         match *op {
 746             interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUndef::Scalar(s))) => {
 747                 s.is_bits()
 748             }
 749             interpret::Operand::Immediate(Immediate::ScalarPair(
 750                 ScalarMaybeUndef::Scalar(l),
 751                 ScalarMaybeUndef::Scalar(r),
 752             )) => l.is_bits() && r.is_bits(),
 753             interpret::Operand::Indirect(_) if mir_opt_level >= 2 => {
 754                 let mplace = op.assert_mem_place(&self.ecx);
 755                 intern_const_alloc_recursive(&mut self.ecx, InternKind::ConstProp, mplace, false)
 756                     .expect("failed to intern alloc");
 757                 true
 758             }
 759             _ => false,
 760         }
 761     }
 762 }
 763
 764 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
 765 #[derive(Clone, Copy, Debug, PartialEq)]
 766 enum ConstPropMode {
 767     /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
 768     FullConstProp,
 769     /// The `Local` can be propagated into but reads cannot be propagated.
 770     OnlyPropagateInto,
 771     /// No propagation is allowed at all.
 772     NoPropagation,
 773 }
 774
 775 struct CanConstProp {
 776     can_const_prop: IndexVec<Local, ConstPropMode>,
 777     // false at the beginning, once set, there are not allowed to be any more assignments
 778     found_assignment: IndexVec<Local, bool>,
 779 }
 780
 781 impl CanConstProp {
 782     /// returns true if `local` can be propagated
 783     fn check(body: &Body<'_>) -> IndexVec<Local, ConstPropMode> {
 784         let mut cpv = CanConstProp {
 785             can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
 786             found_assignment: IndexVec::from_elem(false, &body.local_decls),
 787         };
 788         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 789             // cannot use args at all
 790             // cannot use locals because if x < y { y - x } else { x - y } would
 791             //        lint for x != y
 792             // FIXME(oli-obk): lint variables until they are used in a condition
 793             // FIXME(oli-obk): lint if return value is constant
 794             let local_kind = body.local_kind(local);
 795
 796             if local_kind == LocalKind::Arg || local_kind == LocalKind::Var {
 797                 *val = ConstPropMode::OnlyPropagateInto;
 798                 trace!("local {:?} can't be const propagated because it's not a temporary", local);
 799             }
 800         }
 801         cpv.visit_body(&body);
 802         cpv.can_const_prop
 803     }
 804 }
 805
 806 impl<'tcx> Visitor<'tcx> for CanConstProp {
 807     fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
 808         use rustc_middle::mir::visit::PlaceContext::*;
 809         match context {
 810             // Constants must have at most one write
 811             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 812             // only occur in independent execution paths
 813             MutatingUse(MutatingUseContext::Store) => {
 814                 if self.found_assignment[local] {
 815                     trace!("local {:?} can't be propagated because of multiple assignments", local);
 816                     self.can_const_prop[local] = ConstPropMode::NoPropagation;
 817                 } else {
 818                     self.found_assignment[local] = true
 819                 }
 820             }
 821             // Reading constants is allowed an arbitrary number of times
 822             NonMutatingUse(NonMutatingUseContext::Copy)
 823             | NonMutatingUse(NonMutatingUseContext::Move)
 824             | NonMutatingUse(NonMutatingUseContext::Inspect)
 825             | NonMutatingUse(NonMutatingUseContext::Projection)
 826             | MutatingUse(MutatingUseContext::Projection)
 827             | NonUse(_) => {}
 828             _ => {
 829                 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
 830                 self.can_const_prop[local] = ConstPropMode::NoPropagation;
 831             }
 832         }
 833     }
 834 }
 835
 836 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
 837     fn tcx(&self) -> TyCtxt<'tcx> {
 838         self.tcx
 839     }
 840
 841     fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
 842         trace!("visit_constant: {:?}", constant);
 843         self.super_constant(constant, location);
 844         self.eval_constant(constant, self.source_info.unwrap());
 845     }
 846
 847     fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
 848         trace!("visit_statement: {:?}", statement);
 849         let source_info = statement.source_info;
 850         self.ecx.set_span(source_info.span);
 851         self.source_info = Some(source_info);
 852         if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
 853             let place_ty: Ty<'tcx> = place.ty(&self.local_decls, self.tcx).ty;
 854             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 855                 if let Some(local) = place.as_local() {
 856                     let can_const_prop = self.can_const_prop[local];
 857                     if let Some(()) = self.const_prop(rval, place_layout, source_info, place) {
 858                         if can_const_prop == ConstPropMode::FullConstProp
 859                             || can_const_prop == ConstPropMode::OnlyPropagateInto
 860                         {
 861                             if let Some(value) = self.get_const(local) {
 862                                 if self.should_const_prop(value) {
 863                                     trace!("replacing {:?} with {:?}", rval, value);
 864                                     self.replace_with_const(rval, value, statement.source_info);
 865
 866                                     if can_const_prop == ConstPropMode::FullConstProp {
 867                                         trace!("propagated into {:?}", local);
 868                                     }
 869                                 }
 870                             }
 871                         }
 872                     }
 873                     if self.can_const_prop[local] != ConstPropMode::FullConstProp {
 874                         trace!("can't propagate into {:?}", local);
 875                         if local != RETURN_PLACE {
 876                             self.remove_const(local);
 877                         }
 878                     }
 879                 }
 880             }
 881         } else {
 882             match statement.kind {
 883                 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
 884                     let frame = self.ecx.frame_mut();
 885                     frame.locals[local].value =
 886                         if let StatementKind::StorageLive(_) = statement.kind {
 887                             LocalValue::Uninitialized
 888                         } else {
 889                             LocalValue::Dead
 890                         };
 891                 }
 892                 _ => {}
 893             }
 894         }
 895
 896         self.super_statement(statement, location);
 897     }
 898
 899     fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
 900         let source_info = terminator.source_info;
 901         self.ecx.set_span(source_info.span);
 902         self.source_info = Some(source_info);
 903         self.super_terminator(terminator, location);
 904         match &mut terminator.kind {
 905             TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
 906                 if let Some(value) = self.eval_operand(&cond, source_info) {
 907                     trace!("assertion on {:?} should be {:?}", value, expected);
 908                     let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
 909                     let value_const = self.ecx.read_scalar(value).unwrap();
 910                     if expected != value_const {
 911                         // poison all places this operand references so that further code
 912                         // doesn't use the invalid value
 913                         match cond {
 914                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 915                                 self.remove_const(place.local);
 916                             }
 917                             Operand::Constant(_) => {}
 918                         }
 919                         let msg = match msg {
 920                             AssertKind::DivisionByZero => AssertKind::DivisionByZero,
 921                             AssertKind::RemainderByZero => AssertKind::RemainderByZero,
 922                             AssertKind::BoundsCheck { ref len, ref index } => {
 923                                 let len =
 924                                     self.eval_operand(len, source_info).expect("len must be const");
 925                                 let len = self
 926                                     .ecx
 927                                     .read_scalar(len)
 928                                     .unwrap()
 929                                     .to_machine_usize(&self.tcx)
 930                                     .unwrap();
 931                                 let index = self
 932                                     .eval_operand(index, source_info)
 933                                     .expect("index must be const");
 934                                 let index = self
 935                                     .ecx
 936                                     .read_scalar(index)
 937                                     .unwrap()
 938                                     .to_machine_usize(&self.tcx)
 939                                     .unwrap();
 940                                 AssertKind::BoundsCheck { len, index }
 941                             }
 942                             // Overflow is are already covered by checks on the binary operators.
 943                             AssertKind::Overflow(_) | AssertKind::OverflowNeg => return,
 944                             // Need proper const propagator for these.
 945                             _ => return,
 946                         };
 947                         self.report_assert_as_lint(
 948                             lint::builtin::UNCONDITIONAL_PANIC,
 949                             source_info,
 950                             "this operation will panic at runtime",
 951                             msg,
 952                         );
 953                     } else {
 954                         if self.should_const_prop(value) {
 955                             if let ScalarMaybeUndef::Scalar(scalar) = value_const {
 956                                 *cond = self.operand_from_scalar(
 957                                     scalar,
 958                                     self.tcx.types.bool,
 959                                     source_info.span,
 960                                 );
 961                             }
 962                         }
 963                     }
 964                 }
 965             }
 966             TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
 967                 if let Some(value) = self.eval_operand(&discr, source_info) {
 968                     if self.should_const_prop(value) {
 969                         if let ScalarMaybeUndef::Scalar(scalar) =
 970                             self.ecx.read_scalar(value).unwrap()
 971                         {
 972                             *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
 973                         }
 974                     }
 975                 }
 976             }
 977             //none of these have Operands to const-propagate
 978             TerminatorKind::Goto { .. }
 979             | TerminatorKind::Resume
 980             | TerminatorKind::Abort
 981             | TerminatorKind::Return
 982             | TerminatorKind::Unreachable
 983             | TerminatorKind::Drop { .. }
 984             | TerminatorKind::DropAndReplace { .. }
 985             | TerminatorKind::Yield { .. }
 986             | TerminatorKind::GeneratorDrop
 987             | TerminatorKind::FalseEdges { .. }
 988             | TerminatorKind::FalseUnwind { .. } => {}
 989             //FIXME(wesleywiser) Call does have Operands that could be const-propagated
 990             TerminatorKind::Call { .. } => {}
 991         }
 992     }
 993 }