src/librustc_mir/transform/const_prop.rs

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