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, EarlyBinder, Instance, ParamEnv, ScalarInt, Ty, TyCtxt,
  27     TypeVisitable,
  28 };
  29 use rustc_session::lint;
  30 use rustc_span::Span;
  31 use rustc_target::abi::{HasDataLayout, Size, TargetDataLayout};
  32 use rustc_trait_selection::traits;
  33 use std::cell::Cell;
  34
  35 /// The maximum number of bytes that we'll allocate space for a local or the return value.
  36 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
  37 /// Severely regress performance.
  38 const MAX_ALLOC_LIMIT: u64 = 1024;
  39 pub struct ConstProp;
  40
  41 impl<'tcx> MirLint<'tcx> for ConstProp {
  42     fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
  43         // will be evaluated by miri and produce its errors there
  44         if body.source.promoted.is_some() {
  45             return;
  46         }
  47
  48         let def_id = body.source.def_id().expect_local();
  49         let is_fn_like = tcx.def_kind(def_id).is_fn_like();
  50         let is_assoc_const = tcx.def_kind(def_id) == DefKind::AssocConst;
  51
  52         // Only run const prop on functions, methods, closures and associated constants
  53         if !is_fn_like && !is_assoc_const {
  54             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  55             trace!("ConstProp skipped for {:?}", def_id);
  56             return;
  57         }
  58
  59         let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
  60         // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
  61         // computing their layout.
  62         if is_generator {
  63             trace!("ConstProp skipped for generator {:?}", def_id);
  64             return;
  65         }
  66
  67         // Check if it's even possible to satisfy the 'where' clauses
  68         // for this item.
  69         // This branch will never be taken for any normal function.
  70         // However, it's possible to `#!feature(trivial_bounds)]` to write
  71         // a function with impossible to satisfy clauses, e.g.:
  72         // `fn foo() where String: Copy {}`
  73         //
  74         // We don't usually need to worry about this kind of case,
  75         // since we would get a compilation error if the user tried
  76         // to call it. However, since we can do const propagation
  77         // even without any calls to the function, we need to make
  78         // sure that it even makes sense to try to evaluate the body.
  79         // If there are unsatisfiable where clauses, then all bets are
  80         // off, and we just give up.
  81         //
  82         // We manually filter the predicates, skipping anything that's not
  83         // "global". We are in a potentially generic context
  84         // (e.g. we are evaluating a function without substituting generic
  85         // parameters, so this filtering serves two purposes:
  86         //
  87         // 1. We skip evaluating any predicates that we would
  88         // never be able prove are unsatisfiable (e.g. `<T as Foo>`
  89         // 2. We avoid trying to normalize predicates involving generic
  90         // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
  91         // the normalization code (leading to cycle errors), since
  92         // it's usually never invoked in this way.
  93         let predicates = tcx
  94             .predicates_of(def_id.to_def_id())
  95             .predicates
  96             .iter()
  97             .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
  98         if traits::impossible_predicates(
  99             tcx,
 100             traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
 101         ) {
 102             trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
 103             return;
 104         }
 105
 106         trace!("ConstProp starting for {:?}", def_id);
 107
 108         let dummy_body = &Body::new(
 109             body.source,
 110             body.basic_blocks().clone(),
 111             body.source_scopes.clone(),
 112             body.local_decls.clone(),
 113             Default::default(),
 114             body.arg_count,
 115             Default::default(),
 116             body.span,
 117             body.generator_kind(),
 118             body.tainted_by_errors,
 119         );
 120
 121         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
 122         // constants, instead of just checking for const-folding succeeding.
 123         // That would require a uniform one-def no-mutation analysis
 124         // and RPO (or recursing when needing the value of a local).
 125         let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
 126         optimization_finder.visit_body(body);
 127
 128         trace!("ConstProp done for {:?}", def_id);
 129     }
 130 }
 131
 132 /// Finds optimization opportunities on the MIR.
 133 struct ConstPropagator<'mir, 'tcx> {
 134     ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
 135     tcx: TyCtxt<'tcx>,
 136     param_env: ParamEnv<'tcx>,
 137     source_scopes: &'mir IndexVec<SourceScope, SourceScopeData<'tcx>>,
 138     local_decls: &'mir IndexVec<Local, LocalDecl<'tcx>>,
 139     // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
 140     // the last known `SourceInfo` here and just keep revisiting it.
 141     source_info: Option<SourceInfo>,
 142 }
 143
 144 impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
 145     type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
 146
 147     #[inline]
 148     fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
 149         err
 150     }
 151 }
 152
 153 impl HasDataLayout for ConstPropagator<'_, '_> {
 154     #[inline]
 155     fn data_layout(&self) -> &TargetDataLayout {
 156         &self.tcx.data_layout
 157     }
 158 }
 159
 160 impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
 161     #[inline]
 162     fn tcx(&self) -> TyCtxt<'tcx> {
 163         self.tcx
 164     }
 165 }
 166
 167 impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
 168     #[inline]
 169     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 170         self.param_env
 171     }
 172 }
 173
 174 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 175     fn new(
 176         body: &Body<'tcx>,
 177         dummy_body: &'mir Body<'tcx>,
 178         tcx: TyCtxt<'tcx>,
 179     ) -> ConstPropagator<'mir, 'tcx> {
 180         let def_id = body.source.def_id();
 181         let substs = &InternalSubsts::identity_for_item(tcx, def_id);
 182         let param_env = tcx.param_env_reveal_all_normalized(def_id);
 183
 184         let can_const_prop = CanConstProp::check(tcx, param_env, body);
 185         let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
 186         for (l, mode) in can_const_prop.iter_enumerated() {
 187             if *mode == ConstPropMode::OnlyInsideOwnBlock {
 188                 only_propagate_inside_block_locals.insert(l);
 189             }
 190         }
 191         let mut ecx = InterpCx::new(
 192             tcx,
 193             tcx.def_span(def_id),
 194             param_env,
 195             ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
 196         );
 197
 198         let ret_layout = ecx
 199             .layout_of(EarlyBinder(body.return_ty()).subst(tcx, substs))
 200             .ok()
 201             // Don't bother allocating memory for large values.
 202             // I don't know how return types can seem to be unsized but this happens in the
 203             // `type/type-unsatisfiable.rs` test.
 204             .filter(|ret_layout| {
 205                 !ret_layout.is_unsized() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
 206             })
 207             .unwrap_or_else(|| ecx.layout_of(tcx.types.unit).unwrap());
 208
 209         let ret = ecx
 210             .allocate(ret_layout, MemoryKind::Stack)
 211             .expect("couldn't perform small allocation")
 212             .into();
 213
 214         ecx.push_stack_frame(
 215             Instance::new(def_id, substs),
 216             dummy_body,
 217             &ret,
 218             StackPopCleanup::Root { cleanup: false },
 219         )
 220         .expect("failed to push initial stack frame");
 221
 222         ConstPropagator {
 223             ecx,
 224             tcx,
 225             param_env,
 226             source_scopes: &dummy_body.source_scopes,
 227             local_decls: &dummy_body.local_decls,
 228             source_info: None,
 229         }
 230     }
 231
 232     fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
 233         let op = match self.ecx.eval_place_to_op(place, None) {
 234             Ok(op) => op,
 235             Err(e) => {
 236                 trace!("get_const failed: {}", e);
 237                 return None;
 238             }
 239         };
 240
 241         // Try to read the local as an immediate so that if it is representable as a scalar, we can
 242         // handle it as such, but otherwise, just return the value as is.
 243         Some(match self.ecx.read_immediate_raw(&op, /*force*/ false) {
 244             Ok(Ok(imm)) => imm.into(),
 245             _ => op,
 246         })
 247     }
 248
 249     /// Remove `local` from the pool of `Locals`. Allows writing to them,
 250     /// but not reading from them anymore.
 251     fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
 252         ecx.frame_mut().locals[local] = LocalState {
 253             value: LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit)),
 254             layout: Cell::new(None),
 255         };
 256     }
 257
 258     fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
 259         source_info.scope.lint_root(self.source_scopes)
 260     }
 261
 262     fn use_ecx<F, T>(&mut self, source_info: SourceInfo, f: F) -> Option<T>
 263     where
 264         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 265     {
 266         // Overwrite the PC -- whatever the interpreter does to it does not make any sense anyway.
 267         self.ecx.frame_mut().loc = Err(source_info.span);
 268         match f(self) {
 269             Ok(val) => Some(val),
 270             Err(error) => {
 271                 trace!("InterpCx operation failed: {:?}", error);
 272                 // Some errors shouldn't come up because creating them causes
 273                 // an allocation, which we should avoid. When that happens,
 274                 // dedicated error variants should be introduced instead.
 275                 assert!(
 276                     !error.kind().formatted_string(),
 277                     "const-prop encountered formatting error: {}",
 278                     error
 279                 );
 280                 None
 281             }
 282         }
 283     }
 284
 285     /// Returns the value, if any, of evaluating `c`.
 286     fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 287         // FIXME we need to revisit this for #67176
 288         if c.needs_subst() {
 289             return None;
 290         }
 291
 292         match self.ecx.mir_const_to_op(&c.literal, None) {
 293             Ok(op) => Some(op),
 294             Err(error) => {
 295                 let tcx = self.ecx.tcx.at(c.span);
 296                 let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
 297                 if let Some(lint_root) = self.lint_root(source_info) {
 298                     let lint_only = match c.literal {
 299                         ConstantKind::Ty(ct) => match ct.kind() {
 300                             // Promoteds must lint and not error as the user didn't ask for them
 301                             ConstKind::Unevaluated(ty::Unevaluated {
 302                                 def: _,
 303                                 substs: _,
 304                                 promoted: Some(_),
 305                             }) => true,
 306                             // Out of backwards compatibility we cannot report hard errors in unused
 307                             // generic functions using associated constants of the generic parameters.
 308                             _ => c.literal.needs_subst(),
 309                         },
 310                         ConstantKind::Val(_, ty) => ty.needs_subst(),
 311                     };
 312                     if lint_only {
 313                         // Out of backwards compatibility we cannot report hard errors in unused
 314                         // generic functions using associated constants of the generic parameters.
 315                         err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span));
 316                     } else {
 317                         err.report_as_error(tcx, "erroneous constant used");
 318                     }
 319                 } else {
 320                     err.report_as_error(tcx, "erroneous constant used");
 321                 }
 322                 None
 323             }
 324         }
 325     }
 326
 327     /// Returns the value, if any, of evaluating `place`.
 328     fn eval_place(&mut self, place: Place<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 329         trace!("eval_place(place={:?})", place);
 330         self.use_ecx(source_info, |this| this.ecx.eval_place_to_op(place, None))
 331     }
 332
 333     /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
 334     /// or `eval_place`, depending on the variant of `Operand` used.
 335     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
 336         match *op {
 337             Operand::Constant(ref c) => self.eval_constant(c, source_info),
 338             Operand::Move(place) | Operand::Copy(place) => self.eval_place(place, source_info),
 339         }
 340     }
 341
 342     fn report_assert_as_lint(
 343         &self,
 344         lint: &'static lint::Lint,
 345         source_info: SourceInfo,
 346         message: &'static str,
 347         panic: AssertKind<impl std::fmt::Debug>,
 348     ) {
 349         if let Some(lint_root) = self.lint_root(source_info) {
 350             self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
 351                 let mut err = lint.build(message);
 352                 err.span_label(source_info.span, format!("{:?}", panic));
 353                 err.emit();
 354             });
 355         }
 356     }
 357
 358     fn check_unary_op(
 359         &mut self,
 360         op: UnOp,
 361         arg: &Operand<'tcx>,
 362         source_info: SourceInfo,
 363     ) -> Option<()> {
 364         if let (val, true) = self.use_ecx(source_info, |this| {
 365             let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
 366             let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
 367             Ok((val, overflow))
 368         })? {
 369             // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
 370             // appropriate to use.
 371             assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
 372             self.report_assert_as_lint(
 373                 lint::builtin::ARITHMETIC_OVERFLOW,
 374                 source_info,
 375                 "this arithmetic operation will overflow",
 376                 AssertKind::OverflowNeg(val.to_const_int()),
 377             );
 378             return None;
 379         }
 380
 381         Some(())
 382     }
 383
 384     fn check_binary_op(
 385         &mut self,
 386         op: BinOp,
 387         left: &Operand<'tcx>,
 388         right: &Operand<'tcx>,
 389         source_info: SourceInfo,
 390     ) -> Option<()> {
 391         let r = self.use_ecx(source_info, |this| {
 392             this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?)
 393         });
 394         let l = self.use_ecx(source_info, |this| {
 395             this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?)
 396         });
 397         // Check for exceeding shifts *even if* we cannot evaluate the LHS.
 398         if op == BinOp::Shr || op == BinOp::Shl {
 399             let r = r.clone()?;
 400             // We need the type of the LHS. We cannot use `place_layout` as that is the type
 401             // of the result, which for checked binops is not the same!
 402             let left_ty = left.ty(self.local_decls, self.tcx);
 403             let left_size = self.ecx.layout_of(left_ty).ok()?.size;
 404             let right_size = r.layout.size;
 405             let r_bits = r.to_scalar().ok();
 406             let r_bits = r_bits.and_then(|r| r.to_bits(right_size).ok());
 407             if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
 408                 debug!("check_binary_op: reporting assert for {:?}", source_info);
 409                 self.report_assert_as_lint(
 410                     lint::builtin::ARITHMETIC_OVERFLOW,
 411                     source_info,
 412                     "this arithmetic operation will overflow",
 413                     AssertKind::Overflow(
 414                         op,
 415                         match l {
 416                             Some(l) => l.to_const_int(),
 417                             // Invent a dummy value, the diagnostic ignores it anyway
 418                             None => ConstInt::new(
 419                                 ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
 420                                 left_ty.is_signed(),
 421                                 left_ty.is_ptr_sized_integral(),
 422                             ),
 423                         },
 424                         r.to_const_int(),
 425                     ),
 426                 );
 427                 return None;
 428             }
 429         }
 430
 431         if let (Some(l), Some(r)) = (l, r) {
 432             // The remaining operators are handled through `overflowing_binary_op`.
 433             if self.use_ecx(source_info, |this| {
 434                 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, &l, &r)?;
 435                 Ok(overflow)
 436             })? {
 437                 self.report_assert_as_lint(
 438                     lint::builtin::ARITHMETIC_OVERFLOW,
 439                     source_info,
 440                     "this arithmetic operation will overflow",
 441                     AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
 442                 );
 443                 return None;
 444             }
 445         }
 446         Some(())
 447     }
 448
 449     fn const_prop(
 450         &mut self,
 451         rvalue: &Rvalue<'tcx>,
 452         source_info: SourceInfo,
 453         place: Place<'tcx>,
 454     ) -> Option<()> {
 455         // Perform any special handling for specific Rvalue types.
 456         // Generally, checks here fall into one of two categories:
 457         //   1. Additional checking to provide useful lints to the user
 458         //        - In this case, we will do some validation and then fall through to the
 459         //          end of the function which evals the assignment.
 460         //   2. Working around bugs in other parts of the compiler
 461         //        - In this case, we'll return `None` from this function to stop evaluation.
 462         match rvalue {
 463             // Additional checking: give lints to the user if an overflow would occur.
 464             // We do this here and not in the `Assert` terminator as that terminator is
 465             // only sometimes emitted (overflow checks can be disabled), but we want to always
 466             // lint.
 467             Rvalue::UnaryOp(op, arg) => {
 468                 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
 469                 self.check_unary_op(*op, arg, source_info)?;
 470             }
 471             Rvalue::BinaryOp(op, box (left, right)) => {
 472                 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
 473                 self.check_binary_op(*op, left, right, source_info)?;
 474             }
 475             Rvalue::CheckedBinaryOp(op, box (left, right)) => {
 476                 trace!(
 477                     "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
 478                     op,
 479                     left,
 480                     right
 481                 );
 482                 self.check_binary_op(*op, left, right, source_info)?;
 483             }
 484
 485             // Do not try creating references (#67862)
 486             Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
 487                 trace!("skipping AddressOf | Ref for {:?}", place);
 488
 489                 // This may be creating mutable references or immutable references to cells.
 490                 // If that happens, the pointed to value could be mutated via that reference.
 491                 // Since we aren't tracking references, the const propagator loses track of what
 492                 // value the local has right now.
 493                 // Thus, all locals that have their reference taken
 494                 // must not take part in propagation.
 495                 Self::remove_const(&mut self.ecx, place.local);
 496
 497                 return None;
 498             }
 499             Rvalue::ThreadLocalRef(def_id) => {
 500                 trace!("skipping ThreadLocalRef({:?})", def_id);
 501
 502                 return None;
 503             }
 504
 505             // There's no other checking to do at this time.
 506             Rvalue::Aggregate(..)
 507             | Rvalue::Use(..)
 508             | Rvalue::CopyForDeref(..)
 509             | Rvalue::Repeat(..)
 510             | Rvalue::Len(..)
 511             | Rvalue::Cast(..)
 512             | Rvalue::ShallowInitBox(..)
 513             | Rvalue::Discriminant(..)
 514             | Rvalue::NullaryOp(..) => {}
 515         }
 516
 517         // FIXME we need to revisit this for #67176
 518         if rvalue.needs_subst() {
 519             return None;
 520         }
 521
 522         self.use_ecx(source_info, |this| this.ecx.eval_rvalue_into_place(rvalue, place))
 523     }
 524 }
 525
 526 impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
 527     fn visit_body(&mut self, body: &Body<'tcx>) {
 528         for (bb, data) in body.basic_blocks().iter_enumerated() {
 529             self.visit_basic_block_data(bb, data);
 530         }
 531     }
 532
 533     fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
 534         self.super_operand(operand, location);
 535     }
 536
 537     fn visit_constant(&mut self, constant: &Constant<'tcx>, location: Location) {
 538         trace!("visit_constant: {:?}", constant);
 539         self.super_constant(constant, location);
 540         self.eval_constant(constant, self.source_info.unwrap());
 541     }
 542
 543     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
 544         trace!("visit_statement: {:?}", statement);
 545         let source_info = statement.source_info;
 546         self.source_info = Some(source_info);
 547         if let StatementKind::Assign(box (place, ref rval)) = statement.kind {
 548             let can_const_prop = self.ecx.machine.can_const_prop[place.local];
 549             if let Some(()) = self.const_prop(rval, source_info, place) {
 550                 match can_const_prop {
 551                     ConstPropMode::OnlyInsideOwnBlock => {
 552                         trace!(
 553                             "found local restricted to its block. \
 554                                 Will remove it from const-prop after block is finished. Local: {:?}",
 555                             place.local
 556                         );
 557                     }
 558                     ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
 559                         trace!("can't propagate into {:?}", place);
 560                         if place.local != RETURN_PLACE {
 561                             Self::remove_const(&mut self.ecx, place.local);
 562                         }
 563                     }
 564                     ConstPropMode::FullConstProp => {}
 565                 }
 566             } else {
 567                 // Const prop failed, so erase the destination, ensuring that whatever happens
 568                 // from here on, does not know about the previous value.
 569                 // This is important in case we have
 570                 // ```rust
 571                 // let mut x = 42;
 572                 // x = SOME_MUTABLE_STATIC;
 573                 // // x must now be uninit
 574                 // ```
 575                 // FIXME: we overzealously erase the entire local, because that's easier to
 576                 // implement.
 577                 trace!(
 578                     "propagation into {:?} failed.
 579                         Nuking the entire site from orbit, it's the only way to be sure",
 580                     place,
 581                 );
 582                 Self::remove_const(&mut self.ecx, place.local);
 583             }
 584         } else {
 585             match statement.kind {
 586                 StatementKind::SetDiscriminant { ref place, .. } => {
 587                     match self.ecx.machine.can_const_prop[place.local] {
 588                         ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
 589                             if self
 590                                 .use_ecx(source_info, |this| this.ecx.statement(statement))
 591                                 .is_some()
 592                             {
 593                                 trace!("propped discriminant into {:?}", place);
 594                             } else {
 595                                 Self::remove_const(&mut self.ecx, place.local);
 596                             }
 597                         }
 598                         ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
 599                             Self::remove_const(&mut self.ecx, place.local);
 600                         }
 601                     }
 602                 }
 603                 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
 604                     let frame = self.ecx.frame_mut();
 605                     frame.locals[local].value =
 606                         if let StatementKind::StorageLive(_) = statement.kind {
 607                             LocalValue::Live(interpret::Operand::Immediate(
 608                                 interpret::Immediate::Uninit,
 609                             ))
 610                         } else {
 611                             LocalValue::Dead
 612                         };
 613                 }
 614                 _ => {}
 615             }
 616         }
 617
 618         self.super_statement(statement, location);
 619     }
 620
 621     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
 622         let source_info = terminator.source_info;
 623         self.source_info = Some(source_info);
 624         self.super_terminator(terminator, location);
 625         match &terminator.kind {
 626             TerminatorKind::Assert { expected, ref msg, ref cond, .. } => {
 627                 if let Some(ref value) = self.eval_operand(&cond, source_info) {
 628                     trace!("assertion on {:?} should be {:?}", value, expected);
 629                     let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
 630                     let value_const = self.ecx.read_scalar(&value).unwrap();
 631                     if expected != value_const {
 632                         enum DbgVal<T> {
 633                             Val(T),
 634                             Underscore,
 635                         }
 636                         impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
 637                             fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
 638                                 match self {
 639                                     Self::Val(val) => val.fmt(fmt),
 640                                     Self::Underscore => fmt.write_str("_"),
 641                                 }
 642                             }
 643                         }
 644                         let mut eval_to_int = |op| {
 645                             // This can be `None` if the lhs wasn't const propagated and we just
 646                             // triggered the assert on the value of the rhs.
 647                             self.eval_operand(op, source_info).map_or(DbgVal::Underscore, |op| {
 648                                 DbgVal::Val(self.ecx.read_immediate(&op).unwrap().to_const_int())
 649                             })
 650                         };
 651                         let msg = match msg {
 652                             AssertKind::DivisionByZero(op) => {
 653                                 Some(AssertKind::DivisionByZero(eval_to_int(op)))
 654                             }
 655                             AssertKind::RemainderByZero(op) => {
 656                                 Some(AssertKind::RemainderByZero(eval_to_int(op)))
 657                             }
 658                             AssertKind::Overflow(bin_op @ (BinOp::Div | BinOp::Rem), op1, op2) => {
 659                                 // Division overflow is *UB* in the MIR, and different than the
 660                                 // other overflow checks.
 661                                 Some(AssertKind::Overflow(
 662                                     *bin_op,
 663                                     eval_to_int(op1),
 664                                     eval_to_int(op2),
 665                                 ))
 666                             }
 667                             AssertKind::BoundsCheck { ref len, ref index } => {
 668                                 let len = eval_to_int(len);
 669                                 let index = eval_to_int(index);
 670                                 Some(AssertKind::BoundsCheck { len, index })
 671                             }
 672                             // Remaining overflow errors are already covered by checks on the binary operators.
 673                             AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None,
 674                             // Need proper const propagator for these.
 675                             _ => None,
 676                         };
 677                         // Poison all places this operand references so that further code
 678                         // doesn't use the invalid value
 679                         match cond {
 680                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 681                                 Self::remove_const(&mut self.ecx, place.local);
 682                             }
 683                             Operand::Constant(_) => {}
 684                         }
 685                         if let Some(msg) = msg {
 686                             self.report_assert_as_lint(
 687                                 lint::builtin::UNCONDITIONAL_PANIC,
 688                                 source_info,
 689                                 "this operation will panic at runtime",
 690                                 msg,
 691                             );
 692                         }
 693                     }
 694                 }
 695             }
 696             // None of these have Operands to const-propagate.
 697             TerminatorKind::Goto { .. }
 698             | TerminatorKind::Resume
 699             | TerminatorKind::Abort
 700             | TerminatorKind::Return
 701             | TerminatorKind::Unreachable
 702             | TerminatorKind::Drop { .. }
 703             | TerminatorKind::DropAndReplace { .. }
 704             | TerminatorKind::Yield { .. }
 705             | TerminatorKind::GeneratorDrop
 706             | TerminatorKind::FalseEdge { .. }
 707             | TerminatorKind::FalseUnwind { .. }
 708             | TerminatorKind::SwitchInt { .. }
 709             | TerminatorKind::Call { .. }
 710             | TerminatorKind::InlineAsm { .. } => {}
 711         }
 712
 713         // We remove all Locals which are restricted in propagation to their containing blocks and
 714         // which were modified in the current block.
 715         // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
 716         let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
 717         for &local in locals.iter() {
 718             Self::remove_const(&mut self.ecx, local);
 719         }
 720         locals.clear();
 721         // Put it back so we reuse the heap of the storage
 722         self.ecx.machine.written_only_inside_own_block_locals = locals;
 723         if cfg!(debug_assertions) {
 724             // Ensure we are correctly erasing locals with the non-debug-assert logic.
 725             for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
 726                 assert!(
 727                     self.get_const(local.into()).is_none()
 728                         || self
 729                             .layout_of(self.local_decls[local].ty)
 730                             .map_or(true, |layout| layout.is_zst())
 731                 )
 732             }
 733         }
 734     }
 735 }