compiler/rustc_mir_transform/src/const_prop_lint.rs

   1 //! Propagates constants for early reporting of statically known
   2 //! assertion failures
   3
   4 use crate::const_prop::CanConstProp;
   5 use crate::const_prop::ConstPropMachine;
   6 use crate::const_prop::ConstPropMode;
   7 use crate::MirLint;
   8 use rustc_const_eval::interpret::Immediate;
   9 use rustc_const_eval::interpret::{
  10     self, InterpCx, InterpResult, LocalState, LocalValue, MemoryKind, OpTy, Scalar, StackPopCleanup,
  11 };
  12 use rustc_hir::def::DefKind;
  13 use rustc_hir::HirId;
  14 use rustc_index::bit_set::BitSet;
  15 use rustc_index::vec::IndexVec;
  16 use rustc_middle::mir::visit::Visitor;
  17 use rustc_middle::mir::{
  18     AssertKind, BinOp, Body, Constant, Local, LocalDecl, Location, Operand, Place, Rvalue,
  19     SourceInfo, SourceScope, SourceScopeData, Statement, StatementKind, Terminator, TerminatorKind,
  20     UnOp, RETURN_PLACE,
  21 };
  22 use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
  23 use rustc_middle::ty::InternalSubsts;
  24 use rustc_middle::ty::{self, ConstInt, Instance, ParamEnv, ScalarInt, Ty, TyCtxt, TypeVisitable};
  25 use rustc_session::lint;
  26 use rustc_span::Span;
  27 use rustc_target::abi::{HasDataLayout, Size, TargetDataLayout};
  28 use rustc_trait_selection::traits;
  29 use std::cell::Cell;
  30
  31 /// The maximum number of bytes that we'll allocate space for a local or the return value.
  32 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
  33 /// Severely regress performance.
  34 const MAX_ALLOC_LIMIT: u64 = 1024;
  35 pub struct ConstProp;
  36
  37 impl<'tcx> MirLint<'tcx> for ConstProp {
  38     fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
  39         // will be evaluated by miri and produce its errors there
  40         if body.source.promoted.is_some() {
  41             return;
  42         }
  43
  44         let def_id = body.source.def_id().expect_local();
  45         let is_fn_like = tcx.def_kind(def_id).is_fn_like();
  46         let is_assoc_const = tcx.def_kind(def_id) == DefKind::AssocConst;
  47
  48         // Only run const prop on functions, methods, closures and associated constants
  49         if !is_fn_like && !is_assoc_const {
  50             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  51             trace!("ConstProp skipped for {:?}", def_id);
  52             return;
  53         }
  54
  55         let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
  56         // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
  57         // computing their layout.
  58         if is_generator {
  59             trace!("ConstProp skipped for generator {:?}", def_id);
  60             return;
  61         }
  62
  63         // Check if it's even possible to satisfy the 'where' clauses
  64         // for this item.
  65         // This branch will never be taken for any normal function.
  66         // However, it's possible to `#!feature(trivial_bounds)]` to write
  67         // a function with impossible to satisfy clauses, e.g.:
  68         // `fn foo() where String: Copy {}`
  69         //
  70         // We don't usually need to worry about this kind of case,
  71         // since we would get a compilation error if the user tried
  72         // to call it. However, since we can do const propagation
  73         // even without any calls to the function, we need to make
  74         // sure that it even makes sense to try to evaluate the body.
  75         // If there are unsatisfiable where clauses, then all bets are
  76         // off, and we just give up.
  77         //
  78         // We manually filter the predicates, skipping anything that's not
  79         // "global". We are in a potentially generic context
  80         // (e.g. we are evaluating a function without substituting generic
  81         // parameters, so this filtering serves two purposes:
  82         //
  83         // 1. We skip evaluating any predicates that we would
  84         // never be able prove are unsatisfiable (e.g. `<T as Foo>`
  85         // 2. We avoid trying to normalize predicates involving generic
  86         // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
  87         // the normalization code (leading to cycle errors), since
  88         // it's usually never invoked in this way.
  89         let predicates = tcx
  90             .predicates_of(def_id.to_def_id())
  91             .predicates
  92             .iter()
  93             .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
  94         if traits::impossible_predicates(
  95             tcx,
  96             traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
  97         ) {
  98             trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
  99             return;
 100         }
 101
 102         trace!("ConstProp starting for {:?}", def_id);
 103
 104         let dummy_body = &Body::new(
 105             body.source,
 106             (*body.basic_blocks).clone(),
 107             body.source_scopes.clone(),
 108             body.local_decls.clone(),
 109             Default::default(),
 110             body.arg_count,
 111             Default::default(),
 112             body.span,
 113             body.generator_kind(),
 114             body.tainted_by_errors,
 115         );
 116
 117         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
 118         // constants, instead of just checking for const-folding succeeding.
 119         // That would require a uniform one-def no-mutation analysis
 120         // and RPO (or recursing when needing the value of a local).
 121         let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
 122         optimization_finder.visit_body(body);
 123
 124         trace!("ConstProp done for {:?}", def_id);
 125     }
 126 }
 127
 128 /// Finds optimization opportunities on the MIR.
 129 struct ConstPropagator<'mir, 'tcx> {
 130     ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
 131     tcx: TyCtxt<'tcx>,
 132     param_env: ParamEnv<'tcx>,
 133     source_scopes: &'mir IndexVec<SourceScope, SourceScopeData<'tcx>>,
 134     local_decls: &'mir IndexVec<Local, LocalDecl<'tcx>>,
 135     // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
 136     // the last known `SourceInfo` here and just keep revisiting it.
 137     source_info: Option<SourceInfo>,
 138 }
 139
 140 impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
 141     type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
 142
 143     #[inline]
 144     fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
 145         err
 146     }
 147 }
 148
 149 impl HasDataLayout for ConstPropagator<'_, '_> {
 150     #[inline]
 151     fn data_layout(&self) -> &TargetDataLayout {
 152         &self.tcx.data_layout
 153     }
 154 }
 155
 156 impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
 157     #[inline]
 158     fn tcx(&self) -> TyCtxt<'tcx> {
 159         self.tcx
 160     }
 161 }
 162
 163 impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
 164     #[inline]
 165     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 166         self.param_env
 167     }
 168 }
 169
 170 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 171     fn new(
 172         body: &Body<'tcx>,
 173         dummy_body: &'mir Body<'tcx>,
 174         tcx: TyCtxt<'tcx>,
 175     ) -> ConstPropagator<'mir, 'tcx> {
 176         let def_id = body.source.def_id();
 177         let substs = &InternalSubsts::identity_for_item(tcx, def_id);
 178         let param_env = tcx.param_env_reveal_all_normalized(def_id);
 179
 180         let can_const_prop = CanConstProp::check(tcx, param_env, body);
 181         let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
 182         for (l, mode) in can_const_prop.iter_enumerated() {
 183             if *mode == ConstPropMode::OnlyInsideOwnBlock {
 184                 only_propagate_inside_block_locals.insert(l);
 185             }
 186         }
 187         let mut ecx = InterpCx::new(
 188             tcx,
 189             tcx.def_span(def_id),
 190             param_env,
 191             ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
 192         );
 193
 194         let ret_layout = ecx
 195             .layout_of(body.bound_return_ty().subst(tcx, substs))
 196             .ok()
 197             // Don't bother allocating memory for large values.
 198             // I don't know how return types can seem to be unsized but this happens in the
 199             // `type/type-unsatisfiable.rs` test.
 200             .filter(|ret_layout| {
 201                 ret_layout.is_sized() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
 202             })
 203             .unwrap_or_else(|| ecx.layout_of(tcx.types.unit).unwrap());
 204
 205         let ret = ecx
 206             .allocate(ret_layout, MemoryKind::Stack)
 207             .expect("couldn't perform small allocation")
 208             .into();
 209
 210         ecx.push_stack_frame(
 211             Instance::new(def_id, substs),
 212             dummy_body,
 213             &ret,
 214             StackPopCleanup::Root { cleanup: false },
 215         )
 216         .expect("failed to push initial stack frame");
 217
 218         ConstPropagator {
 219             ecx,
 220             tcx,
 221             param_env,
 222             source_scopes: &dummy_body.source_scopes,
 223             local_decls: &dummy_body.local_decls,
 224             source_info: None,
 225         }
 226     }
 227
 228     fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
 229         let op = match self.ecx.eval_place_to_op(place, None) {
 230             Ok(op) => {
 231                 if matches!(*op, interpret::Operand::Immediate(Immediate::Uninit)) {
 232                     // Make sure nobody accidentally uses this value.
 233                     return None;
 234                 }
 235                 op
 236             }
 237             Err(e) => {
 238                 trace!("get_const failed: {}", e);
 239                 return None;
 240             }
 241         };
 242
 243         // Try to read the local as an immediate so that if it is representable as a scalar, we can
 244         // handle it as such, but otherwise, just return the value as is.
 245         Some(match self.ecx.read_immediate_raw(&op) {
 246             Ok(Ok(imm)) => imm.into(),
 247             _ => op,
 248         })
 249     }
 250
 251     /// Remove `local` from the pool of `Locals`. Allows writing to them,
 252     /// but not reading from them anymore.
 253     fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
 254         ecx.frame_mut().locals[local] = LocalState {
 255             value: LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit)),
 256             layout: Cell::new(None),
 257         };
 258     }
 259
 260     fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
 261         source_info.scope.lint_root(self.source_scopes)
 262     }
 263
 264     fn use_ecx<F, T>(&mut self, source_info: SourceInfo, f: F) -> Option<T>
 265     where
 266         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 267     {
 268         // Overwrite the PC -- whatever the interpreter does to it does not make any sense anyway.
 269         self.ecx.frame_mut().loc = Err(source_info.span);
 270         match f(self) {
 271             Ok(val) => Some(val),
 272             Err(error) => {
 273                 trace!("InterpCx operation failed: {:?}", error);
 274                 // Some errors shouldn't come up because creating them causes
 275                 // an allocation, which we should avoid. When that happens,
 276                 // dedicated error variants should be introduced instead.
 277                 assert!(
 278                     !error.kind().formatted_string(),
 279                     "const-prop encountered formatting error: {}",
 280                     error
 281                 );
 282                 None
 283             }
 284         }
 285     }
 286
 287     /// Returns the value, if any, of evaluating `c`.
 288     fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 289         // FIXME we need to revisit this for #67176
 290         if c.needs_subst() {
 291             return None;
 292         }
 293
 294         self.use_ecx(source_info, |this| this.ecx.eval_mir_constant(&c.literal, Some(c.span), None))
 295     }
 296
 297     /// Returns the value, if any, of evaluating `place`.
 298     fn eval_place(&mut self, place: Place<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 299         trace!("eval_place(place={:?})", place);
 300         self.use_ecx(source_info, |this| this.ecx.eval_place_to_op(place, None))
 301     }
 302
 303     /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
 304     /// or `eval_place`, depending on the variant of `Operand` used.
 305     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 306         match *op {
 307             Operand::Constant(ref c) => self.eval_constant(c, source_info),
 308             Operand::Move(place) | Operand::Copy(place) => self.eval_place(place, source_info),
 309         }
 310     }
 311
 312     fn report_assert_as_lint(
 313         &self,
 314         lint: &'static lint::Lint,
 315         source_info: SourceInfo,
 316         message: &'static str,
 317         panic: AssertKind<impl std::fmt::Debug>,
 318     ) {
 319         if let Some(lint_root) = self.lint_root(source_info) {
 320             self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, message, |lint| {
 321                 lint.span_label(source_info.span, format!("{:?}", panic))
 322             });
 323         }
 324     }
 325
 326     fn check_unary_op(
 327         &mut self,
 328         op: UnOp,
 329         arg: &Operand<'tcx>,
 330         source_info: SourceInfo,
 331     ) -> Option<()> {
 332         if let (val, true) = self.use_ecx(source_info, |this| {
 333             let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
 334             let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
 335             Ok((val, overflow))
 336         })? {
 337             // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
 338             // appropriate to use.
 339             assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
 340             self.report_assert_as_lint(
 341                 lint::builtin::ARITHMETIC_OVERFLOW,
 342                 source_info,
 343                 "this arithmetic operation will overflow",
 344                 AssertKind::OverflowNeg(val.to_const_int()),
 345             );
 346             return None;
 347         }
 348
 349         Some(())
 350     }
 351
 352     fn check_binary_op(
 353         &mut self,
 354         op: BinOp,
 355         left: &Operand<'tcx>,
 356         right: &Operand<'tcx>,
 357         source_info: SourceInfo,
 358     ) -> Option<()> {
 359         let r = self.use_ecx(source_info, |this| {
 360             this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?)
 361         });
 362         let l = self.use_ecx(source_info, |this| {
 363             this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?)
 364         });
 365         // Check for exceeding shifts *even if* we cannot evaluate the LHS.
 366         if op == BinOp::Shr || op == BinOp::Shl {
 367             let r = r.clone()?;
 368             // We need the type of the LHS. We cannot use `place_layout` as that is the type
 369             // of the result, which for checked binops is not the same!
 370             let left_ty = left.ty(self.local_decls, self.tcx);
 371             let left_size = self.ecx.layout_of(left_ty).ok()?.size;
 372             let right_size = r.layout.size;
 373             let r_bits = r.to_scalar().to_bits(right_size).ok();
 374             if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
 375                 debug!("check_binary_op: reporting assert for {:?}", source_info);
 376                 self.report_assert_as_lint(
 377                     lint::builtin::ARITHMETIC_OVERFLOW,
 378                     source_info,
 379                     "this arithmetic operation will overflow",
 380                     AssertKind::Overflow(
 381                         op,
 382                         match l {
 383                             Some(l) => l.to_const_int(),
 384                             // Invent a dummy value, the diagnostic ignores it anyway
 385                             None => ConstInt::new(
 386                                 ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
 387                                 left_ty.is_signed(),
 388                                 left_ty.is_ptr_sized_integral(),
 389                             ),
 390                         },
 391                         r.to_const_int(),
 392                     ),
 393                 );
 394                 return None;
 395             }
 396         }
 397
 398         if let (Some(l), Some(r)) = (l, r) {
 399             // The remaining operators are handled through `overflowing_binary_op`.
 400             if self.use_ecx(source_info, |this| {
 401                 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, &l, &r)?;
 402                 Ok(overflow)
 403             })? {
 404                 self.report_assert_as_lint(
 405                     lint::builtin::ARITHMETIC_OVERFLOW,
 406                     source_info,
 407                     "this arithmetic operation will overflow",
 408                     AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
 409                 );
 410                 return None;
 411             }
 412         }
 413         Some(())
 414     }
 415
 416     fn const_prop(
 417         &mut self,
 418         rvalue: &Rvalue<'tcx>,
 419         source_info: SourceInfo,
 420         place: Place<'tcx>,
 421     ) -> Option<()> {
 422         // Perform any special handling for specific Rvalue types.
 423         // Generally, checks here fall into one of two categories:
 424         //   1. Additional checking to provide useful lints to the user
 425         //        - In this case, we will do some validation and then fall through to the
 426         //          end of the function which evals the assignment.
 427         //   2. Working around bugs in other parts of the compiler
 428         //        - In this case, we'll return `None` from this function to stop evaluation.
 429         match rvalue {
 430             // Additional checking: give lints to the user if an overflow would occur.
 431             // We do this here and not in the `Assert` terminator as that terminator is
 432             // only sometimes emitted (overflow checks can be disabled), but we want to always
 433             // lint.
 434             Rvalue::UnaryOp(op, arg) => {
 435                 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
 436                 self.check_unary_op(*op, arg, source_info)?;
 437             }
 438             Rvalue::BinaryOp(op, box (left, right)) => {
 439                 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
 440                 self.check_binary_op(*op, left, right, source_info)?;
 441             }
 442             Rvalue::CheckedBinaryOp(op, box (left, right)) => {
 443                 trace!(
 444                     "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
 445                     op,
 446                     left,
 447                     right
 448                 );
 449                 self.check_binary_op(*op, left, right, source_info)?;
 450             }
 451
 452             // Do not try creating references (#67862)
 453             Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
 454                 trace!("skipping AddressOf | Ref for {:?}", place);
 455
 456                 // This may be creating mutable references or immutable references to cells.
 457                 // If that happens, the pointed to value could be mutated via that reference.
 458                 // Since we aren't tracking references, the const propagator loses track of what
 459                 // value the local has right now.
 460                 // Thus, all locals that have their reference taken
 461                 // must not take part in propagation.
 462                 Self::remove_const(&mut self.ecx, place.local);
 463
 464                 return None;
 465             }
 466             Rvalue::ThreadLocalRef(def_id) => {
 467                 trace!("skipping ThreadLocalRef({:?})", def_id);
 468
 469                 return None;
 470             }
 471
 472             // There's no other checking to do at this time.
 473             Rvalue::Aggregate(..)
 474             | Rvalue::Use(..)
 475             | Rvalue::CopyForDeref(..)
 476             | Rvalue::Repeat(..)
 477             | Rvalue::Len(..)
 478             | Rvalue::Cast(..)
 479             | Rvalue::ShallowInitBox(..)
 480             | Rvalue::Discriminant(..)
 481             | Rvalue::NullaryOp(..) => {}
 482         }
 483
 484         // FIXME we need to revisit this for #67176
 485         if rvalue.needs_subst() {
 486             return None;
 487         }
 488         if !rvalue
 489             .ty(&self.ecx.frame().body.local_decls, *self.ecx.tcx)
 490             .is_sized(*self.ecx.tcx, self.param_env)
 491         {
 492             // the interpreter doesn't support unsized locals (only unsized arguments),
 493             // but rustc does (in a kinda broken way), so we have to skip them here
 494             return None;
 495         }
 496
 497         self.use_ecx(source_info, |this| this.ecx.eval_rvalue_into_place(rvalue, place))
 498     }
 499 }
 500
 501 impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
 502     fn visit_body(&mut self, body: &Body<'tcx>) {
 503         for (bb, data) in body.basic_blocks.iter_enumerated() {
 504             self.visit_basic_block_data(bb, data);
 505         }
 506     }
 507
 508     fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
 509         self.super_operand(operand, location);
 510     }
 511
 512     fn visit_constant(&mut self, constant: &Constant<'tcx>, location: Location) {
 513         trace!("visit_constant: {:?}", constant);
 514         self.super_constant(constant, location);
 515         self.eval_constant(constant, self.source_info.unwrap());
 516     }
 517
 518     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
 519         trace!("visit_statement: {:?}", statement);
 520         let source_info = statement.source_info;
 521         self.source_info = Some(source_info);
 522         if let StatementKind::Assign(box (place, ref rval)) = statement.kind {
 523             let can_const_prop = self.ecx.machine.can_const_prop[place.local];
 524             if let Some(()) = self.const_prop(rval, source_info, place) {
 525                 match can_const_prop {
 526                     ConstPropMode::OnlyInsideOwnBlock => {
 527                         trace!(
 528                             "found local restricted to its block. \
 529                                 Will remove it from const-prop after block is finished. Local: {:?}",
 530                             place.local
 531                         );
 532                     }
 533                     ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
 534                         trace!("can't propagate into {:?}", place);
 535                         if place.local != RETURN_PLACE {
 536                             Self::remove_const(&mut self.ecx, place.local);
 537                         }
 538                     }
 539                     ConstPropMode::FullConstProp => {}
 540                 }
 541             } else {
 542                 // Const prop failed, so erase the destination, ensuring that whatever happens
 543                 // from here on, does not know about the previous value.
 544                 // This is important in case we have
 545                 // ```rust
 546                 // let mut x = 42;
 547                 // x = SOME_MUTABLE_STATIC;
 548                 // // x must now be uninit
 549                 // ```
 550                 // FIXME: we overzealously erase the entire local, because that's easier to
 551                 // implement.
 552                 trace!(
 553                     "propagation into {:?} failed.
 554                         Nuking the entire site from orbit, it's the only way to be sure",
 555                     place,
 556                 );
 557                 Self::remove_const(&mut self.ecx, place.local);
 558             }
 559         } else {
 560             match statement.kind {
 561                 StatementKind::SetDiscriminant { ref place, .. } => {
 562                     match self.ecx.machine.can_const_prop[place.local] {
 563                         ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
 564                             if self
 565                                 .use_ecx(source_info, |this| this.ecx.statement(statement))
 566                                 .is_some()
 567                             {
 568                                 trace!("propped discriminant into {:?}", place);
 569                             } else {
 570                                 Self::remove_const(&mut self.ecx, place.local);
 571                             }
 572                         }
 573                         ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
 574                             Self::remove_const(&mut self.ecx, place.local);
 575                         }
 576                     }
 577                 }
 578                 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
 579                     let frame = self.ecx.frame_mut();
 580                     frame.locals[local].value =
 581                         if let StatementKind::StorageLive(_) = statement.kind {
 582                             LocalValue::Live(interpret::Operand::Immediate(
 583                                 interpret::Immediate::Uninit,
 584                             ))
 585                         } else {
 586                             LocalValue::Dead
 587                         };
 588                 }
 589                 _ => {}
 590             }
 591         }
 592
 593         self.super_statement(statement, location);
 594     }
 595
 596     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
 597         let source_info = terminator.source_info;
 598         self.source_info = Some(source_info);
 599         self.super_terminator(terminator, location);
 600         match &terminator.kind {
 601             TerminatorKind::Assert { expected, ref msg, ref cond, .. } => {
 602                 if let Some(ref value) = self.eval_operand(&cond, source_info) {
 603                     trace!("assertion on {:?} should be {:?}", value, expected);
 604                     let expected = Scalar::from_bool(*expected);
 605                     let Ok(value_const) = self.ecx.read_scalar(&value) else {
 606                         // FIXME should be used use_ecx rather than a local match... but we have
 607                         // quite a few of these read_scalar/read_immediate that need fixing.
 608                         return
 609                     };
 610                     if expected != value_const {
 611                         enum DbgVal<T> {
 612                             Val(T),
 613                             Underscore,
 614                         }
 615                         impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
 616                             fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
 617                                 match self {
 618                                     Self::Val(val) => val.fmt(fmt),
 619                                     Self::Underscore => fmt.write_str("_"),
 620                                 }
 621                             }
 622                         }
 623                         let mut eval_to_int = |op| {
 624                             // This can be `None` if the lhs wasn't const propagated and we just
 625                             // triggered the assert on the value of the rhs.
 626                             self.eval_operand(op, source_info)
 627                                 .and_then(|op| self.ecx.read_immediate(&op).ok())
 628                                 .map_or(DbgVal::Underscore, |op| DbgVal::Val(op.to_const_int()))
 629                         };
 630                         let msg = match msg {
 631                             AssertKind::DivisionByZero(op) => {
 632                                 Some(AssertKind::DivisionByZero(eval_to_int(op)))
 633                             }
 634                             AssertKind::RemainderByZero(op) => {
 635                                 Some(AssertKind::RemainderByZero(eval_to_int(op)))
 636                             }
 637                             AssertKind::Overflow(bin_op @ (BinOp::Div | BinOp::Rem), op1, op2) => {
 638                                 // Division overflow is *UB* in the MIR, and different than the
 639                                 // other overflow checks.
 640                                 Some(AssertKind::Overflow(
 641                                     *bin_op,
 642                                     eval_to_int(op1),
 643                                     eval_to_int(op2),
 644                                 ))
 645                             }
 646                             AssertKind::BoundsCheck { ref len, ref index } => {
 647                                 let len = eval_to_int(len);
 648                                 let index = eval_to_int(index);
 649                                 Some(AssertKind::BoundsCheck { len, index })
 650                             }
 651                             // Remaining overflow errors are already covered by checks on the binary operators.
 652                             AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None,
 653                             // Need proper const propagator for these.
 654                             _ => None,
 655                         };
 656                         // Poison all places this operand references so that further code
 657                         // doesn't use the invalid value
 658                         match cond {
 659                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 660                                 Self::remove_const(&mut self.ecx, place.local);
 661                             }
 662                             Operand::Constant(_) => {}
 663                         }
 664                         if let Some(msg) = msg {
 665                             self.report_assert_as_lint(
 666                                 lint::builtin::UNCONDITIONAL_PANIC,
 667                                 source_info,
 668                                 "this operation will panic at runtime",
 669                                 msg,
 670                             );
 671                         }
 672                     }
 673                 }
 674             }
 675             // None of these have Operands to const-propagate.
 676             TerminatorKind::Goto { .. }
 677             | TerminatorKind::Resume
 678             | TerminatorKind::Abort
 679             | TerminatorKind::Return
 680             | TerminatorKind::Unreachable
 681             | TerminatorKind::Drop { .. }
 682             | TerminatorKind::DropAndReplace { .. }
 683             | TerminatorKind::Yield { .. }
 684             | TerminatorKind::GeneratorDrop
 685             | TerminatorKind::FalseEdge { .. }
 686             | TerminatorKind::FalseUnwind { .. }
 687             | TerminatorKind::SwitchInt { .. }
 688             | TerminatorKind::Call { .. }
 689             | TerminatorKind::InlineAsm { .. } => {}
 690         }
 691
 692         // We remove all Locals which are restricted in propagation to their containing blocks and
 693         // which were modified in the current block.
 694         // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
 695         let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
 696         for &local in locals.iter() {
 697             Self::remove_const(&mut self.ecx, local);
 698         }
 699         locals.clear();
 700         // Put it back so we reuse the heap of the storage
 701         self.ecx.machine.written_only_inside_own_block_locals = locals;
 702         if cfg!(debug_assertions) {
 703             // Ensure we are correctly erasing locals with the non-debug-assert logic.
 704             for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
 705                 assert!(
 706                     self.get_const(local.into()).is_none()
 707                         || self
 708                             .layout_of(self.local_decls[local].ty)
 709                             .map_or(true, |layout| layout.is_zst())
 710                 )
 711             }
 712         }
 713     }
 714 }