rustc_passes/
dead.rs

1// This implements the dead-code warning pass.
2// All reachable symbols are live, code called from live code is live, code with certain lint
3// expectations such as `#[expect(unused)]` and `#[expect(dead_code)]` is live, and everything else
4// is dead.
5
6use std::mem;
7
8use hir::ItemKind;
9use hir::def_id::{LocalDefIdMap, LocalDefIdSet};
10use rustc_abi::FieldIdx;
11use rustc_data_structures::fx::FxIndexSet;
12use rustc_data_structures::unord::UnordSet;
13use rustc_errors::MultiSpan;
14use rustc_hir::def::{CtorOf, DefKind, Res};
15use rustc_hir::def_id::{DefId, LocalDefId, LocalModDefId};
16use rustc_hir::intravisit::{self, Visitor};
17use rustc_hir::{self as hir, Node, PatKind, QPath, TyKind};
18use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
19use rustc_middle::middle::privacy::Level;
20use rustc_middle::query::Providers;
21use rustc_middle::ty::{self, TyCtxt};
22use rustc_middle::{bug, span_bug};
23use rustc_session::lint::builtin::DEAD_CODE;
24use rustc_session::lint::{self, LintExpectationId};
25use rustc_span::{Symbol, sym};
26
27use crate::errors::{
28    ChangeFields, IgnoredDerivedImpls, MultipleDeadCodes, ParentInfo, UselessAssignment,
29};
30
31// Any local node that may call something in its body block should be
32// explored. For example, if it's a live Node::Item that is a
33// function, then we should explore its block to check for codes that
34// may need to be marked as live.
35fn should_explore(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
36    matches!(
37        tcx.hir_node_by_def_id(def_id),
38        Node::Item(..)
39            | Node::ImplItem(..)
40            | Node::ForeignItem(..)
41            | Node::TraitItem(..)
42            | Node::Variant(..)
43            | Node::AnonConst(..)
44            | Node::OpaqueTy(..)
45    )
46}
47
48/// Returns the local def id of the ADT if the given ty refers to a local one.
49fn local_adt_def_of_ty<'tcx>(ty: &hir::Ty<'tcx>) -> Option<LocalDefId> {
50    match ty.kind {
51        TyKind::Path(QPath::Resolved(_, path)) => {
52            if let Res::Def(def_kind, def_id) = path.res
53                && let Some(local_def_id) = def_id.as_local()
54                && matches!(def_kind, DefKind::Struct | DefKind::Enum | DefKind::Union)
55            {
56                Some(local_def_id)
57            } else {
58                None
59            }
60        }
61        _ => None,
62    }
63}
64
65/// Determine if a work from the worklist is coming from a `#[allow]`
66/// or a `#[expect]` of `dead_code`
67#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
68enum ComesFromAllowExpect {
69    Yes,
70    No,
71}
72
73struct MarkSymbolVisitor<'tcx> {
74    worklist: Vec<(LocalDefId, ComesFromAllowExpect)>,
75    tcx: TyCtxt<'tcx>,
76    maybe_typeck_results: Option<&'tcx ty::TypeckResults<'tcx>>,
77    live_symbols: LocalDefIdSet,
78    repr_unconditionally_treats_fields_as_live: bool,
79    repr_has_repr_simd: bool,
80    in_pat: bool,
81    ignore_variant_stack: Vec<DefId>,
82    // maps from tuple struct constructors to tuple struct items
83    struct_constructors: LocalDefIdMap<LocalDefId>,
84    // maps from ADTs to ignored derived traits (e.g. Debug and Clone)
85    // and the span of their respective impl (i.e., part of the derive
86    // macro)
87    ignored_derived_traits: LocalDefIdMap<FxIndexSet<(DefId, DefId)>>,
88}
89
90impl<'tcx> MarkSymbolVisitor<'tcx> {
91    /// Gets the type-checking results for the current body.
92    /// As this will ICE if called outside bodies, only call when working with
93    /// `Expr` or `Pat` nodes (they are guaranteed to be found only in bodies).
94    #[track_caller]
95    fn typeck_results(&self) -> &'tcx ty::TypeckResults<'tcx> {
96        self.maybe_typeck_results
97            .expect("`MarkSymbolVisitor::typeck_results` called outside of body")
98    }
99
100    fn check_def_id(&mut self, def_id: DefId) {
101        if let Some(def_id) = def_id.as_local() {
102            if should_explore(self.tcx, def_id) || self.struct_constructors.contains_key(&def_id) {
103                self.worklist.push((def_id, ComesFromAllowExpect::No));
104            }
105            self.live_symbols.insert(def_id);
106        }
107    }
108
109    fn insert_def_id(&mut self, def_id: DefId) {
110        if let Some(def_id) = def_id.as_local() {
111            debug_assert!(!should_explore(self.tcx, def_id));
112            self.live_symbols.insert(def_id);
113        }
114    }
115
116    fn handle_res(&mut self, res: Res) {
117        match res {
118            Res::Def(DefKind::Const | DefKind::AssocConst | DefKind::TyAlias, def_id) => {
119                self.check_def_id(def_id);
120            }
121            _ if self.in_pat => {}
122            Res::PrimTy(..) | Res::SelfCtor(..) | Res::Local(..) => {}
123            Res::Def(DefKind::Ctor(CtorOf::Variant, ..), ctor_def_id) => {
124                let variant_id = self.tcx.parent(ctor_def_id);
125                let enum_id = self.tcx.parent(variant_id);
126                self.check_def_id(enum_id);
127                if !self.ignore_variant_stack.contains(&ctor_def_id) {
128                    self.check_def_id(variant_id);
129                }
130            }
131            Res::Def(DefKind::Variant, variant_id) => {
132                let enum_id = self.tcx.parent(variant_id);
133                self.check_def_id(enum_id);
134                if !self.ignore_variant_stack.contains(&variant_id) {
135                    self.check_def_id(variant_id);
136                }
137            }
138            Res::Def(_, def_id) => self.check_def_id(def_id),
139            Res::SelfTyParam { trait_: t } => self.check_def_id(t),
140            Res::SelfTyAlias { alias_to: i, .. } => self.check_def_id(i),
141            Res::ToolMod | Res::NonMacroAttr(..) | Res::Err => {}
142        }
143    }
144
145    fn lookup_and_handle_method(&mut self, id: hir::HirId) {
146        if let Some(def_id) = self.typeck_results().type_dependent_def_id(id) {
147            self.check_def_id(def_id);
148        } else {
149            assert!(
150                self.typeck_results().tainted_by_errors.is_some(),
151                "no type-dependent def for method"
152            );
153        }
154    }
155
156    fn handle_field_access(&mut self, lhs: &hir::Expr<'_>, hir_id: hir::HirId) {
157        match self.typeck_results().expr_ty_adjusted(lhs).kind() {
158            ty::Adt(def, _) => {
159                let index = self.typeck_results().field_index(hir_id);
160                self.insert_def_id(def.non_enum_variant().fields[index].did);
161            }
162            ty::Tuple(..) => {}
163            ty::Error(_) => {}
164            kind => span_bug!(lhs.span, "named field access on non-ADT: {kind:?}"),
165        }
166    }
167
168    #[allow(dead_code)] // FIXME(81658): should be used + lint reinstated after #83171 relands.
169    fn handle_assign(&mut self, expr: &'tcx hir::Expr<'tcx>) {
170        if self
171            .typeck_results()
172            .expr_adjustments(expr)
173            .iter()
174            .any(|adj| matches!(adj.kind, ty::adjustment::Adjust::Deref(_)))
175        {
176            self.visit_expr(expr);
177        } else if let hir::ExprKind::Field(base, ..) = expr.kind {
178            // Ignore write to field
179            self.handle_assign(base);
180        } else {
181            self.visit_expr(expr);
182        }
183    }
184
185    #[allow(dead_code)] // FIXME(81658): should be used + lint reinstated after #83171 relands.
186    fn check_for_self_assign(&mut self, assign: &'tcx hir::Expr<'tcx>) {
187        fn check_for_self_assign_helper<'tcx>(
188            typeck_results: &'tcx ty::TypeckResults<'tcx>,
189            lhs: &'tcx hir::Expr<'tcx>,
190            rhs: &'tcx hir::Expr<'tcx>,
191        ) -> bool {
192            match (&lhs.kind, &rhs.kind) {
193                (hir::ExprKind::Path(qpath_l), hir::ExprKind::Path(qpath_r)) => {
194                    if let (Res::Local(id_l), Res::Local(id_r)) = (
195                        typeck_results.qpath_res(qpath_l, lhs.hir_id),
196                        typeck_results.qpath_res(qpath_r, rhs.hir_id),
197                    ) {
198                        if id_l == id_r {
199                            return true;
200                        }
201                    }
202                    return false;
203                }
204                (hir::ExprKind::Field(lhs_l, ident_l), hir::ExprKind::Field(lhs_r, ident_r)) => {
205                    if ident_l == ident_r {
206                        return check_for_self_assign_helper(typeck_results, lhs_l, lhs_r);
207                    }
208                    return false;
209                }
210                _ => {
211                    return false;
212                }
213            }
214        }
215
216        if let hir::ExprKind::Assign(lhs, rhs, _) = assign.kind
217            && check_for_self_assign_helper(self.typeck_results(), lhs, rhs)
218            && !assign.span.from_expansion()
219        {
220            let is_field_assign = matches!(lhs.kind, hir::ExprKind::Field(..));
221            self.tcx.emit_node_span_lint(
222                lint::builtin::DEAD_CODE,
223                assign.hir_id,
224                assign.span,
225                UselessAssignment { is_field_assign, ty: self.typeck_results().expr_ty(lhs) },
226            )
227        }
228    }
229
230    fn handle_field_pattern_match(
231        &mut self,
232        lhs: &hir::Pat<'_>,
233        res: Res,
234        pats: &[hir::PatField<'_>],
235    ) {
236        let variant = match self.typeck_results().node_type(lhs.hir_id).kind() {
237            ty::Adt(adt, _) => adt.variant_of_res(res),
238            _ => span_bug!(lhs.span, "non-ADT in struct pattern"),
239        };
240        for pat in pats {
241            if let PatKind::Wild = pat.pat.kind {
242                continue;
243            }
244            let index = self.typeck_results().field_index(pat.hir_id);
245            self.insert_def_id(variant.fields[index].did);
246        }
247    }
248
249    fn handle_tuple_field_pattern_match(
250        &mut self,
251        lhs: &hir::Pat<'_>,
252        res: Res,
253        pats: &[hir::Pat<'_>],
254        dotdot: hir::DotDotPos,
255    ) {
256        let variant = match self.typeck_results().node_type(lhs.hir_id).kind() {
257            ty::Adt(adt, _) => adt.variant_of_res(res),
258            _ => {
259                self.tcx.dcx().span_delayed_bug(lhs.span, "non-ADT in tuple struct pattern");
260                return;
261            }
262        };
263        let dotdot = dotdot.as_opt_usize().unwrap_or(pats.len());
264        let first_n = pats.iter().enumerate().take(dotdot);
265        let missing = variant.fields.len() - pats.len();
266        let last_n = pats.iter().enumerate().skip(dotdot).map(|(idx, pat)| (idx + missing, pat));
267        for (idx, pat) in first_n.chain(last_n) {
268            if let PatKind::Wild = pat.kind {
269                continue;
270            }
271            self.insert_def_id(variant.fields[FieldIdx::from_usize(idx)].did);
272        }
273    }
274
275    fn handle_offset_of(&mut self, expr: &'tcx hir::Expr<'tcx>) {
276        let data = self.typeck_results().offset_of_data();
277        let &(container, ref indices) =
278            data.get(expr.hir_id).expect("no offset_of_data for offset_of");
279
280        let body_did = self.typeck_results().hir_owner.to_def_id();
281        let typing_env = ty::TypingEnv::non_body_analysis(self.tcx, body_did);
282
283        let mut current_ty = container;
284
285        for &(variant, field) in indices {
286            match current_ty.kind() {
287                ty::Adt(def, args) => {
288                    let field = &def.variant(variant).fields[field];
289
290                    self.insert_def_id(field.did);
291                    let field_ty = field.ty(self.tcx, args);
292
293                    current_ty = self.tcx.normalize_erasing_regions(typing_env, field_ty);
294                }
295                // we don't need to mark tuple fields as live,
296                // but we may need to mark subfields
297                ty::Tuple(tys) => {
298                    current_ty =
299                        self.tcx.normalize_erasing_regions(typing_env, tys[field.as_usize()]);
300                }
301                _ => span_bug!(expr.span, "named field access on non-ADT"),
302            }
303        }
304    }
305
306    fn mark_live_symbols(&mut self) {
307        let mut scanned = UnordSet::default();
308        while let Some(work) = self.worklist.pop() {
309            if !scanned.insert(work) {
310                continue;
311            }
312
313            let (id, comes_from_allow_expect) = work;
314
315            // Avoid accessing the HIR for the synthesized associated type generated for RPITITs.
316            if self.tcx.is_impl_trait_in_trait(id.to_def_id()) {
317                self.live_symbols.insert(id);
318                continue;
319            }
320
321            // in the case of tuple struct constructors we want to check the item, not the generated
322            // tuple struct constructor function
323            let id = self.struct_constructors.get(&id).copied().unwrap_or(id);
324
325            // When using `#[allow]` or `#[expect]` of `dead_code`, we do a QOL improvement
326            // by declaring fn calls, statics, ... within said items as live, as well as
327            // the item itself, although technically this is not the case.
328            //
329            // This means that the lint for said items will never be fired.
330            //
331            // This doesn't make any difference for the item declared with `#[allow]`, as
332            // the lint firing will be a nop, as it will be silenced by the `#[allow]` of
333            // the item.
334            //
335            // However, for `#[expect]`, the presence or absence of the lint is relevant,
336            // so we don't add it to the list of live symbols when it comes from a
337            // `#[expect]`. This means that we will correctly report an item as live or not
338            // for the `#[expect]` case.
339            //
340            // Note that an item can and will be duplicated on the worklist with different
341            // `ComesFromAllowExpect`, particularly if it was added from the
342            // `effective_visibilities` query or from the `#[allow]`/`#[expect]` checks,
343            // this "duplication" is essential as otherwise a function with `#[expect]`
344            // called from a `pub fn` may be falsely reported as not live, falsely
345            // triggering the `unfulfilled_lint_expectations` lint.
346            if comes_from_allow_expect != ComesFromAllowExpect::Yes {
347                self.live_symbols.insert(id);
348            }
349            self.visit_node(self.tcx.hir_node_by_def_id(id));
350        }
351    }
352
353    /// Automatically generated items marked with `rustc_trivial_field_reads`
354    /// will be ignored for the purposes of dead code analysis (see PR #85200
355    /// for discussion).
356    fn should_ignore_item(&mut self, def_id: DefId) -> bool {
357        if let Some(impl_of) = self.tcx.impl_of_method(def_id) {
358            if !self.tcx.is_automatically_derived(impl_of) {
359                return false;
360            }
361
362            // don't ignore impls for Enums and pub Structs whose methods don't have self receiver,
363            // cause external crate may call such methods to construct values of these types
364            if let Some(local_impl_of) = impl_of.as_local()
365                && let Some(local_def_id) = def_id.as_local()
366                && let Some(fn_sig) =
367                    self.tcx.hir_fn_sig_by_hir_id(self.tcx.local_def_id_to_hir_id(local_def_id))
368                && matches!(fn_sig.decl.implicit_self, hir::ImplicitSelfKind::None)
369                && let TyKind::Path(QPath::Resolved(_, path)) =
370                    self.tcx.hir_expect_item(local_impl_of).expect_impl().self_ty.kind
371                && let Res::Def(def_kind, did) = path.res
372            {
373                match def_kind {
374                    // for example, #[derive(Default)] pub struct T(i32);
375                    // external crate can call T::default() to construct T,
376                    // so that don't ignore impl Default for pub Enum and Structs
377                    DefKind::Struct | DefKind::Union if self.tcx.visibility(did).is_public() => {
378                        return false;
379                    }
380                    // don't ignore impl Default for Enums,
381                    // cause we don't know which variant is constructed
382                    DefKind::Enum => return false,
383                    _ => (),
384                };
385            }
386
387            if let Some(trait_of) = self.tcx.trait_id_of_impl(impl_of)
388                && self.tcx.has_attr(trait_of, sym::rustc_trivial_field_reads)
389            {
390                let trait_ref = self.tcx.impl_trait_ref(impl_of).unwrap().instantiate_identity();
391                if let ty::Adt(adt_def, _) = trait_ref.self_ty().kind()
392                    && let Some(adt_def_id) = adt_def.did().as_local()
393                {
394                    self.ignored_derived_traits
395                        .entry(adt_def_id)
396                        .or_default()
397                        .insert((trait_of, impl_of));
398                }
399                return true;
400            }
401        }
402
403        false
404    }
405
406    fn visit_node(&mut self, node: Node<'tcx>) {
407        if let Node::ImplItem(hir::ImplItem { owner_id, .. }) = node
408            && self.should_ignore_item(owner_id.to_def_id())
409        {
410            return;
411        }
412
413        let unconditionally_treated_fields_as_live =
414            self.repr_unconditionally_treats_fields_as_live;
415        let had_repr_simd = self.repr_has_repr_simd;
416        self.repr_unconditionally_treats_fields_as_live = false;
417        self.repr_has_repr_simd = false;
418        match node {
419            Node::Item(item) => match item.kind {
420                hir::ItemKind::Struct(..) | hir::ItemKind::Union(..) => {
421                    let def = self.tcx.adt_def(item.owner_id);
422                    self.repr_unconditionally_treats_fields_as_live =
423                        def.repr().c() || def.repr().transparent();
424                    self.repr_has_repr_simd = def.repr().simd();
425
426                    intravisit::walk_item(self, item)
427                }
428                hir::ItemKind::ForeignMod { .. } => {}
429                hir::ItemKind::Trait(.., trait_item_refs) => {
430                    // mark assoc ty live if the trait is live
431                    for trait_item in trait_item_refs {
432                        if matches!(trait_item.kind, hir::AssocItemKind::Type) {
433                            self.check_def_id(trait_item.id.owner_id.to_def_id());
434                        }
435                    }
436                    intravisit::walk_item(self, item)
437                }
438                _ => intravisit::walk_item(self, item),
439            },
440            Node::TraitItem(trait_item) => {
441                // mark the trait live
442                let trait_item_id = trait_item.owner_id.to_def_id();
443                if let Some(trait_id) = self.tcx.trait_of_item(trait_item_id) {
444                    self.check_def_id(trait_id);
445                }
446                intravisit::walk_trait_item(self, trait_item);
447            }
448            Node::ImplItem(impl_item) => {
449                let item = self.tcx.local_parent(impl_item.owner_id.def_id);
450                if self.tcx.impl_trait_ref(item).is_none() {
451                    //// If it's a type whose items are live, then it's live, too.
452                    //// This is done to handle the case where, for example, the static
453                    //// method of a private type is used, but the type itself is never
454                    //// called directly.
455                    let self_ty = self.tcx.type_of(item).instantiate_identity();
456                    match *self_ty.kind() {
457                        ty::Adt(def, _) => self.check_def_id(def.did()),
458                        ty::Foreign(did) => self.check_def_id(did),
459                        ty::Dynamic(data, ..) => {
460                            if let Some(def_id) = data.principal_def_id() {
461                                self.check_def_id(def_id)
462                            }
463                        }
464                        _ => {}
465                    }
466                }
467                intravisit::walk_impl_item(self, impl_item);
468            }
469            Node::ForeignItem(foreign_item) => {
470                intravisit::walk_foreign_item(self, foreign_item);
471            }
472            Node::OpaqueTy(opaq) => intravisit::walk_opaque_ty(self, opaq),
473            _ => {}
474        }
475        self.repr_has_repr_simd = had_repr_simd;
476        self.repr_unconditionally_treats_fields_as_live = unconditionally_treated_fields_as_live;
477    }
478
479    fn mark_as_used_if_union(&mut self, adt: ty::AdtDef<'tcx>, fields: &[hir::ExprField<'_>]) {
480        if adt.is_union() && adt.non_enum_variant().fields.len() > 1 && adt.did().is_local() {
481            for field in fields {
482                let index = self.typeck_results().field_index(field.hir_id);
483                self.insert_def_id(adt.non_enum_variant().fields[index].did);
484            }
485        }
486    }
487
488    /// Returns whether `local_def_id` is potentially alive or not.
489    /// `local_def_id` points to an impl or an impl item,
490    /// both impl and impl item that may be passed to this function are of a trait,
491    /// and added into the unsolved_items during `create_and_seed_worklist`
492    fn check_impl_or_impl_item_live(
493        &mut self,
494        impl_id: hir::ItemId,
495        local_def_id: LocalDefId,
496    ) -> bool {
497        if self.should_ignore_item(local_def_id.to_def_id()) {
498            return false;
499        }
500
501        let trait_def_id = match self.tcx.def_kind(local_def_id) {
502            // assoc impl items of traits are live if the corresponding trait items are live
503            DefKind::AssocFn => self.tcx.associated_item(local_def_id).trait_item_def_id,
504            // impl items are live if the corresponding traits are live
505            DefKind::Impl { of_trait: true } => self
506                .tcx
507                .impl_trait_ref(impl_id.owner_id.def_id)
508                .and_then(|trait_ref| Some(trait_ref.skip_binder().def_id)),
509            _ => None,
510        };
511
512        if let Some(trait_def_id) = trait_def_id {
513            if let Some(trait_def_id) = trait_def_id.as_local()
514                && !self.live_symbols.contains(&trait_def_id)
515            {
516                return false;
517            }
518
519            // FIXME: legacy logic to check whether the function may construct `Self`,
520            // this can be removed after supporting marking ADTs appearing in patterns
521            // as live, then we can check private impls of public traits directly
522            if let Some(fn_sig) =
523                self.tcx.hir_fn_sig_by_hir_id(self.tcx.local_def_id_to_hir_id(local_def_id))
524                && matches!(fn_sig.decl.implicit_self, hir::ImplicitSelfKind::None)
525                && self.tcx.visibility(trait_def_id).is_public()
526            {
527                return true;
528            }
529        }
530
531        // The impl or impl item is used if the corresponding trait or trait item is used and the ty is used.
532        if let Some(local_def_id) =
533            local_adt_def_of_ty(self.tcx.hir_item(impl_id).expect_impl().self_ty)
534            && !self.live_symbols.contains(&local_def_id)
535        {
536            return false;
537        }
538
539        true
540    }
541}
542
543impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> {
544    fn visit_nested_body(&mut self, body: hir::BodyId) {
545        let old_maybe_typeck_results =
546            self.maybe_typeck_results.replace(self.tcx.typeck_body(body));
547        let body = self.tcx.hir_body(body);
548        self.visit_body(body);
549        self.maybe_typeck_results = old_maybe_typeck_results;
550    }
551
552    fn visit_variant_data(&mut self, def: &'tcx hir::VariantData<'tcx>) {
553        let tcx = self.tcx;
554        let unconditionally_treat_fields_as_live = self.repr_unconditionally_treats_fields_as_live;
555        let has_repr_simd = self.repr_has_repr_simd;
556        let effective_visibilities = &tcx.effective_visibilities(());
557        let live_fields = def.fields().iter().filter_map(|f| {
558            let def_id = f.def_id;
559            if unconditionally_treat_fields_as_live || (f.is_positional() && has_repr_simd) {
560                return Some(def_id);
561            }
562            if !effective_visibilities.is_reachable(f.hir_id.owner.def_id) {
563                return None;
564            }
565            if effective_visibilities.is_reachable(def_id) { Some(def_id) } else { None }
566        });
567        self.live_symbols.extend(live_fields);
568
569        intravisit::walk_struct_def(self, def);
570    }
571
572    fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
573        match expr.kind {
574            hir::ExprKind::Path(ref qpath @ QPath::TypeRelative(..)) => {
575                let res = self.typeck_results().qpath_res(qpath, expr.hir_id);
576                self.handle_res(res);
577            }
578            hir::ExprKind::MethodCall(..) => {
579                self.lookup_and_handle_method(expr.hir_id);
580            }
581            hir::ExprKind::Field(ref lhs, ..) => {
582                if self.typeck_results().opt_field_index(expr.hir_id).is_some() {
583                    self.handle_field_access(lhs, expr.hir_id);
584                } else {
585                    self.tcx.dcx().span_delayed_bug(expr.span, "couldn't resolve index for field");
586                }
587            }
588            hir::ExprKind::Struct(qpath, fields, _) => {
589                let res = self.typeck_results().qpath_res(qpath, expr.hir_id);
590                self.handle_res(res);
591                if let ty::Adt(adt, _) = self.typeck_results().expr_ty(expr).kind() {
592                    self.mark_as_used_if_union(*adt, fields);
593                }
594            }
595            hir::ExprKind::Closure(cls) => {
596                self.insert_def_id(cls.def_id.to_def_id());
597            }
598            hir::ExprKind::OffsetOf(..) => {
599                self.handle_offset_of(expr);
600            }
601            _ => (),
602        }
603
604        intravisit::walk_expr(self, expr);
605    }
606
607    fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
608        // Inside the body, ignore constructions of variants
609        // necessary for the pattern to match. Those construction sites
610        // can't be reached unless the variant is constructed elsewhere.
611        let len = self.ignore_variant_stack.len();
612        self.ignore_variant_stack.extend(arm.pat.necessary_variants());
613        intravisit::walk_arm(self, arm);
614        self.ignore_variant_stack.truncate(len);
615    }
616
617    fn visit_pat(&mut self, pat: &'tcx hir::Pat<'tcx>) {
618        self.in_pat = true;
619        match pat.kind {
620            PatKind::Struct(ref path, fields, _) => {
621                let res = self.typeck_results().qpath_res(path, pat.hir_id);
622                self.handle_field_pattern_match(pat, res, fields);
623            }
624            PatKind::TupleStruct(ref qpath, fields, dotdot) => {
625                let res = self.typeck_results().qpath_res(qpath, pat.hir_id);
626                self.handle_tuple_field_pattern_match(pat, res, fields, dotdot);
627            }
628            _ => (),
629        }
630
631        intravisit::walk_pat(self, pat);
632        self.in_pat = false;
633    }
634
635    fn visit_pat_expr(&mut self, expr: &'tcx rustc_hir::PatExpr<'tcx>) {
636        match &expr.kind {
637            rustc_hir::PatExprKind::Path(qpath) => {
638                let res = self.typeck_results().qpath_res(qpath, expr.hir_id);
639                self.handle_res(res);
640            }
641            _ => {}
642        }
643        intravisit::walk_pat_expr(self, expr);
644    }
645
646    fn visit_path(&mut self, path: &hir::Path<'tcx>, _: hir::HirId) {
647        self.handle_res(path.res);
648        intravisit::walk_path(self, path);
649    }
650
651    fn visit_anon_const(&mut self, c: &'tcx hir::AnonConst) {
652        // When inline const blocks are used in pattern position, paths
653        // referenced by it should be considered as used.
654        let in_pat = mem::replace(&mut self.in_pat, false);
655
656        self.live_symbols.insert(c.def_id);
657        intravisit::walk_anon_const(self, c);
658
659        self.in_pat = in_pat;
660    }
661
662    fn visit_inline_const(&mut self, c: &'tcx hir::ConstBlock) {
663        // When inline const blocks are used in pattern position, paths
664        // referenced by it should be considered as used.
665        let in_pat = mem::replace(&mut self.in_pat, false);
666
667        self.live_symbols.insert(c.def_id);
668        intravisit::walk_inline_const(self, c);
669
670        self.in_pat = in_pat;
671    }
672}
673
674fn has_allow_dead_code_or_lang_attr(
675    tcx: TyCtxt<'_>,
676    def_id: LocalDefId,
677) -> Option<ComesFromAllowExpect> {
678    fn has_lang_attr(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
679        tcx.has_attr(def_id, sym::lang)
680            // Stable attribute for #[lang = "panic_impl"]
681            || tcx.has_attr(def_id, sym::panic_handler)
682    }
683
684    fn has_allow_expect_dead_code(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
685        let hir_id = tcx.local_def_id_to_hir_id(def_id);
686        let lint_level = tcx.lint_level_at_node(lint::builtin::DEAD_CODE, hir_id).level;
687        matches!(lint_level, lint::Allow | lint::Expect)
688    }
689
690    fn has_used_like_attr(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
691        tcx.def_kind(def_id).has_codegen_attrs() && {
692            let cg_attrs = tcx.codegen_fn_attrs(def_id);
693
694            // #[used], #[no_mangle], #[export_name], etc also keeps the item alive
695            // forcefully, e.g., for placing it in a specific section.
696            cg_attrs.contains_extern_indicator()
697                || cg_attrs.flags.contains(CodegenFnAttrFlags::USED_COMPILER)
698                || cg_attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER)
699        }
700    }
701
702    if has_allow_expect_dead_code(tcx, def_id) {
703        Some(ComesFromAllowExpect::Yes)
704    } else if has_used_like_attr(tcx, def_id) || has_lang_attr(tcx, def_id) {
705        Some(ComesFromAllowExpect::No)
706    } else {
707        None
708    }
709}
710
711// These check_* functions seeds items that
712//   1) We want to explicitly consider as live:
713//     * Item annotated with #[allow(dead_code)]
714//         - This is done so that if we want to suppress warnings for a
715//           group of dead functions, we only have to annotate the "root".
716//           For example, if both `f` and `g` are dead and `f` calls `g`,
717//           then annotating `f` with `#[allow(dead_code)]` will suppress
718//           warning for both `f` and `g`.
719//     * Item annotated with #[lang=".."]
720//         - This is because lang items are always callable from elsewhere.
721//   or
722//   2) We are not sure to be live or not
723//     * Implementations of traits and trait methods
724fn check_item<'tcx>(
725    tcx: TyCtxt<'tcx>,
726    worklist: &mut Vec<(LocalDefId, ComesFromAllowExpect)>,
727    struct_constructors: &mut LocalDefIdMap<LocalDefId>,
728    unsolved_items: &mut Vec<(hir::ItemId, LocalDefId)>,
729    id: hir::ItemId,
730) {
731    let allow_dead_code = has_allow_dead_code_or_lang_attr(tcx, id.owner_id.def_id);
732    if let Some(comes_from_allow) = allow_dead_code {
733        worklist.push((id.owner_id.def_id, comes_from_allow));
734    }
735
736    match tcx.def_kind(id.owner_id) {
737        DefKind::Enum => {
738            let item = tcx.hir_item(id);
739            if let hir::ItemKind::Enum(_, _, ref enum_def) = item.kind {
740                if let Some(comes_from_allow) = allow_dead_code {
741                    worklist.extend(
742                        enum_def.variants.iter().map(|variant| (variant.def_id, comes_from_allow)),
743                    );
744                }
745
746                for variant in enum_def.variants {
747                    if let Some(ctor_def_id) = variant.data.ctor_def_id() {
748                        struct_constructors.insert(ctor_def_id, variant.def_id);
749                    }
750                }
751            }
752        }
753        DefKind::Impl { of_trait } => {
754            if let Some(comes_from_allow) =
755                has_allow_dead_code_or_lang_attr(tcx, id.owner_id.def_id)
756            {
757                worklist.push((id.owner_id.def_id, comes_from_allow));
758            } else if of_trait {
759                unsolved_items.push((id, id.owner_id.def_id));
760            }
761
762            for def_id in tcx.associated_item_def_ids(id.owner_id) {
763                let local_def_id = def_id.expect_local();
764
765                if let Some(comes_from_allow) = has_allow_dead_code_or_lang_attr(tcx, local_def_id)
766                {
767                    worklist.push((local_def_id, comes_from_allow));
768                } else if of_trait {
769                    // FIXME: This condition can be removed
770                    // if we support dead check for assoc consts and tys.
771                    if !matches!(tcx.def_kind(local_def_id), DefKind::AssocFn) {
772                        worklist.push((local_def_id, ComesFromAllowExpect::No));
773                    } else {
774                        // We only care about associated items of traits,
775                        // because they cannot be visited directly,
776                        // so we later mark them as live if their corresponding traits
777                        // or trait items and self types are both live,
778                        // but inherent associated items can be visited and marked directly.
779                        unsolved_items.push((id, local_def_id));
780                    }
781                }
782            }
783        }
784        DefKind::Struct => {
785            let item = tcx.hir_item(id);
786            if let hir::ItemKind::Struct(_, _, ref variant_data) = item.kind
787                && let Some(ctor_def_id) = variant_data.ctor_def_id()
788            {
789                struct_constructors.insert(ctor_def_id, item.owner_id.def_id);
790            }
791        }
792        DefKind::GlobalAsm => {
793            // global_asm! is always live.
794            worklist.push((id.owner_id.def_id, ComesFromAllowExpect::No));
795        }
796        _ => {}
797    }
798}
799
800fn check_trait_item(
801    tcx: TyCtxt<'_>,
802    worklist: &mut Vec<(LocalDefId, ComesFromAllowExpect)>,
803    id: hir::TraitItemId,
804) {
805    use hir::TraitItemKind::{Const, Fn};
806    if matches!(tcx.def_kind(id.owner_id), DefKind::AssocConst | DefKind::AssocFn) {
807        let trait_item = tcx.hir_trait_item(id);
808        if matches!(trait_item.kind, Const(_, Some(_)) | Fn(..))
809            && let Some(comes_from_allow) =
810                has_allow_dead_code_or_lang_attr(tcx, trait_item.owner_id.def_id)
811        {
812            worklist.push((trait_item.owner_id.def_id, comes_from_allow));
813        }
814    }
815}
816
817fn check_foreign_item(
818    tcx: TyCtxt<'_>,
819    worklist: &mut Vec<(LocalDefId, ComesFromAllowExpect)>,
820    id: hir::ForeignItemId,
821) {
822    if matches!(tcx.def_kind(id.owner_id), DefKind::Static { .. } | DefKind::Fn)
823        && let Some(comes_from_allow) = has_allow_dead_code_or_lang_attr(tcx, id.owner_id.def_id)
824    {
825        worklist.push((id.owner_id.def_id, comes_from_allow));
826    }
827}
828
829fn create_and_seed_worklist(
830    tcx: TyCtxt<'_>,
831) -> (
832    Vec<(LocalDefId, ComesFromAllowExpect)>,
833    LocalDefIdMap<LocalDefId>,
834    Vec<(hir::ItemId, LocalDefId)>,
835) {
836    let effective_visibilities = &tcx.effective_visibilities(());
837    // see `MarkSymbolVisitor::struct_constructors`
838    let mut unsolved_impl_item = Vec::new();
839    let mut struct_constructors = Default::default();
840    let mut worklist = effective_visibilities
841        .iter()
842        .filter_map(|(&id, effective_vis)| {
843            effective_vis
844                .is_public_at_level(Level::Reachable)
845                .then_some(id)
846                .map(|id| (id, ComesFromAllowExpect::No))
847        })
848        // Seed entry point
849        .chain(
850            tcx.entry_fn(())
851                .and_then(|(def_id, _)| def_id.as_local().map(|id| (id, ComesFromAllowExpect::No))),
852        )
853        .collect::<Vec<_>>();
854
855    let crate_items = tcx.hir_crate_items(());
856    for id in crate_items.free_items() {
857        check_item(tcx, &mut worklist, &mut struct_constructors, &mut unsolved_impl_item, id);
858    }
859
860    for id in crate_items.trait_items() {
861        check_trait_item(tcx, &mut worklist, id);
862    }
863
864    for id in crate_items.foreign_items() {
865        check_foreign_item(tcx, &mut worklist, id);
866    }
867
868    (worklist, struct_constructors, unsolved_impl_item)
869}
870
871fn live_symbols_and_ignored_derived_traits(
872    tcx: TyCtxt<'_>,
873    (): (),
874) -> (LocalDefIdSet, LocalDefIdMap<FxIndexSet<(DefId, DefId)>>) {
875    let (worklist, struct_constructors, mut unsolved_items) = create_and_seed_worklist(tcx);
876    let mut symbol_visitor = MarkSymbolVisitor {
877        worklist,
878        tcx,
879        maybe_typeck_results: None,
880        live_symbols: Default::default(),
881        repr_unconditionally_treats_fields_as_live: false,
882        repr_has_repr_simd: false,
883        in_pat: false,
884        ignore_variant_stack: vec![],
885        struct_constructors,
886        ignored_derived_traits: Default::default(),
887    };
888    symbol_visitor.mark_live_symbols();
889    let mut items_to_check;
890    (items_to_check, unsolved_items) =
891        unsolved_items.into_iter().partition(|&(impl_id, local_def_id)| {
892            symbol_visitor.check_impl_or_impl_item_live(impl_id, local_def_id)
893        });
894
895    while !items_to_check.is_empty() {
896        symbol_visitor.worklist =
897            items_to_check.into_iter().map(|(_, id)| (id, ComesFromAllowExpect::No)).collect();
898        symbol_visitor.mark_live_symbols();
899
900        (items_to_check, unsolved_items) =
901            unsolved_items.into_iter().partition(|&(impl_id, local_def_id)| {
902                symbol_visitor.check_impl_or_impl_item_live(impl_id, local_def_id)
903            });
904    }
905
906    (symbol_visitor.live_symbols, symbol_visitor.ignored_derived_traits)
907}
908
909struct DeadItem {
910    def_id: LocalDefId,
911    name: Symbol,
912    level: (lint::Level, Option<LintExpectationId>),
913}
914
915struct DeadVisitor<'tcx> {
916    tcx: TyCtxt<'tcx>,
917    live_symbols: &'tcx LocalDefIdSet,
918    ignored_derived_traits: &'tcx LocalDefIdMap<FxIndexSet<(DefId, DefId)>>,
919}
920
921enum ShouldWarnAboutField {
922    Yes,
923    No,
924}
925
926#[derive(Debug, Copy, Clone, PartialEq, Eq)]
927enum ReportOn {
928    TupleField,
929    NamedField,
930}
931
932impl<'tcx> DeadVisitor<'tcx> {
933    fn should_warn_about_field(&mut self, field: &ty::FieldDef) -> ShouldWarnAboutField {
934        if self.live_symbols.contains(&field.did.expect_local()) {
935            return ShouldWarnAboutField::No;
936        }
937        let field_type = self.tcx.type_of(field.did).instantiate_identity();
938        if field_type.is_phantom_data() {
939            return ShouldWarnAboutField::No;
940        }
941        let is_positional = field.name.as_str().starts_with(|c: char| c.is_ascii_digit());
942        if is_positional
943            && self
944                .tcx
945                .layout_of(
946                    ty::TypingEnv::non_body_analysis(self.tcx, field.did)
947                        .as_query_input(field_type),
948                )
949                .map_or(true, |layout| layout.is_zst())
950        {
951            return ShouldWarnAboutField::No;
952        }
953        ShouldWarnAboutField::Yes
954    }
955
956    fn def_lint_level(&self, id: LocalDefId) -> (lint::Level, Option<LintExpectationId>) {
957        let hir_id = self.tcx.local_def_id_to_hir_id(id);
958        let level = self.tcx.lint_level_at_node(DEAD_CODE, hir_id);
959        (level.level, level.lint_id)
960    }
961
962    // # Panics
963    // All `dead_codes` must have the same lint level, otherwise we will intentionally ICE.
964    // This is because we emit a multi-spanned lint using the lint level of the `dead_codes`'s
965    // first local def id.
966    // Prefer calling `Self.warn_dead_code` or `Self.warn_dead_code_grouped_by_lint_level`
967    // since those methods group by lint level before calling this method.
968    fn lint_at_single_level(
969        &self,
970        dead_codes: &[&DeadItem],
971        participle: &str,
972        parent_item: Option<LocalDefId>,
973        report_on: ReportOn,
974    ) {
975        fn get_parent_if_enum_variant<'tcx>(
976            tcx: TyCtxt<'tcx>,
977            may_variant: LocalDefId,
978        ) -> LocalDefId {
979            if let Node::Variant(_) = tcx.hir_node_by_def_id(may_variant)
980                && let Some(enum_did) = tcx.opt_parent(may_variant.to_def_id())
981                && let Some(enum_local_id) = enum_did.as_local()
982                && let Node::Item(item) = tcx.hir_node_by_def_id(enum_local_id)
983                && let ItemKind::Enum(..) = item.kind
984            {
985                enum_local_id
986            } else {
987                may_variant
988            }
989        }
990
991        let Some(&first_item) = dead_codes.first() else {
992            return;
993        };
994        let tcx = self.tcx;
995
996        let first_lint_level = first_item.level;
997        assert!(dead_codes.iter().skip(1).all(|item| item.level == first_lint_level));
998
999        let names: Vec<_> = dead_codes.iter().map(|item| item.name).collect();
1000        let spans: Vec<_> = dead_codes
1001            .iter()
1002            .map(|item| match tcx.def_ident_span(item.def_id) {
1003                Some(s) => s.with_ctxt(tcx.def_span(item.def_id).ctxt()),
1004                None => tcx.def_span(item.def_id),
1005            })
1006            .collect();
1007
1008        let descr = tcx.def_descr(first_item.def_id.to_def_id());
1009        // `impl` blocks are "batched" and (unlike other batching) might
1010        // contain different kinds of associated items.
1011        let descr = if dead_codes.iter().any(|item| tcx.def_descr(item.def_id.to_def_id()) != descr)
1012        {
1013            "associated item"
1014        } else {
1015            descr
1016        };
1017        let num = dead_codes.len();
1018        let multiple = num > 6;
1019        let name_list = names.into();
1020
1021        let parent_info = if let Some(parent_item) = parent_item {
1022            let parent_descr = tcx.def_descr(parent_item.to_def_id());
1023            let span = if let DefKind::Impl { .. } = tcx.def_kind(parent_item) {
1024                tcx.def_span(parent_item)
1025            } else {
1026                tcx.def_ident_span(parent_item).unwrap()
1027            };
1028            Some(ParentInfo { num, descr, parent_descr, span })
1029        } else {
1030            None
1031        };
1032
1033        let encl_def_id = parent_item.unwrap_or(first_item.def_id);
1034        // If parent of encl_def_id is an enum, use the parent ID instead.
1035        let encl_def_id = get_parent_if_enum_variant(tcx, encl_def_id);
1036
1037        let ignored_derived_impls =
1038            if let Some(ign_traits) = self.ignored_derived_traits.get(&encl_def_id) {
1039                let trait_list = ign_traits
1040                    .iter()
1041                    .map(|(trait_id, _)| self.tcx.item_name(*trait_id))
1042                    .collect::<Vec<_>>();
1043                let trait_list_len = trait_list.len();
1044                Some(IgnoredDerivedImpls {
1045                    name: self.tcx.item_name(encl_def_id.to_def_id()),
1046                    trait_list: trait_list.into(),
1047                    trait_list_len,
1048                })
1049            } else {
1050                None
1051            };
1052
1053        let diag = match report_on {
1054            ReportOn::TupleField => {
1055                let tuple_fields = if let Some(parent_id) = parent_item
1056                    && let node = tcx.hir_node_by_def_id(parent_id)
1057                    && let hir::Node::Item(hir::Item {
1058                        kind: hir::ItemKind::Struct(_, _, hir::VariantData::Tuple(fields, _, _)),
1059                        ..
1060                    }) = node
1061                {
1062                    *fields
1063                } else {
1064                    &[]
1065                };
1066
1067                let trailing_tuple_fields = if tuple_fields.len() >= dead_codes.len() {
1068                    LocalDefIdSet::from_iter(
1069                        tuple_fields
1070                            .iter()
1071                            .skip(tuple_fields.len() - dead_codes.len())
1072                            .map(|f| f.def_id),
1073                    )
1074                } else {
1075                    LocalDefIdSet::default()
1076                };
1077
1078                let fields_suggestion =
1079                    // Suggest removal if all tuple fields are at the end.
1080                    // Otherwise suggest removal or changing to unit type
1081                    if dead_codes.iter().all(|dc| trailing_tuple_fields.contains(&dc.def_id)) {
1082                        ChangeFields::Remove { num }
1083                    } else {
1084                        ChangeFields::ChangeToUnitTypeOrRemove { num, spans: spans.clone() }
1085                    };
1086
1087                MultipleDeadCodes::UnusedTupleStructFields {
1088                    multiple,
1089                    num,
1090                    descr,
1091                    participle,
1092                    name_list,
1093                    change_fields_suggestion: fields_suggestion,
1094                    parent_info,
1095                    ignored_derived_impls,
1096                }
1097            }
1098            ReportOn::NamedField => MultipleDeadCodes::DeadCodes {
1099                multiple,
1100                num,
1101                descr,
1102                participle,
1103                name_list,
1104                parent_info,
1105                ignored_derived_impls,
1106            },
1107        };
1108
1109        let hir_id = tcx.local_def_id_to_hir_id(first_item.def_id);
1110        self.tcx.emit_node_span_lint(DEAD_CODE, hir_id, MultiSpan::from_spans(spans), diag);
1111    }
1112
1113    fn warn_multiple(
1114        &self,
1115        def_id: LocalDefId,
1116        participle: &str,
1117        dead_codes: Vec<DeadItem>,
1118        report_on: ReportOn,
1119    ) {
1120        let mut dead_codes = dead_codes
1121            .iter()
1122            .filter(|v| !v.name.as_str().starts_with('_'))
1123            .collect::<Vec<&DeadItem>>();
1124        if dead_codes.is_empty() {
1125            return;
1126        }
1127        // FIXME: `dead_codes` should probably be morally equivalent to `IndexMap<(Level, LintExpectationId), (DefId, Symbol)>`
1128        dead_codes.sort_by_key(|v| v.level.0);
1129        for group in dead_codes.chunk_by(|a, b| a.level == b.level) {
1130            self.lint_at_single_level(&group, participle, Some(def_id), report_on);
1131        }
1132    }
1133
1134    fn warn_dead_code(&mut self, id: LocalDefId, participle: &str) {
1135        let item = DeadItem {
1136            def_id: id,
1137            name: self.tcx.item_name(id.to_def_id()),
1138            level: self.def_lint_level(id),
1139        };
1140        self.lint_at_single_level(&[&item], participle, None, ReportOn::NamedField);
1141    }
1142
1143    fn check_definition(&mut self, def_id: LocalDefId) {
1144        if self.is_live_code(def_id) {
1145            return;
1146        }
1147        match self.tcx.def_kind(def_id) {
1148            DefKind::AssocConst
1149            | DefKind::AssocFn
1150            | DefKind::Fn
1151            | DefKind::Static { .. }
1152            | DefKind::Const
1153            | DefKind::TyAlias
1154            | DefKind::Enum
1155            | DefKind::Union
1156            | DefKind::ForeignTy
1157            | DefKind::Trait => self.warn_dead_code(def_id, "used"),
1158            DefKind::Struct => self.warn_dead_code(def_id, "constructed"),
1159            DefKind::Variant | DefKind::Field => bug!("should be handled specially"),
1160            _ => {}
1161        }
1162    }
1163
1164    fn is_live_code(&self, def_id: LocalDefId) -> bool {
1165        // if we cannot get a name for the item, then we just assume that it is
1166        // live. I mean, we can't really emit a lint.
1167        let Some(name) = self.tcx.opt_item_name(def_id.to_def_id()) else {
1168            return true;
1169        };
1170
1171        self.live_symbols.contains(&def_id) || name.as_str().starts_with('_')
1172    }
1173}
1174
1175fn check_mod_deathness(tcx: TyCtxt<'_>, module: LocalModDefId) {
1176    let (live_symbols, ignored_derived_traits) = tcx.live_symbols_and_ignored_derived_traits(());
1177    let mut visitor = DeadVisitor { tcx, live_symbols, ignored_derived_traits };
1178
1179    let module_items = tcx.hir_module_items(module);
1180
1181    for item in module_items.free_items() {
1182        let def_kind = tcx.def_kind(item.owner_id);
1183
1184        let mut dead_codes = Vec::new();
1185        // Only diagnose unused assoc items in inherient impl and used trait,
1186        // for unused assoc items in impls of trait,
1187        // we have diagnosed them in the trait if they are unused,
1188        // for unused assoc items in unused trait,
1189        // we have diagnosed the unused trait.
1190        if matches!(def_kind, DefKind::Impl { of_trait: false })
1191            || (def_kind == DefKind::Trait && live_symbols.contains(&item.owner_id.def_id))
1192        {
1193            for &def_id in tcx.associated_item_def_ids(item.owner_id.def_id) {
1194                if let Some(local_def_id) = def_id.as_local()
1195                    && !visitor.is_live_code(local_def_id)
1196                {
1197                    let name = tcx.item_name(def_id);
1198                    let level = visitor.def_lint_level(local_def_id);
1199                    dead_codes.push(DeadItem { def_id: local_def_id, name, level });
1200                }
1201            }
1202        }
1203        if !dead_codes.is_empty() {
1204            visitor.warn_multiple(item.owner_id.def_id, "used", dead_codes, ReportOn::NamedField);
1205        }
1206
1207        if !live_symbols.contains(&item.owner_id.def_id) {
1208            let parent = tcx.local_parent(item.owner_id.def_id);
1209            if parent != module.to_local_def_id() && !live_symbols.contains(&parent) {
1210                // We already have diagnosed something.
1211                continue;
1212            }
1213            visitor.check_definition(item.owner_id.def_id);
1214            continue;
1215        }
1216
1217        if let DefKind::Struct | DefKind::Union | DefKind::Enum = def_kind {
1218            let adt = tcx.adt_def(item.owner_id);
1219            let mut dead_variants = Vec::new();
1220
1221            for variant in adt.variants() {
1222                let def_id = variant.def_id.expect_local();
1223                if !live_symbols.contains(&def_id) {
1224                    // Record to group diagnostics.
1225                    let level = visitor.def_lint_level(def_id);
1226                    dead_variants.push(DeadItem { def_id, name: variant.name, level });
1227                    continue;
1228                }
1229
1230                let is_positional = variant.fields.raw.first().is_some_and(|field| {
1231                    field.name.as_str().starts_with(|c: char| c.is_ascii_digit())
1232                });
1233                let report_on =
1234                    if is_positional { ReportOn::TupleField } else { ReportOn::NamedField };
1235                let dead_fields = variant
1236                    .fields
1237                    .iter()
1238                    .filter_map(|field| {
1239                        let def_id = field.did.expect_local();
1240                        if let ShouldWarnAboutField::Yes = visitor.should_warn_about_field(field) {
1241                            let level = visitor.def_lint_level(def_id);
1242                            Some(DeadItem { def_id, name: field.name, level })
1243                        } else {
1244                            None
1245                        }
1246                    })
1247                    .collect();
1248                visitor.warn_multiple(def_id, "read", dead_fields, report_on);
1249            }
1250
1251            visitor.warn_multiple(
1252                item.owner_id.def_id,
1253                "constructed",
1254                dead_variants,
1255                ReportOn::NamedField,
1256            );
1257        }
1258    }
1259
1260    for foreign_item in module_items.foreign_items() {
1261        visitor.check_definition(foreign_item.owner_id.def_id);
1262    }
1263}
1264
1265pub(crate) fn provide(providers: &mut Providers) {
1266    *providers =
1267        Providers { live_symbols_and_ignored_derived_traits, check_mod_deathness, ..*providers };
1268}