rustc_mir_transform/
known_panics_lint.rs

1//! A lint that checks for known panics like overflows, division by zero,
2//! out-of-bound access etc. Uses const propagation to determine the values of
3//! operands during checks.
4
5use std::fmt::Debug;
6
7use rustc_abi::{BackendRepr, FieldIdx, HasDataLayout, Size, TargetDataLayout, VariantIdx};
8use rustc_const_eval::const_eval::DummyMachine;
9use rustc_const_eval::interpret::{
10    ImmTy, InterpCx, InterpResult, Projectable, Scalar, format_interp_error, interp_ok,
11};
12use rustc_data_structures::fx::FxHashSet;
13use rustc_hir::HirId;
14use rustc_hir::def::DefKind;
15use rustc_index::IndexVec;
16use rustc_index::bit_set::DenseBitSet;
17use rustc_middle::bug;
18use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
19use rustc_middle::mir::*;
20use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
21use rustc_middle::ty::{self, ConstInt, ScalarInt, Ty, TyCtxt, TypeVisitableExt};
22use rustc_span::Span;
23use tracing::{debug, instrument, trace};
24
25use crate::errors::{AssertLint, AssertLintKind};
26
27pub(super) struct KnownPanicsLint;
28
29impl<'tcx> crate::MirLint<'tcx> for KnownPanicsLint {
30    fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
31        if body.tainted_by_errors.is_some() {
32            return;
33        }
34
35        let def_id = body.source.def_id().expect_local();
36        let def_kind = tcx.def_kind(def_id);
37        let is_fn_like = def_kind.is_fn_like();
38        let is_assoc_const = def_kind == DefKind::AssocConst;
39
40        // Only run const prop on functions, methods, closures and associated constants
41        if !is_fn_like && !is_assoc_const {
42            // skip anon_const/statics/consts because they'll be evaluated by miri anyway
43            trace!("KnownPanicsLint skipped for {:?}", def_id);
44            return;
45        }
46
47        // FIXME(welseywiser) const prop doesn't work on coroutines because of query cycles
48        // computing their layout.
49        if tcx.is_coroutine(def_id.to_def_id()) {
50            trace!("KnownPanicsLint skipped for coroutine {:?}", def_id);
51            return;
52        }
53
54        trace!("KnownPanicsLint starting for {:?}", def_id);
55
56        let mut linter = ConstPropagator::new(body, tcx);
57        linter.visit_body(body);
58
59        trace!("KnownPanicsLint done for {:?}", def_id);
60    }
61}
62
63/// Visits MIR nodes, performs const propagation
64/// and runs lint checks as it goes
65struct ConstPropagator<'mir, 'tcx> {
66    ecx: InterpCx<'tcx, DummyMachine>,
67    tcx: TyCtxt<'tcx>,
68    typing_env: ty::TypingEnv<'tcx>,
69    worklist: Vec<BasicBlock>,
70    visited_blocks: DenseBitSet<BasicBlock>,
71    locals: IndexVec<Local, Value<'tcx>>,
72    body: &'mir Body<'tcx>,
73    written_only_inside_own_block_locals: FxHashSet<Local>,
74    can_const_prop: IndexVec<Local, ConstPropMode>,
75}
76
77#[derive(Debug, Clone)]
78enum Value<'tcx> {
79    Immediate(ImmTy<'tcx>),
80    Aggregate { variant: VariantIdx, fields: IndexVec<FieldIdx, Value<'tcx>> },
81    Uninit,
82}
83
84impl<'tcx> From<ImmTy<'tcx>> for Value<'tcx> {
85    fn from(v: ImmTy<'tcx>) -> Self {
86        Self::Immediate(v)
87    }
88}
89
90impl<'tcx> Value<'tcx> {
91    fn project(
92        &self,
93        proj: &[PlaceElem<'tcx>],
94        prop: &ConstPropagator<'_, 'tcx>,
95    ) -> Option<&Value<'tcx>> {
96        let mut this = self;
97        for proj in proj {
98            this = match (*proj, this) {
99                (PlaceElem::Field(idx, _), Value::Aggregate { fields, .. }) => {
100                    fields.get(idx).unwrap_or(&Value::Uninit)
101                }
102                (PlaceElem::Index(idx), Value::Aggregate { fields, .. }) => {
103                    let idx = prop.get_const(idx.into())?.immediate()?;
104                    let idx = prop.ecx.read_target_usize(idx).discard_err()?.try_into().ok()?;
105                    if idx <= FieldIdx::MAX_AS_U32 {
106                        fields.get(FieldIdx::from_u32(idx)).unwrap_or(&Value::Uninit)
107                    } else {
108                        return None;
109                    }
110                }
111                (
112                    PlaceElem::ConstantIndex { offset, min_length: _, from_end: false },
113                    Value::Aggregate { fields, .. },
114                ) => fields
115                    .get(FieldIdx::from_u32(offset.try_into().ok()?))
116                    .unwrap_or(&Value::Uninit),
117                _ => return None,
118            };
119        }
120        Some(this)
121    }
122
123    fn project_mut(&mut self, proj: &[PlaceElem<'_>]) -> Option<&mut Value<'tcx>> {
124        let mut this = self;
125        for proj in proj {
126            this = match (proj, this) {
127                (PlaceElem::Field(idx, _), Value::Aggregate { fields, .. }) => {
128                    fields.ensure_contains_elem(*idx, || Value::Uninit)
129                }
130                (PlaceElem::Field(..), val @ Value::Uninit) => {
131                    *val =
132                        Value::Aggregate { variant: VariantIdx::ZERO, fields: Default::default() };
133                    val.project_mut(&[*proj])?
134                }
135                _ => return None,
136            };
137        }
138        Some(this)
139    }
140
141    fn immediate(&self) -> Option<&ImmTy<'tcx>> {
142        match self {
143            Value::Immediate(op) => Some(op),
144            _ => None,
145        }
146    }
147}
148
149impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
150    type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
151
152    #[inline]
153    fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
154        err
155    }
156}
157
158impl HasDataLayout for ConstPropagator<'_, '_> {
159    #[inline]
160    fn data_layout(&self) -> &TargetDataLayout {
161        &self.tcx.data_layout
162    }
163}
164
165impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
166    #[inline]
167    fn tcx(&self) -> TyCtxt<'tcx> {
168        self.tcx
169    }
170}
171
172impl<'tcx> ty::layout::HasTypingEnv<'tcx> for ConstPropagator<'_, 'tcx> {
173    #[inline]
174    fn typing_env(&self) -> ty::TypingEnv<'tcx> {
175        self.typing_env
176    }
177}
178
179impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
180    fn new(body: &'mir Body<'tcx>, tcx: TyCtxt<'tcx>) -> ConstPropagator<'mir, 'tcx> {
181        let def_id = body.source.def_id();
182        // FIXME(#132279): This is used during the phase transition from analysis
183        // to runtime, so we have to manually specify the correct typing mode.
184        let typing_env = ty::TypingEnv::post_analysis(tcx, body.source.def_id());
185        let can_const_prop = CanConstProp::check(tcx, typing_env, body);
186        let ecx = InterpCx::new(tcx, tcx.def_span(def_id), typing_env, DummyMachine);
187
188        ConstPropagator {
189            ecx,
190            tcx,
191            typing_env,
192            worklist: vec![START_BLOCK],
193            visited_blocks: DenseBitSet::new_empty(body.basic_blocks.len()),
194            locals: IndexVec::from_elem_n(Value::Uninit, body.local_decls.len()),
195            body,
196            can_const_prop,
197            written_only_inside_own_block_locals: Default::default(),
198        }
199    }
200
201    fn local_decls(&self) -> &'mir LocalDecls<'tcx> {
202        &self.body.local_decls
203    }
204
205    fn get_const(&self, place: Place<'tcx>) -> Option<&Value<'tcx>> {
206        self.locals[place.local].project(&place.projection, self)
207    }
208
209    /// Remove `local` from the pool of `Locals`. Allows writing to them,
210    /// but not reading from them anymore.
211    fn remove_const(&mut self, local: Local) {
212        self.locals[local] = Value::Uninit;
213        self.written_only_inside_own_block_locals.remove(&local);
214    }
215
216    fn access_mut(&mut self, place: &Place<'_>) -> Option<&mut Value<'tcx>> {
217        match self.can_const_prop[place.local] {
218            ConstPropMode::NoPropagation => return None,
219            ConstPropMode::OnlyInsideOwnBlock => {
220                self.written_only_inside_own_block_locals.insert(place.local);
221            }
222            ConstPropMode::FullConstProp => {}
223        }
224        self.locals[place.local].project_mut(place.projection)
225    }
226
227    fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
228        source_info.scope.lint_root(&self.body.source_scopes)
229    }
230
231    fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
232    where
233        F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
234    {
235        f(self)
236            .map_err_info(|err| {
237                trace!("InterpCx operation failed: {:?}", err);
238                // Some errors shouldn't come up because creating them causes
239                // an allocation, which we should avoid. When that happens,
240                // dedicated error variants should be introduced instead.
241                assert!(
242                    !err.kind().formatted_string(),
243                    "known panics lint encountered formatting error: {}",
244                    format_interp_error(self.ecx.tcx.dcx(), err),
245                );
246                err
247            })
248            .discard_err()
249    }
250
251    /// Returns the value, if any, of evaluating `c`.
252    fn eval_constant(&mut self, c: &ConstOperand<'tcx>) -> Option<ImmTy<'tcx>> {
253        // FIXME we need to revisit this for #67176
254        if c.has_param() {
255            return None;
256        }
257
258        // Normalization needed b/c known panics lint runs in
259        // `mir_drops_elaborated_and_const_checked`, which happens before
260        // optimized MIR. Only after optimizing the MIR can we guarantee
261        // that the `PostAnalysisNormalize` pass has happened and that the body's consts
262        // are normalized, so any call to resolve before that needs to be
263        // manually normalized.
264        let val = self.tcx.try_normalize_erasing_regions(self.typing_env, c.const_).ok()?;
265
266        self.use_ecx(|this| this.ecx.eval_mir_constant(&val, c.span, None))?
267            .as_mplace_or_imm()
268            .right()
269    }
270
271    /// Returns the value, if any, of evaluating `place`.
272    #[instrument(level = "trace", skip(self), ret)]
273    fn eval_place(&mut self, place: Place<'tcx>) -> Option<ImmTy<'tcx>> {
274        match self.get_const(place)? {
275            Value::Immediate(imm) => Some(imm.clone()),
276            Value::Aggregate { .. } => None,
277            Value::Uninit => None,
278        }
279    }
280
281    /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
282    /// or `eval_place`, depending on the variant of `Operand` used.
283    fn eval_operand(&mut self, op: &Operand<'tcx>) -> Option<ImmTy<'tcx>> {
284        match *op {
285            Operand::RuntimeChecks(_) => None,
286            Operand::Constant(ref c) => self.eval_constant(c),
287            Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
288        }
289    }
290
291    fn report_assert_as_lint(
292        &self,
293        location: Location,
294        lint_kind: AssertLintKind,
295        assert_kind: AssertKind<impl Debug>,
296    ) {
297        let source_info = self.body.source_info(location);
298        if let Some(lint_root) = self.lint_root(*source_info) {
299            let span = source_info.span;
300            self.tcx.emit_node_span_lint(
301                lint_kind.lint(),
302                lint_root,
303                span,
304                AssertLint { span, assert_kind, lint_kind },
305            );
306        }
307    }
308
309    fn check_unary_op(&mut self, op: UnOp, arg: &Operand<'tcx>, location: Location) -> Option<()> {
310        let arg = self.eval_operand(arg)?;
311        // The only operator that can overflow is `Neg`.
312        if op == UnOp::Neg && arg.layout.ty.is_integral() {
313            // Compute this as `0 - arg` so we can use `SubWithOverflow` to check for overflow.
314            let (arg, overflow) = self.use_ecx(|this| {
315                let arg = this.ecx.read_immediate(&arg)?;
316                let (_res, overflow) = this
317                    .ecx
318                    .binary_op(BinOp::SubWithOverflow, &ImmTy::from_int(0, arg.layout), &arg)?
319                    .to_scalar_pair();
320                interp_ok((arg, overflow.to_bool()?))
321            })?;
322            if overflow {
323                self.report_assert_as_lint(
324                    location,
325                    AssertLintKind::ArithmeticOverflow,
326                    AssertKind::OverflowNeg(arg.to_const_int()),
327                );
328                return None;
329            }
330        }
331
332        Some(())
333    }
334
335    fn check_binary_op(
336        &mut self,
337        op: BinOp,
338        left: &Operand<'tcx>,
339        right: &Operand<'tcx>,
340        location: Location,
341    ) -> Option<()> {
342        let r =
343            self.eval_operand(right).and_then(|r| self.use_ecx(|this| this.ecx.read_immediate(&r)));
344        let l =
345            self.eval_operand(left).and_then(|l| self.use_ecx(|this| this.ecx.read_immediate(&l)));
346        // Check for exceeding shifts *even if* we cannot evaluate the LHS.
347        if matches!(op, BinOp::Shr | BinOp::Shl) {
348            let r = r.clone()?;
349            // We need the type of the LHS. We cannot use `place_layout` as that is the type
350            // of the result, which for checked binops is not the same!
351            let left_ty = left.ty(self.local_decls(), self.tcx);
352            let left_size = self.ecx.layout_of(left_ty).ok()?.size;
353            let right_size = r.layout.size;
354            let r_bits = r.to_scalar().to_bits(right_size).discard_err();
355            if r_bits.is_some_and(|b| b >= left_size.bits() as u128) {
356                debug!("check_binary_op: reporting assert for {:?}", location);
357                let panic = AssertKind::Overflow(
358                    op,
359                    // Invent a dummy value, the diagnostic ignores it anyway
360                    ConstInt::new(
361                        ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
362                        left_ty.is_signed(),
363                        left_ty.is_ptr_sized_integral(),
364                    ),
365                    r.to_const_int(),
366                );
367                self.report_assert_as_lint(location, AssertLintKind::ArithmeticOverflow, panic);
368                return None;
369            }
370        }
371
372        // Div/Rem are handled via the assertions they trigger.
373        // But for Add/Sub/Mul, those assertions only exist in debug builds, and we want to
374        // lint in release builds as well, so we check on the operation instead.
375        // So normalize to the "overflowing" operator, and then ensure that it
376        // actually is an overflowing operator.
377        let op = op.wrapping_to_overflowing().unwrap_or(op);
378        // The remaining operators are handled through `wrapping_to_overflowing`.
379        if let (Some(l), Some(r)) = (l, r)
380            && l.layout.ty.is_integral()
381            && op.is_overflowing()
382            && self.use_ecx(|this| {
383                let (_res, overflow) = this.ecx.binary_op(op, &l, &r)?.to_scalar_pair();
384                overflow.to_bool()
385            })?
386        {
387            self.report_assert_as_lint(
388                location,
389                AssertLintKind::ArithmeticOverflow,
390                AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
391            );
392            return None;
393        }
394
395        Some(())
396    }
397
398    fn check_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) -> Option<()> {
399        // Perform any special handling for specific Rvalue types.
400        // Generally, checks here fall into one of two categories:
401        //   1. Additional checking to provide useful lints to the user
402        //        - In this case, we will do some validation and then fall through to the
403        //          end of the function which evals the assignment.
404        //   2. Working around bugs in other parts of the compiler
405        //        - In this case, we'll return `None` from this function to stop evaluation.
406        match rvalue {
407            // Additional checking: give lints to the user if an overflow would occur.
408            // We do this here and not in the `Assert` terminator as that terminator is
409            // only sometimes emitted (overflow checks can be disabled), but we want to always
410            // lint.
411            Rvalue::UnaryOp(op, arg) => {
412                trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
413                self.check_unary_op(*op, arg, location)?;
414            }
415            Rvalue::BinaryOp(op, box (left, right)) => {
416                trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
417                self.check_binary_op(*op, left, right, location)?;
418            }
419
420            // Do not try creating references (#67862)
421            Rvalue::RawPtr(_, place) | Rvalue::Ref(_, _, place) => {
422                trace!("skipping RawPtr | Ref for {:?}", place);
423
424                // This may be creating mutable references or immutable references to cells.
425                // If that happens, the pointed to value could be mutated via that reference.
426                // Since we aren't tracking references, the const propagator loses track of what
427                // value the local has right now.
428                // Thus, all locals that have their reference taken
429                // must not take part in propagation.
430                self.remove_const(place.local);
431
432                return None;
433            }
434            Rvalue::ThreadLocalRef(def_id) => {
435                trace!("skipping ThreadLocalRef({:?})", def_id);
436
437                return None;
438            }
439
440            // There's no other checking to do at this time.
441            Rvalue::Aggregate(..)
442            | Rvalue::Use(..)
443            | Rvalue::CopyForDeref(..)
444            | Rvalue::Repeat(..)
445            | Rvalue::Cast(..)
446            | Rvalue::Discriminant(..)
447            | Rvalue::WrapUnsafeBinder(..) => {}
448        }
449
450        // FIXME we need to revisit this for #67176
451        if rvalue.has_param() {
452            return None;
453        }
454        if !rvalue.ty(self.local_decls(), self.tcx).is_sized(self.tcx, self.typing_env) {
455            // the interpreter doesn't support unsized locals (only unsized arguments),
456            // but rustc does (in a kinda broken way), so we have to skip them here
457            return None;
458        }
459
460        Some(())
461    }
462
463    fn check_assertion(
464        &mut self,
465        expected: bool,
466        msg: &AssertKind<Operand<'tcx>>,
467        cond: &Operand<'tcx>,
468        location: Location,
469    ) {
470        let Some(value) = &self.eval_operand(cond) else { return };
471        trace!("assertion on {:?} should be {:?}", value, expected);
472
473        let expected = Scalar::from_bool(expected);
474        let Some(value_const) = self.use_ecx(|this| this.ecx.read_scalar(value)) else { return };
475
476        if expected != value_const {
477            // Poison all places this operand references so that further code
478            // doesn't use the invalid value
479            if let Some(place) = cond.place() {
480                self.remove_const(place.local);
481            }
482
483            enum DbgVal<T> {
484                Val(T),
485                Underscore,
486            }
487            impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
488                fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
489                    match self {
490                        Self::Val(val) => val.fmt(fmt),
491                        Self::Underscore => fmt.write_str("_"),
492                    }
493                }
494            }
495            let mut eval_to_int = |op| {
496                // This can be `None` if the lhs wasn't const propagated and we just
497                // triggered the assert on the value of the rhs.
498                self.eval_operand(op)
499                    .and_then(|op| self.ecx.read_immediate(&op).discard_err())
500                    .map_or(DbgVal::Underscore, |op| DbgVal::Val(op.to_const_int()))
501            };
502            let msg = match msg {
503                AssertKind::DivisionByZero(op) => AssertKind::DivisionByZero(eval_to_int(op)),
504                AssertKind::RemainderByZero(op) => AssertKind::RemainderByZero(eval_to_int(op)),
505                AssertKind::Overflow(bin_op @ (BinOp::Div | BinOp::Rem), op1, op2) => {
506                    // Division overflow is *UB* in the MIR, and different than the
507                    // other overflow checks.
508                    AssertKind::Overflow(*bin_op, eval_to_int(op1), eval_to_int(op2))
509                }
510                AssertKind::BoundsCheck { len, index } => {
511                    let len = eval_to_int(len);
512                    let index = eval_to_int(index);
513                    AssertKind::BoundsCheck { len, index }
514                }
515                // Remaining overflow errors are already covered by checks on the binary operators.
516                AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => return,
517                // Need proper const propagator for these.
518                _ => return,
519            };
520            self.report_assert_as_lint(location, AssertLintKind::UnconditionalPanic, msg);
521        }
522    }
523
524    fn ensure_not_propagated(&self, local: Local) {
525        if cfg!(debug_assertions) {
526            let val = self.get_const(local.into());
527            assert!(
528                matches!(val, Some(Value::Uninit))
529                    || self
530                        .layout_of(self.local_decls()[local].ty)
531                        .map_or(true, |layout| layout.is_zst()),
532                "failed to remove values for `{local:?}`, value={val:?}",
533            )
534        }
535    }
536
537    #[instrument(level = "trace", skip(self), ret)]
538    fn eval_rvalue(&mut self, rvalue: &Rvalue<'tcx>, dest: &Place<'tcx>) -> Option<()> {
539        if !dest.projection.is_empty() {
540            return None;
541        }
542        use rustc_middle::mir::Rvalue::*;
543        let layout = self.ecx.layout_of(dest.ty(self.body, self.tcx).ty).ok()?;
544        trace!(?layout);
545
546        let val: Value<'_> = match *rvalue {
547            ThreadLocalRef(_) => return None,
548
549            Use(ref operand) | WrapUnsafeBinder(ref operand, _) => {
550                self.eval_operand(operand)?.into()
551            }
552
553            CopyForDeref(place) => self.eval_place(place)?.into(),
554
555            BinaryOp(bin_op, box (ref left, ref right)) => {
556                let left = self.eval_operand(left)?;
557                let left = self.use_ecx(|this| this.ecx.read_immediate(&left))?;
558
559                let right = self.eval_operand(right)?;
560                let right = self.use_ecx(|this| this.ecx.read_immediate(&right))?;
561
562                let val = self.use_ecx(|this| this.ecx.binary_op(bin_op, &left, &right))?;
563                if matches!(val.layout.backend_repr, BackendRepr::ScalarPair(..)) {
564                    // FIXME `Value` should properly support pairs in `Immediate`... but currently
565                    // it does not.
566                    let (val, overflow) = val.to_pair(&self.ecx);
567                    Value::Aggregate {
568                        variant: VariantIdx::ZERO,
569                        fields: [val.into(), overflow.into()].into_iter().collect(),
570                    }
571                } else {
572                    val.into()
573                }
574            }
575
576            UnaryOp(un_op, ref operand) => {
577                let operand = self.eval_operand(operand)?;
578                let val = self.use_ecx(|this| this.ecx.read_immediate(&operand))?;
579
580                let val = self.use_ecx(|this| this.ecx.unary_op(un_op, &val))?;
581                val.into()
582            }
583
584            Aggregate(ref kind, ref fields) => Value::Aggregate {
585                fields: fields
586                    .iter()
587                    .map(|field| self.eval_operand(field).map_or(Value::Uninit, Value::Immediate))
588                    .collect(),
589                variant: match **kind {
590                    AggregateKind::Adt(_, variant, _, _, _) => variant,
591                    AggregateKind::Array(_)
592                    | AggregateKind::Tuple
593                    | AggregateKind::RawPtr(_, _)
594                    | AggregateKind::Closure(_, _)
595                    | AggregateKind::Coroutine(_, _)
596                    | AggregateKind::CoroutineClosure(_, _) => VariantIdx::ZERO,
597                },
598            },
599
600            Repeat(ref op, n) => {
601                trace!(?op, ?n);
602                return None;
603            }
604
605            Ref(..) | RawPtr(..) => return None,
606
607            Cast(ref kind, ref value, to) => match kind {
608                CastKind::IntToInt | CastKind::IntToFloat => {
609                    let value = self.eval_operand(value)?;
610                    let value = self.ecx.read_immediate(&value).discard_err()?;
611                    let to = self.ecx.layout_of(to).ok()?;
612                    let res = self.ecx.int_to_int_or_float(&value, to).discard_err()?;
613                    res.into()
614                }
615                CastKind::FloatToFloat | CastKind::FloatToInt => {
616                    let value = self.eval_operand(value)?;
617                    let value = self.ecx.read_immediate(&value).discard_err()?;
618                    let to = self.ecx.layout_of(to).ok()?;
619                    let res = self.ecx.float_to_float_or_int(&value, to).discard_err()?;
620                    res.into()
621                }
622                CastKind::Transmute | CastKind::Subtype => {
623                    let value = self.eval_operand(value)?;
624                    let to = self.ecx.layout_of(to).ok()?;
625                    // `offset` for immediates only supports scalar/scalar-pair ABIs,
626                    // so bail out if the target is not one.
627                    match (value.layout.backend_repr, to.backend_repr) {
628                        (BackendRepr::Scalar(..), BackendRepr::Scalar(..)) => {}
629                        (BackendRepr::ScalarPair(..), BackendRepr::ScalarPair(..)) => {}
630                        _ => return None,
631                    }
632
633                    value.offset(Size::ZERO, to, &self.ecx).discard_err()?.into()
634                }
635                _ => return None,
636            },
637
638            Discriminant(place) => {
639                let variant = match self.get_const(place)? {
640                    Value::Immediate(op) => {
641                        let op = op.clone();
642                        self.use_ecx(|this| this.ecx.read_discriminant(&op))?
643                    }
644                    Value::Aggregate { variant, .. } => *variant,
645                    Value::Uninit => return None,
646                };
647                let imm = self.use_ecx(|this| {
648                    this.ecx.discriminant_for_variant(
649                        place.ty(this.local_decls(), this.tcx).ty,
650                        variant,
651                    )
652                })?;
653                imm.into()
654            }
655        };
656        trace!(?val);
657
658        *self.access_mut(dest)? = val;
659
660        Some(())
661    }
662}
663
664impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
665    fn visit_body(&mut self, body: &Body<'tcx>) {
666        while let Some(bb) = self.worklist.pop() {
667            if !self.visited_blocks.insert(bb) {
668                continue;
669            }
670
671            let data = &body.basic_blocks[bb];
672            self.visit_basic_block_data(bb, data);
673        }
674    }
675
676    fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
677        self.super_operand(operand, location);
678    }
679
680    fn visit_const_operand(&mut self, constant: &ConstOperand<'tcx>, location: Location) {
681        trace!("visit_const_operand: {:?}", constant);
682        self.super_const_operand(constant, location);
683        self.eval_constant(constant);
684    }
685
686    fn visit_assign(&mut self, place: &Place<'tcx>, rvalue: &Rvalue<'tcx>, location: Location) {
687        self.super_assign(place, rvalue, location);
688
689        let Some(()) = self.check_rvalue(rvalue, location) else { return };
690
691        match self.can_const_prop[place.local] {
692            // Do nothing if the place is indirect.
693            _ if place.is_indirect() => {}
694            ConstPropMode::NoPropagation => self.ensure_not_propagated(place.local),
695            ConstPropMode::OnlyInsideOwnBlock | ConstPropMode::FullConstProp => {
696                if self.eval_rvalue(rvalue, place).is_none() {
697                    // Const prop failed, so erase the destination, ensuring that whatever happens
698                    // from here on, does not know about the previous value.
699                    // This is important in case we have
700                    // ```rust
701                    // let mut x = 42;
702                    // x = SOME_MUTABLE_STATIC;
703                    // // x must now be uninit
704                    // ```
705                    // FIXME: we overzealously erase the entire local, because that's easier to
706                    // implement.
707                    trace!(
708                        "propagation into {:?} failed.
709                        Nuking the entire site from orbit, it's the only way to be sure",
710                        place,
711                    );
712                    self.remove_const(place.local);
713                }
714            }
715        }
716    }
717
718    fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
719        trace!("visit_statement: {:?}", statement);
720
721        // We want to evaluate operands before any change to the assigned-to value,
722        // so we recurse first.
723        self.super_statement(statement, location);
724
725        match statement.kind {
726            StatementKind::SetDiscriminant { ref place, variant_index } => {
727                match self.can_const_prop[place.local] {
728                    // Do nothing if the place is indirect.
729                    _ if place.is_indirect() => {}
730                    ConstPropMode::NoPropagation => self.ensure_not_propagated(place.local),
731                    ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
732                        match self.access_mut(place) {
733                            Some(Value::Aggregate { variant, .. }) => *variant = variant_index,
734                            _ => self.remove_const(place.local),
735                        }
736                    }
737                }
738            }
739            StatementKind::StorageLive(local) => {
740                self.remove_const(local);
741            }
742            StatementKind::StorageDead(local) => {
743                self.remove_const(local);
744            }
745            _ => {}
746        }
747    }
748
749    fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
750        self.super_terminator(terminator, location);
751        match &terminator.kind {
752            TerminatorKind::Assert { expected, msg, cond, .. } => {
753                self.check_assertion(*expected, msg, cond, location);
754            }
755            TerminatorKind::SwitchInt { discr, targets } => {
756                if let Some(ref value) = self.eval_operand(discr)
757                    && let Some(value_const) = self.use_ecx(|this| this.ecx.read_scalar(value))
758                    && let Some(constant) = value_const.to_bits(value_const.size()).discard_err()
759                {
760                    // We managed to evaluate the discriminant, so we know we only need to visit
761                    // one target.
762                    let target = targets.target_for_value(constant);
763                    self.worklist.push(target);
764                    return;
765                }
766                // We failed to evaluate the discriminant, fallback to visiting all successors.
767            }
768            // None of these have Operands to const-propagate.
769            TerminatorKind::Goto { .. }
770            | TerminatorKind::UnwindResume
771            | TerminatorKind::UnwindTerminate(_)
772            | TerminatorKind::Return
773            | TerminatorKind::TailCall { .. }
774            | TerminatorKind::Unreachable
775            | TerminatorKind::Drop { .. }
776            | TerminatorKind::Yield { .. }
777            | TerminatorKind::CoroutineDrop
778            | TerminatorKind::FalseEdge { .. }
779            | TerminatorKind::FalseUnwind { .. }
780            | TerminatorKind::Call { .. }
781            | TerminatorKind::InlineAsm { .. } => {}
782        }
783
784        self.worklist.extend(terminator.successors());
785    }
786
787    fn visit_basic_block_data(&mut self, block: BasicBlock, data: &BasicBlockData<'tcx>) {
788        self.super_basic_block_data(block, data);
789
790        // We remove all Locals which are restricted in propagation to their containing blocks and
791        // which were modified in the current block.
792        // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
793        let mut written_only_inside_own_block_locals =
794            std::mem::take(&mut self.written_only_inside_own_block_locals);
795
796        // This loop can get very hot for some bodies: it check each local in each bb.
797        // To avoid this quadratic behaviour, we only clear the locals that were modified inside
798        // the current block.
799        // The order in which we remove consts does not matter.
800        #[allow(rustc::potential_query_instability)]
801        for local in written_only_inside_own_block_locals.drain() {
802            debug_assert_eq!(self.can_const_prop[local], ConstPropMode::OnlyInsideOwnBlock);
803            self.remove_const(local);
804        }
805        self.written_only_inside_own_block_locals = written_only_inside_own_block_locals;
806
807        if cfg!(debug_assertions) {
808            for (local, &mode) in self.can_const_prop.iter_enumerated() {
809                match mode {
810                    ConstPropMode::FullConstProp => {}
811                    ConstPropMode::NoPropagation | ConstPropMode::OnlyInsideOwnBlock => {
812                        self.ensure_not_propagated(local);
813                    }
814                }
815            }
816        }
817    }
818}
819
820/// The maximum number of bytes that we'll allocate space for a local or the return value.
821/// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
822/// Severely regress performance.
823const MAX_ALLOC_LIMIT: u64 = 1024;
824
825/// The mode that `ConstProp` is allowed to run in for a given `Local`.
826#[derive(Clone, Copy, Debug, PartialEq)]
827enum ConstPropMode {
828    /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
829    FullConstProp,
830    /// The `Local` can only be propagated into and from its own block.
831    OnlyInsideOwnBlock,
832    /// The `Local` cannot be part of propagation at all. Any statement
833    /// referencing it either for reading or writing will not get propagated.
834    NoPropagation,
835}
836
837/// A visitor that determines locals in a MIR body
838/// that can be const propagated
839struct CanConstProp {
840    can_const_prop: IndexVec<Local, ConstPropMode>,
841    // False at the beginning. Once set, no more assignments are allowed to that local.
842    found_assignment: DenseBitSet<Local>,
843}
844
845impl CanConstProp {
846    /// Returns true if `local` can be propagated
847    fn check<'tcx>(
848        tcx: TyCtxt<'tcx>,
849        typing_env: ty::TypingEnv<'tcx>,
850        body: &Body<'tcx>,
851    ) -> IndexVec<Local, ConstPropMode> {
852        let mut cpv = CanConstProp {
853            can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
854            found_assignment: DenseBitSet::new_empty(body.local_decls.len()),
855        };
856        for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
857            let ty = body.local_decls[local].ty;
858            if ty.is_async_drop_in_place_coroutine(tcx) {
859                // No const propagation for async drop coroutine (AsyncDropGlue).
860                // Otherwise, tcx.layout_of(typing_env.as_query_input(ty)) will be called
861                // (early layout request for async drop coroutine) to calculate layout size.
862                // Layout for `async_drop_in_place<T>::{closure}` may only be known with known T.
863                *val = ConstPropMode::NoPropagation;
864                continue;
865            } else if ty.is_union() {
866                // Unions are incompatible with the current implementation of
867                // const prop because Rust has no concept of an active
868                // variant of a union
869                *val = ConstPropMode::NoPropagation;
870            } else {
871                match tcx.layout_of(typing_env.as_query_input(ty)) {
872                    Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
873                    // Either the layout fails to compute, then we can't use this local anyway
874                    // or the local is too large, then we don't want to.
875                    _ => {
876                        *val = ConstPropMode::NoPropagation;
877                        continue;
878                    }
879                }
880            }
881        }
882        // Consider that arguments are assigned on entry.
883        for arg in body.args_iter() {
884            cpv.found_assignment.insert(arg);
885        }
886        cpv.visit_body(body);
887        cpv.can_const_prop
888    }
889}
890
891impl<'tcx> Visitor<'tcx> for CanConstProp {
892    fn visit_place(&mut self, place: &Place<'tcx>, mut context: PlaceContext, loc: Location) {
893        use rustc_middle::mir::visit::PlaceContext::*;
894
895        // Dereferencing just read the address of `place.local`.
896        if place.projection.first() == Some(&PlaceElem::Deref) {
897            context = NonMutatingUse(NonMutatingUseContext::Copy);
898        }
899
900        self.visit_local(place.local, context, loc);
901        self.visit_projection(place.as_ref(), context, loc);
902    }
903
904    fn visit_local(&mut self, local: Local, context: PlaceContext, _: Location) {
905        use rustc_middle::mir::visit::PlaceContext::*;
906        match context {
907            // These are just stores, where the storing is not propagatable, but there may be later
908            // mutations of the same local via `Store`
909            | MutatingUse(MutatingUseContext::Call)
910            | MutatingUse(MutatingUseContext::AsmOutput)
911            // Actual store that can possibly even propagate a value
912            | MutatingUse(MutatingUseContext::Store)
913            | MutatingUse(MutatingUseContext::SetDiscriminant) => {
914                if !self.found_assignment.insert(local) {
915                    match &mut self.can_const_prop[local] {
916                        // If the local can only get propagated in its own block, then we don't have
917                        // to worry about multiple assignments, as we'll nuke the const state at the
918                        // end of the block anyway, and inside the block we overwrite previous
919                        // states as applicable.
920                        ConstPropMode::OnlyInsideOwnBlock => {}
921                        ConstPropMode::NoPropagation => {}
922                        other @ ConstPropMode::FullConstProp => {
923                            trace!(
924                                "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
925                                local, other,
926                            );
927                            *other = ConstPropMode::OnlyInsideOwnBlock;
928                        }
929                    }
930                }
931            }
932            // Reading constants is allowed an arbitrary number of times
933            NonMutatingUse(NonMutatingUseContext::Copy)
934            | NonMutatingUse(NonMutatingUseContext::Move)
935            | NonMutatingUse(NonMutatingUseContext::Inspect)
936            | NonMutatingUse(NonMutatingUseContext::PlaceMention)
937            | NonUse(_) => {}
938
939            // These could be propagated with a smarter analysis or just some careful thinking about
940            // whether they'd be fine right now.
941            MutatingUse(MutatingUseContext::Yield)
942            | MutatingUse(MutatingUseContext::Drop)
943            | MutatingUse(MutatingUseContext::Retag)
944            // These can't ever be propagated under any scheme, as we can't reason about indirect
945            // mutation.
946            | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
947            | NonMutatingUse(NonMutatingUseContext::FakeBorrow)
948            | NonMutatingUse(NonMutatingUseContext::RawBorrow)
949            | MutatingUse(MutatingUseContext::Borrow)
950            | MutatingUse(MutatingUseContext::RawBorrow) => {
951                trace!("local {:?} can't be propagated because it's used: {:?}", local, context);
952                self.can_const_prop[local] = ConstPropMode::NoPropagation;
953            }
954            MutatingUse(MutatingUseContext::Projection)
955            | NonMutatingUse(NonMutatingUseContext::Projection) => bug!("visit_place should not pass {context:?} for {local:?}"),
956        }
957    }
958}
rustc_mir_transform/known_panics_lint.rs

rustc_mir_transform/
known_panics_lint.rs
