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::ShallowInitBox(..)
447            | Rvalue::Discriminant(..)
448            | Rvalue::WrapUnsafeBinder(..) => {}
449        }
450
451        // FIXME we need to revisit this for #67176
452        if rvalue.has_param() {
453            return None;
454        }
455        if !rvalue.ty(self.local_decls(), self.tcx).is_sized(self.tcx, self.typing_env) {
456            // the interpreter doesn't support unsized locals (only unsized arguments),
457            // but rustc does (in a kinda broken way), so we have to skip them here
458            return None;
459        }
460
461        Some(())
462    }
463
464    fn check_assertion(
465        &mut self,
466        expected: bool,
467        msg: &AssertKind<Operand<'tcx>>,
468        cond: &Operand<'tcx>,
469        location: Location,
470    ) {
471        let Some(value) = &self.eval_operand(cond) else { return };
472        trace!("assertion on {:?} should be {:?}", value, expected);
473
474        let expected = Scalar::from_bool(expected);
475        let Some(value_const) = self.use_ecx(|this| this.ecx.read_scalar(value)) else { return };
476
477        if expected != value_const {
478            // Poison all places this operand references so that further code
479            // doesn't use the invalid value
480            if let Some(place) = cond.place() {
481                self.remove_const(place.local);
482            }
483
484            enum DbgVal<T> {
485                Val(T),
486                Underscore,
487            }
488            impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
489                fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
490                    match self {
491                        Self::Val(val) => val.fmt(fmt),
492                        Self::Underscore => fmt.write_str("_"),
493                    }
494                }
495            }
496            let mut eval_to_int = |op| {
497                // This can be `None` if the lhs wasn't const propagated and we just
498                // triggered the assert on the value of the rhs.
499                self.eval_operand(op)
500                    .and_then(|op| self.ecx.read_immediate(&op).discard_err())
501                    .map_or(DbgVal::Underscore, |op| DbgVal::Val(op.to_const_int()))
502            };
503            let msg = match msg {
504                AssertKind::DivisionByZero(op) => AssertKind::DivisionByZero(eval_to_int(op)),
505                AssertKind::RemainderByZero(op) => AssertKind::RemainderByZero(eval_to_int(op)),
506                AssertKind::Overflow(bin_op @ (BinOp::Div | BinOp::Rem), op1, op2) => {
507                    // Division overflow is *UB* in the MIR, and different than the
508                    // other overflow checks.
509                    AssertKind::Overflow(*bin_op, eval_to_int(op1), eval_to_int(op2))
510                }
511                AssertKind::BoundsCheck { len, index } => {
512                    let len = eval_to_int(len);
513                    let index = eval_to_int(index);
514                    AssertKind::BoundsCheck { len, index }
515                }
516                // Remaining overflow errors are already covered by checks on the binary operators.
517                AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => return,
518                // Need proper const propagator for these.
519                _ => return,
520            };
521            self.report_assert_as_lint(location, AssertLintKind::UnconditionalPanic, msg);
522        }
523    }
524
525    fn ensure_not_propagated(&self, local: Local) {
526        if cfg!(debug_assertions) {
527            let val = self.get_const(local.into());
528            assert!(
529                matches!(val, Some(Value::Uninit))
530                    || self
531                        .layout_of(self.local_decls()[local].ty)
532                        .map_or(true, |layout| layout.is_zst()),
533                "failed to remove values for `{local:?}`, value={val:?}",
534            )
535        }
536    }
537
538    #[instrument(level = "trace", skip(self), ret)]
539    fn eval_rvalue(&mut self, rvalue: &Rvalue<'tcx>, dest: &Place<'tcx>) -> Option<()> {
540        if !dest.projection.is_empty() {
541            return None;
542        }
543        use rustc_middle::mir::Rvalue::*;
544        let layout = self.ecx.layout_of(dest.ty(self.body, self.tcx).ty).ok()?;
545        trace!(?layout);
546
547        let val: Value<'_> = match *rvalue {
548            ThreadLocalRef(_) => return None,
549
550            Use(ref operand) | WrapUnsafeBinder(ref operand, _) => {
551                self.eval_operand(operand)?.into()
552            }
553
554            CopyForDeref(place) => self.eval_place(place)?.into(),
555
556            BinaryOp(bin_op, box (ref left, ref right)) => {
557                let left = self.eval_operand(left)?;
558                let left = self.use_ecx(|this| this.ecx.read_immediate(&left))?;
559
560                let right = self.eval_operand(right)?;
561                let right = self.use_ecx(|this| this.ecx.read_immediate(&right))?;
562
563                let val = self.use_ecx(|this| this.ecx.binary_op(bin_op, &left, &right))?;
564                if matches!(val.layout.backend_repr, BackendRepr::ScalarPair(..)) {
565                    // FIXME `Value` should properly support pairs in `Immediate`... but currently
566                    // it does not.
567                    let (val, overflow) = val.to_pair(&self.ecx);
568                    Value::Aggregate {
569                        variant: VariantIdx::ZERO,
570                        fields: [val.into(), overflow.into()].into_iter().collect(),
571                    }
572                } else {
573                    val.into()
574                }
575            }
576
577            UnaryOp(un_op, ref operand) => {
578                let operand = self.eval_operand(operand)?;
579                let val = self.use_ecx(|this| this.ecx.read_immediate(&operand))?;
580
581                let val = self.use_ecx(|this| this.ecx.unary_op(un_op, &val))?;
582                val.into()
583            }
584
585            Aggregate(ref kind, ref fields) => Value::Aggregate {
586                fields: fields
587                    .iter()
588                    .map(|field| self.eval_operand(field).map_or(Value::Uninit, Value::Immediate))
589                    .collect(),
590                variant: match **kind {
591                    AggregateKind::Adt(_, variant, _, _, _) => variant,
592                    AggregateKind::Array(_)
593                    | AggregateKind::Tuple
594                    | AggregateKind::RawPtr(_, _)
595                    | AggregateKind::Closure(_, _)
596                    | AggregateKind::Coroutine(_, _)
597                    | AggregateKind::CoroutineClosure(_, _) => VariantIdx::ZERO,
598                },
599            },
600
601            Repeat(ref op, n) => {
602                trace!(?op, ?n);
603                return None;
604            }
605
606            Ref(..) | RawPtr(..) => return None,
607
608            ShallowInitBox(..) => return None,
609
610            Cast(ref kind, ref value, to) => match kind {
611                CastKind::IntToInt | CastKind::IntToFloat => {
612                    let value = self.eval_operand(value)?;
613                    let value = self.ecx.read_immediate(&value).discard_err()?;
614                    let to = self.ecx.layout_of(to).ok()?;
615                    let res = self.ecx.int_to_int_or_float(&value, to).discard_err()?;
616                    res.into()
617                }
618                CastKind::FloatToFloat | CastKind::FloatToInt => {
619                    let value = self.eval_operand(value)?;
620                    let value = self.ecx.read_immediate(&value).discard_err()?;
621                    let to = self.ecx.layout_of(to).ok()?;
622                    let res = self.ecx.float_to_float_or_int(&value, to).discard_err()?;
623                    res.into()
624                }
625                CastKind::Transmute | CastKind::Subtype => {
626                    let value = self.eval_operand(value)?;
627                    let to = self.ecx.layout_of(to).ok()?;
628                    // `offset` for immediates only supports scalar/scalar-pair ABIs,
629                    // so bail out if the target is not one.
630                    match (value.layout.backend_repr, to.backend_repr) {
631                        (BackendRepr::Scalar(..), BackendRepr::Scalar(..)) => {}
632                        (BackendRepr::ScalarPair(..), BackendRepr::ScalarPair(..)) => {}
633                        _ => return None,
634                    }
635
636                    value.offset(Size::ZERO, to, &self.ecx).discard_err()?.into()
637                }
638                _ => return None,
639            },
640
641            Discriminant(place) => {
642                let variant = match self.get_const(place)? {
643                    Value::Immediate(op) => {
644                        let op = op.clone();
645                        self.use_ecx(|this| this.ecx.read_discriminant(&op))?
646                    }
647                    Value::Aggregate { variant, .. } => *variant,
648                    Value::Uninit => return None,
649                };
650                let imm = self.use_ecx(|this| {
651                    this.ecx.discriminant_for_variant(
652                        place.ty(this.local_decls(), this.tcx).ty,
653                        variant,
654                    )
655                })?;
656                imm.into()
657            }
658        };
659        trace!(?val);
660
661        *self.access_mut(dest)? = val;
662
663        Some(())
664    }
665}
666
667impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
668    fn visit_body(&mut self, body: &Body<'tcx>) {
669        while let Some(bb) = self.worklist.pop() {
670            if !self.visited_blocks.insert(bb) {
671                continue;
672            }
673
674            let data = &body.basic_blocks[bb];
675            self.visit_basic_block_data(bb, data);
676        }
677    }
678
679    fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
680        self.super_operand(operand, location);
681    }
682
683    fn visit_const_operand(&mut self, constant: &ConstOperand<'tcx>, location: Location) {
684        trace!("visit_const_operand: {:?}", constant);
685        self.super_const_operand(constant, location);
686        self.eval_constant(constant);
687    }
688
689    fn visit_assign(&mut self, place: &Place<'tcx>, rvalue: &Rvalue<'tcx>, location: Location) {
690        self.super_assign(place, rvalue, location);
691
692        let Some(()) = self.check_rvalue(rvalue, location) else { return };
693
694        match self.can_const_prop[place.local] {
695            // Do nothing if the place is indirect.
696            _ if place.is_indirect() => {}
697            ConstPropMode::NoPropagation => self.ensure_not_propagated(place.local),
698            ConstPropMode::OnlyInsideOwnBlock | ConstPropMode::FullConstProp => {
699                if self.eval_rvalue(rvalue, place).is_none() {
700                    // Const prop failed, so erase the destination, ensuring that whatever happens
701                    // from here on, does not know about the previous value.
702                    // This is important in case we have
703                    // ```rust
704                    // let mut x = 42;
705                    // x = SOME_MUTABLE_STATIC;
706                    // // x must now be uninit
707                    // ```
708                    // FIXME: we overzealously erase the entire local, because that's easier to
709                    // implement.
710                    trace!(
711                        "propagation into {:?} failed.
712                        Nuking the entire site from orbit, it's the only way to be sure",
713                        place,
714                    );
715                    self.remove_const(place.local);
716                }
717            }
718        }
719    }
720
721    fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
722        trace!("visit_statement: {:?}", statement);
723
724        // We want to evaluate operands before any change to the assigned-to value,
725        // so we recurse first.
726        self.super_statement(statement, location);
727
728        match statement.kind {
729            StatementKind::SetDiscriminant { ref place, variant_index } => {
730                match self.can_const_prop[place.local] {
731                    // Do nothing if the place is indirect.
732                    _ if place.is_indirect() => {}
733                    ConstPropMode::NoPropagation => self.ensure_not_propagated(place.local),
734                    ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
735                        match self.access_mut(place) {
736                            Some(Value::Aggregate { variant, .. }) => *variant = variant_index,
737                            _ => self.remove_const(place.local),
738                        }
739                    }
740                }
741            }
742            StatementKind::StorageLive(local) => {
743                self.remove_const(local);
744            }
745            StatementKind::StorageDead(local) => {
746                self.remove_const(local);
747            }
748            _ => {}
749        }
750    }
751
752    fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
753        self.super_terminator(terminator, location);
754        match &terminator.kind {
755            TerminatorKind::Assert { expected, msg, cond, .. } => {
756                self.check_assertion(*expected, msg, cond, location);
757            }
758            TerminatorKind::SwitchInt { discr, targets } => {
759                if let Some(ref value) = self.eval_operand(discr)
760                    && let Some(value_const) = self.use_ecx(|this| this.ecx.read_scalar(value))
761                    && let Some(constant) = value_const.to_bits(value_const.size()).discard_err()
762                {
763                    // We managed to evaluate the discriminant, so we know we only need to visit
764                    // one target.
765                    let target = targets.target_for_value(constant);
766                    self.worklist.push(target);
767                    return;
768                }
769                // We failed to evaluate the discriminant, fallback to visiting all successors.
770            }
771            // None of these have Operands to const-propagate.
772            TerminatorKind::Goto { .. }
773            | TerminatorKind::UnwindResume
774            | TerminatorKind::UnwindTerminate(_)
775            | TerminatorKind::Return
776            | TerminatorKind::TailCall { .. }
777            | TerminatorKind::Unreachable
778            | TerminatorKind::Drop { .. }
779            | TerminatorKind::Yield { .. }
780            | TerminatorKind::CoroutineDrop
781            | TerminatorKind::FalseEdge { .. }
782            | TerminatorKind::FalseUnwind { .. }
783            | TerminatorKind::Call { .. }
784            | TerminatorKind::InlineAsm { .. } => {}
785        }
786
787        self.worklist.extend(terminator.successors());
788    }
789
790    fn visit_basic_block_data(&mut self, block: BasicBlock, data: &BasicBlockData<'tcx>) {
791        self.super_basic_block_data(block, data);
792
793        // We remove all Locals which are restricted in propagation to their containing blocks and
794        // which were modified in the current block.
795        // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
796        let mut written_only_inside_own_block_locals =
797            std::mem::take(&mut self.written_only_inside_own_block_locals);
798
799        // This loop can get very hot for some bodies: it check each local in each bb.
800        // To avoid this quadratic behaviour, we only clear the locals that were modified inside
801        // the current block.
802        // The order in which we remove consts does not matter.
803        #[allow(rustc::potential_query_instability)]
804        for local in written_only_inside_own_block_locals.drain() {
805            debug_assert_eq!(self.can_const_prop[local], ConstPropMode::OnlyInsideOwnBlock);
806            self.remove_const(local);
807        }
808        self.written_only_inside_own_block_locals = written_only_inside_own_block_locals;
809
810        if cfg!(debug_assertions) {
811            for (local, &mode) in self.can_const_prop.iter_enumerated() {
812                match mode {
813                    ConstPropMode::FullConstProp => {}
814                    ConstPropMode::NoPropagation | ConstPropMode::OnlyInsideOwnBlock => {
815                        self.ensure_not_propagated(local);
816                    }
817                }
818            }
819        }
820    }
821}
822
823/// The maximum number of bytes that we'll allocate space for a local or the return value.
824/// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
825/// Severely regress performance.
826const MAX_ALLOC_LIMIT: u64 = 1024;
827
828/// The mode that `ConstProp` is allowed to run in for a given `Local`.
829#[derive(Clone, Copy, Debug, PartialEq)]
830enum ConstPropMode {
831    /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
832    FullConstProp,
833    /// The `Local` can only be propagated into and from its own block.
834    OnlyInsideOwnBlock,
835    /// The `Local` cannot be part of propagation at all. Any statement
836    /// referencing it either for reading or writing will not get propagated.
837    NoPropagation,
838}
839
840/// A visitor that determines locals in a MIR body
841/// that can be const propagated
842struct CanConstProp {
843    can_const_prop: IndexVec<Local, ConstPropMode>,
844    // False at the beginning. Once set, no more assignments are allowed to that local.
845    found_assignment: DenseBitSet<Local>,
846}
847
848impl CanConstProp {
849    /// Returns true if `local` can be propagated
850    fn check<'tcx>(
851        tcx: TyCtxt<'tcx>,
852        typing_env: ty::TypingEnv<'tcx>,
853        body: &Body<'tcx>,
854    ) -> IndexVec<Local, ConstPropMode> {
855        let mut cpv = CanConstProp {
856            can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
857            found_assignment: DenseBitSet::new_empty(body.local_decls.len()),
858        };
859        for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
860            let ty = body.local_decls[local].ty;
861            if ty.is_async_drop_in_place_coroutine(tcx) {
862                // No const propagation for async drop coroutine (AsyncDropGlue).
863                // Otherwise, tcx.layout_of(typing_env.as_query_input(ty)) will be called
864                // (early layout request for async drop coroutine) to calculate layout size.
865                // Layout for `async_drop_in_place<T>::{closure}` may only be known with known T.
866                *val = ConstPropMode::NoPropagation;
867                continue;
868            } else if ty.is_union() {
869                // Unions are incompatible with the current implementation of
870                // const prop because Rust has no concept of an active
871                // variant of a union
872                *val = ConstPropMode::NoPropagation;
873            } else {
874                match tcx.layout_of(typing_env.as_query_input(ty)) {
875                    Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
876                    // Either the layout fails to compute, then we can't use this local anyway
877                    // or the local is too large, then we don't want to.
878                    _ => {
879                        *val = ConstPropMode::NoPropagation;
880                        continue;
881                    }
882                }
883            }
884        }
885        // Consider that arguments are assigned on entry.
886        for arg in body.args_iter() {
887            cpv.found_assignment.insert(arg);
888        }
889        cpv.visit_body(body);
890        cpv.can_const_prop
891    }
892}
893
894impl<'tcx> Visitor<'tcx> for CanConstProp {
895    fn visit_place(&mut self, place: &Place<'tcx>, mut context: PlaceContext, loc: Location) {
896        use rustc_middle::mir::visit::PlaceContext::*;
897
898        // Dereferencing just read the address of `place.local`.
899        if place.projection.first() == Some(&PlaceElem::Deref) {
900            context = NonMutatingUse(NonMutatingUseContext::Copy);
901        }
902
903        self.visit_local(place.local, context, loc);
904        self.visit_projection(place.as_ref(), context, loc);
905    }
906
907    fn visit_local(&mut self, local: Local, context: PlaceContext, _: Location) {
908        use rustc_middle::mir::visit::PlaceContext::*;
909        match context {
910            // These are just stores, where the storing is not propagatable, but there may be later
911            // mutations of the same local via `Store`
912            | MutatingUse(MutatingUseContext::Call)
913            | MutatingUse(MutatingUseContext::AsmOutput)
914            // Actual store that can possibly even propagate a value
915            | MutatingUse(MutatingUseContext::Store)
916            | MutatingUse(MutatingUseContext::SetDiscriminant) => {
917                if !self.found_assignment.insert(local) {
918                    match &mut self.can_const_prop[local] {
919                        // If the local can only get propagated in its own block, then we don't have
920                        // to worry about multiple assignments, as we'll nuke the const state at the
921                        // end of the block anyway, and inside the block we overwrite previous
922                        // states as applicable.
923                        ConstPropMode::OnlyInsideOwnBlock => {}
924                        ConstPropMode::NoPropagation => {}
925                        other @ ConstPropMode::FullConstProp => {
926                            trace!(
927                                "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
928                                local, other,
929                            );
930                            *other = ConstPropMode::OnlyInsideOwnBlock;
931                        }
932                    }
933                }
934            }
935            // Reading constants is allowed an arbitrary number of times
936            NonMutatingUse(NonMutatingUseContext::Copy)
937            | NonMutatingUse(NonMutatingUseContext::Move)
938            | NonMutatingUse(NonMutatingUseContext::Inspect)
939            | NonMutatingUse(NonMutatingUseContext::PlaceMention)
940            | NonUse(_) => {}
941
942            // These could be propagated with a smarter analysis or just some careful thinking about
943            // whether they'd be fine right now.
944            MutatingUse(MutatingUseContext::Yield)
945            | MutatingUse(MutatingUseContext::Drop)
946            | MutatingUse(MutatingUseContext::Retag)
947            // These can't ever be propagated under any scheme, as we can't reason about indirect
948            // mutation.
949            | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
950            | NonMutatingUse(NonMutatingUseContext::FakeBorrow)
951            | NonMutatingUse(NonMutatingUseContext::RawBorrow)
952            | MutatingUse(MutatingUseContext::Borrow)
953            | MutatingUse(MutatingUseContext::RawBorrow) => {
954                trace!("local {:?} can't be propagated because it's used: {:?}", local, context);
955                self.can_const_prop[local] = ConstPropMode::NoPropagation;
956            }
957            MutatingUse(MutatingUseContext::Projection)
958            | NonMutatingUse(NonMutatingUseContext::Projection) => bug!("visit_place should not pass {context:?} for {local:?}"),
959        }
960    }
961}
rustc_mir_transform/known_panics_lint.rs

rustc_mir_transform/
known_panics_lint.rs
