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