src/librustc_mir/transform/const_prop.rs

   1 //! Propagates constants for early reporting of statically known
   2 //! assertion failures
   3
   4 use std::cell::Cell;
   5
   6 use rustc_ast::ast::Mutability;
   7 use rustc_data_structures::fx::FxHashSet;
   8 use rustc_hir::def::DefKind;
   9 use rustc_hir::HirId;
  10 use rustc_index::bit_set::BitSet;
  11 use rustc_index::vec::IndexVec;
  12 use rustc_middle::mir::interpret::{InterpResult, Scalar};
  13 use rustc_middle::mir::visit::{
  14     MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
  15 };
  16 use rustc_middle::mir::{
  17     AggregateKind, AssertKind, BasicBlock, BinOp, Body, ClearCrossCrate, Constant, Local,
  18     LocalDecl, LocalKind, Location, Operand, Place, Rvalue, SourceInfo, SourceScope,
  19     SourceScopeData, Statement, StatementKind, Terminator, TerminatorKind, UnOp, RETURN_PLACE,
  20 };
  21 use rustc_middle::ty::layout::{HasTyCtxt, LayoutError, TyAndLayout};
  22 use rustc_middle::ty::subst::{InternalSubsts, Subst};
  23 use rustc_middle::ty::{self, ConstInt, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeFoldable};
  24 use rustc_session::lint;
  25 use rustc_span::{def_id::DefId, Span};
  26 use rustc_target::abi::{HasDataLayout, LayoutOf, Size, TargetDataLayout};
  27 use rustc_trait_selection::traits;
  28
  29 use crate::const_eval::ConstEvalErr;
  30 use crate::interpret::{
  31     self, compile_time_machine, truncate, AllocId, Allocation, Frame, ImmTy, Immediate, InterpCx,
  32     LocalState, LocalValue, MemPlace, Memory, MemoryKind, OpTy, Operand as InterpOperand, PlaceTy,
  33     Pointer, ScalarMaybeUninit, StackPopCleanup,
  34 };
  35 use crate::transform::{MirPass, MirSource};
  36
  37 /// The maximum number of bytes that we'll allocate space for a local or the return value.
  38 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
  39 /// Severely regress performance.
  40 const MAX_ALLOC_LIMIT: u64 = 1024;
  41
  42 /// Macro for machine-specific `InterpError` without allocation.
  43 /// (These will never be shown to the user, but they help diagnose ICEs.)
  44 macro_rules! throw_machine_stop_str {
  45     ($($tt:tt)*) => {{
  46         // We make a new local type for it. The type itself does not carry any information,
  47         // but its vtable (for the `MachineStopType` trait) does.
  48         struct Zst;
  49         // Printing this type shows the desired string.
  50         impl std::fmt::Display for Zst {
  51             fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
  52                 write!(f, $($tt)*)
  53             }
  54         }
  55         impl rustc_middle::mir::interpret::MachineStopType for Zst {}
  56         throw_machine_stop!(Zst)
  57     }};
  58 }
  59
  60 pub struct ConstProp;
  61
  62 impl<'tcx> MirPass<'tcx> for ConstProp {
  63     fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) {
  64         // will be evaluated by miri and produce its errors there
  65         if source.promoted.is_some() {
  66             return;
  67         }
  68
  69         use rustc_middle::hir::map::blocks::FnLikeNode;
  70         let hir_id = tcx.hir().local_def_id_to_hir_id(source.def_id().expect_local());
  71
  72         let is_fn_like = FnLikeNode::from_node(tcx.hir().get(hir_id)).is_some();
  73         let is_assoc_const = tcx.def_kind(source.def_id()) == DefKind::AssocConst;
  74
  75         // Only run const prop on functions, methods, closures and associated constants
  76         if !is_fn_like && !is_assoc_const {
  77             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  78             trace!("ConstProp skipped for {:?}", source.def_id());
  79             return;
  80         }
  81
  82         let is_generator = tcx.type_of(source.def_id()).is_generator();
  83         // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
  84         // computing their layout.
  85         if is_generator {
  86             trace!("ConstProp skipped for generator {:?}", source.def_id());
  87             return;
  88         }
  89
  90         // Check if it's even possible to satisfy the 'where' clauses
  91         // for this item.
  92         // This branch will never be taken for any normal function.
  93         // However, it's possible to `#!feature(trivial_bounds)]` to write
  94         // a function with impossible to satisfy clauses, e.g.:
  95         // `fn foo() where String: Copy {}`
  96         //
  97         // We don't usually need to worry about this kind of case,
  98         // since we would get a compilation error if the user tried
  99         // to call it. However, since we can do const propagation
 100         // even without any calls to the function, we need to make
 101         // sure that it even makes sense to try to evaluate the body.
 102         // If there are unsatisfiable where clauses, then all bets are
 103         // off, and we just give up.
 104         //
 105         // We manually filter the predicates, skipping anything that's not
 106         // "global". We are in a potentially generic context
 107         // (e.g. we are evaluating a function without substituting generic
 108         // parameters, so this filtering serves two purposes:
 109         //
 110         // 1. We skip evaluating any predicates that we would
 111         // never be able prove are unsatisfiable (e.g. `<T as Foo>`
 112         // 2. We avoid trying to normalize predicates involving generic
 113         // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
 114         // the normalization code (leading to cycle errors), since
 115         // it's usually never invoked in this way.
 116         let predicates = tcx
 117             .predicates_of(source.def_id())
 118             .predicates
 119             .iter()
 120             .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
 121         if traits::impossible_predicates(
 122             tcx,
 123             traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
 124         ) {
 125             trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", source.def_id());
 126             return;
 127         }
 128
 129         trace!("ConstProp starting for {:?}", source.def_id());
 130
 131         let dummy_body = &Body::new(
 132             body.basic_blocks().clone(),
 133             body.source_scopes.clone(),
 134             body.local_decls.clone(),
 135             Default::default(),
 136             body.arg_count,
 137             Default::default(),
 138             tcx.def_span(source.def_id()),
 139             body.generator_kind,
 140         );
 141
 142         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
 143         // constants, instead of just checking for const-folding succeeding.
 144         // That would require an uniform one-def no-mutation analysis
 145         // and RPO (or recursing when needing the value of a local).
 146         let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx, source);
 147         optimization_finder.visit_body(body);
 148
 149         trace!("ConstProp done for {:?}", source.def_id());
 150     }
 151 }
 152
 153 struct ConstPropMachine<'mir, 'tcx> {
 154     /// The virtual call stack.
 155     stack: Vec<Frame<'mir, 'tcx, (), ()>>,
 156     /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
 157     written_only_inside_own_block_locals: FxHashSet<Local>,
 158     /// Locals that need to be cleared after every block terminates.
 159     only_propagate_inside_block_locals: BitSet<Local>,
 160     can_const_prop: IndexVec<Local, ConstPropMode>,
 161 }
 162
 163 impl<'mir, 'tcx> ConstPropMachine<'mir, 'tcx> {
 164     fn new(
 165         only_propagate_inside_block_locals: BitSet<Local>,
 166         can_const_prop: IndexVec<Local, ConstPropMode>,
 167     ) -> Self {
 168         Self {
 169             stack: Vec::new(),
 170             written_only_inside_own_block_locals: Default::default(),
 171             only_propagate_inside_block_locals,
 172             can_const_prop,
 173         }
 174     }
 175 }
 176
 177 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
 178     compile_time_machine!(<'mir, 'tcx>);
 179
 180     type MemoryExtra = ();
 181
 182     fn find_mir_or_eval_fn(
 183         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 184         _instance: ty::Instance<'tcx>,
 185         _args: &[OpTy<'tcx>],
 186         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 187         _unwind: Option<BasicBlock>,
 188     ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> {
 189         Ok(None)
 190     }
 191
 192     fn call_intrinsic(
 193         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 194         _instance: ty::Instance<'tcx>,
 195         _args: &[OpTy<'tcx>],
 196         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 197         _unwind: Option<BasicBlock>,
 198     ) -> InterpResult<'tcx> {
 199         throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
 200     }
 201
 202     fn assert_panic(
 203         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 204         _msg: &rustc_middle::mir::AssertMessage<'tcx>,
 205         _unwind: Option<rustc_middle::mir::BasicBlock>,
 206     ) -> InterpResult<'tcx> {
 207         bug!("panics terminators are not evaluated in ConstProp")
 208     }
 209
 210     fn ptr_to_int(_mem: &Memory<'mir, 'tcx, Self>, _ptr: Pointer) -> InterpResult<'tcx, u64> {
 211         throw_unsup!(ReadPointerAsBytes)
 212     }
 213
 214     fn binary_ptr_op(
 215         _ecx: &InterpCx<'mir, 'tcx, Self>,
 216         _bin_op: BinOp,
 217         _left: ImmTy<'tcx>,
 218         _right: ImmTy<'tcx>,
 219     ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
 220         // We can't do this because aliasing of memory can differ between const eval and llvm
 221         throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
 222     }
 223
 224     fn box_alloc(
 225         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 226         _dest: PlaceTy<'tcx>,
 227     ) -> InterpResult<'tcx> {
 228         throw_machine_stop_str!("can't const prop heap allocations")
 229     }
 230
 231     fn access_local(
 232         _ecx: &InterpCx<'mir, 'tcx, Self>,
 233         frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
 234         local: Local,
 235     ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
 236         let l = &frame.locals[local];
 237
 238         if l.value == LocalValue::Uninitialized {
 239             throw_machine_stop_str!("tried to access an uninitialized local")
 240         }
 241
 242         l.access()
 243     }
 244
 245     fn access_local_mut<'a>(
 246         ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
 247         frame: usize,
 248         local: Local,
 249     ) -> InterpResult<'tcx, Result<&'a mut LocalValue<Self::PointerTag>, MemPlace<Self::PointerTag>>>
 250     {
 251         if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
 252             throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
 253         }
 254         if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
 255             trace!(
 256                 "mutating local {:?} which is restricted to its block. \
 257                 Will remove it from const-prop after block is finished.",
 258                 local
 259             );
 260             ecx.machine.written_only_inside_own_block_locals.insert(local);
 261         }
 262         ecx.machine.stack[frame].locals[local].access_mut()
 263     }
 264
 265     fn before_access_global(
 266         _memory_extra: &(),
 267         _alloc_id: AllocId,
 268         allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
 269         _static_def_id: Option<DefId>,
 270         is_write: bool,
 271     ) -> InterpResult<'tcx> {
 272         if is_write {
 273             throw_machine_stop_str!("can't write to global");
 274         }
 275         // If the static allocation is mutable, then we can't const prop it as its content
 276         // might be different at runtime.
 277         if allocation.mutability == Mutability::Mut {
 278             throw_machine_stop_str!("can't access mutable globals in ConstProp");
 279         }
 280
 281         Ok(())
 282     }
 283
 284     #[inline(always)]
 285     fn init_frame_extra(
 286         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 287         frame: Frame<'mir, 'tcx>,
 288     ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
 289         Ok(frame)
 290     }
 291
 292     #[inline(always)]
 293     fn stack(
 294         ecx: &'a InterpCx<'mir, 'tcx, Self>,
 295     ) -> &'a [Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>] {
 296         &ecx.machine.stack
 297     }
 298
 299     #[inline(always)]
 300     fn stack_mut(
 301         ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
 302     ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>> {
 303         &mut ecx.machine.stack
 304     }
 305 }
 306
 307 /// Finds optimization opportunities on the MIR.
 308 struct ConstPropagator<'mir, 'tcx> {
 309     ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
 310     tcx: TyCtxt<'tcx>,
 311     param_env: ParamEnv<'tcx>,
 312     // FIXME(eddyb) avoid cloning these two fields more than once,
 313     // by accessing them through `ecx` instead.
 314     source_scopes: IndexVec<SourceScope, SourceScopeData>,
 315     local_decls: IndexVec<Local, LocalDecl<'tcx>>,
 316     // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
 317     // the last known `SourceInfo` here and just keep revisiting it.
 318     source_info: Option<SourceInfo>,
 319 }
 320
 321 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
 322     type Ty = Ty<'tcx>;
 323     type TyAndLayout = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
 324
 325     fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyAndLayout {
 326         self.tcx.layout_of(self.param_env.and(ty))
 327     }
 328 }
 329
 330 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
 331     #[inline]
 332     fn data_layout(&self) -> &TargetDataLayout {
 333         &self.tcx.data_layout
 334     }
 335 }
 336
 337 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
 338     #[inline]
 339     fn tcx(&self) -> TyCtxt<'tcx> {
 340         self.tcx
 341     }
 342 }
 343
 344 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 345     fn new(
 346         body: &Body<'tcx>,
 347         dummy_body: &'mir Body<'tcx>,
 348         tcx: TyCtxt<'tcx>,
 349         source: MirSource<'tcx>,
 350     ) -> ConstPropagator<'mir, 'tcx> {
 351         let def_id = source.def_id();
 352         let substs = &InternalSubsts::identity_for_item(tcx, def_id);
 353         let param_env = tcx.param_env_reveal_all_normalized(def_id);
 354
 355         let span = tcx.def_span(def_id);
 356         // FIXME: `CanConstProp::check` computes the layout of all locals, return those layouts
 357         // so we can write them to `ecx.frame_mut().locals.layout, reducing the duplication in
 358         // `layout_of` query invocations.
 359         let can_const_prop = CanConstProp::check(tcx, param_env, body);
 360         let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
 361         for (l, mode) in can_const_prop.iter_enumerated() {
 362             if *mode == ConstPropMode::OnlyInsideOwnBlock {
 363                 only_propagate_inside_block_locals.insert(l);
 364             }
 365         }
 366         let mut ecx = InterpCx::new(
 367             tcx,
 368             span,
 369             param_env,
 370             ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
 371             (),
 372         );
 373
 374         let ret = ecx
 375             .layout_of(body.return_ty().subst(tcx, substs))
 376             .ok()
 377             // Don't bother allocating memory for ZST types which have no values
 378             // or for large values.
 379             .filter(|ret_layout| {
 380                 !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
 381             })
 382             .map(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack));
 383
 384         ecx.push_stack_frame(
 385             Instance::new(def_id, substs),
 386             dummy_body,
 387             ret.map(Into::into),
 388             StackPopCleanup::None { cleanup: false },
 389         )
 390         .expect("failed to push initial stack frame");
 391
 392         ConstPropagator {
 393             ecx,
 394             tcx,
 395             param_env,
 396             // FIXME(eddyb) avoid cloning these two fields more than once,
 397             // by accessing them through `ecx` instead.
 398             source_scopes: body.source_scopes.clone(),
 399             //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
 400             local_decls: body.local_decls.clone(),
 401             source_info: None,
 402         }
 403     }
 404
 405     fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
 406         let op = match self.ecx.eval_place_to_op(place, None) {
 407             Ok(op) => op,
 408             Err(e) => {
 409                 trace!("get_const failed: {}", e);
 410                 return None;
 411             }
 412         };
 413
 414         // Try to read the local as an immediate so that if it is representable as a scalar, we can
 415         // handle it as such, but otherwise, just return the value as is.
 416         Some(match self.ecx.try_read_immediate(op) {
 417             Ok(Ok(imm)) => imm.into(),
 418             _ => op,
 419         })
 420     }
 421
 422     /// Remove `local` from the pool of `Locals`. Allows writing to them,
 423     /// but not reading from them anymore.
 424     fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
 425         ecx.frame_mut().locals[local] =
 426             LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) };
 427     }
 428
 429     fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
 430         match &self.source_scopes[source_info.scope].local_data {
 431             ClearCrossCrate::Set(data) => Some(data.lint_root),
 432             ClearCrossCrate::Clear => None,
 433         }
 434     }
 435
 436     fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
 437     where
 438         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 439     {
 440         match f(self) {
 441             Ok(val) => Some(val),
 442             Err(error) => {
 443                 trace!("InterpCx operation failed: {:?}", error);
 444                 // Some errors shouldn't come up because creating them causes
 445                 // an allocation, which we should avoid. When that happens,
 446                 // dedicated error variants should be introduced instead.
 447                 assert!(
 448                     !error.kind.allocates(),
 449                     "const-prop encountered allocating error: {}",
 450                     error
 451                 );
 452                 None
 453             }
 454         }
 455     }
 456
 457     /// Returns the value, if any, of evaluating `c`.
 458     fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 459         // FIXME we need to revisit this for #67176
 460         if c.needs_subst() {
 461             return None;
 462         }
 463
 464         match self.ecx.const_to_op(c.literal, None) {
 465             Ok(op) => Some(op),
 466             Err(error) => {
 467                 let tcx = self.ecx.tcx.at(c.span);
 468                 let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
 469                 if let Some(lint_root) = self.lint_root(source_info) {
 470                     let lint_only = match c.literal.val {
 471                         // Promoteds must lint and not error as the user didn't ask for them
 472                         ConstKind::Unevaluated(_, _, Some(_)) => true,
 473                         // Out of backwards compatibility we cannot report hard errors in unused
 474                         // generic functions using associated constants of the generic parameters.
 475                         _ => c.literal.needs_subst(),
 476                     };
 477                     if lint_only {
 478                         // Out of backwards compatibility we cannot report hard errors in unused
 479                         // generic functions using associated constants of the generic parameters.
 480                         err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span));
 481                     } else {
 482                         err.report_as_error(tcx, "erroneous constant used");
 483                     }
 484                 } else {
 485                     err.report_as_error(tcx, "erroneous constant used");
 486                 }
 487                 None
 488             }
 489         }
 490     }
 491
 492     /// Returns the value, if any, of evaluating `place`.
 493     fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
 494         trace!("eval_place(place={:?})", place);
 495         self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
 496     }
 497
 498     /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
 499     /// or `eval_place`, depending on the variant of `Operand` used.
 500     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 501         match *op {
 502             Operand::Constant(ref c) => self.eval_constant(c, source_info),
 503             Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
 504         }
 505     }
 506
 507     fn report_assert_as_lint(
 508         &self,
 509         lint: &'static lint::Lint,
 510         source_info: SourceInfo,
 511         message: &'static str,
 512         panic: AssertKind<impl std::fmt::Debug>,
 513     ) -> Option<()> {
 514         let lint_root = self.lint_root(source_info)?;
 515         self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
 516             let mut err = lint.build(message);
 517             err.span_label(source_info.span, format!("{:?}", panic));
 518             err.emit()
 519         });
 520         None
 521     }
 522
 523     fn check_unary_op(
 524         &mut self,
 525         op: UnOp,
 526         arg: &Operand<'tcx>,
 527         source_info: SourceInfo,
 528     ) -> Option<()> {
 529         if let (val, true) = self.use_ecx(|this| {
 530             let val = this.ecx.read_immediate(this.ecx.eval_operand(arg, None)?)?;
 531             let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, val)?;
 532             Ok((val, overflow))
 533         })? {
 534             // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
 535             // appropriate to use.
 536             assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
 537             self.report_assert_as_lint(
 538                 lint::builtin::ARITHMETIC_OVERFLOW,
 539                 source_info,
 540                 "this arithmetic operation will overflow",
 541                 AssertKind::OverflowNeg(val.to_const_int()),
 542             )?;
 543         }
 544
 545         Some(())
 546     }
 547
 548     fn check_binary_op(
 549         &mut self,
 550         op: BinOp,
 551         left: &Operand<'tcx>,
 552         right: &Operand<'tcx>,
 553         source_info: SourceInfo,
 554     ) -> Option<()> {
 555         let r = self.use_ecx(|this| this.ecx.read_immediate(this.ecx.eval_operand(right, None)?));
 556         let l = self.use_ecx(|this| this.ecx.read_immediate(this.ecx.eval_operand(left, None)?));
 557         // Check for exceeding shifts *even if* we cannot evaluate the LHS.
 558         if op == BinOp::Shr || op == BinOp::Shl {
 559             let r = r?;
 560             // We need the type of the LHS. We cannot use `place_layout` as that is the type
 561             // of the result, which for checked binops is not the same!
 562             let left_ty = left.ty(&self.local_decls, self.tcx);
 563             let left_size = self.ecx.layout_of(left_ty).ok()?.size;
 564             let right_size = r.layout.size;
 565             let r_bits = r.to_scalar().ok();
 566             // This is basically `force_bits`.
 567             let r_bits = r_bits.and_then(|r| r.to_bits_or_ptr(right_size, &self.tcx).ok());
 568             if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
 569                 debug!("check_binary_op: reporting assert for {:?}", source_info);
 570                 self.report_assert_as_lint(
 571                     lint::builtin::ARITHMETIC_OVERFLOW,
 572                     source_info,
 573                     "this arithmetic operation will overflow",
 574                     AssertKind::Overflow(
 575                         op,
 576                         match l {
 577                             Some(l) => l.to_const_int(),
 578                             // Invent a dummy value, the diagnostic ignores it anyway
 579                             None => ConstInt::new(
 580                                 1,
 581                                 left_size,
 582                                 left_ty.is_signed(),
 583                                 left_ty.is_ptr_sized_integral(),
 584                             ),
 585                         },
 586                         r.to_const_int(),
 587                     ),
 588                 )?;
 589             }
 590         }
 591
 592         if let (Some(l), Some(r)) = (l, r) {
 593             // The remaining operators are handled through `overflowing_binary_op`.
 594             if self.use_ecx(|this| {
 595                 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
 596                 Ok(overflow)
 597             })? {
 598                 self.report_assert_as_lint(
 599                     lint::builtin::ARITHMETIC_OVERFLOW,
 600                     source_info,
 601                     "this arithmetic operation will overflow",
 602                     AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
 603                 )?;
 604             }
 605         }
 606         Some(())
 607     }
 608
 609     fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
 610         match *operand {
 611             Operand::Copy(l) | Operand::Move(l) => {
 612                 if let Some(value) = self.get_const(l) {
 613                     if self.should_const_prop(value) {
 614                         // FIXME(felix91gr): this code only handles `Scalar` cases.
 615                         // For now, we're not handling `ScalarPair` cases because
 616                         // doing so here would require a lot of code duplication.
 617                         // We should hopefully generalize `Operand` handling into a fn,
 618                         // and use it to do const-prop here and everywhere else
 619                         // where it makes sense.
 620                         if let interpret::Operand::Immediate(interpret::Immediate::Scalar(
 621                             ScalarMaybeUninit::Scalar(scalar),
 622                         )) = *value
 623                         {
 624                             *operand = self.operand_from_scalar(
 625                                 scalar,
 626                                 value.layout.ty,
 627                                 self.source_info.unwrap().span,
 628                             );
 629                         }
 630                     }
 631                 }
 632             }
 633             Operand::Constant(_) => (),
 634         }
 635     }
 636
 637     fn const_prop(
 638         &mut self,
 639         rvalue: &Rvalue<'tcx>,
 640         source_info: SourceInfo,
 641         place: Place<'tcx>,
 642     ) -> Option<()> {
 643         // Perform any special handling for specific Rvalue types.
 644         // Generally, checks here fall into one of two categories:
 645         //   1. Additional checking to provide useful lints to the user
 646         //        - In this case, we will do some validation and then fall through to the
 647         //          end of the function which evals the assignment.
 648         //   2. Working around bugs in other parts of the compiler
 649         //        - In this case, we'll return `None` from this function to stop evaluation.
 650         match rvalue {
 651             // Additional checking: give lints to the user if an overflow would occur.
 652             // We do this here and not in the `Assert` terminator as that terminator is
 653             // only sometimes emitted (overflow checks can be disabled), but we want to always
 654             // lint.
 655             Rvalue::UnaryOp(op, arg) => {
 656                 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
 657                 self.check_unary_op(*op, arg, source_info)?;
 658             }
 659             Rvalue::BinaryOp(op, left, right) => {
 660                 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
 661                 self.check_binary_op(*op, left, right, source_info)?;
 662             }
 663             Rvalue::CheckedBinaryOp(op, left, right) => {
 664                 trace!(
 665                     "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
 666                     op,
 667                     left,
 668                     right
 669                 );
 670                 self.check_binary_op(*op, left, right, source_info)?;
 671             }
 672
 673             // Do not try creating references (#67862)
 674             Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
 675                 trace!("skipping AddressOf | Ref for {:?}", place);
 676
 677                 // This may be creating mutable references or immutable references to cells.
 678                 // If that happens, the pointed to value could be mutated via that reference.
 679                 // Since we aren't tracking references, the const propagator loses track of what
 680                 // value the local has right now.
 681                 // Thus, all locals that have their reference taken
 682                 // must not take part in propagation.
 683                 Self::remove_const(&mut self.ecx, place.local);
 684
 685                 return None;
 686             }
 687             Rvalue::ThreadLocalRef(def_id) => {
 688                 trace!("skipping ThreadLocalRef({:?})", def_id);
 689
 690                 return None;
 691             }
 692
 693             // There's no other checking to do at this time.
 694             Rvalue::Aggregate(..)
 695             | Rvalue::Use(..)
 696             | Rvalue::Repeat(..)
 697             | Rvalue::Len(..)
 698             | Rvalue::Cast(..)
 699             | Rvalue::Discriminant(..)
 700             | Rvalue::NullaryOp(..) => {}
 701         }
 702
 703         // FIXME we need to revisit this for #67176
 704         if rvalue.needs_subst() {
 705             return None;
 706         }
 707
 708         if self.tcx.sess.opts.debugging_opts.mir_opt_level >= 3 {
 709             self.eval_rvalue_with_identities(rvalue, place)
 710         } else {
 711             self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
 712         }
 713     }
 714
 715     // Attempt to use albegraic identities to eliminate constant expressions
 716     fn eval_rvalue_with_identities(
 717         &mut self,
 718         rvalue: &Rvalue<'tcx>,
 719         place: Place<'tcx>,
 720     ) -> Option<()> {
 721         self.use_ecx(|this| {
 722             match rvalue {
 723                 Rvalue::BinaryOp(op, left, right) | Rvalue::CheckedBinaryOp(op, left, right) => {
 724                     let l = this.ecx.eval_operand(left, None);
 725                     let r = this.ecx.eval_operand(right, None);
 726
 727                     let const_arg = match (l, r) {
 728                         (Ok(x), Err(_)) | (Err(_), Ok(x)) => this.ecx.read_immediate(x)?,
 729                         (Err(e), Err(_)) => return Err(e),
 730                         (Ok(_), Ok(_)) => {
 731                             this.ecx.eval_rvalue_into_place(rvalue, place)?;
 732                             return Ok(());
 733                         }
 734                     };
 735
 736                     let arg_value =
 737                         this.ecx.force_bits(const_arg.to_scalar()?, const_arg.layout.size)?;
 738                     let dest = this.ecx.eval_place(place)?;
 739
 740                     match op {
 741                         BinOp::BitAnd => {
 742                             if arg_value == 0 {
 743                                 this.ecx.write_immediate(*const_arg, dest)?;
 744                             }
 745                         }
 746                         BinOp::BitOr => {
 747                             if arg_value == truncate(u128::MAX, const_arg.layout.size)
 748                                 || (const_arg.layout.ty.is_bool() && arg_value == 1)
 749                             {
 750                                 this.ecx.write_immediate(*const_arg, dest)?;
 751                             }
 752                         }
 753                         BinOp::Mul => {
 754                             if const_arg.layout.ty.is_integral() && arg_value == 0 {
 755                                 if let Rvalue::CheckedBinaryOp(_, _, _) = rvalue {
 756                                     let val = Immediate::ScalarPair(
 757                                         const_arg.to_scalar()?.into(),
 758                                         Scalar::from_bool(false).into(),
 759                                     );
 760                                     this.ecx.write_immediate(val, dest)?;
 761                                 } else {
 762                                     this.ecx.write_immediate(*const_arg, dest)?;
 763                                 }
 764                             }
 765                         }
 766                         _ => {
 767                             this.ecx.eval_rvalue_into_place(rvalue, place)?;
 768                         }
 769                     }
 770                 }
 771                 _ => {
 772                     this.ecx.eval_rvalue_into_place(rvalue, place)?;
 773                 }
 774             }
 775
 776             Ok(())
 777         })
 778     }
 779
 780     /// Creates a new `Operand::Constant` from a `Scalar` value
 781     fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
 782         Operand::Constant(Box::new(Constant {
 783             span,
 784             user_ty: None,
 785             literal: ty::Const::from_scalar(self.tcx, scalar, ty),
 786         }))
 787     }
 788
 789     fn replace_with_const(
 790         &mut self,
 791         rval: &mut Rvalue<'tcx>,
 792         value: OpTy<'tcx>,
 793         source_info: SourceInfo,
 794     ) {
 795         if let Rvalue::Use(Operand::Constant(c)) = rval {
 796             if !matches!(c.literal.val, ConstKind::Unevaluated(..)) {
 797                 trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
 798                 return;
 799             }
 800         }
 801
 802         trace!("attepting to replace {:?} with {:?}", rval, value);
 803         if let Err(e) = self.ecx.const_validate_operand(
 804             value,
 805             vec![],
 806             // FIXME: is ref tracking too expensive?
 807             &mut interpret::RefTracking::empty(),
 808             /*may_ref_to_static*/ true,
 809         ) {
 810             trace!("validation error, attempt failed: {:?}", e);
 811             return;
 812         }
 813
 814         // FIXME> figure out what to do when try_read_immediate fails
 815         let imm = self.use_ecx(|this| this.ecx.try_read_immediate(value));
 816
 817         if let Some(Ok(imm)) = imm {
 818             match *imm {
 819                 interpret::Immediate::Scalar(ScalarMaybeUninit::Scalar(scalar)) => {
 820                     *rval = Rvalue::Use(self.operand_from_scalar(
 821                         scalar,
 822                         value.layout.ty,
 823                         source_info.span,
 824                     ));
 825                 }
 826                 Immediate::ScalarPair(
 827                     ScalarMaybeUninit::Scalar(one),
 828                     ScalarMaybeUninit::Scalar(two),
 829                 ) => {
 830                     // Found a value represented as a pair. For now only do cont-prop if type of
 831                     // Rvalue is also a pair with two scalars. The more general case is more
 832                     // complicated to implement so we'll do it later.
 833                     // FIXME: implement the general case stated above ^.
 834                     let ty = &value.layout.ty.kind;
 835                     // Only do it for tuples
 836                     if let ty::Tuple(substs) = ty {
 837                         // Only do it if tuple is also a pair with two scalars
 838                         if substs.len() == 2 {
 839                             let opt_ty1_ty2 = self.use_ecx(|this| {
 840                                 let ty1 = substs[0].expect_ty();
 841                                 let ty2 = substs[1].expect_ty();
 842                                 let ty_is_scalar = |ty| {
 843                                     this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
 844                                         == Some(true)
 845                                 };
 846                                 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
 847                                     Ok(Some((ty1, ty2)))
 848                                 } else {
 849                                     Ok(None)
 850                                 }
 851                             });
 852
 853                             if let Some(Some((ty1, ty2))) = opt_ty1_ty2 {
 854                                 *rval = Rvalue::Aggregate(
 855                                     Box::new(AggregateKind::Tuple),
 856                                     vec![
 857                                         self.operand_from_scalar(one, ty1, source_info.span),
 858                                         self.operand_from_scalar(two, ty2, source_info.span),
 859                                     ],
 860                                 );
 861                             }
 862                         }
 863                     }
 864                 }
 865                 _ => {}
 866             }
 867         }
 868     }
 869
 870     /// Returns `true` if and only if this `op` should be const-propagated into.
 871     fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool {
 872         let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level;
 873
 874         if mir_opt_level == 0 {
 875             return false;
 876         }
 877
 878         match *op {
 879             interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUninit::Scalar(s))) => {
 880                 s.is_bits()
 881             }
 882             interpret::Operand::Immediate(Immediate::ScalarPair(
 883                 ScalarMaybeUninit::Scalar(l),
 884                 ScalarMaybeUninit::Scalar(r),
 885             )) => l.is_bits() && r.is_bits(),
 886             _ => false,
 887         }
 888     }
 889 }
 890
 891 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
 892 #[derive(Clone, Copy, Debug, PartialEq)]
 893 enum ConstPropMode {
 894     /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
 895     FullConstProp,
 896     /// The `Local` can only be propagated into and from its own block.
 897     OnlyInsideOwnBlock,
 898     /// The `Local` can be propagated into but reads cannot be propagated.
 899     OnlyPropagateInto,
 900     /// The `Local` cannot be part of propagation at all. Any statement
 901     /// referencing it either for reading or writing will not get propagated.
 902     NoPropagation,
 903 }
 904
 905 struct CanConstProp {
 906     can_const_prop: IndexVec<Local, ConstPropMode>,
 907     // False at the beginning. Once set, no more assignments are allowed to that local.
 908     found_assignment: BitSet<Local>,
 909     // Cache of locals' information
 910     local_kinds: IndexVec<Local, LocalKind>,
 911 }
 912
 913 impl CanConstProp {
 914     /// Returns true if `local` can be propagated
 915     fn check(
 916         tcx: TyCtxt<'tcx>,
 917         param_env: ParamEnv<'tcx>,
 918         body: &Body<'tcx>,
 919     ) -> IndexVec<Local, ConstPropMode> {
 920         let mut cpv = CanConstProp {
 921             can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
 922             found_assignment: BitSet::new_empty(body.local_decls.len()),
 923             local_kinds: IndexVec::from_fn_n(
 924                 |local| body.local_kind(local),
 925                 body.local_decls.len(),
 926             ),
 927         };
 928         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 929             let ty = body.local_decls[local].ty;
 930             match tcx.layout_of(param_env.and(ty)) {
 931                 Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
 932                 // Either the layout fails to compute, then we can't use this local anyway
 933                 // or the local is too large, then we don't want to.
 934                 _ => {
 935                     *val = ConstPropMode::NoPropagation;
 936                     continue;
 937                 }
 938             }
 939             // Cannot use args at all
 940             // Cannot use locals because if x < y { y - x } else { x - y } would
 941             //        lint for x != y
 942             // FIXME(oli-obk): lint variables until they are used in a condition
 943             // FIXME(oli-obk): lint if return value is constant
 944             if cpv.local_kinds[local] == LocalKind::Arg {
 945                 *val = ConstPropMode::OnlyPropagateInto;
 946                 trace!(
 947                     "local {:?} can't be const propagated because it's a function argument",
 948                     local
 949                 );
 950             } else if cpv.local_kinds[local] == LocalKind::Var {
 951                 *val = ConstPropMode::OnlyInsideOwnBlock;
 952                 trace!(
 953                     "local {:?} will only be propagated inside its block, because it's a user variable",
 954                     local
 955                 );
 956             }
 957         }
 958         cpv.visit_body(&body);
 959         cpv.can_const_prop
 960     }
 961 }
 962
 963 impl<'tcx> Visitor<'tcx> for CanConstProp {
 964     fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
 965         use rustc_middle::mir::visit::PlaceContext::*;
 966         match context {
 967             // Projections are fine, because `&mut foo.x` will be caught by
 968             // `MutatingUseContext::Borrow` elsewhere.
 969             MutatingUse(MutatingUseContext::Projection)
 970             // These are just stores, where the storing is not propagatable, but there may be later
 971             // mutations of the same local via `Store`
 972             | MutatingUse(MutatingUseContext::Call)
 973             // Actual store that can possibly even propagate a value
 974             | MutatingUse(MutatingUseContext::Store) => {
 975                 if !self.found_assignment.insert(local) {
 976                     match &mut self.can_const_prop[local] {
 977                         // If the local can only get propagated in its own block, then we don't have
 978                         // to worry about multiple assignments, as we'll nuke the const state at the
 979                         // end of the block anyway, and inside the block we overwrite previous
 980                         // states as applicable.
 981                         ConstPropMode::OnlyInsideOwnBlock => {}
 982                         ConstPropMode::NoPropagation => {}
 983                         ConstPropMode::OnlyPropagateInto => {}
 984                         other @ ConstPropMode::FullConstProp => {
 985                             trace!(
 986                                 "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
 987                                 local, other,
 988                             );
 989                             *other = ConstPropMode::OnlyInsideOwnBlock;
 990                         }
 991                     }
 992                 }
 993             }
 994             // Reading constants is allowed an arbitrary number of times
 995             NonMutatingUse(NonMutatingUseContext::Copy)
 996             | NonMutatingUse(NonMutatingUseContext::Move)
 997             | NonMutatingUse(NonMutatingUseContext::Inspect)
 998             | NonMutatingUse(NonMutatingUseContext::Projection)
 999             | NonUse(_) => {}
1000
1001             // These could be propagated with a smarter analysis or just some careful thinking about
1002             // whether they'd be fine right now.
1003             MutatingUse(MutatingUseContext::AsmOutput)
1004             | MutatingUse(MutatingUseContext::Yield)
1005             | MutatingUse(MutatingUseContext::Drop)
1006             | MutatingUse(MutatingUseContext::Retag)
1007             // These can't ever be propagated under any scheme, as we can't reason about indirect
1008             // mutation.
1009             | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
1010             | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
1011             | NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
1012             | NonMutatingUse(NonMutatingUseContext::AddressOf)
1013             | MutatingUse(MutatingUseContext::Borrow)
1014             | MutatingUse(MutatingUseContext::AddressOf) => {
1015                 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
1016                 self.can_const_prop[local] = ConstPropMode::NoPropagation;
1017             }
1018         }
1019     }
1020 }
1021
1022 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
1023     fn tcx(&self) -> TyCtxt<'tcx> {
1024         self.tcx
1025     }
1026
1027     fn visit_body(&mut self, body: &mut Body<'tcx>) {
1028         for (bb, data) in body.basic_blocks_mut().iter_enumerated_mut() {
1029             self.visit_basic_block_data(bb, data);
1030         }
1031     }
1032
1033     fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
1034         self.super_operand(operand, location);
1035
1036         // Only const prop copies and moves on `mir_opt_level=3` as doing so
1037         // currently increases compile time.
1038         if self.tcx.sess.opts.debugging_opts.mir_opt_level >= 3 {
1039             self.propagate_operand(operand)
1040         }
1041     }
1042
1043     fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
1044         trace!("visit_constant: {:?}", constant);
1045         self.super_constant(constant, location);
1046         self.eval_constant(constant, self.source_info.unwrap());
1047     }
1048
1049     fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
1050         trace!("visit_statement: {:?}", statement);
1051         let source_info = statement.source_info;
1052         self.source_info = Some(source_info);
1053         if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
1054             let can_const_prop = self.ecx.machine.can_const_prop[place.local];
1055             if let Some(()) = self.const_prop(rval, source_info, place) {
1056                 // This will return None if the above `const_prop` invocation only "wrote" a
1057                 // type whose creation requires no write. E.g. a generator whose initial state
1058                 // consists solely of uninitialized memory (so it doesn't capture any locals).
1059                 if let Some(value) = self.get_const(place) {
1060                     if self.should_const_prop(value) {
1061                         trace!("replacing {:?} with {:?}", rval, value);
1062                         self.replace_with_const(rval, value, source_info);
1063                         if can_const_prop == ConstPropMode::FullConstProp
1064                             || can_const_prop == ConstPropMode::OnlyInsideOwnBlock
1065                         {
1066                             trace!("propagated into {:?}", place);
1067                         }
1068                     }
1069                 }
1070                 match can_const_prop {
1071                     ConstPropMode::OnlyInsideOwnBlock => {
1072                         trace!(
1073                             "found local restricted to its block. \
1074                                 Will remove it from const-prop after block is finished. Local: {:?}",
1075                             place.local
1076                         );
1077                     }
1078                     ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1079                         trace!("can't propagate into {:?}", place);
1080                         if place.local != RETURN_PLACE {
1081                             Self::remove_const(&mut self.ecx, place.local);
1082                         }
1083                     }
1084                     ConstPropMode::FullConstProp => {}
1085                 }
1086             } else {
1087                 // Const prop failed, so erase the destination, ensuring that whatever happens
1088                 // from here on, does not know about the previous value.
1089                 // This is important in case we have
1090                 // ```rust
1091                 // let mut x = 42;
1092                 // x = SOME_MUTABLE_STATIC;
1093                 // // x must now be uninit
1094                 // ```
1095                 // FIXME: we overzealously erase the entire local, because that's easier to
1096                 // implement.
1097                 trace!(
1098                     "propagation into {:?} failed.
1099                         Nuking the entire site from orbit, it's the only way to be sure",
1100                     place,
1101                 );
1102                 Self::remove_const(&mut self.ecx, place.local);
1103             }
1104         } else {
1105             match statement.kind {
1106                 StatementKind::SetDiscriminant { ref place, .. } => {
1107                     match self.ecx.machine.can_const_prop[place.local] {
1108                         ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
1109                             if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
1110                                 trace!("propped discriminant into {:?}", place);
1111                             } else {
1112                                 Self::remove_const(&mut self.ecx, place.local);
1113                             }
1114                         }
1115                         ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1116                             Self::remove_const(&mut self.ecx, place.local);
1117                         }
1118                     }
1119                 }
1120                 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
1121                     let frame = self.ecx.frame_mut();
1122                     frame.locals[local].value =
1123                         if let StatementKind::StorageLive(_) = statement.kind {
1124                             LocalValue::Uninitialized
1125                         } else {
1126                             LocalValue::Dead
1127                         };
1128                 }
1129                 _ => {}
1130             }
1131         }
1132
1133         self.super_statement(statement, location);
1134     }
1135
1136     fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
1137         let source_info = terminator.source_info;
1138         self.source_info = Some(source_info);
1139         self.super_terminator(terminator, location);
1140         match &mut terminator.kind {
1141             TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
1142                 if let Some(value) = self.eval_operand(&cond, source_info) {
1143                     trace!("assertion on {:?} should be {:?}", value, expected);
1144                     let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
1145                     let value_const = self.ecx.read_scalar(value).unwrap();
1146                     if expected != value_const {
1147                         enum DbgVal<T> {
1148                             Val(T),
1149                             Underscore,
1150                         }
1151                         impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
1152                             fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
1153                                 match self {
1154                                     Self::Val(val) => val.fmt(fmt),
1155                                     Self::Underscore => fmt.write_str("_"),
1156                                 }
1157                             }
1158                         }
1159                         let mut eval_to_int = |op| {
1160                             // This can be `None` if the lhs wasn't const propagated and we just
1161                             // triggered the assert on the value of the rhs.
1162                             match self.eval_operand(op, source_info) {
1163                                 Some(op) => {
1164                                     DbgVal::Val(self.ecx.read_immediate(op).unwrap().to_const_int())
1165                                 }
1166                                 None => DbgVal::Underscore,
1167                             }
1168                         };
1169                         let msg = match msg {
1170                             AssertKind::DivisionByZero(op) => {
1171                                 Some(AssertKind::DivisionByZero(eval_to_int(op)))
1172                             }
1173                             AssertKind::RemainderByZero(op) => {
1174                                 Some(AssertKind::RemainderByZero(eval_to_int(op)))
1175                             }
1176                             AssertKind::BoundsCheck { ref len, ref index } => {
1177                                 let len = eval_to_int(len);
1178                                 let index = eval_to_int(index);
1179                                 Some(AssertKind::BoundsCheck { len, index })
1180                             }
1181                             // Overflow is are already covered by checks on the binary operators.
1182                             AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None,
1183                             // Need proper const propagator for these.
1184                             _ => None,
1185                         };
1186                         // Poison all places this operand references so that further code
1187                         // doesn't use the invalid value
1188                         match cond {
1189                             Operand::Move(ref place) | Operand::Copy(ref place) => {
1190                                 Self::remove_const(&mut self.ecx, place.local);
1191                             }
1192                             Operand::Constant(_) => {}
1193                         }
1194                         if let Some(msg) = msg {
1195                             self.report_assert_as_lint(
1196                                 lint::builtin::UNCONDITIONAL_PANIC,
1197                                 source_info,
1198                                 "this operation will panic at runtime",
1199                                 msg,
1200                             );
1201                         }
1202                     } else {
1203                         if self.should_const_prop(value) {
1204                             if let ScalarMaybeUninit::Scalar(scalar) = value_const {
1205                                 *cond = self.operand_from_scalar(
1206                                     scalar,
1207                                     self.tcx.types.bool,
1208                                     source_info.span,
1209                                 );
1210                             }
1211                         }
1212                     }
1213                 }
1214             }
1215             TerminatorKind::SwitchInt { ref mut discr, .. } => {
1216                 // FIXME: This is currently redundant with `visit_operand`, but sadly
1217                 // always visiting operands currently causes a perf regression in LLVM codegen, so
1218                 // `visit_operand` currently only runs for propagates places for `mir_opt_level=3`.
1219                 self.propagate_operand(discr)
1220             }
1221             // None of these have Operands to const-propagate.
1222             TerminatorKind::Goto { .. }
1223             | TerminatorKind::Resume
1224             | TerminatorKind::Abort
1225             | TerminatorKind::Return
1226             | TerminatorKind::Unreachable
1227             | TerminatorKind::Drop { .. }
1228             | TerminatorKind::DropAndReplace { .. }
1229             | TerminatorKind::Yield { .. }
1230             | TerminatorKind::GeneratorDrop
1231             | TerminatorKind::FalseEdge { .. }
1232             | TerminatorKind::FalseUnwind { .. }
1233             | TerminatorKind::InlineAsm { .. } => {}
1234             // Every argument in our function calls have already been propagated in `visit_operand`.
1235             //
1236             // NOTE: because LLVM codegen gives performance regressions with it, so this is gated
1237             // on `mir_opt_level=3`.
1238             TerminatorKind::Call { .. } => {}
1239         }
1240
1241         // We remove all Locals which are restricted in propagation to their containing blocks and
1242         // which were modified in the current block.
1243         // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
1244         let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
1245         for &local in locals.iter() {
1246             Self::remove_const(&mut self.ecx, local);
1247         }
1248         locals.clear();
1249         // Put it back so we reuse the heap of the storage
1250         self.ecx.machine.written_only_inside_own_block_locals = locals;
1251         if cfg!(debug_assertions) {
1252             // Ensure we are correctly erasing locals with the non-debug-assert logic.
1253             for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
1254                 assert!(
1255                     self.get_const(local.into()).is_none()
1256                         || self
1257                             .layout_of(self.local_decls[local].ty)
1258                             .map_or(true, |layout| layout.is_zst())
1259                 )
1260             }
1261         }
1262     }
1263 }