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,
  11     ScalarMaybeUninit, StackPopCleanup,
  12 };
  13 use rustc_hir::def::DefKind;
  14 use rustc_hir::HirId;
  15 use rustc_index::bit_set::BitSet;
  16 use rustc_index::vec::IndexVec;
  17 use rustc_middle::mir::visit::Visitor;
  18 use rustc_middle::mir::{
  19     AssertKind, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, Location, Operand, Place,
  20     Rvalue, SourceInfo, SourceScope, SourceScopeData, Statement, StatementKind, Terminator,
  21     TerminatorKind, UnOp, RETURN_PLACE,
  22 };
  23 use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
  24 use rustc_middle::ty::subst::{InternalSubsts, Subst};
  25 use rustc_middle::ty::{
  26     self, ConstInt, ConstKind, Instance, ParamEnv, ScalarInt, Ty, TyCtxt, TypeVisitable,
  27 };
  28 use rustc_session::lint;
  29 use rustc_span::Span;
  30 use rustc_target::abi::{HasDataLayout, Size, TargetDataLayout};
  31 use rustc_trait_selection::traits;
  32 use std::cell::Cell;
  33
  34 /// The maximum number of bytes that we'll allocate space for a local or the return value.
  35 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
  36 /// Severely regress performance.
  37 const MAX_ALLOC_LIMIT: u64 = 1024;
  38 pub struct ConstProp;
  39
  40 impl<'tcx> MirLint<'tcx> for ConstProp {
  41     fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
  42         // will be evaluated by miri and produce its errors there
  43         if body.source.promoted.is_some() {
  44             return;
  45         }
  46
  47         let def_id = body.source.def_id().expect_local();
  48         let is_fn_like = tcx.def_kind(def_id).is_fn_like();
  49         let is_assoc_const = tcx.def_kind(def_id) == DefKind::AssocConst;
  50
  51         // Only run const prop on functions, methods, closures and associated constants
  52         if !is_fn_like && !is_assoc_const {
  53             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  54             trace!("ConstProp skipped for {:?}", def_id);
  55             return;
  56         }
  57
  58         let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
  59         // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
  60         // computing their layout.
  61         if is_generator {
  62             trace!("ConstProp skipped for generator {:?}", def_id);
  63             return;
  64         }
  65
  66         // Check if it's even possible to satisfy the 'where' clauses
  67         // for this item.
  68         // This branch will never be taken for any normal function.
  69         // However, it's possible to `#!feature(trivial_bounds)]` to write
  70         // a function with impossible to satisfy clauses, e.g.:
  71         // `fn foo() where String: Copy {}`
  72         //
  73         // We don't usually need to worry about this kind of case,
  74         // since we would get a compilation error if the user tried
  75         // to call it. However, since we can do const propagation
  76         // even without any calls to the function, we need to make
  77         // sure that it even makes sense to try to evaluate the body.
  78         // If there are unsatisfiable where clauses, then all bets are
  79         // off, and we just give up.
  80         //
  81         // We manually filter the predicates, skipping anything that's not
  82         // "global". We are in a potentially generic context
  83         // (e.g. we are evaluating a function without substituting generic
  84         // parameters, so this filtering serves two purposes:
  85         //
  86         // 1. We skip evaluating any predicates that we would
  87         // never be able prove are unsatisfiable (e.g. `<T as Foo>`
  88         // 2. We avoid trying to normalize predicates involving generic
  89         // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
  90         // the normalization code (leading to cycle errors), since
  91         // it's usually never invoked in this way.
  92         let predicates = tcx
  93             .predicates_of(def_id.to_def_id())
  94             .predicates
  95             .iter()
  96             .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
  97         if traits::impossible_predicates(
  98             tcx,
  99             traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
 100         ) {
 101             trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
 102             return;
 103         }
 104
 105         trace!("ConstProp starting for {:?}", def_id);
 106
 107         let dummy_body = &Body::new(
 108             body.source,
 109             body.basic_blocks().clone(),
 110             body.source_scopes.clone(),
 111             body.local_decls.clone(),
 112             Default::default(),
 113             body.arg_count,
 114             Default::default(),
 115             body.span,
 116             body.generator_kind(),
 117             body.tainted_by_errors,
 118         );
 119
 120         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
 121         // constants, instead of just checking for const-folding succeeding.
 122         // That would require a uniform one-def no-mutation analysis
 123         // and RPO (or recursing when needing the value of a local).
 124         let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
 125         optimization_finder.visit_body(body);
 126
 127         trace!("ConstProp done for {:?}", def_id);
 128     }
 129 }
 130
 131 /// Finds optimization opportunities on the MIR.
 132 struct ConstPropagator<'mir, 'tcx> {
 133     ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
 134     tcx: TyCtxt<'tcx>,
 135     param_env: ParamEnv<'tcx>,
 136     source_scopes: &'mir IndexVec<SourceScope, SourceScopeData<'tcx>>,
 137     local_decls: &'mir IndexVec<Local, LocalDecl<'tcx>>,
 138     // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
 139     // the last known `SourceInfo` here and just keep revisiting it.
 140     source_info: Option<SourceInfo>,
 141 }
 142
 143 impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
 144     type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
 145
 146     #[inline]
 147     fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
 148         err
 149     }
 150 }
 151
 152 impl HasDataLayout for ConstPropagator<'_, '_> {
 153     #[inline]
 154     fn data_layout(&self) -> &TargetDataLayout {
 155         &self.tcx.data_layout
 156     }
 157 }
 158
 159 impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
 160     #[inline]
 161     fn tcx(&self) -> TyCtxt<'tcx> {
 162         self.tcx
 163     }
 164 }
 165
 166 impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
 167     #[inline]
 168     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 169         self.param_env
 170     }
 171 }
 172
 173 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 174     fn new(
 175         body: &Body<'tcx>,
 176         dummy_body: &'mir Body<'tcx>,
 177         tcx: TyCtxt<'tcx>,
 178     ) -> ConstPropagator<'mir, 'tcx> {
 179         let def_id = body.source.def_id();
 180         let substs = &InternalSubsts::identity_for_item(tcx, def_id);
 181         let param_env = tcx.param_env_reveal_all_normalized(def_id);
 182
 183         let can_const_prop = CanConstProp::check(tcx, param_env, body);
 184         let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
 185         for (l, mode) in can_const_prop.iter_enumerated() {
 186             if *mode == ConstPropMode::OnlyInsideOwnBlock {
 187                 only_propagate_inside_block_locals.insert(l);
 188             }
 189         }
 190         let mut ecx = InterpCx::new(
 191             tcx,
 192             tcx.def_span(def_id),
 193             param_env,
 194             ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
 195         );
 196
 197         let ret_layout = ecx
 198             .layout_of(body.bound_return_ty().subst(tcx, substs))
 199             .ok()
 200             // Don't bother allocating memory for large values.
 201             // I don't know how return types can seem to be unsized but this happens in the
 202             // `type/type-unsatisfiable.rs` test.
 203             .filter(|ret_layout| {
 204                 !ret_layout.is_unsized() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
 205             })
 206             .unwrap_or_else(|| ecx.layout_of(tcx.types.unit).unwrap());
 207
 208         let ret = ecx
 209             .allocate(ret_layout, MemoryKind::Stack)
 210             .expect("couldn't perform small allocation")
 211             .into();
 212
 213         ecx.push_stack_frame(
 214             Instance::new(def_id, substs),
 215             dummy_body,
 216             &ret,
 217             StackPopCleanup::Root { cleanup: false },
 218         )
 219         .expect("failed to push initial stack frame");
 220
 221         ConstPropagator {
 222             ecx,
 223             tcx,
 224             param_env,
 225             source_scopes: &dummy_body.source_scopes,
 226             local_decls: &dummy_body.local_decls,
 227             source_info: None,
 228         }
 229     }
 230
 231     fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
 232         let op = match self.ecx.eval_place_to_op(place, None) {
 233             Ok(op) => op,
 234             Err(e) => {
 235                 trace!("get_const failed: {}", e);
 236                 return None;
 237             }
 238         };
 239
 240         // Try to read the local as an immediate so that if it is representable as a scalar, we can
 241         // handle it as such, but otherwise, just return the value as is.
 242         Some(match self.ecx.read_immediate_raw(&op, /*force*/ false) {
 243             Ok(Ok(imm)) => imm.into(),
 244             _ => op,
 245         })
 246     }
 247
 248     /// Remove `local` from the pool of `Locals`. Allows writing to them,
 249     /// but not reading from them anymore.
 250     fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
 251         ecx.frame_mut().locals[local] = LocalState {
 252             value: LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit)),
 253             layout: Cell::new(None),
 254         };
 255     }
 256
 257     fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
 258         source_info.scope.lint_root(self.source_scopes)
 259     }
 260
 261     fn use_ecx<F, T>(&mut self, source_info: SourceInfo, f: F) -> Option<T>
 262     where
 263         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 264     {
 265         // Overwrite the PC -- whatever the interpreter does to it does not make any sense anyway.
 266         self.ecx.frame_mut().loc = Err(source_info.span);
 267         match f(self) {
 268             Ok(val) => Some(val),
 269             Err(error) => {
 270                 trace!("InterpCx operation failed: {:?}", error);
 271                 // Some errors shouldn't come up because creating them causes
 272                 // an allocation, which we should avoid. When that happens,
 273                 // dedicated error variants should be introduced instead.
 274                 assert!(
 275                     !error.kind().formatted_string(),
 276                     "const-prop encountered formatting error: {}",
 277                     error
 278                 );
 279                 None
 280             }
 281         }
 282     }
 283
 284     /// Returns the value, if any, of evaluating `c`.
 285     fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 286         // FIXME we need to revisit this for #67176
 287         if c.needs_subst() {
 288             return None;
 289         }
 290
 291         match self.ecx.mir_const_to_op(&c.literal, None) {
 292             Ok(op) => Some(op),
 293             Err(error) => {
 294                 let tcx = self.ecx.tcx.at(c.span);
 295                 let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
 296                 if let Some(lint_root) = self.lint_root(source_info) {
 297                     let lint_only = match c.literal {
 298                         ConstantKind::Ty(ct) => match ct.kind() {
 299                             // Promoteds must lint and not error as the user didn't ask for them
 300                             ConstKind::Unevaluated(ty::Unevaluated {
 301                                 def: _,
 302                                 substs: _,
 303                                 promoted: Some(_),
 304                             }) => true,
 305                             // Out of backwards compatibility we cannot report hard errors in unused
 306                             // generic functions using associated constants of the generic parameters.
 307                             _ => c.literal.needs_subst(),
 308                         },
 309                         ConstantKind::Val(_, ty) => ty.needs_subst(),
 310                     };
 311                     if lint_only {
 312                         // Out of backwards compatibility we cannot report hard errors in unused
 313                         // generic functions using associated constants of the generic parameters.
 314                         err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span));
 315                     } else {
 316                         err.report_as_error(tcx, "erroneous constant used");
 317                     }
 318                 } else {
 319                     err.report_as_error(tcx, "erroneous constant used");
 320                 }
 321                 None
 322             }
 323         }
 324     }
 325
 326     /// Returns the value, if any, of evaluating `place`.
 327     fn eval_place(&mut self, place: Place<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 328         trace!("eval_place(place={:?})", place);
 329         self.use_ecx(source_info, |this| this.ecx.eval_place_to_op(place, None))
 330     }
 331
 332     /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
 333     /// or `eval_place`, depending on the variant of `Operand` used.
 334     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 335         match *op {
 336             Operand::Constant(ref c) => self.eval_constant(c, source_info),
 337             Operand::Move(place) | Operand::Copy(place) => self.eval_place(place, source_info),
 338         }
 339     }
 340
 341     fn report_assert_as_lint(
 342         &self,
 343         lint: &'static lint::Lint,
 344         source_info: SourceInfo,
 345         message: &'static str,
 346         panic: AssertKind<impl std::fmt::Debug>,
 347     ) {
 348         if let Some(lint_root) = self.lint_root(source_info) {
 349             self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
 350                 let mut err = lint.build(message);
 351                 err.span_label(source_info.span, format!("{:?}", panic));
 352                 err.emit();
 353             });
 354         }
 355     }
 356
 357     fn check_unary_op(
 358         &mut self,
 359         op: UnOp,
 360         arg: &Operand<'tcx>,
 361         source_info: SourceInfo,
 362     ) -> Option<()> {
 363         if let (val, true) = self.use_ecx(source_info, |this| {
 364             let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
 365             let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
 366             Ok((val, overflow))
 367         })? {
 368             // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
 369             // appropriate to use.
 370             assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
 371             self.report_assert_as_lint(
 372                 lint::builtin::ARITHMETIC_OVERFLOW,
 373                 source_info,
 374                 "this arithmetic operation will overflow",
 375                 AssertKind::OverflowNeg(val.to_const_int()),
 376             );
 377             return None;
 378         }
 379
 380         Some(())
 381     }
 382
 383     fn check_binary_op(
 384         &mut self,
 385         op: BinOp,
 386         left: &Operand<'tcx>,
 387         right: &Operand<'tcx>,
 388         source_info: SourceInfo,
 389     ) -> Option<()> {
 390         let r = self.use_ecx(source_info, |this| {
 391             this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?)
 392         });
 393         let l = self.use_ecx(source_info, |this| {
 394             this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?)
 395         });
 396         // Check for exceeding shifts *even if* we cannot evaluate the LHS.
 397         if op == BinOp::Shr || op == BinOp::Shl {
 398             let r = r.clone()?;
 399             // We need the type of the LHS. We cannot use `place_layout` as that is the type
 400             // of the result, which for checked binops is not the same!
 401             let left_ty = left.ty(self.local_decls, self.tcx);
 402             let left_size = self.ecx.layout_of(left_ty).ok()?.size;
 403             let right_size = r.layout.size;
 404             let r_bits = r.to_scalar().ok();
 405             let r_bits = r_bits.and_then(|r| r.to_bits(right_size).ok());
 406             if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
 407                 debug!("check_binary_op: reporting assert for {:?}", source_info);
 408                 self.report_assert_as_lint(
 409                     lint::builtin::ARITHMETIC_OVERFLOW,
 410                     source_info,
 411                     "this arithmetic operation will overflow",
 412                     AssertKind::Overflow(
 413                         op,
 414                         match l {
 415                             Some(l) => l.to_const_int(),
 416                             // Invent a dummy value, the diagnostic ignores it anyway
 417                             None => ConstInt::new(
 418                                 ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
 419                                 left_ty.is_signed(),
 420                                 left_ty.is_ptr_sized_integral(),
 421                             ),
 422                         },
 423                         r.to_const_int(),
 424                     ),
 425                 );
 426                 return None;
 427             }
 428         }
 429
 430         if let (Some(l), Some(r)) = (l, r) {
 431             // The remaining operators are handled through `overflowing_binary_op`.
 432             if self.use_ecx(source_info, |this| {
 433                 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, &l, &r)?;
 434                 Ok(overflow)
 435             })? {
 436                 self.report_assert_as_lint(
 437                     lint::builtin::ARITHMETIC_OVERFLOW,
 438                     source_info,
 439                     "this arithmetic operation will overflow",
 440                     AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
 441                 );
 442                 return None;
 443             }
 444         }
 445         Some(())
 446     }
 447
 448     fn const_prop(
 449         &mut self,
 450         rvalue: &Rvalue<'tcx>,
 451         source_info: SourceInfo,
 452         place: Place<'tcx>,
 453     ) -> Option<()> {
 454         // Perform any special handling for specific Rvalue types.
 455         // Generally, checks here fall into one of two categories:
 456         //   1. Additional checking to provide useful lints to the user
 457         //        - In this case, we will do some validation and then fall through to the
 458         //          end of the function which evals the assignment.
 459         //   2. Working around bugs in other parts of the compiler
 460         //        - In this case, we'll return `None` from this function to stop evaluation.
 461         match rvalue {
 462             // Additional checking: give lints to the user if an overflow would occur.
 463             // We do this here and not in the `Assert` terminator as that terminator is
 464             // only sometimes emitted (overflow checks can be disabled), but we want to always
 465             // lint.
 466             Rvalue::UnaryOp(op, arg) => {
 467                 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
 468                 self.check_unary_op(*op, arg, source_info)?;
 469             }
 470             Rvalue::BinaryOp(op, box (left, right)) => {
 471                 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
 472                 self.check_binary_op(*op, left, right, source_info)?;
 473             }
 474             Rvalue::CheckedBinaryOp(op, box (left, right)) => {
 475                 trace!(
 476                     "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
 477                     op,
 478                     left,
 479                     right
 480                 );
 481                 self.check_binary_op(*op, left, right, source_info)?;
 482             }
 483
 484             // Do not try creating references (#67862)
 485             Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
 486                 trace!("skipping AddressOf | Ref for {:?}", place);
 487
 488                 // This may be creating mutable references or immutable references to cells.
 489                 // If that happens, the pointed to value could be mutated via that reference.
 490                 // Since we aren't tracking references, the const propagator loses track of what
 491                 // value the local has right now.
 492                 // Thus, all locals that have their reference taken
 493                 // must not take part in propagation.
 494                 Self::remove_const(&mut self.ecx, place.local);
 495
 496                 return None;
 497             }
 498             Rvalue::ThreadLocalRef(def_id) => {
 499                 trace!("skipping ThreadLocalRef({:?})", def_id);
 500
 501                 return None;
 502             }
 503
 504             // There's no other checking to do at this time.
 505             Rvalue::Aggregate(..)
 506             | Rvalue::Use(..)
 507             | Rvalue::CopyForDeref(..)
 508             | Rvalue::Repeat(..)
 509             | Rvalue::Len(..)
 510             | Rvalue::Cast(..)
 511             | Rvalue::ShallowInitBox(..)
 512             | Rvalue::Discriminant(..)
 513             | Rvalue::NullaryOp(..) => {}
 514         }
 515
 516         // FIXME we need to revisit this for #67176
 517         if rvalue.needs_subst() {
 518             return None;
 519         }
 520
 521         self.use_ecx(source_info, |this| this.ecx.eval_rvalue_into_place(rvalue, place))
 522     }
 523 }
 524
 525 impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
 526     fn visit_body(&mut self, body: &Body<'tcx>) {
 527         for (bb, data) in body.basic_blocks().iter_enumerated() {
 528             self.visit_basic_block_data(bb, data);
 529         }
 530     }
 531
 532     fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
 533         self.super_operand(operand, location);
 534     }
 535
 536     fn visit_constant(&mut self, constant: &Constant<'tcx>, location: Location) {
 537         trace!("visit_constant: {:?}", constant);
 538         self.super_constant(constant, location);
 539         self.eval_constant(constant, self.source_info.unwrap());
 540     }
 541
 542     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
 543         trace!("visit_statement: {:?}", statement);
 544         let source_info = statement.source_info;
 545         self.source_info = Some(source_info);
 546         if let StatementKind::Assign(box (place, ref rval)) = statement.kind {
 547             let can_const_prop = self.ecx.machine.can_const_prop[place.local];
 548             if let Some(()) = self.const_prop(rval, source_info, place) {
 549                 match can_const_prop {
 550                     ConstPropMode::OnlyInsideOwnBlock => {
 551                         trace!(
 552                             "found local restricted to its block. \
 553                                 Will remove it from const-prop after block is finished. Local: {:?}",
 554                             place.local
 555                         );
 556                     }
 557                     ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
 558                         trace!("can't propagate into {:?}", place);
 559                         if place.local != RETURN_PLACE {
 560                             Self::remove_const(&mut self.ecx, place.local);
 561                         }
 562                     }
 563                     ConstPropMode::FullConstProp => {}
 564                 }
 565             } else {
 566                 // Const prop failed, so erase the destination, ensuring that whatever happens
 567                 // from here on, does not know about the previous value.
 568                 // This is important in case we have
 569                 // ```rust
 570                 // let mut x = 42;
 571                 // x = SOME_MUTABLE_STATIC;
 572                 // // x must now be uninit
 573                 // ```
 574                 // FIXME: we overzealously erase the entire local, because that's easier to
 575                 // implement.
 576                 trace!(
 577                     "propagation into {:?} failed.
 578                         Nuking the entire site from orbit, it's the only way to be sure",
 579                     place,
 580                 );
 581                 Self::remove_const(&mut self.ecx, place.local);
 582             }
 583         } else {
 584             match statement.kind {
 585                 StatementKind::SetDiscriminant { ref place, .. } => {
 586                     match self.ecx.machine.can_const_prop[place.local] {
 587                         ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
 588                             if self
 589                                 .use_ecx(source_info, |this| this.ecx.statement(statement))
 590                                 .is_some()
 591                             {
 592                                 trace!("propped discriminant into {:?}", place);
 593                             } else {
 594                                 Self::remove_const(&mut self.ecx, place.local);
 595                             }
 596                         }
 597                         ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
 598                             Self::remove_const(&mut self.ecx, place.local);
 599                         }
 600                     }
 601                 }
 602                 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
 603                     let frame = self.ecx.frame_mut();
 604                     frame.locals[local].value =
 605                         if let StatementKind::StorageLive(_) = statement.kind {
 606                             LocalValue::Live(interpret::Operand::Immediate(
 607                                 interpret::Immediate::Uninit,
 608                             ))
 609                         } else {
 610                             LocalValue::Dead
 611                         };
 612                 }
 613                 _ => {}
 614             }
 615         }
 616
 617         self.super_statement(statement, location);
 618     }
 619
 620     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
 621         let source_info = terminator.source_info;
 622         self.source_info = Some(source_info);
 623         self.super_terminator(terminator, location);
 624         match &terminator.kind {
 625             TerminatorKind::Assert { expected, ref msg, ref cond, .. } => {
 626                 if let Some(ref value) = self.eval_operand(&cond, source_info) {
 627                     trace!("assertion on {:?} should be {:?}", value, expected);
 628                     let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
 629                     let value_const = self.ecx.read_scalar(&value).unwrap();
 630                     if expected != value_const {
 631                         enum DbgVal<T> {
 632                             Val(T),
 633                             Underscore,
 634                         }
 635                         impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
 636                             fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
 637                                 match self {
 638                                     Self::Val(val) => val.fmt(fmt),
 639                                     Self::Underscore => fmt.write_str("_"),
 640                                 }
 641                             }
 642                         }
 643                         let mut eval_to_int = |op| {
 644                             // This can be `None` if the lhs wasn't const propagated and we just
 645                             // triggered the assert on the value of the rhs.
 646                             self.eval_operand(op, source_info).map_or(DbgVal::Underscore, |op| {
 647                                 DbgVal::Val(self.ecx.read_immediate(&op).unwrap().to_const_int())
 648                             })
 649                         };
 650                         let msg = match msg {
 651                             AssertKind::DivisionByZero(op) => {
 652                                 Some(AssertKind::DivisionByZero(eval_to_int(op)))
 653                             }
 654                             AssertKind::RemainderByZero(op) => {
 655                                 Some(AssertKind::RemainderByZero(eval_to_int(op)))
 656                             }
 657                             AssertKind::Overflow(bin_op @ (BinOp::Div | BinOp::Rem), op1, op2) => {
 658                                 // Division overflow is *UB* in the MIR, and different than the
 659                                 // other overflow checks.
 660                                 Some(AssertKind::Overflow(
 661                                     *bin_op,
 662                                     eval_to_int(op1),
 663                                     eval_to_int(op2),
 664                                 ))
 665                             }
 666                             AssertKind::BoundsCheck { ref len, ref index } => {
 667                                 let len = eval_to_int(len);
 668                                 let index = eval_to_int(index);
 669                                 Some(AssertKind::BoundsCheck { len, index })
 670                             }
 671                             // Remaining overflow errors are already covered by checks on the binary operators.
 672                             AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None,
 673                             // Need proper const propagator for these.
 674                             _ => None,
 675                         };
 676                         // Poison all places this operand references so that further code
 677                         // doesn't use the invalid value
 678                         match cond {
 679                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 680                                 Self::remove_const(&mut self.ecx, place.local);
 681                             }
 682                             Operand::Constant(_) => {}
 683                         }
 684                         if let Some(msg) = msg {
 685                             self.report_assert_as_lint(
 686                                 lint::builtin::UNCONDITIONAL_PANIC,
 687                                 source_info,
 688                                 "this operation will panic at runtime",
 689                                 msg,
 690                             );
 691                         }
 692                     }
 693                 }
 694             }
 695             // None of these have Operands to const-propagate.
 696             TerminatorKind::Goto { .. }
 697             | TerminatorKind::Resume
 698             | TerminatorKind::Abort
 699             | TerminatorKind::Return
 700             | TerminatorKind::Unreachable
 701             | TerminatorKind::Drop { .. }
 702             | TerminatorKind::DropAndReplace { .. }
 703             | TerminatorKind::Yield { .. }
 704             | TerminatorKind::GeneratorDrop
 705             | TerminatorKind::FalseEdge { .. }
 706             | TerminatorKind::FalseUnwind { .. }
 707             | TerminatorKind::SwitchInt { .. }
 708             | TerminatorKind::Call { .. }
 709             | TerminatorKind::InlineAsm { .. } => {}
 710         }
 711
 712         // We remove all Locals which are restricted in propagation to their containing blocks and
 713         // which were modified in the current block.
 714         // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
 715         let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
 716         for &local in locals.iter() {
 717             Self::remove_const(&mut self.ecx, local);
 718         }
 719         locals.clear();
 720         // Put it back so we reuse the heap of the storage
 721         self.ecx.machine.written_only_inside_own_block_locals = locals;
 722         if cfg!(debug_assertions) {
 723             // Ensure we are correctly erasing locals with the non-debug-assert logic.
 724             for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
 725                 assert!(
 726                     self.get_const(local.into()).is_none()
 727                         || self
 728                             .layout_of(self.local_decls[local].ty)
 729                             .map_or(true, |layout| layout.is_zst())
 730                 )
 731             }
 732         }
 733     }
 734 }