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