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::const_eval::ConstEvalErr;
   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, ConstantKind, Local, LocalDecl, Location, Operand, Place,
  19     Rvalue, SourceInfo, SourceScope, SourceScopeData, Statement, StatementKind, Terminator,
  20     TerminatorKind, UnOp, RETURN_PLACE,
  21 };
  22 use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
  23 use rustc_middle::ty::subst::{InternalSubsts, Subst};
  24 use rustc_middle::ty::{
  25     self, ConstInt, ConstKind, Instance, ParamEnv, ScalarInt, Ty, TyCtxt, TypeVisitable,
  26 };
  27 use rustc_session::lint;
  28 use rustc_span::Span;
  29 use rustc_target::abi::{HasDataLayout, Size, TargetDataLayout};
  30 use rustc_trait_selection::traits;
  31 use std::cell::Cell;
  32
  33 /// The maximum number of bytes that we'll allocate space for a local or the return value.
  34 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
  35 /// Severely regress performance.
  36 const MAX_ALLOC_LIMIT: u64 = 1024;
  37 pub struct ConstProp;
  38
  39 impl<'tcx> MirLint<'tcx> for ConstProp {
  40     fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
  41         // will be evaluated by miri and produce its errors there
  42         if body.source.promoted.is_some() {
  43             return;
  44         }
  45
  46         let def_id = body.source.def_id().expect_local();
  47         let is_fn_like = tcx.def_kind(def_id).is_fn_like();
  48         let is_assoc_const = tcx.def_kind(def_id) == DefKind::AssocConst;
  49
  50         // Only run const prop on functions, methods, closures and associated constants
  51         if !is_fn_like && !is_assoc_const {
  52             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  53             trace!("ConstProp skipped for {:?}", def_id);
  54             return;
  55         }
  56
  57         let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
  58         // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
  59         // computing their layout.
  60         if is_generator {
  61             trace!("ConstProp skipped for generator {:?}", def_id);
  62             return;
  63         }
  64
  65         // Check if it's even possible to satisfy the 'where' clauses
  66         // for this item.
  67         // This branch will never be taken for any normal function.
  68         // However, it's possible to `#!feature(trivial_bounds)]` to write
  69         // a function with impossible to satisfy clauses, e.g.:
  70         // `fn foo() where String: Copy {}`
  71         //
  72         // We don't usually need to worry about this kind of case,
  73         // since we would get a compilation error if the user tried
  74         // to call it. However, since we can do const propagation
  75         // even without any calls to the function, we need to make
  76         // sure that it even makes sense to try to evaluate the body.
  77         // If there are unsatisfiable where clauses, then all bets are
  78         // off, and we just give up.
  79         //
  80         // We manually filter the predicates, skipping anything that's not
  81         // "global". We are in a potentially generic context
  82         // (e.g. we are evaluating a function without substituting generic
  83         // parameters, so this filtering serves two purposes:
  84         //
  85         // 1. We skip evaluating any predicates that we would
  86         // never be able prove are unsatisfiable (e.g. `<T as Foo>`
  87         // 2. We avoid trying to normalize predicates involving generic
  88         // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
  89         // the normalization code (leading to cycle errors), since
  90         // it's usually never invoked in this way.
  91         let predicates = tcx
  92             .predicates_of(def_id.to_def_id())
  93             .predicates
  94             .iter()
  95             .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
  96         if traits::impossible_predicates(
  97             tcx,
  98             traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
  99         ) {
 100             trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
 101             return;
 102         }
 103
 104         trace!("ConstProp starting for {:?}", def_id);
 105
 106         let dummy_body = &Body::new(
 107             body.source,
 108             body.basic_blocks().clone(),
 109             body.source_scopes.clone(),
 110             body.local_decls.clone(),
 111             Default::default(),
 112             body.arg_count,
 113             Default::default(),
 114             body.span,
 115             body.generator_kind(),
 116             body.tainted_by_errors,
 117         );
 118
 119         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
 120         // constants, instead of just checking for const-folding succeeding.
 121         // That would require a uniform one-def no-mutation analysis
 122         // and RPO (or recursing when needing the value of a local).
 123         let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
 124         optimization_finder.visit_body(body);
 125
 126         trace!("ConstProp done for {:?}", def_id);
 127     }
 128 }
 129
 130 /// Finds optimization opportunities on the MIR.
 131 struct ConstPropagator<'mir, 'tcx> {
 132     ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
 133     tcx: TyCtxt<'tcx>,
 134     param_env: ParamEnv<'tcx>,
 135     source_scopes: &'mir IndexVec<SourceScope, SourceScopeData<'tcx>>,
 136     local_decls: &'mir IndexVec<Local, LocalDecl<'tcx>>,
 137     // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
 138     // the last known `SourceInfo` here and just keep revisiting it.
 139     source_info: Option<SourceInfo>,
 140 }
 141
 142 impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
 143     type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
 144
 145     #[inline]
 146     fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
 147         err
 148     }
 149 }
 150
 151 impl HasDataLayout for ConstPropagator<'_, '_> {
 152     #[inline]
 153     fn data_layout(&self) -> &TargetDataLayout {
 154         &self.tcx.data_layout
 155     }
 156 }
 157
 158 impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
 159     #[inline]
 160     fn tcx(&self) -> TyCtxt<'tcx> {
 161         self.tcx
 162     }
 163 }
 164
 165 impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
 166     #[inline]
 167     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 168         self.param_env
 169     }
 170 }
 171
 172 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 173     fn new(
 174         body: &Body<'tcx>,
 175         dummy_body: &'mir Body<'tcx>,
 176         tcx: TyCtxt<'tcx>,
 177     ) -> ConstPropagator<'mir, 'tcx> {
 178         let def_id = body.source.def_id();
 179         let substs = &InternalSubsts::identity_for_item(tcx, def_id);
 180         let param_env = tcx.param_env_reveal_all_normalized(def_id);
 181
 182         let can_const_prop = CanConstProp::check(tcx, param_env, body);
 183         let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
 184         for (l, mode) in can_const_prop.iter_enumerated() {
 185             if *mode == ConstPropMode::OnlyInsideOwnBlock {
 186                 only_propagate_inside_block_locals.insert(l);
 187             }
 188         }
 189         let mut ecx = InterpCx::new(
 190             tcx,
 191             tcx.def_span(def_id),
 192             param_env,
 193             ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
 194         );
 195
 196         let ret_layout = ecx
 197             .layout_of(body.bound_return_ty().subst(tcx, substs))
 198             .ok()
 199             // Don't bother allocating memory for large values.
 200             // I don't know how return types can seem to be unsized but this happens in the
 201             // `type/type-unsatisfiable.rs` test.
 202             .filter(|ret_layout| {
 203                 !ret_layout.is_unsized() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
 204             })
 205             .unwrap_or_else(|| ecx.layout_of(tcx.types.unit).unwrap());
 206
 207         let ret = ecx
 208             .allocate(ret_layout, MemoryKind::Stack)
 209             .expect("couldn't perform small allocation")
 210             .into();
 211
 212         ecx.push_stack_frame(
 213             Instance::new(def_id, substs),
 214             dummy_body,
 215             &ret,
 216             StackPopCleanup::Root { cleanup: false },
 217         )
 218         .expect("failed to push initial stack frame");
 219
 220         ConstPropagator {
 221             ecx,
 222             tcx,
 223             param_env,
 224             source_scopes: &dummy_body.source_scopes,
 225             local_decls: &dummy_body.local_decls,
 226             source_info: None,
 227         }
 228     }
 229
 230     fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
 231         let op = match self.ecx.eval_place_to_op(place, None) {
 232             Ok(op) => op,
 233             Err(e) => {
 234                 trace!("get_const failed: {}", e);
 235                 return None;
 236             }
 237         };
 238
 239         // Try to read the local as an immediate so that if it is representable as a scalar, we can
 240         // handle it as such, but otherwise, just return the value as is.
 241         Some(match self.ecx.read_immediate_raw(&op) {
 242             Ok(Ok(imm)) => imm.into(),
 243             _ => op,
 244         })
 245     }
 246
 247     /// Remove `local` from the pool of `Locals`. Allows writing to them,
 248     /// but not reading from them anymore.
 249     fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
 250         ecx.frame_mut().locals[local] = LocalState {
 251             value: LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit)),
 252             layout: Cell::new(None),
 253         };
 254     }
 255
 256     fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
 257         source_info.scope.lint_root(self.source_scopes)
 258     }
 259
 260     fn use_ecx<F, T>(&mut self, source_info: SourceInfo, f: F) -> Option<T>
 261     where
 262         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 263     {
 264         // Overwrite the PC -- whatever the interpreter does to it does not make any sense anyway.
 265         self.ecx.frame_mut().loc = Err(source_info.span);
 266         match f(self) {
 267             Ok(val) => Some(val),
 268             Err(error) => {
 269                 trace!("InterpCx operation failed: {:?}", error);
 270                 // Some errors shouldn't come up because creating them causes
 271                 // an allocation, which we should avoid. When that happens,
 272                 // dedicated error variants should be introduced instead.
 273                 assert!(
 274                     !error.kind().formatted_string(),
 275                     "const-prop encountered formatting error: {}",
 276                     error
 277                 );
 278                 None
 279             }
 280         }
 281     }
 282
 283     /// Returns the value, if any, of evaluating `c`.
 284     fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 285         // FIXME we need to revisit this for #67176
 286         if c.needs_subst() {
 287             return None;
 288         }
 289
 290         match self.ecx.mir_const_to_op(&c.literal, None) {
 291             Ok(op) => Some(op),
 292             Err(error) => {
 293                 let tcx = self.ecx.tcx.at(c.span);
 294                 let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
 295                 if let Some(lint_root) = self.lint_root(source_info) {
 296                     let lint_only = match c.literal {
 297                         ConstantKind::Ty(ct) => match ct.kind() {
 298                             // Promoteds must lint and not error as the user didn't ask for them
 299                             ConstKind::Unevaluated(ty::Unevaluated {
 300                                 def: _,
 301                                 substs: _,
 302                                 promoted: Some(_),
 303                             }) => true,
 304                             // Out of backwards compatibility we cannot report hard errors in unused
 305                             // generic functions using associated constants of the generic parameters.
 306                             _ => c.literal.needs_subst(),
 307                         },
 308                         ConstantKind::Val(_, ty) => ty.needs_subst(),
 309                     };
 310                     if lint_only {
 311                         // Out of backwards compatibility we cannot report hard errors in unused
 312                         // generic functions using associated constants of the generic parameters.
 313                         err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span));
 314                     } else {
 315                         err.report_as_error(tcx, "erroneous constant used");
 316                     }
 317                 } else {
 318                     err.report_as_error(tcx, "erroneous constant used");
 319                 }
 320                 None
 321             }
 322         }
 323     }
 324
 325     /// Returns the value, if any, of evaluating `place`.
 326     fn eval_place(&mut self, place: Place<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 327         trace!("eval_place(place={:?})", place);
 328         self.use_ecx(source_info, |this| this.ecx.eval_place_to_op(place, None))
 329     }
 330
 331     /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
 332     /// or `eval_place`, depending on the variant of `Operand` used.
 333     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 334         match *op {
 335             Operand::Constant(ref c) => self.eval_constant(c, source_info),
 336             Operand::Move(place) | Operand::Copy(place) => self.eval_place(place, source_info),
 337         }
 338     }
 339
 340     fn report_assert_as_lint(
 341         &self,
 342         lint: &'static lint::Lint,
 343         source_info: SourceInfo,
 344         message: &'static str,
 345         panic: AssertKind<impl std::fmt::Debug>,
 346     ) {
 347         if let Some(lint_root) = self.lint_root(source_info) {
 348             self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
 349                 let mut err = lint.build(message);
 350                 err.span_label(source_info.span, format!("{:?}", panic));
 351                 err.emit();
 352             });
 353         }
 354     }
 355
 356     fn check_unary_op(
 357         &mut self,
 358         op: UnOp,
 359         arg: &Operand<'tcx>,
 360         source_info: SourceInfo,
 361     ) -> Option<()> {
 362         if let (val, true) = self.use_ecx(source_info, |this| {
 363             let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
 364             let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
 365             Ok((val, overflow))
 366         })? {
 367             // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
 368             // appropriate to use.
 369             assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
 370             self.report_assert_as_lint(
 371                 lint::builtin::ARITHMETIC_OVERFLOW,
 372                 source_info,
 373                 "this arithmetic operation will overflow",
 374                 AssertKind::OverflowNeg(val.to_const_int()),
 375             );
 376             return None;
 377         }
 378
 379         Some(())
 380     }
 381
 382     fn check_binary_op(
 383         &mut self,
 384         op: BinOp,
 385         left: &Operand<'tcx>,
 386         right: &Operand<'tcx>,
 387         source_info: SourceInfo,
 388     ) -> Option<()> {
 389         let r = self.use_ecx(source_info, |this| {
 390             this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?)
 391         });
 392         let l = self.use_ecx(source_info, |this| {
 393             this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?)
 394         });
 395         // Check for exceeding shifts *even if* we cannot evaluate the LHS.
 396         if op == BinOp::Shr || op == BinOp::Shl {
 397             let r = r.clone()?;
 398             // We need the type of the LHS. We cannot use `place_layout` as that is the type
 399             // of the result, which for checked binops is not the same!
 400             let left_ty = left.ty(self.local_decls, self.tcx);
 401             let left_size = self.ecx.layout_of(left_ty).ok()?.size;
 402             let right_size = r.layout.size;
 403             let r_bits = r.to_scalar().to_bits(right_size).ok();
 404             if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
 405                 debug!("check_binary_op: reporting assert for {:?}", source_info);
 406                 self.report_assert_as_lint(
 407                     lint::builtin::ARITHMETIC_OVERFLOW,
 408                     source_info,
 409                     "this arithmetic operation will overflow",
 410                     AssertKind::Overflow(
 411                         op,
 412                         match l {
 413                             Some(l) => l.to_const_int(),
 414                             // Invent a dummy value, the diagnostic ignores it anyway
 415                             None => ConstInt::new(
 416                                 ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
 417                                 left_ty.is_signed(),
 418                                 left_ty.is_ptr_sized_integral(),
 419                             ),
 420                         },
 421                         r.to_const_int(),
 422                     ),
 423                 );
 424                 return None;
 425             }
 426         }
 427
 428         if let (Some(l), Some(r)) = (l, r) {
 429             // The remaining operators are handled through `overflowing_binary_op`.
 430             if self.use_ecx(source_info, |this| {
 431                 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, &l, &r)?;
 432                 Ok(overflow)
 433             })? {
 434                 self.report_assert_as_lint(
 435                     lint::builtin::ARITHMETIC_OVERFLOW,
 436                     source_info,
 437                     "this arithmetic operation will overflow",
 438                     AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
 439                 );
 440                 return None;
 441             }
 442         }
 443         Some(())
 444     }
 445
 446     fn const_prop(
 447         &mut self,
 448         rvalue: &Rvalue<'tcx>,
 449         source_info: SourceInfo,
 450         place: Place<'tcx>,
 451     ) -> Option<()> {
 452         // Perform any special handling for specific Rvalue types.
 453         // Generally, checks here fall into one of two categories:
 454         //   1. Additional checking to provide useful lints to the user
 455         //        - In this case, we will do some validation and then fall through to the
 456         //          end of the function which evals the assignment.
 457         //   2. Working around bugs in other parts of the compiler
 458         //        - In this case, we'll return `None` from this function to stop evaluation.
 459         match rvalue {
 460             // Additional checking: give lints to the user if an overflow would occur.
 461             // We do this here and not in the `Assert` terminator as that terminator is
 462             // only sometimes emitted (overflow checks can be disabled), but we want to always
 463             // lint.
 464             Rvalue::UnaryOp(op, arg) => {
 465                 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
 466                 self.check_unary_op(*op, arg, source_info)?;
 467             }
 468             Rvalue::BinaryOp(op, box (left, right)) => {
 469                 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
 470                 self.check_binary_op(*op, left, right, source_info)?;
 471             }
 472             Rvalue::CheckedBinaryOp(op, box (left, right)) => {
 473                 trace!(
 474                     "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
 475                     op,
 476                     left,
 477                     right
 478                 );
 479                 self.check_binary_op(*op, left, right, source_info)?;
 480             }
 481
 482             // Do not try creating references (#67862)
 483             Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
 484                 trace!("skipping AddressOf | Ref for {:?}", place);
 485
 486                 // This may be creating mutable references or immutable references to cells.
 487                 // If that happens, the pointed to value could be mutated via that reference.
 488                 // Since we aren't tracking references, the const propagator loses track of what
 489                 // value the local has right now.
 490                 // Thus, all locals that have their reference taken
 491                 // must not take part in propagation.
 492                 Self::remove_const(&mut self.ecx, place.local);
 493
 494                 return None;
 495             }
 496             Rvalue::ThreadLocalRef(def_id) => {
 497                 trace!("skipping ThreadLocalRef({:?})", def_id);
 498
 499                 return None;
 500             }
 501
 502             // There's no other checking to do at this time.
 503             Rvalue::Aggregate(..)
 504             | Rvalue::Use(..)
 505             | Rvalue::CopyForDeref(..)
 506             | Rvalue::Repeat(..)
 507             | Rvalue::Len(..)
 508             | Rvalue::Cast(..)
 509             | Rvalue::ShallowInitBox(..)
 510             | Rvalue::Discriminant(..)
 511             | Rvalue::NullaryOp(..) => {}
 512         }
 513
 514         // FIXME we need to revisit this for #67176
 515         if rvalue.needs_subst() {
 516             return None;
 517         }
 518
 519         self.use_ecx(source_info, |this| this.ecx.eval_rvalue_into_place(rvalue, place))
 520     }
 521 }
 522
 523 impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
 524     fn visit_body(&mut self, body: &Body<'tcx>) {
 525         for (bb, data) in body.basic_blocks().iter_enumerated() {
 526             self.visit_basic_block_data(bb, data);
 527         }
 528     }
 529
 530     fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
 531         self.super_operand(operand, location);
 532     }
 533
 534     fn visit_constant(&mut self, constant: &Constant<'tcx>, location: Location) {
 535         trace!("visit_constant: {:?}", constant);
 536         self.super_constant(constant, location);
 537         self.eval_constant(constant, self.source_info.unwrap());
 538     }
 539
 540     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
 541         trace!("visit_statement: {:?}", statement);
 542         let source_info = statement.source_info;
 543         self.source_info = Some(source_info);
 544         if let StatementKind::Assign(box (place, ref rval)) = statement.kind {
 545             let can_const_prop = self.ecx.machine.can_const_prop[place.local];
 546             if let Some(()) = self.const_prop(rval, source_info, place) {
 547                 match can_const_prop {
 548                     ConstPropMode::OnlyInsideOwnBlock => {
 549                         trace!(
 550                             "found local restricted to its block. \
 551                                 Will remove it from const-prop after block is finished. Local: {:?}",
 552                             place.local
 553                         );
 554                     }
 555                     ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
 556                         trace!("can't propagate into {:?}", place);
 557                         if place.local != RETURN_PLACE {
 558                             Self::remove_const(&mut self.ecx, place.local);
 559                         }
 560                     }
 561                     ConstPropMode::FullConstProp => {}
 562                 }
 563             } else {
 564                 // Const prop failed, so erase the destination, ensuring that whatever happens
 565                 // from here on, does not know about the previous value.
 566                 // This is important in case we have
 567                 // ```rust
 568                 // let mut x = 42;
 569                 // x = SOME_MUTABLE_STATIC;
 570                 // // x must now be uninit
 571                 // ```
 572                 // FIXME: we overzealously erase the entire local, because that's easier to
 573                 // implement.
 574                 trace!(
 575                     "propagation into {:?} failed.
 576                         Nuking the entire site from orbit, it's the only way to be sure",
 577                     place,
 578                 );
 579                 Self::remove_const(&mut self.ecx, place.local);
 580             }
 581         } else {
 582             match statement.kind {
 583                 StatementKind::SetDiscriminant { ref place, .. } => {
 584                     match self.ecx.machine.can_const_prop[place.local] {
 585                         ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
 586                             if self
 587                                 .use_ecx(source_info, |this| this.ecx.statement(statement))
 588                                 .is_some()
 589                             {
 590                                 trace!("propped discriminant into {:?}", place);
 591                             } else {
 592                                 Self::remove_const(&mut self.ecx, place.local);
 593                             }
 594                         }
 595                         ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
 596                             Self::remove_const(&mut self.ecx, place.local);
 597                         }
 598                     }
 599                 }
 600                 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
 601                     let frame = self.ecx.frame_mut();
 602                     frame.locals[local].value =
 603                         if let StatementKind::StorageLive(_) = statement.kind {
 604                             LocalValue::Live(interpret::Operand::Immediate(
 605                                 interpret::Immediate::Uninit,
 606                             ))
 607                         } else {
 608                             LocalValue::Dead
 609                         };
 610                 }
 611                 _ => {}
 612             }
 613         }
 614
 615         self.super_statement(statement, location);
 616     }
 617
 618     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
 619         let source_info = terminator.source_info;
 620         self.source_info = Some(source_info);
 621         self.super_terminator(terminator, location);
 622         match &terminator.kind {
 623             TerminatorKind::Assert { expected, ref msg, ref cond, .. } => {
 624                 if let Some(ref value) = self.eval_operand(&cond, source_info) {
 625                     trace!("assertion on {:?} should be {:?}", value, expected);
 626                     let expected = Scalar::from_bool(*expected);
 627                     let value_const = self.ecx.read_scalar(&value).unwrap();
 628                     if expected != value_const {
 629                         enum DbgVal<T> {
 630                             Val(T),
 631                             Underscore,
 632                         }
 633                         impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
 634                             fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
 635                                 match self {
 636                                     Self::Val(val) => val.fmt(fmt),
 637                                     Self::Underscore => fmt.write_str("_"),
 638                                 }
 639                             }
 640                         }
 641                         let mut eval_to_int = |op| {
 642                             // This can be `None` if the lhs wasn't const propagated and we just
 643                             // triggered the assert on the value of the rhs.
 644                             self.eval_operand(op, source_info).map_or(DbgVal::Underscore, |op| {
 645                                 DbgVal::Val(self.ecx.read_immediate(&op).unwrap().to_const_int())
 646                             })
 647                         };
 648                         let msg = match msg {
 649                             AssertKind::DivisionByZero(op) => {
 650                                 Some(AssertKind::DivisionByZero(eval_to_int(op)))
 651                             }
 652                             AssertKind::RemainderByZero(op) => {
 653                                 Some(AssertKind::RemainderByZero(eval_to_int(op)))
 654                             }
 655                             AssertKind::Overflow(bin_op @ (BinOp::Div | BinOp::Rem), op1, op2) => {
 656                                 // Division overflow is *UB* in the MIR, and different than the
 657                                 // other overflow checks.
 658                                 Some(AssertKind::Overflow(
 659                                     *bin_op,
 660                                     eval_to_int(op1),
 661                                     eval_to_int(op2),
 662                                 ))
 663                             }
 664                             AssertKind::BoundsCheck { ref len, ref index } => {
 665                                 let len = eval_to_int(len);
 666                                 let index = eval_to_int(index);
 667                                 Some(AssertKind::BoundsCheck { len, index })
 668                             }
 669                             // Remaining overflow errors are already covered by checks on the binary operators.
 670                             AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None,
 671                             // Need proper const propagator for these.
 672                             _ => None,
 673                         };
 674                         // Poison all places this operand references so that further code
 675                         // doesn't use the invalid value
 676                         match cond {
 677                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 678                                 Self::remove_const(&mut self.ecx, place.local);
 679                             }
 680                             Operand::Constant(_) => {}
 681                         }
 682                         if let Some(msg) = msg {
 683                             self.report_assert_as_lint(
 684                                 lint::builtin::UNCONDITIONAL_PANIC,
 685                                 source_info,
 686                                 "this operation will panic at runtime",
 687                                 msg,
 688                             );
 689                         }
 690                     }
 691                 }
 692             }
 693             // None of these have Operands to const-propagate.
 694             TerminatorKind::Goto { .. }
 695             | TerminatorKind::Resume
 696             | TerminatorKind::Abort
 697             | TerminatorKind::Return
 698             | TerminatorKind::Unreachable
 699             | TerminatorKind::Drop { .. }
 700             | TerminatorKind::DropAndReplace { .. }
 701             | TerminatorKind::Yield { .. }
 702             | TerminatorKind::GeneratorDrop
 703             | TerminatorKind::FalseEdge { .. }
 704             | TerminatorKind::FalseUnwind { .. }
 705             | TerminatorKind::SwitchInt { .. }
 706             | TerminatorKind::Call { .. }
 707             | TerminatorKind::InlineAsm { .. } => {}
 708         }
 709
 710         // We remove all Locals which are restricted in propagation to their containing blocks and
 711         // which were modified in the current block.
 712         // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
 713         let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
 714         for &local in locals.iter() {
 715             Self::remove_const(&mut self.ecx, local);
 716         }
 717         locals.clear();
 718         // Put it back so we reuse the heap of the storage
 719         self.ecx.machine.written_only_inside_own_block_locals = locals;
 720         if cfg!(debug_assertions) {
 721             // Ensure we are correctly erasing locals with the non-debug-assert logic.
 722             for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
 723                 assert!(
 724                     self.get_const(local.into()).is_none()
 725                         || self
 726                             .layout_of(self.local_decls[local].ty)
 727                             .map_or(true, |layout| layout.is_zst())
 728                 )
 729             }
 730         }
 731     }
 732 }